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Radio sources observed in the decametric range during type II and type III solar radio bursts are believed to be produced by coherent plasma emission due to electrostatic plasma oscillations induced by propagating suprathermal electrons (e.g. Ginzburg & Zhelezniakov 1958).
This type of emission is a valuable tool for observational diagnostics of the upper corona. Produced at the local plasma frequency, $f_{pe}\rm{[kHz]}= 8.93 (n_e\rm{[cm}^{-3}\rm{]})^{-1/2}$
or its harmonic, plasma emission can reveal the electron density structure of the upper corona. Early estimations of heliocentric distances of the sources in solar radio bursts showed they often appear further away from the Sun than expected from canonical coronal density models. This effect was interpreted as a result of enhanced density in the corona over the active regions (e.g. Wild et al. 1959). However, some argued that abnormal positions of the observed sources are due to scattering and refraction of radio-waves in the corona (e.g. Aubier et al. 1971). Indeed, the corona is not fully transparent for lower frequency radio waves: density inhomogeneities can scatter and refract propagating waves, affecting the apparent positions and sizes of the observed sources. Recently, McCauley et al. (2018) showed that coronal density enhancement cannot fully explain positions of radio sources in some type III radio-bursts observed with the MWA radio array in the range 80-240 MHz. Another recent work, by Chrysaphi et al. (2018), demonstrated that apparent positions of solar radio sources in a type II burst observed by LOFAR are consistent with the presence of strong radio-wave scattering. The aim of this study is to evaluate heliocentric distances of solar radio-sources at different frequencies, and compare them with the positions predicted by the Newkirk coronal density model.

Results

We use LOFAR observations of 12 different solar sources in the frequency range 30-48 MHz. Positional measurements are calibrated using several Tau A observations performed with the same observational set up. Calibration shows that, after applying the ionospheric refraction correction, the error of measurements of centroid position is 150 arcsec or less (Figure 1), which is applied as an error value for all solar observations at all frequencies.

Figure 1. Centroid positions of Tau A observed at different zenith angles at different frequencies. Black symbols show actual positions, colour symbols show positions corrected for ionospheric refraction. Different colours correspond to different frequencies, from 30 MHz (red) to 48 MHz (blue). 

Our analysis shows that 3 of the 12 observed solar sources appear at heliocentric distances, which are substantially larger than those predicted by the Newkirk density model (Figure 2) or other canonical models (see Gordovskyy et al. 2019). The projection effect cannot explain these abnormal distances: non-90o projection angles would mean that the sources are located even further away from the Sun. There are three possible explanations for the observed effect: enhanced coronal densities, scattering and refraction of the radio-waves, and harmonic radio emission. The latter is very unlikely: in two of the “abnormal” sources emission is strongly polarised, with the polarisation degree up to 70-80%, which is inconsistent with the harmonic emission. Scattering of radio-waves in the turbulent corona can “shift” the apparent source positions by the observed value (0.2-0.7 Rsun), although it is not clear whether it can explain the observed flattening of the frequency-distance functions. The latter, however, can be explained by the density changes in the corona: active events in the corona should result in fast plasma motions and evaporation, which, in turn, would enhance the coronal density and reduce stratification. Enhanced hydrodynamic scale length in the upper corona would explain the flattening of the frequency-distance functions for the observed sources.

Figure 2. Frequency-distance functions for three “abnormal” sources. Symbols show apparent source positions with the error bars. Solid black lines shows positions predicted by the Newkirk density model. Solid blue lines show positions predicted by the Newkirk model in presence of strong turbulence. Corresponding dotted lines show positions predicted by the densities x2 higher and lower than the mean densities (solid lines). 

One of the “abnormal” events, observed on 25 June 2015, has been previously studied by Chrysaphi et al. (2018). They analysed this event at ~10:45UT, when its dynamic spectrum revealed band-splitting, indicating the presence of a shock. We, however, consider a different stage of this event, observed approximately 1 hour later, consisting of numerous type-III-like bursts (Figure 3). Interestingly, both studies yield very similar frequency-distance diagrams. This makes the radio-wave scattering in the corona the most viable interpretation for the observed effect: indeed, in the presence of strong scattering, apparent heliocentric distances to the sources should depend mostly on the characteristics of plasma turbulence, while the intrinsic positions of the sources are not important.

Figure 3. The dynamic spectrum (left) and the intensity map (right) for the 25 June 2015 event observed at 12:08UT.

Conclusions Based on our analysis, we conclude that the observed abnormal locations of solar radio-sources observed by LOFAR are likely to be caused by strong radio-wave scattering due to plasma turbulence in the active corona, as well as the enhanced coronal density.

Based on the recent paper by M.Gordovskyy, E.P. Kontar, P.K. Browning and A.A. Kuznetsov “Frequency-Distance Structure of Solar Radio Sources Observed by LOFAR”, 2019, Astrophysical Journal, 873, 48, doi: 10.3847/1538-4357/ab03d8

References
Aubier, M., Leblanc, Y. and Boischot, A., 1971, A&A, 12, 435.
Chrysaphi, N., Kontar, E.P., Holman, G.D. and Temmer, M., 2018, ApJ, 868, 79.
Ginzburg, V.L., Zhelezniakov, V.V., 1958, Sov.Astr, 2, 653.
McCauley, P.I., Cairns, I.H. and Morgan, J., 2018, Sol.Phys., 293, 132.
Wild, J.P., Sheridan, K.V. and Neylan, A.A., 1959, Aust.J.Phys., 12, 369.

Full list of authors: Mykola Gordovskyy, Eduard Kontar, Philippa Browning and Alexey Kuznetsov.

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Long before satellite communications, high frequency (HF, 3–30 MHz) radio was the primary method for long distance, over-the-horizon wireless communications. HF signals are able to travel long distances by refracting off of the ionosphere in what is known as “skip” or “skywave” propagation. Aside from a transmitter and receiver, no additional technological infrastructure is needed. Because of this, even in the modern age of space‐borne relays and widely distributed Internet availability, HF radio remains a key technology for long‐distance communications. It is actively used by aircraft, ships at sea, military operations, disaster relief efforts, and amateur (ham) radio operators.

The ionosphere is a weakly ionized plasma located between ~60 to 1000 km above the Earth’s surface. It is primarily generated by solar ultraviolet energy ionizing the neutral atmosphere, but ionization can also result from auroral particle precipitation, solar energetic particles, solar x-ray radiation, and more. Additionally, geomagnetic activity and lower atmospheric processes can affect ionospheric density and composition. All of these factors affect HF radio propagation, which may either be advantageous or harmful from a communications point of view. All of this is advantageous in terms of science, as these changes in radio propagation provide an opportunity for remote sensing not only the ionosphere, but the entire coupled geospace system.

Traditionally, the ionosphere is studied using research-grade instruments such as ionosondes, SuperDARN radars, incoherent scatter radars, riometers, and networks of GPS total electron content (GPS TEC) receivers. However, even with all of this instrumentation, the ionosphere is vast and remains undersampled. Recently, it has been shown that the amateur radio community has been making global-scale observations of HF communications for almost an entire solar cycle, and these observations can be used for both ionospheric study and for investigating the impacts of space weather on the terrestrial HF communications systems. Frissell et al. (2019) looks at the impacts of solar flares and geomagnetic storms during a particularly disturbed period in September 2017. Here, we highlight the solar flare/radio blackout event shown in that paper.

Amateur Radio Observations

Amateur (ham) radio operators are licensed hobbyists with an interest in radio communications, engineering, and science. Hams are of all ages and come from all walks of life, and earn their license by passing a knowledge exam given by their respective national government. There are about 820,000 hams in the United States, and over 3 million worldwide. Using technologies such as software define radios, personal computers, and the Internet, hams have built systems that will automatically observe and log certain types of global HF radio communications. These systems include the Reverse Beacon Network (RBN, reversebeacon.net), Weak Signal Propagation Reporter Network (WSPRNet, wsprnet.org), and Phase Shift Keying Reporter (PSKReporter, pskreporter.info). Data from these networks is available either directly from their website, or by contacting the network’s owner. The plots below combine data from both the RBN and WSPRNet.

6 September 2017 Radio Blackout

Solar flares cause radio blackouts because their high amounts of X-ray energy cause a sudden increase in ionization in the lowest layers of the ionosphere, primarily the D region (~60-90 km altitude) (Dellinger, 1937; Chakraborty et al., 2018). Unlike ionospheric regions at higher altitudes which are often approximated as collisionless and generally refract HF rays back to Earth, the D region electron-neutral collision frequency is high causing the HF radio waves to be absorbed when D region densities are elevated (Benson, 1964; McNamara, 1979). Therefore, the X-ray energy from large solar flares can cause HF radio blackouts through a process known as collisional damping.

Figure 1: Amateur radio reporting network results for the (a) 15 min prior to and (b) 15 min following the X9.3 solar flare on 6 September 2017 1153 UT. The propagation paths are color‐coded based on the amateur radio frequency on which the report occurred. The gray and white background shows the diurnal boundary. A reduction in reports can be seen across all frequencies with 7 MHz (dark orange), 14 MHz (bluish green), and 21 MHz (light orange) being the most affected across Europe. (Figure from Frissell et al. 2019)

Figure 1 shows the dramatic impact a solar flare radio blackout can have on HF radio communications. Figure 1a shows RBN and WSPRNet amateur radio communications observed globally for a 15 minute period prior to an X9.3 class flare observed by the GOES spacecraft on 6 Sept 2017 at 1153 UT. A total of 12824 HF radio paths are observed across 6 different frequency bands. A dramatic decrease (82% reduction) in the number of communications can be seen in Figure 1b, the 15 minute period following the flare.

Figure 2 shows a detailed time series of communications over Europe for both the 1153 UT X9.3 class flare and an X2.2 class flare that occurred at 0857 UT, just a few hours earlier. Figure 2a uses the Kp and Sym-H index to show that these flares took place during a period of geomagnetic quiet. Figure 2b shows GOES observations of the solar flares. Figure 2c-f shows histograms of communications distances versus time and spatial maps for each of four major amateur radio bands: 28, 21, 14, and 7 MHz. The location of each communication is chosen as the midpoint between the transmitter and receiver. Immediately following the two X-ray flares, communications cease almost entirely on all bands. Europe was in daylight for both of these events; the white dashed line on each of the time series histograms shows the solar zenith angle for the center of the observations (marked by a yellow star on each map).

Figure 2: Space weather environment and HF radio response over Europe on 6 September 2017 0600–1800 UT. (a) SYM‐H (black line) and Kp (colored stems). (b) GOES‐13 (blue) and GOES‐15 (orange) XRS 0.1–0.8 nm X‐ray measurements. Flares are observed at 0857 UT (X2.2) and 1153 UT (X9.3) and indicated with dotted vertical lines. (c–f) Two‐dimensional contour histograms of RBN and WSPRNet spot data for the 28‐, 21‐, 14‐, and 7‐MHz amateur radio bands, respectively. Bin size is 250 km × 10 min. To the left of each histogram is a map showing the log density of TX‐RX midpoints of all spots used in the histogram. The white dashed lines on the histograms show the solar zenith angle computed for (51° N, 8° E), the point indicated by the yellow star on each map. Radio blackouts across the HF bands can be seen in response to the solar flares in the GOES data. GOES = Geostationary Operational Environmental Satellite; NOAA = United States National Oceanic and Atmospheric Administration; RBN = Reverse Beacon Network; WSPRNet = Weak Signal Propagation Reporting Network. (Figure from Frissell et al. 2019)

Both Figure 1 and Figure 2 show how quickly and dramatically a solar flare can impact HF communications. Radio blackouts are particularly difficult because they are generally impossible to predict. Since the flare X-ray energy travels at the speed of light, we can only know the flare has occurred once it has already arrived. Fortunately, the recombination time of the D region is relatively fast, and communications can resume within just a few hours. Also, solar flares primarily affect only the dayside ionosphere; Frissell et al. (2019) shows a corresponding figure to Figure 2 that shows United States communications were barely affected by the flares because the US was on the dawn flank.

Conclusions

Amateur radio networks provide a powerful method for observing global-scale space weather impacts on the ionosphere and terrestrial HF communications. Here, we highlighted the effects of a solar flare. But Frissell et al. (2019) also shows the more complicated ionospheric response to geomagnetic storms. Similarly, Frissell et al. (2018) used the same data sets and a large-scale, coordinated citizen science campaign to study the ionospheric impacts of the 21 August 2017 solar eclipse. Here, the amateur radio data were combined with forward modeling techniques to relate the observations to physical ionospheric parameters.

To continue and improve upon this work, the Ham Radio Science Citizen Investigation (hamsci.org) has been created. HamSCI is an international collective of both professional researchers and amateur radio operators working together to solve problems in ionospheric and space physics, as well as bring added enjoyment to the amateur radio hobby.

Based on the recent paper: Frissell, N. A., Vega, J. S., Markowitz, E., Gerrard, A. J., Engelke, W. D., Erickson, P. J., et al. (2019). High‐frequency communications response to solar activity in September 2017 as observed by amateur radio networks. Space Weather, 17, 118– 132. https://doi.org/10.1029/2018SW002008

Acknowledgements

NAF acknowledges the support of NSF Grant AGS‐1552188/479505‐19C75. We are especially grateful to the amateur radio community who voluntarily produced and provided the HF radio observations used in this paper, especially the operators of the Reverse Beacon Network (RBN, reversebeacon.net), the Weak Signal Propagation Reporting Network (WSPRNet, wsprnet.org), qrz.com, and hamcall.net. The Kp index was accessed through the OMNI database at the NASA Space Physics Data Facility (https://omniweb.gsfc.nasa.gov/). The SYM‐H index was obtained from the Kyoto World Data Center for Geomagnetism (http://wdc.kugi.kyoto-u.ac.jp/). GOES data are provided by NOAA NCEI (https://satdat.ngdc.noaa.gov/).

References

Benson, R. F. (1964). Electron collision frequency in the ionospheric D region. Journal of Research of the National Bureau of Standards, Section D: Radio Science, 68D(10), 1123–1126.

Chakraborty, S., Ruohoniemi, J. M., Baker, J. B. H., & Nishitani, N. (2018). Characterization of short‐wave fadeout seen in daytime SuperDARN ground scatter observations. Radio Science, 53, 472–484. https://doi.org/10.1002/2017RS006488

Dellinger, J. H. (1937). Sudden disturbances of the ionosphere. Proceedings of the Institute of Radio Engineers, 25(10), 1253–1290. https://doi.org/10.1109/JRPROC.1937.228657

Frissell, N. A., Katz, J. D., Gunning, S. W., Vega, J. S., Gerrard, A. J., Earle, G. D., et al. (2018). Modeling amateur radio soundings of the ionospheric response to the 2017 great American eclipse. Geophysical Research Letters, 45, 4665– 4674. https://doi.org/10.1029/2018GL077324

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Coronal mass ejections (CMEs) are sometimes accompanied by continuum emission at decimetric and metric wavelengths, called Type IV radio bursts. Of particular interest to determining CME properties are moving Type IV radio bursts, which are broadband radio sources moving outwards from the Sun. First studies suggested that moving Type IV bursts are emitted by synchrotron or gyro-synchrotron emitting electrons that are trapped inside CME loops (Dulk 1973). However, since their discovery, a number of stationary Type IV radio sources have also been observed, believed to be generated by the plasma emission mechanism (Benz & Tarnstrom 1976), while some moving Type IV bursts have also been identified as plasma emission (Gary et al. 1985). So far, the emission mechanism of Type IV bursts seems to vary on a case-by-case basis and it is not yet clear what leads to the generation of various Type IV radio bursts.

Figure 1 – Radio bursts associated with the 22 September 2011 flare and CME. (a) Type II and continuum emission observed by the ARTEMIS spectrograph at frequencies <200 MHz. (b) Type IV bursts observed by e-CALLISTO BLEIEN at 200–400. (c) Microwave burst observed by the Ondrejov Spectrograph (0.8–1 GHz) and Phoenix3 spectrometer (1–2 GHz). (d) GOES X-ray light curve of the X1.4 flare on 22 September 2011

On 22 September 2011, an X1.4 class flare occurred on the eastern limb of the visible solar disc, which was accompanied by a fast CME. The flare and CME were associated with radio emission at frequencies ranging from a few hundred kHz to tens of GHz, observed by multiple instruments. The flare and CME were accompanied by Type III radio bursts, a Type II radio burst and continuum-like emission at frequencies <100 MHz, and a broad continuum at higher frequencies (Figure 1). In this study, we analyse the emission mechanism and origin of a Type IV radio burst at frequencies of 200–400 MHz observed in radio images from the Nançay Radioheliograph (NRH; Figure 2).

We identified two components in the Type IV radio burst: an earlier stationary Type IV showing a large extended source located on top of the post-eruption flare loops (first Type IV in Figure 2 at 10:40-10:46 UT), and a later moving Type IV burst covering the same frequency band with a more compact source, moving outwards behind the CME (second Type IV in Figure 2 after 10:46 UT). The first Type IV has a low flux density (∼100 sfu, where 1 sfu = 10−22 W m2 Hz−1 ) and it is unpolarised. The second Type IV emission is significantly brighter (∼600 sfu), consists of bursty components across NRH frequencies and it is up to 80% circularly polarised.

Figure 2 – Radio source locations observed by the NRH associated with the 22 September 2011 CME. The radio sources (filled contours) are overlaid on AIA 211 Å running-difference images. The radio sources are shown from top to bottom at three different frequencies of 228, 270, and 327MHz and from left to right at three different times of 10:36, 10:44, and 10:52 UT.

Flux density spectra of the Type IV burst were computed through time in order to determine its emission mechanism. We combined the flux density of the individual Type IV sources from NRH images with flux calibrated data from the Phoenix3 spectrometer and RSTN of the full solar disc (Figure 3). The high frequency emission (>1000 MHz), corresponding to the microwave burst in Figure 1, shows a typical gyro-synchrotron spectrum in all panels of Figure 3. The Type IV observed at NRH frequencies appears to be unrelated to the microwave burst and shows changing behavior through time.

Figure 3 – Flux density spectrum during the occurrence of the Type IV radio burst at (a) 10:41 UT, (b) 10:44 UT, and (c) 10:49 UT. The NRH flux densities are represented by red circles, fitted with a power-law function to obtain the spectral index. The RSTN data are represented by orange triangles fitted by a gyro-synchrotron function. The Phoenix3 data points are represented by blue inverted triangles.

The flux density spectra confirm that there are indeed two separate components as the emission mechanism of the Type IV continuum appears to change over time (Figure 3). At 10:41:00 UT, the flux density of the first Type |V component increases with frequency (red circles in Figure 3a), while at 10:49:00 UT, the flux density of the second Type IV component decreases with frequency (red circles in Figure 3c). The increase and decrease of flux with frequency has a power-law behaviour. The spectral index of the Type IV in the NRH frequency range is initially negative with a value of $\alpha$=1.56, which falls in the range of gyro-synchrotron spectral indices (Nita et al., 2004) and is therefore indicative of gyro-synchrotron emission. The spectral index then progressively flattens until it becomes positive in the case of the second Type IV component, which is indicative of plasma emission.

The first component of the Type IV burst, which is stationary, is most likely emitted by gyro-synchrotron electrons that are trapped inside post-eruption flare loops. The second component of the Type IV burst has opposite characteristics: it is moving, it has consistently negative spectral indices over time, higher brightness temperature and bursty, highly-polarised emission. Therefore, the second Type IV component is most likely emitted by a coherent emission mechanism following the acceleration of electrons in the wake of the CME, such as plasma emission or electron-cyclotron maser (ECM) emission.

Conclusions

The flare and CME on 22 September 2011 were accompanied by numerous radio bursts, including a prominent Type IV continuum that showed both stationary and moving components. Most interestingly, the emission mechanism of the observed Type IV continuum was found to change over time from showing gyro-synchrotron emission of electrons trapped inside post eruption loops to plasma or ECM emission from electrons accelerated behind the CME. 

Additional info

Based on recent publication, Morosan et al., 2019, A&A, 623, A63. https://doi.org/ 10.1051/0004-6361/201834510

References

Dulk, G. A. 1973, Sol. Phys., 32, 491

Benz, A. O., & Tarnstrom, G. L. 1976, ApJ, 204, 597

Gary, D. E., Dulk, G. A., House, L. L., et al. 1985, A&A, 152, 42

Nita, G. M., Gary, D. E., & Lee, J. 2004, ApJ, 605, 528

*Full list of authors: Diana E. Morosan, Emilia K. J. Kilpua, Eoin P. Carley and Christian Monstein

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Solar activity is often accompanied by solar radio emission. At low frequencies, radio bursts with short durations of <1 s, known as solar S-bursts, have been identified. These intriguing, low frequency bursts were first identified by McConnell (1982) who named them solar S-bursts, owing to their similarity to Jovian S-bursts: the S stands for short or storm. S-bursts appear as narrow tracks on a dynamic spectrum that usually drift from high to low frequencies, and in rarer cases, from low to high frequencies.

Here, we present complimentary observations from the Ukrainian T-shaped Radio telescope (UTR-2) and the LOw Frequency ARray (LOFAR) which give us new insight into their nature. The exact emission mechanism behind the generation of S-bursts remains a topic of debate, however, they are believed to be emitted at the plasma frequency. There are currently two leading interpretations of S-burst generation that were proposed by Zaitsev & Zlotnik (1986) and Melnik et al. (2010).

The model put forward by Zaitsev & Zlotnik (1986), suggests that plasma waves near the upper hybrid resonance frequency are excited owing to electrons propagating through an anisotropic plasma within a quasihomogeneous magnetic field. These plasma waves are then scattered by thermal ions to produce electromagnetic radiation at the local plasma frequency, fp. More recently, Melnik et al. (2010) proposed a new mechanism of S-burst generation involving beams of particles (electrons or protons) in resonance with right-hand (RH) circularly polarised waves. This takes place against a background of Langmuir turbulence. The RH waves interact with the Langmuir waves to produce radio emission close to the local plasma frequency that we register as S-bursts.

In this work, the spectral properties of over 3000 S-bursts were analysed to test the validity of the proposed emission mechanisms. We present evidence that the model of Melnik et al. (2010) can account for the observed spectral properties of S-bursts. It is shown that S-bursts can provide an alternative method of estimating the coronal magnetic field strength at various altitudes of the solar atmosphere.

Observations & Results

Figure 1 shows images of the instruments used and a sample of our data from UTR-2 that contains S-bursts. The following properties of the bursts were recorded: start time, end time, duration, flux, start frequency, end frequency, bandwidth, drift rate, full-width-half-max duration, and instantaneous bandwidth. S-bursts were found to have short durations of 0.5-0.9 s. For the first time, we show a linear relation between the instantaneous bandwidth and frequency of S-bursts over a wide frequency band, extending the previous result of Melnik et al. (2010). The flux calibration and high sensitivity of UTR-2 enabled measurements of their fluxes, which yielded 11±3 solar flux units (1 SFU ≡ 104 Jy). The source particle velocities of S-bursts were found to be ∼0.07 c.

Figure 1 – Panel (a): Dipole elements that make up UTR-2. UTR-2 consists of 2040 of these dipoles. In the N-S direction, 1440 elements are spread over 240 rows, while 600 elements are spread over the E-W direction giving the telescope a total area of ∼1.4 ×105 m2 (Konovalenko et al. 2016). Panel (b): The heart of the LOFAR core, known as the Superterp (Haarlem et al. 2013). Panel (c): Dynamic spectrum obtained from UTR-2 on 9 July 2013 containing examples of type III, type IIIb, and S-bursts. The highlighted region is shown in greater detail below, indicating the short duration of the bursts (<1 s).

It was found that the model of Zaitsev & Zlotnik (1986) is unable to account for the characteristics of S-bursts that are commonly observed at decametre wavelengths. The bursts modelled by Zaitsev & Zlotnik (1986) represent very narrow-band bursts (<1 MHz) that have central frequencies of ∼254 MHz and total bandwidths that are approximately equal to their instantaneous bandwidths. The model assumes a static source that attributes the frequency drift rate to the electromagnetic wave group delay. However, the S-bursts presented in this work display long lasting saber-shaped features which may extend in frequency by up to 12 MHz (Melnik et al. 2010). Additionally, their total bandwidths are much greater than their instantaneous bandwidths, indicating a dynamic source.

Our observations support for the core assumptions contained within the model of Melnik et al. (2010). For example, the long-lasting sabre shaped structure of S-bursts and their appearance against the background of other types of radio activity (indicating a turbulent atmosphere) were consistent throughout the data. The linear relation between frequency and instantaneous bandwidth provided further support. Additionally, evidence was observed that S-bursts are produced by moving particles, as opposed to the stationary sources proposed by Zaitsev & Zlotnik (1986).

The Melnik et al. (2010) model can enable us to remote sense of the coronal magnetic field. Given that the model assumes emission close to the plasma frequency, we can estimate the source heights for each burst. Figure 2 shows how the plasma frequency varies with height for various density models. Figure 2b gives an example of how we estimated the source height of an S-burst with a frequency of ∼19 MHz. As shown, we calculated a minimum and maximum height and then used the average (dashed line) of these heights as our estimation. Figure 2c shows the magnetic field strengths found using the model of Melnik et al. (2010) at each of the source height estimates. These magnetic field strengths ranged from 0.9-5.8 G between heights of ∼1.3 R⊙ to 2 R⊙.

Figure 2 – Panel (a): Electron density vs. heliocentric height for the various active region electron density models. Panel (b): The plasma frequency vs. heliocentric height for each density model. An example of how we estimated the source height of an S-burst with a frequency of ∼19 MHz is shown. Panel (c): The magnetic field strengths predicted by Melnik et al. (2010) at each of the source height estimates according to our data.

To verify the accuracy of the Melnik et al. (2010) model magnetic field strengths, we compared the magnetic field at 1.02 R⊙ according to a potential field source surface (PFSS) extrapolation with the value found at the same height obtained via the extrapolation of the fitted data. The values were found to be ∼332 G and ∼389 G, respectively. These fields are in good agreement indicating that the fit to the data may provide us with the ability to conduct remote sensing of the coronal magnetic field on the day of the observations.

Conclusions

The spectral properties of S-bursts were measured using UTR-2 and LOFAR. Leading theories of S-burst generation were investigated. It was noted that the model of Zaitsev & Zlotnik (1986) is unable to account for the properties of S-bursts that are commonly observed at decametre wavelengths. It was found that the magnetic field strengths at the source heights of S-bursts ranged from 0.9-5.8 G. This Melnik et al. (2010) model can account for the observed spectral properties of S-bursts and produced magnetic fields that are in good agreement with observations and coronal magnetic field models.

References

Konovalenko, A., Sodin, L., Zakharenko, V., et al. 2016, Exp. Astron., 42, 11

McLean, D. J. D., Dulk, G. A. G., & McLean, D. J. D. 1978, Sol. Phys., 57, 51 279

McConnell, D. 1982, Sol. Phys., 78, 253

Melnik, V. N., Konovalenko, A. A., Rucker, H. O., et al. 2010, Sol. Phys., 264, 53 103

Newkirk, Jr., G. 1961, ApJ, 133, 983

Newkirk, Jr., G. 1967, Annu. Rev. Astron. Astrophys., 5, 213

Zaitsev, V. V., & Zlotnik, E. Y. 1986, Sov. Astron. Lett., 12, 311

Zucca, P., Carley, E. P., Bloomfield, D. S., et al. 2014, A&A, 564, A47

Based on: Brendan P. Clarke et al., 2019, A&A, 622, A204

*Full list of authors: Brendan P. Clarke, Diana E. Morosan, Peter T. Gallagher, Vladimir V. Dorovskyy, Alexander A. Konovalenko, & Eoin P. Carley

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Solar flares involve the sudden release of magnetic energy in the solar corona. Accelerated nonthermal electrons have been often invoked as the primary means for transporting the bulk of the released energy to the lower solar atmosphere. However, significant challenges remain for this scenario, especially in accounting for the large number of accelerated electrons inferred from observations. Propagating magnetohydrodynamics (MHD) waves, particularly those with subsecond/second-scale periods, have been proposed as an alternative means for transporting the released flare energy likely alongside the electron beams[1-3], while observational evidence remains elusive. Here we report possible observational evidence for these subsecond-period fast-mode MHD waves[4] in the late impulsive phase of a two-ribbon flare. This is based on ultra-high cadence dynamic imaging spectroscopic observations of a peculiar type of decimetric (“dm-λ”) radio bursts obtained by the Karl G. Jansky Very Large Array (VLA).

Decimetric Radio Bursts Associated with (E)UV Brightenings

The dm-λ bursts were observed by the VLA in 1.0-1.5 GHz during a GOES-class C7.2 solar flare [5], shown as Figure 1(A). Two main episodes can be distinguished in the dynamic spectrum (Figure 1(B)). The bursts appear as multiple arch-shaped emission lanes in the dynamic spectrum, which display a low-high-low frequency drift pattern with a moderate relative frequency drift rate of $\dot{\nu}/\nu\lesssim 0.2s^{-1}$. The drift rate is about one order of magnitude lower than type III radio bursts emitted by beams of fast electrons, but similar to fiber bursts and lace bursts in the same frequency range. Such bursts with an intermediate frequency drift rate are sometimes referred to as “intermediate drift bursts”[6].

Figure 1: (A) Composite EUV image of SDO/AIA showing the flare context. Contours are 12-25 keV HXR emission by RHESSI during the early flare impulsive phase. (B) VLA cross-power dynamic spectrum showing the dm-λ bursts. (C) AIA 304 Å background-detrended image sequence at times marked by the black vertical arrows in (B), showing the EUV ribbon brightenings near the radio sources. Red contours are the radio sources that correspond to the bursts (the time and frequency are marked in the dynamic spectrum of panel A as red circles). Green arrows indicate the location of the transient EUV brightening. (D-G) Detrended dynamic spectra of emission lanes of the second dm-λ burst, showing the subsecond-scale oscillations in the emission frequency.

Radio imaging of the bursts places the burst source (red contours) near the northern flare ribbon, as shown in Figure 1(C). Background in Figure 1(C) is the detrended AIA 304 Å image sequence during the same time interval of the radio dynamic spectrum shown in panel B. During this period, the northern ribbon is featured by the appearance of two transient EUV brightenings during radio bursts, and the location of the brightenings is very close to the radio source. The appearance of the radio source during the flare impulsive phase, as well as its close spatial and temporal association with the ribbon brightenings, suggests that the radio source is intimately related to the release and transport of the flare energy.

More detailed inspection of the dynamic spectral features of the stronger burst reveals multitudes of very short, subsecond-scale fine structures on each emission lane. Figure 1(D)-(G) show emission lanes for the burst that have been detrended to remove their overall frequency drift pattern. The bursts appear to oscillate in their emission frequency around the central “ridge” of the emission lane quasi-periodically with period of ~0.3-1 s and damping times of ~0.5-5 s in amplitude.

Radio Dynamic Spectroscopic Imaging

The capability of simultaneous imaging and dynamic spectroscopy offered by the VLA allows each pixel in the dynamic spectrum to form a radio image. The radio source first moves toward the flare ribbon as frequency increases until it reaches the maximum frequency at the lowest height, and then bounces back to the opposite direction away from the ribbon as frequency decreases. The average speed in projection is ~1000-2000 km s-1, which is typical for propagating Alfvén or fast-mode magnetosonic waves in the low corona. This is a strong indication for the radio emission being associated with a propagating Alfvén or fast-mode MHD disturbance in a magnetic tube in the close vicinity of the flare ribbon.

Nature of the propagating radio source

We interpret the observed radio bursts as short-period, weakly compressible fast-mode magnetosonic wave packets, likely triggered by the impulsive flare energy release, propagating along newly reconnected magnetic flux tubes (serving as waveguides) linking to the flare ribbon (see Figure 2 for a schematic). The observed reflection of the waves at or near the flare ribbon may be due to sharp gradients at and/or below the transition region. The small-amplitude oscillations in frequency can be interpreted as weak density perturbations associated with the propagating MHD waves. As stated in the Introduction section, subsecond-period MHD waves may be a viable mechanism responsible for transporting a substantial amount of the magnetic energy released in the corona downward to the lower atmosphere, resulting in intense plasma heating and/or particle acceleration. From the observed density oscillations and the source speed, we estimate that these wave packets carry an energy flux of 2-8×108 erg s-1 cm-2, which is comparable to the average energy flux required for driving the flare heating during the late impulsive phase of the flare estimated from the UV ribbon brightenings.

Figure 2: Schematic illustration of the observed radio bursts of interest. The impulsive energy release associated with the filament eruption and the two-ribbon flare generates ubiquitous MHD disturbances, some of which propagate along newly reconnected field lines in the form of MHD wave packets that contain multiple subsecond-period oscillations. Electrons trapped or accelerated within these wave packets generate Langmuir waves and convert to radio emission. Some of the wave packets can reflect at or near the flare ribbon due to sharp gradients, resulting in the observed spatial motion of the radio source and the low-high-low frequency drift pattern of the radio burst in the dynamic spectrum. The (E)UV brightenings at the flare ribbon may be associated with heating by the precipitated energetic electrons or the deposited wave energy.

Conclusions

We report possible detection of subsecond-period propagating MHD Waves in post-reconnection magnetic loops during the late impulsive phase of a two-ribbon flare, based on imaging spectroscopic observations of a peculiar type of dm-λ radio bursts recorded by the VLA. The radio source, propagating at 1000–2000 km s-1 in projection, shows close spatial and temporal association with transient brightenings on the flare ribbon. In addition, multitudes of subsecond-period oscillations are present in the radio emission. We interpret the observed radio bursts as short-period fast-mode MHD wave packets propagating along newly reconnected magnetic flux tubes linking to the flare ribbon. The estimated energy flux (2-8×108 erg s-1 cm-2) carried by the waves is comparable to that needed to account for the plasma heating during the late impulsive phase of this flare.

Recently published in The Astrophysical Journal, 2019, 872, 1, id. 71 (16 pp.), doi: 10.3847/1538-4357/aaff6d

Reference

[1] Temperature minimum heating in solar flares by resistive dissipation of Alfven waves, Emslie, A. G., & Sturrock, P. A. 1982, SoPh, 80, 99

[2] Impulsive Phase Flare Energy Transport by Large-Scale Alfvén Waves and the Electron Acceleration Problem, Fletcher, L., & Hudson, H. S. 2008, ApJ, 675, 1645

[3] Solar flares and focused energy transport by MHD waves, Russell, A. J. B., & Stackhouse, D. J. 2013, A&A, 558, A76

[4] Origin of the Modulation of the Radio Emission from the Solar Corona by a Fast Magnetoacoustic Wave, Kolotkov, Dmitrii Y.; Nakariakov, Valery M.; Kontar, Eduard P, 2017, ApJ, 861, 33

[5] Possible Detection of Subsecond-Period Propagating Magnetohydrodynamics Waves in Post-Reconnection Magnetic Loops during a Two-Ribbon Solar Flare. Yu, Sijie & Chen, Bin. 2019, ApJ, 872, 1

[6] Intermediate drift bursts and the coronal magnetic field. Benz, A. O., & Mann, G. 1998, A&A, 333, 1034

Full list of authors: Sijie Yu and Bin Chen

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Solar flares are sudden explosive processes, that convert the energy of the magnetic field into the kinetic energy of electrons and ions. Since the beginning of the century, millimeter observations of solar flares became routinely possible at a few frequencies with limited spatial resolution (see Kaufmann 2012, as a review). One of the most puzzling aspects of the observations at millimeter wavelengths (200-400 GHz) is the presence, in some flares, of a bright spectral component that grows with frequency. This emission is about hundred trillion times more powerful than the power from the active millimeter full body scanners used in airports around the world.

The large flux of ~104 solar flux units (s.f.u.) at 400 GHz in some flares and a noticeable correlation with hard X-ray emission quickly led to the proposal that the emission is likely associated with accelerated non-thermal electrons (Kaufmann et al. 2001). The measurement of radio emission source sizes could provide additional observational constraints. However, there are currently no reliable source size measurements near 400 GHz and there is a long list of proposed emission mechanisms (e.g. Kaufmann 2012, Fleishman & Kontar, 2011, Zaitsev et al, 2014), which, unfortunately, have several assumptions that cannot be verified observationally.

However, the recent analyses of the relationship between the area of flare ribbons and the flare millimeter component, suggests that a thermal emission model, in which radio emission originates from the transition region of solar flare ribbons perturbed by flare-accelerated electron heating can explain the puzzling observations.

Observations and model comparison

A total of 17 solar flares with radio flux observations at millimeter range have been used in the study. For the selected events, the spectral indexes determined by radio fluxes at 212 GHz and 405 GHz are found to be consistent with several emission mechanisms including the optically thick free-free emission.

Figure 1 – The flux density spectrum showing the rising millimeter component (shown by green ellipse) above 200 GHz (left) and UV solar flare ribbon observed by TRACE satellite (right). The figure from Kontar et al, 2018.

The observed spectral flux density is proportional to the area of the emitting source due to the Rayleigh-Jeans relation. Therefore, the area is an important parameter for a thermal emission model. If the millimeter emission originates from optically thick thermal plasma in the upper chromosphere/transition region, then the area of the heated plasma (flare ribbon area) should be sufficient to provide the observed radio flux.

To evaluate flare ribbon area, UV images have been studied at the 1600 Å passband, obtained from the Transition Region and Coronal Explorer (TRACE) and from the Solar Dynamics Observatory Atmospheric Imaging Assembly (SDO/AIA). Figure 2 shows that all radio-fluxes observed can be explained by radiation from an optically thick plasma with the temperature between 104 and 106 Kelvin, which is typical for the transition region of solar atmosphere.

Figure 2 – Observed spectral flux density (crosses with error bars) and the flux density (solid lines) at 212 GHz (left) and 405 GHz (right) predicted by the flare ribbon emission model for transition region temperatures are shown by pink, dark yellow and dark blue lines respectively.

It is important to note that relatively dense plasma heated by energetic electrons to temperatures 0.1-1 million Kelvin (MK) leads to enhanced radiation, so that the radiation losses would lead to effective cooling. The estimates of radiative cooling time suggest that the plasma can quickly (at sub-second scale) cool if the heating time is greater than the radiation loss time. Hence, the interaction between non-thermal electron heating and the radiative cooling of dense plasma can explain the observed sub-second variability of flare millimeter emission.

Conclusions

The large spectral fluxes of the observed millimeter range (or sub-THz) emission are proposed to be associated with the large areas of these flare ribbons. Then, the millimeter range emission is produced by thermal plasma at the heated flare ribbons. Flares which demonstrate extended flare ribbons should give large fluxes at millimeter frequency range, which is consistent with the observations. Then, the thermal emission from an optically thick transition region and/or low coronal plasma, with temperatures between 0.1-2~MK produces a spectrum growing with frequency as required by the observations. The comparison of flare-ribbon model with existing observations show the millimeter spectral flux density (200-400 GHz) in all flares studied can be explained by the model.

Based on the recently published paper: Kontar, E. P.; Motorina, G. G.; Jeffrey, N. L. S.; Tsap, Y. T.; Fleishman, G. D.; Stepanov, A. V. Astronomy and Astrophysics, 620, id. A95 (2018), DOI: 10.1051/0004-6361/201834124

References

Fleishman, G. D., & Kontar, E. P. 2010, ApJ, 709, L127

Kaufmann, P., Raulin, J.-P., Correia, E., et al. 2001, ApJ, 548, L95

Kaufmann, P. 2012, Astrophys. Space Sci. Proc., 30, 61

Zaitsev, V. V.; Stepanov, A. V.; Kaufmann, P. 2014, Solar Physics, 289, 3017

*Full list of authors: Motorina, G. G.; Kontar, E. P.; Jeffrey, N. L. S.; Tsap, Y. T.; Fleishman, G. D.; Stepanov, A. V.

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T-IV solar radio bursts are wideband continuum observed with dynamic spectrographs at metric to decametric wavelengths. They can be classified into static and moving ones, according to whether the sources remain basically static or move outward. The static ones are related to solar flares with frequencies extending up to GHz, while the moving ones (T-IVms) are related to coronal mass ejections (CMEs) with frequencies sometimes shifting to tens of MHz.

Earlier studies, mainly based on the Culgoora radio-heliograph data, reveal various morphologies of T-IVm sources, in form of plasmoid, advancing front, or loop-like structure (e.g., Wild 1970). In addition, T-IVms were found to be closely associated with coronal transients, including eruptive prominence and flare surges, long before CMEs were discovered. Due to scarcity of simultaneous imaging data at different wavelengths, it remains elusive about which part of CMEs releases the radio bursts.

It is also controversial regarding what emission mechanism accounts for T-IVms. Both incoherent gyrosynchrotron emission and coherent plasma emission have been suggested. Earlier studies found that T-IV sources can be traced as far as 5 solar radii at 80 MHz, much higher than the corresponding plasma level. In addition, no dispersion of T-IV sources with observing frequencies was found in some studies on the basis of earlier interferometric data. These observations were considered to be against plasma emission, and in line with gyrosynchrotron from mildly relativistic electrons.

Observations and data analysis

Here we investigate a T-IVm burst (Figure 1). It is a very clean event without interference from other bursts. The event is well observed by a tandem of instruments (NRH, AIA/SDO, LASCO/SOHO, EUVI/STEREO). The observations, together with the limb perspective, make the event a nice candidate for studies on T-IVms.

Figure 1. The composite of dynamic spectra of the T-IVm burst recorded by ORFEES (300-144 MHz, upper part) and NDA (80-10 MHz, lower part). A data gap exists between 144 MHz and 80 MHz. (b) The GOES SXR light curves.

In Figure 2, the T-IVm sources observed by NRH are overlaid onto corresponding images at 94 Å. Initially, the sources are well co-spatial with a gradually-rising and twisted EUV structure, observable only through hot channels like 94 and 131 Å, indicating the presence of high-temperature plasmas. Evidence of magnetic reconnection is observed with the formation of the structure. The reconnection may have generated energetic electrons that are then injected into and trapped by the eruptive structure.

Figure 2. T-IVm sources observed by NRH, superposed onto the AIA-94 Å images at closet times. NRH sources are represented by 85% and 95% contours of corresponding maxima of brightness temperature.

The hot EUV structure later develops into a CME as observed by LASCO-C2. In the AIA FOV, the T-IVm sources lie in the upper part of the EUV structure, and in the LASCO-C2 FOV, one source at 150 MHz is still observable by NRH, located at the upper part of the bright core of the CME. This suggests that the CME bright core, at least partially, originates from the hot EUV structure (usually, the CME bright core is attributed to dense eruptive filament).

We find that T-IVm sources line up together to form a section of an arcade-like structure (see Figure 2d), with clear spatial dispersion with frequencies. The burst lasts for at least 2.5h. In addition, its brightness temperature varies from 107 to 109 K, and the polarization levels remain weak in general. DEM analysis of AIA data shows that the electron density within the EUV structure is at the order of 108 cm-3 and the temperature is several MK.

These observations, especially the relatively high brightness temperature and the obvious spatial dispersion of sources with frequencies, do not support gyrosynchrotron to be the emission mechanism. Gyrosynchrotron simulations using density values given above with reasonable strength of magnetic field (e.g., 5 Gauss) reveal a similar conclusion. We therefore suggest that the T-IVm burst is given by plasma emission. One likely process to excite such emission is through z-mode maser driven by loss-cone distribution of energetic electrons trapped by the source structure. The instability shall grow in the parameter regime of ωpe>>ωce within a hot plasma background (~ several MK).

Conclusions

We found that T-IVm sources in the inner corona correlate with the upper part of a hot and twisted eruptive EUV structure, while in the outer corona the sources are associated with the top front of the bright core of a white light CME, indicating the CME core stems from the hot EUV structure. We also found that the T-IVm sources manifest a clear spatial dispersion with frequencies, with brightness temperature varying form 107-109 K and polarization level being in-general weak. Multi-wavelength data of AIA are used to infer plasma properties of T-IVm sources, such as density and temperature. Our observations support that the radiation mechanism of T-IVms is likely the coherent plasma emission excited by energetic electrons trapped within the source structure.

Based on a recently published article: V. Vasanth, Yao Chen, Maoshui Lv, Hao Ning, Chuangyang Li, Shiwei Feng, Zhao Wu, and Guohui Du (2019), Source Imaging of a Moving Type IV Solar Radio Burst and Its Role in Tracking Coronal Mass Ejection from the Inner to the Outer Corona, The Astrophysical Journal, 870:30 (11pp), https://doi.org/10.3847/1538-4357/aaeffd.

References

Wild, J. P. 1970, Proc. Astr. Soc. Australia, 1, 365

*Full list of authors: V. Vasanth, Yao Chen, Maoshui Lv, Hao Ning, Chuangyang Li, Shiwei Feng, Zhao Wu, and Guohui Du

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While the solar chromosphere has been routinely observed in high-resolution from ground-based optical telescopes such as the Swedish Solar Telescope (SST), and more recently in the UV from space-borne telescopes such as the Interface Region Imaging Spectrograph (IRIS), radio observations lag behind despite their great potential.

Distinctly from photospheric lines, most chromospheric diagnostics such as Ca II 854.2 nm and Mg II h and k form under non-LTE conditions, therefore they are weakly coupled to the local conditions of the plasma (e.g. de la Cruz Rodríguez et al. 2017, Leenaarts et al. 2013, Bjørgen et al. 2018). On the other hand, 1.2 mm and 3 mm continuum emission as probed by ALMA forms in the chromosphere and its source function can be treated in LTE, so it can be used to estimate chromospheric temperatures ( e.g. Wedemeyer et al 2016 ). However, proper use of ALMA data requires successful imaging at very high resolution which is a rather challenging endeavor. In fact, ALMA observations will be most useful when combined with other spectral ranges, but the combined diagnostic potential is not yet fully assessed.

Figure 1 – Snapshot of gas temperature is a 3D radiation-MHD simulations of the solar atmosphere at different heights (top row) and inferred temperatures using a combination of different diagnostics.

In da Silva Santos et al (2018) we we find that coordinated observations with SST, IRIS and ALMA will permit us to estimate with greater accuracy the full thermodynamical state of the plasma as a function of optical depth $\log \tau$ based on experiments with a snapshot of a three-dimensional magnetohydrodynamics (MHD) simulation of the Sun’s atmosphere.

Inversions of ultraviolet lines and radio continuum

Figure 1 shows that state-of-the-art non-LTE inversion codes such as STiC ( de la Cruz Rodríguez et al 2016 ) are able to infer gas temperatures with relatively good accuracy depending on the available chromospheric diagnostics. In particular, the mm continua are expected to provide stronger constraints on the gas temperatures higher up in the atmosphere, improving the diagnostic value of the Mg II h, k and triplet lines. This is of great usefulness for the large community that makes use of IRIS observations since IRIS usually provides support for ALMA campaigns. We look forward to invert actual ALMA data to learn more about the solar chromosphere.

Response functions

The radio continua in the mm responds to temperature perturbations at low optical depths in the chromosphere, that is approximately the same range of heights as the cores of the Mg II and Ca II lines (Figure 2) though there might be systematic differences.

By tuning into slightly different wavelength bands we can thus probe the entire chromosphere in high-resolution with the aid of interferometry (see also Loukitcheva et al. 2015).

At the moment of writing only band 3 (100 GHz) and band 6 (250 GHz) are available, but the inclusion of even higher mm-bands would certainly be useful for constraining even higher layers up to the base of the transition region where we lack suitable diagnostics.

Fig. 2: Response functions to temperature perturbations for the mm-continua and at the core of a few chromospheric lines (da Silva Santos et al, 2018)

Conclusions

Inversions with ALMA will help us obtaining more accurate gas temperatures for better quantifying heating and its location in the chromosphere, and hopefully bringing us to a closer understanding of its source.

Additional info

Based on the recently published article by da Silva Santos, J. M., de la Cruz Rodríguez, J. & Leenaarts, J. 2018, A&A 620, A124 DOI: https://doi.org/10.1051/0004-6361/201833664

Free copy of the paper available in the author page.

References

Bjørgen, J. P., Sukhorukov, A.V. & Leenaarts, J. et al 2018, A&A, 611, A62

de la Cruz Rodríguez, J. & van Noort, M. 2017, SSRv, 210, 109

de la Cruz Rodríguez, J. Leenaarts, J. & Asensio Ramos, A. 2016, ApJ, 830, L30

Leenaarts, J., Pereira, T. M. D. & Carlsson, M. et al 2013, ApJ, 772, 90

Loukitcheva, M., Solanki, S. K., Carlsson, M. & White, S. M. 2015, A&A 575, A15

Wedemeyer, S., Bastian, T. & Brajša, R. et al 2016, SSRv, 200, 1

*Full list of auhtors: da Silva Santos, J. M., de la Cruz Rodríguez, J. & Leenaarts, J.

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Coronal Mass Ejections (CMEs) are often viewed as the major drivers of space weather disturbances in the Sun-Earth system. Shocks driven by CMEs can excite radio emissions characterised by a slow frequency drift across dynamic spectra. These radio emissions are known as Type II solar radio bursts and can consist of two bands with a frequency ratio of 1:2. Each of these bands can split into two thinner sub-bands, a phenomenon known as “band splitting”. The cause of band splitting has been under debate for several decades. The two most widely-accepted interpretations are the Smerd et al. (1974, 1975) model and the Holman & Pesses (1983) model, but make opposing predictions for the true location of the emission sources of the split-band Type II components. The Smerd et al. (1974, 1975) model attributes the band splitting to emission occurring from the upstream (undisturbed) and downstream (shocked) regions of a shock front, two virtually co-spatial locations. The frequency split observed is thus explained by the steep density jump between the upstream and the downstream sides of the shock front. The Holman & Pesses (1983) model, on the other hand, explains band splitting as emission originating from two spatially separated parts of the upstream region of the shock front. In this case, the frequency split is the expected result of the different densities that the two sources encounter due to their separated positions along the curved shock as the shock propagates through the gradually and radially decreasing coronal density.

Intriguingly, Smerd et al. (1974, 1975) were aware of previously-observed large separations between sub-band sources (approximately 0.06 – 0.25 Rsun) challenging their model which requires the sub-band sources to originate from virtually the same region. They provided several reasons as to why these observed separations might not be representative of the true physical relation of the sub-band sources. Suggestions included the time delay due to the inability to image the two sub-bands at the same time, meaning that a source propagated with the shock during that time, and the frequency-dependent shift that sources could experience due to radio-wave scattering effects in the corona. Hence, the dominant mechanism of band splitting is a topic of ongoing discussion in solar physics.

We utilised the imaging capabilities of the LOw-Frequency ARray (LOFAR) to study the spatial relation of the higher- and lower-frequency sub-band sources. LOFAR has the unprecedented capability to image the emissions at any and all frequency and time points. We have thus obtained, for the first time, simultaneous images of the two sub-bands enabling us to estimate the locations of the sources without any time-delay ambiguities.

 Observations & Results

A split-band Type II burst was observed by LOFAR between 30 and 40 MHz (see Figure 1). The separation between the sources of the two sub-bands was estimated to be approximately 0.2+/-0.05 Rsun. The locations of both sub-band sources were compared to the Newkirk (1961) coronal density model and it was found that both sub-bands are described by the 4.5xNewkirk model. The description of both sub-bands by the same density model is inconsistent with the Smerd et al. (1974, 1975) model as a density jump between the sub-band sources is not observed.

Figure 1. Top panel: a combination of data from SDO/AIA, SOHO/LASCO/C2, and LOFAR. Blue crosses represent centroids of the higher-frequency sub-band sources, red crosses represent centroids of the lower-frequency sub-band sources, and green crosses represent Type III centroids. Yellow and magenta lines indicate fits through the Type II and Type III centroids, respectively. Bottom panel: dynamic spectrum recorded by LOFAR on 25 June 2015 around 10:45 UT showing the split-band Type II burst between 30 and 42 MHz. Black dashed lines indicate moments of equal time, red crosses indicate points from the higher- and lower-frequency Type II sub-bands, and black crosses indicate points from a Type III burst. (Figure adapted from Chrysaphi et al. (2018).)

In light of recent investigations (Kontar et al. (2017)) that highlighted the dominance of scattering effects on low-frequency observations, we quantitatively estimated, for the first time, the extent of scattering on split-band Type II bursts. Scattering shifts sources radially father from the Sun, with lower frequencies experiencing a higher degree of shift. We find that a 32 MHz source appears as radially shifted by ~0.3 Rsun from a 40 MHz source. Therefore, the imaged separation of 0.2+/-0.05 Rsun is consistent with radio-wave scattering effects, indicating that the sub-band sources could in reality be virtually co-spatial.

Since scattered sources are observed radially farther from their true locations, the deduced coronal density will appear to be larger than its true value. We find that coronal densities are exaggerated by a factor of ~4.3, meaning that the 4.5xNewkirk model obtained from the observations is an over-estimation. We also find that once scattering effects are considered, the sub-band sources are no longer described by the same density model, but a density jump between the sub-bands occurs. These effects are schematically illustrated in Figure 2.

Figure 2. Schematic illustration of the effects of scattering on radio-wave emission sources. True sources (shown in circles) are shifted radially outwards from the Sun, with the higher frequency source (red) shifted more than the lower frequency source (blue). This frequency-dependent shift causes the apparent emission sources (shown in crosses) to appear as spatially separated, despite that they could originate from virtually co-spatial regions on the shock. The radial displacement of the sources also causes the apparent coronal density to appear larger than its true value. (Figure adapted from Chrysaphi et al. (2018).)

Conclusions

We showed that the large separations observed between the components of a split-band Type II burst are consistent with radio-wave scattering effects, highlighting the importance of considering radio-wave propagation effects in the analysis of radio emissions. As a consequence, the true emission sources of a split-band Type II burst could originate in virtually co-spatial regions, and hence the emission could be produced in the upstream and the downstream shock front parts.

Based on the recently published paper: Nicolina Chrysaphi et al., 2018, ApJ, 868, 79, DOI: 10.3847/1538-4357/aae9e5

References

Holman, G. D., & Pesses, M. E. 1983, ApJ, 267, 837

Kontar, E. P., Yu, S., Kuznetsov, A. A., et al. 2017, NatCo, 8, 1515

Newkirk, G., Jr. 1961, ApJ,133, 983

Smerd, S. F., Sheridan, K. V., & Steward, R. T. 1974, in IAU Symp. 57, 389

Smerd, S. F., Sheridan, K. V., & Steward, R. T. 1975, ApL, 16, 23

*Full list of authors: Nicolina Chrysaphi, Eduard P. Kontar, Gordon D. Holman, and Manuela Temmer

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A diverse set of radio production mechanisms makes the radio wavelengths a unique observational window into the universe. Radio emission can be produced by both quiescent and eruptive phenomena, e.g. flares and coronal mass ejections (CMEs). The morphology of the quiet Sun in radio depends on macroscopic MHD quantities, such as the density, the temperature and the magnetic field, and the type of the emission. Lee at al. (2009) showed that higher radio frequencies can probe stronger magnetic fields, as they penetrate deeper into solar atmospheric layers.

While there is a plethora of quiet Sun observations, observing radio loud CMEs is a rather difficult task as they become diffuse soon after the eruption. Only a handful of CMEs have been observed in low radio frequencies and only at the early stages of their evolution, e.g. Zucca et al. (2018). We developed and included in our wind and coronal simulations a radio synthetic imaging capability, which computes the radio intensity of a specific radio emission mechanism along the curved ray paths. We present the first results with this ray-tracing algorithm and compare them with solar low and high frequency observations.

Figure 1. Left: Synthesized Bremsstrahlung radiation as captured in our simulation at 17 GHz for CR2107 (centered at March 7, 2011). Right: March 7, 2011 Observations from Nobeyama at 17GHz with the overplotted contours corresponding to the simulation.

Our simulations are performed with the state-of-the-art code BATS-R-US van der Holst et al (2014), which provides a realistic description of the solar corona, wind and CMEs, calculating the plasma and magnetic field conditions in a time-dependent manner. For the radio synthetic images we implemented in BATS-R-US a ray tracing algorithm based on Benkevitch et al. (2010), which calculates the actual radio ray trajectories in space plasmas with well determined MHD plasma characteristics. For this study we focus on Bremsstrahlung emission, due to its dependance on the density and temperature which are provided by our MHD simulation. The solar corona is a highly non-uniform medium, which means that the refractive index is a strongly spatially varying quantity between media of different densities. For that reason the role of refraction is critical for locating in 3D space the locations and origins of different radio sources. Specifically, for each radio frequency there is a critical surface below which the ray cannot penetrate and which defines a magnified size of the solar surface. This is a well known property of radio observations.

Results

We have validated and tested our algorithm by running a simulation for a dipolar solar magnetic field and recovering the expected radio flux as a function of frequency for the quiescent Sun (Zarka 2007).

Time-averaged over a solar rotation (synoptic) photospheric magnetic fields are used to drive our simulations. In this study, we simulated the quiet Sun during Carrington Rotation (CR) 2107. In Figure 1 we show the synthesized (left) and the Nobeyama Radioheliograph observed radio emission at 17 GHz. Our results are in good agreement with the observations.

We also simulated the CME event that took place on 7 March 2011 at 19:40. Figure 2 shows synthetic radio images of that event in 6 different frequencies. The results demonstrate that refraction is playing an important role with rays of higher frequencies probing stronger magnetic fields in active regions as they penetrate in layers closer to the photosphere. With increasing frequency the total radio flux is gradually dominated by the active regions, with the transition happening around the GHz frequencies. The CME morphology captured is in good qualitative agreement with the CME observed by Bastian et al. (2001). The dominant emission of that event was attributed to synchrotron emission, which is not accounted for here. However, the synchrotron emission strongly depends on the magnetic field, which is computed by our 3D MHD simulation in a self-consistent manner.

Figure 2. Bremsstrahlung multi-frequency radio synthetic views at 50 MHz, 100 MHz, 500 MHz, 1 GHz, 5 GHz and 10 GHz. Higher frequencies probe deeper into the solar atmosphere and capture the finer scales of the magnetic field such as ARs. The radio intensity of the solar disk and the CME is higher for higher frequencies

Conclusions

We have incorporated a synthetic radio capability in state-of-the-art 3D MHD solar corona simulations. The main focus of this study is the Bremsstrahlung emission, which depends on the density squared ($\rho^2$), similar to EUV and X-ray line dominated emission. The radio intensity of the Bremsstrahlung emission is computed along curved ray paths accounting for refraction. Our simulations captured the radio counterpart of EUV waves propagating toward the Sun due to the CME exhaust. This is the first time that refraction has been accounted for so systematically in a realistic solar corona through forward modeling in 3D.

This ray-tracing algorithm can provide guidance for the next generation radio missions. In the future, we plan to account for scattering, and include non thermal radio emission processes in our simulations in order to study the evolution of radio bursts. Recently, Cohen et al. (2018) used this algorithm to predict the detectability of exoplanets by quantifying the modulation of the stellar radio mission by an exoplanetary magnetosphere through a transit.

Additional info

Contact info:

email: sofia.moschou@cfa.harvard.edu

twitter: @SofiaMoschou

Full paper can be retrieved at: Moschou, S.-P.; et al.: 2018, ApJ, 867, 51

References

Bastian, T.; et al: 2001, ApJ, 558, L65

Benkevitch, L.; et al.: ArXiV

Cohen, O.; Moschou, S.-P.; et al.: 2018, AJ, 156, 202

Lee, J.; et al.: 2009, ApJ 510, 413

van der Holst, B.; et al.: 2014, ApJ, 782, 81

Zarka, P.: 2006, PlanSS, 55, 598

Zucca, P.; et al.: 2018, A&A, 615, 89

*Full list of authors: S.-P. Moschou, I. Sokolov, O. Cohen, J. J. Drake, D. Borovikov, J. C. Kasper, J. D. Alvarado-Gomez, & C. Garraffo

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