Hard x-ray and Metric/Decimetric Radio Observations of the 20 February 2002 Solar Flare

The GOES C7.5 flare on 20 February 2002 at 11:07 UT is one of the first solar flares observed by RHESSI at X-ray wavelengths. It was simultaneously observed at metric/decimetric wavelengths by the Nançay radioheliograph (NRH) which provided images of the flare between 450 and 150 MHz. We present a first comparison of the hard X-ray images observed with RHESSI and of the radio emission sites observed by the NRH. This first analysis shows that: (1) there is a close occurrence between the production of the HXR-radiating most energetic electrons and the injection of radio-emitting non-thermal electrons at all heights in the corona, (2) modifications with time in the pattern of the HXR sources above 25 keV and of the decimetric radio sources at 410 MHz are observed occurring on similar time periods, (3) in the late phase of the most energetic HXR peak, a weak radio source is observed at high frequencies, overlying the EUV magnetic loops seen in the vicinity of the X-ray flaring sites above 12 keV. These preliminary results illustrate the potential of combining RHESSI and NRH images for the study of electron acceleration and transport in flares.     

Are There Radio-quiet Solar Flares?

Some 15% of solar flares having a soft X-ray flux above GOES class C5 are reported to lack coherent radio emission in the 100 – 4000 MHz range (type I – V and decimetric emissions). A detailed study of 29 such events reveals that 22 (76%) of them occurred at a radial distance of more than 800″ from the disk center, indicating that radio waves from the limb may be completely absorbed in some flares. The remaining seven events have statistically significant trends to be weak in GOES class and to have a softer non-thermal X-ray spectrum. All of the non-limb flares that were radio-quiet above 100 MHz were accompanied by metric type III emission below 100 MHz. Out of 201 hard X-ray flares, there was no flare except near the limb (R>800″) without coherent radio emission in the entire meter and decimeter range. We suggest that flares above GOES class C5 generally emit coherent radio waves when observed radially above the source.     

Correlation of solar decimetric radio bursts with X-ray flares

Several hundred radio bursts in the decimetric wavelength range (300–1000 MHz) have been compared with simultaneous soft and hard X-ray emission. Long lasting (type IV) radio events have been excluded. The association of decimetric emission with hard X-rays has been found to be surprisingly high (48%). The association rate increases with bandwidth, duration, number of structural elements, and maximum frequency. Type III-like bursts are observed up to the upper limit of the observed band. This demonstrates that the corona is transparent up to densities of about 10¹⁰ cm^–3, contrary to previous assumptions. This can only be explained in an inhomogeneous corona with the radio source being located in a dense structure. The short decimetric bursts generally occur during the impulsive phase, i.e. simultaneously with hard X-rays. The times of maximum flux are well correlated (within 2 s). The HXR emission lasts 4 times longer then the radio emission in the average. This work finds a close relationship between decimetric and HXR emission with sufficient statistics offering additional information on the flare process.     

CHANGES IN SUNSPOT AND FLOCCULAR SOURCES OF RADIO EMISSION PRECEDING AN IMPORTANCE 2N FLARE ON 23 AUGUST 1988

Based on observations from the Siberian solar radio telescope, and invoking data from other observatories, we investigate preflare changes in the sunspot and floccular sources of radio emission and the development of an importance 2N flare in the chromosphere and corona in the active region on August 23, 1988.It has been ascertained that preflare changes became observable six hours prior to the flare onset and manifested themselves in intense flux fluctuations above the sunspot and in an enhancement of the source emission flux above the flocculus.It is shown that the flare onset is associated with a newly emerged magnetic flux in the form of a pore near the filament and with the appearance of radio sources above the filament. The flare was accompanied by type III radio bursts and a noise storm at meter wavelengths. Coronal mass ejection parameters are estimated from type III burst observations.     

Very-Large-Array Observations of Evolving Type I Noise Storms and Nonthermal Bursts In Association With Flares And Coronal Mass Ejections

Very-Large-Array (VLA) observations of the Sun at 20, 91 and 400 cm have been combined with data from the SOHO, TRACE and Wind solar missions to study the properties of long-lasting Type I noise storms and impulsive metric and decimetric bursts during solar flares and associated coronal mass ejections. These radio observations provide information about the acceleration and propagation of energetic electrons in the low and middle corona as well as their interactions with large-scale magnetic structures where energy release and transport takes place. For one flare and its associated CME, the VLA detected impulsive 20 and 91 cm bursts that were followed about ten minutes later by 400 cm burst emission that appeared to move outward into the corona. This event was also detected by the Waves experiment on Wind which showed intense, fast-drifting interplanetary Type III bursts following the metric and decimetric bursts detected by the VLA. For another event, impulsive 91 cm emission was detected about a few minutes prior to impulsive bursts at 20.7 cm, suggesting an inwardly propagating beam of electrons that excited burst emission at lower levels and shorter wavelengths. We also find evidence for significant changes in the intensity of Type I noise storms in the same or nearby active region during impulsive decimetric bursts and CMEs. These changes might be attributed to flare-initiated heating of the Type I radio source plasma by outwardly-propagating flare ejecta or to the disruption of ambient magnetic fields by the passage of a CME.     

Multi-Wavelength Analysis of High-Energy Electrons in Solar Flares: A Case Study of the August 20, 2002 Flare

A multi-wavelength spatial and temporal analysis of solar high-energy electrons is conducted using the August 20, 2002 flare of an unusually flat (γ₁ = 1.8) hard X-ray spectrum. The flare is studied using RHESSI, Hα, radio, TRACE, and MDI observations with advanced methods and techniques never previously applied in the solar flare context. A new method to account for X-ray Compton backscattering in the photosphere (photospheric albedo) has been used to deduce the primary X-ray flare spectra. The mean electron flux distribution has been analysed using both forward fitting and model-independent inversion methods of spectral analysis. We show that the contribution of the photospheric albedo to the photon spectrum modifies the calculated mean electron flux distribution, mainly at energies below ∼100 keV. The positions of the Hα emission and hard X-ray sources with respect to the current-free extrapolation of the MDI photospheric magnetic field and the characteristics of the radio emission provide evidence of the closed geometry of the magnetic field structure and the flare process in low altitude magnetic loops. In agreement with the predictions of some solar flare models, the hard X-ray sources are located on the external edges of the Hα emission and show chromospheric plasma heated by the non-thermal electrons. The fast changes of Hα intensities are located not only inside the hard X-ray sources, as expected if they are the signatures of the chromospheric response to the electron bombardment, but also away from them.     

The solar radio event of January 14, 1971

A study has been made of the behaviour in time of the positions, flux density and polarization for the radio emissions at frequencies of 237 and 408 MHz (from Trieste observations), taking into account also the published results at 169 and 160–200 MHz (from Nançay and Utrecht) connected with the solar flare which occurred on January 14, 1971, at 11^h21^m UT.The flare and the subsequent emissions may have been excited by a fall of coronal matter along the lines of force of the magnetic field in the relevant active region, exciting radio emissions of two different types: a type II-like emission and a type III group apparently located at different positions.This discrepancy can be easily explained if we assume that the type III group was due to second harmonic emission in backward direction, strongly refracted by the coronal plasma.The observed spike emission was due to a source high in the corona, excited by the type II-like perturbation which originated in the flare region, and this fact clearly points out the relationship between all the observed peculiar emissions from type IV sources, as has already been stated by this author.     

MICROWAVE EMISSION FROM CORONAL HEIGHTS: STUDY OF A NON-THERMAL RADIO FLARE

Two small radio flares following the great gamma-ray burst on 11 June 1991 are studied. We analyse the different association of emission features at microwaves, decimeter waves, and soft and hard X-rays for the events. The first flare has well-defined emission features in microwaves and soft and hard X-rays, and a faint decimetric signature well after the hard X-ray burst. It is not certain if the decimetric event is connected to the burst features. The second event is characterized by an almost simultaneous appearance of hard X-ray burst maxima and decimetric narrowband drift bursts, but soft X-ray emission is missing from the event. With the exception of the possibility that the soft X-ray emission is absorbed along the way, the following models can explain the reported differences in the second event: (1) Microwave emission in the second event is produced by 150 keV electrons spiraling in the magnetic field relatively low in the corona, while the hard X-ray emission is produced at the beginning of the burst near the loop top as thick-target emission. If the bulk of electrons entered the loop, the low-energy electrons would not be effectively mirrored and would eventually hit the footpoints and cause soft X-ray emission by evaporation, which was not observed. The collisions at the loop top would not produce observable plasma heating. The observed decimetric type III bursts could be created by plasma oscillations caused by electron beams traveling along the magnetic field lines at low coronal heights. (2) Microwave emission is caused by electrons with MeV energies trapped in the large magnetic loops, and the electrons are effectively mirrored from the loop footpoints. The hard X-ray emission can come both from the loop top and the loop footpoints as the accelerated lower energy electrons are not mirrored. The low-energy electrons are not, however, sufficient to create observable soft X-ray emission. The type III emission in this case could be formed either at low coronal heights or in local thick regions in the large loops, high in the corona.     

Very Large Array,SOHO, and RHESSI Observations of Magnetic Interactions and Particle Propagation across Large-Scale Coronal Loops

Very Large Array (VLA) observations at wavelengths of 20 and 91 cm have been combined with data from the SOHO and RHESSI solar missions to study the evolution of transequatorial loops connecting active regions on the solar surface. The radio observations provide information about the acceleration and propagation of energetic electrons in these large-scale coronal magnetic structures where energy release and transport take place. On one day, a long-lasting Type I noise storm at 91 cm was seen to intensify and shift position above the northern hemisphere region following an impulsive hard X-ray burst in the southern hemisphere footpoint region. VLA 20-cm observations as well as SOHO EIT EUV images showed evolving coronal plasma that appeared to move across the solar equator during this time period. This suggests that the transequatorial loop acted as a conduit for energetic particles or fields that may have triggered magnetic changes in the corona where the northern noise storm region was seen. On another day, a hard X-ray burst detected at the limb was accompanied by impulsive 20- and 91-cm burst emission along a loop connecting to an active region in the same hemisphere but about 5′ away, again suggesting particle propagation and remote flare triggering across interconnecting loops.     

Radio Pulsating Structures with Coronal Loop Contraction

We present a multi-wavelength study of a solar eruption event on 20 July 2004, comprising observations in H??, EUV, soft X-rays, and in radio waves with a wide frequency range. The analyzed data show both oscillatory patterns and shock wave signatures during the impulsive phase of the flare. At the same time, large-scale EUV loops located above the active region were observed to contract. Quasi-periodic pulsations with ???10 and ???15 s oscillation periods were detected both in microwave??C?millimeter waves and in decimeter??C?meter waves. Our calculations show that MHD oscillations in the large EUV loops ?C but not likely in the largest contracting loops ?C could have produced the observed periodicity in radio emission, by triggering periodic magnetic reconnection and accelerating particles. As the plasma emission in decimeter??C?meter waves traces the accelerated particle beams and the microwave emission shows a typical gyrosynchrotron flux spectrum (emission created by trapped electrons within the flare loop), we find that the particles responsible for the two different types of emission could have been accelerated in the same process. Radio imaging of the pulsed decimetric??C?metric emission and the shock-generated radio type II burst in the same wavelength range suggest a rather complex scenario for the emission processes and locations. The observed locations cannot be explained by the standard model of flare loops with an erupting plasmoid located above them, driving a shock wave at the CME front.     

Location of Decimetric Pulsations in Solar Flares

This work investigates the spatial relation between coronal X-ray sources and coherent radio emissions, both generally thought to be signatures of particle acceleration. Two limb events were selected during which the radio emission was well correlated in time with hard X-rays. The radio emissions were of the type of decimetric pulsations as determined from the spectrogram observed by Phoenix-2 of ETH Zurich. The radio positions were measured from observations with the Nançay Radioheliograph between 236 and 432 MHz and compared to the position of the coronal X-ray source imaged with RHESSI. The radio pulsations originated at least 30?–?240 Mm above the coronal hard X-ray source. The altitude of the radio emission increases generally with lower frequency. The average positions at different frequencies are on a line pointing approximately to the coronal hard X-ray source. Thus, the pulsations cannot be caused by electrons trapped in the flare loops, but are consistent with emission from a current sheet above the coronal source.     

一个复杂太阳射电爆发及其微波源和远紫外冕环特征的初步研究

利用太阳射电宽带频谱仪(0．7-7．6GHz)于2001年10月19日观测到的复杂太阳射电大爆发，呈现出许多有趣的特征，结合NoRH(Nobeyama Radio Heliograph)的高空间分辨率射电成像观测及TRACE(Transition Region and Coronal Explorer)在远紫外(EUV)波段的高空间分辨率成像观测资料，分析了该爆发的射电频谱特征和微波射电源的演化以及它们与复杂的EUV日冕环系统的关系，该爆发是一个双带大耀斑的射电表征．前一部分以宽带(从厘米到米波)爆发为主，机制是回旋同步辐射，所对应的是环足源的辐射；后一部分以窄带(分米到米波)分米波爆发为主，机制是等离子体辐射和回旋共振辐射的联合，对应的是环顶源的辐射。     

Seismic Emissions from a Highly Impulsive M6.7 Solar Flare

On 10 March 2001 the active region NOAA 9368 produced an unusually impulsive solar flare in close proximity to the solar limb. This flare has previously been studied in great detail, with observations classifying it as a type 1 white-light flare with a very hard spectrum in hard X-rays. The flare was also associated with a type II radio burst and coronal mass ejection. The flare emission characteristics appeared to closely correspond to previous instances of seismic emission from acoustically active flares. Using standard local helioseismic methods, we identified the seismic signatures produced by the flare that, to date, is the least energetic (in soft X-rays) of the flares known to have generated a detectable acoustic transient. Holographic analysis of the flare shows a compact acoustic source strongly correlated with the impulsive hard X-rays, visible continuum, and radio emission. Time?–?distance diagrams of the seismic waves emanating from the flare region also show faint signatures, mainly in the eastern sector of the active region. The strong spatial coincidence between the seismic source and the impulsive visible continuum emission reinforces the theory that a substantial component of the seismic emission seen is a result of sudden heating of the low photosphere associated with the observed visible continuum emission. Furthermore, the low-altitude magnetic loop structure inferred from potential-field extrapolations in the flaring region suggests that there is a significant anti-correlation between the seismicity of a flare and the height of the magnetic loops that conduct the particle beams from the corona.     

Repeated Flaring from Loop–Loop Interaction

A series of solar flares was observed near the same location in NOAA active region 8996 on 18–20 May 2000. A detailed analysis of one of these flares is presented where the emitting structures in soft and hard X-rays, EUV, H, and radio at centimeter wavelengths are compared. Hard X-rays and radio emission were observed at two separate loop footpoints, while soft X-rays and EUV emission were observed mainly above the nearby positive polarity region. The flare was confined although the observed type III bursts at the time of the flare maximum indicate that some field lines were open to the corona. No flux emergence was evident but moving magnetic features were observed around the sunspot region and within the positive polarity (plage) region. We suggest that the flaring was due to loop–loop interactions over the positive polarity region, where accelerated electrons gained access to the two separate loop systems. The repeated radio flaring at the footpoint of one loop was visible because of the strong magnetic fields near the large sunspot region while at the footpoint of the other loop the electrons could precipitate and emit in hard X-rays. The simultaneous emission and fluctuations in radio and X-rays – in two different loop ends – further support the idea of a single acceleration site at the loop intersection.     

Statistical Analysis of Decimetric Type III Bursts

Detailed statistics and analysis of 264 type III bursts observed with the 625–1500 MHz spectrograph during the 23rd solar cycle (from July 2000 to April 2003) are carried out in the present article. The main statistical results are similar to those of microwave type III bursts presented in the literature cited, such as the correlation between type III bursts and flares, polarization, duration, frequency drift rate (normal and reverse slopes), distribution of type III bursts and frequency bandwidth. At the same time, the statistical results also point out that the average values of the frequency drift rates and degrees of polarization increase with the increase in frequency and the average value of duration decreases with the increase in frequency. Other statistical results show that the starting frequencies of the type III bursts are mainly within the range from 650 to 800 MHz, and most type III bursts have an average bandwidth of 289 MHz. The distributions imply that the electron acceleration and the place of energy release are within a limited decimetric range. The characteristics of the narrow bandwidth possibly involve the magnetic configuration at decimetric wavelengths, the location of electron acceleration in the magnetic field nearto the main flare, the relevant runaway or trapped electrons, or the coherent radio emission produced by some secondary shock waves. In addition, the number of type III bursts with positive frequency drift rates is almost equal to that with negative frequency drift rates. This is probably explained by the hypothesis that an equal number of electron beams are accelerated upwards and downwards within the range of 625 to 1500 MHz. The radiation mechanism of type III bursts at decimetric wavelengths probably includes these microwave and metric mechanisms and the most likely cause of the coherent plasma radiation are the emission processes of the electron cyclotron maser.     

Solar radio pulsations at 410 MHz

On 6 September, 1982 very regular, narrow-band radio pulsations of solar origin were observed on the 410 MHz solar radiometer at the Learmonth Solar Observatory. Initial low-amplitude pulsations with a period of about 3 min gave way to large-amplitude pulsations with a period of about 5 min following a 1B solar flare. Position measurements at 327 MHz with the Culgoora Radioheliograph indicated two sources: a strong, extended source located above a unipolar magnetic region near the centre of the disk and a much weaker source near the west limb. Polarisation measurements indicate the burst to be plasma emission.The radio pulsations were unique in their association with both sympathetic radio emission and optical flares at widely different locations. Interpretation of the observations in terms of sausage mode standing oscillations in a coronal flux tube leads to an estimate of the magnetic flux density B = 45 G at the 400 MHz plasma level. Also a 2.8-fold density increase in the loop after the 1B flare is inferred.     

Multiple-loop structure of a solar flare from Microwave,EUV and X-ray Imaging Data

We present the results of an analysis of a flare event of importance M2.8 that occurred at 00:56 UT 28 August 1999. The analysis is based upon observations made with the Nobeyama radioheliograph (NoRH) and polarimeters (NoRP), TRACE, SOHO/MDI, EIT, and Yohkoh/SXT. The images show a very complex flaring region. Pre-flare TRACE and EIT images at 00:24 UT show a small brightening in the region before the flare occurred. The active region in which the flare occurred had evolving magnetic fields, and new magnetic flux seems to have emerged. The X-ray and radio time profiles for this event show a double-peaked structure. The polarimeter data showed that the maximum radio emission (1200 s.f.u.) occurred at 9.4 GHz. At 17 GHz the NoRH images appear to show four different radio sources including the main spot and the main flare loop. Most of the microwave emission seems to originate from the main flare loop. Comparison of BATSE and microwave time profiles at 17 and 34 GHz from the main sunspot source shows that these profiles have similar structures and they coincide with the hard X-ray peaks. The maximum of the flare loop emission was delayed by 10 s relative to the second maximum of the sunspot associated flare emission. Analysis of SXT images during the post-flare phase shows a complex morphology – several intersecting loops and changes in the shape of the main flare loop.     

Broadband Radio Bursts and Fine Structures during the Great Solar Event on 14 July 2000

25 MHz–7.6 GHz global and detailed (fine structure – FS) radio spectra are presented, which were observed in the NOAA 9077 active region for the Bastille Day (14 July 2000) flare at 10:10–11:00 UT. Besides broadband radio bursts, high-resolution dynamic spectra reveal metric type II burst, decimetric type IV burst and various decimetric and microwave FSs, such as type III bursts, type U bursts, reverse-slope (RS)-drifting burst, fiber bursts, patch and drifting pulsation structure (DPS). The peak-flux-density spectrum of the radio bursts over the range 1.0–7.6 GHz globally appears as a U-shaped signature. Analyzing the features of backbone and herringbones of the type II burst, the speeds of shock and relevant energetic electron beams were estimated to be 1100 km s⁻¹ and 58 500 km s⁻¹, respectively. Also the time sequence of the radio emission is analyzed by comparing with the hard X-rays (HXRs) and the soft X-rays (SXRs) in this flare. After the maxima of the X-rays, the radio emission in the range 1.0–7.6 GHz reached maxima first at the higher frequency, then drifted to the lower frequency. This comparison suggested that the flare included three successive processes: firstly the X-rays rose and reached maxima at 10:10–10:23 UT, accompanied by fine structures only in the range 2.6–7.6 GHz; secondly the microwave radio emission reached maxima accompanied by many fine structures over the range 1.0–7.6 GHz at 10:23–10:34 UT; then a decimetric type IV burst and its associated FSs (fibers) in the range 1.0–2.0 GHz appeared after 10:40 UT.     

Fast Magnetoacoustic Waves in a Fan Structure Above a Coronal Magnetic Null Point

We analyze the 26 November 2005 solar radio event observed interferometrically at frequencies of 244 and 611 MHz by the Giant Metrewave Radio Telescope (GMRT) in Pune, India. These observations are used to make interferometric maps of the event at both frequencies with the time cadence of 1 s from 06:50 to 07:12 UT. These maps reveal several radio sources. The light curves of these sources show that only two sources at 244 MHz and 611 MHz are well correlated in time. The EUV flare is more localized with flare loops located rather away from the radio sources. Using SoHO/MDI observations and potential magnetic field extrapolation we demonstrate that both the correlated sources are located in the fan structure of magnetic field lines starting from a coronal magnetic null point. Wavelet analysis of the light curves of the radio sources detects tadpoles with periods in the range P=10?–?83 s. These wavelet tadpoles indicate the presence of fast magnetoacoustic waves that propagate in the fan structure of the coronal magnetic null point. We estimate the plasma parameters in the studied radio sources and find them consistent with the presented scenario involving the coronal magnetic null point.     

On the wave mode of subsecond pulses in the cm-range

In this paper we determined the wave mode of subsecond pulses (SSP). We used data on pulses with a degree of polarization over 30%, with the sources located at −60 to +60 deg from the central meridian, for the period 2000–2002. The superposition of SSRT radio maps and MDI magnetograms has shown that radio SSP sources are typically located near the polarity inversion line of the active region magnetic field. Such an arrangement indicates that SSP sources are located at the tops of magnetic loops. The ordinary mode of electromagnetic radiation is recorded in SSP sources located from the inversion line by no less than about 10 arc sec.