Origin of coronal and interplanetary shock and particle acceleration of a flare/CME event

By using radio data from ground-based telescopes (from 270 MHz to 25 MHz), and from the Radio and Plasma Wave experiment (WAVES) on board the WIND spacecraft (1–14 MHz and several kHz-11 MHz), as well as FY -2 satellite data, the origin of coronal and interplanetary shock and particle acceleration of the 14 July 2000 flare/CME event (the Bastille day event) have been studied. Main conclusions are as follows: (1) We investigate the causal relationship between metric type 11 bursts observed by the digital IZMIRAN radio spectrograph and type II radio emissions in the frequency range from 1–14 MHz and several kHz-11 MHz observed by the WAVES/WIND. The analysis indicate that the fast CME is the origin of both coronal and interplanetary shocks. (2)According to the time profiles of Hard X-ray, and energetic particles (include proton, ³He, and ⁴He) from FY-2 satellite, it is obvious that the Bastille day event is the event, in which both impulsive and gradual phenomena occur. The energetic particles accelerated not only in flare but also in CME.     

Evidence for a solar coronal thick-target hard X-ray source observed by RHESSI

We study a solar flare hard X-ray (HXR) source observed by the Reuven Ramaty high energy solar spectroscopic imager (RHESSI) in which the HXR emission is almost entirely in a coronal loop so dense as to be collisionally thick at electron energies up to ∼45−60 keV. This contrasts with most events previously reported in which the HXR emission is primarily from the loop footpoints in the collisionally dense chromosphere. In particular, we show that the high loop column densities inferred from the GOES and RHESSI soft X-ray emission measure and the volume of the flare loop are consistent with the coronal thick-target interpretation of the HXR images and spectra. The high column densities observed already at the very beginning of the impulsive phase are explained by chromospheric evaporation during a preflare which, as Nobeyama 17 GHz radio images reveal, took place in the same set of nested loops as the main flare.     

Spectroscopic analysis of the solar flare event on 2002 August 3 with the use of RHESSI and RESIK data

We use simultaneous observations from RESIK and RHESSI instruments to compare plasma properties of a major solar flare in its rise and gradual phase. This event occurred on 2002 August 3 (peak time at 19:06 UT). The flare had a very good coverage with RESIK data and well-resolved soft and hard X-ray sources were seen in RHESSI images. Spectra of X-ray radiation from RHESSI images are studied and compared with RESIK measurements in different flare phases. Result shows large differences in flare morphology and spectra between flare rise and gradual phase.     

Modulations of broad-band radio continua and X-ray emissions in the large X-ray flare on 03 November 2003

The GOES X3.9 flare on 03 November 2003 at ∼09:45 UT was observed from metric to millimetric wavelengths by the Nançay Radioheliograph (NRH), the Radio Solar Telescope Network (RSTN) and by radio instruments operated by the Institute of Applied Physics (University of Bern). This flare was simultaneously observed and imaged up to several 100 keV by the RHESSI experiment. The time profile of the X-ray emission above 100 keV and of the radio emissions shows two main parts, impulsive emission lasting about 3 min and long duration emission (partially observed by RHESSI) separated in time by 4 min. We shall focus here on the modulations of the broad-band radio continua and of the X-ray emissions observed in the second part of the flare. The observations suggest that gyrosynchrotron emission is the prevailing emission mechanism even at decimetric wavelengths for the broad-band radio emission. Following this interpretation, we deduce the density and the magnetic field of the decimetric sources and briefly comment on possible interpretations of the modulations.     

Onset study of impulsive solar energetic particleevents

Impulsive solar energetic particle (SEP) events are associated with impulsive X-ray flares, energetic electrons,and enhanced heavy ion abundances. Using instruments on ACE, we have examined the composition and origin of twelve impulsive SEP events from November 1997 to June 2000. All selected impulsive SEP events have enhanced ³He/⁴He ratios compared with the solar wind values. The range of ³He/⁴He ratios varies from 0.01 to 7.8. By assuming scatter-free propagation at zero degree pitch-angle, we fitted the minimum particle path lengths (from 1.2 to 1.4 AU, as expected), and estimated the ion event release time back at the Sun to within better than 30 minutes in most cases. We found only four events in which the release times agree for both 38–50 keV electrons and <1 MeV/nucleon ions. Five of our events have significant differences (>40 minutes) between the electron and ion onset times, all with ions injected later. Three impulsive ion events have no association with any impulsive electron event. Seven events have associated solar electromagnetic signatures (Type III radio bursts and/or X-ray flares).     

RHESSI and radio imaging observations of microflares

We present an analysis of five microflares, three observed simultaneously by RHESSI in hard X-rays and Nobeyama RadioHeliograph (NoRH) in microwaves (17 GHz) and two observed by RHESSI and Nancay RadioHeliograph (NRH) at metric wavelengths (150–450 MHz). Since we have no radio imaging telescopes simultaneously operating at microwave and meter wavelengths in the same time zone, we are obliged to use a different set of metric events in contrast to that used for comparison with the two radio wavelengths. We are interested in using the locations and other imaging characteristics of the events from both RHESSI and radio observations instead of just temporal correlation. So we have used the Nancay (France) metric radioheliograph at 150–450 MHz for this purpose. Here we describe the properties of five events – three in microwaves and two at metric wavelengths. We discuss the brightness temperatures, emission measures and the hard X-ray spectral properties of these microevents. One sees small (mini) flaring loops clearly in NoRH and RHESSI images. The microwave emission often seems to come from the RHESSI foot points (for higher energies), and from the entire small (mini) flaring loop (for lower energies).The RHESSI microflares seem to be associated in position with metric type III bursts. Frequently, the hard X-ray spectrum of the microwave associated RHESSI microflares can be fit by a thermal component at low energies (∼3–12 keV) and a nonthermal component at higher energies (∼12–20 keV).     

Collaborative VLA,SOHO and RHESSI observations of evolving sources of energy release in the corona above active regions

Very Large Array (VLA) observations at 20 and 91 cm wavelength are compared with data from the SOHO (EIT and MDI) and RHESSI solar missions to investigate the evolution of decimetric Type I noise storms and Type III bursts and related magnetic activity in the photosphere and corona. The combined data sets provide clues about the mechanisms that initiate and sustain the decimetric bursts and about interactions between thermal and nonthermal plasmas at different locations in the solar atmosphere. On one day, frequent, low-level hard X-ray flaring observed by RHESSI appears to have had no clear affect on the evolution of two closely-spaced Type I noise storm sources lying above the target active region. EIT images however, indicate nearly continuous restructuring of the underlying EUV loops which, through accompanying low-level magnetic reconnection, might give rise to nonthermal particles and plasma turbulence that sustain the long-lasting Type I burst emission. On another day, the onset of an impulsive hard X-ray burst and subsequent decimetric burst emission followed the gradual displacement and coalescence of a small patch of magnetic magnetic polarity with a pre-existing area of mixed magnetic polarity. The time delay of the impulsive 20 and 91 cm bursts by up to 20 min suggests that these events were unlikely to represent the main sites of flare electron acceleration, but instead are related to the rearrangement of the coronal magnetic field after the main flare at lower altitude. Although the X-ray flare is associated with the decimetric burst, the brightness and structure of a long-lasting Type I noise storm from the same region was not affected by the flare. This suggests that the reconfiguration of the coronal magnetic fields and the subsequent energy release that gave rise to the impulsive burst emission did not significantly perturb that part of the corona where the noise storm emission was located.     

RHESSI observation of the M4.0 flare on 17 March 2002

The M4.0/SF flare on 17 March 2002 is a good example of the early observations with RHESSI. We presenthard X-ray images, light curves and energy spectra of individual hard X-ray sources, the spatial relationship between the hard X-ray sources and the H emission regions, and comparisons of light curves observed by RHESSI and GOES. We found that the picture exhibited by RHESSI is consistent with the general cartoon of a solar flare. In particular, we showed that the hard X-ray image spectra could be explained by a power-law electron beam with a lower energy cutoff E_c. The derived E_c could be as high as 40 keV, larger than the usually value of 20 keV.     

RHESSI observations of the coronal component ofsolar flare hard X-ray sources

In flares that occur behind the limb, the intense chromospheric (foot-point) part of the hard X-ray source is occulted, thus permitting good observations of the coronal component. Between 15 and 18 April 2002, RHESSI observed a series of small (GOES Class C) flares produced by the active region NOAA 9905 as it rotated behind the west limb. A preliminary analysis of the observed hard X-ray sources in the 17–18 April 2002 flares has confirmed that flare-associated sources of gradual 12–25 keV X-ray emission can exist in the corona at heights up to 27000 km.     

Evidence of the radio-quiet hard X-ray precursor of the 13 December 2006 solar flare

We report multi-wavelength investigation of the pre-impulsive phase of the 13 December 2006 X-class solar flare. We use hard X-ray data from the anticoincidence system of spectrometer onboard INTEGRAL (ACS) jointly with soft X-ray data from the GOES-12 and Hinode satellites. Radio data are from Nobeyama and Learmonth solar observatories and from the Culgoora Solar Radio Spectrograph. The main finding of our analysis is a spiky increase of the ACS count rate accompanied by surprisingly gradual and weak growth of microwave emission and without detectable radio emission at meter and decimeter wavelengths about 10 min prior to the impulsive phase of the solar flare. At the time of this pre-flare hard X-ray burst the onset of the GOES soft X-ray event has been reported, positive derivative of the GOES soft X-ray flux started to rise and a bright spot has appeared in the images of the Hinode X-ray telescope (XRT) between the flare ribbons near the magnetic inversion line close to the sources of thermal and non-thermal hard X-ray emission observed by Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) during the flare. These facts we consider as evidences of solar origin of the increased pre-flare ACS count rate. We briefly discuss a possible cause of the pre-flare emission peculiarities.     

Narrowband dm-spikes in the frequency range above 1 GHz and hard X-ray emission

Narrowband dm-spikes observed in nine intervals during five solar flares in the 1–2 GHz range were analyzed together with the RHESSI and HXRS observations. It was found that the over-frequency integrated radio flux during the spike period is closely related with the hard X-ray bursts (the correlation coefficient was 0.7–0.9) and their time delays after X-rays were 2–14 s, with one exception (March 18, 2003) where the time delay was opposite −15 s. Association of spikes with X-ray spectral characteristics enabled us to divide the spikes into two groups: (a) those observed before the soft X-ray flare maximum and, (b) those observed after this maximum. While for the spikes observed after the flare maximum no systematic spectral characteristics were found, the spikes, observed before the flare maximum were at their beginning associated with relatively hard X-ray spectra and their hardness decreased with time. The RHESSI X-ray sources were compact, only in the March 18, 2003 event an additional X-ray source appeared just at the time of the dm-spikes observation. Fourier transformation of the dynamic spectra of spikes was done to compare their dynamics with the X-ray spectral indices. No correlation between power-law spike and X-ray indices were found. It indicates that the MHD turbulence, if it plays a role, does not represent a strong connection between the spectral characteristics of the dm-spikes and associated X-ray bursts. Furthermore, the results were compared with those obtained by (Aschwanden, M.J., Güdel, M. The coevolution of decimetric millisecond spikes and hard X-ray emission during solar flares. Astrophys. J. 401, 736–753, 1992) for spikes observed on lower radio frequencies. Contrary to their results, no monotonic dependence between time delays and X-ray intensities were found. Finally, the results were discussed using the model of the narrowband dm-spikes and model of electron acceleration in the collapsing magnetic trap.     

Hard X-ray and metric/decimetric spatially resolvedobservations of the 10 April 2002 solar flare

The GOES M8.2 flare on 10 April 2002 at 1230 UT was observed at X-ray wavelengths by RHESSI and atmetric/decimetric wavelengths by the Nançay Radioheliograph (NRH). We discuss the temporal evolution of X-ray sources together with the evolution of the radio emission sites observed at different coronal heights by the NRH. While the first strong HXR peak at energies above 50 keV arises from energy release in compact magnetic structures (with spatial scales of a few 10⁴ km) and is not associated with strong radio emission, the second one leads to energy release in magnetic structures with scales larger than 10⁵ km and is associated with intense decimetric/metric and dekametric emissions. We discuss these observations in the context of the acceleration sites of energetic electrons interacting at the Sun and of escaping ones.     

Hard X-ray footpoint motion and acceleration mechanism

An intense (X9.4/2B) flare, which occurred on 6 November 1997, was observed with the hard X-ray telescope on board Yohkoh. In the M2- (33 – 53 keV) and H-band (53 – 93 keV), This flare clearly show double footpoint sources during its impulsive phase. We have analyzed the locations and motions of these sources in detail. It is found that, at 11:53:06 UT, one of the footpoint sources in the M2-band moved to a new position earlier than the corresponding source in the H-band. The time-lag is about one second and the separation between the old and new positions is 5 arcsec. This happened between two major spikes in the time profile of hard X-ray intensity. This apparent motion might indicate that an epoch of energy release finished somewhere high in the corona and the next epoch started in another magnetic field system. This observation clearly shows that higher energy electrons precipitate into the footpoint region later. We try to interpret this particular phenomenon under the two assumptions, which are the direct precipitation model and the trap-and-precipitation model.     

Solar flare electron acceleration: Comparing theories and observations

A popular scenario for electron acceleration in solar flares is transit-time damping of low-frequency MHD waves excited by reconnection and its outflows. The scenario requires several processes in sequence to yield energetic electrons of the observed large number. Until now there was very little evidence for this scenario, as it is even not clear where the flare energy is released. RHESSI measurements of bremsstrahlung by non-thermal flare electrons yield energy estimates as well as the position where the energy is deposited. Thus quantitative measurements can be put into the frame of the global magnetic field configuration as seen in coronal EUV line observations. We present RHESSI observations combined with TRACE data that suggest primary energy inputs mostly into electron acceleration and to a minor fraction into coronal heating and primary motion. The more sensitive and lower energy X-ray observations by RHESSI have found also small events (C class) at the time of the acceleration of electron beams exciting meter wave Type III bursts. However, not all RHESSI flares involve Type III radio emissions. The association of other decimeter radio emissions, such as narrowband spikes and pulsations, with X-rays is summarized in view of electron acceleration.     

HALE 17590 活动区的爆发辐射

本文根据1981年HALE 17590 太阳活动区的观测资料,着重分析了它的射电辐射特性后发现:(1)在光学活动区发展的上升阶段,每串射电爆发的强度也有由弱到强的变化,其频谱由单调谱变成不规则谱和U型谱;(2)对大的耀斑爆发而言,射电爆发的先兆相比X射线爆早。在射电先兆相期间常伴有谱斑增亮和暗条激活等现象;(3)大耀斑爆发的脉冲极大时刻在射电8毫米波段到来最早。      

Energetic electrons in solar flares as viewed in X-rays

Hard X-ray observations provide the most direct diagnostic we have of the suprathermal electrons and the hottest thermal plasma present in solar flares. The Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is obtaining the most comprehensive observations of individual solar flares ever available in hard X-rays. For the first time, high-resolution spectra are available for a large number of flares that accurately display the spectral shape and its evolution and, in many cases, allow us to identify the transition from the bremsstrahlung X-rays produced by suprathermal electrons to the bremsstrahlung at lower energies emitted by thermal plasma. Also, for the first time, images can be produced in arbitrary energy bands above 3–4 keV, and spectra of distinct imaged components can be obtained.I review what we have learned from RHESSI observations about flare suprathermal electron distributions and their evolution. Next, I present computations of the energy deposited by these suprathermal electrons in individual flares and compare this with the energy contained in the hot thermal plasma. I point out unsolved problems in deducing both suprathermal electron distributions and the energy content of the thermal plasma, and discuss possible solutions. Finally, I present evidence that electron acceleration is associated with magnetic reconnection in the corona.     

Radio fine structures in dm–cm wavelength range associated with magnetic reconnection processes

Radio bursts with fine structures in decimetric–centimetric wave range are generally believed to manifest the primary energy release process during flare/CME events. By spectropolarimeters in 1–2 GHz, 2.6–3.8 GHz, and 5.2–7.6 GHz at NAOC/Huairou with very high temporal (1.25–8 ms) and spectral (4–20 MHz) resolutions, the zebra patterns, spikes, and new types of radio fine structures with mixed frequency drift features are observed during several significant flare/CME events. In this paper we will discuss the occurrence of radio fine structures during the impulsive phase of flares and/or CME initiations, which may be connected to the magnetic reconnection processes.     

A question raised from the observation of dynamic cusp formation: When and where does particle acceleration occur?

We present observations of a C9.4 flare on 2002 June 2 in EUV (TRACE) and X-rays (RHESSI). The multiwavelength data reveal: (1) the involvement of a quadrupole magnetic configuration; (2) loop expansion and ribbon motion in the pre-impulsive phase; (3) gradual formation of a new compact loop with a long cusp at the top during the impulsive phase of the flare; (4) appearance of a large, twisted loop above the cusp expanding outward immediately after the hard X-ray peak; and (5) X-ray emission observed only from the new compact loop and the cusp. In particular, the gradual formation of an EUV cusp feature is very clear. The observations also reveal the timing of the cusp formation and particle acceleration: most of the impulsive hard X-rays (>25 keV) were emitted before the cusp was seen. This suggests that fast reconnection occurred during the restructuring of the magnetic configuration, resulting in more efficient particle acceleration, while the reconnection slowed after the cusp was completely formed and the magnetic geometry was stabilized. This observation is consistent with the observations obtained with Yohkoh/Soft X-ray Telescope (SXT) that soft X-ray cusp structures only appear after the major impulsive energy release in solar flares. These observations have important implications for the modeling of magnetic reconnection and particle acceleration.     

The temperature map of the loop and coronal sources of an occulted solar flare

An occulted solar flare occurred at about 06:07 UT on 2002, November 2. The RHESSI X-ray images show two separate parts. The lower part consists of a complete loop and the upper part a coronal source which well extends above the solar limb. The loop source shrank for about 3 min with a speed of ∼24 km s⁻¹ during the early impulsive phase and then expanded at ∼7 km s⁻¹, while the coronal source presented an upward motion at about 6 km s⁻¹. We obtained the temperature map of the loop source from RHESSI image spectrum. The temperature of the loop increases with altitude, indicating that the reconnection X-point of this flare is located above the loop source. However, the apparent coronal source is the top of another independent large-scale loop.     

Reorganization of the solar corona following a C4.7 flare

Yohkoh X-ray images, multifrequency two-dimentional observations of the Nancay Radioheliograph, Kitt Peak and Mees magnetograms provide a unique set of data with which to study a C4.7 long-duration flare that was observed close to the equator (S07, W11) on 25 Oct. 1994 at 09:49 UT. Linear force-free field extrapolations indicate a very high degree of non-potentiality in the active region. The X-ray flare started with the expansion of spectacular twisted loops. Fifteen minutes after the flare onset sporadic radio (type III) bursts were observed spreading over an area of almost 1/3 of the solar disc and two remote X-ray brightenings appeared over quiet regions of opposite magnetic polarity located in on opposite hemispheres of the Sun. In the close vicinity of these remote brightenings two coronal holes formed. The timing and location of these events combined with the overall magnetic configuration provide evidence for a large-scale magnetic reconnection occurring between the expanding twisted loops and the overlying huge loops which inter-connect quiet solar regions.