Electron Spikes,Type III Radio Bursts and EUV Jets on 22 February 2010

The Solar Electron Proton Telescope on board the twin STEREO spacecraft measures electrons and ions in the energy range from 30 to above 400 keV with an energy resolution better than 10%. On 22 February 2010 during a short interval of 100 minutes, a sequence of impulsive energetic electron events in the range below 120 keV was observed with the STEREO-A/SEPT instrument. Each of the four events was associated with a type III radio burst and a narrow EUV jet. All the events show nearly symmetric “spike”-like time profiles with very short durations ≃ 5 min. The estimated electron injection time for each individual event shows a small time delay between the electron spike and the corresponding type III radio emission and a close coincidence with an EUV jet. These observations reveal the existence of spike-like electron events showing nearly “scatter-free” propagation from the Sun to STEREO-A. From the time coincidence we infer that the mildly relativistic electrons are accelerated at the same time and at the same location as the accompanying type III emitting electrons and coronal EUV jets. The characteristics of the spikes reflect the injection and acceleration profiles in the corona rather than interplanetary propagation effects.     

Sources of SEP Acceleration during a Flare – CME Event

A high-speed, halo-type coronal mass ejection (CME), associated with a GOES M4.6 soft X-ray flare in NOAA AR 0180 at S12W29 and an EIT wave and dimming, occurred on 9 November 2002. A complex radio event was observed during the same period. It included narrow-band fluctuations and frequency-drifting features in the metric wavelength range, type III burst groups at metric – hectometric wavelengths, and an interplanetary type II radio burst, which was visible in the dynamic radio spectrum below 14 MHz. To study the association of the recorded solar energetic particle (SEP) populations with the propagating CME and flaring, we perform a multi-wavelength analysis using radio spectral and imaging observations combined with white-light, EUV, hard X-ray, and magnetogram data. Velocity dispersion analysis of the particle distributions (SOHO and Wind in situ observations) provides estimates for the release times of electrons and protons. Our analysis indicates that proton acceleration was delayed compared to the electrons. The dynamics of the interplanetary type II burst identify the burst source as a bow shock created by the fast CME. The type III burst groups, with start times close to the estimated electron-release times, trace electron beams travelling along open field lines into the interplanetary space. The type III bursts seem to encounter a steep density gradient as they overtake the type II shock front, resulting in an abrupt change in the frequency drift rate of the type III burst emission. Our study presents evidence in support of a scenario in which electrons are accelerated low in the corona behind the CME shock front, while protons are accelerated later, possibly at the CME bow shock high in the corona.     

Particle Acceleration and Propagation in Strong Flares without Major Solar Energetic Particle Events

Solar energetic particles (SEPs) detected in space are statistically associated with flares and coronal mass ejections (CMEs). But it is not clear how these processes actually contribute to the acceleration and transport of the particles. The present work addresses the question why flares accompanied by intense soft X-ray bursts may not produce SEPs detected by observations with the GOES spacecraft. We consider all X-class X-ray bursts between 1996 and 2006 from the western solar hemisphere. 21 out of 69 have no signature in GOES proton intensities above 10 MeV, despite being significant accelerators of electrons, as shown by their radio emission at cm wavelengths. The majority (11/20) has no type III radio bursts from electron beams escaping towards interplanetary space during the impulsive flare phase. Together with other radio properties, this indicates that the electrons accelerated during the impulsive flare phase remain confined in the low corona. This occurs in flares with and without a CME. Although GOES saw no protons above 10 MeV at geosynchronous orbit, energetic particles were detected in some (4/11) confined events at Lagrangian point L1 aboard ACE or SoHO. These events have, besides the confined microwave emission, dm-m wave type II and type IV bursts indicating an independent accelerator in the corona. Three of them are accompanied by CMEs. We conclude that the principal reason why major solar flares in the western hemisphere are not associated with SEPs is the confinement of particles accelerated in the impulsive phase. A coronal shock wave or the restructuring of the magnetically stressed corona, indicated by the type II and IV bursts, can explain the detection of SEPs when flare-accelerated particles do not reach open magnetic field lines. But the mere presence of these radio signatures, especially of a metric type II burst, is not a sufficient condition for a major SEP event.     

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.     

太阳米波和分米波Ⅱ型、Ⅲ型射电暴及其精细结构观测研究进展

太阳米波和分米波的射电观测是对太阳爆发过程中耀斑和日冕物质抛射现象研究的重要观测手段。米波和分米波的太阳射电暴以相干等离子体辐射为主导,表现出在时域和频域的多样性和复杂性。其中Ⅱ型射电暴是激波在日冕中运动引起电磁波辐射的结果。在Ⅱ型射电暴方面,首先对米波Ⅱ型射电暴的激波起源问题和米波Ⅱ型射电暴与行星际Ⅱ型射电暴的关系问题进行了讨论;其次,结合Lin-Forbes太阳爆发理论模型对Ⅱ型射电暴的开始时间和起始频率进行讨论:最后,对Ⅱ型射电暴信号中包含的两种射电精细结构,Herringbone结构(即鱼骨结构)和与激波相关的Ⅲ型射电暴也分别进行了讨论。Ⅲ型射电暴是高能电子束在日冕中运动产生电磁波辐射的结果。在Ⅲ型射电暴方面,首先介绍了利用Ⅲ型射电暴对日冕磁场位形和等离子体密度进行研究的具体方法;其次,对利用Ⅲ型射电暴测量日冕温度的最新理论进行介绍;最后,对Ⅲ型射电暴和Ⅱ型射电暴的时间关系、Ⅲ型射电暴和粒子加速以及Ⅲ型射电暴信号中包含的射电精细结构(例如斑马纹、纤维爆发及尖峰辐射)等问题进行讨论并介绍有关的最新研究进展。     

The participation of nuclei in type-III-related electron streams

We study 27 increases of the flux of 300–800 keV electrons on board HELIOS A or B, associated with intense type III radio bursts close to perihelion passages of the two spacecraft, during the solar minimum. Electrons can be detected inside cones with an angular width between 30° and 60°. Though only intense type III bursts are associated with recognizable electron events in space, such an association does not exist for all of them; this fact and great differences in fluxes of the individual events indicate that, apart from the intensity, also some other charactefistic of the type III burst acceleration or propagation process determines the resulting flux of electrons in space; the energy spectrum of the accelerated electrons is one of the likely candidates. A comparison of the electron flux in these events with the flux of 1.7–3.7 MeV nucl^–1 helium reveals very large variations of the helium/electron flux ratio, by a factor of at least 15 and possibly much higher. We demonstrate that these variations are not caused by propagation effects in interplanetary space. Therefore, they must be due either to propagation effects in the solar corona or, more likely, to intrinsic variations in the relative production of electrons and nuclei in the type III burst process. An extrapolation of the observed fluxes to 1 AU shows that in only 7 of the 27 electron events studied might a marginal > 1.7 MeV helium flux be recognized ar the Earth distance.     

Electron Acceleration in the Corona

The series of nine impulsive, highly collimated beams of near-relativistic electrons seen by ACE/EPAM on 26 and 27 June 2004 occurred at a quiet time with respect to solar flare and CME production. However, they were accompanied by decametric type III radio bursts observed by WIND/WAVES, which had progressively higher starting frequencies, suggestive of coronal acceleration. There were no CMEs seen by SOHO/LASCO in association with any of the type III bursts except possibly the first. The energy spectrum of the electrons was soft, typically E^−4.5 but extended up to at least ∼200 keV. We suggest that the source region for these events is in the high corona. We discuss this result in the context of solar electron acceleration at other times.     

Type III Bursts Associated with Millisecond Spikes Which Occurred Within the Frequency Range from 230 to 300 MHz

The metric spectral data obtained at the Yunnan Observatory from July 1990 to December 1991 are analysed and some type III bursts associated with millisecond spikes are found. Their different morphologies reveal the relation between the type III bursts and the millisecond spikes. Based on the occurrence time and the characteristics of continuity and changes of the morphologies in the spikes and the type III bursts of two typical events, we verify that the acceleration region of the coronal electrons is located above the emitting region of the millisecond spikes and the type III bursts. The observations show that the interface frequency of a pair of type III bursts lies near 250 MHz. Finally the authors attempt to explain qualitatively the generating mechanism of the metric millisecond spike-type III bursts by means of the plasma hypothesis.     

Comparative Analysis of Type ii Bursts and of Thermal and non-Thermal Flare Signatures

The relationship between metric type II radio bursts and solar flares is studied. Well-defined correlations between the properties of type II bursts and the characteristics of associated microwave and soft X-ray bursts are established in two entirely independent data sets. It is shown that the correlations are strongly affected by the wide range of coronal Alfvén velocities involved, comprising values from only 150 up to 800 km s^–1, with a typical value of 400 km s^–1. After careful data analysis it was inferred that type II bursts are more closely related to the soft X-ray bursts than they are to microwave bursts. The correlations indicate that type II burst shocks are preferably generated by flares with a relatively strong thermal component, and that the shocks are probably ignited by the plasma expansion associated with the 'evaporation' process in the transition region. Although the results imply that the majority of metric type II bursts are caused by flares, a simple geometrical consideration shows that a fraction of non-flare type II bursts cannot be explained by behind-limb events and that roughly 10% of metric type II bursts should be attributed to non-flare coronal mass ejections.     

Interplanetary phenomena and solar radio bursts

The relationship between solar radio emissions and transient interplanetary phenomena is reviewed. It is believed that the most significant advance in recent years has come from coordinated studies of coronal mass ejections and moving type IV bursts, where the evidence appears to favour the Langmuir wave hypothesis as the emission mechanism. Type II bursts are not generally a signature of the main energetic particle acceleration in flares. They do, however, occasionally propagate to 1 AU, and beyond, where they are normally accompanied by protons in the 20 MeV region. Apart from the impulsive microwave burst, there is no reliable radio signature associated with energetic particle acceleration in flares, although many phenomena have high correlations with radio emissions. The exceptions suggest that such correlations may be incidental. Therefore, it is concluded that attention should also be given to events with a positive absence of radio emission in order to make progress in understanding solar processes.Proceedings of the Workshop on Radio Continua during Solar Flares, held at Duino (Trieste), Italy, 27–31 May, 1985.     

A Type IV Radio Burst Associated with Type III Burst,Pulsation and Spike Emissions

A type IV radio burst accompanied by several normal- and reverse-drifting type III bursts, multiple long-term quasi-periodic pulsations and spikes was observed with the radio spectrometers (1.0–2.0 and 2.6–3.8 GHz) at National Astronomical Observatories of China (NAOC) on 23 September 1998. In combination with the images of Siberian Solar Radio Telescope (SSRT) of Russia, the complex and multiple magnetic structures inferred from the radio bursts reveal the existence of both large-scale and small-scale magnetic structures. This event suggests that the geometries of coronal magnetic fields contain multiple discrete electron acceleration/injection sites at different heights, and extended open and closed magnetic field lines. It can be shown that the energetic electrons gain access to open, diverging and closed field lines thus producing different types of radio bursts. From the characteristics of position, polarization, dispersion and displacement of the sources, the model of the type IV event is supported, which involves synchrotron emission from the electrons confined by the rapid scattering through the interaction of hydromagnetic wave with particles.     

Characteristics of flares producing metric type II bursts and coronal mass ejections

We attempt to study the origin of coronal shocks by comparing several flare characteristics for two groups of flares: those with associated metric type II bursts and coronal mass ejections (CMEs) and those with associated metric type II bursts but no CMEs. CMEs accompany about 60% of all flares with type II bursts for solar longitudes greater than 30°, where CMEs are well observed with the NRL Solwind coronagraph. H flare areas, 1–8 Å X-ray fluxes, and impulsive 3 cm fluxes are all statistically smaller for events with no CMEs than for events with CMEs. It appears that both compact and large mass ejection flares are associated with type II bursts. The events with no CMEs imply that at least many type II shocks are not piston-driven, but the large number of events of both groups with small 3 cm bursts does not support the usual assumption that type II shocks are produced by large energy releases in flare impulsive phases. The poor correlation between 3 cm burst fluxes and the occurrence of type II bursts may be due to large variations in the coronal Alfvén velocity.Sachs/Freeman Associates, Inc., Bowie, MD 20715, U.S.A.     

Characteristics of shock-associated fast-drift kilometric radio bursts

The existence of a class of fast-drift, shock-associated (SA), kilometric radio bursts which occur at the time of metric type II emission and which are not entirely the kilometric continuation of metric type III bursts has been reported previously (Cane et al., 1981). In this paper, we establish unambiguous SA event criteria for the purpose of statistically comparing SA events with conventional kilometric type III bursts. We apply these criteria to all long-duration, fast-drift bursts observed by the ISEE-3 spacecraft during a 28-month interval and find that more than 70% of the events satisfying the criteria are associated with the radio signatures of coronal shocks. If a given event in our sample is associated with a metric type II or type IV burst, it is 13 times more likely to satisfy the SA criteria than an event associated only with metric type III activity. Compared with conventional kilometric type III bursts, the characteristics of these SA events are longer duration, higher maximum intensity, and a larger number of components. Differences in these characteristics for the two classes of events are not sufficient to distinguish all SA events from conventional type III bursts. The consistent lack of reported metric type III activity during the latter part of the candidate events suggests that some of the electrons are accelerated high in the corona, at or near the altitude of the shock.     

Characteristics of solar coronal source regions producing 3He-rich particle events

We use H, X-ray, and kilometric radio data to examine the solar coronal activity associated with energetic (1 MeV/nucl^–1) ³He-rich particle events observed near Earth. The basis of the study is the 12 ³He-rich events observed in association with impulsive 2 to 100 keV electron events reported by Reames et al. (1985). We find that when H and X-ray brightenings can be associated with ³He/electron events, they have onsets coinciding to within 1 min of that of the associated metric type III bursts. In three or four events we found no associated H or X-ray flares, and in two events even the metric type III bursts were weak or absent. The measured low-energy (2 keV) electron spectra for these events show no evidence of a flattening due to Coulomb collisional losses. These results and several other recent findings are consistent with the idea that the ³He/electron events are due to particle acceleration in the corona well above the associated H and X-ray flares.     

Energetic Particle Acceleration and Propagation in Strong CME-Less Flares

Flares and coronal mass ejections (CMEs) contribute to the acceleration and propagation of solar energetic particles (SEP) detected in the interplanetary space, but the exact roles of these phenomena are yet to be understood. We examine two types of energetic particle tracers related with 15 CME-less flares that emit bright soft X-ray bursts (GOES X class): radio emission of flare-accelerated electrons and in situ measurements of energetic electrons and protons near 1 AU. The CME-less flares are found to be vigorous accelerators of microwave-emitting electrons, which remain confined in low coronal structures. This is shown by unusually steep low-frequency microwave spectra and by lack of radio emission from the middle and high corona, including dm?–?m wave type IV continua and metre-to-hectometre type III bursts. The confinement of the particles accelerated in CME-less flares agrees with the magnetic field configuration of these events inferred by others. Two events produced isolated metric type II bursts revealing coronal shock waves. None of the seven flares in the western hemisphere was followed by enhanced particle fluxes in the GOES detectors, but one, which was accompanied by a type II burst, caused a weak SEP event detected at SoHO and ACE. Three of the CME-less flares were followed within some hours by SEP-associated flares from the same active region. These SEP-producing events were clearly distinct from the CME-less ones by their association with fast and broad CMEs, dm?–?m wave radio emission, and intense DH type III bursts. We conclude that radio emission at decimetre and longer waves is a reliable indication that flare-accelerated particles have access to the high corona and interplanetary space. The absence of such emission can be used as a signal that no SEP event is to be expected despite the occurrence of a strong soft X-ray burst.     

Interplanetary type III radiobursts and relativistic electrons

For the time periods 1979 April 22–May 17 and 1980 May 9–June 10, when the HELIOS spacecraft were located inside 0.5 AU, we compared the antenna temperature T _A of the 466 kHz type III bursts measured by the SBH instrument on ISEE 3 with the fluxes of 0.5 MeV electrons measured by HELIOS. For 51 flare-associated kilometric type III bursts (FAIII bursts) with log(T _A) > 10 we find: (1) 25 bursts (49%) are accompanied by a relativistic electron event in interplanetary space, (2) the probability for detection of an electron event decreases from more than 74% inside a cone of ± 20 ° to 56% inside a cone of ± 60° around the flare site, (3) there is only a small correlation between the brightness temperature of the radio burst and the size of the electron event, and (4) despite the broad scatter of these values there is a clear indication that for a given size of the relativistic electron event the intensity of the type III burst is about a factor of 5 higher if it is accompanied by a type II burst. These results give evidence (a) that at least part of the relativistic electrons frequently is accelerated together with non-relativistic electrons and (b) that the coronal shock associated with the metric type II burst has a weaker effect on relativistic than on non-relativistic electrons.Now at DFVLR, Oberpfaffenhofen, Germany.     

Post-Impulsive-Phase Acceleration in a Wide Range of Solar Longitudes

We have analyzed five solar energetic particle (SEP) events observed aboard the SOHO spacecraft during 1996–1997. All events were associated with impulsive soft X-ray flares, Type II radio bursts and coronal mass ejections (CMEs). Most attention is concentrated on the SEP acceleration during the first 100 minutes after the flare impulsive phase, post-impulsive-phase acceleration, being observed in eruptions centered at different solar longitudes. As a representative pattern of a (nearly) well-connected event, we consider the west flare and CME of 9 July 1996 (S10 W30). Similarities and dissimilarities of the post-impulsive-phase acceleration at large heliocentric-angle distance from the eruption center are illustrated with the 24 September 1997 event (S31 E19). We conclude that the proton acceleration at intermediate scales, between flare acceleration and interplanetary CME-driven shock acceleration, significantly contributes to the production of ≳10 MeV protons. This post-impulsive-phase acceleration seems to be caused by the CME lift-off.     

Velocities of type II solar radio events

Radial velocities for 144 simple but representative type II bursts were determined from measured frequency time histories. The velocity distribution is peaked in the region between 500 and 700 km s ^–1 (with the exact value dependent upon the coronal density model assumed) and skewed towards the larger velocities. In 85 % of the cases it was found that the velocities were constant with height. In the remaining 15 % the drift rate decreased drastically at low frequencies. This tended to occur for events having high initial velocities. The measured velocity is dependent upon the properties of the flare event but does not appear to be related to other characteristics of the radio burst. Comparisons show that the group of type II events studied had a velocity distribution which was comparable with that for coronal mass ejection events seen in association with type II bursts. The measured velocities were however statistically smaller than those of interplanetary type II bursts.     

Sharp Decreases of Solar Metric Radio Storm Emission

We discuss a little-known variety of sharp decreases of long-duration meter-wavelength noise storms and type IV bursts. A survey of the IZMIRAN and AIP radio observations shows that a decrease or nearly complete disappearance of the continuum and bursts developing over tens of minutes without a subsequent recovery of the radio flux occasionally occurs. The decrease is usually preceded by a short-duration (several tens of minutes) enhancement of the radio emission. In these events, the onset of the flux decrease drifts from high to low frequencies with a rate of –(0.05–0.35) MHz s^–1, comparable to the drift rates of noise-storm onsets and of chains of type I bursts. White-light coronagraph observations, as well as the characteristics of the accompanying microwave and soft X-ray emissions, provide evidence that such radio decreases appear to be associated with coronal mass ejections (CMEs) and post-CME phenomena. Yohkoh/SXT images show radio flux decrease events which are accompanied by significant rearrangements of coronal structures. We suggest that the radio flux variations are caused by CME interactions with pre-existing coronal arcade structures which are sources of noise storms and energetic electron acceleration. The fact that the noise-storm decreases develop with delays of several tens of minutes relative to the associated microwave burst peak, when the corresponding CME front is located at heights of several R , however, is not explained.     

Second-stage acceleration in a limb-occulted flare

We analyze hard and soft X-ray, microwave and meter wave radio, interplanetary particle, and optical data for the complex energetic solar event of 22 July 1972. The flare responsible for the observed phenomena most likely occurred 20° beyond the NW limb of the Sun, corresponding to an occultation height of 45 000 km. A group of type III radio bursts at meter wavelengths appeared to mark the impulsive phase of the flare, but no impulsive hard X-ray or microwave burst was observed. These impulsive-phase phenomena were apparently occulted by the solar disk as was the soft X-ray source that invariably accompanies an H flare. Nevertheless essentially all of the characteristic phenomena associated with second-stage acceleration in flares - type II radio burst, gradual second stage hard X-ray burst, meter wave flare continuum (FC II), extended microwave continuum, energetic electrons and ions in the interplanetary medium - were observed. The spectrum of the escaping electrons observed near Earth was approximately the same as that of the solar population and extended to well above 1 MeV.Our analysis of the data leads to the following results: (1) All characteristics are consistent with a hard X-ray source density n _i 10⁸ cm^–3 and magnetic field strength 10 G. (2) The second-stage acceleration was a physically distinct phenomenon which occurred for tens of minutes following the impulsive phase. (3) The acceleration occurred continuously throughout the event and was spatially widespread. (4) The accelerating agent was very likely the shock wave associated with the type II burst. (5) The emission mechanism for the meter-wave flare continuum source may have been plasma-wave conversion, rather than gyrosynchrotron emission.