Solar hard X-ray bursts

The major results from SMM are presented as they relate to our understanding of the energy release and particle transportation processes that lead to the high-energy X-ray aspects of solar flares. Evidence is reviewed for a 152–158 day periodicity in various aspects of solar activity including the rate of occurrence of hard X-ray and gamma-ray flares. The statistical properties of over 7000 hard X-ray flares detected with the Hard X-Ray Burst Spectrometer are presented including the spectrum of peak rates and the distribution of the photon number spectrum. A flare classification scheme introduced by Tanaka is used to divide flares into three different types. Type A flares have purely thermal, compact sources with very steep hard X-ray spectra. Type B flares are impulsive bursts which show double footpoints in hard X-rays, and soft-hard-soft spectral evolution. Type C flares have gradually varying hard X-ray and microwave fluxes from high altitudes and show hardening of the X-ray spectrum through the peak and on the decay. SMM data are presented for examples of type B and type C events. New results are presented showing coincident hard X-rays, O v, and UV continuum observations in type B events with a time resolution of 128 ms. The subsecond variations in the hard X-ray flux during 10% of the stronger events are discussed and the fastest observed variation in a time of 20 ms is presented. The properties of type C flares are presented as determined primarily from the non-imaged hard X-ray and microwave spectral data. A model based on the association of type C flares and coronal mass ejections is presented to explain many of the characteristics of these gradual flares.     

High energy observations of June 1980 solar flares

The paper presents a detailed study of the high energy X-ray observation of the most unusual solar events observed on 4 and 7 June, 1980 with the Hard X-Ray Burst Spectrometer (HXRBS) on Solar Maximum Mission (SMM) satellite. The hard X-ray data of the events are also compared with the radio microwave fluxes.The X-ray time profiles of these flares are characterized by the occurrence of impulsive phase superposed with a number of narrow spikes before the occurrence of the main energetic events. Studies of the temporal and spectral properties of these events indicated a quasi-oscillatory nature of the sources. Various models for explaining the evolution of the events are considered and the sequential firing loop model seems to be consistent with the observations of the events.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

The Neupert effect: What can it tell us about the impulsive and gradual phases of solar flares?

The Neupert effect is the name given to the correlation observed in many flares between the time-integrated microwave and hard X-ray emissions and the soft X-ray emission light curve. We have used hard X-ray data from the Hard X-Ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM) and soft X-ray data from the detector on GOES to determine what fraction of all events show this correlation and how the correlation changes from the impulsive to the gradual phase. We have found that of 66 HXRBS events observed in 1980 with a peak rate of > 1000 counts s^-1, 58 (80%) showed good correlations with peaks in the GOES time derivative plot corresponding to peaks in the hard X-ray (HXR) plots to within ±20 s. In 20 of these good-correlation cases (30%), the soft X-ray (SXR) time derivative stays high after the HXR emission has decreased suggesting that the later emissions result from energy release in a loop already affected by the initial energy release. In 8 of the 13 flares that showed poor correlation, the SXR time derivative shows no peak corresponding to the initial HXR impulsive peak that has structure on a time scale of 1 s. This suggests that in these events the initial impulsive energy release results primarily in electron acceleration, and that the secondary plasma heating from the accelerated electrons contributes relatively little compared to the more gradual plasma heating already taking place at the same time. The more gradually varying events, commonly referred to as type C flares, tend to show poorer correlation between the SXR time derivative and the HXR time profile. Of 26 GOES X1 or greater flares recorded between 1980 and 1989 with HXR peaks lasting over 10 rain, 13 (50%) showed poor correlation with the gradual HXR peaks either not registering at all in the SXR time derivative plots or showing up as very broad peaks. In one case, on 1981 April 26, the SXR time derivative peak was delayed by 13 rain. Only 17 (65%) of the 26 X-flares had an earlier, impulsive component and of those, 12 (71%) showed good correlation between the impulsive peaks.     

Comparison of mm-wave and X-ray diagnostics of flare plasma

To compare mm-wave and X-ray diagnostics of solar flare plasma, five flares observed in 1980–1991 in Metsähovi at 22 and 37 GHz and with GOES, SMM, and GRO are studied. The first impulsive peak of the mm-wave bursts under investigation coincides in time with hard X-ray emission. The second gradual component in mm-wave emission coincides with the maximum of the soft X-ray emission measure. The bremsstrahlung mm-wave radiation from hot chromospheric plasma and gyrosynchrotron radiation driven by common population of superthermal electrons are calculated. It is shown that for mm-wave events with the first peak intensity 100 s.f.u., the thermal bremsstrahlung is more important than the gyrosynchrotron emission. The total energy of fast electrons deduced from the first peak of mm-wave bursts is one to two orders of magnitude less than that determined from the hard X-ray emission in the approximation of a thick-target nonthermal model. That can testify in favour of the hybrid thermal/nonthermal model proposed by Holman and Benka (1992). The emission measure and the energy of evaporated plasma using both mm-wave and soft X-ray data are also determined. For events investigated here the energy of evaporated chromospheric plasma is larger than the total energy of fast electron beams. We have concluded that, for evaporation, additional energy release in the chromosphere is needed. The possibility of such energy release in the framework of an advanced circuit model for solar flares is discussed.     

The <Emphasis Type="Italic">Ulysses</Emphasis> Catalog of Solar Hard X-Ray Flares

Ulysses was launched in October 1990, and its Solar X-ray/Cosmic Gamma-Ray Burst Experiment (GRB) has provided more than 13 years of uninterrupted observations of solar X-ray flare activity. Due to the large variation of the relative solar latitude and longitude of the spacecraft orbit with respect to the Earth, the perspective of the GRB instrument often differed significantly from that of X-ray instruments on Earth-orbiting satellites. During extended periods the GRB experiment made direct observations of flares on the hidden face of the Sun, providing a unique record of events not visible to other instruments. The small detector area of GRB and its optimization for very high counting rates minimized the effects of pulse pile-up. We interpret the spectra, time histories, and occurrence distribution patterns of GRB data in terms of “thermal feed-through”, the confusion of thermal soft X-rays and non-thermal hard X-rays. This effect is a systematic problem for scintillation-counter spectrometers observing the solar hard X-ray spectrum. This paper provides a definitive catalog of the Ulysses X-ray flare observations and discusses various features of this unique database. For the equivalent GOES range X2 – X25, we find a power-law fit for the (differential) occurrence frequency at >25 keV with slope −1.61±0.04, with no evidence for a downturn at the highest event magnitudes (for the relatively small sample of such events available in this study). If the nine most intense events are excluded because of concerns about the effects of pulse pile-up, the slope steepens to −1.75±0.08.     

A multiwavelength study of a double impulsive flare

Extensive data from the Solar Maximum Mission (SMM) and ground-based observatories are presented for two flares; the first occurred at 12:48 UT on 31 August, 1980 and the second just 3 min later. They were both compact events located in the same part of the active region. The first flare appeared as a typical X-ray flare: the Caxix X-ray lines were broadened ( 190±40 km s^-1) and blue shifted ( 60±20 km s^-1) during the impulsive phase, and there was a delay of about 30 s between the hard and soft X-ray maxima. The relative brightness of the two flares was different depending on the spectral region being used to observe them, the first being the brighter at microwave and hard X-ray wavelengths but fainter in soft X-rays. The second flare showed no significant mass motions, and the impulsive and gradual phases were almost simultaneous. The physical characteristics of the two flares are derived and compared. The main difference between them was in the pre-flare state of the coronal plasma at the flare site: before the first flare it was relatively cool (3 × 10⁶ K) and tenuous (4 × 10⁹ cm^-3), but owing to the residual effects of the first flare the coronal plasma was hotter (5 × 10⁶ K) and more dense (3 × 10¹¹ cm^-3) at the onset of the second flare. We are led to believe from these data that the plasma filling the flaring loops absorbed most of the energy released during the impulsive phase of the second flare, so that only a fraction of the energy could reach the chromosphere to produce mass motions and turbulence.A simple study of the brightest flares observed by the SMM shows that at least 43% of them are multiple. Thus, the situation studied here may be quite common, and the difference in initial plasma conditions could explain at least some of the large variations in observed flare parameters. We draw a number of conclusions from this study. First, the evolution of the second flare is substantially affected by the presence of the first flare. Secondly, the primary energy release in the second event is in the corona. Thirdly, the flares occur in a decaying magnetic region, probably as a result of the interaction of existing sheared loops; there is no evidence of emerging magnetic flux. Also, magnetic structures of greatly varying size participate in the flare processes. Lastly, there is some indication that the loops are not symmetrical or stable throughout the flares, i.e. the magnetic field does not act as a uniform passive bottle for the plasma, as is often assumed in flare models.NOAA/Space Environment Laboratory, currently at NASA/MSFC, Ala., U.S.A.Now at Sacramento Peak Observatory, Tucson, Ariz., U.S.A.     

Hard X-ray dynamic spectrum of flares observed by Hinotori

Time variations of the hard X-ray spectrum in solar flares are observed by the hard X-ray spectrometer (HXM) aboard the Hinotori satellite. With a new presentation of the dynamic spectrum we have studied the differences between impulsive and gradual hard X-ray bursts. In the impulsive events a “bent” spectrum up to some hundred keV persists at least until the main peak. In the gradual events, on the other hand, a power-law spectrum augmented by a low-energy excess is dominant.     

Coronal oscillations as revealed by solar hard X-ray bursts

During the 21st solar activity cycle the HXRBS aboard SMM satellite and the HXM on HINOTORI spacecraft detected several thousand hard X-ray solar flares. Studies of the temporal properties of these events had revealed hundreds of examples of fast spikes with durations of less than 1 sec. We analysed part of these observations and found that they have four common characteristics. Among these characteristics, quasi-periodic oscillations led us to believe the possibility of oscillations existing in the corona. We have studied the characteristics of the oscillations and derived their periods. The conditions of trapping the oscillations are also discussed.     

Hard X-ray imaging evidence of nonthermal and thermal burst components

We analyze hard X-ray imaging observations of three flares, showing widely different characteristics, in order to try and discriminate the relative efficiency of heating and acceleration in the primary energy release. Using a simplified approach, we compute the hard X-ray distribution and energy deposition due to accelerated electrons, with beam and ambient plasma parameters appropriate for each of the observed events. The results are convolved with the Hard X-Ray Imaging Spectrometer (HXIS) instrumental response and compared with observations. We find that: (a) Many observations are compatible with thick target processes, and with the possibility that flares may have high (>20%) acceleration efficiency. (b) Single hard X-ray sources should be very common in the data available at present (HXIS and HINOTORI), as it is the case, as well as a transition from chromospheric footpoints to single source structures. The latter cannot then be directly interpreted as thermal sources. (c) In the particular case of a limb flare, associated with a rather weak high energy burst, we show that the spatial and spectral behavior of the hard X-ray emission is incompatible with pure nonthermal processes. We thus propose that the observed emission was principally due to the strong heating intrinsic to a reconnection process within the region of interaction between two magnetic structures which are seen in the soft X-ray data. (d) We also study the heating effect of a beam, due to Coulomb losses, during its passage through the flare loops. In some cases, rather large and localized temperature increases can be expected to appear within short timescales ( 1 s), leading to a combination of nonthermal plus thermal output in the hard X-ray spectrum, which renders virtually impossible the determination of the underlying beam parameters. We finally discuss the extent to which our conclusions are valid, considering the instrumental limitations as well as the simple physical treatment that we apply.     

Hard X-ray bursts from flares behind the solar limb

The determination of the location of the region of origin of hard X-rays is important in evaluating the importance of 10–100 keV electrons in solar flares and in understanding flare particle acceleration. At present only limb-occulted events are available to give some information on the height of X-ray emission. In fifteen months of OSO-7 operation, nine major soft X-ray events had no reported correlated Hα flare. We examine the hard X-ray spectra of eight of these events with good candidate X-ray flare producing active regions making limb transit at the time of the soft X-ray bursts. All eight bursts had significant X-ray emission in the 30–44 keV range, but only one had flux at the 3σ level above 44 keV. The data are consistent with most X-ray emission occurring in the lower chromosphere, but some electron trapping at high altitudes is necessary to explain the small nonthermal fluxes observed.     

Magnetic field structures of hard X-ray flares observed by HINOTORI spacecraft

The magnetic field structure of five flares observed by HINOTORI spacecraft is studied. The double source structure of impulsive flares seems to indicate hard X-ray emission from the two footpoints of a flaring loop, but the potential field computation does not reproduce a loop connecting the two sources. Therefore the magnetic field could be in a sheared configuration and the force-free field modeling would be the next step to examine. On the other hand gradual flares are characterized by hard X-ray sources located in the corona, 2–4 x 10⁴ km above the photosphere. The potential field modeling is found to give a reasonable fitting in this type of flares, and the hard X-ray sources are located at the top of the magnetic loop or arcade. This configuration is consistent with the thick-target trap model of the hard X-ray bursts.     

High-temperature flares observed in broadband soft X-rays

Broadband soft solar X-rays monitored by the GOES satellites have been used to detect high-temperature flares (> 25 MK). The data suggest that there are two general categories of high-temperature flares: those that are intrinsically hot and recur repeatedly in particular active regions and those that show enhanced temperatures because of their proximity to the solar limb. Intrinsically hot flares associate with gamma-ray flares and impulsive hard X-ray flares. Hot flares show a small incidence with gradual hard X-ray flares, but those cases are either extremely intense flares or limb flares. The apparently hot flares occur near the visible limb, which suggests the strong thermal stratification of flare plasmas as demonstrated by over-the-limb events; even on the visible disk near the limb, the lower, cooler plasmas are somehow partially occulted.     

Spectral evolution of microwaves and hard X-rays in the 1989 March 18 flare and its interpretation

We analyze the time variation of microwave spectra and hard X-ray spectra of 1989 March 18, which are obtained from the Solar Array at the Owens Valley Radio Observatory (OVRO) and the Hard X-Ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM), respectively. From this observation, it is noted that the hard X-ray spectra gradually soften over 50–200 keV on-and-after the maximum phase while the microwaves at 1–15 GHz show neither a change in spectral shape nor as rapid a decay as hard X-rays. This leads to decoupling of hard X-rays from the microwaves in the decay phase away from their good correlation seen in the initial rise phase. To interpret this observation, we adopt a view that microwave-emitting particles and hard X-ray particles are physically separated in an inhomogeneous magnetic loop, but linked via interactions with the Whistler waves generated during flares. From this viewpoint, it is argued that the observed decoupling of microwaves from hard X-rays may be due to the different ability of each source region to maintain high energy electrons in response to the Whistler waves passing through the entire loop. To demonstrate this possibility, we solve a Fokker-Planck equation that describes evolution of electrons interacting with the Whistler waves, taking into account the variation of Fokker-Planck coefficients with physical quantities of the background medium. The numerical Fokker-Planck solutions are then used to calculate microwave spectra and hard X-ray spectra for agreement with observations. Our model results are as follows: in a stronger field region, the energy loss by electron escape due to scattering by the waves is greatly enhanced resulting in steep particle distributions that reproduce the observed hard X-ray spectra. In a region with weaker fields and lower density, this loss term is reduced allowing high energy electrons to survive longer so that microwaves can be emitted there in excess of hard X-rays during the decay phase of the flare. Our results based on spectral fitting of a flare event are discussed in comparison with previous studies of microwaves and hard X-rays based on either temporal or spatial information.     

On the Lower Energy Cutoff of Nonthermal Electrons in Solar Flares

1 INTRODUCTIONThe lower energy cutoff of nonthermal electron beams is an important quantity. Not only isit related to the acceleration mechanism, but it also determines the total number of acceleratedelectrons and the energy they carry. The power-law of electron beams cannot extend to lowerenergies indefinitely for if it did, it would imply an indeflnite1y large nuInber of electrons.A lower energy cutoff (E.), therefore, must exist, to keep the number of electrons within areasonable rang…     

The super-hot thermal component in the decay phase of solar flares

Solar X-ray observations from balloons and from the SMM and HINOTORI spacecraft have revealed evidence for a super-hot thermal component with a temperature of 3 × 10⁷ K in many solar flares, in addition to the usual 10–20 × 10⁶ K soft X-ray flare plasma. We have systematically studied the decay phase of 35 solar flare X-ray events observed by ISEE-3 during 1980. Based on fits to the continuum X-ray spectrum in the 4.8–14 keV range and to the intensity of the 1.9 Å feature of iron lines, we find that 15 (about 43%) of the analyzed events have a super-hot thermal component in the decay phase of the flare. In this paper the important properties of the super-hot thermal component in the decay phase are summarized. It is found that an additional input of energy is required to maintain the super-hot thermal components. Finally, it is suggested that the super-hot thermal component in the decay phase is created through the reconnection of the magnetic field during the decay phase of solar flares.     

X-ray imaging of three flares during the impulsive phase

The impulsive phases of three flares that occurred on April 10, May 21, and November 5, 1980 are discussed. Observations were obtained with the Hard X-ray Imaging Spectrometer (HXIS) and other instruments aboard SMM, and have been supplemented with Hα data and magnetograms. The flares show hard X-ray brightenings (16–30 keV) at widely separated locations that spatially coincide with bright Hα patches. The bulk of the soft X-ray emission (3.5–5.5 keV) originates from in between the hard X-ray brightenings. The latter are located at different sides of the neutral line and start to brighten simultaneously to within the time resolution of HXIS. Concluded is that:

1. The bright hard X-ray patches coincide with the footpoints of loops.
2. The hard X-ray emission from the footpoints is most likely thick target emission from fast electrons moving downward into the dense chromosphere.
3. The density of the loops along which the beam electrons propagate to the footpoints is restricted to a narrow range (10⁹ < n < 2 × 10¹⁰ cm^-3), determined by the instability threshold of the return current and the condition that the mean free path of the fast electrons should be larger than the length of the loop.
4. For the November 5 flare it seems likely that the acceleration source is located at the merging point of two loops near one of the footpoints.

It is found that the total flare energy is always larger than the total energy residing in the beam electrons. However, it is also estimated that at the time of the peak of the impulsive hard X-ray emission a large fraction (at least 20%) of the dissipated flare power has to go into electron acceleration. The explanation of such a high acceleration efficiency remains a major theoretical problem.     

Initial low/hard state, multiple jet ejections and X-ray/radio correlations during the outburst of XTE J1859+226

We have studied the 1999 soft X-ray transient outburst of XTE J1859+226 at radio and X-ray wavelengths. The event was characterized by strong variability in the disc, corona and jet – in particular, a number of radio flares (ejections) took place and seemed well-correlated with hard X-ray events. Apparently unusual for the canonical 'soft' X-ray transient, there was an initial period of low/hard state behaviour during the rise from quiescence but prior to the peak of the main outburst – we show that not only could this initial low/hard state be a ubiquitous feature of soft X-ray transient outbursts, but also it could be extremely important in our study of outburst mechanisms.     

Fast transients in hard X-ray solar flares

We present the results of a search for fast spikes in 5483 hard X-ray solar flares as observed with the Hard X-Ray Burst Spectrometer on the Solar Maximum Mission (SMM). Hundreds of fast spikes with durations of less than 1 second have been detected at time resolutions of 128 ms and 10 ms. Fast spikes have been detected with rise and decay times as short as 20 ms and with widths as short at 45 ms that represent the fastest hard X-ray variations yet seen from the Sun. The observations of such fast variations place new constraints on the physical nature of the source.     

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.     

Temporal correlation of solar hard X-ray bursts with chromospheric evaporation

During the impulsive phase of many solar flares, blueshifted emission wings are observed on the soft X-ray spectral lines of highly excited ions that are produced in the flare plasma. This emission has been commonly interpreted as chromospheric evaporation of material from the footpoints of coronal loops by non-thermal particle beams, although the question of whether the bulk of the energy is carried by electrons or ions (protons) has been the subject of much debate. The precise temporal relationship between the onsets of the blueshifted emission and the hard X-ray bursts is particularly important in resolving the mechanism of energy transfer to the hot plasma in the impulsive phase. A sample of flares observed with the Bragg Crystal Spectrometer (BCS) onYohkoh has been analysed for blueshifted emission and the results compared with hard X-ray light turves obtained with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO). In some flares, the blueshifted emission precedes the onset of the hard X-rays by up to 100 s. There is no evidence for a temporal correlation between the maximum energy input to the hard X-ray bursts and the maximum blueshifted intensity. These results lend support to those models favouring protons as the dominant energy carrier in the impulsive phase of flares and are inconsistent with the hypothesis that the bulk of the energy resides in electron beatos, although some other energy input, while unlikely, cannot be completely eliminated.