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Particle acceleration is intrinsic to the primary energy release in the impulsive phase of solar flares, and we cannot understand flares without understanding acceleration. New observations in soft and hard X-rays, γ-rays and coherent radio emissions are presented, suggesting flare fragmentation in time and space. X-ray and radio measurements exhibit at least five different time scales in flares. In addition, some new observations of delayed acceleration signatures are also presented. The theory of acceleration by parallel electric fields is used to model the spectral shape and evolution of hard X-rays. The possibility of the appearance of double layers is further investigated.

We report findings concerning energy transport and dynamics in flares during the impulsive and gradual phases based on new ground-based and space observations (notably fromYohkoh). A preheating sometimes occurs during the impulsive phase. Caxix line shifts are confirmed to be good tracers of bulk plasma motions, although strong blue shifts are not as frequent as previously claimed. They often appear correlated with hard X-rays but, forsome events, the concept that electron beams provide the whole energy input to the thermal component seems not to apply. Theory now yields: new diagnostics of low-energy proton and electron beams; accurate hydrodynamical modeling of pulse beam heating of the atmosphere; possible diagnostics of microflares (based on X-ray line ratio or on loop variability); and simulated images of chromospheric evaporation fronts. For the gradual phase, the continual reorganization of magnetic field lines over active regions determines where and when magnetic reconnection, the mechanism favoured for energy release, will occur. Spatial and temporal fragmentation of the energy release, observed at different wavelengths, is considered to be a factor as well in energy transport and plasma dynamics.

The Nobeyama Radioheliograph, originally constructed as a 17 GHz system, was upgraded into a dual-frequency system operating at 17 and 34 GHz on a time sharing basis. For each of the 84 antennas, a frequency-selective sub-reflector, which reflects 17 GHz radio waves into the Cassegrain focus while transmits 34 GHz waves into the primary focus, was installed and a 34 GHz frontend receiver system was mounted in parallel with the existing 17 GHz system. No major modification was introduced to the backend system. Neither were antennas added nor their arrangement changed. With this minimal modification, we have obtained (1) an angular resolution of ∼5″ (at 34 GHz) and (2) a spectral diagnostic capability of cm- to mm-wave emissions from solar flares with temporal resolution up to 100 ms. Daily 8-hour (from ∼22:45 to ∼6:45 UT) operation at dual frequencies started late October, 1995. Final tuning of the new system, such as the calibration and development of image synthesis software tools is still under way. Flare images taken at the dual frequencies are presented and compared with that from the Yohkoh SXT as an example.

The primary scientific objective of the High Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission selected by NASA is to investigate the physics of particle acceleration and energy release in solar flares. Observations will be made of x-rays and (gamma) rays from approximately 3 keV to approximately 20 MeV with an unprecedented combination of high resolution imaging and spectroscopy. The HESSI instrument utilizes Fourier- transform imaging with 9 bi-grid rotating modulation collimators and cooled germanium detectors. The instrument is mounted on a Sun-pointed spin-stabilized spacecraft and placed into a 600 km-altitude, 38 degrees inclination orbit.It will provide the first imaging spectroscopy in hard x-rays, with approximately 2 arcsecond angular resolution, time resolution down to tens of ms, and approximately 1 keV energy resolution; the first solar (gamma) ray line spectroscopy with approximately 1-5 keV energy resolution; and the first solar (gamma) -ray line and continuum imaging,with approximately 36-arcsecond angular resolution. HESSI is planned for launch in July 2000, in time to detect the thousands of flares expected during the next solar maximum.

The Nobeyama Radioheliograph was completed in two years by 1992 March. It has been put into regular observations in late June 1992 for eight hours a day from 23h UT to 07h UT. It has caught some part of maximum activities of the Sun, but not so large as the flares in 1991 June. Observational results are described for faint structures on the Sun; an introduction to previous reviews is also given. Recent developments of image deconvolution software at Nobeyama are very promising to produce images of high‐dynamic range or of high quality. Examples of improved images with Steer algorithm are demonstrated. This image synthesis program will be released at Nobeyama for open use of the observed correlation data. The first light of the Dual‐frequency observations is also presented.

We have found remarkably different manifestations of a bright point flare in X‐ray and radio (microwave) wavelengths, unlike previous observations. In X‐rays, the BP flare was relatively simple while in radio, the bright point flare had a large scale component and a transient moving component. The large scale structure may be the radio counterpart of large scale structures sometimes seen during X‐ray BP flares. The transient component was also compact and moved away from the location of the X‐ray BP flare with a speed of ∼60 km s−1. The compact source also showed fast time structure which suggests nonthermal emission mechanism for the transient sources.

We have studied the spectacular 1991 June X-class flares using gamma-ray data from the Charged Particle Detectors (CPDs) of the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) and 80 GHz millimeter data from Nobeyama, Japan. The CPDs were the only CGRO instrument that did not saturate during the extremely intense 1991 June 4 flare. We have shown that for this flare the CPDs respond to MeV photons, most of which are due to bremsstrahlung produced by relativistic electrons at the Sun. We have further shown that the gamma-ray and millimeter observations agree numerically if the 80 GHz radiation is gyrosynchrotron radiation produced by trapped electrons and the gamma rays are thick-target bremsstrahlung due to electrons precipitating out of the trap. The requirement that the trapping time obtained from the numerical comparison be consistent with the observed time profiles implies a magnetic field between about 200 and 300 G and an electron spectral index between about 3 to 5. By comparing the CPD observations with both the 80 GHz data and nuclear line data from the Energetic Gamma Ray Experiment Telescope (EGRET) and the Oriented Scintillation Spectroscopy Experiment (OSSE) on CGRO for the flares of June 4, 6, 9, and 11, we found that the ratio of the CPD counts to both the millimeter flux densities and the nuclear line fluences decreases with decreasing flare heliocentric angle. All of these flares were produced in the same active region. We interpreted this result in terms of a loop model in which the gyrosynchrotron emission is produced in the coronal portion of the loop where the electrons are kept isotropic by pitch angle scattering due to plasma turbulence, while the bremsstrahlung is produced by precipitating electrons that interact anisotropically. We found that the trapping time in the coronal portion is time dependent, reaching a minimum of about 10 s at the peak of the CPD count rate. We suggested the damping of the turbulence as a possible reason for the variation of the trapping time. turbulence as a possible reason for the variation of the trapping time.

An unusually high speed (10,000 km/s) type II radio burst was observed in the rising phase of a strong gamma-ray burst which started at 2359:58.7 UT on April 24, 1984. The high-speed type II burst decelerated suddenly to 3600 km/s and, after that, activated an advancing front moving type IV radio burst and an H-alpha Moreton wave. Comparisons of the type II burst with X-ray and gamma-ray emissions suggest that shock acceleration did not play a major role in acceleration of high-energy electrons and ions which were responsible for production of the strong X-ray and gamma-ray emissions in the impulsive phase of the April 24 flare. 13 refs.

Results of simultaneous high-resolution microwave and X-ray two-dimensional imaging observations are briefly reviewed. It is shown that seven events published in the literature are not homogeneous but rather diverse with respect to spatial structure, mutual relations on position or shape. An outlook is presented for the next solar cycle to obtain a large data set for extensive study of energetic solar phenomena.

A study of the relative timing of solar flare emissions at different wavelengths can provide valuable insights into the associated particle-acceleration mechanisms. We report here that the significant systematic differences between the times of occurrence of the peak intensities of microwave, hard X-ray and γ-ray line emissions, which were found in two impulsive flares on 7 and 21 June 1980 at 0312 and 0118 UT respectively, suggest that two kinds of acceleration take place successively within a few seconds.

Theoretical models have been proposed, stimulated by a successful account of the jovian decametric radiation1, for a maser mechanism, or a normal relativistic cyclotron resonance mechanism, operating in solar flares, and for transport of energy across magnetic fields to the electron gas by maser-amplified radio waves2. Much effort has been directed towards observations of solar-flare radio spike bursts3–8. Here we compare solar narrow-band decimetre-wave spike bursts with corresponding X-ray events for 25 solar flares. We find that decimetre spike bursts are 102–103 times stronger than normal impulsive microwave bursts for the same observed amount of hard X-ray emission. This feature, along with the observation of 100% circular polarization, strongly supports a maser origin for the decimetre spike bursts.

AT Toyokawa Observatory we have been mapping the brightness and polarization distribution across the Sun at intervals of 10 s by a quick scanning system installed at the 8 cm radio interferometer since the end of August 1968 (ref. 1). The base line of the 32 element adding interferometer is 213.28 m or 2,666 wavelengths, which corresponds to a half-power beam width of 1.1 min of arc. This letter describes two examples of bursts with double structure which have been observed during the past eight months.

Active regions on the Sun in the 20th solar cycle are studied with special reference to their association with proton flares based on microwave interferometric observations at Toyokawa Observatory. It has been reconfirmed that the active regions associated with intense S-component emission with a high 3-cm to 8-cm flux ratio are likely to produce proton flares.

Bursts with peculiar time evolution and narrow-band microwave spectra are studied in detail based on X-ray data of HINOTORI and microwave data of Toyokawa Observatory. Discussions are given for emission mechanism and burst scenario.

It is shown that the long time delays more than five seconds between the peaks of intense hard X-ray and microwave bursts are concerned with two independent phenomena. One is the energy dependent time delays in X-rays and the other is the frequency dependent time delays in microwaves. The time delays of 5 s to 10 s between the peaks of solar hard X-ray burst (≲100 keV) obtained with Hinotori spacecraft and microwave burst at 17 GHz were observed exceptionally in three intense events with a spectral maximum at about 17 GHz. It is found that the peak of harder X-rays (≳300 keV) also delays in these events by about the same amount with respect to the softer X-rays (≲100 keV), so that the peak at 17 GHz nearly coincides (≲4s) with that of the harder X-rays. This is quite reasonable because the gyro-synchrotron emissions from the electrons below about 100 keV in the solar flares are generally negligible at high microwave frequencies (≳10 GHz). The optical thickness of the radio source decreases with frequency and is unity generally at about 10–20 GHz in intense bursts as inferred from the radio spectrum. Further delay of the peaks at the lower microwave frequencies is attributed to the temporal increase in the effective size of radio source which is optically thick at the lower frequencies.

In the astrophysics community, “the solar flare problem” is generally considered to be how to accumulate sufficient magnetic energy in one active region and to subsequently release it on a sufficiently short time scale. Satisfactory solution of the solar flare problem will require at least two achievements by the solar physics community: first, convincing theoretical demonstration that one or more mechanisms of energy storage and release can occur; second, convincing observational demonstration that one (or more) of these theoretical processes actually does occur in the solar atmosphere. The contents of this Chapter essentially relate to the second problem, being largely concerned with how the energy released from magnetic form is transported through the solar atmosphere before escaping in the form of the radiant and mechanical energy signatures which we must interpret.

A brief description of an astronomy satellite for solar flares, HINOTORI, is given on observations, data handling, data acquisition, SOX and SXT.

Pulsations with large amplitude and duration have been observed during the hard X-ray and microwave radio bursts associated with the 1980 June 7 solar flare. The high time resolution measurements of 20-800 keV X-rays were made with the X-ray spectrometers aboard the ISEE 3 and P78-1 spacecraft. The radio measurements, covering metric to microwave wavelengths, were made at the Nobeyama and Toyokawa observatories in Japan. The temporal evolution of the X-ray and radio spectra and the polarization and spatial structure of the microwave source have been examined. The following interpretation is found to be consistent with the observations: (1) the variations in the electron acceleration/injection spectrum are responsible for the observed variations in the hard X-ray and microwave emissions; (2) the locations of the hard X-ray and microwave sources are probably different, the X-ray source being located at a lower altitude.