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Discussion of the heating of the ambient plasma by high-energy electrons in solar flares. It is shown that for large flares the heating is enough to produce a thermal plasma with a temperature up to 10 to the 7th K rapidly in the initial phase of the flares. Thus thermal bremsstrahlung in addition to nonthermal bremsstrahlung should be considered for the X-ray emission of solar flares in the initial phase.-
The observations of solar wind helium enhancements following major solar flares are reviewed, and the hypothesis that helium enhancements often mark flare piston plasma is confirmed. Helium enhancements were observed during each of the three periods (March 1966, July 1966, August/September 1966) of major solar activity that occurred from October 1965 to October 1966. No enhancements were seen during the long quiet periods that occurred that year. At 1 AU, the helium-enhanced plasma pistons had slowed so that the velocity was 80 percent of the mean transit velocity, in general agreement with theoretical models of the propagation of flare disturbances. A qualitative model, in which the piston plasma is accelerated from the flare site deep in the corona, is discussed briefly. If the model is valid in general outline, the piston plasmas provide samples of material from the lower levels of the corona.
Analysis is made of observations of the August, 1972 flares at Big Bear and Tel Aviv, involving monochromatic movies, magnetograms, and spectra. In each flare the observations fit a model of particle acceleration in the chromosphere with emission produced by impart and by heating by the energetic electrons and protons. The region showed twisted flux and high gradients from birth, and flares appear due to strong magnetic shears and gradients across the neutral line produced by sunspot motions. Post flare loops show a strong change from sheared, force-free fields parallel to potential-field-like loops, perpendicular to the neutral line above the surface.
The mechanism responsible for the modulation of synchrotron radiation in the meter wave range by a solar flare is examined. The proposed explanation is based on the interaction of a flare generated shock front with a magnetic flux tube extending out into the corona over the flare. As the shock wave propagates through the flux tube, Alfven waves are developing and travelling up the tube toward each other, thereby accelerating electrons and protons by the Fermi mechanism and thus filling the top of the flux tube with energetic electrons. Radial oscillations develop as the shock front becomes parallel to the magnetic field direction and the intensity of the synchrotron radiation is modulated by these oscillations. An OSO-5 experiment detected a hard X-ray burst associated with the flare that produced modulated radio emission and X-ray modulations that are attributed to density fluctuations in the X-ray bursts.
Simultaneous observations of flares in H-alpha and a band 15 A wide centered on 3835 A show no change whatever in 3835 A at the time of several flares, although the chromospheric network is easily visible. Flares are therefore transparent in this wavelength. Since the flare represents a test heat input from above, and no brightening of the network appears, we conclude it is impossible for the corona to produce the bright network by downward convection.
A study has been made of the variation in hard (E greater than 10 keV) X-radiation, H-alpha and microwave emission during the impulsive phase of solar flares. Analysis shows that the rise-time in the 20-30-keV X-ray spike depends on the electron hardness. The impulsive phase is also marked by an abrupt, very intense increase in H-alpha emission in one or more knots of the flare. Properties of these H-alpha kernels include: (1) a luminosity several times greater than the surrounding flare, (2) an intensity rise starting about 20-30 sec before, peaking about 20-25 sec after, and lasting about twice as long as the hard spike, (3) a location lower in the chromosphere than the remaining flare, (4) essentially no expansion prior to the hard spike, and (5) a position within 6000 km of the boundary separating polarities, usually forming on both sides of the neutral line near both feet of the same tube of force. Correspondingly, impulsive microwave events are characterized by: (1) great similarity in burst structure with 20-32 keV X-rays but only above 5000 MHz, (2) typical low frequency burst cutoff between 1400-3800 MHz, and (3) maximum emission above 7500 MHz.
Examination of a moderately active sunspot group, McMath 9735, showed that 15 of 16 flares observed on Oct. 20-21, 1968, occurred near, and were preceded by, at least one of several EFR's (emerging flux regions) in the area. Flares were larger and more numerous when: (1) the EFR appeared close to already existing spots, (2) a large amount of filament reorientation was occurring, and when (3) the EFR was most active - i.e., it was increasing in area and brightness and was accompanied by violent surging and great brightness fluctuations at the feet of the dark fibrils. Only two flares occurred at an 'inverted' EFR - i.e., a leading spot with f polarity; however the largest event (2B) of the 15 quickly spread to this region after starting in a different EFR. A sunspot appeared in the inverted emerging flux region less than three hours after the flares. However, this is thought to be merely an indication of the growing EFR and therefore a secondary effect.
Observations of impulsive hard X-rays and other flash phase emissions from small solar flares are analyzed in order to determine the characteristics of energetic electrons accelerated during the flash phase. The electron spectrum and its time variation are deduced from a model of the X-ray source in which the electron injection is continuous, and the electron energy loss is primarily due to collisions with the ambient plasma and escape into the corona. The instantaneous electron spectrum in the X-ray source, as well as the acceleration spectrum, are found to be nonthermal. The observations are consistent with a flare model having the following properties: (1) the acceleration region is located in the lower corona where the ion density is less than 10 to the 9th power per cubic centimeter, (2) the total kinetic energy of the nonthermal electron is approximately 10 percent of the total flare energy, so that the efficiency of the acceleration process is very high, (3) the nonthermal electrons (and protons) provide energy for all flash phase emissions, (4) the acceleration is a continuous process with a time constant less than 1 second, or a continuous series of impulses each lasting less than 1 second and a total duration of approximately 100 seconds, (5) the electron spectrum continuously hardens during the increasing phase of the X-ray burst and softens during the decreasing phase and, (6) the more energetic flares to not necessarily produce a harder electron spectrum.
Ground-based observations have established that impulsive extreme-ultraviolet (EUV) flashes occur during solar flares, in close association with hard X-ray bursts and certain impulsive portions of the optical flare. The energy flux radiated in the 10 to 1030 Angstroms wavelength range exceeds that in hard X-rays (greater than or equal to 10 keV) by about 100,000, and is roughly comparable to the energy flux radiated at all optical wavelengths. In this paper our current knowledge of EUV flare radiation and ground-based observations of the radiations are reviewed. Several hypotheses are proposed about the interrelation between impulsive EUV emissions and other flare radiations, their spatial location, and association with magnetic fields.
Large solar flares produce intense soft X-ray emission, indicating the existence of high temperature plasmas that coexist in time with the plasmas responsible for the normally observed brightenings in H-alpha. The time behavior of the X-ray flux, as revealed, for example, by ion chamber detectors on the series of Solrad monitoring satellites, appears to roughly mimic the intensity-time behavior of the H-alpha flare, insofar as start times, times of maximum flux, and approximate decay times are concerned. In recent years, soft X-ray spectra of both active regions and solar flares have been obtained by instruments flown on spacecraft such as the Orbiting Solar Observatory (OSO) series. The disbursing elements used were Bragg crystals, and in the 8 Angstrom region the resolution is typically approximately 1200. This paper discusses the observed characteristics of X-ray flare spectra and spectroscopic diagnostics for determining electron temperatures, electron densities, and departures from ionization equilibrium within the soft X-ray emitting plasma.
The propagation in the corona of the magnetohydrodynamic (MHD) disturbance possibly emitted at the explosive stage in the initial phase of a flare is considered. The behavior of the MHD fast-mode wavefront, whose source is located at the flare, is calculated by using eiconal-characteristic method in the High Altitude Observatory (HAO) realistic models of coronal magnetic field and density for the days of some particular flare events. It is shown as the result that the peculiar behavior of Moreton's surface wave and the peculiar appearance in the shape and position of the type 2 burst sources can be consistently understood by considering the refraction, focussing, and formation of shocks of MHD fast-mode disturbance in the actual distribution of Alfven velocity in the corona. Moreton waves seem to appear only when the flare explosion happens to occur at the edge of an active region and faces a low-Alfven-velocity region lying on the surface. The wave, which is initially emitted isotropically is refracted into a direction in which the condition for down-refraction holds to allow chromospheric reentrance of disturbance.
Shock waves are indirectly observed as the source of type II radio bursts, whereas magnetic bottles are identified as the source of moving metric type IV radio bursts. The difference between the expansion speeds of these waves and bottles is examined during their generation and propagation near the flare regions. It is shown that, although generated in the explosive phase of flares, the bottles behave quite differently from the waves and that the bottles are generally much slower than the waves. It has been suggested that the waves are related to flare-associated interplanetary disturbances which produce SSC geomagnetic storms. These disturbances may, therefore, be identified as interplanetary shock waves. The relationship among magnetic bottles, shock waves near the sun, and flare-associated disturbances in interplanetary space is briefly discussed.
The origin of a large corotating solar particle event in August 1970 is discussed. Proton data from spacecraft at five widely separated heliocentric longitudes are used to identify two distinct release points which are over 100 deg apart in solar longitude. Optical flare data show a high incidence of time-overlapping flares between plage regions close to the two release points, indicating a good connection between them. Unusual X-ray and radio emissions are also observed from these regions. The spectrum of the relativistic electrons in the corotating particle event is represented by a power law with an index gamma equal to approximately -4, considerably steeper than that usually observed from a solar flare. It is concluded that there is a large magnetic loop structure connecting points over 100 deg apart on the sun which is able to trap energetic protons and electrons from an earlier solar flare. Subsequent release of these particles establishes an intense, long-lived corotating event.
Using sounding rockets, the differential energy spectra of Fe in three solar flares between 0.3 and 50 MeV/nuc were measured. In one of these flares, the geomagnetic field was undisturbed, and the Fe spectrum showed a peak at approximately 2 MeV/nuc. Interpreting this observation with the established quiet time geomagnetic cutoffs, it was calculated that the charge of 2 MeV/nuc Fe is 22 plus or minus 1 instead of the equilibrium value of about 16 for Fe in neutral matter. This nonequilibrium state of ionization imposes a constraint on the flare mechanism and suggests the importance of the flash-phase flare plasma for ionizing heavy nuclei prior to acceleration.
Using Lexan and glass detectors, the composition of solar particles with atomic number 2 or greater was determined as a function of energy from approximately 0.2 to 50 MeV/nucleon in four flares of quite different intensity and during a time when the sun was nearly quiet. Fe is nearly completely stripped. The enhancement factor increases with atomic number at a given energy but decreases with energy. Heavy element enhancements are detectable at higher energies in stronger flares. They occurred in ancient as well as in present-day flares. We have observed Li and Be in one flare. Incomplete ionization, preferential leakage of heavy ions, energy loss by ionization, and nuclear reactions appear to be involved in solar particle production.
A superposed epoch analysis of 1964-1970 solar flares shows a marked increase in flare occurrence within a day of (-+) solar sector boundaries, as well as a local minimum in flare occurrence near (+-) sector boundaries. This perference for (-+) boundaries is more noticeable for Northern Hemisphere flares, where these polarities match the Hale polarity law, but is not reversed in the south. Plage regions do not show such a perference.
The time dependences of the 2.2 MeV and 0.51 MeV gamma ray lines from solar flares are calculated and the results are compared with observations of the 1972, August 4 and 7 flares. Time lag between the nuclear reactions and the formation of these two lines are caused, respectively, by capture of the neutrons, and by deceleration of the positrons and decay of the radioactive nuclei. Results show that the calculation is consistent with the observed rise of the 2.2 MeV line on August 4, and it does not require different time dependences for the accelerated protons and electrons in the flare region. The above lags can explain the delayed gamma ray emission observed on August 7. Positrons of energies greater than about 10 MeV could be detected in interplanetary space following large solar flares.
A search of MIT/OSO-7 data was made for evidence of X-ray emission from flares of UV Ceti flare stars. Observations from McDonald Observatory were used to identify the times of optical flares. The only instance of coincident coverage occurred on 1974 January 21 UT at 03:43:26 GMT for delta m(u)=0.86 flare of YZ CMi. No radio coverage of this particular event was obtained. Upper limits (3 sigma) of 0.8, 1.0, and 0.7 photons/sq cm-sec on the observed X-ray flux were set for the energy ranges greater than or approximately equal to 15, greater than or approximately equal to 3, and 1-10 keV, respectively.