- The ~10–102 keV electrons accelerated during the flash phase constitute the bulk of the total flare energy.
- The flare can be divided into two regions depending on whether the electron energy input goes into radiation or explosive heating. The computed energy input to the radiative quasi-equilibrium region agrees with the observed flare energy output in optical, UV, and EUV radiation.
- The electron energy input to the explosive heating region can produce evaporation of the upper chromosphere needed to form the soft X-ray flare plasma.
- Very intense energetic electron fluxes can provide the energy and mass for interplanetary shock wave by heating the atmospheric gas to energies sufficient to escape the solar gravitational and magnetic fields. The threshold for shock formation appears to be ~1031 ergs total energy in >20 keV electrons, and all of the shock energy can be supplied by electrons if their spectrum extends down to 5–10 keV.
- High energy protons are accelerated later than the 10–102 keV electrons and most of them escape to the interplanetary medium. The energetic protons are not a significant contributor to the energization of flare phenomena. The observations are consistent with shock-wave acceleration of the protons and other nuclei, and also of electrons to relativistic energies.
- The flare white-light continuum emission is consistent with a model of free-bound transitions in a plasma with strong non-thermal ionization produced in the lower solar chromosphere by energetic electrons. The white-light continuum is inconsistent with models of photospheric heating by the energetic particles. A threshold energy of ~5×1030 ergs in >20 keV electrons is required for detectable white-light emission.
- The overall magnetic field configuration in which the flare occurred was a fairly simple, closed arch containing nonpotential substructure.
- The flare occurred spontaneously within the arch; it was not triggered by emerging magnetic flux.
- The impulsive energy release occurred in two major spikes. The second spike took place within the flare arch heated in the first spike, but was concentrated on a different subset of field lines. The ratio of Ov emission to hard X-ray emission decreased by at least a factor of 2 from the first spike to the second, probably because the plasma density in the flare arch had increased by chromospheric evaporation.
- The impulsive energy release most likely occurred in the upper part of the arch; it had three immediate products:
- An increase in the plasma pressure throughout the flare arch of at least a factor of 10. This is required because the Fexxi emission was confined to the feet of the flare arch for at least the first minute of the impulsive phase.
- Nonthermal energetic (~ 25 keV) electrons which impacted the feet of the arch to produce the hard X-ray burst and impulsive brightening in Ov and D3. The evidence for this is the simultaneity, within ± 2 s, of the peak Ov and hard X-ray emissions.
- Another population of high-energy (~100keV) electrons (decoupled from the population that produced the hard X-rays) that produced the impulsive microwave emission at 22 GHz. This conclusion is drawn because the microwave peak was 6 ± 3 s later than the hard X-ray peak.
- between energies and frequencies of flares on stars of different luminosities;
- between total radiation energies of flares and quiet stars both in X-ray and Balmer emission lines;
- between flare decay rates just after the maxima and flare luminosities at maxima.
- Decimetric pulsations, interpreted as plasma emission at densities of 109–1010 cm?3, and soft X-rays are observed before any Hα or hard X-ray increase.
- Some of the metric type III radio bursts appear close in time to hard X-ray peaks but delayed between 0.5 and 1.5 s, with the shorter delays for the bursts with the higher starting frequencies.
- The starting frequencies of these type III bursts appear to correlate with the electron temperatures derived from isothermal fits to the hard X-ray spectra. Such a correlation is expected if the particles are released at a constant altitude with an evolving electron distribution. In addition to this effect we find evidence for a downward motion of the acceleration site at the onset of the flash phase.
- In some cases the earlier type III bursts occurred at a different location, far from the main position during the flash phase.
- The flash phase is characterized by higher hard X-ray temperatures, more rapid increase in X-ray flux, and higher starting frequency of the coincident type III bursts.
- One or more well defined loops were the only structures of flare intensity observed during the rise phase and near flare maximum, except for knots which were close to the resolution of the telescope in size (≈2 arc seconds) and whose structure can therefore not be determined.
- The flare core features were always sharply defined during the rise phase.
- For the twenty events which contain loops, the geometry of the structure near maximum was that of a loop in ten cases, a loop with a spike at the top in four cases, a cusp or triangle in four cases, and a cusp combined with a spike in another two cases.
- Of the fifteen cases in which sufficient data were available to allow us to follow a flare's evolution, five showed no significant geometrical deviation from a loop structure, one displayed little change except for a small scale short-lived perturbation on one side of the loop 10 seconds before a type III radio burst was observed, eight underwent a large scale deformation of the loop or loops on a time scale comparable to that of the flare itself and one double loop event changed in a complex and undetermined manner, with reconnection being one possibility.
- The broad-wing Hα kernels characteristic of the impulsive phase of flares occur within 10″ of neutral lines encircling features of isolated magnetic polarity (‘satellite sunspots’).
- Photospheric field changes intimately associated with several importance 1 flares and one importance 2B flare are confined to satellite sunspots, which are small (10″ diam). They often correspond to spot pores in white-light photographs.
- The field at these features appears to strengthen in the half hour just before the flares. During the flares the growth is reversed, the field drops and then recovers to its previous level.
- The magnetic flux through flare-associated features changes by about 4 × 1019 Mx in a day. The features are the same as the ‘Structures Magnétiques Evolutives’ of Martres et al. (1968a).
- An upper limit of 1021 Mx is set for the total flux change through McMath Regions 10381 and 10385 as the result of the 2B flare of 24 October, 1969.
- Large spots in the regions investigated did not evince flux changes or large proper motions at flare time.
- The results are taken to imply that the initial instability of a flare occurs at a neutral point, but the magnetic energy lost cannot yet be related to the total energy of the subsequent flare.
- No unusual velocities are observed in the photosphere at flare time.
We describe the instrumental corrections which have to be incorporated for reliable correction and deconvolution of images obtained in the 16–22 keV and 22–30 keV energy bands of the Hard X-Ray Imaging Spectrometer (HXIS) aboard the Solar Maximum Mission (SMM). These corrections include amplifier gain and collimator hole size variations across the field of view, amplifier/filter efficiency, variation in effective collimator hole size and angular response with photon energy, dead-time, and hard X-ray plate transmission. We also emphasise the substantial Poisson noise in these energy bands, and describe the maximum entropy deconvolution/correction routine we have developed to establish the spatial structure which can be reliably inferred from HXIS data.
Next we discuss the results of application of our routine to the three impulsive flare phases reported by Duijveman et al. (1982) as exhibiting hard X-ray ‘footpoints’, namely 1980, April 10, May 21, and November 5. Our main conclusions are:
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(1)
Maximum entropy smoothing and Poisson noise data perturbations do not remove the main footpoint features in 16–30 keV nor change their basic morphology. However the results emphasise the asymmetry in footpoint size in the May 21 flare and confirm its possible presence in April 10. They also reveal the 3rd weak distant footpoint in the May 21 flare at an earlier time than found by Duijveman et al.
When the 16–22 and 22–30 keV bands are analysed separately, however, it is found that the footpoints are much less visible above noise in the harder band - i.e. the footpoint spectra are steep. In the April 10 and November 5 flares they are steeper than either the spectrum of intervening pixels or the spectrum at higher energies measured for the whole flare by the SMM Hard X-Ray Burst Spectrometer (HXRBS).
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(2)
The footpoint contrast with surroundings is less than found by Duijveman et al., despite image deconvolution, because of the maximum entropy smoothing of noise.
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(3)
The 16–30keV HXIS footpoint fluxes in the three flares are respectively 28%, 17%, and 15% of the simultaneous HXRBS flare power-law spectrum extrapolated into this energy range.
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(4)
Where Poisson noise is taken into account we find, by cross-correlating pixel count rates, that footpoint synchronism was either not provable at all, or substantially less close than reported by Duijveman et al.
Next we considered the implications of these results for models of the footpoint emission. Contrary to Duijveman et al. we do not consider the HXIS ‘footpoint’ data as supporting a conventional thick target beam interpretation since:
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(A)
The footpoint photon (and electron) fluxes are much less than expected from HXRBS extrapolation. This result casts doubt on recent models of chromospheric heating by electron beams which usually assume all of the HXRBS emission to come from HXIS footpoints.
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(B)
The footpoint spectra for the April 10 and November 5 flares are much softer than the HXRBS spectrum and than the spectrum of intervening pixels, contrary to thick target predictions.
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(C)
Contrary to Duijveman et al. footpoint synchronism does not demand an unreasonable Alfvén speed and so does not require non-thermal particles.
In spite of these objections we also re-considered the constraints placed on the acceleration site conditions in a beam interpretation by return current stability and footpoint contrast in the summed 16–30 keV range. Using the smoothed maximum entropy contrast and taking explicit account of coronal thermal emission, we find maximum densities somewhat larger than Duijveman et al. estimated, and much higher maximum values of T e /T i .
Regarding thermal interpretations we found:
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(a)
Models involving continuous production of short-lived hot kernels in the arch top with Maxwellian tail electrons escaping to the footpoints could explain the 16–30 keV contrast with a rather higher energetic efficiency than a pure beam model. However, whatever the temperature distribution of hot kernel production, the model predicts footpoints harder than the arch summit, contrary to HXIS data.
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(b)
A model with hot kernels produced in one limb of an arch can explain the asymmetry in footpoint size observed in May 21, and probably April 10, and is energetically even more efficient than (a) but is also inconsistent with the spectral data.
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(c)
Finally we point out that HXIS footpoint data may be consistent with a purely geometric interpretation in an almost uniform arch filled with hot plasma.
- The observed energy of X-rays which vary in proportion to the 17 GHz emission concentrates mostly below 100 keV with a median energy of 70 keV. Since the mean energy of electrons emitting 70 keV X-rays is ?130 keV or ?180 keV, depending on the assumed hard X-ray emission model (thin-target and thick-target, respectively), this photon energy strongly suggests that the 17 GHz emission comes mostly from electrons with an energy of less than a few hundred keV.
- Correspondingly, the magnetic field strength in the microwave source is calculated to be 500–1000 G for the thick-target case and 1000–2000 G for the thin-target case. Finally, judging from the values of the source parameters required for the observed microwave fluxes, we conclude that the thick-target model in which precipitating electrons give rise to both X-rays and microwaves is consistent with the observations for at least 16 out of 22 flares examined.
- The type III's have a dual chromospheric association. Sometimes they are related to a flare, sometimes to a perturbation of a different kind. The latter is seen in absorption in the Hα core and ±0.75 Å away. It is interpreted as a rather dense and cool material in motion in the chromosphere or the low corona. A part of which moves downward, the other upward. The type III's are more closely related to the downward motion.
- The type III associated absorbing features take place at the border of an active center and along an H∥ = 0 line. At the present time this appears as the most conspicuous property for marking them off from the great variety of the Hα absorbing features commonly observed on the Sun.
- Most of the type III associated flares are related to an absorbing feature of the same kind, which appears before the flare itself. This indicates that the initial instability which is responsible for the type III emission is basically the same, whether the bursts are flare associated or not.
- The disruption of the magnetic configuration at the flare onset, as indicated by prominence eruption or activation and by associated white-light coronal transients;
- a continuous energy deposition, presumably at the top of loops, during a large fraction of the flare development and well after the intensity peak;
- a continuous supply of additional material to the top of loops, with subsequent downflows and out-of-hydrostatic equilibrium conditions.
- the appearance of satellites of forbidden components in the flares spectrum, due to turbulent electric fields, is the most probable for Hei 3819.606 Å lines;
- the Baranger-Mozer method is more sensitive to the high-frequency component of turbulent fields than to the low-frequency ones;
- the upper limit of the turbulent oscillation level in flares is evaluated.
- Multi-thermal Diagnostic of 6.7 and 8.0 keV Fe and Ni lines
- Multi-thermal Conduction Cooling Delays
- Chromospheric Altitude of Hard X-Ray Emission
- Evidence for Dipolar Reconnection Current Sheets
- Footpoint Motion and Reconnection Rate
- Evidence for Tripolar Magnetic Reconnection
- Displaced Electron and Ion Acceleration Sources.
- The solar neutrino problem
- Structure of the solar interior (helioseismology)
- The solar magnetic field (dynamo, solar cycle, corona)
- Hydrodynamics of coronal loops
- MHD oscillations and waves (coronal seismology)
- The coronal heating problem
- Self-organized criticality (from nanoflares to giant flares)
- Magnetic reconnection processes
- Particle acceleration processes
- Coronal mass ejections and coronal dimming
- The interplanetary blast model is based on subsurface, helically twisted flux ropes which erupt to form spots and then transfer their twists and energy by Alfvén-twist waves into the atmospheric magnetic fields. The blast is triggered by the prior-commencing flash phase or by a coronal wave.
- The thermal flare is explained in terms of Alfvén waves travelling up numerous ‘flux strands’ (Figure 3) which have frayed away from the two flux ropes. The waves originate in interaction (collisions, bending, twisting, rubbing) between subsurface flux strands; the sudden flash is caused by a collision. The classical twin-ribbon flare results from the collision of a flux rope with a tight bunch of S-shaped flux strands.
- The impulsive acceleration of electrons (hard X-ray, EUV, Hα and radio bursts) is tentatively attributed to magnetic reconnection between fields in two parallel, helically twisted flux strands in the low corona.
- Flare (Moreton) waves in the corona have the same origin as the interplanetary blast. Sympathetic flares represent only the start of enhanced activity in a flare event already in the slow phase. Filament activation also occurs during the slow phase as twist Alfvén waves store their energy in the atmosphere.
- Flare ejecta are caused by Alfvén waves moving up flux strands. Surges are attributed to packets of twist Alfvén waves released into bundles of flux strands; the waves become non-linear and drive plasma upwards. Spray-type prominences result from accumulations of Alfvén wave energy in dome-shaped fields; excessive energy density eventually explodes the field.
- The negative infrared flares discovered around 8000 Å is not in agreement with the theory in the case of the flare star UV Cet. Some traces of negative flares have been noted for a number of less powerful flares of EV Lac.
- The amplitudes of the recorded positive flares of UV Cet and EV Lac on λ8000 Å are in good agreement with the magnitudes predicted by the fast-electron hypothesis (non-thermal bremsstrahlung).
- In the future the negative flares around 8000 Å should be looked for in early-type flare stars of types M0-K5.
- For a positive discovery of negative flares, future observations must be carried out in the wavelength region of 1–3 μm.