Time delay between Hα and hard X-ray emissions during impulsive solar flares

This investigation shows that statistically there are significant time delays between H and hard X-ray (HXR) emissions during solar flares; most impulsive flares produce HXR emissions up to 1 min before and up to 2 min after the onset of H emission. HXR emissions are also found to be peaked up to 2 min before the H emissions.     

The acceleration and propagation of solar cosmic rays as deduced from the relative abundance of protons to helium nuclei

Except for protons, the chemical composition of solar cosmic rays is very similar to the abundance of the elements at the photosphere of the Sun. If we consider the relative abundance ratio of protons to -particles (P/) at constant rigidity, this ratio is highly variable from one solar cosmic ray event to another. This ratio observed at the Earth, however, decreases monotonically with time from the onset of solar flares and, furthermore, is dependent on the heliocentric distance of the parent flares from the central meridian of the solar disk. P/'s which have been measured before the onset of SC geomagnetic storms change from 1.5 to 50 or more, being a function of the westward position of the source from the east limb of the Sun. These variations with respect to time and heliocentric distance suggest that the propagation of solar cosmic rays is strongly modulated in the interplanetary space. The major part of the -particles seem to propagate as if they are trapped within the magnetic clouds which produce SC geomagnetic and cosmic ray storms at the earth.The chemical composition and rigidity spectra of solar cosmic rays suggest that solar cosmic rays are mainly accelerated by the Fermi mechanism in solar flares. The observed variation of P/'s is produced mainly through the difference between the propagation characteristics of protons and -particles.NAS-NRC Associate with NASA.     

Kα line emission during solar X-ray bursts

K X-ray line emission from S, Ar, Ca and Fe is calculated for conditions likely to exist in solar flares. We consider both the non-thermal and thermal phases of flares as indicated by X-ray observations. Impulsive non-thermal events seen at the onset of a flare at photon energies > 20 keV generally give rise to small K line fluxes (<250 photons cm^-2 s^-1) on the basis of data presented by Kane and Anderson. The amount of S K radiation in particular depends sensitively on the lower-energy bound of the non-thermal electron distribution giving rise to the impulsive burst, offering a possible means of determining this. Thermal K emission is significant for only Fe ions. For S, Ar and Ca, the temperatures required for a sizeable number of electrons with energies greater than the K-ionization potential will also strip these elements to ionization stages too high for K transitions to be possible. Comparison of thermal K emission from iron during an intense solar flare leads to a very high emission measure on the basis of these calculations, but such a value seems to be compatible with an analysis of the 1–3 Å continuum during the same event.NAS/NRC Resident Research Associate.Visiting Scientist, High Altitude Observatory, NCAR, Boulder, Colo. 80302.     

The excitation of the iron Kα feature in solar flares

We evaluate the relationship between the hard X-ray photon spectrum and the flux of iron K emission in a thick-target electron bombardment model. Results are presented for various power-law hard X-ray spectra. We then apply these results to two events observed with the Hard X-Ray Burst Spectrometer and the K channel of the X-Ray Polychromator Bent Crystal Spectrometer on the Solar Maximum Mission satellite. For one of the events, on 29 March, 1980, at 09:18 UT, the K flux predicted for a thick-target non-thermal process is significant compared to the background fluorescent component, and the data are indeed consistent with an enhancement of the predicted amount. For the other event, on 14 October, 1980 at 06:09 UT, the hard X-ray spectrum is so steep that no significant Ka flux is predicted for this process, and no enhancement is seen. We conclude that the agreement between the predicted K flux and the observed magnitude of the K enhancement above the fluorescent background at the time of the large hard X-ray bursts lends support to a thick-target non-thermal interpretation of impulsive hard X-ray emission in solar flares.     

Spatial relationship between λ5303 and Hα components of a loop prominence system

There are remarkable similarities in the structure of loop prominences when observed in H and coronal lines, although the lines arise from extremely different excitation conditions. This leads to the consideration of a multi-component model, where different emission lines come from different elements of the structure. The late phases of a large west limb loop prominence system followed by a surge were recorded at Haleakala Observatory on March 6, 1970. Simultaneous filter-grams in H and 5303 were obtained, together with spectra at three heights in the prominence over the range 3850–5950 Å. The positions of bright knots of emission as seen in the green line are compared with associated H knots. With these relative positions determined, the filtergrams are superimposed to demonstrate the two dimensional spatial relationship between H and 5303 structures. The results support a model of cool loops within a closely associated hot loop system.     

The relations between chromospheric features and photospheric magnetic fields

High resolution photographic magnetograms are compared with H filtergrams (both on- and off - band) for a wide variety of solar features. It is verified that H filaments overlie neutral lines or bands and that H plages always occur at magnetic field clumps. However, the brightness of H plages bear no relation to magnetic field strength or polarity, and the direction of the magnetic field with respect to threads and filaments remains obscure. Counter-examples can be found for virtually every rule that has been formulated so far.Basic questions about the usefulness and final research goal of filtergrams and magnetograms are raised. It is shown that neither filtergram or magnetogram alone is capable of furnishing a unique solution. It is suggested that the proper direction for research is to use magnetograms, together with (as yet unspecified) additional sources of data, to understand H structures.     

Relative timing of solar flares observed at different wavelengths

The timing of 503 solar flares observed simultaneously in hard X-rays, soft X-rays and H is analyzed. We investigated the start and the peak time differences in different wavelengths, as well as the differences between the end of the hard X-ray emission and the maximum of the soft X-ray and H emission. In more than 90% of the analyzed events, a thermal pre-heating seen in soft X-rays is present prior to the impulsive flare phase. On average, the soft X-ray emission starts 3 min before the hard X-ray and the H emission. No correlation between the duration of the pre-heating phase and the importance of the subsequent flare is found. Furthermore, the duration of the pre-heating phase does not differ for impulsive and gradual flares. For at least half of the events, the end of the non-thermal emission coincides well with the maximum of the thermal emission, consistent with the beam-driven evaporation model. On the other hand, for 25% of the events there is strong evidence for prolonged evaporation beyond the end of the hard X-rays. For these events, the presence of an additional energy transport mechanism, most probably thermal conduction, seems to play an important role.     

An example for solar flares caused by magnetic field non-equilibrium

Using a photospheric magnetogram of the solar activity complex HR 16862, 16863, 16864 at the end of May 1980 as boundary data a sequence of three-dimensional force-free magnetic fields with spatially constant ( defined by the equation × B = B) is calculated, including numerical field-line tracing. The field is assumed not to be frozen-in and ¦¦, which is a measure of the magnetic free energy, is assumed to increase with time due to some dynamo mechanism. Variation of , starting from = 0, produces a catastrophe-like change of the topology of a field-line system corresponding to an arch filament system connecting the leading spot of HR 16863 with the trailing spot of HR 16862. This topological change is interpreted as causing a series of large homologous flares with synchronous flaring in the two spots. The catastrophe-like behaviour of the field topology is attributed to the nonlinear field-line equations.     

Study of alpha component dynamics in the solar wind using the Prognoz satellite

The analysis of solar wind He⁺⁺ and H⁺ ion distribution functions, collected over five months by the satellite Prognoz 1, shows that these are in general maxwellian but that often tails appear at higher speeds. The existing relation V-T, the observation of ratios of T/Tp 3.83 and V/Vp 1.035 give evidence of preferential He⁺⁺ ion heating and acceleration. The criteria for heating by dissipation of hydromagnetic waves proposed by Barnes and Hung (1973) are tested experimentally. Finally, multifluid models are likely to predict certain observations such as dependence of the velocity ratio V/Vp on the solar wind flux.     

Thermal runaway as the solar flare trigger mechanism

It is postulated that the solar flare trigger mechanism is a thermal runaway which occurs in the inner corona. This runaway is the result of a radiative power function which decreases with increasing temperature. Relationships of the onsets of H flares, hard X-ray, soft X-ray and centimeter radio bursts are consistent with the model. Flares are shown to be common solar phenomena which occur preferentially in hot and dense active regions.     

Hα line profile observations of solar flares with high temporal resolution

H line profile observations of solar flares with high temporal resolution are an important tool for the analysis of the energy transport mechanism from the site of the flare energy release to the chromosphere. A specially designed instrument (imaging spectrograph) allows two-dimensional imaging of an active region simultaneously in 15 spectral channels along the H line profile with a temporal resolution of 5.4 s. Two flares have been observed in November 1982. The first one shows H signatures which one would typically expect in the case of explosive chromospheric evaporation produced by massive injection of non-thermal electrons. The observations of the other flare indicate that the heating of the upper chromosphere is dominated by thermal conduction, although during the impulsive hard X-ray burst there are also signatures of heating by non-thermal electrons.     

The solar Lα flux near solar minimum

After being turned off in 1972 the OSO-5 satellite was reactivated during the summer of 1974 for one year. The University of Paris experiment designed to monitor the solar L flux operated almost perfectly during that period which occurred near a minimum in solar activity. This new set of data is presented here, showing that neither the total L flux nor the flux emitted at the center of the line correlate with the solar activity indices in the same manner as previously found at higher levels of activity.These new observations confirm that the solar L flux varies approximately by a factor of two from solar maximum to solar minimum.Furthermore, the lower boundary of the transition region seems to be strongly perturbated near solar minimum, since the flux variations observed at the center of the L line are drastically different from all those previously reported. This seems to be related to the presence of large coronal holes over the Sun.     

Solar soft X-rays and solar activity

Soft solar X-rays (8 gl 12 Å) were observed from OSO-III. An analysis of the X-ray enhancements associated with 165 solar flares revealed that there is a tendency for a weak soft X-ray enhancement to precede the cm- burst and H flare. The peak soft X-ray flux follows the cm- peak by about 4 min, on the average. Additionally, it was found that flare-rich active centers tend to produce flares which are stronger X-ray and cm- emitters than are flares which take place in flare-poor active centers.     

Magnetic-cloud chamber effects behind flare-generated shock waves

A mechanism explaining the generation of the helium-enriched plasma-condensation colud (HAE-events) behind the front of shock waves associated with mass-ejecting flares is presented. The mechanism is based on the occurence of physical conditions, analogous to those in a Wilson cloud chamber in a magnetic field, behind the front of a flare-generated shock wave propagation out into interplanetary space. Consequently, if the solar atmosphere above the flare active region is saturated with ejected helium plasma, conditions are created for the forming of the helium-enriched plasma-condensation colud in the temperature-depressed region behind the shock wave front.     

The solar-flare infrared continuum: Observational techniques and upper limits

Exploratory observations at 20 and 350 have determined detection thresholds for solar flares in these wavelengths. In the 20 range solar atmospheric fluctuations (the temperature field) set the basic limits on flare detectability at 5K; at 350 the extinction in the Earth's atmosphere provides the basic limitation of 30K. These thresholds are low enough for the successful detection of several infrared-emitting components of large flares. Limited observing time and lack of solar activity have prevented observations of large flares up to the present, but the techniques promise to be extremely useful in the future.The upper limits obtained thus far, for subflares, indicate that the thickness of the H flare region does not exceed 10 km. This result confirms the conclusion of Suemoto and Hiei (1959) regarding the small effective thickness of the H-emitting regions in solar flares.     

Iron Kα-fluorescence in solar flares: A probe of the photospheric iron abundance

The X-ray line at 6.4 keV has been observed from solar flares. It is found that K-fluorescence of neutral iron in the photosphere due to thermal (T 10⁷ K) X-rays of the gradual phase is its dominant production mechanism. For a given flux and energy spectrum of incident X-rays, the flux at 1 AU of iron K-photons depends on the photospheric iron abundance, the height of the X-ray source, and the helio-centric angle between the flare and the observer. Therefore, the flux of iron K-photons, when measured simultaneously with the flux and energy spectrum of the X-ray continuum and the flare location, can give us information on the height of the X-ray source and the photospheric iron abundance. Here we present our Monte Carlo calculations of iron K-fluorescence efficiencies, so that they might be useful for interpretations of future measurements of the 6.4 keV line (e.g., by a detector to be flown on the Solar Maximum Mission).     

Acceleration of electrons in absence of detectable optical flares deduced from type III radio bursts,Hα activity and soft X-ray emission

The relationship between H absorption features, type III radio bursts and soft X-ray emission has been examined in order to determine the characteristics of the particle acceleration process operating when a H-flare may not be detectable. It is found that transient H activity observed in the absence of reported flares is associated with production of relatively weak type III radio and soft X-ray emission. Since such optical phenomena are much more frequent than flares themselves, it is concluded that instabilities generating fast particles may be produced in the corona in a quasi-continuous way with coincident perturbations in the lower solar atmosphere.The soft X-ray component, which is similar to the precursor in flares, is not necessarily the direct product of fast particles, but is probably associated with some type of heating since both the soft X-ray emission and the H features exhibit a similar evolution, the type III bursts occurring near the maximum of this perturbation. The observations are consistent with a model in which the electron acceleration region is located at an altitude where the ion density is 10⁹ cm^–3 and most of the accelerated electrons( 20 keV) are confined to coronal altitudes where the ion density is 10¹⁰ cm^–3.     

High resolution observation of Hα solar flares and temporal relation between Hα and X-ray,microwave emission

We studied the evolutional characteristics of fine structures in H flare emitting regions and their relation to X-ray and microwave emissions for selected events observed with the 60 cm Domeless Solar Telescope at Hida Observatory, University of Kyoto. The principal conclusions of this investigation are: (1) H kernel consists of some finer bright points or Hflare points whose individual size is less than 1 arc sec. (2) Impulsive brightnenings of H flare points occurred simultaneously with the spikes of the hard X-ray and microwave bursts within the time resolution of our H observations which varied from 1 to 10 s. (3) It is concluded that fast electron beams must be the principal mechanism of heating H flares during the impulsive phase of a flare.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984. Contributions from the Kwasan and Hida Observatories, University of Kyoto, No. 265.     

Chromospheric downflow velocities as a diagnostic in solar flares

We explore the dynamics of chromospheric condensations driven by evaporation during the impulsive phase of solar flares. Specifically, we find that the maximum chromospheric downflow speed obeys the approximate relation _d = 0.4 (F/ _ch )^1/3, where F is that part of the flare energy flux driving chromospheric evaporation, and _ch is the mass density in the preflare chromosphere just below the preflare transition region. This implies that chromospheric downflows as measured by H asymmetries may be a powerful probe of flare energetics.     

Heating and acceleration of α-particles in the solar wind

The non-linear saturation of whistler mode instability in the solar wind is considered here. The resulting heating and acceleration are calculated. It is shown that this instability heats predominantly the -particles and the ratio of heating rates of -particles to ions have been calculated. This instability is shown to be effective in accelerating -particles than ions. However, this process cannot account for V /V _i 1.