Particle acceleration in impulsive solar flares

A model for second-step electron acceleration in impulsive solar flares is presented. We have extended the theory of stochastic particle acceleration to include Coulomb energy losses which become important at low coronal heights. This inclusion successfully explains the observed steepening of interplanetary electron spectra below 3 MeV following impulsive solar flares taking place at low coronal heights. It also explains the observed spectral differences of relativistic electrons in long-duration and impulsive flares.     

Heavy Ion Abundances in Impulsive Solar Flares: Influence of Pre-Acceleration in a Current Sheet

Competition between stochastic energy gains and collisional energy losses is known to lead to preferential acceleration of heavy ions in flare loops. Ion acceleration in a reconnecting current sheet is shown to mitigate the influence of collisional energy losses on stochastic particle acceleration in impulsive solar flares. This effect decreases the sensitivity of the resulting abundance ratios on initial ion charge states. The resulting abundances are determined by the fact that the energy loss rate falls off rapidly with increasing energy. As an example, the expected Fe/O enhancement ratios are computed and shown to be comparable with those observed with ACE SEPICA in several impulsive flares in 1998. One consequence of the model is that the preferential acceleration of heavy ions can occur only when the plasma gas pressure is large enough, m _e/m _p, which may explain the observed correlation between the heavy ion enrichment and selective ³He acceleration in impulsive flares.     

Exact solution of the equation for the problem of diffuse reflection and transmission with a scattering function ω0(1 +x cos⊙) by the method of laplace transform and theory of linear singular integral equation

Neutrinos couple through a weak neutral current to the density of matter, in particular to the neutron density. Density fluctuations, or phonons, in the neutron fluid may be emitted or absorbed by neutrinos passing through the matter. At high densities, temperatures and neutrino energies the neutrino mean free paths for phonon emission and absorption can be 10⁶ cm. Significant changes in the neutrino momentum and energy accompany these processes. We present a model calculation for neutrino scattering by phonons, and representative numerical results for the neutrino mean free path and mean energy and momentum changes fork _B T andE _v both ranging from 1 to 27 MeV.Research supported by the National Research Council of Canada.     

Coulomb losses and the nuclear composition of the solar flare accelerated particles

The influence of Coulomb collisions in two-component plasma on the nuclear composition and the charge-state of accelerated particles is investigated. The main characteristics are the location and value of the two loss maxima. It is shown that the maximum of energy losses on the electron component of plasma for flares is a high energy threshold which prevents the penetration of a large particle flux into the range 10 MeV nucl^–1. The low-energy range up to the maximum is considered in detail. At preliminary or initial stage of acceleration the nuclear composition of accelerated particles is strongly dependent on their energy losses on the proton component of plasma which prevails at low energies. The conditions under which the equilibrium charge is reached are investigated.     

Estimation of an upper limit for the solar neutron emission during large flares

Solar neutron emission during large flares is investigated by using neutron monitor data from the mountain stations Chacaltaya (Bolivia), Mina Aguilar (Argentine), Pic-du-Midi (France) and Jungfraujoch (Switzerland). Registrations from such days on which large flares appeared around the local noon time of the monitor station are superimposed with the time of the optical flare as reference point.No positive evidence for a solar neutron emission was found with this method, However, by using an extrapolation of the neutron transport functions given by Alsmiller and Boughner a rough estimation of mean upper limits for the solar neutron flux is possible. The flux limits are compared with Lingenfelter's model calculations.From the Chacaltaya measurements it follows: N ₀2.8 × 10^{–3 N} cm^–2 s^–1 per proton flare, E > 50 MeV, if P0 = 125 MV N ₀1.4 × 10^{–2 N} cm^–2 s^–1 per proton flare, E > 50 MeV, if P ₀ = 60 MV and from Pic-du-Midi measurements: N ₀6.7 × 10^{–3 N} cm^–2 s^–1 per proton flare, E > 50 MeV, if P ₀ = 125 MV N ₀4 × 10^{–2 N} cm^–2 s^–1 per proton flare, E > 50 MeV, if P ₀ = 60 MV P ₀ = characteristic rigidity of the producing proton spectrum on the Sun.The flux limits estimated for some special proton flares are consistent with Lingenfelter's predictions for the acceleration phase but are too small for the slowing down phase. Therefore it is believed that Lingenfelter's assumption of isotropic proton emission from the flare region is not fulfilled.     

Effect of Coulomb collisions on the particle acceleration in collapsing magnetic traps

The problem of particle acceleration in collapsing magnetic traps in the solar corona has been solved by taking into account the particle scattering and braking in the high-temperature plasma of solar flares. The Coulomb collisions are shown to be weak in traps with lifetimes t _l < 10 s and strong for t _l > 100 s. In the approximation of strong collisions, collapsing magnetic traps are capable of confining up to 20% of the injected particles in the corona for a long time. In the collisionless approximation, this value exceeds 90%. The question about the observational manifestations of collisions is examined. For collision times comparable to t _l , the electron spectrumat energies above 10 keV is shown to be a double-power-law one. Such spectra were found by the RHESSI satellite in flares.     

Interplanetary acceleration of energetic particles at 1 and 5 AU

Energetic particle (0.1 to 100 MeV protons) acceleration is studied by using high resolution interplanetary magnetic field and plasma measurements at 1 AU (HEOS-2) and at 5 AU (Pioneer 10). Energy changes of a particle population are followed by computing test particle trajectories and the energy changes through the particle interaction with the time varying magnetic field. The results show that considerable particle acceleration takes place throughout the interplanetary medium, both in the corotating interaction regions (CIR) (5 AU), and in quiet regions (1 AU). Although shocks may contribute to acceleration we suggest statistical acceleration within the CIRs is sufficient to explain most energetic particle observations (e.g., McDonaldet al., 1975; Barnes and Simpson, 1976).The first and second order statistical acceleration coefficients which include transit time damping and Alfvén resonance interactions, are found to be well represented byD _T 8.5×10^–6 T ^0.5 MeV s^–1 andD _TT 4×10^–6 T ^1.5 MeV² s^–1 at 5 AU.By comparison, Fisk's estimates (1976), based on quasi-linear theory for transit-time damping, gaveD _TT 5×10^–7 T MeV² s^–1 at 1 AU.     

Prompt proton and electron acceleration to relativistic energies in solar flares

As a possible mechanism for particle acceleration in the impulsive phase of solar flares, a new particle acceleration mechanism in shock waves is proposed; a collisionless fast magnetosonic shock wave can promptly accelerate protons and electrons to relativistic energies, which was found by theory and relativistic particle simulation. The simultaneous acceleration of protons and electrons takes place in a rather strong magnetic field such that _{ce pe} . For a weak magnetic field ( _{ce pe} ), strong acceleration occurs to protons only. Resonant protons gain relativistic energies within the order of the ion cyclotron period (much less than 1 s for solar plasma parameters). The electron acceleration time is shorter than the ion-cyclotron period.     

The phase of particle acceleration in the flare development

Evidence is given that the particle acceleration in flares is confined to the initial phase of the flare development preceding the H flare maximum and lasting for less than 10 min. The impulsive acceleration process is confined to a relatively small limited volume of about 5 × 10²⁷ cm³ in the region of highest magnetic gradient in the flare, and its size represents about 0.05 or less of the total extent of the hot condensation which produces the soft X-ray and gradual microwave bursts. About one in fifty particles in this volume is accelerated to energy exceeding 100 keV, the total particle density being 10¹⁰ cm^–3. The accelerated electrons produce the impulsive hard X-ray burst, but synchrotron losses greatly reduce the number of relativistic electrons participating in the bremsstrahlung process. Protons above 20 MeV penetrate to the lowest chromosphere and upper photosphere and temporarily increase the temperature in the bombarded region. As the result a flash of continuous emission appears, which should be most expressive below 1527 Å. The associated white-light emission shows the bottom of the region where the impulsive acceleration process occurs.     

Excitation of whistler waves driven by an electron temperature anisotropy

A 3-D particle simulation of excitation of whistler waves driven by an electron temperature anisotropy (T > T ) is presented. Results show that whistler waves can have appreciable growth driven by the anisotropy. The maximum intensity of the excited whistler waves increases as a quadratic function of the anisotropy. Due to the presence of a threshold, one needs a relatively large electron temperature anisotropy above threshold to generate large-amplitude whistler waves. The average amplitude of turbulence in the context of whistler waves is up to as large as about 1% of the ambient magnetic field when T /T . The total energy density of the whistler turbulence is adequate for production of relativistic electrons in solar flares through stochastic acceleration.     

A model of impulsive loop flares revisited

A critical examination of the components of the recent impulsive loop flare model of Takakura is made. It is found that his analysis of the stability of the electron distribution resulting from anomalous heat conduction is in error and electron plasma waves would not be excited. Rather, in the regions where the electron/proton temperature ratioT _e/T _i 10, electrostatic ion-cyclotron waves would be excited and in the regions whereT _e 10, ion-acoustic waves would be excited. Ratios ofT _e/T _i 10 occur only in the late time development behind the conduction fronts. Since the anomalous resistivity due to electrostatic ion-cyclotron waves is fortuitously about 70% of the one used by Takakura, the general development will follow closely the one calculated by him. Because the anomalous resistivity due to ion-acoustic waves is about 95 times the one used by Takakura, the development in the parts of the loop whereT _e/T _i 10 for late times would be altered considerably.Also Guest Worker at NOAA Space Environment Laboratory, Boulder, Colorado, U.S.A.     

Prompt particle acceleration around moving X-point magnetic field during impulsive phase of solar flares

We present a model for high-energy solar flares to explain prompt proton and electron acceleration, which occurs around moving X-point magnetic fields during the implosion phase of the current sheet. We derive the electromagnetic fields during the strong implosion of the current sheet, which is driven by the converging flow toward the center of the magnetic arcade. We investigated a test particle motion in the strong electromagnetic fields derived from the MHD equations. It is shown that both protons and electrons can be promptly (within 1 s) accelerated to 70 and 200 MeV, respectively. This acceleration mechanism can be applicable for the impulsive phase of the gradual gamma-ray and proton flares (gradual GR/P flare), which have been called two-ribbon flares.     

Characteristics of solar coronal source regions producing 3He-rich particle events

We use H, X-ray, and kilometric radio data to examine the solar coronal activity associated with energetic (1 MeV/nucl^–1) ³He-rich particle events observed near Earth. The basis of the study is the 12 ³He-rich events observed in association with impulsive 2 to 100 keV electron events reported by Reames et al. (1985). We find that when H and X-ray brightenings can be associated with ³He/electron events, they have onsets coinciding to within 1 min of that of the associated metric type III bursts. In three or four events we found no associated H or X-ray flares, and in two events even the metric type III bursts were weak or absent. The measured low-energy (2 keV) electron spectra for these events show no evidence of a flattening due to Coulomb collisional losses. These results and several other recent findings are consistent with the idea that the ³He/electron events are due to particle acceleration in the corona well above the associated H and X-ray flares.     

Second-stage acceleration in a limb-occulted flare

We analyze hard and soft X-ray, microwave and meter wave radio, interplanetary particle, and optical data for the complex energetic solar event of 22 July 1972. The flare responsible for the observed phenomena most likely occurred 20° beyond the NW limb of the Sun, corresponding to an occultation height of 45 000 km. A group of type III radio bursts at meter wavelengths appeared to mark the impulsive phase of the flare, but no impulsive hard X-ray or microwave burst was observed. These impulsive-phase phenomena were apparently occulted by the solar disk as was the soft X-ray source that invariably accompanies an H flare. Nevertheless essentially all of the characteristic phenomena associated with second-stage acceleration in flares - type II radio burst, gradual second stage hard X-ray burst, meter wave flare continuum (FC II), extended microwave continuum, energetic electrons and ions in the interplanetary medium - were observed. The spectrum of the escaping electrons observed near Earth was approximately the same as that of the solar population and extended to well above 1 MeV.Our analysis of the data leads to the following results: (1) All characteristics are consistent with a hard X-ray source density n _i 10⁸ cm^–3 and magnetic field strength 10 G. (2) The second-stage acceleration was a physically distinct phenomenon which occurred for tens of minutes following the impulsive phase. (3) The acceleration occurred continuously throughout the event and was spatially widespread. (4) The accelerating agent was very likely the shock wave associated with the type II burst. (5) The emission mechanism for the meter-wave flare continuum source may have been plasma-wave conversion, rather than gyrosynchrotron emission.     

Electron acceleration in solar flares

For the period September 1978 to December 1982 we have identified 55 solar flare particle events for which our instruments on board the ISEE-3 (ICE) spacecraft detected electrons above 10 MeV. Combining our data with those from the ULEWAT spectrometer (MPI Garching and University of Maryland) electron spectra in the range from 0.1 to 100 MeV were obtained. The observed spectral shapes can be divided into two classes. The spectra of the one class can be fit by a single power law in rigidity over the entire observed range. The spectra of the other class deviate from a power law, instead exhibiting a steepening at low rigidities and a flattening at high rigidities. Events with power-law spectra are associated with impulsive (&#x003C;1 hr duration) soft X-ray emission, whereas events with hardening spectra are associated with long-duration (&#x003c;1 hr) soft X-ray emission. The characteristics of long-duration events are consistent with diffusive shock acceleration taking place high in the corona. Electron spectra of short-duration flares are well reproduced by the distribution functions derived from a model assuming simultaneous second-order Fermi acceleration and Coulomb losses operating in closed flare loops.     

Formation of SCR energy spectra during stochastic acceleration with allowance for Coulomb losses

The stochastic acceleration of heavy ions by Alfvén turbulence is considered with allowance for Coulomb losses. The pattern of energy dependence of these losses gives rise to characteristic features in the energy spectra of the accelerated particles at energies of the order of several MeV nucleon^?1. The manifestation of these features in the spectra is sensitive to the temperature and density of the medium, which can serve as a basis for plasma diagnostics in the flare region. Some impulsive solar energetic particle events during which features in the spectra of ³He and ⁴He were observed are considered as an example.     

Impulsive and hot thermal solar flares

Some X-class flares (hot thermal flares, HTF) observed with the Hinotori satellite show unique behavior: slow time variability, a compact hard X-ray source containing dense (n > 10¹¹ cm^–3) and hot (T > 3 × 10⁷ K) plasma, and unusually weak microwave emission in spite of the intense magnetic field (B > 330 G) required theoretically to sustain the hot plasma. These observations show that HTF's have essentially thermal characteristics throughout the flare evolution, while in impulsive flares, there is a transition in the energy release mode from particle acceleration (impulsive phase) to plasma heating (gradual phase). This behavior can be explained in a unified manner by employing parallel DC electric field acting over large distances.     

Enhancement of solar heavy nuclei at high energies in the 4 July 1974 event

Relative abundances of energetic nuclei in the 4 July 1974 solar event are presented. The results show a marked enhancement of abundances that systematically increase with nuclear charge numbers in the range of the observation, 6 Z 26 for energies above 15 MeV nucl.^–1 While such enhancements are commonly seen below 10 MeV nucl^–1, most observations at higher energies are found to be consistent with solar system abundances. The energy spectrum of oxygen is observed to be significantly steeper than most other solar events studied in this energy region. It is proposed that these observations are characteristic of particle populations at energies 1 MeV nucl^–1, and that the anomalous features observed here may be the result of the high energy extension of such a population that is commonly masked by other processes or populations that might occur in larger solar events.     

Role of anisotropy of the initial particle distribution in the acceleration in collapsing solar-flare traps

We consider the behavior of charged particles with an anisotropic initial velocity distribution in a magnetic trap with approaching mirrors in connection with the problem of particle acceleration in solar flares. We show that, irrespective of the charge sign, the efficiency of confinement and acceleration increases with increasing anisotropy factor of the initial distribution α = (T_⊥/T_∥)^1/2. For a positive electric potential of the trap plasma relative to the mirrors, the emerging additional effect of ion expulsion form the trap increases with α_i. The derived estimate of the electric potential suggests an amplification of the initial perturbation and the development of instability.     

On the coronal lines in the chromosphere at the 1970 eclipse

Spectrographic observations of the flash spectrum were made by the Kwasan Observatory at the total solar eclipse on 7 March, 1970. The integrated intensities of Fexiv 5303, Fex 6374, and the continuum were measured on the spectrograms as a function of height above the Sun's limb. It was found that a large amount of emission in the coronal lines originates in the interspicular regions of the chromosphere. Analysis of the data yielded that the interspicular regions consist of coronal material of T _e = 1.6 × 10⁶–1.2 × 10⁶ and log N _e = 8.5–9.5, and that a decrease in T _e and an increase in N _e occur with decreasing height.