Dean F. Smith

First phase acceleration mechanisms and implications for hard X-ray burst models in solar flares

Requirements for the number of nonthermal electrons which must be accelerated in the impulsive phase of a flare are reviewed. These are uncertain by two orders of magnitude depending on whether hard X-rays above 25 keV are produced primarily by hot thermal electrons which contain a small fraction of the flare energy or by nonthermal streaming electrons which contain > 50% of the flare energy. Possible acceleration mechanisms are considered to see to what extent either X-ray production scenario can be considered viable. Direct electric field acceleration is shown to involve significant heating. In addition, candidate primary energy release mechanisms to convert stored magnetic energy into flare energy, steady reconnection and the tearing mode instability, transfer at least half of the stored energy into heat and most of the remaining energy to ions. Acceleration by electron plasma waves requires that the waves be driven to large amplitude by electrons with large streaming velocities or by anisotropic ion-acoustic waves which also require streaming electrons for their production. These in turn can only come from direct electric field acceleration since it is shown that ion-acoustic waves excited by the primary current cannot amplify electron plasma waves. Thus, wave acceleration is subject to the same limitations as direct electric field acceleration. It is concluded that at most 0.1% of the flare energy can be deposited into nonthermal streaming electrons with the energy conversion mechanisms as they have been proposed and known acceleration mechanisms. Thus, hard X-ray production above 10 keV primarily by hot thermal electrons is the only choice compatible with models for the primary energy release as they presently exist.

Solar Radio Emission

The subject of solar radio emission is quite broad and has been reviewed repeatedly in articles and books (Wild et al., 1963; Kundu, 1965; Zheleznyakov, 1970; Wild and Smerd, 1972; Rosenberg, 1976; Smerd, 1976a; Kruger, 1979; Melrose, 1980a). In addition, recent conference proceedings include issue 9 of Radiophysics and Quantum Electronics 20 (1977) and ‘Radio Physics of the Sun’, IAU Symposium 86 (Reidel, Dordrecht, 1980). Entire books exist on specialized topics, and we shall refer to them as necessary.

Stochastic acceleration of electrons in solar flares

The generation of lower-hybrid waves by cross-field currents is applied to reconnection processes proposed for solar flares. Recent observations on fragmentation of energy release and acceleration, and on hard X-ray (HXR) spectra are taken into account to develop a model for electron acceleration by resonant stochastic interactions with lower-hybrid turbulence. The continuity of the velocity distribution is solved including collisions and escape from the turbulence region. It describes acceleration as a diffusion process in velocity space. The result indicates two regimes that are determined by the energy of the accelerating electrons which may explain the double power-law often observed in HXR spectra. The model further predicts an anticorrelation between HXR flux and spectral index in agreement with observations.

Spatial scales of tropical precipitation inferred from TRMM microwave imager data

The local spatial scales of tropical precipitating systems were studied using Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) rain rate imagery from the TRMM satellite. Rain rates were determined from TMI data using the Goddard Profiling (GPROF) Version 5 algorithm. Following the analysis of Ricciardulli and Sardeshmukh (RS), who studied local spatial scales of tropical deep convection using global cloud imagery (GCI) data, active precipitating months were defined alternatively as those having greater than either 0.1 mm/h or 1 mm/h of rain for more than 5% of the time. Spatial autocorrelation values of rain rate were subsequently computed on a 55/spl times/55 km grid for convectively active months from 1998 to 2002. The results were fitted to an exponential correlation model using a nonlinear least squares routine to estimate a spatial correlation length at each grid cell. The mean spatial scale over land was 90.5 km and over oceans was 122.3 km for a threshold of 0.1 mm/h of rain with slightly higher values for a threshold of 1 mm/h of rain. An error analysis was performed which showed that the error in these determinations was of order 2% to 10%. The results of this study should be useful in the design of convective schemes for general circulation models and for precipitation error covariance models for use in numerical weather prediction and associated data assimilation schemes. The results of the TMI study also largely concur with those of RS, although the more direct relationship between the TMI data and rain rate relative to the GCI imagery provide more accurate correlation length estimates. The results also confirm the strong impact of land in producing short spatial scale convective rain.

Weak turbulence theory of intense beam microwave experiments

Recent intense beam experiments in microwave radiation are considered within the context of strong turbulence theory of the beam‐plasma instability. A weak turbulence model for radiation at the plasma fundamental gives qualitatively correct power levels and suggests that the radiation itself is an important dynamical entity in the beam‐plasma instability. Radiation in the region ω∼2ωp is weak, in agreement with experiment. Plasma inhomogeneities in the experiment of size approximately 0.3 cm can explain the low‐frequency emission power level, and its dependence on the plasma density and external magnetic field. Understanding these processes may lead to use of radiation as a versatile diagnostic measurement of beam‐plasma systems.

Martens-Kuin models of normal and inverse polarity filament eruptions and coronal mass ejections

An analysis is made of the Martens-Kuin filament eruption model in relation to observations of coronal mass ejections (CMEs). The field lines of this model are plotted in the vacuum or infinite resistivity approximation with two background fields. The first is the dipole background field of the model and the second is the potential streamer model of Low. The assumption is made that magnetic field evolution dominates compression or other effects which is appropriate for a low-β coronal plasma. The Martens-Kuin model predicts that, as the filament erupts, the overlying coronal magnetic field lines rise in a manner inconsistent with observations of CMEs associated with eruptive filaments. Initially, the bright arc of a CME broadens in time much more slowly than the dark cavity between it and the filament, whereas in the model they broaden at the same rate or the bright arc broadens more rapidly than the dark cavity, depending on the background field. Thus, this model and, by generalization the whole class of so-called Kuperus-Raadu configurations in which a neutral point occurs below the filament, are of questionable utility for CME modeling. An alternate case is considered in which the directions of currents in the Martens-Kuin model are reversed resulting in a so-called normal polarity configuration of the filament magnetic field. In this case, a neutral line occurs above the current-carrying filament. The background field lines now distort to support the filament and help eject it. While the vacuum field results make this configuration appear very promising, a full two- or more-dimensional MHD simulation is required to properly analyze the dynamics resulting from this configuration.

Nonlinear Effects Involved in the Generation of Type III Solar Radio Bursts

Recent observations of the simultaneous measurement of plasma waves and radiation, and electrons and radiation during type III bursts are reviewed. One-dimensional quasilinear relaxation results are considered and found to be in reasonable agreement with observations, but often inconsistent when weak turbulence wave-wave and strong turbulence processes are included. Strong turbulence processes are considered and soliton collapse is found to be important, but the steady-state plasma wave distribution in a plasma of three-dimensionally collapsing solitons is presently unknown. Radiation processes are considered from both weak and strong turbulence. Subsidiary problems such as the origin of observed and inferred plasma wave clumping are briefly considered and directions for future research are suggested.

A mechanism for producing plasma radiation in the gigahertz range by precipitating high-energy protons

Gamma-ray observations are discussed to determine the density of protons of about 1 MeV precipitating to the photosphere. It is shown that Coulomb collisions will produce a positive slope in the proton distribution for energies less than 0.1 MeV for traversed column depths greater than 1018 cm −2. This could lead to plasma wave emission and radiation near the plasma frequency for densities ∼ 3.1 × 1010 cm−3 and temperatures ∼ 4.0 × 104 K where collisional and collisionless damping of the plasma waves is sufficiently weak. It is expected that these conditions will only be satisfied sporadically which leads to stationary radio emission limited in frequency and time. Recent radio observations of impulsive phase non-drifting patches in the 1–3 GHz range with duration 2–4 s are presented which could be produced by this mechanism.

Shock versus stochastic acceleration of impulsive solar flare protons

The two major candidates for proton acceleration in impulsive γ-ray producing flares, shock and stochastic acceleration, are considered in light of recent observations of mass motions and turbulence in flares. Starting with the basic problem of energies required, energy storage and the currents which must be involved, it is concluded that the primary energy release must occur close to the temperature minimum region. It is shown that energy can propagate upwards in the form of fast magnetosonic waves which become evanescent in the transition region, converting a large fraction of their energy to mass motions and turbulence. Present observations are mostly of rather coarse (7000 km) spatial resolution and it is quite possible that significantly higher velocities than those observed were present. Using the results of recent simulations of parallel shocks and the well tested theory of Lee (1983) for parallel shock acceleration in the interplanetary medium, it is shown that shock acceleration is a viable candidate at velocities slightly higher than present observations. It is also shown that shocks must be driven by a mass of material which would be visible in coronal lines such as Caxix for them to be energetically important in proton acceleration.Stochastic acceleration is examined using the hypothesis that there is an equipartition of energy between observed turbulence and magnetic field fluctuations. It is shown that this is a viable acceleration mechanism within a large range of presently observed turbulence provided that the above equipartition hypothesis is valid and the turbulent elements are of small scale (1–200 km). Since turbulence is observed in many flares without any evidence of γ-rays, one of the above conditions must not be satisfied in general. It is concluded that although present observations favor stochastic acceleration, no definitive conclusion can be made without higher spatial resolution observations and additional theoretical work.

Evidence for two hard X-ray components in double power-law fits to the 1980 June 7 flare

The June 7, 1980 flare at 0312 UT was analyzed with double power-law fits on the basis of SMM hard X-ray burst spectrometer data. The flare is found to consist of seven peaks of characteristic time scale of about 8 sec followed by seven valleys which may contain significant peak components because of overlap. It is suggested that the possibility of thermal spectra for the peaks is unlikely. An investigation of the double power-law parameters through the third and fourth peaks revealed a hysteresis effect in the fourth peak. The present results have been interpreted in terms of a trap plus precipitation model. 12 references.