arXiv subjects

Danny Summers

Publications and source records attributed to Danny Summers.

4 recordsLinked to original sources

arXiv2026

Ion firehose and ion cyclotron instability with subtracted-Kappa distributions

In the present paper we discuss numerical solutions of the dispersion relation for electromagnetic waves propagating along the lines of an ambient magnetic field, considering parameters representative of space plasma environments, and considering the ion population described by a subtracted-Kappa distribution. We consider situations in which the ion thermal anisotropy is such that $T_{i\perp} T_{i\parallel}$, and investigate the effects of the same parameters on the ion-cyclotron instability. For both forms of instability, we also investigate the effect of the thermal anisotropy of the electron distribution, and the effect of the occurrence of a drift velocity in the electron distribution function. Among the results obtained, we show that the increase of the loss-cone feature in a bi-kappa distribution leads to a decrease of the growth rates of the firehose instability, and to an increase of the growth rates of the ion-cyclotron instability.

physics.plasm-ph↗

arXiv2000

Rapid acceleration of electrons in the magnetosphere by fast-mode MHD waves

During major megnetic storms, enhanced flux of relativistic electrons in the inner magnetosphere have been observed to correleated with ULF waves. The enhancements can take place over a period of several hours. In order to account for such a rapid generation of relativistic electrons, we examine the mechanism of transit-time acceleration of electrons by low-frequency fast-mode MHD waves, here the assumed form of ULF waves. Calcaulations of the acceleration timescales in the model show that fast-mode waves in the Pc4 to Pc5 frequency range, with typically observed wave amplitudes 10--20 nT, can accelerate the seed electrons to energies of order MeV in a period of a few hours.

physics.plasm-ph↗

arXiv1999

A Model for Generating Relativistic Electrons in the Earth's Inner Magnetosphere Based on Gyroresonant Wave-Particle Interactions

During the recovery phase of a magnetic storm, fluxes of relativistic ($>1$ MeV) electrons in the inner magnetosphere ($3\le L \le 6$) increase to beyond pre-storm levels, reaching a peak about 4 days after the initiation of the storm. In order to account for the generation of these "killer electrons", a model is presented primarily based on stochastic acceleration of electrons by enhanced whistler-mode chorus. In terms of a quasi-linear formulation, a kinetic (Fokker-Planck) equation for the electron energy distribution is derived, comprising an energy diffusion coefficient based on gyroresonant electron-whistler-mode wave interaction and parallel wave propagation; a source term representing substorm-produced (lower energy) seed electrons; and a loss term representing electron precipitation due to pitch-angle scattering by whistler-mode waves and EMIC waves. Steady-state solutions for the electron energy distribution are constructed, and fitted to an empirically-derived relativistic Maxwellian distribution for the high energy "hard" electron population at geosynchronous orbit. The mechanism is expected to be particularly effective for the class of small and moderate storms possessing a long-lasting recovery phase during which many substorms occur.

physics.plasm-ph↗

arXiv1998

Formation of Power-law Energy Spectra in Space Plasmas by Stochastic Acceleration due to Whistler-Mode Waves

A non-relativistic Fokker-Planck equation for the electron distribution function is formulated incorporating the effects of stochastic acceleration by whistler-mode waves and Coulomb collisions. The stationary solution $f$ to the equation, subject to a zero-flux boundary condition, is found to be a generalized Lorentzian (or kappa) distribution, which satisfies $f\propto v^{-2(κ+1)}$ for large velocity $v$, where $κ$ is the spectral index. The parameter $κ$ depends strongly on the relative wave intensity $R$. Taking into account the critical energy required for resonance of electrons with whistlers, we calculate a range of values of $R$ for each of a number of different space plasmas for which kappa distributions can be expected to be formed. This study is one of the first in the literature to provide a theoretical justification for the formation of generalized Lorentzian (or kappa) particle distribution functions in space plasmas.

physics.plasm-ph↗