Planetary Radiation Belts Research Articles

Effects of nonlinear wave growth on extreme radiation belt electron fluxes

[1] We present new relativistic formulae for the self-limiting electron integral and differential fluxes in a planetary radiation belt. The formulae depend on the relativistic particle anisotropy, the power gain of electromagnetic whistler mode waves assumed to be generated near the equator, and the associated length scale for convective wave growth. The theoretical limit for the electron differential flux is compared with measured fluxes at geosynchronous orbit. Next we incorporate the effects of nonlinear wave growth into the calculation of extreme radiation belt electron fluxes. We assume that whistler mode waves undergo a linear growth phase, during which the frequency is constant, followed by a nonlinear growth phase during which the frequency is time increasing. We determine the total power gain GTOT as the sum of the linear wave gain gL during linear growth and the nonlinear wave gain gN during nonlinear growth. GTOT depends on the maximum values of the linear and nonlinear wave growth rates at the equator and is hence a function of the energetic electron number density Nh. For a specified value of GTOT, we can determine the value of Nh that corresponds to an extreme value for the electron (integral/differential) flux. Our new result for the extreme electron flux in the nonlinear regime can be regarded as a generalization of the Kennel-Petschek limit, which is based on linear wave growth. The extreme value for the electron flux depends on the parameters that characterize the plasma and the assumed particle distribution, as well as the convective length scales λLH and λNH for linear and nonlinear wave growth.

Limit on stably trapped particle fluxes in planetary magnetospheres

We reexamine the Kennel‐Petschek concept of self‐limitation of stably trapped particle fluxes in a planetary magnetosphere. In contrast to the original Kennel‐Petschek formulation, we carry out a fully relativistic analysis. In addition, we replace the wave reflection criterion in the Kennel‐Petschek theory by the condition that the limit on the stably trapped particle flux is attained in the steady state condition of marginal stability when electromagnetic waves generated at the magnetic equator acquire a specified gain over a given convective growth length. We derive relativistic formulae for the limiting electron integral and differential fluxes for a general planetary radiation belt at a given L shell. The formulae depend explicitly on the spectral index and pitch angle index of the assumed particle distribution and on the ratio of the electron gyrofrequency to the electron plasma frequency. We compare the theoretical limits on the trapped flux with observed energetic electron fluxes at Earth, Jupiter, and Uranus.

Introduction to plasma physics: with space and laboratory applications

The emphasis of this text is on basic plasma theory, with applications to both space and laboratory plasmas. All mathematical concepts beyond those normally covered in an advanced calculus course are fully explained. Topics covered include single-particle motions, kinetic theory, magnetohydrodynamics, small amplitude waves in both cold and hot plasmas, nonlinear phenomena and collisional effects. Applications include planetary magnetospheres and radiation belts, the confinement and stability of plasmas in fusion devices, the propagation of discontinuities and shock waves in the solar wind, and the analysis of various types of plasma waves and instabilities that can occur in planetary magnetospheres and laboratory plasma devices. This book is structured as a text for a one- or two-semester introductory course in plasma physics at the advanced undergraduate or first-year graduate level. It can also serve as a resource book on the basic principles of plasma physics.

Variations in planetary radiation belt fluxes and their radiative observations

The variations in the fluxes of the relativistic electrons in the planetary radiation belts are due to a set of different physical processes which violate one or more of the adiabatic invariants. We survey the mechanisms which break down these invariants and investigate the time scales for the processes and the resulting effects on the observed fluxes. The mechanisms include (a) sudden deformation of the magnetic field configuration, (b) radial diffusion due to low-frequency electromagnetic oscillations, (c) transit-time damping due to fast waves and (d) diffusion due to electromagnetic ion-cyclotron (emic) or whistler waves. It is indicated how the waves which interact resonantly with the relativistic electrons are responsible for enhancement in the radiative spectra of the gyrosynchrotron emissions in the GHz frequency range and the X-ray bremsstrahlung emissions at the MeV energy range.

Effective saturation of absorption in a magnetospheric plasma maser

The properties of the quasilinear interaction of whistler and Alfven waves with planetary radiation belts are discussed. It is shown that quasilinear relaxation can lead to an increase in the increment of cyclotron instability at the leading front of an electromagnetic pulse. This corresponds to the effective saturation of absorption and makes it energetically advantageous for the noise emission to be divided into separate electromagnetic pulses. Peculiarities of the manifestation of fast and slow, compared with the pulse length, effective saturation of absorption are discussed.

Existence and Role of Amorphous Grains in the Solar System

Consideration of the nucleation and growth processes and observational arguments suggest that besides crystalline grains in the solar system and interplanetary space there are also amorphous H20-ice, Si02 and C grains. Of particular importance are effects of the bombardment of these grains by protons in the solar wind and in the planetary radiation belts and the ensuing various erosion processes. Differences in the physical properties of crystalline and amorphous grains lead to interesting astrophysical consequences.