Charged particle behavior in low‐frequency geomagnetic pulsations 1. Transverse waves

Abstract

The behavior of charged particles in low-frequency geomagnetic pulsations is examined with particular emphasis on what a spacecraft-borne detector would observe. We concentrate on the effects of purely transverse electromagnetic signals. The time scale of a particle's motion relative to the wave period is shown to determine the nature of its response. For low-energy particles, the acceleration in the last gyroperiod before detection is what matters. At higher energies, what has occurred over recent bounce and drift motions becomes increasingly important and convection of gradients by the wave E x B drift must be considered. Distinguishing features such as phase differences between signals in back-to-back detectors or between channels of different energy are catalogued. In particular, we assess the detectability of resonance effects in the light of detector characteristics and finite signal bandwidth. Recent observations are used to illustrate the ideas developed.

The interaction of ultra-low-frequency pc3-5 waves with charged particles in Earth’s magnetosphere

“…In situ observations of drift-bounce-resonance between energetic electrons and ULF waves following IP shocks have been reported by (Zong et al 2007(Zong et al , 2009aTan et al 2004Tan et al , 2011. As mentioned earlier, this is possible because of the comparable periods of energetic particle drift motion and ULF oscillations (e.g., Southwood and Kivelson 1981;Takahashi et al 1985Takahashi et al , 1990Takahashi et al , 1992Hudson et al 2008;Zong et al 2007Zong et al , 2009a. Drift-resonance with poloidal mode ULF waves (Zong et al 2009a, b) and compressional waves (Tan et al 2011) induced by IP shock impact is associated with fast electron acceleration and even the formation of new radiation belts .…”

“…The other possible scenario whereby poloidal ULF waves can be excited is through drift-bounce resonance (Southwood and Kivelson 1981). suggested that energetic ring current ions can excite poloidal ULF waves of Schematic showing how a pressure enhancement associated with an IP shock drives reconfiguration in the nightside magnetosphere.…”

“…Subsequently, Kivelson (1981, 1982) developed theory that has proved useful for interpreting observational features of ULF waves and the modulations of ion and electron flux produced by them (Zong et al 2007(Zong et al , 2009a. The Southwood-Kivelson theory has recently been revisited by , who accounted for finite growth and decay of ULF waves previously not included in the Southwood and Kivelson (1981) formalism.…”

“…ULF waves excited by solar wind dynamic pressure variations have much larger amplitudes than VLF waves (Claudepierre et al 2008;Zong et al 2009a, b; and hence the former can accelerate energetic particles at a much faster rate. The comparable timescales associate with drift and bounce motion of energetic particles and ULF wave periods makes drift-bounce-resonance (e.g., Southwood et al 1981;Hudson et al 2008 possible. This process can adiabatically accelerate inner-magnetosphere charged particles and significantly enhance their radial diffusion (e.g., Elkington et al 2003;Loto'aniu et al 2006).…”

THEMIS observations of compressional pulsations in the dawn-side magnetosphere: a case study

“…the pulsations are antisymmetric with respect to the.equator (second harmonic). Southwood and Kivelson (1981) showed mathematically that in the case where a signal electric field is antisymmetric about the equator, resonance is possible only for particles that detect odd harmonics of the bounce frequency. Therefore, we set N=±1 and the values for m derived using the finite gradient techniques above as a function of radius.…”