Timescale for radiation belt electron acceleration by whistler mode chorus waves

Abstract

[1] Electron acceleration inside the Earth's magnetosphere is required to explain increases in the $MeV radiation belt electron flux during magnetically disturbed periods. Recent studies show that electron acceleration by whistler mode chorus waves becomes most efficient just outside the plasmapause, near L = 4.5, where peaks in the electron phase space density are observed. We present CRRES data on the spatial distribution of chorus emissions during active conditions. The wave data are used to calculate the pitch angle and energy diffusion rates in three magnetic local time (MLT) sectors and to obtain a timescale for acceleration. We show that chorus emissions in the prenoon sector accelerate electrons most efficiently at latitudes above 15°for equatorial pitch angles between 20°a nd 60°. As electrons drift around the Earth, they are scattered to large pitch angles and further accelerated by chorus on the nightside in the equatorial region. The timescale to accelerate electrons by whistler mode chorus and increase the flux at 1 MeV by an order of magnitude is approximately 1 day, in agreement with satellite observations during the recovery phase of storms. During wave acceleration the electrons undergo many drift orbits and the resulting pitch angle distributions are energy-dependent. Chorus scattering should produce pitch angle distributions that are either flat-topped or butterfly-shaped. The results provide strong support for the wave acceleration theory.

Relativistic electron response to the combined magnetospheric impact of a coronal mass ejection overlapping with a high‐speed stream: Van Allen Probes observations

“…It is known that chorus activity can energize electrons on a rapid timescale, often less than hours . Furthermore, characteristically chorus energization leads to a delayed flux increase at higher electron energies [Horne et al, 2005]. Thus, both episodes of the enhanced chorus activity could play a role in energizing electrons and account for the energy-dependent flux enhancements.…”

New evidence for generation mechanisms of discrete and hiss‐like whistler mode waves

“…The whistler mode wave emissions are generally believed to be excited in the vicinity of the geomagnetic equatorial plane [LeDocq et al, 1998;Lauben et al, 2002;Santolik et al, 2005;Li et al, 2009], but another source region may also be present at high latitudes in the dayside magnetosphere [Tsurutani and Smith, 1977;Tsurutani et al, 2009]. Because they provide efficient energy diffusion through cyclotron resonances, whistler mode waves are fundamentally important for accelerating seed electrons to highly relativistic energies [Summers et al, 1998;Meredith et al, 2001;Horne et al, 2005;Cattell et al, 2008;Thorne et al, 2013]. On the other hand, they can also scatter electrons into the loss cone leading to precipitation into the upper atmosphere, which is an important loss process in the radiation belts and provides a major source of energy for the diffuse and pulsating aurora [Horne and Thorne, 2003;Ni et al, 2008Ni et al, , 2011Nishimura et al, 2010Nishimura et al, , 2013Thorne et al, 2005Thorne et al, , 2010.…”

Effects of amplitude modulation on nonlinear interactions between electrons and chorus waves

“…[3] The effects of chorus waves on energetic electrons have been mainly modeled by quasilinear theory, which assumes wideband and small amplitude wave fields [e.g., Horne et al, 2005b]. High resolution observations of chorus waves, however, have revealed the discrete and coherent nature of chorus wave packets [see, e.g., Santolík et al, 2004;Tsurutani et al, 2009].…”