Outward radial diffusion driven by losses at magnetopause

Abstract

[1] Loss mechanisms responsible for the sudden depletions of the outer electron radiation belt are examined based on observations and radial diffusion modeling, with L*-derived boundary conditions. SAMPEX data for October-December 2003 indicate that depletions often occur when the magnetopause is compressed and geomagnetic activity is high, consistent with outward radial diffusion for L* > 4 driven by loss to the magnetopause. Multichannel Highly Elliptical Orbit (HEO) satellite observations show that depletions at higher L occur at energies as low as a few hundred keV, which excludes the possibility of the electromagnetic ion cyclotron (EMIC) wave-driven pitch angle scattering and loss to the atmosphere at L* > 4. We further examine the viability of the outward radial diffusion loss by comparing CRRES observations with radial diffusion model simulations. Model-data comparison shows that nonadiabatic flux dropouts near geosynchronous orbit can be effectively propagated by the outward radial diffusion to L* = 4 and can account for the main phase depletions of outer radiation belt electron fluxes.

The Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) on RBSP

“…During the main phase of the storm a strong ring current will distort the magnetic field and allow current sheet scattering to move to lower L-shells. Similarly, the outward motion of the radiation belt particles due to the adiabatic effect causes electrons to move to larger L, thus increasing the losses through the magnetopause (Shprits et al 2006;Jordanova et al 2008;. Accurate measurements of magnetic field distortions during geomagnetic storms are required to compute the effectiveness of such loss.…”

“…ULF waves, with periods comparable to tens of minutes, cause a violation of the third invariant, resulting in radial diffusion. Since the power in ULF waves is considerably enhanced during magnetic storms (Mathie and Mann 2000), radial diffusion is a potentially important mechanism for energetic electron acceleration (Rostoker et al 1998;Elkington et al 1999;Hudson et al 2001;O'Brien et al 2001;Shprits and Thorne 2004) or loss (Shprits et al 2006;Jordanova et al 2008;) during storm conditions, dependent on the radial gradient in phase space density. Higher frequency ELF and VLF waves cause violation of the first two invariants and lead to pitch angle scattering loss to the atmosphere (Thorne and Kennel 1971;Lyons et al 1972;Thorne 1998a, 1998b) or local stochastic energy diffusion (Horne and Thorne 1998;Summers et al 1998;Miyoshi et al 2003).…”

Butterfly pitch angle distribution of relativistic electrons in the outer radiation belt: Evidence of nonadiabatic scattering

“…The solar wind flow velocity decreased from 450 km/s to 380 km/s. Thus, for this event, we do not expect that the magnetopause shadowing effect can play an important role in loses of the equatorial (pitch angle ∼ 90 ∘ ) electrons [e.g., Shprits et al, 2006;Turner et al, 2012].…”

Relativistic electron flux dropouts in the outer radiation belt associated with corotating interaction regions

“…Drift loss of electrons occurs when the magnetopause is compressed inward and intersects the drift path of electrons in the outer radiation belt [Shprits et al, 2006;Turner et al, 2012;Tu et al, 2013Tu et al, , 2014.…”

“…There are currently three types of loss mechanisms to explain the electron flux dropout in the outer radiation belt: adiabatic loss or "Dst effect" [Kim and Chan, 1997]; precipitation loss to the atmosphere via pitch angle scattering with plasma waves Abel and Thorne, 1998;O'Brien et al, 2003;Voss et al, 1998]; and drift loss to the magnetopause and outward radial diffusion [Brautigam and Albert, 2000;Miyoshi et al, 2003;Shprits et al, 2006;Ohtani et al, 2009;Matsumura et al, 2011;Turner et al, 2012;Tu et al, 2014].…”