Pitch angle scattering of energetic electrons by bursty bulk flows (BBFs)

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Tuesday, November 13, 2018

Mary Hudson and others have studied field line curvature scattering by the magnetic field structure associated with Bursty Bulk Flows (BBFs), using simulated output fields from the Lyon‐Fedder‐Mobarry (LFM) global magnetohydrodynamic (MHD) code for specified solar wind input. There are weak magnetic field strength (B) regions adjacent to BBFs observed in the simulations. We show that these regions can cause strong scattering where the first adiabatic invariant changes by several factors within one equatorial crossing of energetic electrons of a few keV when the BBFs are beyond 10RE geocentric in the tail. Scattering by BBFs decreases as they move towards the Earth or when the electron energy decreases.

Graphic image showing the pitch angle scattering of electrons in high resolution LFM fields
Pitch angle scattering of electrons in high resolution LFM fields from Wiltberger et al. JGR 2015 has been investigated and shown to compete with inward radial transport of electrons by BBFs investigated in a separate study.

For radiation belt electrons near or inside geosynchronous orbit we demonstrate that the fields associated with BBFs can cause weak scattering where the fractional change of the first invariant (μ0) within one equatorial crossing is small, but the change due to several crossings can accumulate. For the weak scattering case we developed a method of calculating the pitch angle diffusion coefficient Dαα. Dαα for radiation belt electrons for one particular BBF were calculated as a function of initial energy, equatorial pitch angle and radial location. These Dαα values were compared to calculated Dαα for a dipole field with no electric field. We further compared Dαα values with that of stretched magnetic fields calculated by Artemyev et al. [2013] at r≈7RE. Results show that scattering by BBFs can be comparable to the most highly stretched magnetic field they studied.

Publication Name: JGR Space Physics

First HAO Author's Name: Mary K. Hudson