Quasi-steady, marginally unstable electron cyclotron harmonic wave amplitudes

Zhang, Xiaojia; Angelopoulos, Vassilis; Ni, Binbin; Thorne, Richard M.; Horne, Richard B. ORCID: https://orcid.org/0000-0002-0412-6407. 2013 Quasi-steady, marginally unstable electron cyclotron harmonic wave amplitudes. Journal of Geophysical Research: Space Physics, 118 (6). 3165-3172. 10.1002/jgra.50319

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Abstract/Summary

Electron cyclotron harmonic (ECH) waves have long been considered a potential driver of diffuse aurora in Earth's magnetotail. However, the scarcity of intense ECH emissions in the outer magnetotail suggests that our understanding of the amplification and the relative importance of these waves for electron scattering is lacking. We conduct a comprehensive study of wave growth and quasi-linear diffusion to estimate the amplitude of loss-cone-driven ECH waves once diffusion and growth balance but before convection or losses alter the background hot plasma sheet population. We expect this to be the most common state of the plasma sheet between episodes of fast convection. For any given wave amplitude, we model electron diffusion caused by interaction with ECH waves using a 2-D bounce-averaged Fokker-Planck equation. After fitting the resultant electron distributions as a superposition of multicomponent subtracted bi-Maxwellians, we estimate the maximum path-integrated gain using the HOTRAY ray-tracing code. We argue that the wave amplitude during quasi-steady state is the inflection point on a gain-amplitude curve. During quasi-steady state, ECH wave amplitudes can be significant (~1 mV/m) at L ~ 8 but drop to very low values (<~0.1 mV/m) in the outer magnetotail (L ~ 16) and likely fall below the sensitivity of typical instrumentation relatively close to Earth mainly because of the smallness of the loss cone. Our result reinforces the potentially important role of ECH waves in driving diffuse aurora and suggests that careful comparison of theoretical wave amplitude estimates and observations is required for resolving the equatorial scattering mechanism of diffuse auroral precipitation.

Item Type:	Publication - Article
Digital Object Identifier (DOI):	10.1002/jgra.50319
Programmes:	BAS Programmes > Polar Science for Planet Earth (2009 - ) > Climate
ISSN:	2169-9402
Additional Keywords:	ECH wave; diffuse aurora; magnetotail; wave-particle interaction; ray tracing
NORA Subject Terms:	Space Sciences
Date made live:	02 Jul 2013 13:21 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/502342

Item Type:

Publication - Article

Digital Object Identifier (DOI):

10.1002/jgra.50319

Programmes:

BAS Programmes > Polar Science for Planet Earth (2009 - ) > Climate

ISSN:

2169-9402

Additional Keywords:

ECH wave; diffuse aurora; magnetotail; wave-particle interaction; ray tracing

NORA Subject Terms:

Space Sciences