Upper limit of proton anisotropy and its relation to EMIC waves in the
inner magnetosphere
Abstract
Proton anisotropy in velocity space has been generally accepted as a
major parameter for exciting electromagnetic ion cyclotron (EMIC) waves.
In this study, we estimate the proton anisotropy parameter as defined by
the linear resonance theory using data from the Van Allen Probes
mission. Our investigation uses the measurements of the inner
magnetosphere (L < 6) from January 2013 to February 2018. We
find that the proton anisotropy is always clearly limited by an upper
bound and it well follows an inverse relationship with the parallel
proton b (the ratio of the plasma pressure to the magnetic pressure)
within a certain range. This upper bound exists over wide spatial
regions, AE conditions, and resonance energies regardless of the
presence of EMIC waves. EMIC waves occur when the anisotropy lies below
but close to this upper bound within a narrow plasma b range: The lower
cutoff b is due to an excessively high anisotropy threshold and the
upper cutoff b is possibly due to the predominant role of a
faster-growing mirror mode instability. We also find that the anisotropy
during the observed EMIC waves is unstable, leading to the linear ion
cyclotron instability. This result implies that the upper bound of the
anisotropy is due to nonlinear processes.