Statistical study of heavy ion energization in the near-Earth
magnetotail using a cross-correlation approach: Magnetospheric
Multiscale (MMS) Observations
Abstract
We present a statistical study of energetic heavy ion acceleration in
the near-Earth magnetotail using observations from the Energetic Ion
Spectrometer (EIS) onboard the Magnetospheric Multiscale (MMS)
spacecraft. Although the EIS instrument does not measure ion charge
state directly, we have inferred the dominant charge state of the
suprathermal heavy ions (i.e., ~60-1000 keV He and
C-N-O), using a previously-developed correlation analysis of the
time-dependent flux response between different energy channels of
different ion species. For specific events we have also distinguished
adiabatic (charge-dependent) energization from non-adiabatic
(mass-dependent) energization. This work uses observations from the MMS
“Bursty Bulk Flows (BBF) Campaign” in August 2016, when
high-energy-resolution “burst”-mode data are more frequently
available, to examine the relative occurrence of adiabatic energization
versus preferential energization of heavy ions. The results of this
study demonstrate the utility and limitations of the cross-correlation
technique that was applied. We find that the technique is consistently
able to discern coarse charge states for heavy ions such as
O+/6+, He+/++ (i.e., ionospheric
versus solar wind sources), but that the more subtle job of uniquely
determining adiabatic versus non-adiabatic behaviors for the ionospheric
component (O+) is only sometimes achievable. The
dynamics of Earth’s magnetotail are apparently too complex and variable
to consistently accommodate our simple assumption for adiabatic behavior
of energy/charge-ordered transport from a common source of particles.