2.3 Particle flux measurements
Electron flux measurements from the Medium Energy Proton and Electron
Detector (MEPED) instruments on the POES/MetOp satellites (NOAA15,
NOAA18, NOAA19, METOP01, and METOP02) provide a measure of strong
electron flux in the bounce loss cone (BLC). This flux is expected to
increase in regions where strong microburst precipitation is occurring.
While the 2-second cadence of these measurements cannot resolve
microbursts, their excellent spatial coverage is invaluable for
identifying the likely size of the precipitation region over time. It
should be noted that POES/MetOp has limited spatial coverage between 10
and 15 MLT (see Figure S1) in years after the MEPED instrument aboard
NOAA16 was no longer in operation (2005). The POES/MetOp Space
Environment Monitor (SEM/2) MEPED data used in this study are calibrated
as described by Pettit et al. [2021] and is referred to as
the MEPED Precipitating Electron (MPE) data set. The POES MEPED data
uses three overlapping integral energy channels of with energies of 30
-2500 keV, 100-2500 keV and 300-2500 keV respectively. An additional,
virtual high-energy channel (> 700 keV) can be utilized by
using one of the proton channels aboard MEPED that is contaminated with
relativistic electrons [Yando et al., 2001]. From there, a
differential electron flux spectrum using the 4 electron channels
spanning an energy range of 27 keV – 8.9 MeV with 27 differential
energy levels is created. A flux vs. energy threshold profile for the
satellites was developed, which was then compared to the estimated BLC
fluxes for energies between 84.7 keV - 320.8 keV (energies commonly
scattered by chorus). The threshold profile was generated by filtering
all the MPE spectral data from 2014 - 2020 using a threshold daily
Kp-index of 5 or greater. From the filtered data a median was calculated
for each energy level available in the MPE spectral data set. We then
applied the threshold on the days of interest by removing all BLC flux
that did not meet or exceed the threshold median. A median was chosen in
lieu of a mean since BLC flux does not follow a Gaussian distribution.
Since the MPE data set measures all precipitating flux occurring on a
given day, the purpose of this analysis was to remove lower levels of
BLC flux in an attempt to isolate precipitation caused my chorus waves.
However, precipitation caused by other means (EMIC waves, etc.) may
still show up in the results.