4. Discussion and Conclusions
Constraining the extent of the microburst-producing chorus region is
vital to understanding whether microburst precipitation is a significant
loss source for relativistic outer radiation belt electrons
(Breneman et al. , 2017). We have presented an event study for a
30-hour period (00:00 UT on 5 December 2017 to 06:00 UT on 6 December
2017) where simultaneous chorus wave (from RBSP, Arase, and ground-based
VLF stations), microburst (from FIREBIRD and AC6-A), and electron
precipitation observations (from POES/MetOp) were made.
Chorus wave normal angles varied from quasi-parallel at low magnetic
latitudes to quasi-perpendicular at higher magnetic latitudes. These
observations are consistent with Colpitts et al. [2020] who
compared RBSP and Arase observations of the same chorus wave packets,
along with a ray tracing analysis, to verify unducted propagation from
the near-equatorial source to higher latitudes. Additional studies have
shown both ducted [Chen et al. , 2020] and nonducted
[Chen et al. , 2021; Ozaki et al., 2021] propagation can
occur. Our observations support the theory that near equatorial (RBSP)
chorus propagates to off-equatorial (Arase) latitudes, where the chorus
waves become more oblique and can resonate with hundreds of keV
electrons to produce microbursts.
We estimate that the microburst-producing chorus region extends from 4
to 8 MLT and 2 to 8.5 L. Both our lower bound of the size of the
region (MLT of 4 and L of 2) and our upper bound (MLT of 8
and L of 8.5) are larger than the bounds (MLT from 1 to 5 hours)
found by Breneman et al. [2017]. Using our extents and the
microburst flux assumption of Breneman et al. [2017], we find
the loss timescales range from 8 to 12 hours, suggesting microburst
precipitation is likely a major loss source of outer radiation belt
electrons for this event. However, because there were multiple
injections throughout the day, comparing the calculated loss rate to
other loss processes becomes difficult. We conclude, as didBreneman et al. [2017], that microburst precipitation
constitutes a major source of electron loss from the outer radiation
belt.
To further constrain the importance of microburst loss, we plan to
perform a statistical study of all days with magnetic conjunction events
between RBSP and FIREBIRD, incorporating additional satellite-borne and
ground-based data when available, similar to the study presented here.
Including all events will help to constrain the size of the region
through improved spacecraft coverage and determine if constraints depend
on geomagnetic activity.