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.