3.1 Chorus wave observations
Starting near the beginning of the 5 December 2017 event, RBSP, Arase,
and ground-based VLF stations observe >30 hours of
near-continuous lower-band chorus activity, with enhancements associated
with the substorm injections. We focus on three nine-hour periods,
selected to provide sufficient spatial coverage of the inner
magnetosphere outside of the plasmasphere while still allowing insight
into the overall time evolution of the microburst producing region. Wave
observations during the first period (~02:00 - 11:00 UT
on 5 December 2017) were made primarily by Arase, with the strongest
from ~03:20 to 06:30 UT. Waves during the second period
(~12:00 - 21:00 UT on 5 December 2017) were mainly
observed by RBSP, with the strongest from ~11:20 to
18:10 UT (~14:20 to 20:42 UT) on RBSP-A (RBSP-B). The
third period (~21:00 UT on 5 December 2017 to 06:00 UT
on 6 December 2017) consisted of weak chorus observed by both RBSP and a
separate chorus region observed by Arase.
Throughout the entire period, electric and magnetic wave amplitudes of
>15 mV/m and >95 pT were measured on RBSP.
Chorus wave normal angles (the angle of the wave vector with the
background magnetic field) at low magnetic latitudes as observed on RBSP
and Arase were mostly quasi field aligned (<45°) with a mix of
quasi-perpendicular (>45°) for weaker waves. At higher
magnetic latitudes of 25° - 35°, where the chorus waves likely create
relativistic microbursts, wave normal angles observed on Arase varied
from quasi-parallel to quasi-perpendicular.
Figure 1 shows multipoint observations of the L/MLT distribution of
lower-band chorus waves, microburst signatures, and strong electron
precipitation. Because the plots each include 9 hours of data, a
magnetic conjunction cannot be implied based solely on overlapping
tracks. Care should also be taken when analyzing the strength of chorus
waves, particularly on Arase, because there are other waves in the same
frequency band which may not be chorus (see Figure S2). Chorus waves
initiate around MLT of 6 for the first two time periods and around 7 MLT
for the later period, however, due to limited early MLT coverage, these
cutoffs are not exact, and we are therefore potentially underestimating
the size of the chorus region. At later MLTs however, Arase observed
chorus wave cutoffs around 15.5 MLT for the 02:00 to 11:00 UT period and
14 MLT for the 12:00 to 21:00 UT period. Because Arase coverage extends
beyond these MLTs, these boundaries better reflect the later MLT extent
of the region. For the 12:00 to 06:00 UT on 6 December 2017 period,
chorus waves are not observed past 11 MLT from RBSP. Chorus waves were
observed by Arase between 15 and ~16 MLT. It should be
noted however that there is some indication from the VLF ground stations
that there may be chorus in between the observations from RBSP and Arase
(see Figure 1).