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).