3.3 Multipoint observations and results
Figures 2a and 2b show the same observations as Figure 1 but plots the MLT and L data separately versus time for the entire event study (from 5 December 2017 00:00 UT to 6 December 2017 06:00 UT). Figures 2c and 2d show the measured electron fluxes from MagEIS and HEP. Data are only plotted for regions where simultaneous observations of chorus and microbursts/precipitation could have been made (i.e. the black regions have no coverage from RBSP and/or Arase). Figures 2a and 2b demonstrate the consistency in the size (MLT and L) of the microburst and chorus regions throughout the day. Figures 2c and 2d show that the locations of strong electron precipitation and increased microburst flux coincide with regions of high electron fluxes. For example, from 14:00 to 20:00 UT, the electron fluxes increase, and FIREBIRD and AC6-A observe a number of microbursts. Another region of high electron and microburst/precipitation fluxes occurs between 10:00 and 12:00 UT. Both of these regions also have strong electron precipitation observed by POES/MetOp. Electron flux is also lower earlier in the day, which is likely contributing to the lack of strong electron precipitation observed by POES/MetOp. POES/MetOp does observe precipitation in these regions (see Figures S3 and S4), but it is not strong enough to exceed our threshold profile. The RBSP-A and FU4 conjunction observations indicate that the microburst flux was considerably higher at 23:00 UT than 16:30 UT even though average chorus wave amplitudes were larger at the earlier time. Determining the reason for this discrepancy is beyond the scope of this study.
To constrain the region size we analyze the MLT and L extent of chorus and precipitation/microbursts for the three 9-hour periods discussed above. Figures 3a, 3c, and 3e present the chorus and microburst/precipitation extent for these intervals and show clear overlap between the chorus wave and microburst/precipitation observations. There are other regions where microburst/precipitation is observed; however, there was no spacecraft coverage to detect chorus waves. Figures 3b, 3d, and 3f show the upper/lower bounds on the size of the microburst-producing chorus regions for these three time periods. We find that the microburst-producing chorus region ranges from 4 to 8 MLT and 2 to 8.5 L.
It should be noted that due to there being more than one large precipitation region (for example, the three precipitation regions during the 12:00 – 21:00 UT time period in Figure 1b described above) we must put boundaries on our calculations of the region size to exclude precipitation regions that are likely not caused by chorus wave interactions. Therefore, the green regions in Figure 3 show the MLT extent of the precipitation between 4 and 14 MLT, excluding the precipitation region observed from 14 to 22 MLT that is likely caused by other sources. It should also be noted that assumptions must be made about the continuity of chorus waves between two observational points. Therefore, we assume chorus waves are active throughout the region if observations show chorus wave activity when the satellites are in the inner magnetosphere outside of the plasmasphere.
These three long-lived (~9 hour) events provide the first accurate (lower bound) estimates for the microburst-producing chorus region. Note that all three periods had limited chorus coverage prior to 6 MLT and beyond 6 L, suggesting we may be underestimating the extent of the chorus region.