A reexamination of latitudinal limits of substorm‐produced energetic electron precipitation

Abstract

The primary sources of energetic electron precipitation (EEP) which affect altitudes <100 km (>30 keV) are expected to be from the radiation belts and during substorms. EEP from the radiation belts should be restricted to locations between L = 1.5 and 8, while substorm‐produced EEP is expected to range from L = 4 to 9.5 during quiet geomagnetic conditions. Therefore, one would not expect any significant D region impact due to electron precipitation at geomagnetic latitudes beyond about L = 10. In this study we report on large unexpectedly high‐latitude D region ionization enhancements, detected by an incoherent scatter radar at L ≈ 16, which appear to be caused by electron precipitation from substorms. We go on to reexamine the latitudinal limits of substorm‐produced EEP using data from multiple low‐Earth orbiting spacecraft, and demonstrate that the precipitation stretches many hundreds of kilometers poleward of the previously suggested limits. We find that a typical substorm will produce significant EEP over the International Geomagnetic Reference Field L shell range L = 4.6 ± 0.2–14.5 ± 1.2, peaking at L = 6–7. However, there is significant variability from event to event; in contrast to the median case, the strongest 25% of substorms have significant EEP in the range spanning L = 4.1 ± 0.1–20.7 ± 2.2, while the weakest 25% of substorms have significant EEP in the range spanning L = 5.5 ± 0.1–10.1 ± 0.7. We also examine the occurrence probability of very large substorms, focusing on those events which appear to be able to disable geostationary satellites when they are located near midnight magnetic local time. On average, these large substorms occur approximately one to six times per year, a significant rate, given the potential impact on satellites.

An Updated Model Providing Long‐Term Data Sets of Energetic Electron Precipitation, Including Zonal Dependence

“…Higher A p levels correspond to greater geomagnetic disturbances, which are likely to involve multiple substorms. It has previously been shown that substorms lead to strong precipitation over a wide L shell range (Cresswell‐Moorcock et al, ), which would explain the EEP enhancement seen in Figure for those A p conditions.…”

Long‐term determination of energetic electron precipitation into the atmosphere from AARDDVARK subionospheric VLF observations

“…This time period spans an extended period of low solar activity, in which the trapped low Earth orbit relativistic electron fluxes reported by SAMPEX [ Russell et al ., ] and the geosynchronous GOES observations both fell to noise floor levels. Similar decreases in the POES‐trapped relativistic electrons have been reported, which were noted as being “unprecedented in the ~14 years of SEM‐2 observations” [ Cresswell‐Moorcock et al ., ]. In the same time period, that study noted the outer belt >100 keV POES‐trapped electron fluxes decreased by 1–1.5 orders of magnitude, recovering to the typical long‐term average in 2010.…”

Magnetic Local Time‐Resolved Examination of Radiation Belt Dynamics during High‐Speed Solar Wind Speed‐Triggered Substorm Clusters

“…This is undertaken for a series of MLT range: 0–3, 3–6, 6–9, through to 21–24 MLT. A more detailed description of the dataset and the processing undertaken can be found in Rodger, Clilverd, et al () and Cresswell‐Moorcock et al ().…”