Electron precipitation characteristics during isolated, compound, and multi-night substorm events

Abstract

Abstract. A set of 24 isolated, 46 compound, and 36 multi-night substorm events from the years 2008–2013 have been analysed in this study. Isolated substorm events are defined as single expansion–recovery phase pairs, compound substorms consist of multiple phase pairs, and multi-night substorm events refer to recurring substorm activity on consecutive nights. Approximately 200 nights of substorm activity observed over Fennoscandian Lapland have been analysed for their magnetic disturbance magnitude and the level of cosmic radio noise absorption. Substorm events were automatically detected from the local electrojet index data and visually categorized. We show that isolated substorms have limited lifetimes and spatial extents as compared to the other substorm types. The average intensity (both in absorption and ground-magnetic deflection) of compound and multi-night substorm events is similar. For multi-night substorm events, the first night is rarely associated with the strongest absorption. Instead, the high-energy electron population needed to cause the strongest absorption builds up over 1–2 additional nights of substorm activity. The non-linear relationship between the absorption and the magnetic deflection at high- and low-activity conditions is also discussed. We further collect in situ particle spectra for expansion and recovery phases to construct median precipitation fluxes at energies from 30 eV up to about 800 keV. In the expansion phases the bulk of the spectra show a local maximum flux in the range of a few keV to 10 keV, while in the recovery phases higher fluxes are seen in the range of tens of keV to hundreds of keV. These findings are discussed in the light of earlier observations of substorm precipitation and their atmospheric effects.

The Predictive Capabilities of the Auroral Electrojet Index for Medium Energy Electron Precipitation

“…There is increasing evidence that substorms are key in driving MEE precipitation (Beharrell et al, 2015;Partamies et al, 2021). Nevertheless, the substorms remain partly unresolved in the existing proxies when considering the MEE precipitation (van de Kamp et al, 2016).…”

“…Recent studies, however, also point out that the role of substorms in driving MEE precipitation is not readily explained by a single magnetic index value. Hence, substorms is one of the main unknowns in the existing proxies when considering the MEE precipitation and especially its high energy tail (≳ 300 keV) (Partamies et al, 2021).…”

“…Further acceleration also occurs as the injection of source particles (tens of keV) gives rise to Very Low Frequency (VLF) wave growth, which may resonantly interact and accelerate radiation belt electrons to MEE (Borovsky & Yakymenko, 2017). Repeated substorm activity has been shown to be particularly important for MEE fluxes (Rodger et al, 2016;Partamies et al, 2021), possibly due the induction electric field directly energizing the trapped electrons as they drift across the nightside during substorm expansion phases (Dai et al, 2014). Partamies et al (2021) identified the substorm occurrence by a regional AE index and used cosmic noise absorption enhancement as a measure of the MEE precipitation.…”

“…Repeated substorm activity has been shown to be particularly important for MEE fluxes (Rodger et al, 2016;Partamies et al, 2021), possibly due the induction electric field directly energizing the trapped electrons as they drift across the nightside during substorm expansion phases (Dai et al, 2014). Partamies et al (2021) identified the substorm occurrence by a regional AE index and used cosmic noise absorption enhancement as a measure of the MEE precipitation. They showed that for multi-night substorm events, the first night was rarely associated with the most intense absorption.…”

A New MEPED‐Based Precipitating Electron Data Set

“…The advantage of using Delaunay triangulation to map the data is that it can capture small‐scale features such as substorm activity that would be averaged out in other data sets. There is a disadvantage in that substorm activity will effectively persist for a day at a time since the maps have 1‐day resolution; in reality, these storms most likely have durations of minutes to hours (Beharrell et al., 2015 ; Cresswell‐Moorcock et al., 2013 ; Partamies et al., 2021 ). On the other hand, it is likely that not all substorms are observed, particularly in a year like 2003 when only three POES satellites were operational.…”