Energetic electron precipitation (EEP, energies ~30-300 keV) is an important substorm manifestation which increases electron concentration in the D region, affects radio communications and navigation and, on long term, also affects ozone concentration and climate, making EEP diagnostic and forecast an actual problem. A remarkable EEP manifestation is Auroral Absorption (AA, subtype of Cosmic Noise Absorption) which provides an integral measure of surplus D-region ionization and is monitored by global riometer network. Though the AA morphology has been intensively studied since 1960-s, no dynamic EEP model yet exists reflecting extremely complicated evolution of trapped electron populations (both ambient and injected) and of their pitch-angle scattering into the ionosphere during substorms. We develop and investigate an empirical EEP model (here mostly based on observed AA) based on following assumptions:
(1) Energetic electrons are accelerated (primarily by betatron-like mechanism) and injected near midnight but then drift eastward, precipitating and creating auroral absorption. The EEP intensity at different MLTs is determined by initial conditions at midnight, the response being different for different MLTs, but stable in each MLT sector reflecting stability of drift delays in the Radiation Belt.
(2). As EE injection temporally/spatially coincides with magnetic dipolarization (which may be responsible for electron energization), the dipolarization magnitude (which is proportional to SCW intensity) can be used to initialize the injection strength. Here the intensity of elementary injection is estimated as the positive increment of MPB1/2 (t) at 5-min steps, where MPB is the mid-latitude positive bay index (after McPherron&Chu, Space Sci. Rev 2017) also related to SCW.
(3). Appreciating multiplicity of injections during substorms and suggesting 4-hour- long response of AA to each elementary (5-min scale) injection, we represent AA value as a sum of contributions of injections occurring during 4 preceding hours. Prediction filter-type response functions are then determined from AA and MPB time series (inputs) separately for each MLT using data sets, each including dozens of substorm events.
This approach was realized and investigated using Canadian NORSTAR riometer data as well as EISCAT D-region electron densities Ne measured by EISCAT radar. (a) We found that in the same MLT sector at the same latitude the response functions are similar, justifying our approach and giving the opportunity to use several stations in common. Also for AA and Ne (at 90km altitude) the filters have similar shapes in different geographical zones. (b) Absorption values predicted by the model, were compared with Ne measured at different altitudes. Rather good correlation (up to CC~0.7-0.8) together with reasonable CC dependence on altitude and MLT justify our approach. (c) Comparison of filter dependence on MLT with RCM simulation of MLT variation of drifting cloud intensity during short injection event showed a nice agreement of precipitation front dynamics in the inner magnetosphere. (d) Surprisingly, the filters reconstructed for isolated and clustered substorm data sets appeared almost identical for riometers in the center of the actual auroral zone. “Universal” response functions for different substorm types probably point on similar processes acting inside those types.
We conclude that the approach based on drifting electron cloud energized by betatron-like mechanism is viable. Currently it provides a quantitative dynamical portrait of EE precipitation during substorm events of various complexity (excluding strong storms). The correlation of model predictions with observations has CC ~0.6-0.7, so a substantial part of energetic electron precipitation has a “short memory“ of 3-4 hours and can be reconstructed based on magnetic variation data. Also, usefulness of MPB index is greatly supported by our study.
The work is supported by Russian Science Foundation grant 22-27-00169 and by Russian Ministry of Science and High Education grant N075-15-2021-583