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.

Highlights

  • Substorms are key energy transfer and reconfiguration elements in the magnetosphere–ionosphere system

  • Compound, and multi-night substorm events with different magnetic disturbance magnitudes, examine any cumulative effects the multi-night events may have on cosmic radio noise absorption (CNA), and compare in situ particle precipitation spectra to previously observed reference values of energetic electron forcing

  • Out of the 25 events covered by both optical and spacecraft data, 5 events showed optical signatures of Pulsating aurora (PsA) during the spacecraft overpass. If this is a representative fraction for the whole data set, it should not bias the median spectrum of all recovery-phase overpasses. Another possible bias to the results presented here could be caused by the local time distribution of the spacecraft overpasses, since a precipitation energy increase has been observed in the morning MLT hours (Hosokawa and Ogawa, 2015)

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Summary

Introduction

Substorms are key energy transfer and reconfiguration elements in the magnetosphere–ionosphere system. These relativistic particles can be lost in the atmosphere through pitch angle scattering, resulting in a more spread-out high-energy drizzle during the recovery phase after the initial substorm onset In the ionosphere, this is related to diffuse and pulsating aurorae, which are primarily observed in the magnetic morning sector The substorm detection routines have slightly different definitions and threshold values, and the previous studies have indicated both temporal delays and non-linear magnitude dependence between the magnetic disturbances and the energetic particle precipitation signatures, as outlined above. Compound, and multi-night substorm events with different magnetic disturbance magnitudes, examine any cumulative effects the multi-night events may have on CNA, and compare in situ particle precipitation spectra to previously observed reference values of energetic electron forcing

Magnetic activity indices and substorm phase detection
From cosmic noise absorption to a regional absorption index
Space-borne particle precipitation measurements
Event selection and substorm categories
The relationship between CNA and magnetic disturbances
Particle precipitation spectra
Discussion
Conclusions

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