How does auroral electron precipitation near the open–closed field line boundary compare to that within the auroral oval during substorm onset?

Abstract

Abstract. Auroral electron precipitation during a substorm exhibits complex spatiotemporal variations which are still not fully understood, especially during the very dynamic phase immediately following the onset. Since during disturbed times the auroral oval typically extends across several hundreds of kilometers in the latitudinal direction, one may expect that precipitating electron spectra differ at locations close to the open–closed field line boundary (OCB) compared to the central part of the auroral oval. We carry out a statistical study based on 57 auroral breakups associated with substorm onsets observed above Tromsø (66.7° N geomagnetic latitude, i.e. central oval) and 25 onsets occurring above Svalbard (75.4° N geomagnetic latitude, i.e. poleward boundary) between 2015 and 2022. The events were selected based on the availability of both optical observations and field-aligned incoherent scatter radar measurements. Those are two sets of different substorms; hence we compare solar wind driving conditions and geomagnetic indices for the two event lists in the statistical sense. Using the ELSPEC method (based on the inversion of the electron density profile) on the radar data, we retrieve precipitating electron fluxes within 1–100 keV around each onset time, and we apply the superposed epoch analysis method on the electron spectra at each location. We compare the statistical precipitation characteristics above both sites in terms of peak differential flux, energy of the peak, integrated energy flux, and their time evolution during the minutes following the onset. We find that the integrated energy flux associated with events occurring in the central part of the auroral oval (Tromsø) exhibit a sharp peak up to 25 mW m^-2 in the first two minutes following the auroral breakup, before decreasing and reaching stable values around 7 mW m^-2 for at least 20 min. In turn, no initial peak is seen near the open–closed field line boundary (Svalbard), and values remain low throughout (1–2 mW m^-2). A comparison of the median spectra indicates that the precipitating flux of > 10 keV electrons is lower above Svalbard than above Tromsø by a factor of at least 10, which may partly explain the differences. However, it proves difficult to conclude whether the differences originate from the latitude at which the auroral breakup takes place or from the fact that the breakups seen from Svalbard occur equatorward from the radar beam, which only sees expansion-phase precipitation after a few minutes.