Energetic electron precipitation associated with pulsating aurora: EISCAT and Van Allen Probe observations

Abstract

Pulsating auroras show quasi-periodic intensity modulations caused by the precipitation of energetic electrons of the order of tens of keV. It is expected theoretically that not only these electrons but also subrelativistic/relativistic electrons precipitate simultaneously into the ionosphere owing to whistler mode wave-particle interactions. The height-resolved electron density profile was observed with the European Incoherent Scatter (EISCAT) Tromsø VHF radar on 17 November 2012. Electron density enhancements were clearly identified at altitudes >68 km in association with the pulsating aurora, suggesting precipitation of electrons with a broadband energy range from~10 keV up to at least 200 keV. The riometer and network of subionospheric radio wave observations also showed the energetic electron precipitations during this period. During this period, the footprint of the Van Allen Probe-A satellite was very close to Tromsø and the satellite observed rising tone emissions of the lower band chorus (LBC) waves near the equatorial plane. Considering the observed LBC waves and electrons, we conducted a computer simulation of the wave-particle interactions. This showed simultaneous precipitation of electrons at both tens of keV and a few hundred keV, which is consistent with the energy spectrum estimated by the inversion method using the EISCAT observations. This result revealed that electrons with a wide energy range simultaneously precipitate into the ionosphere in association with the pulsating aurora, providing the evidence that pulsating auroras are caused by whistler chorus waves. We suggest that scattering by propagating whistler simultaneously causes both the precipitations of subrelativistic electrons and the pulsating aurora.

Secondary electron emission from meteoric smoke particles inside the polar ionosphere

“…The causal primary electron flux for the atmospheric secondary flux shown in Fig. 3 is only comparable to the differential flux measured by Miyoshi et al (2015) within a pulsating aurora (cf. Fig.…”

“…We have studied three precipitation cases, of which the weak and medium cases happen in the polar atmosphere and the strong case has more the nature of a thought experiment with extraordinarily severe electron precipitation. The electron precipitation for pulsating aurora (weak case), caused by energetic electrons with tens of kiloelectronvolts of energy (Miyoshi et al, 2015), occurs frequently during minor geomagnetic activity. Furthermore, electrons trapped within the Earth's magnetosphere can be accelerated to relativistic energies (e.g., Reeves et al, 2013), which can be injected into the Earth's atmosphere during geomagnetic storms; this scenario is comparable to our medium precipitation case.…”

“…This quantity can be derived from measurements of incoherent-scatter radars and by instruments on board satellites and sounding rockets (e.g., Rees, 1969;Miyoshi et al, 2015, and references therein) and is usually given for the top of the atmosphere. While penetrating the atmosphere, energetic electrons collide with atmospheric molecules; they lose energy and can get absorbed completely.…”

“…3 the spectra for three different cases of electron precipitation are plotted (different line shapes), namely at the top of the atmosphere, at 90 km altitude and 70 km altitude (different colors). The three cases are defined as follows: the "weak" electron precipitation case is the spectrum measured during a pulsating aurora event over Tromsø (Miyoshi et al, 2015). The "medium" precipitation case corresponds to the "Hard" spectra shown in Osepian and Kirkwood (1996).…”

“…Pfister (1967) also shows similar atmospheric secondary differential fluxes for higher altitudes. We assume that this atmospheric secondary electron (Left) Electron precipitation spectra for weak (dashdotted, data published in Miyoshi et al, 2015), medium (solid) and strong (dashed) conditions; spectra at the top of the atmosphere (black) and remnants at 90 km (green) and 70 km (red); and atmospheric secondary electron spectrum measured at 105 km by Doering (data taken from Fig. 4.…”