Electron Energy Spectrum and Auroral Power Estimation From Incoherent Scatter Radar Measurements

Abstract

Differential energy flux of electrons precipitating into the high‐latitude ionosphere can be estimated from incoherent scatter radar observations of the ionospheric electron density profile. We present a method called ELSPEC for electron spectrum estimation from incoherent scatter radar measurements, which is based on integration of the electron continuity equation and spectrum model selection by means of the Akaike information criterion. This approach allows us to use data with almost arbitrary time resolutions, enables spectrum estimation with dense energy grids, avoids noise amplifications in numerical derivatives, and yields statistical error estimates for all the output parameters, including the number and energy fluxes and upward field‐aligned currents carried by the precipitating electrons. The technique is targeted for auroral energies, 1–100 keV, which ionize the atmosphere mainly between 80 and 150 km altitudes. We validate the technique by means of a simulation study, which shows that Maxwellian, kappa, and mono‐energetic spectra, as well as combinations of those, can be reproduced. Comparison study for two conjugate satellite measurements to the EISCAT UHF radar are shown, for Reimei and Swarm, showing an agreement with the results. Finally, an example of a 2‐hr measurement by the EISCAT radar is shown, during which we observe a variety of precipitation characteristics, from soft background precipitation to mono‐energetic spectra with peak energies up to 60 keV. The upward field‐aligned current varies from 0 to 10 μAm−2 and the total energy flux from 0 to 250 mWm−2.

A new auroral phenomenon, the anti-black aurora

“…The figures in brackets are the standard deviation. The radar inversion uncertainty is 20% 12 whereas the optical modelling uncertainty is 10% 17 . Previous studies have determined a mean of 4.4 16 and 6.7 keV 4 for black aurora energies, which is consistent with our findings.…”

“…1 , 2 and 3 , a key parameter is the mean energy of the precipitating electrons creating the optical phenomenon. This can be determined uniquely by inverting the electron density altitude profile as observed by the EISCAT radar because new ionisation in the atmosphere depends on the flux of precipitating electrons from space whose final deposition altitude depends on their initial energy above the atmosphere 12 . The EISCAT arc1 code effectively covers all precipitating electron energies below 30 keV due to the altitude range covered when pointing into the (near vertical) magnetic zenith 11 .…”

A new auroral phenomenon: The anti-black aurora

“…ELSPEC also requires the atmospheric neutral and ion composition density as a function of altitude, which is derived from the MSIS 24 and IRI models 25 . Uncertainty in the composition estimates may cause about 10% error in the total energy flux estimates 12 . The overall inversion uncertainty is ∼20% 12 .…”

“…In addition to the morphology of the anti-black and black auroras, shown in Figures 1-3, a key parameter is the mean energy of the precipitating electrons creating the optical phenomenon. This can be determined uniquely by inverting the electron density altitude profile as observed by the EISCAT radar because new ionisation in the atmosphere depends on the flux of precipitating electrons from space whose final deposition altitude depends on their initial energy above the atmosphere 12 . The EISCAT arc1 code effectively covers all precipitating electron energies below ∼30 keV due to the altitude range covered when pointing into the (near vertical) magnetic zenith 11 .…”

Daedalus: a low-flying spacecraft for in situ exploration of the lower thermosphere–ionosphere

“…The high oxygen density in the Daedalus orbital environment, in particular near perigee, will cause EFI probe surfaces to oxidize. Oxidation of probe surfaces can create an electrically resistive layer on the probe surface (Ergun et al, 2015), or it can erode probe coatings entirely (Visentine, 1983;Visentine et al, 1985). Either effect can degrade or destroy the ability of EFI to measure DC-coupled electric fields (Mozer et al, 2016).…”