Ionospheric response to wave‐accelerated electrons at the poleward auroral boundary

Abstract

The local ionospheric response to an auroral intensification at the poleward auroral boundary has been investigated using the incoherent scatter radar (ISR) and optical instrumentation at Sondrestrom, Greenland, in conjunction with space‐borne measurements by the IMAGE and FAST satellites. ISR elevation scans through the illuminated region revealed filamentary columns of enhanced plasma density, ∼5 km in latitude by ∼200 km in altitude. Column densities were typically 5 × 1011/m3 above background and often constant over a broad range of altitudes. The brightness of the O+ 732–733 nm multiplet, monitored simultaneously by a near‐infrared spectrometer, exceeded 1.2 kR during one 4‐min period (a factor of ∼4 brighter than previously reported auroral measurements). A time‐dependent model was developed to relate O+ emission intensities to O+ column densities for a given illumination time. The results suggested that the electron source was composed of kilometer‐scale flux tubes locked in the E × B flow for several minutes whose average energy varied temporally between <100 eV and >1 keV over their illumination lifetime. Conjugate electrons measured by the FAST satellite at 1700 km showed evidence for energization by inertial Alfvén waves. Ionization rates computed from these spectra were sufficient to account for the observed filamentary ionospheric structure. The implications of such ionization patterns for electrodynamic coupling with the magnetosphere are discussed.