Large Perpendicular Velocity Fluctuations Observed in the Topside Ionosphere

We present observations by the European incoherent scatter radar (EISCAT) of a sporadic event (less than 20 s) occurring near a substorm onset, characterized by a strong apparent ion temperature increase in the auroral F region and associated with significant anisotropy and distorted incoherent scatter spectra. The simultaneously observed electric field is too weak to account for the temperature increase by frictional heating. Indeed, it is interpreted as a spurious effect of the standard analysis procedure in the presence of short‐lived ion drift velocity fluctuations, perpendicular to the background magnetic field, which are unresolved by the radar because of space or timescales smaller than the measurement resolution. A standard analysis of such spectra overestimates the ion temperature and underestimates the electron temperature. Similar observation and interpretation were first reported by Swartz et al. [1988] and Knudsen et al. [1993]. In this paper we develop a modified analysis of incoherent scatter spectra to infer, self‐consistently, the real temperatures and the amplitude of velocity fluctuations. This modified analysis can be applied systematically on a routine basis. The first results confirm the earlier works by Swartz et al. [1988] and Knudsen et al. [1993] and show that the geomagnetic orientation of the velocity structure is mainly in the north‐south direction and its maximum amplitude reaches up to about 2000 m/s at 350 km altitude. Coordinated satellite and ground‐based data tend to argue in favor of a time‐varying electric field (possibly a standing Alfvén wave) as the origin of the observed EISCAT spectral distortions.