Incoherent scatter radar observations of the cusp acceleration region and cusp field-aligned currents (original) (raw)

1998, Journal of Geophysical Research

Incoherent scatter radar measurements are used to study the E region and lower F region in the cusp proper. At the equatorward edge of the cusp a region of enhanced ionization is found, indicating precipitation of either electrons of about 1 keV energy or protons of about 3 keV. The signature is found at the same altitude, around 150 kin, as the maximum of the solar EUV produced ionization, but is clearly recognizable. The position and likely proton energies indicate that this is probably the ionospheric signature of the cusp acceleration region. Regions of low electron densities are also found in the same altitude region, which may be explained by downward field-aligned currents causing depletions in the E and lower F region. First-order estimates show that dense field-aligned currents are needed to cause the observed depletions, possibly as dense as 30/•A/m = . The ionization in the cusp acceleration region, and the depletion caused by downward field-aligned currents both influence the Pedersen conductivity of the ionosphere, and the resulting conductivity gradients strongly influence the small-scale field-aligned currents flowing in the cusp region. 1. Introduction Ionospheric effects of magnetosheath particles precipitating in the cusp region have been described in a number of studies, most based on radar and optical observations. Optical observations can give good temporal and spatial resolution but have a number of limitations. These include that the emission altitude is uncertain and that ground-based optical measurements can be obtained only during mid winter [e.g., Sandholt et al., 1986]. HF radars can study convection and morphology of the cusp precipitation region [e.g., Baker et al., 1995], even though the precise reason for the HF radar spectra from the cusp remains unclear. However, only incoherent scatter radars can give detailed information on the state of the ionosphere. Incoherent scatter (IS) radar can give altitude profiles of electron densities, electron temperatures, ion temperatures, and ion drift velocities. The altitude resolution is important because it means that the energy of precipitating particles causing electron density and tempera-ture enhancements can be estimated [e.g., Kirkwood and Osepian, 1995]. Previous IS radar studies [e.g., Lockwood et al., 1993; Nilsson et al., 1994, 1996] have shown that a clear signature is seen in the ionospheric F region from the 100 eV cusp electrons. The ions cause ionization at about the same altitude as the solar EUV and did not give a distinguishable signature in the study of Nilsson et al. [1994]. However, a careful study of the E region in the data presented by Nilsson et al. [1996] reveals that regions of enhanced E region electron densities can be identified. Most notably, there is a region of such enhancements just at the equatorward edge of the cusp. As will be discussed later, this is the obvious signature of the cusp acceleration region as reported by, for example, Woch and Lundin [1992] and Newell and Meng [1991]. The characteristic charged particle population observed in this region is usually interpreted as the accelerated flows away from the reconnection region predicted by reconnection theories (referred to as the smoking gun evidence of reconnection by among others $onnerup et al. [1995]). Satellite-based measurements of such accelerated flows seem to imply that they are not always present even if reconnection seems to be going on (for a review, see Cowley [1982], and for more recent work, see Phan and Paschmann [1995]). Continuous ground-based observation of the ionospheric signature of this region could shed more light on the nature and persistence of active reconnection. Specifically, 26,721