On the possible role of cusp/cleft precipitation in the formation of polar-cap patches

Abstract

Abstract. The work describes experimental observations of enhancements in the electron density of the ionospheric F-region created by cusp/cleft particle precipitation at the dayside entry to the polar-cap convection¯ow. Measurements by meridian scanning photometer and all-sky camera of optical red-line emissions from aurora are used to identify latitudinally narrow bands of soft-particle precipitation responsible for structured enhancements in electron density determined from images obtained by radio tomography. Two examples are discussed in which the electron density features with size scales and magnitudes commensurate with those of patches are shown to be formed by precipitation at the entry region to the anti-sunward ow. In one case the spectrum of the incoming particles results in ionisation being created, for the most part below 250 km, so that the patch will persist only for minutes after convecting away from the auroral source region. However in a second example, at a time when the plasma density of the solar wind was particularly high, a substantial part of the particle-induced enhancement formed above 250 km. It is suggested that, with the reduced recombination loss in the upper F-region, this structure will retain form as a patch during passage in the anti-sunward¯ow across the polar cap.

Polar patches observed by ESR and their possible origin in the cusp region

“…Modelling studies by Sojka and Schunk (1986) indicated that densities might increase by a factor of two above background within a¯ux tube that convected through cusp-precipitation for some 10±15 min. An electron density enhancement within the cusp region, with characteristics consistent with those of a polar patch, was observed by Walker et al (1999) using radio tomography. Auroral observations con®rmed that soft precipitation had played an important role in the creation of this structure at an altitude where the enhanced plasma densities would be expected to have su ciently long lifetimes to preserve the patch in the cross-polar¯ow.…”

“…The durations of the enhancements were in broad agreement with those of increases in F-region backscatter observed by the CUTLASS Iceland radar, with the leading edge of the events observed by ESR lagging the onset of those seen by the HF radar by about 3 min. With the likelihood of increased precipitation accompanying the increased backscatter, it is proposed that the polar patches were created by intensi®cations of soft-particle precipitation within the cusp region in-keeping with the conclusion of Walker et al (1999). Whilst it is not possible to be conclusive about the driving mechanism of the temporal variation in the precipitation, it is likely that it is due to temporal magnetospheric changes.…”

“…The modelling study of Sojka et al (1994) indicated the presence of patches in all seasons with their occurrence modulated by UT. Pederson et al (1998) observed patch structures on horizontal scales of some 200±300 km using the Sondrestrom incoherent scatter radar, while Walker et al (1999) imaged a patch-like structure of latitudinal dimension some 2.5°using radio tomography.…”

An evaluation of International Reference Ionosphere electron density in the polar cap and cusp using EISCAT Svalbard radar measurements

“…Soft particle precipitation in the cusp region where particles precipitate directly from the magnetosheath can produce polar cap patches that can be convected over the polar cap (e.g., Walker et al, 1999;Oksavik et al, 2006;Goodwin et al, 2015). As a result of plasma transport and particle precipitation, the high-latitude F region ionosphere is nonuniform and highly dynamic.…”

Interplanetary magnetic field control of polar patch velocity

“…MacDougall, private communication, 2002). They are created either in the sunlit F-region equatorward of the cusp or by precipitation in the cusp and are subsequently convected across the polar cap [Lockwood and Carlson, 1992;Valladares et al, 1994;McEwen et al, 1995;Ma and Schunk, 1997;Walker et al, 1999;Smith et al, 2000]. The patches have been studied by using both ground-based instruments (such as optical imagers [Steele and Cogger, 1996] and radars ) and satellite-based in situ detectors [Coley and Heelis, 1998].…”