Localized electron density enhancements in the high-altitude polar ionosphere and their relationships with storm-enhanced density (SED) plumes and polar tongues of ionization (TOI)

Y. Kitanoya,N. Nishitani,T. Hori,A. W. Yau,T. Abe

doi:10.5194/angeo-29-367-2011

Abstract

Abstract. Events of localized electron density increase in the high-altitude (>3000 km) polar ionosphere are occasionally identified by the thermal plasma instruments on the Akebono satellite. In this paper, we investigate the vertical density structure in one of such events in detail using simultaneous observations by the Akebono and DMSP F15 satellites, the SuperDARN radars, and a network of ground Global Positioning System (GPS) receivers, and the statistical characteristics of a large number (>10 000) of such events using Akebono data over half of an 11-year solar cycle. At Akebono altitude, the parallel drift velocity is remarkably low and the O+ ion composition ratio remarkably high, inside the high plasma-density regions at high altitude. Detailed comparisons between Akebono, DMSP ion velocity and density, and GPS total electron content (TEC) data suggest that the localized plasma density increase observed at high altitude on Akebono was likely connected with the polar tongue of ionization (TOI) and/or storm enhanced density (SED) plume observed in the F-region ionosphere. Together with the SuperDARN plasma convection map these data suggest that the TOI/SED plume penetrated into the polar cap due to anti-sunward convection and the plume existed in the same convection channel as the dense plasma at high altitude; in other words, the two were probably connected to each other by the convecting magnetic field lines. The observed features are consistent with the observed high-density plasma being transported from the mid-latitude ionosphere or plasmasphere and unlikely a part of the polar wind population.

Highlights

The plasma density in the ionosphere in general decreases with altitude, and is lower at high latitude than at middle and low latitudes at a given altitude
We present a detailed case study of the relationships between local plasma density enhancements at high-altitudes and storm enhanced density” (SED) and tongue of ionization (TOI) plumes in the F-region, using simultaneous data from the Akebono and Defense Meteorological Satellite Program (DMSP) F-15 satellites, the SuperDARN radars, and a network of ground Global Positioning System (GPS) receivers in one of the observed events; and a statistical study of the characteristic features of localized plasma density enhancements, using the Akebono data in a large number (>10 000) of observed events
Ichikawa et al (2002) suggested that dense plasma produced by solar EUV photo-ionization in the dayside or by particle precipitation in the cusp can be a possible source for the observed high-density and low-temperature plasma in the highaltitude polar cap

Summary

Introduction

The plasma density in the ionosphere in general decreases with altitude, and is lower at high latitude than at middle and low latitudes at a given altitude. Ichikawa et al (2002) analyzed Akebono plasma density data over a 10-year period, and reported the occasional occurrence of local plasma density enhancement (>1000 cm−3) above 4000 km altitude in the polar cap ionosphere, where the electron density is usually on the order of and below 100 cm−3 near solar maximum and minimum, respectively. They showed that such high-density regions are characterized by a distinctively low electron temperature (10 000) of observed events

Akebono observations of local plasma density enhancements

Statistical features of localized plasma density enhancements

Findings

Discussions and conclusion