The O⁺ density trough at 5000 km altitude in the polar cap

Abstract

[1] At altitudes near 5000 km over the southern polar cap region of the terrestrial magnetosphere/ionosphere, the Thermal Ion Dynamics Experiment (TIDE) on board the Polar satellite has observed O + ion density trough regions, in which the O + densities were at least one order of magnitude lower than the surrounding O + densities. In the O + density trough regions, the estimated O + densities were generally lower than 0.01 per cc. The boundaries between normal density level regions and the trough density regions were usually abrupt transitions. From 1 December 1997 to 30 November 1998, polar cap O + troughs in Polar/TIDE observations occurred at a frequency of about 48%. Statistical examination of the Polar perigee observations from 1 December 1997 to 30 November 1998 shows that the Polar perigee passes evenly covered the southern polar cap region, while the O + density trough was always located on the nightside portion of the polar cap magnetosphere/ionosphere, and that invariant latitude spans of such troughs could be as large as 23°in extent. The trough occurrence displayed a strong seasonal dependence; in the winter season (e.g., for July in the Southern Hemisphere) the O + ion density trough occurrence frequency ranged up to 92%, while in the summer season (e.g., for January in the Southern Hemisphere) it decreased to as low as 15%. Our statistical results show that the trough occurrence was generally anticorrelated with solar wind dynamic pressure in the solar wind dynamic pressure range 0.8-2.6 nanopascal. The O + ion density trough occurrence appeared relatively independent of the geomagnetic Kp index, IMF Bz, and By conditions. However, as suggested by the seasonal dependence, the O + ion density trough occurrence was strongly related to the solar zenith angle (SZA). In the SZA range 50°to 125°, the trough occurrence increased monotonically with SZA. In addition, we sought to determine consistent density and velocity signatures at lower altitudes associated the O + ion density trough at 5000 km by examining the nearsimultaneous O + densities and vertical velocities observed by the DMSP satellite group orbiting at 840 km altitude. However, consistent correlations between the dual altitudes were not reliably established from the present examinations.INDEX TERMS: 2467 Ionosphere: Plasma temperature and density; 2431 Ionosphere: Ionosphere/magnetosphere interactions (2736);

Simulation of the formation of O+ density troughs at 6000 km altitude in the polar cap ionosphere‐magnetosphere region

“…The resulting simulation O + density at the Polar satellite altitude of ∼7644 km was about 1.9 × 10 −3 cm −3 . The corresponding observed O + densities were both about 10 −3 cm −3 and within O + trough regions as discussed in the work of Zeng et al [2004]. The simulated O + density profiles for the Polar track intersection times are displayed in Figures 5b and 5d.…”

“…Correspondingly, the pressure gradient force which would support high altitude O + density plumes [e.g., Ganguli , 1996] was decreased, and thus the densities at high altitudes declined. Hence, very low O + densities in such regions should occur in both in situ observations [ Horita et al , 1993; Zeng et al , 2004] and simulations (cf. Figure 4).…”

“…In this case, the estimated O + densities were above 10 O + ions cm −3 for the period 1517–1530 UT, then sharply declined to less than 0.01 O + ions cm −3 for the period ∼1530–1540 UT, and returned to elevated values of around 10 O + ions cm −3 for the remaining displayed period of 1540–1543 UT. As per the discussion in the work of Zeng et al [2004], the region of very low (<0.01 O + ions cm −3 ) estimated O + densities is designated as an O + trough region.…”

“…The ion depletion zones reported by Horita et al [1993] were primarily located in the nightside region of the polar cap magnetosphere at invariant latitudes above 70° and altitudes between 8000 and 10,000 km. The statistical study by Zeng et al [2004] indicated that the occurrence of the O + density troughs over the polar cap region at 5000 km altitude could be influenced by factors such as the solar wind dynamic pressure and the solar illumination. The trough occurrence was found to decrease with increasing solar wind dynamic pressure but to increase with decreasing solar illumination.…”

“… Horita et al [1993] suggested that the ion depletion zones might be caused by convection vortices which could form during magnetically quiet periods, where recombination of ions and electrons in a darkened F region and topside ionosphere could proceed for extended intervals. Zeng et al [2004] suggested that effects of the E × B vertical component and the recombination of ions and electrons in darkened F region source locations might contribute to the formation of the observed O + density trough.…”