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Abstract
Abstract. Using spacecraft potential measurements of the Polar electric field experiment, we investigate electron density variations of key plasma regions within the magnetosphere, including the polar cap, cusp, trough, plasmapause, and auroral zone. The statistical results were presented in the first part of this study, and the present paper reports detailed structures revealed by individual satellite passes. The high-altitude (> 3 RE) polar cap is generally one of the most tenuous regions in the magnetosphere, but surprisingly, the polar cap boundary does not appear as a steep density decline. At low altitudes (1 RE) in summer, the polar densities are very high, several 100 cm-3 , and interestingly, the density peaks at the central polar cap. On the noonside of the polar cap, the cusp appears as a dense, 1–3° wide region. A typical cusp density above 4 RE distance is between several 10 cm-3 and a few 100 cm-3 . On some occasions the cusp is crossed multiple times in a single pass, simultaneously with the occurrence of IMF excursions, as the cusp can instantly shift its position under varying solar wind conditions, similar to the magnetopause. On the nightside, the auroral zone is not always detected as a simple density cavity. Cavities are observed but their locations, strengths, and sizes vary. Also, the electric field perturbations do not necessarily overlap with the cavities: there are cavities with no field disturbances, as well as electric field disturbances observed with no clear cavitation. In the inner magnetosphere, the density distributions clearly show that the plasmapause and trough densities are well correlated with geomagnetic activity. Data from individual orbits near noon and midnight demonstrate that at the beginning of geomagnetic disturbances, the retreat speed of the plasmapause can be one L-shell per hour, while during quiet intervals the plasmapause can expand anti-earthward at the same speed. For the trough region, it is found that the density tends to be an order of magnitude higher on the day-side (~1 cm-3) than on the nightside (~0.1–1 cm-3), particularly during low Kp.Key words. Magnetospheric physics (auroral phenomena; plasmasphere; polar cap phenomena)
Highlights
The modeling of the plasma number density in the magnetosphere is a difficult task for a number of reasons
Using spacecraft potential measurements of the Polar electric field experiment, we investigate electron density variations of key plasma regions within the magnetosphere, including the polar cap, cusp, trough, plasmapause, and auroral zone
Differential potential ( V ) measurements from the Polar electric field instrument have been used to study electron density profiles in some specific regions of the magnetosphere between 2–9 RE geocentric distances. This technique provides a measure of the thermal plasma density to values even below 0.1 cm−3 with high time resolution
Summary
The modeling of the plasma number density in the magnetosphere is a difficult task for a number of reasons. Using high-resolution data from individual orbits, the present paper investigates several plasma regions that have distinct density signatures, such as the polar cap, cusp, trough, plasmapause, and auroral zone. A few hours later, a significant, large-scale density enhancement is observed when Polar enters the northern cusp Poleward of this region, the density becomes very low as the satellite encounters the polar cap. The density variations are quite similar from orbit to orbit, there are numerous differences between orbits which reveal fascinating and complex features, such as large-scale density patterns in the polar cap, the structure of the cusp, density cavitation at the auroral zone, characteristics of the density decline at the plasmapause and so on Their structure and dynamics are revealed to the fullest extent when the data are studied with high time resolution, with respect to invariant variables, such as L-shell and invariant latitude
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