Evidence of small-scale field aligned current sheets from the low and middle altitude cusp continuing in the ionosphere

TL;DR

This study provides evidence that small-scale field-aligned current sheets observed in the magnetospheric cusp extend into the ionosphere, highlighting their potential commonality and the challenges in detecting them due to their rarity.

Contribution

It demonstrates the continuation of small-scale field-aligned current sheets from the magnetosphere into the ionosphere using satellite conjunction data from 2008.

Findings

Small-scale field-aligned current sheets extend from the cusp into the ionosphere.

Ionospheric current densities are relatively weak, around a few micro A/m2.

Such events are likely common but difficult to detect due to satellite conjunction rarity.

Abstract

We investigate kilometer-scale field-aligned currents that were detected both in the magnetospheric cusp at a few Earth radii altitude and in the ionosphere by satellites that were, according to the Tsyganenko model, within a few tens of kilometers and minutes on the same magnetic field line. Also thermosphere up-welling that often accompanies the dayside field-aligned currents in the inner cusp was seen. We used Cluster and CHAMP satellites, and searched for conjunctions during the whole year of 2008, as then the Cluster spacecrafts were mostly at mid-altitudes when crossing the cusps. We focus on two case studies from this period. Evidence is presented that sheets of small scale field-aligned current continue through the low altitude cusp and ionosphere. The ionospheric current densities are not particularly strong, a few micro A/m2 at about 340 km, and several tens of nA/m2 at about 20000 km, implying that these currents might be relatively common events, but are hard to detect due to rareness of suitable locations of at least two satellites from different missions.