The characteristics of field-aligned currents associated with equatorial plasma bubbles as observed by the CHAMP satellite

Abstract

Abstract. Field-aligned currents (FACs) generate magnetic deflections perpendicular to the ambient Earth magnetic field. We investigate the characteristics of FACs associated with equatorial plasma bubbles (EPBs) as deduced from magnetic field measurements by the CHAMP satellite. Meridional magnetic deflections inside EPBs show a clear hemispheric anti-symmetry for events observed before 21:00 LT: inward in the Northern Hemisphere and outward in the Southern Hemisphere. When an eastward electric field is assumed the magnetic signature signifies a Poynting flux directed downward along the magnetic field lines. This means that FACs are driven by a high-altitude equatorial source. Such a scheme cannot be drawn as strictly from our observations after 22:00 LT, possibly because of a westward turning of the electric field inside EPBs and/or a decay of EPBs later at night. The perpendicular magnetic deflection is tilted by 40 • from the magnetic meridional plane in westward direction, which implies that the depleted volume of EPBs, as well as the FAC structure, is tilted westward by 40 • above the magnetic equator. The peak-to-peak amplitude of the FAC density is found to range typically between 0.1-0.5 µA/m 2 . The field-aligned sheet current density and the diamagnetic current strength show no correlation.

Field-aligned current associated with low-latitude plasma blobs as observed by the CHAMP satellite

“…It was also found that the perpendicular magnetic deflections are not purely inward/outward as suggested by Aggson et al (1992), but tilted westward by ∼ 40 • , which is consistent with the known EPB geometries (Mendillo and Tyler, 1983;Mukherjee, 2003;Makela and Kelley, 2003). Using Ampère's law Park et al (2009) estimated the FAC density to be of the order of 0.1 µA/m 2 , which is consistent with the result of Pottelette et al (2007) using E-and B-field measurements.…”

“…However, a significant fraction of examples were inconsistent with the model suggested by Aggson et al (1992) and Bhattacharyya and Burke (2000). Park et al (2009) showed that the model is consistent with CHAMP observations only before ∼22:00 LT. They argued that the assumption of an eastward E-field (used in Aggson et al, 1992;Bhattacharyya andBurke, 2000, andStolle et al, 2006) might not be valid after ∼22:00 LT.…”

“…In panel (g) the black curve represent the perpendicular magnetic deflection along the direction of maximum variance (hereafter, "maximum variance component") and the red curve that of minimum variance axis (hereafter, "minimum variance component"). As the maximum variance component is much larger than the minimum variance component, FAC density can be estimated by Ampère's law from a single satellite (Park et al, 2009):…”

“…It is well-known that FACs flow along EPB walls (Aggson et al, 1992;Bhattacharyya and Burke, 2000;Stolle et al, 2006;Pottelette et al, 2007;Park et al, 2009). Aggson et al (1992) observed for the first time magnetic fluctuations perpendicular to the ambient magnetic field (hereafter, "perpendicular magnetic deflections"), which indicate the existence of FACs.…”

Determining the boundaries of the auroral oval from CHAMP field-aligned current signatures – Part 1

“…POMME, Potsdam Magnetic Model of the Earth) for defining the mean field (e.g. Park et al, 2009). In order to avoid the effect of attitude noise, introduced by the uncertainty of the star trackers, when transforming the magnetic field readings into the NEC frame, we make use of the original field measurements in the sensor frame.…”