Filamentary Alfvénic structures excited at the edges of equatorial plasma bubbles

Abstract

Abstract. Recent observations performed by the French DEMETER satellite at altitudes of about 710 km suggest that the generation of equatorial plasma bubbles correlates with the presence of filamentary structures of field aligned currents carried by Alfvén waves. These localized structures are located at the bubble edges. We study the dynamics of the equatorial plasma bubbles, taking into account that their motion is dictated by gravity driven and displacement currents. Ion-polarization currents appear to be crucial for the accurate description of the evolution of plasma bubbles in the high altitude ionosphere. During their eastward/westward motion the bubbles intersect gravity driven currents flowing transversely with respect to the background magnetic field. The circulation of these currents is prohibited by large density depressions located at the bubble edges acting as perfect insulators. As a result, in these localized regions the transverse currents have to be locally closed by field aligned currents. Such a physical process generates kinetic Alfvén waves which appear to be stationary in the plasma bubble reference frame. Using a two-dimensional model and "in situ" wave measurements on board the DEMETER spacecraft, we give estimates for the magnitude of the field aligned currents and the associated Alfvén fields.

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.…”

“…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.…”

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

“…EPB strength) might be indirectly related. Pottelette et al (2007) attributed FACs to the interception of gravity-driven perpendicular currents by insulating EPBs. Yet, there has been no decisive observational evidence to determine which term of the cross-field current dominates FAC generation.…”

“…the magnetic deflection was not always outward (inward) in the Southern (Northern) Hemisphere. Recently, Pottelette et al (2007) observed electromagnetic fluctuation at EPB edges, and estimated the edge thickness to be ∼1 km. They also estimated FAC density for one example, which amounted to 0.2µA/m 2 .…”

“…There is a consensus that they result from the divergence of zonal crossfield currents associated with EPBs (see Aggson et al, 1992;Pottelette et al, 2007). The ionospheric current, j , consists of a number of contributions: gravity-driven current, pressure gradient current, and current related with the ambient E-field (Kelley, 1989;Stolle et al, 2006):…”