Equatorial bubbles updrafting at supersonic speeds

Abstract

We present plasma and electric field observations from two satellite encounters with equatorial plasma bubbles updrafting at velocities of-2 km/s. These large, upward velocities are consistent with an adaptation of Chandrasekhar's model for the motion of plasma blobs supported against gravity by a magnetic field; that is,/fz • -g. Vector magnetic field measurements, available during one of the bubble encounters show a perturbation of -150 nT, directed radially outward from the Earth, near the western wall of deepest plasma depletion. This magnetic variation is too large to be caused by simple shunting of the g x B current along the bubble's edge. Rather, it is Alfv6nic in nature, radiating from a generator located near the magnetic equator, in the plasma outside the bubble's leading edge. A heuristic model of a depleted flux tube with constant circular cross section moving upward through a background plasma predicts most of the measurements' qualitative features. INTRODUCTION Plasma bubbles, responsible for equatorial spread F originate in the bottomside of the F layer where plasma density gradients are unstable to the growth of Rayleigh-Taylor turbulence [Hudson and Kennel, 1975]. In a local approximation the linear growth rate reverses sign at the peak of the F layer [Balsley et al., 1972]. G. Haerendel (unpublished manuscript, 1973) stressed that plasma bubbles act as twodimensional entities whose line-integrated characteristics determine the motion of the entire flux tube. Upward moving density depletions have been detected below and near the peak of the F layer [McClure et al., 1977]. They are, however, also found well above the F layer peak [Burke et al., 1979; Young et al., 1984]. Computer simulations show that with proper background conditions Rayleigh-Taylor turbulence grows into the nonlinear regime and penetrates through to the topside ionosphere [Ossakow et al., 1979; Anderson and Haerendel, 1979; Zalesak et al., 1982]. An analytical model for the nonlinear evolution of plasma bubbles was developed by Ott [1978] and Kelley and Ott [1978]. Plasma bubbles are field-aligned density depletions that percolate through the equatorial ionosphere after sunset. They are represented as two-dimensional structures whose buoyancy is supplied by eastward electric fields within the depletions. The electric fields result from steps in g x B currents at the edges of depleted flux tubes. In the absence of dissipation, positive (negative) charge accumulates on the west (east) wall of the flux tube until it reaches a flotation level. During the charge accumulation phase a bubble's E x B velocity should increase at a rate nearly opposite to the acceleration due to gravity [Chandrasekhar, 1960]. The model of Chandrasekhar [1960] concerns plasma enhancements, rather than depletions, supported against gravity by a magnetic field. Ossakow and Chaturvedi [1978] pointed out a formal similarity between the equations of single [Ossakow and Chaturvedi, 1978] and multiple [Chen et al., 1983] bubbles have been obtained assuming that ...

Filamentary Alfvénic structures excited at the edges of equatorial plasma bubbles

“…Due to the large surface area covered by the bubbles the amount of collected gravity currents is not negligible in spite of the weakness of these currents. The present observations are consistent with the role of Alfvén waves in connecting the equatorial F region and the conjugate E regions (Aggson et al, 1992;Bhattacharyya and Burke, 2000). It is worth mentioning that these waves influence the dynamics of a plasma bubble by providing, in the bubble reference frame, an additional quasi-static electric field component transverse to the magnetic field.…”

“…In the present paper, we show wave data recorded by the French DEMETER (Detection of ElectroMagnetic Emissions Transmitted from Earthquake Regions) polar satellite at altitudes of ∼700 km. As previously reported from the San Marco D and DE2 satellite data (Aggson et al, 1992;Aggson et al, 1996), the DEMETER data illustrate some of the characteristics of the turbulence generated by such equatorial bubbles propagating in collisionless background plasmas.…”

“…A more systematic analysis of DEMETER data reveals that such fluctuations are usually anti-correlated with broadband electrostatic emissions taking place in the lowdensity inner regions of the bubbles in conjunction with small scale (10-100 m) density gradients. Magnetic-field fluctuations coupled with the presence of plasma bubbles have already been reported from both the DE-2 and the CHAMP satellite observations (Aggson et al, 1992;Luehr et al, 2002;Stolle et al, 2006). Moreover, it has been suggested that when a plasma bubble starts to grow, field aligned currents supported by Alfvén waves join the equatorial F-region with the conjugate E-regions (Bhattacharyya and Burke, 2000).…”

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

“…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.…”

“…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. It was suggested that FACs originate from the divergence of ambient zonal currents at EPB walls, and they close at the poleward bottomside ionosphere (see Fig.…”