Adiabatic acceleration induced by convection in the plasma sheet

[1] We study two main types of ion energy dispersions observed in the energy range $1 to 14 keV on board the Interball-Auroral (IA) satellite at altitudes 2-3 R E at the poleward boundary of the plasma sheet. The first type of structure is named velocity dispersed ion structures (VDIS). It is known that VDIS represent a global proton structure with a latitudinal width of $0.7-2.5°, where the ion overall energy increases with latitude. IA data allow to show that VDIS are made of substructures lasting for $1-3 min. Inside each substructure, high-energy protons arrive first, regardless of the direction of the plasma sheet boundary crossing. A near-continuous rise of the maximal and minimal energies of consecutive substructures with invariant latitude characterizes VDIS. The second type of dispersed structure is named time-of-flight dispersed ion structures (TDIS). TDIS are recurrent sporadic structures in H + (and also O + ) with a quasi-period of $3 min and a duration of $1-3 min. The maximal energy of TDIS is rather constant and reaches !14 keV. During both poleward and equatorward crossings of the plasma sheet boundary, inside each TDIS, high-energy ions arrive first. These structures are accompanied by large fluxes of upflowing H + and O + ions with maximal energies up to 5-10 keV. In association with TDIS, bouncing H + clusters are observed in quasi-dipolar magnetic field tubes, i.e., equatorward from TDIS. The electron populations generally have different properties during observations of VDIS and TDIS. The electron flux accompanying VDIS first increases smoothly and then decreases after Interball-Auroral has passed through the proton structure. The average electron energy in the range $0.5-2 keV is typical for electrons from the plasma sheet boundary layer (PSBL). The electron fluxes associated with TDIS increases suddenly at the polar boundary of the auroral zone. Their average energy, reaching $5-8 keV, is typical for CPS. A statistical analysis shows that VDIS are observed mainly during magnetically quiet times and during the recovery phase of substorms, while sporadic and recurrent TDIS are observed during the onset and main phases of substorms and magnetic storms and, although less frequently, during substorm recovery phases. From the slope of the (velocity) À1 versus time dispersions of TDIS, we conclude that they have a sporadic source located at the outer boundary of the central plasma sheet, at distances from 8 to 40 R E in the equatorial plane. The disappearance of the PSBL associated with TDIS can be tentatively linked to a reconfiguration of the magnetotail, which disconnects from the Earth the field lines forming the ''quiet'' PSBL. We show that VDIS consist of ion beams ejected from an extended current sheet at different distances. These ion beams could be formed in the neutral sheet at distance ranging from $30 R E to $100 R E from the Earth. Inside each substructure the time-offlight dispersion of ions generally dominate over any latitudinal dispersion induced by a dawn-dusk electric field. T...

Section: Observationsmentioning

confidence: 57%

Two types of energy‐dispersed ion structures at the plasma sheet boundary

Sauvaud

2004

“…In this analysis we are less interested in the boundary than in looking at the ion characteristics as a function of latitude well removed from the boundary. As pointed out above, previous research has shown [Galpernin et al, 1976[Galpernin et al, , 1978Hultqvist, 1979;Sauvaud et al, 1981] that over a range of latitudes, for individual passes in the evening and morning sectors and during quiet times, the average energy of the precipitating ions varies with L shell approximately as would be expected if the ions were undergoing quasi-static, adiabatic compression.…”

Section: Eave(90 ø) = Eaveo(90ø)(lo/l) 3 (7)mentioning

confidence: 83%

“…Specifically, we will discuss the degree to which the variation in ion characteristics, at least over some range of latitudes, MLTs, and activities, is consistent with a simple physical model of adiabatic compression and the creation of Alfven layers as the ions are transported within the magnetosphere. The chief motivation for this is to determine the degree to which such adiabatic effects, reported by earlier researchers in case studies, are repeated in the average study [Hultqvist, 1974;Galpernin et al, 1976Galpernin et al, , 1978Hultqvist, 1979 , and references therein; Sauvaud et al, 1981]. In addition, we will consider the relationship of the precipitating ions to the various boundary regions in the magnetosphere, the implications of the variation of the average energy and integral number flux in the cusp region, and the degree to which the pattern of ion precipitation agrees with models for the motion of ions in the magnetosphere.…”

Section: Discussionmentioning

confidence: 99%

“…observations have shown that in the nightside auroral region, during quiet times the latitudinal variation in ion characteristics is consistent, over a range of latitudes, with adiabatic compression of the plasma as it is convected toward the Earth from the tail and the possible formation of Alfven layers nearer to the Earth [Hultqvist, 1974;Galpernin et al, 1976Galpernin et al, , 1978Hultqvist, 1979, and references therein; Sauvaud et al, 1981]. These observations are at some variance with measurements made at higher altitudes that show the inner magnetosphere often to be populated by the sudden injection of hot plasma [Mcllwain, 1974].…”

Section: Paper Number 88ja03169mentioning

confidence: 99%

See 1 more Smart Citation

A statistical model of auroral ion precipitation

Hardy¹,

Gussenhoven²,

Brautigam³

1989

294

313

The average global characteristics of precipitating auroral ions were determined using the data from the SSJ/4 detectors on the F6 and F7 satellites of the Defense Meteorological Satellite Program (DMSP). For this study the high‐latitude region was divided into spatial elements in magnetic local time (MLT) and corrected geomagnetic latitude (CGL). One such spatial matrix was created for each of seven levels of magnetic activity as defined by Kp. Approximately 26.5 million, individual, 1‐s spectra were used to determine the average ion spectrum over the energy range from 30 eV to 30 keV in each spatial zone and at each level of activity. Where appropriate, the spectra were extrapolated to 100 keV to provide a more complete estimate of the total integral energy flux, number flux, and average energy. The global patterns of the integral energy flux, integral number flux, and average energy derived from the average spectra vary smoothly with latitude, MLT, and activity. For a given Kp value the higher levels of integral energy flux occur in C‐shaped regions symmetric about a meridian running prenoon to premidnight. The maximum integral energy flux is found premidnight; the minimum peak integral energy flux in latitude is found prenoon. Except in the prenoon region, the overall level of integral energy flux increases with Kp with the premidnight maximum increasing by a factor of 6.1 from Kp = 0 to Kp ≥ 6−. The maximum integral number flux is centered at noon and is taken to be the center of the cusp. The maximum value does not vary significantly with activity, although its location moves to lower latitude with increasing Kp. A minimum in the average energy also occurs in the noon meridian, several degrees poleward of the integral number flux maximum. The separation in latitude between the integral flux maximum and the average energy minimum of the cusp increases with Kp. The maximum average energy occurs on the eveningside of the oval near the equatorward boundary of the ion precipitation and shifts toward noon with increasing activity. The average integral number flux for the precipitating auroral ions is 1‐2 orders of magnitude less than that for the precipitating auroral electrons at all latitudes, MLTs, and activities. The integral energy flux of the ions, on the eveningside of the oval, can equal or exceed that for the electrons near the equatorward edge of the auroral region. Integrated globally, the ratio of the ion to electron number flux varies from 0.024 to 0.015 over the range of activity from Kp = 0 to Kp ≥ 6−. The ratio for the integral energy flux ranges from 0.11 to 0.17 with no trend in activity.

“…In both plates, the results of the simulations were sorted according to pitch angle, namely, from 5 ø to 15 ø (left panels), from 30 ø to 60 ø (middle panels) and from 75 ø up to 85 ø (right panels). Distinct initial temperatures were considered in the two upper panels, and the case with two spectral maxima [e.g., Galperin et al, 1978;Lennartsson, 1992] is shown in the bottom panels.…”

Section: Postdipolarization Evolutionmentioning

confidence: 99%

Plasma sheet ion energization during dipolarization events

Delcourt¹,

Sauvaud²

1994

Self Cite

The energization of plasma sheet particles during storm‐time dipolarization of the magnetospheric field lines is examined by means of three‐dimensional particle codes. In the direction perpendicular to the magnetic field, owing to appreciable temporal variations of the field within a cyclotron turn, particles can experience large magnetic moment changes. A detailed analysis of this effect reveals an “effective pick‐up velocity” at substorm onset, which depends upon the magnitude of the magnetic transition and delineates particle motions at nearly constant or highly variable magnetic moment. In the parallel direction, the large but short‐lived electric field yields an intense acceleration of the particles traveling in the equatorial region. It is demonstrated that significant decelerations can also be achieved off‐equator if the particles drift against the surging electric field. Such decelerations occur in regions where the curvature of the E × B paths substantially exceeds that of the magnetic field lines (namely, at relatively large L‐ shells). Systematic calculations of model plasma sheet distribution functions clearly exemplify the earthward injection of energetic particles during dipolarization events. Owing to phasing between particle motion and the transient electric field, the computed spectrograms display the formation of characteristic bouncing ion patterns with fine structures on fast time scales. To some extent, the simulations support the convection surge mechanism of particle acceleration put forward by Mauk [1986]. However, distinct collapse characteristics as well as the inclusion of nonadiabatic effects result in significantly higher energization rates, particularly at high mass‐to‐charge ratios.