W. J. Burke

Equatorial density depletions observed at 840 km during the great magnetic storm of March 1989

Early on March 14, 1989, a thermal plasma probe on the Defense Meteorological Satellite Program (DMSP) F9 spacecraft detected extensive and dramatic decreases in the ion density at 840 km, near 2130 LT, during two consecutive transequatorial passes over South America. The order of magnitude decreases in the ion density extended more than 4000 km along the satellite track. The depletions were accompanied by upward and westward plasma drifts, both in excess of 100 m/s. Their onsets and terminations were marked by extremely sharp density gradients. DMSP F9 observed no similar depletions over the Atlantic during preceding orbits. A partial depletion was detected over the eastern Pacific during the following orbit. The DMSP F9 ground track passed slightly west of a Brazilian total electron content (TEC) station and two Brazilian ionosondes during the first depletion encounter. The TEC fell far below normal during the night of March 13–14. The ionosonde measurements indicate that, in the hour after sunset, before DMSP passed through the depletions, the F2 layer rose rapidly and disappeared, but at the time of the first depletion encounter, hmF2 was decreasing over one of the stations. The DMSP F8 satellite, which orbits in the dawn-dusk meridian, made related measurements on March 13 and 14. Crossing the equator at dust on March 13, at the same longitude where DMSP F9 encountered the first depletion, DMSP F8 detected upward and westward drifts, but it measured extremely large rather then depleted ion densities. During two dawn passes over the eastern Pacific on March 14, DMSP F8 observed depletions somewhat similar to those detected by DMSP F9. Large westward drifts accompanied the depletions detected by DMSP F8. It is quite probable that the morningside depletions detected on March 14 are remnants of those detected earlier by DMSP F9 in the evening sector. We develop a phenomenological model reconciling DMSP F8, F9, and ground-based measurements. Our calculations show that rapid upward drifts sustained for several hours can produce depletions in the equatorial ion density with sharp gradients at their high-latitude boundaries, consistent with the data. We discuss possible contributing mechanisms for generating these upward drifts. These include direct penetration of the magnetospheric electric field to low latitudes, the electric fields generated by the disturbance dynamo, and the effects of conductivity gradients near the dusk terminator and the South Atlantic anomaly.

Electromagnetics of substorm onsets in the near‐geosynchronous plasma sheet

A search of the CRRES database identified 20 events in which the satellite was located within the local-time sector spanned by the substorm current wedge (SCW) as it formed. Poynting vectors for low-frequency waves are derived from the electric and magnetic field measurements. In 19 of the events, data are inconsistent with the notion that the SCW initiates from the braking of earthward bulk flows emanating from a near-Earth X line. Rather, the data support drift-Alfven ballooning in the near-geosynchronous plasma sheet as being responsible for initiation of the SCW and substorm onset. Dipolarization at CRRES is preceded by eastward excursions of the electric field (trigger waves), at which time the first significant electromagnetic energy is observed flowing toward the ionosphere. Dipolarization and the SCW appear before ground onset, following one or more of these trigger waves. The so-called “explosive growth phase” occurs in association with explosive growth of the trigger waves soon after onset. Seven characteristic features of substorm onsets and expansions observed at CRRES are described. Among these are two stages of expansion. The first expansion stage is initiated by the trigger waves (ballooning) in the near-geosynchronous plasma sheet. Approximately 10 minutes later a second stage begins consistent with the arrival of earthward bulk flows emanating from a near-Earth X line. Near-geosynchronous substorm onsets can explain the observed increase in the occurrence rate of fast bulk flows earthward of its minimum value near X = −12 RE. Drift-Alfven ballooning also provides a possible causal link between observed reductions of the solar wind driver and substorm onsets.

Ionospheric signatures of dayside magnetopause transients: A case study using satellite and ground measurements

A case study is presented of coordinated ground and space measurements featuring a set of transient, auroral fragments located on the poleward side of a stable cusp/cleft arc. Optical ground data from Ny Alesund (Svalbard) and Heiss Island (Franz Josef Land) were combined with DMSP F9 satellite measurements to examine the characteristics of these auroral features. A stable red arc stretched across most of the dayside auroral zone in a region dominated by westward convection in accordance with the orientation of the IMF. Poleward of the red arc were several, westward moving auroral jets having characteristics similar to midday auroral breakup events. Such events may be ground signatures of transitory magnetic merging at the dayside magnetopause. If so, the driven convective motion of these structures should contribute to the polar cap potential. Within this limited data set we find that although the transitory structures have an inherent potential associated with the motion of the optical signatures the structures on the whole contribute a small fraction of the total polar cap potential.

DMSP F8 observations of the mid‐latitude and low‐latitude topside ionosphere near solar minimum

The retarding potential analyzer on the DMSP F8 satellite measured ion density, composition, temperature, and ram flow velocity at 840-km altitude near the dawn and dusk meridians close to solar minimum. Nine days of data were selected for study to represent the summer and winter solstices and the autumnal equinox under quiet, moderately active, and disturbed geomagnetic conditions. The observations revealed extensive regions of light-ion dominance along both the dawn and dusk legs of the DMSP F8 orbit. These regions showed seasonal, longitudinal, and geomagnetic control, with light ions commonly predominating in places where the subsatellite ionosphere was relatively cold. Field-aligned plasma flows also were detected. In the morning, ions flowed toward the equator from both sides. In the evening, DMSP F8 detected flows that either diverged away from the equator or were directed toward the northern hemisphere. The effects of diurnal variations in plasma pressure gradients in the ionosphere and plasmasphere, momentum coupling between neutral winds and ions at the feet of field lines, and E {times} B drifts qualitatively explain most features of these composition and velocity measurements. 23 refs., 5 figs., 2 tabs.

Mapping ionospheric convection patterns to the magnetosphere

While convection patterns in the high-latitude ionosphere are usually presented in a corotating frame of reference, those of the magnetosphere are given in inertial coordinates. In the corotating representation the convection throat, which is frequently associated with the cusp, opens between 1000 and 1100 MLT. Cusp precipitation, however, centers about noon. We find that transforming the convection patterns of Heppner and Maynard (1987) (hereinafter H-M) into inertial coordinates aligns the throat region with local noon. We present projections of the H-M patterns to the magnetosphere in both corotating and inertial coordinates using the magnetic field model of Tsyganenko (1989). In inertial coordinates the mapped H-M convection throat opens at noon. Consistent with predictions of the Rice convection model for magnetospheric electric fields late in the substorm cycle, only a small fraction of the equipotential contours penetrate to the subsolar region. This suggests that a significant portion of flux tube merging occurs on magnetic field lines whose equatorial mapping is on the flanks of the magnetosphere. Nonconjugacy between the mapping of H-M patterns for both positive and negative interplanetary magnetic field BY, especially in the 1400-1600 LT sector, may explain the BY dependence of the electron precipitation “hot spot” discovered by Evans (1985). A separate lobe cell is not required to explain the central, equipotential contours of the large convection cell.

Pitch angle scattering of diffuse auroral electrons by whistler mode waves

Resonant electron-whistler interactions in the plasma sheet are investigated as possible explanations of the nearly isotropic fluxes of low-energy electrons observed above the diffuse aurora. Whistler mode waves, propagating near the resonance cone with frequencies near or larger than half the equatorial electron cyclotron frequency, can interact with low-energy plasma sheet electrons. A Hamiltonian formulation is developed for test particles interacting with the coherent chorus emission spectra. We consider the second-order resonance condition which requires that inhomogeneities in the Earths magnetic field be compensated by a finite bandwidth of wave frequencies to maintain resonance for extended distances along field lines. These second-order interactions are very efficient in scattering the electrons toward the atmospheric loss cone. Numerical calculations are presented for the magnetic shell L = 5.5 for wave amplitudes of ∼ 10−6 V/m, using different frequency and magnetospheric conditions.

Relativistic particle acceleration by obliquely propagating electromagnetic fields

The relativistic equations of motion are analyzed for charged particles in a magnetized plasma and externally imposed electromagnetic fields (ω, k), which have wave vectors k that are at arbitrary angles. The particle energy is obtained from a set of nonlinear differential equations, as a function of time, initial conditions, and cyclotron harmonic numbers. For a given cyclotron resonance, the energy oscillates in time within the limits of a potential well; stochastic acceleration occurs if the widths of different Hamiltonian potentials overlap. The net energy gain for a given harmonic increase with the angle of propagation, and decreases as the magnitude of the wave magnetic field increases. Potential applications of these results to the acceleration of ionsopheric electrons are presented.

Diffusion of radiation belt protons by whistler waves

Whistler waves propagating near the quasi-electrostatic limit can interact with energetic protons ({approximately}80-500 keV) that are transported into the radiation belts. The waves may be launched from either the ground or generated in the magnetosphere as a result of the resonant interactions with trapped electrons. The wave frequencies are significant fractions of the equatorial electron gyrofrequency, and they propagate oliquely to the geomagnetic field. A finite spectrum of waves compensates for the inhomogeneity of the geomagnetic field allowing the protons to stay in gyroresonance with the waves over long distances along magnetic field lines. The Fokker-Planck equation is integrated along the flux tube considering the contributions of multiple-resonance crossings. The quasi-linear diffusion coefficients in energy, cross energy/pitch angle, and pitch angle are obtained for second-order resonant interactions. They are shown to be proportional to the electric fields amplitudes. Numerical calculations for the second-order interactions show that diffusion dominates near the edge of the loss cone. For small pitch angles the largest diffusion coefficient is in energy, although the cross energy/pitch angle term is also important. This may explain the induced proton precipitation observed in active space experiments. 24 refs., 12 figs.

A rotating, midday auroral event with northward interplanetary magnetic field

We present ground-based observations of an isolated, daytime auroral event that was detected above Svalbard by means of an all-sky TV camera and a multichannel, meridian scanning photometer. This 5-min event occurred near magnetic noon, poleward of a stable cusp aurora. It emitted ∼20 kR of 557.7-nm light, with little if any admixture of 630.0-nm radiation. The structure rotated rapidly then underwent a sudden breakup. The spectral distribution of emitted light indicates that electrons responsible for this auroral structure were accelerated though several kilovolts between the magnetosheath and the ionosphere. Such a potential structure requires that the precipitating, arc electrons constitute a field-aligned current out of the ionosphere. Current continuity and Ohms law require radial electric fields within the arc. From the structures rotational speed, we estimate that the accelerated electrons carried an upward field-aligned current of order 100µA/m². Satellite observations of polar cap convection patterns suggest that the interplanetary magnetic field had turned northward prior to the event. The auroral structure may be explained as resulting from a transitory magnetic merging of interplanetary and lobe magnetic field lines at and/or a penetration of plasma across the high-latitude magnetopause.

Some Considerations of near-Earth Space as a Gaseous Dielectric

My assigned topic, the gaseous dielectric properties of matter resident in near-Earth space, is unusual. It is necessary first to identify the diverse plasma populations, then explain the sense in which they may be regarded as gaseous dielectrics. We focus specifically on regions called the ionosphere and the magnetosphere. The ionosphere is a layer of cold plasma that extends from IOOO km above the Earth. This plasma is created by solar ultraviolet light and by energetic particles from the magnetosphere. It is usually divided into D, E, and F layers. The D and E layers range in altitude from 60–95 km and 100–150 km, respectively. Outside the auroral oval they are mostly dayside features that disappear due to ion-electron recombination after sunset. Maximum plasma densities of ~106 cm−3 occur at F layer altitudes between 300 and 400 km on the dayside.