Cynthia A. Cattell

Satellite measurements and theories of low altitude auroral particle acceleration

Several previous and new S3-3 satellite results on DC electric fields, field-aligned currents, and waves are described, interpreted theoretically, and applied to the understanding of auroral particle acceleration at altitudes below 8000 km. These results include the existence of two spatial scale sizes (less than 0.1 degree and a few degrees invariant latitude) in both the perpendicular and parallel electric fields; the predominance of S-shaped rather than V-shaped equipotential contours on both spatial saales; the correlated presence of field-aligned currents, low frequency wave turbulence, coherent ion cyclotron wave emissions and accelerated upmoving ions and downgoing electrons; intense waves inside electrostatic shocks and important wave-particle interactions therein; correlations of field-aligned currents with magnetospheric boundaries that are determined by convection electric field measurements; electron acceleration producing discrete auroral arcs in the smaller scale fields and producing inverted-V events in the larger scale fields; ion and electron acceleration due to both wave-particle interactions and the parallel electric fields. Further analyses of acceleration mechanisms and energetics are presented.

Quasistatic electric field measurements with spherical double probes on the GEOS and ISEE satellites

Spherical double probes for measurements of electric fields on the GEOS-1, GEOS-2, and ISEE-1 satellites are described. An essential feature of these satellites is their conductive surfaces which eliminate errors due to differential charging and enable meaningful diagnostic experiments to be carried out. The result of these experiments is a good understanding of interactions between the plasma, the satellite and the probes, including photo-electron emission on satellite and probes. Electric field measurements are compared with measurements of plasma drift perpendicular to the magnetic field in the solar wind and the magnetosphere and the error bar for the absolute values of the electric field is found to be in the range ±(0.5–1.0) mV m-1 whereas relative variations can be determined with much better accuracy. A useful by-product from a spherical double probe system is the determination of satellite floating potential which is related to the plasma electron flux. This measurement allows high time resolution studies of boundary crossings. Examples of electric field measurements, which reflect the recent scientific results, are given for different regions of the magnetosphere from the bow shock, the inner magnetosphere and the tail. Several examples of simultaneous GEOS-ISEE observations are described.

Wave properties near the subsolar magnetopause: Pc 3–4 energy coupling for northward interplanetary magnetic field

Strong slow mode waves in the Pc 3–4 frequency range are found in the magnetosheath close to the magnetopause. We have studied these waves at one of the ISEE subsolar magnetopause crossings using the magnetic field, electric field, and plasma measurements. We use the pressure balance at the magnetopause to calibrate the Fast Plasma Experiment data versus the magnetometer data. When we perform such a calibration and renonnalization, we find that the slow mode structures are not in pressure balance and small scale fluctuations in the total pressure still remain in the Pc 3–4 range. Energy in the total pressure fluctuations can be transmitted through the magnetopause by boundary motions. The Poynting flux calculated from the electric and magnetic field measurements suggests that a net Poynting flux is transmitted into the magnetopause. The two independent measurements show a similar energy transmission coefficient. The transmitted energy flux is about 18% of the magnetic energy flux of the waves in the magnetosheath. Part of this transmitted energy is lost in the sheath transition layer before it enters the closed field line region. The waves reaching the boundary layer decay rapidly. Little wave power is transmitted into the magnetosphere.

Geotail observations of spiky electric fields and low‐frequency waves in the plasma sheet and plasma sheet boundary

Electric field data from the Geotail spacecraft provide an opportunity to extend the observations of spiky fields made by ISEE-1 to a region of the magnetosphere where quasistatic electric field measurements have not previously been made, to examine their possible importance in the dynamics of the middle and distant tail, and to test some hypotheses about their formation. In this paper, examples of large fields in the plasma sheet and its boundary at radial distances up to {approximately}90 R{sub E} are presented. It is shown that three different types of large electric fields can occur: (1) spiky fields; (2) {open_quotes}DC{close_quotes} fields; and (3) waves at frequencies comparable to the lower hybrid frequency. There is usually a gradation between (1) and (3), and often large electric field spikes are embedded in regions of lower amplitude waves. The waves tend to occur in short (few to 10`s or seconds) packets whose start and stop times are not always correlated with changes in the magnetic field and/or density (as indicated by the spacecraft potential). The peak frequency is often less than but comparable to the lower hybrid frequency in agreement with theories of lower hybrid drift waves in the magnetotail. The largest spikesmorexa0» are not always associated with the largest changes in the spacecraft potential and/or magnetic field. It is suggested that the spiky fields may represent the nonlinear development of the waves. 21 refs., 4 figs.«xa0less

The MHD structure of the plasmasheet boundary: (1) Tangential momentum balance and consistency with slow mode shocks

Differences between observations in the near and distant tail and MHD simulations have motivated a statistical study of the MHD structure of the plasmasheet boundary in the near tail, utilizing data from the AMPTE/IRM spacecraft. The relationship between the change in the tangential velocity predicted by the Rankine-Hugoniot relations for a discontinuity with a normal magnetic field and the measured change in the tangential velocity for ∼80 crossings is presented. The measured change is almost always much less than the predicted value. This suggests that either: (1) There is not usually a normal component of the magnetic field across the boundary; combined with previous work on pressure balance, this implies that the boundary is a tangential discontinuity; or (2) The boundary is not usually well-modeled as a planar, time-stationary MHD discontinuity. This is very different from the case of the deep tail where the plasmasheet boundary is usually a slow mode shock tailward of the neutral line. It agrees with MHD simulations, including the effects of the ionosphere, which do not have slow mode shocks earthward of the neutral line. These results suggest that the magnetic connection to the earth inhibits the formation of slow mode shocks. There is a small subset of the crossings which are consistent with the identification of part of the plasmasheet boundary as a slow mode shock. These may be cases where the substorm neutral line formed in close proximity to the satellite.

A search for upstream pressure pulses associated with flux transfer events: An AMPTE/ISEE case study

On September 19, 1984, the Active Magnetospheric Particle Tracers Explorers (AMPTE) United Kingdom Satellite (UKS) and Ion Release Module (IRM) and ISEE 1 and 2 spacecraft passed outbound through the dayside magnetopause at about the same time. The AMPTE spacecraft pair crossed first and were in the near-subsolar magnetosheath for more than an hour. Meanwhile the ISEE pair, about 5 RE to the south, observed flux transfer event (FTE) signatures. We use the AMPTE UKS and IRM plasma and field observations of magnetosheath conditions directly upstream of the subsolar magnetopause to check whether pressure pulses are responsible for the FTE signatures seen at ISEE. Pulses in both the ion thermal pressure and the dynamic pressure are observed in the magnetosheath early on when IRM and UKS are close to the magnetopause, but not later. These large pulses appear to be related to reconnection going on at the magnetopause nearby. AMPTE magnetosheath data far from the magnetopause do not show a pressure pulse correlation with FTEs at ISEE. Moreover, the magnetic pressure and ten- sion effects seen in the ISEE FTEs are much larger than any pressure effects seen in the magne- tosheath. A superposed epoch analysis based on small-amplitude peaks in the AMPTE magne- tosheath total static pressure (nkT + B2/2lao) hint at some boundary effects, <5 nT peak-to-peak variations in the ISEE 1 and 2 BN signature starting about I min after the pressure peak epoch. However, these variations are much smaller than the standard deviations of the BN field compo- nent. Thus the evidence from this case study suggests that upstream magnetosheath pressure pulses do not give rise to FTEs, but may produce very small amplitude signatures in the mag- netic field at the magnetopause.

Large Electric Fields in the Magnetosphere

In several regions of the magnetosphere, perpendicular and/or parallel electric fields are found to be orders-of-magnitude larger than expected from simple considerations. Problems associated with these large fields that may be amenable to study through computer simulations are discussed. Regions in which large electric fields are observed include: a) The auroral ionosphere, where Langmuir soliton-like structures have been measured to contain plasma frequency oscillations as large as 500 mV/m, the envelopes of which have parallel electric fields of ∼100 mV/m lasting for fractions of a millisecond; b) The auroral acceleration region, where electrostatic shocks have been observed to contain perpendicular fields as large as 1000 mV/m and parallel fields as large as 100 mV/m, and where double layers having parallel fields up to 10 mV/m have been observed; c) The high latitude boundary of the plasma sheet, where turbulent electric fields as large as 100 mV/m have been seen along with quasi-static fields of 5–10 mV/m; d) Inside the plasma sheet, where fields of 5–10 mV/m have frequently been observed; e) The bow shock, where turbulent fields as large as 100 mV/m and d.c. fields of ∼5 mV/m normal to the shock have been seen.

High-time resolution measurements of upstream magnetic field and plasma conditions during flux transfer events at the Earth's dayside magnetopause

We present preliminary results of a study of upstream magnetic field and plasma conditions measured by IRM during flux transfer events observed at the Earths magnetopause by CCE. This study was designed to determine the importance of various upstream factors in the formation of bipolar magnetic field signatures called flux transfer events (FTEs). Six FTE encounters were examined. Three cases were when the two satellites were on similar magnetic field lines. Preliminary investigation showed that fluctuations occurred in the Bz component of the interplanetary magnetic field (IMF) resulting in a southward field preceding the FTE in all three of these cases. In two of these cases, the changes were characterized by a distinct rotation from a strong southward to a strong northward field. There were also accompanying changes in the dynamic and thermal pressure in the solar wind immediately before the FTE was encountered. Examination of the 3d plasma distributions showed that these pulses were due to the addition of energetic upstreaming foreshock particles, There were no consistent changes in either Bz or the plasma pressure at IRM for the three events when the satellites were not connected by the IMF.

High time resolution measurements in the magnetosphere

Summary form only given. The physics of the magnetosphere are characterized by the formation of thin boundaries. It is within these narrow regions that energy conversion and particle acceleration processes occur. These processes are often controlled by nonlinear microphysics occurring on time scales as fast as a few microseconds. To understand these phenomena it is necessary to measure the plasma distributions and time-domain electric and magnetic fields with appropriately high time resolution. Satellites have recently measured such phenomena as the dissipation mechanisms in the collisionless bow shock; large, spiky fields in the plasma sheet boundary layer; and the double layers which accelerate particles in the auroral zone