L. A. Frank

CDAW 9 analysis of magnetospheric events on May 3, 1986: Event C

The ninth Coordinated Data Analysis Workshop (CDAW 9) focused upon several intervals within the PROMIS period (March-June 1986). Event interval C comprised the period 0000-1200 UT on May 3, 1986, which was a highly disturbed time near the end of a geomagnetic storm interval. A very large substorm early in the period commenced at 0111 UT and had a peak AE index value of ∼1500 nT. Subsequent activity was lower, but at least three other substorms occurred at 2-3 hour intervals. The substorms on May 3 were well observed by a variety of satellites including ISEE 1, 2, and IMP 8 in the magnetotail plus SCATHA, GOES, GMS, and LANL spacecraft at or near geostationary orbit. A particularly important feature of the 0111 UT substorm was the simultaneous imaging of the southern auroral oval by DE 1 and of the northern auroral oval by Viking. The excellent constellation of spacecraft near local midnight in the radial range 5–9 RE made it possible to study the strong cross-tail current development during the bstorm growth phase and the current disruption and current wedge development during the expansion phase. We use a time-evolving magnetic field model to map observed auroral features out into the magnetospheric equatorial plane. There was both a dominant eastward and a weaker westward progression of activity following the expansion phase. A clear latitudinal separation (≳10°) of the initial region of auroral brightening and the region of intense westward electrojet current was identified. The combined ground, near-tail, and imaging data for this event provided an unprecedented opportunity to investigate tail current development, field line mapping, and substorm onset mechanisms. We find evidence for strong current diversion within the near-tail plasma sheet during the late growth phase and strong current disruption and field-aligned current formation from deeper in the tail at substorm onset. We conclude that these results are consistent with a model of magnetic neutral line formation in the late growth phase which causes plasma sheet current diversion before the substorm onset. The expansion phase onset occurs considerably later due to reconnection of lobelike magnetic field lines and roughly concurrent cross-tail disruption in the inner plasma sheet region.

Preliminary results on the plasma environment of Saturn from the Pioneer 11 plasma analyzer experiment

The Ames Research Center Pioneer 11 plasma analyzer experiment provided measurements of the solar wind interaction with Saturn and the character of the plasma environment within Saturns magnetosphere. It is shown that Saturn has a detached bow shock wave and magnetopause quite similar to those at Earth and Jupiter. The scale size of the interaction region for Saturn is roughly one-third that at Jupiter, but Saturns magnetosphere is equally responsive to changes in the solar wind dynamic pressure. Saturns outer magnetosphere is inflated, as evidenced by the observation of large fluxes of corotating plasma. It is postulated that Saturns magnetosphere may undergo a large expansion when the solar wind pressure is greatly diminished by the presence of Jupiters extended magnetospheric tail when the two planets are approximately aligned along the same solar radial vector.

On open and closed field line regions in Tsyganenko's field model and their possible associations with horse collar auroras

Using the empirical Tsyganenko (1987) long model as a prime example of a magnetospheric field model, we have attempted to identify the boundary between open and closed field lines. We define as “closed” all field lines that are connected with the Earth at both ends and cross the equatorial plane earthward of x = −70RE, the tailward validity limit of the Tsyganenko model. We find that the form of the open/closed boundary at the Earths surface, identified with the polar cap boundary, can exhibit the arrowhead shape, pointed toward the Sun, observed in “horse collar auroras” (Hones et al., 1989). The “polar cap” size in the Tsyganenko model increases with increasing Kp values, and it becomes rounder and less pointed. The superposition of a net By field, which is the expected consequence of an IMF By, rotates the polar cap pattern and, for larger values, degrades the arrowhead shape, resulting in polar cap configurations consistent with known asymmetries in the aurora. The pointedness of the polar cap shape also diminishes or even completely disappears if the low-latitude magnetopause is assumed open and located considerably inside of the outermost magnetic flux surface in the Tsyganenko model. The arrowhead shape of the polar cap is found to be associated with a strong increase of Bz from midnight toward the tail flanks, which is observed independently, and is possibly related to the NBZ field-aligned current system, observed during quiet times and strongly northward IMF Bz. The larger Bz values near the flanks of the tail cause more magnetic flux to close through these regions than through the midnight equatorial region. Since field lines at the flanks primarily map to the dayside polar regions, it becomes plausible that the closed field line region extends to higher latitudes on the dayside than on the nightside, when the increase of Bz becomes more pronounced. By comparison with a different field model we demonstrate that this association is not unique to the Tsyganenko model. The similarity of the quiet symmetric polar cap pattern to “horse collar” auroras suggests that the bright “bars,” observed at the sides of the arrowhead shaped polar cap, are connected with the separatrix layers (or plasma sheet boundary layers) extending to the distant X line or separator, while the adjacent “web” regions, located between the bars and the main auroral oval, are connected with the low-latitude boundary regions along the flanks of the magnetotail.

Venus - An upper limit on intrinsic magnetic dipole moment based on absence of a radiation belt

On the basis of the absence of energetic electrons (Ee 〉 45 kiloelectron volts) and protons (Ep 〉 320 kiloelectron volts) associated with Venus to within a radial distance of 10,150 kilometers from the center of the planet and using a physical similitude argument and the observational and theoretical knowledge of the magnetosphere of Earth, we conclude that the intrinsic magnetic dipole moment of Venus is almost certainly less than 0.01 and probably less than 0.001 of that of Earth. Corresponding upper limits on the magnetic field at the equatorial surface of Venus are about 350 and 35 x 10-5 gauss, respectively.

THE LOWEST POSSIBLE LATITUDE OF THE WESTWARD ELECTROJET DURING SEVERELY DISTURBED PERIODS

Abstract. It has been reported that the AE index cannot, at times, adequately monitor the auroral electrojets because as magnetic activity increases, it shifts equatorward from the standard AE stations, resulting in a serious underestimation of the auroral electrojet intensity. To evaluate quantitatively the equatorial shift of the westward electrojet, an extensive database obtained from CANOPUS, Alaska and IMAGE chains of magnetometers are utilized in this study. The data thus assembled enable us to determine how the westward electrojet shifts equatorward with increased magnetic activity. We are particularly interested in the latitude of the center of the westward electrojet during intense magnetic storms. The tendency of equatorward shift is confirmed from this study. However, the peak of the westward electrojet seems to shift only approximately 60° in magnetic latitude, regardless of magnetic activity levels. Therefore the current AE network, which covers as low as about 62°, does not have any serious problems in monitoring the auroral electrojet. The relative location of the westward electrojet with respect to the global auroral image taken from the Polar satellite is also examined. It is found that the center of the westward electrojet does not flow over the brightest auroral region but slightly poleward of it, with less luminous region. It indicates that the electric field is more important in intensifying the auroral electrojet than the ionospheric conductivity.