The thinning of the near-Earth current sheet during the growth phase of the CDAW 6 magnetospheric substerm is studied. The exp,'msion onset of the substerm occurred at 1054 UT, March 22, 1979. During the growth phase, two spacecraft, ISEE 1 and ISEE 2, were within the current sheet approximately 13R e from the Earth and obtained simultaneous highresolution magnetic dam at two points in the current sheet. Plasma data were also provided by the ISEE spacecraft and solar wind dam by IMP 8. To facilitate the analysis, the GSM magnetic field data are transformed to a "neutral sheet coordinate system" in which the new x axis is parallel to the average magnetic field above m•d below the neutral sheet and the new y axis lies in the GSM equatorial pl,-me. A model based on the assumption that the current sheet is a •ne-invariant structure fails to predict neutral sheet crossing times. Consequently, the Harris sheet model, which allows one to remove the restriction of time invariancy, is used instead. It is found that during the growth phase, a model p,-tr,'uneter corresponding to the thickness of the current sheet decreased exponentially from about 5R e to IR e with a time constant of about 14 min. In addition, the ISEE 1 ,'red ISEE 2 neutral sheet crossings after expansion onset indicate that the neutral sheet was moving upward at 7 km/s relative to the spacecraft. Since both crossings occurred in approximately 80 s, the current sheet thickness is estimated to be about 500 km. These results demonstrate that the near-Earth current sheet undergoes dramatic thinning during the substerm growth phase m•d expansion onset.

[1] CRRES plasma wave receiver density data were used to study the distribution and properties of dense plasmaspheric-like plasma observed outside the plasmapause. Our study indicates that outer plasmaspheric structure, often called plasmaspheric plumes, blobs, tails, or detached plasma regions, can exist at all local times under all levels of geomagnetic activity. Of the 558 CRRES orbits that had at least one clearly defined plasmapause, 169 (or 30%) had plasmaspheric-like density structures at higher L shells than the plasmapause. Most of the occurrences of plasmaspheric-like plasma observed by CRRES were in the noon-to-dusk sector in the aftermath of enhanced geomagnetic activity consistent with plasmaspheric plume models.

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The AMPTE CCE satellite frequently observed transient (1 < r < 8 min) events marked by magnetic field strength increases and bipolar magnetic field signatures (peak-topeak amplitudes > 4 nT) while in the outer dayside magnetosphere. We report a survey of 59 prominant events observed from August to November 1984. The bipolar signatures and minimum variance analysis reveal that most events move poleward and antisunward, except in the immediate vicinity of local noon. Here the motion of the events appears to be better governed by the spiral/orthospiral interplanetary magnetic field (IMF) orientation than magnetic curvature forces associated with IMF By. The IMF orientation appears to have little or no influence on event occurrence or orientation. We interpret the events in terms of solar wind/foreshock pressure pulse induced ripples on the magnetopause surface. Our results can be reconciled with those obtained in previous studies which made use of ISEE 1/2, AMPTE IRM, and AMPTE UKS observations if pressure pulses produce large-amplitude events and bursty merging (or reconnection) produces small-amplitude events.Paper number 95JA03063. 0148-0227/96/95JA-03063505.00 vidual events and their statistical occurrence patterns (see review by Sibeck [1994]). Distinguishing between these two mechanisms is important. Under the merging interpretation, the prevalence of transient events implies that bursty merging is a major contributor to the solar wind-magnetosphere interaction [e.g., Cowley, 1982]. Under the pressure pulse interpretation, the prevalence of transient events implies that the magnetosphere is constantly being buffeted by highly variable solar wind parameters, perhaps with preconditioning in the foreshock [Sibeck, 1990].Since the bursty merging and pressure pulse models predict differing patterns for event occurrence, orientation, and direction of motion as a function of IMF orientation, local time, and latitude, statistical studies should help determine the relative significance of each model in the production of transient events. We begin by outlining the contrasting predictions of the two models concerning event occurrence patterns as a function of IMF orientation. We then present a case study and a statistical survey of transient events observed by the AMPTE CCE satellite in the outer dayside magnetosphere, and conclude with an attempt to reconcile the new results with those obtained in previous studies. Predictions of the Bursty Merging ModelMerging occurs when any one of several current layer instabilities [e.g., Schindler, 1974;Huba et al., 1977;Lui et al., 1991] enhances resistivities, disrupts currents, and allows magnetosheath and magnetospheric magnetic field lines to interconnect across the magnetopause. Since instability growth rates increase with increasing current density and there is no clear dependence of the current layer thickness upon the magnetosheath magnetic field orientation [Berchem and Russell, 1982], we expect growth rates to increase as the shear between the magnetosheath and magnetosp...

A system which enables fast and reliable measurements of the dielectric constant over continuous microwave frequency ranges for both solid and liquid low-loss materials is described. The main thrust of this work is the application of the open-ended coaxial-line probe technique, which has been used previously for soft biological materials, to low-loss solid samples. Using the instrumentation and procedure presented here, the dielectric constant for low-loss solids can be measured absolutely to ±2%–3% with routine care. The uncertainty can be reduced by about a factor of 2 by averaging several measurements. It is also smaller for liquid samples. This application features the use of relatively simple and readily available microwave components. Also, it is shown that a simple empirical relationship can be used to obtain the bulk dielectric constant from samples of a material in the form of thin slabs. The experimental results which are presented here for kapton, Teflon, Corning glass No. 0211, soda lime glass, magnesium oxide, sapphire, silicon, alumina, and lanthanum aluminate, as well as carbon tetrachloride, compare favorably with existing literature values.

Niobium triselenide, NbSe 3 , shows two phase transitions, 1 at 145 K (7\) and 59 K (T 2 ) associated with the development of incommensurate charge-density waves (CDW). These transitions lead to the increase of resistivity which is ascribed to the reduction of the Fermi surface; 2 ' 3 the CDW condensate is assumed to be pinned by impurities to give negligible contribution to the dc conductivity at low electric fields. The conductivity increases when the electric field is larger than a threshold field E T and then saturates at high electric fields. 4 " 6 This and the observation of noise in the nonlinear region 6 * 7 together with the x ray observation of the superstructure at high electric fields 8 demonstrate that the depinning of the CDW is responsible for the increase of the conductivity with increasing electric field. The resistive anomalies are also suppressed at microwave frequencies 2 suggesting that the pinning frequency is less than 10 10 Hz.In this paper we report the first observation of frequency-dependent conductivity associated with the incommensurate CDW states in NbSe 3 . We show that the CDW anomalies are suppressed at extremely low frequencies and also lead to a gigantic dielectric constant. The transition between the low-frequency (/<1 MHz) and high-frequency (/>100 MHz) limit is smooth with no edited by M. L. Klein and J.The observation of a frequency-dependent conductivity (o) and dielectric constant (e) in NbSe 3 is reported. In both charge-density-wave phases a strong frequency dependence and huge dielectric constant are observed below 100 MHz, with greatest effects observed at 42 K. The conductivity cr increases smoothly from the dc value to the high-frequency (f = 100 MHz) limit; this increase is accompanied by the reduction of e. A resistancecapacitance network model is suggested to account for the observed frequency dependence.

[1] We present a survey of the variability of the geosynchronous magnetic field strength on the dayside using observations by the GOES satellites over a period exceeding 4 years. Only intervals of reduced geomagnetic activity, as defined by Dst > À20 nT, were considered in this study. The magnetic field strength data were filtered with a passband of 1.7 mHz to 17 mHz (1-10 minutes), a process that eliminates the diurnal variation of the field strength and the effects of most of the higher frequency (>17 mHz) ultralowfrequency (ULF) waves. The geosynchronous field strength appears to exhibit the greatest variability in the prenoon sector for spiral interplanetary magnetic fields (IMF) and in the postnoon sector for orthospiral IMF, suggesting that pressure pulses generated in the foreshock/bow shock region may have a significant influence on the geosynchronous field. The seasonal dependence of the variability was determined to be positively correlated to the seasonal dependence of ground-based observations of magnetic impulse events. The response of the variability of the geosynchronous magnetic field strength around local noon to solar wind parameters was also studied. Here, we observed that the variability was strongly affected by changes in the solar wind dynamic pressure but was seemingly independent of the northward/southward direction of the interplanetary magnetic field. However, for high solar wind dynamic pressures, the variability was found to be greater for northward IMF than for southward IMF.

Plasmaspheric plumes have ionospheric signatures and are observed as storm‐enhanced density (SED) in global positioning system (GPS) total electron content (TEC). These ionospheric signatures have been primarily observed over the American sector and in a few limited examples over the European sector. This study examines the longitudinal occurrence frequency of plasmaspheric plumes. We analyzed all images from the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) Extreme Ultraviolet Imager (EUV) databases for the first half of 2001 and identified a total of 31 distinct plume intervals observed during different storm events. Out of the total IMAGE EUV plumes that we identified, 12 were projected over North America, 10 over Asia, and the remaining 9 were over Europe and the Atlantic Ocean. Using ground‐based GPS TEC from MIT's Madrigal database, we searched for corresponding SED/TEC plumes at different longitudinal sector and found 12 ionospheric SED plume signatures over North America, 4 over Europe, and 2 over Asia. This indicates that the observation probability of an ionospheric SED plume when a plasmaspheric plume is seen is 100% in the American sector, 50% in the European sector, and 20% in the Asian sector. This could be due to the fact that the plumes may be either positioned beyond the limit of the ground‐based GPS field of view, which happens mainly when there is less plasmaspheric erosion, or are too weak to be detected by the sparse number of GPS receivers over Asia. The in situ plasma densities from the available coincident defense metrological satellite program (DMSP) satellites were also used to study the characteristics of SED/TEC plume at DMSP orbiting altitude (i.e., ∼870 km). The TOPographic EXplorer (TOPEX) altimeter TEC also is used to identify the conjugate SED/plume signature over the Southern Hemisphere.