Jeff Sanny

Growth-Phase Thinning of the Near-Earth Current Sheet During the Cdaw-6 Substorm

The thinning of the near-Earth current sheet during the growth phase of the CDAW 6 magnetospheric substorm is studied. The expansion onset of the substorm occurred at 1054 UT, March 22, 1979. During the growth phase, two spacecraft, ISEE 1 and ISEE 2, were within the current sheet approximately 13RE from the Earth and obtained simultaneous high-resolution magnetic data at two points in the current sheet. Plasma data were also provided by the ISEE spacecraft and solar wind data 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 and below the neutral sheet and the new y axis lies in the GSM equatorial plane. A model based on the assumption that the current sheet is a time-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 parameter corresponding to the thickness of the current sheet decreased exponentially from about 5RE to 1RE with a time constant of about 14 min. In addition, the ISEE 1 and 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 substorm growth phase and expansion onset.

A statistical study of transient events in the outer dayside magnetosphere

The AMPTE CCE satellite frequently observed transient (1 ≤ τ ≤ 8 min) events marked by magnetic field strength increases and bipolar magnetic field signatures (peak-to-peak 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.

Open‐ended coaxial‐line technique for the measurement of the microwave dielectric constant for low‐loss solids and liquids

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...

A statistical study of the magnetic signatures of FTEs near the dayside magnetopause

During magnetopause crossings, the AMPTE CCE satellite frequently observed flux transfer events (FTEs) characterized by fluctuations in the magnetic field strength (B) and bipolar signatures in the field component (BN) normal to the nominal magnetopause. In this study, we survey 110 events observed from October to December 1984 and during January 1986. Nearly all events exhibited increases in B, and although the majority of events exhibited a symmetric bipolar signature in BN, a significant number (31 of 110) had asymmetric bipolar signatures in which the trailing pulse was dominant. Most of the asymmetric events were observed near the magnetic equator. This is consistent with an explanation in which FTEs form via merging along a single subsolar X line with strongly asymmetric signatures but that these signatures evolve into the familiar symmetric bipolar form with distance from the merging line.

A statistical study of transient event motion at geosynchronous orbit

The geosynchronous GOES 5 and GOES 6 satellites frequently observe transient events marked by magnetic field strength increases and bipolar magnetic field signatures lasting several minutes. In this study we report a survey of 87 events observed simultaneously by both GOES spacecraft (for a total of 174 individual observations) from August to December 1984. Events detected in the prenoon sector outnumbered those in the postnoon sector by about a 3 to 1 ratio. The distribution of the events versus local time exhibited a significant prenoon peak like the distribution of magnetic impulse events observed in high-latitude ground magnetometers. A cross-correlation analysis of the two GOES data sets indicated lags that range from 0 to over 2 min, with the majority of the events moving antisunward. The short lags correspond to azimuthal speeds of hundreds of kilometers per second, greater than flow speeds in the magnetosheath, but less than fast mode waves. The short lags may indicate that the events move primarily latitudinally and/or that transient events are seldom localized, but rather occur over extended, if not global, regions. Investigations of event occurrence versus interplanetary magnetic field (IMF) Bz, event motion versus IMF By, and correspondence between upstream plasma data and the events all indicate that pressure pulses are the likely source of many of the events. About 27% of the events with simultaneous solar wind data were preceded by sharp reversals in one or more IMF components, and nearly all of this particular group of events occurred in the dawn sector. This suggests that the pressure pulses may be commonly generated in the foreshock/bow shock region, since the prenoon magnetopause lies generally behind the quasi-parallel bow shock where such pulses are thought to be triggered by IMF discontinuities. Finally, several events in the data set were also observed by the AMPTE/CCE. These are presented as case studies.

Measurement of the microwave dielectric constant for low‐loss samples with finite thickness using open‐ended coaxial‐line probes

This work addresses the effect of finite sample thickness on microwave dielectric constant measurements for thin, planar, low‐loss samples using the open‐ended coaxial‐line probe method. Detailed measurements of the dielectric constant were carried out on a wide range of thicknesses of air samples which were backed by infinitely thick teflon and alumina dielectric media. The measurements were made at room temperature for various (50 Ω) coaxial‐line dimensions, microwave frequencies 4–8 GHz, and power levels near a fraction of a mW. The results provide strong support for previously published theoretical calculations based on a boundary value problem which uses a spectral domain formulation for the aperture fields. From thin, planar samples, values of 10.4±0.5 and 25.9±1.3 were obtained at 5 GHz and 300 K for the bulk dielectric constant of MgO and LaAl2O3, respectively. The applicability of a simple empirical model based on an exponential fit is discussed.

Beyond the Point Charge: Equipotential Surfaces and Electric Fields of Various Charge Configurations

A laboratory experiment often performed in an introductory electricity and magnetism course involves the mapping of equipotential lines on a conductive sheet between two objects at different potentials. In this article, we describe how we have expanded this experiment so that it can be used to illustrate the electrostatic properties of conductors. Different configurations of electrodes can be used to show that the electric field is zero inside a conductor as well as within a cavity, the electric field is perpendicular to conducting surfaces, and the charge distribution on conducting surfaces can vary.