Feasibility of hydromagnetic wave measurements on space shuttle

The feasibility of using a hydromagnetic wave sensor on the space shuttles was investigated. It was found that although existing sensors are inadequate in terms of resolution, dynamic range, and frequency range, they can be modified to make the necessary measurements. It is shown that since the sensor cannot be mounted on the shuttle itself because of high levels of magnetic noise, a free subsatellite that can be positioned and stabilized may be used for locating the hydromagnetic wave sensor. Other results show that studies of long period waves would require either an array of sensors in shuttle orbit or a long-term mapping of the crustal anomalies, and that effective wave studies would require at least two variably spaced sensors in shuttle orbit and one ground station

Magnetotail changes in relation to the solar wind magnetic field and magnetospheric substorms

An attempt is made to understand some of the magnetotail dynamics by using simultaneous observations from several satellites: Explorers 33 and 35 in the solar wind, IMP 4 in the near magnetotail (30 RE), ATS 1, and OGO 5 in the magnetosphere. It was observed that in the main lobes of the tail the magnetic field increases slowly when the interplanetary magnetic field turns southward, and can decrease slowly after a substorm. The plasma sheet changes indicate a thinning when the interplanetary magnetic field turns southward and an expansion when it turns northward. When combined with the plasma sheet expansion, which has been observed to follow a substorm, these results allow a schematic view of the relations between the changes in the orientation of the solar wind magnetic field, the substorms, and the changes in the tail parameters to be developed

Relative timing of substorm onset phenomena

[1] In this paper we examine the temporal ordering of midtail flow bursts, Pi2 pulsations, and auroral arc brightening at substorm onset. We present three substorm events for which the Geotail spacecraft was situated at local midnight, near the inner edge of the plasmasheet. We show that high-speed, convective Earthward directed plasma flows observed by Geotail occurred 1–3 min before auroral onset as observed by the Polar Visible Imaging System and Ultraviolet Imager auroral imagers on board the Polar spacecraft. We also show that the onsets of both nightside Pi2 pulsations and magnetic bay variations were simultaneous with auroral onset. We argue that these observations lend strong support to the flow burst-driven model of magnetotail dynamics. We also examine a high-latitude magnetic precursor to onset and show that it is likely due to the currents expected from the passage of a flow burst through the plasmasheet prior to substorm onset. Finally, we calculate an analytic expression for this current and show that it is unlikely to generate discrete auroral structures

A procedure for accurate calibration of the orientation of the three sensors in a vector magnetometer

Procedures are described for the calibration of a vector magnetometer of high absolute accuracy. It is assumed that the calibration will be performed in the magnetic test facility of Goddard Space Flight Center (GSFC). The first main section of the report describes the test equipment and facility calibrations required. The second presents procedures for calibrating individual sensors. The third discusses the calibration of the sensor assembly. In a final section recommendations are made to GSFC for modification of the test facility required to carry out the calibration procedures

Multiple-satellite studies of magnetospheric substorms: Plasma sheet recovery and the poleward leap of auroral-zone activity

Particle observations from pairs of satellites (Ogo 5, Vela 4A and 5B, Imp 3) during the recovery of plasma sheet thickness late in substorms were examined. Six of the nine events occurred within about 5 min in locations near the estimated position of the neutral sheet, but over wide ranges of east-west and radial separations. The time of occurrence and spatial extent of the recovery were related to the onset (defined by ground Pi 2 pulsations) and approximate location (estimated from ground mid-latitude magnetic signatures) of substorm expansions. It was found that the plasma sheet recovery occurred 10 - 30 min after the last in a series of Pi bursts, which were interpreted to indicate that the recovery was not due directly to a late, high latitude substorm expansion. The recovery was also observed to occur after the substorm current wedge had moved into the evening sector and to extend far to the east of the center of the last preceding substorm expansion

Modeling Harris Current Sheets with Themis Observations

Current sheets are ubiquitous in nature. occurring in such varied locations as the solar atmosphere. the heliosphere, and the Earth's magnetosphere. The simplest current sheet is the one-dimensional Harris neutral sheet, with the lobe field strength and scale-height the only free parameters. Despite its simplicity, confirmation of the Harris sheet as a reasonable description of the Earth's current sheet has remained elusive. In early 2009 the orbits of the 5 THEMIS probes fortuitously aligned such that profiles of the Earth's current sheet could be modeled in a time dependent manner. For the few hours of alignment we have calculated the time history of the current sheet parameters (scale height and current) in the near-Earth region. during both quiet and active times. For one particular substorm. we further demonstrate good quantitative agreement with the diversion of cross tail current inferred from the Harris modeling with the ionospheric current inferred from ground magnetometer data

Evaluation of the Tail Current Contribution to \u3cem\u3eDst\u3c/em\u3e

The Dst index is produced using low-latitude ground magnetic field measurements and frequently is used as an estimate of the energy density of the ring current carried mainly by energetic (∼10-200 keV) ions relatively close to the Earth. However, other magnetospheric current systems can cause field perturbations at the Earth\u27s surface: for example, dayside magnetopause currents are known to contribute to the Dst index. It has also been suggested that the nightside tail current sheet can significantly affect the Dst index during high magnetic activity periods when the currents are intense and flow relatively close to the Earth. In this study, several disturbed periods are input into Tsyganenko magnetic field models. From the time series of the external and internal fields an artificial Dst index is computed using the same procedure followed in the actual Dst calculation. A tail region in the magnetosphere is explicitly defined and the T96 and T89 models are used to calculate the effect of current within this tail region on ground measurements and therefore on Dst. The results are then compared with the measured Dst to determine the tail current contribution to Dst. It is found that for a geomagnetic storm and a storm-time substorm with Dst of ~80 nT the tail current contribution is between 22 and 26 nT. The same analysis is also applied to several isolated non-storm-time substorms, yielding a nearly linear relationship between Dst and the tail current contribution. This contribution is approximately one quarter of Dst

Reply to Comment on Evaluation of the Tail Current Contribution to \u3cem\u3eDst\u3c/em\u3e

Turner et al. [2000] analyzed the contribution of cross-tail currents to the Dst index. In order to estimate this contribution we used modified versions of the Tsyganenko models which had been adjusted to match spacecraft data in the tail, and we isolated a tail region and calculated its influence. We concluded that the tail currents were responsible for around 25% of the Dst response during moderately disturbed times. Maltsev and Ostapenko [2002] conclude that our estimate was low by a factor of 2, owing to that fact that we neglected dayside currents and that the model we used systematically underestimates the cross-tail current system. We appreciate their insightful analysis of our work, but we disagree with their conclusions. The models we used were modified to match spacecraft data in the tail, so we do not feel they underestimate the tail currents, and we consider the tail currents to be primarily located in the magnetotail, so we feel our decision to neglect dayside currents was justified. Additionally, we feel that some of the discrepancies between our results and theirs are due to different definitions of tail and ring currents and our decisions on whether to include the induced ground current contribution in our estimates of the tail current contribution to Dst. Here we respond briefly to their arguments and conclude that we still find the approximate magnitude of the tail current contribution to Dst to be around 25%. Additionally, Maltsev and Ostapenko include their own analysis of the tail current contribution to Dst, but we will limit our response to those comments which directly relate to our work

Magnetospheric Response to Solar Wind Variations

The time lagged response of the magnetosphere to solar wind variations has been determined using the linear prediction filtering method and 34 intervals of high time resolution IMP-8 solar wind data and auroral electrojet AL index data. The linear prediction filtering method is a powerful time series analysis technique which is utilized to produce a filter of time lagged response coefficients which estimates the most general linear relationship between magnetospheric activity and solar wind variations. This study uses the AL index to monitor the magnetosphere's response and VB/sub s/ to monitor the solar wind input. Before analysis, the median value of the AL index for each of the 34 intervals was utilized to rank the intervals according to the level of geomagnetic activity. It is found that the VB/sub s/-AL filters are composed of two response pulses peaking at time lags of 20-minutes and 60-minutes. Our interpretation associates the 20-minute pulse with activity driven directly by solar wind-magnetosphere interaction and it associates the 60-minute pulse with activity driven by the release of stored energy from the magnetotail. Thus, the filter results suggest that both the directly driven and the unloading models of magnetospheric response are important in describing the time lagged response of the magnetosphere to solar wind variations. 11 refs., 3 figs