L. A. Weiss

A new class of forward‐reverse shock pairs in the solar wind

A new class of forward-reverse shock pairs in the solar wind has been discovered using Ulysses observations at high heliographic latitudes. These shock pairs are produced by expansion of coronal mass ejections, CMEs, that have internal pressures that are higher than, and speeds that are comparable to, that of the surrounding solar wind plasma. Of six certain CMEs observed poleward of S31°, three have associated shock pairs of this nature. We suggest that high internal CME pressures may exist primarily for events that have high speeds close to the surface of the Sun.

Evolution of plasmaspheric ions at geosynchronous orbit during times of high geomagnetic activity

The evolution of the plasmasphere, the region of relatively dense cold plasma surrounding the Earth, is strongly dependent on magnetospheric activity. Here we report on plasmaspheric evolution as observed at geosynchronous orbit in association with magnetopause crossings and storm sudden commencements (SSCs). The occurrence frequency distributions at geosynchronous orbit of both magnetopause-associated and SSC-associated plasmaspheric ions is peaked near 1400 LT, with an overall range from 1000 LT to beyond 1800 LT. This is greatly skewed from the average plasmaspheric distribution at 6.6 RE, which peaks closer to 1800 LT. The evolution of SSC-associated plasmaspheric ions is tracked using a superposed epoch analysis: lower-activity SSCs produce minor changes from the pre-SSC local time distribution; after geomagnetically-effective SSCs, the ions appear almost immediately at earlier local times, spanning the late morning to dusk local time sector for hours. These observations are consistent with (1) a push of plasmaspheric material inward over the spacecraft due to magnetospheric compression and (2) the prompt penetration of a convection electric field.

An observational test of the Tsyganenko (T89a) model of the magnetospheric field

The range of magnetic field tilt angles observed at several satellites in geosynchronous orbit is compared with the ranges predicted for the same locations by the different Kp parameterizations of the Tsyganenko 1989a magnetic field model as a function of local time and season. The data are examined separately for satellite locations near the magnetic equator and slightly off the equator. The model predicts reasonably well the observed basic variation in the tilt angle with location, and it permits a range of field inclinations adequate to encompass the majority of the observed angles for the dawn, dusk, and night quadrants. On the day side of the magnetosphere the model exhibits very little variation in tilt over the entire range of parameterizations and cannot reproduce the observed range of tilt angles. Near the magnetic equator the majority of observed tilt angles lie within the model range, with roughly equal numbers of cases that are overstretched or understretched with respect to the model range. Off the equator the models tend to be more stretched than is generally observed. With some modest season-to-season differences these results are valid for all four seasons.

Global modeling of the plasmasphere following storm sudden commencements

We examine the dynamics of the outer plasmasphere during 10 post-ssc events by comparing observations of cold, dense ions from Los Alamos magnetospheric plasma analyzers on board three widely spaced geosynchronous satellites with output from the Magnetospheric Specification and Forecast Model (MSFM). The MSFM is a data-driven, operational space weather specification and forecast code originally designed to facilitate U.S. Air Force spacecraft operations. For this study we modified the MSFM to include a cold plasmaspheric ion population that was subject to the effects of ionospheric refilling. We utilized the electron density model of Carpenter and Anderson [1992] and the assumption of charge neutrality to initialize the plasmaspheric proton density within a specified plasmapause. This configuration was then allowed to evolve under the effects of E × B drift and refilling. The modified MSFM clearly shows the development and westward transport of duskside plasmaspheric plumes/tails during periods of enhanced convection and the eastward transport of these structures during decreasing activity. We present a detailed comparison between the data and the model output for one case and a “statistical” analysis of the comparison for all 10 cases. We also compare the model results with previously published observations of plasmapsheric ions and models of plasmaspheric dynamics. The MSFM was able to systematically reproduce the geosynchronous observations with good accuracy in both local time placement and density level. We find the modified MSFM to be an improvement upon previous plasmaspheric models and a useful tool in the interpretation of spatially and temporally separated geosynchronous observations.

Observational determination of magnetic connectivity of the geosynchronous region of the magnetosphere to the auroral oval

This is a report of a program to study the magnetic connectivity between the auroral region of the ionosphere and the equatorial geosynchronous region of the magnetosphere. The program uses plasma measurements made with polar orbiting Defense Meteorological Satellite Program (DMSP) satellites and several geosynchronous satellites, seeking time intervals when nearly identical plasma electron spectra (32 eV to 30 keV) indicate magnetic connectivity between a polar/geosynchronous satellite pair. When such signatures of connectivity are found, the locations of the relevant satellite pair are compared with the locations that would be predicted by a magnetospheric model. Here we report results from the initial application of this program that uses data from DMSP F-8, F-9 and F-10 polar satellites and the synchronous satellites 1989-046 and 1990-095 acquired in the 6-day interval March 7–12, 1991. The results are compared with predictions of the T89a model [Tsyganenko, 1989; Peredo et al., 1993]. Orbital calculations predicted 96 close conjunctions among these satellites during the interval. Of those we have made spectral comparisons for 47, finding 20 for which close spectral similarity occurred during few-second intervals. Ionospheric footpoints of the satellite pairs calculated by the T89a model for these intervals revealed eight for which the discrepancy between the DMSP latitude at “best spectral match” and the model projection of the synchronous satellite was smaller than 1°. However, there were five intervals for which the discrepancy was greater than 6°. The latter were all found in the local time interval 0700–0900, perhaps indicating a particular inadequacy of the T89a model in that region. This initial work also supports the view that the auroral oval of discrete arcs is an ionospheric mapping of the full thickness of the plasma sheet (e.g., Feldstein and Galperin, 1985) and not just of the plasma sheet boundary layer (e.g., Eastman et. al., 1985). During just its first few years of concurrent operation this constellation of well-instrumented synchronous and polar-orbiting satellites has provided data for literally thousands of close conjunctions such as we analyze in this report. Thus an algorithm is developed enabling the verification and testing of proposed mappings between the ionosphere and magnetosphere (such as T89a).

The appearance of plasmaspheric plasma in the outer magnetosphere in association with the substorm growth phase

Short intrevals ( 10 cm{sup {minus}3}), cold (< 10 eV) plasmaspheric plasma are observed at synchronous orbit in association with the growth phase of geomagnetic substorms. The authors interpret the strong correlation between these phenomena in terms of the reconfiguration of the duskside/nightside magnetosphere during the growth phase of geomagnetic substorms. This reconfiguration moves plasmapheric plasma outward to synchronous orbit, occasionally even in the midnight region. 24 refs., 4 fig.

A quantitative test of different magnetic field models using conjunctions between DMSP and geosynchronous orbit

We report here on a study which tests the magnetic field line mapping between geosynchronous orbit and the ionosphere. The mapping is determined both observationally and fiom five magnetospheric magnetic field models. The mapping is tested observationally by comparing electron energy spectra obtained by the Magnetospheric Plasma Analyzer (MPA) at geosynchronous orbit and by the DMSP spacecraft. Because the orbits are nearly perpendicular, in general, the spectra match well for only a few seconds providing a good determination of when DMSP crosses the geosynchronous drift shell. In this way the mapping between geosynchronous orbit and the ionosphere can be determined to better than one degree. We then compare the measured magnetic footpoints of geosynchronous orbit with the footpoints predicted by five magnetospheric field models: Tsyganenko-89, Tsyganenko-87, Tsyganenko-82, Oslen-Pfitzer, and Hilmer-Voigt. Based on a set of over 100 measured magnetic conjunctions we find that, in general, there are significant differences between the mappings predicted by various magnetic field models but that there is no clear “winner” in predicting the observed mapping. We find that the range of magnetic latitudes at which we measure conjunctions is much broader than the range of latitudes which the models can accommodate. This lack of range is common to all magnetic field models tested. Although there are certainly cases where the models are not sufliciently stretched, we find that on average all magnetic field models tested are too stretched. This technique provides a n excellent opportunity for testing future magnetic field models and for determining the appropriate parameterizations for those models.

Long-term energetic-particle databases from geosynchronous and GPS orbits

The Los Alamos National Laboratory has flown thirteen energetic-particle instruments on geosynchronous satellites since 1976 and on seven GPS satellites since 1983. These instruments measure electrons and protons over a wide range of energies. The various instruments and the particles and energies that they measure are described. The measured fluxes are stored at Los Alamos in several databases that are available to outside users.

Measuring the magnetic connectivity of the geosynchronous region of the magnetosphere

This is the final report of a three-year, Laboratory-Directed Research and Development (LDEtD) project at the Los Alamos National Laboratory (LANL). The purpose of this project was to determine the magnetic connectivity of the geosynchronous region of the magnetosphere to the auroral zone in the polar ionosphere in order to test and refine current magnetospheric magnetic field models. We used plasma data from LANL instruments on three geosynchronous satellites and from USAF instruments on three low-altitude, polar-orbiting, DMSP satellites. Magnetic connectivity is tested by comparing plasma energy spectra at DMSP and geosynchronous satellites when they are in near conjunction. The times of closest conjugacy (Le., best spectral match) are used to evaluate the field models. We developed the tools for each step of the process and applied them to the study of a one-week test set of conjunctions. We automated the analysis tools and applied them to four months of twosatellite observations. This produced a database of about 130 definitive magnetic conjunctions. We compared this database with the predictions of the widely-used Tsyganenko magnetic field model and showed that in most cases one of the various parameterizations of the model could reproduce the observed field line connection. Further, we explored various measurables (e.g., magnetospheric activity indices or the geosynchronous field orientation) that might point to the appropriate parameterization of the model for these conjunctions, and ultimately, for arbitrary times. 1. Background and Research Objectives One of the most urgent problems of magnetospheric research is that of magnetic field line mapping between the low-altitude ionosphere and atmosphere and the high-altitude *Principal investigator, e-mail: mthomsen@lad.gov