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MPA-related ISTP Studies


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SCIENCE NUGGETS

  • Plasma Flow through the Magnetosphere
  • The Super Dense Plasmasheet
  • Testing Magnetic Field Models
  • Modeling the Outer Plasmasphere
  • Pressure gradients

  • Multiple-Satellite Measurements of Plasma Flow into and through the Earth's Magnetosphere

    NASA/ISTP and pre-ISTP multiple-satellite configurations were used by Los Alamos researchers to inter-compare the plasma in the solar wind with the plasma at various locations within the Earth's magnetosphere. Strong connections were found between the solar wind and the magnetospheric plasmas and information was obtained about the timescales for the entry of material into the magnetosphere and for the movement of material through the magnetosphere.

    As the plasma wind from the sun blows past the Earth's magnetic field, the magnetic field is distorted into a long magnetotail, much like a wind sock. The long magnetotail plus the region near the Earth are known as the magnetosphere. This magnetosphere shields the Earth from the direct solar wind, but some material from the solar wind leaks into the magnetosphere. Using the coordinated measurements from nine satellites, the leakage of this solar-wind material into and through the magnetosphere was monitored. The movement of material was detected statistically on a case-by-case basis.

    These studies found that, as suspected, the flow of plasma through the magnetosphere is backward; i.e. wind material appears to enter the magnetosphere in the magnetotail and the material is later exhausted from the magnetosphere on the upwind end. This backward movement is owed to the complex electrical interaction between the solar wind and the Earth's magnetic field. The studies determined that material flows quite rapidly through the magnetosphere. In these studies the movement of material between pairs of satellites was timed and transport speeds exceeding 10 km/sec (23,000 miles/hr) were found within the Earth's magnetosphere. It was observed that in four hours or less solar-wind plasma can reach deep within the magnetosphere to geosynchronous orbit, where most of mankind's communications satellites reside. This plasma interacts with spacecraft there to cause a number of electronic problems. This diagram schematically illustrates the sunward transport and approximate travel times of plasma through the magnetosphere.

    This multiple-satellite NASA-sponsored study is providing direct observations of solar-wind material moving through the Earth's magnetosphere, providing information about the timescales for the entry and the transport of this material, and providing information about the pathways by which this material moves.

    Contact at LANL: Joe Borovsky (505)667-8368 jborovsky@lanl.gov or Michelle Thomsen (505)667-1210 mthomsen@lanl.gov


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    The Superdense Plasma Sheet as the Source of the Ring Current during Geomagnetic Storms

    Synergistically using computer calculations and multi-satellite observations, researchers have uncovered a new piece to the puzzle of how geomagnetic storms happen.

    A geomagnetic storm occurs when the solar wind drives large amounts of energy into the Earth's magnetosphere. A storm may last one or two days. During a severe storm communications can be disrupted, satellite electronics can be damaged, electrical-power grids can be disabled, and the Earth's radiation belts can build up. The Dst index, which is constructed from measurements made by ground-based magnetic-field detectors, is a primary indicator of the magnitude of a storm. The Dst index measures the perturbation of the Earth's magnetic field caused by the ring current that builds up in space in the Earth's dipole field.

    Researchers at the University of Michigan have developed specialized computer models to study the growth and decay of the ring current. As input information, these computer models use measurements from satellites located at geosynchronous orbit. Geosynchronous orbit is on the outer edge of the ring current, so satellites located there are able to measure and monitor the plasma (ionized matter) moving into that region. Using these models, the University of Michigan researchers have determined that the superdense plasma that appears at the outer edge of the ring current in the early phases of a storm is an important ingredient for making a strong ring current and a strong Dst signature.

    The plasma in the outer portion of the dipole is part of a large structure known as the plasma sheet. The plasma sheet extends from near the Earth (geosynchronous orbit) to about half a million miles down the Earth's magnetotail; the volume of the plasma sheet can be 5000 times the volume of the Earth. Researchers examining data from satellites in the magnetotail and at geosynchronous orbit recently discovered that the plasma sheet can occasionally be superdense (having densities several times larger than normal). Using the NASA-ISTP configuration of satellites, researchers at Los Alamos have determined that the origin of the anomalously high densities of the plasma sheet is high-density solar wind.

    Putting these discoveries together, researchers conclude that during a storm high-density solar wind produces a superdense plasma sheet in the magnetotail, which moves into the dipole to produce a superdense plasma sheet there. Then the strong driving of the magnetosphere by the solar wind energizes the superdense-plasma-sheet material to create a strong ring current, which produces the large Dst signature of a storm. The whole chain of events, illustrated in this figure, comes to pass in a few hours.

    This result is surprising in that the solar-wind driving of the Earth's magnetosphere is not purely electromagnetic: the high density of the solar wind is necessary to produce a strong storm-time ring current. This means that the density of the solar wind plays a role in how "geoeffective" the wind is.

    Contact at the University of Michigan: Janet Kozyra (313)747-3550 jukozyra@umich.edu
    Contact LANL: Joe Borovsky (505)667-8368 jborovsky@lanl.gov or Michelle Thomsen (505)667-1210 mthomsen@lanl.gov

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    Observational testing of magnetospheric magnetic field models at geosynchronous orbit

    The magnetic field configuration linking the ionosphere and the near-Earth magnetosphere is a crucial element of many magnetospheric and space weather studies. In particular, ISTP studies often rely on global magnetospheric magnetic field models to relate satellite measurements from different regions of the magnetosphere. Because of the obvious value of such field models and because of the wide variety of applications for which they are being used, it is extremely important that they be tested quantitatively so that we better understand 1) the magnetospheric conditions under which the models work well (or poorly); 2) how the models differ from each other (and therefore which might be most appropriate for a particular task); and 3) how they might be improved.

    We have undertaken a series of studies aimed at evaluating a number of widely used and readily accessible magnetospheric magnetic field models. In these studies we have used measurements from several satellites at geosynchronous orbit and in low-Earth orbit to pursue two different approaches:

    1) In the first approach, using magnetic field directions determined from the symmetry axis of the observed electron distributions, a large statistical comparison has been made between the range of observed magnetic field tilt angles and the ranges predicted for the same locations by various parameterizations of the Tsyganenko 1989a (T89a) and the Hilmer-Voigt (HV) magnetic field models as a function of local time and season. We found [Thomsen et al., 1996a] that the T89a 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 most of the geosynchronous satellite locations. Off the magnetic equator the T89a models tend to be more stretched than is generally observed.

    A similar study of the HV magnetic field model [Thomsen et al., 1996b] suggests that during certain seasons the hinge point for the model tail current sheet lies too far away from the earth so that the model places a near-equatorial satellite on the wrong side of the current sheet. An activity-dependent hinging distance is currently being incorporated into the HV model, which is one of the core components of the Rice Magnetospheric Specification Model.

    2) In the second approach, we evaluate a model by comparing the observed magnetic connection between two widely-spaced satellites with the model's prediction. Using plasma electron measurements from two geosynchronous satellites and three low-altitude DMSP satellites carrying similar plasma instrumentation, we have compiled a database of 138 magnetic field conjunctions between pairs of these satellites. The conjunctions were determined using an automated spectral comparison and selection technique which allowed the identification of definitive intervals of close match between the electron distributions at the two satellites. This database, covering a wide range of magnetospheric activity, local times, and season, reveals that Kp is not a reliable indicator of which of the T89a stretching levels best reproduces an observed conjunction [Hones et al., 1996; Weiss et al., 1997]. Moreover, comparison of the observed and model mappings shows that in over half the cases the observed mappings fell completely outside the stretching range of T89a, indicating that the real field may at times be considerably more distorted than allowed for in that model. Reeves et al. [1997] assessed five different magnetic field models with the same database and found that they all typically predicted a too-stretched configuration at geosynchronous orbit. These results suggest that future models should allow a greater range of field line stretching, including less-stretched configurations. Finally, in investigating how well the observed degree of field stretch was ordered by various magnetospheric indices, we found that both the tilt of the field at geosynchronous orbit and the equatorward edge of the diffuse aurora showed a strong correlation with the degree of stretching needed to reproduce the conjunctions. Indices based on these parameters are thus promising candidates for incorporation into future models [Weiss et al., 1997].

    Hones, E. W., M. F. Thomsen, G. D. Reeves, L. A. Weiss, and D. J. McComas, Observational determination of magnetic connectivity of the geosynchronous region of the magnetosphere to the auroral oval, J. Geophys. Res., 101, 2629, 1996.
    Reeves, G. D., L. A. Weiss, M. F. Thomsen, and D. J. McComas, A quantitative test of different magnetic field models using conjunctions between DMSP and geosynchronous orbit, in Radiation Belt Models & Standards, AGU Monograph 97, edited by J. F. Lemaire, D. Heynderickx, and D. N. Baker, pp. 167-172, 1997.

    Thomsen, M. F., D. J. McComas, G. D. Reeves, and L. A. Weiss, An Observational Test of the Tsyganenko (T89a) Model of the Magnetospheric Field, J. Geophys. Res., 101, 24,827, 1996a.
    Thomsen, M. F., L. A. Weiss, D. J. McComas, G. D. Reeves, and R. Hilmer, An Observational Test of the Hilmer-Voigt Magnetic Field Model at Geosynchronous Orbit, presentation to 1996 Fall AGU meeting, San Francisco, 1996b.
    Weiss, L. A., M. F. Thomsen, G. D. Reeves, and D. J. McComas, An examination of the Tsyganenko (T89a) field model using a database of two-satellite magnetic conjunctions, in press, J. Geophys. Res., 1997.

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    Modeling the evolution of the outer plasmasphere

    New observations of plasmaspheric ions in the post-noon sector using multiple ISTP geosynchronous satellites show that these ions are seen at progressively earlier local times following periods of enhanced convection following sudden commencements (SCs) [Elphic et al., 1996]. We have examined the dynamics of the outer plasmasphere during a 3-day interval in February, 1992 by comparing observations from the Los Alamos magnetospheric plasma analyzers (MPAs) onboard these satellites with the predictions of the Magnetospheric Specification and Forecast Model (MSFM) [Weiss et al., 1997]. We modified the MSFM to include a cold, plasmaspheric population subject to the effects of corotation, convection, and ionospheric refilling. Unlike previous simulations, the MSFM models the dynamics of the outer plasmasphere using electric and magnetic field models which are adjusted by actual input data. Because the field models are responsive to changes in magnetospheric activity on time scales of 15-30 minutes, the MSFM can be used to model specific events, and its predictions can be compared to the multi-satellite MPA observations. The model does a very good job of reproducing the geosynchronous cold ion observations; in particular, it clearly shows the formation and westward transport of duskside plasmaspheric plumes during convection enhancements, supporting Elphic et al.'s [1996] conclusion that entrained plasma tails account for the earlier local time shift of geosynchronous cold ion encounters following SCs. The MSFM's ability to model dynamically the evolution of the outer plasmasphere under varying solar wind conditions improves upon previous plasmaspheric models and has aided our understanding of how the outer layer of the plasmasphere is stripped away during enhanced convection and evacuated from the dayside magnetosphere.

    Elphic, R. C., L. A. Weiss, M. F. Thomsen, D. J. McComas and M. B. Moldwin, Evolution of plasmaspheric ions at geosynchronous orbit during time of high geomagnetic activity, Geophys. Res. Lett., 23, 2189, 1996.

    Weiss, L. A., R. L. Lambour, R. C. Elphic, and M. F. Thomsen, Study of plasmaspheric evolution using geosynchronous obserations and global modeling, in press, Geophys. Res. Lett., 1997.

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    Azimuthal pressure gradients at geosynchronous orbit

    Since late fall of 1995, two of the three geosynchronous satellites carrying Los Alamos plasma and energetic particle instruments have been separated by only 2.3 hours of local time. We are using this advantageous configuration to examine the azimuthal gradient of the particle pressure at geosynchronous orbit as a function of local time and substorm phase [Weiss et al., 1996]. The ion pressures are computed using both MPA (100 eV - 40 keV) and SOPA (50 keV - 670 keV) contributions. The primary objectives of this work are to determine 1) the "average" cross-tail pressure gradient using simultaneous ion measurements from the two satellites; and 2) the dependence of the azimuthal pressure gradient on substorm phase -- in particular, to assess the Lyons [1995] theory that a pressure minimum near midnight (due to a rapid decrease in the magnetospheric electric field) triggers the expansion phase.

    Our initial study using one month of two-satellite gradient measurements revealed: 1) that the ion pressure typically peaks near local midnight; 2) on the dawnside, the observed pressure gradient has the same magnitude (~2 nPa/km) and direction as the statistical cross-tail pressure gradient measured by ISEE 1 and predicted by the Spence and Kivelson magnetotail convection model (~3 nPa/km); 3) on the duskside, the background pressure gradient is opposite to what is frequently assumed or what has been suggested by some models (e.g., Spence and Kivelson); and 4) the background pressure gradient should act to produce field-aligned currents in the same sense as the Region 2 currents.

    Using two substorm onsets occurring when the satellites were centered at different local times (24 and 03 LT) we found: 1) the post-onset pressure gradients are dominated by the SOPA-energy injections of particles and, in particular, the position and subsequent motion of the injection region with respect to the satellites. The delay in one of the events of the ion pressure peak from one satellite to the other is consistent with the suggestion of displaced ion and electron injection fronts [e.g., Reeves et al., 1991; Birn et al., 1996]; 2) the growth-phase pressure gradients are dominated by the MPA-energy particles. In both events, the growth-phase pressure gradients were westward, thus increasing the pre-existing westward pressure gradient on the dawnside; and 3) a growth-phase westward pressure gradient in the post-midnight region is in the opposite direction of that predicted by the Lyons substorm theory.

    Birn, J., M. F. Thomsen, J. E. Borovsky, G. D. Reeves, D. J. McComas, and R. D. Belian, Characteristic plasma properties during dispersionless substorm injection at geosynchronous orbit, in press, J. Geophys. Res., 1997.

    Lyons, L. R., A new theory for magnetospheric substorms, J. Geophys. Res., 100. 19069, 1995.

    Reeves, G. D., R. D. Belian, and T. A. Fritz, Numerical tracing of energetic particle drifts in a model magnetosphere, J. Geophys. Res., 96. 13997, 1991.

    Weiss, L. A., G. D. Reeves, M. F. Thomsen, J. E. Borovsky, J. Birn, and D. J. McComas, Azimuthal pressure gradients at geosynchronous orbit, presented, Fall meeting, American Geophysical Union, Dec., 1996.

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