Mark B. Moldwin

An empirical plasmasphere and trough density model: CRRES observations

Combined Release and Radiation Effects Satellite (CRRES) sweep frequency receiver data were used to develop an empirical model of the plasmasphere and trough number density. The over 1000 CRRES orbits provided good statistical coverage of all local times between an L shell of 3 to 7. The CRRES density data were separated into plasmaspheric-like and trough-like by assuming a minimum density value for the plasmasphere as a function of L shell. For the plasmasphere the average number density (in cm−3) as a function of L shell (3 ≤ L ≤ 7) was found to be: np = 1390 (3/L)4.8 ± 440 (3/L)3.6. For the trough the average number density (in cm−3) as a function of L-shell (3 ≤ L ≤ 7) and magnetic local time (0 ≤ LT ≤ 24) was found to be nt = l24 (3/L)4.0 + 36(3/L)3.5 cos({LT-[7.7(3/L)2.0+12]}π/12) ± {78 (3/L)4.7 + 17 (3/L)3.7 cos[(LT - 22)π/12]}. No clear dependence on magnetic activity was found for either density model. This empirical model is an improvement over earlier models in that it is continuous in local time and can be used to track densities based on refilling history. The model standard deviations are representative of either early time or late time refilling of the trough or newly filled or saturated plasmaspheric densities.

Geotail observations of magnetic flux ropes in the plasma sheet

[1] Examination of Geotail measurements in the near-tail (X > � 30 RE) has revealed the presence of small flux ropes in the plasma sheet. A total of 73 flux rope events were identified in the Geotail magnetic field measurements between November 1998 and April 1999. This corresponds to an estimated occurrence frequency of � 1 flux rope per 5 hours of central plasma sheet observing time. All of the flux ropes were embedded within high-speed plasma sheet flows with 35 directed Earthward, hVxi = 431 km/s, and 38 moving tailward, hVxi = � 451 km/s. We refer to these two populations as ‘‘BBF-type’’ and ‘‘plasmoid-type’’ flux ropes. The flux ropes were usually several tens of seconds in duration, and the two types were readily distinguished by the sense of their quasisinusoidal Bz perturbations, i.e., � for the ‘‘BBF’’ events and ± for the ‘‘plasmoid’’ events. Most typically, a flux rope was observed to closely follow the onset of a high-speed flow within � 1–2 min. Application of the Lepping-Burlaga constant-a flux rope model (i.e., J = aB) to these events showed that approximately 60% of each class could be acceptably described as cylindrical, force-free flux ropes. The modeling results yielded mean flux rope diameters and core field intensities of 1.4 RE and 20 nT and 4.4 RE and 14 nT for the BBF and plasmoid-type events, respectively. The inclinations of the flux ropes were small relative to the GSM X–Y plane, but a wide range of azimuthal orientations were determined within that plane. The frequent presence of these flux ropes in the plasma sheet is interpreted as strong evidence for multiple reconnection X-lines (MRX) in the near-tail. Hence, our results suggest that reconnection in the near-tail may closely resemble that at the dayside magnetopause where MRX reconnection has been hypothesized to be responsible for the generation of flux transfer events. INDEX TERMS: 2740 Magnetospheric Physics: Magnetospheric configuration and dynamics; 2764 Magnetospheric Physics: Plasma sheet; 2744 Magnetospheric Physics: Magnetotail; 2788 Magnetospheric Physics: Storms and substorms

Magnetospheric plasma analyzer: Initial three‐spacecraft observations from geosynchronous orbit

The first three magnetospheric plasma analyzer (MPA) instruments have been returning data from geosynchronous orbit nearly continuously since late 1989, 1990, and 1991. These identical instruments provide for the first time simultaneous plasma observations from three widely spaced geosynchronous locations. The MPA instruments measure the three-dimensional velocity space distributions of both electrons and ions with energies between ∼1 eV/q and ∼40 keV/q. MPA capabilities and observations are summarized in this paper. We use the simultaneous observations from three longitudinally separated spacecraft to synthesize a synoptic view of the morphology of the magnetosphere at geosynchronous orbit over a 6-week interval in early 1992. The MPA observations indicate that the spacecraft encountered seven regions with characteristic plasma populations during this period: (1) the cool, dense plasmasphere (13.1% of the data); (2) a warmer, less dense plasma trough (22.5%); (3) the hot plasma sheet (40.3%); (4) a combination of plasma trough and plasma sheet (18.6%); (5) an empty trough region, devoid of plasma sheet, plasmasphere, or plasma trough populations (4.3%); (6) the magnetosheath and/or low-latitude boundary layer (0.7%); and (7) the lobe (0.3%). The local time distributions of these regions are examined. For example, as suggested by previous authors, we find that at geomagnetically quiet times (Kp < 2) geosynchronous orbit can lie entirely within the plasmasphere while at more active times only the afternoon to evening portions of the orbit are typically within the plasmasphere. We also find that the plasma convection inside the plasmasphere is generally sunward in the corotating (geosynchronous spacecraft) reference frame, independent of activity level, in contrast to previous studies. In addition to such statistical results, the simultaneous data sets at different local times allow us to at least partially separate spatial from temporal variations. In particular, we use these observations to examine the instantaneous shapes of the plasmapause and magnetopause as they pass over geosynchronous orbit. As expected, the plasmapause is found to have a highly variable shape, at various times showing (1) a stable dusk side bulge, (2) a variable bulge which expands, contracts, and moves, (3) an overall expansion and contraction of the plasmasphere, and (4) even more complicated behavior which is best accounted for by large-scale structure of the plasmapause and/or disconnected plasma blobs. During the 6 weeks of data examined, the magnetosheath was encountered on several occasions at synchronous orbit, preferentially on the prenoon side of the magnetosphere. For the first time, simultaneous prenoon and postnoon observations confirm this asymmetry and demonstrate that the magnetopause shape can be highly asymmetric about the Earth-Sun line.

Electron scattering by whistler‐mode ELF hiss in plasmaspheric plumes

Nonadiabatic loss processes of radiation belt energetic electrons include precipitation loss to the atmosphere due to pitch-angle scattering by various magnetospheric plasma wave modes. Here we consider electron precipitation loss due to pitch-angle scattering by whistler-mode ELF hiss in plasmaspheric plumes. Using wave observations and inferred plasma densities from the Plasma Wave Experiment on the Combined Release and Radiation Effects Satellite (CRRES), we analyze plume intervals for which well-determined hiss spectral intensities are available. We then select 14 representative plumes for detailed study, comprising 10 duskside plumes and 4 nonduskside plumes, with local hiss amplitudes ranging from maximum values of above 300 pT to minimum values of less than 1 pT. We estimate the electron loss timescale τ loss due to pitch-angle scattering by hiss in each chosen plume as a function of L-shell and electron energy; τ loss is calculated from quasi-linear theory as the inverse of the bounce-averaged diffusion rate evaluated at the equatorial loss cone angle. We find that pitch-angle scattering by hiss in plumes can be efficient for inducing precipitation loss of outer-zone electrons with energies throughout the range 100 keV to 1 MeV, though the magnitude of τ loss can be highly dependent on wave power, L-shell, and electron energy. For 100- to 200-keV electrons, typically τ loss ∼ 1 day while the minimum loss timescale (τ loss ) min ∼ hours. For 500-keV to 1-MeV electrons, typically (τ loss ) min ∼ days, while (τ loss ) min < 1 day in the case of large wave amplitude (∼100s pT). Apart from inducing direct precipitation loss of MeV electrons, scattering by hiss in plumes may reduce the generation of MeV electrons by depleting the lower energy electron seed population. Models of the dynamical variation of the outer-zone electron flux should incorporate electron precipitation loss induced by ELF hiss scattering in plasmaspheric plumes.

On the formation and evolution of plasmoids: A survey of ISEE 3 geotail data

ISEE 3 magnetometer and electron plasma measurements from the 1983 Geotail Mission were surveyed to determine the magnetic and plasma properties of plasmoids and their evolution with distance downtail. Events were selected on the basis of a bipolar magnetic signature in either the geocentric solar magnetospheric Bz and/or By component; most had Bz bipolar signatures. We found 366 events consistent with this signature while ISEE 3 was in the plasma sheet. ISEE 3 observed plasmoids all along its trajectory whenever it was in the plasma sheet. Plasmoids are characterized by high-speed plasma flow. Plasmoid length was determined using both the magnetometer and the electron plasma velocity data. We found the average length of plasmoids is 16.7 ± 13.0 RE, significantly smaller than previous estimates. Many plasmoids have a well-defined magnetic core field, characterized by a field strength maximum at the center of the pass through the structure. Plasmoids appear to be relatively stable structures once their formation process is complete. The size, velocity, magnetic core strength, and Bz field amplitude of plasmoids do not depend on distance beyond 100 RE downtail. The average electron temperature inside plasmoids drops by a factor of 2 and the electron density increases by a factor of 2 as plasmoids propagate from near Earth distances (within 100 RE of the Earth) to the deep tail. We conclude that the stable size of the plasmoids, the density increase and the temperature decrease are consistent with a flux of cold electrons into the plasmoid. The strong correlation of interplanetary magnetic field By an hour before the event with the strength and direction of By observed inside plasmoids, the existence of events with the bipolar signature in both the By and Bz components, and the possible mass flux all are consistent with plasmoids being “open” magnetic structures.

Small‐scale magnetic flux ropes in the solar wind

Small-scale magnetic flux ropes have been discovered in the solar wind at 1 AU in observations from the IMP 8 and WIND spacecraft. These small magnetic structures (diameter of 270 RE, on average) have some similar properties to magnetic clouds (diameters of 0.2–0.3 AU or about 6000–8000 RE), which are well known large-scale magnetic flux ropes, but have durations of 10s of minutes as opposed to many hours or days for most magnetic clouds. The presence of these small helical field structures suggests that solar wind flux ropes may have a wide-range of scale sizes, or possibly have a bimodal size distribution, and are perhaps more common than previously estimated. Similarities and differences with magnetic clouds will be discussed. We suggest that these small scale magnetic flux ropes are signatures of magnetic reconnection in the solar wind as opposed to in the solar corona.

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 examination of the structure and dynamics of the outer plasmasphere using multiple geosynchronous satellites

The structure and the dynamics of the plasmaspheric bulge are examined using in situ three-dimensional plasma observations from magnetospheric plasma analyzers onboard multiple geosynchronous satellites. We identify the plasmasphere by the presence of high fluxes of low-energy (≈ few eV) ions (corresponding to densities of ≈10s up to ≈100 cm−3). The results from one year (1991) of nearly continuous plasma measurements from two longitudinally and latitudinally separated spacecraft are presented. This study corroborates many of the features and statistical behavior of the plasmaspheric bulge evidenced in past ground-based and single spacecraft data sets, except we often find a more complex outer plasmasphere than earlier studies have suggested. By using multipoint, simultaneous observations to separate spatial from temporal changes, this study extends previous examinations of the plasmasphere at synchronous orbit. We find that the width and location of the plasmaspheric bulge can differ significantly for the two spacecraft (separated by 6-8 hours in time), particularly during quiet geomagnetic conditions. The very different plasmaspheric morphologies seen by the two spacecraft lead us to conclude that the outer plasmasphere is often highly structured even during steady geomagnetic conditions and that the simple teardrop model of the bulge rarely, if ever, adequately describes the duskside plasmasphere.

Geomagnetic substorm association of plasmoids

The relationship of geomagnetic substorms and plasmoids is examined by determining the correlation of the 366 plasmoids identified by Moldwin and Hughes (1992) with ground auroral zone magnetograms and geosynchronous particle data signatures of substorm onsets. Over 84% of the plasmoid events occurred between 5 and 60 min after a substorm onset. We also find near one-to-one correlation between large isolated substorm signatures in the near-Earth region and signatures consistent with a passing plasmoid in the distant tail (i.e., a traveling compression region, or an actual plasmoid observation). However, there does not appear to be an absolute correspondence of every substorm onset to a plasmoid signature in the deep tail especially, for periods of prolonged disturbance that have multiple substorm insets. A correlation of inter-planetary magnetic field B. south with plasmoid observations was also found. The locations of the near- and far-Earth reconnection sites are estimated using the time of flight of the plasmoids from substorm onset to their observation at ISEE 3. The estimates of the near- and far-Earth reconnection sites are highly variable and range from 10 to 140 RE, 32 refs., 4 figs. 2 tabs.

Observations of Earthward and tailward propagating flux rope plasmoids: Expanding the plasmoid model of geomagnetic substorms

A survey of IMP 8 magnetometer data for plasmoid signatures during magnetospheric intervals from 1981 through 1983 found 16 plasmoids and 37 traveling compression regions as well as two earthward propagating flux ropes and 19 south-north bipolar lobe signatures. The properties of these relatively near-Earth plasmoids, traveling compression regions, and earthward propagating flux ropes and a qualitative model for their formation are presented. The plasmoids have estimated sizes, durations, magnetic field signatures, downtail velocities, and substorm associations very similar to those of the plasmoids identified in ISEE 3 deep-tail observations. The occurrence frequency of these near-Earth plasma sheet plasmoids is significantly smaller than that of plasmoids found in the mid- and deep tail with ISEE 3. The earthward propagating flux ropes are characterized by a south-north bipolar turning in the GSM Bz component, are localized near the noon-midnight meridional plane, and are strongly correlated with interplanetary magnetic field Bz north and small isolated high latitude geomagnetic substorms. These events are also apparently very rare and/or spatially localized. We propose that these structures are “proto-plasmoids,” i.e., plasmoids for which near-Earth magnetic reconnection stopped before all the closed plasma sheet field lines were reconnected. The proto-plasmoids are then “trapped” inside closed magnetic field lines and propagate earthward owing to the effect of the distant X-lines earthward plasma flow. We suggest that the two different “types” of plasmoids are due to the different energy states of the magnetosphere during periods of southward and northward interplanetary magnetic field.