D. Larson

A three-dimensional plasma and energetic particle investigation for the wind spacecraft

This instrument is designed to make measurements of the full three-dimensional distribution of suprathermal electrons and ions from solar wind plasma to low energy cosmic rays, with high sensitivity, wide dynamic range, good energy and angular resolution, and high time resolution. The primary scientific goals are to explore the suprathermal particle population between the solar wind and low energy cosmic rays, to study particle accleration and transport and wave-particle interactions, and to monitor particle input to and output from the Earths magnetosphere.Three arrays, each consisting of a pair of double-ended semi-conductor telescopes each with two or three closely sandwiched passivated ion implanted silicon detectors, measure electrons and ions above ∼20 keV. One side of each telescope is covered with a thin foil which absorbs ions below 400 keV, while on the other side the incoming <400 keV electrons are swept away by a magnet so electrons and ions are cleanly separated. Higher energy electrons (up to ∼1 MeV) and ions (up to 11 MeV) are identified by the two double-ended telescopes which have a third detector. The telescopes provide energy resolution of ΔE/E≈0.3 and angular resolution of 22.5°×36°, and full 4π steradian coverage in one spin (3 s).Top-hat symmetrical spherical section electrostatic analyzers with microchannel plate detectors are used to measure ions and electrons from ∼3 eV to 30 keV. All these analyzers have either 180° or 360° fields of view in a plane, ΔE/E≈0.2, and angular resolution varying from 5.6° (near the ecliptic) to 22.5°. Full 4π steradian coverage can be obtained in one-half or one spin. A large and a small geometric factor analyzer measure ions over the wide flux range from quiet-time suprathermal levels to intense solar wind fluxes. Similarly two analyzers are used to cover the wide range of electron fluxes. Moments of the electron and ion distributions are computed on board.In addition, a Fast Particle Correlator combines electron data from the high sensitivity electron analyzer with plasma wave data from the WAVE experiment (Bougeretet al., in this volume) to study wave-particle interactions on fast time scales. The large geometric factor electron analyzer has electrostatic deflectors to steer the field of view and follow the magnetic field to enhance the correlation measurements.

THEMIS observations of an earthward‐propagating dipolarization front

[1] We report THEMIS observations of a dipolarization front, a sharp, large-amplitude increase in the Z-component of the magnetic field. The front was detected in the central plasma sheet sequentially at X = -20.1 R E (THEMIS P1 probe), at X = -16.7 R E (P2), and at X = -11.0 R E (P3/P4 pair), suggesting its earthward propagation as a coherent structure over a distance more than 10 R E at a velocity of 300 km/s. The front thickness was found to be as small as the ion inertial length. Comparison with simulations allows us to interpret the front as the leading edge of a plasma fast flow formed by a burst of magnetic reconnection in the midtail.

Tail Reconnection Triggering Substorm Onset

Magnetospheric substorms explosively release solar wind energy previously stored in Earths magnetotail, encompassing the entire magnetosphere and producing spectacular auroral displays. It has been unclear whether a substorm is triggered by a disruption of the electrical current flowing across the near-Earth magnetotail, at ∼10 RE (RE: Earth radius, or 6374 kilometers), or by the process of magnetic reconnection typically seen farther out in the magnetotail, at ∼20 to 30 RE. We report on simultaneous measurements in the magnetotail at multiple distances, at the time of substorm onset. Reconnection was observed at 20 RE, at least 1.5 minutes before auroral intensification, at least 2 minutes before substorm expansion, and about 3 minutes before near-Earth current disruption. These results demonstrate that substorms are likely initiated by tail reconnection.

A magnetic cloud containing prominence material: January 1997

This work discusses the relations among (1) an interplanetary force-free magnetic cloud containing a plug of cold high-density material with unusual composition, (2) a coronal mass ejection (CME), (3) an eruptive prominence, and (4) a model of prominence material supported by a force-free magnetic flux rope in a coronal streamer. The magnetic cloud moved past the Wind spacecraft located in the solar wind upstream of Earth on January 10 and 11, 1997. The magnetic field configuration in the magnetic cloud was approximately a constant-α, force-free flux rope. The 4He++/H+ abundance in the most of the magnetic cloud was similar to that of the streamer belt material, suggesting an association between the magnetic cloud and a helmet streamer. A very cold region of exceptionally high density was detected at the rear of the magnetic cloud. This dense region had an unusual composition, including (1) a relatively high (10%) 4He++/He+ abundance (indicating a source near the photosphere), and (2) 4He+, with an abundance relative to 4He++ of ∼1%, and the unusual charge states of O5+ and Fe5+ (indicating a freezing-in temperature of (1.6–4.0) × 105 °K, which is unusually low, but consistent with that expected for prominence material). Thus we suggest that the high-density region might be prominence material. The CME was seen in the solar corona on January 6, 1997, by the large angle and spectrometric coronagraph (LASCO) instrument on SOHO shortly after an eruptive prominence. A helmet streamer was observed near the latitude of the eruptive prominence a quarter of a solar rotation before and after the eruptive prominence. These observations are consistent with recent models, including the conceptual model of Low and Hundhausen [1995] for a quasi-static helmet streamer containing a force-free flux rope which supports prominence material and the dynamical model of Wu et al. [1997] for CMEs produced by the disruption of such a configuration.

Quantitative prediction of radiation belt electrons at geostationary orbit based on solar wind measurements

Solar wind measurements are used to predict the MeV electron radiation belt flux at the position of geostationary orbit. Using a model based on the standard radial diffusion equation, a prediction efficiency of 0.81 and a linear correlation of 0.90 were achieved for the years 1995–1996 for the logarithm of average daily flux. Model parameters based on the years 1995-1996 gave a prediction efficiency and a linear correlation for the years 1995–1999 of 0.59 and 0.80, respectively. The radial diffusion equation is solved after making the diffusion coefficient a function of the solar wind velocity and interplanetary magnetic field. The solar wind velocity is the most important parameter governing relativistic electron fluxes at geostationary orbit. The model also provides a physical explanation to several long standing mysteries of the variation of the MeV electrons.

Kinetic structure of the sharp injection/dipolarization front in the flow-braking region

[1] Observations of three closely-spaced THEMIS spacecraft at 9-11 Re near midnight and close to the neutral sheet are used to investigate a sharp injection/ dipolarization front (SDF) propagating inward in the flow-braking region. This SDF was a very thin current sheet along the North-South direction embedded within an Earthward-propagating flow burst. A short-lived depression of the total magnetic field (down to 1 nT), devoid of wave activity and intense particle fluxes, stays ahead of the SDF. Clear finite proton gyroradius effects, which help visualize the geometry and sub-gyroscale of the SDF, are seen centered at the thin current sheet. The SDF nearly coincides with the narrow interface between plasmas of different densities and temperatures. At that interface, we observed strong (40―60 mV/m peak) E-field bursts of the lower-hybrid time scale that are confined to a localized region of density depletions. This sharp dipolarization/injection front propagating in the flow-braking region appears to be a complicated kinetic-scale plasma structure that combines a number of small-scale elements (Bz drops, thin current sheets, LH cavities, injection fronts) previously discussed as separate objects.

Magnetotail flow bursts: Association to global magnetospheric circulation, relationship to ionospheric activity and direct evidence for localization

A series of bursty bulk flow events (BBFs) were observed by GEOTAIL and WIND in the geomagnetotail. IMP8 at the solar wind showed significant energy coupling into the magnetosphere, while the UVI instrument on POLAR evidenced significant energy transfer to the ionosphere during two substorms. There was good correlation between BBFs and ionospheric activity observed by UVI even when ground magnetic signatures were absent, suggesting that low ionospheric conductivity at the active sector may be responsible for this observation. During the second substorm no significant flux transport was evidenced past WIND in stark contrast to GEOTAIL and despite the small intersatellite separation ((3.54, 2.88, −0.06) RE). Throughout the intervals studied there were significant differences in the individual flow bursts at the two satellites, even during longitudinally extended ionospheric activations. We conclude that the half-scale-size of transport-bearing flow bursts is less than 3 RE.

On the Origin of Impulsive Electron Events Observed at 1 AU

A statistical survey of 12 impulsive electron events detected at energies down below 1 keV and 58 events detected above 25 keV observed at 1 AU by the 3-D Plasma and Energetic Particles experiment on the Wind spacecraft is presented. Timing analysis of the velocity dispersion reveals two different kinds of electron events: (1) events released from the Sun at the onset of a radio type III burst, which suggest that these electrons are part of the population producing the type III radio emission; and (2) events in which the electrons are released up to half an hour later than the onset of the type III burst. These electrons therefore may be produced by a different acceleration mechanism than the population producing the radio emission. Both types of behavior can be observed during the same impulsive electron event at different energies, but most events show the same timing at all energies. At lower energies ( 25 keV), events not related to type III bursts are more numerous (41 of 58). However, events of both classes are observed below 1 keV. Impulsive electron events not related to type III radio bursts are observed to be proton rich, with an order-of-magnitude lower electron-to-proton ratio than events related to type III bursts. For roughly 3/4 of the events not related to type III bursts, large-scale coronal transient waves, also called EIT waves or coronal Moreton waves, are observed by the Extreme Ultraviolet Imaging Telescope (EIT) on board SOHO. Temporal and spatial correlations together with hydromagnetic simulations show that at least some impulsive electron events are more likely related to the propagating Moreton wave than to the flare phenomenon itself.

THEMIS analysis of observed equatorial electron distributions responsible for the chorus excitation

[1] A statistical survey of plasma densities and electron distributions (0.5–100 keV) is performed using data obtained from the Time History of Events and Macroscale Interactions During Substorms spacecraft in near‐equatorial orbits from 1 July 2007 to 1 May 2009 in order to investigate optimum conditions for whistler mode chorus excitation. The plasma density calculated from the spacecraft potential, together with in situ magnetic field, is used to construct global maps of cyclotron and Landau resonant energies under quiet, moderate, and active geomagnetic conditions. Statistical results show that chorus intensity increases at higher AE index, with the strongest waves confined to regions where the ratio between the plasma frequency and gyrofrequency, fpe/fce, is less than 5. On the nightside, large electron anisotropies and intense chorus emissions indicate remarkable consistency with the confinement to 8 RE. Furthermore, as injected plasma sheet electrons drift from midnight through dawn toward the noon sector, their anisotropy increases and peaks on the dayside at 7 6) on the dayside. In addition, very isotropic distributions at a few keV, which may be produced by Landau resonance and contribute to the formation of the typical gap in the chorus spectrum near 0.5 fce, are commonly observed on the dayside. Citation: Li, W., et al. (2010), THEMIS analysis of observed equatorial electron distributions responsible for the chorus excitation, J. Geophys. Res., 115, A00F11, doi:10.1029/2009JA014845.

Tracing the topology of the October 18–20, 1995, magnetic cloud with ∼0.1–10² keV electrons

Five solar impulsive ∼1–10² keV electron events were detected while the WIND spacecraft was inside the magnetic cloud observed upstream of the Earth on October 18–20, 1995. The solar type III radio bursts produced by these electrons can be directly traced from ∼1 AU back to X-ray flares in solar active region AR 7912, implying that at least one leg of the cloud was magnetically connected to that region. Analysis of the electron arrival times shows that the lengths of magnetic field lines in that leg vary from ∼3 AU near the cloud exterior to ∼1.2 AU near the cloud center, consistent with a model force-free helical flux rope. Although the cloud magnetic field exhibits the smooth, continuous rotation signature of a helical flux rope, the ∼0.1-1 keV heat flux electrons and ∼1–10² keV energetic electrons show numerous simultaneous abrupt changes from bidirectional streaming to unidirectional streaming to complete flux dropouts. We interpret these as evidence for patchy disconnection of one end or both ends of cloud magnetic field lines from the Sun.