Charles W. Carlson

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.

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.

The Plasma Instrument for AMPTE IRM

The AMPTE IRM plasma instrument package consists of three sensors. Two of them measure complete 3-D velocity-distribution functions of ions and electrons every spacecraft revolution (i.e., 4.35 s). The third sensor is a retarding potential analyzer (RPA) for low-energy electron measurements. The 3-D measurements consist of countrates at 30 energies and 128 angles evenly distributed over the 4xr solid-angle sphere. The energy range is 15 eV-30 keV for electrons and 20 eV/q-40 keV/q for ions. The RPA extends the electron measurements to lower energies. Three microcomputers within the experinent perform extensive on-board data-processing functions. Two of them compute the moments (density, velocity, temperature tensor, and heat flux vector) of the distribution functions of ions and electrons in real time. The third compresses the RPA data and computes an indication for the spacecraft potential.

The Toroidal Imaging Mass-Angle Spectrograph (TIMAS) for the POLAR Mission

The science objectives of the Toroidal Imaging Mass-Angle Spectrograph (TIMAS) are to investigate the transfer of solar wind energy and momentum to the magnetosphere, the interaction between the magnetosphere and the ionosphere, the transport processes that distribute plasma and energy throughout the magnetosphere, and the interactions that occur as plasma of different origins and histories mix and interact. In order to meet these objectives the TIMAS instrument measures virtually the full three-dimensional velocity distribution functions of all major magnetospheric ion species with one-half spin period time resolution. The TIMAS is a first-order double focusing (angle and energy), imaging spectrograph that simultaneously measures all mass per charge components from 1 AMU e−1 to greater than 32 AMU e−1 over a nearly 360° by 10° instantaneous field-of-view. Mass per charge is dispersed radially on an annular microchannel plate detector and the azimuthal position on the detector is a map of the instantaneous 360° field of view. With the rotation of the spacecraft, the TIMAS sweeps out very nearly a 4π solid angle image in a half spin period. The energy per charge range from 15 eV e−1 to 32 keV e−1 is covered in 28 non-contiguous steps spaced approximately logarithmically with adjacent steps separated by about 30%. Each energy step is sampled for approximately 20 ms;14 step (odd or even) energy sweeps are completed 16 times per spin. In order to handle the large volume of data within the telemetry limitations the distributions are compressed to varying degrees in angle and energy, log-count compressed and then further compressed by a lossless technique. This data processing task is supported by two SA3300 microprocessors. The voltages (up to 5 kV) for the tandem toroidal electrostatic analyzers and preacceleration sections are supplied from fixed high voltage supplies using optically controlled series-shunt regulators.

Precipitation of auroral protons in detached arcs

[1]xa0Recent global-scale observations by the IMAGE-FUV instrument demonstrate the existence of regions of particle precipitation at sub-auroral latitudes on the dayside. The signature of this precipitation is seen infrequently, but when so, it is clear in all 3 channels of the FUV instrument. A conjugate hemisphere conjunction with the FAST satellite demonstrates the presence of precipitating protons and the notable absence of precipitating electrons in these arcs. With this knowledge, one can determine the mean energy and energy flux of the precipitating protons by intercomparison of the response in the three FUV channels. Assuming that the protons have a kappa energy distribution, the mean energy is found to be ∼20 keV, with a peak in total energy flux of ∼1 mW/m2/sec, consistent with fits to the FAST ion measurements. These phenomena are observed mainly during times of high solar wind dynamic pressure and variable interplanetary magnetic field, and are associated with earlier nightside enhancements in the brightness and latitudinal extent of the proton aurora.

Ion and electron characteristics in auroral density cavities associated with ion beams: No evidence for cold ionospheric plasma

Low-density cavities associated with upgoing ion beams were identified in the auroral regions over two decades ago. In order to understand the waves, double layers, and solitary structures observed within these cavities, accurate measurements of the plasma distribution function are required. Although measurements by DE 1 indicated that these cavities were composed primarily of hot plasma in the form of ion beams, plasma sheet ions, and inverted V electrons, later reports from Viking showed these cavities contained a cold plasma component whose density was an order of magnitude larger than the hot component. Recent measurements by the FAST satellite contrast sharply with the Viking results and support the earlier DE 1 observations. Regions of upgoing ion beams observed by FAST are shown to contain little or no cold plasma. The hot electron densities (>100 eV) and the combined plasma sheet ion and upgoing ion beam densities (>30 eV) agree remarkably well. Furthermore, no cold ions (0–30 eV) are measured at low energies by the mass spectrometer, which precludes the presence of significant (>20%) cold electrons to preserve charge neutrality. Characteristics of the plasma for 11 ion beam events are tabulated.

Observation of an impulsive solar electron event extending down to ∼0.5 keV energy

We present the first observation of a solar impulsive electron event spanning the entire solar wind-suprathermal particle energy range (few eV to hundreds of keV), obtained with the 3-D Plasma and Energetic Particle experiment on the WIND spacecraft. The electron energy spectrum fits to a power-law ∼ E−3 from ∼40 keV down to a peak at ≲ 1 keV, with significant flux detected down to ∼0.5 keV. Since the range of such low energy electrons in ionized hydrogen is much less than the column density of the corona, they must be accelerated high, ∼1 R⊙ (solar radius) above the photosphere, for typical active coronal density models.

The subsolar magnetosheath and magnetopause for high solar wind ram pressure: WIND observations

On a rapid inward pass through the subsolar magnetosheath (MSH) and magnetopause (MP), the WIND spacecraft initially encountered a moderately-compressed low-magnetic shear MP (at a radial distance of 8.6 RE), followed by multiple crossings of a high-shear MP (at 8.2 RE). The large shear resulted from a southward turning of the external MSH field. Strong magnetic field pile-up, a plasma depletion layer (PDL), and plasma flow acceleration and rotation to become more perpendicular to the local magnetic field were observed in the MSH on approach to the low-shear MP. At the high-shear MP, magnetic reconnection flows were detected, and there are some indications that plasma depletion effects were weak or absent in the adjacent MSH. We attribute the changes in the MP and MSH properties to the sudden rotation of the MSH field direction. In essence, the structure of the MP regions under the unusually high solar wind ram pressure condition in this case does not seem to be qualitatively different from that observed under more typical (less compressed) conditions. Also similar to previous observations, the mirror mode is marginally unstable in the MSH proper, but is stable in the PDL. In this region, the proton temperature anisotropy is inversely correlated with βp∥. Finally, the electron distributions are observed to be anisotropic (Te⟂/Te∥ ∼1.3) throughout the entire MSH.

Species dependent energies in upward directed ion beams over auroral arcs as observed with FAST TEAMS

Upward flowing field-aligned ion beams over auroral arcs have been observed with the 3-dimensional ion mass spectrograph TEAMS on FAST. We have performed a statistical study on a sample of 77 ion beams from the auroral campaign in early 1997. All observed beams contain substantial amounts of H+, He+ and O+. A clear ordering of the total energies according to mass is found, with H+ having the lowest and O+ the highest energy. The composition varies significantly from beam to beam, with O+/H+ ratios ranging from ≈ 0.1 to 10. No variation of the energy ratio between species is observed as a function of relative abundance. These results are discussed in the light of earlier observations of higher energies for O+ in statistical studies of beams during solar minimum and attempts to explain this behavior in terms of beam instabilities.

Surface waves and field line resonances: A THEMIS case study

[1]xa0Using magnetic field and plasma observations from four of the five Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft, a surface wave at the dawn flank of the magnetopause was identified on 25 April 2007. The wave had an amplitude of about 1 RE and propagated tailward with a velocity of about 190–240 km/s. Its azimuthal wavelength was in the range of 9–11 RE. Magnetosheath velocity values support the hypothesis that this surface wave was generated by the Kelvin-Helmholtz instability. Simultaneously, an ultralow-frequency (ULF) pulsation event was detected by the fifth THEMIS spacecraft deeper in the magnetosphere, at a distance of about 5–7 RE from the magnetopause. This ULF event showed all the signatures predicted for waves generated by the classical field line resonance process. Frequency and phases of the detected ULF oscillations were found to be in close agreement with the magnetopause surface periodic disturbances. We conclude that the observed ULF wave event was most likely directly generated by the magnetopause surface wave and thus represents one of the few known manifestations of the classical field line resonance process in space directly observed, a conclusion made possible due to the special configuration of the THEMIS mission and its five spacecraft.