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The THEMIS ESA Plasma Instrument and In-flight Calibration
The THEMIS plasma instrument is designed to measure the ion and electron distribution functions over the energy range from a few eV up to 30 keV for electrons and 25 keV for ions. The instrument consists of a pair of “top hat” electrostatic analyzers with common 180°×6° fields-of-view that sweep out 4
π
steradians each 3 s spin period. Particles are detected by microchannel plate detectors and binned into six distributions whose energy, angle, and time resolution depend upon instrument mode. On-board moments are calculated, and processing includes corrections for spacecraft potential. This paper focuses on the ground and in-flight calibrations of the 10 sensors on five spacecraft. Cross-calibrations were facilitated by having all the plasma measurements available with the same resolution and format, along with spacecraft potential and magnetic field measurements in the same data set. Lessons learned from this effort should be useful for future multi-satellite missions.
Journal Article
First Results from the THEMIS Mission
THEMIS was launched on February 17, 2007 to determine the trigger and large-scale evolution of substorms. During the first seven months of the mission the five satellites coasted near their injection orbit to avoid differential precession in anticipation of orbit placement, which started in September 2007 and led to a commencement of the baseline mission in December 2007. During the coast phase the probes were put into a string-of-pearls configuration at 100 s of km to 2 R
E
along-track separations, which provided a unique view of the magnetosphere and enabled an unprecedented dataset in anticipation of the first tail season. In this paper we describe the first THEMIS substorm observations, captured during instrument commissioning on March 23, 2007. THEMIS measured the rapid expansion of the plasma sheet at a speed that is commensurate with the simultaneous expansion of the auroras on the ground. These are the first unequivocal observations of the rapid westward expansion process in space and on the ground. Aided by the remote sensing technique at energetic particle boundaries and combined with ancillary measurements and MHD simulations, they allow determination and mapping of space currents. These measurements show the power of the THEMIS instrumentation in the tail and the radiation belts. We also present THEMIS Flux Transfer Events (FTE) observations at the magnetopause, which demonstrate the importance of multi-point observations there and the quality of the THEMIS instrumentation in that region of space.
Journal Article
Evaluation of whistler-mode chorus intensification on the nightside during an injection event observed on the THEMIS spacecraft
The intensification of the nightside whistler‐mode chorus emissions is observed in the low‐density region outside the plasmapause during the injection of anisotropic plasma sheet electrons into the inner magnetosphere. Time History of Events and Macroscale Interactions During Substorms data of the electron phase space density over the energy range between 0.1 keV and 30 keV are used to develop an analytical model for the distribution of injected suprathermal electrons. The path‐integrated gain of chorus waves is then evaluated with the HOTRAY code by tracing whistler‐mode chorus waves in a hot magnetized plasma. The simulated wave gain is compared to the observed wave electric field and magnetic field, respectively. The results indicate that lower‐energy (<1 keV) plasma sheet electrons can penetrate deeper toward the Earth but cause little chorus intensification, while higher‐energy (1 keV to tens of kiloelectron volts) electrons can be injected at relatively higher L‐shells and are responsible for the intensification of lower‐band and upper‐band whistler‐mode chorus. Compared to the lower‐band chorus, a relatively higher electron anisotropy is required to generate upper‐band chorus. In addition, higher plasma density results in stronger wave intensity and a broader frequency band of chorus waves.
Journal Article
First Results of the THEMIS Search Coil Magnetometers
We present the first data from the THEMIS Search Coil Magnetometers (SCM), taken between March and June 2007 while the THEMIS constellation apogee moved from the duskside toward the dawnside. Data reduction, especially the SCM calibration method and spurious noise reduction process, is described. The signatures of magnetic fluctuations in key magnetospheric regions such as the bow shock, the magnetopause and the magnetotail during a substorm, are described. We also discuss the role that magnetic fluctuations could play in plasma transport, acceleration and heating.
Journal Article
Cross-scale coupling in the auroral acceleration region
High resolution imaging within regions of auroral luminosity reveal complex, highly structured dynamic and often vortical forms which evolve on time scales of the order of several seconds and less. These features are inherently multi‐scale in nature with different sizes moving and evolving at different rates. Recent analyses have shown how the scale dependency of these motions can provide new insights into the nature of energy transport across scales occurring in current sheets through the auroral acceleration region. However the processes driving this transport and thus facilitating particle acceleration and the formation of bright and dynamic aurora remain unknown. This is a basic issue not only for advancing understanding of auroral arc formation but moreover for understanding dissipation and particle acceleration in current sheets generally. In this Frontier article we show how dedicated space‐borne auroral imagery combined with magnetically conjugate field and particle measurements can be used to advance understanding of this universal physical process. By coupling these measurements with numerical simulations we show how flow shear, magnetic reconnection and tearing may launch a cascade toward smaller scales and conspire to form, shape and structure auroral forms. The simulations show that these processes evolve toward a robust scaling of structured magnetic fields (Bx) with wavenumber (ky) perpendicular to the geomagnetic field where Bx2(ky)/Δky ∼ ky−7/3 as observed. Key Points Magnetic tearing and flow drive auroral arc structuring Tearing drives a cascade to small scales Spectra of magnetic field observations through aurora may result from tearing
Journal Article
Global Distribution of Crustal Magnetization Discovered by the Mars Global Surveyor MAG/ER Experiment
Vector magnetic field observations of the martian crust were acquired by the Mars Global Surveyor (MGS) magnetic field experiment/electron reflectometer (MAG/ER) during the aerobraking and science phasing orbits, at altitudes between ∼100 and 200 kilometers. Magnetic field sources of multiple scales, strength, and geometry were observed. There is a correlation between the location of the sources and the ancient cratered terrain of the martian high-lands. The absence of crustal magnetism near large impact basins such as Hellas and Argyre implies cessation of internal dynamo action during the early Naochian epoch (∼4 billion years ago). Sources with equivalent magnetic moments as large as 1.3 × 10$^{17}$ ampere-meter$^2$ in the Terra Sirenum region contribute to the development of an asymmetrical, time-variable obstacle to solar wind flow around Mars.
Journal Article
Magnetic Field and Plasma Observations at Mars: Initial Results of the Mars Global Surveyor Mission
The magnetometer and electron reflectometer investigation (MAG/ER) on the Mars Global Surveyor spacecraft has obtained magnetic field and plasma observations throughout the near-Mars environment, from beyond the influence of Mars to just above the surface (at an altitude of ∼100 kilometers). The solar wind interaction with Mars is in many ways similar to that at Venus and at an active comet, that is, primarily an ionospheric-atmospheric interaction. No significant planetary magnetic field of global scale has been detected to date (<2 × 10$^{21}$ Gauss-cubic centimeter), but here the discovery of multiple magnetic anomalies of small spatial scale in the crust of Mars is reported.
Journal Article
Extended magnetic reconnection at the Earth's magnetopause from detection of bi-directional jets
Magnetic reconnection is a process that converts magnetic energy into bi-directional plasma jets; it is believed to be the dominant process by which solar-wind energy enters the Earth's magnetosphere
1
,
2
. This energy is subsequently dissipated by magnetic storms and aurorae
3
,
4
. Previous single-spacecraft observations
5
,
6
,
7
revealed only single jets at the magnetopause—while the existence of a counter-streaming jet was implicitly assumed, no experimental confirmation was available. Here we report
in situ
two-spacecraft observations of bi-directional jets at the magnetopause, finding evidence for a stable and extended reconnection line; the latter implies substantial entry of the solar wind into the magnetosphere. We conclude that reconnection is determined by large-scale interactions between the solar wind and the magnetosphere, rather than by local conditions at the magnetopause.
Journal Article
Ionospheric convection signatures of tail fast flows during substorms and Poleward Boundary Intensifications (PBI)
Tail fast flows have been associated both with the onset of substorms and with auroral Poleward Boundary Intensifications (PBIs) that extend equatorward as streamers. We study here a series of bursts of fast tail flow that occurred on 5 March 2008 when four of the THEMIS probes were aligned in the tail from mid‐tail to inner magnetosphere and were in good conjunction with the Sondrestrom Incoherent Scatter Radar. The series of burst are identified as two separate events. We find that the first event is associated with a small substorm onset, and the second with a PBI and then possibly another onset. The ionospheric flow signatures of the substorm and the PBI are distinctly different: the substorm onset is characterized by flow enhancement in the polar cap several minutes before onset and by sudden ionospheric flow reduction at onset, while the PBI is accompanied by a flow enhancement directed primarily equatorward and intruding from the polar cap into the plasma sheet. Key Points Distinct tail and ionospheric flow signatures for substorm onsets and PBIs Enhanced flow in polar cap min before onset During PBI upward FAC at separatrix, equatorward flow from open to closed field
Journal Article