THEMIS Science Objectives and Mission Phases

Sibeck, D. G.; Angelopoulos, V.

doi:10.1007/s11214-008-9393-5

Cited by 188 publications

(130 citation statements)

References 101 publications

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“…The main goal of this mission is to conduct multi-point investigations of substorm phenomena in the tail of the terrestrial magnetosphere (Sibeck and Angelopoulos, 2008). For the current study, Search Coil Magnetometer (SCM) observations (Le Contel et al, 2008) and plasma measurements of the Electric Field Instrument (EFI) (Bonnell et al, 2008) were analyzed.…”

Section: Data Description and Processing Techniquementioning

confidence: 99%

Observations and modeling of forward and reflected chorus waves captured by THEMIS

et al. 2011

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Abstract. Discrete ELF/VLF chorus emissions are the most intense electromagnetic plasma waves observed in the radiation belts of the Earth's magnetosphere. Chorus emissions, whistler-mode wave packets propagating roughly along magnetic field lines from a well-localized source in the vicinity of the magnetic equator to polar regions, can be reflected at low altitudes. After reflection, wave packets can return to the equatorial plane region. Understanding of whistler wave propagation and reflection is critical to a correct description of wave-particle interaction in the radiation belts. We focus on properties of reflected chorus emissions observed by the THEMIS (Time History of Events and Macroscale Interactions During Substorms) spacecraft Search Coil Magnetometer (SCM) and Electric Field Instrument (EFI) at ELF/VLF frequencies up to 4 kHz at L ≥ 8. We determine the direction of the Poynting flux and wave vector distribution for forward and reflected chorus waves. Although both types of chorus waves were detected near the magnetic equator and have similar, discrete structure and rising tones, reflected waves are attenuated by a factor of 10-30 and have 10% higher frequency than concurrently-observed forward waves. Modeling of wave propagation and reflection using geometrical optics ray-tracing allowed us to determine the chorus source region location and explain observed propagation characteristics. We find that reflected wave attenuation at a certain spatial region is caused by divergence of the ray paths of these non-ducted emissions, and that the frequency shift is caused by generation of the reflected waves at lower L-shells where the local equatorial gyrofrequency is larger.

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Section: Data Description and Processing Techniquementioning

confidence: 99%

Observations and modeling of forward and reflected chorus waves captured by THEMIS

et al. 2011

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“…The observations presented here were obtained from the THEMIS satellites (Angelopoulos, 2008;Sibeck and Angelopoulos, 2008;Bester et al, 2008) and THEMIS groundbased observatories Mende et al, 2008;Russell et al, 2008). Each THEMIS spacecraft carries an identical set of instruments.…”

Section: Instrumentationmentioning

confidence: 99%

THEMIS ground-space observations during the development of auroral spirals

et al. 2009

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Abstract.A simultaneous observation of an auroral spiral and its generator region in the near-Earth plasma sheet is rather unlikely. Here we present such observations using the THEMIS spacecraft as well as the THEMIS ground network of all-sky imagers and magnetometers. Two consecutive auroral spirals separated by approximately 14 min occurred during a substorm on 19 February 2008. The spirals formed during the expansion phase and a subsequent intensification, and were among the brightest features in the aurora with diameters of 200-300 km. The duration for the formation and decay of each spiral was less than 60 s. Both spirals occurred shortly after the formation of two oppositely rotating plasma flow vortices in space, which were also accompanied by dipolarizations and ion injections, at ∼11 R E geocentric distance. Observations and model calculations also give evidence for a magnetic-field-aligned current generation of approximately 0.1 MA via the flow vortices, connecting the generator region of the spirals with the ionosphere, during the formation of both spirals. In the ionosphere, a pair of equivalent ionospheric current (EIC) vortices with opposite rotations (corresponding to upward and downward currents) was present during both auroral spirals with enhanced EICs and ionospheric flows at the locations of the auroral spirals and along the auroral arcs. The combined ground and space observations suggest that each auroral spiral was powered by two oppositely rotating plasma flow vortices that caused a current enhancement in the substorm current wedge.

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“…Moreover, evidence is given that the space vortices contributed to the field-aligned current of the SCW by applying equation (2) and by comparison with model results of a reconstructed SCW. Properties of one space vortex are described using multipoint measurement from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft [Angelopoulos, 2008;Sibeck and Angelopoulos, 2008;Bester et al, 2008;Auster et al, 2008;McFadden et al, 2008;D. Larson et al, Solid state telescope for THEMIS, manuscript in preparation, 2009).…”

Section: Introductionmentioning

confidence: 99%

Substorm current wedge driven by plasma flow vortices: THEMIS observations

et al. 2009

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[1] A multipoint analysis of conjugate magnetospheric and ionospheric flow vortices during the formation of the substorm current wedge (SCW) on 19 February 2008 is presented. During the substorm, four Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft were located close to the neutral sheet in the premidnight region between 9 and 12 R E geocentric distance, of which three closely ($1-2 R E ) clustered at $23 MLT and one was farther west at $21 MLT. The closely clustered spacecraft were engulfed by a counterclockwise plasma flow vortex, while the single spacecraft recorded a clockwise plasma flow vortex. Simultaneously, a pair of conjugate flow vortices with clockwise and counterclockwise rotation appeared in the ionosphere, as inferred from equivalent ionospheric currents. The counterclockwise space vortex, which corresponded to a downward field-aligned current, was at least 1-2 R E in diameter and had rotational flow speeds of up to 900 km/s. Current density estimates associated with the formation of the space vortex in the first 30 s yielded 2.8 nA/m 2 (14 mA/m 2 mapped to the ionosphere), or a total current of 1.1 Â 10 5 A. Model calculations based on midlatitude ground magnetometer data show a gradual increase of the field-aligned current, with 1-2 Â 10 5 A within the first minute and a peak value of 7 Â 10 5 A after 10 min, associated with the SCW, and a matching meridian of the downward current of the SCW and the downward current (counterclockwise) space vortex. The combined ground and space observations, together with the model results, present a scenario in which the space vortices generated the field-aligned current of the SCW at the beginning of the substorm expansion phase and coupled to the ionosphere, causing the ionospheric vortices.

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THEMIS Science Objectives and Mission Phases

Cited by 188 publications

References 101 publications

Observations and modeling of forward and reflected chorus waves captured by THEMIS

Observations and modeling of forward and reflected chorus waves captured by THEMIS

THEMIS ground-space observations during the development of auroral spirals

Substorm current wedge driven by plasma flow vortices: THEMIS observations

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