A three-dimensional model is utilized to derive the electrie potential induced across the magnetosphere by reconnection between the geomagnetic lield and an arbitrary interplanetary mugnetie field. AssumpLions [hat Lhe magnetosheath ffow is uniform and ihat reconncction is not Iimited by processes occurring at Lhe magnetopause are made so that Lhe model calculation provides an upper limit to Lhe magnetospheric potential produced by reconnection. The magnitude of Lhe polar cap ionospheric electric tield compuLed from this model is larger than that measured on bailoons by an average factor of about 3, and Lhe model reproduces Lhe temporal variations of Lhe experimental data. Some magnetospheric properties related to Lhe magnetopause reconnection une and potential are discussed as a tirst suggestion toward using the model in studies of relevant magnetospheric problems. The model potential induced across Lhe magnetosphere becomes less dependent on the direction of the inLerplaneLary magnetic lield as Lhe strength of às transverse componeni increases. Thus a reduced correlation heiwcen magnetospheric phenomena and Lhe southward component of Lhe interplanetarN lield should exisi when the Lransverse interplanetary field component exceeds about 8 ,y. Observed dependences oU Lhe polar cap ionospheric current system on B. are explained by Lhe model, and similar variations with B x are predictcd. Graphs oU Lhe model potential as functioris of Lhe interplanetary magnetic iield components are given as an aid in its application to other problems.Considerable evidence nas accumulated to suggest that many features of magnetospheric dynamics are controlled by the interaction between the interplanetary and terrestrial magnetic tields. Dungey [1961] suggested that the mechanism responsible for this interaction is magnetic field reconoection taking place at the magnetopause, where the two fields meet. Thc general properties of this interaction are a change in Lhe topology of Lhe reconnecting fields and magnetic energy conversion into thermal and bulk energies of the plasma LDungey, 1953: Sweei, 1958Parker, 1963; Peischek, 1964; Yeh andAxford, 1970; Sonnerup, 19721. Direct evidence for reconnection between the geomagnetic and interpianetary lields arises from observations of energetic solar particles. The first results of this nature were described by Lin and Anderson [1966], and a recent review of subsequent observations has been presented by Morfihi and Schoier [1973]. Further experimental evidence has been obtained through correlations of geomagnetic disturbance aL ground levei with the southward component of Lhe interplanetary magnetic field [Fairfieid and Cahill, 1966; Rosioker and Faiihammar, 1967: Schaiien andWilcox, 1967; Nishida, 1968; Arno/dy. 1971], through ground magnetometer correlations with Lhe azimuthal component of the interplanetary magnetic field [Svaigaard. 1968;Mansurov, 1969; Friis-Chris:ensen et ai. 1972; Berihelier and Guerin, 19721, through studies of sateilite measurements of the tail magnetic fie...
Measurements of the cross polar cap electric potential, by the double probe electric field experiment aboard S3‐3, from 55 orbits in the dawn‐dusk plane are compared with the reconnection electric fields predicted by a variety of models, both theoretical and experimental. The purpose of these comparisons is to understand the extent to which nonreconnection contributions to the polar cap potential must be included, to determine the time response of the polar cap potential to time varying reconnection rates, and to determine the efficiency and saturation levels of the reconnection process. It is found that (1) After several hours of northward interplanetary magnetic field, the cross polar cap potential declines to progressively lower values than those after 1 hour of northward interplanetary magnetic field. This suggests that it requires several hours for the ionospheric polar cap potential to respond to the ‘turning off’ or ‘turning down’ of the reconnection process. (2) The decay of the polar cap potential is used to demonstrate that contributions to the polar cap potential not associated with the reconnection process can be limited to less than 20 kV. It is shown that contributions to the polar cap potential that scale with the dynamic pressure of the solar wind are limited to less than 1 kV. (3) The cross polar cap electric potential is best predicted by a weighted sum of contributions from interplanetary magnetic field parameters over the 4 hours previous to the measurement. The weighting functions have the form of an exponential decay of 2–3 hours with the strongest weight on interplanetary parameters over the 1 hour previous to the measurement. (4) For values of the dawn dusk component of the interplanetary electric field less than about 0.5 mV/m, the measured polar cap potential is consistent with reconnection of all the interplanetary magnetic flux incident on a 30 RE wide frontside magnetopause. For larger values of the dawn dusk component of the interplanetary electric field, the proportion of field lines reconnecting is less, indicating saturation of the reconnection process. The above results are obtained when the extended response time of the magnetosphere to changes in interplanetary parameters is considered. They are independent of the detailed reconnection model assumed, and they are quite different from the viscous contribution, saturation levels of reconnection, and reconnection efficiencies inferred from comparisons of polar cap potentials to single hour averages of interplanetary parameters.
Abstract. THEMIS was launched onDuring the coast phase the probes were put into a string-of-pearls configuration at 100s of km to 2R 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.2
Abstract. On 26 January 2001, the Cluster spacecraft detected high-speed plasma jets at multiple crossings of the high-latitude duskside magnetopause (MP) and boundary layer (BL) over a period of more than 2 h. The 4 spacecraft combined spent more than half of this time in the MP/BL and jets were observed whenever a spacecraft was in the MP. These observations were made under steady southward and dawnward interplanetary magnetic field (IMF) conditions. The magnetic shear across the local MP was ∼100 • and β∼1 in the adjacent magnetosheath. The jet velocity is in remarkable agreement with reconnection prediction throughout the entire interval, except for one crossing that had no ion measurements inside the current layer. The flow speed measured in the deHoffmann Teller frame is 90% of the Alfvén speed on average for the 10 complete MP current layer crossings that are resolved by the ion measurements. These findings strongly suggest that reconnection was continuously active for more than two hours. The jets were directed persistently in the same northward and anti-sunward direction, implying that the X-line was always below the spacecraft. This feature is inconsistent with patchy and random reconnection or convecting multiple X-lines. The majority of MP/BL crossings in this two-hour interval were partial crossings, implying that they are caused by bulges sliding along the MP, not by inward-outward motion of a uniformly thin MP/BL. The presence of the bulges suggests that, although reconnection is continuously active under steady IMF conditions, its rate may be modulated. The present investigation also reveals that (1) the predicted ion D-shaped distributions are absent in all reconnection jets on this day, (2) the electric field fluctuations are larger in the reconnecting MP than in the magnetosheath proper, but their amplitudes never exceed 20 mV/m, (3) the ion-electron differential motion is ∼20 km/s for the Correspondence to: T. D. Phan (phan@ssl.berkeley.edu) observed MP current density of ∼50 nA/m 2 (deduced from ∇×B), thus inconsequential for the deHoffmann-Teller and Walén analyses, (4) flows in an isolated flux transfer event (FTE) are directed in the same direction as the MP jets and satisfy the Walén relation, suggesting that this FTE is also generated by reconnection. Finally, the present event cannot be used to evaluate the validity of component or anti-parallel merging models because, although the magnetic shear at the local MP was ∼100 • ( 180 • ), the X-line may be located more than 9 R E away (in the opposite hemisphere), where the shear could be substantially different.
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