R. C. Fear

University of Leicester, Leiscester, England, United Kingdom

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Publications (38)62.03 Total impact

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    ABSTRACT: Nightside polar arcs are some of the most puzzling auroral emissions at Earth. They are features which extend from the nightside auroral oval into the open magnetic field line region (polar cap) and they represent optical signatures of magnetotail dynamics. Here we report the first observation of an arc at Saturn, which is attached at the nightside main oval and extends into the polar cap region, resembling a terrestrial transpolar arc. We show that Earth-like polar arcs can exceptionally occur in a fast rotational and internally influenced magnetosphere such as Saturn's. Finally, we discuss the possibility that the polar arc at Saturn is related to tail reconnection and we address the role of solar wind in the magnetotail dynamics at Saturn.
    Geophysical Research Letters. 09/2014;
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    R. C. Fear, S. E. Milan, K. Oksavik
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    ABSTRACT: Flux transfer events are bursts of dayside reconnection, which give rise to local perturbations of the magnetic field that can be observed by spacecraft near the magnetopause. Although it is commonly accepted that flux transfer events are caused by reconnection, various models exist to explain their structure. A key difference between mechanisms is that when the magnetic shear across the magnetopause is close to 180°, some models will give rise to a structure whose axis is oriented north-south, whereas others will result in a dawn-dusk-oriented structure. Several techniques for determining the axial direction of such structures have been suggested: minimum variance analysis on the magnetic field, minimization of the axial electric field, and Grad-Shafranov reconstruction. We apply these techniques to a series of flux transfer events observed by Cluster at a high-shear magnetopause crossing on 27 March 2007. Minimum variance analysis on the signatures caused by the draping of unreconnected magnetic flux around the flux transfer events consistently results in an axial direction that is dawn-dusk oriented. However, the electric field technique, applied to flux transfer events that are penetrated by the spacecraft, results in a mixture of north-south and dawn-dusk axes. Testing these axial directions with Grad-Shafranov reconstruction suggests that the axes of events which appear to be oriented dawn-dusk might be more reliably determined than the axes of events which appear to be north-south. Using the Grad-Shafranov method alone to determine the axial direction of the penetrated events gives dawn-dusk axial directions, consistent with the results of minimum variance analysis on the draping events. Overall, these observations are consistent with the formation of flux transfer events by processes involving relatively long reconnection lines (e.g., single or multiple X-line reconnection) rather than elbow-shaped flux tubes, as originally envisaged. Furthermore, examination of one particular flux transfer event that was observed by Cluster 1 but not by Cluster 2 (which was closer to the magnetopause) allows us to preclude the elbow-shaped model for this event.
    Journal of Geophysical Research Atmospheres 09/2012; 117(A9):9220-. · 3.44 Impact Factor
  • R. C. Fear, S. E. Milan
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    ABSTRACT: Transpolar arcs are large-scale auroral features which are observed within the polar cap when the IMF has a northward component. One leading candidate formation mechanism proposes that they are formed by reconnection in the magnetotail some time after a period of dayside reconnection with a non-zero IMF BY component which introduces a twist into the magnetotail. As a result of the twist, the mechanism predicts that the return flows of the newly closed magnetic field lines are asymmetric about midnight; their direction should depend upon the IMF BY component in the hours beforehand and should be opposite in the northern and southern hemispheres. In this paper, we use data from the SuperDARN network of high-latitude ionospheric radars to examine whether such ionospheric flows are present before the formation of 33 transpolar arcs. We find that the flows are present and in a manner that is consistent with the reconnection mechanism for 76% of the events; in the remaining few, the discrepancy can be attributed either to an uncertainty in the formation time determined for the arc (due to previous polar cap activity in the same local time sector) or due to the geometry of the radars which observe the backscatter.
    Journal of Geophysical Research Atmospheres 09/2012; 117(A9):9230-. · 3.44 Impact Factor
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    ABSTRACT: During intervals of southward IMF, magnetic reconnection can result in the formation of flux transfer events (FTEs) on the dayside magnetopause which travel along the magnetopause in the anti-sunward direction. Of particular interest is their fate and the role they play transporting solar wind plasma into the magnetosphere. We present the discovery of FTEs far along the distant tail magnetopause (x = -67 Earth radii) using data from ARTEMIS on the dusk flank magnetopause under southward/duskward IMF conditions. The identification of several events is further supported by excellent fits to a force-free flux rope model. The axis of each structure is principally north-south, i.e., perpendicular to the Sun-Earth line. Simultaneous observations by THEMIS on the dayside magnetopause indicate that FTEs are being produced there, although perhaps 2-4 times smaller in size. The convection time from the dayside magnetopause to ARTEMIS is 30 min, and the FTEs have a flux content comparable to those typically observed on the dayside magnetopause, indicating that these features are in quasi-equilibrium as they are convected downtail. By considering the relative orientations of the FTEs observed by THEMIS and ARTEMIS, the magnetic field geometry is consistent with the FTEs being produced on the dayside magnetopause along an extended X-line in the presence of IMF By and bending as they are convected to the flanks.
    Journal of Geophysical Research Atmospheres 08/2012; 117(A8):8222-. · 3.44 Impact Factor
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    ABSTRACT: We examine the orientation and current density of the current sheet during current sheet crossings from Cluster's 2001-2007 tail seasons. The curlometer technique is used to estimate the current density and is combined with Minimum Variance Analysis (MVA) to calculate the direction of the current sheet normal. The SYM-H and AE indices at the time of each crossing are employed to assess how the tilt angle (the angle the normal makes with the Z axis in the GSM YZ plane) and current density depend on geomagnetic conditions. Our results indicate a larger current sheet tilt in the YZ plane during intervals of stronger and/or more prolonged substorm activity, as indicated by the AE index. There is also evidence that when the ring current is enhanced during magnetic storms, the current sheet is less tilted even though the AE index is also disturbed. In addition larger current densities are seen during times of both magnetic storms and substorms, compared to crossings during only substorms and a quiet ring current. We conclude that increased substorm activity disrupts the current sheet structure resulting in greater motion of the current sheet (as found by Davey et al. (2012)) and a greater local tilt to the current sheet. We propose that the increased open flux in the tail during magnetic storms stabilizes the current sheet such that the tilt angle of the current sheet is reduced. The increased amount of open flux during magnetic storms also results in larger current densities within the current sheet.
    Journal of Geophysical Research Atmospheres 07/2012; 117(A7):7217-. · 3.44 Impact Factor
  • R. C. Fear, S. E. Milan
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    ABSTRACT: Transpolar arcs (TPAs) are auroral features which extend into the polar cap from the night side of the main auroral oval. In their most developed form, TPAs and the main auroral oval resemble a Greek 'theta', hence their alternative name of theta auroras. Observations from low-altitude spacecraft have reported that the plasma distribution above a TPA is similar to that above the main auroral oval, indicating that TPAs exist on closed magnetic field lines embedded within the open polar cap, but very few simultaneous observations have been reported of TPAs and conjugate points further out in the magnetotail. A major candidate mechanism for TPA formation invokes the closure of lobe flux in a twisted magnetotail, where the closed flux is prevented from returning to the dayside as the twist causes the northern and southern hemisphere footprints of the closed field lines to straddle the midnight meridian. In this mechanism, closed flux builds up on the night side, so plasma similar to typical plasma sheet distributions should be observed at high latitudes embedded within the lobe. We present preliminary observations of three cases where the Cluster spacecraft observes plasma-sheet plasma embedded within the lobes, and at much higher latitudes than those at which the plasma sheet is usually observed. The plasma distributions are indicative of closed field lines, and the locations of the spacecraft map to a point on the TPA that is significantly poleward of the main auroral oval. These observations are consistent with TPAs being formed by the proposed reconnection/twisted magnetotail mechanism.
    04/2012;
  • R. C. Fear, M. Palmroth, S. E. Milan
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    ABSTRACT: Most models of flux transfer event (FTE) formation produce pairs of structures, which in general move away from the subsolar region and give rise to signatures which can be observed in both the northern and southern hemispheres. The multiple reconnection line (X-line) model is unique as a reconnection-based model that is capable of producing a single flux rope if only two X-lines are present. Raeder [2006] reported the results of an MHD simulation where he studied the effect of the Earth's dipole tilt on reconnection at the dayside magnetopause for a southward IMF orientation; in his simulations, flux ropes were formed by the sequential formation of X-lines, and when the dipole tilt was set to a value representative of solstice the flux ropes moved preferentially towards the winter hemisphere. Some observational evidence has previously been presented for a bias towards FTE signatures being observed in the winter hemisphere; in this presentation, we show further observational evidence for this phenomenon, using an independently-derived data set. Once the seasonal bias is taken into account, we find that the IMF clock angle controls the location of FTE signatures. We also find that the effective dipole tilt (combining the geomagnetic dipole tilt with the IMF tilt angle) provides no clear control of the location of FTE signatures.
    Journal of Geophysical Research Atmospheres 04/2012; 117(A4):7705-. · 3.44 Impact Factor
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    ABSTRACT: We present a study of the magnetotail lobes during different geomagnetic conditions. We have employed data from the Cluster spacecraft from 2001 to 2007. Specifically, we have selected orbits through the magnetotail which contain either one or multiple crossings of the current sheet (indicated by a change in BX from 5 nT to -5 nT or vice versa). SYM-H and AE indices are used to ascertain the storm and substorm conditions during the orbits. We show that the current sheet is more dynamic and highly tilted with lower current densities during substorms than compared to quiet times. We also show that during magnetic storms, the current sheet is less active, with reduced tilt angles and larger current densities, even though the AE index indicates that substorms are occurring. We will also present results to give a more complete picture of the region, showing how the lobe magnetic field behaves during quiet, substorm and storm times.
    04/2012;
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    M. Palmroth, R. C. Fear, I. Honkonen
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    ABSTRACT: We examine the spatial variation of magnetospheric energy transfer using a global magnetohydrodynamic (MHD) simulation (GUMICS-4) and a large data set of flux transfer events (FTEs) observed by the Cluster spacecraft. Our main purpose is to investigate whether it is possible to validate previous results on the spatial energy transfer variation from the GUMICS-4 simulation using the statistical occurrence of FTEs, which are manifestations of magnetospheric energy transfer. Previous simulation results have suggested that the energy transfer pattern at the magnetopause rotates according to the interplanetary magnetic field (IMF) orientation, and here we investigate whether a similar rotation is seen in the locations at which FTE signatures are observed. We find that there is qualitative agreement between the simulation and observed statistics, as the peaks in both distributions rotate as a function of the IMF clock angle. However, it is necessary to take into account the modulation of the statistical distribution that is caused by a bias towards in situ FTE signatures being observed in the winter hemisphere (an effect that has previously been predicted and observed in this data set). Taking this seasonal effect into account, the FTE locations support the previous simulation results and confirm the earlier prediction that the energy transfers in the plane of the IMF. In addition, we investigate the effect of the dipole orientation (both the dipole tilt angle and its orientation in the plane perpendicular to the solar wind flow) on the energy transfer spatial distribution. We find that the energy transfer occurs mainly in the summer hemisphere, and that the dayside reconnection region is located asymmetrically about the subsolar position. Finally, we find that the energy transfer is 10% larger at equinox conditions than at solstice, contributing to the discussion concerning the semiannual variation of magnetospheric dynamics (known as "the Russell-McPherron effect").
    Annales Geophysicae 03/2012; 30(3):515-526. · 1.52 Impact Factor
  • R. C. Fear, S. E. Milan
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    ABSTRACT: Transpolar arcs are auroral features that extend from the nightside auroral oval into the polar cap. It is well established that they occur predominantly when the interplanetary magnetic field (IMF) has a northward component (Bz > 0). Results concerning how the magnetic local time at which transpolar arcs form might depend upon the IMF dawn-dusk component (BY) are more mixed. Some studies have found a correlation between these two variables, with Northern Hemisphere arcs forming predominantly premidnight when BY > 0 and postmidnight when BY < 0 and vice versa in the Southern Hemisphere. However, a more recent statistical study found that there was no significant correlation, and other studies find that the formation of moving arcs is triggered by a change in the sign of the IMF BY component. In this paper, we investigate the relationship between the magnetic local time at which transpolar arcs form and the IMF BY component. It is found that there is indeed a correlation between the magnetic local time at which transpolar arcs form and the IMF BY component, which acts in opposite senses in the Northern and Southern hemispheres. However, this correlation is weak if the IMF is only averaged over the hour before the first emergence of the arc and becomes stronger if the IMF is averaged 3-4 h beforehand. This is consistent with a mechanism where the magnetic local time at which the arc first forms depends on the BY component in the magnetotail adjacent to the plasma sheet, which is determined by the IMF BY component during intervals of dayside reconnection in the hours preceding the first emergence of the arc. We do not find evidence for the triggering of arcs by an IMF BY sign change.
    Journal of Geophysical Research Atmospheres 03/2012; 117(A3):3213-. · 3.44 Impact Factor
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    ABSTRACT: Passes through the mid-region of the magnetotail by the Cluster spacecraft from 2001 to 2007 have been examined to study the dynamics of the cross-tail current sheet. Cluster is ideally placed to study this region due to the orientation of the orbit in the magnetotail, such that the current sheet is sampled at distances downtail from about 8-19 RE. Multiple fluctuations of the X component of the magnetic field (BX) from positive to negative and vice versa, measured by Cluster as it crosses the nominal location of the current sheet, indicate that the current sheet is in motion. In this study we use the number of crossings of the current sheet by the Cluster 3 spacecraft as a measure of the dynamics of the magnetotail. The effects of substorm and magnetic storm activity on the dynamics during these orbits have been investigated using the AE and SYM-H indices. Our results indicate that the current sheet is more often in motion during orbits when there is greater than average substorm activity and a quiet ring current. Results suggest that internal processes within the tail that initiate substorms may also initiate the flapping motion of the current sheet. In addition the more dipolar field that results from an enhanced ring current during magnetic storms may inhibit tail dynamics during substorm events.
    Journal of Geophysical Research Atmospheres 02/2012; 117(A2):2202-. · 3.44 Impact Factor
  • R. Fear, S. E. Milan
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    ABSTRACT: Transpolar arcs or polar cap arcs are auroral features which are observed within the polar cap. They occur predominantly during intervals of northward IMF (Berkey et al., 1976). There is mixed evidence for IMF BY control of the local time at which the arcs initially form; Gussenhoven (1982) found that polar cap arcs formed preferentially post-midnight when BY < 0 (evaluated over 1 or 2 hours preceding the start of the arc) and pre-midnight when BY > 0, whereas Valladares et al (1991) found no clear dependency. The only previous statistical study of globally-imaged transpolar arcs (Kullen et al., 2002) found differing results for moving and non-moving arcs, concluding that three different models were required to identify (i) moving arcs, (ii) stationary arcs near the dawn/dusk portion of the main oval, and (iii) stationary arcs in the midnight sector. In this presentation, we show the results of a statistical study of 131 transpolar arcs observed by the FUV cameras on the IMAGE satellite between June 2000 and September 2005. We find that arcs tend to form following the same dependency on BY as identified by Gussenhoven (1982), whether moving or not. We find that the correlation between the magnetic local time at which the arc forms and the IMF BY component is relatively weak if the IMF is only averaged over the hour preceding the arc formation, but becomes stronger if the IMF is evaluated between 1 and 4 hours before the arc first forms. This is consistent with the timescale that is expected for newly-opened magnetospheric flux to reach the magnetotail plasma sheet (Dungey, 1961; Milan et al., 2007), and is therefore consistent with the suggestion that transpolar arcs map to the plasma sheet. We suggest that the similar dependence of stationary and moving arcs on the IMF BY component might imply that it is possible to explain both types of arc in terms of a single mechanism.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: Elizabeth Davey and colleagues survey the behaviour of the cross-tail current sheet, a presentation that won a Rishbeth prize at the NAM/UKSP/MIST meeting in April this year.
    Astronomy & Geophysics 07/2011; 52(4):4.17 - 4.19. · 0.34 Impact Factor
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    ABSTRACT: We present observations of auroral omega bands on 28 September 2009. Although generally associated with the substorm recovery phase and typically observed in the morning sector, the features presented here occurred just after expansion phase onset and were observed in the midnight sector, dawnward of the onset region. An all‐sky imager located in northeastern Iceland revealed that the omega bands were ∼150 × 200 km in size and propagated eastward at ∼0.4 km s−1 while a colocated ground magnetometer recorded the simultaneous occurrence of Ps6 pulsations. Although somewhat smaller and slower moving than the majority of previously reported omega bands, the observed structures are clear examples of this phenomenon, albeit in an atypical location and unusually early in the substorm cycle. The THEMIS C probe provided detailed measurements of the upstream interplanetary environment, while the Cluster satellites were located in the tail plasma sheet conjugate to the ground‐based all‐sky imager. The Cluster satellites observed bursts of 0.1–3 keV electrons moving parallel to the magnetic field toward the Northern Hemisphere auroral ionosphere; these bursts were associated with increased levels of field‐aligned Poynting flux. The in situ measurements are consistent with electron acceleration via shear Alfvén waves in the plasma sheet ∼8 RE tailward of the Earth. Although a one‐to‐one association between auroral and magnetospheric features was not found, our observations suggest that Alfvén waves in the plasma sheet are responsible for field‐aligned currents that cause Ps6 pulsations and auroral brightening in the ionosphere. Our findings agree with the conclusions of earlier studies that auroral omega bands have a source mechanism in the midtail plasma sheet.
    Journal of Geophysical Research Atmospheres 05/2011; 116(A00I30). · 3.44 Impact Factor
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    ABSTRACT: 1] Several conceptual models have been proposed for the formation of flux transfer events (FTEs), including models based on reconnection at a single reconnection line (X line) and at multiple X lines. Two‐dimensional magnetohydrodynamic models have previously been used to simulate both scenarios and have found a tendency for FTEs generated by single X line reconnection to exhibit an asymmetry in the bipolar B N signature that is the major in situ signature of FTE structures, with the leading peak being substantially smaller than the trailing peak. On the other hand, simulated FTEs generated by multiple X line reconnection led to more symmetric signatures. We present a comparison of these simulation results with observations made at the Earth's magnetopause by the Cluster spacecraft, using a data set of 213 FTEs which were observed by all four spacecraft in 2002/2003 at the high‐latitude magnetopause near local noon and at low latitudes on the flanks, and 36 FTEs which were observed by one or more Cluster spacecraft near the subsolar point in 2007 and 2008. A tendency is found for the B N signatures to be asymmetric but with the leading peak larger in amplitude than the trailing peak, opposite to the prediction made by the 2‐D single X line simulations. This tendency is weaker in the subsolar FTEs. Therefore, the observations are not consistent with 2‐D MHD simulations of single X line reconnection. The signatures observed near the subsolar point are more consistent with those predicted by 2‐D simulations of multiple X line reconnection, although the multiple X line simulation studies did not report any net asymmetry. We propose that the observed asymmetry can be explained by a compression of magnetic flux ahead of the propagating FTE structure and a rarefaction behind it. The weaker tendency nearer the subsolar point is consistent with a weaker compression and rarefaction due to lower FTE velocities.
    Journal of Geophysical Research Atmospheres 01/2010; 115. · 3.44 Impact Factor
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    ABSTRACT: Previous observations have allowed the scale size of flux transfer events (FTEs) to be determined both normal to the magnetopause and in the direction of motion of the FTE, but a key difference between some different models of FTE structure is their azimuthal scale size. Previous ground-based observations of the ionospheric signatures of FTEs indicated that magnetic reconnection can occur coherently over large extents of the magnetopause, but in situ determination of the azimuthal scale size of FTEs has not been possible until recent Cluster magnetopause crossing seasons when the separation of the spacecraft was ∼10, 000 km. In this paper, we present Cluster observations of flux transfer events from the 27th March 2007, along with observations of the conjugate ionospheric signatures. We highlight two magnetospheric FTEs which were consistent with long X-line FTE models, but note also several FTEs with considerably smaller azimuthal scale.
    12/2009: pages 389-398;
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    ABSTRACT: The nature of the interaction of the solar wind and associated magnetic field benefits from simultaneous coverage over a range of different magnetopause sites. We consider Cluster, Double Star and THEMIS conjunctions which allow exploration of the conjugate response of the dayside magnetopause and dawn/dusk flanks. In particular, during the April to July 2007 epoch, the array of four Cluster spacecraft, separated at large distances (10,000 km), were traversing the dawn-side magnetopause at high and low latitudes; the five THEMIS spacecraft were often in a 4+1 configuration, traversing the low latitude, dusk-side magnetosphere, and the Double star, TC-1 spacecraft was in an equatorial orbit between the local times of the THEMIS and Cluster orbits. This combination of 10 spacecraft provided simultaneous monitoring across a wide range of local times. The distribution and grouping of spacecraft allow multi-scale analysis of local phenomena operating on both flanks of the magnetopause, such as the occurrence and location of reconnection sites; Kelvin-Helmholtz waves, and boundary layer extent. Other configurations have shown evidence for a high-latitude reconnection site, closely following a period of low latitude reconnection, which is located on initially closed field-lines, where the magnetic field orientations inside and outside the magnetopause are close to anti-parallel.
    AGU Fall Meeting Abstracts. 12/2009;
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    ABSTRACT: Poleward Moving Radar Auroral Forms (PMRAFs) have been identified as the signature in Super Dual Auroral Radar Network (SuperDARN) of flux transfer events. As such they are particularly important in understanding the time dependence of reconnection at the dayside magnetopause. Few intervals where more than one radar observe these events have been published, however. The interval on 1 October 2002 is unusual in at least three respects: one is that at least 3 of the southern hemisphere SuperDARN radars simultaneously observe the events, a second is that there are observations in both hemispheres of these events, and the other is that the interval is unusually long over which these events are observed, over 4 hours. In this paper we report the observations of these events. Further, by comparison with the overall SuperDARN estimates of the ionospheric convection and the motion of the polar cap boundary from global auroral images, we estimate the contribution to the total transpolar voltage each individual PMRAF event.
    04/2009;
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    ABSTRACT: Observations from previous satellite missions have allowed the scale size of flux transfer events (FTEs) to be determined both normal to the magnetopause and in the direction of motion of the FTE (polewards when the IMF is strongly southward). However, a key difference between some different models of FTE structure is their azimuthal scale size, which will also determine what proportion FTEs contribute to the the global reconnection voltage. Whilst previous ground-based observations of the ionospheric signatures of FTEs have indicated that magnetic reconnection can occur coherently over large extents of the magnetopause, in situ determination of the azimuthal scale size of FTEs has not been possible until recent Cluster magnetopause crossing seasons when the separation of the spacecraft was ~10,000 km. In this talk, we present Cluster observations of flux transfer events from 27th January 2006 and 27th March 2007. We find some FTEs which extend further azimuthally than they do poleward, consistent with longer X-line models of FTE formation, but also some more spatially limited structures.
    04/2009;
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    ABSTRACT: We present simultaneous observations of flux transfer events (FTEs) made by the THEMIS and Cluster spacecraft on the 3rd May 2007, along with supporting observations of fast ionospheric flows made by the SuperDARN radar network. The THEMIS spacecraft were in a string-of-pearls formation approximately 20,000 km long, and crossed the post-noon magnetopause at low latitudes between 12:00 UT (TH-C) and 14:30 UT (TH-E). The Cluster spacecraft were in a triangular formation tangential to the magnetopause with a maximum separation of ~9,000 km, and were situated in the magnetosheath at high latitudes in the southern hemisphere, approaching the magnetopause which was crossed at about 16:00 UT. THEMIS observed 'standard' polarity FTEs between 11:00 and 15:00 UT, whilst Cluster observed 'reverse' polarity FTEs mainly between 11:00 and 14:00 UT. The two sets of FTEs are consistent with being generated at the same small region of a subsolar reconnection line. Fast poleward flows were observed in the 12:00 MLT sector, near the magnetic footprints of both Cluster and THEMIS, between 13:00 and 14:00 UT. However, in this interval the Double Star TC-1 satellite (situated nearer noon MLT) observed only one FTE suggesting either that the reconnection line was less active in this region or that some FTEs near to the subsolar point are not fully formed (Russell et al., 1985; Southwood et al., 1986). The large separation of the THEMIS spacecraft also allows us to track the motion of two FTEs across larger distances than the maximum separation of the Cluster spacecraft, and we find their acceleration to be negligible on this scale, adding confidence to the results of previous multi-spacecraft analyses of FTEs.
    04/2009;