Ann. Geophysicae 18, 1118±1127 62000) Ó EGS ± Springer-Verlag 2000

Multi-instrument study of footprints of reconnection in the summer

S. E. Pryse, A. M. Smith, I. K. Walker, L. Kersley Department of Physics, University of Wales, Aberystwyth, Ceredigion SY23 3BZ, UK

Received: 10 January 2000 / Revised: 17 April 2000 / Accepted: 18 April 2000

Abstract. Results are presented from a multi-instrument Bz, the ®eld merging is preferentially in the magneto- investigation of the signatures of equatorial reconnec- spheric lobe, and the opened ®eld lines initially move tion in the summer, sunlit ionosphere. Well-established sunward. These magnetospheric processes can extend ion dispersion signatures measured during three DMSP over very large volumes of space but their signatures satellite passes were used to identify footprints in are brought into close proximity when they are mapped ionospheric observations made by radio tomography, down the converging geomagnetic ®eld into the vicinity and both the EISCAT ESR and mainland . Under of the cusp. Equatorial reconnection drives a two-cell the prevalent conditions of southward IMF with the Bz high-latitude convective ¯ow, comprising antisunward component increasing in magnitude, the reconnection cross- ¯ow and return sunward ¯ows at lower footprint was seen to move equatorward through the latitudes in the dawn and dusk sectors 6Rei€ and ESR ®eld-of-view. The most striking signature was in Burch, 1985; Cowley, 1998). By contrast, lobe recon- the electron temperatures of the F2 region measured by nection is thought to generate a pair of lobe convection the EISCAT mainland that revealed signi®cantly cells with sunward cross-polar ¯ow. Tension forces enhanced temperatures with a steep equatorward edge, caused by the IMF By component distort the symmetry in general agreement with the leading edge of the ion of the convection cells, displacing the polar ¯ow from dispersion. It is suggested that this sharp transition in the solar meridian 6Cowley et al., 1991). Signatures of the electron temperature may be an indicator of the the velocity ®lter dispersion 6Rosenbauer et al., 1975), boundary, mapping from the reconnection site, between in energy ¯ux measurements of precipitating ions by closed geomagnetic ®eld lines and those opened along low-altitude satellites 6Newell and Meng, 1995), are which ions precipitate. characteristic of reconnection. The fastest particles arrive ®rst along newly opened ®eld-lines, with the less energetic ions being found subsequently when the Key words: Ionosphere 6ionosphere± opened ®eld-lines have convected downstream with interactions; particle precipitation; temperature the plasma. For southward IMF, with antisunward and density) ¯ow in the polar cap, the lower energy ions are found poleward of the dispersion high-energy edge, but in lobe reconnection the dispersion signature is reversed in the sunward convection. The energy deposition of cusp precipitation with particle ¯uxes of energy less than a few hundred eV produces enhancements in F-region 1 Introduction electron temperature 6Nilsson et al., 1994). Optical observations allowed Sandholt et al. 61998) to divide was proposed by Dungey 61961) auroral activity in the cusp into three main con®gura- as the key coupling process between the interplanetary tions, depending on the IMF clock angle. Type 1 magnetic ®eld and the geomagnetic ®eld. With a occur at typically 70° to 74° MLAT under the southward orientation of the IMF, reconnection is conditions of equatorial reconnection, with clock angles likely near the equatorial plane and the newly opened between 90° and 180°. Type 2 aurora are found at ®eld-lines are swept antisunward. For strongly positive 78°±79° MLAT under strongly northward IMF conditions, when the clock angle is in the range 0° to 45°. There is a coexistence of type 1 and type 2 auroral forms for clock angles between 45° and 90°. Footprint Correspondence to: S. E. Pryse signatures of reconnection processes in the spatial S. E. Pryse et al.: Multi-instrument study of footprints of magnetopause reconnection 1119 distribution of the ionospheric plasma density were reported by Walker et al. 61998) and Pryse et al. 61999) for Bz negative and positive respectively. The former study identi®ed a density enhancement co-located with type 1 aurora, with a steep density gradient on its equatorward side. A tilt in the F layer peak height was found with the altitude of maximum density increasing with latitude in keeping with the softening ion precip- itation. The latter companion paper for steady, north- ward IMF revealed the reverse dispersion signature, with the height of the F region peak increasing with decreasing latitude. The structure was bounded on its equatorward edge by a footprint of the adiaroic wall across which no plasma ¯ows. In a subsequent multi- instrument study, it was found that the EISCAT radar showed evidence of increased electron temperatures within the region of ion dispersion 6Pryse et al., 2000). Identi®cation of ionospheric plasma signatures of reconnection has relied heavily to date on comparisons with optical auroral forms. Of necessity the observa- tions were for the dark winter months of the polar ionosphere under a clear sky. The current work uses observations from many instruments to identify foot- prints of equatorial reconnection in the plasma during summer daylight. Measurements by particle detectors aboard satellites, radio tomography, and both incoher- ent and coherent scatter radars are presented, from experiments made in the magnetic noon sector on 20 August, 1998.

Fig. 1. Bx, By and Bz components of the Interplanetary Magnetic 2 Observations Field intensity measured by the ACE satellite upstream in the solar between 0700 and 0900 UT on 20 August, 1998 2.1 Interplanetary magnetic ®eld

The three components of the IMF measured upstream in the by the ACE satellite north of Svalbard, as expected for the negative Bz.In between 0700 and 0900 UT are shown in the three addition, there are hints of ¯ow toward the radar at the panels of Fig. 1. The estimated delay between the edges of the ®eld-of-view in-keeping with sunward observations and ionospheric response is about return ¯ows at lower latitudes. 70 min 6Lockwood et al., 1989). Of particular rele- vance is the relatively strong negative Bz component, initially steady at approximately )7 nT, but increasing gradually in magnitude after about 0730 UT and then 2.3 DMSP more abruptly around 0800 UT to reach some )12 nT. In comparison, both the Bx and By components were Three DMSP satellite passes traversed the noon-sector relatively weak, with the former being positive and polar cap during the time interval of interest. Each set of the latter mainly negative though with two positive observations showed clear signatures of ion dispersion excursions at the time of the decrease in Bz at around with a sharp low-latitude cut-o€ of the incoming particle 0800 UT. ¯ux. The electron and ion ¯ux spectra for the pass of satellite F14 around 0853 UT are shown in Fig. 3, where the equatorward edge for the ion dispersion can be identi®ed at 73.5 MLAT and 1240 MLT. Similar 2.2 Plasma convection dispersion signatures were revealed in two other satellite passes at 0832 and 1038 UT, with the MLT extent of the The SuperDARN radar 6CUTLASS) measured signatures depending on the geometry of the satellite line-of-sight plasma velocities that were relatively con- trajectory. The dispersion signatures for each of the stant throughout the time interval of interest. Figure 2 passes are illustrated in the MLAT/MLT plot of Fig. 4 for 0842:45 UT is representative of the period. The as triangular shapes where the base line indicates the velocities show antisunward ¯ow in the polar region cut-o€ of the high-energy ions at the open/closed 1120 S. E. Pryse et al.: Multi-instrument study of footprints of magnetopause reconnection

Fig. 2. Line-of-sight velocities measured by the Finland radar of the CUTLASS SuperDARN facility at 0842:45 UT on a geographic grid. Positive values represent ¯ows towards the radar and negative values ¯ows away from the radar

Fig. 3. Electron and ion ¯ux spectra for the pass of the DMSP F14 top of the electron ¯ux panel but the ion energies largest at the bottom satellite at around 0853 UT. The energy scales are in the opposite of the ion ¯ux panel senses for the two panels, with the electron energies being largest at the magnetic ®eld-line boundary and the apex indicates the 2.4Radio tomography tail of the lower-energy incoming ions observed along the satellite trajectory. An equatorward progression of A radio tomography satellite in the NIMS constellation the boundary can be seen. 6Navy Ionospheric Monitoring System) that crossed a S. E. Pryse et al.: Multi-instrument study of footprints of magnetopause reconnection 1121

Fig. 4. Plot showing the map- ping of the observations on a versus MLT grid. The locations of the ion dispersion signatures mea- sured during the three DMSP satellite passes at 0832, 0853 and 1038 UT are shown by the black triangles. The base of the trian- gles indicates the high-energy cut-o€ of the signatures and the apex the extent of the tail of softer precipitation. Also shown is the intersection at 300 km of the ESR radar beam between 0800 and 0950 UT. The pro- gression of the position of the equatorward edge of the region of elevated electron temperatures measured by the EISCAT mainland radar during OMOT observations between 0720 and 1030 UT is also marked

2.5 EISCAT Svalbard radar

The EISCAT Svalbard incoherent scatter radar 678.2°N) was operating as part of the OMOT UK Special Programme experiment with the radar pointing along the geomagnetic ®eld in the F region. The viewing region of the EISCAT Svalbard radar 6ESR) is illustrated in Fig. 4 where the intersection of the radar beam with an altitude of 300 km is shown between 0800 and 0950 UT. The DMSP ion dispersion signa- tures around 0832 UT were north of the ESR intersection latitude of about 75° MLAT 677.8°N) but to the south by 1038 UT. Comparison with Fig. 5 shows that the plasma enhancement in the tomographic was north of the ESR ®eld-of-view at around 0821 UT. The upper and lower panels of Fig. 6 show the Fig. 5. Tomographic reconstruction of the spatial distribution of electron density and electron temperature respectively, ionospheric electron density measured during a NIMS satellite pass measured by the ESR radar between 0800 and 0950 UT that crossed 75°Nat0821UT and analysed using a post-integration time interval of 30 s. At the beginning of the time of interest, when the DMSP dispersion signature and tomography density latitude of 75°N at 0821 UT yielded the broad structure enhancement were north of the radar viewing region, of the spatial density distribution of ionospheric plasma the densities and electron temperatures observed by shown in Fig. 5, where the image plane maps to ESR were low. However, at about 0837 UT there was approximately 1050 MLT at 75°N. In the southern an abrupt onset of increased values in both parameters region of the ®eld-of-view the increasing e€ect of solar- that can be attributed to the precipitation region production with decreasing latitude is seen in what is moving equatorward and coming into the ®eld-of-view essentially a summer mid-latitude ionosphere. However, of the radar in response to the increasingly negative Bz. north of 78°N there is a density enhancement separated It can be noted that during the period of enhanced from the ionosphere to the south by a density trough. densities and temperatures lasting until about 0930 UT The location of the region of elevated density was to the there are variations in both parameters on time scales of north of the most northerly tomography receiver so a few minutes, though a detailed discussion of this ®ne that the ®ner details of the plasma density signatures of structure is beyond the scope of the present study. The reconnection are not resolved, though the steeper densities and temperatures reverted to near former gradient on the equatorward edge of the feature can background levels after about 0930 UT with contours be identi®ed 6Walker et al., 1998). rising to greater altitudes in the bottomside. The return 1122 S. E. Pryse et al.: Multi-instrument study of footprints of magnetopause reconnection

Fig. 6. Electron densities 6upper panel) and electron temperatures 6lower panel) measured by the ESR radar between 0800 and 0950 UT observing with the beam aligned along the geomag- netic ®eld in the F region. These measurements were degraded by interference for a few minutes around 0920 UT to background levels can be interpreted as being linked becoming increasingly apparent as the feature came into to the precipitation region associated with the ion the centre of the viewing area. The latitudinal movement dispersion having moved further to the south, out of the of the equatorward edge, de®ned at an electron temper- ESR ®eld-of-view. ature of 3500 K and mapped down ®eld lines to an altitude of 350 km, is also plotted in geomagnetic terms in Fig. 4. A gradual southward motion of the gradient is 2.6 EISCAT mainland radar seen to start at about 0820 UT, with the progression becoming more rapid at about 0915 UT. While it is not The most striking e€ect of the southward progression of possible to be precise about the exact timing, because of the reconnection signature can be seen in the electron the 20 min cycle time of the EISCAT scans, it is likely temperatures observed by the EISCAT mainland radar that the relatively slow latitudinal motion occurred in 669.6°N) which was also observing in the OMOT UK response to the gradually increasingly-negative Bz Special Programme mode, scanning in the geographic measured in the IMF, and that the more rapid motion meridian in cycles of 20 min duration. In each cycle, the was linked to the later sharper change in Bz. It is also of radar started pointing in the zenith, and then scanned note that the motion of the equatorward edge of the northward in latitudinal steps of 0.125° at 300 km with elevated temperatures is in general agreement with the a dwell time of 20 s at each pointing direction. The equatorward edge of the dispersion signatures observed electron temperatures measured between 0800 and 1030 by the second and third DMSP satellite passes. UT are shown in the sequence of panels in Fig. 7. The The electron densities measured by the scans between ®rst plot contains a hint of increased temperatures at the 0800 and 0920 UT, though not shown here, revealed extremity of the ®eld-of-view. These furthest range gates only the characteristics of solar-controlled F layer in the most northerly pointing directions represent plasma with a peak height at about 300 km. The returns from the topside ionosphere to the north of absence of footprints of the reconnection process is the ESR ®eld-of-view. The region of elevated electron not surprising as the precipitation, apparent in the temperatures moved progressively to lower latitudes elevated electron temperatures at higher altitudes, maps with time, with the sharpness of the equatorial edge to the F region at latitudes north of the viewing volume. S. E. Pryse et al.: Multi-instrument study of footprints of magnetopause reconnection 1123 1124 S. E. Pryse et al.: Multi-instrument study of footprints of magnetopause reconnection

Fig. 8. Electron densities mea- sured by the EISCAT mainland radar observing in the OMOT UK Special Programme mode for the scans between 0920 and 1020 UT

However, structure is observed near the layer peak at exceeded those of the solar-ionisation at latitudes to its the northern extremity of the scan starting at 0920 UT, south. In addition, the altitude of the layer peak within and this can be seen to move progressively southward in the enhancement was marginally lower than that of the the following plots 6Fig. 8). Close inspection of the solar-produced plasma. These characteristics of the bottom panel, for the scan starting at 1000 UT, shows enhancement, and its co-location with the elevated that the densities in the enhancement in the north electron temperatures at the F-peak altitude, suggest that some of the ionisation originated from the soft b precipitation and that it formed at the equatorward edge Fig. 7. Electron temperatures measured by the EISCAT mainland of the ion dispersion signature of the reconnection. radar observing in the OMOT UK Special Programme mode between However, it must be noted that this precipitation- 0800 and 1030 UT produced ionisation constitutes only part of the total S. E. Pryse et al.: Multi-instrument study of footprints of magnetopause reconnection 1125

Fig. 9. Ion temperatures mea- sured by the EISCAT mainland radar observing in the OMOT UK Special Programme mode during the scan starting at 1000 UT density within the enhancement, with solar radiation during the summer daytime, the interpretation has relied also playing an important role in the sunlit summer on DMSP particle data. However, it can be noted that ionosphere. Berry et al. 61999) have already linked DMSP measure- The density trough separating the enhanced density ments of particle ¯uxes and spectra to discrete structures structure to the north and the strictly solar-produced in the ionospheric plasma in the post-noon sector ionosphere to the south can be related to a region of imaged by radio tomography. elevated ion temperatures. It can be seen from the scan During the course of observations the IMF condi- starting at 1000 UT, shown in Fig. 9, that a ®eld-aligned tions were such that the ®eld-of-view of the ESR radar region of higher ion temperatures at altitudes above the changed from being on closed geomagnetic ®eld lines F-region peak is located between 73.5° and 74.0°N. It is with the signature of ion dispersion completely to the also of note that EISCAT tristatic westward velocities north, to being on open ®eld-lines. It might be expected measured in this region were found to be in excess of that a signature of the dispersion could be found in the 1kms)1. The narrow trough in electron density was ESR measurements, as the region moved equatorward thus likely to be a consequence of ion frictional heating through the ®eld-of-view. For example, it is possible that resulting in an enhanced loss rate. Comparing the an increase in peak height might be observed due to electron densities at the peak, it can be seen that a the most energetic particles coming ®rst into the ®eld-of- depletion of some 0.8 ´ 1011 m)3 occurs in the back- view followed by the slower particles in the dispersion ground density at about 73.5°N in the 40 min between tail. In general, inspection of the ESR measurements the top and bottom panels of Fig. 8. A simple calcula- within the precipitation shows evidence for many tion shows that increased chemical recombination rates, transient events, over time scales of minutes and much for an ion temperature of 2000 K and a velocity of less than that of the general trend in the southward 1.2 km s)1 6Rodger et al., 1992) can result in a depletion motion. The ongoing pulsed reconnection throughout of this magnitude on such a time scale, though in the period comprised many individual events whose practice the actual situation was undoubtedly more varying characteristics serve to mask any possible complex. signature in the layer height of the slow progression of the reconnection region and the associated ion disper- sion. However, it can be noted that in the later stages of 3 Discussion and conclusions the observations shown in Fig. 6 the mapping of the reconnection site moved south of the ®eld-of-view. The work concerns the interpretation of ionospheric Consistent with this motion is an increase in the height signatures of equatorial reconnection under sunlit of the contours of the bottomside electron density and summer conditions. Emphasis has been placed on the the topside electron temperature after about 0930 UT. gross behaviour of the plasma in response to a trend in The ESR results shown in Fig. 6 are in broad qualitative Bz over a period of more than an hour, rather than agreement with the predicted signatures of pulsed transient events on time scales of a few minutes. A study reconnection modelled by Davis and Lockwood by McCrea et al. 6submitted 2000) for wintertime 61996). In addition, the sequence of reconnection events observations of the large-scale response of the iono- suggested here is in accord with the meridian scanning sphere to changes in IMF orientation is also presented photometer results of Sandholt et al. 61992), who in this issue. In the current study footprints in the F showed that the 630.0 nm luminosity was characterised region plasma, consistent with equatorial reconnection by a series of poleward-moving auroral forms repre- during a period of increasingly negative Bz, have been senting the signatures of ongoing reconnection around identi®ed by the EISCAT ESR and mainland radars, midday in winter when such observations were possible. radio tomography, and the CUTLASS radar. In the Comment should be made on the discrepancy between enforced absence of measurements of optical emissions the electron densities measured by the ESR and the 1126 S. E. Pryse et al.: Multi-instrument study of footprints of magnetopause reconnection

EISCAT radars, with the former being greater by some circulating on ¯ux-tubes in the night side, being carried 70% for a common observing volume. Con®rmation in the sunward return ¯ow into the dayside ionosphere. that it is the ESR densities that are high is obtained by However, in the present case there is clear evidence to comparison with the tomographic image. It has not indicate a local formation mechanism for this high- proved possible to check the calibration of the ESR latitude dayside trough as the plasma is eroded in a radar in this case as no observations are available from latitudinally con®ned region of ion frictional heating. the ionosonde at for the period under Central to the investigation were signatures of study. However, the absolute values of the electron incoming ion ¯ux along newly opened ®eld-lines that densities are of little concern to the work presented here. were being swept antisunward. The DMSP passes Unlike the investigation by Walker et al. 61998), the showed a region of precipitation extending over about current study did not provide evidence for signatures in two degrees in latitude with an energy dispersion the ionospheric E region relating to the reconnection signature. The spatial distribution of ionospheric plasma process. The operating modes of the radars were not measured by radio tomography showed an enhancement designed for E layer studies, but there was little in the F layer with a sharp equatorward edge that was indication of the elevated electron temperatures extend- also some two degrees in latitudinal extent and that ing down to the lowest altitudes. This absence of mapped to the region of ion dispersion. Signi®cantly E region e€ects is not surprising within the general cusp elevated electron temperatures in the topside ionosphere region where the incoming particles are generally of low measured by the EISCAT mainland radar were also energy, and indeed in the present observations DMSP attributed to the e€ect of the incoming precipitation. measured only soft-electron precipitation. These results The asymmetry in the latitudinal distribution, with an are in accord with the modelling work of Millward et al. abrupt low-latitude edge and a di€use northern extrem- 61999) who predicted that ion precipitation into the cusp ity to the enhanced temperatures, supported the disper- region produced ionisation at heights in the vicinity of sion interpretation. It is possible that the sharp the F1 ledge, with the plasma production due to soft- temperature transition at the equatorward edge may be electron precipitation maximising near the F2 peak. an identi®er in the ionosphere of the boundary between The most striking ionospheric signature of the open and closed magnetic ®eld lines. A narrow latitu- dispersion region in the present study was found in the dinally localised enhancement in ion temperature was electron temperatures measured by the mainland observed by the EISCAT radar just south of the limit of EISCAT radar that is attributed to energy deposition the region of elevated electron temperatures resulting in of the incoming precipitation 6Nilsson et al., 1994). The a density trough formed by frictional heating. progress of the reconnection footprint was marked by an abrupt equatorward edge with a tail of elevated, Acknowledgements. Financial support for the project has been but decreasing, temperatures extending to the north. provided by the UK Particle Physics and Astronomy Research Figure 4 indicates good general agreement between the Council under grant PPA/G/S/1997/00269. The assistance of the University of Tromsù, UNIS Longyearbyen and the Norwegian motion of the southern boundary of this region of Polar Research Institute at Ny AÊ lesund in the tomographic increased electron temperature and the equatorward measurements is gratefully acknowledged. EISCAT is an interna- edge of the dispersion signatures seen at the di€erent tional facility supported by the national science councils of universal times by the DMSP satellites. The temperature Finland, France, , , , and the boundary appears to be indicative of the position of the . The DMSP data were provided by F. Rich at footprint of the reconnection site. It appears that this AFRL, Hanscom, USA. The CUTLASS radar is a UK national facility funded by the Particle Physics and Astronomy Research may represent a signature of use in identifying the Council; the data used here have been supplied by T. Yeoman. The separatrix between the open and closed magnetic ®eld- ACE data were obtained from the CDAWeb, NASA. lines. Indeed it may be that a detailed investigation of Topical Editor M. Lester thanks S. W. H. Cowley for his help the slope of such a boundary under steady southward in evaluating this paper. IMF conditions, could mark the inclination of the newly opened magnetic ®eld-lines. A possible study could be References one reminiscent to that carried out by Jones et al. 61997), who used localised signatures of discrete soft Berry, S. T., L. Kersley, J. Moen, and W. F. Denig, Ionospheric precipitation to measure the inclination of the geomag- signatures of magnetospheric boundaries in the post-noon netic ®eld above EISCAT. sector, Ann. Geophysicae, 18, 74±80, 2000. 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