Proceedings Cluster/Double Star Conference

SPACE AND GROUND-BASED INVESTIGATIONS OF DAYSIDE RECONNECTION: CLUSTER, DOUBLE STAR AND SUPERDARN OBSERVATIONS

J. A. Wild1, S. E. Milan2, J. A. Davies3, S. W. H. Cowley2, M. W. Dunlop3, C. J. Owen6, J. M. Bosqued5, M. Lester2, A. Balogh4, C. M. Carr4, A. N. Fazakerley6, and H. R`eme5

1Department of Communication Systems, Infolab21, Lancaster University, Lancaster, LA1 4WA, UK 2Department of Physics & Astronomy, University of Leicester, Leicester, LE1 7RH, UK 3Rutherford Appleton Laboratory, Didcot, Oxfordshire, OX11 0QX, UK 4Blackett Laboratory, Imperial College, London SW7 2BZ, UK 5CESR/CNRS, 9 Avenue du Colonel Roche BP 4346, 31028 Toulouse, Cedex 4, France 6Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Surrey RH5 6NT, UK

ABSTRACT a larger spatial scale, but at what point would the spacecraft cease to act as a single experiment and revert to four individual point-measurements, each In this paper, we present an overview of several in- with the spatial/temporal ambiguities that the Clus- vestigations that have exploited Cluster, Double Star ter mission was intended to overcome? One solution and SuperDARN radar data in order to scrutinise might be to deploy more spacecraft, nesting clusters the coupling of the solar wind, magnetosphere and of spacecraft inside larger clusters in order to investi- ionosphere. The studies introduced have drawn upon gate multiple spatial scales simultaneously. An alter- simultaneous space- and ground-based data in or- native would be to exploit the pre-existing network der to overcome the inherent shortcomings of the of ground-based experiments to provide remotely- in situ (space-based) and remotely-sensed (ground- sensed measurements of the magnetosphere in sup- based) measurement techniques. In particular, we port of the Cluster mission. This paper will address shall highlight the results of studies that investi- the latter of these options. gate the dynamics arising from magnetic reconnection at the dayside magnetopause and the resulting Ground-based investigations have much to offer the ionospheric responses. Cluster mission. Typically, ground-based instruments enjoy extensive fields-of-view when mapped Key words: Cluster; Double Star; SuperDARN; Re- from the Earth into the magnetosphere, due ei- connection; Ground-based. ther to the large viewing area of each instrument (e.g. ionospheric radars, all sky cameras, riome- ters) or due to the networking of numerous indi- 1. INTRODUCTION vidual diagnostics (e.g. magnetometers, ionoson- des). Furthermore, many ground-based experiments operate continuously and allow the monitoring of Cluster is the first magnetospheric mission designed ionospheric/magnetospheric conditions over many to harness the power of multi-point measurements in hours, thus providing context to the multi point in order to resolve the spatial/temporal ambiguities in- situ measurements. herent in single spacecraft investigations. This multipoint measurement capability is crucial if the bound- In this paper, we shall focus upon Cluster/ground- aries, structure and dynamics of the complex terres- based investigations that exploit the Super-Dual Au- trial magnetosphere are to be understood. However, roral Radar Network (SuperDARN; Greenwald et al. Cluster can only measure these characteristics over 1995) in order to study the signatures of dayside the spatial scale of the inter-spacecraft separation. reconnection. The SuperDARN network currently During the first five years of the Cluster mission, this comprises 17 high-frequency (HF) coherent-scatter has typically been in the range of a few hundred to radars, 10 located in the northern hemisphere and 7 several thousand kilometers, i.e. small compared to in the southern hemisphere. The radars are nearly the scale of the magnetosphere. Of course, it is pos- identical and operate in the 8-20 MHz range, rou- sible to expand the separation of the Cluster space- tinely measuring the line-of-sight (l-o-s) Doppler ve- craft in order to characterise features that exist on locity and spectral width of, and the backscattered

______Proceedings Cluster and Double Star Symposium – 5th Anniversary of Cluster in Space, Noordwijk, The Netherlands, 19 - 23 September 2005 (ESA SP-598, January 2006) 2

power from, decametre scale plasma irregularities ary layer. Subsequently, the outward motion of the which have been shown to drift with the ionospheric boundary caused the spacecraft to re-enter the mag- E×B velocity (Ruohoniemi et al. 1987; Villain et al. netopause at ∼10:22 UT before departing the mag- 1985). In normal operation the field of view (f-o- netosphere for the final time (during this interval) at v) of each radar is formed by scanning sequentially ∼10:33 UT. through 16 beams of separation 3.24o, each beam gated into 75 ranges of 45 km each. Typically, the Prior to and following the magnetopause crossings, dwell time on each beam is 3 or 7 s resulting in a the four Cluster spacecraft observed a series of mag- scan of the complete f-o-v covering 52o in azimuth netospheric and then magnetosheath flux transfer and over 3000 km in range every 1 or 2 min. Con- events (FTEs). Each was characterised by a nor- sequently, the 17 SuperDARN radars routinely mon- mal polarity (positive-negative) bipolar perturba- itor the ionospheric convection pattern over a sig- tion to the component of the magnetic field normal nificant fraction of the auroral ionosphere in both to the local magnetopause, an enhancement in the hemispheres. overall magnetic field strength, and mixing of the magnetospheric and magnetosheath plasmas. The field tilting effects in the plane of the magnetopause 2. INTERHEMISPHERIC OBSERVA- (the αLM parameter) were interpreted as evidence of TIONS OF FLUX TRANSFER EVENTS open northern hemisphere magnetic flux tubes moving poleward and dawnward, following reconnection somewhere duskward and equatorward of the space- In a pair of papers drawing upon data from the craft, as shown in Figure 2. In Figure 2a, the motion first month of Cluster science operations, Wild and of an open flux tube (solid lines marked ’1’-’4’) fol- co-authors investigated dayside reconnection under lowing a low-latitude reconnection event in the dusk southward and duskward interplanetary magnetic sector (near ’1’) and (b) the temporal evolution of field conditions (Wild et al. 2001, 2003). Between 09– an open flux tube (solid lines marked ’i’-’iv’) follow- 11 UT on 14 February 2001, the Cluster spacecraft ing a low-latitude reconnection event in the dawn traverse the high latitude magnetopause in the post- sector (near ’i’) are shown. The approximate loca- noon sector (∼14 MLT). Figure 1 presents data from tion of the Cluster spacecraft is also indicated. The the Cluster 1 spacecraft and the ACE upstream solar grey arrows in the figure show the direction of open wind monitoring spacecraft between 09:15–11:15 UT field line motion, including westward motion in the on this day. The top panel shows the clock an- northern cusp (and eastward motion in the south- gle of the interplanetary magnetic field measured ern cusp). The arrowed short-dashed lines indicate at the ACE spacecraft and lagged by 55 min such magnetospheric field lines within the magnetopause that comparisons can be made with Cluster observa- boundary region tions at the front of the magnetosphere. The next three panels present the component of the magnetic This interpretation was entirely consistent with the field normal to the magnetopause, the total mag- simultaneous ionospheric observations. While the netic field strength, and the angle of the magnetic Cluster spacecraft were located within the mag- field in the plane of the magnetopause (the αLM netosphere, their northern hemisphere footprints parameter) measured by the FGM instrument on- mapped to the field-of-view of the CUTLASS pair board the Cluster 1 (Rumba) spacecraft. The fifth of SuperDARN radars (Lester et al. 2004). Figure and sixth panels present the total ion density profile 3 presents l-o-s Doppler velocity and backscattered and ion energy-time spectrogram (all pitch angles) power measurements along three beams of the Fin- observed by the CIS (HIA) instrument on Cluster land CUTLASS radar. In the power panels black 1. Finally, the lower panel presents the electron en- indicates insufficient signal-to-noise ratio to deter- ergy distribution measured in the field-parallel direc- mine the spectral characteristics of the backscat- tion by the PEACE HEEA sensor, also on Cluster 1. ter. In the Doppler shift panels, velocities are only Overlaid on this figure (dashed lines) are the times shown where significant power is observed. Nega- of events discussed below. Specifically the centre tive velocities represent Doppler shifts away from times of four magnetospheric FTEs, the entry into the radar along the line-of-sight. During this inter- the boundary layer (BL), three crossings of the mag- val the radar’s poleward and westward field-of-view netopause (MP), and four magnetosheath FTEs are straddled the noon sector high latitude ionosphere indicated. in the northern hemisphere. These beams look in the westward (dawnward) direction. Superimposed Throughout the interval, the solar wind clock an- on each panel are the times of note identified in the gle was typically between 900–180o (i.e. southward Cluster observations, as in Figure 1, specifically the and duskward) with only occasional northward or centre times of the four magnetospheric FTEs, the dawnward excursions. Motion of the magnetopause entry into the boundary layer (BL), three crossings relative to Cluster resulted in the spacecraft encoun- of the magnetopause (MP), and four magnetosheath tering the boundary three times during this inter- FTEs. Throughout the interval, the Finland radar val. The first (outward) traversal at ∼10:17 UT was observed a series of poleward-moving radar auroral preceded by an encounter with a complex bound- forms (PMRAFs). These features, indicated by ar- 3 α Log Differential Energy Flux 10 (ev cm sr s ev ) Ions 2- -1-1 -1 -2 ons ctr Ele

Figure 1. Plot of ACE and Cluster data for the interval 09:15-11:15 UT on 14 February 2001. From Wild et al. (2003). rows in the upper (beam 1) power panel, are the observed by Cluster and the pulsed enhancements accepted ionospheric counterparts to FTEs. Re- of convection (pulsed ionospheric flows - PIFs) and lated events are observed in each of the other beams power observed by the Finland radar in the (74o– shown. These observations correspond to regions 76o Mlat) latitude range. These observations sug- of ionospheric plasma being dragged polewards and gested that the reconnection events had a large spa- dawnward across the noon sector at slightly higher tial scale and that the low-latitude flow region corre- latitude (≥76o Mlat) than the Cluster footprint. sponded to the footprint of newly-opened flux tubes. Ionospheric motion in the lower latitude region (74o– The poleward convection enhancements observed at 76o Mlat) was characterised by low velocity flow higher latitudes were the fossil-like signatures of the essentially westward and northward, in agreement reconnected flux tubes in the ionosphere. with the positive IMF BY , and detailed study of this region revealed pulses of ionospheric flow and The ionospheric convection geometry provided by of backscatter power. Figure 4a presents the flow the CUTLASS radars confirmed the dusk location measurements in more detail. of the magnetopause reconnection site inferred from the Cluster data. The subsequent study of magnet- Detailed examination of the Cluster and Super- ically conjugate Syowa East SuperDARN radar ob- DARN data indicated a clear one-to-one correla- servations in the Southern Hemisphere (Wild et al. tion between the signatures of magnetospheric FTEs 2003) again revealed modulations of the convection 4

a. 4

3 2

i Figure 3. Three pairs of backscatter power and Doppler shift measurements from beams 1, 2, and ii 3 of the Finland SuperDARN radar during the same iii interval as presented in Figure 1. From Wild et al. (2001).

iv to the right (left) of the plot. In each case the open- Figure 2. Sketch of the Earth’s dayside magne- closed field line boundary (OCFLB) is represented topause in a view looking from the Sun, showing the by a solid black line. The field-of-view of the Fin- temporal evolution of open flux tubes. From Wild land (Syowa East) radar is indicated in the northern et al. (2003). (southern) hemisphere by the grey shaded region at approximately 10 UT. Where a perturbation (i.e. a region of newly-opened flux tubes appended to the flow at low latitudes and poleward convection en- OCFLB) is shown, the quasi-equilibrium position of hancements at high latitudes, as presented in Figure the boundary is indicated by a short dashed line. 4b and c. These flow modulations were well corre- Following integration into the (open) polar cap, the lated with the flow pulsations observed by the Su- region of newly-opened flux is indicated by a dot- perDARN Finland radar and with the magnetic per- dashed line whilst the ionospheric flow that brings turbations observed by Cluster. As expected, the about the inclusion of this region into the polar cap reconnected flux tubes in the Southern Hemisphere is indicated by a shaded arrow. The expected north- propagated poleward and eastward, the asymmetry ern and southern hemisphere responses to the addi- between the hemispheres being due to the non-zero tion of open flux in the post-noon sector are shown BY component of the IMF. However, the location in (a) and (b) respectively. Similarly, the expected of this radar around noon MLT implied that the northern and southern hemisphere responses to the reconnected flux tubes had to be generated on the addition of a spatially extended region open flux that dawn magnetopause in order to propagate duskward straddles noon are shown in (c) and (d). The convec- though the field-of-view of the radar. Consequently, tion flow modulations observed simultaneously in the the reconnected flux tubes observed in the southern two hemispheres in both the pre- and post-noon sec- ionosphere were not the same as the reconnected flux tors supported that the reconnection occurred along tubes seen simultaneously by the Finland radar and a single reconnection line extending over at least 4 h Cluster. This is illustrated schematically in Figure 5. MLT. The ground-based data in the Southern Hemi- In each figure, the view is from a location above the sphere were thus necessary to infer the large-scale northern magnetic pole with the noon meridian indi- geometry of the magnetopause reconnection in this cated by a long dashed line and dawn (dusk) located case. 5

a. b. NOON NOON a.

DUSK DAWN DUSK DAWN

NORTHERN HEMISPHERE SOUTHERN HEMISPHERE c. d. b. NOON NOON

DUSK DAWN DUSK DAWN

NORTHERN HEMISPHERE SOUTHERN HEMISPHERE c. Figure 5. Schematic indicating the expected perturbations to the ionospheric open-closed field line boundary (OCFLB) and the resulting ionospheric flows due to the addition of open flux during a flux transfer event. From Wild et al. (2003).

magnetic field normal to the magnetopause, enhance- Figure 4. (a) Velocity data from beam 3 of the CUT- ments in the overall magnetic field strength, and field LASS Finland SuperDARN radar, in a format simi- tilting effects in the plane of the magnetopause whilst lar to that in Figure 3, but with a revised colour scale the satellites were located on the magnetosheath side which reveals the pulsing of the line-of-sight flow in of the boundary. These observations are summarised the band of lower-latitude scatter. (c) Velocity data in Figure 6. Whilst a subset of the FTE signatures from beam 0 of the Syowa East SuperDARN radar, observed could be identified as being either normal presented in the format of (a), albeit with a differ- or reverse polarity, the rapid succession of events ob- ent colour scale and a reversed latitude axis. (b) The served made it difficult to classify some of the signa- mean velocity averaged over the latitude range indi- tures unambiguously. By employing the flux tube cated by dashed horizontal lines in (a) and (c) (and model of Cooling et al. (2001), the source region given positive values in this case, though in each case and motion of flux tubes opened by magnetic recon- the flow is directed away from the radars). The in- nection at low latitudes (i.e. between Cluster and digo trace corresponds to the Finland average veloc- Geotail) was investigated. It was demonstrated that ities whilst the red trace corresponds to the Syowa the spacecraft observations were consistent with the East average velocities. Superimposed on each panel motion of northward (southward) and tailward mov- are the times of note identified in the Cluster obser- ing flux tubes anchored in the northern (southern) vations, as in Figure 1. From Wild et al. (2003). hemisphere passing in close proximity to the Cluster (Geotail) satellites. The multi-spacecraft approach, coupled with a realistic model of flux tube motion in the magnetosheath, enabled the authors to infer 3. MULTIPOINT IN-SITU AND the approximate position of the reconnection site, GROUND-BASED OBSERVATIONS which in this case was located at near-equatorial lat- OF FTES itudes. Unfortunately, this favourable conjunction of spacecraft occurred over the Siberian sector, a region where the coverage provided by ground-based The launch of the first Double Star spacecraft in experiments is sparse. late 2003 presented scientists with an opportunity to make coordinated multipoint in situ measurements In one of the first results to come from the Dou- at differing spatial scales. As a precursor to such ble Star mission, Wild et al. (2005b) exploited mea- investigations, Wild et al. (2005a) exploited a for- surements from a very similar conjunction to that in tunate and favourable conjunction of the Cluster Wild et al. (2005a) that occurred on 25 March 2005. and Geotail spacecraft on 17 February 2003 during At ∼07 UT this day, the equatorial Double Star which Geotail skimmed the sub-solar magnetopause spacecraft (TC1) entered the magnetosphere slightly as Cluster traversed the high-latitude magnetopause. southward of the subsolar point at ∼11.5 MLT (Fig- This conjunction culminated in the observation of a ure 7). Around one hour later (∼8 UT) the four series of flux transfer events (FTEs), characterised Cluster spacecraft, separated from one another by by bipolar perturbations in the component of the ∼250 km, traversed the mid-latitude magnetopause 6

Figure 7. The locations of the Cluster 1 (circles) and equatorial Double Star (stars) spacecraft in the noon-midnight meridian during the interval under Figure 6. A comparison of |B| and BN measure- scrutiny. The location of a Shue et al. (1997) magne- ments from Cluster 3 and Geotail on 17 February topause is also indicated. From Wild et al. (2005b). 2003. FTEs indicated by arrows, labelled i–vii (Clus- ter) and a–d (Geotail). From Wild et al. (2005a).

of the Cooling model that support the data interpretation during this interval. This figure shows six in the outbound direction at very similar magnetic views of the dayside magnetopause in the GSM Y-Z local time to Double Star’s inbound crossing. Dur- plane as viewed from the Sun. The concentric dotted ing the interval 06–09 UT (i.e. during the Dou- circles indicate the magnetopause in the GSM Y-Z ble Star and Cluster magnetopause encounters), the plane at X positions of X=+5 RE, 0 RE, −5 RE, IMF was generally dominated by the duskward com- and −10 RE while the cusps are represented by the ponent with a small and variable north-south com- diamond symbol. In this model, the cusps are po- ponent. While still in the magnetosheath, Double sitioned at the GSM locations [0.5 RMP , 0, ±RMP ] Star observed several (at least three) normal polar- where RMP is the radius of the model magnetopause ity FTEs as indicated in Figure 8. Similarly, shortly at the subsolar point. In this case, RMP has been set after entering the magnetosheath, the four Cluster to 10 RE, roughly the value predicted by the model spacecraft also observed a series of normal polarity of Shue et al. (1997) during this interval. Figure 9a magnetosheath FTEs. The polarity of these signa- shows the boundary layer (BL) flow stream lines re- tures therefore indicated of open flux tubes anchored sulting from an X-line ∼10 RE in length that is cen- in the northern hemisphere passing by the spacecraft. tred upon the subsolar point and oriented along the direction of the magnetopause current (akin to the In order to investigate the likely source region of the component reconnection hypothesis (e.g. Cowley open flux tubes (FTEs), Wild et al. (2005c) have also 1976)). Blue (red) streamlines indicated BL flow as- employed the Cooling et al. (2001) model of open flux sociated with newly reconnected field lines anchored tube motion in the magnetosheath (often referred in the northern (southern) hemisphere. The IMF di- to as “the Cooling model”). This model employs rection employed corresponds to the appropriately the simple stress balance computation presented in lagged IMF observed by the ACE spacecraft The re- Cowley & Owen (1989) in order to calculate the ve- sulting IMF clock angle, corresponding to 06:50 UT locity of open magnetic flux tubes at the point that at the magnetopause, is indicated in the upper right- the tube penetrates the surface of the magnetosphere hand corner of the figure. This corresponds to the (i.e. the de Hoffmann–Teller (DHT) velocity of the interval when the Double Star spacecraft was tra- flux tube.) In this case, the Cooling model has also versing the magnetopause boundary layer just prior been used to calculate the plasma velocity in the to crossing the magnetopause. In this case, the Cool- magnetopause boundary layer (i.e. the velocity of ing model indicates that a low latitude reconnection plasma inside an open flux tube in the rest frame of line would result in a boundary layer plasma flow the magnetosphere). Figure 9 presents the results that is directed mainly dawnward at the location of 7

reconnection X-lines. Indeed, it should also be noted that ﬂux tubes are dragged across the X-line in both hemispheres, indicating that steady reconnection at these X-lines is not possible.

A similar comparison between Cluster measurements and model is presented in figures 9d, e and f. Fig- ure 9d shows the boundary layer flow streamlines corresponding to the IMF orientation at 07:55 UT (when Cluster traversed the magnetopause boundary layer) while Figure 9e shoes the loci of open flux tubes over the surface of the magnetopause at 08:25 UT (when Cluster was observing FTEs). The location of the Cluster quartet is indicated by the black filled circle. The direction of poleward and dawnward boundary layer plasma flow observed by Cluster (not shown) was almost identical to the expected BL plasma flow predicted by the Cooling model, although in this case, the speed of the modelled BL plasma flow (∼180 km s−1) was larger than the observed flow (∼120 km s−1) by ∼50%. Of course, the multipoint Cluster measurements allow the speed of the open flux tubes to be calculated as the characteristic bipolar signature convected over the four spacecraft. In this case, multi-spacecraft analysis techniques indicated that the open flux tubes were were travelling at ∼65 km s−1 in the poleward and dawnward direction. At the location of the Figure 8. Magnetic field measurements from the (a) Cluster spacecraft, the Cooling model predicted that Double Star TC1 and (b) Cluster 1 spacecraft. The flux tubes would be travelling in exactly the direction component of the magnetic field normal to the local observed, but at much greater speed (∼190 km s−1). magnetopause and the overall magnetic field strength As was the case previously (and despite a rotation are presented. Magnetopause crossings and FTEs of the IMF at this time to a duskward and slightly are indicated by dashed lines and arrows respectively. southward orientation) open flux tubes originating From Wild et al. (2005b). from the X-line defined by the antiparallel reconnection hypothesis (Figure 9f) would be expected to be dragged tailward, away from the location of the Clus- Double Star (indicated by a black star). The Double ter and Double Star spacecraft. Star plasma measurements (not shown) indicate that at this time the spacecraft briefly passed through From these observations, it is concluded that while a layer of ∼130 km s−1 dawnward directed plasma in the vicinity of the magnetopause between 07– flow, exactly as predicted by the Cooling model. 08 UT on 25 March 2005, the Cluster and Double By ∼07:15 UT Double Star had entered the mag- Star spacecraft observed open flux tubes that were netosphere, re-entered the magnetosheath (where it anchored in the northern hemisphere retreating from observed the FTEs presented in 8) and was about the a low-latitude X-line which was probably tilted with re-enter the magnetosphere for the final time. Due to respect to the magnetic equatorial plane. The Cool- the single-point nature of the Double Star observa- ing model is able to accurately reproduce the direc- tions, it is not possible the estimate the direction of tion of boundary layer plasma flow and flux tube propagation of the flux tubes as they passed over the motion observed by the spacecraft during this inter- spacecraft. However, based upon the results of the val. However, at higher latitudes, the Cooling model Cooling model ( 9b), the open northern hemisphere consistently over-estimates the plasma and flux tube flux tubes launched from a low latitude reconnection speeds. This might be explained by the recent find- would pass over the location of Double Star. This ings of Longmore et al. (2005) who, based upon is consistent with the normal polarity bipolar signa- 4 years of Cluster data, reported sub-alfvénicflow tures observed by the spacecraft. Conversely, if the in the vicinity of the magnetospheric cusps. This location of the X-line(s) is defined by the antipar- sub-alfvénicflow region is not described by the gas- allel reconnection hypothesis (e.g. Crooker 1979), as dynamic model of magnetosheath plasma flow on in Figure 9c, newly-opened flux tubes would move which the Cooling model is based. tailwards under the action of the solar wind flow in which one end of each field line remains embedded. Figure 10 presents an overview of the ground-based As such, it seems unlikely that flux tubes (FTEs) ob- data during this interval. At 07:30 UT, the Dou- served by Double Star originate from high-latitude ble Star and Cluster spacecraft were all located 8

Figure 9. Modelled magnetopause boundary layer streamlines and flux tube loci for the FTEs observed by Double Star and Cluster on 25 March 2005. Adapted from Wild et al. (2005c). within the magnetosphere, TC1 having recently by detailed measurements of the highly time-varying traversed the low latitude magnetopause from the structure of the polar ionosphere north of the Sval- magnetosheath (where FTEs were observed), while bard archipelago as observed by the EISCAT Sval- Cluster 1 was approaching the mid-latitude magne- bard Radar (ESR), discussed in more detail in Wild topause (where FTE signatures were also observed). et al. (2005c). The satellites’ northern hemisphere ionospheric footprints were located within the fields-of-view of the Ionospheric measurements made at lower lat- easternmost CUTLASS pair of SuperDARN radars. itudes reveal further evidence of solar wind– Figure 10 presents the ionospheric convection pat- magnetosphere–ionosphere coupling. Figure 11 tern at this time, inferred from all available north- presents data from beam 5 of the CUTLASS Ice- ern hemisphere SuperDARN data. The estimated land SuperDARN radar. This beam points in a ionospheric flow pattern is as expected for convec- poleward and eastward (duskward) direction and BY the data, corresponding to the early morning sector tion driven by a dominant IMF component (in o this case duskward) with ionospheric plasma in the ionosphere (08-09 MLT) in the region ≤ 76 Mlat, noon sector flowing dawnward and poleward into the are interpreted as follows. The Doppler spectral polar cap as newly reconnected magnetic field lines, width of the echoes observed by the radar are small one end of which remains embedded in the solar suggesting that the observed ionospheric irregulari- wind, respond to magnetic tension forces and the in- ties are associated with closed magnetic field lines fluence of the anti-sunward directed solar wind flow. (large Doppler spectral widths being an accepted sig- Detailed examination of high-latitude (≥ 75o Mlat) nature of the ionospheric projection of the magne- line-of-sight velocity measurements from individual tospheric cusp (e.g. Baker et al. 1995)). The line-of- radars located in the noon and pre-noon sector re- sight Doppler velocity measurements observed dur- veals pulsed ionospheric flows - bursts of high speed ing this interval were generally all negative (i.e. away (∼1000 m s−1) ionospheric plasma flow directed into from the radar), the chosen colour-coding therefore the polar cap. These are entirely consistent with the discriminates between small and large eastward ve- interpretation of newly opened magnetic flux tubes locities. Immediately apparent are periodic fluctuations of the eastward flow, varying between 0– passing in a dawnward and poleward/tailward direc- −1 tion over the Cluster spacecraft in the noon sector, 500 m s with a timescale between 5–10 minutes. Closer investigation of the velocity fluctuations re- their footprints dragging ionospheric plasma dawn- −1 ward and poleward as they are incorporated into the veals them to be almost sinusoidal ±250 m s open polar cap. These observations are supported perturbations superimposed upon a steady eastward background flow of ∼250 m s−1. These are inter- 9

Figure 10. SuperDARN northern hemisphere ionospheric convection pattern at 07:30 UT on 25 March 2004, derived using the technique of Ruo- honiemi & Baker (1998). The location of the magnetic footprints of the Cluster 1 and TC1 spacecraft are also indicated. Adapted from Wild et al. (2005b).

preted as the ionospheric signatures of ultra-low frequency (ULF) wave activity in the Pc5 frequency band (e.g. Provan & Yeoman 1997). The morning sector radar data were supported by further ev- Figure 11. Measurements from the CUTLASS Ice- idence of ULF wave activity throughout the morn- land SuperDARN radar during the interval 06– ing and daytime ionosphere. Although not shown 08 UT on 25 March 2005. Backscattered power (top here, the signatures of ULF waves could be seen in panel), line-of-sight Doppler velocity (middle panel) magnetograms recorded by the IMAGE and SAM- and Doppler spectral width (bottom panel) are colour- NET magnetometer arrays that span the Icelandic– coded and presented as functions of magnetic lati- Scandinavian sector and the ionospheric parameters tude and universal time. Adapted from Wild et al. measured by the mainland UHF EISCAT incoher- (2005c). ent scatter radar. These, and other datasets, are described in detail in Wild et al. (2005c).

The simultaneous space- and ground-based data mechanism by which ULF wave activity modulates recorded on 25 March 2004 highlight several of the the reconnection rate at the dayside magnetopause mechanisms of solar wind-magnetosphere-ionosphere (Prikryl et al. 1997). However, in this case, there is coupling. Magnetic reconnection was occurring in no evidence of a solar wind driver of the ULF waves, the vicinity of the sub-solar point during an inter- indeed the source of the driving mechanism remains val of duskward oriented IMF. The newly opened unclear. Therefore, the relationship between ULF flux tubes retreated away from the reconnection wave dynamics and dayside magnetic reconnection site and passed over the Double Star and Cluster will form the basis of future space- and ground-based spacecraft located near to the low- and high-latitude investigations. magnetopause respectively. In the high-latitude ionosphere, in the ionospheric projection of the cusp, ionospheric plasma associated with the newly opened flux tubes was observed to rapidly move dawnward 4. SUMMARY and poleward as the open flux tubes were incorporated in polar cap. Equatorward, in a region that is magnetically separate from the reconnec- In this paper, we have highlighted the results of sev- tion driven dynamics at higher latitudes, ionospheric eral Cluster- and ground-based investigations of day- flows excited by global-scale ULF wave activity dom- side solar wind–magnetosphere–ionosphere coupling. inate the motion of the morning and daytime sector These results, which illustrate the powerful synergy ionosphere. In the past, similar observations have of multi-point space- and ground-based experiments, been interpreted as evidence of a forward coupling include: 10

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ACKNOWLEDGMENTS

JAW wishes to thanks the many individuals who contributed to the papers discussed above. Special thanks also to the Cluster and SuperDARN PIs (and their teams), the ACE Science Center, the EISCAT scientiﬁc association and the IMAGE and SAMNET magnetometer networks for the data exploited in the referenced publications.