An ionospheric signature of possible enhanced magnetic field merging on the dayside magnetopause

An ionospheric signature of possible enhanced magnetic field merging on the dayside magnetopause M.

PINNOCK,* A. S. ROIXER,*

J. R. DUDEMY,*

R. A. GREENWALD,? J. M. RUOHONIEMI~

K. B. BAKER?

and

*British Antarctic Survey, Natural Environment Research Council, Madingley Road, Cambridge, CB3 OET, U.K. ; i_The Johns Hopkins

University, Applied 20707, U.S.A.

Physics Laboratory,

Laurel,

MD

Abstract-Identifying the causative mechanisms at the magnetopause that produce a variety of transient plasma velocity signatures in the high latitude ionosphere is dificult. Correct identification is offundamental importance in determining how solar wind energy is coupled to the magnetosphere. Observations in conjugate hemispheres offer the chance to distinguish between events triggered by merging and those initiated by solar wind pressure variations, if the direction of travel of the ionospheric signatures can be determined. Using data from two conjugate HF radars. high temporal resolution measurements of the Fregion plasma convection in the vicinity of the cusp are presented for 22 April 198X. In a previous study of this datasct [GREENWALD ('Icd..1990.J. qc~ph~~.v. Rm. 95, 80571, the authors identified a particular

region of the cusp ionosphere as being the footprint of the antIparallel merging line on the magnetopause. Following an enhancement in the Interplanetary Magnetic Field’s (IMF) southward component the inferred ionospheric footprint of the magnetopause merging line moved equatorward for 20 min. In this period. two poleward-directed bursts of high plasma velocities (c 2000 m/s) were observed in the southern hemisphere. occurring close to the ionospheric footprint of the merging line, with a weaker (_ 1000 m/s) response in the northern hemisphere. Poleward-directed flow at lower latitudes, crossing the inferred polar cap boundary, was also observed. We interpret the flow bursts as a signature ofan event driven by magnetic merging at the magnetopause, primarily because the motion of the flow burst features in each hemisphere was conditioned by the prevailing east&west component of the IMF. The azimuthal variation of the radar’s line-of-sight velocities may be interpreted as showing the presence of vortices which is consistent with theoretical models advanced for the ionospheric signature of patchy reconnection. However, the presence of vortices was not detected by the radar until at least 3 min after the onset of the poleward flow burst. The hemispherical differences in the plasma velocities of the flow burst events may be explained in terms of the differences in the mapping of the magnetopause merging lines to the conjugate hemispheres.

1. INTRODUCTION

In recent years, considerable progress has been made in determining the high latitude ionospheric convection pattern for periods when the Interplanetary Magnetic Field (IMF) has a southward component. The daysidc patterns synthesized from satellite data (HEPPNER and MAYNARD, 1987) are in substantial agreement

with

steady-state

patterns

derived

theo-

(CROOKER, 1988; REIFF and BURCH, 1985). For IMF southward conditions it is now widely accepted that merging of solar wind and magnetospheric field lines at the dayside magnetopause, and subsequent reconnection in the magnetotail, is the prime means by which the convection pattern is driven. The exact nature of the energy transfer from the magnetopause into the ionosphere is not fully understood. Theoretical models have been advanced that describe the nature of the ionospheric signature of merging (SAUNDERS et al., 1984; SOUTHWOOD, retically

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1987; KAN, 1988) but convincing observational cvidence to support any of these models is still very limited. Recent radar and ground-based magnetometer observations have revealed direct control of the dayside ionospheric convection pattern by the IMF, particularly the southward (B_) component (see LOCKWOOD and COWLEY, 1988, and references therein). However, observations of merging signatures are sparse because of the high spatial and temporal resolution required. The search for patchy merging phenomena [or Flux Transfer Events (FTEs)] in the ionosphere has yielded one excellent radar image (GOERTZ et d., 1985) supported by satellite obscrvations near the magnetopause. However, many more observations are required to establish the spatial and temporal characteristics of ionospheric merging signatures and to link them to the evolution of the largescale convection pattern. It is now becoming clear that transient signatures

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in the dayside convection pattern can be caused by processes other than merging of magnetic field lines at the dayside magnetopause. For example, solar wind pressure pulses of varying time-scales can cause compression of the magnetopause, which produces signatures on ground-based magnetometers similar to those attributed to FTEs (FARRLKZA et rd., 1989, SIBIXK et al., 1989). To discriminate between the various phenomena. without simultaneous solar wind data, is dificult. Pressure-driven events propagate with the magnetosheath flow (SIBECK et al.. 1989), whilst merging phenomena experience the vector addition of magnctosheath flow and the flow associated with the release of magnetic field line tension following merging (SAUNDERS, 1989). Thus pressuredriven events would be expected to produce conjugate signatures that propagate in the same direction in both hemispheres. Observational evidence suggests that the direction of travel would be around the polar cap boundary (FRIIS-CHRISTENSENrt al., 1988). Merging signatures, in the presence of an IMF east-west component (B,), will also propagate with a dawnward or duskward component, but oppositely directed in the two hemispheres. Thus conjugate ground-based observations offer the chance to discriminate between events driven by pressure pulses and those driven by merging. HF backscatter radars of the type described by GREENWALD Ed ul. (1985) offer a chance to observe large areas of the noon polar cap boundary ( - 3 h of magnetic local time) within which merging signatures should occur. To date only limited bistatic HF radar observations of the ionospheric cusp region are available from which unambiguous plasma flows can be measured. but considerable success has been achieved in utilizing line-of-sight velocity information obtained from a monostatic radar to derive the large-scale convcction pattern (RLJOHONIEMIet al.. 1989). For smallscale merging signatures, for example the <300 km scale size features deduced by RUSSELL and ELPHIC (1978). the application of the monostatic velocity mapping technique is difficult owing to the high spatial and temporal variability of the phenomena. GRIXNWALD it al. (1990) (hereafter referred to as paper [I]). using the Polar Anglo-American Conjugate Experiment (PACE) HF radars (BAKER c>t(II., l989), described the influence of B, on F-region ionospheric flows in the cusp region. They agreed with findings of FREEMAN et ul. (1990) that the flows responded to changes in the sense of B,. on a timescale (- 2 min) similar to that found for B_ changes (ET~MADI c’t uI., 1988). Examination of the ionospheric flow transition sequences in [I] provided the first experimental evidence to support the existence of

the ‘cusp wedge’ (CROOKER, 1988). The sides of the cusp wedge are the ionospheric projections of the antiparallel merging lines on the magnetopause (hereafter referred to as the merging lines). In this paper the work of [I] is extended to examine small-scale ( -200 km) ionospheric flow features observed in the vicinity of the merging lines. The signature ofenhanccd merging resulting from an increase in the southward component of the IMF is identified in the southern hemisphere, with a weaker response in the northern hemisphere. The key feature of the signature is a poleward-directed burst of high velocity (2000 m/s) plasma. The radar’s line-of-sight velocity measurements provide evidence for the presence of vortices as described by the SOUTHWOOD(1987) model of FTEs, although this evidence is not apparent until at least 3 min after the onset of the polcward burst.

2. INSTRUMENTATION PACE uses two conjugate coherent HF backscatter radars of the type described by GREENWALD rt cd. (1985), one at Goose Bay, Labrador (53”N, 60 W). the other at Halley, Antarctica (76 S. 27 W). The radars measure the power and the line-of-sight Doppler spectral characteristics of signals backscattered from decametre-scale irregularities in the polar E- and F-regions. Together, the two radars provide a means for simultaneous studies of convection in both hcmispheres over the range from 6.5 to 85 invariant latitude and spanning up to 4 h of magnetic local time. For a more complete description. set BAKER et al. (1989). For the datasets described below. the radars scanned their entire fields-of-view every 96 s. The radars’ range gating was set at 45 km and the beam width of each beam is approximately 100 km at the ranges of interest in this paper. This configuration allowed the evolution of a substantial part of the cusp region flow pattern to be followed, whilst still permitting the imaging of merging signatures. The irregularities observed by the radars arise from a variety of plasma instabilities prevalent in the high latitude ionosphere (TSUNODA, 1988). This paper does not address the physics of the irregularities, but we note that many of the features associated with merging signatures (velocity shears, field aligned currents. geomagnetic pulsations) are potential sources of irregularities. The data presented here are from F-region irregularities which, as RUOHONIEMIcutal. (I 987) have shown, propagate at the bulk plasma velocity. The data are plotted on a grid of geomagnetic latitude and longitude whose derivation is based on the

Enhanced

magnetic

International Geomagnetic Reference Field 1985 updated to 1988 (see BAKER and WING, 1989, for more details). The vector plots of ionospheric convection have been derived from the line-of-sight velocity measurements in a number of beams using the algorithm described in RUOHO~IEMI et al. (1989). In essence. it is a sophisticated implementation of the commonly used radar beam-swinging technique which utilizes information from between 5 and I6 01 the beams during one scan.

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3. OBSER~~ATJONS

In [I], PACE data from 1430 to 1530 UT (I 130-1230 MLT) on 22 April 1988 were presented. The geophysical conditions in this period were disturbed, with K,, varying from 4+ to 5. Solar wind magnetic field measurements from the IMP-8 satellite are available for this period and are shown in the three upper panels of Fig. I. The satellite was located in the dawn local time sector at .Y - 11 R,, y h - 29 R,. and z - 6 R,, (GSM coordinates). In this period, IMF B_ was negative with a typical value of - 8 nT ; B, changed sign several times, whilst B, remained negative with a typical value of about - 7 nT. The delay between IMP changes observed at IMP-8 and the onset of a change in the ionospheric flow was found to be 8 i 3 min ; see [I] for a detailed discussion of the derivation of this delay. The results presented in [I] showed that, during quasi-steady-state IMF 8,. conditions, the conjugate convection patterns in the vicinity of the c~tsp were similar to those presented by HEPPNER and MAYNARU (1987). Changes in the sense of 4, produced rapid reconfigurations of the convection pattern. The precisc manner in which this reconfiguration took place was described in detail. The key finding was that the rc~on~guration was initiated from a IatitLld~nally narrow (- 1 ) but longitudinally extended (-2 h MLT) region which propagated polcwards until all the plasma motion in the field-of-view (-5 of latitude) reconfigured to the new sense of B,. typically taking 6 mm to do so. The latitudinally narrow band of perturbed velocity vectors was interpreted as the tow latitude side of the cusp wedge (CROOKER, 1988), the sides of the cusp wedge being the merging lines. The wedge should mark the polar cap boundary (the boundary between open and closed field lines). As I$. changed sense several times during the study period, it was possible to track the temporal changes in latitude of the merging line. Prior to 1445 UT, the convection pattern had not expanded equatorward sufficiently for the merging line to be monitored. After the expansion from 1500

.e

1430

,

1500

Universal Time(H) Fig. I. Panels (a)--(c) show magnetometer data from the IMP-8 satellite for 22 April 198X for the period from 1430 to 1530 UT. Magnetic field strengths are in nano Tesht and the GSM coordinate system is used where z is positive northwards, _r is positive eastwards and x is positive sunwards. Panel (d) shows the maximum line-of-sight Doppler velocity (V,,J measured by the Halley radar during the same period in the area from - 72 to - 74’ geomagnetic latitude and 20. 40” geomagnetic longitude. Panel (e) has the same format as (d) but is for the Goose Bay radar and an area from 75 to 77“ geomagnetic latitude and 20-40” geomagnetic longitude.

to 1530 UT, the southern hemisphere low latitude cusp merging line was typically located near -72’ latitude, while the northern hemisphere merging line was at 75’. No explanation was offered for the hemispherical asymmetry in the latitude of the merging line, but such asymmetries are frequently found in satellite observations of the latitude of the cusp precipitation in the two hemispheres (CANDIDI and MENC;,1988). Between 1450 and 1510 UT, two bursts of high. negative (po1eward) line-of-sight velocity (V,,,) were found in the Halley radar data. The Bow bursts originated at -73’ latitude but spread rapidly polewards. In Fig. 1d, the maximum V,,, in the area between - 72 and -74”’ geomagnetic latitude and 20-40’ gcomagnetic longitude is plotted as a function of time, This rather large arca of coverage has been chosen in order to track the movement of the feature, described below. The background flow produces Doppler vclocities in the range from -400 to -800 m/s for both senses of B,.. The flow bursts are clearly seen as large (> 1500m/s) negative (polcward) velocities that occur between 14.50 and 1509 UT. The first starts at = 1450 UT and leaves the radar’s field-of-view by 1500 UT; the next flow burst develops by 1502 UT. This explains the drop in V,,, to below 1500 m/s at around 1500 UT. The start of a third flow burst occurs at 1521 UT and may be a similar event; the reason for its short duration will be discussed in the next section. At 1445 UT the R, component (Fig. lc) became mom negative and B, (Fig. 1b) changed sign from positive to negative, both changes occurring as step functions. For the next 20 min there were rapid fluctuations of both components. The extreme, but shortlived values of Z&that occur at 1451 UT (I?_= - 11nT) and 1503 UT (B_ = - 9 nT) may not be as important as the general trend. In this period the field had an average orientation (b 140 ‘) similar to that prior to 1445 UT but with an increased negative B= component of between 1 and 2 nT. The flow bursts follow these changes in the IMF conditions, the time delay from IMP-8 sensing the change to the ionospheric response being - 6 min. No flow burst was observed after the change in IMF conditions which occurred at 1503 UT when the IMP-8 magnetometer showed that B,. switched from negative to positive and the B,component reduced in magnitude to about -4 nT. Goose Bay radar data show no dramatic flow burst features, although isolated patches of slightly enhanced V,,, do occur. Figure le was produced by the same analysis technique used for Fig. Id but the area covered is 75 77 geomagnetic latitude (to allow for the higher latitude of the northern hemisphere

merging line) and 20--40” geomagnetic longitude. An increase in the maximum poleward VI,, at 1450 UT, occurs but only to a value of - 1000 m/s. The enhancement in velocities appears to cease at about 1515 UT although lack of data, possibly caused by the radar operator changing sounding parameters rather than a lack of scatter, makes it difficult to determine the time accurately. For the Halley flow burst, the very rapid temporal and spatial variation of velocity does not permit the use of the vector a1gorithn~ used to derive the velocity plots presented in [I]. Indeed some of the missing vectors in those plots are coincident with the location of the flow bursts. With only line-of-sight velocity Ineasurem~nts available, the analysis is concentrated on the evolution of the spatial scale and movement of the flow burst features, although some features of the plasma flow can be deduced. The flow burst beginning at 1502 UT has been examined in detail because the spatial coverage of scatter for this cv’ent is more complete than for the preceding burst at 1450 UT. The amount of scatter poleward of -72 increases rapidly during the first flow burst. The I502 UT event also occurs during a period of quasi-steady negative 8, (any effects from the positive Byspike at 1450 UT should have occurred in the ionosphere by I500 UT). Figure 2 is reproduced from [I] to illustrate the background flow conditions in the two hcmisphcres around I502 UT. averaged over a I2-min period. The Goose Bay radar data (Fig. 2a) show a smooth rotation of the dusk cell flow from poleward and westward to poleward and castward with increasing latitude. The flow reversal is centrcd at about 75“ geomagnetic latitude. The Halley radar data (Fig. 2b) show fairly uniform polcward and westw~lrd flow across the entire noon local time sector. The change to B, positive at 1503 UT. detected in the ionosphere at 1511 UT, produced a merging line signature at -72 latitude. Both convection patterns are what would be expected for B,. negative conditions in the respective hemispheres (HEPPNERand MAYNARD, 1987). Figure 3 gives a sequence of scans from the Halley radar for the period from I50058 to 15 1043 UT ; each panel shows the line-of-sight Doppler velocities mapped in geomagnetic latitude and longitude. The operating parameters of the radar determined that aliasing of the measured velocity would occur for velocities exceeding t 1600 m/s. Dealiasing of the data has only been done for clear cases where the velocity increases rapidly with latitude up to the aliasing boundary and then changes sign; this occurs predominantly in the centre of the flow burst. Isolated

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Fig. 2. Simultaneous time averaged (- 12 min) convection patterns in the vicinity of the cusp as observed with the Goose Bay (a) and Halley (b) radars for a period centred on I502 UT on 22 April I988.

scatter showing high positive velocity values remain in Fig. 3, as there is no way of determining whether they are aliased. The scan commencing at 150058 UT exhibits the line-of-sight velocities that most closely correspond to the background convection pattern shown in Fig. 2b. Note that the velocities are predominantly away (yellows and reds) from the radar, increasing in magnitude toward the western edge of the field-of-view. Two ‘hot spots’ in velocity exist; the one at -73”S, 16”E is the remnant of the previous flow burst. The other one, at -72.5% 32”E, is the embryonic second

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flow burst whose development is now described. At this stage the enhanced velocities associated with this second flow burst occupy 2 beams and 4 range gates, corresponding to a spatial scale of - 200 km, and the maximum V,,, measured is 1200 m/s, an increase of -500 m/s over the background flow. Because the radar’s beamwidth (4”) is greater than the separation between beam centres (3.3”), small features occupying the overlap region between two adjacent beams will be smeared out in longitude. Thus a scale size of 200 km represents an upper limit. In the next scan (96 s later) the most intense flows (1800 m/s) have moved westwards by 4- and the enhanced negative velocities have spread polewards and westwards, extending to - 74.5’ latitude and 22” longitude. The feature has now expanded to a square of side 300 km, but the most intense flows are sited on its equatorward edge and still have a spatial scale of 200 km. This suggests that the velocities at the poleward limit of the feature are a response to either the energetic Aows at the core of the event or a weaker accelerating force. If the ionospheric plasma were uniformly accelerated between -72 and -74. latitude and was flowing in a poleward and westward direction, simple geometry determines that the highest I’,,, would be at the poleward and westward limit of the feature. which is not the case. The line-of-sight velocity signature of the flow burst is not consistent with a purely poleward flow at all longitudes. This would maximize in the central beams (looking along the magnetic meridian) and decrease either side of them, but the measured decrease is found to be too large to satisfy this condition. In the later scans it will be shown that, as the flow burst moves poleward and westwards, the highest velocities in the event are seen in the most western beams. This is consistent with the beam direction becoming more closely aligned with the direction of the plasma flow within the IIow burst, i.e. polewards and westwards. In Fig. 3b, the equatorward area bordering the flow burst also shows an enhancement of V,,, (having changed from - -400 to N - 550 m/s). This low latitude enhancement extends down to about -70 latitude. Note that both this enhancement and the flow burst are confined east and west by regions showing the normal background flow. By 150546 (Fig. 3~) the region of enhanced velocities has expanded to a scale size of - 500 km. The line-of-sight velocities on the eastern flank of the flow burst (- 74 to - 75 latitude and 23 to 3 I ’ longitude) are reduced significantly, from - -800 m/s to velocities in the range of rt 200 m/s. Evidence for a comparable reduction in the line-of-sight velocities on the western flank is harder to find, although velocities in

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the most western beam between - 7 1 and - 72 latitude are lower than would be expected for the background convective flow. The westward propagation of the merging signature, combined with a lack of scatter at key locations, makes it difficult to establish this conclusively. If the flow burst features are considered as being superposed upon the background flow (Fig. 2b), the flow is enhanced by _ 1200 m/s at the centre of the feature whilst at the eastern flank it is reduced by 6001000 m/s; the western flank may be experiencing a similar reduction. The presence of these velocity enhancements and reductions is consistent with a vortcx flow being present, possibly a twin vortex pattern. However, the reduction in velocities on the flanks is not detected by the radar until at least 3 min after the enhancement at the centre of the event started. At 150858 UT (Fig. 3d) the largest velocities ( -2200 m/s) are in the far western beams at - 73 latitude and IO longitude, whilst the flows in the vicinity of the origin of the flow burst are very confused. The enhanced velocities at lower latitudes have also tracked westward with the motion of the flow burst. The velocities east of longitude 34 , whilst somewhat more chaotic and showing the reduction described previously, are still predominantly negative and very similar to those shown in Fig. 3a at the start of the event. The scan at I5 1034 UT (not shown) is complex because of the disruption caused by the start of the convection pattern reconfiguration initiated by the B, change at 1503 UT; see [I] for a more detailed description. Only a remnant of the flow burst remains in the western beams and by the scan beginning at 151219 the flow burst has disappeared completely. The line-of-sight velocities associated with the change of B, are quite different in character from the flow burst just presented, being typically around - 1000 m/s. They are also much closer to describing a uniform flow at all longitudes for a constant latitude. In Fig. 4 the geomagnetic position of the maximum k’,,,, within the flow burst, taken to be the centre ol the disturbance, is plotted to examine the relationship between the background flow and the motion of the how burst. The absence of scatter at key latitudes in Fig. 3d,e. may mean that the maximum occurs poleward of the value plotted. Figure 4 confirms that the averaged motion at the centre of the event from 1502 to 1508 UT is in the same sense as the background flow. The latter typically has a direction that is 55 west of poleward at -74’ latitude (Fig. 2b), whilst the motion deduced from the maximum V,,, is 52 west of poleward, at a velocity of I700 m/s. However. the motion of the peak v,<,, between 1500 and I502

UT (Fig. 3a, b) is westward and slightly equatorward. There also appears to have been some disruption of this motion around 1505 UT. The flow burst line-of-sight velocities maximize (- -2000 m/s) in the most western beams (Fig. 3d) and are close to the actual plasma velocities, since the beam geometry means that the beam direction here is almost aligned with the direction of the motion of the flow burst feature. Taking this velocity and the minimum scale size observed (200 km). a minimum potential of 20 kV is associated with the flow burst. It also shows that the bulk velocity of the feature derived from Fig. 4 is comparable with the velocity of the plasma within the flow burst. The maximum velocity errors (i.e. the errors arising from the radar measurement of I’,,,,) associated with the data prcscntcd in Fig. 3 are of the order of IO0 m/s whilst typical errors are 30 m/s. Unfortunately the Goose Bay dataset for this pcriod is interrupted by operator interventions. and only three full scans of data exist. Figure 5a shows the scans beginning at 150150, the line-of-sight velocities being generally consistent with the averaged flow pattern shown in Fig. 2a. The largest lint-of-sight vclocities are located at - 77 latitude and 30 longitude and are a consequence of the radar’s viewing geometry with respect to the flow reversal of the afternoon cell. Three minutes later (Fig. 5b), these maximum V,,,, values show an increase which is of the order of 300 m/s. The enhanced velocity values extend eastward, but lack of scatter at higher latitudes prevents the polcward component from being determined. The enhanced velocities thus appear to bc aligned in a direction similar to the background flow. This velocity signature is certainly moving castward. From the foregoing observations. a summary of events can be produced. (1) Following an increase in the southward component of the IMF, two separate bursts of large poleward line-of-sight velocity, separated by - IO min, were observed in the southern hemisphere cusp region, each lasting approximately 8 min. The bursts started at the latitude ( - 72 ) of the inferred ionospheric footprint of the magnetopause merging line (dctcrmined at Halley at I5 11 UT). The second event may have ended with the transition from negative to positive B, and a coincident reduction in the magnitude of B, to its value prior to 1445 UT. (2) The two flow bursts occurred in a period when the polar cap boundary was moving equatorward, reaching - 72 latitude in the southern hemisphere by I511 UT. (3) For the second flow burst. the initial source region was a square of side -200 km and the arca

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Fig. A scqucncc scam from IIallcy radar the mcasurcd Doppler vcllocity V,,,. culuur ~LJIa liwi CJ~ Lield-of-view. Positive are towards and negative velocities away from the radar. The data arc plotted on a grid of geomagnetic latitude and longitude Relow the longitude coordinate is shown the megnetic locsrl time. The time shown ut the top is the start of’ the scan. For clarity pan4 (a) culour coding spans a different velocity range than the other panels. The solid black lines show the limits of the radar’s field-of-view. The purple lme shows the position of the solar terminator at 110 km altitude; the parallel black line indicates the side which is in darkness.

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from four other days during which scatter occurred in the cusp region. Such flow bursts occurred at least once during periods in which strong cusp scatter persists for a period of an hour or so on either side of magnetic noon. A future study will look at their frequency of occurrence. their distribution in magnetic local time and their relationship with solar wind conditions. data

3. DlSCUSSlON

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Longitude

(deg)

Fig. 4. Plotted on the same grid as Fig. 3, the data show the position of the maximum line-of-sight velocity (V,,,) within the flow burst at the Universal Time (UT) indicated. Typical error bars are shown on the data point piotted for 150316 UT.

containing the most energetic flows continued to have a comparable spatial scale through the event. However. the area encompassing all the perturbed velocities expanded to a scale size of at least 500 km. (4) Except at the start of the event, the bulk motion of the feature moved with a velocity of - 1700 m/s in a direction similar to that of the background ionospheric convection. The line-of-sight velocity signature of the plasma flow within the feature can be interpreted as being closely aligned to the same direction, polewards and westwards. (5) Eastward of the flow burst are flows (600-1000 m/s) against the background flow- conditions, providing evidence for a vortex flow pattern. Westward of the flow burst there is some evidence for a second vortex, but this is not conclusive. Neither of these velocity signatures appeared until the scan commcncing at 150546, at least 3 min (allowing for the 96-s time resolution) after the start of the flow burst. (6) At latitudes lower than the ionospheric merging line, weaker spatially confined enhancements of poleward flow were observed. (7) In the conjugate, northern hemisphere ionosphere a much weaker response is observed, with velocity enhancements of only a few hundred metres per second, but also aligned with the background Row direction. Having identified the characteristics of this flow burst a brief examination has been made of PACE

A fundamental question that can be addressed with the data presented here is whether these flow burst events are the ionospheric signature of magnetopause phenomena triggered by solar wind pressure pulses, Kelvin-Helmholtz instabilities, flux transfer (patchy reconnection) events or some as yet unidentified phenomena. Solar wind pressure variations and KelvinHelmholtz waves are thought of as unlikely to have triggered this event. The features observed propagate into the polar cap in the northern and southern hemispheres with duskward and dawnward components, respectively, showing that the y-component of the IMF influenced the direction of motion. This is consistent with the event being triggered by merging, but not by pressure pulses, as discussed in the Introduction. For the same reason it is unlikely that this event was triggered by impulsive penetration of solar wind plasma (HIXULA et al., 1989). Furthermore, the observation that the bulk velocity of the flow burst feature is comparable with the velocity of the plasma within it is consistent with a merging driven event. Experimental observations show that this is not the case for pressure pulse induced features (FKIISCHRISTEKSEKst al., 1988). The flow bursts start -6 min after the B_ component of the IMF became more negative. as measured at the IMP-8 spacecraft. This will lead to an increased merging rate (SONNERUP.1974). It is noted that the rcpctition rate of the flow bursts is approximately 10 min, similar to that reported for FTEs at the magnetopause (RIJNBEEKrt nl.,1984) and for daysidc flow bursts and aurora1 activity reported by LoCKW~OD~~U~. (1989). Many of the characteristics of this event match the predictions of the various theoretical models that have described ionospheric signatures of magnetopause merging. At the beginning of the event, the region of enhanced velocities had an area (- 200 km square) comparable with that calculated by Som~woon (1987) for the footprint of a recently merged flux tube. The subsequent bulk motion of the feature fits the

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description given by SAUNDERS(I 989) for the motion of a recently merged flux tube as it travels through the cusp region. Immediately following merging, the motion of the tube is dominated by the release of magnetic field tension ; for negative B,-in the southern hemisphere this motion is almost purely westward. As the field tension is reduced, magnetosheath flow plays an increasing role and the poleward component to the motion grows. From Fig. 4, it is seen that the motion appears to be dominated by field line tension for ~lpproxilnately one scan, or around - 100 s. As yet no quantitative estimates of the duration of tensiondominated flow have been produced, but it must be shorter than the transit time of a flux tube through the cusp (~3 min). This motion has similarities to the optical signatures of transient dayside aurorae described by LXKWOOD et nl. (1989), which they associated with magnetopause FTEs. The key feature of the actual flow burst is a jet of poleward and westward flow reaching -2000 m/s. This is consistent with the models of SOUTHW~OD (1987) and KAN (1988) who termed this ‘an enhanced convection channel’. There is evidence for a vortex on the eastern flank of the plasma jet. Additional weaker evidence also indicates a second vortex may have been present on the western flank of the flow burst. KAN (1988) does not predict the development of vortices ; this is one of the major differences between his model and that of Southwood. The fact that evidence for the presence of vortices occurs at least 3 min after the Bow burst starts, which would not be expected from the Southwood model. may be misleading. The viewing geometry of the radar and the configuration of the vortices may prevent the return flows being readily detected at the start of the event. From Fig. 4 the initial motion of the feature is almost purely westward and the central plasma jet of an FTE signature would be similarly aligned. At this time any return flow would be aligned almost purely eastward ; it would also occur over a larger area than the central jet and should thus be weaker. This situation exists for Fig. 3a. b. when the feature occupies the central beams. It may be that the return flow cannot be detected by the radar at this time because it is weak and orthogonal to the radar beam. However, one might expect to see the return flow at the eastern end of the feature, where it rotates to join the central plasma jet. The scale size of the event at this time and the spatial resolution of the radar may not permit this to bc resolved; its only effect could be to contribute to the very large Doppler spectral widths observed. As the feature starts to move poleward and westward (by Fig. 3~). and grows in spatial extent, the return flow becomes more parallel to the beam direction

et al.

of the radar and a significant line-of-sight velocity component is generated [see LOCKWOOD(1990), his fig. 1I for further discussion of this scenario]. In summary, we feel that there is not necessarily any inconsistency between the observations concerning vortices and the SOUTBW~~D(1987) FTE model, but further work is required on this point. These observations may well be expected to deviate from the SOUTHWOOD (1987) model because the latter assumes uniform ionospheric conductivity. The solar terminator at I10 km (the purple line in Fig. 3) is at 71.5 latitude. A gradient in Pedersen conductivity must exist, with lower conductivity at the higher latitudes. If the transient aurora1 features reported by Lm~wttot) er fzl. (1989) are associated with FTEs then they too would lead to conductivity gradients. The consequences of this have not yet been evaluated. The enhanced poleward flow at lower latitudes represents How across the polar cap boundary and is predicted by all the models. It is consistent with the modelling work of FREEMANand SOUTHWOQD (1988) for the response of ionospheric flows in the vicinity of a bulge in the polar cap boundary caused by the erosion of closed field lines. This study complements the SABRE radar data presented by Freeman and Southwood, which came from the post-noon sector. The fact that the flow at lower latitudes is considerably weaker than that of the flow burst is consistent with the expansion of the polar cap boundary at this time (KAN. 1988). The interruptions to the Goose Bay radar scans could be responsible for the apparently weaker merging signature in the northern hemisphere. By 1504 UT (Fig. 5b) the flow burst signature may be polcward of the area in which backscatter was obtained. However. the ionospheric merging line (and the convection rcversai, see Fig. 2b) was inferred to be at 75 at this time [I]. The southern hemisphere flow burst had moved 1’ poleward of the merging line by 1504 UT (Fig. 4). If the motion of the northern hemisphere signature was similar it would be at 76 . which is covered by the backscatter. If the weaker response in the northern hemisphere is genuine it could be explained by the predictions of the antiparallel merging model. When the northern and southern magnetopause merging lines are mapped into the ionosphere, CR~OKER (1979) has shown that two ionospheric flow vectors, of differing magnitude. are produced in the cusp. Figure 6 is a sketch giving an approximate representation of the cusp wedge geometry (CROOKER,1988) in each hemisphere at I SO5 UT and the radars’ fields-of-view. The bold arrows represent the velocity vectors derived by CROOKEK(I 979). If the average flow patterns shown

Enhanced magnetic field merging Nonhern

Southern

hemwphere

/

‘\

\

I/ I

\

-;

t

I

\

c

a---/

/

/

/

reduction at the latitude of the flow burst and an increase poleward of it. The poleward movement of scatter may be associated with polar patches (WEBER et al., 1984) which occur in this dataset and which were described by BAKER et al. (1989). The sequence at this period is not as conclusive as those presented by BAKEK rf al. (1989) ; there is not the steady poleward a

hemisphere

-

\

\\ \

:

.._

’

I/ 18

f

\ \

I

\

I

‘-*

migration of scatter over _ 6 min. However, it is noted that KAIT (1988) has speculated that polar patches

~~ 17 MLT

711

12 MU

Fig. 6. Sketch, taken from CROOKER(1979, 198X), showing the conditions arevaiiing at 1505 UT in both hemisoheres. The dashed lines show the polar cap boundary, in~~rp~ratin~ the cusp wedge, and solid lines the convection pattern. The arrows in the noon sector indicate the relative values of the velocity vectors mapping into the ionosphere from the magnetopause as a result of dayside merging. The radars’ fields-of-view are indicated by the shaded area.

(Fig. 2) were plotted in polar projection they show excellent correspondence with the post-noon patterns derived by CROOKER (1979). The Halley radar contains the merging line associated with the more vigourous flow, whilst the field-of-view of the Goose Bay radar contains the merging line associated with less vigourous flow. It could be that a clear signature of enhanced merging is only present in regions where the most vigourous flow is produced. If our conjecture is correct, clear merging signatures would be detected by the Halley radar for negative B,., and Goose Bay for positive B,. conditions. The southern hemisphere data support this hypothesis since clear merging signatures are absent after B,, becomes positive at - 1504 UT (mapping to the ionosphere at - I.512 UT). The only other flow burst signature in the Halley radar field-of-view occurred at 1520 UT (see Fig. 1d), possibly corresponding LOa very short period when B, went negative around I5 14 UT (Fig. lb). Unfortunately Goose Bay data do not provide corollary evidence, but this is primarily because scatter is not in the right location to observe any flow bursts that may have occurred. The location of the scattering regions changes through the second flow burst event. Figure 3 shows

may be a consequence of merging. This dataset provides limited evidence to support that idea.

4.

CONCLLfSlONS

The high temporal and spatial resolution of the PACE radar data have allowed the formation and evolution of a transient plasma velocity signature in the cusp ionosphere to be studied. The most likely cause of this event, evidenced by the direction of travel of the feature in each hemisphere, is enhanced field line merging at the magnetopause. The signature satisfies many of the predictions of theoretical models that require the formation of vortices. The northern hemisphere showed a much weaker enhancement in velocities, consistent with the vector supe~osition arguments presented by CROAKER (1979) for the antiparallel merging model. Acknowledgements-The authors acknowledge discussions with Dr M. Lockwood. The Goose supported in part by the National Science

very helpful Bay radar is

Foun~tion (NSF), Division of Atmospheric Sciences, and the Air Force Office of Scientific Research, Directorate of Atmospheric and

Chemical Sciences under grant ATM-8713982. The Halley radar was developed under support from the NSF Division of Polar Programs, Grant DPP-8602975. and the Natural Environment Research Council of Great Britain. This work was supported by the above grants as well as by NASA grant NAGS-1099. The authors wish to thank Dr R. P. Lepping for providing IMF data from the IMP-8 satellite. We also thank the staff of the AFGL High-latitude Ionospheric Observatory for their help in the daily operations of the Goose Bay radar as well as the Ionospheric Effects Branch of the Air Force Geophysics Laboratory for permission and support in operating from the Goose Bay site. Grateful thanks are also extended to the base members of Halley Station for their considerable assistance in the operation of the Halley radar. One of the authors, MP. wishes to thank Dr A. Etemadi for helpful discussions.

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198X 198X 19x5

1985 I990

1989 1987 198X I990 1988 1989 I985 1984 1989 1987 197x 1989 1984 1989

1974 1987 1988 1984