Multistage substorm expansion: Auroral dynamics in relation to plasma sheet particle injection, precipitation, and plasma convection

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. A11, 1342, doi: /2001ja900116, 2002 Multistage substorm expansion: Auroral dynamics in relation to plasma sheet particle injection, precipitation, and plasma convection Per Even Sandholt, 1 Charles J. Farrugia, 2 Mark Lester, 3 Stan Cowley, 3 Steve Milan, 3 William F. Denig, 4 Bjørn Lybekk, 1 Espen Trondsen, 1 and Vjacheslav Vorobjev, 5 Received 14 May 2001; revised 12 July 2001; accepted 20 July 2001; published 2 November [1] We present observations of the auroral expansions during two substorms, focusing on multistage intensifications and the morphology of the poleward boundary, and relate these auroral observations to the local plasma convection and plasma sheet dynamics. The observations are made by meridian scanning photometers and an all-sky camera (ASC) at Ny Ålesund, Svalbard (76 magnetic latitude (MLAT)), an ASC in Lovozero, Russia (64 MLAT), the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer chain in Svalbard and Scandinavia, the HYDRA instrument on Polar located at the inner edge of the plasma sheet, particle detectors on DMSP F13 and DMSP F14 traversing the ionospheric projection of the plasma sheet, and the CUTLASS Finland HF radar. In each substorm the aurora between 70 and 80 MLAT consisted of two branches separated by 5 in MLAT. The higher-latitude branch (at MLAT) was subject to a sequence of short-lived (1 2 min) intensifications, so-called poleward boundary intensifications (PBIs), recurring at 3-min intervals. Subsequent to each brightening, auroral forms traveled equatorward at a speed of km s 1. On Polar the PBIs are related on a one-to-one basis with injections of electrons in the 5- to 20-keV energy range at the inner edge of the equatorial plasma sheet with predominantly a trapped distribution, delayed by 5 min. Electron precipitation within MLAT, corresponding to a large radial extent of the plasma sheet, is documented by DMSP flights in the magnetic local time (MLT) sector. In discussing the branches of the high-latitude aurora within the context of current understanding of the relation of bursty bulk flows to substorm expansion phase dynamics, we note the following: (1) the initial auroral breakup located at MLAT near the equatorward edge of plasma sheet precipitation, which was followed by (2) two successive brightenings/auroral expansions appearing within MLAT/2100 MLT, separated by 14 min, (3) a 20-min-long brightening sequence in the poleward auroral branch (75 78 MLAT), consisting of six discrete events (PBIs) within the boundary plasma sheet precipitation, and (4) the presence of auroral vortex motion/strong field-aligned current sheets in some of these PBIs, which were accompanied by (5) electron injections at the inner edge of the plasma sheet, (6) brightenings of the lower-latitude auroral branch when equatorward moving auroral forms (EMAFs/streamers) arrive there, and (7) localized bursts of equatorward ionospheric convection at speeds of km s 1 in the latitude range of the EMAFs/streamers. The documented associations between PBIs/EMAFs, plasma sheet injections, and the local convection events are explained in terms of a substorm scenario involving bursty bulk flows in the late expansion phase. INDEX TERMS: 2704 Magnetospheric Physics: Auroral phenomena (2407); 2740 Magnetospheric Physics: Magnetospheric configuration and dynamics; 2764 Magnetospheric Physics: Plasma sheet; 2788 Magnetospheric Physics: Storms and substorms; KEYWORDS: substorm, aurora, plasma convection, plasma sheet injections, poleward boundary intensifications (PBIs), bursty bulk flows (BBFs) 1 Department of Physics, University of Oslo, Oslo, Norway. 2 Space Science Center, University of New Hampshire, Durham, New Hampshire, USA. Copyright 2002 by the American Geophysical Union /02/2001JA Department of Physics and Astronomy, University of Leicester, Leicester, UK. 4 Space Vehicles Directorate, Air Force Research Laboratory, Hanscom AFB, Massachusetts, USA. 5 Polar Geophysical Institute, Kola, Russia. SMP 4-1

2 SMP 4-2 SANDHOLT ET AL.: MULTISTAGE SUBSTORM EXPANSION AURORAL DYNAMICS Citation: Sandholt, P. E., C. J. Farrugia, M. Lester, S. Cowley, S. Milan, W. F. Denig, B. Lybekk, E. Trondsen, and V. Vorobjev, Multistage substorm expansion: Auroral dynamics in relation to plasma sheet particle injection, precipitation, and plasma convection, J. Geophys. Res., 107(A11), 1342, doi: /2001ja900116, Introduction [2] There are important unresolved issues concerning the temporal-spatial evolution of magnetotail dynamics during substorms [Lui et al., 2000]. One important source of information on substorm processes is the aurora. Great efforts have been made to describe the various aspects of the auroral substorm based on ground and/or satellite observations [Akasofu, 1964; Nakamura et al., 1993; Elphinstone et al., 1995a, 1995b, 1995c]. The association between the magnetotail dynamics and the aurora has been investigated by combining coordinated observations from different altitudes [Sergeev et al., 2000b; Lazutin et al., 2000]. It has thus been documented that the substorm onset is tied to an auroral breakup arc located well equatorward of the open-closed field line boundary [Samson et al., 1992; Elphinstone et al., 1995b; Lui and Murphree, 1998]. The subsequent poleward expansion with brightenings along the oval poleward boundary, so-called poleward boundary intensifications (PBIs), has been the topic of several recent studies [Petrukovich et al., 1998; Henderson et al., 1998; Sergeev et al., 1999; Lyons et al., 1999; Fairfield et al., 1999; Lui et al., 1998]. Ionospheric ion drift observations in the vicinity of the poleward boundary of the nightside red line aurora have been used to derive information on magnetotail reconnection rates, assuming that the red line emission boundary can be used to determine the location of the magnetic separatrix [Gallagher et al., 1993; de la Beaujardiere et al., 1994; Blanchard et al., 1996]. [3] In situ magnetotail observations during the last decade indicate that magnetotail convection takes the form of 5- to 10-min-long groups of short (1 min) high-speed flow events called bursty bulk flows [Angelopoulos et al., 1992]. It has been furthermore suggested that these short flow bursts in the magnetotail (observed at 8 15 R E radial distance) constitute the smallest fundamental process that makes up substorms [Sergeev et al., 1996; Fairfield et al., 1999]. A recent search for ionospheric signatures of these magnetotail flow bursts indicates that they have a distinct footprint in the form of the poleward boundary intensifications (PBIs) in the aurora [Petrukovich et al., 1998; Lyons et al., 1999; Fairfield et al., 1999], which seems at times to be characterized by the formation of north-south-aligned auroral forms, also called auroral streamers [Henderson et al., 1994; Sergeev et al., 1999; Henderson et al., 1998; Sergeev et al., 2000a; Zesta et al., 2000]. [4] Since the phenomenon of PBIs is considered to be such an important aspect of magnetospheric activity, more detailed studies directed toward understanding their morphology and processes leading to their formation have been recommended [Lyons et al., 1999; Sergeev et al., 2000b]. With this background we started an investigation of the detailed morphology of PBIs and their relation to substorms and plasma sheet particle injections during intervals when essential features of the evolution of the auroral substorm occurred within the field of view (FOV) of our optical instruments in Ny Ålesund, Svalbard (76 magnetic latitude (MLAT)). Thus, in this report we describe the auroral morphology of two consecutive substorms on 12 December The first substorm, observed in the dusk sector, which was triggered by an interplanetary shock, was limited to high latitudes (70 80 MLAT). The initial breakup, the poleward expansion, and the associated formation of two latitudinally separated branches occurred within the FOV of the optical instruments in Ny Ålesund. The second substorm, which occurred 2.5 hours after the first one, is a typical, strong substorm with initial breakup at lower latitudes (63 64 MLAT). We concentrate on the later phase of the highlatitude expansion of this substorm, when two latitudinally separated branches were formed in the magnetic local time (MLT)/70 78 MLAT sector. The major high-latitude activity was initiated by a breakup arc in the southern part of the field of view, at MLAT, followed by a rapid poleward expansion. Then a 20-minlong sequence of six high-latitude PBIs in the northern auroral branch was observed. The breakup arc/poleward expansion at 1900 UT represents the third stage in the stepwise poleward advance of the actual substorm activity, taking place 20 min after the first onset at MLAT. According to Elphinstone et al. [1995a, p. 12,091] the temporal development of features during the double-oval configuration are fundamental to understanding the lowaltitude signatures of particle precipitation traditionally referred to as boundary plasma sheet (BPS) and central plasma sheet (CPS) [Winningham et al., 1975]. The northernmost and southernmost branches of the double-oval configuration were found to correspond to BPS and CPS precipitations, respectively [Elphinstone et al., 1995a]. [5] Taking advantage of the potential of continuous ground observations for resolving the finer details of the auroral activity associated with substorms, we focus on distinct features associated with the double-oval configuration. We find that a sequence of short-lived (2 min) brightening events is a characteristic feature of the northern branch activity (PBIs), which in our case was located within the regime of boundary plasma sheet precipitation (75 78 MLAT). The double-oval configuration is also characterized by equatorward moving auroral forms (EMAFs)/structures emanating from the poleward boundary. Novel is our further observation that when these forms subsequently reach the latitude of the southern auroral branch (at MLAT), a rebrightening is observed there. Figure 1. (opposite) Solar wind plasma and interplanetary magnetic field (IMF) data obtained from spacecraft Wind during the interval UT on 12 December Plots from top to bottom show plasma density, proton temperature, bulk speed, dynamic pressure, IMF magnitude, and the field components B x, B y, B z, and the IMF clock angle. The interplanetary shock at 1550 UT and the time of the substorm onset at 1839 UT have been marked by vertical dasheddotted lines.

3 SANDHOLT ET AL.: MULTISTAGE SUBSTORM EXPANSION AURORAL DYNAMICS SMP 4-3

4 SMP 4-4 SANDHOLT ET AL.: MULTISTAGE SUBSTORM EXPANSION AURORAL DYNAMICS [6] The strongest events in this sequential brightening of the higher- and lower-latitude auroral branches were accompanied by ground magnetic deflections (westward electrojet) propagating equatorward. The detailed two-dimensional evolution of the auroral events in the context of the local ionospheric convection pattern is as inferred from the CUTLASS Finland radar. Convection observations in the vicinity of the nightside polar cap boundary have previously been used to derive information on the magnetotail reconnection process [Watanabe et al., 1998]. Bursts of equatorward flow have been observed during the late expansion phase by Yeoman and Luhr [1997], who interpreted these as possible signatures of reconnection in the tail. Yeoman et al. [1998] then demonstrated that concurrent with these signatures were observations by Geotail reminiscent of bursty bulk flows. In this paper we focus on the association between the auroral dynamics (PBIs/streamers) and events of localized equatorward convection. [7] The present ground and satellite observations are discussed within the framework of recent studies of earthward moving flux tubes/bubbles and activations of fieldaligned current wedgelets accompanying the high-speed bursty bulk flows (BBFs), a dynamical process which spans a large radial distance in the plasma sheet [Chen and Wolf, 1993; Shiokawa et al., 1998; Fairfield et al., 1999; Lu et al., 2000; Sergeev et al., 2000a; Zesta et al., 2000]. [8] The large radial extent of the dynamical plasma sheet involved in the process is confirmed in this study by the observed close correlation of the high-latitude (75 78 MLAT) auroral activity with plasma dynamics in the inner equatorial plasma sheet at a radial distance of 6 R E, observed from the Polar spacecraft. Thus a central aspect of this substorm study concerns the association between PBIs, presumably mapping to the X line at radial distances beyond 25 R E, and electron injections at 6 R E, with emphasis placed on time delays [Fairfield et al., 1999]. In our case, electron injections in the inner plasma sheet, as recorded by the Polar, were observed to be delayed by 5 min with respect to the high-latitude auroral brightening events (PBIs). This confirms that the tail dynamics associated with the poleward boundary intensifications can extend throughout the entire radial extent of the plasma sheet [Lyons et al., 1999, p. 4485; Zesta et al., 2000; Sergeev et al., 2000a]. In our case the low-altitude precipitation signature of the entire plasma sheet from its inner edge to the plasma sheet boundary layer/x line spanned the latitude range MLAT, as inferred from north-south traversals of DMSP spacecraft. 2. Observations 2.1. Interplanetary Observations [9] Field and plasma observations from the Magnetic Fields Investigation and Solar Wind Experiment on the Wind spacecraft are shown in Figure 1 for the interval UT on 12 December The plots display from top to bottom the proton density, temperature, bulk speed, dynamic pressure, the total magnetic field intensity and its geocentric solar magnetospheric (GSM) components, and the interplanetary magnetic field (IMF) clock angle ( polar angle in the GSM Y-Z plane. Positional information is given at the bottom (GSE coordinates). The Figure 2. Locations of magnetometers belonging to the International Monitor for Auroral Geomagnetic Effects (IMAGE) chain in Scandinavia, ranging from Ny Ålesund (NAL), Svalbard, to Uppsala (UPS), corresponding to the latitude range from 76 to 56 MLAT. spacecraft is in the solar wind following a petal orbit. It is on the dawnside of Earth and tailward of the terminator. [10] An interplanetary shock passes the spacecraft at 1552 UT, marked by the first vertical guideline, when the solar wind dynamic pressure underwent a strong and sharp increase from 0.1 to 1 npa. The effect of the increased dynamic pressure behind this shock on the ground is recorded instantaneously as a magnetic sudden impulse (SI) event in International Monitor for Auroral Geomagnetic Effects (IMAGE) stations in Svalbard and Scandinavia (see Figures 2 and 3) and was immediately followed by the activation of a high-latitude substorm. Behind the shock and up to the two field directional discontinuities (DDs) between 1730 and 1800 UT, the IMF is characterized by average values of B10 nt, B x 5 nt, B y 7 nt, and B z 1 nt. The dynamic pressure, having decreased from 1 npa to 0.2 npa after the shock, increased slowly to 1.5 npa during the interval of the IMF DDs within UT and stayed at that level until 2000 UT. During the interval UT the field is more southward oriented, with the B z component fluctuating within the range 6 to 10 nt and a large B y positive component. A second, major substorm was activated at 1839 UT, i.e., after a 40-min interval of strongly southward IMF orientation Ground Magnetic Observations [11] Figure 2 shows a map of the magnetometer stations of the IMAGE chain in Svalbard Scandinavia, and Russia, which are used in this study.

5 SANDHOLT ET AL.: MULTISTAGE SUBSTORM EXPANSION AURORAL DYNAMICS SMP 4-5 of equatorward expanding negative deflections are marked by tilted lines. Major poleward boundary intensifications in the aurora (see section 2.3) are marked by arrows in the top panel. The auroral and convection signatures of the event at UT will be discussed in some detail. After 1930 UT the activity decreased to a quiet level Auroral Observations From the Ground [14] Figure 5 shows meridian scanning photometer (MSP) observations (line of sight intensities versus zenith angle) at and nm from Ny Ålesund (NAL), Svalbard (76 MLAT), for the interval UT, covering the first substorm reported. [15] An initial brightening occurred near the southern horizon at 1557 UT, followed by a major activation at both wavelengths starting at 1604 UT. Then a poleward leap occurred at 1613 UT, followed by gradual poleward expansion of the poleward boundary during the interval UT. From 1630 to 1720 UT the aurora configures as two branches located at the poleward and equatorward boundaries of the field of view and separated by 8 10 MLAT. Distinct brightenings of the high-latitude branch at 1646, 1650, and 1659 UT gave rise to forms emanating from this branch and moving equatorward toward the lower- Figure 3. X component magnetograms (1-min averages) from the IMAGE chain in Svalbard-Scandinavia for the interval UT on 12 December The chain of stations from Ny Ålesund (NAL) to Uppsala (UPS) spans the latitude range from 76 to 56 MLAT. Vertical line marks the sudden impulse at 1552 UT. Auroral events (poleward boundary intensifications (PBIs)) are indicated by arrows in the top panel. [12] Figure 3 shows X component magnetograms from the IMAGE chain for the interval UT. We note the following features: (1) the sudden impulse (SI) recorded at all stations at 1552 UT, marked by vertical line, (2) a sharp, substorm electrojet deflection, maximizing ( 300 nt) at station BJN (71 MLAT) at 1600 UT, which is followed by (3) a series of smaller deflections at the Svalbard stations during the period UT. Brightenings of the aurora near the polar cap boundary (see below), as observed from Ny Ålesund, are marked by arrows in the top panel. [13] Figure 4 shows X component deflections during the following interval UT, showing the sharp onset of a major substorm at stations LOZ and SOR, located at MLAT, at 1839 UT, followed by a poleward leap into the latitude regime of the Svalbard stations (BJN and NAL), manifested by sharp electrojet activations at 1846 and 1900 UT (marked by vertical lines), centered at the latitudes of BJN and HOP (71 72 MLAT) and HOR and LYR (73 75 MLAT), respectively. The large negative bay contains some substructure at the northernmost stations during UT, as marked in Figure 4. We note that each high-latitude electrojet intensification corresponds to a positive deflection at low and middle latitudes. A series Figure 4. X component magnetograms (1-min averages) from the IMAGE chain in Svalbard-Scandinavia for the interval UT on 12 December The chain of stations from Ny Ålesund (NAL) to Uppsala (UPS) spans the latitude range from 76 to 56 MLAT. Substorm intensifications are marked by vertical lines at 1839, 1846, and 1900 UT. Auroral events (PBIs) are marked by arrows in the top panel.

6 SMP 4-6 SANDHOLT ET AL.: MULTISTAGE SUBSTORM EXPANSION AURORAL DYNAMICS latitude branch visible in both lines. A rebrightening is observed at both wavelengths when these forms approach the lower branch, 10 min after the initial brightening in the north. We note in particular the brightening of the lower branch during UT and its associated magnetic deflection (electrojet) at the closest stations HOR and HOP (see Figure 3). [16] Figure 6 shows MSP data at and nm from Ny Ålesund (NAL), Svalbard (76 MLAT), for the second substorm ( UT). The activity corresponds to the stages of electrojet activations marked in Figure 4. Stage 1 of the substorm expansion phase consists of a poleward auroral expansion which encroached into the FOV of the optical instruments from the south at 1843 UT. Stage 2 is the auroral intensification/poleward expansion taking place during UT, which fades during UT. Stage 3 consists of the new auroral breakup arc at UT and the activity that follows up to 1930 UT: (1) a breakup arc within the field of view of the optical instruments at MLAT/2100 MLT at UT (see below), (2) a rapid poleward movement of the poleward boundary at UT, (3) fading of the brightening arc in the southern part of the FOV at UT, (4) a sequence of six high-latitude (75 77 MLAT) brightenings at UT, each of which is followed by (5) equatorward motion of auroral luminosity, spanning the whole latitude range from 77 to 72 MLAT, and (6) auroral poleward boundary retreating back to the latitude of the breakup arc ( UT) at UT. We note that the 2-min-long poleward boundary intensifications observed during UT recur at 3-min intervals. [17] Figure 7 shows a sequence of all-sky camera (ASC) images at nm for the interval UT, which includes the second high-latitude brightening event shown in Figure 6, i.e., the third stage of substorm expansion. Images (2-s integration time) are shown at the following times: 1858:00, 1859:00, 1901:00, 1903:00, 1906:00, and 1910:00 UT. The initial brightening event appears as a small bright spot to the west of the MSP scanning meridian (dashed line through M, almost along the vertical axis) at 1858 UT (near 2100 MLT) before it expanded in the east- Figure 5. Meridian scanning photometer (MSP) observations (line of sight intensities versus zenith angle) from Ny Ålesund (NAL), Svalbard (76 MLAT), at (top) nm and (bottom) nm, respectively, for the interval UT. Relative intensities are given by the color code at the bottom. Figure 6. MSP observations (line of sight intensities versus zenith angle) from Ny Ålesund (NAL), Svalbard (76.1 MLAT), at (top) nm and (bottom) nm, respectively, for the interval UT. Relative intensities are given by the color code at the bottom.

7 SANDHOLT ET AL.: MULTISTAGE SUBSTORM EXPANSION AURORAL DYNAMICS SMP 4-7 Figure 7. All-sky camera observations at nm from Ny Ålesund during the interval UT: ((a) 1858, (b) 1859, (c) 1901, (d) 1903, (e) 1906, and (f ) 1910 UT). The images are shown in a zenith angle azimuth angle coordinate system. Zenith angles 30, 60, and 90 are marked by dashed circles. The MSP scanning meridian is marked by an almost vertical dashed line. Magnetic north is up, and west is to the left. The camera horizon is at 80 zenith angle. west dimension during UT (Figures 7b and 7c). The rapid poleward expansion during UT was followed by a fading during UT (Figure 7e). A higher-latitude auroral form expanded into the camera FOV from the eastern side (Figure 7d), reaching the MSP meridian slightly south of zenith of the station at 1903 UT. This form expanded farther westward during the interval UT before fading from the eastern side during UT. A spiral form is formed during the maximum phase ( UT). [18] Further details on the auroral morphology are documented by showing the green line images of Figure 8. They are taken at 1-min intervals during UT. Bright forms are evident near the southern horizon (lower-latitude branch) in the two first images ( UT), which is followed by the activation of the high-latitude auroral branch during UT (compare Figure 6). The latter aurora develops a strong counterclockwise vorticity (Figures 8d and 8e). This form appears as north-southaligned forms in the MSP meridian (dashed line marked M) during the interval UT (Figure 6). In its maximum phase ( UT) this aurora spanned a zenith angular range from 30 south of zenith to north of zenith. It is one of the strongest poleward boundary intensifications in the photometer data in Figure 6. [19] In order to document the first auroral breakup at UT we show in Figure 9 an auroral keogram obtained from a white light TV camera located at station Lovozero (LOZ) on the Kola Peninsula (see Figure 2). Station LOZ is located at 64.1 MLAT. We note the initial equatorward motion of the aurora (a well-known growth phase phenomenon), which was followed by a strong intensification (breakup arc) and poleward expansion during UT. The auroral breakup occurring at south of zenith (63 64 MLAT) was accompanied by a rapid negative X component deflection recorded in the local magnetogram (station LOZ in Figure 4). Other features to notice are the activations of strong north-southaligned auroral forms (streamers) at 1850, , and UT. The two former events extended down to 60 south of zenith, corresponding to 62 MLAT. The three events are related to distinct auroral activities at higher latitudes, so-called poleward boundary intensifications (PBIs), as reported above. [20] A simple illustration of the observation geometry with the fields of view of the optical instruments (MSP and all-sky camera) in Ny Ålesund and a schematic illustration of the auroral activity during the interval UT is given in Figure 10. The FOV of the camera is the circle. Four different latitudes, referring to central aspects of the observations, are marked A, B, C, and D in Figure 10. The latitude corresponding to the footprint of spacecraft Polar at 1900 UT is marked D. This also marks the latitudinal location of the initial auroral breakup at LOZ (at 1839 UT). The initial electrojet activation is centered within the latitude range from C to D. Positions of stations MAS and LOZ are marked in Figure 10. The initial breakup of the auroral substorm was observed in the close vicinity of LOZ (Figure 9). The hatched belt B1 in the southern part of the camera FOV marks the second stage of auroral expansion at 1846 UT. The approximate location of the third expansion stage ( breakup arc observed at 1859 UT) is marked B2. The auroral belt A marks the high-latitude auroral branch observed during UT whose initial westward

8 SMP 4-8 SANDHOLT ET AL.: MULTISTAGE SUBSTORM EXPANSION AURORAL DYNAMICS Figure 8. All-sky camera observations at nm from Ny Ålesund during the interval (a f ) UT. The images are shown in a zenith angle azimuth angle coordinate system. Zenith angles 30, 60, and 90 are marked by dotted circles. The MSP scanning meridian is marked by an almost vertical dashed line. Magnetic north is up, and west is to the left. The camera horizon is at 80 zenith angle. expansion during UT is marked by the arrow in belt A. Equatorward moving auroral forms (EMAFs) between branches A and B are marked by two arrows. The trajectories of satellites F 13 (with positions at 1917 and 1920 UT) and F 14 (Southern Hemisphere pass during UT) are also shown Polar Plasma Observations [21] Figure 11 shows observations of electron fluxes acquired by the HYDRA experiment on Polar during the interval UT on 12 December In this interval, Polar moved radially outward from a radial distance of 4.56 R E inside the plasmasphere to 7.00 R E inside the plasma sheet, crossing the GSM equatorial plane traveling north near 1900 UT. The local time position of the spacecraft varied from 1930 to 1950 MLT at 1830 and 1940 UT, respectively, and the magnetic latitude changed from 0 to 15. [22] Figure 11 shows differential electron energy fluxes with intensities according to the color codes on the right. Figures 11a 11d display the omnidirectional flux, the perpendicular flux (pitch angle range ), the flux antiparallel to the field (pitch angle range ), and the flux parallel to the field (pitch angle range 0 30 ), respectively. [23] An initial enhancement of low-energy (<1 kev) electron fluxes took place at 1830 UT. At this time the density of the thermal plasma decreased significantly [Sandholt and Farrugia, 2001], consistent with the traversal of the boundary between the plasmasphere and the plasma sheet during the outward motion of the spacecraft. A minor enhancement in fluxes below 2 kev occurred at 1845 UT, i.e., 6 min after the initial substorm breakup, as inferred from the ground observations. [24] The first strong enhancement of the flux of higherenergy (0.2 5 kev) electrons is observed at 1852 UT, some 6 min after the onset of the second stage of substorm expansion was observed from the ground, as the spacecraft moved toward larger radial distances at the inner edge of the plasma sheet. The next major flux enhancement (in the energy range 1 10 kev) occurred at 1905 UT, 5 min after the onset of the third stage of substorm expansion observed from the ground on Svalbard. From 1905 UT to 1922 UT, background number fluxes of energetic electrons generally increase. On top of this trend, there is evidence of further injections, each lasting 1 2 min. Between 1905 and 1925 Figure 9. Line of sight auroral intensities versus zenith angle along the magnetic meridian obtained by a TV camera in Lovozero (LOZ), Russia, for the interval UT on 12 December North is up.

9 SANDHOLT ET AL.: MULTISTAGE SUBSTORM EXPANSION AURORAL DYNAMICS SMP 4-9 Figure 10. Observation geometry with approximate fields of view of the MSP (arrowed meridian line) and the all-sky camera (circle) in Ny Ålesund, for the nm emission observed during the interval UT. Auroral forms located at different latitudes, corresponding to the different stages of the substorm expansion, are marked in Figure 10 (see text). The high-latitude auroral branch during the interval UT is marked A. Trajectories of satellites F 13 (with positions at 1917 and 1920 UT) and F 14 (Southern Hemisphere pass during UT) are marked by straight lines. Approximate latitudes of auroral intensifications at 1846 and 1900 UT are marked B1 and B2, respectively. The footprint of spacecraft Polar at 1900 UT is indicated by a circled cross at latitude D. Station Lovozero (LOZ) is close to the site of the first auroral breakup at 1839 UT. UT one can make out 6 7 injections, appearing as corrugations in the peak differential energy fluxes, affecting mostly the perpendicular fluxes (Figures 11b and 11d), indicating that the injected electrons have predominantly a trapped distribution. The electron events observed during the interval UT correspond to the brightenings at the poleward boundary of the aurora (PBIs) but follow these high-latitude disturbances at a 5-min delay. The events typically recur at 3-min intervals Observations From DMSP Spacecraft: Particle Precipitation and Field-Aligned Currents [25] The strong field-aligned current at the poleward boundary of the plasma sheet precipitation, corresponding to the activity of the discrete aurora reported in section 2.3, may be seen in data obtained from the DMSP spacecraft F 13 and F 14. [26] Data from DMSP F 13 during the interval UT are shown in Figure 12. Figures 12a 12d show differential electron fluxes, differential proton fluxes, horizontal (violet) and vertical (green) ion drift speeds, and local magnetic field deflections B x, B y, and B z, respectively. The universal time (UT), MLAT, and MLT of the trajectory are given at the bottom. The pass intersected the dusk sector (18 MLT) auroral zone ( UT), the polar cap ( UT), and the dawn sector auroral zone ( UT). The location of the satellite pass with respect to the ground observations is as indicated in Figure 10. [27] We shall focus on the interval UT when the satellite mapped particle precipitation from the plasma sheet in the dusk sector (18 MLT) during the interval of poleward boundary intensifications in the aurora recorded from Svalbard near 2130 MLT. The zone of plasma sheet precipitation spans the latitude range MLAT. The poleward boundary of precipitation of plasma sheet particles, recorded at 1921 UT, is sharp. This same boundary, located at 77 MLAT, is the site of strong electron precipitation within the whole energy range of the electron detector (30 ev to 30 kev). A reversal from sunward to antisunward convection is detected at 1920 UT (74.2 MLAT/18.0 MLT). Thus the poleward part of the preciptation, within MLAT, is located on field lines convecting antisunward. [28] Similar observations from DMSP spacecraft F 14 on a Southern Hemisphere overpass during the interval UT are shown in Figure 13. The pass intersected the prenoon sector cusp precipitation during UT, the polar cap during UT, and the evening sector auroral zone during UT. The location of the satellite pass with respect to the ground observations is indicated in Figure 10. We shall focus on the mapping of the plasma sheet precipitation in the MLT sector during the interval UT, which corresponds to the early phase of the high-latitude auroral activations recorded from the ground at 2130 MLT. We note the strong electron precipitation in the vicinity of the sharp polar cap boundary recorded at approximately 75 MLAT (1902 UT). The plasma sheet precipitation spans the latitude range from 60 to 75 MLAT, with the poleward part corresponding to the boundary plasma sheet (BPS). The latter precipitation is accompanied by a system of oppositely directed field-aligned currents, as inferred from the gradients in the magnetic field deflections in Figure 13d. B x (black) points radially down, B y (violet) is closely aligned to the satellite s motion vector, and B z (green) completes the left-handed system and points in a generally antisunward direction CUTLASS Radar Observations of Ionospheric Convection in Relation to the Auroral/Magnetic Activity [29] Figure 14 shows line of sight velocities, color coded such that toward (away) velocities are blue/green (red/ yellow), obtained by the CUTLASS HF radar in Hankasalmi, Finland. Beam 9 intersects the MSP scanning meridian slightly south of Ny Ålesund and is therefore very useful for comparisons with the MSP data in Figure 6. Beam 4 is 16 west of beam 9. [30] The vertical line at 1839 UT marks a disappearance of radar backscatter associated with the initial breakup of the auroral substorm at this time. We note the following features: (1) equatorward moving belts of flow with large equatorward component (blue) during the interval UT (the growth phase), (2) a latitudinally wide zone of flow with large equatorward component (blue), particularly strong along beam 4, during UT, just before the substorm expansion at 1900 UT (marked by the second vertical line in Figure 14), and (3) a series of equatorward expanding flow bursts during the interval

10 SMP 4-10 SANDHOLT ET AL.: MULTISTAGE SUBSTORM EXPANSION AURORAL DYNAMICS Figure 11. Electron spectrograms from the HYDRA experiment on Polar during the interval UT for the (a) omnidirectional differential energy flux, (b) perpendicular flux, (c) antiparallel flux, and (d) flux parallel to the field. Color codes are given to the right UT, each of which is initiated by an auroral PBI (marked by vertical dashed lines in Figure 14). One of the latter events is that at UT. In this case, localized equatorward flows, particularly strong in beam 4, were observed within geographic latitude GLAT. [31] Figures 15 and 16 show spatial plots of ionospheric ion drift and aurora for two selected intervals during the Figure 12. Observations from spacecraft DMSP F 13 during the interval UT: (a) electron precipitation, (b) proton precipitation, (c) horizontal (violet) and vertical (green) ion drift, and (d) magnetic field deflections B x, B y, and B z. The pass intersected the dusk sector auroral zone ( UT), the polar cap ( UT), and the dawn sector auroral zone ( UT).

11 SANDHOLT ET AL.: MULTISTAGE SUBSTORM EXPANSION AURORAL DYNAMICS SMP 4-11 Figure 13. Observations from spacecraft DMSP F 14 during the interval UT: (a) electron precipitation, (b) proton precipitation, (c) horizontal (violet) and vertical (green) ion drift, and (d) magnetic field deflections B x, B y, and B z. The pass intersected the prenoon sector cusp precipitation during UT, the polar cap during UT, and the evening sector auroral zone during UT in the Southern Hemisphere. second substorm. Ion drift vectors have been inferred by using a beam-swinging algorithm given by Villain et al. [1987] and Ruohoniemi et al. [1989]. Ion drift velocities in Figures 15a, 15b, 16a, and 16b are color-coded with the same code as in Figure 14. Blue is toward (equatorward) and red is away (antisunward) from the radar. The velocity scale for the beam-swung vectors is given to the left of the plot. Figures 15c, 15d, 16c, and 16d show the aurora at wavelength nm (red), and Figures 15e, 15f, 16e, and 16f show the aurora at wavelength nm (green), with ion drift vectors superimposed. The auroral images are obtained by the all-sky camera in Ny Ålesund. Auroral intensities are color coded (intensity increasing from blue to red). The sector shown is MLT/60 90 MLAT. [32] Interval 1 ( UT, Figure 15) represents the third stage of auroral brightening/poleward expansion in this substorm. The aurora expanded east-west and north, emanating from an initial bright spot. This was accompanied by a rapid negative X component deflection, maximizing at HOR (74 MLAT), near the poleward boundary of the aurora. The flow at 1855 UT is dominated by equatorward flow, with a small westward component at higher latitudes, poleward of the auroral emission, and a small eastward component in the region of the aurora. The velocities are typically of the order of 1 km s 1 as estimated from the beam-swinging algorithm. By 1900 UT, the time of the auroral brightening, the convection poleward of the aurora is still dominated by equatorward flow but now with an eastward component, while within and equatorward of the aurora the convection is now dominated by eastward flow. The inferred eastward convection extends across a broad band between Svalbard and the Scandinavian mainland (68 75 MLAT). [33] Interval 2 ( UT, Figure 16) includes the formation of the poleward auroral branch at 1903 UT and the strong PBI at 1910 UT. At the latter time the equatorward branch also brightened, as is seen in the green line plot (Figure 16f ). The latter event seems to extend down to the Scandinavian mainland, appearing as a strong north-southaligned form, as recorded by the TV camera in Lovozero, Russia (see Figure 9). At 1903 UT the convection is still mainly eastward at latitudes between Svalbard and mainland Scandinavia, although it is weaker than at 1900 UT. However, there is a gap in the scatter over Svalbard, which had started soon after the initial brightening at 1900 UT, which results in the beam-swinging analysis being less efficient at these latitudes. The area over which there is a loss of scatter continues to increase in size thereafter, such that at 1910 UT the scatter is mainly equatorward of Svalbard, apart from a region to the west of the island. Here the flow is dominated by large velocities toward the radar, which is spatially localized to 2100 MLT/70 73 MLAT in the region where EMAFs are observed. This flow burst is the event marked by arrows in Figure 14 and the slanted line in the magnetograms in Figure Discussion [34] This paper is based on multipoint and multitechnique observations relating to the substorm process. The observations will be discussed in relation to present ideas on

12 SMP 4-12 SANDHOLT ET AL.: MULTISTAGE SUBSTORM EXPANSION AURORAL DYNAMICS Figure 14. Line of sight ion drift velocities obtained by beams (top) 4 and (bottom) 9 of the Finland (Hankasalmi) antenna of the CUTLASS radar during the interval UT on 12 December Velocities are color coded according to the scale to the right. Positive (blue/dark green) is toward the radar (equatorward) and negative (red) is away from the radar (poleward). Geographic latitudes of the scatter volumes are given along the vertical axis. The equatorward expansion of equatorward flow (blue) during the growth phase and the initial auroral breakup at 1839 UT are marked by arrow and vertical line, respectively. The vertical line at 1900 UT marks the auroral breakup/poleward expansion giving rise to the formation of a double-oval configuration. The auroral poleward boundary intensifications (PBI) sequence during UT is marked by vertical dashed lines in both plots. The PBI event centered at 1910 UT is marked by arrows in both plots. The latitude of the optical site in Ny Ålesund (NAL) is marked by horizontal dashed line in the bottom plot. Figure 15. Super DARN (Super Dual Auroral Radar Network) plots of line of sight ion drift velocity and drift vectors, obtained by the Finland (Hankasalmi) antenna of the CUTLASS radar for (a) 1855 UT and (b) 1859 UT on 12 December Overlaid plots of ion drift and (c and d) red and (e and f ) green line auroras at approximately 1856 UT (Figures 15c and 15e) and approximately 1900 UT (Figures 15d and 15f ). The coordinate system is MLT/MLAT. Meridians in the MLTsector and the 60,70, and 80 MLAT circles are shown by dashed lines and curves, respectively. Ion drift velocity scale is given on the left side.

13 SANDHOLT ET AL.: MULTISTAGE SUBSTORM EXPANSION AURORAL DYNAMICS SMP 4-13 Figure 16. Same as Figure 15 for the times 1903 and 1909 and 1910 UT. magnetotail topology and dynamics. While a major topic in present substorm research is about the possible role of BBFs as trigger of the substorm onset, we focus on the association of BBFs/intermittent plasma injections with low-altitude ionospheric auroral and current features in the later stages of the expansion phase. A useful framework for our discussion is Figure 17 (after Fairfield et al. [1999] and Kennel [1995]). In Figure 17a the linkage of magnetotail phenomena to the ionosphere is indicated by field lines threading the plasma sheet at three different radial distances: A, the reconnection X line at 25 R E or beyond; B, an intermediate distance marking the source region of fieldaligned currents (the current wedge), and C and D, the location in the inner part of the plasma sheet, where the initial substorm breakup is expected to take place [Lui, 1991; Lui et al., 1992], which is the L shell where spacecraft Polar was located in our case, i.e., at the plasmasphereplasma sheet boundary. Ionospheric projections of the four radial distances in the plasma sheet are indicated in Figure 10. The cartoon suggests that field-aligned energetic particles (beamlets) ejected into the plasma sheet boundary layer, along field line marked A, from the X line, gives rise to poleward boundary intensifications in the aurora. Figure 17b shows the equatorial plasma sheet with a radially elongated channel of bursty bulk flow in the premidnight sector [after Kennel, 1995]. The position of spacecraft Polar at 1900 UT on 12 December 1999, is shown, indicating the location where plasma injections were observed. [35] In the case of the second substorm on 12 December 1999, which is a typical strong substorm, the initial auroral breakup occurred at MLAT, corresponding to the inner edge of the plasma sheet, as inferred from the particle precipitation data in Figure 12. The poleward (outward) expansion of the substorm activity, which occurred in a three-stage scenario [Wiens and Rostoker, 1975; Erickson et al., 2000], is particularly evident in the auroral and ground magnetic data in our case. In the framework of Figure 17 the initial poleward expansion, during UT, corresponds to a radial expansion from C toward B. Figure 17. (a) Schematic illustration of magnetotail topology in the plasma sheet during substorms [after Fairfield et al., 1999]. Field lines threading the plasma sheet at four different radial distances are marked A, B, C, and D (compare Figure 10). Lower (b) Schematic illustration of localized high-speed plasma flow bursts in the equatorial plasma sheet [from Kennel, 1995] (used by permission of Oxford University Press, Inc.). The location of spacecraft Polar at the inner edge of the plasma sheet, at 1930 MLT, is marked.

14 SMP 4-14 SANDHOLT ET AL.: MULTISTAGE SUBSTORM EXPANSION AURORAL DYNAMICS [36] Our observations support the current view that the substorm breakup occurs near the inner edge of the plasma sheet and that it is accompanied by the development of a substorm current wedge [Lui et al., 1992]. In the wedge model, downward field-aligned currents flow from the magnetotail to the ionosphere in the dawnward portion of the substorm region, continue in the ionosphere to form a westward electrojet, and return to the magnetotail by upward field-aligned currents in the duskward part of the activity region. In this model the poleward expansion of the substorm at the ionospheric level (see Figure 6) corresponds to the tailward expansion of the cross-tail current disruption [Jackuey et al., 1991; Lui et al., 2000]. [37] Next we focus on the later stages of the poleward expansion of the substorm activity, which were initiated by clear auroral brightenings at 1846 and 1900 UT. The following 25-min interval ( UT) is characterized by a sequence of intensifications in the high-latitude branch of the aurora. The auroral intensifications/poleward expansions at 1846 and 1900 UT were both accompanied by an 5 -wide channel of eastward convection on the equatorward side of the poleward auroral boundary. According to Hamza et al. [2000] the eastward convection jet around the poleward boundary of the auroral oval is found to be a remarkable and important element of the magnetospheric substorm. In this study we find that a transition to eastward flow (immediately equatorward of the polar cap boundary) is observed in association with each individual brightening of the poleward auroral boundary. [38] According to Maynard et al. [1997] the rapid (explosive) poleward motion of the poleward boundary of the aurora in the later stage of poleward expansion, when the double-oval configuration is formed, marks the time when the newly activated near-earth neutral line, marked A in Figure 17, begins to reconnect lobe magnetic flux. The rapid auroral expansion into the polar cap is taken as an indication of a high rate of reconnection of lobe flux. Thus previous studies indicate that the auroral poleward bulge reaches the open-closed boundary toward the end of the expansion phase, at which time the poleward boundary brightens and becomes more active while the aurora in the middle of the bulge often fade, leading to a double-oval configuration [Frank, 1988; Elphinstone et al., 1995a; Maynard et al., 1997; Fox et al., 1999]. The source of energy for this high-latitude activity is thought to be the tail lobe magnetic field, released by enhanced reconnection at a neutral line [Rostoker, 1998]. The particle precipitation corresponding to the northernmost auroral branch is the so-called BPS [Winningham et al., 1975; Frank, 1988; Elphinstone et al., 1995a]. [39] Following the previous suggestions on the auroramagnetotail connection in the later stage of substorm expansion [Fairfield et al., 1999], it is natural to attribute the brightening sequence observed in the high-latitude branch during the interval UT (six events in 20 min) to the intermittency of the magnetotail reconnection process. More specifically, we suggest that the auroral PBIs represent an ionospheric signature of particle beams (socalled beamlets ) in the plasma sheet boundary layer ejected from the X line, associated with bursts of magnetotail reconnection (see Figure 17). The auroral signature and its association with plasma convection are reported here with better temporal-spatial resolution than in previous work. [40] The clear counterclockwise vortical auroral motions, observed within a circular area of diameter 500 km during UT (see Figure 8), is direct evidence of strong field-aligned currents (FACs) flowing in and out of the ionosphere. The spiral auroral form at the polar cap boundary is consistent with the satellite observations reported in Figure 13 and also with MHD modeling and theoretical considerations of FACs generated by magnetotail reconnection [Yamada et al., 2000; Burke et al., 1994]. [41] The EMAFs emanating from the poleward boundary are clearly connected to the individual poleward boundary intensifications (PBIs) (see the green line emission in Figure 6). The EMAFs span the latitude range between the two oval branches observed within the FOV of the optical instruments on Svalbard; that is, they range from 78 to 72 MLAT. The velocity of EMAFs is typically km s 1, which is larger than the convection speed observed by the radar in the same region. This means that they are different from the equatorward drift at 0.5 km s 1 of electron field-aligned temperature structures (European Incoherent Scatter measurements) reported at lower latitudes by Persson et al. [1994]. The latter was primarily a growth phase phenomenon, while our observations refer to the late expansion phase. [42] We suggest that PBIs are a nightside phenomenon representing the ionospheric signature of bursty reconnection in the tail in a way similar to how the equatorward boundary intensifications in the dayside cusp region represent the signature of pulsed dayside magnetopause reconnection [Sandholt et al., 1990; Sandholt and Farrugia, 1999]. Then it is natural to consider EMAFs/streamers as representing earthward moving high-speed flows modeled as bubbles [Chen and Wolf, 1993] and/or hydromagnetic waves in the plasma sheet boundary layer, emanating from the X line in the tail [Semenov et al., 1999; Allan and Wright, 1998, 2000; Wright et al., 1999]. [43] Next, we focus attention on the brightenings of the lower branch of the double-oval configuration ocurring when the EMAFs reach that latitude (see the green line panels (557.7-nm panels) of Figures 5 and 6). The brightening sequence is observed at the equatorward boundary of the FOV during the interval UT. These brightenings at the lower branch may have a natural explanation within the magnetotail scenario of Shiokawa et al. [1998], assuming that the poleward boundary intensifications (PBIs) mark the release of bursty bulk flows (BBFs) from the reconnection site [Lyons et al., 1999; Fairfield et al., 1999]. According to Shiokawa et al. [1998] the braking of BBFs in the inner plasma sheet may form FAC wedgelets, as schematically indicated in Figure 17, with associated low-altitude particle acceleration and auroral brightening. [44] Chen and Wolf [1993] proposed a model according to which BBFs consist of magnetic flux tubes which, by virtue of their lower density with respect to ambient plasma, are propelled earthward by magnetic bouyancy. Modeling quantitatively the decelerating phase close to Earth, the authors find that currents flow on the edges of these flux tubes into the ionosphere at the dawnside and out of it at the duskside [see also Shiokawa et al., 1997]. The ionospheric projection of these tubes (diameter 1 2 R E ) is estimated to