The double auroral oval in the dusk-midnight sector: Formation ...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, A08203, doi:10.1029/2011JA017501, 2012

The double auroral oval in the dusk-midnight sector: Formation, mapping and dynamics S. Ohtani,1 H. Korth,1 S. Wing,1 E. R. Talaat,1 H. U. Frey,2 and J. W. Gjerloev3 Received 31 December 2011; revised 12 June 2012; accepted 12 June 2012; published 2 August 2012.

[1] The present study observationally examines the double auroral oval structure in the evening-premidnight sector. The results are summarized as follows; (1) The poleward auroral branch can be attributed to accelerated auroral precipitation, occasionally accompanied by Alfvénic precipitation, and it is collocated with the upward R1 current; (2) The equatorward auroral branch, which is embedded in the downward R2 current, very often if not always consists of the diffuse precipitation of energetic electrons and extends mostly poleward of the b2i boundary collocated with energetic ion precipitation; (3) The associated ionospheric convection is consistent with the two-cell pattern, and the poleward branch is located at the zonal flow shear. It is inferred that the equatorward branch is mapped to the geosynchronous region and outside, whereas the poleward branch is mapped farther down the tail to the source region of the R1 system. It is suggested that the intense ring current and injection of energetic electrons are necessary for the equatorward branch to be distinct, but the substorm expansion phase is not favorable for the formation of the emission gap. This explains why the double oval often takes place during magnetospheric storms and during the recovery phase of intense substorms. The equatorial magnetic field is expected to be strong in the mid-tail region, which might be related to the auroral gap region. It is also found that the poleward branch occasionally intensifies in an extended MLT sector in a few minutes, which may be explained in terms of the sudden enhancement of the return (sunward) convection flow. Citation: Ohtani, S., H. Korth, S. Wing, E. R. Talaat, H. U. Frey, and J. W. Gjerloev (2012), The double auroral oval in the dusk-midnight sector: Formation, mapping and dynamics, J. Geophys. Res., 117, A08203, doi:10.1029/2011JA017501.

1. Introduction [2] The auroral emission is distributed along a circular band in the polar region, the auroral oval [Akasofu, 1964; Feldstein and Starkov, 1967]. A part of the night-side auroral oval sometimes splits into two branches, equatorward and poleward ones with a gap (or a region of weak emission) between them. This feature is widely known as double auroral oval [Elphinstone et al., 1995a, 1995b]. Despite the term, the two emission branches co-exist usually in a limited MLT range rather than throughout the entire oval. The double auroral oval very often forms during magnetospheric storms or during the recovery phase of intense substorms. [3] The double oval forms as an active auroral region expands poleward followed by the fading of emission in the middle of the thickened oval [e.g., Elphinstone et al., 1993; 1 Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA. 2 Space Science Laboratory, University of California, Berkeley, California, USA. 3 Department of Physics and Technology, University of Bergen, Bergen, Norway.

Corresponding author: S. Ohtani, Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Rd., Laurel, MD 20723, USA. ([email protected]) ©2012. American Geophysical Union. All Rights Reserved. 0148-0227/12/2011JA017501

Gjerloev et al., 2008]. This typical sequence, however, does not mean that the double auroral oval is a transient recovery feature of preceding auroral activity. On the contrary, the double oval configuration often stays for a long time, for example, during steady magnetospheric convection (SMC) events [Sergeev et al., 1996] and sawtooth storm events [Henderson et al., 2006]. It is therefore suggested that this auroral structure represents a unique state of the magnetosphere during geomagnetically active periods. [4] However, despite its frequent occurrence during active times, there remain fundamental issues about the double auroral oval to be understood. One such issue is concerned with the latitudinal structures of particle precipitation and field-aligned currents (FACs), which should be critical for addressing the mapping of the two auroral branches. This is especially the case for the dusk-to-midnight sector since the previous such studies investigated the morning [Elphinstone et al., 1995a] and midnight [Mende et al., 2002] sectors; Elphinstone et al. [1995a] examined precipitation observed by DMSP satellites in different local time sectors, but examined FAC signatures only for one Viking pass on the morning side. Since the polarities of FACs and the characteristics of precipitation are different in different MLT sectors, it is important to address what is unique, or common, with the double oval structure in the dusk-to-midnight sector. Note also that this is the local time sector where substorm activity is most noticeable.

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OHTANI ET AL.: THE DOUBLE AURORAL OVAL

[5] Another issue is the auroral intensification and the associated dynamics in the magnetosphere. During magnetospheric storms, substorms start on the equatorward branch [Henderson et al., 2006], and the associated particle injection and dipolarization can be observed in the middle of the ring current, at r 5 RE [Ohtani et al., 2007]. Aurora also intensifies on the poleward branch as identified as poleward boundary intensifications (PBI’s) [e.g., Lyons et al., 2000]. In general, the PBI’s are localized in longitude and are associated with transient bursts of the plasma sheet convection or bursty bulk flows (BBF’s) [Zesta et al., 2000]. However, the poleward branch often intensifies simultaneously in a wide longitudinal range (as will be shown in this study). The simultaneous observations of convection flows and magnetic disturbances in the plasma sheet should be helpful for considering the formation and intensification of the double auroral oval, especially its poleward branch, in the context of tail dynamics. [6] In the present study we examine double auroral oval events observed on 2 March 2001 and 15 March 2004. In section 2 we describe data sets we use in this study. In section 3 we examine those events with an emphasis on the 2 March 2001 event. For each event we examine auroral dynamics, precipitation and FAC signatures, and tail dynamics. For the 2 March 2001 event we also address an intensification of the poleward branch in terms of the ionospheric convection. In section 4 we discuss the mapping of the two auroral branches, the occurrence conditions of the double auroral oval, and the intensification of each branch. Section 5 is a summary.

2. Data Set [7] We use FUV auroral images taken by the Wideband Imaging Camera (WIC) [Mende et al., 2000] onboard the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite [Burch, 2000]. The IMAGE satellite was launched in March 2000 into a 7.2 RE 1000 km orbit with an orbital period of 13.5 h. The orbit precessed during the mission, and the satellite was at northern vantage points in the 2 March 2001 event and at southern vantage points in the 15 March 2004 event. All WIC auroral images are remapped into the Altitude Adjusted Corrected Geomagnetic (AACGM) Coordinate system [Baker and Wing, 1989]. The WIC instrument takes images approximately every two minutes with an exposure period of 24 s. In the following data analysis we refer to each image by the start time of its exposure period, and caution needs to be exercised since each image has uncertainty of timing. Especially we should keep in mind that any change of auroral distribution and intensity between two consecutive images can happen at any time between the two corresponding exposure periods or even during either exposure period. [8] For examining field-aligned currents (FACs) and particle signatures we use magnetometer data and particle precipitation data from DMSP satellites, -F14 and -15 for the 2 March 2001 event and -F15 and -F16 for the 15 March 2004 event. Each of those DMSP satellites has a Sunsynchronous orbit with a prenoon-premidnight orientation at 835–850 km in altitude. The orbital period is approximately 100 min. The DMSP satellites are three-axis stabilized.

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[9] The vector magnetic field is measured by a triaxial fluxgate magnetometer onboard with a time resolution of 1s, a range of 65535 nT and a one-bit resolution of 2 nT [Rich et al., 1985]. In this study we present magnetic variations in the satellite coordinate system, in which the X axis is directed vertically downward, the Y axis is in the direction of the satellite motion and the Z axis completes a righthand orthogonal system. The Z axis is approximately in the azimuthal direction. If the satellite crosses a FAC sheet extending in the east-west direction, the associated magnetic variation can be seen primarily in the Z component. [10] Particle precipitation is measured by the particle (SSJ4) instrument package onboard, which uses curved plate electrostatic analyzers to measure ions and electrons from 32 eV to 30 keV in logarithmically spaced steps [Hardy et al., 1984]. One complete 19-point electron and ion spectra are obtained each second. The detector apertures always point toward local zenith. In the polar region the magnetic field is approximately vertical and therefore, measured particles are precipitating into the ionosphere. [11] We examine tail dynamics with 12-s magnetic field, electric field and plasma measurements made by the magnetic field (MGF) [Kokubun et al., 1994], electric field (EFD) [Tsuruda et al., 1994], and the Low Energy Particle (LEP) [Mukai et al., 1994] instruments onboard the Geotail satellite [Nishida, 1994]. The energy range of the LEP instrument is 32 eV/Q to 39 keV/Q. The GSM coordinate system is adopted for examining plasma velocity and magnetic field, whereas we use the original satellite coordinate system for the electric field measurements, which can be regarded as the GSE system for any practical purpose. In addition, we use IMF and Solar wind data obtained from the MAG [Smith et al., 1998] and SWEPAM [McComas et al., 1998] instruments, respectively, onboard the ACE satellite at the L1 point.

3. Data Analysis 3.1. The 2 March 2001 Event [12] The first event we examine occurred on 2 March 2001. Figure 1 shows from the top the solar wind dynamic pressure and IMF BZ component, which were measured by the ACE satellite at the L1 point but were propagated to 1 AU, the AU (blue) and AL (red) indices, and Sym-H (red) and Asy-H (blue) indices for 1000–2200 UT on 2 March 2001. The interval of interest is 1700–1930 UT, which is marked by the horizontal bar in Figure 1c. Following the southward turning of IMF BZ slightly before 15 UT (Figure 1b), AL started to decrease reaching 400 nT at 1630 UT and then stayed around that level for a few hours (Figure 1c). AL reached its minimum, 700 nT at 1847 UT and then recovered sharply after IMF BZ turned northward. This is not a storm-time event; Sym-H remained above 5 nT throughout the interval (Figure 1d). The solar wind dynamic pressure did not show any noticeable change during the interval (Figure 1a). 3.1.1. Global Auroral Images [13] Figure 2 shows global auroral images in the Northern Hemisphere taken by the IMAGE/WIC instrument at selected times. In some images a DMSP pass is marked if the satellite crossed the night-side auroral oval in the corresponding time frame (section 3.1.2). The effect of dayglow is subtracted. The auroral oval is wide in latitude in the image of 17:18:50 UT

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Figure 1. (a) Solar wind dynamic pressure, Pdyn, (b) IMF BZ, (c) AU (blue) and AL (red) indices, and (d) Asy-H (blue) and Sym-H (red) indices for the 12-h interval of 10:00– 22:00 UT on 2 March 2001. The IMF and solar wind data were acquired by the ACE satellite around the L1 point and are time-shifted to 1 AU. (Figure 2a), which was taken a few hours after the start of geomagnetic activity (Figure 1). The image of 17:49:31 UT (Figure 2e) shows that the aurora emission intensified suddenly in the pre-midnight sector; compare with the image of 17:47:28 UT (Figure 2d). A mid-latitude positive bay started at 1747 UT along with a westward magnetic deflection at Irkutsk (GLat: 52.2 ; GLon: 104.5 ) in the midnight sector (not shown), which suggests the formation of a substorm current wedge centered in the premidnight sector. Here the positive bay onset should be regarded as simultaneous with the auroral intensification within the time resolutions of auroral images (2 min) and ground data (1 min). Simultaneously, dispersionless injection was observed by the LANL geosynchronous satellite 1994–084 (not shown). The auroral activity continued especially in the early evening sector (Figure 2f). [14] The auroral oval thickened in the pre-midnight sector (22–24 in MLT) at 1807 UT with enhanced emission along the poleward edge, which is separated from the pre-existing equatorward branch (compare Figures 2g and 2h). That is, the double oval was formed. The new poleward branch intensified subsequently (Figure 2i), whereas the auroral emission equatorward changed very little. Then the poleward branch weakened (Figure 2j), but it strengthened again at 1818 UT

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(Figure 2k). The image of 18:20:12 UT shows a new activity region in the midnight sector (Figure 2l), which expanded in both westward and eastward as well as poleward and developed into an auroral bulge (Figures 2m and 2n). At the same time the auroral emission at the evening-side poleward edge intensified in a wide MLT sector. A positive bay onset and a Pi2 onset were observed at 1821 UT at Kakioka (GLat: 36.2 ; GLon: 140.2 ) in the postmidnight sector (not shown). Then the auroral oval narrowed and the emission faded especially in the midnight sector (Figure 2o), but in the evening sector the double oval structure stayed (Figure 2p). 3.1.2. Precipitation and FACs [15] DMSP-F14 and -F15 crossed the night-side auroral oval one after another with a time separation of less than 10 min around 1720, 1825, 1905 UT. For the first and third intervals the satellites crossed the auroral oval in the Northern Hemisphere, in the same hemisphere as the auroral images, whereas for the second interval the oval crossings occurred in the Southern Hemisphere. Figure 3 shows, in the top part of each panel, an IMAGE/WIC auroral image with an exposure time closest to the center of the corresponding DMSP auroral oval crossing and, in the bottom part, DMSP magnetic field and particle precipitation data at each auroral oval crossing. The DMSP part of each panel shows, from the top, two horizontal magnetic components, the energy flux and average energy of precipitating electrons (black dots, labeled on the left-hand side) and ions (red dots, labeled on the right-hand side), and the E-t diagrams of precipitating electrons and ions. The vertical axis of the ion E-t diagram is inverted. The satellite orbital segment is shown in the auroral image, along which the position at each minute tick of the bottom axis is marked by a node. If the satellite crossed the auroral oval in the opposite hemisphere, its conjugate location is shown by changing the sign of the magnetic latitude. (Each horizontal axis is 5-min long. If the plot starts at a fraction of minute, there are only 5 nodes along the satellite pass and small segments before the first minute tick and after the last minute tick are omitted from the orbital segment.) [16] In Figure 3 we plot the along-track, BY (blue), and cross-track, BZ (red) components. BZ serves as a good proxy of the azimuthal magnetic component, and it is the primary component of magnetic variations if the satellite crosses a FAC sheet extending in the east-west direction (section 2). The polarity of each large-scale FAC, which can be deduced from the slope (increase or decrease) of the BZ plot and the direction of the satellite motion, is also shown by an open arrow. In this event as well as in the 15 March 2004 event two DMSP satellites crossed the auroral oval of the same hemisphere within 10 min in time. Although some differences in FAC and precipitation signatures are noticeable along the two satellite tracks, we will rather focus on similarities in the following. [17] Figure 3a (Figure 3b) shows the DMSP-F15 (-F14) crossings of the auroral oval in the Northern Hemisphere, the same hemisphere as the auroral images, which occurred around 1719 (1726 UT) at MLT 20.3 (MLT 19.8). The global auroral images show active auroral ovals, but neither of them has a double-oval structure along the DMSP passes. Both satellites crossed the auroral oval from equatorward to poleward. The latitudinal FAC structure is well approximated by a pair of downward R2 and upward R1 currents. The

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Figure 2. Selected auroral images taken by IMAGE/WIC in the Northern Hemisphere on 2 March 2001. DMSP footprints are also shown in Figures 2a, 2b, 2c, 2m, 2n, 2o, and 2p. Each image is labeled with the start of the 24 s exposure time. 4 of 26

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Figure 3. Night-side auroral oval crossings by (a, c, e) DMSP-15 and (b, d, f) -14 around 1720, 1825, and 1905 UT on 2 March 2001. In each panel the top part shows an IMAGE/WIC auroral image with the satellite orbital segment labeled with the start of the 24s exposure time. The bottom part shows, from top to bottom, the along-track (BY, blue) and cross-track (BZ, red) magnetic components along with open arrows indicating the polarities of large-scale FACs, the energy flux and average energy of precipitating electrons (black dots, labeled on the left-hand side) and ions (red dots, labeled on the right-hand side), and the E-t diagrams of precipitating electrons and ions. The vertical axis for the ion E-t diagram is inverted. The small red circles at the top of the electron E-t diagrams of Figures 3c–3f indicate the latitudes of the b2i boundaries. 5 of 26

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Figure 3. (continued)

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downward R2 current is collocated with the precipitation of energetic (>10 keV) ions, whereas the upward R1 current is well collocated with accelerated electron precipitation. [18] DMSP-F15 (Figure 3c) and -14 (Figure 3d) crossed the evening-side (MLT: 19.1 20.6) auroral oval again at 1820 1830 UT, but from poleward to equatorward in the Southern Hemisphere this time. The corresponding auroral image shows a double oval structure along each satellite orbit. The equatorward branch is faint but it is clearly separated from the poleward branch. The accelerated electron precipitation was collocated with an upward R1 current along both satellite tracks as we saw in Figures 3a and 3b. On its equatorward side, diffuse electron precipitation extends over several degrees in latitude, which is mostly collocated with energetic ion precipitation; the b2i ion precipitation boundary [Newell et al., 1996] is marked by the red solid circle at the top of the electron E-t diagram (for all DMSP crossings of double ovals), which will be later discussed in section 4 in terms of mapping and tail configuration. The extended diffuse electron precipitation is the most noticeable difference from the previous oval crossings and can be attributed presumably to the injection of fresh particles associated with the substorm activity that started at 1749 UT (section 3.1). The region of this diffuse electron precipitation coincided with a downward R2 current along the DMSP-F15 orbit, but the agreement is less clear along the DMSP-F14 orbit because of the plateau of the BZ component between the large-scale upward (R1) and downward (R2) FACs. The energy flux of the diffuse electron precipitation has its peak in the middle of the precipitation, which is therefore well separated in latitude from the accelerated electron precipitation, and its intensity is weaker than the peak energy flux of the accelerated electron precipitation. These features are consistent with the aforementioned latitudinal structure of auroral emission. A close visual inspection, however, indicates that the emission peaks are slightly equatorward of the electron precipitation peaks, which may be attributed to the fact that the auroral images and electron precipitation were observed in different hemispheres. [19] About 40 min later, DMSP-F15 (Figure 3e) and -14 (Figure 3f) crossed the evening-side (MLT: 20.2 20.6) auroral oval in the Northern Hemisphere from equatorward to poleward. The observed precipitation and magnetic signatures are qualitatively very similar to what were observed at the previous auroral oval crossings in the Southern Hemisphere, although both FACs and electron precipitation are noticeably weaker. On the poleward side the precipitation of accelerated electrons is collocated with an upward R1 current. Interestingly, for these DMSP passes, broadband, or Alfvénic, precipitation can be found along with monoenergetic precipitation. On the equatorward side, diffuse electron precipitation extends several degrees in latitude. This diffuse electron precipitation is collocated with a downward R2 current and also with energetic ion precipitation except for its equatorward potion. Corresponding to the two local peaks of electron energy flux, one for the accelerated electron precipitation and the other for the diffuse electron precipitation, the auroral image shows along each DMSP pass two latitudinally separated emission peaks (humps), that is, the double oval structure.

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3.1.3. Ionospheric Convection [20] In this section we examine the ionospheric convection measured by SuperDARN. We focus on the intensification of the poleward auroral branch of the double-oval structure at 1822 UT. A comparison between the two consecutive images at 1820:12 and 1822:15 UT (Figures 2l and 2m) reveals that the auroral emission enhanced in an extremely wide range of MLT, from dusk to post-midnight, within 2 min. [21] Figures 4a and 4b show the two consecutive northern polar maps of convection flows measured by SuperDARN for the 2-min intervals starting at 1820 and 1822 UT, respectively. In Figures 4c and 4d the close-ups of the duskto-midnight quadrant are overlaid over the IMAGE/WIC auroral images taken closest in time. The flow velocities are fitted based on the measurements of the line-of-sight velocity. However, for the Scandinavian sector in Figures 4c and 4d, the fields of view of by the Pykkvibær (Iceland) and Hankasalmi (Finland) radars overlap significantly, and both radars received excellent echoes for the corresponding intervals. Thus the shown velocity vectors there are most reliable. [22] There are three points to be noted. First, as shown in Figures 4a and 4b, the global convection is consistent with the two-cell pattern. Second, the convection flow is directed mostly eastward on the poleward side of the poleward branch and westward on the equatorward side (Figures 4c and 4d). That is, the poleward auroral branch is located at the convection flow shear, more precisely, near the poleward edge of the westward (return) flow. The corresponding electric fields have a finite divergence, which should be collocated with an upward field-aligned current as we found in Figure 3c. [23] Finally, corresponding to the auroral intensification, the westward return flow on the equatorward side of the poleward auroral branch enhanced noticeably, whereas the eastward flow on the poleward side changed very little. That is, the convection shear strengthened in association with the auroral intensification mostly because of the enhancement of the westward return flow. It is widely recognized that auroral acceleration is closely related to upward FACs, but not to downward FACs and therefore, in general, the auroral distribution does not match the ionospheric projection of the responsible process in the magnetotail. This event provides a good example for that as the enhanced return flow was clearly skewed equatorward of the intensified poleward auroral branch. [24] This enhanced return flow is presumably the ionospheric projection of an earthward convection flow in the near-Earth tail, which will be supported by the Geotail observation in section 3.1.4. The enhanced return flow may be also related to the substorm onset at 1821 UT (as identified by the Pi2 and positive onsets at Kakioka (not shown)), which was immediately followed by the formation of the auroral bulge in the midnight sector (Figures 2m and 2n). 3.1.4. Tail Dynamics [25] In this event the Geotail satellite was in the magnetotail around its apogee distance, 31 RE, slowly moving duskward crossing the midnight meridian at 1700 UT. Figure 5 shows, from the top, three magnetic components in GSM, the X component of plasma flow velocity, the Y-component ⇀ ⇀ electric field and ðV B ÞY , and ion beta (bi) measured by

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Figure 4. Convection patterns in the entire northern polar region measured by SuperDARN during the 2-min intervals of (a) 1820–1822 and (b) 1822–1824 UT on 2 March 2001. The close-ups of the duskto-midnight sector on the top of IMAGE/WIC auroral images at (c) 1820:12 and (d) 1822:15 UT. Geotail for 1700–2000 UT. Both magnetic field and flow velocity changed wildly during the interval reflecting high geomagnetic activity (Figure 1). [26] The flow was initially directed earthward, and its velocity, even the perpendicular component (V?,X, hatched area in Figure 5b), occasionally exceeded 1000 km/s. After subsiding around 1735 UT, a tailward flow started at 1749 UT (the 1st vertical dotted line). The peak tailward velocity, which occurred at 1751 UT, reached 570 km/s. BZ was initially positive and then it became negative transiently. It should be reasonable to conclude that new activity started before

1749 UT on the earthward side of Geotail, which was presumably related to the substorm initiation at 1748 UT (section 3.1.1). [27] After an interval of relatively weak earthward flow, the next fast tailward flow arrived at Geotail at 1815 UT (the 2nd vertical dotted line), which suggests the formation of a new active region on the earthward side of Geotail before 1815 UT. This tailward flow was observed when Geotail was exiting from the plasma sheet (as indicated by the sharp decrease in bi; see Figure 5d). Geotail observed a tailward flow again when it transiently approached the plasma

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Figure 5. (a) The BX (solid), BY (dotted) and BZ (shaded) magnetic components in GSM, (b)⇀the X (V?,X; ⇀ shaded area; VX: dotted line) component of plasma flow velocity, (c) EY (solid) and ðV B ÞY (dotted), and (d) ion beta (bi) observed by Geotail on 2 March 2001. Geotail was around its apogee distance near midnight moving from (30.3, 0.0, 1.9) to (30.6, 1.4, 1.5) RE in GSM from 1700 to 1930 UT. Three vertical dotted lines mark 1749, 1815, and 1825 UT. sheet (or plasma sheet boundary layer) around 1825 UT (the 3rd vertical dotted line). EY was at elevated levels until 1830 UT even when Geotail was off the equator, indicating that this tailward flow lasted for about 15 min. [28] In contrast to the start of the fast tailward flow at 1749 UT, the auroral activity associated with this new tailward flow is difficult to identify (section 3.1.1). One possibility is the intensification of the poleward branch of the double oval structure in the premidnight sector, which started after 1816 UT (Figure 3). Another possibility is the substorm onset at 1821 UT, about 6 min later than the start of the tailward flow, which was followed by the auroral bulge formation in the midnight sector. [29] At Geotail the fast tailward flow, or the enhanced EY, continued for about 15 min until 1830 UT, during the same interval of the poleward auroral branch intensification. The enhancement of the ionospheric return flow in the late evening sector, which we examined in section 3.1.3, occurred at 1820–1824 UT in the middle of this interval. It is therefore suggested that the poleward auroral branch is closely associated with the convection in the magnetotail and for this interval, the enhanced convection was driven on the earthward side of the Geotail position, that is, within 30 RE from Earth. [30] After spending some short intervals in the tail lobe or boundary layer, Geotail returned to the plasma sheet around

1900 UT (see the plot of bi in Figure 5d) and observed several bursts of earthward flows. Obviously the convection driver, presumably the reconnection site, moved tailward of Geotail by then. This is around the time when DMSP-F15 and -14 crossed the double oval structure. Interestingly, despite the fact that the plasma flow was very active at Geotail, the nightside aurora, especially the poleward branch of the double oval was faint, and both FACs and electron precipitation were significantly weaker than those observed at the preceding auroral oval crossings (section 3.1.2). It is possible that the poleward branch of the double oval intensifies only if the convection enhances in the near-Earth tail (as confirmed for the intensification after 1815 UT), and the fast earthward flows observed by Geotail might have very little, if at all, impact on the near-Earth tail [Ohtani et al., 2006]. 3.2. The 15 March 2004 Event 3.2.1. Overall Geomagnetic Activity [31] In this section we examine an event that occurred on 15 March 2004 in the middle of the recovery phase of a magnetospheric storm. The Sym-H index reached a minimum of 91 nT at 2250 UT on 9 March, and then it recovered gradually over almost 10 days (not shown). The AL index shows the frequent occurrence of substorms throughout this prolonged storm interval (not shown), and therefore

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Figure 6. (a) Solar wind dynamic pressure, Pdyn, (b) IMF BZ, (c) AU (blue) and AL (red) indices, and (d) Asy-H (blue) and Sym-H (red) indices for the 6-h interval of 10:00– 16:00 UT on 15 March 2004. The IMF and solar wind data were acquired by the ACE satellite around the L1 point and are time-shifted to 1 AU. this storm is regarded as a High-Intensity, Long-Duration, Continuous Aurora Event, Activity (HILDCAA) event [e.g., Tsurutani and Gonzalez, 1987]. Figure 6 shows, in the same format as Figure 1, the external and internal conditions of the 6-h interval of 1000–1600 UT on 15 March 2004. Sym-H was around 30 nT throughout the interval (Figure 6d). In the following we focus on the interval of 1200–1300 UT. Following the recovery of the preceding activity the AL index started to decrease around 1130 UT (Figure 6c). IMF BZ turned southward at 1118 UT and stayed negative for about an hour (Figure 6b). The dynamic pressure did not change noticeably during the interval (Figure 6a). 3.2.2. Particle Precipitation and Field-Aligned Currents [32] In this subsection we examine FACs and particle precipitation observed by DMSP-F15 and -F16 when they crossed the auroral oval at different MLTs but almost simultaneously on 15 March 2004. The first crossings we examine occurred in the Southern Hemisphere, in the same hemisphere as the IMAGE/WIC images, at 1223 UT, when AL started to recover temporarily (Figure 6c). DMSP-F15 (MLT 22.7) was about two hours later in MLT than -F16 (MLT 20.8). The auroral image at 12:23:13 UT shows that both satellites crossed the double oval structure. Figures 7a and 7b show IMAGE/WIC auroral images, magnetic field and particle precipitation data observed by DMSP-F15 and

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-F16, respectively, in the same way as Figure 3. Note that the color scale for the auroral emission adopted for this event is different from that for the 2 March 2001 event. [33] First, we examine the DMSP-F16 signatures (Figure 7b). This satellite observed features very similar to what we saw for the 2 March 2001 event, that is, intense precipitation of accelerated electrons in the upward R1 current and diffuse high-energy electron precipitation along with energetic ion precipitation farther equatorward. The overall polarity of FAC in this equatorward part is downward as expected for the R2 current in this MLT sector, but many small FACs are embedded as indicated by BZ fluctuations, which appear to be related to structured precipitation of lowenergy (