JournalofGeophysicalResearch: SpacePhysics

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1 JournalofGeophysicalResearch: SpacePhysics RESEARCH ARTICLE Key Points: We present two examples of auroral fragmentation in all-sky auroral images The auroral fragmentation is probably caused by pressure-driven instability The auroral fragmentation is a process that creates auroral patches Auroral fragmentation into patches Kazuo Shiokawa 1, Ayumi Hashimoto 1, Tomoaki Hori 1, Kaori Sakaguchi 2, Yasunobu Ogawa 3, Eric Donovan 4, Emma Spanswick 4, Martin Connors 5, Yuichi Otsuka 1, Shin-Ichiro Oyama 1, Satonori Nozawa 1, and Kathryn McWilliams 6 1 Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan, 2 National Institute of Information and Communications Technology, Koganei, Japan, 3 National Institute of Polar Research, Tachikawa, Japan, 4 Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada, 5 Center for Science, Athabasca University, Athabasca, Alberta, Canada, 6 Institute of Space and Atmospheric Studies, University of Saskatchewan, Saskatoon, Saskatchewan, Canada Supporting Information: Readme Animation S1 Animation S2 Animation S3 Animation S4 Animation S5 Correspondence to: K. Shiokawa, shiokawa@stelab.nagoyau.ac.jp Citation: Shiokawa, K., et al. (2014), Auroral fragmentation into patches, J. Geophys. Res. Space Physics, 119, , doi:. Abstract Auroral patches in diffuse auroras are very common features in the postmidnight local time. However, the processes that produce auroral patches are not yet well understood. In this paper we present two examples of auroral fragmentation which is the process by which uniform aurora is broken into several fragments to form auroral patches. These examples were observed at Athabasca, Canada (geomagnetic latitude: 61.7 N), and Tromsø, Norway (67.1 N). Captured in sequences of images, the auroral fragmentation occurs as finger-like structures developing latitudinally with horizontal-scale sizes of km at ionospheric altitudes. The structures tend to develop in a north-south direction with speeds of m/s without any shearing motion, suggesting that pressure-driven instability in the balance between the earthward magnetic-tension force and the tailward pressure gradient force in the magnetosphere is the main driving force of the auroral fragmentation. Therefore, these observations indicate that auroral fragmentation associated with pressure-driven instability is a process that creates auroral patches. The observed slow eastward drift of aurora during the auroral fragmentation suggests that fragmentation occurs in low-energy ambient plasma. Received 11 APR 2014 Accepted 9 SEP 2014 Accepted article online 12 SEP 2014 Published online 21 OCT Introduction Aurora is a manifestation of plasma dynamics in the Earth s magnetosphere, in which auroral emissions are caused by electrons precipitating from the magnetosphere. Study of the formation of auroral structures helps to deepen our understanding of magnetospheric plasma processes and their connection to the Earth s atmosphere. The two major auroral structure types are represented by the discrete and diffuse auroras [e.g., Davis, 1978]. Discrete auroras (auroral arcs) show longitudinally extended curtain-like features; these features often appear as folds or rays and are caused mainly by field-aligned potential differences [e.g., Frank and Ackerson, 1971; Reiff et al., 1988]. Such discrete auroras are likely the ionospheric footprint of plasma sheet structures that are radially localized and azimuthally extended, with one candidate being flow shear [e.g., Lyons, 1980; Haerendel, 2007]. Flow shear in the plasma sheet induces the curtain-like extended features of auroral arcs. In contrast, diffuse aurora consists primarily of fragments of aurora, that is, small-scale auroral patches, often exhibits a pulsating intensity. The diffuse aurora also can assume various shapes with persistent geometry [e.g., Stenbaek-Nielsen, 1980]. Field-aligned current is intensified at the edge of auroral patches [Oguti and Hayashi, 1984]. Sergienko et al. [2008] reported a detailed study of fine structures of diffuse auroras, using high-resolution ground cameras and auroral particle measurements taken by the FAST satellite. However, the processes that form auroral patches are not yet well understood. Pulsating auroral patches are probably caused by pitch angle scattering of 10 kev electrons by whistler mode waves [e.g., Nishimura et al., 2010; Lessard, 2012]. Thus, the patch structure may be a manifestation of the spatial distribution of whistler mode waves controlled by background low-energy plasma density [e.g., Kennel and Petschek, 1966]. Recently Shiokawa et al. [2010] reported small-scale (5 25 km) finger-like structures at the western boundary of auroral patches, suggesting macroscopic Rayleigh-Taylor-type plasma instability arising in the magnetospheric equatorial plane from force balancing of the (eastward) magnetic-tension force and the (westward) pressure gradient force. They suggested that this instability may be one factor in producing auroral-patch SHIOKAWA ET AL American Geophysical Union. All Rights Reserved. 8249

2 (a) (b) (c) (d) (e) (f) Figure 1. (a f) IMF Bz, solar wind speed, solar wind density, AL and AU indices, and SYM-H index at UT on 26 January The interval of auroral fragmentation is shown by the horizontal arrow in Figure 1d. structures in diffuse aurora, because it divides larger-scale auroral patches into smaller-scale structures. However, they also reported the development of small-scale finger-like structures at the boundary of already existing auroral patches. Sakaguchi et al. [2011] reported periodic black patches of auroras with a scale size of about 50 km and suggested that the pressure-driven interchange instability induced the beaded shape of black patches through the action of field-aligned currents. In this paper, we present two examples of large-scale auroral fragmentation, during which highly uniform auroras are fragmented into auroral patches by development of larger-scale finger-like structures. This process can be a major cause of auroral-patch formation during substorms. From the observed slow eastward drift velocity of these structures, we suggest that they develop as structures of low-energy ambient plasma, rather than as structures of the 10 kev order high-energy plasma in the magnetosphere. 2. Observation 2.1. Event 1 Observed at Athabasca The first event reported in this paper (event 1) was observed at Athabasca, Canada (54.7 N, E, geomagnetic latitude (MLAT) 61.7 N) at subauroral latitudes on 26 January 2009 during a magnetically active period. The magnetic local time at Athabasca is 8 h behind UT. Figure 1 shows solar wind parameters, AU and AL auroral electrojet indices, and the SYM-H index during the event. The interval of the event is shown by the horizontal arrow in Figure 1d. A major substorm was observed at around Universal Time (UT) after the southward turning of interplanetary magnetic field (IMF) Bz. The minimum AL index was lower than 600 nt at 1240 UT. The fragmentation was observed at UT and associated with a small negative excursion of the AL index. The SYM-H index was 20 to 30 nt, indicating that this event was associated with a weak storm-like interval that started at 0720 UT, which time is associated with sudden density enhancement in the solar wind. SHIOKAWA ET AL American Geophysical Union. All Rights Reserved. 8250

3 Journal of Geophysical Research: Space Physics Figure 2. Auroral fragmentation in sequential all-sky auroral images at (left) nm and (right) nm, observed at Athabasca, Canada, on 26 January Figure 3. Panchromatic auroral images with finger-like structures in the geomagnetic latitude-longitude coordinates obtained at 1340:00 UT on 26 January 2013 at Athabasca, Canada. SHIOKAWA ET AL American Geophysical Union. All Rights Reserved. Figure 2 and Animation S1 in the supporting information show the auroral fragmentation observed at Athabasca in auroral images at nm (left) and nm (right). These images were obtained by a Narrow-band All-Sky Camera for Auroral Monitoring operated by the NORSTAR project with a time resolution of 12 s. The auroral structures are more distinct in the nm images. At the beginning of the imaged event shown in Animation S1, before 1250 UT (0450 magnetic local time (MLT)), a diffuse-pulsating aurora appeared at the northwestern edge of the images, and then gradually expanded southeastward. At UT ( MLT), the diffuse aurora became brighter in the region further southeastward, beyond the zenith of Athabasca; simultaneously, finger-like dark structures invaded southeastward into the brightened diffuse aurora. The development of these finger-like structures appears to be 8251

4 Figure 4. (a h) Cross sections (keograms) of the panchromatic auroral images in geomagnetic longitudes at eight different geomagnetic latitudes from N to N to show latitudinal dependence of eastward drift speed of aurora. The auroral images were obtained at Fort Smith and Athabasca, Canada, on 26 January a fluid-like motion, suggesting some instability on the magnetohydrodynamics (MHD) scales. Then, the structures drifted eastward. More smaller-scale finger-like structures developed at the western boundary of the bright regions at around 1345 UT, suggesting that a secondary instability was being generated. Figure 3 shows panchromatic auroral images obtained at UT at Athabasca by a Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky imager (ASI) [Donovan et al., 2006b; Mende et al., 2008]. This image has been converted into geomagnetic latitude-longitude coordinates. From this image, we estimate the longitudinal separation of the dark bands to be 1 2 in longitude ( km), giving longitudinal wavelength of km. The scale size of the secondary instability seen at (62 63 N, E) is about 10 km, which is comparable to the scale sizes reported by Shiokawa et al. [2010]. Using similar images in geomagnetic coordinates, we estimated that the location of the equatorward boundary of the finger-like structures at 1310 UT and 1335 UT were at 63 N and 61 N, respectively, giving an average equatorward velocity of development of about 150 m/s (= 220 km/25 min). Animation S2 shows a broader view of auroral dynamics over North America during this event; the images used in this animation were obtained by multipoint panchromatic THEMIS ASIs. The auroral fragmentation at Athabasca (the southernmost station in central Canada) took place at the equatorward boundary of the auroral oval at subauroral latitudes. At higher latitudes, the aurora continuously drifted eastward. However, at Athabasca, this eastward drift velocity clearly slowed down and almost stopped; this occurred just before the development of the finger-like structures. Unfortunately, due to sunrise, the two stations east of Athabasca stopped the observations before the fragmentation started. However, the eastward auroral drift also slowed down at these stations, just before the development of finger-like structures at Athabasca. In contrast, the higher-latitude auroras kept drifting eastward. To measure this eastward drift velocity quantitatively, we made longitudinal (east-west) keograms for fixed geomagnetic latitudes from N to N. These keograms (Figure 4) were made from panchromatic all-sky images of THEMIS ASIs at Fort Smith (60.0 N, E) and at Athabasca with the same longitude. In the keograms, the eastward drift motions of auroral structures are clearly seen as traces of increasing longitude and the drift speed is clearly faster above 65 N MLAT. By linear fitting on these traces, we obtain the eastward drift speed of each auroral structure, as shown in Figure 5 (red lines). Red and blue dots indicate results obtained from keograms at Fort Smith and Athabasca, respectively. The eastward drift speeds of auroral structures are more than 1000 m/s above 65 N MLAT; they are mostly less than 500 m/s below 64 N MLAT. It is noteworthy that the auroral structures at the beginning SHIOKAWA ET AL American Geophysical Union. All Rights Reserved. 8252

5 Figure 5. Latitudinal variation of eastward drift speed of aurora at UT on 26 January The auroral fragmentation was observed at latitudes below 65 N where the drift speed is slower than at higher latitudes. of the finger-like structure development were almost stationary, as shown in Figure 4f at UT and in Animations S1 and S2. Animation S3 indicates the plasma drift velocities obtained by a Super Dual Auroral Radar Network (Super- DARN) high-frequency (HF) radar at Prince George, which is located west of Athabasca. Figure 6 shows four snapshots of Animation S3. The radar drift velocities are plotted on the THEMIS ASI images. The Doppler velocities measured by the radar are clearly positive (blue indicates toward the radar) in the western half and negative (orange indicates away from the radar) in the eastern half of the field of view, respectively. The white bars at the center of the radar echoes indicate the plasma velocities as calculated by fitting a uniform velocity vector to radar echoes at N MLAT. The values of these velocities and their angles from the east are indicated at the bottom left of each panel. These velocities are obtained by applying beam-swinging technique for each two-dimensional scan by assuming a uniform velocity vector without assuming any flow direction (equivalent to the method by Makarevich and Dyson [2007]). These values are about m/s eastward, consistent with the lowest values obtained by the auroral motion at latitudes below 64 N MLAT in Figure 5. It should be cautioned that because of the short-range distances, these velocities are measured from the E region echoes of the radar, for which the Doppler velocity tends to become smaller than the actual E B drift velocity [Haldoupis, 1989;Koustov et al., 2005]. Anyhow, the estimated velocity ( m/s) is far smaller than the eastward auroral velocities at higher latitudes (more than 1000 m/s) and closer to the eastward auroral velocity at this latitude (less than 500 m/s), as shown in Figure 5. Figure 7 shows the location of the finger-like structures relative to the equatorward boundary of proton aurora. Proton aurora was observed by a meridian scanning photometer (MSP) at Fort Smith, as shown in the keogram of Figure 7a. The panchromatic keograms obtained by THEMIS ASIs at Fort Smith and Athabasca are shown in Figure 7b. The red curves in these two keograms indicate the equatorward boundary of proton aurora, taken from Figure 7a. The development of finger-like structures at Athabasca took place at UT at the equatorward boundary of aurora, as shown by the red arrow. These images clearly show that this finger-like structure developed at latitudes well south of the equatorward boundary of proton aurora. The proton auroral boundary gradually moved equatorward at UT before heading back poleward after 1250 UT. The development of finger-like structures occurs in conjunction with this poleward motion of the proton aurora. Figure 8 shows the magnetic field variations obtained by various observations: (a c) taken by the GOES 11 satellite at a geographical longitude of E( 20 west of Athabasca); (d g) were obtained by auroral zone ground stations from west to east; and (h i) were obtained from subauroral and middle-latitude ground stations at the longitude of Athabasca. The He, Hn, and Hp in GOES 11 magnetic data indicate SHIOKAWA ET AL American Geophysical Union. All Rights Reserved. 8253

6 Figure 6. Plasma drift velocities obtained by a SuperDARN HF radar at Prince George at (a) 1310 UT, (b) 1320 UT, (c) 1330 UT, and (d) 1340 UT on 26 January The Doppler velocities obtained by the radar are plotted in the fan-shaped radar field of view (FOV) on the THEMIS ASI all-sky images. The white bars at the center of the radar FOV indicate the plasma velocities calculated by fitting a uniform velocity vector for radar echoes at N MLAT. The values of these velocities and their angle from the east (90 = northward) are indicated at the bottom left of each panel. (a) (b) Figure 7. North-south cross sections (keograms) of (a) proton aurora at a wavelength of nm observed at Fort Smith and (b) panchromatic aurora in visible wavelengths observed at Fort Smith and Athabasca. The red curves indicate the low-latitude boundary of proton aurora in both Figures 7a and 7b. The horizontal red arrow indicates the interval when the auroral fragmentation was observed at Athabasca at latitudes below 65 N. SHIOKAWA ET AL American Geophysical Union. All Rights Reserved. 8254

7 (a) (b) approximately earthward, eastward, and northward, respectively. The ground-based magnetic field data were obtained by the THEMIS Ground Based Observatory (GBO) array [Russell et al., 2008]. (c) (d) (e) (f) (g) (h) (i) Figure 8. (a c) Magnetic field variations observed by the GOES 11 satellite at geosynchronous orbit; (d i) H component geomagnetic field variations observed at six ground-based stations. The He, Hn, and Hp directions in GOES 11 magnetic data are approximately earthward, eastward, and northward, respectively. KIAN, INUV, YKC, and RANK indicate the auroral zone stations, from east (Alaska) to west (eastern Canada). The YKC, ATH, and HOTS stations are along the meridian of Athabasca from the auroral zone to subauroral latitudes. The longitude of GOES 11 is near that of RANK. Auroral fragmentation was observed at UT at Athabasca as finger-like structures developing equatorward. The H component magnetic field at Athabasca showed a temporary negative excursion at that time, indicating an enhancement and a decay of westward electrojet current. The decrease in He with the increase in Hp at UT indicates that the magnetic field dipolarized in this interval. The auroral zone magnetic field at Yellowknife (station ID: YKC) at the longitude of Athabasca showed a decrease at UT, while the field showed a continuous increase from 1240 UT at Kiana, Alaska (KIAN), and a sudden decrease at 1320 UT at Inuvik (INUV), both at longitudes west of Athabasca. The middle latitude magnetic field at Hot Springs, Montana (HOTS), increased from 1245 UT. These overall magnetic signatures indicate a localized dipolarization and enhancement of westward current at longitudes near Athabasca, suggesting a localized substorm expansion. Also of note is that the auroral keogram at Fort Smith in Figure 7 indicates that an auroral poleward expansion occurred at UT in the northern sky of Fort Smith. Auroral fragmentation at the equatorward edge of the auroral oval occurred during this localized auroral expansion Event 2 at Tromsø The second event reported in this paper (event 2) was observed at Tromsø, Norway (69.6 N, 19.2 E, 67.1 MLAT), at auroral latitudes on 26 January 2012 during a magnetically active time. The magnetic local time at Tromsø is 2.5 h ahead of UT. Figure 9 shows solar wind parameters and the AU, AL, and SYM-H indices during the event. The interval of the event is shown by the horizontal arrow in Figure 9d. A small substorm took place at approximately 0100 UT. The minimum AL index was about 150 nt at 0105 UT. The event was observed at UT, when this substorm turned to the recovery phase. The SYM-H index was 40 to 30 nt, indicating that this event was also associated with a weak storm-like interval. Figure 10 and Animation S4 show the auroral fragmentation observed at Tromsø in color auroral images. These images were obtained by a commercial digital color camera with a fisheye lens with an exposure SHIOKAWA ET AL American Geophysical Union. All Rights Reserved. 8255

8 Journal of Geophysical Research: Space Physics (a) (b) (c) (d) (e) (f) Figure 9. (a f ): IMF Bz, solar wind speed, solar wind density, AL and AU indices, and SYM-H index from 18 UT on 25 January 2012 to 07 UT on 26 January The interval of auroral fragmentation is shown by the horizontal arrow in Figure 9d. time of 1 min. The orange line in the west of the images is the beam from a sodium lidar. At the beginning, a diffuse aurora appeared over the whole sky and drifted continuously eastward. Then, small dips of brighter aurora appeared at the western part of the images at 0112 UT (0312 MLT). These brightening auroras developed poleward and became large finger-like structures of bright aurora with some pulsations in them at around 0130 UT. A dark band of finger-like structures can also be seen to have simultaneously developed equatorward. After the development of these finger-like structures, the auroral patches covered the whole sky, indicating that this finger-like structuring caused auroral fragmentation, creating auroral patches from uniform diffuse aurora. (a) (d) (g) (b) (e) (h) (c) (f) (i) Figure 10. All-sky auroral images of auroral fragmentation obtained by a digital camera at Tromsø on 26 January The orange line in the images is light from a colocated sodium lidar. SHIOKAWA ET AL American Geophysical Union. All Rights Reserved. Animation S5 shows a color auroral movie obtained during this event with a time resolution of 1 s. This movie was obtained by an all-sky-parallel imager system at the European Incoherent Scatter Tromsø site. The system consists of three TV imagers (WAT-120N+), three fisheye lenses (f = 1.4 mm, F1.4), three-color filters with transmission wavelengths of nm, nm, and nm, and a gigabit network video encoder (Axis Q7404). Three-color images simultaneously taken by the three TV imagers are composed and stored to make each frame of the color auroral movies [Ogawa et al., 2013]. 8256

9 Journal of Geophysical Research: Space Physics (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l) Figure 11. Sequential auroral images of auroral fragmentation (in geographical coordinates) obtained by an all-sky airglow imager at a wavelength of nm at Tromsø on 26 January The dotted lines labeled (1), (2), and (3) indicate the development of high- and low-latitude boundaries of finger-like structures to show their development speeds. The difference of the lidar beam direction between Animations S4 and S5 is due to the difference of relative location of the buildings of two cameras to the lidar building. Looking at these time-resolved images, it can be recognized that the equatorward half of the diffuse aurora was pulsating significantly before the beginning of the auroral fragmentation. Small dips, seeds of the fragmentation, developed in this pulsating auroral region at UT. As the fragmentation expanded poleward, the pulsating auroral region also expanded poleward; finally, the pulsating patches covered the whole sky. Figure 11 shows nm auroral images obtained by an all-sky-cooled charge-coupled device airglow imager with an exposure time of 5 s at Tromsø on the same night. Details of this airglow imager, which is a part of the Optical Mesosphere Thermosphere Imagers (OMTIs), are given by Shiokawa et al. [1999, 2009]. The images are converted to geographical coordinates with an assumed emission altitude of 110 km; this is done to measure the horizontal-scale sizes and the speeds of poleward and equatorward development of the finger-like structures. The horizontal-scale size of a finger-like bright structure was estimated to be km in longitude. The speeds were estimated to be 420 m/s (poleward), 280 m/s (equatorward), and 220 m/s (poleward) in the figure; these are labeled (1), (2), and (3), respectively. Figure 12. East-west cross sections (keograms) of the nm auroral images in geographic longitudes at a latitude of Tromsø at 69.6 N to show the eastward drift speed of aurora. The auroral images were obtained at Tromsø on 26 January SHIOKAWA ET AL American Geophysical Union. All Rights Reserved. Figure 12 shows a longitudinal (east-west) keogram for the nm aurora, obtained by the OMTIs airglow imager. The aurora tended to move eastward during the plotted interval. 8257

10 The eastward drift speed slowed down as time passed after 0130 UT. The drift speed was estimated to be 260 m/s (10 longitude per 25 min) for UT, which was during the auroral fragmentation. Figure 13 shows the X component magnetic field variations obtained by the IMAGE magnetometer array in the magnetic meridian of Tromsø, arranged from high to low latitudes. The time interval of auroral fragmentation is indicated by the two vertical lines and a horizontal arrow. This interval clearly corresponds to the recovery phase of a small substorm. Figure 13. X component geomagnetic field variations obtained by the IMAGE magnetometer chain in the longitudes near Tromsø (TRO). The time interval of auroral fragmentation is indicated by two vertical lines and a horizontal arrow. 3. Discussion We reported on two events of auroral fragmentation, which is characterized by development of finger-like auroral structures with longitudinal-scale sizes of km developing poleward or equatorward with speeds of m/s. The auroral fragmentation is a cause to create auroral patch structures. The auroral fragmentation occurs at both auroral latitudes (Tromsø, 67.1 N MLAT) and subauroral latitudes (Athabasca, 61.7 N MLAT). The observed finger-like structures without shear motion indicate that this auroral fragmentation is caused by pressure-driven (Rayleigh-Taylor-type) instability, rather than by shear (Kelvin-Helmholtz) instability in the magnetosphere. Pressure-driven instability can be ballooning instability or interchange instability. Interchange instability has zero wave number along the magnetic field line, and the whole magnetic flux tube moves together. Ballooning instability has a finite wave number along the magnetic field line. Both instability types develop in the region where force is balanced between earthward magnetic-tension force and tailward pressure gradient force in the MHD fluid approximation [e.g., Gold, 1959; Voronkov et al., 1997; Cheng, 2004; Xing and Wolf, 2007; Kozlovsky et al., 2007; Miura, 2009]. The fluid-like behavior of aurora observed during the reported events also suggests the development of these MHD instabilities. Pressure-driven instability, where the pressure gradient force P = P z works analogously to a gravitational force in Rayleigh-Taylor instability, has a growth rate which may be expressed as (P n n 2 m) 1 2, where n = n z is the spatial gradient of the density n and m is the ion mass [e.g., Chen, 1974; Treumann and Baumjohann, 1997]. Thus, this instability grows as the radial gradients of plasma pressure or plasma density become larger. Earthward magnetic-tension force generally balances with tailward pressure gradient force, so this instability will also develop under high magnetic curvature. The dipolarization of the magnetic field observed by GOES 11 during event 1 may indicate the opposite, since the magnetic field curvature decreases during dipolarization. The poleward motion of proton aurora during event 1 also suggests the dipolarization. However, the particle injection associated with the dipolarization and the local expansion onset during event 1 may have enhanced the plasma pressure gradient. Event 2 was observed when a substorm entered the recovery phase. Changes in magnetic field configuration when a substorm shifts phase from expansion to recovery is likely to set up favorable conditions for instability growth. SHIOKAWA ET AL American Geophysical Union. All Rights Reserved. 8258

11 It has also been suggested that ballooning and interchange instabilities occur during substorm expansion onset, visualized by the beaded structures of brightening aurora [e.g., Donovanetal., 2006a; Liang et al., 2008; Motoba et al., 2012]. The present events show that the growth of pressure-driven instability appears over a much longer time scale (10 20 min) with larger latitudinal extent (more than 1 latitude) than the beaded structures along the brightening arc during substorm onset. This may be because the radial pressure profile that evolves during the substorm growth phase is dramatically different under different geomagnetic conditions. It is interesting to note that the eastward drift speed during auroral fragmentation is rather slow (event 1: m/s; event 2: 260 m/s) compared with the typical eastward drift rate of auroras in the postmidnight sector. The SuperDARN HF radar data in Figure 6 also indicates slow drift speeds of about m/s though that is inferred from the E region echoes. If we assume that Earth has a dipole field at 6.6 R E, then the gradient and curvature drift speed of electrons with energies of 2 kev and 10 kev will be about 200 m/s and 900 m/s eastward at ionospheric altitudes. Thus, the observed slow drift speeds of auroras during the auroral fragmentation suggest that the fragmentation is occurring in the ambient plasma in the magnetosphere rather than in the hot ( 10 kev) electrons which usually cause the pulsating aurora. The slow plasma-drift velocity also suggests that high drift speeds may suppress pressure-driven instability. Nakamura and Oguti [1987] compared the drift velocities of auroral structures to ambient plasma drift velocities measured by radar and concluded that the drift of auroral structures, including patches, is due to the convection electric field. Their conclusion supports the idea that the pulsating auroral patches are also the ionospheric projections of cold-plasma irregularities in the magnetosphere because hot high-energy plasma drift is caused by both the electric field and the gradient and curvature of the magnetic field. The cold-plasma density controls generation of plasma waves [e.g., Li et al., 2011], which scatter high-energy plasma sheet electrons to smaller pitch angles and cause the observed aurora as precipitation of the electrons in the ionosphere. The pressure gradient force that contributes to the pressure-driven instability would be supported by the main component of the plasma sheet plasma rather than the ambient cold plasma. The contribution from electron pressure is usually much smaller than that from the ion pressure in the plasma sheet. Thus, the pressure-driven instability probably occurs in the region of balance between the earthward magnetic-tension force and the tailward force of ion pressure gradients. Instability then creates finger-like structures at the boundary of two plasma regions. The ambient plasma characteristics may also differ between these two regions; it is likely that the plasma density decreases outside of the high-tension region. 4. Summary and Concluding Remarks We have reported two events of auroral fragmentation on the basis of ground auroral observations. The observed features are summarized as follows. 1. Event 1 was observed at Athabasca at subauroral latitudes (61.7 N MLAT) at UT ( MLT). Event 2 was observed at Tromsø at auroral latitudes (67.1 N MLAT) at UT ( MLT). 2. Both events were characterized by latitudinal development of finger-like structures invading into a highly uniform diffuse auroral region and causing auroral fragmentation into patches. Secondary fragmentation also occurred in the east-west direction at the boundary of the north-south finger-like structures during event The finger-like structures can occur as either dark or bright regions of the auroras. These structures showed fluid-like behavior during development, suggesting that MHD instability is the cause of fragmentation. 4. The longitudinal-scale sizes of finger-like structures were km (wavelength: km), giving an azimuthal wave number of for the instability. The poleward/equatorward development speeds were m/s. 5. The eastward drift speeds of the finger-like structures were notably slow (event 1: m/s; event 2: 260 m/s). For event 1, colocated SuperDARN HF radar also observed slow eastward plasma drift from the E region echoes, with a speed of about m/s, near Athabasca. SHIOKAWA ET AL American Geophysical Union. All Rights Reserved. 8259

12 6. Both events were observed during the recovery phase of substorms, as characterized by an increase in the AL index. However, event 1 seems to have been associated with a localized substorm expansion characterized by field dipolarization at geosynchronous orbit, auroral brightening and poleward expansion at higher latitudes, and positive and negative H bays in the high- and middle-latitude magnetograms. Event 1 was observed well south of the equatorward boundary of proton aurora. From these observations, we suggest that the pressure-driven instabilities, both ballooning and interchange types, are a likely cause of the observed auroral fragmentation. The observed slow eastward drift velocity suggests that the instability develops as structures of low-energy ambient plasma, rather than in the 10 kev high-energy plasma in the magnetosphere. Auroral fragmentation is likely a cause of auroral patches during the recovery phase of substorms. However, we note that this sort of auroral fragmentation is not always observed during substorms. A statistical study of auroral fragmentation will be conducted in the near future. Acknowledgments WethankY.Katoh,H.Hamaguchi,Y. Yamamoto, and T. Kawabata of the Solar-Terrestrial Environment Laboratory (STEL) at Nagoya University for their skillful support of the observations at Athabasca and Tromsø. The OMNI data were obtained from the GSFC/SPDF OMNIWeb interface at The solar wind and IMF data were provided by the ACE satellite. The SYM-H, AU, andal indices were provided by WDC-C2 for Geomagnetism at Kyoto University. The THEMIS ASI and GMAG data were provided courtesy of H. Frey, S. Mende, C. T. Russell, I. R. Mann, D. K. Milling, and with financial support of the Canadian Space Agency and National Science Foundation through grants AGS and AGS Operation of imagers at the Athabasca University Geophysical Observatory was supported by the Canada Foundation for Innovation. 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