Ann. Geophys., 27, 1743–1754, 2009 www.ann-geophys.net/27/1743/2009/ Annales © Author(s) 2009. This work is distributed under Geophysicae the Creative Commons Attribution 3.0 License. Evolution of dipolarization in the near-Earth current sheet induced by Earthward rapid flux transport R. Nakamura1, A. Retino`1, W. Baumjohann1, M. Volwerk1, N. Erkaev2, B. Klecker3, E. A. Lucek4, I. Dandouras5, M. Andre´6, and Y. Khotyaintsev6 1Space Research Institute, Austrian Academy of Sciences, 8042 Graz, Austria 2Institute of Computation Modelling, Russian Academy of Sciences, Siberian Branch, Krasnoyarsk, Russia 3Max-Planck-Institut fur¨ extraterrestrische Physik, P.O. Box 1312, Garching, 85741, Germany 4Imperial College, London, SW7 2BZ, UK 5CESR/CNRS, 9 Ave. du Colonel Roche, B.P. 4346, 31028 Toulouse Cedex 4, France 6Swedish Institute of Space Physics, P.O. Box 537, 75121 Uppsala, Sweden Received: 6 October 2008 – Revised: 5 February 2009 – Accepted: 18 February 2009 – Published: 9 April 2009 Abstract. We report on the evolution of dipolarization dipolarization pulses could differ, depending on the configu- and associated disturbances of the near-Earth current sheet ration of the current sheet. during a substorm on 27 October 2007, based upon Clus- Keywords. Magnetospheric physics (Plasma convection; ter multi-point, multi-scale observations of the night-side Plasma sheet; Storms and substorms) plasma sheet at X∼−10 RE. Three dipolarization events were observed accompanied by activations on ground mag- netograms at 09:07, 09:14, and 09:22 UT. We found that all these events consist of two types of dipolarization signatures: 1 Introduction (1) Earthward moving dipolarization pulse, which is accom- panied by enhanced rapid Earthward flux transport and is One of the essential signatures of substorm onset observed in followed by current sheet disturbances with decrease in BZ the magnetosphere is the magnetic field dipolarization, i.e., and enhanced local current density, and subsequent (2) in- enhancement(s) in BZ, indicating that the distribution of the crease in BZ toward a stable level, which is more prominent tail current has changed locally and/or globally. Statistical at Earthward side and evolving tailward. During the 09:07 studies of dipolarization events using satellites between 6.4 event, when Cluster was located in a thin current sheet, the and 17 RE downtail showed that more events were observed dipolarization and fast Earthward flows were also accom- further away from Earth (Lopez et al., 1988; Sigsbee et al., panied by further thinning of the current sheet down to a 2005). Many studies reported that dipolarization are associ- half-thickness of about 1000 km and oscillation in a kink- ated with Earthward flows exceeding 100 km/s and attributed like mode with a period of ∼15 s and propagating duskward. to magnetic flux transported Earthward (e.g., Angelopoulos Probable cause of this “flapping current sheet” is shown to et al., 1994; Baumjohann et al., 1999; Sigsbee et al., 2005). be the Earthward high-speed flow. The oscillation ceased as The dipolarization happens first in a limited region and prop- the flow decreased and the field configuration became more agates azimuthally (e.g., Nagai, 1982) as expected also from dipolar. The later rapid flux transport events at 09:14 and the finite width of the fast flows and accompanying magnetic 09:22 UT took place when the field configuration was ini- disturbances (e.g., Sergeev et al., 1996; Nakamura et al., tially more dipolar and were also associated with BZ distur- 2002, 2004). On a long time scale (∼45 min), however, the bance and local current density enhancement, but to a lesser dipolarization propagates tailward recovering globally from degree. Hence, current sheet disturbances induced by initial a thin current sheet state to a dipolar configuration (Baumjo- hann et al., 1999). One mechanism for dipolarization is that the near-Earth Correspondence to: R. Nakamura neutral line (NENL) causes large amounts of magnetic flux ([email protected]) to be transported Earthward, which eventually starts to pile Published by Copernicus Publications on behalf of the European Geosciences Union. 1744 R. Nakamura et al.: Dipolarization and near-Earth current sheet 20071027 nT Bz 6 ACE: +49 min 18 Geotail 0 0 ACE -6 a -18 400nT/tickm. H 0 (high-lat) KIAN INUV BETT FSIM -800 FYKN FSMI EAGL GILL geograph. b WHIT (lat., lon.) 1nT/tickm SHU (55 161) Pi2 CCNV (39 240) FYTS 20nT/tickm (46 259) KIAN (67 200) BETT c (67 208) 20nT/tickm. H SHU CCNV d 40 Cluster nT Bx 0 -40 e 8 9 10 11 Fig. 1. (a) IMF BZ from ACE and BZ component from Geotail in the magnetosheath at (X, Y, Z)=(−2, 16, 13)RE, (b) H component of the high-latitude magnetograms from north-American stations, (c) mid-latitude and high-latitude Pi2 (filtered H component between 40 and 150 s), (d) positive bay, and (e) BX component of C3. The three positive bay onsets discussed in the text are shown by the vertical line. The gray hatched time shows the third substorm event when Cluster crossed the center of the current sheet. up in the near magnetotail region where the flows brake in bances. While a pair of spacecraft was separated by only a strong field/high pressure region (Shiokawa et al., 1997; several 10s of km, the other two spacecraft and the pair were Baumjohann, 2002). Such pile-up effect is expected to prop- separated by 10 000 km from each other. This configuration agate tailward (e.g., Birn and Hesse, 1996). Tailward and allows to monitor the evolution of the fast flows associated azimuthal propagation of the magnetic disturbance is also a disturbances at multi-scales. In this study we use Cluster natural direction, when instabilites such as ballooning (Roux data from the FluxGate Magnetometer (FGM) experiment et al., 1991) or cross-field current instabilities (Lui et al., (Balogh et al., 2001), the Electric Field and Wave (EFW) 1991) take place near Earth dipolar region. To study the evo- instrument (Gustafsson et al., 2001), and by the Cluster ion lution of the near-Earth current sheet disturbances during a spectrometry (CIS) experiment (Reme` et al., 2001). substorm dipolarization and to examine the role of the flux transport from the tail, propagation properties of the field and those of the flow disturbances need to be examined indepen- 2 Overview of the event dently. Between 08:00 and 11:00 UT on 27 October 2007, there are Since July 2007 Cluster started to observe the magneto- mainly three Pi2 activities accompanied by electrojet activa- tail neutral sheet regions Earthward of 11 RE, where fast tions starting at around 08:08, 08:38, and 09:06 UT, as shown flow braking is considered to take place (Shiokawa et al., in Fig. 1. The 09:06 UT activation has the strongest electro- 1997). A result from the four spacecraft analysis of the jet and Pi2 when Cluster crossed the center of the current current sheet disturbance induced by the rapid flux trans- sheet (Fig. 1e) and will be discussed in this study. Cluster port observed from 09:07 UT on 27 October 2007 is stud- was located premidnight at XGSM=−9.2 RE, YGSM=5.5 RE, ied in this paper. By analyzing the plasma and magnetic ZGSM=−1.6 RE with their foot points in the Alaskan sec- field data from Cluster we succeeded to measure the motion tor. The relative position of the four spacecraft and loca- of the dipolarization front associated with the flow distur- tion of the foot points are given in Fig. 2. The midlatitude Ann. Geophys., 27, 1743–1754, 2009 www.ann-geophys.net/27/1743/2009/ R. Nakamura et al.: Dipolarization and near-Earth current sheet 1745 +X C3 C4 260 275 +Y 20071027 09:00 50 km C3 C4 C1 INUV GSM Y Δ C2 C2 KIAN 10000 km/tickm BETT 70 C4 FYKN Δ a X GSM ( ) C3 C1 C3 EAGL C4 50 km GSM Z Δ C2 C1 C3 C4 60 (+ ~2MLT) +Z 10000 km/tickm (geomagnetic coordinate) CCNV SHU ΔY +Y (b) GSM (c) Fig. 2. Relative position of Cluster in (a) X-Y , (b) Y -Z plane, and (c) location of the foot-points of Cluster. positive bay at SHU, located near the local time sector of the 09:07 event are discussed by Asano et al. (2009) using Cluster, started around 09:06 UT with further enhancement Polar UV images. A localized short-lived auroral activation at 09:25 UT, while the positive bay at CCNV, located about started dusk side of the foot point of C2 between 09:04:18 1.5 MLT east of WHI, started around 08:40 and 09:30 UT. and 09:04:55 UT and weakened while a next stronger acti- This suggests that while the 08:38 UT onset is localized in vation started between 09:06:45 and 09:07:22 UT around the Central American sector, the 09:06 UT onset is localized ini- Cluster foot point regions. The strongest westward electro- tially in Alaskan sector, but extends later to a larger local time jet for the 09:14 UT and 09:21 UT activations were observed sector involving Central American sector. This substorm oc- at FYKN and EAGL, respectively, indicating that the center curred during a prolonged interval of negative IMF BZ based of the electrojet region moved eastward of the Cluster foot on ACE and Geotail data (Fig. 1a). point region. (Note that the foot points of Cluster during the 09:21 UT events are located about 5◦ west from those Figure 3 shows the ground magnetogram and Cluster ob- given in Fig.