1. Cluster observations of a reconnection site at high- latitude magnetopause Y. Khotyaintsev (1), A. Vaivads (1), Y. Ogawa (1,2), M. André(1), S. Buchert(1), N. Cornilleau-Wehrlin(3), P. Décréau(4), A. Fazakerley(5), D. Gurnett(6), H. Rème(7), A. Retino(1) (1)IRF-Uppsala, Sweden (2)Solar-Terrestrial Environment Laboratory, Nagoya University, Japan (3)CETP, CNRS, France (4)LPCE, CNRS, France (5)Mullard Space Science Laboratory, University College London, United Kingdom (6)Department of Physics and Astronomy, The University of Iowa, USA (7)CESR, France e-mail: yuri@irfu.se Yuri Khotyaintsev May 13, 2003

2. Abstract We present high-resolution observations of high-latitude magnetopause by Cluster for conditions of mainly northward IMF (with significant Y component). In the vicinity of magnetopause Cluster observes accelerated earthward plasma flows with their direction being consistent with an ongoing reconnection tailward from Cluster. Later, in association with southward turning of the IMF, the flow changes from being earthward to being tailward. The turning point contains a region of depleted magnetic field with magnitude down to several nT which is surrounded from both sides by narrow layers of strong wave activity with frequency about local electron plasma frequency. As a possible cause of the flow reversal and depletion in magnetic field we suggest a scenario where the reconnection site, which is moving due to changes in the IMF, is passing very close to Cluster; high frequency waves can be generated by electron beams at separatrices coming from the reconnection site. Using multiple Cluster spacecraft observations we study the detailed structure of the reconnection site. Yuri Khotyaintsev May 13, 2003

3. Event 1 (2002-03-04) Figure 1 shows IMF and solar wind data from ACE satellite which is shifted in time to Cluster location. We concentrate on time period around 09:40 UT, when IMF is turning southward, but keeps large Y component. Cluster is located in the outer northern cusp, slightly on the dusk flank (Figure 2). Magnetic field (FGM) for outbound MP crossing by Cluster is shown on Figure 3; the region of interest with strong depression of magnetic field is marked by a circle. Observation of accelerated tailward plasma flows at 09:38:30 UT and 09:42:30 UT (Figure 4) with a significant component opposite to the IMF By direction indicate ongoing reconnection sunward from Cluster. However, the flow direction changes to sunward at 09:39:40 UT, meaning location of the reconnection site changed to tailward from Cluster at this time. Yuri Khotyaintsev May 13, 2003

4. Figure 1: IMF and Solar wind Yuri Khotyaintsev May 13, 2003

5. Figure 2: Cluster location Yuri Khotyaintsev May 13, 2003 Outbound crossing Position GSE : (2.95,2.95,7.75) [Re] Altitude : 8.8 Re Distance to model MP : 2.5 Re x x z z y y

6. Figure 3: Magnetic field Yuri Khotyaintsev May 13, 2003

7. Figure 4: Large scale Yuri Khotyaintsev May 13, 2003

8. Wave observations Figure 5 shows WHISPER spectrograms from four Cluster spacecraft for the period of sunward plasma flow indicated on Figure 4. There is a noticeable burst of wave activity around local upper hybrid frequency (~30 kHz) at 09:40:15 UT seen on all spacecraft, however, the amplitude is significantly higher on sc 3 and 4. It appears that the burst is located just outside the region of the strogest magnetic field depression shown on Figure 6. The total magnetic field goes as low as 1 nT on sc 4. The upper panel shows measured amplitude of B. Shifting the structure in time (lower panel) yields a velocity of V=74.3*[0.46,-0.81,-0.37] km/s in GSE. The time axis on the lower panel is recalculated into distance using this velocity. Local proton gyroradius is about 10 3 km and ion inertial length is 60 km. Yuri Khotyaintsev May 13, 2003

9. Figure 6: Structure Yuri Khotyaintsev May 13, 2003 09:40:18 09:40:16

10. Wave observations (cont) Spacecraft potential (~density), WHISPER spectrogram, and STAFF spectrograms for E&B for sc 4 are shown on Figure 7. One can see that the burst of plasma waves is located before at the edge of magnetic cavity and there are no low frequency waves seen at this time. There is no wave activity inside the cavity itself, and the most of activity below 200 Hz (whistlers) is located on the other side of the cavity. Figure 8 shows electron (PEACE) data for the period of interest. Despite a low temporal resolution of the measurements, one can probably conclude that there is an indication of electron beam at 09:40:15-16 UT seen as a dominance of parallel flux (0 degrees) at this time. Similar configurations are observed on other spacecraft. Yuri Khotyaintsev May 13, 2003

11. Figure 8: Electrons Yuri Khotyaintsev May 13, 2003

12. Micro scale Figure 9 shows the result of minimum variance analysis (MVA) performed around the region of min B for sc 4. There is a good quality MV frame with L1=106, L2=13.1 and L3=0.83, and the direction of minimum variance is [-0.18, -0.98, -0.05] in GSE. The magnetic field rotates by approximately 145 0. The strongest gradient of magnetic field is located close to the minimum. Possible interpretation of the observation is shown on Figure 10. We compare the observations to a schematic of a reconnection region usually seen in simulations. The schematic presented is asymmetric due to presence of a finite guide field. Electron beam and plasma waves are located on one of separatrices, while the strongest current and magnetic field depression are located close to the other separatrix. An enhancement of B mean between the separatrices is observed instead of quadrupole pattern, and the resulting hodograms are not S-shaped. Yuri Khotyaintsev May 13, 2003

13. Figure 9: MVA Yuri Khotyaintsev May 13, 2003

14. Figure 10: Schematic Yuri Khotyaintsev May 13, 2003 Jy e-  H+ Spacecraft path Ion outflow Waves Depression of B Separatrixes Electron beam

15. Event 2 (2001-12-03) Yuri Khotyaintsev May 13, 2003 Figure 11 shows another outbound magnetopause crossing by Cluster on December 3, 2001. A steep density gradient and a reversal of plasma flow direction are observed at 10:58 UT. WBD spectrogram for the same period is shown on Figure 12. There is a region of high wave activity at the high density side of the boundary (10:58:01--10:58:14 UT). The wave frequency extends from the lowest frequencies up to local upper hybrid frequency (30-40 kHz). There is an indication of parallel electron beam (dominance of flux at 0 degrees) in electron data (PEACE, Figure 13). Figure 14 shows an example of a waveform (10:58:01.34 UT). One can see packets of plasma waves, as well as electrostatic solitary waves.

16. Figure 12: WBD spectra Yuri Khotyaintsev May 13, 2003

17. Figure 14: Waveforms Yuri Khotyaintsev May 13, 2003

18. Summary Yuri Khotyaintsev May 13, 2003 ● We have studied high frequency waves around a region of strong magnetic depression (B tot in minimum is ~1 nT). The size of this region is of order of several ion inertial length and several times smaller than proton gyroradius. ● The region of magnetic depression is associated with thin current sheet. Low value of the magnetic field in the minimum and plasma flow behavior indicate that Cluster is passing close to a reconnection site. The magnetic field rotates by approximately 145 0 implying the existence of a guide field of about 5 nT (B tot ~ 15 nT). ● The observed structure is compared to a structure of a reconnection region which is usually seen in simulations with a finite guide field.

19. Yuri Khotyaintsev May 13, 2003 ● Strong wave activity around upper hybrid frequency is dominant on one side of the structure. There is no significant current seen in FGM data in association with this waves, which makes electron-beam instability preferable over Buneman instability as a source of the waves. ● High resolution WBD data (for a similar MP encounter) indicate that the wave activity measured by WHISPER may consist of electrostatic solitary waves (electron holes) in addition to Langmuir waves. ● Whistler waves are not observed simultaneously with a peak of plasma waves, but rather on the other side of the current structure. ● Region of low magnetic field (B tot < 10 nT) contains no significant wave activity.