Cluster observations of waves in the whistler frequency range associated with magnetic reconnection in the Earth s magnetotail

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi:10.1029/2006ja011771, 2007 Cluster observations of waves in the whistler frequency range associated with magnetic reconnection in the Earth s magnetotail X. H. Wei, 1,2 J. B. Cao, 1 G. C. Zhou, 1 O. Santolík, 3 H. Rème, 4 I. Dandouras, 4 N. Cornilleau-Wehrlin, 5 E. Lucek, 6 C. M. Carr, 6 and A. Fazakerley 7 Received 9 April 2006; revised 11 April 2007; accepted 11 July 2007; published 31 October 2007. [1] On 21 August 2002 the Cluster spacecraft encountered a quasi-collisionless magnetic reconnection event when it crossed the plasma sheet. Prior to the southward turning of magnetic field in the tailward flow, the weak whistler waves were first observed. The burst of whistler waves appeared about 30 s earlier than the southward turning. During magnetic reconnection event, the waves in the whistler frequency range were greatly enhanced. For the waves in the reconnection process, those in the higher-frequency range are mainly quasi-parallel propagating right-hand polarized whistler waves, while those in the lowerfrequency range are quasi-perpendicular propagating linear polarized waves, which may be a superposition of several linearly polarized waves. The combined observations of energetic electrons and waves show that after the southward turning of magnetic field, the waves in the reconnection process are greatly enhanced by energetic electron beams. Citation: Wei, X. H., J. B. Cao, G. C. Zhou, O. Santolík, H. Rème, I. Dandouras, N. Cornilleau-Wehrlin, E. Lucek, C. M. Carr, and A. Fazakerley (2007), Cluster observations of waves in the whistler frequency range associated with magnetic reconnection in the Earth s magnetotail, J. Geophys. Res., 112,, doi:10.1029/2006ja011771. 1. Introduction [2] The magnetic reconnection is the major process responsible for energy transfers from the solar wind to the Earth s magnetosphere and energy release in the magnetotail [Dungey, 1961; Vasyliunas, 1975]. How magnetic energy can be released so quickly has perplexed scientists for several decades. The breaking of the magnetic field lines actually takes place in a narrow region around the X line. In this narrow region the magnetohydrodynamic description fails and the diffusion region is found to develop a multiscale structure based on the electron and ion inertial lengths [Drake, 1995; Drake et al., 1997]. When the reconnection takes place at ion inertial length, the wave-particle interaction plays an important role in reconnection process. Both the whistler dynamics and kinetic Alfvén waves can strongly influence the structure of the dissipation region during magnetic reconnection. Hall term in the generalized Ohm s law brings the dynamics of whistler waves into the fluid 1 Key Laboratory of Space Weather, Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing, China. 2 Graduate School, Chinese Academy of Sciences, Beijing, China. 3 Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic. 4 Centre d Etude Spatiale des Rayonnements, Toulouse, France. 5 Centre d Etudes des Environnements Terrestre et Planétaires, L Université de Versailles Saint-Quentin-en-Yvelines, Centre National de la Recherche Scientifique, Velizy, France. 6 Blackett Laboratory, Imperial College, London, London, UK. 7 Mullard Space Science Laboratory, University College London, London, London, UK. Copyright 2007 by the American Geophysical Union. 0148-0227/07/2006JA011771 equations [Drake, 1995]. The fast reconnection also depends on the dynamics of whistler and kinetic Alfvén waves at small scales [Rogers et al., 2001]. In the absence of guiding field, oblique Alfvén-whistler waves are the dominant waves in the reconnection layer [Wang et al., 2000, 2001]. Linear MHD theories also show that there are obliquely propagating whistler and whistler instabilities in the ion inertial region [Zhou et al., 2004, 2005; Weietal., 2005]. Using linearized Vlasov equation, Akimoto and Gray [1987] found that there exist obliquely propagating whistler instabilities in magnetotail, and these instabilities can be driven by electron beams, ion beams, and field-aligned currents. [3] Whistler is often observed in the magnetosphere. Imp8 observed the whistler waves in the region near the neural sheet in the magnetotail at radial distances ranging from 46.3 to 23.1 R e [Gurnett et al., 1976]. Gurnett et al. [1976] suggested that the whistler waves are most likely produced by current-driven plasma instabilities. The whistler waves are also observed by ISEE3 in the plasma sheet [Scarf et al., 1984] and magnetotail flux ropes [Kennel et al., 1986]. It is suggested that superthermal electrons with highly anisotropic pitch angle distributions generate the whistler waves. Zhang et al. [1999] analyze whistler waves observed by Geotail in the magnetotail at radial distance ranging from 210 R e to 10 R e, and they found that whistler waves can exist in both plasma sheet and plasma sheet boundary layer and propagate quasi-parallel to the ambient magnetic field with an average propagation angle of 23. They thought that it is the energetic electron beams that generate the whistler waves. Whistler waves associated with reconnection in the Earth s magnetopause were also observed by Geotail [Deng and Matsumoto, 2001; Drake et 1of8

Figure 1. Plasma parameters observed by C1 (black), C3 (green), and C4 (blue) during the interval of 0750 0800 UT on 21 August 2002. From top to bottom: plasma flow (V x ), magnetic field components (B XGSM, B YGSM, B ZGSM ), ion density, total magnetic field (B), and plasma beta. al., 1997]. Wind observed low-frequency Alfvén/whistler waves associated with the low hybrid drift instability (LHDI) in the near-vicinity of the X-line of reconnection far from the Earth at about 57 R e in the magnetotail [Farrell et al., 2002, 2003]. Thus up to present, the satellite observations have shown that whistlers exist in reconnection process and are most likely excited by electron beams generated by magnetic reconnection. However, the whistler waves prior to reconnection are rarely reported. Therefore in order to get a more confirmative conclusion about the relation between whistler waves and reconnection, much more detailed observational evidences and theoretical studies are needed. [4] Recently, the new observations show that many other waves are also associated with reconnection process, such as the electrostatic solitary waves (ESWs) [Matsumoto et al., 2003; Cattell et al., 2002; Farrell et al., 2002, 2003] and lower hybrid waves (LHW) [Bale et al., 2002; Øieroset et al., 2002]. Cluster observed many quasi-collisionless magnetic reconnection events in the magnetotail [Alexeev et al., 2005; Zhou et al., 2003] and ESWs (electron holes) associated with reconnection far from the Earth at about 18 R e in the Earth s magnetotail [Drake et al., 2003; Cattell et al., 2005]. However, the relations between waves and reconnection are not yet fully understood. [5] This paper is organized as follows. In section 2 we first present the reconnection event on 21 August 2002 and associated waves. Walén test is used here for the tailward ion flow with southward magnetic field. Section 3 summarizes the main conclusions of this study. [6] The data used in this study are from Cluster instruments. Ion velocity data are from CODIF/CIS [Rème et al., 2001], magnetic field data from FGM [Balogh et al., 2001], wave data from STAFF [Cornilleau-Wehrlin et al., 2003], and electron data from PEACE instrument [Johnstone et al., 1997].The time resolutions of ions, electrons, and magnetic field are 4 s. The time resolution of wave data from STAFF- SA is 0.125 s. Since the data from CIS/C2 are not available, 2of8

Figure 2. Schematic of magnetic reconnection configuration and Cluster satellite locations at the same time in Figure 1. The red stars represent the locations of Cluster satellites during the reconnection. here only the data from three satellites (C1, C3, and C4) are used. All the data are given in the GSM coordinates. 2. Cluster Observations of Magnetic Reconnection Event on 21 August 2002 2.1. Magnetic Reconnection Event on 21 August 2002 [7] Cluster crossed the magnetotail plasma sheet from 0700 to 0900 UT on 21 August 2002. Figure 1 gives the ion flow velocity components (V x ), magnetic field components (B x, B y, B z ), plasma density, total magnetic field (B), and plasma Beta, which are observed by C1 (black), C3 (green), and C4 (blue) during the interval of 0750 0800 UT on 21 August 2002. At 0750:00 UT, C1 was approximately located at ( 18.36, 4.27, 0.25) R e,c3at( 18.81, 4.29, 0.01) R e, and C4 at ( 19.0, 4.40, 0.62) R e. At 0800:00 UT, C1 was located at ( 18.35, 4.27, 0.17) R e,c3at ( 18.81, 4.3, 0.09) R e, and C4 at ( 19.0, 4.40, 0.54) R e. All Cluster spacecrafts were on the dawnside and in plasma sheet. It can be seen that a high-speed tailward ion flow V x < 0 accompanied by southward magnetic field component was observed by three satellites and it lasted about 5 min. The tailward ion flow with southward magnetic field component appeared at 0753:50 UT on C1 and C3 and at 0754:05 UT on C4. The velocity of tailward ion flow was very large and its maximum value even exceeded 1500 km/s. The maximum southward magnetic field component reached 25 nt. Generally, such a high-speed tailward ion flow with a large southward magnetic field component is produced by magnetic reconnection. The tailward ion flow with southward magnetic field component disappeared at 0758:30 UT. Figures 1e and 1f give the ion density and plasma beta (b) observed by C1 and C4. From 0750 UT to 0756 UT, the plasma b was larger than 0.7, with a peak value 3.17 at 0754:45 UT, where the ion density was about 0.3/cm 3, the total magnetic field was about 15 nt, and the proton temperature was about 5897 ev. Thus according to general identification criterion of plasma sheet [Hughes, 1995], the Cluster satellites were located in the plasma sheet at least between 0750 and 0756 UT. [8] Throughout the magnetic reconnection event, the B x observed by three satellites was basically positive, indicating that Cluster satellites were located on the northern side of magnetic equator. B y was basically negative, which coincides with Hall magnetic field polarity of collisionless reconnection. During the period of 0700 0900 UT, the IMF B y observed by ACE was very stable and approximately equal to 2 nt. Therefore the possibility that the observed B y polarity comes from solar wind can be excluded. [9] Figure 2 shows the schematic of the basic reconnection configuration, the coordinate system, and the spacecraft locations. Three red stars mark the locations of the Cluster satellites. The magnetic reconnection configuration in Figure 2 is consistent with the physical picture of reconnection shown in Figure 1. Three satellites were located on the tailward side of X line of reconnection and thus they all observed a tailward high-speed ion flow with southward magnetic field component. This reconnection event occurred in the expansion phase of a large substorm, during which the AE index even reaches 1250 nt. 2.2. Walén Analysis [10] For a rotational discontinuity at magnetopause, the outflow produced in the reconnection is Alfvénic in the dehoffmann-teller (HT) frame of reference and should satisfy the Walén relation [Sonnerup et al., 1987; Khrabrov and Sonnerup, 1998]. Thus Walén test is a very useful tool to check the magnetopause reconnection [Phan et al., 2001, 2004; Pu et al., 2005]. Recently, Walén test are also used for tail reconnection [Øieroset et al., 2000; Eriksson et al., 2004]. It is found that the sub-alfvénic flows in the HT frame are consistent with the presence of slow-mode shocks connected to the diffusion region. Therefore the Walén test can also be used in identifying the stationary reconnection structures in the tail. The Walén test slope is negative for earthward flow and positive for tailward flow in northern hemisphere. [11] In this paper, Walén test is applied to above-mentioned high-speed tailward flow. We calculate the tailward flow velocity in the dehoffmann-teller frame and apply Walén analysis as in the work of Øieroset et al. [2000] and Eriksson et al. [2004]. Figure 3 shows the scatter plot of x, y, and z component of the flow velocity in the HT frame versus the Alfvén velocity observed by C4 for the periods of 0754:22 0754:42 UT. The corresponding dehoffmann- Teller velocity is 656.3, 154.0, 36.6 km/s. The ~V HT is mainly in the antisunward ( x) direction. 3of8

Figure 3. Walén analyses for the period of 0754:22 0754:42 UT. The X GSM, Y GSM, and Z GSM components of flow velocity are represented by plus sign, asterisk, and diamond, respectively. [12] The obtained regression slope for the tailward flow interval is 64% of the Alfvén velocity, which is similar to those obtained by Øieroset et al. [2000]. The correlation coefficient is 0.92, indicating the existence of a good dehoffmann-teller frame. The Walén slope is positive, which is in agreement with Cluster observing tailward plasma flows on the northern side of the neutral sheet under the assumption that a near-earth neutral line exists earthward of Cluster and that the plasma is accelerated across a Petschek-type slow-mode shock connected to the diffusion region. [13] The combined analysis of tailward ion flow with southward magnetic field, Hall magnetic field polarity, and Walén test show that Cluster encountered a magnetic reconnection event, and this magnetic reconnection event had Hall magnetic field signatures. 2.3. Waves Associated With the Magnetic Reconnection Event [14] The waves in the frequency range 10 Hz to 4 khz observed by STAFF are analyzed by means of the Propagation Analysis of STAFF-SA Data With Coherency Tests (PRASSADCO) tool [Santolík et al., 2003]. All three satellites (C1, C3, and C4) observed the whistler waves prior to southward turning of B z component. The wave characteristics observed by C1, C2, and C3 were nearly identical. However, the wave characteristics observed by C4 were different from those of other three satellites. Thus only the wave characteristics observed by C1 and C4 are displayed here. Figure 4 shows the wave characteristics observed by C1 and C4 during the interval of 0750 0800 UT on 21 August 2002. The black curves represent the electron cyclotron frequency. Figures 4a and 4b show the power- 4of8

Figure 4 5of8

Figure 5. Electron energy-time spectrograms observed by PEACE/C1 during the period of 0750 0820 UT for three directions: (a) antiparallel, (b) perpendicular, and (c) parallel to ambient magnetic field. The vertical black line denotes the start of reconnection. spectral densities of magnetic field. Figures 4c and 4d represent the polar angles (Theta) of the wave normal direction with respect to the ambient magnetic field. These angles are obtained from the magnetic power spectral with the method of SVD [Santolík et al., 2003]. Figures 4e and 4f represent the sense of polarization, in which the values of c B > 0 indicate a right-hand polarized wave and the values of c B < 0 indicate a left-hand polarized wave. During the period of 0750 0800 UT, weak (yellow) and enhanced (red) wave activities were observed. The frequency range of waves was between ion cyclotron frequency and electron cyclotron frequency. [15] Before the appearance of strong wave activities, some weaker intermittent electromagnetic wave activities were already observed by Cluster in the period of 0750:00 0753:20 UT. The frequency range of weaker waves is from 20 to 200 Hz. The weaker electromagnetic waves propagated in directions of about 20 or 80. The polarization sense of these weaker intermittent waves is positive (see the light yellow spots in Figure 4). These weaker intermittent waves are right-hand polarized, quasi-parallel, and should be whistler mode. [16] The first intense electromagnetic wave event observed by C1 started at about 0753:20 UT, about 30 s earlier than the southward turning of B z component of the tailward ion flow (0753:50 UT). Intense electromagnetic waves lasted longer than tailward ion flows. The waves prior to the southward turning of B z component also propagated at q 40 and its sense of polarization was right-hand polarized. [17] During magnetic reconnection, the waves in the higher-frequency range 60 800 Hz propagated at q 40, and the waves of lower-frequency range (from 10 to 60 Hz) propagated at q 80. The sense of polarization of waves in the higher-frequency range was right-hand polarized. They should be a mixture of whistler-mode waves and general right-hand polarized waves. Here the waves with frequency very close to the electron cyclotron frequency are possibly electron cyclotron waves. The sense of polarization of waves in the lower-frequency range was almost zero, indicating that the waves were linearly polarized. [18] Since the waves prior to the southward turning of B z component and the waves in the higher-frequency range during the magnetic reconnection event were quasi-parallel (q 40 ) propagating and right-hand polarized waves, they are typical whistler modes. The waves in the lower-frequency range during the magnetic reconnection event were quasi-perpendicular propagating and linearly polarized wave, and they may result from a superposition of several linearly polarized waves [Santolík et al., 2002]. Figure 4. Wave characteristics observed by C1 and C4 during the period of 0750 0800 UT on 21 August 2002, showing (a,b) the dynamic spectra of total field turbulence B-power, (c,d) the polar angles (THETA) of the wave normal direction with respect to ambient magnetic field, and (e,f) the sense of polarization. Black curves on the dynamic spectra are the electron cyclotron frequency. 6of8

[19] The above analysis shows that the intense whistler activities appeared about 30 s prior to the southward turning of magnetic field. The whistler waves were greatly enhanced after the southward turning of magnetic field. In addition, as reconnection proceeded, the wave frequency became higher and higher. When tailward flows reached the maximum, the wave frequency got closer to the electron cyclotron frequency. 2.4. Electron Behavior in the Magnetic Reconnection Event [20] Figure 5 shows the energy-time spectrograms of electrons within energy range 10 20 kev for three directions (antiparallel (Figure 5a), perpendicular (Figure 5b), and parallel (Figure 5c) to the ambient magnetic field), which were observed by PEACE of C1. The solid curve close to the bottom of each figure shows the potential between the EFW probes and the spacecraft. Since this potential is close to the spacecraft potential, the electrons with energy below or a few ev above the solid curve are not from the plasma but from the spacecraft and should be ignored. Before the southward turning of magnetic field in the tailward flow (at 0753:50 UT), there existed background weak electron fluxes with energy from 800 ev to 6 kev in three directions, and the energy spectra of parallel and antiparallel electron fluxes were basically similar to that of perpendicular electron flux. After the southward turning of magnetic field (at 0753:50 UT), many bursts of parallel and antiparallel 3 4 kev electron fluxes emerged, while no bursts of perpendicular 3 4 kev electron fluxes were found. These enhanced energetic (3 4 kev) parallel and antiparallel electron fluxes lasted the whole reconnection event. [21] It can be seen that the whistler existed both before and after the southward turning of magnetospheric magnetic field for the reconnection event investigated here. However, the characteristics of whistler waves before and after the southward turning of magnetic field were different, which suggest that they were produced by different mechanism. The whistler wave frequency during the reconnection event is from 0.1 W e to W e, indicating that this whistler belongs to high-frequency whistler. The close relation between whistler waves and energetic electrons during the reconnection process strongly suggest that the whistler waves are generated and/or enhanced by bursts of counter streaming energetic electron beams. This kind of parallel and antiparallel directions electrons may exist in the reconnection regions. Generally speaking, these bidirection particles are on closed field lines, that is, magnetic island, or dipolar field of Earth (if the satellite is located earthward of reconnection region). 3. Conclusions [22] Cluster satellites (C1 C4) crossed the plasma sheet in the magnetotail during the period of 0700 0900 UT on 21 August 2002. All three satellites C1, C3, and C4 observed quasi-collisionless reconnection event during the interval of 0750:00 0800:00 UT. Walén test shows that the flow speed in the dehoffmann-teller frame is 63% of the Alfvén speed, similar to the results of Walén test of reconnection in the tail obtained by Øieroset et al. [2000]. [23] This reconnection event was accompanied by wave activities. The whistler wave activities were observed before and after the southward turning of B z component. However, their characteristics of waves are obviously different. The observations of electrons also show that during the reconnection process, there are many burst of energetic electrons. [24] We used the PRASSADCO program to analyze properties of observed waves. The results show that there are several kinds of wave modes accompanying magnetic reconnection, which can be classified into two categories by wave frequency. The waves in the high-frequency range are right-hand polarized and quasi-parallel propagating whistler, while the waves in the lower-frequency range are linearly polarized and quasi-perpendicular propagating waves, which may be a superposition of several linearly polarized waves [Santolík et al., 2002]. [25] The whistler waves in the reconnection process are also observed by Geotail [Deng and Matsumoto, 2001; Deng et al., 2004]. Our results show that in the plasma sheet, the whistler wave can exist before the southward turning of B z component. Therefore it is possible that whistler waves appeared before the reconnection. The large B x and parallel and antiparallel electron beams, as well as the increase of parallel velocity and temperature show that the satellites observed the whistler waves near boundary or seperatrix region. [26] Up to present, the problem of triggering of magnetic reconnection in the tail is still not fully understood. The MHD theory of collisionless reconnection requires that anomalous resistivity exist in the reconnection layer. The whistler waves prior to the reconnection may play an important role in the triggering of reconnection. The satellite observations show that besides whistler waves, there are LHWs [Bale et al., 2002; Øieroset et al., 2002] and solitary waves [Matsumoto et al., 2003; Cattell et al., 2002, 2005; Farrell et al., 2002, 2003] in magnetopause and magnetotail reconnections. Also during dayside magnetopause reconnection, whistler modes could be converted to LHW modes [Daughton, 2003; Carter et al., 2002, 2005]. 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