PUBLICATIONS. Journal of Geophysical Research: Space Physics. Two types of whistler waves in the hall reconnection region

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1 PUBLICATIONS Journal of Geophysical Research: Space Physics RESEARCH ARTICLE Key Points: Two types of whistler waves are observed in the reconnection diffusion region First one is in pileup region, and second one is around separatrix Whistlers are the consequences of magnetic reconnection Two types of whistler waves in the hall reconnection region S. Y. Huang 1,2,H.S.Fu 3, Z. G. Yuan 1,A.Vaivads 4,Y.V.Khotyaintsev 4,A.Retino 2,M.Zhou 5, D. B. Graham 4, K. Fujimoto 6,F.Sahraoui 2, X. H. Deng 5,B.Ni 1,Y.Pang 5,S.Fu 1,D.D.Wang 1, and X. Zhou 7 1 School of Electronic Information, Wuhan University, Wuhan, China, 2 Laboratoire de Physique des Plasmas, CNRS-Ecole Polytechnique-UPMC, Palaiseau, France, 3 School of Space and Environment, Beihang University, Beijing, China, 4 Swedish Institute of Space Physics, Uppsala, Sweden, 5 Institute of Space Science and Technology, Nanchang University, Nanchang, China, 6 Division of Theoretical Astronomy, National Astronomical Observatory of Japan, Mitaka, Japan, 7 School of Physics, Liaoning University, Shenyang, China Correspondence to: S. Y. Huang, shiyonghuang@whu.edu.cn Citation: Huang, S. Y., et al. (2016), Two types of whistler waves in the hall reconnection region, J. Geophys. Res. Space Physics, 121, , doi: / 2016JA Received 11 MAR 2016 Accepted 29 JUN 2016 Accepted article online 4 JUL 2016 Published online 25 JUL American Geophysical Union. All Rights Reserved. Abstract Whistler waves are believed to play an important role during magnetic reconnection. Here we report the near-simultaneous occurrence of two types of the whistler-mode waves in the magnetotail Hall reconnection region. The first type is observed in the magnetic pileup region of downstream and propagates away to downstream along the field lines and is possibly generated by the electron temperature anisotropy at the magnetic equator. The second type, propagating toward the X line, is found around the separatrix region and probably is generated by the electron beam-driven whistler instability or Čerenkov emission from electron phase-space holes. These observations of two different types of whistler waves are consistent with recent kinetic simulations and suggest that the observed whistler waves are a consequence of magnetic reconnection. 1. Introduction Magnetic reconnection occurs in a crucial region, called the diffusion region (including ion diffusion region where electrons can be demagnetized and electron diffusion region where ions can be demagnetized), where magnetic energy is transferred into plasma kinetic and thermal energy. Strong wave activity and wave-particle interactions in the diffusion region may play an important role in the triggering and development of the reconnection process [e.g., Vaivads et al., 2006; Fujimoto, 2006; Fujimoto et al., 2011]. Many different types of waves have been observed in or near the reconnection diffusion region, such as electrostatic solitary waves [Matsumoto et al., 2003; Cattell et al., 2005; Khotyaintsev et al., 2010], Langmuir waves [Farrell et al., 2002; Vaivads et al., 2004], electron cyclotron waves [Viberg et al., 2013], whistler waves [Deng and Matsumoto, 2001; Wei et al., 2007; Petkaki et al., 2006; Tang et al., 2013], lower hybrid waves [Zhou et al., 2009, 2014], kinetic Alfvén waves or Alfvén-whistler waves [Chaston et al., 2009; Huang et al., 2010, 2012a], and slow magnetosonic waves [Cao et al., 2013]. In particular, we focus on whistler waves in this paper. Recent kinetic simulations have pointed out that whistler waves do not control the process of energy dissipation but are a consequence of the reconnection process itself [Fujimoto and Sydora, 2008; Fujimoto, 2014]. From the observational view point, Deng and Matsumoto [2001] first reported whistler waves in the vicinity of a reconnection region in the Earth s magnetopause and treated their observations as evidence of whistler-mediated reconnection. Wei et al. [2007] have observed weak whistler waves before the detection of reconnection and strong whistler waves during the reconnection process, leading to the suggestion that the whistler waves may play an important role in triggering the reconnection. Tang et al. [2013] have reported whistler waves in the electron diffusion region at the magnetopause and suggested the whistler waves could heat the electrons. Apart from Tang et al. [2013], previous observations have reported whistler waves associated with magnetic reconnection but have not confirmed the precise locations of the whistler waves and hence the precise roles of these waves during the reconnection process remain unclear. In this paper, we report the near-simultaneous observations of whistler waves by the Cluster spacecraft in the pileup region and around the separatrix region inside the Hall reconnection region. In the following section we describe the data analysis of a crossing of Hall reconnection region (In our study, Hall reconnection region corresponds to the classical ion diffusion region and can be identified with a quadrupole structure of the outof-plane magnetic field.) and report near-simultaneous observations of whistler waves. Then we compare our observations to recent kinetic simulations and suggest that the observed whistler waves are a consequence of magnetic reconnection. HUANG ET AL. WHISTLERS IN RECONNECTION REGION 6639

2 Figure 1. Diagram showing two DFs inside Hall reconnection region and the associated whistler waves (see Fu et al. [2013a] for details). The pink dashed shades display the regions where the whistler waves can be observed. The blue dots represent the source regions of whistler waves. The red arrows display the whistler waves that propagate away from the source regions to downstream. The green and blue curves show the trajectory of C3 and C4 spacecraft, respectively. 2. Observations The data from several instruments on board the Cluster spacecraft are used in this study. The magnetic field data come from the FGM instrument [Balogh et al., 2001], the ion plasma data are obtained from the CIS instruments [Rème et al., 2001], the electron data are taken from the PEACE experiment [Johnstone et al., 1997], while the high frequency electric and magnetic field spectra data are measured by the STAFF instrument [Cornilleau-Wehrlin et al., 2003]. We revisit the Cluster event on 19 September 2002 studied by Fu et al. [2013a]. Two dipolarization fronts (DFs) are detected inside the Hall reconnection region, as illustrated in the schematic of Figure 1b. This gives in situ evidence that transient reconnection can produce DFs [Fu et al., 2013a]. DFs, also called jet fronts, are characterized by the sharp increase of magnetic field B z component (GSM coordinates) embedded inside the high-speed plasma flows in the magnetotail [e.g., Nakamura et al., 2002; Runov et al., 2009; Sitnov et al., 2009; Fu et al., 2011, 2012a, 2012b; Huang et al., 2015a, 2015b]. In this event, the Cluster separation was about 3000 km (Figure 1a). Figure 2 presents the observations of the DFs and wave emissions in GSM coordinates measured by Cluster 3 (C3). We can see that C3 first detect tailward flow (V x < 0 before 13:12:30 UT in Figure 2c), then earthward flow (V x > 0 after 13:12:30 UT in Figure 2c) accompanied by the reversal of B z from negative to positive (Figure 2a) and the well-defined Hall magnetic field B y (Figure 2b). This event can be identified as a crossing of Hall reconnection region. Two jumps of magnetic field B z (marked as DF1 and DF2 by vertical dashed lines in HUANG ET AL. WHISTLERS IN RECONNECTION REGION 6640

3 Journal of Geophysical Research: Space Physics Figure 2. C3 spacecraft measurements of DFs and whistler waves. (a and b) Magnetic field Bz and Bx and By components from the FGM instrument. (c) Plasma flow Vx from the CIS experiments; (d and e) power spectra densities of the magnetic and electric components of the waves. (f) Polarization degrees, (g) ellipiticities (red: right hand; blue: left hand), (h) propagation angles, and (i) field-aligned Poynting flux. The parameters in Figures 2d 2i are from the STAFF instrument. Two vertical dashed lines mark two DFs. The dashed white ellipses emphasize the observations of whistler waves. Figure 2) were found inside the tailward and earthward flow of the Hall reconnection region, respectively. These two jumps are identified as DFs [Fu et al., 2013a]. Detailed identification of this Hall reconnection region and the associated DF crossing has been presented by Fu et al. [2013a]. Figures 2d and 2e show the power spectral densities (PSDs) of wave emissions (from 8 to 4000 Hz) during this period. The properties of these waves, including the polarization degrees, ellipticities, propagation angles, and field-aligned Poynting flux are displayed in Figures 2f 2i, respectively. All these parameters are derived from singular value decomposition (SVD) method by STAFF team [Santolik et al., 2003]. It can be seen that intense electromagnetic waves (marked by white ellipses in Figure 2) are observed behind both DFs with their frequencies generally below the electron cyclotron frequency fce, the large polarization degrees (>0.4 in Figure 2f) and positive ellipticities (>0.7 in Figure 2g), and relatively small propagation angles (<40 in Figure 2h). These waves should correspond to the right-hand polarized and quasi-parallel whistler waves. There are no clear whistler waves in the other regions. Therefore, the whistler waves are only observed in the enhancement of magnetic field Bz in the pileup region behind the DFs [Hoshino et al., 2001; Fu et al., 2013b]. Field-aligned Poynting fluxes of whistler waves are mainly positive (Figure 2i), implying that the observed whistler waves propagate parallel to the magnetic field. Given the values of Bx (Figure 2b), C3 is southward of the current sheet (Bx < 0) behind DF1, and northward of the current sheet (Bx > 0) behind DF2), we infer that the whistler waves propagate away from the center of current sheet. For zero guide field reconnection case, the pileup region is generally south-north symmetric, so we can deduce that the source region of the whistler waves should be in the magnetic equator, i.e., the central plane of the current sheet. Figure 1 illustrates these features of the observed whistler waves. In the present work, the STAFF instrument provides waveform data with a sampling frequency of 25 Hz (normal mode) [Cornilleau-Wehrlin et al., 2003], while the FGM waveforms have a sampling frequency of HUANG ET AL. WHISTLERS IN RECONNECTION REGION 6641

4 Journal of Geophysical Research: Space Physics Figure 3. The same format as Figure 2 but for the C4 spacecraft. 23 Hz (normal mode). Those sampling rates do not allow us to capture the whistler waves that occur at higher frequencies. The time resolution of STAFF data derived from SVD method is 4 s. The time duration of the detection of the whistler waves depend on the crossing of the Cluster spacecraft. If the spacecraft crossing is very fast, the whistler wave would appear only in 1 or 2 bins as shown in our event. Figure 3 shows the observations of the whistler waves detected by Cluster 4 (C4). Two DFs are also observed by C4 (marked as DF1 and DF2 by vertical dashed lines in Figure 3) inside the tailward flow and earthward flow of the Hall reconnection region, respectively. The jumps in Bz are not very sharp because C4 is far from the current sheet center. Interestingly, electromagnetic whistler waves are also detected and characterized by their frequencies below the electron cyclotron frequency fce, large polarization degrees (>0.35 in Figure 3f) and right-hand polarized (>0.8 in Figure 3g), and quasi-parallel propagation angles (<40 in Figure 3h). These observations of the whistler waves are highlighted by the white ellipses in Figure 3. Considering the large Bx (Bx ~ 18 nt behind DF1 and Bx ~ 25 nt behind DF2 in Figure 3b) and small outflow Vx ( Vx < 100 km/s in Figure 3c), we infer that C4 detects the whistler waves far away from the center of the current sheet and around the separatrix region. Field-aligned Poynting fluxes of the whistler waves are positive behind DF1 (northward of the current sheet in the tailward flow) and negative behind DF2 (northward of the current sheet in the earthward flow), indicating that these whistler waves propagate toward the X line. In this event, C1 did not capture the reconnection signature because it was far away from the current sheet (most northward, see Figure 1a). C2 was near the current sheet center but did not capture the reconnection signature either, probably it had a large separation from C3 and C4 in Y direction (Figure 1a), and was out of the Hall reconnection region due to the limit spatial scale of Hall reconnection region in Y direction. To understand the electron dynamics associated with the whistler waves, Figure 4 displays the electron pitch angle distributions during the detections of the whistlers. We find that the electron distributions detected by C3 corresponding to the whistler waves behind DF1 are field aligned (PSDs enhance at ~0 and ~180 in Figure 4a, and PSDs at 180 are slightly larger than those at 0 ), while the electron distributions corresponding to the whistler waves behind DF2 are more perpendicular (90 pitch angle in Figure 4b) at the high energy HUANG ET AL. WHISTLERS IN RECONNECTION REGION 6642

5 Figure 4. Pitch angle distributions (PSDs) of the electrons measured by the PEACE instrument during the observations of whistler waves detected by (a and b) C3 and (c and d) C4. (>2 kev) and nearly isotropic at low energy. Thus, the electrons behave differently when C3 detects the whistler waves. This may be attributed to the locations of C3 (C3 is far from the center of current sheet behind DF1 but close to the center of the current sheet behind DF2, as seen in Figure 1b). The electron distribution is usually more perpendicular at the center of current sheet and becomes more field aligned when moving away from the center of current sheet [Walsh et al., 2011]. Perpendicular-enhanced distribution of the electrons can provide favorable conditions for the generation of the whistler waves [e.g., Gary and Karimabadi, 2006]. We used three bi-maxwellians to fit the electron distribution behind DF2 when C3 is close to the center of the current sheet (Figure 5a). Based on the fit to the electron distribution, the theoretical result of our dispersion solver (Waves in Homogeneous Anisotropic Multicomponent Magnetized Plasma [Ronnmark, 1982]) is shown in Figure 5b. One can see a positive growth rate due to the electron temperature anisotropy with maximum growth γ/ω ce = occurring at ω/ω ce = From the gyroresonance condition, the resonant energy is estimated to be 7 kev. The fit to the electron observations agrees very well for the electros near the resonant energies. At these energies the temperature anisotropy is approximately T /T = 1.4, which produces the observed whistler waves. Thus, the simulated result implies that the whistlers in the pileup region can be generated by an electron temperature anisotropy. However, regarding the whistlers detected by C4, the electron distributions are field aligned at low-energy level and antiparallel (Figure 4c) at the high energy (>1 kev) behind DF1 and nearly isotropic and slightly field aligned at low-energy behind DF2 (Figure 4d), which is not very favorable condition for the excitation of the whistler waves. Thus, these whistler waves may be generated around the remote separatrix region and then propagate toward the X line along the separatrix. C4 detect the whistler waves from the source region at the separatrix region. HUANG ET AL. WHISTLERS IN RECONNECTION REGION 6643

6 Figure 5. (a) The fit to the electron distribution behind DF2 when C3 detected the whistler waves (cycles: observed; solid curves: fitted); (b) dispersion relations (blue) and growth rates (red) of the theoretical whistlers based on the fit to the electron distributions. Figure 1b summarizes these features of the two types of whistlers observed by Cluster and shows that only C3 behind DF2 observed the whistler waves in the source region. 3. Discussion and Summary Whistler waves have previously been reported associated with DF events or dipolarization processes [Le Contel et al., 2009; Deng et al., 2010; Khotyaintsev et al., 2011; Huang et al., 2012b; Viberg et al., 2014; Fu et al., 2014; Li et al., 2015]. These observations, however, are far from the reconnection sites and, therefore, may not be important for reconnection process itself in the diffusion region and also not be relevant for our study. We should note that the physics of the generation of such whistlers in the flux pileup region is the same regardless if the pileup is close or away from the Hall reconnection region. Whistler waves have been suggested to be associated with magnetic reconnection [Deng and Matsumoto, 2001; Petkaki et al., 2006; Wei et al., 2007; Tang et al., 2013]. Apart from Tang et al. [2013]; other studies have not confirmed precisely where whistler waves are detected. Fujimoto and Sydora [2008] have shown via kinetic simulations that whistler waves can be driven by an electron temperature anisotropy at the magnetic equator inside the pileup region of outflow with enhanced magnetic field, and they propagate downstream along the field lines. No obvious whistler waves are found in the electron diffusion region. Thus, they argued that whistler waves do not control the dissipation process in the electron diffusion region but behave as one consequence of magnetic reconnection. In addition, Fujimoto [2014] has simulated the whistler waves generated by the electron beam-driven whistler instability at the separatrix region, which propagate toward the X line. Goldman et al. [2014] have revealed the whistlers originating near the separatrix and propagating toward the lower inflow region (i.e., toward the X line) via kinetic simulations of magnetotail reconnection, which is not generated by a linear instability, but rather by Čerenkov emission of whistlers from nonlinear coherent electron phase-space holes. In our event, Cluster observed two types of whistler waves: one is in the pileup region and propagates away from the center of current sheet, and the source regions are probably at the magnetic equator; the other is around the separatrix and propagates toward the X line. This is consistent with previous simulations [Fujimoto and Sydora, 2008; Fujimoto, 2014; Goldman et al., 2014], hence inferring that the observed whistler waves are a consequence of magnetic reconnection in our case. We cannot investigate the generation mechanism of these whistlers since C4 detects the whistler waves far from the source region, and the waveform data are HUANG ET AL. WHISTLERS IN RECONNECTION REGION 6644

7 not available in our event. We suggest that the whistlers observed by C4 may be generated by the electron beam-driven whistler instability [Fujimoto, 2014] or Čerenkov emissions from electron phase-space holes [Goldman et al., 2014]. In summary, we observed two types of whistler waves in the Hall reconnection region. The first type of whistler wave in the pileup region is possibly generated at the magnetic equator by an electron temperature anisotropy and then propagates away from the source region along the field lines to downstream. The second type of whistler wave around the separatrix is possibly triggered by the electron beam-driven whistler instability or Čerenkov emissions from electron phase-space holes and propagates toward the X line. These results are consistent with recent kinetic simulations and therefore suggest that the observed whistler waves are a result of magnetic reconnection. The high-resolution particle and waveform data from MMS mission will be very useful to address the generation mechanism of the whistler waves in the separatrix region and the resulting wave-particle interactions in the future. Acknowledgments This work was supported by the National Natural Science Foundation of China ( , , , , , , and ), research fund for the Doctoral Program of Higher Education of China ( ), Program for New Century Excellent Talents in University (NCET ), China Postdoctoral Science Foundation Funded Project (2014M and 2015T80830), and the Fundamental Research Fund for the Central Universities ( kf0017). S.Y.H. and F.S. acknowledge financial support from the project THESOW, grant ANR-11-JS and from LABEX Plas@Par through a grant managed by the Agence Nationale de la Recherche (ANR), as part of the program Investissements d Avenir under the reference ANR-11-IDEX We thank the Cluster teams and Cluster Science Archive ( for the high-quality data. 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