Ftes, Pressure Pulses Or Surface Waves on the Magnetopause Surface?

WDS'10 Proceedings of Contributed Papers, Part II, 168–175, 2010. ISBN 978-80-7378-140-8 © MATFYZPRESS

FTEs, Pressure Pulses or Surface Waves on the Magnetopause Surface?

O. Tkachenko, J. Safr´ankov´a,ˇ and Z. Nˇemeˇcek Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic.

Abstract. The paper analyzes one pass (August 26, 2007) of the THEMIS spacecraft across the dayside low–latitude magnetopause. A gradually decreasing upstream dynamic pressure caused an outward magnetopause motion and the outbound orbiting spacecraft spent more than two hours in this region. THEMIS B serving partly as a magnetosheath monitor observed several magnetic field rotations, whereas THEMIS A scanned the inner magnetosphere. We discuss the plasma and magnetic field data with motivation to identify sources of observed quasiperiodic plasma transients that are accompanied with bipolar BN signatures. Such events at the magnetopause are usually attributed to pressure pulses coming from the solar wind, foreshock fluctuations, flux transfer events or surface waves.

Introduction Quasiperiodic fluctuations of magnetic field and plasma parameters at the magnetopause are often attributed to flux transfer events (FTEs), surface waves or recurrent pressure pulses coming from the solar wind or from the foreshock. FTEs were identified in the dayside magnetopause by the ISEE 1 and 2 [Russell and Elphic, 1978, 1979] and by HEOS 2 spacecraft [Haeren- del et al., 1978] as regularly occurring magnetic field signatures. Their characteristic features are a bipolar oscillation in the boundary normal component of the magnetic field (BN ), mixtures of magnetosheath and magnetospheric plasmas, and either enhancements or crater–like variations of the magnetic field strength at the event center. Statistical surveys of the occurrence of FTEs showed that they are observed predominantly when the magnetosheath or interplanetary magnetic field (IMF) points southward [e.g., Berchem and Russell, 1984; Kuo et al., 1995], strongly suggesting an association with the time–dependent magnetic reconnection process that was proposed as fundamental to the coupling of mass and energy between the solar wind and magnetosphere [Dungey, 1961]. Follow–up studies [e.g., Lockwood, 1991; Russell et al., 1996] have found that the mean time interval between FTE signatures is of order of a few minutes. The similar characteristic particle and field signatures at the dayside magnetopause were attributed to magnetopause motions in response to transient changes in the dynamic pressure of the solar wind [e.g., Sibeck, 1990, 1992; Sibeck and Smith, 1992]. Solar wind/magnetosheath pressure pulses may play a role in the triggering of reconnection at the magnetopause, and thus they may be related to the formation of FTEs [Potemra et al., 1992]. The properties and structure of FTEs have been a subject of many studies in the last years, however, a significant progress started with new spacecraft missions as Cluster and THEMIS. For example, Cluster contributed: (1) to discussion of the differences in the signatures between closely separated (∼ 600 km) spacecraft that indicated the substructure of the FTE on this scale [Owen et al., 2001]; (2) to a better understanding of the FTE formation and to determine the cross–sectional profiles [Fuselier et al., 2005; Hasegawa et al., 2006]; and (3) to studies of the diffusion region of magnetic reconnection at the magnetopause [e.g., Vaivads et al., 2004; Retino et al., 2006]. Multipoint measurements by the THEMIS spacecraft allowed the identification of FTEs, investigation of their properties, and their comparison with simulation results as it was shown in first papers from this mission [Sibeck et al., 2008; Liu et al., 2008; Zhang et al., 2008]. Furthermore, an example of magnetopause surface waves excited by the Kelvin–Helmholtz

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(KH) instability was referred by Eriksson et al. [2009]. The authors reported that the trailing edges of KH waves are commonly related to small–scale magnetic islands or FTEs during the growth phase of these surface waves. The FTEs typically show a characteristic bipolar BN structure with an enhanced total pressure at their center. The authors confirmed the presence of a magnetic island within the low-latitude boundary layer adjacent to the magnetopause. The island was associated with a small plasma vortex and both features appeared between two large–scale plasma vortices. The authors propose that the observed magnetic islands may have been generated from a time–varying reconnection process in a low ion plasma beta and low field shear environment at the sunward edge of the growing KH waves where the local magnetopause current sheet may be compressed by the converging flow of the large–scale plasma vortices as suggested by numerical simulations of the KH instability [e.g., Nykyri and Otto, 2001; Nykyri et al., 2006]. In this paper, we present a study of one-hour pass of the THEMIS spacecraft through the dayside low-latitude magnetopause and provide the investigation of the nature of three transient events on the magnetopause exhibiting bipolar fluctuations in the magnetic field. The registered plasma transients are usually associated with FTEs but our analysis proposes another possible scenario and discuss both explanations in detail.

Overview of THEMIS observations and solar wind conditions We used a fleet of the THEMIS spacecraft launched into a near–equatorial orbit on Febru- ary 17, 2007 [Angelopoulos, 2008]. All five spacecraft aligned in a line crossed the low–latitude magnetopause and the adjacent layers twice a day with short–time lags between the spacecraft before a modification of their orbits at the end of 2007. Each THEMIS spacecraft carries an identical instrumentation including a fluxgate magnetometer (FGM), an electrostatic analyzer (ESA), a solid state telescope (SST), a search coil magnetometer (SCM), and an electric field instrument (EFI). In our analysis, we used magnetic field measurements provided by the FGM instrument [Auster et al., 2008] and plasma measurements of the ESA spectrometer [McFadden et al., 2008]. For our investigation we selected three of magnetopause crossings from the plasma sheet to the magnetosheath that were identified on August 26, 2007. To analyze such observations, an actual IMF orientation is needed, thus our investigation is supplemented by the data from ACE spacecraft. Figure 1 shows the part of the THEMIS orbit during the selected events (from 0740 to 0840 UT). Figure 2a shows an overview of measurements of the solar wind monitor (dynamic pressures, p and IMF propagated to the THB location in 2th and 3th panels from the ACE spacecraft, respectively). Since the investigated processes would be determined by the magnetic field at a close proximity of the magnetopause, we will use THB observations whenever located in the magnetosheath. This measurement is shown in the top panel of Fig. 2a.

Detailed analysis of one event The original idea behind this analysis was an investigation of FTE properties, so we have recalculated THEMIS magnetic fields into boundary normal coordinates. Several bipolar structures in the BN component can be identified from 0740 to 0840 UT. The most distinct of them are numbered (numbers 1, 2, and 3) and shadowed in Fig. 2b. Events 1-3 exhibit clear bipolar BN signatures that suggest that they can be considered as possible FTE candidates. Unfortunately, THEMIS changed its operational mode between events 1 and 2 and starting from 0800 UT, the high-resolution plasma data are not available, thus we will concentrate our attention to the event 1. Magnetic field and plasma observations of THB are shown in Fig. 3a, THC data in Fig. 3b. THD and THE observations resemble those of THC, thus they are not shown. THA was in the magnetosphere and did not observe notable variations of plasma and magnetic field. THB was in the magnetosheath, probably in a close proximity of the magnetopause because the velocity is rather small. The BZ component of the magnetic field was

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THEMIS 2007-08-26 -40

-20

[Re] 0 GSE Y

-8.5 BCD

20 -8.0 E -7.5 A -7.0 9.0 8.5 8.0 7.5 7.0 6.5 6.0 40 60 40 20 0 -20 XGSE [Re] Figure 1. Projections of one day spacecraft orbits onto the equatorial plane with the nominal 3 location (B=5 nT, BZ =0 nT, V =400 km/s, n=5 cm− ) of the bow shock [Jerab et al., 2005] and the magnetopause [Shue et al., 1998]. An enlarged view of the THEMIS spacecraft location at 0800 UT is inserted at the bottom part of the figure. small and negative but it turns to positive values at ≈ 0750 UT. This turn is complemented with an increase of the magnetic field magnitude and velocity components. Since the spacecraft is near the equatorial plane, an increase of the vX and vY components could be connected with the overall increase of the magnetosheath speed. On the other hand, vZ values of the order of 100 km/s suggest an encounter with the reconnection flows. Nevertheless, a principal IMF component is BY (see Fig. 2a) and it is also the largest component of the magnetosheath magnetic field at the THB location. According to Nemecek et al. [2003], such IMF orientation displaces the reconnection site to high latitudes dawn the cusp in the northern hemisphere, even if the

40 THEMIS 2007-08-26 from 07:40:00 to 08:40:00 2007-08-26 from 07:40:00 to 08:40:00 20 60 N 40 0 THB B 20 -20 -40 1 2 3 0 40 -20 20 THEMIS B [nT] N -40 0 6 4 THC B -20 2 -40

0 40 20

-2 N ACE B [nT] -4 0

-6 THD B -20 2.0 -40 1.5 40 20 1.0 N 0 0.5 THE B ACE P [nPa] -20 0.0 -40 0740 0800 0820 0840 0740 0800 0820 0840 UT UT a b

Figure 2. The left panels from top to bottom: the magnetic ﬁeld from THEMIS and ACE, respectively (gray color BX , dotted line BY , black color BZ ), the solar wind dynamic pressure measured by ACE. The right panels: BN components of the magnetic ﬁelds of THB, THC, THD, and THE. The shadowed numbered areas correspond to selected events.

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2007-08-26 2007-08-26 60 60 40 40 20 20 0 0 THC B GSM

THB B GSM -20 -20 -40 -40 60 60 40 40 20 20 0

0 THC B LMN THB B LMN -20 -20 -40 -40 150 150 100 100 50 50 0 0

THC V GSM -50 THB V GSM -50 -100 -100 -150 -150 10 20 8 15 6 THC N

THB N 10 4 5 2 0 0 5 105 107 10 4 6 104 10 10 106 3 5 103 10 10 5 10 2 102 10 104 THC Ei THB Ei 4 1 10 1 3 10 10 10 100 103 100 0748 0750 0752 0754 0756 0758 0748 0750 0752 0754 0756 0758 UT UT a b

Figure 3. Detail observations of magnetic field and plasma parameters of THEMIS B (a) and C (b) during the event 1. From top to bottom: three components of the magnetic field in GSM and LMN coordinate systems in nT; three components of the velocity in km/s; the ion density 3 in cm− ; and ion spectra in eV (gray color BX , BM , VX ; dotted line BY , BL, VY ; black color BZ ,BN , VZ) magnetosheath magnetic field has a minor southward component (note that upstream monitor in Fig. 2a shows a weak northward component). In such a case, the turn of BZ that is a minor magnetosheath component from the southward to northward orientation would displace the merging site only slightly around the cusp. Since a possible reconnection site would be at high altitudes, i.e., far away from THB, one would expect to see southward streaming ions, whereas THB observed changes of the sign of the vZ velocity component. We will return to this point later in the text. By contrast, THC (Fig. 3b) in the magnetosphere observed very similar patterns in the magnetic field but the velocity behaves differently. The spacecraft started in a region of the low plasma density and velocity and high temperature. At ≈ 0750:30 UT, it entered a boundary layer occupied by the cold dense plasma. The vX and vY velocity components are consistent with the expected convection but the vZ component suggests reconnection northward of the spacecraft. This flow continues for more than 3 minutes, whereas the bipolar patterns in the BN component of the magnetic field last for 1 minute only. Moreover, a very similar flow is encountered at ≈ 0754 UT, without any signature in the magnetic field. THD and THE observed exactly the same structures in the flow and magnetic field as THC but it is not surprising because a mutual separation of these three spacecraft is less than 0.2 RE. A comparison of the BN components of magnetic field from all four spacecraft in the boundary normal coordinate system is shown in Fig. 4. It can be seen that the normal components exhibit bipolar structures of the same amplitude, phase, and duration.

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THEMIS 2007-08-26 from 07:48:00 to 07:58:00 MP 30

20 LLBL N 10 MSH 0 THB B -10 -20 E 30 C 20 B D A z

N 10

0 x THC B -10

-20 30 (b) 20

N 10 MP

0 THD B -10 LLBL -20 MSH 30 20 E A

N 10 C D B z 0 THE B -10 x -20 0750 0752 0754 0756 UT (a) (c)

Figure 4. The left panels show BN components of the magnetic fields of THB, THC, THD, and THE (a). The right two panels illustrate the actual configuration of the magnetospheric field prior to (b) and after (c) the change of the BZ direction in the XY plane. The points with corresponding letters stand for the locations of the THEMIS spacecraft.

There are two possible interpretations of the observed changes: the first scenario expects that a turn of the magnetosheath BZ component displaced the high-latitude reconnection site and the reconnection outflow crossed the spacecraft locations. The bipolar structures in the THEMIS magnetic field are caused by a deformation of the magnetopause connected with the transient decrease of the magnetosheath density (see the fourth panel in Fig. 3a) that is associated with the turn of the magnetic field. The most peculiar feature of the event 1 is change of the vZ sign observed by THB during the event. Since this change is observed by THB only, it is the magnetosheath feature. We already suggested that a source of accelerated flows is reconnection northward of the spacecraft. This flow encounters the magnetopause deformation and it can change its direction. A similar effect was observed at the dayside magnetopause by Shue et al. [2009]. The authors show that an interaction of the magnetosheath flow with the magnetopause deformation can lead to the flow reversal, and even the sunward flow can be observed. Since only one spacecraft was in the magnetosheath during our event and none crossed the magnetopause, we cannot perform a similar analysis but the Shue et al. [2009] paper clearly shows that the change of the velocity direction of the flow around the magnetopause due to its deformation can be expected. It should be stressed out that spacecraft were located on the morning side near magnetic equator (ZGSM ∼ 0) where magnetosheath plasma flow spread out to both hemispheres from the subsolar point. In such case THB could observe both northward and southward ion flow. This

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flows are accelerated at the magnetopause deformation caused by the turn of the BZ component in the magnetosheath. Bipolar fluctuations of the BN component detected by THC, D, E were the result of motion of the magnetopause deformation along spacecraft. The second possible interpretation of our event is that a FTE crossed the location of the spacecraft. A sketch of the actual configuration of the magnetospheric field during such scenario is presented in Fig. 4b. Magnetic field signatures are consistent with such explanation because THB-E observed bipolar changes of the BN component accompanied with the increase of the magnetic field strength. The region of a depressed density correspond to a region of enhanced magnetic field. A typical dimension of FTE would be about 1 RE and the duration of observations (∼ 80 s) together with the mean velocity (∼ 100 km/s) lead to a similar value. Consequently, the spacecraft separated by ∼ 0.15 RE can observe very similar changes of the velocity and magnetic field. On the other hand, the duration of the enhanced plasma flow in THC data is much longer than the duration of the flux rope deduced from the magnetic field observation and this feature contradicts to the interpretation of the event in terms of FTEs. THEMIS probably encountered a burst of reconnection followed by an interval of more steady reconnection. Another feature that is apparently inconsistent with a FTE encounter is the change of the vZ sign observed by THB in the magnetosheath. However, the change of the BZ sign from southward to northward could induce reconnection of the IMF and FTE flux tube lines. The magnetospheric geometry under such assumption is shown in Fig. 4c. We can conclude that the observed features are almost consistent with both possible interpretations. However, we prefer the explanation in terms of the magnetopause deformation because it includes a rotation of BZ in the magnetosheath and can explain enhanced flows of low energy plasma that are not accompanied with the magnetic field changes. Such structures are observed frequently between 0730 and 0830 UT (see Fig. 1).

Conclusion We have identified all observed transients as crossings of the reconnection outflows. The event number 1 is characterized by bipolar changes of the BN component and by the enhance- ment of the magnetic field strength that are considered as typical FTE signatures [Hasegawa et al., 2006] but we concluded that this event is an encounter with the reconnection layer that is reformed due to the change of the sign of the magnetosheath BZ magnetic field component observed by THB. We would like to point out that other two events with bipolar structures (numbers 2 and 3 in Fig. 2b) could not be analyzed in detail due to lack of fast plasma moments but the magnetic field signatures are very similar including reversal of the magnetosheath BZ component. We believe that their nature is the same as that discussed for the event 1.

Acknowledgments. We acknowledge NASA contract NAS5-02099 and V. Angelopoulos for use of data from the THEMIS Mission. Specifically, we acknowledge C. W. Carlson and J. P. McFadden for use of ESA data, K. H. Glassmeier, U. Auster and W. Baumjohann for the use of FGM data provided under the lead of the Technical University of Braunschweig and with financial support through the German Ministry for Economy and Technology and the German Center for Aviation and Space (DLR) under contract 50 OC 0302. The present work was partly supported by the Czech Grant Agency under Contracts 205/09/0170, 205/09/0112, and 202/08/H057 and partly by the Research Plan MSM 0021620860 that is financed by the Ministry of Education of the Czech Republic. O. Tkachenko thanks also to the Grant Agency of Charles University for a support (GAUK 163810).

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