The Plasma Flow Burst Observations in the Distant Magnetotail

WDS'12 Proceedings of Contributed Papers, Part II, 182–187, 2012. ISBN 978-80-7378-225-2 © MATFYZPRESS

K. Grygorov, L. Pˇrech, J. Safr´ankov´a,ˇ and Z. Nˇemeˇcek Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic.

Abstract. In the paper we present a statistical study of so-called fast plasma ﬂow (FPF) events, which periodically occurs along a wide distance range in the Earth’s magnetotail, especially when the spacecraft crosses the central plasmasheet and plasmasheet boundary layer regions. Between November 2003 and January 2004 the Wind spacecraft voyaged behind ≈ −200 RE to the L2 point and recorded far magnetotail dynamics, including such kind of events, as a response to the solar wind activity. The goal of this study is to detect and to separate the FPF events from over magnetotail dynamics data, to obtain their main parameters (velocity, plasma beta, magnetic ﬁeld magnitude, etc.). Also, we try to explain evolution of probability of their observation along the WIND spacecraft orbit.

Introduction The first model of the Earth’s magnetotail appeared in the second half of the 20th century. Reconnection-based models of the magnetosphere, proposed by Dungey [1965] required a long, around 1000 RE, magnetotail. A simple model of Milan [2004] shows that the length of the tail can vary between 400 and 4000 RE in just a few hours. The distant geomagnetic tail of the Earth’s magnetosphere began to be studied experimentally by ISEE-3 spacecraft and then by GEOTAIL, WIND and STEREO A,B spacecraft missions. Slavin et al. [1985] investigated magnetotail using the ISEE-3 magnetic field and plasma data both for average and substorm conditions in the distant magnetotail. Authors determined a low-latitude tail with diameter of 60±5 RE at distances |X| = 130–225 RE and showed correlations between VX, BY and AE index [Kamei et al., 1983], in the plasmasheet with respect to reconnection (dynamics in this region is associated with substorms). Also authors concluded that during quiet times a ”distant” X-line is located around 150 RE in downtail. During substorms the X-line is located near the Earth at 10–30 RE [e.g., Russell and McPherron, 1973; Baumjohann et al., 1999; Angelopoulos et al., 2008; Petrukovich et al., 2009]. Hones et al. [1984] has shown that fast tailward flows are often observed in the distant tail with substorm expansion. These flows are often accompanied with plasmoids [e.g. Ieda et al., 1998] and energetic particles flux increase [Richardson et al., 1987]. Nagai et al. [1997] made a statistical survey of the tailward flows associated with substorms. According to GEOTAIL data base, the duration of these flows on the average is 28 minutes (16 min for median) between −180 and −210 RE. Nagai et al. [1997] also suggested that beyond X = −100 RE the earthward flows are infrequent and only tailward flows appear in the substorm active time. Angelopoulos et al. [1994] showed a positive correlation between bursty bulk flow events (BBFs) in the inner plasma sheet and AE index. From his data set he concluded that the ratio between the tailward and earthward BBFs increases with distance from the Earth.

Observations We present a research of observations in the distant magnetotail near the L2 point between November 2003 and January 2004. Figure 1 shows the WIND trajectory in the XY plane (a) and in the YZ plane (b) in the GSM coordinate system, respectively. The black arrow indicates the direction of the spacecraft motion. At the same time, the ACE spacecraft was located between (232.2, 41, −19.9)GSE and (224, −37.2, 19.6)GSE near the L1 point in front of the Earth. Using ratio between the total velocity from WIND and ACE spacecraft we selected the fast plasma ﬂows (FPF) in the far magnetotail according to the best resolution of the ACE plasma data

182 GRYGOROV ET AL.: THE PLASMA FLOW BURST OBSERVATIONS WIND 11/2003 - 01/2004 60 a 40 The first day of the each month The interplanetary shock events ] 20 E The fast plasma flow events [R 0 GSM

Y -20 -40 -60 -240 -220 -200 -180 -160 60 XGSM[RE] 60 c b 40 40 ] ] 20 20 E E [R [R 0 0 GSE GSM Z -20 Z -20 -40 -40 -60 -60 -60 -40 -20 0 20 40 60 -60 -40 -20 0 20 40 60

YGSE[RE] YGSM[RE] Figure 1. Orbit of the Wind spacecraft in the XY plane GSM (panel a), YZ plane GSM (b) and YZ plane GSE (c) during the studied period of time. The fast plasma ﬂows events are marked by squares. The aberrated mean far magnetopause position between −160 and −240 RE XGSM is indicated by the two dash-dotted line (panel a) and by the circles (panel b and c).

(1 minute). From a statistical survey we obtained main parameters of FPF events (density, velocity, etc.), their duration, and location in relation to ZGSM. The fast plasma flows in the Earth’s magnetotail were observed during both geomagnetically active and quiet times. The magnetic reconnection in the magnetotail has been believed as one of the most possible mechanisms to generate the fast plasma flows regardless of the geomagnetic conditions. The typical durations of these fast plasma flows is 10 minutes that is identified as the Bursty Bulk Flow (BBF), but the fast plasma flows with much shorter duration (less than 5 minutes) were also observed. This transient fast plasma flows are called Plasma Flow Burst (PFB). For example, the tailward flows can vary from 8 minutes in near magnetotail to almost 30 minutes in far tail (see [Ieda et al., 1998]) according to the GEOTAIL data base. Rouquette et al. [2000] reported about the BBF event during 2.5 hours in the middle tail region. This event had maximum velocity up to 1500 km/s. The authors separated flow in the outer layers of the plasmasheet where the beta parameter is less than 1 and the central layer where beta is greater than 1. We use this suggestion in our research for identification the fast plasma flow events in the plasmasheet boundary layer (outer region) and the central plasmasheet (central layer) as well. We have calculated the total numbers of the both kinds of events and their distribution along the ZGSM coordinate in the histogram plot (Figure 4).

Methodology In our study, we used data from the MFI instruments for the magnetic ﬁeld measurements from both ACE and WIND spacecrafts, the SWE instrument onboard the ACE spacecraft and the 3DP data from the WIND spacecraft for plasma data. For the purpose of our study we compared the data from both WIND instruments with the best resolution of ACE (1 minute), taking into account the solar wind drift time between the ACE and WIND locations (see below).

183 GRYGOROV ET AL.: THE PLASMA FLOW BURST OBSERVATIONS Calibration between ACE and WIND instruments 600

400

Numbers 200

0 0.6 0.8 1.0 1.2 1.4

VWIND/VACE (solid line), BWIND/BACE (dashed line) Figure 2. Ratio between the total velocity and the total magnetic ﬁeld from WIND and ACE spacecraft near the L1 point during 10 days in October, 2005.

The FPF events duration 150

100 ______PSBL region

_ _ _ _ CPS region

Numbers 50

0 0 5 10 15 20 Minutes

Figure 3. The fast plasma ﬂow events duration in the studied period of time. Events, related to plasmasheet boundary layer (where plasma beta is less than 1) are marked by the solid line. Central plasmasheet (where plasma beta is greater than 1) is marked by the dashed line.

We also compared the data we used from both spacecraft when they were close to each other near the L1 point during 10 days in October, 2005. Figure 2 presents the ratios between total velocities (solid line) and total magnetic field (dashed line) from WIND and ACE, respectively. Based on this result, we can conclude that both spacecraft measured almost the same parameters of the plasma and their calibration is good enough. Using plasma data for protons and alpha-particles from the 3DP instrument and for elec- trons from the SWE instrument onboard WIND spacecraft we have calculated the plasma beta parameter similar to Mullan and Smith [2006]. For determination of this parameter with good time resolution we used the proton density from the 3DP instrument. It is in good agreement with the density from the SWE instrument in the studied period of time (the center of distribution between the ratio from 3DP and SWE instruments onboard WIND spacecraft lies at the value 0.97, not shown). The distance between the ACE and WIND spacecraft were around 400 RE in the GSE coordinate system along the X-axis during the studied period of time. In our study, we shifted in time the plasma and magnetic field data from the ACE spacecraft according to the XGSE position of the WIND. As a selection criterion for the FPF events we have taken the ratio between the total velocity from WIND and ACE spacecraft VWIND/VACE > 1.6. It is not too large for the registration of the FPF events with low velocity and not too small for the identification (screening) of some local fluctuation of the plasma.

184 GRYGOROV ET AL.: THE PLASMA FLOW BURST OBSERVATIONS The FPF occurrences in relation to Z GSM The FPF duration in relation to ZGSM 40 120 a b 100 CPS 30 80 PSBL

20 60 Minute Numbers 40 10 20

0 0 -20 -10 0 10 20 -20 -10 0 10 20 β β β β ZGSM[RE], <1 (Solid line), >1 (Dashed line) ZGSM[RE], <1 (Solid line), >1 (Dashed line)

The amount of time for given ZGSM 8000 c

6000

4000 Minute

2000

0 -20 -10 0 10 20

ZGSE [RE] Figure 4. Panel (a): the FPF events occurrences; (b): the FPF events duration (in minutes); (c): the amount of the time vs. the location of the WIND spacecraft in ZGSM coordinates.

Data Processing Figure 1 depicts the orbit of the WIND spacecraft during three months near the L2 point in the GSM coordinate system (a, b) and in the GSE system (c). The ﬁrst day of each month is marked by asteriks. The plus sign depicts the Earth locations. The crosses indicate the interplanetary shock events, observed by the Proton Monitor sensor onboard the SOHO spacecraft between November, 2003 and January, 2004 (available at http://umtof.umd.edu/pm/FIGS.HTML). Both kinds of the FPF events in the central plasmasheet (CPS) and in the plasmasheet boundary layer (PSBL) are marked by squares. The dot-dashed line in Figure 1(a) and circles in Figure 1 (b, c) indicate ±4.5◦ aberration of the magnetopause due to the motion of the Earth. The fast plasma ﬂow events duration (in minutes) is shown in Figure 3. The events we attribute to the PSBL region are marked by the solid line and the CPS region by the dashed line. Figure 4 depicts number of the FPF events (panel a), the total duration of the jets (panel b) and the amount of time of the spacecraft (panel c) was located near a particular ZGSM coordinate, respectively. The main parameters of the FPF events are shows in Figure 5.

Discussion and Conclusion In our paper we present the study of the plasma ﬂow event observations in the far magnetotail (between −170 and −240 RE in GSM coordinate system). For determination of the inner and outer region in the far magnetotail we have calculated the plasma beta parameter from the proton, alpha particle and electron number densities. Using the ratio between the total velocities from the WIND and ACE spacecraft VWIND/VACE > 1.6 as the selection cri-

185 GRYGOROV ET AL.: THE PLASMA FLOW BURST OBSERVATIONS Parameters of the FPF events 100

0 0.01 0.10 1.00 10.00 100.00 Plasma β 1000

100

1 0.0 0.1 0.2 0.3 0.4 Proton density cm^(-3) 100

1 400 600 800 1000 1200 1400 Total Velocity, km/s 100

1 0 200 400 600 800 T proton, eV. β<1 (Solid line), β>1 (Dashed line) Figure 5. Parameters of the FPF events. From top to bottom: Plasma beta parameter, proton number density, total velocity, proton temperature. terion we obtained the total number of both plasma flow burst and bursty bulk flow events. According to Rouquette et al. [2000] we divided our selected data set into the two group: in the outer PSBL region our method selected 266 PFB events and 32 BBF events; in the inner CPS region we obtained 159 PFB events and 27 BBF events. Their spatial distributions are presented in Figure 1 and dependent to the ZGSM in Figure 4 (panels a and b). Interestingly, that in Figure 4 (panel b) for negative ZGSM values we can see an alternation of the PSBL (solid arrows) and the CPS (dashed arrow) regions. Also it is clearly seen, that in the CPS region (for the same part of distribution as described above) the WIND spacecraft registered the long-lived BBF events. The peak of distribution around −17 RE ZGSM is correlated with the long dwell time of the spacecraft in this region. The local peak around 3 RE is connected with escaping of the WIND spacecraft from the L2 point region during January, 2004. In this time, instruments onboard spacecraft continuously observed crossings of the far magnetotail boundary and recorded the FPF and BBF events as well. During January, 2004 the trajectory of the spacecraft was (−180 .. − 235) RE lying closely to the aberrated magnetopause position (see Figure 1). During November, 2003 we observed 5.6 % from the total number of both kinds of events, during December, 2003 51.4 % and 43 % in January, 2004, respectively (see Figure 1 (panel a)). Almost all FPF events are lying between dash-dotted lines, i.e. inside the mean aberrated far tail region. The proton number density distribution in Figure 5 (second panel) is typical for CPS and PSBL regions in the far magnetotail [Slavin et al., 1985]. Both kinds of events have wide range of the total velocity and proton temperatures. There is no significant difference in parameters between the CPS and PSBL regions in Figure 5. It may be connected with weak variation of plasma pressures across the tail and small magnetic field amplitudes at such long distance from the Earth. But from the spatial distributions of the beta parameter for the FPF events according to the ZGSM we can see that far magnetotail can still keep own

186 GRYGOROV ET AL.: THE PLASMA FLOW BURST OBSERVATIONS structure with respect to the aberrated magnetopause (±4.5◦). Based on results, described above we can conclude, that:

• Both the BBF and PFB events are often observed in the distant magnetotail by WIND spacecraft.

• The main parameters of the both kinds of events are similar.

• Distribution of the BBF and PFB events is connected with magnetotail structure (by the alternation PSBL-CPS-PSBL) along the ZGSM coordinate of the trajectory of the WIND spacecraft.

Acknowledgment. The authors thank all spacecraft teams for the magnetic ﬁeld and plasma data. The data were obtained through the CDAWeb service. The present work was supported by the Czech Grant Agency under contract 205/09/0170. We also thanks to the Grant Agency of Charles University for the support (GAUK 163810).

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