Introduction
Flux transfer events (FTEs), defined by bipolar perturbations of the magnetic field component normal to the local magnetopause, can exist in two forms, which are referred to as 'typical FTEs' (T-FTEs) and 'crater FTEs' (C-FTEs), respectively. These two types of FTEs differ in the field magnitude profiles recorded when a spacecraft crosses them on a trajectory that passes close to their central regions. For both types, the field magnitude increases in conjunction with the bipolar perturbation, but for T-FTEs the magnitude peaks near the center of the bipolar signature whereas for a C-FTE there is a crater-like dimple in the field magnitude near the zero-crossing of the bipolar signature. T-FTEs are interpreted as magnetic flux ropes with strong core fields and small transverse plasma pressure gradients and C-FTEs are interpreted as flux ropes with a significant thermal pressure near the central axis and a weaker core field.
Previous explanations of the differences of structure have suggested control by the IMF clock angle or by the magnetosheath beta (ratio of plasma pressure to magnetic pressure). It has been suggested that when the clock angle is large and favorable to quasi anti-parallel reconnection, the weak guide field produces a weak core field, resulting in a C-FTE, while for smaller clock angle component reconnection occurs and the strong guide field produces the strong core field of a T-FTE. Alternatively, it has been suggested that when the magnetosheath beta is high, more plasma can be incorporated into an FTE and plasma pressure can partially balance the magnetic tension force, forming a C-FTE. However, our THEMIS study indicates that neither the IMF clock angle nor the solar wind beta relates systematically to differences of structure. Instead we show (see discussion below) that FTEs initially form as C-FTEs and evolve into T-FTEs with a reduction of central plasma pressure resulting from transport of plasma along their axes.
Observations
An FTE can be encountered by spacecraft at a variety of impact parameters or can be sensed remotely by spacecraft passing nearby. The magnetic field signatures, especially the variation of field magnitude, are distinct along trajectories with different impact parameters (Figure 1). Therefore, by using these distinct signatures, we can infer the approximate impact parameter of an observing spacecraft. Particularly useful for distinguishing C-FTEs and T-FTES are trajectories of the T-2 type in the figure that take a spacecraft very close to the central axis of an FTE, where the central plasma properties can be analyzed.
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| Figure 1. The distinct signatures on trajectories with differing impact parameters relative to an FTE. (a-d) Typical perturbations in BN (red) and BT (black) on trajectories of types T1, T2, T3 and T4. The top two panels show schematically the typical signatures, and the bottom panels show data from actual events as examples. (e) The configuration of the plasma layers near the magnetopause and four trajectories with different impact parameters: T1, T2, T3 and T4. The plane shown is perpendicular to the axis of the FTE (ellipse), which is embedded within an expanded and distorted magnetopause current layer within which the field magnitude is weak (gray region) corresponding to the field strength dips on T2 and T3. Blue and yellow represent the magnetosheath and the magnetosphere, respectively. (f) Plots of the superposed-epoch analyses of BN, BT and N (number density) for all 3701 events on the four types of trajectories. The time interval is 6 minutes, and the reference time corresponds to the zero-crossing of BN. The color coding (green for T1, blue for T2, purple for T3 and red for T4) corresponds to that used in Figure 2e. The averaged BN and BT reproduce the expected properties of the four trajectories as shown in Figure 2a, 2b, 2c and 2d.
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On July 30, 2007, a newly-formed C-FTE was recorded simultaneously by four THEMIS spacecraft on the duskside magnetopause. Spacecraft P2 traversed the C-FTE through its center (T2 trajectory). Before P2 reached the center (i.e, before the red vertical line in Figure 2), the plasma and field signatures were those of a C-FTE in a steady state and the magnetic field and plasma data fit an equilibrium flux rope model quite well. However, the C-FTE began to evolve as P2 approached the center and evolved into a T-FTE by the time P2 reached the center (the blue vertical line). During this period of evolution, the magnetic field and plasma properties do not fit an equilibrium flux rope model. The flow along the FTE axis, which started as the C-FTE began to evolve (the red vertical line in Figure 2), appears to have transported plasma out of the C-FTE along its axis and changed the character of the FTE.
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| Figure 2. The plasma and magnetic field data from P2 for the July 30, 2007 event. From top to bottom the plots on the left show the magnetic field in an LMN coordinate system (BN in red, BM in green, BL in blue and the field magnitude BT in black), the ion number density, the ion temperature, the magnetic pressure (in blue) and thermal pressure (in red) and the total pressure (in black), the ion beta and the ion bulk velocity also in LMN coordinate system (VN in red, VM in green and VL in blue). The bottom four panels display the ion and electron energy spectra from two THEMIS instruments: SST and ESA, respectively. On the top of the plots, ‘0’ stands for the magnetosphere; ‘1’ for LLBL; ‘2’ for the transition region between the FTE and LLBL; and ‘3’ for the FTE. The red vertical line marks the beginning of the flow along the FTE axis and the blue vertical line denotes the reversal point of the BN component. Four distinct velocity distributions from different regions are shown on the right, and the times at which the measurements were acquired are marked as short blue lines on the top of the plots on the left. The coexistence of the magnetosheath plasma and magnetospheric plasma inside the FTE indicates that the C-FTE was observed not long after its formation.
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For comparison’s sake, we present a T-FTE event on May 20, 2007, which formed quite long before it was observed as is evident from absence of high energy magnetospheric particles (Figure 3). This T-FTE structure was quite steady, had no strong ion bulk flows along its axis, and fitted an equilibrium flux rope model without plasma pressure gradients.
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| Figure 3. As in Figure 2, the plasma and field data from P3 for the T-FTE encounter of May 20, 2007.
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We selected 34 T-FTEs and 22 C-FTEs identified by signatures on T2-type trajectories for a statistical study. We applied a special technique to make sure that the spacecraft trajectories passed close to the centers of these structures in order to confirm that the T-FTEs are really T-FTEs rather than grazing encounters of C-FTEs. Then we determined the IMF and solar wind conditions (Figure 4) pertinent to each case. We found that neither the IMF clock angle nor the solar wind beta relate systematically to differences of structure (C-FTE or T-FTE).
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| Figure 4. (a) The IMF clock angle distributions for the 22 C-FTEs (red) and the 34 T-FTEs. (b) The IMF clock angles for the C-FTEs (in red) and for T-FTEs (in blue) are plotted vs. the magnetosheath ion beta. All the IMF and the solar wind data were measured by ACE, Geotail or Wind when they were located in the solar wind. All the data have been shifted to the nose of the magnetopause.
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The plasma and field properties in the core regions of the 34 T-FTEs and 22 C-FTEs are compared in Figure 5. Significant differences in the properties of the two classes of FTEs are consistent with the suggestion that C-FTEs are recently formed and T-FTEs arise through evolution. The number density inside a recently formed FTE is expected to be larger than half the magnetosheath density. Loss of plasma reduces the number density. Indeed, the number density near the center of C-FTEs usually exceeds half of the background magnetosheath number density while that inside a T-FTE is typically less than half of the sheath density. The maximum field magnitudes inside most T-FTEs are stronger than the background magnetospheric fields while inside C-FTEs they are weaker than the background magnetospheric fields. The field-aligned plasma flows are stronger within C-FTEs than T-FTEs. All of these observations are consistent with an aging model.
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| Figure 4. The cumulative distributions of FTEs (typical FTEs in blue and C-FTEs in red) by (a) the ratio of number density inside FTE to that in the background magnetosheath, (b) the ratio of field strength inside FTE to that in the background magnetosphere, and (c) the parallel ion bulk velocity inside FTEs. The internal parameters of an FTE are sampled within 12 seconds of the reversal point of BN. The background sheath or magnetospheric properties are the averaged values within 60 minutes around FTE (30 min before and 30 min after). |
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Conclusions
In this paper, we present the observational evidence, based on THEMIS measurements on the dayside magnetopause, that flux transfer events (FTEs) initially form as crater FTEs and evolve into typical FTEs with a reduction of central plasma pressure resulting from transport of plasma along their axes.
Source
Zhang, H., M.G. Kivelson, K.K. Khurana, J. McFadden, R.J. Walker, V. Angelopoulos, J.M. Weygand, T. Phan, D. Larson, K.-H. Glassmeier, U. Auster (2010), Evidence that Crater FTEs are Initial Stages of Typical FTEs, J. Geophys. Res., 115, A08229, doi:10.1029/2009JA015013.
Biographical Note
Hui Zhang is an Assistant Researcher at the Institute of Geophysics and Planetary Physics, University of California at Los Angeles. His current research interests are flux transfer event associated phenomena and magnetic reconnection on the dayside magnetopause of the Earth.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu
