Introduction
Dynamics of the magnetosphere are multi-scale and involve coupling between macro- (global), meso- (typical time scale 1-10 min), and micro-scale (time scale less than 10 s, spatial scale less than or equal to ion gyroradius) processes. The global interaction between solar wind plasma, carrying the frozen-in interplanetary magnetic field (IMF) and the magnetosphere includes reconnection on the day-side and increase in the magnetic flux through the magnetotail lobes. The latter leads to thinning of the magnetotail current sheet, separating the lobes, and an increase in the current density within it, which in turn makes the current sheet unstable against tearing, current driven, and/or drift instabilities (i.e., involves micro-scale, kinetic plasma processes). The instability development leads to reconnection or current disruption in the magnetotail current sheet. These processes convert the magnetic energy stored in the current sheet to plasma kinetic and thermal energies. As a consequence, meso-scale bursts (typical time scale is from 1 to several minutes) of fast convection of hot plasma with significant superthermal population appear, known as bursty bulk flows (BBFs). Reconnection and current disruption in the magnetotail current sheet also leads to global-scale reconfiguration of the magnetic field in the near-Earth plasma sheet, known as dipolarization.
In this paper we studied multi-point observations of a dipolarization front (DF), a kinetic scale boundary, separating hot, tenuous plasma of a BBF from the ambient plasma sheet. Although the DF is a micro-scale object, with typical thickness comparable to an ion thermal gyroradius, it appears as a result of the meso-scale interaction between the BBF and the plasma sheet, and, in turn, is involved in the global reconfiguration of the near-Earth plasma sheet. The goal of this paper is to combine global, meso- and micro-scale signatures to construct a simple phenomenological model of DF.
Observations
On February 27, 2009, the THEMIS probes were in the major conjunction with P1 at X=-20, P2 at X=-16, and a cluster of P3, P4, and P5 at X=-10 RE (Fig. 1). This configuration allows us to track a magnetic field disturbance (DF, inside dashed box) during its propagation along the magnetotail. Relative timing of DF detections by different probes gives us a front propagation velocity estimate of 300 km/s. Since the DF was not detected by the geostationary spacecraft in conjunction with THEMIS probes, it likely was stopped or deflected somewhere in between -11 and -7 RE. Ground-based magnetometer detected onset of a negative variation in the magnetic field X component roughly simultaneously with the DF detection in space. All-ski imagers at around footprints of the THEMIS probes observed a north-south arc appeared in a minute later than the DF was detected by P1.
Figure 2 shows detailed observations of magnetic field and plasma parameters, including the flux-tube entropy function, observed by P2 and P4 at X=-16 and -10 RE, respectively. The observations indicate that the probes observed the same entropy-depleted flux tube, propagating earthward. These objects are known as "plasma bubbles."
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| Figure 1. THEMIS probe locations in GSM XZ and XY planes and T96-model field lines at 07:50 UT on February 27, 2009. Space-born and ground-based magnetic field time series: THEMIS pseudo-AL, dBx at FSMI (blue) and YKC (red) ground stations, absolute value and Z-component of the magnetic fieldf rom THEMIS spacecraft, absolute value and p-component at the geosynchronous GOES-13 satellite between 0700 and 0900 UT. |
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| Figure 2. THEMIS-B observations from the near-Earth solar wind. The top plot shows the magnetic field strength (black) and components (colors) in the GSM frame. The second and third plots from the top show the ion and electron energy-flux spectrograms, respectively. Ion velocity (components in GSM), density, and temperature are shown in the next three plots, and the bottom plot shows the dynamic pressure. The foreshock is apparent in the data between shortly after 17:20 UT and 17:30 UT, and the foreshock cavity is the feature in the ion density between ~17:28 and 17:31 UT. |
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The multi-point observations allowed us to convert time delays into spatial distances assuming the DF propagation at a constant velocity. Thus, spatial profiles of magnetic field and its gradient may be reconstructed from the time series. Moreover, measurements of the spacecraft potential gave us an estimate of electron density with the time resolution enough to resolve kinetic structure of the DF. Figure 3 shows the reconstructed profiles and electron distribution functions, obtained ahead of the DF, on it, and behind it. The DF thickness was found to be of 500 km, which is comparable with an ion thermal gyroradius. Thus, the DF is a thin layer of enhanced current density: a vertical thin current sheet. Electron populations ahead and behind the front have different properties: electrons behind the front are more energetic. Interestingly, pitch-angle distributions of energetic electrons are different at X=-16 RE and at X=-10 RE.
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| Figure 3. Sketch of the scenario in which a foreshock cavity is swept along the magnetopause and bow shock by the discontinuity in the solar wind. The discontinuity normal direction was calculated using a minimum variance analysis from the TH-B field observations. Here, the foreshock region is shown with gray shading. The rarefied density region of the foreshock cavity is colored red, while the compressed regions flanking the cavity are colored dark blue. IMF fieldlines tangent to the bow shock are also indicated before and after the event. |
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Conclusion
We presented a THEMIS study of a dipolarization front (DF) associated with a bursty bulk flow (that was observed in the central plasma sheet sequentially at X=-20, -16, and -10 RE. The main conclusion of our study is that the DF is a kinetic-scale boundary (a thin vertical current sheet) separating the energized plasma with low flux tube entropy (a plasma bubble) from the ambient plasma sheet. The boundary thickness is comparable to an ion thermal gyroradius. The life time of this structure may exceed several minutes, i.e., is much larger than an ion gyroperiod. The DF may travel macroscale distance of hundreds of the ion gyroradii. Therefore, DF is an example of a micro-scale structure which is involved in macro-scale dynamics.
Source
Runov, A., V. Angelopoulos, M. Sitnov, V.A. Sergeev, R. Nakamura, Y. Nishimura, H.U. Frey, J.P. McFadden, D. Larson, J. Bonnell, K.-H. Glassmeier, U. Auster, M. Connors, C.T. Russell, and H.J. Singer (2010), Dipolarization fronts in the magnetotail plasma sheet, Planet. Space Sci., 59(7): 517-525, doi:10.1016/j.pss.2010.06.006.
Biographical Note
Andrei Runov is a research space physicist with the Institute of Geophysics and Planetary Physics at the University of California, Los Angeles. His current research interests are physics of magnetospheric substorms, dynamics of space current sheets, and particle energization in space plasmas.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu
