Introduction
Dipolarization fronts (DFs), earthward propagating structures with abrupt enhancements of the magnetic field Bz component, are often observed in the magnetotail current sheet in association with earthward bursty bulk flows (BBFs). However, the onset of earthward flows does not coincide with the arrival of the dipolarization front. In fact, observations have shown that enhancement of earthward plasma velocity, usually accompanied by enhancements of plasma density and plasma pressure, typically appears ~1 min before the dipolarization front.
The main objective of the present study is to understand the nature of precursor flows upstream of advancing dipolarization fronts. We propose that the precursor flows are caused by the gradual emergence of a new ion population composed of ions that have been accelerated and reflected by the incoming front. After that, the reflected ions would be confined to a region with the size comparable to their gyroradius over the background Bz upstream of the front. This idea is supported by both numerical simulations and statistical surveys of THEMIS observations.
Observations
THEMIS observations of ion distribution functions have already shown that earthward flows in advance of dipolarization front arrival are often caused by the appearance of a new ion population moving earthward atop a pre-existing plasma sheet component. One of the observational examples is shown below:
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| Figure 1. THEMIS P4 observations of the 26 February 2008 dipolarization front event. (a) GSM magnetic field Bx By, and Bz components. (b) Plasma flow Vx. (c) Plasma density. (d) Ion differential energy fluxes v.s. azimuthal angle in the probe spin plane, as measured by SST in the 30-45 keV range. Here 0° and 90° correspond to earthward and duskward fluxes, respectively. |
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The strong Bz enhancement, indicating the dipolarization front arrival, was at 0453:25 UT. Before front arrival, P4 had already observed gradual increases in earthward flow velocity and plasma density, as well as the appearance of an earthward streaming ion population superimposed over a pre-existing steady duskward anisotropic ion population.
All these features are typical signatures of dipolarization front precursor flows.
We make use of numerical simulation tools to understand these observed signatures. The initial condition is a kinetic equilibrium 2-D current sheet model (with a finite Bz), which self-consistently provides magnetic field and particle distribution functions everywhere within the current sheet. Test-particle simulations can then be carried out to simulate the evolution of ion distributions. Here the association between ion distributions in the initial equilibrium and those at later times is given by Liouville’s theorem. In other words, the ion distributions f(r, v, t) can be obtained by tracing ion trajectories backward in time to identify their initial locations r0 and velocities v0 within the modeled equilibrium current sheet, and equating f(r, v, t) with the corresponding f(r0, v0, t0) values.
The magnetic field adopted in the trajectory computations is assumed to be the same as the field in the initial equilibrium, except for the superposition of a hyperbolic-tangent Bz enhancement associated with the earthward propagating front. Also superposed in the model is a dawn-dusk electric field downstream of the front built up in accordance with Faraday’s law. The simulation results are as follows:
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| Figure 2. Results of our four simulation runs with different parameters. Note that all of the simulations stop as front arrives the virtual spacecraft. (a, c, e, and g) Simulated ion differential energy flux versus equatorial azimuthal angle and time. (b, d, f and h) Earthward flow velocity and plasma density shown as solid and dashed lines, respectively. The vertical dotted lines correspond to the time when earthward flow velocity becomes no less than 20% of the front propagating speed. |
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The simulations well reproduce the key observational features: a gradual appearance of the superposed ion component over pre-existing patterns, as well as increasing values of the plasma density and earthward velocity before dipolarization front arrivals. The pre-existing duskward anisotropic patterns could be understood as diamagnetic behavior in the initial equilibrium current sheet, and the appearance of the earthward-streaming ion population suggests that the ion had been accelerated by the approaching front.
The acceleration took place at the front due to the presence of the front-associated electric field when the ion started to gyrate duskward around the enhanced Bz field. After that, the ion was reflected back to the upstream region of the front. Instead of streaming earthward unrestrictedly, the ion would execute a gyration around the background Bz, which means that the superimposed ion population, as well as the excursion of the related earthward flow and enhanced plasma density, would be observed only within the ion accessibility region, which is approximately one thermal gyroradius earthward of the front.
Therefore, one would expect the time duration of the precursor flows upstream of the approaching front be proportional to the ion thermal speed, inversely proportional to the magnitude of the background Bz field, and inversely proportional to the earthward propagating speed of the dipolarization front. In other words,
δt ∝ mvT / eBnvf
The validity of the predicted relationship is examined by statistical surveys of THEMIS observations. We select dipolarization front observed by THEMIS inner probes in an automated fashion, to find 36 dipolarization front events within the 2008 and 2009 tail seasons. The relevant parameters collected from each events are used to construct the scatterplots shown below, where abscissa and ordinate of each dot are the measured precursor flow durations δt and mvT / eBnvf values of each event.
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| Figure 3. Scatterplots of 36 dipolarization front events found by THEMIS inner probes. (a) The x and y locations of each dot correspond to the δt and mvT /eBnvf values of the corresponding event. The solid line is the fitted linear regression line of these data points. (b) Same format, except that the dt values in abscissa eliminate the effects of the Bz dips preceding the fronts. |
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The linear regression fit to the data suggests a correlation coefficient of 0.71, which supports the prediction and lend further credence to the suggestion the precursor flows in advance of dipolarization fronts are composed of reflected ions confined within a thermal gyroradius around the background normal field ahead of the front.
Conclusion
One of the most interesting observational signatures associated with the earthward propagating dipolarization fronts is the appearance of earthward plasma flows well before front arrival. Both THEMIS observations and numerical simulations suggest that the precursor flows are caused by the gradual emergence of a new ion population composed of ions that have been accelerated and reflected by the front. The reflected ions would be confined to a region comparable to their thermal gyroradius over the background Bz upstream of the advancing front.
Source
Zhou, X.-Z., V. Angelopoulos, V. A. Sergeev, and A. Runov (2011), On the nature of precursor flows upstream of advancing dipolarization fronts, J. Geophys. Res., doi:10.1029/2010JA016165.
Biographical Note
Xu-Zhi Zhou is an assistant researcher with the Institute of Geophysics and Planetary Physics at the University of California, Los Angeles. His research interests focus on the particle dynamics of the thin current sheet, of the cusp region, and ULF wave-particle interactions.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu
