2009 THEMIS NUGGETS

SUMMARIES OF THEMIS RESEARCH

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Shasha Zou, 2 December 2009. Nightside Ionospheric Electrodynamics Associated with Substorms: PFISR and THEMIS ASI Observations

In this paper, we focus on observations of ionospheric electrodynamics during the substorm expansion phase using data from the newly available Poker Flat Incoherent Scatter Radar (PFISR) and THEMIS all-sky imagers (ASIs). Read more.


Andrei Runov, 26 June 2009. THEMIS observations of an earthward-propagating dipolarization front.

Multi-point measurements are needed to separate temporal and spatial effects. The four-point ESA/NASA Cluster mission has shown an abundance of traveling spatial boundaries in the magnetosphere. Because of its polar orbit, Cluster, however, spent relatively short time inside the plasma sheet. The recent NASA THEMIS mission was designed to monitor different regions of the plasma sheet simultaneously with a fleet of five spacecraft on nearly-equatorial orbits, widely separated along the magnetotail. Read more.


Andreas Keiling, 27 May 2009. Field-Aligned Current Generation by Plasma Flow Vortices.

Vorticity in the magnetosphere is of importance because it has been associated with field-aligned current (FAC) systems flowing during substorms. One such current system is the substorm current wedge (SCW). In this study, we present a comparison of magnetospheric and ionospheric vortices, and we demonstrate that the ionospheric vortices were driven by the space vortices. Additionally, evidence is given that the space vortices contributed to the field-aligned current of the SCW, and we estimate the current generation. Read more.


Robert Ergun, 16 April 2009. Observations of Double Layers in Earth's Plasma Sheet.
The THEMIS spacecraft have made the first direct observations of double layers in the plasma sheet. These double layers are observed during periods of strong magnetic fluctuations. This widespread observation of double layers not only suggests that parallel electric fields are universal in collisionless plasmas, but that many active plasma regions, including astrophysical plasmas, may be subject to strongly-nonlinear and non-ideal behavior. Read more


Reconstruction of one magnetic island observed by the TH-A probe S. Eriksson, H. Hasegawa, W.-L. Teh, and B. Sonnerup, 21 February 2009. Magnetic Island Formation at a Wavy Flank Magnetopause.

It is well known that large-scale surface waves may be excited along the Earth's flank magnetopause. Using THEMIS observations and a novel reconstruction technique, we report for the first time that small flux ropes or magnetic islands are more commonly observed on the sunward-facing sides of such magnetopause waves than on the opposite sides. The results suggest that the flank magnetopause can open up locally to the solar wind. A local compression of the barrier resulting from converging flow of neighboring large-scale flow vortices may be required for this process to proceed. Read more.


Cold Dense Magnetopause Boundary Layer Under Northward IMFWen Li, 23 March 2009. Global distribution of whistler-mode chorus waves observed on the THEMIS spacecraft.

Whistler-mode chorus waves have been received intense scientific attention due to their important roles in both acceleration and loss processes of radiation belt electrons. A new global survey of whistler-mode chorus waves is performed using magnetic field filter bank data from the THEMIS spacecraft in near-equatorial orbits. Our results confirm earlier analyses of the strong dependence of wave amplitudes on geomagnetic activity, confinement of nightside emissions to low magnetic latitudes, and extension of dayside emissions to high latitudes. An important new finding is the strong occurrence rate of chorus on the dayside at L > 7, where moderate dayside chorus is present >10% of the time and can persist even during periods of low geomagnetic activity. Read more.


Schematic diagram showing the MLT distribution of plasma waves and an example of the chorus emission Wen Li, 3 February 2009. Evaluation of Whistler-mode Chorus Intensification on the Nightside During an Injection Event Observed on the THEMIS Spacecraft.

Chorus waves are generated by the injection of plasmasheet electrons during geomagnetically disturbed conditions. THEMIS spacecraft can measure wave magnetic field, electric field, and pitch angle distribution of particles simultaneously, thus providing excellent opportunity to study generation mechanism and characteristics of whistler-mode chorus waves. We used HOTRAY code to calculate wave growth rates based on the measured electron distribution and compared the simulation results to the observed wave intensity. Read more.


The magnetopause and discontinuity observed by all five THEMIS probes H. Zhang, 31 January 2009. Interaction of a weak interplanetary shock with the Earth's bow shock.

It is well known that the interaction of an interplanetary shock with the Earth's bow shock launches a fast shock into the magnetosheath and forms a new discontinuity where the magnetic field strength and density increase, the temperature decreases and the velocity remains unchanged. Observations from the five THEMIS spacecraft showed that after interaction with the bow shock, a weak interplanetary shock turned to a discontinuity which propagated towards the Earth at around 90 km/s. Read more.


Interaction of a weak interplanetary shock with the Earth’s bow shock Jonathan Rae, 22 January 2009. Timing and localization of ionospheric signatures associated with substorm expansion phase onset.

In general, substorm onset is difficult to time and locate in the ionosphere, either through determining the first brightening of auroral forms, or by attempting to determine the first magnetic signatures of onset in inherently noisy magnetic time series. Using ground-based magnetometers and all-sky imagers, we demonstrate the capability to accurately and consistently time and locate the formation of small-scale auroral features prior to auroral break-up and during multiple activation events. Read more.


THEMIS observations from 0100 UT to 0300 UT on 20 April 2007 Xu-Zhi Zhou, 15 January 2009. Thin current sheet in the substorm late growth phase: Modeling of THEMIS observations.

During the late growth phase of the 26 February 2008 substorm event [Angelopoulos et al., 2008], THEMIS P1 and P2 satellites both clearly observed non-gyrotropic particle distributions right before the reconnection onset. Here we present that the observed ion distributions can be reproduced by the SGS model, and based on the model, the current sheet profile can be obtained by single-point observations of the particle distributions. Read more.


THEMIS observations from 0100 UT to 0300 UT on 20 April 2007 Ching-Chang Cheng, 12 January 2009. THEMIS observations of consecutive bursts of Pi2 pulsations: The 20 April 2007 event.

In the past decades, successive onsets of Pi2 pulsations in a substorm-related disturbance sequence are a common phenomenon. But some studies showed that a train of Pi2 pulsations can occur at high and low latitudes during quiet times. Thus, it is an important issue to distinguish whether quiet-time Pi2s are internally or externally excited. On 20 April 2007, the THEMIS-E spacecraft moved inbound at the dusk flank of the nightside magnetosphere where was coincidentally located to compare with ground magnetic measurements. This provides us an opportunity to attack the aforementioned issue. Read more.


Pressure Gradient Effect on a Particle Velocity Distribution Anthony Lui, 9 January 2009. Pressure Gradient Effect on a Particle Velocity Distribution.

Measurements of the three-dimensional particle velocity distribution are routinely used to examine the dynamical evolution of plasmas in space missions. An accurate determination of the bulk flow of space plasmas is crucial if the frozen-in-condition is judged by E + V x B = 0, where E is the electric field, V is the bulk flow, and B is the magnetic field. We show that the first velocity moment computed from the measured particle velocity distribution can deviate substantially from the bulk flow of the particle population when a significant pressure gradient exists near the measurement location. Read more.


All-sky camera data from Fort Yukon, Alaska Nicholas Bunch, 1 January 2009. Auroral medium frequency burst radio emission associated with the 23 March 2007 THEMIS study substorm.

On 23 March 2007 an auroral medium frequency (MF) burst radio emission was detected by the Dartmouth radio interferometer near Toolik Lake, Alaska. This MF burst temporally coincides with the onset of the March 23 THEMIS study substorm. Direction of arrival calculations show the MF burst to coincide spatially with auroral arcs to the south, observed by all-sky camera at Fort Yukon, Alaska. These observations represent the first direction of arrival measurements for MF burst, which is consistent with the direction to the eastern edge of the substorm onset location and suggests that location of MF burst radio emissions may be an effective method of locating substorm onsets, much as radio atmospherics are used to locate lightning. Read more.


Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu