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).
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.