2008 THEMIS NUGGETS
SUMMARIES OF THEMIS RESEARCH
>>View 2009 THEMIS nuggets
31 December 2008.
Auroral signatures and their association with plasma sheet activities.
Thirty years ago, it was noted that the westward traveling surge of substorms is not always a continuous progression
but also occurs in discrete steps of individual intensifications. It was suggested that these auroral intensifications
could be connected to different regions in space, a suggestion that has not been confirmed through observations.
Here we present multipoint observations, using THEMIS and imagery from Polar, showing this connection. Furthermore,
we show evidence for a connection between pre-onset azimuthally spaced auroral forms and plasma sheet waves.
Wenhui Li, 8 December 2008.
Cold Dense Magnetopause Boundary Layer Under Northward IMF.
The THEMIS spacecraft detected a thick layer of relatively cold and dense
solar wind particles inside the magnetopause when the Interplanetary
Magnetic Field oriented northward, indicating that many more solar wind
particles enter the magnetosphere under such conditions, overthrowing the
previous views about the solar wind entry. Using the global magnetosphere
MHD model OpenGGCM, we were able to reproduce this event successfully
and show how the solar wind particles enter the magnetosphere
and consequently form this dense plasma layer.
Andrei Runov, 19 December 2008.
Multi-point In-situ and Ground-based Observations During Auroral Intensifications.
We present results of a THEMIS case study, showing a two-step development of a substorm. First, signatures of
magnetic reconnection were observed at X=-17 and -22 RE; 5 minutes later, a burst-like collapse of the magnetic field and
plasma energization were observed at X=-8 and -9 RE, while probes at -17 and -22 RE detected fast tailward flow.
According to a model scenario, the Earthward flow, resulting from mid-tail reconnection, triggers the instability in
the near-Earth plasma sheet, which leads to the substorm.
Sibeck, Øieroset, Raeder, and Li, 15 December 2008.
THEMIS Discovers Breach in Earth's Magnetosphere.
THEMIS team scientists recently discovered that contrary to longstanding views on how and when solar plasma enters the
Earth's magnetosphere, 20 times more solar wind plasma penetrates Earth's magnetosphere when the sun's magnetic field is
aligned with that of the Earth.
V. Sergeev, 6 August 2008.
Magnetotail response to changes of solar wind direction.
The Earth's magnetosphere, formed by the solar wind flow interaction with the Earth's magnetic field, is almost always
changing due to the permanently variable solar wind. The details of the tail response to these changes are as yet very
little known for most types of variations, although their knowledge is important to interpret THEMIS observations in the
magnetotail. The configurational changes of the current sheet central surface (the so-called 'neutral sheet', where the
tail-aligned Bx-component of the magnetic field changes its sign) are especially valuable when addressing the substorm
onset problem. We performed global MHD simulations to explore the magnetotail response to the solar wind flow directional
changes (Vz-variations), which cause significant variations of the neutral sheet shape and location.
V. Angelopoulos, 24 July 2008.
Tail reconnection triggering substorm onset.
Magnetospheric substorms explosively release solar wind energy previously stored in Earth's magnetotail, encompassing the entire magnetosphere
and producing spectacular auroral displays. It has been unclear whether a substorm is triggered by a disruption of the electrical current flowing
across the near-Earth magnetotail, at ~10 RE (RE = Earth Radius, or 6374 km), or by the process of magnetic reconnection typically seen farther
out in the magnetotail, at ~20 to 30 RE. We report on simultaneous measurements in the magnetotail at multiple distances, at the time of substorm
onset. Reconnection was observed at 20 RE, at least 1.5 min before auroral intensification, at least 2 min before substorm expansion, and about 3 min before near-Earth current disruption. These results demonstrate that substorms are likely initiated by tail reconnection.
M. Usanova, 16 July 2008.
Multipoint Observations of Magnetospheric Compression-related EMIC Pc1 Waves.
We show that enhancements in solar wind density can drive electromagnetic ion cyclotron (EMIC)
waves in the inner magnetosphere. Using three THEMIS satellites, we were able to demonstrate that
the cyclotron wave-particle interaction was radially localized to a region of high plasma density
approximately one Earth radius wide. As EMIC waves are believed to be responsible for loss of
energetic particles in near-Earth space, these results can be used to predict intensities of energetic
particle fluxes under conditions when magnetosphere is highly compressed, most importantly,
during magnetic storms. Read more.
J. Liu, 12 July 2008.
THEMIS observations of the dayside traveling compression region and flows surrounding flux transfer events.
A series of flux transfer events (FTEs) (and/or the vicinity signatures of FTEs) were captured by the five THEMIS
satellites on August 18, 2007. With the THEMIS satellites in a string-of-pearls configuration, a reconstruction of the
vicinity structure of one of the observed FTEs can be done using the data from all five spacecraft. This reconstruction
shows that the FTE perturbs the nearby magnetosphere like an obstacle moving through an otherwise stationary plasma.
A. Keiling, 11 June 2008.
Correlation of substorm injections, auroral modulations, and ground Pi2.
Auroral breakup, substorm injections in the near-Earth plasma sheet and
ground Pi2 pulsations are phenomena associated with substorms and are
the result of various physical processes. A complete understanding of
how they are interrelated is still outstanding. For a substorm, it is
found that all three phenomena showed one-to-one correlated (albeit
time-delayed) periodic oscillations in the Pi2 frequency range, clearly
demonstrating that they had a common source which controlled their
periodicity. As of now, it remains open as to what this source was.
C. Gabrielse, 11 June 2008.
Propagation characteristics of plasma sheet oscillations during a small storm.
During the first stage of its mission, the THEMIS spacecraft were
perfectly lined up in their orbits to observe and study flapping waves
in the Earth's neutralsheet. At the same time THEMIS saw a wave pass
by, it also saw fast-moving particles zoom past. As a result, this
study is the first to directly discover the cause of these seemingly
random flapping waves: bursts of fast moving particles, called bursty
bulk flows (BBFs).
V. Angelopoulos, 12 May 2008. Westward propagation of a substorm in space, ground.
Active aurorae surge poleward and westward during the course of a substorm.
The THEMIS satellites captured for the first time the westward expansion of the most intense currents flowing in space
above the westward propagating surge. The multi-satellite observations of THEMIS's string-of-pearls configuration allow
us to determine the spatial extent of the flows and currents in space.
J. Eastwood, 25 April 2008.
Explosions at the Edge of the Magnetosphere.
THEMIS satellites, along with the array of ground-based observatories, allow for the first coordinated observations of
the origin and demise of a hot flow anomaly, a deflected plasma flow caused by a discontinuity in the solar wind near the
bow shock. Observations like those from July 4, 2007 will help to uncover the effects that hot flow anomalies have on the magnetopause specifically
and the magnetosphere in general. Read more.
C-P. Wang, 18 April 2008. Penetration of the plasma sheet into the ring current region during a magnetic storm.
THEMIS satellites provide unprecedented end-to-end measurements of the equatorial plasma sheet from its outer boundary at the magnetopause
to its earthward boundary, which allows us to obtain an unambiguous determination of the radial extent of the plasma sheet and how its earthward
penetration contributes to the ring current during a geomagnetic storm.
A.T.Y. Lui, 16 April 2008. Reconstruction of a magnetic flux rope from THEMIS observations.
Magnetic flux ropes at the Earth's magnetopause play an important role in coupling the solar wind with the magnetosphere and the ionosphere. The Grad-Shafranov reconstruction technique is applied to THEMIS measurements of a magnetic flux rope at the Earth's magnetopause to reveal the surrounding current densities. It is found that it had an intense axial current density linking the solar wind with the magnetosphere and the ionosphere. Read more.
J. Liang, 8 April 2008.Intensification of Preexisting Auroral Arc at Substorm Expansion Phase Onset: Wave-like Disruption During the First Tens of Seconds.
With the deployment of the all-sky imager array of the THEMIS mission we were able to track the auroral substorm onset in unprecedentedly
great details. We find that the auroral breakup occurs near-simultaneously (within seconds) over an arc segment of ~15° longitudinal extent
and leads to wave-like structuring of the arc. The growth time of intensification is on order of 10 seconds, wavelength on order of 1000 km.
These new observations strongly suggest that plasma instability waves in the near-Earth tail are the cause of substorm expansion onset.
D. Sibeck, 4 April 2008. Flux Ropes at the Earth's Magnetopause.
According to some estimates, flux ropes at the Earth's magnetopause,
known as FTE's, may play a major role in the overall solar wind-
magnetosphere interaction. During periods of southward interplanetary magnetic field
orientation, reconnection removes magnetic flux from the dayside
magnetopause and deposits it in the Earth's magnetotail, setting the
stage for geomagnetic substorms. If sufficiently large and sufficiently
frequent, FTEs may transport the bulk of this flux. Whether or or not
they dominate the overall interaction, FTEs provide an opportunity
for magnetosheath plasma to stream into the magnetosphere, and
radiation belt particles to escape into the solar wind.
Observations from dates such as May 20, 2007 provide a closer look at flux ropes from inside, outside, and at the magnetopause.
These observations offer the opportunity to understand the response of the magnetopshere to differing solar wind conditions.
C. Chaston, 2 April 2008. Wave-driven transport of the solar wind into the magnetosphere.
The origin of matter populating near-Earth space is a basic issue necessary to understand our near-Earth environment. The outer boundary of
the Earth's magnetic field provides a natural barrier preventing the entry of solar wind particles into near-Earth space, yet these particles are
commonly observed close to Earth. However, multiple point observations from the THEMIS spacecraft have revealed the persistent occurrence of waves
at this boundary which can drive solar wind plasmas into near-Earth space at a rate sufficient to account for observations.
C.T. Russell, 29 February 2008.
THEMIS Ground-Based Magnetometers.
UCLA has provided many of the newly installed ground-based magnetometers for the THEMIS project. These magnetometers
have involved students and allowed them to make a meaningful contribution to THEMIS science. These data are now being
broadcast worldwide, in many cases, in near real time.
Please send comments/suggestions to
Emmanuel Masongsong / firstname.lastname@example.org