2008 THEMIS NUGGETS

SUMMARIES OF THEMIS RESEARCH

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Auroral signatures A. Keiling, 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. Read more.


Cold Dense Magnetopause Boundary Layer Under Northward IMF 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. Read more.


Multi-point In-situ and Ground-based Observations During Auroral Intensifications 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. Read more.


The Earth's magnetosphere 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. Read more.


Magnetotail response to changes of solar wind direction 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. Read more.


Reconnection 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. Read more.


THEMIS 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. Read more.


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. Read more.


Gabrielse nugget 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). Read more.


Westward propagation of a substorm 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. Read more.


Hot Flow Anomalies caused by discontinuities in the solar wind 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.


The ring current and plasma sheet in the Earth's magnetosphere. 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. Read more.


Maps obtained from Grad-Shafranov reconstruction 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.


A sequence of THEMIS ASI images showing a substorm auroral breakup on Feburary 22, 2006. 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. Read more.


A schematic of a flux transfer event 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. Read more.


Transport 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. Read more.


Locations of GBO and EPO magnetometers. 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. Read more.


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