2010 THEMIS NUGGETS

SUMMARIES OF THEMIS RESEARCH

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Jenni Kissinger, 19 November 2010. Steady magnetospheric convection and stream interfaces: Relationship over a solar cycle.

The solar wind output from the Sun can sometimes 'reconnect' with the Earth's magnetic field, allowing plasma and energy to enter our magnetosphere. The magnetosphere responds to this input in different ways. One way is a substorm, which builds up energy and then quickly releases it, similar to a short evening thunderstorm. A steady magnetospheric convection, or SMC, is analogous to a constant, steady rain throughout the day. These intervals can be many hours long, and are thought to result from balanced reconnection between the day and night sides of the magnetosphere. Read more.




Wai-Leong Teh, 9 November 2010. Secondary Magnetic Island Observed at the Magnetopause Diffusion Region.

We present the first (THEMIS) observation of a small-scale secondary magnetic island within the ion diffusion region at the dayside magnetopause for anti-parallel field conditions. This finding indicates that magnetic islands may play an important role in the reconnection process of the diffusion regions, as suggested by recent reconnection simulation studies. Read more.



Toshi Nishimura, 26 October 2010. Categorization of the time sequence of events leading to substorm onset based on THEMIS all-sky imager observations.

We present a variation of the onset sequence we have previously reported and two less frequently observed types of onset time sequence: poleward boundary contact and Harang aurora deformation. While poleward boundary contact events also start with PBIs, the auroral oval width becomes much narrower (~2 deg MLAT) prior to onset, indicating that the plasma sheet is thin and the nightside magnetic separatrix is located closer to the near-Earth onset region. Harang auroral deformation events are not associated with an observed PBI, but the equatorward portion of a pre-existing Harang aurora bends equatorward, which indicates a rapid convection change leading to onset. All of those three categories of events suggest that new plasma intrusion toward onset location may change the pressure profile in the near-Earth region leading to onset of a near-Earth instability. Read more.



Harald U. Frey, 7 October 2010. Small and meso-scale properties of a substorm onset auroral arc.

A fortunate coincidence of a substorm onset arc observation by the Japanese Reimei spacecraft, the THEMIS satellites, and the THEMIS ground-based system allowed for a detailed study of the small- and meso-scale properties of the onset arc. A very good agreement between measured, deduced, and theoretically predicted arc properties was achieved. Read more.


Hui Zhang, 6 October 2010. Hot Flow Anomalies: Explosions at the Earth’s bow shock.

Hot flow anomalies (HFAs) are kinetic phenomena occurring near the bow shock, marked by greatly heated solar wind plasmas and substantial flow deflection. HFAs are produced by the interaction of certain upstream discontinuities with the bow shock. One outstanding question concerns how ions and electrons are heated inside HFAs. Observations of a series of HFAs and their proto-HFAs by multiple THEMIS spacecraft showed convincingly that HFAs start with two ion populations, a solar wind beam and a reflected ion distribution. Read more.


Kazue Takahashi, 17 September 2010. Plasmaspheric trapping of compressional MHD waves.

The propagation speed of MHD waves is lower in the plasmasphere than in the plasmatrough and theory had predicted that the spatial variation of the wave speed leads to trapping of compressional MHD waves in the plasmasphere. The trapped waves could be the source of magnetic field oscillations commonly observed on the ground and known as Pc3 and Pc4 pulsations. Read more.



Toshi Nishimura, 1 September 2010. Pre-onset time sequence of auroral substorms: Coordinated observations by ASIs, satellites and radars.

Using two conjunction events of the THEMIS imagers and spacecraft as well as the SuperDARN and PFISR radars, we show that longitudinally narrow flow bursts in the nightside polar cap can precede PBIs that are followed by equatorward moving N-S arcs, including those leading to substorm onset instability within the near-Earth plasma sheet. We also investigated differences between N-S arc sequences which do and do not lead to substorm expansion onset. Each N-S arc leads to small intensification of the growth phase arc, and the pre-existing growth phase arc is much brighter than at the times of non-onset related N-S arcs, when the onset-related N-S arc reaches the equatorward portion of the auroral oval. Read more.


Hiroshi Hasegawa, July 2010. Magnetic Flux Rope Generation by Magnetopause Reconnection at Multiple Sites.

The THEMIS spacecraft for the first time detected a magnetic flux rope that emerged from multiple reconnection sites on the dayside magnetopause. Such a flux rope could be regulating the transport of solar wind energy into the magnetosphere during summer/winter when the geomagnetic dipole axis is tilted toward or away from the Sun. Read more.


Andrei Runov, 9 June 2010. Dipolarization fronts in the magnetotail plasma sheet.

In this paper we studied multi-point observations of a dipolarization front (DF), a kinetic scale boundary, separating hot, tenuous plasma of a bursty bulk flow (BBF) from the ambient plasma sheet. Although the DF is a micro-scale object, with typical thickness comparable to an ion thermal gyroradius, it appears as a result of the meso-scale interaction between the BBF and the plasma sheet, and, in turn, is involved in the global reconfiguration of the near-Earth plasma sheet. The goal of this paper is to combine global, meso- and micro-scale signatures to construct a simple phenomenological model of DF. Read more.


Oleksiy Agapitov, 8 June 2010. Chorus source region localization in the Earth's outer magnetosphere using THEMIS measurement.

The technique proposed allows one to distinguish the source properties from the effect of wave propagation through the media with random fluctuations. An application of this technique makes it possible to estimate the spatial scale of the wave phase coherence. Evaluation of these characteristics is very important for the study of propagation of whistler waves that play significant role in the processes of wave-particle interaction in the radiation belts. Read more.


Jonathan Rae, 26 May 2010. Optical characterisation of the growth and spatial structure of a substorm onset arc.

We used THEMIS All-Sky Imager data to look in detail at the auroral forms that are the first ionospheric signatures of substorm expansion phase onset, revealing a wave instability operating in the near-Earth magnetosphere. Read more.


Hui Zhang, 30 March 2010. Evidence that crater flux transfer events are initial stages of typical flux transfer events.

Using data from multiple THEMIS spacecraft at the dayside magnetopause, we present observational evidence that flux transfer events (FTEs) initially form as crater FTEs, within which non-uniform plasma pressure peaks near the flux rope axis. Subsequently, as a result of transport of plasma along their axes, they evolve into typical FTEs in which transverse plasma pressure gradients are not significant. Read more.


Miho Saito, 24 March 2010. Magnetic field minimum in the magnetotail.

The THEMIS in-formation flight identified the minimum of the equatorial magnetic field in the magnetotail for the first time. Read more.


Matt Fillingim, 1 March 2010. Polar UVI and THEMIS GMAG observations of the ionospheric response to a hot flow anomaly.

We present observations of the ionospheric response to a hot flow anomaly interacting with the magnetosphere. Surprisingly, we find a very large difference bewteen the speed at which the field aligned current signature propagates and the speed at which the region of auroral emission propagates. Read more.


Zuyin Pu, 26 February 2010. THEMIS Observations of Magnetotail Reconnection Initiated Substorms on February 26, 2008.

This paper presents further studies of two substorms. The measurements of bi-directional anisotropy of energetic electrons indicate that midtail reconnection clearly occurred ahead of auroral signatures of both two substorms. The pseudobreakups appeared about 1-2 min immediately after the reconnection onsets; while the expansion phase onsets occurred within about one min when earthward flows arrived at the near-Earth tail. Read more.


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