2019 THEMIS SCIENCE NUGGETS
THEMIS reveals a puzzling asymmetry in near-Earth space
by Stein Haaland
Max Planck Institute
University of Bergen, Norway
Introduction
When the stream of plasma emitted from the Sun - the solar wind - encounters the Earth's magnetic field, it is slowed down and diverted around it. As a result, a magnetic cavity - the magnetosphere is formed as illustrated in Figure 1. The boundary of this cavity is the magnetopause, a thin current sheet separating the Earth's magnetosphere on one side from the shocked solar wind on the other side.
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| Figure 1. Two dimensional view of the Earth's magnetosphere and its boundaries. The solar wind comes in from the left, is shocked at the bow shock, and slows down as it encounter the Earth. A region of compressed, slowed down and diverted solar wind plasma - the magnetosheath (gray shaded area) is formed between the shock front and the magnetopause (orange boundary). Behind the magnetopause, a magnetic cavity - the magnetosphere is formed. |
The magnetopause current sheet is home to a number of fascinating plasma processes such as particle acceleration, wave generation and turbulence and magnetic reconnection. Due to its importance for magnetospheric dynamics, like geomagnetic storms and substorms, the dayside magnetopause has been extensively studied by a number of spacecraft missions.
The first unambiguous in-situ observations of the Earth's magnetopause were made by the Explorer 12 spacecraft in the early 1960s. Later, in the 1980s, UCLA researchers Jean Berchem and Christopher Russell used observations from the ISEE 1 and 2 spacecraft to characterize the magnetopause [1]. They found a highly dynamic magnetopause, constantly moving back and forth in response to changes in the solar wind. Observed thicknesses were in the range 400-800 km, i.e., only a fraction of an Earth radius (ca 6400 km).
One would expect a fairly symmetric deflection of the solar wind stream around the Earth, and a magnetopause with similar properties both at dawn and dusk. Results from THEMIS, however, show an intriguing and interesting difference between the two flanks.
THEMIS contributions: Dawn-dusk asymmetries
Although the THEMIS mission was primarily designed to study magnetospheric substorm processes inside the magnetosphere, two of the probes, THEMIS B and THEMIS C, regularly crossed the dayside magnetopause during their elliptical orbit around the Earth in the years 2007 to 2009. (Later, the orbit of these two probes were lifted and eventually became the ARTEMIS mission.) Due to the orbital precession, the two probes traverse the dayside magnetopause from dawn to dusk within 6 months, thus covering the dusk flank around May and June, and the dawn flank during October and November as indicated in the Figure 1. During the period 2007 to 2009, the two THEMIS probes recorded more than 1000 crossings of the magnetopause current layer.
A group of researchers from the University of Bergen, Norway and UCLA used this extensive dataset to investigate dawn-dusk asymmetries [2]. Figure 2 shows an example of THEMIS measurements during a magnetopause crossing. The magnetic field shows a rotation indicating the magnetopause current sheet. There are also abrupt changes in the plasma density, velocity and temperature as the spacecraft crosses the magnetopause. From these measurements, it is possible to estimate the thickness and in-out motion of the magnetopause.
Results
By systematically examining the THEMIS magnetopause crossings, it was found that the magnetopause current sheet is significantly thicker at the flanks than at noon. While the thickness and motion found at the dayside magnetopause largely corroborate earlier results, the THEMIS observations show a persistent dawn-dusk asymmetry. The magnetopause is thicker at dawn, where the current sheet is approximately 1400 km wide, compared to dusk, where the current sheet is approximately 1150 km wide. The motion of the magnetopause at dawn is also more variable, with faster back-and-forth motion (average velocity was 67 km/s) at dawn than at dusk (50 km/s).
An dawn-dusk asymmetry in plasma parameters exists already in the upstream magnetosheath plasma, and probably related to large scale orientation of the interplanetary magnetic field [3]. Still, it was not possible to identify a single, unique mechanism or process responsible for the dawn-dusk asymmetries in thickness or motion of the magnetopause.
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| Figure 2. THEMIS measurements of magnetic field (panel a), plasma density, velocity and temperature (panels b to d) during a magnetopause crossing at the dawn flank in November 2007. Panel e) and f) shows th location of the magnetopause crossing in two different projections. The magnetopause current sheet can be identified from the strong rotation in the magnetic field, combined with changes in plasma parameters. Vertical dashed lines indicate intervals used to determine magnetopause thickness and velocity. |
Conclusion
The THEMIS mission has provided us with new and interesting knowledge about the terrestrial magnetopause. As a key region for transfer of momentum and energy from the solar wind to the magnetosphere, knowledge about the magnetopause structure is very important for space weather phenomena such as magnetospheric storms and substorms. A thinner magnetopause current sheet is more susceptible to instabilities and consequently allow a more frequent, and possibly larger transfer of energy from the solar wind to the magnetosphere.
These new THEMIS results highlight the importance of the dayside magnetopause, but also reveal a puzzling dawn-dusk asymmetry for which we have no simple explanation. This dawn-dusk asymmetry poses an interesting challenge for models and theories.
Reference
1. Berchem, J., and Russell, C. T. ( 1982), The thickness of the magnetopause current layer: ISEE 1 and 2 observations, J. Geophys. Res., 87( A4), 2108– 2114, doi:10.1029/JA087iA04p02108.2. Haaland, S., Runov, A., Artemyev, A., & Angelopoulos, V. ( 2019). Characteristics of the flank magnetopause: THEMIS observations. J. Geophys. Res. 124, 3421– 3435. https://doi.org/10.1029/2019JA026459
3. Stein Haaland, Andrei Runov, Colin Forsyth (eds) Dawn-Dusk Asymmetries in Planetary Plasma Environments AGU Monograph series, ISBN: 978-1-119-21632-2 October 2017 Wiley Publishers, New York.
Biographical Note
Stein Haaland is a researcher at the Max-Planck Institute for Solar Systems Research in Germany, and the Birkeland Centre for Space Science at the University of Bergen, Norway. He also teaches space weather in the high Arctic at the University Centre in Svalbard, Norway.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu