2015 THEMIS SCIENCE NUGGETS
Conjugate observations of traveling convection vortices associated with transient events at the magnetopause
by Hyomin Kim
Center for Space Science and Engineering Research, Virginia Tech
Now at Center for Solar-Terrestrial Research, New Jersey Institute of Technology
Introduction
Transient events with durations of approximately a few minutes to half an hour are common in dayside magnetometer data and observed as magnetic impulse events (MIEs) at high latitudes. A particular type of MIE is traveling convection vortices (TCVs), characterized by the vertical structure of ionospheric convection propagating longitudinally from the dayside to tailside. In a magnetogram from a single high-latitude ground station, TCVs are typically seen as a bipolar deflection (positive to negative or vice versa) mainly in the horizontal (H) component with frequencies approximately in the Pc5 range (2-7 mHz). The ionospheric pattern during TCVs is due to the effect of circular Hall currents produced by a pair of oppositely directed moving field-aligned current (FAC) system.
Generation mechanisms of transient TCVs have been attributed to interaction processes between the solar wind and the magnetosphere. Although multiple mechanisms are likely to generate TCVs, a number of observations and numerical simulations support the idea that solar wind impulsive dynamic pressure events are a major source of TCVs. Transient compression of the magnetosphere due to pressure changes at the magnetopause or the inner edge of the low-latitude boundary layer (LLBL) produce FACs into and out of the high-latitude ionosphere. Where FACs associated with TCVs map in the magnetosphere is still, however, a challenging question.
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| Figure 1. THEMIS (A, D, and E) magnetic field data (left) and satellite orbits (right) during the transient event on January 19, 2013. The symbols in the orbit plot indicate the initial locations of each satellite during the time period shown on top of the panel. All the spacecraft, which were located near the magnetopause during this event, measured inward motion of the magnetopause. THEMIS-D only briefly moved into the magnetosheath at the peak of the compression while THEMIS-A and THEMIS-E observed fluctuating motion of the magnetopause, moving back and forth between the magnetosphere and magnetosheath. |
Results
On January 19, 2013, a transient event was observed by the three THEMIS satellites (A, D, and E) near the magnetopause (Figure 1). Magnetic field data from THEMIS show the radial motion of the magnetopause at around 14:30 UT: THEMIS-D only briefly moved into the magnetosheath at the peak of the compression while THEMIS-A and THEMIS-E observed fluctuating motion of the magnetopause, moving back and forth between the magnetosphere and magnetosheath. During this event, a solar wind pressure increase (1.5 nPa over 5 min) and magnetospheric compression (8 nT over 20 min) were observed from ACE data and SYM-H index (not shown here), respectively.
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| Figure 2. Magnetic field data from ground stations in the Northern Hemisphere (left) and their conjugate stations in the Southern Hemisphere (right) during the transient event on January 19, 2013. |
Figure 2 presents stacked plots of ground magnetometer data from the Greenland network (one exception is IQA, Canada) (left panel) and their conjugate stations in Antarctica (right panel). Geomagnetic field signatures in response to the transient event begin at around 14:33 UT, showing a clear bipolar structure in both hemispheres. The stations are stacked in order of magnetic latitude, the top plot being the highest (northern stations) and the lowest (southern stations).
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| Figure 3. Ground equivalent convection inferred from the magnetic field data during the transient event on January 19, 2013. Latitudinally interpolated convection vectors are displayed in black lines overplotted with the observed convection patterns in red lines. The grey arrows indicate the approximate duration of TCV event. . |
A temporal structure of TCV signatures during this event is clearly seen in ionospheric convection flow patterns inferred from the ground magnetometer data, which represent E x B convection (Figure 3). A twin vortex is clearly seen in the Northern Hemisphere while only one vortex is observed in the Southern Hemisphere. The asymmetry of conjugacy (number of vortex, duration, latitudinal/longitudinal locations, propagation speed, etc.) is found, which needs to be further explored.
Conclusion
This study shows a transient event associated with a solar wind dynamic pressure impulse near the magnetopause and their responses in conjugate hemispheres. The transient event showing radial fluctuations of the magnetopause was observed by the THEMIS spacecraft. Geomagnetic signatures seen as TCVs in response to the transient event were observed by the ground magnetometer array in Greenland and Canada and their conjugate locations in Antarctica. This study focuses mainly on the spatial and temporal features of the TCVs in conjugate hemispheres in relation to the transient event at the magnetopause. While interhemispheric conjugacy is expected with an assumption that TCV signatures are created by FACs directed in both hemispheres, the observations suggest that there might be more complex mechanisms contributing to the asymmetrical features, perhaps due to field line mapping and/or conductivity differences.
Reference
Kim, H., C. R. Clauer, M. J. Engebretson, J. Matzka, D. G. Sibeck, H. J. Singer, C. Stolle, D. R. Weimer, and Z. Xu (2015), Conjugate observations of traveling convection vortices associated with transient events at the magnetopause, J. Geophys. Res. Space Physics, 120, doi:10.1002/2014JA020743.Biographical Note
Dr. Hyomin Kim is Assistant Research Professor at the Center for Solar-Terrestrial Research at New Jersey Institute of Technology. The work shown above was done as part of autonomous adaptive low-power instrument platform (AAL-PIP) project led by Virginia Tech for space weather research in Antarctica.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu