Introduction
Large-scale disturbances of Earth's magnetopause can result from pressure variations in the solar wind, flux transfer events, Kelvin-Helmholz surface waves and vortices, and several types of events resulting from phenomena in the foreshock region. The foreshock is the region upstream of the quasi-parallel bow shock characterized by suprathermal plasma traveling upstream along interplanetary magnetic field (IMF) fieldlines. This suprathermal plasma interacts with the solar wind, resulting in a variety of waves and other kinetic phenomena [e.g. Omidi et al., 2010]. One type of event is known as a foreshock cavity [e.g. Sibeck et al. 2002], which is a relatively small region of foreshock that results in compressions in the disconnected (i.e. non-foreshock) plasma around it. In other words, a foreshock cavity is a localized density cavity region surrounded by regions of enhanced density. Such features have been observed previously and are apparently not uncommon events [Sibeck et al., 2001]. Additionally, foreshock cavities can penetrate through the magnetosheath to impinge upon the magnetopause [e.g. Fairfield et al., 1990], and they can be transported by discontinuities in the IMF, making them appear as transient features as observed by spacecraft [e.g. Sibeck et al., 2000]. Here, we use multipoint THEMIS observations of a magnetopause disturbance along the dawn-side, equatorial flank on 21 November 2008. We argue that the disturbance resulted from a foreshock cavity. Interestingly, the event exhibits: 1) distinct flows in the magnetospheric plasma being displaced around it, which are very similar to those around FTEs [Korotova et al., 2008], 2) evidence of a complex boundary layer possibly involving reconnection, and 3) an abnormally strong compression of the plasma density, which peaks at >7 times the density of the near-Earth solar wind.
|
|
| Figure 1. THEMIS-E observations of the event along the magnetopause. The top plot shows magnetic field strength (black) and components (colors) in the boundary-normal, LMN frame. The second plot from the top shows the ion energy-flux spectrogram, which clearly reveals the period between ~17:21:30 and 17:23:00 that TH-E was in the sheath. The middle plot shows the ion velocity magnitude (black) and components (colors) in the LMN frame. The bottom two plots show the ion density and pressures, with the total pressure in black, magnetic pressure in green, and perpendicular plasma pressure in dark red. |
|
Click each image to enlarge.
|
Results
Four of the five THEMIS spacecraft are used in this study. In the magnetosphere, TH-A, -D, and -E, which were all near the magnetopause around 08:00 MLT at the time of the event, provide observations of the disturbance along the magnetopause. Figure 1 shows TH-E observations of the event. This spacecraft was the closest (compared to TH-A and -D) to the magnetopause and actually crossed into the sheath during the initial disturbance between ~17:21:30 and 17:23:30 UT. After this initial disturbance, TH-E observed an overall enhancement followed by a depression in the total field strength and pressure signatures that are evidence of it moving away from and then back towards the magnetopause between ~17:23:30 and 17:26:00 UT. Associated with these changes in the field, TH-E observed very fast plasma flows, with the strongest component being in the magnetopause normal direction (which is good for determining the direction of magnetopause motion) and magnitudes greater than 400 km/s. This is indicative of the sudden and extreme magnetopause motion associated with this event. The magnetopause first moved in over TH-E, then very rapidly out, bringing TH-E back into the magnetosphere, and then in once again bringing TH-E close to the magnetopause. Such motion indicates that the plasma in the sheath consisted of a rarefied density region between two compressed regions. Indeed, when TH-E is in the sheath during the initial disturbance, it observed enhanced density that peaked along the boundaries and was more than 7 times greater than the density in the solar wind. This compression factor is significantly higher than the factor of 4 expected from the Rankine-Hugoniot jump conditions. Finally, all three THEMIS spacecraft in the magnetosphere also observed very distinct and fast plasma flows, which are clear evidence of the magnetospheric plasma being diverted around the disturbance and indicate that the disturbance was moving primarily down the tail.
|
|
| Figure 2. THEMIS-B observations from the near-Earth solar wind. The top plot shows the magnetic field strength (black) and components (colors) in the GSM frame. The second and third plots from the top show the ion and electron energy-flux spectrograms, respectively. Ion velocity (components in GSM), density, and temperature are shown in the next three plots, and the bottom plot shows the dynamic pressure. The foreshock is apparent in the data between shortly after 17:20 UT and 17:30 UT, and the foreshock cavity is the feature in the ion density between ~17:28 and 17:31 UT. |
|
Click each image to enlarge.
|
THEMIS-B (TH-B) provided measurements of the near-Earth solar wind, as it was located along the dawn side near the dawn-dusk meridian just upstream of the bow shock. From Figure 2, TH-B observed clear evidence of the foreshock region and a distinct feature that is consistent with a foreshock cavity. The IMF direction, suprathermal ions, lower density, and magnetic field fluctuations observed from shortly after 17:20 UT until 17:30 UT are all consistent with TH-B being in the foreshock. At ~17:30 UT, the discontinuity in the IMF disconnected this region from the foreshock, but associated with the discontinuity was a cavity feature in the ion density consisting of a low-density region flanked by enhanced density regions. Performing a timing analysis using the velocity of the solar wind in the direction normal to the IMF discontinuity, this feature could have also resulted in the magnetopause disturbance observed by the other three spacecraft.
|
|
| Figure 3. Sketch of the scenario in which a foreshock cavity is swept along the magnetopause and bow shock by the discontinuity in the solar wind. The discontinuity normal direction was calculated using a minimum variance analysis from the TH-B field observations. Here, the foreshock region is shown with gray shading. The rarefied density region of the foreshock cavity is colored red, while the compressed regions flanking the cavity are colored dark blue. IMF fieldlines tangent to the bow shock are also indicated before and after the event. |
|
Click each image to enlarge.
|
Conclusion
Based on the evidence presented here, we conclude that the magnetopause disturbance observed by THEMIS on 21 Nov. 2008 was most likely the result of a foreshock cavity being swept along the magnetosphere by a discontinuity in the IMF (see Figure 3 for a sketch of this scenario). The effects of this foreshock cavity apparently penetrated through the sheath and impinged upon the magnetopause, resulting in its distinct in-out-in displacement as observed by TH-A, -D, and -E. The nature of a foreshock cavity, with its rarefied plasma region flanked by regions of enhanced plasma density, can explain this magnetopause motion, and just such a cavity was observed upstream of the bow-shock by TH-B after the observations by TH-A, -D, and -E. The timing of the observations is consistent with the feature moving with the IMF discontinuity in the solar wind, which is a scenario that was presented by Sibeck et al. [2000]. Interestingly, the magnetospheric plasma flows around the initial magnetopause disturbance were very similar to those previously reported around flux transfer events [e.g. Korotova et al., 2008]. Using these simultaneous THEMIS observations from the magnetosphere, magnetosheath, and solar wind, we propose that the abnormal density enhancement observed by TH-E was the result of either a combination of compression effects due to the magnetosheath and the cavity's leading-flank compression region or from some complex interaction, like reconnection, near the magnetopause along the event's clear boundary layer (see disturbed magnetic fields associated with highest densities observed by TH-E in Figure 1). Further study of this event is ongoing since foreshock cavities are common features, and their effects on the magnetosphere are still not well understood.
References:
1. Fairfield, D. H., W. Baumjohann, G. Paschmann, H. Luhr, and D. G. Sibeck (1990), Upstream pressure variations associated with the bow shock and their effects on the magnetosphere, J. Geophys. Res., 95, A4, 3773-3786.
2. Korotova, G. I., D. G. Sibeck, and T. Rosenberg (2009), Geotail observations of FTE velocities, Ann. Geophys., 27, 83-92.
3. Omidi, N., J. P. Eastwood, and D. G. Sibeck (2010), Foreshock bubbles and their global magnetospheric impacts, J. Geophys. Res., 115, A06204, doi:10.1029/2009JA014828.
4. Sibeck, D. G., K. Kudela, R. P. Lepping, R. Lin, Z. Nemecek, M. N. Nozdrachev, T.-D. Phan, L. Prech, J. Safrankova, H. Singer, and Y. Yermolaev (2000), Magnetopause motion driven by interplanetary magnetic field variations, J. Geophys. Res., 105, A11, 25155-25169.
5. Sibeck, D. G., R. B. Decker, D. G. Mitchell, A. J. Lazarus, R. P. Lepping, and A. Szabo (2001), Solar wind preconditioning in the flank foreshock: IMP 8 observations, J. Geophys. Res., 106, A10, 21675-21688.
6. Sibeck, D. G., T.-D. Phan, R. Lin, R. P. Lepping, and A. Szabo (2002), Wind observations of foreshock cavities: A case study, J. Geophys. Res., 107, A10, doi:10.1029/2001JA007539.
Source
Turner, D.L., S. Eriksson, T. D. Phan, V. Angelopoulos, W. Tu, W. Liu, W.-L. Teh, X. Li, J. P. McFadden, and K. -H. Glassmeier (2011), Multi-spacecraft observations of a foreshock-induced magnetopause disturbance exhibiting distinct plasma flows and an intense density compression, J. Geophys. Res., 116, A04230, doi:10.1029/2010JA015668.
Biographical Note
Drew L. Turner is an Assistant Researcher with IGPP at UCLA. He completed his undergraduate degree in Engineering Physics from Embry-Riddle Aeronautical University in 2005 and his Master of Science and Ph.D. in Aerospace Engineering Science from the University of Colorado at Boulder in 2008 and 2010 respectively. At UCLA, his current research interests include Earth's radiation belts and magnetopause and bow shock phenomena. He also works on space mission design, with a focus on energetic particle instrumentation.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu
