2018 ARTEMIS SCIENCE NUGGETS
Impact of Transient Foreshock Perturbations on Mid-tail Magnetopause at X ~ –55 RE
by Chih-Ping Wang
UCLA AOS
Introduction
There are many different types of transient ion foreshock perturbations (time scale of a few minutes) generated by the kinetic interaction of an interplanetary magnetic field (IMF) directional discontinuity (DD) with the quasi-parallel bow shock, including hot flow anomalies (e.g., Zhang et al., 2010), foreshock bubbles (e.g., Liu et al., 2015), and foreshock cavities. These foreshock transients consist of a low-density core, thus resulting in a perturbation of the dynamic pressure (Pdyn). As a foreshock transient propagates into the magnetosheath, its Pdyn change causes a transient magnetopause deformation. The foreshock transients are expected to propagate tailward within the magnetosheath. Previous observations indirectly suggested that they may propagate to beyond X ~ 50 RE and retain the associated Pdyn perturbations. To confirm such suggestions, here we present an event with multispacecraft observations to unambiguously show that a foreshock transient can propagate along the tail magnetosheath and have an impact on the mid-tail magnetopause at X ~ –55 RE (Wang et al., 2018).
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| Figure 1. The event on 17 April 2014. (a) The X-Y and X-Z projections of the satellite locations at the time of observing the DD. The black and purple curves indicate the projections of the magnetopause and the bow shock, respectively, at Z = 0 (top) and Y = 0 (bottom). The black arrow indicates the normal to the DD boundary and the black dotted lines indicate cuts of the DD boundary plane. The orange arrows indicate the IMF directions. (b) From top to bottom: magnetic field components, magnetic field clock (φB) and cone angles (θBx), flow velocities, ion density, and ion temperature observed by Wind, Cluster C4, Geotail (GT), and ARTEMIS P2. The vertical purple lines indicate the time of the DD. |
Observations
The foreshock transient event was on 17 April 2014. Figure 1 shows observations of WIND, Geotail, Cluster, and ARTEMIS from ~01:00 to 03:30 UT. WIND observed a DD at 01:16 UT but no transient changes in the solar wind plasma parameters. Both Geotail and Cluster C4 were in the foreshock with Geotail observing the DD at 02:52 UT and C4 observing the DD at 02:56 UT, giving an estimate of the normal direction to the discontinuity boundary, n, (0.13, 0.26, 0.95). Associated with the DD, both Geotail and C4 observed transient perturbations with a low-density and low- field-strength core, deflected flows, and heated plasma, which are consistent with the perturbations of a foreshock transient. ARTEMIS P2 was in the dawnside magnetosheath at X ~ 54REand observed the DD at 03:22 UT, together with transient perturbations in plasma moments, consistent with the dayside perturbations observed by C4 being propagated to this mid-tail location along with the DD.
As shown in Figure 1, around the time C4 observed the foreshock perturbations, THEMIS satellite P4 was quite close to C4 but was within the magnetosheath. Figures 2a–2c show the foreshock perturbations in density, flow speed, and Pdyn observed by C4. The Pdyn dropped quickly from 3.1 nPa at ~02:50 UT to its minimum of 1.7 nPa at ~02:53 UT and then quickly bounced back to its maximum of 3.5 nPa at ~02:59 UT. From the orientation of the DD shown in Figures 1a and 1b, it is expected that this Pdyn perturbation observed at C4 would affect the magnetopause near P4 at an earlier time. Figures 2d–2h show that P4 indeed observed a sudden and short (~1 min) appearance of the magnetosphere at ~02:50 UT. The magnetosphere was identified by its hot keV ion and electron thermal populations, low densities (< 1 cm–3) and low flow speeds, and large magnetic field strengths (indicated by the yellow shaded region). About ~6 min later, P4 then observed a sudden and short (~2 min) appearance of the solar wind (the blue shaded region). The solar wind can be verified by comparing the densities and flow speeds with those observed by C4 shown in Figures 2e and 2f. This 6-min difference is the same as the time difference between the Pdyn minimum and maximum observed by C4. Therefore, the brief encounters of the regions of the magnetosphere and the solar wind indicate that the magnetopause and the bow shock was moving sharply outward then inward in response to the large Pdyn changes associated with the foreshock perturbations.
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| Figure 2. (a) Number density, flow speed, and Pdyn observed by C4 around the time of DD (indicated by the purple vertical line). (b) Energy spectrum of ion and electron energy flux (eV/(cm2-s-sr-eV)), number density, flow velocity components, and magnetic field components observed by THEMIS P4. |
As shown in Figure 1a, at the time ARTEMIS P2 observed the DD, ARTEMIS P1 was also in the magnetosheath at almost the same X (~ –55 RE) as P2 but was ~2 RE closer to the magnetopause. Figure 3 compares the observations from P1 and P2. From ~03:15 to 03:21 UT when P2 observed the drop of Pdyn from 3 to 1 nPa associated with the low-density core of the transient perturbations, almost simultaneously P1 encountered the magnetosphere. This can be identified by sudden changes in plasma to very low densities and flow speeds, sharp changes in BZ due to magnetopause crossing, and the appearance of the magnetospheric keV ion and electron thermal populations. Therefore, this event unambiguously shows that as a foreshock transient propagated from the dayside magnetosheath to the tail magnetosheath at X ~ 55 RE, the resulting Pdyn perturbations continue to cause a transient outward motion of the tail magnetopause for at least 30 min.
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| Figure 3. (a) Ion number density, flow velocity Vx, Pdyn, and Bz observed by ARTEMIS P1 (red lines) and P2 (red lines). (b) Energy spectrum of ion and electron energy fluxes (eV/(cm2-s-sr-eV)) observed by P2 (top) and P1 (bottom). |
Conclusion
We presented an event from multiple spacecraft observations to show that a DD-associated foreshock transient was first observed on the dayside and later in the tail magnetosheath at X ~ 55 RE. The time difference was consistent with the estimated propagation times of the DD to different spacecraft locations, indicating the tailward propagation of the foreshock transient within the tail magnetosheath along with the DD. We showed that the associated Pdyn perturbations were significant and caused transient outward motion of the mid-tail magnetopause. Therefore, despite that a transient foreshock perturbation is only mesoscale (<~15 min and a few RE), its tailward propagation along the tail magnetosheath would allow it to have a broader global effect by impacting both the dayside and nightside magnetosphere over a considerable long duration, for example, >30 min in this event.
References
Wang, C.-P., Liu, T. Z., Xing, X., & Masson, A. (2018). Multispacecraft observations of tailward propagation of transient foreshock perturbations to mid-tail magnetosheath. Journal of Geophysical Research: Space Physics, 123. https://doi.org/10.1029/2018JA025921Liu, Z., Turner, D. L., Angelopoulos, V., & Omidi, N. (2015). THEMIS observations of tangential discontinuity-driven foreshock bubbles. Geophysical Research Letters, 42, 7860–7866. https://doi.org/10.1002/2015GL065842
Zhang, H., Sibeck, D. G., Zong, Q.-G., Gary, S. P., McFadden, J. P., Larson, D., Glassmeier, K. H., et al. (2010). Time History of Events and Macroscale Interactions during Substorms observations of a series of hot flow anomaly events. Journal of Geophysical Research, 115, A12235. https://doi.org/10.1029/2009JA015180
Biographical Note
Chih-Ping Wang is a Researcher in Dept. of Atmospheric and Oceanic Sciences at UCLA. His main research interests are the Earth’s mid-tail dynamics, particle sources and transport within the plasma sheet, the low-latitude boundary layer, the magnetosphere-ionosphere coupling, ring current physics, ULF waves, and reconnection.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu