Browsing by Author "Angelopoulos, Vassilis"

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    Direct observations of a surface eigenmode of the dayside magnetopause
    Archer, M.O.; Hietala, H.; Hartinger, Michael D.; Plaschke, F.; Angelopoulos, Vassilis (Nature Publishing Group, 2019-02-12)
    The abrupt boundary between a magnetosphere and the surrounding plasma, the magnetopause, has long been known to support surface waves. It was proposed that impulses acting on the boundary might lead to a trapping of these waves on the dayside by the ionosphere, resulting in a standing wave or eigenmode of the magnetopause surface. No direct observational evidence of this has been found to date and searches for indirect evidence have proved inconclusive, leading to speculation that this mechanism might not occur. By using fortuitous multipoint spacecraft observations during a rare isolated fast plasma jet impinging on the boundary, here we show that the resulting magnetopause motion and magnetospheric ultra-low frequency waves at well-defined frequencies are in agreement with and can only be explained by the magnetopause surface eigenmode. We therefore show through direct observations that this mechanism, which should impact upon the magnetospheric system globally, does in fact occur. © 2019, The Author(s).
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    Geospace Plume and Its Impact on Dayside Magnetopause Reconnection Rate
    Zou, Ying; Walsh, Brian M.; Shi, Xueling; Lyons, Larry; Liu, Jiang; Angelopoulos, Vassilis; Ruohoniemi, John M.; Coster, Anthea J.; Henderson, Michael G. (2021-06)
    The role a geospace plume in influencing the efficiency of magnetopause reconnection is an open question with two contrasting theories being debated. A local-control theory suggests that a plume decreases both local and global reconnection rates, whereas a global-control theory argues that the global reconnection rate is controlled by the solar wind rather than local physics. Observationally, limited numbers of point measurements from spacecraft cannot reveal whether a local change affects the global reconnection. A distributed observatory is hence needed to assess the validity of the two theories. We use THEMIS and Los Alamos National Laboratory spacecraft to identify the occurrence of a geospace plume and its contact with the magnetopause. Global evolution and morphology of the plume is traced using GPS measurements. SuperDARN is then used to monitor the distribution and the strength of dayside reconnection. Two storm-time geospace plume events are examined and show that as the plume contacts the magnetopause, the efficiency of reconnection decreases at the contact longitude. The amount of local decrease is 81% and 68% for the two events, and both values are consistent with the mass loading effect of the plume if the plume's atomic mass is similar to 4 amu. Reconnection in the surrounding is enhanced, and when the solar wind driving is stable, little variation is seen in the cross polar cap potential. This study illuminates a pathway to resolve the role of cold dense plasma on solar wind-magnetosphere coupling, and the observations suggest that plumes redistribute magnetopause reconnection activity without changing the global strength substantially. Plain Language Summary A variety of magnetospheric plasma populations exist at the interface where the solar wind encounters the magnetosphere, and they can impact the efficiency of the energy transfer from the solar wind to the magnetosphere. One population of particular interest is geospace plumes, which are plumes of cold dense plasma of ionospheric origin drifting sunward toward the magnetopause during enhanced geomagnetic activity. Plumes often have a density much higher than the other magnetospheric populations and can therefore mass load the dayside magnetopause slowing down magnetic reconnection. However, whether reconnection is slowed at a local or global scale is under debate. Observationally, point measurements from spacecraft cannot reveal whether a local change affects the global reconnection and a distributed observatory is hence needed. In this study we strategically coordinate measurements made by THEMIS and Los Alamos National Laboratory spacecraft, GPS network, and SuperDARN to investigate the effect of plumes on reconnection. Our results suggest that plumes decrease the local reconnection rate at the plume longitude and increase the reconnection rate in regions adjacent to the plume. When the solar wind is stable, the global reconnection remains unchanged. Such observations illuminate a pathway to resolve the role of cold dense plasma on solar wind-magnetosphere coupling.
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    Local time extent of magnetopause reconnection using space-ground coordination
    Zou, Ying; Walsh, Brian M.; Nishimura, Yukitoshi; Angelopoulos, Vassilis; Ruohoniemi, J. Michael; McWilliams, Kathryn A.; Nishitani, Nozomu (European Geosciences Union, 2019-04-10)
    Magnetic reconnection can vary considerably in spatial extent. At the Earth's magnetopause, the extent generally corresponds to the extent in local time. The extent has been probed by multiple spacecraft crossing the magnetopause, but the estimates have large uncertainties because of the assumption of spatially continuous reconnection activity between spacecraft and the lack of information beyond areas of spacecraft coverage. The limitations can be overcome by using radars examining ionospheric flows moving anti-sunward across the open-closed field line boundary. We therefore infer the extents of reconnection using coordinated observations of multiple spacecraft and radars for three conjunction events. We find that when reconnection jets occur at only one spacecraft, only the ionosphere conjugate to this spacecraft shows a channel of fast anti-sunward flow. When reconnection jets occur at two spacecraft and the spacecraft are separated by < 1 Re, the ionosphere conjugate to both spacecraft shows a channel of fast anti-sunward flow. The consistency allows us to determine the reconnection jet extent by measuring the ionospheric flows. The full-width-at-half-maximum flow extent is 200, 432, and 1320 km, corresponding to a reconnection jet extent of 2, 4, and 11 Re. Considering that reconnection jets emanate from reconnections with a high reconnection rate, the result indicates that both spatially patchy (a few Re) and spatially continuous and extended reconnections (> 10 Re) are possible forms of active reconnection at the magnetopause. Interestingly, the extended reconnection develops from a localized patch via spreading across local time. Potential effects of IMF B-x and B-y on the reconnection extent are discussed.