A wedge of magnetic field lines can grow in Earth’s nightside magnetosphere during periods when the interplanetary magnetic field (IMF) points northward, and this wedge can then tear open and reconnect directly with the IMF—a process that fundamentally changes how the magnetosphere behaves. This matters because the magnetosphere’s response to the solar wind is poorly understood when the IMF is northward, even though this condition occurs roughly half the time. Most models assume the IMF simply stirs magnetic flux; the discovery that it can instead open field lines at unexpected latitudes means those models may be wrong. The researchers will use data from the Cluster and ARTEMIS spacecraft to map the wedge’s spatial extent, plasma structure, and evolution, and to test for upstream triggers. The work is fundamental science—it aims to understand a basic driver of Earth’s space environment. There is no immediate practical application. But a deeper grasp of how the magnetosphere responds to the solar wind could eventually improve predictions of space weather, which affects satellite orbits, communications, and power grids. Past fundamental research on magnetic reconnection, for example, led directly to models now used to forecast geomagnetic storms.
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Earth’s magnetosphere is driven by its interaction with the solar wind, and in particular by its response to the interplanetary magnetic field (IMF). The IMF is highly variable; the most important controller of the magnetosphere’s response to the solar wind is whether the IMF is directed “northward” or “southward”. In recent years, there have been a series of advances in our understanding of how the magnetosphere responds to periods of northward IMF, but overall our understanding remains poor. During periods of northward IMF, a “wedge” of magnetic field lines can build up in the nightside magnetosphere (the magnetotail), which is associated with a structure in the aurora (northern lights) called a transpolar arc. The “wedge” field lines are “closed”, i.e. connected in both directions to the planet, unlike higher latitude field lines in the magnetotail which are “open” (connected to the IMF). The “wedge” of closed field lines can grow to fill the magnetotail in a given sector; we have discovered that it can then interact directly with the IMF through a process called reconnection. This is an exciting discovery, because it means that the IMF then opens magnetic field lines at these latitudes, rather than simply “stirring” flux as is typically the case. Our proposed work will capitalize on these recent discoveries to develop a better understanding of the way the magnetosphere behaves when the IMF is northward. Specifically, we will: Determine the spatial extent of reconnection between the IMF and the wedge Determine the impact that the wedge plasma has on the lobe reconnection process Characterise “wedge” plasma structure three times further downtail than previously possible Investigate how the wedge evolves, once formed Test for upstream “triggers” of wedge events in order to provide valuable modelling constraints We will achieve these objectives by exploiting a large dataset of “wedge” events that we have developed, based on data from a spacecraft mission called Cluster. We will extend the spatial coverage of wedge observations by surveying data from a mission called ARTEMIS, which crosses the magnetotail much further downtail than previous wedge observations (most likely at a significant fraction of the distance downtail to the end of the wedge). Our investigation of potential “triggers” will have significance for the validity of magnetospheric models during periods of norward IMF. Collectively, these will provide a major advance in our understanding of how the magnetosphere behaves.
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