Uranus’ Coupling with the Solar Wind Through Magnetic Reconnection

A central question that remains unresolved is whether global magnetic reconnection processes continue to dominate as drivers of the outer planetary magnetospheres. Uranus offers a unique opportunity to investigate solar wind-magnetospheric interactions in the outer solar system. Here, we assess the effectiveness of magnetic reconnection in driving Uranus’ magnetospheric dynamics by modeling the associated voltage applied to the planet’s dayside magnetopause. We present theoretical predictions of these reconnection voltages under various solar wind and magnetospheric configurations using models with much heritage from the Earth and other planets [1-9] and inputs based on Voyager 2 data. Figure 1 illustrates example outputs of the model viewed from along the upstream solar wind flow direction.

Figure 1a shows the planetary magnetic field confined by the magnetopause at a selected rotational phase during Uranus’ solstice season and Figure 1b shows the magnetosheath field that results from a draped northward interplanetary magnetic field (IMF). Figure 1c identifies regions on the magnetopause where reconnection is permitted (in green) and suppressed (in black). The solid red lines indicate maxima of magnetic shear angles between the reconnecting fields – proxies for reconnection X-line locations. The reconnection voltage is then calculated using the electric field, shown in Figure 1d, to determine the potential difference along these X-lines.

In this study, we investigate how variability in the structure of the internal field with season and solar cycle modulation of the solar wind conditions impact the reconnection voltages. Voltages predicted for a full Uranian year are summarized in Figure 2. Voltages depicted in each box are calculated at eight phases over one Uranus rotation and under 100 distinct solar wind conditions that have an 11-year solar cycle dependence. Figure 2 reveals that increasing the IMF strength –mimicking conditions during solar maximum – can increase the median voltage from approximately 17 kV to 31 kV. Across one Uranian year, the median predicted dayside reconnection voltage is relatively low at 22 kV. We also note that there is no clear seasonal dependence of the reconnection voltages between solstice and equinox.

Our results suggest that any potential seasonal variations are obscured by large dependencies of the voltages on prevailing solar wind conditions and diurnal phase, as showcased by the large range of each box in Figure 2 that spans multiple orders of magnitude [10].  In the future, these results could be tested with new in situ observations of the Uranian system that could aid in revealing whether global magnetic reconnection processes dominate solar wind-magnetospheric interactions.

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