Sophia Zomerdijk-Russell

Mass and energy fluxes from the solar wind can access a planet's magnetosphere through dayside magnetic reconnection. The plasma and magnetic field conditions on either side of the magnetopause determine whether reconnection is suppressed. However, magnetosheath conditions are sensitive to the shape of the magnetopause, and deviations from axisymmetry manifest in the field and flow patterns. This effect is, therefore, not captured in analytical models that assume axisymmetric boundary shapes. Here we utilize the BATSRUS MHD model to study the extent to which nonaxisymmetry alters the conditions for reconnection at Uranus's dayside magnetopause and how this varies on diurnal and seasonal timescales. Specifically, in this work we study conditions based on plasma and magnetic field parameters near the magnetopause that are known to suppress reconnection, rather than modeling reconnection itself. The magnetopause boundary arises naturally in MHD simulations instead of being prescribed. We demonstrate quantitatively the deviation of Uranus's magnetopause shape from a paraboloid and capture the resulting modification in magnetosheath field and flow patterns. Further, we show that suppression of reconnection is highly diurnal, though reconnection is never suppressed on the entire dayside magnetopause. Finally, we outline the limitations of axisymmetric analytical models and present the various features not captured in those models.

Mercury’s massive metallic core, volatile-rich surface, and extreme space environment establish it as a key end-member in understanding terrestrial planet formation and evolution. With the imminent arrival of the BepiColombo mission, the planet now stands at the threshold of a transformative exploration phase.

Over the last 40 yrs, Uranus' thermospheric temperature has decreased by ∼8 K/yr. Here we show that this is less likely to be driven by solar wind kinetic power than previously thought. We determine that the solar wind kinetic power at Uranus has been increasing since the start of solar cycle 24 in 2009, while Uranus's thermospheric temperature has continued to decline. A comparison of the ascending phases of solar cycles 24 and 25 shows an increase in solar wind power of 15%, while Uranus's thermospheric temperature decreased by 18% in the same timeframe. If the solar wind was a driver of thermospheric temperature at Uranus it would be expected that Uranus would have reversed its declining temperature trend in recent years. We have also estimated that a decline in energy sourced from the solar wind would not be sufficient to change the thermospheric temperature by the level observed.

Uranus' magnetosphere presents a unique system to examine global magnetic reconnection processes due to its location far from the Sun. We assess reconnection effectiveness during Voyager 2's Uranus flyby, using a physics‐based analytical model to calculate reconnection voltages applied to the magnetopause. This assessment of reconnection conditions at Uranus uses real‐time varying solar wind conditions as opposed to steady conditions or parameter studies. Here we show that reconnection effectiveness varied considerably over the 2 weeks before Voyager 2 passed into the magnetosphere, with voltages fluctuating between ∼0 and 76 kV; peaking and then sharply dropping off shortly before Voyager 2's entry. Variability in the voltages was found to be strongly influenced by Uranus' extreme magnetic obliquity driving diurnal modulation. Furthermore, changes in solar wind conditions, in particular density and interplanetary magnetic field strength were found to drive additional variations which disrupts the expected ∼17‐hr periodicity of the voltages.

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. References: [1] Cooling, B. M. A., et al. (2001). JGR: Space Phys, 106(A9), 18763–18775. [2] Doss, C. E., et al. (2015). JGR: Space Phys, 120(9), 7748–7763. [3] Kobel, E., & Flückiger, E. O. (1994). JGR: Space Phys, 99(12), 23617–23622. [4] Masters, A. (2014). JGR: Space Phys, 119(7), 5520–5538. [5] Masters, A. (2015b). GRL, 42(8), 2577–2585. [6] Masters, A. (2017). JGR: Space Phys, 122(11), 11,154-11,174. [7] Petrinec, S. M., et al. (1997). JGR: Space Phys, 102(A12), 26943–26959. [8] Swisdak, M., et al. (2003). JGR: Space Phys, 108(A5). [9] Zomerdijk- Russell, S., et al. (2023). JGR: Space Phys, 128(11), e2023JA031810. [10] Zomerdijk-Russell, S., et al. (under review). JGR: Space Phys.

Uranus provides a key missing piece for fundamentally understanding solar wind‐magnetospheric interactions due to its location in the outer solar system. Whether the viscous‐like interaction overtakes global magnetic reconnection as the dominant process at the magnetopause of the outer planets remains unresolved. Here, we present theoretical predictions of dayside reconnection voltages applied to the Uranian system under different magnetospheric configurations to assess the effectiveness of global magnetic reconnection in the driving of Uranus' magnetosphere. We find the median model‐predicted dayside reconnection voltage applied to Uranus' magnetosphere is 22.4 kV. Over just one full planetary rotation, the reconnection voltages are found to vary by tens of kV under Uranus' magnetospheric configuration during its solstice and equinox seasons with fixed solar wind conditions. However, we do not find a significant difference between average voltages at the different seasons, despite the large differences in magnetospheric configuration between solstice and equinox at Uranus. An increase from ∼17 to ∼31 kV in the modeled reconnection voltages is observed when the strength of the interplanetary magnetic field is increased corresponding to expected conditions during solar maximum. Our results suggest that variability resulting from the planet's diurnal rotation and changing solar wind conditions, are more important in controlling the reconnection voltages than seasonal dependencies.

Planet Mercury, with its weak internal magnetic field, is continuously exposed to an intense solar wind. This interaction becomes particularly dynamic during coronal mass ejections, resulting in a strong compression of the magnetosphere. Such events drive electrical currents within the planet, which, depending on the planetary conductivity structure, lead to secondary magnetic fields detectable outside. Analysis of these induced fields provides insights into Mercury’s interior structure.Here, we utilized data from the Helios-1 probe, recorded during a CME at 0.31 AU, to evaluate changes in solar wind conditions and their impact on Mercury's magnetosphere. We applied a semi-empirical model to estimate the external field variations and employed endmembers of radial symmetric conductivity models to calculate the range of induced magnetic fields. Our analysis highlights the influence of these variations on Mercury's upper mantle layers, taking into account both dipolar and quadrupolar components of the magnetic field. Eventually, we predict the potential induced magnetic fields at the future location of the BepiColombo spacecraft, currently en route to Mercury.

Observations of Uranus in the near‐infrared by ground‐based telescopes from 1992 to 2018 have shown that the planet's upper atmosphere (thermosphere) steadily cooled from ∼700 to ∼450 K. We explain this cooling as due to the concurrent decline in the power of the solar wind incident on Uranus' magnetic field, which has dropped by ∼50% over the same period due to solar activity trends longer than the 11‐year solar cycle. Uranus' thermosphere appears to be more strongly governed by the solar wind than any other planet where we have assessed this coupling so far. Uranus' total auroral power may also have declined, in contrast with the power of the radio aurora that we expect has been predominantly modulated by the solar cycle. In the absence of strong local driving, planets with sufficiently large magnetospheres may also have thermospheres predominantly governed by the stellar wind, rather than stellar radiation.