Abstract
AbstractSaturn's dayside aurora displays a number of morphological features poleward of the main emission region. We present an unusual morphology captured by the Hubble Space Telescope on 14 June 2014 (day 165), where for 2 h, Saturn's FUV aurora faded almost entirely, with the exception of a distinct emission spot at high latitude. The spot remained fixed in local time between 10 and 15 LT and moved poleward to a minimum colatitude of ~4°. It was bright and persistent, displaying intensities of up to 49 kR over a lifetime of 2 h. Interestingly, the spot constituted the entirety of the northern auroral emission, with no emissions present at any other local time—including Saturn's characteristic dawn arc, the complete absence of which is rarely observed. Solar wind parameters from propagation models, together with a Cassini magnetopause crossing and solar wind encounter, indicate that Saturn's magnetosphere was likely to have been embedded in a rarefaction region, resulting in an expanded magnetosphere configuration during the interval. We infer that the spot was sustained by reconnection either poleward of the cusp or at low latitudes under a strong component of interplanetary magnetic field transverse to the solar wind flow. The subsequent poleward motion could then arise from either reconfiguration of successive open field lines across the polar cap or convection of newly opened field lines. We also consider the possible modulation of the feature by planetary period rotating current systems.
Highlights
Detections of dayside reconnection signatures at Saturn provide evidence of the solar wind influence on magnetospheric and ionospheric dynamics
We present an unusual morphology captured by the Hubble Space Telescope on 14 June 2014, where for 2 h, Saturn’s FUV aurora faded almost entirely, with the exception of a distinct emission spot at high latitude
In this case study we report on Hubble Space Telescope (HST) observations of Saturn’s northern FUV auroras on 14 June 2014, together with in situ Cassini measurements of the upstream solar wind conditions and magnetopause location
Summary
Detections of dayside reconnection signatures at Saturn provide evidence of the solar wind influence on magnetospheric and ionospheric dynamics. Gérard et al [2005] investigated a cusp spot signature fixed at noon local time for at least 30 min during an interval of intermediate IMF strength following a minor compression of the solar wind These observations were generally consistent with the model of Bunce et al [2005], which estimates reconnection-driven flows and resulting polar cusp UV auroral emissions for different IMF conditions. This case was attributed to lobe reconnection, consistent with the high latitude of the emission and the expected location of the magnetopause reconnection site during southward IMF at Saturn In addition to these solar wind-driven signatures, Saturn’s auroral emissions display a rotational modulation in intensity and location. We discuss the possible modulation of the auroral signature by the rotating planetary-period current system
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