Analysis of HST, VLT and Gemini coordinated observations of Uranus late 2017 : a multi-spectral search for auroral signatures

TL;DR

This study analyzes multi-spectral observations of Uranus during a solar storm in 2017, revealing widespread H3+ emissions but no localized auroras, enhancing understanding of Uranus's magnetospheric response.

Contribution

It provides new high-resolution NIR images of Uranus's ionosphere, comparing them with FUV data to improve understanding of auroral activity during a solar event.

Findings

Detected bright southern auroral spot in FUV images.

Mapped H3+ emission across Uranus's disc, brighter near the south pole.

Found no evidence of localized auroral emission in NIR images.

Abstract

On 6 Sept. 2017, an exceptional coronal mass ejection departed from the Sun toward the Earth and Uranus, whose magnetospheres are sensitive to the solar wind. The resulting interplanetary shock triggered geomagnetic storm and intense aurora at Earth the next day and was predicted by MHD models to reach Uranus around 10-11 Nov. This event provided a unique opportunity to investigate the auroral response of the asymmetric Uranian magnetosphere in its intermediate equinox-to-solstice configuration. Coordinated multi-spectral observations were acquired with the Hubble Space Telescope (HST) in the far-UV (FUV), with the Very Large Telescope (VLT) and Gemini North in the near-IR (NIR) and with Chandra in the X-ray domain. In this study, we focus on the analysis of NIR images obtained between 9 and 17 Nov. 2017 which are compared to one FUV image acquired on 11 Nov. The latter reveals a bright southern auroral spot in the H2 bands, which we use as a reference to locate auroral precipitations. The NIR images were aimed at mapping H3+ emission from the Uranian ionosphere and at updating the results built from a couple of pioneer images taken 25 years ago. These new high resolution images reveal H3+ from the whole disc although brighter near the southern pole, but show no evidence of localized auroral emission.