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Title: Uranus' aurorae past equinox
Author: L. Lamy, R. Prangé, K. C. Hansen, C. Tao, S. W. H. Cowley, T. Stallard, H. Melin, N. Achilleos, P. Guio, S. V. Badman, T. Kim, N. Pogorelov

The aurorae of Uranus were recently detected in the far ultraviolet with the Hubble Space Telescope (HST) providing a new, so far unique, means to remotely study the asymmetric Uranian magnetosphere from Earth. We analyse here two new HST Uranus campaigns executed in Sept. 2012 and Nov. 2014 with different temporal coverage and under variable solar wind conditions numerically predicted by three different MHD codes. Overall, HST images taken with the Space Telescope Imaging Spectrograph reveal auroral emissions in three pairs of successive images (one pair acquired in 2012 and two in 2014), hence six additional auroral detections in total, including the most intense Uranian aurorae ever seen with HST. The detected emissions occur close the expected arrival of interplanetary shocks. They appear as extended spots at southern latitudes, rotating with the planet. They radiate 5-24 kR and 1.3-8.8 GW of ultraviolet emission from H2, last for tens of minutes and vary on timescales down to a few seconds. Fitting the 2014 observations with model auroral ovals constrains the longitude of the southern (northern) magnetic pole to 104±26° (284±26°) in the Uranian Longitude System. We suggest that the Uranian near-equinoctial aurorae are pulsed cusp emissions possibly triggered by large-scale magnetospheric compressions.

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 Hubble Spots Aurorae on the Planet Uranus

hs-2012-21-a-small_web.jpg

These are among the first clear images, taken from the distance of Earth, to show aurorae on the planet Uranus. This composite image combines 2011 Hubble observations of the aurorae in visible and ultraviolet light, 1986 Voyager 2 photos of the cyan disk of Uranus as seen in visible light, and 2011 Gemini Observatory observations of the faint ring system as seen in infrared light.
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Title: Earth-based detection of Uranus' aurorae
Authors: Laurent Lamy, Renee Prange, Kenneth C. Hansen, John T. Clarke, Philippe Zarka, Baptiste Cecconi, Jean Aboudarham, Nicolas André, Graziella Branduardi-Raymont, George Randall Gladstone, Mathieu Barthelemy, Nick Achilleos, Patrick Guio, Michele K. Dougherty, Henrik Melin, Stanley W. H. Cowley, Tom S. Stallard, Jonathan D. Nichols, Gilda E. Ballester

This study is based on multi-planet multi-wavelength observations of planetary aurorae throughout the heliosphere, acquired along the propagation path of a series of consecutive interplanetary shocks. The underlying motivation to track the shocks was to increase the probability of detection of auroral emissions at Uranus. Despite several Earth-based attempts in the past few years, at Far-UV (FUV) and Near-IR (NIR) wavelengths, such emissions have never been unambiguously re-observed since their discovery by Voyager 2 in 1986. Here, we present a campaign of FUV observations of Uranus obtained in November 2011 with the Hubble Space Telescope (HST) during active solar wind conditions. We positively identify auroral signatures in several of these HST measurements, together with some obtained in 1998, representing the first images of Uranus' aurorae. We analyse their characteristics and discuss the implications for the asymmetric Uranian magnetosphere and its highly variable interaction with the solar wind flow from near-solstice (1986) to near-equinox (2011) configurations.

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