Alice Far-ultraviolet Spectrograph Research Articles

Stellar Occultation by Comet 67P/Churyumov–Gerasimenko Observed with Rosetta's Alice Far-ultraviolet Spectrograph

Abstract Following our previous detection of ubiquitous and absorption against the far-ultraviolet continuum of stars located near the nucleus of Comet 67P/Churyumov–Gerasimenko, we present a serendipitously observed stellar occultation that occurred on 2015 September 13, approximately one month after the comet’s perihelion passage. The occultation appears in two consecutive 10-minute spectral images obtained by Alice, Rosetta’s ultraviolet (700–2100 Å) spectrograph, both of which show absorption with column density >1017.5 cm−2 and significant absorption ( / ≈ 5%–10%). Because the projected distance from the star to the nucleus changes between exposures, our ability to study the column density profile near the nucleus (impact parameters <1 km) is unmatched by our previous observations. We find that the and column densities decrease with increasing impact parameter, in accordance with expectations, but the column decreases ∼3 times more quickly than . When combined with previously published results from stellar appulses, we conclude that the and column densities are highly correlated, and / decreases with the increasing column.

H2O and O2 absorption in the coma of comet 67P/Churyumov–Gerasimenko measured by the Alice far-ultraviolet spectrograph on Rosetta

We have detected H$_2$O and O$_2$ absorption against the far-UV continuum of stars located on lines of sight near the nucleus of Comet 67P/Churyumov-Gerasimenko using the Alice imaging spectrograph on Rosetta. These stellar appulses occurred at impact parameters of $\rho=4$-20 km, and heliocentric distances ranging from $R_h=-1.8$ to 2.3 AU (negative values indicate pre-perihelion observations). The measured H$_2$O column densities agree well with nearly contemporaneous values measured by VIRTIS-H. The clear detection of O$_2$ independently confirms the initial detection by the ROSINA mass spectrometer; however, the relative abundance of O$_2$/H$_2$O derived from the stellar spectra (11%-68%, with a median value of 25%) is considerably larger than published values found by ROSINA. The cause of this difference is unclear, but potentially related to ROSINA measuring number density at the spacecraft position while Alice measures column density along a line of sight that passes near the nucleus.

An investigation into potential causes of the anomalistic feature observed by the Rosetta Alice spectrograph around 67P/Churyumov–Gerasimenko

The Alice far-ultraviolet spectrograph in operation around the comet 67P/Churyumov–Gerasimenko on the Rosetta spacecraft experiences an anomalistic feature (AF) that is ubiquitous at comet separations less than 450km. This feature is highly temporally variable and displays no relation to any studied parameters with the exception of comet separation. This paper tests several possible causes with simulations and finds that positive ions produce a partial explanation for the anomaly, but still finds no definitive source of the AF.

Measurements of the near-nucleus coma of comet 67P/Churyumov-Gerasimenko with the Alice far-ultraviolet spectrograph on Rosetta

Aims. The Alice far-ultraviolet spectrograph onboard Rosetta is designed to observe emissions from various atomic and molecular species from within the coma of comet 67P/ Churyumov-Gerasimenko and to determine their spatial distribution and evolution with time and heliocentric distance. Methods. Following orbit insertion in August 2014, Alice made observations of the inner coma above the limbs of the nucleus of the comet from cometocentric distances varying between 10 and 80 km. Depending on the position and orientation of the slit relative to the nucleus, emissions of atomic hydrogen and oxygen were initially detected. These emissions are spatially localized close to the nucleus and spatially variable with a strong enhancement above the comet's neck at northern latitudes. Weaker emission from atomic carbon and CO were subsequently detected. Results. Analysis of the relative line intensities suggests photoelectron impact dissociation of H2O vapor as the source of the observed H I and O I emissions. The electrons are produced by photoionization of H2O. The observed C I emissions are also attributed to electron impact dissociation, of CO2, and their relative brightness to H I reflects the variation of CO2 to H2O column abundance in the coma.