By Paul D. Feldman, Department of Physics and Astronomy, Johns Hopkins University
On February 22, 2016, Alessandra Rotundi, the PI of the GIADA (Grain Impact Analyzer and Dust Accumulator) instrument on Rosetta, sent out a report describing a large increase in the number of dust particles detected between UT 10:00 at 13:00 on February 19, asking whether or not any of the other Rosetta instruments had concurrent observations of an outburst event at this time. In the following weeks reports came in from multiple instruments as well as the spacecraft’s startrackers, with the OSIRIS imagers clearly showing a strong brightening of the coma in front of the nucleus beginning at UT 09:40 spreading to the entire field-of-view in successive images. The Alice far-ultraviolet spectrograph, one of the instruments on board Rosetta provided by NASA, also observed an increase in long-wavelength UV (1750-1950 Å) reflected solar radiation, both against the bright nucleus and from the coma directly above the sunward limb of the comet.
While outbursts of both dust and gas had been regularly observed in the few months surrounding perihelion in August 2015, what made this outburst unusual was that it occurred at a heliocentric distance of 2.40 AU on the comet’s outbound leg from the Sun. Moreover, eight of Rosetta’s instruments reported seeing the effects of the outburst. This led Eberhard Gruen, one of Rosetta’s Interdisciplinary Scientists, to propose a coordinated analysis of the event, and this was kicked off at a brief meeting on March 17 in Leiden, the Netherlands, during a symposium at which all of the Rosetta instrument teams were represented, and has culminated in a multi-authored paper submitted to The Monthly Notices of the Royal Astronomical Society last week.
Amongst the questions that data from Alice is able to address is the timing and duration of the event. The Alice observations were made with the small lobe of the nucleus in the lower half of the slit and the coma above the sunward limb in the upper half as shown in the NAVCAM image from UT 10:14:15, about one half hour after the commencement of the outburst (Figure 1). The distance of Rosetta from the center of the comet was 34.5 km and the solar phase angle was 63°. A light curve, derived from the Alice spectra, is shown in Figure 2. In two successive 10- minute histograms beginning UT 09:45:59 Alice observed a ~50% increase in brightness in the direction of the small lobe of the nucleus. Considering the 10- minute integration time of the Alice data, the onset time is consistent with that reported by OSIRIS. At the same time the dust coma brightness increased by a factor of ~5 and remained at that level for ~30 minutes. The following histograms had contamination of the coma from a bright star in the field-of-view leading to a 20-minute data gap. From UT 11:10 until the end of the observation sequence at ~12:30 the coma brightness remained at a value of ~2.5 times the prior quiescent level. There were no further Alice observations with the same viewing geometry on February 19. The same data do not show any significant gas emissions at the time of the outburst.
Work is currently underway to understand the nature of the ejected dust from a comparison of its scattering properties in the ultraviolet and visible.