Summer outbursts in the coma of 67P/Churyumov-Gerasimenko as observed by VIRTIS-M

In this work, we present an analysis of the outbursts observed by VIRTIS-M for the dates of August 10, September 13 and 14 2015, when the comet was at heliocentric distances of -1.2 AU (inbound) to 1.3 AU (outbound), 2 day before and one month after the perihelion passage respectively. They are characterized by a sudden and short increase of the dust emission, from localized areas with variable degree of collimation, followed by a sweet decrease of activity. The lifetime is between 5 and 15 minutes and the corresponding radiance at 0.55 μm is typically of the order of 10-2 - 1 Watt/m2/str/μm, i.e. 10-20 times larger than the surrounding coma. VIRTIS-M identified 2 main dust plumes morphologies associated to these events: a narrow jet and a broad plumes. We found clear evidence of different colour values in the ouburst with respect to the surrounding coma. This implies that we observed dust with different physical characteristics both in the structures and in the coma. The comparison between IR (VIRTIS-H) and VIS (VIRTISM) observations allows to derive the projected velocity of the small dust grain obtaining 23 -40 m/s. It is also possible to estimate the total mass loss in the outburst, by converting the radiance over an image to a dust cross section, and then to mass assuming a dust size distribution with a power law index of -3. For September 14 large event we estimated a total ejected dust mass of few tens tons as found by Vincent et al. 2016.