Solar-insolation-induced changes in the coma morphology of comet 67P/Churyumov-Gerasimenko - Optical monitoring with the Nordic Optical Telescope

Context. 67P/Churyumov-Gerasimenko (67P/C-G) is a short-period Jupiter family cometwith an orbital periodof 6.55 years. Being the target cometof ESA’s Rosetta mission, 67P/C-G has become one of the most intensively studied minor bodies of the Solar System. The Rosetta Orbiter and the Philae Lander have brought us unique information about the structure and activity of the comet nucleus, as well as its activity along the orbit, composition of gas, and dust particles emitted into the coma. However, as Rosetta stayed in very close proximity to the cometary nucleus (less than 500 km with a few short excursions reaching up to 1500 km), it could not see the global picture of a comaat the scales reachable by telescopic observations (10 3 - 10 5 km). Aims. In this work we aim to connect in-situ observations made by Rosetta with the morphological evolution of the comastructures monitored by the ground-based observations. In particular, we concentrate on causal relationships between the comamorphology and evolution observed with the Nordic Optical Telescope(NOT) in the Canary Islands, and the seasonal changes of the insolationand the activity of the cometobserved by the Rosetta instruments. Methods. Comet67P/C-G was monitored with the NOT in imaging mode in two colors. Imaging optical observations were performed roughly on a weekly basis, which provides good coverage of short- and long-term variability. With the three dimensional modeling of the comaproduced by active regions on the Southern Hemisphere, we aim to qualify the observed morphology by connecting it to the activity observed by Rosetta. Results. During our monitoring program, we detected major changes in the comamorphology of comet67P/C-G. These were longterm and long-lasting changes. They do not represent any sudden outburst or short transient event, but are connected to seasonal changes of the surface insolationand the emergence of new active regions on the irregular shaped comet nucleus. We have also found significant deviations in morphological changes from the prediction models based on previous apparitions of 67P/C-G, like the time delay of the morphology changes and the reduced activity in the Northern Hemisphere. According to our modeling of comastructures and geometry of observations, the changes are clearly connected with the activity in the Southern Hemisphereobserved by the Rosetta spacecraft.