The RPC-ICA instrument onboard Rosetta has been watching the early stages of how a magnetosphere forms around Comet 67P Churyumov-Gerasimenko as it moves closer to the Sun along its orbit and begins to interact with the solar wind.

As the comet gets warmer, volatile substances, mainly water, evaporate from the surface and form an atmosphere around the comet. The Sun's ultraviolet radiation and collisions with the solar wind ionizes some of the comet's atmosphere. The newly formed ions are affected by the solar wind electric and magnetic fields and can be accelerated to high speeds. When the comet gets close enough to the Sun, its atmosphere becomes so dense and ionized that it becomes electrically conductive. When this happens, the atmosphere starts to resist the solar wind and a comet's magnetosphere is born – a region surrounding the comet that is shielded from the solar wind.

"The comet environment is a laboratory for scientists; we can see what happens when the solar wind streams through an atmosphere," says Associate Professor Hans Nilsson at the Swedish Institute of Space Physics (IRF) in Kiruna. Hans Nilsson is responsible for the instrument Ion Composition Analyser, ICA, developed and built in Kiruna. The instrument measures positively charged ions and is one of five instruments in the Rosetta Plasma Consortium.

Hans explained that studying the interaction of the solar wind with a comet's atmosphere in this way could shed light on how the solar wind interacted with the planets during their early formation. "In the young Solar System, the planet’s upper atmospheres were strongly heated and extended far into space. The solar wind should have streamed through the outer part of these extended atmospheres in a way that resembles its passage through the comet atmosphere for this low activity stage," he explains.

At 67P/C-G, Rosetta’s RPC-ICA instrument detected low velocity water ions in data collected on 7 August 2014, one day after arriving at the comet at a distance of 100 km.

"It was an unambiguous signature of the comet, a clear detection of ions from the comet's atmosphere," confirmed Hans.

Rosetta is a unique space mission. Previous spacecraft that have studied comets have rushed past them at a great distance and at speeds of tens of kilometres per second. At the time of these previous spacecraft-comet encounters, the comet magnetosphere had been fully developed. By comparison, Rosetta is currently orbiting around 67P/C-G at a distance of just a few tens of kilometres, and at low speed. Importantly, the mission arrived at the comet in time to watch the very early stages of the magnetosphere formation.

“For the first time, we can see what happens before the comet atmosphere resists the solar wind," says Hans. "We discovered that the comet atmosphere affects the solar wind more than we thought it would at this early stage. We are also surprised how much structure we see in our data – the comet atmosphere appears to be very unevenly distributed around the nucleus.

"We are still in the early stages of analysing and modelling our data, but perhaps the energy transfer from the solar wind to the atmosphere is less efficient in removing an atmosphere than we originally thought.
As the comet moves closer to the Sun along its orbit, Hans and his colleagues will see the transition from this early phase to the growth of a well established comet magnetosphere.

"That transition is likely the most exciting part," he says. "It can happen any day now. Every morning we keep looking at the new data which have just arrived. What will we find today?"

"Birth of a comet magnetosphere: a spring of water ions," by Hans Nilsson et al., is published in 23 January issue of Science