This artist's impression shows the Rosetta orbiter at comet 67P/Churyumov-Gerasimenko. The image is not to scale. Credit: ESA/ATG Medialab

Rosetta is making good progress in one of its key investigations, which concerns the interaction between the comet and the solar wind.

The solar wind is the constant stream of electrically charged particles that flows from the Sun, carrying its magnetic field out into the Solar System. Like all comets, 67P/Churyumov–Gerasimenko must navigate this flow in its orbit around the Sun.

It is the constant battle fought between the comet and the solar wind that helps to sculpt the comet's ion tail. Rosetta's instruments are monitoring the fine detail of this process.

Using the Rosetta Plasma Consortium Ion Composition Analyzer, Hans Nilsson from the Swedish Institute of Space Physics and his colleagues have been studying the gradual evolution of the comet's ion environment. They have seen that the number of water ions – molecules of water that have been stripped of one electron – accelerated away from the comet increased hugely as 67P/C-G moved between 3.6AU (about 538 million km) and 2.0AU (about 300 million km) from the Sun. Although the day-to-day acceleration is highly variable, the average 24-hour rate has increased by a factor of 10 000 during the study, which covered the period August 2014 to March 2015.

The water ions themselves originate in the coma, the atmosphere of the comet. They are placed there originally by heat from the Sun liberating the molecules from the surface ice. Once in gaseous form, the collision of extreme ultraviolet light displaces electrons from the molecules, turning them into ions. Colliding particles from the solar wind can do this as well. Once stripped of some of their electrons, the water ions can then be accelerated by the electrical properties of the solar wind.

This 3D simulation models the plasma interactions between comet 67P/Churyumov-Gerasimenko and the solar wind. The simulated conditions represent those expected at 1.3 AU from the Sun, close to perihelion, where the comet is strongly active – a gas production rate of 5 × 1027 molecules/s is assumed here. The solar wind approaches from the left at ~400 km/s, carrying with it the embedded interplanetary magnetic field with a strength of about 5 nT. The material from the comet's nucleus forms an extensive envelope, the coma, several million km in size (not shown here). Part of the neutral gas molecules in the coma gets ionised by solar UV radiation or by charge exchange with the solar wind particles. These cometary ions are picked up by the approaching solar wind, a process known as mass loading, and cause it to slow down. In the model simulation enough ions are produced and picked up by the solar wind to slow it down from supersonic speed to sub sonic speed, causing a bow shock to form in front of the comet. (0:00 - 0:12) At the start of this simulation, the shape of the cometary bow shock is depicted by the curved semi-transparent surface. It cuts off at the edges of the simulation box (which is about 10 000 km across). At the sub-solar point it stands 2000 km from the comet nucleus. The density of cometary ions at the comet is shown in purple, the brighter the denser. (0:13 - 0:26) Stream lines of the solar wind coming in from the left are shown in orange. The flow deflects at the bow shock. (0:27 - 1:03) The magnetic field lines (in yellow) at a given time are shown from different angles, in a rotating view. Coming in from the left, at a 52° slant, the field lines are deformed at the bow shock and wrap around the comet. Behind the bow shock, closer to the comet nucleus, the field lines pile up due to the deceleration of the mass loaded solar wind as it encounters the increasingly denser inner coma. The draped field lines shape the comet's plasma tail. (1:04 - 1:13) The simulation zooms in on the central region of the coma, a few hundred km across. The comet nucleus (not shown) measures only ~4 km. The Sun now is to the lower left. The density of cometary ions is again shown in purple. This fades to show the motion of two populations of cometary ions close to the nucleus. (1:14 - 1:42) Purple lines depict the flow of incoming cometary ions which were picked up by the solar wind. The flow of outward bound ions originating in the region close to the comet nucleus are shown in blue. The ions originating in this inner region, where the neutral gas (not shown) is most dense, are pushed outward due to collisions with the neutral gas. The resulting outward force is strong enough to withstand the mass loaded solar wind and generate a small diamagnetic cavity. The pick-up ions approaching the cavity are deflected around it. The boundary of the cavity is called the ionopause. It has a subsolar stand-off distance of about 25 km in this simulation and 45 km at the terminator. (1:43 - 2:14) The mass loaded plasma, carrying the interplanetary magnetic field, is decelerated towards the cometary ionopause. The magnetic field cannot pass this boundary and wraps around it (yellow lines). It piles up on the dayside leaving the diamagnetic cavity magnetic field free. In this simulation the field strength peaks at 78 nT at about 45 km in front of the nucleus. The movie is based on descriptions provided in the paper "Dynamical features and spatial structures of the plasma interaction region of 67P/Churyumov–Gerasimenko and the solar wind" by C. Koenders et al., Planetary and Space Science (105) January 2015. dx.doi.org/10.1016/j.pss.2014.11.014. Credit: Modelling and simulation: Technische Universität Braunschweig and Deutsches Zentrum für Luft- und Raumfahrt; Visualisation: Zuse-Institut Berlin

Not all of the ions are accelerated outwards, some will happen to strike the comet's surface. Solar wind particles will also find their way through the coma to hit home. When this happens, they cause a process called sputtering, in which they displace atoms from material on the surface – these are then 'liberated' into space.

Peter Wurz from the University of Bern, Switzerland, and colleagues have studied these sputtered atoms with Rosetta's Double Focussing Mass Spectrometer (DFMS), which is part of the ROSINA experiment.

They have so far discovered sodium, potassium, silicon and calcium, which are all present in a rare form of meteorites called carbonaceous chondrites. There are differences in the amounts of these atoms at the comet and in these meteorites, however. While the abundance of sodium appears the same, 67P/C-G shows an excess of potassium and a depletion of calcium.

Most of the sputtered atoms come from the winter side of the comet. Although this is the hemisphere that is mostly facing away from the Sun at present, solar wind particles can end up striking the surface because they are deflected during interactions with ions in the comet's coma. This can be a significant process so long as the density of the coma ions is not too large. But at some point the comet's atmosphere becomes dense enough to be a major defence, protecting the icy surface.

As the comet gets closer to the Sun, the sputtering will eventually stop because the comet will release more gas and the coma will become impenetrable. When this happens, the solar wind ions will always collide with atoms in this atmosphere or be deflected away before striking the surface.

The first evidence that this deflection is taking place at 67P/C-G has been measured with the Rosetta Plasma Consortium Ion and Electron Sensor, by Thomas Broiles of the Southwest Research Institute (SwRI) in San Antonio, Texas, and colleagues.

Their observations began on 6 August 2014 when Rosetta arrived at the comet, and have been almost continuous since. The instrument has been measuring the flow of the solar wind as Rosetta orbits 67P/C-G, showing that the solar wind can be deflected by up to 45° away from the anti-solar direction.

The deflection is largest for the lighter ions, such as protons, and not so much for the heavier ions derived from helium atoms. For all ions the deflection is set to increase as the comet gets closer to the Sun and the coma becomes ever denser.

As all this happens, Rosetta will be there to continue monitoring and measuring the changes. This was the raison d'être for the rendezvous with this comet. Previous missions have taken snapshots during all too brief fly-bys but Rosetta is showing us truly how a comet behaves as it approaches the Sun.

Explore further Image: Increasingly active Comet 67P

More information: This article is based on four papers: This article is based on four papers: "Evolution of the ion environment of comet 67P/Churyumov-Gerasimenko: Observations between 3.6 and 2.0 AU" by H. Nilsson et al., accepted for publication in Astronomy and Astrophysics "Rosetta observations of solar wind interaction with the comet 67P/Churyumov-Gerasimenko" by T.W. Broiles et al., accepted for publication in Astronomy and Astrophysics "Solar wind sputtering of dust on the surface of 67P/Churyumov-Gerasimenko" by P. Wurz et al., accepted for publication in Astronomy and Astrophysics, dx.doi.org/10.1051/0004-6361/201525980 "Dynamical features and spatial structures of the plasma interaction region of 67P/Churyumov–Gerasimenko and the solar wind" by C. Koenders et al., published in Planetary and Space Science, January 2015. dx.doi.org/10.1016/j.pss.2014.11.014 Journal information: Astronomy and Astrophysics , Planetary and Space Science