Simulation of plasma interactions between comet 67P/C-G and the solar wind around perihelion
29 July 2015
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. http://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
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