Hale-Bopp comet

Icy visitor: A view of comet Hale-Bopp, as seen from California in 1997, shows the matchhead and feathery tail.
Historically, the shape of comets revealed the presence of the solar wind in the 1950s

In our exploration of the space and the Sun, an important aspect is to understand how emanated gases behave when they are close to the sun. To study this, a simulation of a comet interacting with solar wind has been made by researchers. The work, published in Physical Review Letters, reveals how the electrons and ions in the comet's body interact with the electrons and protons in the solar wind. This is interesting as comet observations have a history of revealing even the existence of the solar wind.

Solar wind consists of a plasma of electrons and protons flowing away from the sun at hypersonic speeds. Its existence was first inferred indirectly in the 1950s by observing the shapes of comet tails. Only later was its existence confirmed by the Mariner 2 spacecraft.

Comets, on the other hand, are icy wanderers that travel far into space and make an appearance periodically in the skies when they pass close to the Sun. Their shape is characteristic — a small rounded match-head-like halo followed by a long tail — and dictated by its interaction with the solar wind. The halo and the tail consist of material that has sublimated from its icy nucleus and has been dragged out by the solar wind.

Four-fluid model

The material of the comet and the particles in the solar wind do not actually collide, they interact via the electromagnetic field surrounding these charged electrons, protons and ions. Jan Deca and collaborators modelled the system as four-fluid plasma, with the four components being the electrons and protons spewed out by the Sun, and the electrons and ions in the comet's halo.

They were able to reproduce several features observed by the Rosetta Mission, which followed and studied the comet 67P/Churyumov-Gerasimenko for over two years in its orbit.

The electrons and protons belonging to the solar wind react differently to comet's magnetic field. The electrons, being lighter are deflected easily and flow along the outer contours while the protons penetrate deeper and interact with the inner regions of the comet. Thus solar-wind protons are neutralised by the inner electrons of the comet, and the solar-wind electrons flowing down the outer contours neutralise the positive water ions of the comet. This causes a charge transfer and momentum transfer leading to the changes in the shape of the water-ion tail.

The study can help scientists to understand the nature of the solar wind and also the structure and behaviour of comets. In comet 67P, a phenomenon which was labelled "singing comet" oscillations was observed. This is related to a fan-like filamentous structure in the lower portion of the tail. The researchers speculate that further probing their simulation and model may be able to explain this feature.