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یک سری مقاله ها از ویلاگ Rosetta
This blog post is based on the papers “Evolution of the ion environment of comet 67P/Churyumov-Gerasimenko: Observations between 3.6 and 2.0 AU ” by H. Nilsson et al.; “Rosetta observations of solar wind interaction with the comet 67P/Churyumov-Gerasimenko” by T.W. Broiles et al.; and “Solar Wind Sputtering of Dust on the Surface of 67P/Churyumov-Gerasimenko ” by Peter Wurz et al., which have all been accepted for publication in Astronomy and Astrophysics, and “Dynamical
features and spatial structures of the plasma interaction region of
67P/Churyumov–Gerasimenko and the solar wind” by C. Koenders et al,
which is published in Planetary and Space Science.
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.
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
تصویر و توضیح این قسمت تو ادرس زیر .
http://blogs.esa.int/rosetta/2015/07/29/rosetta-shows-how-comet-interacts-with-the-solar-wind/
Screenshot from a simulation of plasma interactions between Comet
67P/C-G and the solar wind around perihelion. Click for full animation
and detailed caption. Credit: Modelling and simulation: Technische
Universität Braunschweig and Deutsches Zentrum für Luft- und Raumfahrt;
Visualisation: Zuse-Institut Berlin
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.
Read “Evolution of the ion environment of comet 67P/Churyumov-Gerasimenko: Observations between 3.6 and 2.0 AU” by H. Nilsson et al. here.
Read “Rosetta observations of solar wind interaction with the comet 67P/Churyumov-Gerasimenko” by T.W. Broiles et al. here.
Read “Solar Wind Sputtering of Dust on the Surface of 67P/Churyumov-Gerasimenko” by Peter Wurz et al. here.
Read “Dynamical features and spatial structures of the
plasma interaction region of 67P/Churyumov–Gerasimenko and the solar
wind” by C. Koenders et al. here.
