From Newsgroup: sci.space.news


http://sci.esa.int/rosetta/57576-rosetta-finds-magnetic-field-free-bubble-at-comet/

Rosetta finds magnetic field-free bubble at comet
European Space Agency
11 March 2016

ESA's Rosetta spacecraft has revealed a surprisingly large region
around its host comet devoid of any magnetic field.

When ESA's Giotto flew past Comet Halley three decades ago, it
found a vast magnetic-free region extending more than 4000 km from
the nucleus. This was the first observation of something that
scientists had until then only thought about but had never seen.

Interplanetary space is pervaded by the solar wind, a flow of
electrically charged particles streaming from the Sun and carrying
its magnetic field across the Solar System. But a comet pouring
lots of gas into space obstructs the solar wind <http://sci.esa.int/rosetta/56238-rosetta-shows-how-comet-interacts-with-the-solar-wind/>.

At the interface between the solar wind and the coma of gas around
the active comet, particle collisions as well as sunlight can
knock out electrons from the molecules in the coma <http://sci.esa.int/rosetta/55963-ultraviolet-study-reveals-surprises-in-comet-coma/>,
which are ionised and picked up by the solar wind. This process
slows the solar wind, diverting its flow around the comet and
preventing it from directly impacting the nucleus.

Along with the solar wind, its magnetic field is unable to
penetrate the environment around the comet, creating a region
devoid of magnetic field called a diamagnetic cavity.

Prior to Rosetta arriving at Comet 67P/Churyumov-Gerasimenko,
scientists had hoped to observe such a magnetic field-free region
in the environment of this comet. The spacecraft carries a
magnetometer as part of the Rosetta Plasma Consortium suite of
sensors (RPC-MAG), whose measurements were already used to
demonstrate that the comet nucleus is not magnetised <http://sci.esa.int/rosetta/55788-rosetta-and-philae-find-comet-not-magnetised/>.

However, since Rosetta's comet is much less active than Comet
Halley, the scientists predicted that a diamagnetic cavity could
form only in the months around perihelion - the closest point to
the Sun on the comet's orbit - but that it would extend only
50-100 km from the nucleus.

During 2015, the increased amounts of dust dragged into space by
the outflowing gas became a significant problem for navigation
close to the comet. To keep Rosetta safe, trajectories were chosen
such that by the end of July 2015, a few weeks before perihelion,
it was some 170 km away from the nucleus. As a result, scientists
considered that detecting signs of the magnetic field-free bubble
would be impossible.

"We had almost given up on Rosetta finding the diamagnetic
cavity, so we were astonished when we eventually found it," says
Charlotte Gotz of the Institute for Geophysics and
extraterrestrial Physics in Braunschweig, Germany.

Charlotte is the lead author of a new study, published in the
journal Astronomy and Astrophysics, presenting the detection of
a diamagnetic cavity obtained by RPC-MAG on 26 July. The paper
describes one of the most spectacular measurements from almost 700
detections of regions with no magnetic field made by Rosetta at
the comet since June 2015.

"We were able to detect the cavity, and on many occasions,
because it is much bigger and dynamic than we had expected," adds
Charlotte.

To investigate why the magnetic field-free cavity is so much
bigger than predicted, Charlotte and her colleagues looked at
measurements performed around the same time by other instruments,
such as Rosetta's scientific camera, OSIRIS, and the Rosetta
Orbiter Spectrometer for Ion and Neutral Analysis instrument,
ROSINA, to verify whether any anomalous changes in the comet's
activity could be pushing the cavity away from the nucleus.

While one of the cavity detections, on 29 July, occurred in
conjunction with a strong outburst of gas and dust <http://sci.esa.int/rosetta/56325-comets-firework-display-ahead-of-perihelion/> recorded by other instruments on Rosetta, this seems to be an
isolated case. Almost all of the other observations of magnetic
field-free regions, including the one recorded on 26 July, were
not accompanied by any appreciable increase of outgassing.

"To account for such a big cavity in the simulations, we would
need the outgassing rate to be 10 times higher than was measured
at the comet by ROSINA," says co-author Karl-Heinz Glassmeier
from Technische Universitat Braunschweig, Germany, principal
investigator of RPC-MAG.

The most likely explanation seems to lie, instead, in the
dynamical nature of the cavity boundary.

Boundaries between plasma regions with different properties are
often unstable, and small oscillations can arise in the pile-up
region of the solar wind, where it encounters the magnetic
field-free region, on the Sun-facing side of the comet. If these
oscillations propagate and get amplified along the boundary, in
the direction opposite the Sun, they could easily cause the cavity
to grow in size.

Such a moving instability would also explain why the measurements
of magnetic field-free regions are sporadic and mainly span
several minutes, with the 26 July one lasting 25 minutes and the
longest one, recorded in November, about 40 minutes. The short
duration of the detections is not a result of Rosetta crossing the
cavity - the spacecraft moves much too slowly with respect to the
comet - but of the magnetic field-free regions repeatedly passing
through the spacecraft.

"What we are seeing is not the main part of the cavity but the
smaller pockets at the cavity boundary, which are occasionally
pushed farther away from the nucleus by the waves propagating
along the boundary," adds Charlotte.

Scientists are now busy analysing all the magnetic field-free
events recorded by Rosetta, to learn more about the properties of
the plasma in the comet environment and its interaction with the
solar wind. After perihelion, as the comet moved away from the Sun
and its outgassing and dust production rate declined, the
spacecraft was able to move closer to the nucleus, and the
magnetometer continued detecting magnetic field-free regions for
several months, until the latest detection in February 2016.

"Three decades ago, Giotto's detection at Comet Halley was a
great success, because it was the first confirmation of the
existence of a diamagnetic cavity at a comet," says Matt Taylor,
Rosetta Project Scientist at ESA.

"But that was only one measurement, while now we have seen the
cavity at Rosetta's comet come and go hundreds of times over many
months. This is why Rosetta is there, living with the comet and
studying it up close."



Notes for Editors

"First detection of a diamagnetic cavity at comet
67P/Churyumov-Gerasimenko
<http://dx.doi.org/10.1051/0004-6361/201527728>," by C. Gotz et
al. is published in the journal /Astronomy & Astrophysics/. The
results will be presented at the 50th ESLAB Symposium "From Giotto
to Rosetta <http://www.congrexprojects.com/2016-events/16a07/>",
held 14-18 March in Leiden, the Netherlands.



For more information contact:

Charlotte Gotz
Institute for Geophysics and extraterrestrial Physics
Technische Universitat Braunschweig, Germany
Email: c.goetz@tu-bs.de

Karl-Heinz Glassmeier
RPC-MAG principal investigator
Institute for Geophysics and extraterrestrial Physics
Technische Universitat Braunschweig, Germany
Email: kh.glassmeier@tu-bs.de

Matt Taylor
ESA Rosetta Project Scientist
Email: matt.taylor@esa.int

Markus Bauer
ESA Science Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer@esa.int

SEEN-BY: 154/30 2320/100 0 1 227/0