Dec 26, 2018 by News Staff / Source
The ‘bow shock’ is the first boundary the solar wind encounters as it approaches planets or comets. From 2014 to 2016, ESA’s Rosetta orbiter studied 67P/Churyumov-Gerasimenko and its surroundings from near and far. The spacecraft flew directly through the bow shock several times both before and after the comet reached its closest point to the Sun along its orbit. According to a new study, published in the journal Astronomy & Astrophysics, Rosetta observed an infant bow shock — the first ever seen forming anywhere in the Solar System — around 50 times closer to the comet’s nucleus than anticipated in the case of 67P/Churyumov-Gerasimenko. This boundary was observed to be asymmetric, wider than the fully developed bow shocks observed at other comets, and moving in unexpected ways.
An artist’s impression of the infant bow shock detected by ESA’s Rosetta spacecraft at Comet 67P/Churyumov-Gerasimenko. Image credit: ESA.
Comets offer scientists an extraordinary way to study the plasma in the Solar System.
Plasma is a hot, gaseous state of matter comprising charged particles, and is found in the Solar System in the form of the solar wind: a constant stream of particles flooding out from our star into space.
As the supersonic solar wind flows past objects in its path, such as planets or smaller bodies, it first hits a boundary known as a bow shock. As the name suggests, this phenomenon is somewhat like the wave that forms around the bow of a ship as it cuts through choppy water.
Bow shocks have been found around comets, too — Halley’s comet being a good example. Plasma phenomena vary as the medium interacts with the surrounding environment, changing the size, shape, and nature of structures such as bow shocks over time.
Rosetta looked for signs of such a feature over its two-year mission, and ventured over 1,500 km away from the center of 67P/Churyumov-Gerasimenko on the hunt for large-scale boundaries around the comet — but apparently found nothing.
“We looked for a classical bow shock in the kind of area we’d expect to find one, far away from the comet’s nucleus, but didn’t find any, so we originally reached the conclusion that Rosetta had failed to spot any kind of shock,” said Dr. Herbert Gunell of the Royal Belgian Institute for Space Aeronomy and Umeå University.
“However, it seems that the spacecraft actually did find a bow shock, but that it was in its infancy. In a new analysis of the data, we eventually spotted it around 50 times closer to the comet’s nucleus than anticipated in the case of 67P/Churyumov-Gerasimenko. It also moved in ways we didn’t expect, which is why we initially missed it.”
On March 7, 2015, when the comet was over twice as far from the Sun as the Earth and heading inwards towards our star, Rosetta data showed signs of a bow shock beginning to form.
The same indicators were present on its way back out from the Sun, on February 24, 2016. This boundary was observed to be asymmetric, and wider than the fully developed bow shocks observed at other comets.
“Such an early phase of the development of a bow shock around a comet had never been captured before Rosetta,” said Dr. Charlotte Goetz, from the Institute for Geophysics and Extraterrestrial Physics in Germany.
The researchers analyzed ion spectra from the Ion Composition Analyzer of the Rosetta Plasma Consortium (RPC-ICA), ion and electron spectra from the Ion and Electron Sensor (RPC-IES), and magnetic flux density measurements from the magnetometer instrument (RPC-MAG).
They found that, when the forming bow shock washed over Rosetta, the comet’s magnetic field became stronger and more turbulent, with bursts of highly energetic charged particles being produced and heated in the region of the shock itself.
Beforehand, particles had been slower-moving, and the solar wind had been generally weaker — indicating that Rosetta had been ‘upstream’ of a bow shock.
“These observations are the first of a bow shock before it fully forms, and are unique in being gathered on-location at the comet and shock itself,” said Rosetta project scientist Dr. Matt Taylor, from ESA.
“This finding also highlights the strength of combining multi-instrument measurements and simulations. It may not be possible to solve a puzzle using one dataset, but when you bring together multiple clues, as in this study, the picture can become clearer and offer real insight into the complex dynamics of our Solar System — and the objects in it, like 67P/Churyumov-Gerasimenko.”
Herbert Gunell et al. 2018. The infant bow shock: a new frontier at a weak activity comet. A&A 619, L2; doi: 10.1051/0004-6361/201834225
Thanks to: http://www.sci-news.com