{"id":5559,"date":"2015-08-11T10:03:02","date_gmt":"2015-08-11T10:03:02","guid":{"rendered":"http:\/\/revoscience.com\/en\/?p=5559"},"modified":"2015-08-11T10:03:02","modified_gmt":"2015-08-11T10:03:02","slug":"comets-firework-display-ahead-of-prihelion","status":"publish","type":"post","link":"https:\/\/www.revoscience.com\/en\/comets-firework-display-ahead-of-prihelion\/","title":{"rendered":"Comet’s firework display ahead of prihelion"},"content":{"rendered":"
\n
\"Outburst<\/a>
Outburst in action<\/figcaption><\/figure>\n

In the approach to perihelion over the past few weeks, Rosetta has been witnessing growing activity from Comet 67P\/Churyumov\u2013Gerasimenko, with one dramatic outburst event proving so powerful that it even pushed away the incoming solar wind.<\/span><\/p>\n

The comet reaches perihelion on Thursday, the moment in its 6.5-year orbit when it is closest to the Sun. In recent months, the increasing solar energy has been warming the comet\u2019s frozen ices, turning them to gas, which pours out into space, dragging dust along with it.<\/span><\/p>\n

The period around perihelion is scientifically very important, as the intensity of the sunlight increases and parts of the comet previously cast in years of darkness are flooded with sunlight.<\/span><\/p>\n

Although the comet\u2019s general activity is expected to peak in the weeks following perihelion, much as the hottest days of summer usually come after the longest days, sudden and unpredictable outbursts can occur at any time \u2013 as already seen earlier in the mission.<\/span><\/p>\n<\/div>\n

\n
\"Discovery<\/a>
Discovery of diamagnetic cavity<\/figcaption><\/figure>\n

On 29 July, Rosetta observed the most dramatic outburst yet, registered by several of its instruments from their vantage point 186 km from the comet. They imaged the outburst erupting from the nucleus, witnessed a change in the structure and composition of the gaseous coma environment surrounding Rosetta, and detected increased levels of dust impacts.<\/span><\/p>\n

Perhaps most surprisingly, Rosetta found that the outburst had pushed away the solar wind magnetic field from around the nucleus.<\/span><\/p>\n<\/div>\n

\n

A sequence of images taken by Rosetta\u2019s scientific camera OSIRIS show the sudden onset of a well-defined jet-like feature emerging from the side of the comet\u2019s neck, in the Anuket region. It was first seen in an image taken at 13:24 GMT, but not in an image taken 18 minutes earlier, and has faded significantly in an image captured 18 minutes later. The camera team estimates the material in the jet to be travelling at 10 m\/s at least, and perhaps much faster.<\/span><\/p>\n

\u201cThis is the brightest jet we\u2019ve seen so far,\u201d comments Carsten G\u00fcttler, OSIRIS team member at the Max Planck Institute for Solar System Research in G\u00f6ttingen, Germany.<\/span><\/p>\n

\u201cUsually, the jets are quite faint compared to the nucleus and we need to stretch the contrast of the images to make them visible \u2013 but this one is brighter than the nucleus.\u201d<\/span><\/p>\n

\n

\"Gas_changes_during_29_July_outburst_medium\"<\/a>Soon afterwards, the comet pressure sensor of ROSINA detected clear indications of changes in the structure of the coma, while its mass spectrometer recorded changes in the composition of outpouring gases.<\/span><\/p>\n

For example, compared to measurements made two days earlier, the amount of carbon dioxide increased by a factor of two, methane by four, and hydrogen sulphide by seven, while the amount of water stayed almost constant.<\/span><\/p>\n

\u201cThis first \u2018quick look\u2019 at our measurements after the outburst is fascinating,\u201d says Kathrin Altwegg, ROSINA principal investigator at the University of Bern. \u201cWe also see hints of heavy organic material after the outburst that might be related to the ejected dust.<\/span><\/p>\n

\u201cBut while it is tempting to think that we are detecting material that may have been freed from beneath the comet’s surface, it is too early to say for certain that this is the case.\u201d<\/span><\/p>\n<\/div>\n

\n

Meanwhile, about 14 hours after the outburst, GIADA was detecting dust hits at rates of 30 per day, compared with just 1\u20133 per day earlier in July. A peak of 70 hits was recorded in one 4-hour period on 1 August, indicating that the outburst continued to have a significant effect on the dust environment for the following few days.<\/span><\/p>\n

\u201cIt was not only the abundance of the particles, but also their speeds measured by GIADA that told us something \u2018different\u2019 was happening: the average particle speed increased from 8 m\/s to about 20 m\/s, with peaks at 30 m\/s \u2013 it was quite a dust party!\u201d says Alessandra Rotundi, principal investigator at the \u2018Parthenope\u2019 University of Naples, Italy.<\/span><\/p>\n<\/div>\n

\n

Perhaps the most striking result is that the outburst was so intense that it actually managed to push the solar wind away from the nucleus for a few minutes \u2013 a unique observation made by the Rosetta Plasma Consortium\u2019s magnetometer.<\/span><\/p>\n

The solar wind is the constant stream of electrically charged particles that flows out from the Sun, carrying its magnetic field out into the Solar System. Earlier measurements made by Rosetta and Philae had already shown that the comet is not magnetised,\u00a0 so the only source for the magnetic field measured around it is the solar wind.<\/span><\/p>\n

But it doesn\u2019t flow past unimpeded. Because the comet is spewing out gas, the incoming solar wind is slowed to a standstill where it encounters that gas and a pressure balance is reached.<\/span><\/p>\n

\u201cThe solar wind magnetic field starts to pile up, like a traffic jam, and eventually stops moving towards the comet nucleus, creating a magnetic field-free region on the Sun-facing side of the comet called a \u2018diamagnetic cavity\u2019,\u201d explains Charlotte G\u00f6tz, magnetometer team member at the Institute for Geophysics and extraterrestrial Physics in Braunschweig, Germany.<\/span><\/p>\n

Diamagnetic cavities provide fundamental information on how a comet interacts with the solar wind, but the only previous detection of one associated with a comet was made at about 4000 km from Comet Halley as ESA\u2019s Giotto flew past in 1986.\u00a0<\/span><\/p>\n

Rosetta\u2019s comet is much less active than Halley, so scientists expected to find a much smaller cavity around it, up to a few tens of kilometres at most, and prior to 29 July, had not observed any sign of one.<\/span><\/p>\n

But, following the outburst on that day, the magnetometer detected a diamagnetic cavity extending out at least 186 km from the nucleus. This was likely created by the outburst of gas, which increased the neutral gas flux in the comet\u2019s coma, forcing the solar wind to \u2018stop\u2019 further away from the comet and thus pushing the cavity boundary outwards beyond where Rosetta was flying at the time.<\/span><\/p>\n

“Finding a magnetic field-free region anyway in the Solar System is really hard, but here we’ve had it served to us on a silver platter \u2013 this is a really exciting result for us,” adds Charlotte.<\/span><\/p>\n

\u201cWe\u2019ve been moving Rosetta out to distances of up to 300 km in recent weeks to avoid problems with navigation caused by dust, and we had considered that the diamagnetic cavity was out of our grasp for the time being. But it seems that the comet has helped us by bringing the cavity to Rosetta,\u201d says Matt Taylor, Rosetta Project Scientist.<\/span><\/p>\n

\u201cThis is a fantastic multi-instrument event which will take time to analyse, but highlights the exciting times we\u2019re experiencing at the comet in this \u2018hot\u2019 perihelion phase.\u201d<\/span><\/p>\n<\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

In the approach to perihelion over the past few weeks, Rosetta has been witnessing growing activity from Comet 67P\/Churyumov\u2013Gerasimenko, with one dramatic outburst event proving so powerful that it even pushed away the incoming solar wind. The comet reaches perihelion on Thursday, the moment in its 6.5-year orbit when it is closest to the Sun. […]<\/p>\n","protected":false},"author":6,"featured_media":5560,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[20],"tags":[],"class_list":["post-5559","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-space-news"],"featured_image_urls":{"full":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/08\/Outburst_in_action_large.jpg",625,208,false],"thumbnail":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/08\/Outburst_in_action_large-150x150.jpg",150,150,true],"medium":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/08\/Outburst_in_action_large-300x99.jpg",300,99,true],"medium_large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/08\/Outburst_in_action_large.jpg",625,208,false],"large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/08\/Outburst_in_action_large.jpg",625,208,false],"1536x1536":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/08\/Outburst_in_action_large.jpg",625,208,false],"2048x2048":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/08\/Outburst_in_action_large.jpg",625,208,false],"ultp_layout_landscape_large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/08\/Outburst_in_action_large.jpg",625,208,false],"ultp_layout_landscape":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/08\/Outburst_in_action_large.jpg",625,208,false],"ultp_layout_portrait":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/08\/Outburst_in_action_large.jpg",600,200,false],"ultp_layout_square":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/08\/Outburst_in_action_large.jpg",600,200,false],"newspaper-x-single-post":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/08\/Outburst_in_action_large.jpg",625,208,false],"newspaper-x-recent-post-big":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/08\/Outburst_in_action_large.jpg",550,183,false],"newspaper-x-recent-post-list-image":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/08\/Outburst_in_action_large.jpg",95,32,false],"web-stories-poster-portrait":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/08\/Outburst_in_action_large.jpg",625,208,false],"web-stories-publisher-logo":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/08\/Outburst_in_action_large.jpg",96,32,false],"web-stories-thumbnail":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/08\/Outburst_in_action_large.jpg",150,50,false]},"author_info":{"info":["Amrita Tuladhar"]},"category_info":"Space\/ AstroPhysics<\/a>","tag_info":"Space\/ AstroPhysics","comment_count":"0","_links":{"self":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts\/5559","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/users\/6"}],"replies":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/comments?post=5559"}],"version-history":[{"count":0,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts\/5559\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/media\/5560"}],"wp:attachment":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/media?parent=5559"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/categories?post=5559"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/tags?post=5559"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}