{"id":8276,"date":"2017-10-04T16:17:49","date_gmt":"2017-10-04T16:17:49","guid":{"rendered":"https:\/\/blogs.futura-sciences.com\/feldmann\/?p=8276"},"modified":"2019-10-10T16:40:18","modified_gmt":"2019-10-10T16:40:18","slug":"saison-aurores-polaires-ouverte","status":"publish","type":"post","link":"https:\/\/blogs.futura-sciences.com\/feldmann\/2017\/10\/04\/saison-aurores-polaires-ouverte\/","title":{"rendered":"La saison des aurores polaires est ouverte"},"content":{"rendered":"

Les semaines qui encadrent les \u00e9quinoxes voient augmenter le nombre d’ aurores polaires. Une co\u00efncidence que l’on commence \u00e0 mieux comprendre. <\/strong><\/p>\n

En 2007 le physicien solaire David Hathaway (du Marshall Space Flight Center<\/em><\/a>) pr\u00e9senta un surprenant graphique\u00a0: en compilant 75 ans de donn\u00e9es concernant les perturbations g\u00e9omagn\u00e9tiques, il avait d\u00e9couvert deux pics d’activit\u00e9 correspondant aux \u00e9quinoxes<\/a>.<\/p>\n

\"\"<\/a><\/p>\n

Ce fut un casse-t\u00eate\u00a0: on connaissait d\u00e9j\u00e0 le lien entre aurores polaires et activit\u00e9 solaire, mais le Soleil<\/a> se moque bien des saisons terrestres. Alors, pourquoi observait-on plus d’aurores aux \u00e9quinoxes ?<\/p>\n

\u00c0\u00a0la m\u00eame \u00e9poque furent lanc\u00e9s les satellites Themis<\/a> (abr\u00e9viation de Time History of Events and Macroscale Interactions during Substorms<\/em>) sp\u00e9cialement con\u00e7us pour \u00e9tudier les aurores (ces satellites sont toujours op\u00e9rationnels). Ces engins spatiaux (cinq microsatellites pesant \u00e0 peine 70 kg chacun) furent plac\u00e9s sur des orbites comprises entre 12 et 30 rayons terrestres \u00e0 l\u2019int\u00e9rieur de la magn\u00e9tosph\u00e8re<\/a>. Gr\u00e2ce \u00e0 eux les scientifiques purent localiser la source des temp\u00eates g\u00e9omagn\u00e9tiques\u00a0: elle se d\u00e9veloppent \u00e0 partir d’explosions d’\u00e9nergie (appel\u00e9es sous-orages magn\u00e9tiques) qui se produisent \u00e0 un tiers de la distance Terre-Lune<\/a>.<\/p>\n

Le r\u00e9seau Themis a \u00e9galement d\u00e9couvert l’existence de cordes magn\u00e9tiques reliant la haute atmosph\u00e8re terrestre directement au Soleil. Themis a pu d\u00e9tecter des reconnexions entre ces cordes magn\u00e9tiques g\u00e9antes juste avant l’intensification des aurores. Ces reconnexions sont favoris\u00e9es par le passage des particules \u00e9nerg\u00e9tiques qui composent le vent solaire. Des r\u00e9sultats qui faisaient dire alors \u00e0 Vassilis Angelopoulos<\/a>, responsable scientifique de la mission\u00a0: \u00ab Nous avons d\u00e9couvert le myst\u00e8re de la danse des aurores bor\u00e9ales<\/em> \u00bb.<\/p>\n

\"\"<\/a><\/p>\n

Mais pourquoi ces orages magn\u00e9tiques sont-ils plus nombreux aux \u00e9quinoxes\u00a0? Bien qu’il reste encore beaucoup \u00e0 apprendre sur ce sujet, les chercheurs pensent que c’est une question de g\u00e9om\u00e9trie : le mouvement de la Terre sur son orbite provoque un oscillation des p\u00f4les magn\u00e9tiques si on les observe depuis le Soleil. Autour de l’\u00e9quinoxe le champ magn\u00e9tique terrestre est mieux orient\u00e9 pour se connecter avec notre \u00e9toile et le vent solaire s’\u00e9coule plus facilement le long des cordes magn\u00e9tiques d\u00e9tect\u00e9es par Themis.<\/p>\n

Depuis le d\u00e9but de l’automne, il ne se passe pas une nuit sans que la danse des aurores n’illumine le ciel nocturne au niveau du cercle polaire. Un spectacle que l’on peut suivre dans la galerie du site Spaceweather<\/a> dont est extraite cette superbe image de Ron et Marketa Murray<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"

Les semaines qui encadrent les \u00e9quinoxes voient augmenter le nombre d’ aurores polaires. Une co\u00efncidence que l’on commence \u00e0 mieux comprendre. En 2007 le physicien solaire David Hathaway (du Marshall Space Flight Center) pr\u00e9senta un surprenant graphique\u00a0: en compilant 75 ans de donn\u00e9es concernant les perturbations g\u00e9omagn\u00e9tiques, il avait d\u00e9couvert deux pics d’activit\u00e9 correspondant aux … Continuer la lecture de La saison des aurores polaires est ouverte<\/span> →<\/span><\/a><\/p>\n","protected":false},"author":8,"featured_media":8279,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":"","_links_to":"","_links_to_target":""},"categories":[85],"tags":[1136,787,2326,2324,378,1907,2323,1349,2325,2327,618],"class_list":["post-8276","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-meteores","tag-aurore-polaire","tag-champ-magnetique","tag-corde-magnetique","tag-david-hathaway","tag-equinoxe","tag-marketa-s-murray","tag-spaceweather","tag-tempete-geomagnetique","tag-themis","tag-vassilis-angelopoulos","tag-vent-solaire"],"yoast_head":"\nLa saison des aurores polaires est ouverte<\/title>\n<meta name=\"description\" content=\"Les semaines qui encadrent les \u00e9quinoxes voient augmenter le nombre d'aurores polaires. 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