Cluster reveals secrets of black aurora
Data from the European space agency (ESA) Cluster spacecraft has revealed detailed information on the nature of black aurorae, dark shapes which appear amongst the glowing colours of the northern and southern lights. Swedish and British researchers used the four Cluster spacecraft to make detailed observations of the regions where black aurorae are generated. The Cluster data showed that the dark patches occur where there are holes in the ionosphere, a layer of the atmosphere filled with billions of positively and negatively charged particles. Here, the particles that make up the ionosphere shoot upwards into space in regions known as 'positively charged electric potential structures.' This is the opposite process to that which creates visible aurorae, where electrons spiral down from space into the atmosphere with similar, but negatively charged, structures. Black aurorae are therefore a kind of 'anti-aurora' where the conditions are the exact opposite of those in the normal aurora. 'The black aurora isn't actually an aurora at all, it's a lack of auroral activity in a region where electrons are 'sucked' from the ionosphere,' explained Professor Göran Marklund of the Alfvén laboratory in Sweden. 'Now, with the aid of the four Cluster spacecraft, we have been able to study for the first time the complex physical processes that create these auroral holes,' he said. 'Cluster has allowed us to discover how the huge vertical structures associated with the black aurora form, how long they last and how they vary with altitude.' He added: 'Understanding the development and growth of these dynamic structures associated with the aurora is a major goal of the Cluster mission, and something which cannot be solved by single satellite measurements.'