Multipoint measurements of magnetospheric substorms: onset timing and tail reconnection rates

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The Earth possesses a magnetic field which is approximately dipolar in shape - very similar to the magnetic field produced a simple bar magnet. Magnetic field lines emerge from the planet at one magnetic pole and extend out of the atmosphere and many thousands of kilometres into space, before returning to the magnetic pole in the opposite hemisphere. Rather than being a vacuum, the region of space that these field lines pass through is filled with plasma - an electrically conducting gas made up charged particles. Most of these particles originate in the Earth's atmosphere having been produced by ultraviolet sunlight which ionises gases in the high altitude atmosphere. The Sun also possesses a strong magnetic field. As nuclear processes generate energy in the solar interior, the outer layer of the solar atmosphere expands outwards through the solar system (forming the solar wind), and carries with it remnants of the Sun's magnetic field (the interplanetary magnetic field). When the solar wind and interplanetary magnetic field arrive at the Earth, they collide with the Earth's magnetic field and are diverted around the planet. The cavity carved out of the solar wind by the Earth's magnetic field is called the magnetosphere. Inside the magnetosphere the plasma and magnetic field originate mainly from the Earth. Outside of the magnetosphere, they originate from the Sun. At the boundary between the interplanetary and terrestrial magnetic fields on the dayside of the Earth, the field lines sometimes orient themselves in opposite directions. When this happens, the field lines can merge or 'reconnect' across the boundary. In other words, closed magnetic field lines that start and finish at the Earth's surface in opposite hemispheres can be opened so that one end stays fixed to the Earth while the other extends outwards into the solar wind. Since the solar wind is constantly streaming away from the Sun, the newly-opened magnetic field line is dragged and stretched away from the Earth. Therefore, because of the process of magnetic reconnection at the dayside boundary, the Earth's dipolar magnetic field is stretched out on the planet's nightside to form a long magnetic tail that points away from the Sun. If the Earth's magnetic field was continuously being peeled away and dragged into the tail, eventually there would be no field left on the dayside of the planet. However, a process in the tail periodically acts to reduce the amount open magnetic field in the tail and return closed field to the dayside - this process is magnetic reconnection. By reconnecting two open magnetic field lines a closed magnetic field is produced (like typing together the two loose ends of a piece of elastic). However, the resulting closed field is highly stretched and, just like a stretched elastic band, it contracts back towards the Earth, catapulting some of the magnetospheric plasma Earthward. The reconnection process in the tail is not steady. Generally magnetic field builds up in the tail until some critical point is reached. Somehow, reconnection is triggered and stretched magnetic field is removed from the tail and returned to the Earth. The period when tail field is building is known as the substorm growth phase, while the explosive release of energy in the tail associated with reconnection and the closure of open field lines is known as the substorm expansion phase. However, the processes that cause the triggering of the expansion phase (i.e. that mechanisms that trigger the catapult) remain unclear - it is one of the biggest uncertainties in solar-terrestrial physics. This investigation will use measurements from instruments on spacecraft located in the tail and observations made from the Earth in order to determine the triggering mechanism of magnetospheric substorms.