THEMIS is a multi-spacecraft and ground-based programme to study the aurora borealis, or northern lights. This was my field of expertise as a research scientist, and in years past I worked closely with some of those involved with the THEMIS mission.
Auroras are the result of energetic particles ejected by the Sun impacting on the Earth’s upper atmosphere. The colourful displays are due to electrons which collide with and transfer energy to atmospheric constituents such as nitrogen and oxygen at altitudes of between 100 and 300 kilometres.
When the affected atoms and molecules relax back to their ground state, they release energy in the form of light of various colours. Diffuse red light is from atomic oxygen at higher altitudes, while the more defined blue and green structures relate to molecular nitrogen and atomic oxygen lower down.
As a space physicist, my main area of interest was fine-scale structure in the aurora. THEMIS will look more at macro-scale mechanisms out in the Earth’s magnetosphere that trigger the large-scale displays known as auroral substorms.
Magnetic field lines carried by the solar wind interact with the Earth’s field in a process known as magnetic reconnection. Through reconnection, field lines on the sunward side of the magnetosphere (see graphic) are swept behind the planet into the tail of the magnetosphere, which is drawn out like the tail of a comet.
During magnetic substorms, the solar wind dumps vast amounts of energy into the Earth’s magnetosphere, which leads to the snapping of field lines at a second reconnection site in the tail. Through reconnection, streams of charged particles in the magnetotail are accelerated Earthwards and precipitate into the upper atmosphere, causing the aurora.
But what exactly triggers a substorm? Space physicists have been arguing for decades about substorm triggering, the problem being that without a three-dimensional perspective, it is impossible to resolve the temporal and spatial changes taking place in the magnetosphere that give rise to magnetic storms.
With four spacecraft flying in formation, it is possible to visualise what is happening in four dimensions: three of space and one of time. The fifth spacecraft in the THEMIS flotilla is a spare in case one of the others should fail.
THEMIS is not the first multi-spacecraft mission looking at space plasmas. Between 2000 and 2003, I worked on the European Space Agency (ESA) Cluster mission, which consists of four identical satellites flying through the magnetosphere and solar wind in a tetrahedral formation. And in collaboration with ESA, the Chinese have a two-spacecraft mission called Double Star.
Along with the five THEMIS spacecraft, 20 ground stations in Alaska and Canada equipped with all-sky cameras and magnetometers will observe auroral light shows and electric currents flowing through the magnetosphere and upper atmosphere of the Earth. My good friend and former research collaborator, Trond Trondsen, who works with the University of Calgary and Keo Scientific Ltd., designed the cameras that are part of the ground-based component of THEMIS.
Working on space missions is exciting, but, for those like me who prefer fieldwork to being chained full-time to a desk and computer in some city-based research institute, ground-based observations are far more interesting.
Trond Trondsen and I used to record auroral displays from the Svalbard archipelago that lies midway between Norway’s north coast and the north pole. When not watching out for polar bears, we studied how filaments of auroral light less than 100 metres across relate to plasma instabilities detected in EISCAT radar observations of the ionosphere.
The dayside aurora Trond and I observed from Svalbard are not the same as those of interest to the THEMIS community, but the underlying physical principles are the same.
Associated with auroral arcs and magnetic substorms are intense and transient electric currents that flow along magnetic field lines and short-circuit through the ionosphere. The currents drive instabilities in the ionised upper atmosphere, and the instabilities lead to the kind of intricate and dynamic structures we see in the aurora. Click here for a series of still images from the Portable Auroral Imager (PAI) showing examples of Kelvin-Helmholtz instabilities.
It was challenging work Trond and I did on Svalbard, requiring state-of-the-art image-intensified video cameras, and some especially detailed analysis of the radar data. Trond has since focused on developing further his auroral cameras, and is marketing them through Keo Scientific Ltd.
With THEMIS, the Canadian aurora watchers are now deploying their imaging systems on a large scale. The THEMIS all-sky imagers are highly-specialised video cameras with special fish-eye lenses and image intensifiers that enable the gathering of light too feint to be detected by the unaided eye.
Beautiful though auroras are to watch and film, there is a practical reason for studying substorms and identfying the mechanisms that give rise to such explosive releases of energy in the Earth’s space environment.
When the Sun is particularly active, massive eruptions on its surface lead to magnetic storms that can have severe economic consequences here on Earth.
Electric currents in space induce currents on the ground, and there have been numerous cases of magnetic storms causing power outages, frying satellite electronics and leading to communications blackouts.
THEMIS will provide space scientists with the first global look at substorms from the Earth’s upper atmosphere through to the magnetosphere, and hopefully show where, when and how substorm initiation begins.
* THEMIS: “Time History of Events and Macroscale Interactions during Substorms”. In Greek mythology, Themis is the goddess of justice, wisdom and good counsel, and the guardian of oaths.