The THEMIS mission,short for the Time History of Events and Macroscale Interactions During Substorms,studied what powers geomagnetic substorms – atmospheric events that appear in the Northern Hemisphere as a rapid brightening of the northern lights,or aurora borealis.
During a substorm,massive amounts of solar energy previously trapped in the Earth's magnetic field are released,causing the aurora,usually seen as a greenish-white band of light,to break apart and brighten brilliantly.
The goal of the THEMIS mission was to pinpoint what was triggering these energy releases,said Nicky Fox,deputy project scientist for the Living With a Star,Radiation Belt Storm Probe Mission. Themis is the Greek goddess of justice.
“This question is one that has puzzled scientists since the beginning of the space age,” THEMIS principal investigator Vassilis Angelopoulos said.
Data from the five THEMIS satellites revealed that substorms are sparked by an explosion of magnetic energy known as magnetic reconnection occurring in a region of space about a third of the way from Earth to the moon.
“Reconnection occurs when the Earth's magnetic field absorbs energy from the sun and the solar wind,and it does that by stretching the Earth's magnetic field like rubber bands far,far off into the distance,” THEMIS project scientist David Sibeck said. “You can only stretch them so far before they snap.”
When the Earth's stretched magnetic field “snaps” back together,Sibeck said,energy is flung back to the Earth and can create a substorm,which causes the sudden brightening of auroras.
The THEMIS mission marked the largest number of scientific satellites launched into orbit by a single rocket,in February 2007. The five satellites – each about the size of a washing machine – orbited at different points in space and timed substorm actions to determine their trigger.
Twenty ground observatories across Alaska and Canada coordinated with the satellites to provide additional data,including digital images of auroras and measurements of the changes in Earth's magnetic field.
The effects of space weather on Earth can be violent,Fox said,disrupting power lines,pipelines and communications.
This further understanding of substorms will help scientists create a physics-based model that can predict when and where substorms will occur,and at what intensity,Angelopoulos said.
“Our goal is to understand the basic physics of these processes by understanding where reconnection is about to happen – what location that is and what time it is about to happen – and put them within our large scale models of the sun-Earth interaction,” Angelopoulos said.