People look upon the geomagnetic substorm today as a bad thing that disrupts their lives, but still it is a fascinating process. What exactly is a substorm anyway? The term "magnetic substorm" is used to describe build-up and release of magnetic energy in the magnetosphere which occurs over approximately two hours.  At the end of the substorm conditions return to normal. 

Evidence of a substorm is seen from the Earth as an explosive increase in auroral luminosity on the night side.  This luminosity intensifies as it expands spatially.   The stable aurora red arc is distinct from the polar auroras and makes its appearance at lower latitudes when there are substorms.  Stable means that it remains for the period of a geomagnetic substorm. It is red and very difficult to see from Earth; however optical instruments make it visible to us.

During a substorm the cross-tail current is disrupted and diverted to the substorm current wedge area of the magnetosphere.  Substorms release magnetic pulsations causing electrical currents to move closer to the Earth connecting the ionosphere of the polar areas with the magnetosphere.  Scientist on Earth use these pulsations to monitor the substorm process.

Current systems are an important contribution to substorm activity. 

The Field Aligned Currents (FAC) provide a structure to guide the energy movement of the substorm along the direction of the magnetic field toward the polar area.  FAC’s are over each of the polar areas of the Earth.  Two large sheets of electric current, one coming down on the morning side of the auroral zone, one going up on the evening side. Typically, each sheet carries a million amperes or more.  Also equatorward of each current sheet there are a less intense parallel sheets flowing in the opposite direction finding their way out of the ionosphere and back out into space.

So at each pole there are four sheets of current two heading toward the Earth and two heading away.

The auroras are the most noticeable consequence of a substorm on the Earth.  High energy electrons strike oxygen atoms high in the atmosphere.  These atoms are first energized and later emit the red light at wavelengths around 630 nanometers as they return to their original energy state.  Green light is created when atoms emit a wavelength around 558 nanometers. When the red emissions are present they usually sit on top of the green emissions.

A large number of current strands are located in the substorm current wedge, and they look like the multiple strands of a necklace. This 2D model show where these current strands are located within the inner magnetosphere. 

By N.A. Tsyganenko

This electric current model by N.A. Tsyganenko (2000) illustrates how the electrical current strands in the current wedge.  During a substorm these current flow lines appear.

Notice that when looking at the South Pole auroras from space, it looks like an eye.  The North Pole is similar. The line down the middle of the eye is called a theta.

Scientists continue to study the geomagnetic substorms in an effort to avoid some of the outstanding risks they present to space bound humans and their equipment. 

Geomagnetic substorm from space

REFERENCE:

http://www.space-plasma.qmw.ac.uk/SPGweb/SPGsubstorm.html