substorm

sub- +‎ storm

substorm (plural substorms)

1. (astronomy) A phenomenon in which a magnetotail becomes swollen and unstable.
   • 2007 January 23, Warren E. Leary, “Five New Satellites With a Mission of Finding a Source of Color in Space”, in New York Times‎:

     At times, however, energy builds up within the magnetic field until it suddenly breaks loose into what is called a substorm, releasing a burst of electrical current that turns the auroras into pulsing red, purple and white colors.

• strombus