When? -- does it occur

• Solar cycle with AA index:  
• Great auroral substorm:  
  A Great Auroral Substorm. The great auroral substorm over the Southern Hemisphere as observed with

Dynamics Explorer 1 from 0202 to 0517 UT on 13 June 1983 is shown here in a 16-image sequence. Substorm onset is observed with the first image of the sequence, at upper left. The aurora advances poleward in the midnight sector for ~ 2 ½ hours (the first 12 images) as the spacecraft approaches auroral latitudes and lower altitudes. With the last four images the aurora is observed to decline in luminosity and the area of the dark polar cap increases as the aurora retreats to lower latitudes in the midnight sector. The passband of the optical filter is 136-165 nm, for which the dominant responses are from emissions in the LBH bands of molecular nitrogen. The terrestrial ring current, as measured with the Dst index, is observed to grow rapidly during the expansive phase of this substorm. Hourly averages of the Dst index decrease by 150 nT (nanoteslas) in the four-hour period beginning with substorm onset.
• A magnetic substorm:  
  Auroras occur during magnetic substorms, an example of which is shown here. The northward component of the Earth's magnetic field measured at a series of magnetic obseratories across Canada (top four lines:  Great Whale River to Yellowknife) and Alaska (bottom two lines:  College and Barrow).  Each vertical tick mark is 100 nT.  At about 0850 UT (universal time, or Greenwich mean time) the Canadian stations show a sudden decrease in intensity of the magnetic field due to an electric current nearly overhead in the westward direction (at an altitude of about 100 km).  Such a "line current" creates a magnetic field that is southward below the current.  At about 0920 UT the two Alaskan stations observe a sudden change, with the College trace going first positive (an eastward current), then negative.  These magnetic variations are limited in location to the auroral oval; thus the name substorm in contrast to a magnetic storm, which is worldwide.
• A magnetic storm:  
  A magnetic storm is observed worldwide. Large substorms often occur during the development of the magnetic storm main phase. Magnetic variations at both College (near Fairbanks), Alaska and Honolulu, Hawaii.  Note the long period of magnetic activity at College, and especially the identification of substorms when the station rotated to local midnight at 1000 UT.  In both the Honolulu H trace and the College H trace, note the sudden increase in the H component at 0000 UT.  This is called a "sudden commencement" and is seen worldwide.  It is caused by a sudden compression of the entire magnetosphere due to an increase in pressure of the solar wind.  At about 0330 UT, the H trace at Honolulu decreases to a minumum at about 0530 UT.  This decrease is called the main phase of a magnetic storm.  Such a decrease would be observed at mid and low latitude locations around the globe.  It takes many hours, even days, for the Earth's magnetic field to recover.  This large decrease in the magnetic field is due to an increase in the intensity of the Earth's trapped radiation, mainly protons, creating a westward electric current in a loop around the Earth near the equator at a distance of about 3 Earth radii.  This current is called the "ring current".  Such a current loop creates a magnetic field with a southward component inside the loop (on the ground), opposite to the Earth's field from internal sources.

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Last updated:  7/26/00