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When? -- does it occur
cycle with AA index:
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.
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 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
Above is background material for archival reference only.