Substorm
Encyclopedia
A substorm, sometimes referred to as a magnetospheric substorm or an auroral substorm, is a brief disturbance in the Earth's magnetosphere that causes energy to be released from the "tail" of the magnetosphere and injected into the high latitude ionosphere. Visually, a substorm is seen as a sudden brightening and increased movement of auroral arcs
. Substorms were first described by the Norwegian scientist Kristian Birkeland
which he called polar elementary storms. Sydney Chapman used the term substorm about 1960 which is now the standard term. The morphology of a substorm was first described by Japanese geophysicist Syun-Ichi Akasofu
in 1964 using data collected during the International Geophysical Year
.
Substorms are distinct from geomagnetic storm
s in that the latter take place over a period of several days, are observable from anywhere on Earth, inject a large number of ion
s into the outer radiation belt, and occur once or twice a month during the maximum of the solar cycle and a few times a year during solar minimum. Substorms, on the other hand, take place over a period of a few hours, are observable primarily at the polar regions, do not inject many particles into the radiation belt, and are relatively frequent—often occurring only a few hours apart from each other. Substorm occurrence becomes more frequent during a geomagnetic storm when one substorm may start before the previous one has completed. The source of the magnetic disturbances observed at the Earth's surface during geomagnetic storms is the ring current
, whereas the sources of magnetic disturbances observed on the ground during substorms are electric currents in the ionosphere
at high latitudes.
Substorms can cause magnetic field disturbances in the auroral zones
up to a magnitude of 1000 nT
, roughly 2% of the total magnetic field strength in that region. The disturbance is much greater in space, as some geosynchronous satellites have registered the magnetic field dropping to half of its normal strength during a substorm. The most visible indication of a substorm is an increase in the intensity and size of polar auroras
. Substorms occur roughly six times per day, though they are more intense and more frequent during a geomagnetic storm. They are divided into three phases: the growth phase, the expansion phase, and the recovery phase.
Aurora (astronomy)
An aurora is a natural light display in the sky particularly in the high latitude regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere...
. Substorms were first described by the Norwegian scientist Kristian Birkeland
Kristian Birkeland
Kristian Olaf Birkeland was a Norwegian scientist. He is best remembered as the person who first elucidated the nature of the Aurora borealis. In order to fund his research on the aurorae, he invented the electromagnetic cannon and the Birkeland-Eyde process of fixing nitrogen from the air...
which he called polar elementary storms. Sydney Chapman used the term substorm about 1960 which is now the standard term. The morphology of a substorm was first described by Japanese geophysicist Syun-Ichi Akasofu
Syun-Ichi Akasofu
is the Founding Director of the International Arctic Research Center of the University of Alaska Fairbanks , serving in that position from the center's establishment in 1998 until January 2007. Previously he had been director of the university's Geophysical Institute from 1986.-Background:Akasofu...
in 1964 using data collected during the International Geophysical Year
International Geophysical Year
The International Geophysical Year was an international scientific project that lasted from July 1, 1957, to December 31, 1958. It marked the end of a long period during the Cold War when scientific interchange between East and West was seriously interrupted...
.
Substorms are distinct from geomagnetic storm
Geomagnetic storm
A geomagnetic storm is a temporary disturbance of the Earth's magnetosphere caused by a disturbance in the interplanetary medium. A geomagnetic storm is a major component of space weather and provides the input for many other components of space weather...
s in that the latter take place over a period of several days, are observable from anywhere on Earth, inject a large number of ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s into the outer radiation belt, and occur once or twice a month during the maximum of the solar cycle and a few times a year during solar minimum. Substorms, on the other hand, take place over a period of a few hours, are observable primarily at the polar regions, do not inject many particles into the radiation belt, and are relatively frequent—often occurring only a few hours apart from each other. Substorm occurrence becomes more frequent during a geomagnetic storm when one substorm may start before the previous one has completed. The source of the magnetic disturbances observed at the Earth's surface during geomagnetic storms is the ring current
Ring current
A ring current is an electric current carried by charged particles trapped in a planet's magnetosphere. It is caused by the longitudinal drift of energetic particles.-Earth's ring current:...
, whereas the sources of magnetic disturbances observed on the ground during substorms are electric currents in the ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
at high latitudes.
Substorms can cause magnetic field disturbances in the auroral zones
Aurora (astronomy)
An aurora is a natural light display in the sky particularly in the high latitude regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere...
up to a magnitude of 1000 nT
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
, roughly 2% of the total magnetic field strength in that region. The disturbance is much greater in space, as some geosynchronous satellites have registered the magnetic field dropping to half of its normal strength during a substorm. The most visible indication of a substorm is an increase in the intensity and size of polar auroras
Aurora (astronomy)
An aurora is a natural light display in the sky particularly in the high latitude regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere...
. Substorms occur roughly six times per day, though they are more intense and more frequent during a geomagnetic storm. They are divided into three phases: the growth phase, the expansion phase, and the recovery phase.