- an outburst of high-energy particles headed directly toward Earth may produce a severe or extreme
- transformer damage and widespread blackouts are possible at higher
latitudes, though power outages are the most severe of the possible effects.
- spacecraft are also at risk, and radio-communication disruptions are
- aurorae are thus seen maximally from the time of arrival at earth and for
the next few hours depending on their intensity and the effect on the
interplanetary magnetic field (IMF) which may be rotated by the incoming
- intense southern aurorae may be seen from the most southern parts of
Victoria away from light pollution
- Geomagnetic storms that ignite auroras actually happen more often
during the months around the equinoxes -- that is, early Autumn and
Spring. Aurora activity in the northern hemisphere tends to be maximal
in the northern autumn (ie. October)
- Geomagnetic storms erupt when solar wind gusts or coronal mass ejections (CMEs)
hit Earth's magnetosphere -- a magnetic bubble around our planet that
protects us from the relentless solar wind. The magnetosphere is filled with
electrons and protons. Normally these particles are trapped by lines of
force (so-called "magnetic bottles") that prevent them from
escaping to space or descending to the planet below.
- Still frames from a digital movie (click image to view movie) showing how coronal mass ejections
compress Earth's magnetosphere and trigger auroras.
- When a CME hits the magnetosphere, the impact knocks loose some of those
trapped particles. They rain down on Earth's atmosphere and cause the air to
glow where they hit. Precipitating particles mostly follow magnetic field
lines that lead to Earth's poles, the auroral ovals (circular regions of
auroral light around the magnetic poles) expand during magnetic storms -
Sometimes they grow so large that people at middle latitudes such as
southern Victoria can see the light.
- Factors affecting geomagnetic activity:
- Variability in the Sun itself that is reflected in the solar wind/IMF
- the 11- and 22-year solar cycles
- The 22-year double-solar-cycle variation in geomagnetic
activity was identified by Chernosky (1966). Activity is higher
in the second half of even-numbered solar cycles and in the
first half of odd-numbered cycles.
- The 11-year variability of the geomagnetic activity can be
divided into three peaks:
- Shortly before sunspot maximum. Linked with transient
solar activity, and seen with relatively larger amplitude in
ring current (storm) activity than in substorm activity.
- About 2 years after sunspot maximum. Largest peak compound
of transient and recurrent magnetic activity (the former
- Descending phase of the solar cycle. Largely recurrent,
and seen with larger amplitude in substorm activity than in
ring current (storm) activity.
- the 1.3
- The Earth's orbit around the Sun taking it to different solar
latitudes (annual variability)
- The annual geomagnetic variation relates to the Earth's orbit. Due
to the 7.2 degrees tilt of the solar rotation axis with respect to
the normal of ecliptic, the Earth reaches the highest northern and
southern heliographic latitude (where solar wind speed is higher by
50km/s on average) on September 6 and March 5, respectively, and
crosses the equator twice a year between these dates. Thus, when
observed from Earth, one should expect a semiannual variation in
solar wind speed with maxima around these dates. However, annual
variation is often more clear, and this is because the solar wind
distribution is asymmetric or shifted with respect to equator.
- The Earth's orbit around the Sun that changes the orientation of
systems (semi-annual variation)
- widespread storms are usually nurtured by what scientists call "Bz"
(pronounced "Bee sub Zee") -- in other words, the component of the
interplanetary magnetic field (IMF)
that lies along Earth's magnetic axis. At the magnetopause, the part of our
planet's magnetosphere that fends off the solar wind, Earth's magnetic field
points north. If the IMF tilts south (i.e., Bz becomes
large and negative) it can partially cancel Earth's magnetic field at the
point of contact. At such times the two fields (Earth's and the IMF) link
up, you can then follow a magnetic field line from Earth directly into the
solar wind. South-pointing Bz's open a door through which energy
from the solar wind can reach Earth's inner magnetosphere.
- In the early 1970's Russell and colleague R. L. McPherron recognized a
connection between Bz and Earth's changing seasons: The average
size of Bz is greatest each year in early Spring and Autumn.
- It's a result of geometry, explains Russell. The interplanetary magnetic
field comes from the Sun; it's carried outward from our star by the solar
wind. Because the Sun rotates (once every 27 days) the IMF has a spiral
shape -- named the "Parker spiral" after the scientist who first
described it. Earth's magnetic dipole axis is most closely aligned with the
Parker spiral in April and October. As a result, southward (and northward)
excursions of Bz are greatest then.
- the north-south component of the IMF controls the
energy flow of the solar wind into our magnetosphere. Northward fields have
little effect, but southward Bz's can set the stage for
substantial geomagnetic activity.
- Rotation of the Sun around its axis:
- which can lead to periodicities at T = 27 days and T = 13-14 days
this activity is called "recurrent" as opposed to
"transient" that the other types are
- The recurrent storm 27 day activity is due to coronal
holes that cause fast solar wind streams.
- In addition, long intervals exists when two high-speed streams per
solar rotation can be seen creating a 13.5-day periodicity.