Australian Antarctic Division Cosmic Ray Real Time Data at IPS
Historical Australian Antarctic Division Cosmic Ray data from Kingston in Tasmania, and Mawson in Antarctica are
available in the IPS World Data Centre.
Background
Cosmic rays consist mainly of protons. They can originate from
galactic cosmic radiation or from the Sun. Cosmic rays are
observed indirectly by a device known as a neutron monitor. When cosmic ray
particles enter the Earth's atmosphere they interact with the
nuclei of the air molecules to produce secondary radiation. This
consists of pions (which decay to muons) and a shower of protons and
neutrons. The neutrons predominate in this secondary radiation because
the protons, being charged are more easily attenuated in subsequent travel.
The cosmic ray detector actual detects these secondary neutrons and as
a consequence is referred to as a neutron monitor.
Cosmic Ray Data Applications to Space Weather Forecasting
Forbush Decrease Event
The magnetic fields entrapped in and around coronal mass ejections exert
a shielding effect on the galactic cosmic radiation (GCR) which is detected
by the neutron monitors. This causes a reduction in the count rate from
the monitor. The reduction is typically from about 3 to 20%. The reduction
occurs typically over a timescale of several hours to a few days.
Forbush decrease events must be at least 3% for a Forbush decrease alert to be issued.
The reduction in the GCR due to a coronal mass ejection (CME) is dependent upon:
- the size of the CME
- the strength of the magnetic fields in the CME
- the proximity of the CME to the Earth
Because the reduction is dependent on three factors (rather than one), it is difficult
to forecast the time from a Forbush Decrease to the arrival of a coronal mass ejection
at the Earth. However, previous experience in IPS is that a Forbush Decrease is
a reliable indicator of a geomagnetic storm, and that warning times of up to
24 hours or more may be made. The Forbush Decrease can be used in conjunction with
other indications (eg coronagraph imagery) to further confirm the event. Detection of
a Forbush Decrease is in use at the IPS ASFC for assistance in prediction of
geomagnetic storms.
Ground Level Event
In this case, an increase in detector count rate is not due to galactic cosmic
radiation, but to the addition of solar cosmic (high energy) radiation (solar
cosmic rays) from an previous solar particle event (SPE). The increase in the
count rate may be from about 3 to 10,000%. The increase from ambient to peak
count rate may take only a few minutes (fast high energy event) to an hour or two
(slow and usually lower energy event). The return if the count rate to galactic
cosmic ray background may take anywhere from a few hours to a few days.
Neutron Monitor Data from Mawson, Antarctica
Data is transferred at 5 minute intervals, 1 record per minute. For slow ground
level event detection (increase in count rate) and forbush decrease (decrease in count rate)
hourly averages of the minute data are used. Alert thresholds (above and below) have been set
at 5 times the standard deviation obtained over a 48 hour period. In addition Forbush decrease
events must be at least 3% for a IPS Forbush Decrease alert to be issued. The threshold shown in the
plots are the 5 times standard deviation threshold. The requirement of at least a 3% decrease
is necessary for when the cosmic ray data exhibits very little variation, bringing the 5 times
standard variation threshold potentially very close in counts to small data count fluctuations, increasing
the risk of false alarms being produced.
For fast ground level events the minute data is used with a increase
perentage threshold of 10%.
The above thresholds are experimental and may need to be adjusted with experience.
The data displayed below is provided by the Australian Antarctic program
in near real time. A neutron monitor located in Antarctica has no geomagnetic
(lower) cutoff energy (although there is atmospheric cutoff energy), and thus
provides the most sensitive indication of cosmic radiation of any sensor located
on the Earth's surface.
Realtime Cosmic Ray detectors (Neutron Monitors) operated by other organizations are also available:
Fast Ground Level Event Detection Plots:
Geophysical 
MAG: Normal