Milky Way
All but the highest energy cosmic rays originate in our galaxy.
It is thought that supernovae are the primary source of these high energy particles.


Cosmic radiation is the name given to high energy particulate radiation that pervades the solar system (and the rest of the galaxy). The primary radiation consists mostly of high energy protons with energies ranging up to 1021 electrons volts. When this radiation passes through the Earth's atmosphere it interacts with the nuclei of air molecules and produces a range of secondary particles. The most prolific of these are mu-mesons (muons), and these can be detected at the Earth's surface.

Cosmic ray interaction with the Earth's atmosphere
Primary cosmic radiation intereacts with the Earth's atmosphere producing
secondary particles which are those detected on the Earth's surface.

The primary galactic cosmic radiation (GCR) influx to the solar system is relatively constant. The flux measured at ground level, however, can vary due to three factors. The first is the Earth's magnetic field which deflects the lower energy particles and thus acts as a partial shield. This affect varies according to the position on the Earth.

The next effect is a shielding due to the Sun's magnetic field. This varies according the magnetic flux carried into the interstellar medium by the solar wind. In particular, a large coronal mass ejection (CME) carries with it a large magnetic field, and this can shield the Earth from the lower energy GCR. As such a CME travels out from the Sun past the Earth it produces what is called a Forbush decrease in the GCR flux. This is recorded by an Earth based GCR monitor and can bhe used to give advance warning of a CME.

The third and last effect is due to a solar particle event (SPE) where protons and a few higher mass nuclei are accelerated away from the Sun. Only the highest energy SPEs have sufficient energy to penetrate the Earth's magnetic field and reach the surface where they produce a ground level event (GLE), and increase in surface radiation that can be detected by a cosmic ray monitor. These types of events are very dangerous to astronauts and spacecraft.


A mu-meson monitor is installed at the ASA Meckering campus. This consists of a large volume GM tube. The pulses produced by this sensor are fed to a digital interface and counted by an Arduino microcontroller. When a count of 10,000 is reached the data is sent to a Raspberry Pi single board computer (SBC) which computes the count rate and the standard deviation of the count rate (assuming Poisson statisics). This data is archived and graphed.

As the mu-mesons are produced by interaction with the Earth's atmosphere, the flux rate is dependent on the atmospheric pressure at the production height. This pressure is not available, but a correction for pressure is made using ground level pressure which is monitored by the system.


The data from the cosmic ray monitor is presented as a daily (UT) graph and uploaded at 0030 UT. Error bars (2 sigma) are also plotted along with the count rate. The current daily graph is shown below.

Mu-meson count rate


ASAAustralian Space Academy