Cosmic rays are highly accelerated protons which bombard the
atmosphere constantly. They are high energy radiation which are produced mainly
outside the Solar System. Though there are a wide range of potential sources
such as active galactic nuclei or quasars the only two sources which have been
identified with higher certainty are gamma ray bursts and supernovae. The Fermi
Observatory’s data has been analysed to show that the supernovae produces
approximately between 3 * 1042 and 3 * 1043.
These cosmic rays can be subdivided into two categories:
Galactic Cosmic Rays (GCR) or Solar energetic particles (SEP). GCR is produced
external to our solar system while the SEP are emitted by our sun. However,
when we talk about cosmic rays, we refer mainly to the GCR because the sun’s
emitted particles are too few to make a difference. These rays are composed of
99% protons and alpha particles (hydrogen and helium nuclei respectively) as
well as around 1% of heavier nuclei and less than 1% of positrons and
antiprotons.
And within the cosmic rays (CR), there are 2 sub-categories
of rays: primary and secondary. The primary CRs are the ones consisting mainly
of protons and alpha particles while the secondary CRs are what is caused when
the primary rays collide with nuclei in the atmosphere. Here is a diagram of
the “shower” of particles which are a result of the collisions:
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Once the proton collides with the nucleus of an atom in the
atmosphere, it produces pions, which are incredibly unstable and decay quickly
into muons. The flux (number) of muons can be measured at sea level through the
use of some relatively easy methods. As the muon is also unstable, it has a
short lifetime but travels at a speed of c/10. If the trajectory of a muon was
considered non-relativistically, then, taking into account the distance from
the atmosphere to sea level and the half-life of a muon, there should only be
0.3 muons being detected at sea level out of every 1 million generated in the
atmosphere. However, this is much less than the value we measure. But if
considered relativistically, since the muon is travelling at a speed comparable
to the speed of light, there will be time dilation occurring and length
contraction. So, factoring these in as well, we arrive at the much closer value
of 49,000 per every million muons reaching sea level.
As these cosmic rays mostly originate from outside the Solar
System, the flux varies with the solar wind, which is plasma emitted by the
Sun. The solar winds, at around a distance of 94 AU (astronomical units) away
from the sun, undergo a reduction in motion called the termination shock. The
solar winds change from travelling at supersonic speeds to subsonic speeds. The
region between the termination shock and the heliopause (the outermost parts of
the area over which the sun has influence over with its solar winds), acts as a
barrier to cosmic rays. So when the solar wind activity dips, this barrier will
drop temporarily and thus more cosmic rays (GCR) will enter. The flux blocked,
however, is only at low energies (less than 1 GeV). Also, the Earth’s magnetic
field blocks cosmic rays from hitting the atmosphere so, because the magnetic
field strength varies depending on the latitude and longitude, the flux will
also vary with these parameters. These cosmic rays are deflected towards the
poles by the magnetic field lines, which causes the Aurora Borealis.
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