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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Cosmic ray | 3/8 | https://en.wikipedia.org/wiki/Cosmic_ray | reference | science, encyclopedia | 2026-05-05T13:32:02.154183+00:00 | kb-cron |
=== Energy distribution === Measurements of the energy and arrival directions of the ultra-high-energy primary cosmic rays by the techniques of density sampling and fast timing of extensive air showers were first carried out in 1954 by members of the Rossi Cosmic Ray Group at the Massachusetts Institute of Technology. The experiment employed eleven scintillation detectors arranged within a circle 460 metres in diameter on the grounds of the Agassiz Station of the Harvard College Observatory. From that work, and from many other experiments carried out all over the world, the energy spectrum of the primary cosmic rays is now known to extend beyond 1020 eV. A huge air shower experiment called the Auger Project is currently operated at a site on the Pampas of Argentina by an international consortium of physicists. The project was first led by James Cronin, winner of the 1980 Nobel Prize in Physics from the University of Chicago, and Alan Watson of the University of Leeds, and later by scientists of the international Pierre Auger Collaboration. Their aim is to explore the properties and arrival directions of the very highest-energy primary cosmic rays. The results are expected to have important implications for particle physics and cosmology, due to a theoretical Greisen–Zatsepin–Kuzmin limit to the energies of cosmic rays from long distances (about 160 million light years) which occurs above 1020 eV because of interactions with the remnant photons from the Big Bang origin of the universe. Currently the Pierre Auger Observatory is undergoing an upgrade to improve its accuracy and find evidence for the yet unconfirmed origin of the most energetic cosmic rays. High-energy gamma rays (>50 MeV photons) were finally discovered in the primary cosmic radiation by an MIT experiment carried on the OSO-3 satellite in 1967. Components of both galactic and extra-galactic origins were separately identified at intensities much less than 1% of the primary charged particles. Since then, numerous satellite gamma-ray observatories have mapped the gamma-ray sky. The most recent is the Fermi Observatory, which has produced a map showing a narrow band of gamma ray intensity produced in discrete and diffuse sources in our galaxy, and numerous point-like extra-galactic sources distributed over the celestial sphere.
== Solar modulation == Solar modulation theory explains how the intensity of cosmic rays changes as they travel through the heliosphere, influenced by the solar wind and magnetic field. The solar cycle causes variations in the magnetic field of the solar wind through which cosmic rays propagate to Earth. Solar modulation is a quasiperiodical change in cosmic rays intensity caused by 11- and 22-year cycles of solar activity.
== Parker transport equation == The Parker transport equation (also called the Parker equation, for Eugene Parker) is a kinetic equation that describes the acceleration and transport of energetic particles in astrophysical plasmas. The equation comprises diffusion terms both in coordinate space and in momentum space. The equation is used for studying energetic particle transport, and the mechanism of cosmic ray acceleration can be derived from it (diffusive shock acceleration). The equation is also used "to study the acceleration of cosmic rays at supernova remnant shocks, the acceleration and transport of solar energetic particles at shocks driven by coronal mass ejections, and the acceleration and transport of energetic particles at the solar wind termination shock". The Parker transport equation in one dimension is:
∂
f
∂
t
+
U
∂
f
∂
x
−
∂
∂
x
(
k
∂
f
∂
x
)
−
p
3
∂
U
∂
x
∂
f
∂
p
−
1
p
2
∂
∂
p
(
p
2
D
p
p
∂
f
∂
p
)
=
Q
{\displaystyle {\frac {\partial f}{\partial t}}+U{\frac {\partial f}{\partial x}}-{\frac {\partial }{\partial x}}\left(k{\frac {\partial f}{\partial x}}\right)-{\frac {p}{3}}{\frac {\partial U}{\partial x}}{\frac {\partial f}{\partial p}}-{\frac {1}{p^{2}}}{\frac {\partial }{\partial p}}\left(p^{2}D_{pp}{\frac {\partial f}{\partial p}}\right)=Q}
where:
f
(
x
,
p
,
t
)
{\displaystyle f(x,p,t)}
is the omni-directional distribution function of energetic particles
U
{\displaystyle U}
is the plasma (fluid) bulk velocity
k
(
x
,
p
)
{\displaystyle k(x,p)}
is the spatial diffusion coefficient
D
p
p
{\displaystyle D_{pp}}
is the diffusion coefficient in momentum space
Q
{\displaystyle Q}
is the source term
p
{\displaystyle p}
is the momentum
x
{\displaystyle x}
is the position
t
{\displaystyle t}
is the time
== Sources == Early speculation on the sources of cosmic rays included a 1934 proposal by Baade and Zwicky suggesting cosmic rays originated from supernovae. A 1948 proposal by Horace W. Babcock suggested that magnetic variable stars could be a source of cosmic rays. Subsequently, Sekido et al. (1951) identified the Crab Nebula as a source of cosmic rays. Since then, a wide variety of potential sources for cosmic rays began to surface, including supernovae, active galactic nuclei, quasars, and gamma-ray bursts.