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Cosmic-Ray Scintillations


Owens, Aaron James (1973) Cosmic-Ray Scintillations. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/3NFQ-SF03.


This Thesis reports the observation and theoretical interpretation of a new physical phenomenon. Cosmic-ray "scintillations" are temporal fluctuations in the counting rate of a detector pointing in a fixed direction in space. Power-spectral analyses of energetic-particle counting rate data are used to demonstrate that scintillations are a statistically significant, persistent, and interesting feature of the cosmic-ray flux observed near earth for a wide range of frequencies (10-7 Hz to 10-4 Hz) and energies (~1 MeV to 10 GeV protons, 3 to 12 MeV electrons). The observed power spectra of cosmic-ray scintillations are approximately power laws in frequency with exponents of -1.5 to -2.0, and for protons the relative scintillations are a rapidly-decreasing function of energy.

Quantitative theoretical models, based on generalized quasilinear solutions of the collisionless Liouville equation with a stochastic magnetic field, are presented for the production of cosmic-ray scintillations by random magnetic fields in the magnetosheath and in interplanetary space. It is shown that the ~1-10 MeV proton scintillations observed during quiet times inside the magnetosphere are probably caused by fluctuations in the magnetic field of the magnetosheath. Scintillations of high-energy particles (≳1 GeV/nucleon) are probably generated by the stochastic interplanetary magnetic field. The detailed theoretical prediction for the power spectrum of the flux from neutron monitors, including the effect of the earth's rotation on the interplanetary scintillations model, is calculated and shown to be in excellent agreement with observations from the Alert and Deep River Neutron Monitors. The shape and amplitude of the observed spectra, and in particular a broad enhancement in the Deep River spectrum near one cycle per day, are explained by the theory.

This investigation gives relations for the power spectrum Pj(f) of the cosmic-ray flux of the form

Pj(f)/j02 = A(f) (PB(f)/B02) δ2

where j0 is the average flux, PB(f) is the power spectrum of a component of the magnetic field, B0 is the average magnetic field strength, and δ is the cosmic-ray anisotropy. The factor A(f) is a frequency-dependent function which exhibits enhancements near frequencies corresponding to cyclotron resonances (and near 1 cycle per day for neutron monitors) but which is essentially constant for 1 MeV - 10 GeV proton scintillations at frequencies ≾10-4 Hz. The cosmic-ray scintillations thus can provide information about magnetic fluctuations, and neutron-monitor power spectra can give information about the interplanetary magnetic field from ground-based measurements. The shape of the theoretical spectrum near cyclotron resonances depends strongly on non-linear terms in the generalized quasi-linear equations, so scintillations may provide a useful test of non-linear plasma theories. The agreement of the theory of scintillations with observation supports the standard theory of cosmic-ray diffusion near earth and the relation between the diffusion coefficient and magnetic-field fluctuations. Thus the previously ignored "noise" in the cosmic-ray intensity may contain much useful information.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Physics; Economics
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Physics
Minor Option:Economics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Jokipii, J. Randolph
Thesis Committee:
  • Unknown, Unknown
Defense Date:21 May 1973
Funding AgencyGrant Number
Record Number:CaltechTHESIS:08222018-111448400
Persistent URL:
Related URLs:
URLURL TypeDescription adapted for Part II.
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:11155
Deposited On:27 Aug 2018 17:08
Last Modified:21 Dec 2019 04:28

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