Physics – Space Physics
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........14b&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF KANSAS, 1995.Source: Dissertation Abstracts International, Volume: 56-07, Section: B, page: 3818.
Physics
Space Physics
Interplanetary Medium, Solar Flares, Shock Waves
Scientific paper
Low energy charged particles with energies ranging from 0.3 to 2 MeV are nearly always present in the environment of the Earth. Specific solar flare events and interplanetary shock waves are identified as producing or enhancing these fluxes. However, interplanetary particles are observed even in the absence of solar flares. The explanation of the presence of these proton fluxes in the interplanetary medium and accounting for their variations is a major problem in space physics. Observations of interplanetary proton fluxes have been made continuously at 1 AU with IMP 8 from 1973 to the present and in the 1-5 AU range by Voyager 1 and 2 (1977-78), and Ulysses (1990-91). Daily-averaged proton fluxes of IMP 8, Voyager 1 and 2, and Ulysses have been carefully interpolated to matching energy passbands so that fluxes in the same passbands at two radial distances could be compared. These daily averaged fluxes were then compared as ratios, autocorrelation, and cross correlation as functions of time delay. The radial gradient, the energy spectra and the distribution of these proton fluxes were also examined. The results showed that protons in the 0.3 to 5 MeV energy range using the Voyager 1/IMP 8, Voyager 2/IMP 8, and Ulysses/IMP 8 paired observations in the 1 to 5 AU in-ecliptic region tend to "decorrelate" within increasing radial separation and become uncorrelated by about 4 or 5 AU. Higher energy fluxes decorrelate less rapidly, and lower energy proton fluxes have smaller radial gradients than higher energy. The radial gradient of 0.3 to 0.5 MeV proton fluxes is dominantly positive for 1-5 AU, whereas the radial gradient of 2 to 4 MeV proton fluxes is negative. We conclude that 0.3 to 0.5 MeV protons are much more subject to interplanetary acceleration than 2 to 5 MeV protons. The results also showed that radial gradients are robust and persist even if all heliolongitude coherence is purposely removed by shuffling the time order. Our interpretation of this is that modulation of particle fluxes and spectra are dominated by interplanetary acceleration. Interplanetary shocks are the most likely agents to produce this acceleration. We suspect that "SDA" (Shock Drift Acceleration) is more effective here than diffusive shock acceleration because of the IMF (Interplanetary Magnetic Field) geometry producing the high shock normal angle for which "SDA" is most efficient. This interplanetary process or "SDA" maintains and redistributes the intensities of 0.3 to 1 MeV protons, whereas most of the intensities of higher energy protons (>1 MeV) are unaffected by this mechanism and originate within 1 AU (as would be due to solar flares).
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