Radial and longitudinal dependence of solar 4-13 MeV and 27-37 MeV proton peak intensities and fluences: Helios and IMP-8 observations

Details Radial and longitudinal dependence of solar 4-13 MeV and 27-37 MeV proton peak intensities and fluences: Helios and IMP-8 observations Radial and longitudinal dependence of solar 4-13 MeV and 27-37 MeV proton peak intensities and fluences: Helios and IMP-8 observations

May 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agusmsh44a..05l&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #SH44A-05

Physics

7514 Energetic Particles (2114), 7984 Space Radiation Environment, 2114 Energetic Particles (7514), 2139 Interplanetary Shocks

Scientific paper

We study the radial and longitudinal dependence of the 4-13 MeV and 27-37 MeV proton peak intensities and fluences measured during intense solar energetic particle events of the solar cycle 21 by the Interplanetary Monitoring Platform-8 (IMP-8) and the Helios -1 and -2 spacecraft. Simultaneous measurements of individual events by these spacecraft show that the dominant parameter that determines the peak intensity and fluence is not the heliocentric radial distance of the spacecraft (which we designate as R) but the longitudinal distance (designated as φ) between the parent active region and the footpoint of the magnetic field line that connects each observer with the Sun. Radial distributions of peak intensities and fluences measured during those events observed by at least two or three spacecraft show, on average over the ensemble of events, radial variations that range from R-2.5 to R-1.6 for 4-13 MeV and 27-37 MeV proton peak intensities, respectively, and R-1.9 to R-0.9 for 4-13 MeV and 27-37 MeV proton event fluences, respectively. These radial dependences contrast with those inferred from diffusion transport models (i.e., R-3.3 for peak intensities and R-2.1 for event fluences). We approximate the longitudinal distributions of peak intensities and fluences by a functional form e-k(φ-φ0)2 where φ0 is the centroid of the distribution and k is found to be about ~1/rad2. Longitudinal distributions are not centered around the footpoint of the magnetic field line connecting the observers with the Sun, but toward the east with an angular distance with respect to the observers magnetic footpoints that decreases with the energy of the particles. Individual events observed by two spacecraft with close nominal magnetic connection but at different radial distances show peak intensities and fluences that follow radial dependences less steep than the recommendations given to extrapolate peak intensities and fluences from 1 AU to inner radial distances. These recommended radial dependences (i.e., R-3 and R-2.5 for peak intensities and fluences, respectively) lead to an overestimation of peak intensities and fluences at heliocentric radial distances R<1 AU.

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