Effective atmospheric losses for 125-Mev protons in South Atlantic Anomaly

Abstract

Orbit calculations are carried out to evaluate the effective atmospheric densities for geomagnetically trapped 125‐Mev protons whose guiding centers mirror between 200‐ and 560‐km altitude in the South Atlantic anomaly. Such orbits are limited to the region of B‐L space L = 1.38 and 0.2043 < Bm < 0.2355 gauss. Calculated are the effective atmospheric densities and scale heights experienced by the particle and its guiding center. Rates of energy loss by ionization are also calculated, taking into account atmospheric composition and the ionization energies for each constituent. Atmospheres used in the computations are the Harris and Priester S = 100 and S = 200 models, each diurnally averaged, and the Johnson model for solar minimum. The geomagnetic field is described by the 48‐term spherical harmonic expansion of Jensen and Cain. The numerical results show that for the trajectories considered, the effective atmospheric density averaged over the particle's motion is about twice the density averaged over the guiding center's trajectory. Also, the effective atmospheric scale‐heights can be three to four times the scale height of the model atmosphere, depending upon altitude and time in the solar cycle.

Low-altitude trapped protons during Solar Minimum Period, 1962-1966

“…The calculational results ·of Heckman and Brady [1966] show that within the range of altitudes we are considering the effective atmospheric density scale heigHt lS not constant, but increases approximately in proportion to altitude. The proportionality factor depends on, and increases with, solar activity, and is demonstrated in Figure 4a.…”

“…[Heckman and Brady, 1966}. mIn Measurements of proton.flux scale heights via east-west asymmetry measurements in the anomaly thus can be directly compared with model atmospheric scale heights.…”

Solar modulation of dose rate onboard the Mir station

“…Since the proton lifetimes are determined by energy losses to electrons and collisions with atmospheric nuclei, it is the average atmospheric density along a trapped particle trajectory that should best correlate with proton fluxes. Such calculations were first done by Heckman and Brady [12] and have been stressed recently by Lemaire et al [17]. Kern [18] extended the work of Ray [15] and showed that the trajectory averaged density, in a dipole field, is proportional to the mirror point density .…”

“…Proton lifetimes within the belts are determined primarily by energy losses to electrons and by collisions with atmospheric nuclei. Cornwell et al [11] and Heckman and Brady [12] computed the average atmospheric density for a model atmosphere. Dragt [13] extended these studies to provide the variation of trapped particle fluxes as a function of solar activity.…”

Space Weather Quarterly Volume 4, Issue 3, 2007

“…Their model fundamentally influenced radiation belt proton research for a number of years. Blanchard and Hess [1964;Hess, 1968] and others [Cornwall et al, 1965;Heckman and Brady, 1966;Dragt, 1971] subsequently used the Harris and Priester model to conclude that the inner-belt proton flux J varied inversely with density r, that is J(r) � r…”