On the heliolatitudinal variation of the galactic cosmic-ray intensity. Comparison with Ulysses measurements

G Exarhos,X Moussas

doi:10.5194/angeo-21-1341-2003

Abstract

Abstract. We study the dependence of cosmic rays with heliolatitude using a simple method and compare the results with the actual data from Ulysses and IMP spacecraft. We reproduce the galactic cosmic-ray heliographic latitudinal intensity variations, applying a semi-empirical, 2-D diffusion-convection model for the cosmic-ray transport in the interplanetary space. This model is a modification of our previous 1-D model (Exarhos and Moussas, 2001) and includes not only the radial diffusion of the cosmic-ray particles but also the latitudinal diffusion. Dividing the interplanetary region into "spherical magnetic sectors" (a small heliolatitudinal extension of a spherical magnetized solar wind plasma shell) that travel into the interplanetary space at the solar wind velocity, we calculate the cosmic-ray intensity for different heliographic latitudes as a series of successive intensity drops that all these "spherical magnetic sectors" between the Sun and the heliospheric termination shock cause the unmodulated galactic cosmic-ray intensity. Our results are compared with the Ulysses cosmic-ray measurements obtained during the first pole-to-pole passage from mid-1994 to mid-1995.Key words. Interplanetary physics (cosmic rays; interplanetray magnetic fields; solar wind plasma)

Highlights

Before the Ulysses’ mission our knowledge about the heliolatitudinal variation of the cosmicray particles was limited to low heliographic latitudes arround the ecliptic plane
From mid-1994 to mid-1995 Ulysses completed its first pole-to-pole journey from 80◦ S to 80◦ N over the poles of the Sun, giving us for the first time the opportunity to study the latitudinal variation of many solar wind parameters and the variation of cosmic-ray intensity
We have shown that the heliolatitudinal profile of the galactic cosmic-ray intensity that has been observed by the Ulysses spacecraft can be reproduced if we apply a 2-D diffusionconvection model that includes the radial and latitudinal diffusion of the cosmic-ray particles

Summary

Introduction

Before the Ulysses’ mission (launched October 1990) our knowledge about the heliolatitudinal variation of the cosmicray particles was limited to low heliographic latitudes arround the ecliptic plane. From mid-1994 to mid-1995 Ulysses completed its first pole-to-pole journey from 80◦ S to 80◦ N over the poles of the Sun, giving us for the first time the opportunity to study the latitudinal variation of many solar wind parameters (e.g. velocity, density, temperature, magnetic field) and the variation of cosmic-ray intensity During this time period, close to solar minimum conditions, the temporal variations of the cosmic-ray intensity and the modulation level were relatively small. The latitudinal gradients were very small, < 1%/deg for the galactic cosmic-rays (Simpson et al, 1996; Heber et al, 1998; Heber and Potgieter, 2000), the background magnetic field tends to be nearly radial close to the poles, which means that the access of the cosmic-ray particles to the inner heliosphere would have been easier.

The 2-D shell-model

The model results

Findings

Conclusions