1. Galactic Cosmic Ray Propagation in the 3D Heliosphere
Bernd Heber, Jan Gieseler, Phillip Dunzlaff, Oliver Sternal, Reinhold Müller-Mellin, Andreas Klassen, Raul Gomez-Herrero, and Horst Kunow — Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität zu Kiel, Leibnizstr.11, D Kiel
Abstract
The on-going Ulysses mission provides a unique opportunity to study the propagation and modulation of galactic cosmic rays (GCRs) in detail in the three-dimensional heliosphere. GCR particles are scattered by irregularities in the interplanetary magnetic field and undergo convection and adiabatic deceleration in the expanding solar wind. The large-scale heliospheric magnetic field leads to drifts of GCRs in the interplanetary medium. Therefore the spatial distribution and the time history of electrons and protons are a suitable tool to investigate the importance of drifts and diffusion in heliospheric modulation. In 1995 Ulysses moved from 800S to 800N, showing a positive latitudinal gradient for galactic cosmic ray protons and no latitudinal gradient for electrons. In 2007 the spacecraft will again span the same latitudinal range. Becasue the heliospheric magnetic field reversed from an A>0- to an A<0-magnetic epoch in 2001, the drift motion of particles is reversed. Therefore we expect no latitudinal gradient for protons and a small positive latitudinal gradient for electrons. In this contribution we will present first Ulysses COSPIN/KET measurements from the current southern polar scan.
Radial gradients
With the known galactic cosmic ray intensities measured by Ulysses (I(U)) and at 1AU (I(A), the radial distance of Ulysses and ACE from the Sun (Δr), we can determine the radial gradient of the galactic cosmic rays (Gr) and the normalisation factor f between both channels, by fitting
to the data. The best fit results in a radial gradient of Gr = %/AU
Introduction
The modulation of galactic cosmic rays is caused by a number of physical processes, including spatial diffusion in the turbulent heliospheric magnetic field, convection and adiabatic deceleration in the expanding solar wind, gradient and curvature drift in the large scale magnetic fields. The strength and relative importance of these processes varies with the location in the heliosphere and with the 22-year solar cycle. Since its launch in October 1990 Ulysses reached high southern heliographics latitudes three times, around 1994 close to solar minimum around 2000 close to solar maximum, and again in 2007 during the declining phase of the solar cycle (see figure 1). Since Ulysses is not only moving in latitude but also in radial distance, the determination of the latitudinal gradients requires not only a correction for temporal variation but also for the radial intensity gradient.
Figure 2: The radial gradient of the galactic cosmic ray intensity for alpha particles with energies from 116 to 125 MeV/n in the inner heliosphere. Also plotted is the line of best fit from which we get the radial gradient:
Gr = 4.5 ± 0.6%/AU.
The temporal variation of galactic cosmic rays at Earth is given by a combination of carbon energy channels, as summarized in fig. 1 and tab. 1 which will be compared to the 1.2 GV Helium channel from Ulysses.
We now need to find the ACE measurement which is corresponding best to the quiet time Ulysses helium channel. The best agreement is given, if the ratio of the intensity ratios in 2004/2005 and 1998/1999 is close to one (see 5th column of tab. 1)..
Latitudinal gradients
With the known galactic cosmic ray intensities measured by Ulysses (I(U)) and at 1AU (I(A), the radial distance of Ulysses and ACE from the Sun (Δr), we can determine the radial gradient of the galactic cosmic rays (Gr) and the normalisation factor f between both channels, by fitting
to the data. The best fit results in a radial gradient of Gr = %/AU
Figure 1. From top to bottom: Ulysses radial distance and heliographic latitude, the sunspot number, Ulysses daily averaged count rates of 125 to 250 MeV/nuc Helium and 27 day averaged intensities of 126 to 196 MeV/nuc Carbon. The red curve in panel four reflects quiet time helium intensities. Marked by shading are two time periods in 1998 and 2004 when Ulysses was at about 5 AU from the Sun.
Table 1: Ulysses KET and ACE CRIS accumulated intensities and ratios for the time-intervals and , as well as the ratio of the ratios of column 4 and 3.