Ulysses Cosmic Ray and Solar Particle Investigation/Kiel Electron Telescope observations: Charge sign dependence and spatial gradients during the 1990–2000 A > 0 solar magnetic cycle

Abstract

Ulysses, launched in October 1990, began its second out‐of‐ecliptic orbit in September 1997. From 1991 to mid‐2000, solar polar magnetic field observations indicate a magnetic field pointing outward over the northern polar region (A > 0 epoch). While the first fast latitude scan in 1994/1995 was performed around solar minimum, the second Ulysses orbit covering high latitudes is being performed around solar maximum. We present data of ∼2.5 GV electrons and protons from the Kiel Electron Telescope up to southern heliographic latitudes of 80°, reached by Ulysses in November 2000. Spatial gradients are determined by comparison with a near‐Earth baseline. The radial gradient for an integral proton channel is found to be 2.2 ± 0.6%/AU before early 1998 and 3.5%/AU thereafter. Until mid‐1999 the latitudinal variation is consistent with a zero gradient at low latitudes and an average gradient of about 0.25%/degree for latitudes above about ±25°. In contrast, the latitudinal gradient decreases markedly after mid‐1999, leading to a spherically symmetric intensity distribution with the approach to solar maximum conditions. The electron to proton (e/p) ratio is used to study charge sign dependent modulation. According to drift‐dominated modulation models, galactic cosmic ray protons and electrons respond differently to the global heliospheric magnetic field. Comparison of the e/p ratio over the period 1991 to 2000 with model predictions indicates that drift effects are important over a large part of an 11‐year period with A > 0. With the approach to solar maximum the e/p ratio increases, indicative for the fading out of drift effects and the transition to a diffusion‐dominated modulation.

Modelling cosmic ray intensities along the Ulysses trajectory

“…It was shown that in the equatorial regions, and for solar minimum conditions, the 2.5-GV electrons have a radial gradient of ∼2.9%/AU while for the protons it is ∼1.9%/AU for the A>0 polarity cycle. The latter is in good agreement with values reported by Heber et al (2002). For solar maximum there is a general increase in the radial gradient for both particle species in the equatorial regions due to the small amount of drifts present.…”

“…The values are also quite similar for protons and electrons, primarily caused by the small levels of drifts present. For these extreme solar activity periods both the 2.5-GV electrons and protons have a very fluctuating gradient, varying between 3-4%/AU (see also Heber et al, 2002), up to even 5%/AU on shorter time scales. Similar values were found by Heber et al (1993) for the solar maximum periods of 1990-1992, where the radial gradient was found to be 4.9±1.3%/AU.…”

“…Concerning the observations, Clem et al (2002) reported that the radial gradient of cosmic ray electrons in the heliosphere at rigidities of 1.2 and 2.5 GV from 1 to 5 AU appears to be the same as those for positive particles of the same rigidity. Furthermore, Heber et al, 2002 showed that the radial gradient of 2.5-GV protons increased from 2.2%/AU to 3.5%/AU from solar minimum to maximum, respectively, making a time-dependent determination of the electron radial gradients mandatory. It was also shown by, for example, Fujii and McDonald (2001) that the radial gradient for protons varies with polarity; it is larger in an A<0 than in an A>0 solar magnetic epoch Figure 6 shows the time-dependent radial gradients in the inner heliosphere, computed with the compund approach.…”

“…6 that this model can also compute radial gradients of which the features are comparable to observations. To compute this, the procedure as described in Heber et al (2002) is followed. Concerning the observations, Clem et al (2002) reported that the radial gradient of cosmic ray electrons in the heliosphere at rigidities of 1.2 and 2.5 GV from 1 to 5 AU appears to be the same as those for positive particles of the same rigidity.…”

“…For solar maximum there is a general increase in the radial gradient for both particle species in the equatorial regions due to the small amount of drifts present. For these extreme solar activity periods both the 2.5-GV electrons and protons have a large fluctuating gradient varying between 3-4%/AU (see also Heber et al, 1993;Heber et al, 2002), increasing to even 5%/AU for shorter time scales. After 2000.2 when the HMF polarity changes in the model, from A>0 to A<0, there is a general decrease in the computed radial gradient, with the radial gradient of electrons becoming larger than that for protons, indicating that the gradients vary with HMF polarity.…”