Modelling cosmic ray intensities along the Ulysses trajectory

Abstract

Abstract. Time dependent cosmic ray modulation in the inner heliosphere is studied by comparing results from a 2-D, time-dependent cosmic ray transport model with Ulysses observations. A compound approach, which combines the effects of the global changes in the heliospheric magnetic field magnitude with drifts to establish a realistic timedependence, in the diffusion and drift coefficients, are used. We show that this model results in realistic cosmic ray modulation from the Ulysses launch (1990) until recently (2004) when compared to 2.5-GV electron and proton and 1.2-GV electron and Helium observations from this spacecraft. This approach is also applied to compute radial gradients present in 2.5-GV cosmic ray electron and protons in the inner heliosphere. The observed latitude dependence for both positive and negative charged particles during both the fast latitude scan periods, corresponding to different solar activity conditions, could also be realistically computed. For this an additional reduction in particle drifts (compared to diffusion) toward solar maximum is needed. This results in a realistic charge-sign dependent modulation at solar maximum and the model is also applied to predict charge-sign dependent modulation up to the next expected solar minimum.

Drift Effects on the Galactic Cosmic Ray Modulation

“…Results of the variation in time of the total drift contribution show that it is almost zero in some periods nearing the sunspot maximum. Moreover, during periods of no well-defined polarity of the solar magnetic field, in the sunspot maximum phase, the drift contribution to the CR modulation is estimated to be generally less than 0.30 (see Table 3), reduced with respect to its value over the whole period, which is in good agreement with Ndiitwani et al (2005) and Minnie et al (2007).…”

“…In general, we observe a decrease of the drift contribution for higher energy particles, being the p values are lower for HH than RM and CLI and a sign coherence is observed among the three data sets. Moreover, the obtained values for the drift contribution are generally lower than those computed for the whole period (0.34, 0.37, and 0.31 for CLI, RM, and HH data, respectively; see Table 2), indicating that the drift effects are decreased during the solar maxima, as proposed by Ndiitwani et al (2005). …”

Time-Dependent Modulation of Cosmic Rays in the Heliosphere

“…This resulted in diffusion coefficients and drifts changing over a solar cycle, with smaller values for solar maxima compared to solar minima. As shown by , Ndiitwani et al (2005), Ferreira and Scherer (2006), and this compound approach incorporated in a numerical modulation model yields results in good agreement with spacecraft observations (Ulysses in particular) at various energies.…”

Interstellar-Terrestrial Relations: Variable Cosmic Environments, The Dynamic Heliosphere, and Their Imprints on Terrestrial Archives and Climate

“…Equation (51) use as time-dependent input parameters the observed tilt angle and HMF magnitude. This function results in transport parameters which is roughly a factor of ∼10 smaller for solar minima compared to solar maxima, see also Cummings and Stone (2001) and results in realistic time-dependent modulation Ndiitwani et al, 2005). Figure 26 shows the results from our hybrid model in the form of computed 30 MeV ACR and GCR combined intensities in the meridional plane of the heliosphere.…”

“…Also shown is that for solar maximum conditions the computed combined spectra for both polarity cycles are almost the same for all distances. This is expected because of the reduction of drifts in the model via the compound approach which is essential in explaining chargesign dependent modulation Ndiitwani et al, 2005).…”