Spatial gradients of GCR protons in the inner heliosphere derived fromUlyssesCOSPIN/KET and PAMELA measurements

Abstract

Context. During the transition from solar cycle 23 to 24 from 2006 to 2009, the Sun was in an unusual solar minimum with very low activity over a long period. These exceptional conditions included a very low interplanetary magnetic field (IMF) strength and a high tilt angle, which both play an important role in the modulation of galactic cosmic rays (GCR) in the heliosphere. Thus, the radial and latitudinal gradients of GCRs are very much expected to depend not only on the solar magnetic epoch, but also on the overall modulation level. Aims. We determine the non-local radial and the latitudinal gradients of protons in the rigidity range from ∼0.45 to 2 GV. Methods. This was accomplished by using data from the satellite-borne experiment Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) at Earth and the Kiel Electron Telescope (KET) onboard Ulysses on its highly inclined Keplerian orbit around the Sun with the aphelion at Jupiter's orbit. Results. In comparison to the previous A > 0 solar magnetic epoch, we find that the absolute value of the latitudinal gradient is lower at higher and higher at lower rigidities. This energy dependence is therefore a crucial test for models that describe the cosmic ray transport in the inner heliosphere.

Modulation of Galactic Cosmic Rays in the Inner Heliosphere, Comparing with PAMELA Measurements

“…It is shown that the radial gradients are always positive while the latitudinal gradients are always negative as expected but with less magnitude than that predicted by earlier works (Potgieter et al 2001). Following de Simone et al (2011) and Gieseler & Heber (2016), Vos & Potgieter (2016) also used a numerical model to compute the spatial gradients of protons from 2006 to 2009. They concluded that although the drift effects were weaker than the predictions from those drift-dominated works due to the suppression by the excess diffusion, they still played an important role due to the significant decrease of HMF magnitude until the end of 2009.…”

Global Gradients for Cosmic-Ray Protons in the Heliosphere During the Solar Minimum of Cycle 23/24

“…In Figure 9 the modelling results from Figure 8 are compared to the observational gradients calculated in this study and to observational gradients from De Simone et al (2011) and the very recent study of Gieseler and Heber (2016). The latter investigation used a more sophisticated approach, producing two sets of results for global gradients based on two statistical fitting methods, as indicated in the figure.…”

Propagation of cosmic rays in heliosphere: The HelMod model

“…In fact, this spectrometer allowed one to determine the most accurate differential intensities of protons (Aguilar et al, 2015b), helium nuclei (Aguilar et al, 2015a), antiprotons (Aguilar et al, 2016), electrons and positrons (Aguilar et al, 2014). The high precision of these experimental data together with those from Ulysses spacecraft (e.g., see Simpson et al, 1992;Simpson et al, 1996;Ferrando et al, 1996;De Simone et al, 2011;Gieseler and Heber, 2016) constitute a challenge for any modulation model of the inner part of heliosphere. Actually such a treatment has to reproduce the observed GCR spectra transported -during different solar activity phases -down to Earth and, also, outside the ecliptic plane at distances from Sun ranging from about 1 to 5 AU.…”

“…These effects are treated in the Parker transport equation through the terms including the drift velocity. The analysis on Ulysses out-of-ecliptic observations (e.g., see Simpson, 1996;Simpson et al, 1996;Heber et al, , 1998Heber et al, , 2008Ferrando et al, 1996;De Simone et al, 2011;Gieseler and Heber, 2016) provided, so far, a unique point of view highlighting the presence of latitudinal gradients in the spatial distribution of GCRs, during period of low solar activity, i.e., when the combination of particle charge (q) and solar magnetic polarity (A) is positive (qA > 0); while a more uniform distribution of GCRs in the inner part of the heliosphere occurs for qA < 0.…”