Charge-sign dependent modulation in the heliosphere over a 22-year cycle

Abstract

Abstract.A time-dependent model based on a numerical solution of Parker's transport equation is used to model the modulation of cosmic ray protons, electrons and helium for full 11-year and 22-year modulation cycles using a compound approach. This approach incorporates the concept of propagating diffusion barriers based on global increases in the heliospheric magnetic field as they propagate from the Sun throughout the heliosphere, combined with gradient, curvature and current sheet drifts and the other basic modulation mechanisms. The model results are compared to the observed 11-year and 22-year cycles for 1.2 GV electrons and 1.2 GV Helium at Earth for the period 1975-1998. The model solutions are also compared to the observed chargesign dependent modulation along Ulysses' trajectory for the period 1990-1998. This compound approach to long-term modulation, especially charge-sign dependent modulation, is found to be remarkably successful. It is shown that the model can easily account for the latitude dependence for cosmic ray protons and the lack thereof for cosmic ray electrons by assuming large perpendicular diffusion in the polar direction. This approach contributes to an improved understanding of how diffusion and drifts vary from solar minimum to maximum modulation, and what the time-dependence of the heliospheric diffusion coefficients may be.

Oscillations in the open solar magnetic flux with a period of 1.68 years: imprint on galactic cosmic rays and implications for heliospheric shielding

“…Galactic comic electrons also show this 22-yr cycle and, having the opposite charge q, the drift theory predicts that this should be in antiphase with the nucleon GCR variation (Evenson, 1998). Measurements of 2.5 GV protons and electrons during the first fast latitudinal scan (1994.7-1995.6) revealed a small but clear latitudinal gradient in the time series not observed in the electron data (Heber et al, 1999) and in agreement with a drift-mode according to Ferreira et al (2003) (where protons are expected to drift in from the poles during these years of A>0). In the same paper Heber et al showed that between 1995.5 and 1998 the electron time series presented greater sensitivity to variations in the tilt of the HCS than the proton time series in agreement with negative particles propagating inward along the current sheet (Heber et al, 1999).…”

“…This is true during most of the solar cycle phases of both magnetic field polarities and could point to a more significant role of the open solar flux in cosmic ray flux modulation than has been attributed to date. We notice an interesting fact, that is, the alternate peaked and rounded maxima seen in the neutron GCR flux time series (giving a 22-year cycle), successfully attributed to the polarity-dependent drift component of the propagation mechanisms of cosmic rays in the heliosphere (see Heber, 2000;Ferreira et al, 2003), is also seen in the variation of the two 1-AU open flux estimates presented here. This implies that the Sun has, as well as a twenty-two-year cycle in the direction of its dipolar field, a twenty-two-year pattern in the quantity of open field lines threading a sphere passing at 1 AU.…”

“…Longer recovery times are therefore expected for periods of A<0 when particle inflows are along the heliospheric current sheet than for A>0, where inflows are expected from over the poles. The recent work by Ferreira et al (2003) to include the interplay between these diffusive barriers and large-scale drifts in a full time-dependent model has shown very promising results concerning the charge-sign dependent modulation effects predicted by drift theory. The model used is based on a numerical solution of Parker's time-dependent transport equation Eq.…”

Heliospheric Modulation of Cosmic‐Ray Positrons and Electrons: Effects of the Heliosheath and the Solar Wind Termination Shock

“…As a reference, these ratios are compared to the corresponding LIS ratios at 120 AU. For a review of the modeling of e À /p over 22 yr without a TS and observations with the Ulysses spacecraft, see Ferreira et al (2003) and Heber et al (2002Heber et al ( , 2003, respectively. Figure 6 is similar to Figure 5, except that the intensity ratios are shown at the TS (r s ¼ 90 AU).…”

The heliospheric modulation of cosmic ray boron and carbon

“…Concerning modulation mechanisms, large-scale gradient, curvature and current sheet drifts that charged particles experience in the global heliospheric magnetic (HMF) are most prominent, if not the dominating mechanism at solar minimum, at least for charged nuclei. Drift models predict, apart from a clear charge-sign dependence Ferreira et al, 2003), different spectra for sequential solar minima based on the polarity of the solar magnetic field (Reinecke and Potgieter, 1994). For recent reviews on heliospheric modulation, see Potgieter et al (2001);Heber (2001);Zhang (2003);and Ferreira and Potgieter (2004a).…”