The heliomagnetic and solar-cycle related variations of the cosmic ray flux modeled with the BoPo-hybrid code

Abstract

Based on the five species hybrid model by Scherer & Ferreira (2005, ASTRA, 1, 17) we describe the observations of energetic cosmic ray particles by the Voyager 1 spacecraft for solar minimum and maximum conditions during both the A < 0 as well the A > 0 heliomagnetic cycle. Without going into a detailed data and model analysis, we will show that the model (BoPo-code) is well suited to explain the longterm trend in the observed proton data as this spacecraft moves into the outer heliosphere. By comparison of model results to the observations along the Voyager trajectory we predict, depending on cosmic ray energy, a smooth transition in observed radial profiles after the crossing of the termination shock. From a cosmic ray particle perspective the effect of the shock might be seen in the very low energy data (<30 MeV), where a change in radial gradient occurs over the shock. Also, as Voyager 1 moves into the heliosheath, we show that the solar-cycle related changes in cosmic ray observations, which are clearly observed inside the termination shock, are expected to decrease toward the heliopause beyond which the LISM spectra is assumed to be unperturbed.Key words. heliosphere -interstellar medium -cosmic rays MotivationIn the past years a lot of effort was put in the development of highly advanced models describing the interaction between the interstellar medium and the solar wind: hydrodynamic models (see e. On the other hand, the emphasis in modeling the energydependent cosmic ray flux (see Potgieter 1998, for a review), was put on kinetic modulation models (e.g. (1993), where at low energies, the radial gradient changes abruptly from a lower value inside the shock to a higher value outside, where as at high energies, the higher value of the gradient are inside the shock.Recently, both approaches have been combined by using a hybrid code, the so-called BoPo-model: a hydrodynamical model for the interaction between the solar wind and interstellar protons as well as the interaction with the interstellar neutral hydrogen. Due to charge exchange processes in the inner heliosphere between these particle species new ions are created, the pickup ions (PUIs), which are then transported by the solar wind and its magnetic field to the termination shock. This latter species is also described by a hydrodynamic continuity equation. The acceleration of ACRs at the termination shock and their transport as well as that of GCRs into the inner heliosphere is then selfconsistently described by a kinetic transport model based on solving the Parker (1965) transport equation in the meridional plane of the heliosphere. Because this is a 2-D axisymmetric model, the description in the ecliptical-crosswind direction is not correct. On the other hand the main modulation of the CRs take place at the nose and over the poles, so that in the meridional plane our model adequately describes the physical situation. The advantage of the 2-D model is that it is fast and one can use it modeling the solar cycle variation dynamically includ...

The Effect of a Dynamic Inner Heliosheath Thickness on Cosmic-Ray Modulation

“…Later, Wang & Belcher (1999) proposed that the location of the TS oscillates approximately 13 AU per solar cycle in response to the ∼11 yr solar cycle, and it moves outward faster than it moves inward. It was shown by Scherer & Ferreira (2005a, 2005b that such changes in the geometry of the heliosphere also influence the cosmic-ray particle distribution inside the heliosphere.…”

“…Since the heliosphere is a dynamic structure, the TS position, r ts , responds to variations of the solar wind speed and solar wind density, which change over a ∼11 yr solar cycle (Scherer & Fahr 2003). An increase in either the density or the velocity of the solar wind (or LISM) would result in a TS forming at larger (or smaller in the case of LISM density or velocity increase) heliocentric distances (see, e.g., Wang & Belcher 1999;Scherer & Fahr 2003;Whang et al 2004;Scherer & Ferreira 2005a, 2005bWashimi et al 2011). Whang et al (1995) proposed that the location of the TS is anti-correlated with solar activity, i.e., the TS is located farther away from the Sun during solar minimum periods and closer to the Sun during solar maximum periods.…”

Transport and acceleration of anomalous cosmic rays in the inner heliosheath

“…Alternative models for particle acceleration at the flanks of the heliosphere were suggested by Chalov [1993] and McComas and Schwadron [2006]. Modelling these aspects, including the acceleration mechanisms discussed above, as well as dynamic effects [e.g., Scherer and Ferreira , 2005a, 2005b; Florinski and Zank , 2006] will be the focus of modulation models in future.…”

Calculation of the energetic neutral atom flux from a 3D time-dependent model heliosphere

“…the boundary region between the undisturbed solar wind and the LISM. It contributes significantly to the shielding effect, particularly with respect to the modulation of galactic cosmic ray spectra (Florinski et al 2003;Scherer & Ferreira 2005;Langner et al 2006a). These high-energy particles are able to influence the climate on Earth (Svensmark 1998(Svensmark , 2006bFichtner et al 2006;Scherer et al 2007).…”