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

Abstract

The time-dependent modulation of galactic cosmic rays in the heliosphere is studied over different polarity cycles by computing 2.5 GV proton intensities using a two-dimensional, time-dependent modulation model. By incorporating recent theoretical advances in the relevant transport parameters in the model, we showed in previous work that this approach gave realistic computed intensities over a solar cycle. New in this work is that a time dependence of the solar wind termination shock (TS) position is implemented in our model to study the effect of a dynamic inner heliosheath thickness (the region between the TS and heliopause) on the solar modulation of galactic cosmic rays. The study reveals that changes in the inner heliosheath thickness, arising from a time-dependent shock position, does affect cosmic-ray intensities everywhere in the heliosphere over a solar cycle, with the smallest effect in the innermost heliosphere. A time-dependent TS position causes a phase difference between the solar activity periods and the corresponding intensity periods. The maximum intensities in response to a solar minimum activity period are found to be dependent on the time-dependent TS profile. It is found that changing the width of the inner heliosheath with time over a solar cycle can shift the time of when the maximum or minimum cosmic-ray intensities occur at various distances throughout the heliosphere, but more significantly in the outer heliosphere. The time-dependent extent of the inner heliosheath, as affected by solar activity conditions, is thus an additional time-dependent factor to be considered in the long-term modulation of cosmic rays.

Evidence for a Time Lag in Solar Modulation of Galactic Cosmic Rays

“…Extreme values of r ± ts = 95/75 AU have also been tested. They lead to a ± 10 % shift in the near-Earth flux intensity, for fixed parameters, in agreement with Manuel et al (2015). However, this shift is re-absorbed by different best-fit parametersâ andb, while consistent ∆T values are inferred in all cases.…”

“…The drift velocity components v r and v θ are proportional to 2rβp qA , thus the sign of v d depends on the product qA (Burger & Potgieter 1989). We account for the presence of the termination shock (TS), placed at a default distance r ts ∼ = 85 AU, and for its time dependence over the solar cycle (Manuel et al 2015;Vos & Potgieter 2016). Beyond the TS, plasma density and B increase by a factor s ts = 3, the TS compression ratio, while V and K decrease by 1/s ts .…”

Solution of Heliospheric Propagation: Unveiling the Local Interstellar Spectra of Cosmic-ray Species

“…As discussed in Section 2.1 of Bobik et al (2012), the diffusion parameter, K 0 , is a scaling factor for the overall modulation intensity. It defines the global behavior of the modulation of the particle flux in the heliosphere and its dependence on time reflects the variations of properties of the interplanetary medium (like the actual solar magnetic field transported by SW and its turbulence) during the different phases of solar cycles (e.g., see Equation 4 in Manuel et al 2014). K 0 is expressed in terms of the monthly Smoothed Sunspot Numbers (SSN); such a relationship was demonstrated to be adequate for the description on how the diffusion parameter depends on solar activity and polarity 15 (see also discussion in Section 2.3 of Boschini et al 2017).…”

“…In the current calculation, we assume a static and spherical heliosphere with TS located at 100 AU (Bobik et al 2012). Note that Voyager 1, 2 observations point to a dynamic TS that is moving inward/outward in the heliosphere (Stone et al 2005;Richardson & Wang 2011), while numerical models indicate that this TS movement could be as large as ∼20 AU over a complete solar cycle (see a discussion in Manuel et al 2015, and references therein). In the HelMod framework, this is equivalent to a rescaling of the real size of the heliosphere to a reference size of 100 AU.…”

Propagation of cosmic rays in heliosphere: The HelMod model

“…A more complete asymmetrical structure of the heliosphere can be evaluated using magneto-hydrodynamic (MHD) models (e.g., see Florinski and Pogorelov, 2009;Guo and Florinski, 2014) that include transport description in outer heliosphere, heliosheath and co-rotating interaction regions. Moreover, Voyager 1, 2 observations point to a dynamic TS that is moving inward/outward in the heliosphere (Stone et al, 2005;Richardson and Wang, 2011), while numerical models indicate that this TS movement could be as large as ∼20 AU over a complete solar cycle (see the discussion in Manuel et al, 2015, and references therein). Finally, even though variations of the real size of the heliosphere may be important for the analysis of CR propagation near the TS, we do not consider them in this work.…”