The heliospheric modulation of cosmic ray boron and carbon

Abstract

Abstract. The observed boron to carbon ratio (B/C) at Earth provides a good measure of the overall secondary to primary ratio of galactic cosmic rays. This makes B/C an important constraint and test for the validity and general applicability of theoretical and numerical models of galactic propagation and heliospheric modulation. For this purpose, the modulation of boron and carbon in the heliosphere must be understood in greater detail. The latest approach to heliospheric modulation, using a numerical model containing a termination shock, a heliosheath and particle drifts, is used to the study the modulation of the two species. This model also includes a more comprehensive set of diffusion coefficients. From this and previous work follows that the model is compatible with a variety of observations, for seven species, i.e. protons, anti-protons, electrons, positrons, helium, boron, and carbon, with the same set of parameters for both solar magnetic polarity cycles. Despite the rather flat interstellar spectrum for carbon below 100 MeV/nuc, the modulated spectra at 1 AU look very similar for boron and carbon, caused by adiabatic energy losses, implying that the carbon modulation should have a much larger radial gradient in the outer heliosphere below ∼200-500 MeV/nuc than boron. Significant modulation can be caused by the heliosheath but it is strongly dependent on energy and on the field polarity, with almost no effect at high energies to the largest effect at low energies. The solar wind termination shock has an important effect on the B to C ratio in the heliosphere, although small at Earth, during the A<0 cycle, with E<∼600 MeV/nuc, but it seems less significant for the A>0 cycle and with increasing tilt angles. Drift models produce different spectra for consecutive solar minimum conditions which may account for the modulation level differences between observations around 100 MeV/nuc compared to around 500 MeV/nuc. All factors taken into account, heliospheric modeling indicates that the interstellar spectra for B and C need further refinement around 1 GeV/nuc, in order to fit observations over a wide energy range at Earth and that this refinement probably has to take into account the proposed contribution of a local interstellar carbon component. These results confirm that this numerical model with a TS can reasonably reproduce the B and C modulation between the outer boundary and Earth, making it a reasonableCorrespondence to: M. S. Potgieter (fskmsp@puk.ac.za) approximation for both polarity cycles from solar minimum to moderate solar activity.

Modelling the effects of scattering parameters on particle-drift in the solar modulation of galactic cosmic rays

“…The consequence is that in the equatorial plane the A < 0 spectra are higher than the A > 0 spectra at all energies which is unexpected from a classical drift modelling point of view, and contrary to observations (see e.g. Langner et al, 2003Langner et al, , 2004Potgieter and Langner, 2004;Potgieter, 2013). This feature persists for modulation in the equatorial plane even when the enhancement of K \h is neglected as long as xs has a latitude dependence.…”

“…As such, the effect of changing xs on the weak scattering drift coefficient has always been included in numerical modulation models (as applied by e.g. Langner et al, 2004;Potgieter and Langner, 2004;Potgieter, 2008, 2014). Evidently, the spatial dependence of xs has been neglected.…”

Deciphering Residual Emissions: Time-dependent Models for the Nonthermal Interstellar Radiation from the Milky Way

“…Spatial diffusion, convection with the solar wind, drifts, and adiabatic cooling are the main mechanisms that determine transport of CRs to the inner heliosphere. These effects have been incorporated into realistic (time-dependent, three-dimensional) models (e.g., Florinski et al 2003;Langner et al 2006;Potgieter & Langner 2004;Boschini et al 2017Boschini et al , 2018a. The "force-field" approximation that is ordinarily used (Gleeson & Axford 1968), instead characterises the modulation effect as it varies over the solar cycle using a single parameter -the "modulation potential".…”

Solar cycle variations and cosmic rays

“…These spectra do not take into account contributions from the local interstellar medium (e.g., Potgieter and Langner, 2004); in fact, the very local interstellar spectra below $10 GeV required for detail modulation studies are difficult to be determined. Obviously, variations on the very long term (millennium scale) of these spectra influence the CR flux reaching Earth.…”