On the importance of the local interstellar spectrum for the solar modulation parameter

Abstract

[1] Cosmogenic Isotopes are produced in the Earth's atmosphere due to the interaction of galactic cosmic rays with nuclei of atmospheric atoms. Among others, the 10 Be concentration in ice cores depends on the galactic cosmic ray flux outside of the Earth's magnetosphere and provides therefore a unique tool to investigate the solar modulation over very long time periods. In this study we investigate the importance of different local interstellar proton spectra often used in literature obtained outside of the Earth's magnetosphere. In order to parameterize the heliospheric modulation we apply the forcefield solution using individual local interstellar proton spectrum (LIS) model dependent values. Thus among atmospheric and magnetospheric processes, the 10 Be concentration depends on an interplay of the different LIS and their modulation parameters. Since 10 Be measurements do not provide any spectral resolution, PAMELA data have been used for a comparison with the calculated spectra and to provide the model dependent modulation parameters during the solar minimum in July 2006. Within the limitation of the force-field solution and the freedom in parameter space, all LIS lead to a reasonable agreement with the data. Taking the LIS dependency of the modulation parameter into account, we derive linear equations to convert the individual between the different LIS. The conversions used here are then applied to a long-term reconstruction of derived from a record of the cosmogenic radionuclide 10 Be. By using the derived LIS conversions, we show that the occasionally observed negative values in the reconstruction of Steinhilber et al. (2008) vanish if another LIS model is used. In order to estimate other processes which alter this conclusion, the influence of the palaeo-magnetic field has been included. Thus, if all inner-heliospheric effects on the 10 Be flux would be known, this investigation would have the potential to rule out certain LIS.

Anisotropic diffusion of Galactic cosmic ray protons and their steady-state azimuthal distribution

“…For further comparison, the LIS given by Webber & Higbie (2009;WH09 hereafter) is included in the plots. We used the parametrization given in Herbst et al (2010), where a comparison between a few proposed LIS can be found as well. In face of the still imprecisely known modulation effects on measured spectra inside the heliosphere (see, e.g., Florinski et al 2011;Scherer et al 2011), such an LIS parametrization can give only a rough orientation for what to expect for Galactic CR propagation studies.…”

Assessment of different sunspot number series using the cosmogenic isotope 44 Ti in meteorites

“…The exact value of the modulation potential is not important since it can be defined only relatively, depending on the primary assumption on the local interstellar spectrum (LIS) so that the same GCR spectrum near Earth can be parametrized by different values of φ for different LIS assumptions (Usoskin et al 2005;Herbst et al 2010). However, once the LIS is fixed, the value of φ unambiguously determines the GCR energy spectrum.…”

Modulation of galactic cosmic rays during the unusual solar minimum between cycles 23 and 24

“…Nevertheless, the true LIS is still far from conclusive [Webber et al, 2013]. In addition, different LIS models can produce the observed spectrum with LIS model-dependent modulation parameters [Herbst et al, 2010]. Furthermore, recent studies show that remarkable modulation exists in the outer heliosphere and even beyond the heliopause [e.g., Scherer et al, 2011;Strauss et al, 2013].…”