With Voyager1 crossing the outer boundary of our solar system at the end of 2012, for the first time in the instrumental era an unmodulated local interstellar spectrum (LIS) at galactic particle energies below ~500 MeV has been measured. On the basis of these as well as Payload for Antimatter Matter Exploration and Light‐nuclei Astrophysics (PAMELA) and Alpha Magnetic Spectrometer (AMS02) measurements, most recently, three new LIS models have been proposed in the literature. In this study we compare the newest LIS models to previously most often used ones. Thereby, we investigate and discuss the influence of these LIS models on the terrestrial production rates of the cosmogenic radionuclides 10Be and 14C, which are produced due to the interaction of galactic and solar cosmic rays with atmospheric constituents. After being transported within the atmosphere they are preserved in natural archives such as, e.g., ice sheets or tree rings, forming a unique tool to study the solar modulation of thousands of years back in time. To parameterize the heliospheric modulation we apply the force‐field approximation for the individual LIS models from which LIS‐dependent solar modulation parameter (ϕ) values are derived. Furthermore, we present updated sets of linear regression functions containing the opportunity to convert the LIS‐dependent ϕ values between the investigated LIS models. The results are then applied to a long‐term reconstruction of the solar modulation parameter.

In order to quantify the optimal radiation shielding depth on Mars in preparation for future human habitats on the red planet, it is important to understand the Martian radiation environment and its dependence on the planetary atmospheric and geological properties. With this motivation we calculate the absorbed dose and equivalent dose rates induced by galactic cosmic ray particles at varying heights above and below the Martian surface considering various subsurface compositions (ranging from dry rock to water‐rich regolith). The state‐of‐the‐art Atmospheric Radiation Interaction Simulator based on GEometry And Tracking Monte Carlo method has been employed for simulating particle interaction with the Martian atmosphere as well as subsurface materials. We calculate the absorbed dose in two different phantoms: a thin silicon slab and a water sphere. The former is used to validate our model against the surface measurement by the Radiation Assessment Detector on the Curiosity rover, while the later is used to approximate a human torso, also for evaluation of the biologically weighted equivalent dose. We find that the amount of hydrogen contained in the water‐rich regolith plays an important role in reducing the equivalent dose through modulation of neutron flux (below 10 MeV). This effective shielding by underground water is also present above the surface, providing an indirect shielding for potential human explorations at this region. For long‐term habitats seeking the Martian natural surface material as protection, we also estimate the optimal shielding depth, for different given subsurface compositions, under maximum, average, and minimum heliospheric modulation conditions.

We present an analytical diffusion-expansion Forbush decrease (FD) model ForbMod which is based on the widely used approach of the initially empty, closed magnetic structure (i.e. flux rope) which fills up slowly with particles by perpendicular diffusion. The model is restricted to explain only the depression caused by the magnetic structure of the interplanetary coronal mass ejection (ICME). We use remote CME observations and a 3D reconstruction method (the Graduated Cylindrical Shell method) to constrain initial boundary conditions of the FD model and take into account CME evolutionary properties by incorporating flux rope expansion. Several flux rope expansion modes are considered, which can lead to different FD characteristics. In general, the model is qualitatively in agreement with observations, whereas quantitative agreement depends on the diffusion coefficient and the expansion properties (interplay of the diffusion and the expansion). A case study was performed to explain the FD observed 2014 May 30. The observed FD was fitted quite well by ForbMod for all expansion modes using only the diffusion coefficient as a free parameter, where the diffusion parameter was found to correspond to expected range of values. Our study shows that in general the model is able to explain the global properties of FD caused by FR and can thus be used to help understand the underlying physics in case studies.

We present a statistical analysis of near-relativistic (NR) solar energetic electron event spectra near 1 au. We use measurements of the STEREO Solar Electron and Proton Telescope (SEPT) in the energy range of 45-425 keV and utilize the SEPT electron event list containing all electron events observed by STEREO A and STEREO B from 2007 through 2018. We select 781 events with significant signal to noise ratios for our analysis and fit the spectra with single or broken power law functions of energy. We find 437 (344) events showing broken (single) power laws in the energy range of SEPT. The events with broken power laws show a mean break energy of about 120 keV. We analyze the dependence of the spectral index on the rise times and peak intensities of the events as well as on the presence of relativistic electrons. The results show a relation between the power law spectral index and the rise times of the events with softer spectra belonging to rather impulsive events. Long rise-time events are associated with hard spectra as well as with the presence of higher energy (>0.7 MeV) electrons. This group of events cannot be explained by a pure flare scenario but suggests an additional acceleration mechanism, involving a prolonged acceleration and/or injection of the particles. A dependence of the spectral index on the longitudinal separation from the parent solar source region was not found. A statistical analysis of the spectral indices during impulsively rising events (rise times < 20 minutes) is also shown.