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

2015

ApJ

158

156

During the recent prolonged solar minimum of cycle 23/24, the PAMELA detector measured 27-day averaged Galactic proton energy spectra over the energy range that is important for solar modulation. By comparing these spectra to computed spectra from a three-dimensional model that contains all of the important heliospheric modulation processes, the recent minimum can be studied in detail from a modulation perspective. This was done by setting up a realistic heliosphere in the model, and reproducing a representative selection of seven intermittent PAMELA spectra, separated by approximately six months, from 2006 July to 2009 December. Additionally, a new very local interstellar proton spectrum was constructed using measurements below 600 MeV from Voyager 1, taken beyond the heliopause, combined with PAMELA and AMS-02 measurements above 30 GeV at the Earth. As a result of the extreme minimum modulation conditions that governed the recent solar minimum, the highest ever Galactic cosmic ray spectrum at Earth was observed by PAMELA at the end of 2009. It was found that, apart from the self-consistent changes in the heliospheric current sheet and the heliospheric magnetic field over time, additional increases in the mean free paths during this period were required below~4 GV in order to reproduce the intensities observed by PAMELA.

Section: Rigidity and Spatial Dependence Of The Dcsmentioning

confidence: 99%

New Modeling of Galactic Proton Modulation During the Minimum of Solar Cycle 23/24

Vos

2015

ApJ

158

156

“…The quantity κ 0 A is a constant which can be used as a scaling factor, having values between 0.0 (zero drift) and 1.0 (full drift); in this work, κ 0 A = 1.0, which is known as the so-called weak scattering maximal value for the drift coefficient when P P 0 . For a detailed discussion of this aspect, see Ngobeni and Potgieter (2015).…”

Section: Particle Driftsmentioning

confidence: 99%

New insights from modeling the neutral heliospheric current sheet

Raath

Strauss

2015

Astrophys Space Sci

Recently, the modulation of cosmic rays in the heliosphere has increasingly been studied by solving the well known transport equation via an approach based on stochastic differential equations. This approach, which is now wellestablished and published, allows for an in depth study of the modulation effects of the wavy heliospheric current sheet, in particular as its waviness increases with solar activity up to extreme maximum conditions. This is possible because of the numerical stability of the approach as well as its ability to trace pseudo-particles so that insightful trajectories of how they respond to the wavy heliospheric current sheet can be computed and displayed. Utilising such a stochastic model, we present valuable new insights into how the geometry of the wavy current sheet can affect the modulation of cosmic rays, especially at the highest levels of solar activity. This enables us to show, from a modeling perspective, why a certain choice for the current sheet profile is more suited than another at these high solar activity levels. We emphasise the importance of an effective tilt angle and illustrate how this concept can be employed effectively in interpreting results pertaining to the wavy current sheet as well as the modulation associated with this important heliospheric structure.

“…which reduces to k = pv qB 3 T if weak scattering is assumed (e.g., Forman et al 1974;Engelbrecht & Burger 2015;Ngobeni & Potgieter 2015), with q and v the cosmic-ray particle's charge and speed. Here, ( ) K s is the diffusion tensor, and it can be expressed as…”

Section: T B Dmentioning

confidence: 99%

A Numerical Study of Forbush Decreases with a 3D Cosmic-Ray Modulation Model Based on an SDE Approach

Luo

Zhang

et al. 2017

ApJ

Based on the reduced diffusion mechanism for producing Forbush decreases (Fds) in the heliosphere, we constructed a three-dimensional (3D) diffusion barrier,and by incorporating it into a stochastic differential equation (SDE) based time-dependent, cosmic-ray transport model, a 3D numerical model for simulating Fds is built and applied to a period of relatively quiet solar activity. This SDE model generally corroborates previous Fd simulations concerning the effects of the solar magnetic polarity, the tilt angle of the heliospheric current sheet (HCS), and cosmic-ray particle energy. Because the modulation processes in this 3D model are multi-directional, the barrier's geometrical features affect the intensity profiles of Fds differently. We find that both the latitudinal and longitudinal extent of the barrier have relatively fewereffects on these profiles than its radial extent and the level of decreased diffusion inside the disturbance. We find,with the 3D approach, that the HCS rotational motion causes the relative location from the observation point to the HCS to vary, so that aperiodic pattern appears in the cosmicray intensity at the observing location. Correspondingly, the magnitude and recovery time of an Fd change, andthe recovering intensity profile contains oscillation as well. Investigating the Fd magnitude variation with heliocentric radial distance, we find that the magnitude decreasesoveralland,additionally, that the Fd magnitude exhibits an oscillating pattern as the radial distance increases, which coincides well with the wavy profile of the HCS under quiet solar modulation conditions.