Modulation

The flux (flow rate) of cosmic rays incident on the Earth’s upper atmosphere is modulated (varied) by two processes; the sun’s solar wind and the Earth's magnetic field. The Solar wind is expanding magnetized plasma generated by the sun, which has the effect of decelerating the incoming particles, as well as excluding some of the particles with energies below about 1 GeV. The amount of solar wind is not constant due to changes in solar activity, for instance over its regular eleven-year cycle. Hence the level of modulation varies in anticorrelation with solar activity. Also the Earth's magnetic field deflects some of the cosmic rays, giving rise to the observation that the intensity of cosmic radiation is dependent on latitude, longitude, and azimuth angle. The cosmic flux varies from eastern and western directions due to the polarity of the Earth's geomagnetic field and the positive charge dominance in primary cosmic rays. (This is called the "east-west effect"). The cosmic ray intensity at the Equator is lower than at the poles as the geomagnetic cutoff value is greatest at the equator. This is because charged particles tend to move in the direction of field lines and not across them, so that they are concentrated in the polar regions (where field lines are closest together). This is the reason the auroras occur at the poles, since the field lines curve down towards the Earth’s surface there. Finally, the longitude dependence arises from the fact that the geomagnetic dipole axis is not parallel to the Earth's rotation axis.

This modulation which describes the change in the interstellar intensities of cosmic rays as they propagate in the heliosphere is highly energy and spatial dependent, and it is described by the Parker's Transport Equation in the heliosphere. At large radial distances, far from the Sun (~94 AU), there exists the region where the solar wind undergoes a transition from supersonic to subsonic speeds called the "solar wind termination shock". The region between the termination shock and the heliopause (the boundary marking the end of the heliosphere) is called the heliosheath. This region acts as a barrier to cosmic rays, decreasing their intensity at lower energies by about 90%; thus it is not only the Earth's magnetic field that protects us from cosmic ray bombardment.

From a scientific modeling point of view, there is a challenge in determining the Local Interstellar Spectra (LIS) due to large adiabatic energy changes these particles experience owing to the diverging solar wind in the heliosphere. However, significant progress has been made in the field of cosmic ray studies with the development of an improved state-of-the-art 2D numerical model that includes the simulation of the solar wind termination shock, drifts and the heliosheath coupled with fresh descriptions of the diffusion tensor, see Langner et al. (2004). But challenges also exist because the structure of the solar wind and the turbulent magnetic field in the heliosheath is not well understood indicating the heliosheath as the region unknown beyond. With lack of knowledge of the diffusion coefficient perpendicular to the magnetic field our knowledge of the heliosphere and from the modelling point of view is far from complete. There exist promising theories like ab initio method approaches, but the drawback is that such theories produce poor compatibility with observations (Minnie, 2006) indicating their failure in describing the mechanisms influencing the cosmic rays in the heliosphere.