Where does the heliospheric modulation of galactic cosmic rays start?
Abstract
The long outstanding question of where the heliospheric (solar) modulation of galactic cosmic rays actually begins, in terms of spatial
position, as well as at what high kinetic energy, can now be answered. Both answers are possible by using the results of an advanced
numerical model, together with appropriate observations. Voyager 1 has been exploring the outskirts of the heliosphere and is presently
entering what can be called the very local interstellar medium. It has been generally expected, and accepted, that once the heliopause is
crossed, the local interstellar spectrum (LIS) should be measured in situ by the Voyager spacecraft. However, we show that this may not
be the case and that modulation effects on galactic cosmic rays can persist well beyond the heliopause. For example, proton observations
at 100 MeV close to the heliopause can be lower by 25% to 40% than the LIS, depending on solar modulation conditions. It is also
illustrated quantitatively that significant solar modulation diminishes above 50 GeV at Earth. It is found that cosmic ray observations
above this energy contain less that 5% solar modulation effects and should therefore reflect the LIS for galactic cosmic rays. Input spectra,
in other words the very LIS, for solar modulation models are now constrained by in situ observations and can therefore not any
longer be treated arbitrarily. It is also possible for the first time to determine the lower limit of the very LIS from a few MeV/nuc to
very high energies.