The solar wind shock termination

We analyse the possibility that the shock termination of the solar wind should occur at a heliocentric distance of 50 a.u. It is concluded that this is possible if the density of the interstellar medium near the Sun is of the order of 0.5 cm^-3, or the pressure of low energy cosmic rays (less than about 300 MeV/nucleon) is of the order of 6×10^-12 dyn cm^-2, or the interstellar magnetic field strength is of the order of 8 microgauss, or some suitable combination of these. Such conditions would not normally be expected to prevail in the interstellar medium in the neighbourhood of the Sun. However, the possibility that a supernova explosion occurred nearby some 10⁵-10⁶ years ago must be taken seriously and in such circumstances the required conditions may be relatively easily achieved.