The Heliosphere in Time

Because of the dynamic nature of the interstellar medium, the Sun should have encountered a variety of different interstellar environments in its lifetime. As the solar wind interacts with the surrounding interstellar medium to form a heliosphere, different heliosphere shapes, sizes, and particle contents result from the different environments. Some of the large possible interstellar parameter space (density, velocity, temperature) is explored here with the help of global heliosphere models, and the features in the resulting heliospheres are compared and discussed. The heliospheric size, expressed as distance of the nose of the heliopause to the Sun, is set by the solar wind-interstellar pressure balance, even for extreme cases. Other heliospheric boundary locations and neutral particle results correlate with the interstellar parameters as well. If the H 0 clouds identified in the Millennium Arecibo survey are typical of clouds encountered by the Sun, then the Sun spends ∼99.4% of the time in warm low density ISM, where the typical upwind heliosphere radii are up to two orders of magnitude larger than at present.