![]() Stellar evolution models predict that the Sun has evolved, since its pre-main sequence phase, from a spectral type K to its current G2 classification. In a broad sense, these other “suns” belong to the group of solar-like stars. To understand the past, and future, evolution of the Sun, including its wind, magnetism, activity, rotation, and irradiation, astronomers rely on information from other “suns” in the Universe at different evolutionary stages. However, all this information just tells us about how the Sun looks like now. It is fair to say that the Sun is the best studied star in the whole Universe: we can measure its rotation, magnetic activity, composition, size, irradiation, and wind properties with accuracies like no other star in the Universe. I argue that studying exoplanetary systems could open up new avenues for progress to be made in our understanding of the evolution of the solar wind. I conclude this review then by discussing implications of the evolution of the solar wind on the evolving Earth and other solar system planets. I then link these observational properties (including, rotation, magnetism and activity) with stellar wind models. I overview some clever detection methods of winds of solar-like stars, and derive from these an observed evolutionary sequence of solar wind mass-loss rates. Given that we cannot access data from the solar wind 4 billion years ago, this review relies on stellar data, in an effort to better place the Sun and the solar wind in a stellar context. After the first crossing, such models of shock motion will be useful for predicting the timing of subsequent crossings.How has the solar wind evolved to reach what it is today? In this review, I discuss the long-term evolution of the solar wind, including the evolution of observed properties that are intimately linked to the solar wind: rotation, magnetism and activity. The results of this study suggest that the position of the termination shock can vary by as much as 10 AU in a single year, depending on the nature of variations in the ram pressure, and that multiple crossings of the termination shock by a given outer heliosphere spacecraft are likely. Using a simple kinematic model, we study the nonequilibrium location of the termination shock as it responds to these ram pressure changes. ![]() Such rapid changes in the solar wind ram pressure can cause large perturbations in the location of the termination shock. In addition to the global falloff with distance, there are large variations in ram pressure on relatively short time scales (tens of days), due primarily to large variations in solar wind density at a given radius. This distance scales inversely as the assumed field strength, i.e., for a 7 μG field, the termination shock will be located on average at 64 AU. Assuming that the interstellar pressure is due to a 5 μG magnetic field draped over the upstream face of the heliopause, this radial variation of ram pressure implies that the termination shock will be located at an average distance near 89 AU. ![]() A least squares fit of proton ram pressure to heliocentric distance R over this distance yields a ram pressure equal to (1.67×10 -8 dynes cm -2) R -2.00+/-0.02, where R is measured in astronomical units. We use these observations to discuss the probable location and motion of the termination shock of the solar wind. The Plasma Science experiment on the Voyager 2 spacecraft has measured to date the properties of solar wind protons from 1 to 40.4 AU.
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