The proposed explanation for these SWIA observations is that the charged solar wind ions are neutralized by “charge exchange” reactions in the ionosphere. As neutrals, they are not pushed around the planet but can penetrate deeply. At lower altitudes, these same charge-exchange reactions can turn them back into ions, still traveling at the same speed as the initial solar-wind ions. This process has not been observed before, and will allow us to determine properties of the solar wind even when we cannot detect it in the upstream region. There may also be implications for the behavior of the upper atmosphere. (Courtesy NASA/GSFC)
