What effects would a lack of a magnetosphere have on a planet's atmosphere, specifically in relation to the planet's ability to sustain life?
It seems that the primary issue would involve atmospheric escape. As I understand it, induction of magnetic moments in the ionosphere by solar winds would be sufficient to prevent atmospheric stripping from being the primary form of atmospheric escape. Loss of atmosphere would therefore primarily be due to thermal escape and electric force field acceleration which, if I'm not mistaken, are unaffected by the presence or absence of a magnetic field. At any rate, atmospheric escape is a fairly slow process, and I would think it would be an extremely long time before atmospheric loss would begin to have any adverse effect on life on the planet -- sufficiently long enough to make it a non-issue.
Another issue, and more relevant to the survival of life, would be radiation. A magnetosphere does nothing to stop electromagnetic radiation, but does deflect charged particles. But so does an ionosphere, as discussed above. And even if the ionosphere isn't as effective at blocking particulate radiation as a magnetosphere, a sufficiently thick atmosphere should, it seems, provide adequate protection.
First, am I correct in the above, and second, what other issues might a lack of magnetosphere involve?
(Specifically, I'm thinking of a hypothetical planet similar to Mars, but with an atmosphere thick enough to have a pressure at sea level equivalent to that of Earth's. Like Mars, this planet would be geologically inactive and therefore possess no atmosphere. It would be a similar or slightly greater distance from its star, and therefore receive somewhat less solar radiation than Earth. It's for a story I'm writing, so any help would be greatly appreciated.)