<"Fortunately, particles emitted by solar flares don't travel at the speed of light (more like a million MPH), so a crew on Mars would have several days warning. SOHO or a similar satellite would monitor the Sun for flares."

Supprisingly enough, as the result of a solar flare a large population of electrons (beta radiation) do travel asymptotically close to the speed of light and arrive at detectors just an instant after the light reaches the same location. I don't think this is the major player in terms of dose.

Protons and light charged nuclei, in a flare large and intense enough to be a real threat to humans in Mars orbit, would probably mostly show up in the c/4 to c/2 velocity range. Here I'm generalizing immensely because nearly every flare is unique, but from some of the cases I've studied, this should hold.

"To determine the extent of the danger, it wouldn't be difficult to have sensors on robotic landers to measure the radiation levels from solar flares. This information could be used to design safety measures for the crew."

There was one slated for the 2001 lander before it was canceled in the backlashes following the disaster that was the Mars '98 program.

"On the surface, you'd have a little bit of protection from Mar's thin atmosphere. It wouldn't be much but it'd be better than being up in space. I suppose that one of the first things a crew would do after landing would be to set up living quarters, perhaps covered with soil for additional shielding. If water ice does exist on Mars, they might be able to use that for shielding, too."

The thing is, solar flares produce on the most part, soft x-rays and charged particles. There isn't enough x-ray flux directly from the sun to be worrysome. There MAY be a problem if you get caught in orbit in one of these electron storms and they start producing x-rays from the hull of your spacecraft.

Charged particles from solar flares are simply not energetic (fast) enough to span the Martian atmosphere without colliding very early on, probably in the upper atmosphere.

The real threat are those pesky Galactic Cosmic Rays (GCR) which are relatively heavy nuclei moving at .999999999999999999999999999c Producing Neutrons and exotic particles in the regolith and lower atmosphere. There are not as many GCR events as SPE but the flux is constant and from all parts of the sky, not just a point source like the sun.