I've been given a practice question for an exam and I can't quite get it started. The question is: Intelligent beings on a distant planet try and communicate with earth by sending powerful radio waves swept in frequency from 10 to 50MHz per minute. The linearly polarized emissions must pass through a radiation belt plasma in such a way that E and k are perpendicular to B0. It is found that during solar flares on their sun, frequencies between 24.25 and 28MHz do not get through their radiation belt. From this deduce the plasma frequency and magnetic field there. I think I have to figure out the group velocity but does it also have to do with the cut off region of a plasma frequency? and how do I connect it all together to find out the plasma density and magnetic field? I'm so confused.