" Since AM is more susceptible to magnetic disruptions, I'd figure, and
AM radio is more likely to pick up interferences from outer space,
than an FM radio. The question is, which frequency is best for this?
Frequencies that are too low are sealed out of the ionosphere, while
frequencies that are too high rapidly disappear [line of sight].


I think you need to understand the basics a little more.

AM and FM are types of modulation and are not susceptible in themselves to
magnetic disruptions. You are getting confused by the fact that "medium
wave" broadcasts are referred to as "AM" when describing which frequency
band they are transmitted on. Although they do use Amplitude Modulation
(AM) there is nothing technically to stop you using FM on the same
frequency. The same applies the band known as FM, which is a vhf band, you
could use AM here if you wished.

300GHz is an extremely high microwave frequency anis absorbed by the
atmosphere very quickly, there are few uses for such a high frequency which
is difficult and expensive to use. So most likely you would hear nothing.

Your comments about the atmosphere are correct, low frequencies, below vhf,
are reflected and generally signals from space do not penetrate that well.
vhf and above will. The modulation, AM or FM have no bearing on this.

Jeff