The earths magnetic field traps charged particles (mostly protons and electrons) from the solar winds into large donut-shaped belts around the Earth, well outside of the extent of the atmosphere.
It's these particle belts which filter cosmic rays (gamma radiation) and as the magnetic field fluctuates so too does the spectrum of high energy gamma radiation that makes it through the charged particle shielding, which is further filtered by the atmosphere.
You're correct that there's no direct magnetic transformation of the incoming gamma spectrum, I was casually referencing the very real indirect effect.
It's the first time I ever hear about such an effect and I admit I have a hard time believing it since the particle density is very low in the Van Allen belt.
Do you have any source for that? (Google couldn't help)
The inner Van Allen donut is some 8,000 km thick (from ~1,000km above ground surface stretching out to ~8,500 km or so) and denser than the much much larger outer donut that (IIRC) stretches from 3 earth radii to 10 earth radii.
Low density but a lot of particles, many with high energy, sourced mostly from solar ejection, with cosmic particle and cosmic gamma strikes on the upper atmosphere sourcing a significant chunk of additional charged particles for the inner belt, and both receiving decay particles from cosmic high energy gamma and cosmic charged particles strikes on particles already within the belts (at a proportionatly lower rate than those from upper atmosphere interaction).
It's a Pachinko game, the density is low but the strikes do happen.
The effect I refer to is studied on again | off again (eg: iirc, Robert Forward looked at "draining the Van Allen belts" to remove charged particles and there were papers looking at the sheilding factor for cosmic that would be lost) but largely tossed in as an "and also" .. eg:
our magnetosphere shields us from erosion of our atmosphere by the solar wind (charged particles our Sun continually spews at us), erosion and particle radiation from coronal mass ejections (massive clouds of energetic and magnetized solar plasma and radiation), and cosmic rays from deep space.
Most of the focus tends to be on the cosmic charged particles with mass rather than the massless photons.
I admit I've largely only concerned myself with the fall off of the gamma spectrum through the atmosphere, but I've had past discussions with some on the fall off of the (cosmic) gamma spectrum as it approaches Earth and figure there must be papers on this about.
The Chandra X-ray Observatory calibration procedure papers would likely be our best bet for a good source, the satellite has an eccentric obit ( Perigee altitude 14,307.9 km, Apogee altitude 134,527.6 km ) which puts it fully outside the denser high energy inner belt but half in | half out the less dense outer belt.
There would have to be a procedure to factor out the variation in "local" X-Ray events as it moves about the planet in order to sharpen the data from the two Grating Spectrometers
Thanks for mentioning Chandra (RIP), it shook me out of this weird hypnotic fog that had me almost believe the Van Allen belts are important absorbers of gamma rays.
However, Chandra I know something about: it needs densities over three orders of magnitude larger than what is found in the Van Allen belt to even detect absorption lines, so that the belt would do anything important at all to the gamma ray continuum is just impossible.
I didn't say important at all, check the qualifiers, they absorb some gamma events .. how much would ideally show up in Chandra calibration procedures (if at all) .. it's the thinner outer belt that it rose and fell through after all, not the denser inner belt.
It's these particle belts which filter cosmic rays (gamma radiation) and as the magnetic field fluctuates so too does the spectrum of high energy gamma radiation that makes it through the charged particle shielding, which is further filtered by the atmosphere.
You're correct that there's no direct magnetic transformation of the incoming gamma spectrum, I was casually referencing the very real indirect effect.