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For the observation of solar or galactic gamma rays, one should ideally be above the atmosphere. Quoting the Wikipedia article on Gamma-ray astronomy:

Most gamma rays coming from space are absorbed by the Earth's atmosphere, so gamma-ray astronomy could not develop until it was possible to get detectors above all or most of the atmosphere using balloons and spacecraft.

However, satellites do observe terrestrial gamma rays, either from natural or anthropogenic sources.

If the atmosphere absorbs gamma rays and solar or galactic gamma rays are best observed from space, how can terrestrial gamma rays be observed from above the Earth's atmosphere?

In other words, what does the atmospheric optical depth look like at very high frequencies? Is the following diagram inaccurate?

Atmospheric opacity
Atmospheric opacity, from Wikimedia Commons, originally from NASA.

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It is a matter of flux.

Two factors enter the ability to detect gammas: 1) the flux of the gammas, i.e. how many per meter square per second, and 2) the cross section of interaction. The cross section is dependent on the energy, and for a given energy is the same for extraterrestrial and terrestrial gamma rays.

The flux is not the same for terrestrial and extra terrestrial gamma rays.

Extra terrestrial gamma rays, no matter how high was the flux when they were created, reach us from large to enormous distances, the flux spreading like 1/$r^{2}$ from the distant point (for us) source. Terrestrial gamma rays are close to the satellites in comparison and the flux much higher than the extra-galactic one. The probability of finding terrestrial gammas is measurable, a substantial number survive the trip through the atmosphere. Extra-terrestrial gammas are few and the probability of surviving to the surface very small to zero.

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  • $\begingroup$ I see. I thought the optical depth of the atmosphere for gamma rays was much greater than 1, but clearly it isn't. I'm quite familiar with the atmospheric optical depth for visible, infrared, and microwave radiation, but I don't really know what it looks like for UV, X-Rays, and gamma rays, beyond "most gets absorbed" (no windows). This figure shows optical depth drops off again with higher frequencies. A similar figure but in terms of optical depth would be cool (: $\endgroup$
    – gerrit
    Aug 14, 2013 at 12:48
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    $\begingroup$ @gerrit: You could also look at this: physics.nist.gov/PhysRefData/Xcom/html/xcom1.html $\endgroup$
    – user4552
    Aug 14, 2013 at 13:56
  • $\begingroup$ @BenCrowell Right, that looks like cross sections for particular elements. While relevant, it takes some more steps to get the effective cross section for the colums of the U.S. Standard Atmosphere. $\endgroup$
    – gerrit
    Aug 14, 2013 at 18:03

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