Earth observation from space occurs throughout the electromagnetic spectrum. With my meteorological bias, I until recently believed it wasn't at frequencies higher than UV, but the X-Ray detector AXIS proves me wrong. What is the highest frequency instrument that is dedicated to Earth observation from space? The RHESSI spacecraft apparently measured γ-rays from Earth, but rather as a side-effect to observing γ-rays from the Sun. AXIS measures up to 300 KeV (12 picometre, 25 exahertz). Is this the highest frequency for any dedicated Earth observation from space, or are there even higher-frequency sensors? I'm interested in either historical instrument, current instruments, or instruments confirmed to be launched soon.
tl;dr: 2.4E+22 Hz
Terrestrial Gamma-ray Flashes are natural sources of gamma rays from Earth observed by satellites in space.
The science behind the phenomenon is far from figured out completely, but it involves lightning plus acceleration due to the strong static electric fields themselves.
From the AstroMEV website:
Terrestrial gamma-ray flashes (TGFs) are high-energy photons originating from the Earth's atmosphere in association with thunderstorm activity. TGFs were serendipitously discovered by BATSE detectors aboard the Compton Gamma-Ray Observatory. TGFs have also been detected and further studied by the RHESSI, AGILE and Fermi satellites. Their emission extends up to 100 MeV and exhibits an e+ – e- annihilation line. TGFs were utterly unexpected and as of now they are not fully understood. They are believed to be the product of particles acceleration inside thunderstorms. As they are produced in the Earth’s atmosphere, they potentially have a tremendous impact on our understanding of thunderstorms and atmospheric electrodynamics in general.
100 MeV is quite large for a terrestrial process. Nuclear gamma rays are typically in the 100 keV to few MeV range for nuclear transitions, though there can be larger ones from exotic processes in nuclear collisions.
$$E = h \ f$$
where $h$ is the Planck Constant of about 4.1E-15 eV seconds, 100 MeV energy photons (1E+08 electron Volts) correspond to a frequency of about 2.4E+22 Hz.
I guess that's 24 Zetta-Hertz, (but who's counting?)
The highest frequency is gamma rays, dedicated to detecting nuclear explosions on Earth. These have provided some knowledge of space events as well, due to their low selectivity, but they are intended and dedicated to detecting nuclear explosions.
The first series of satellites to do this detection was the Vela satellites, which detected x-rays, gamma rays, neutrons, and looked for quick flashes of light. There have been many others over the years.