The Little Prince, the main character of a beloved French novella by the same name, departs his home asteroid and visits neighboring worlds, including Earth. Fermi's Large Area Telescope (LAT) detects gamma rays from several places within the solar system, including the Sun , the moon and Earth's atmosphere.
Much of this emission results from interactions by high-energy charged particles called cosmic rays. These particles, typically protons, have been accelerated to near the speed of light and come to us from beyond the solar system. When a cosmic ray collides with another atom, the result is a shower of fast-moving charged particles. Some of these particles decay (for example, pions) directly into gamma rays, while others produce gamma rays through additional interactions.
Although we normally think of the moon as shining by reflected sunlight alone, at higher energies it produces its own glow thanks to cosmic rays. In fact, at gamma-ray energies between about 100 and 400 MeV — energies tens of millions of times higher than the bluest visible light — the moon usually looks brighter than the Sun . Because cosmic rays are charged particles, the Sun 's strong magnetic field can deflect many of these particles away, resulting in fewer collisions and fewer gamma rays.
The Sun regains its throne when seen in gamma-ray energies above 400 MeV. This is partly because higher-energy cosmic rays cannot be deflected as easily and partly because another gamma-ray producing process becomes important. When a fast-moving cosmic ray strikes a visible light photon near the Sun , the impact can boost the photon up to gamma-ray energies.
At times, our star can rival the brightest gamma-ray sources in the sky thanks to explosions called solar flares. Flares occur near sunspots where the Sun 's magnetic fields are most intense and twisted, storing energy like a wound-up rubber band. When oppositely-directed magnetic fields come in contact, they release their stored energy in a titanic eruption. With an explosive equivalent reaching 20 billion megatons of TNT, solar flares are the most violent events in our solar system.
The explosion accelerates subatomic particles to near the speed of light, and these accelerated particles produce gamma rays and other forms of light when they interact with particles in the Sun 's atmosphere and surface.
The blast may excite some atomic nuclei to release gamma rays with energies unique to each kind of atom. Other interactions produce radioactive nuclei that subsequently decay, releasing antimatter positrons. When these positrons encounter an electron, the particles annihilate and produce gamma rays.
Fermi's Gamma-ray Burst Monitor (GBM) has observed gamma rays from these processes during many flares. Solar physicists use the observations to determine the conditions at the site of each flare. The LAT has also detected emission from pion decay. In some flares, this emission continues long after the other flare emissions have faded away. Solar physicists are still trying to understand this surprising behavior.
Finally, Earth's atmosphere is a bright gamma-ray source for Fermi. Most of this emission is a result of cosmic ray bombardment, but there's one important exception. Thunderstorms produce brief pulses of gamma rays called Terrestrial Gamma-ray Flashes (TGFs). These events occur unpredictably and fleetingly, with durations less than a thousandth of a second, yet they are surprisingly common. Data from Fermi's GBM indicate that at least a thousand TGFs occur each day. Studies using GBM observations show that TGFs are associated with the early phase of cloud-to-cloud lightning, but the phenomenon remains poorly understood.