Aurora in Planetary Atmospheres

Planetary aurorae are some of the most iconic and brilliant (in all senses of the word) indicators that not only are we all interconnected on our own planet Earth, but that we are connected throughout the entire solar system as well. They are testimony to the centrality of the Sun, not just in providing the essential sunlight that drives weather systems and makes habitability possible, but in generating a high-velocity wind of electrically charged particles—known as the solar wind—that buffets each of the planets in turn as it streams outward through interplanetary space. In some cases, those solar-wind particles actually cause the aurorae; in others, their pressure prompts and modifies what is already happening within the planetary system as a whole. Aurorae are created when electrically charged particles—predominantly negatively charged electrons or positive ions such as protons, the nuclei of hydrogen—crash into the atoms and molecules of a “planetary” atmosphere. They are guided and accelerated to high energies by magnetic field lines that tend to concentrate them toward the (magnetic) poles. Possessing energies usually measured in hundreds and thousands, all the way up to many millions, of electron Volts (eV), these energetic particles excite the atoms and molecules that constitute the atmosphere. At these energies, such particles can excite the electrons in atoms and molecules from their ground state to higher levels. The atoms and molecules that have been excited by these high-energy collisions can then relax, emitting light immediately after the collision, or after they have been “thermalized” by the surrounding atmosphere. Either way, the emitted radiation is at certain well-defined wavelengths, giving characteristic colors to the aurorae. Just how many particles, how much atmosphere, and what strength of magnetic field are required to create aurorae is an open question. Earth has a moderately sized magnetic field, with a magnetic moment measured at 7.91x1015 Tesla m3 (T m3). It has a moderate atmosphere, too, giving a standard sea-level pressure of 101,325 Pascal (Pa), or 1.01325 bar. The density of the solar wind at Earth is about 6 million per cubic meter (6x106 m-3). Earth has very bright aurorae. Mercury has a magnetic moment 0.7% of that of Earth and no atmosphere to speak of, and consequently no aurorae. But aurorae have been reported on both Venus and Mars, even though they both have surface magnetic fields much less than Mercury: they both have atmospheres, albeit Mars is very rarefied. The giant planets—Jupiter, Saturn, Uranus, and Neptune—have magnetic moments tens, hundreds, and (in the case of Jupiter) thousands of times that of Earth. They all have thick atmospheres, and all of them have aurorae (although Neptune’s has not been seen since the days of the Voyager spacecraft). The aurorae of the solar system are very varied, variable, and exciting.