Scientists have put forward a theory that explains the emergence of steves - similar to the aurora phenomena of mauve color. The authors came to the conclusion that it is necessary to consider steves and the often accompanying green glow as separate phenomena. The former are associated with the heating of particles moving in the atmosphere and are not at all similar to ordinary auroras, and the latter, although they arise in uncharacteristic places, have much in common with standard phenomena, the researchers write in the journal Geophysical Research Letters.
Auroras arise as a result of the interactions of particles captured by the Earth's magnetic field with air molecules in the upper atmosphere. Typically, aurora is caused by electrons in the solar wind, which excite atoms and molecules such as oxygen and nitrogen. Auroras are directly related to the planet's magnetic field, so they usually appear only near the magnetic poles.
In addition to the usual auroras in the northern hemisphere, there are also mauve bands of light about 20-30 kilometers wide and up to thousands of kilometers long. They became known as steves (STEVE, Strong Thermal Emission Velocity Enhancement - a strong increase in speed by thermal radiation). They have been known to amateur astrophotographers for at least several decades, but the first scientific work on this topic was published only in 2018. Steves do not look like standard auroras not only in color, but also in place of appearance, because they were observed farther from the pole. Also, steves are sometimes accompanied by an unusual alternating green glow, reminiscent of a fence.
Scientists have already tried to figure out the mechanism of formation of steves, but could not finally figure it out. To put an end to this question, a team of researchers led by Yukitoshi Nishimura of Boston University collected data on three cases of observation of steves. The authors used data from ground-based instruments that study the magnetosphere of the THEMIS and Swarm satellites, as well as from the DMSP meteorological apparatus, which made it possible to obtain information on electric currents, magnetic fields, and energy spectra of particles in the immediate vicinity of the steves origin area.
Unlike previous studies, the authors of the new work analyzed data from satellites in high orbits for the first time. As a result, the researchers concluded that the steves and the green fence represent two separate phenomena generated by different processes. Scientists were able to confirm the idea expressed in previous works about the connection of steves with subauroral ion drift. This term refers to narrow streams of fast charged particles in the Earth's ionosphere, moving in the area adjacent to the auroral oval from the equator, where the opening of the geomagnetic field lines passes. Steves are actually the glow of electrically heated particles, that is, in a sense, they are similar to the light of the filaments of ordinary incandescent lamps.
The "green fence" was associated with the fallout of electrons into the atmosphere from a height of several thousand kilometers, which is confirmed by simultaneous observations of this phenomenon near both poles.
"The aurora is determined by the fallout of electrons and protons on the atmosphere, while the glow of the Steves is the result of heating without the fallout of particles," says co-author Beatriz Gallardo-Lacourt of the University of Calgary. "The Green Fence is generated by the fallout of electrons and is thus a kind of aurora, although it occurs outside the auroral zone, which makes it unique."
Scientists still do not know a lot about not only simple things, but also about auroras. In particular, only recently has there been clarity about the nature of the pulsating auroras. NASA also conducted an exceptionally beautiful experiment this month, pouring barium and strontium into the aurora. Auroras are also observed on other planets: in particular, the MAVEN probe recorded it on Mars.
Promotional video:
Timur Keshelava
