The next annular solar eclipse will take place on October 14, 2023, during which the moon will partially obscure the sun. Although the eclipse will not be visible in France, it will still be easily observed in North America, and NASA intends to use this to launch its Atmospheric Perturbations around the Eclipse Path (Apep) mission. The latter aims to use three probes to study the effects of the sudden drop in solar brightness during a solar eclipse on the atmosphere and in particular on the ionosphere, the dynamics of which are determined by the ultraviolet rays coming from the Sun.
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It’s an expected event for Americans who love the sky: On October 14, the Earth, Moon, Moon and SunSun will be in perfect alignment. This alignment will temporarily eclipse the Sun, which will then remain hidden behind the Moon. This phenomenon is called an annular solar eclipse and is different from a total solar eclipse. During a solar eclipse, a total solar eclipse, the moon completely covers the sun’s disk, so that it is replaced by a very dark lunar silhouette that only reveals the sun’s crown.
In an annular solar eclipse, on the other hand, the apparent size of the moon in the sky is slightly smaller than that of the sun: observers who look into the sky will then see a very bright ring appear around the disk at the peak of the eclipse. Moon, also called the “Ring of Fire.” The annular solar eclipse on October 14th will only be visible from the Americas, where the apparent brightness of our Sun will drop to 10% of its usual brightness.
Effects on the ionosphere, an atmospheric layer controlled by sunlight
The ionosphere of a planet corresponds to the part of its atmosphere in which the gases are ionized, that is, the atoms, atoms and molecules lose or gain one or more electrons, electrons and thus become charged particles (ions). On Earth, the ionosphere takes up a large part of the atmosphere and is between 60 and 1,000 kilometers high.
It is the ultraviolet radiation emitted by the Sun that is at the origin of the ionosphere: this reacts with atmospheric molecules, sometimes allowing their dissociation (for example, ultraviolet radiation can break a dioxygen molecule into two oxygen atoms). but also to ionize them. Ultraviolet radiation can actually strip a molecule or atom of an electron, creating a soup of ions (lacking an electron) and free electrons in equal amounts. Through these processes, gases in the ionosphere act as an absorbing filter and help limit the amount of ultraviolet radiation that reaches the Earth’s surface. During the day, constant sunlight ensures that these particles remain separated; But as night falls, many recombine into neutral molecules or atoms before breaking away again at daybreak.
A still poorly understood dynamic during a solar eclipse
During a solar eclipse, the sun’s light disappears and reappears almost immediately on a small portion of the Earth’s surface. In a very short time, the temperature and density of the ionosphere decrease, only to rise again, creating waves that move through matter, like a body of water in which a boat is moving, creating ripples (or ripples) in its wake .
To study these phenomena, NASA plans to launch its Atmospheric Perturbations around the Eclipse Path (Apep) program, conducted in collaboration with researchers at Embry-Riddle Aeronautical University in Florida. Little anecdote: The acronym “Apep” refers to the Egyptian god Apophis (Apep in Egyptian), a deity depicted as a snake who, according to mythology, would have pursued the sun god Ra to the point of causing him to almost completely disappear during a solar eclipse.
The American space agency therefore plans to send three probes into the ionosphere, the first 35 minutes before the peak of the eclipse, the second during the peak of the eclipse and the last 35 minutes afterwards. They will travel through our atmosphere at altitudes between 70 and 325 kilometers to deploy four scientific instruments that measure fluctuations in the ionosphere’s electric and magnetic fields, as well as changes in temperature and density. During their trajectories (ascending and descending), these probes can precisely measure changes that occur at different altitudes. The probes will be recovered at the end of the mission and launched again during the next total solar eclipse on April 8, 2024.
The influence of the ionosphere on telecommunications satellites
At the same time, several ground observations will complete the mission. Researchers at Haystack Observatory, led by the Massachusetts Institute of Technology (MIT), will use their instruments to measure ionospheric disturbances slightly further from the path of the eclipse. A team of students from Embry-Riddle Aeronautical University will use high-altitude probe balloons to measure weather changes during the solar eclipse.
All of these measurements will help create an accurate model of the dynamics of the ionosphere. Understanding it is becoming increasingly important as all of our satellite communications pass through the ionosphere before reaching Earth. It therefore seems important to fully understand the behavior of the ionosphere and to be able to predict the disturbances that occur there.