The ionosphere is a unique region of Earth’s upper atmosphere that spans between 75 and 1000 km altitude and peaks at about 300 km, near where the International Space Station flies. During the day, the sun’s radiation ionizes the region (stripping electrons from atoms in the atmosphere) to form a plasma, composed of positively charged ions and negatively charged electrons. As the sun sets, that process of ionization decreases until just a trickle of radiation remains for ionization. Most of the electrons and ions recombine to return to a non-charged state.
That is, of course, unless there is a solar eclipse.
The amount of ionization in the atmosphere can vary from day to day and is especially high during periods of increased solar activity (like solar flares). Researchers and scientists have devised a number of ways to measure the ionosphere. One such way happens to be aboard every one of Spire’s satellites: an ultra-sensitive, dual-frequency GPS sensor. Typically, Spire’s GPS Radio Occultation sensor is used to measure things like temperature, pressure, and humidity of the atmosphere but in order to do that, it must also measure the ionosphere. Spire satellites collect the total number of electrons between the Spire satellite and the GPS satellites – often called Total Electron Content or TEC.
During a solar eclipse, the moon moves into a position where it blocks light and solar radiation from reaching Earth. It is similar to nighttime but is a localized and fleeting event. Compared to a sunset, it’s like flicking a light switch off then right back on again. That light switch effect is also observed in the ionosphere.
One of Spire’s satellites, LEMUR2-LYNSEY-SYMO, intersected with the eclipse. Over the course of about 11 minutes, it was able to take TEC measurements of the ionosphere under eclipse conditions every second. The figure below shows the location of these TEC measurements at 6 particular time stamps.
The result below actually came as a surprise to our team. When compared to a climatological model, a reasonable assumption of what the ionosphere looks like at a normal day at that time and location, the measured total electron content during the eclipse was decreased compared to the climatological model. Our LEMUR2 satellite picked up the decreased electron content caused by Nature’s unique experiment of shutting off the ionization source (the Sun!) during the eclipse.
So to answer the question of what happens to the ionosphere during an eclipse: the eclipsed section of the ionosphere rapidly loses its electron density resulting in a localized section of the ionosphere that behaves much like it would in the dark of night.