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A massive atmospheric experiment is planned for August solar eclipse

The Hinode spacecraft captured this image of the Jan. 6, 2011, solar eclipse. (NASA)

On Aug. 21, as the moon passes in front of the sun and casts a shadow across the United States, millions are expected to gaze at the totality. Meanwhile, a smaller crowd will be glued to 150 custom-made radio receivers set up across the country.

The project, called EclipseMob, is the largest experiment of its kind in history. By recording changes in the radio signal, these citizen scientists will collect data on the ionosphere — the region of the atmosphere where, miles above Earth's surface, cosmic and solar radiation bumps electrons free from atoms and molecules. It plays a crucial role in some forms of long-distance communication: Like rocks skipped across a pond, radio waves can bounce along the top of the ionosphere to travel farther around the globe. But signals passing through the ionosphere sometimes behave in unpredictable ways, and scientists still have a lot of questions about its properties and behavior.

“Any solar eclipse is a good opportunity to study the ionosphere,” said Jill K. Nelson, an expert in signal processing at George Mason University in Virginia. The level of ions in the ionosphere fluctuates from day to night, decreasing in the absence of sunlight. But this change happens gradually during normal sunrises and sunsets. The sudden light-to-dark switch as it occurs during the eclipse, then, is an opportune moment to observe this layer of the atmosphere.

“We’re using the radio signal strength to understand what’s going on in the ionosphere,” Nelson said. The first attempt to study radio signals during an eclipse occurred in 1912, Nelson said. But EclipseMob has a few advantages over past studies: the number of people involved, the consistency of the radio signals and, by hooking receivers to smartphones, accurate data on location and time.

And because the eclipse passes over populous regions in the continental United States, the August totality will be particularly useful. EclipseMob participants are scattered, with some close to the path of the eclipse and others located in far corners of the country.

If the moon rises in the east and sets in the west, why does the eclipse shadow travel from west to east? Dear Science is on the case. (Video: Daron Taylor, Jorge Ribas/The Washington Post)

This experiment will test low- and medium-frequency signals. But high-frequency radio waves play an important role in the ionosphere, too. They can be transmitted into the upper level of the ionosphere where, in a layer called the F region, they bounce back down to Earth. These far-reaching lines of communication are useful when, say, hurricanes knock out cell towers, per George Mason University's description of the project. But fluctuations in the ionosphere can also disrupt radio waves. Increased free electrons in lower regions of the ionosphere during the day alters radio wave transmission, which is why AM radio signals are stronger at night, Nelson said.

During the Aug. 21 eclipse, the citizen scientists will use their radio receivers to listen to two signals. One comes from a Navy transmitter in California. The other is the 60 kHz broadcast from the National Institute of Standards and Technology facility in Colorado, the radio station WWVB, which transmits digital time codes across the United States. (Consumer electronics set their clocks by WWVB time.)

The radio receivers, developed by K.C. Kerby-Patel, an electromagnetics expert at the University of Massachusetts at Boston, and her students, will measure the strength of these signals during the eclipse. Previous experiments indicate that, as the moon's shadow sweeps across the country, the radio signal strength will undergo “fairly dramatic” changes, Nelson said. She expects the signal will look different depending on whether the waves have to cross through the path of totality to reach the receivers. Those close to the center path of the eclipse may see a more dramatic change.

The eclipse could have wide-reaching effects on radio signals, as suggested by earlier studies. In 1999, British citizens listened in for a 639 kHz radio station, broadcast from northern Spain, during an eclipse. Radio La Coruna was typically heard only at night in southern England. But, in the middle of the eclipse, residents reported receiving it much further to the north.

Though all 150 receiver kits created with a National Science Foundation grant have been claimed, it is not too late to join the EclipseMob, Nelson said. The project's website includes instructions for ordering parts and constructing your own radio receiver.

Read more:

Dear Science: Is the eclipse moving backward?

That time a city scared itself silly over a total solar eclipse

Scientists are gearing up to take the longest-ever video of a solar eclipse