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Do radio waves from the Sun come into contact with Earth? If so, do they penetrate the atmosphere or are they reflected, absorbed, or scattered?

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    $\begingroup$ Of course they do. It is part of the noise one can hear on, say, shortwave radio. It is what Penzias and crew were looking at when they found the CMB. $\endgroup$ – Jon Custer Jul 15 '16 at 2:40
  • $\begingroup$ @JonCuster -- The radio waves from the Sun are very distinct from the cosmic microwave background. $\endgroup$ – David Hammen Jul 15 '16 at 3:31
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    $\begingroup$ @DavidHammen - absolutely! The point is that Bell Labs was tasked with measuring and identifying noise sources across the radio spectrum (and in to the microwave regime) to find the right frequencies to use for long-haul telephone connections (what became the microwave links). As part of that Penzias et al. found this weird noise that was the same every direction they looked and independent of time of day. They were not looking for the CMB, it just happened to be there. $\endgroup$ – Jon Custer Jul 15 '16 at 13:40
  • $\begingroup$ Yep, which is precisely what the Wind and STEREO spacecraft were meant to do... $\endgroup$ – honeste_vivere Jul 15 '16 at 16:56
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Do radio waves from the Sun reach Earth?

Of course they do. It's just another form of electromagnetic radiation.


If so, do they penetrate the atmosphere or are they reflected, absorbed, or scattered?

That depends on frequency (or wavelength). The atmosphere reflects, absorbs, or scatters most incoming electromagnetic radiation. There's a window in the visible range, some partial windows in the infrared, and a big broad window in the radio frequencies (including some frequencies that are classified as microwave). This is depicted below.

Image showing the opacity of the Earth's atmosphere as a function of wavelength. Ultraviolet and shorter wavelengths are absorbed by the atmosphere, as are most infrared and microwave wavelengths. Long wavelength radio waves bounce off the ionosphere. Light in the visible wavelengths and much of the radio wavelengths, plus a few infrared and microwave bands pass through the atmosphere.
Image credit: NASA public domain

In the radio wavelengths, long wavelength radio waves (longer than 20-30 meters or so) don't pass through the atmosphere. They instead reflect off the ionosphere. Extremely short wavelength radio waves (shorter than 2-3 cm or so) also have a hard time passing through the atmosphere. They are absorbed by various molecules in the atmosphere such as water (both liquid and vapor) and molecular oxygen. In between, there's a nice radio window that lets scientists see into space, and to see the Sun in radio frequencies.

Radar was first used during World War II to detect enemy planes. Most of the scientists who worked on this during wartime returned to their civilian scientific jobs when hostilities ceased. Arthur Covington was one of those scientists. When he returned to his civilian work at the National Research Council in Ottawa, Canada, he cobbled together a radio telescope using spare military parts.

His antenna operated at 10.7 centimeters (2800 MHz) because those were the parts he had available to him. This turned out to be very serendipitous. He aimed his antenna at several objects, including Jupiter, the Milky Way, the aurora borealis, and the Sun. The instrument was too insensitive to pick up any source except for the Sun.

What he saw from the Sun was completely unexpected: The flux changed from day to day, month to month. He aimed the antenna at the sun during solar eclipse in November 1946. The flux dropped just as the Moon blocked a large sunspot. The correlation between sunspot number and the 10.7 centimeter solar radio flux is incredibly strong:

Image that portrays the monthly mean F10.7 solar radio flux and monthly mean sunspot number as a function of time, from 1947 to 2000. The correlation is incredibly strong.
Image credit: NASA public domain

As mentioned, this choice of 10.7 centimeters was serendipitous. It turns out that radio waves in the vicinity of 10 centimeters are predominantly produced in the Sun by sunspots, coronal ejections, and other high solar activity processes. The F10.7 solar radio flux is used to this day as a proxy for solar activity. One problem: A solar radio observatory that uses this frequency band needs to be far away from an airport. Airports also tend to use 10.7 centimeter radio waves in their radars because that's what was used back in World War II.

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Offhand I'm not sure where to find information about how much is absorbed, reflected and scattered, but the waves certainly do reach Earth, and some, at least, penetrate the atmosphere and end up in solar radio observatory detectors, otherwise we wouldn't have so many of them.

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