Wednesday, March 18, 2015

How Loud is the Sun?

Tuesday morning was our last eclipse of the Spring 2015 eclipse season, so SDO is back to solar data 24/7.

A recent post on asked whether the inside of the Sun would be the loudest place in the solar system.

We see millions of sound waves moving across and through the Sun. They are used to tell us about the inside of the Sun. The waves are excited by convective blobs running into the surface, like the pop you hear when a bubble hits a wall. Blobs are hitting the surface all the time, so sound waves can always be seen in the Sun. Large solar flares can also cause sound waves to ripple across the surface. Other things, such as material falling back onto the surface, might also cause sound waves, but they have not yet been detected. We don’t hear the solar sound waves at Earth because sound cannot travel through a vacuum and space is a really good vacuum. But we do see the waves as changes in the brightness or velocity of the solar surface.

After looking though the post, I interpreted this question as: What would you hear if you could put a microphone into the atmospheres of the solar system and listen to the sound waves? Where is the loudest place in the solar system?

We can and do listen to the sounds in our atmosphere, oceans, and crust. Each has rumbles at frequencies below normal sound (less than 20 Hz) or infrasound. Winds are an important source of infrasound in the atmosphere and oceans. This is easiest to imagine in the oceans. A wind pushes water into a mound on the ocean, which collapses when it gets too heavy. That falling back emits sound waves that can be measured. You also “hear” ships moving along the surface, animals communicating, and earthquakes. Although you don’t hear this when you put your head underwater, this noise is a growing problem, especially for passive sonar. Another example, the infrasound signal from the Chelyabinsk meteor entry was the loudest meteor yet recorded. It was the study of how the waves from the Krakatoa eruption of 1883 were measured in Europe and the U.S.A. that gave us the first theory of sound waves in the Sun.

You don’t hear infrasound because your ears are tuned to the sound waves needed to talk and survive. Below about 1 Hz you don’t really hear the wave so much as feel the wave. So even though the infrasound is loud, you do not sense it as it passes by unless it is really loud. This also means that the threshold of hearing for these sound waves is almost the same as the threshold of pain.

The Sun does not have fast moving winds to generate noise, but it does have convective blobs banging into the surface. They create infrasound at about 3 mHz, a frequency almost 1 million times lower what we use to talk. These sounds waves are observed moving as ripples in the photosphere, the apparent surface of the Sun in visible light.

The loudness of a sound wave can be calculated if either the pressure change or velocity of the sound wave can be measured. Solar sound waves have a measured velocity of about 10 cm/s at the photosphere. Combining that with the density (2.8 × 10-7 g) and sound speed (8.1 × 105 cm/s) at the photosphere, the loudness of these waves is about 103 dB, below the threshold of pain (130 dB), but possibly below the threshold of hearing as well. Even though there are many of these modes, you would not think the Sun is any louder than the ocean.

What is the loudest atmosphere in the solar system? The wind generates infrasound in the atmosphere and ocean. The faster the wind, the louder the noise and the lower the frequency of the noise. That means the 400 mph winds of Jupiter or the 1000 mph winds of Saturn would generate a lot of infrasound in their atmospheres. Rather than the Sun, it seems that Saturn would have the loudest atmosphere.

End note: The simplest formula to estimate the loudness of a sound wave is L = 10 log(I/I0) = 10 log(ρ c u2/10-9).