Expert: Philip Stahl - 4/7/2011

Question
Hi, I have a quick question regarding the solar wind, I keep reading that it is supersonic, which by my understanding means that it is faster than the speed of sound, but how can the speed of sound be relevant outside of the atmosphere? What is it supersonic with respect to?

Thanks.

Answer
Hello,

You are technically correct that the terrestrial sound speed isn't relevant outside our atmosphere. However, there is still what is called the ion sound speed, since in a plasma (electrically charged or ionized gas - such as the solar wind is) ions can still transmit vibrations based on their charges. Thus, what we call "ion acoustic waves" can occur through the aid of an electric field, E. (Also present in ionized plasmas).

The ion acoustic speed is given by:

c_s = {kT_e/ m}^1/2

where k = 1.38 x 10^-23 J/K, the Boltzmann constant, T_e is the electron temperature, and m the ion mass (for H, it is m = 1.66 x 10^-27 kg).

Using this one can obtain an ion sound speed of c_s ~ 28 km/s for a medium with an electron temperature of T_e = 10^5 K. However, we know solar wind velocities can reach 400-500 km/s and hence are "supersonic" in the sense of exceeding the ambient ion sound speed.

Hope this helps!