In our daily lives, fluids that have different temperatures mix in a straightforward manner. When you pour cold cream into a hot cup of coffee, the molecules of the two fluids collide and quickly redistribute the heat, cooling your coffee down to a uniform, drinkable temperature.
If only things were as easy in space.
There, the extremely low densities of gases mean that plasmas with different temperatures can physically overlap but not collide for a while. If such a cup of coffee existed, a thermometer might read an overall lukewarm 40°C, but it might still burn us. This may be unfamiliar to us, but it’s common in space, for instance, where the hot solar wind runs into Earth’s relatively cool magnetic bubble.
Such interactions give rise to a lot of interesting physics. One example is a particular kind of plasma wave called an electron acoustic wave (EAW), the plasma equivalent of sound waves. These waves are found only in mixed plasmas with large differences in temperature. To find out where they might appear in the solar system, Rehman et al. solved the equations to derive the waves’ behavior under different conditions. Their results should help space physicists better understand the role of such waves in phenomena like aurora and space weather.
In general, their calculations show that such waves can exist within only a certain window of frequencies. If the frequencies are too low or too high, they resonate with either the hot or cold electrons, which will damp them out.
But the authors also found the waves are so sensitive to the surrounding conditions that to model them accurately, they needed to add another twist: the quantum effect of the orbital angular momentum of the electrons, which twists the passing waves into helices. Accounting for this effect shifts that frequency window significantly higher. It also reveals that the difference in temperature between the hot and cold electrons has to be more extreme than previously thought.
Other conditions are also important to this dynamic. The balance of hot and cold electrons must be relatively even. Also, the amount of twist in the waves must fall within a certain range; the most favorable appears to be a perfect, symmetrical helix.
As a real-world test, the authors applied their model to Saturn’s magnetic field, where satellites, including NASA’s Cassini, have found mixed-temperature electrons. The model shows just how sensitive EAWs are: Conditions were favorable for them in a narrow region in Saturn’s outer magnetosphere at a distance 13 times Saturn’s radius. However, at 14 Saturn radii, the temperatures of the plasmas were too close together to support EAWs, and at 12 Saturn radii or closer, there weren’t enough hot electrons to support them either.
EAWs may also be important closer to home: The mixed-temperature boundary where Earth’s magnetic field meets the solar wind is where the energy for hazardous geomagnetic storms is unleashed. The techniques developed by the authors could examine whether EAWs have an effect on such storms. (Journal of Geophysical Research: Space Physics, https:/doi.org/10.1002/2016JA023627, 2017)
—Mark Zastrow, Freelance Writer