In a pot of boiling soup, light fat droplets rise to the top, while heavy vegetable chunks sink to the bottom. Particles in the tenuous hot gas around the Sun and other stars behave much the same way — except instead of broth, there's a mixture of charged particles: electrons, protons, and alpha particles.
Gravity pulls all particles downward, but light electrons resist more strongly than heavy protons. This creates a weak electric field that pushes particles apart by altitude. Scientists derived equations describing this field and tested them on a computer. It turned out that the field depends on how many alpha particles, helium nuclei heavier than protons, are in the gas.
When the gas is heated unevenly (for example, when the base of the atmosphere is hotter than the top) the picture becomes more complex. Particles of different masses respond differently to temperature variations, and an additional "thermal" component appears in the electric field. This component can change the field's direction at different altitudes.
This is like a multi-story building where each floor has its own temperature and its own mix of residents. At the bottom, near the hot "stove," there are more heavy particles, while at the top, where it's cooler, light ones accumulate. Such stratification changes the properties of the entire atmosphere — for example, how it conducts heat and how the solar wind escapes from it.
as written by the authors arXiv:2608.12014v1
We investigate stationary states of a collisionless, gravitationally stratified plasma atmosphere composed of electrons, protons, and alpha particles by extending Pannekoek--Rosseland theory to multispecies and multi-temperature plasmas. Starting from Liouville's theorem, we derive the self-consistent ambipolar electric field from kinetic equilibrium and charge neutrality. For a single-temperature atmosphere, we obtain analytical expressions for the ambipolar field, show its dependence on alpha-particle abundance, and determine the relative stratification of the three species. A first-order analytical approximation to the electrostatic potential accurately reproduces the numerical solution. We then generalize the formalism to multi-temperature plasmas generated by stochastic boundary heating, representing the stationary distribution as a superposition of Maxwellian populations. Gravitational filtering produces non-exponential density profiles and increasing temperatures with altitude, while preserving the relative species stratification, with alpha particles most strongly stratified and protons least. The ambipolar electric field contains a dominant gravitational contribution, corresponding to the generalized Pannekoek--Rosseland field, and a thermoelectric contribution arising from species-dependent temperature gradients, which accounts for its non-monotonic structure. These results provide a framework for studying the combined effects of plasma composition and stochastic heating in gravitationally stratified astrophysical plasmas.
In the solar corona, temperature increases with altitude: near the surface it's about 6000 degrees, but a few thousand kilometers away it's already millions of degrees. This paradox is still explained in various ways, and the electric field that stratifies particles is one of the keys.
In Arthur C. Clarke's novel "A Solar Storm," heroes fly through the solar corona, where plasma behaves as described in the article — with particle stratification and complex electric fields.