# Entropy Rain: Dilution and Compression of Thermals in Stratified Domains

## TL;DR

This paper develops an analytical theory and simulations to understand how dense, negatively buoyant thermals, or 'entropy rain', can effectively transport solar luminosity by falling through stratified atmospheres, challenging traditional large-scale flow models.

## Contribution

It introduces a new analytical framework for dense thermals in stratified atmospheres and verifies it with simulations, proposing entropy rain as a key solar luminosity transport mechanism.

## Key findings

- Dense thermals fall in two regimes: stalling and falling.
- Solar downflows are in the falling regime, maintaining entropy perturbations.
- Entropy rain could be an effective nonlocal solar luminosity transport process.

## Abstract

Large-scale convective flows called giant cells were once thought to transport the Sun's luminosity in the solar convection zone, but recent observations have called their existence into question. In place of large-scale flows, some authors have suggested the solar luminosity may instead be transported by small droplets of rapidly falling, low entropy fluid. This "entropy rain" could propagate as dense vortex rings, analogous to rising buoyant thermals in the Earth's atmosphere. In this work, we develop an analytical theory describing the evolution of dense, negatively buoyant thermals. We verify the theory with 2D cylindrical and 3D cartesian simulations of laminar, axisymmetric thermals in highly stratified atmospheres. Our results show that dense thermals fall in two categories: a stalling regime in which the droplets slow down and expand, and a falling regime in which the droplets accelerate and shrink as they propagate downwards. We estimate that solar downflows are in the falling regime and maintain their entropy perturbation against diffusion until they reach the base of the convection zone. This suggests that entropy rain may be an effective nonlocal mechanism for transporting the solar luminosity.