The role of ambipolar heating in the energy balance of solar prominences

TL;DR

This study investigates how ambipolar diffusion influences the energy balance and structure of solar prominences using one-dimensional models, highlighting its significance in prominence physics.

Contribution

It introduces a simplified model demonstrating the importance of ambipolar heating in prominence energy balance and structure, suggesting its inclusion in future complex models.

Findings

Ambipolar diffusion provides significant heating balancing radiative losses.

Models produce prominence-like structures with realistic thread lengths.

Stationary flows due to ambipolar diffusion relate to gravitational drainage.

Abstract

Solar prominence threads are typically located around magnetic dips, where cold and dense plasma is suspended against gravity in the hot corona thanks to the upward magnetic force. Because prominences are partially ionized, ambipolar diffusion can deposit part of the energy of their non-force-free magnetic field into the plasma. This ambipolar heating may therefore play a role in the energy balance of prominences. In this proof-of-concept work, we explore the effect of ambipolar diffusion in one-dimensional models that satisfy both mechanical equilibrium and energy balance. The magnetic configuration is based on the classic Kippenhahn-Schl\"uter model, incorporating a sheared magnetic field. The temperature profile along the magnetic field is computed numerically by balancing radiative losses, thermal conduction, and ambipolar heating. The resulting models consistently consist of a cold, dense, partially ionized thread with prominence core conditions, a very thin prominence-corona transition region, and an extended, hot, fully ionized region with coronal conditions. In addition to providing heating that partly compensates for radiative losses, ambipolar diffusion also gives rise to stationary flows associated with the gravitational drainage of neutrals in the partially ionized region. We investigate how the length of the cold threads depends on the central temperature, central pressure, magnetic field strength, and shear angle, and show that thread lengths compatible with observations are obtained for realistic values of these parameters. Therefore, we demonstrate that ambipolar diffusion plays a relevant role in this simple configuration, indicating that this effect should be incorporated into more elaborate multi-dimensional models and simulations.