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

¹ Departament de Física, Universitat de les Illes Balears, E-07122 Palma de Mallorca, Spain
² Institut d’Aplicacions Computacionals de Codi Comunitari (IAC3), Universitat de les Illes Balears, E-07122 Palma de Mallorca, Spain

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 4 February 2026
Accepted: 13 March 2026

Abstract

Solar prominence threads are typically located around magnetic dips, where cold and dense plasma is suspended against gravity in the hot corona due to the upward magnetic force. Because prominences are partially ionized, ambipolar diffusion has the capacity to deposit part of the energy of their non-force-free magnetic field into the plasma. This ambipolar heating could 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–Schlüter model, incorporating a sheared magnetic field. The temperature profile along the magnetic field was computed numerically by balancing radiative losses, thermal conduction, and ambipolar heating. The resulting models are consistently comprised 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. Here, we investigate how the length of the cold threads depends on the central temperature, central pressure, magnetic field strength, and shear angle. We show that thread lengths compatible with observational results can be obtained for realistic values of these parameters. Finally, we demonstrate that ambipolar diffusion plays a relevant role in this simple configuration, indicating that this effect should be incorporated into more elaborate multidimensional models and simulations.