Solar Prominences: Filaments of Matter in the Solar Atmosphere

Solar Prominences: Magnetic Structures

Solar prominences are spectacular magnetic structures that appear in the solar corona, composed mainly of dense and relatively cool plasma (about 10,000 K) compared to the coronal environment (1 to 3 million K). These filaments of solar material can persist for several weeks before disintegrating or being ejected into space as solar flares.

Formation and Magnetic Structure

Prominences form in the active regions of the Sun where magnetic fields are particularly intense. Their stability is ensured by the balance between several forces:

• The gravitational force that tends to make the plasma fall back
• The Lorentz magnetic force that supports the material
• The thermal pressure of the plasma

The magnetic configuration can be described by the Lorentz force equation: \[ \mathbf{F} = q(\mathbf{E} + \mathbf{v} \times \mathbf{B}) \] where \( q \) is the charge of the plasma, \( \mathbf{v} \) its velocity, \( \mathbf{E} \) the electric field, and \( \mathbf{B} \) the magnetic field.

Classification of Prominences

There are mainly two types of prominences:

1. Quiescent prominences: Stable, can last several months
2. Eruptive prominences: Unstable, often lead to coronal mass ejections

The Enigma of Coronal Heating

Solar prominences play a key role in one of the great enigmas of solar physics: the mechanism of coronal heating that reaches temperatures of 1 to 3 million kelvins, while the visible surface (photosphere) does not exceed 5800 K. This inverse thermal gradient defies the classical laws of thermodynamics.

Explanatory Hypotheses

Several mechanisms have been proposed to explain this phenomenon:

• Magnetohydrodynamic (MHD) waves: Turbulence in the photosphere generates Alfvén waves that propagate their energy along magnetic field lines.
• Magnetic reconnection: Prominences are the site of magnetic reconnections where the energy stored in twisted magnetic fields is abruptly converted into thermal energy.
• Nanoflare micro-eruptions: Millions of local micro-eruptions (energy ~10²⁴ J) continuously heat the corona.

Role of Prominences

Prominences act as tracers of coronal processes:

• Their cold plasma (10⁴ K) coexists with the hot corona (10⁶ K), revealing exceptional thermal insulation
• Their filamentary structure shows the fine organization of the coronal magnetic field
• Their sudden disappearance releases significant amounts of magnetic energy

Source: Living Reviews in Solar Physics (2013)

Observation and Scientific Importance

Prominences are mainly observed:

• In H-alpha light (656.28 nm) for detailed studies
• In extreme ultraviolet by satellites such as SDO
• In coronagraphy for extended structures

Their study allows a better understanding of:

• The dynamics of solar magnetic fields
• Coronal heating mechanisms
• Processes leading to coronal mass ejections

Solar prominences, although studied for over a century, continue to fascinate solar physicists with their magnetic complexity and their role in space weather. Their complete understanding still requires theoretical and observational advances, particularly thanks to new space missions dedicated to the study of the Sun.