The formation of solar prominences by magnetic reconnection and condensation

Abstract

A model for solar quiescent prominences nested in a ' Figure 8' magnetic field topology is developed. This topology is argued to be the natural consequence of the distention of bipolar regions upward into the corona. If this distention is slow enough so that hydrostatic equilibrium holds approximately along the field lines, the transverse gas pressure forces fall exponentially with height whereas the inward Lorentz forces fall as a power law. At a low height in the corona, the pressure forces cannot balance the Lorentz forces provided the field lines remain tied to the photosphere and an inward collapse with subsequent reconnection at the point of closest approach should occur. Because of initial shear in the magnetic field, the reconnection would produce isolated helices above the point of reconnection since field lines would not interact with themselves but with their neighbors. This resulting topology produces a field above the elevated neutral line which is opposite in polarity to that of the photospheric field as in the current sheet models of Kuperus and Tandberg-Hanssen (1967). Raadu and Kuperus (1973), Raadu (1974), andRaadu (1979) and in agreement with recent observations of Leroy (1982), and Leroy et aL (1983).Assuming the isolated helices formed by reconnection are insulated from coronal thermal conduction and heating, the radiative cooling process and condensation is considered for the temperature range of 104-6000 K. This condensation results in a steady downflow to the bottom of the helices as the temperature scale-height falls, thus forming a dense, cool, prominence at the bottom of the helical configuration resting on the elevated neutral line with the remainder of the helix being essentially evacuated of material. We identify this neutral line at the bottom of the prominence with the sharp lower edge often seen when viewing quiescent prominences side-on and the evacuated helix with the coronal cavity observed around prominences when seen during total eclipses. Downflow speeds associated with the condensation process are calculated for prominence temperatures and yield velocities in the range of the observed downfiows of about 1 km s -1.

Physics of Solar Prominences: II—Magnetic Structure and Dynamics

“…Such force-free fields can contain dips in the field lines even when the underlying photospheric field is dipolar. In fact, many authors have suggested that filaments are supported by nearly force-free flux ropes that lie horizontally above the PIL (e.g., Kuperus and Raadu, 1974;Pneuman, 1983;van Ballegooijen and Martens, 1989;Rust and Kumar, 1994;). An alternative force-free configuration with dips, the sheared arcade, has also been studied as a description of the filament channel magnetic structure (Antiochos et al, 1994;DeVore and Antiochos, 2000;.…”

An active region filament studied simultaneously in the chromosphere and photosphere

“…Previous findings about prominence magnetic structure have shown models with dipped field lines in a normal (Kippenhahn & Schlüter 1957) or inverse 1 (Kuperus & Raadu 1974;Pneuman 1983) polarity configuration, the latter having a helical structure. From a large sample of quiescent prominence observations, Leroy et al (1984) found both types of configurations and a strong correlation between the magnetic field topology and the height of the prominence.…”

“…On the one hand, there is the sheared arcade model that, by large-scale photospheric footpoint motions (such as shear at, and convergence towards the PIL, and subsequent reconnection processes), is able to form dips and even helical structures, where plasma can be gravitationally confined (e.g., Pneuman 1983;van Ballegooijen & Martens 1989;Antiochos et al 1994;Aulanier & Demoulin 1998;DeVore & Antiochos 2000;Martens & Zwaan 2001;Welsch et al 2005;Karpen 2007). This model is capable of reproducing both inverse and normal polarity configurations in the same filament (Aulanier et al 2002).…”

Neutral Atom Diffusion in a Partially Ionized Prominence Plasma

“…Several different models have been suggested to explain how cool, dense objects like prominences can be supported in and thermally isolated from the surrounding hot, tenuous coronal plasma (e.g., Tandberg-Hanssen 1995), and these models generally fall into one of two main categories: dip models (Kippenhahn & Schlüter 1957;Antiochos & Klimchuk 1991), and flux-rope models (Hirayama 1985;Leroy 1989;Kuperus & Raadu 1974;Pneuman 1983;Anzer 1989; see also Fig. 1).…”