CHROMOSPHERIC EVAPORATION VIA ALFVEN WAVES

Journal Article · Sun Dec 20 04:00:00 EST 2009 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/707/2/903· OSTI ID:21392595

Haerendel, Gerhard ^[1]

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1. Max Planck Institute for Extraterrestrial Physics, P.O. Box 1312, 85741 Garching (Germany)

This paper presents a scenario for the chromospheric evaporation during solar flares, which is inspired by the chain of events leading to the formation of auroral arcs and ionospheric evacuation during magnetospheric substorms. The plasma, ejected from high coronal altitudes during a flare reconnection event, accumulates at the tops of coronal loops by braking of the reconnection flow, possibly by fast shock formation. A high-beta layer forms and distorts the magnetic field. Energy contained in magnetic shear stresses is transported as Alfven waves from the loop-top toward the chromosphere. It is shown that under these conditions the Alfven waves carry enough energy to feed the chromospheric evaporation process. The second subject of this investigation is identification of the most effective energy dumping or wave dissipation process. Several processes are being analyzed: ion-neutral collisions, classical and anomalous field-aligned current dissipation, and critical velocity ionization. All of them are being discarded, either because they turn out to be insufficient or imply very unlikely physical properties of the wave modes. It is finally concluded that turbulent fragmentation of the Alfven waves entering the chromosphere can generate the required damping. The basic process would be phase mixing caused by a strongly inhomogeneous distribution of Alfvenic phase speed and laminar flow breakup by Kelvin-Helmholtz (K-H) instability. The filamentary (fibril) structure of the chromosphere thus appears to be essential for the energy conversion, in which the K-H instability is the first step in a chain of processes leading to ion thermalization, electron heating, and neutral particle ionization. Quantitative estimates suggest that a transverse structure with scales not far below 100 km suffices to produce strong wave damping within a few seconds. Nonthermal broadening of some metallic ion lines observed during the pre-impulsive rise phase of a flare might be a residue of the turbulent breakup process.

• OSTI ID: 21392595
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 707; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ALFVEN WAVES
ATMOSPHERES
CHARGED PARTICLES
CHROMOSPHERE
CONVERSION
CRITICAL VELOCITY
ELECTRONS
ELEMENTARY PARTICLES
ENERGY CONVERSION
EVAPORATION
FERMIONS
FLUID FLOW
HIGH-BETA PLASMA
HYDROMAGNETIC WAVES
IONIZATION
IONS
LAMINAR FLOW
LEPTONS
MAGNETIC FIELDS
MAIN SEQUENCE STARS
NEUTRAL PARTICLES
PHASE TRANSFORMATIONS
PHYSICAL PROPERTIES
PLASMA
SLOWING-DOWN
SOLAR ACTIVITY
SOLAR ATMOSPHERE
SOLAR FLARES
STARS
STELLAR ACTIVITY
STELLAR ATMOSPHERES
STELLAR FLARES
SUN
THERMALIZATION
VELOCITY