Coronal Alfven speeds in an isothermal atmosphere.I. Global properties

Research output: Contribution to journalArticlepeer-review

19 Citations (Scopus)

Abstract

Aims. Estimating Alfven speeds is of interest in modelling the solar corona, studying the coronal heating problem and understanding the initiation and propagation of coronal mass ejections (CMEs).

Methods. We assume here that the corona is in a magnetohydrostatic equilibrium and that, because of the low plasma beta, one may decouple the magnetic forces from pressure and gravity. The magnetic field is then described by a force-free field for which we perform a statistical study of the magnetic field strength with height for four different active regions. The plasma along each field line is assumed to be in a hydrostatic equilibrium. As a first approximation, the coronal plasma is assumed to be isothermal with a constant or varying gravity with height. We study a bipolar magnetic field with a ring distribution of currents, and apply this method to four active regions associated with different eruptive events.

Results. By studying the global properties of the magnetic field strength above active regions, we conclude that (i) most of the magnetic flux is localized within 50 Mm of the photosphere; (ii) most of the energy is stored in the corona below 150 Mm; (iii) most of the magnetic field strength decays with height for a nonlinear force-free field slower than for a potential field. The Alfven speed values in an isothermal atmosphere can vary by two orders of magnitude (up to 100 000 km s(-1)). The global properties of the Alfven speed are sensitive to the nature of the magnetic configuration. For an active region with highly twisted flux tubes, the Alfven speed is significantly increased at the typical height of the twisted flux bundles; in flaring regions, the average Alfven speeds are above 5000 km s(-1) and depart highly from potential field values.

Conclusions. We discuss the implications of this model for the reconnection rate and inflow speed, the coronal plasma beta and the Alfven transit time.

Original languageEnglish
Pages (from-to)297-309
Number of pages13
JournalAstronomy & Astrophysics
Volume491
DOIs
Publication statusPublished - Nov 2008

Keywords

  • Sun: magnetic fields
  • Sun: corona
  • Sun: atmosphere
  • Sun: flares
  • Sun: coronal mass ejections (CMEs)
  • IMAGING VECTOR MAGNETOGRAPH
  • ELECTRIC-CURRENT SYSTEMS
  • FREE MAGNETIC-FIELDS
  • SOLAR ACTIVE-REGION
  • FORCE-FREE
  • MAGNETOSTATIC ATMOSPHERES
  • 3-DIMENSIONAL STRUCTURES
  • MASS EJECTIONS
  • RESONANT ABSORPTION
  • JULY 14

Fingerprint

Dive into the research topics of 'Coronal Alfven speeds in an isothermal atmosphere.I. Global properties'. Together they form a unique fingerprint.

Cite this