Particle acceleration at shocks propagating in inhomogeneous magnetic fields

Free access article

Issue		A&A Volume 455, Number 2, August IV 2006


Page(s)		685 - 695
Section		The Sun
DOI		http://dx.doi.org/10.1051/0004-6361:20054754

A&A 455, 685-695 (2006)
DOI: 10.1051/0004-6361:20054754

Particle acceleration at shocks propagating in inhomogeneous magnetic fields

A. Sandroos¹ and R. Vainio²

¹ Finnish Meteorological Institute, PO Box 503, 00101 Helsinki, Finland
e-mail: arto.sandroos@helsinki.fi
² Department of Physical Sciences, PO Box 64, 00014 University of Helsinki, Finland

(Received 22 December 2005 / Accepted 27 March 2006 )

Abstract
We consider particle acceleration at the scatter-free limit in quasi-planar shock waves propagating in inhomogeneous magnetic fields. It is shown that both non-constant magnetic field intensity and field-line curvature may lead to efficient acceleration of particles at shocks propagating through the structure. Shocks propagating towards increasing magnetic field intensity trap energetic particles, and as the field increases at the shock front the particles, by conserving their magnetic moment $\mu_B=E_\perp/B$ , increase their perpendicular energy by the ratio of maximum field magnitude to the field magnitude at the point of injection, $E_{\perp,\rm max} = E_{\perp, \rm inj} B_{\rm max}/B_{\rm inj}$ . This may result in energy gains by factor of 100 in the solar corona. In addition, shocks propagating in curved magnetic fields may trap particles and accelerate them to high energies on field lines on which the shock-normal angle gradually increases toward 90°. Suitable field-line geometries should be common in many astrophysical objects, such as stellar coronae and quasi-perpendicular parts of supernova shocks.

Key words: acceleration of particles -- ISM: cosmic rays -- shock waves -- Sun: particle emission

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