A stability property of a force-free surface bounding a vacuum gap
Abstract
A force-free surface (FFS) S is a sharp boundary separating a void from a region occupied by a charge-separated force-free plasma. It is proven here under very general assumptions that there is on S a simple relation between the charge density μ on the plasma side and the derivative of δ=E\cdotB along B on the vacuum side (with E denoting the electric field and B the magnetic field). Combined with the condition δ=0 on S, this relation implies that a FFS has a general stability property, already conjectured by Michel (1979, ApJ 227, 579): S turns out to attract charges placed on the vacuum side if they are of the same sign as μ. In the particular case of a FFS existing in the axisymmetric stationary magnetosphere of a ``pulsar'', the relation is given a most convenient form by using magnetic coordinates, and is shown to imply an interesting property of a gap. Also, a simple proof is given of the impossibility of a vacuum gap forming in a field B which is either uniform or radial (monopolar).
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- January 2005
- DOI:
- 10.1051/0004-6361:20041624
- arXiv:
- arXiv:1005.4698
- Bibcode:
- 2005A&A...429..779A
- Keywords:
-
- stars: pulsars: general;
- magnetic fields;
- plasmas;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- Astron. Astrophys. 429, 779-784 (2005)