A CRITICAL SHOCK MACH NUMBER FOR PARTICLE ACCELERATION IN THE ABSENCE OF PRE-EXISTING COSMIC RAYS: M = √5

被引:32
作者
Vink, Jacco [1 ]
Yamazaki, Ryo [2 ]
机构
[1] Univ Amsterdam, Astron Inst Anton Pannekoek, Gravitat & AstroParticle Phys Amsterdam GRAPPA, NL-1098 XH Amsterdam, Netherlands
[2] Aoyama Gakuin Univ, Coll Sci & Engn, Dept Math & Phys, Sagamihara, Kanagawa 2525258, Japan
关键词
acceleration of particles; galaxies: clusters: intracluster medium; shock waves; Sun: coronal mass ejections (CMEs); Sun: particle emission; RADIO RELICS; SOLAR CORONA; WAVES; MODEL;
D O I
10.1088/0004-637X/780/2/125
中图分类号
P1 [天文学];
学科分类号
0704 ;
摘要
It is shown that, under some generic assumptions, shocks cannot accelerate particles unless the overall shock Mach number exceeds a critical value M > root 5. The reason is that for M <= root 5 the work done to compress the flow in a particle precursor requires more enthalpy flux than the system can sustain. This lower limit applies to situations without significant magnetic field pressure. In case that the magnetic field pressure dominates the pressure in the unshocked medium, i.e., for low plasma beta, the resistivity of the magnetic field makes it even more difficult to fulfill the energetic requirements for the formation of shock with an accelerated particle precursor and associated compression of the upstream plasma. We illustrate the effects of magnetic fields for the extreme situation of a purely perpendicular magnetic field configuration with plasma beta beta = 0, which gives a minimum Mach number of M = 5/2. The situation becomes more complex, if we incorporate the effects of pre-existing cosmic rays, indicating that the additional degree of freedom allows for less strict Mach number limits on acceleration. We discuss the implications of this result for low Mach number shock acceleration as found in solar system shocks, and shocks in clusters of galaxies.
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页数:13
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