Oblique Hydromagnetic Shock Waves

An experiment is described in which shock waves are propagated in a magnetized hydrogen plasma with Alfvén Mach numbers up to MA = 5 and at angles a to the magnetic field from 90° to 40°. The plasma of density ∼5 x 1014 cm3 and temperature ∼1 eV is created in a pyrex chamber of 45 cm diameter and 93 cm length by means of an oscillatory z -discharge. The initial magnetic field is 500-1000 G. A curved magnetic piston is driven into the plasma by exciting a rapidly rising θ -current in a single-turn loop of short axial length arranged round the tube on its midplane. A curved shock front propagates ahead of the piston in the manner of a bow wave. Magnetic and electric probes are used to study the development and structure of the shock front. On the midplane the front has the monotonie field jump typical of propagation perpendicular to the field, but for propagation at an angle to the field a radically different structure is seen. Ahead of the main jump in field and density there is a large-amplitude, circularly polarized oscillation which is identified as a highfrequency Alfvén ('whistler') wave propagating at the same speed as the shock. The wavelength λ is given by λ ≈ (2π cosα)/(M22A-1)½ * (c)/ωpi. The results are compared with numerical computations of shock structure using hydromagnetic equations. To account for the observed structure, it is necessary to invoke collisionless effects. (author)