Study of EMHD waves in a magnetic bubble

Summary form only given. Whistlers are shown to undergo drastic changes when propagated across a null point. These include, manifestation of nonlinearity, attenuation of their wave energy, selective transmission of wave components, helicity reversal etc. It is not clear whether the null point, or the field gradient prevailing around, plays the governing role in invoking such modifications. Experimentally it is difficult to resolve the null region as its characteristic length is of the order of the probe size. In order to delineate what is happening around the null region, we have set up an experimental arrangement in the Large Volume Plasma Device (LVPD)
³
, to produce an extended (~0.5 m
³
), nearly field free (B
_o
~ 250 mG) volume of afterglow plasma (n
_e
= 2 × 10
¹⁷
m
^-3
, T
_e
~ 1 eV), with the gradients pushed to the boundaries. The behaviour of EMHD waves (0.09 - 0.9 ω
_ce
) excited in this plasma are compared with those excited in finite B
_o
. At B=250 mG, the waves exhibit distinctly different dispersion and a reversal in polarization from right handed to left handed, as compared to their whistler counterparts. The results indicate propagation of Helicon like waves. The boundary to the plasma for the excited waves is provided not by walls of the device, but by the sharp gradients in the magnetic field.