Low-β Plasma Penetration across a Magnetic Field

Self‐consistent solutions for the problem of a collisionless plasma streaming across a magnetic field whose dimensions are small compared to ${\mathrm{(r}}_{\mathrm{ce}}{r}_{\mathrm{ci}}{)}^{\text{1/2}}$ show that the electrons can accompany the ions across the field (over distances large compared to $r_{\mathrm{ce}}$ ) provided ${\omega }_{\mathrm{ce}}{\mathrm{\hspace{0.17em}<\hspace{0.17em}\alpha \hspace{0.17em}\omega }}_{\mathrm{pe}}$ , where $\text{α\hspace{0.17em}≤\hspace{0.17em}1}$ is a function of the electron distribution function ($\text{α\hspace{0.17em}=\hspace{0.17em}1}$ when the electron thermal velocities are neglected). This appears to explain the threshold, ${\omega }_{\mathrm{ce}}{\text{\hspace{0.17em}≃\hspace{0.17em}0.9\hspace{0.17em}ω}}_{\mathrm{pe}}$ , observed in recent experiments on plasma penetration across a radial magnetic field.