Stimulation of the Harris instability in the ionosphere

Satellite soundings of the topside ionosphere reveal diffuse signals on the low frequency side of ${\mathrm{nf}}_H$ when ${\mathrm{nf}}_H{\mathrm{\hspace{0.17em}<\hspace{0.17em}f}}_T,$ where $n$ is an integer and $f_H$ and $f_T$ are the electron cyclotron and upper hybrid frequencies, respectively. They extend no further than the midway point between ${\text{(n\hspace{0.17em}−\hspace{0.17em}1)f}}_H$ and ${\mathrm{nf}}_H.$ The present observing conditions allow detection up to $\text{n\hspace{0.17em}=\hspace{0.17em}10.}$ The origin of the signals is attributed to the Harris instability of longitudinal plasma waves when $T_{\perp }{\mathrm{/T}}_{\Vert }\text{\hspace{0.17em}>\hspace{0.17em}2n,}$ where $T_{\perp }$ and $T_{\Vert }$ refer to the temperature corresponding to the electron motion perpendicular and parallel to the earth's magnetic field, respectively. The large anisotropies are caused by collisionless cyclotron damping of energy from the high power sounder pulse. This energy is absorbed in a region which extends radially from the antenna, a distance which is small compared with the antenna length but large compared with the antenna sheath region.