Heater enhanced topside plasma line

In a recent experiment at Arecibo, the ionospheric topside plasma line was enhanced by the high power, high frequency, radiowave, HF heater. The enhancement was a few orders of magnitude over the normal daytime photoelectron‐enhanced plasma lines. This is the first observation of an orders of magnitude enhancement of the plasma line in the topside ionosphere induced by the heater. The critical frequency of the F‐region (f0F2) was reasonably close to (approximately 10% higher than) the HF heater frequency of 5.1 MHz. Possible explanations include propagation via the magneto‐ionic Z‐mode with some unknown excitation mechanism, some severe inhomogenities in the ionosphere, or heater excited electrons streaming upward along the magnetic field lines to excite the plasma.