Phenomenological study of LHR hiss

Abstract

The 1963 and 1964 Alouette 1 records from the Ottawa station were studied to determine the statistical characteristics of VLF hiss exhibiting a sharp lower-cutoff frequency associated with the lower-hybrid resonance (LHR). A particular class of midlatitude hiss events were emphasized and designated ‘S-type hiss.’ These events have lower-cutoff frequencies that vary smoothly with time, have limited bandwidths, and do not appear to be associated with other VLF phenomena. Their diurnal time of peak occurrence provides evidence that their sources may be connected with the type of magnetospheric activity usually associated with auroral-zone phenomena; yet, they are definitely midlatitude events. A likely means for transporting the electromagnetic energy is a horizontal duct that occurs as a result of the presence of more than one ion species in the topside ionosphere. Nonducted whistler-mode signals may be trapped in the duct, and satellites within the region observe signals with characteristics determined by this guiding structure. Other forms of LHR hiss, such as whistler-triggered events, may also be associated with these ducts. The characteristics of noise at altitudes above such ducts, or in the absence of ducts, may be different from that of S-type hiss. Some of the observed Alouette 1 events may be such nontrapped noises.