Whistler evidence of a ‘knee’ in the magnetospheric ionization density profile

Study of a new whistler phenomenon shows that the magnetospheric ionization profile often exhibits a ‘knee,’ that is, a region at several earth radii in which the ionization density drops rapidly from a relatively normal level to a substantially depressed one. The new whistler phenomenon (called, for convenience, the ‘knee whistler’) is compared with ordinary whistlers and is illustrated by a number of examples recorded at middle- and high-latitude stations. It is suggested that the knee exists at all times in the magnetosphere, and that its position varies, moving inward with increasing magnetic activity. There are indications that conditions of whistler-mode propagation may be unusually favorable on the low-latitude side of the knee and that the region on the high-latitude side may be favorable for the production of triggered ionospheric noise. It is pointed out that knee whistlers account for a substantial number of the observations of deep density depressions during magnetic storms. Several questions of interpretation are raised, and the direction of future investigations is indicated.