The Role of the DNT Resonance in the Classification of Magnetospheric and Ionospheric Resonances

A prominent sounder-stimulated plasma resonance with a frequency between the upper-hybrid frequency f_uh (f_T in ionospheric notation) and the electron plasma frequency f_pe ( f_N in ionospheric notation) was first identified and designated as the DNT resonance in ionospheric topside sounder ionograms [Benson, Radio Sci., 17, 167, 1982]. The letter D indicates that it has a diffuse appearance on the topside ionograms. Recently this resonance has also been observed to be stimulated in the magnetosphere by the Radio Plasma Imager (RPI) on the IMAGE satellite [Benson et al., JGR, 108(A5),1207, doi:10.1029/2002JA009589, 2003]. No theory has been offered to explain the formation of the DNT resonance. We will present the theoretical foundation for the following formula for the frequency of DNT which is a function of only f_pe and the electron gyrofrequency f_ce: F_DNT=√ {(F_{p^{2+1+√ {F_p^4+1})/2}}} where F_{DNT}=f{DNT}/f_{ce} and F_{p=fpe}/f_{ce}. We will show that this formula reproduces the Alouette 2, ISIS-1 and IMAGE/RPI observations. The results indicate that the use of this formula will provide a valuable additional independent confirming element to the spectral identification of sounder-stimulated plasma resonances in certain F_p ranges. This confidence in spectral classification, in turn, leads to confidence in determining the electron density N_e to within a few per cent even in space plasmas with N_e < 10 cm^-3 where determinations to this accuracy are difficult to achieve.