Structures in ionospheric number density and velocity associated with polar cap ionization patches


  • Ö. Kivanç,

  • R. A. Heelis


Spectral characteristics of polar cap F region irregularities on large density gradients associated with polar ionization patches are studied using in situ measurements made by the Dynamics Explorer 2 (DE 2) spacecraft. The 18 patches studied in this paper were identified by the algorithm introduced by Coley and Heelis [1995], and they were encountered during midnight-noon passes of the spacecraft. Density and velocity spectra associated with these antisunward convecting patches are analyzed in detail. Observations indicate the presence of structure on most patches regardless of the distance between the patch and the cusp where they are believed to develop. Existence of structure on both leading and trailing edges is established when such edges exist. Results, which show no large dependence of ΔN/N power on the sign of the edge gradient ∇N, do not allow the identification of leading and trailing edges of the patch. The ΔN/N is an increasing function of ∇N regardless of the sign of the gradient. The correlation between ΔN/N and ΔV is generally poor, but for a given intensity in ΔV, ΔN/N maximizes in regions of large gradients in N. There is evidence for the presence of unstructured patches that seem to coexist with unstructured horizontal velocities. Slightly smaller spectral indices for trailing edges support the presence of the E × B drift instability. Although this instability is found to be operating in some cases, results suggest that stirring may be a significant contributor to kilometer-size structures in the polar cap.