An Empirical Orthogonal Function (EOF) Analysis Of Ionospheric Electron Density Profiles Based On The Observation Of Incoherent Scatter Radars

The ionospheric electron density height profile describes the most important characteristic of the ionosphere. The electron density profile of the F2-layer, where most of the electron contents of the ionosphere is concentrated in, has become one of the most representative projects in the basic researches of ionosphere and the applications of the radio wave propagation. The intensive study of the ionospheric electron density profile depends on the development of the means of observing and analysis. The incoherent scatter radar systems has been proved to be a powerful probing technique to study the electron density profiles of the F2-layer, representing the most advanced technique of probing the ionospheric electron density profile at present. In this article, we have applied the empirical orthogonal function (EOF) analysis to investigate the ionospheric electron density profiles of the F2-layer, which are based on the data obtained from incoherent scatter radars. The purpose of this article is to use this powerfully and effective method to reveal the most important distribution and variation of the ionospheric electron density from the huge amount of the observed data.The EOF analysis is applied to analyze the data of two incoherent scatter radars during about three solar cycles. One is Millstone Hill ( 42. 6N, 28 8.5E), and the other is Arecibo ( 18. 3N, 66. 8W). The main results are summarized as follows:1. The corresponding empirical orthogonal basis function Ek of the electron density profiles of the F2-layer over Millstone Hill and Arecibo has been acquired. The first there basis functions have obvious physical meanings. The principal component represented by the first basis function mainly controls the ionospheric F2 peak density NmF2, the component represented by the second basis function controls both F2 peak height hmF2 and effective scale height, Hm and the component represented by the third basis function mainly controls effective scale height Hm. This result shows sufficiently the effectiveness of the EOF analysis in the physical application.2. Furthemore, we have analyzed the cyclic variations of different scales of the corresponding EOF coefficients Ak. These variations reflect the patterns of the climatic variations of NmF2, hmF2, and Hm.(1) Diurnal variations. The diurnal variations of the coefficients of EOF are distinct and have quasi-sinusoidal variations with different phases, which reflect the diurnal variations of NmF2, hmF2, and Hm.(2) Seasonal variations. The seasonal variations of the coefficients of EOF show the characteriscs of the annual and semiannual variations and reflect the winter anomaly and semiannual anomaly of NmF2, hmF2, and Hm.(3) Solar cycle variations. The solar cycle variations of the coefficients of EOF are obvious, reflecting the dependence of NmF2, hmF2, and Hm on the solar activity.EOF series converge rapidly in analyzing electron density profiles, the first several modes are able to represent the main characteristics. So we can use EOF analysis to extract the main distributional characteristics as well as the diurnal and climatic variation characteristics of the electron density profiles, which can be used to develop the empirical model for further study.