Absorption of radio waves occurs when electrons responding to the wave fields collide with and transfer energy to the neutral particles. A study of ionospheric absorption yields information on both the D and E regions, where both electrons and neutral particles exist in sufficient quantity to produce significant absorption. Knowledge of D region characteristics is unattainable from normal sweep-frequency ionograms because absorption prevents the return of a detectable reflected signal. In practice, absorption characteristics are studied primarily in connection with radio communications applications. A riometer (relative ionospheric opacity meter) is a detector that measures the absorptive properties of the ionosphere. This directional detector is oriented in a fixed direction relative to the Earth (usually vertical) and measures the incident cosmic radio noise at a single frequency between 8 MHz and 120 MHz (usually 20, 30, or 60 MHz). During the course of each sidereal day, the riometer receives noise from all celestial longitudes. Due to the general constancy of cosmic radio noise, the signal will be constant except for a large-scale, small-amplitude, daily solar effect which regresses through the observations at the rate of 1 day per year (4 minutes earlier each day). Deviations of observed riometer signal power are primarily attributable to ionospheric variations particularly in the D and E regions. This method is particularly useful during radio blackout (high absorption) since it involves the use of higher frequency signals that are not completely absorbed. The Alaskan Chain Riometer data was discontinued, in analog form, December 31, 1990. The Data Center still receives riometer data from Kiruna. The publication includes magnetograms, k-indices, all-sky camera data, and riometer data. It is published quarterly. We also receive some riometer data from Syowa Station. The data are collections of several years of data and the publication (JARE DATA REPORTS) are sent by the National Institute of Polar Research, Tokyo Japan. These publications only have numeric values. All riometer data are in paper form and are stored off site. Most of the data are ''strip charts'' which record the variations on paper rolls. This data base is mostly a collection of the different charts and publications. No additional quality control is done by the NOAA National Centers for Environmental Information (formerly National Geophysical Data Center) here in Boulder. Data requested are electrostatic copies of the appropriate portion of a chart of publication.
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