thumb|30 MHz HAARP riometer. thumb|HAARP antennas. thumb|Chart of the VHF riometer at HAARP from UTC 2007-01-28, 12:00 until UTC 2007-01-30, 00:00. It shown an event which causes an increased ionospheric absorption.
thumb|30 MHz HAARP riometer. thumb|HAARP antennas. thumb|Chart of the VHF riometer at HAARP from UTC 2007-01-28, 12:00 until UTC 2007-01-30, 00:00. It shown an event which causes an increased ionospheric absorption.
A riometer (commonly relative ionospheric opacity meter, although originally: Relative Ionospheric Opacity Meter for Extra-Terrestrial Emissions of Radio noise) is an instrument used to quantify the amount of electromagnetic-wave ionospheric absorption in the atmosphere. As the name implies, a riometer measures the "opacity" of the ionosphere to radio noise emanating from cosmic origin. In the absence of any ionospheric absorption, this radio noise, averaged over a sufficiently long period of time, forms a quiet-day curve. Increased ionization in the ionosphere will cause absorption of radio signals (both terrestrial and extraterrestrial), and a departure from the quiet-day curve. The difference between the quiet-day curve and the riometer signal is an indicator of the amount of absorption, and is measured in decibels. Riometers are generally passive radio antenna operating in the VHF radio frequency range (~30-40 MHz). Electromagnetic radiation of that frequency is typically Galactic synchrotron radiation and is absorbed in the Earth's D region of the ionosphere.
Discovered by embedding cosine similarity (sentence-transformers MiniLM, 384-dim).