Ionospheric Characteristics and Models

Abstract

The ionized region roughly between 60 and 1200 km above the earth's surface is generally referred to as the ionosphere, even though some ionization persists above and below these bounds. Since most practical problems relate to the free electron number density N e , it is convenient to specify a restricted altitude regime since many legacy systems such as LEO satellite and HF communications, HFDF (i.e., direction finding), and surveillance systems such as OTHR (over‐the‐horizon radar) depend upon this restricted region in a substantive way. But there are other applications to be considered. For example, exoionospheric electrons will augment the ionospheric total electron content (TEC) encountered by GNSS signal waveforms and will introduce additional group path delay errors if compensation schema are not introduced. As a result, many ionospheric models include the plasmaspheric contribution to the TEC. This article examines basic theory of the ionosphere, but much of the material details ionospheric structure, ranging from kilometer‐scale variations causing scintillation or spread‐F to the mesoscale features associated with electrodynamic forces, an imbedded geomagnetic field and upper atmospheric wind patterns. We look at these structures in both space and time for pertinent geographic regions, and identify relationships with solar–terrestrial physics, or space weather phenomena in the more modern vernacular. We examine traveling ionospheric disturbances (TID), an ionospheric tracer of atmospheric gravity waves (AGW), their properties, and their possible sources. Fundamental properties of the more relevant ionospheric models are identified and details are referenced. We cover real‐time forecasting and novel data assimilation schemes. We conclude with a section on challenges facing the ionospheric specialist.