||Kirtland Air Force Base, NM 871175776
The specification of ionospheric characteristics has a major impact on the operation of many important remote sensing and communication systems. The utility of climatological models in capturing the large scale features of the ionosphere, such as bulk seasonal or diurnal variations, is clear. More recently assimilative models of the ionosphere have been used extensively in this context and provide better real time or hindcast capabilities. It is now well known that beyond the large scale features of the ionosphere there exists a hierarchy of transient perturbations, some of which appear as coherent wave structures. The advent of high spatial density networks of GPS receivers, from which the total electron content (TEC) between the satellite and receiver can be determined, has made the observation such structures in TEC maps almost routine. Such wave structures have also been observed in many other ionospheric observational platforms such as all sky optical imager, Ionosonde, Incoherent Scatter Radar (ISR), and HF Doppler sounders. While there have been a few studies comparing the phenomenology of the different observational systems an integrated understanding of these features is still lacking.
Historically such coherent wave structures have been deemed Traveling Ionospheric Disturbances (TID). Such ionospheric disturbances are thought to originate from similar disturbances in the neutral atmosphere often called Traveling Atmospheric Disturbances (TAD). TIDs have typically been categorized into large scale (LSTID) and medium scale (MSTID). In general LSTIDs propagate quasi-horizontally with phase velocities in the 400-600 m/s range and wavelengths of 1000 km or more. They are associated with step changes to Joule heating in the auroral zone and have a strong correlation to the AE index. MSTIDs may have a vertical component to their propagation with phase velocities of 100-200 m/s and wavelengths of a few hundred kilometers. Their origins are typically described as stratospheric or below, e.g., orographic wind, convective systems, earthquakes, tsunamis. Their occurrence rates are still under investigation and range from a few percent to ubiquitous. Some of this disagreement stems from the different definitions and observational systems used to construct the climatologies. There are even smaller irregularities, traveling and otherwise and all of these structures coexist creating complex structures. Interpretation of such measurement prior to assimilation into a model requires some knowledge or assumptions about ionospheric structures, e.g., conversion of slant TEC into vertical TEC or performing an Abel transform on Radio Occultation data. Such interpretation can be complicated by the presence of TIDs and depends in part on the spatial or temporal characteristics of the measurement.
At AFRL we have recently executed and have planned a number of field experiments where multiple ionospheric measurement devices have been fielded together including multiple Digisondes, all-sky imagers, a Fabre Perot Interferometer, Radio Beacon Receivers, HF spectrum analyzers, and GPS receivers. The Digisondes were often run at relatively high cadence and in oblique, or otherwise coordinated, modes. The proper multi-phenomenological interpretation of these data sets in terms of TIDs and TADs is of great interest for an improved understanding of their sources, vertical structure, and propagation characteristics as well as developing methods to understand their impact on assimilative models (or to assimilate them directly as coherent structures) and thereby their impact on RF propagation.
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Ionosphere; HF Propagation; Traveling ionospheric disturbance; Digisonde; GPS; Assimilative modeling; TEC; Vertical structures; System impacts;