A review of theoretical and observational results describing atmospheric gravity wave (AGW)/traveling ionospheric disturbance (TID) phenomena at high latitudes is presented. Some recent experimental studies of AGW's using the Chatanika incoherent scatter radar and other geophysical sensors are reported. Specifically, the following features are described in detail: (1) cause/effect relations between aurorally generated AGW's and TID's detected at mid‐latitudes, including probable ‘source signature’ identification, (2) AGW source phenomenology, particularly a semiquantitative assessment of the relative importance of Joule heating, Lorentz forces, intense particle precipitation, and other mechanisms in generating AGW's, and (3) detection of TID's in the auroral ionosphere. Several instances of F region electron density, temperature, and plasma periodicities accompanied by horizontal plasma velocities which were consistent with theoretical AGW/TID models are documented.

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The physical properties of the ionized layer in the Earth's upper atmosphere enable us to use it to support an increasing range of communications applications. This book presents a modern treatment of the physics and phenomena of the high latitude upper atmosphere and the morphology of radio propagation in the auroral and polar regions. Chapters cover the basics of radio propagation and the use of radio techniques in ionospheric studies. Many investigations of high latitude radio propagation have previously only been published in Conference Proceedings and organizational reports. This book includes many examples of the behavior of quiet and disturbed high latitude HF propagation. Ample cross-referencing, chapter summaries and reference lists make this book an invaluable aid for graduate students, ionospheric physicists and radio engineers.

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A special campaign was conducted at the Sondre Stromfjord incoherent scatter radar facility in October 1985 to study hydromagnetic and atmospheric gravity wave phenomena near the cusp regions of the magnetosphere. During a day when the convection reversal boundary was measured just to the north of the radar, a field‐aligned current filament of inferred amplitude ∼2 × 105 A was measured at the boundary. Both ion and electron heating appeared to accompany the current filament. Magnetic field data acquired in the southern conjugate hemisphere are used to conclude that the current filament was probably on closed field lines. The current filament may be evidence of the magnetosphere boundary layer on closed field lines, perhaps produced by the injection of magnetosheath plasmoids onto the field lines by a flux transfer‐type reconnection process.

On October 18, 1985, moderate geomagnetic activity (Kp = 4+) near 1200 UT was followed by observations of a large-scale traveling ionospheric disturbance (TID) at observatories in Greenland, eastern North America, and Europe. Estimates of the speed and direction of the TID indicate that it was caused by an atmospheric gravity wave expanding from a localized source over the Arctic Ocean north of eastern Siberia. Auroral imaging shows that the source region was located near the westward edge of an expanding auroral bulge and may have been associated with a westward traveling surge (WTS). The largest TID amplitudes were observed along a line perpendicular to the orientation of the auroral oval, and TID periods increased with distance from the source region. This set of observations provides one of the most complete cause and effect relationships found thus far in the study of atmospheric gravity waves. INTRODUCTION Atmospheric gravity waves (AGWs) are motions in the neutral atmosphere having periods greater than about 10 min at ionospheric altitudes (100-400 kin). The perturbation of the neutral atmosphere induces disturbances in the ionization which travel with the AGW, and are thus called traveling ionospheric disturbances (TIDs). AGWs (and TiDs) are often classified according to period: waves having periods of less than 45 rain are termed "medium-scale", while those with periods greater than 45 min are called "large-scale" AGWs. The discussion that follows deals with the latter class.AGWs in the ionosphere have been studied theoretically and observationally for nearly 30 years. The progress in these studies is chronicled in review papers [Francis, 1975;Hunsucker, 1982] and books [Hines et al., 1974]. These reviews conclude that large-scale AGWs are frequently associated with substorms in the auroral oval; the energy source is believed to be either Joule heating or Lorentz force motions in the oval, but uncertainty exists concerning the details of the AGW generation mechanism. Hajkowicz and Hunsucker [1987] have also presented evidence that conjugate auroral

[1] A two-pronged study is under way to improve understanding of the D region response to space weather and its effects on HF propagation. One part, the HF Investigation of D region Ionospheric Variation Experiment (HIDIVE), is designed to obtain simultaneous, quantitative propagation and absorption data from an HF signal monitoring network along with solar X-ray flux from the NOAA GOES satellites. Observations have been made continuously since late December 2002 and include the severe disturbances of October--November 2003. GOES satellite X-ray observations and geophysical indices are assimilated into the Data-Driven D Region (DDDR) electron density model developed as the second part of thisproject. ACE satellite proton observations, the HIDIVE HF observations, and possibly other real-time space weather data will be assimilated into DDDR in the future. Together with the Ionospheric Forecast Model developed by the Space Environment Corporation, DDDR will provide improved specification of HF propagation and absorption characteristics when supplemented by near-real-time propagation observations from HIDIVE.