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Shock waves and the associated phenomena generated by strongly ablating meteoroids with sizes greater than a few millimeters in the lower transitional flow regime of the Earth's atmosphere are the least explored aspect of meteor science. In this paper, we present a comprehensive review of literature covering meteor generated shock wave phenomena, from the aspect of both meteor science and hypersonic gas dynamics. The primary emphasis of this review is placed on the mechanisms and dynamics of the meteor shock waves. We discuss key aspects of both shock generation and propagation, including the great importance of the hydrodynamic shielding that develops around the meteoroid. In addition to this in-depth review, the discussion is extended to an overview of meteoroid fragmentation, followed by airburst type events associated with large, deep penetrating meteoroids. This class of objects has a significant potential to cause extensive material damage and even human casualties on the ground, and as such is of great interest to the planetary defense community. To date, no comprehensive model exists that accurately describes the flow field and shock wave formation of a strongly ablating meteoroid in the non-continuum flow regime. Thus, we briefly present the current state of numerical models that describe the comparatively slower flow of air over non-ablating bodies in the rarefied regime. In respect to the elusive nature of meteor generated shock wave detection, we also discuss relevant aspects and applications of meteor radar and infrasound studies as tools that can be utilized to study meteor shock waves and related phenomena. In particular, infrasound data can provide energy release estimates of meteoroids entering the Earth's atmosphere. We conclude with a summary of unresolved questions in the domain of meteor generated shock waves; topics which should be a focus of future investigations in the field. © 2018 COSPAR

Measurement capabilities of five meteor radars are assessed and compared to determine how well radars having different transmitted power and antenna configurations perform in defining mean winds, tidal amplitudes, and gravity wave (GW) momentum fluxes. The five radars include two new-generation meteor radars on Tierra del Fuego, Argentina (53.8°S) and on King George Island in the Antarctic (62.1°S) and conventional meteor radars at Socorro, New Mexico (34.1°N, 106.9°W), Bear Lake Observatory, Utah (∼41.9°N, 111.4°W), and Yellowknife, Canada (62.5°N, 114.3°W). Our assessment employs observed meteor distributions for June of 2009, 2010, or 2011 for each radar and a set of seven test motion fields including various superpositions of mean winds, constant diurnal tides, constant and variable semidiurnal tides, and superposed GWs having various amplitudes, scales, periods, directions of propagation, momentum fluxes, and intermittencies. Radars having higher power and/or antenna patterns yielding higher meteor counts at small zenith angles perform well in defining monthly and daily mean winds, tidal amplitudes, and GW momentum fluxes, though with expected larger uncertainties in the daily estimates. Conventional radars having lower power and a single transmitting antenna are able to describe monthly mean winds and tidal amplitudes reasonably well, especially at altitudes having the highest meteor counts. They also provide reasonable estimates of GW momentum fluxes at the altitudes having the highest meteor counts; however, these estimates are subject to uncertainties of ∼20 to 50% and uncertainties rapidly become excessive at higher and lower altitudes. Estimates of all quantities degrade somewhat for more complex motion fields.

Operation of a Meteor Wind Radar (MWR) at Eureka, Ellesmere Island (80° N, 86° W) began in February 2006; this is the location of the Polar Environmental and Atmospheric Research Laboratory (PEARL), operated by the "Canadian Network for the Detection of Atmospheric Change" (CANDAC). The first 36 months of wind data (82–97 km) are here combined with contemporaneous winds from the Meteor Wind Radar at Adventdalen, Svalbard (78° N, 16° E), to provide the first evidence for substantial interannual variability (IAV) of longitudinally spaced observations of mean/background winds and waves at such High Arctic latitudes. The influences of "Sudden Stratospheric Warmings" (SSW) are also apparent. Monthly meridional (north-south, NS) 3-year means for each location/radar demonstrate that winds (82–97 km) differ significantly between Canada and Norway, with winter-equinox values generally northward over Eureka and southward over Svalbard. Using January 2008 as case study, these oppositely directed meridional winds are related to mean positions of the Arctic mesospheric vortex. The vortex is from the Canadian Middle Atmosphere Model, with its Data Assimilation System (CMAM-DAS). The characteristics of "Sudden stratospheric Warmings" SSW in each of the three winters are noted, as well as their uniquely distinctive short-term mesospheric wind disturbances. Comparisons of the mean winds over 36 months at 78 and 80° N, with those within CMAM-DAS, are featured. E.g. for 2007, while both monthly mean EW and NS winds from CMAM/radar are quite similar over Eureka (82–88 km), the modeled autumn-winter NS winds over Svalbard (73–88 km) differ significantly from observations. The latter are southward, and the modeled winds over Svalbard are predominately northward. The mean positions of the winter polar vortex are related to these differences.

Operation of a Meteor Radar (MWR) at Eureka, Ellesmere Island (80° N, 86° W) began in February 2006: this is the location of the Polar Environmental and Atmospheric Research Laboratory (PEARL), operated by the "Canadian Network for the Detection of Atmospheric Change" (CANDAC). The first 36 months of tidal wind data (82–97 km) are here combined with contemporaneous tides from the Meteor Radar (MWR) at Adventdalen, Svalbard (78° N, 16° E), to provide the first significant evidence for interannual variability (IAV) of the High Arctic's diurnal and semidiurnal migrating (MT) and non-migrating tides (NMT). The three-year monthly means for both diurnal (DT) and semi-diurnal (SDT) winds demonstrate significantly different amplitudes and phases at Eureka and Svalbard. Typically the summer-maximizing DT is much larger (~24 m s−1 at 97 km) at Eureka, while the Svalbard tide (5–24 m s−1 at 97 km)) is almost linear (north-south) rather than circular. Interannual variations are smallest in the summer and autumn months. The High Arctic SDT has maxima centred on August/September, followed in size by the winter features; and is much larger at Svalbard (24 m s−1 at 97 km, versus 14–18 m s−1 in central Canada). Depending on the location, the IAV are largest in spring/winter (Eureka) and summer/autumn (Svalbard). Fitting of wave-numbers for the migrating and non-migrating tides (MT, NMT) determines dominant tides for each month and height. Existence of NMT is consistent with nonlinear interactions between migrating tides and (quasi) stationary planetary wave (SPW) S=1 (SPW1). For the diurnal oscillation, NMT s=0 for the east-west (EW) wind component dominates (largest tide) in the late autumn and winter (November–February); and s=+2 is frequently seen in the north-south (NS) wind component for the same months. The semi-diurnal oscillation's NMT s=+1 dominates from March to June/July. There are patches of s=+3 and +1, in the late fall-winter. These wave numbers are also consistent with SPW1-MT interactions. Comparisons for 2007 of the observed DT and SDT at 78–80° N, with those within the Canadian Middle Atmosphere Model Data Assimilation System CMAM-DAS, are a major feature of this paper. The diurnal tides for the two locations have important similarities as observed and modeled, with seasonal maxima in the mesosphere from April to October, and similar phases with long/evanescent wavelengths. However, differences are also significant: observed Eureka amplitudes are generally larger than the model; and at Svalbard the modeled tide is classically circular, rather than anomalous. For the semi-diurnal tide, the amplitudes and phases differ markedly between Eureka and Svalbard for both MWR-radar data and CMAM-DAS data. The seasonal variations from observed and modeled archives also differ at each location. Tidal NMT-amplitudes and wave-numbers for the model differ substantially from observations.

Multiwavelength airglow image data depicting a short-period (4.9 min) atmospheric gravity wave characterized by a sharp leading front have been analyzed together with synoptic meteor radar wind data recorded simultaneously from Bear Lake Observatory, Utah (41.6N, 111.6W). The wind data suggest the presence of a semidiurnal tide with horizontal winds peaking at around 60 m/s along the SSE direction of motion (170 from north) of this short-period wave. It was found that the gravity wave was most probably ducted because of the Doppler shift imposed by this wind structure. A marked 180 phase shift was observed between the near-infrared OH and the OI (557.7 nm) emissions. Numerical simulation results for similar ducted waves excited by idealized model sources suggest that the phase shift between the wave-modulated airglow intensities may be explained simply by chemical processes rather than by wave dynamics. Phase velocities of simulated waves, however, appear higher than those of observed waves, suggesting the importance of tidal thermal structure in determining the Doppler-ducted wave characteristics.

There has been some debate over the years concerning the accuracy of mesospheric wind observations made using the imaging Doppler interferometer (IDI) technique. The high potential and increasing use of IDI wind data in joint studies with spaced-antenna MF and meteor radar systems make it important to quantify the IDI results. This paper presents a novel comparison of wind measurements between a dynasonde implementation of IDI and winds derived from an all-sky meteor radar system, a widely-accepted standard for such measurements. Both radars were located at the USU Bear Lake Observatory and operated almost continuously for a four-month period. The winds and tides derived from IDI were found to closely match those measured by meteor radar, not only during the day but also at night, and at all overlapping heights from 80–95 km.