Browsing by Author "Oppenheim, Meers M."
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Item Open Access Day to night variation in meteor trail measurements: Evidence for a new theory of plasma trail evolution(American Geophysical Union, 2008-02-06) Oppenheim, Meers M.; Sugar, Glenn; Bass, Elizabeth; Dimant, Yakov S.; Chau Chong Shing, Jorge LuisA recent theory of meteor trail plasma diffusion made the prediction that meteors will generate more and longer lasting non‐specular echoes at night than during the day. This letter presents the first evidence of a dramatic day to night difference in non‐specular meteor trail occurrence rates and their duration. These observations were made by the 50MHz radar at the Jicamarca Radio Observatory (JRO) in Peru. In one 20 minute period starting 95 minutes before sunrise, this radar detected 1288 head echoes and 341 trails while a similar time after dawn, it measured 1240 head echoes but only 50 trails. Also, the duration of the nighttime trails greatly exceeded the daytime ones. This pattern was confirmed by a second experiment in July 2007. This data provides strong evidence that it is necessary to account for the effect of the ionospheric plasma density to explain meteor diffusion.Item Restricted First 3‐D simulations of meteor plasma dynamics and turbulence(American Geophysical Union, 2015-01-11) Oppenheim, Meers M.; Dimant, Yakov S.Millions of small but detectable meteors hit the Earth's atmosphere every second, creating trails of hot plasma that turbulently diffuse into the background atmosphere. For over 60 years, radars have detected meteor plasmas and used these signals to infer characteristics of the meteoroid population and upper atmosphere, but, despite the importance of meteor radar measurements, the complex processes by which these plasmas evolve have never been thoroughly explained or modeled. In this paper, we present the first fully 3‐D simulations of meteor evolution, showing meteor plasmas developing instabilities, becoming turbulent, and inhomogeneously diffusing into the background ionosphere. These instabilities explain the characteristics and strength of many radar observations, in particular the high‐resolution nonspecular echoes made by large radars. The simulations reveal how meteors create strong electric fields that dig out deep plasma channels along the Earth's magnetic fields. They also allow researchers to explore the impacts of the intense winds and wind shears, commonly found at these altitudes, on meteor plasma evolution. This study will allow the development of more sophisticated models of meteor radar signals, enabling the extraction of detailed information about the properties of meteoroid particles and the atmosphere.Item Restricted Meteor induced ridge and trough formation and the structuring of the nighttime E‐region ionosphere(American Geophysical Union, 2006-12-28) Oppenheim, Meers M.; Dimant, YakovWhen meteor‐generated plasma trails diffuse into the ionosphere they create large ambipolar electric fields mainly perpendicular to the Earth's geomagnetic field (B) and extending many kilometers from the trail along B. These fields will cause ionospheric plasma to collect into ridges extending from the trail along B, enhancing the density by as much as a factor of 2, and they will also remove up to 90% of the plasma on each side of the ridge. We predict that meteor‐induced density perturbations may fill as much as 20% of the ionosphere between 95 and 120 km altitude. This paper presents simulations and theory to show how meteors produce plasma ridges and troughs. We estimate the extent of these as a function of altitude and meteor density. This process may explain observations of extensive nighttime E‐region density structures made by rockets and radars.Item Restricted Photoelectron‐induced waves: A likely source of 150 km radar echoes and enhanced electron modes(American Geophysical Union, 2016-04-27) Oppenheim, Meers M.; Dimant, Yakov S.VHF radars near the geomagnetic equator receive coherent reflections from plasma density irregularities between 130 and 160 km in altitude during the daytime. Though researchers first discovered these 150 km echoes over 50 years ago and use them to monitor vertical plasma drifts, the underlying mechanism that creates them remains a mystery. This paper uses large‐scale kinetic simulations to show that photoelectrons can drive electron waves, which then enhance ion density irregularities that radars could observe as 150 km echoes. This model explains why 150 km echoes exist only during the day and why they appear at their lowest altitudes near noon. It predicts the spectral structure observed by Chau (2004) and suggests observations that can further evaluate this mechanism. It also shows the types and strength of electron modes that photoelectron‐wave interactions generate in a magnetized plasma.Item Open Access Remote sensing lower thermosphere wind profiles using non-specular meteor echoes(American Geophysical Union, 2009-05-12) Oppenheim, Meers M.; Sugar, Glenn; Slowey, Nicholas O.; Bass, Elizabeth; Chau Chong Shing, Jorge Luis; Close, SigridThis article describes a new method of measuring wind velocity profiles between 93 km and 110 km altitude by tracking non‐specular meteor echoes as neutral winds transport the plasma trails. This requires a large VHF radar with interferometric capability able to point nearly perpendicular to the geomagnetic field. A small data sample from the Jicamarca Radio Observatory allows the measurement of horizontal wind speeds and directions with a range resolution of a few hundred meters. These observations show speeds reaching 150 m/s and sometimes changing by as much as 100 m/s over a 6 km altitude range. At the best times, these measurements can be made with only a few minutes of data. With some refinement of the data collection and analysis techniques, this technique should produce high resolution images of lower thermospheric winds as they change in both altitude and time.