Browsing by Author "Mathews, John D."
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Item Restricted A radio science perspective on long-duration meteor trails(American Geophysical Union, 2007-12-27) Malhotra, Akshay; Mathews, John D.; Urbina, JulioNonclassical radar meteor echoes or range spread trail echoes (RSTEs), lasting from tens of seconds to over 15 min, have been a subject of considerable interest and speculation in the community ever since they were first observed in the 1940s. Using data collected from the Jicamarca Radio Observatory 50 MHz VHF radar in June 2006, we find that many properties of RSTEs can be explained largely from a radio science perspective. On a statistical basis, RSTEs of duration >15 s are observed exclusively from scatterers in the k⊥B (k = radar wave number; B = geomagnetic field) region apparently as the result of trail evolution parallel to B, forming approximately Fresnel-zone sized scattering regions thus narrowing the scattering pattern. This result implies that the initial irregularity structures in the meteor trail exhibit a wide scattering pattern that can be seen from anywhere in the radar beam given sufficient radar sensitivity but once the trail significantly elongates along B, it can be detected only from the k⊥B region of the radar. This conclusion has far-reaching implications to current interpretations of instability development in RSTEs as the location of the meteoroid trajectory relative to the narrow k⊥B region strongly determines observed RSTE properties such as onset time relative to the head echo and trail lifetime as functions of altitude. That is, a RSTE event viewed by two closely spaced identical radars would have different properties. Meteoroid size, energy, and fragmentation as well as radar properties such as frequency, beam pattern, and absolute sensitivity play obvious roles as well.Item Restricted Aspect sensitivity considerations in determining day‐to‐night variations in meteor trail durations(American Geophysical Union, 2009-11-24) Malhotra, Akshay; Mathews, John D.; Ray, KimberlyRecently there has been an interest in the day‐to‐night variations in the duration of non‐specular meteor trails or Range Spread Trail Echoes (RSTE). The reported differences between the day and night durations of meteor trails have been used to arrive at conclusions on the role played by background plasma concentration and background electric field and/or winds in the formation and evolution of RSTEs in general. However, the work done to‐date does not account for the aspect sensitivity of meteor trails. We present the duration distribution of meteor trails — the most comprehensive study yet on RSTE durations — considering the aspect sensitivity of these trails. In contrast to the previous reports, the day‐night trail durations are statistically similar in our observations. These results stress the importance of considering aspect sensitivity while performing any such study and how a failure to do so might result in misleading results and conclusions.Item Restricted Aspect sensitivity considerations in interpreting radar meteor range-spread trail echo durations(Springer, 2007-12) Malhotra, Akshay; Mathews, John D.; Urbina, JulioInitial results from June 2006 50 MHz radar campaigns at the Jicamarca Radio Observatory (JRO) reveal that full understanding of Range-Spread Trail Echoes (RSTEs) must involve knowledge of the trail viewing angle relative to kB (radar pointing direction perpendicular to the geomagnetic field) as a function of location along the meteoroid trajectory. A study of RSTEs using the JRO main array demonstrates that an overwhelming majority of long duration ([15 s) trails occur in the kB region of the radar. Short duration (5 s) trails constitute a large majority of the trails and are seen throughout the illuminated volume although with a statistically significant kB preference indicating that observed trail duration is also a function of factors such as meteoroid mass and energy. Given the apparent radar scattering aspect sensitivity of the trail plasma as it expands parallel to the geomagnetic field, we hypothesize that short duration echoes away from the kB zone might be longer duration echoes if viewed from the correct geometry. We conclude that an RSTE event viewed by two closely spaced identical radars would exhibit different properties due to the differing viewing angles. Meteoroid size, energy, and fragmentation as well as radar properties such as frequency, beam pattern, and absolute sensitivity play obvious roles as well.Item Open Access Effect of meteor ionization on sporadic-E observed at Jicamarca(American Geophysical Union, 2008-08-09) Malhotra, Akshay; Mathews, John D.; Urbina, JulioRelatively little progress has been made in the study of equatorial Sporadic-E when compared to the study of mid-latitude Sporadic-E. Indeed, it is unclear if Sporadic-E has been observed at all near the geomagnetic equator using any technique other than the ionosonde. In particular, there have been no reports of Sporadic-E observed using the Jicamarca Radio Observatory (JRO) 50 MHz radar. The overwhelming—in SNR terms—presence of the nearly ubiquitous Equatorial Electrojet (EEJ) is the likely reason Sporadic-E has not been reported at JRO as well as at other similar radars near the geomagnetic equator. We present here what we believe to be the first Sporadic-E (defined here as altitude-narrow E-region layers that last tens of minutes) observations from JRO. The structure and characteristics of these equatorial Sporadic-E layers is compared with their mid-latitude counterparts. We also demonstrate the immediate effect of meteor-produced ionization on the formation and evolution of the equatorial Sporadic-E layers.Item Restricted High-altitude radar meteors observed at Jicamarca Radio Observatory using a multibaseline interferometric technique(Royal Astronomical Society, 2015-08-11) Gao, Boyi; Mathews, John D.A new unambiguous, multibaseline interferometric technique was recently employed for meteor observations at Jicamarca Radio Observatory (JRO). These observations largely confirm high-altitude radar meteors (HARMs). The 50 MHz JRO array is arranged in contiguous quarter-arrays (Q) each of which is comprised of 4 × 4 sub-arrays (M), which are referred to as square modules in the Ochs’ manual. In these observations the radar transmission was from two quarter-arrays sharing a common diagonal. Signal reception was via three, quarter-array (Q) receivers and three adjacent (M) module receivers all of the same polarization. This arrangement offered the usual Q–Q and M–M interferometric baseline-pairs as well as new Q–M baselines that were rotated ∼6° from the Q–Q and M–M baselines. For relatively high signal-to-noise ratio meteors, this arrangement yields ambiguity resolution to the horizon and confirms the existence of HARM events. We report results from 2014 August 4 to 5 observations that include interesting new HARM events and also suggest the meteoric origin of high-altitude, altitude-extended transient events we named ‘Dragons’ in our earlier report (Gao & Mathews 2015a). We hope to extend this new technique with yet more baselines and higher sensitivity in near future observations.Item Restricted Low‐altitude meteor trail echoes(American Geophysical Union, 2009-11-12) Malhotra, Akshay; Mathews, John D.We present here the first observations of a new type of meteor echo. These Low‐Altitude Trail Echoes (LATE), observed almost exclusively below 90 km altitude using the Jicamarca Research Observatory 50 MHz radar, exhibit very different features and characteristics from the other well studied and reported meteor trails ‐ the specular meteor trails and the Range Spread Trail Echoes. Some of these features include strong altitude dependence, low aspect sensitivity and no gap between the head echo and the trail. Perhaps most importantly, almost 80% of the head echoes below 85 km altitude in our data set exhibit LATE trails. The study of these echoes provides valuable insights into the role of diffusion and plasma instabilities in the formation and evolution of meteor trails.Item Open Access Multi-static, common volume radar observations of meteors at Jicamarca(American Geophysical Union, 2007-12-28) Malhotra, Akshay; Mathews, John D.; Urbina, J. V.Multi‐static, common volume radar (MSCVR) observations have long been considered necessary for meteor observations, especially for the study of Range Spread Trail Echoes (RSTE). We present preliminary results ‐ in the form of a case study ‐ from the first MSCVR observations that were carried out at the Jicamarca Radar Observatory (JRO) in June 2007. A second antenna array, of similar sensitivity to a single JRO receive module, was constructed and operated at Carapongo, approximately 5 kilometers geomagnetically south of JRO. The JRO main array was used for transmission. Receiving was done using sub‐arrays at JRO and with the array at Carapongo. The results provide new insights not only into the aspect sensitivity of RSTEs but also into the physical structure of the plasma giving rise to these echoes. These observations also establish a firmer basis for the modeling of the plasma processes that cause meteor trails to become field‐aligned.Item Open Access Solving Long-Standing meteor mysteries(The Pennsylvania State University, 2009-08) Malhotra, Akshay; Mathews, John D.Millions of meteoroids strike and disintegrate in the Earth's upper atmosphere every day and may be seen as visible or radar meteors. Although very few of the these foreign bodies actually make it to the surface of the Earth, they have been known to cause damage to satellites and are also the only known source (except for spacecraft debris) of metallic ions in the upper atmosphere, thus being responsible for various atmospheric phenomena. Despite these meteor echoes being studied using radars for close to a century now, some long-standing unsolved mysteries still remain in the field-what causes some of the meteor trails to last for so much longer compared with the other trails? Are meteors really responsible for the formation of Sporadic-E, an altitude-thin ionospheric layer? If yes, then why has there been no evidence of a direct relationship between the two despite numerous attempts since the 1930's? Using data collected from the jicamarca Radar Observatory in Peru, we answer all of the above questions and solve these long-standing mysteries. Our results emphasize the importance of paying careful attention and due consideration to radio science issues while analyzing radar meteor echoes. It is found that the viewing geometry, i.e. the aspect sensitivity of meteor echoes, is also the primary constraint in observing long-duration meteor trails and almost all of the long duration echoes seem to originate from the K_B region (K=wave vector, B= geomagnetic field) of the radar. This result is extremely significant because it raises questions about the observed durations of all Range Spread Trail Echoes (RSTE): The same meteor event could be simultaneously observed as a long duration trail (greater than 15 seconds) from one radar and a short duration trail from another radar. These claims are supported by observations from the first-ever multi-static common volume radar observations of RSTEs. These results also provide new insights into the physical structure of the plasma giving rise to these echoes and also establish a firmer basis for the modeling of the plasma processes that cause meteor trails to become field-aligned while underlining the importance of carefully distinguishing event radio science as a prelude to specifying the role of these plasma processes.