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Venkateswara Rao N.,Kyoto University | Tsuda T.,Kyoto University | Riggin D.M.,Colorado Research Associates | Gurubaran S.,Indian Institute of Geomagnetism | And 2 more authors.
Journal of Geophysical Research: Space Physics | Year: 2012

Long-term variations of monthly mean zonal and meridional winds in the Mesosphere and Lower Thermosphere (MLT) at low-latitudes are analyzed using four medium frequency (MF) radars and three meteor radars located in the Asia-Oceania region. Radar data taken at close-by latitudes are appended to construct long-term data sets. With this, we have long-term data from five distinct latitudes within 22 (viz., 22N, ∼9N, 0-2N, 6-7S and 21S). The data length varies at different latitudes and spans a maximum of two decades during 1990-2010. The zonal winds show semiannual oscillation (SAO) at all locations with westward (eastward) winds during equinoxes (solstices). The month height pattern of SAO is similar within 9 and is different at 22. The westward winds in the March equinox were enhanced every two or three years during 1990-2002. We define this phenomenon as Mesospheric Quasi-Biennial Enhancement (MQBE). Such signature is not clear after 2002. The meridional winds show annual oscillation (AO), with northward and southward winds during the December and June solstices, respectively. However, the timing at which the wind direction changes does not coincide at all latitudes. The amplitude of the AO is enhanced after 2004 and 2008 at ∼9N and ∼7S, respectively. Orthogonal components of SAO and AO are detected with persistent phase relation, which suggests that the zonal and meridional winds are coupled. The meridional winds show long-term trends at latitudes of ∼9N and ∼6-7S, but not at other latitudes . The zonal winds do not show significant long-term trends. © 2012. American Geophysical Union. All Rights Reserved.

Stockwell R.G.,Colorado Research Associates | Taylor M.J.,Utah State University | Nielsen K.,Utah State University | Jarvis M.J.,British Antarctic Survey
Journal of Geophysical Research: Atmospheres | Year: 2011

All-sky CCD observations of mesospheric gravity waves have been made from Halley Station Antarctica (75.5°S, 26.7°W) as part of a collaborative research program between British Antarctic Survey, U.K. and Utah State University, USA. A mesospheric bore event was observed in the nightglow emissions over a period of several hours on the 27th of May, 2001. Two dimensional S-Transform (ST) analysis is applied to the airglow images of this bore event. This local spectral technique allows one to calculate the wave parameters as a function of time and space. It is observed that the horizontal phase speed and wavelength decrease over time as the amplitude attenuates. Simultaneously with this wave event the background wind experiences a large acceleration in the direction of the wave propagation. Mesospheric bore theory calculations are used to estimate the bore duct depth and it is shown that as the wave packet evolves, the bore duct collapses (decreasing in its vertical extent). As the bore duct shrinks, the wave's group velocity decelerates, the amplitude attenuates, and the wave dissipates. © 2011 by the American Geophysical Union.

Paulino I.,National Institute for Space Research | Takahashi H.,National Institute for Space Research | Vadas S.L.,Colorado Research Associates | Wrasse C.M.,Vale Solucoes em Energia | And 4 more authors.
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2012

Medium-scale gravity waves (MSGWs) observed during the Conjugate Point Experiment (COPEX) at Boa Vista (2.8°N; 60.7°S, dip angle 21.7°) have been ray-traced and studied based on zero wind and model wind conditions. Wind profiles have been used from the TIE-GCM and HWM-07 models. Temperature profiles were used from the NRLMSISE-00 and TIE-GCM models, and TIMED/SABER satellite data. Doppler up-shifted MSGWs, at ~ 87 km of altitude, propagated to higher altitudes into the thermo-sphere-ionosphere domain than waves that were un-shifted. Most MSGWs propagated upwards up to ~ 140 km of altitude and were seen to be unlikely candidates to trigger equatorial plasma bubbles (EPBs) at the F layer bottom side. However, three of them propagated up to heights close to the F layer bottom side, where it could act in the EPB seeding directly. Moreover, three MSGWs, which propagated equatorward, could act on EPB seeding by field-line-integrated effects. © 2012 Elsevier Ltd.

Takahashi H.,National Institute for Space Research | Abdu M.A.,National Institute for Space Research | Taylor M.J.,Utah State University | Pautet P.-D.,Utah State University | And 11 more authors.
Geophysical Research Letters | Year: 2010

Bottom-type spread F events were observed in the south American equatorial region by a VHF coherent radar and an ionosonde at São Luís (2.5°S, 44.3°W), an ionosonde at Fortaleza (3.9°S, 38.4°W) and an airglow OI 630.0 nm imager at Cariri (7.4°S, 36.5°W) and Brasilia (14.8°S, 47.6°W). In the evening of September 30, 2005, a long duration (∼70 minutes) bottom side scattering layer, confined in a narrow height region, was observed. At the same time all-sky imager observed sinusoidal intensity depletions in the zonal plane extending more than 1500 km and elongated along the magnetic meridian. No strong spread F structures developed during the period. Subsequently well developed plasma bubbles were observed. This suggests that the observed bottom-type spread F is an initial phase of the plasma bubbles. We report, for the first time, longitudinal and latitudinal extension of the bottom-type spread F as diagnosed by optical imagers. Copyright © 2010 by the American Geophysical Union.

Martinez-Oliveros J.C.,University of California at Berkeley | Lindsey C.,Colorado Research Associates | Bale S.D.,University of California at Berkeley | Krucker S.,University of California at Berkeley | Krucker S.,University of Applied Sciences and Arts Northwestern Switzerland
Solar Physics | Year: 2012

Low-frequency solar and interplanetary radio bursts are generated at frequencies below the ionospheric plasma cutoff and must therefore be measured in space, with deployable antenna systems. The problem of measuring both the general direction and polarization of an electromagnetic source is commonly solved by iterative fitting methods such as linear regression that deal simultaneously with both directional and polarization parameters. We have developed a scheme that separates the problem of deriving the source direction from that of determining the polarization, avoiding iteration in a multi-dimensional manifold. The crux of the method is to first determine the source direction independently of concerns as to its polarization. Once the source direction is known, its direct characterization in terms of Stokes vectors, in a single iteration if desired, is relatively simple. This study applies the source-direction determination to radio signatures of flares received by STEREO. We studied two previously analyzed radio type III bursts and found that the results of the eigenvalue decomposition technique are consistent with those obtained previously by Reiner et al. (Solar Phys. 259, 255, 2009). For the type III burst observed on 7 December 2007, the difference in travel times from the derived source location to STEREO A and B is the same as the difference in the onset times of the burst profiles measured by the two spacecraft. This is consistent with emission originating from a single, relatively compact source. For the second event of 29 January 2008, the relative timing does not agree, suggesting emission from two sources separated by 0. 1 AU, or perhaps from an elongated region encompassing the apparent source locations. © 2012 Springer Science+Business Media B.V.

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