[1] We analyze three-years of data collected by a fieldaligned airglow imaging system located at the Cerro Tololo Inter-American Observatory near La Serena, Chile to determine the occurrence of equatorial plasma bubbles (EPBs). On 317 of the 552 predominately clear nights of observations, structure indicative of EPBs is present. On 123 of these nights, multiple EPBs with periodic spacings were recorded with 88 nights showing 3 or more consecutive bubbles. We suggest that the periodic spacing of EPBs could be related to the properties of an underlying seed mechanism, namely gravity waves (GWs). The distribution of spacings compares favorably to the spectrum of GW induced traveling ionospheric disturbances (TIDs) measured by Vadas and Crowley (2010) from a similar g e o g r a p h i c l at i t u d e i n t h e no rth ern h emis ph er e. Furthermore, the distribution of spacings decreases from 2006 through 2009, tracking the corresponding decrease in the thermospheric neutral temperature, T n . As T n decreases, GWs with larger horizontal wavelengths have smaller initial amplitudes and cannot propagate as easily to EPB seeding altitudes. Thus, our observations are consistent with GW theory.
Abstract. We present occurrence rate statistics for nighttime medium-scale traveling ionospheric disturbances (MSTIDs) in the Central Pacific and South American sectors using data collected by 630.0 nm filtered CCD imaging systems. The data were collected from September 2006 through December 2012. In general, the statistics are in good agreement with the basic linear theory of MSTIDs, with observations coinciding with low F10.7A values, representative of solar minimum. Overall, MSTIDs are observed in approximately 68 % of the usable nights near the solstices at mid-latitudes and approximately 20 % of the usable nights for equinox periods. Observations closer to the geomagnetic equator yielded a maximum occurrence rate of about 10-20 % during the solstices and about 0-3 % during the equinoxes. The lower number of MSTID observations near the low latitudes is attributed to limitations of MSTID growth rate, propagation, and/or geometrical observational effects. The relatively large number of MSTID occurrences during the solstices can be accounted for by the neutral wind contribution to the MSTID growth rate either at the local or magnetic conjugate point.
Traveling ionospheric disturbances (TIDs) have been detected using various measurement techniques, including HF sounders, incoherent scatter radars, in situ measurements, and optical techniques. However, observations of TIDs have tended to be sparse and there is a need for additional observations to provide new scientific insight into the geophysical source phenomenology and wave propagation physics. The dense network of GPS receivers around the globe offers a relatively new data source to observe and monitor TIDs. In this paper, we use total electron content (TEC) measurements from ~4000 GPS receivers throughout the continental United States to observe TIDs associated with the 11 March 2011 Tohoku tsunami. The tsunami propagated across the Pacific to the U.S. west coast over several hours, and we show that corresponding TIDs were observed in the US. Using this network of GPS receivers we present a 2D imaging of TEC perturbations and calculate various TID parameters, including horizontal wavelength, speed, and period. Well‐formed, planar TIDs were detected over the west coast of the U.S. ~10 h after the earthquake. Fast Fourier transform analysis of the observed waveforms revealed that the period of the wave was 15.1 min with a horizontal wavelength of 194.8 km, phase velocity of 233.0 m/s, and an azimuth of 105.2° (propagating nearly due east in the direction of the tsunami wave). These results are consistent with the TID observations in airglow measurements from Hawaii earlier in the day and with other GPS TEC observations.
In this study, we use the three-dimensional, physics-based numerical model, SAMI3 (Sami3 is Another Model of the Ionosphere), to self-consistently generate nighttime, electrified, medium-scale traveling ionospheric disturbances (MSTIDs) at midlatitudes. These are the first numerical simulations to use the fundamental, physics-based equations in a full flux tube model for the self-consistent generation of MSTIDs. We show that a random perturbation results in the development of modes consistent with the Perkins instability and that the growth rate of a specified k perturbation agrees well with linear theory. We also present synthetic observations of MSTIDs: total electron content, integrated 630.0 nm airglow emission, E × B drift, and electron density. The modeling results show the signature of the instability in the geomagnetic conjugate hemisphere, which has been previously observed experimentally. The qualitative descriptions of the E × B drift and electron density profiles of the MSTIDs provided by SAMI3 are found to be consistent with experimental studies of MSTIDs found in the literature.
LST and is followed by eastward propagation with an average speed of $90-120 m/s prior to local midnight, rapidly decreasing around the midnight and postmidnight periods. The Ascension results are compared with similar observations from Christmas Island in order to examine the longitudinal variations of EPB development and propagation. The observed EPB velocities from Ascension Island are also compared with the results of a plasma drift model. In a case study during the night of 4-5 April, the velocity reveals unusual latitudinal shear, up to 0.12 m/s/km, with a reversal to westward flow at low latitudes while eastward flow is maintained at higher latitudes. Consequently, the bubble rotates counterclockwise and tilts eastward, significantly away from alignment with the geomagnetic field lines. The westward reversal of the drift motion near the geomagnetic equator is most likely the result of a reversal in the F region dynamo or from a large increase in the altitude of the shear node in the F region plasma drift at the geomagnetic equator.Citation: Chapagain, N. P., M. J. Taylor, J. J. Makela, and T. M. Duly (2012), Equatorial plasma bubble zonal velocity using 630.0 nm airglow observations and plasma drift modeling over Ascension Island,
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