Ionospheric irregularity zonal velocities over Cachoeira Paulista
ABSTRACT We have studied the zonal drift velocity of nighttime ionospheric irregularities from Cachoeira Paulista (22.41°S,45°W, dip latitude −17.43°), a station under the Equatorial Anomaly, from December 1998 to February 1999 using L1 band GPS receivers and OI all-sky images. The average decimetric solar flux index for this period of increasing solar activity was about 145 and magnetically quiet days with ΣKp<24 were selected. The GPS technique used receivers spaced in the magnetic east–west direction and probed small scale plasma structures (scale size about ) at altitudes near . The zonal irregularity drift velocities measured by this technique were eastward with values of about at 20 LT, about around midnight, and decreased further in the post-midnight sector. The variability of these drifts decreased significantly after midnight. The zonal velocities of large scale plasma structure were obtained using OI all-sky images from a region located about 24.1°S and 45°W at a nominal height of which corresponds to the bubble projection along the magnetic field lines to over Cachoeira Paulista. These all-sky imager derived zonal drifts are also eastward, but have magnitudes smaller than the spaced GPS eastward drifts, particularly in the pre-midnight sector. We will discuss these two drift measurement techniques and the interpretation of our results.
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ABSTRACT: In this study we have used VHF and GPS-SCINDA receivers located at Nairobi (36.8°E, 1.3°S, dip −24.1°) in Kenya, to investigate the ionospheric scintillation and zonal drift irregularities of a few hundred meter-scale irregularities associated with equatorial plasma density bubbles for the period 2011. From simultaneous observations of amplitude scintillation at VHF and L-band frequencies, it is evident that the scintillation activity is higher during the post sunset hours of the equinoctial months than at the solstice. While it is noted that there is practically no signatures of the L-band scintillation in solstice months (June, July, December, January) and after midnight, VHF scintillation does occur in the solstice months and show post midnight activity through all the seasons. VHF scintillation is characterized by long duration of activity and slow fading that lasts till early morning hours (05:00 LT). Equinoctial asymmetry in scintillation occurs with higher occurrence in March–April than in September–October. The occurrence of post midnight VHF scintillation in this region is unusual and suggests some mechanisms for the formation of scintillation structure that might not be clearly understood. Zonal drift velocities of irregularities were measured using cross-correlation analysis with time series of the VHF scintillation structure from two closely spaced antennas. Statistical analyses of the distribution of zonal drift velocities after sunset hours indicate that the range of the velocities is 30–160 m/s. This is the first analysis of the zonal plasma drift velocity over this region. Based on these results we suggest that the east–west component of the plasma drift velocity may be related to the evolution of plasma bubble irregularities caused by the prereversal enhancement of the eastward electric fields. The equinoctial asymmetry of the drift velocities and scintillation could be attributed to the asymmetry of neutral winds in the thermosphere that drives the eastward electric fields.Advances in Space Research 05/2013; 51(9):1715–1726. · 1.18 Impact Factor
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ABSTRACT: We present OI (630.0 nm) airglow image data from Ascension Island (geographic: 7.9°S, 14.4°W dip latitude: 16°S) in the southern Atlantic Ocean taken with the Utah State University all-sky CCD camera during 20 March to 7 April 1997 in order to study plasma bubbles occurring in the low-latitude nighttime ionosphere. The initial plasma bubble onset occurs in the early evening hours at ˜19:15-20:00 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.Journal of Geophysical Research Atmospheres 06/2012; 117(A6):6316-. · 3.44 Impact Factor
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ABSTRACT: Based on the data obtained from a network of GPS L1 band receivers deployed in Brazil, are presented here the characteristics of the 400 m ionospheric irregularities during magnetically quiet and disturbed conditions. The network is composed of 12 GPS scintillation monitors and covers the latitudinal region from the magnetic equator up to the southern crest of the Equatorial Ionization Anomaly (EIA), which is characterized by large horizontal gradients in the electron density distribution. Some results on equatorial spread-F statistics obtained from digisonde data over Cachoeira Paulista (22.41o S, 45o W, dip latitude 14.89o S) and from ionosonde data over Tucumán (64.5o W, 27o S, dip latitude 13.71o S) are also used in this work to complement the results from GPS network. The effects of local time, season, latitude, longitude, background ionization, solar cycle and magnetic activity on the ionospheric irregularities are presented. The ionospheric irregularity zonal velocities determined by magnetically east-west spaced GPS receivers are also presented. The influence of the ionospheric irregularities on GPS based navigational systems is discussed. These observations, complemented by computational simulations, may improve our understanding of the factors responsible for the generation, growth and dynamics of the equatorial F-region plasma irregularities.Indian Journal of Radio & Space Physics. 08/2007; 36(4):268-277.