Browsing by Author "Basu, S."
Now showing 1 - 6 of 6
Results Per Page
Sort Options
Item Restricted C/NOFS satellite observations of equatorial ionospheric plasma structures supported by multiple ground‐based diagnostics in October 2008(American Geophysical Union, 2011-10-28) Nishioka, M.; Basu, Su.; Basu, S.; Valladares, C. E.; Sheehan, R. E.; Roddy, P. A.; Groves, K. M.In early October 2008, the C/NOFS satellite orbited near the magnetic equator at its perigee altitude of ∼400 km at dusk in the Peruvian sector. This provided an ideal opportunity for a comparison, under the current very low solar flux condition, of equatorial ionospheric disturbances observed with the Communication/Navigation Outage Forecasting System (C/NOFS) in situ measurements and ground‐based observations available near Jicamarca Observatory. The primary objective was the comparison of plasma density disturbances measured by a Planar Langmuir Probe (PLP) instrument on the C/NOFS satellite with VHF scintillation activity at Ancon near Jicamarca for this period. Here we discuss in detail two extreme cases: one in which severe in situ disturbances were accompanied by mild scintillation on a particular day, namely, 10 October while there was little in situ disturbance with strong scintillation on 5 October. This apparent contradiction was diagnosed further by a latitudinal ground‐based GPS network at Peruvian longitudes, a Digisonde, and the incoherent scatter radar (ISR) at Jicamarca. The crucial distinction was provided by the behavior of the equatorial ionization anomaly (EIA). The EIA was well‐developed on the day having severe in situ disturbances (10 Oct). This led to lower equatorial plasma density and total electron content (TEC) at the equator and consequently reduced the scintillations detected at Ancon. On the other hand, on the day with severe scintillations (5 Oct), the EIA was not so well developed as on 10 October, leading to relatively higher equatorial plasma density and TEC. Consequently the severe scintillations at Ancon were likely caused by ionospheric structure located below the altitude of C/NOFS. The NRL SAMI2 model was utilized to gain a greater understanding of the role of neutral winds and electric fields in reproducing the TEC as a function of latitude for both classes of irregularities. Spectral studies with high resolution in situ PLP data were also performed. The power law spectra within the plasma bubbles showed two slopes: the low frequency slope being ∼−5/3 and the high frequency ∼−5 with a break around λ = 70 m. This particular type of two‐slope spectra may be related to the extremely low solar activity and its impact on ion composition and temperature.Item Restricted Effect of magnetic activity on the dynamics of equatorial F region irregularities(American Geophysical Union, 2002) Bhattacharyya, A.; Basu, S.; Groves, K. M.; Valladares, C. E.; Sheehan, R.Two different aspects of the effect of magnetic activity on the dynamics of equatorial spread F (ESF) irregularities are studied here using spaced receiver scintillation observations. The first one deals with the question of how magnetic activity affects the generation of ESF irregularities. For this, a parameter designated the “random velocity,” which is a measure of random changes in the irregularity drift velocity, is evaluated from the data. In past studies, this parameter has been found to have large values in the early phase of evolution of ESF irregularities during the postsunset period, with a steep decline to a low value by 22 LT. This behavior is attributed to the decline in the height of the F region. Therefore, a sudden increase in the “random velocity” in the postmidnight period is attributed to an increase in the height of the F region due to the ionospheric zonal electric field turning from westward to eastward due to the effect of magnetic activity, which may also generate fresh irregularities that produce the observed scintillations. This idea has been used to suggest that for two of the magnetically active days considered in the present study the irregularities may be freshly generated in the postmidnight period. The second aspect is the identification of geomagnetically disturbed plasma drifts, which is generally possible only after 22 LT, when the estimated irregularity drift velocities are close to that of the background plasma. The pattern of the estimated drift after 22 LT (3 UT) is found to be well defined for magnetically quiet days with scintillations during a period of a month. This allows the identification of a superimposed westward perturbation in the drift, produced by a disturbance dynamo due to magnetic activity, for all the three events studied here. On 19 February and 1 March 1999, the eastward drift velocities show an identical decrease of about 50 m/s from the undisturbed drift at 0440 UT. On 1 March, the decay phase of the storm sets in later, and the eastward velocity continues to decrease until 0530 UT, turning westward with a maximum decrease of about 80 m/s from the undisturbed drift. On 22 October 1999, which was more disturbed than these two days, the westward perturbation was larger, causing the drift velocity to turn westward around 5 UT and a decrease of nearly 150 m/s from the quiet time drift at 8 UT. The results are in broad agreement with some of the recent empirical models of the evolution, with storm time, of equatorial disturbance dynamo electric fields.Item Open Access Equatorial scintillation and systems support(American Geophysical Union, 1997-09) Groves, K. M.; Basu, S.; Weber, E. J.; Smitham, M.; Kuenzler, H.; Valladares, C. E.; Sheehan, R.; MacKenzie, E.; Secan, J. A.; Ning, P.; McNeill, W. J.; Moonan, D. W.; Kendra, M. J.The need to nowcast and forecast scintillation for the support of operational systems has been recently identified by the interagency National Space Weather Program. This issue is addressed in the present paper in the context of nighttime irregularities in the equatorial ionosphere that cause intense amplitude and phase scintillations of satellite signals in the VHF/UHF range of frequencies and impact satellite communication, Global Positioning System navigation, and radar systems. Multistation and multifrequency satellite scintillation observations have been used to show that even though equatorial scintillations vary in accordance with the solar cycle, the extreme day-to-day variability of unknown origin modulates the scintillation occurrence during all phases of the solar cycle. It is shown that although equatorial scintillation events often show correlation with magnetic activity, the major component of scintillation is observed during magnetically quiet periods. In view of the day-to-day variability of the occurrence and intensity of scintillating regions, their latitude extent, and their zonal motion, a regional specification and short-term forecast system based on real-time measurements has been developed. This system, named the Scintillation Network Decision Aid, consists of two latitudinally dispersed stations, each of which uses spaced antenna scintillation receiving systems to monitor 250-MHz transmissions from two longitudinally separated geostationary satellites. The scintillation index and zonal irregularity drift are processed on-line and are retrieved by a remote operator on the Internet. At the operator terminal the data are combined with an empirical plasma bubble model to generate three-dimensional maps of irregularity structures and two-dimensional outage maps for the region.Item Open Access Measurements of the latitudinal distributions of total electron content during equatorial spread F events(American Geophysical Union, 2001-12) Valladares, C. E.; Basu, S.; Groves, K.; Hagan, M. P.; Hysell, D.; Mazella Jr., A. J.; Sheehan, R. E.We have constructed latitudinal profiles of the total electron content (TEC) using measurements from six GPS receivers conducted during 1998. The TEC profiles have been divided into two groups: One corresponds to days when plumes or equatorial spread F (ESF) develops, and the second group portrays days of no-ESF condition. The presence/absence of ESF is based on the signature of the coherent echoes measured by the Jicamarca Unattended Long-Term Investigation (JULIA) radar and records of scintillations from two sites spaced in latitude. One scintillation station is located near the magnetic equator (Ancon) and the other 12° southward (Antofagasta). The TEC profiles display the typical day-to-day and seasonal variability seen at low latitudes. During the equinoxes, we observed quite often the crests of the anomaly located between 12° and 20° away from the magnetic equator and a trough in-between. The monthly distribution of the appearance of the anomaly and the local time of their appearance are in very good agreement with the reported variability of the upward vertical drifts and the current theory of the equatorial fountain effect. During the equinoxes and the December solstice, the TEC anomaly is observed almost every day, sometimes when there is no ESF activity. Nevertheless, fine inspection of the TEC latitudinal profiles suggests the existence of a close relationship between the temporal evolution of the TEC profiles near sunset and the onset of ESF. We have examined the TEC latitudinal distributions in two different ways. First, we calculated time difference profiles using the distributions corresponding to 1800 and 2000 LT. Second, we used a parameterization of the TEC distributions obtained at 2000 LT. The first method indicates quite drastic increases of the crest values and sharp decreases near the trough during ESF days. In contrast, during days of no ESF there exist almost uniform TEC decreases at all latitudes. The second method displays a preferred high crest/trough ratio (2), small TEC values at the trough, and large latitudinal integrated values during ESF events.Item Restricted Specification of the occurrence of equatorial ionospheric scintillations during the main phase of large magnetic storms within solar cycle 23(American Geophysical Union, 2010-10-05) Basu, S.; Basu, Su.; MacKenzie, E.; Bridgwood, C.; Valladares, C. E.; Groves, K. M.; Carrano, C.Satellite communication and navigation systems operating at low latitudes suffer outages due to ionospheric scintillations during large magnetic storms that are not currently specified by any model. This paper describes and demonstrates how in the framework of an eastward electric field penetration from high to low latitudes at dusk during the main phase of a large storm, for which the rate of change of Dst ≤ −50 nT/h and the Dst minimum index ≤ −100 nT, it is possible to specify the longitude interval within the low‐latitude ionosphere where scintillations and plasma bubbles are most likely to occur. It is known that the eastward prompt penetration electric field becomes enhanced near sunset due to the day‐to‐night conductivity gradient. Such enhanced eastward electric fields generally set off the Rayleigh‐Taylor plasma instability at F region heights and cause the formation of plasma bubbles and irregularities of electron density that give rise to scintillations of satellite signals. We first discuss two individual magnetic storms that satisfy the criterion of large magnetic storms mentioned above and for which the onsets of the main phase are about 15 h apart. We show that the dusk sectors corresponding to these two storms are such that irregularities and scintillations were observed in the Atlantic‐Peruvian longitude sector for one storm and in the Pacific sector for the other. We then present a statistical study with 30 large magnetic storms during solar cycle 23 which satisfy the two criteria of large magnetic storms and we attempt to specify the longitude interval of irregularity and scintillation occurrence during the main phase of such storms. We have tracked globally the occurrence of equatorial scintillations during magnetic storms by the use of scintillation observations made by the Air Force Research Laboratory's Scintillation Network Decision Aid (SCINDA) network and the DMSP satellite in situ measurements of plasma bubbles at 840 km. The statistical study reveals that during large magnetic storms, scintillations and plasma bubbles occur over a specific longitude sector for which the local dusk corresponds to the time interval of the main phase of storms. The magnetic storm induced scintillations may enhance the general seasonal/longitudinal pattern of quiet time scintillations at the station but may also occur where it is least expected in accordance with climatology. The storm time response of the equatorial ionosphere discussed in this paper will be implemented in the SCINDA algorithm to enhance its capability to specify scintillations during large magnetic storms.Item Restricted The multi‐instrumented studies of equatorial thermosphere aeronomy scintillation system: Climatology of zonal drifts(American Geophysical Union, 1996-12-01) Valladares, C. E.; Sheehan, R.; Basu, S.; Kuenzler, H.; Espinoza, J.A spaced-antenna scintillation system was installed at Ancon, Peru, in May 1994 to measure scintillation of 250-MHz signals from a geostationary satellite by three antennas spaced in the magnetic east-west direction. These measurements were used to establish the climatology of the zonal drift of the irregularities which cause equatorial scintillations. The major objective of this study is to compare this drift climatology to the climatology of zonal neutral wind which is the driver of the equatorial electrodynamics. A comparison of these two climatologies in conjunction with scintillation statistics may provide some clues regarding factors which help or hinder the formation of equatorial spread-F (ESF). With these objectives in mind, the first year's drift and scintillation statistics have been presented as a function of local time, season and magnetic activity and compared with the statistics of ion drift published earlier from incoherent scatter radar observations. The scintillation drift is in good agreement with the Jicamarca radar observations except for the fact that the local time dependence of our drift observations exhibit a broader maximum. The broad maximum may be attributed to lower ion drag experienced in the presence of ESF due to sustained uplifting of the ionosphere. During magnetically active periods, the scintillation drift often exhibits east to west reversals presumably because of the disturbance dynamo effects. The westward drifts during such reversals may be as large as 100 m/s. We have also modeled the zonal drifts as a seasonal basis by using Hedin's neutral wind model and Anderson's fully analytical ionospheric model. The modeled zonal drifts present good quantitative agreement with the drifts obtained with the scintillation technique.