Browsing by Author "Su, S. Y."
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Item Restricted Concurrent study of bottomside spread F and plasma bubble events in the equatorial ionosphere during solar maximum using digisonde and ROCSAT-1(European Geosciences Union, 2005-12-21) Lee, C. C.; Su, S. Y.; Reinisch, B. W.Data from the Jicamarca digisonde and the ROCSAT-1 satellite are employed to study the equatorial ionosphere on the west side of South America during April 1999-March 2000 for the concurrent bottomside spread F (BSSF) and plasma bubble events. This study, using digisonde and ROCSAT-1 concurrently, is the first attempt to investigate the equatorial spread F. Results show that BSSF and plasma bubble observations appear frequently respectively in the summer (January, February, November, and December) and in the equinoctial (March, April, September, and October) months, respectively, but are both rarely observed in the winter (May-August) months. The upward drift velocity during the concurrent BSSF and bubble observations has been determined to study the driving mechanism. This analysis shows that large vertical drift velocities favor BSSF and bubble formations in the equinoctial and summer months. Conversely, the smaller upward velocities during the winter months cause fewer BSSF and bubble occurrences. For the geomagnetic effect, the BSSF/bubble occurrence decreases with an increasing Kp value in the equinoctial months, but no such correlation is found for the summer and winter months. Moreover, the anti-correlations between Kp and dh'F/dt are apparent in the equinoctial months, but not in the summer and winter months. These results indicate that in the equinoctial months the BSSF/bubble generations and the pre-reversal drift velocity can be suppressed by geomagnetic activity, because the disturbance dynamo effects could have decreased the eastward electric field near sunset. However, BSSF and bubble occurrences may not be suppressed by the geomagnetic activity in the summer and winter months.Item Restricted Quiet time equatorial F region vertical plasma drift model derived from ROCSAT‐1 observations(American Geophysical Union, 2008-05-03) Fejer, B. G.; Jensen, J. W.; Su, S. Y.We have used five years of measurements on board the ROCSAT‐1 satellite to develop a detailed quiet time global empirical model for equatorial F region vertical plasma drifts. This model describes the local time, seasonal and longitudinal dependence of the vertical drifts for an altitude of 600 km under moderate and high solar flux conditions. The model results are in excellent agreement with measurements from the Jicamarca radar and also from other ground‐based and in situ probes. We show that the longitudinal dependence of the daytime and nighttime vertical drifts is much stronger than reported earlier, especially during December and June solstice. The late night downward drift velocities are larger in the eastern than in the western hemisphere at all seasons, the morning and afternoon December solstice drifts have significantly different longitudinal dependence, and the daytime upward drifts have strong wave number‐four signatures during equinox and June solstice. The largest evening upward drifts occur during equinox and December solstice near the American sector. The longitudinal variations of the evening prereversal velocity peaks during December and June solstice are anti‐correlated, which further indicates the importance of conductivity effects on the electrodynamics of the equatorial ionosphere.Item Restricted Quiet-time variations of F2-layer parameters at Jicamarca and comparison with IRI-2001 during solar minimum(Elsevier, 2008) Lee, C. C.; Reinisch, B. W.; Su, S. Y.; Chen, W. S.We analyze Jicamarca ionograms to study the quiet-condition variations in the peak electron density (NmF2), its height (hmF2), and F2-layer thickness parameter (B0) of the equatorial F2 layer during solar minimum. The sunrise peak is found in hmF2 and B0 for all months. During daytime and nighttime, the variation in the hmF2 value is mainly responsible for that in NmF2 and B0. The sunset peaks of hmF2 and B0 exist in the equinoctial months, but not in the winter months. Moreover, the observed values of hmF2, NmF2, and B0 are generally similar to the modeled values of IRI-2001.Item Restricted Topside ionospheric effective scale heights (HT) derived with ROCSAT‐1 and ground‐based ionosonde observations at equatorial and midlatitude stations(American Geophysical Union, 2009-10-16) Tulasi Ram, S.; Su, S. Y.; Liu, C. H.; Reinisch, B. W.; McKinnell, Lee AnneIn this study we propose the assimilation of topside in situ electron density data from the Republic of China Satellite (ROCSAT‐1) along with the ionosonde measurements for accurate determination of topside ionospheric effective scale heights (HT) using an α‐Chapman function. The reconstructed topside electron density profiles using these scale heights exhibit an excellent similitude with Jicamarca incoherent scatter radar (ISR) profiles and are much better representations than the existing methods of Reinisch‐Huang method and/or the empirical International Reference Ionosphere–2007 model. The main advantage with this method is that it allows the precise determination of the effective scale height (HT) and the topside electron density profiles at a dense network of ionosonde/Digisonde stations where no ISR facilities are available. The demonstration of the method is applied by investigating the diurnal, seasonal, and solar activity variations of HT over the dip‐equatorial station Jicamarca and the midlatitude station Grahamstown. The diurnal variation of scale heights over Jicamarca consistently exhibits a morning time descent followed by a minimum around 0700–0800 LT and a pronounced maximum at noon during all the seasons of both high and moderate solar activity periods. Further, the scale heights exhibit a secondary maximum during the postsunset hours of equinoctial and summer months, whereas the postsunset peak is absent during the winter months. These typical features are further investigated using the topside ion properties obtained by ROCSAT‐1 as well as Sami2 is Another Model of the Ionosphere (SAMI2) model simulations. The results consistently indicate that the diurnal variation of the effective scale height (HT) does not closely follow the plasma temperature variation and at equatorial latitudes is largely controlled by the vertical E × B drift.