Browsing by Author "Reinisch, B. W."
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Item Open Access Abnormal evening vertical plasma drift and effects on ESF and EIA over Brazil-South Atlantic sector during the 30 October 2003 superstorm(American Geophysical Union, 2008-11) Abdu, M. A.; De Paula, E. R.; Batista, I. S.; Reinisch, B. W.; Matsuoka, M. T.; Camargo, P. O.; Veliz, Oscar; Denardini, C. M.; Sobral, J. H. A.; Kherani, E. A.; De Siqueira, P. M.Equatorial F region vertical plasma drifts, spread F and anomaly responses, in the south American longitude sector during the superstorm of 30 October 2003, are analyzed using data from an array of instruments consisting of Digisondes, a VHF radar, GPS TEC and scintillation receivers in Brazil, and a Digisonde and a magnetometer in Jicamarca, Peru. Prompt penetrating eastward electric field of abnormally large intensity drove the F layer plasma up at a velocity 1200 ms−1 during post dusk hours in the eastern sector over Brazil. The equatorial anomaly was intensified and expanded poleward while the development of spread F/plasma bubble irregularities and GPS signal scintillations were weaker than their quiet time intensity. Significantly weaker F region response over Jicamarca presented a striking difference in the intensity of prompt penetration electric field between Peru and eastern longitudes of Brazil. The enhanced post dusk sector vertical drift over Brazil is attributed to electro-dynamics effects arising energetic particle precipitation in the South Atlantic Magnetic Anomaly (SAMA). These extraordinary results and their longitudinal differences are presented and discussed in this paper.Item Open Access Assimilation of sparse continuous near-earth weather measurements by NECTAR model morphing(American Geophysical Union, 2020-11) Galkin, I. A.; Reinisch, B. W.; Vesnin, A. M.; Bilitza, D.; Fridman, S.; Habarulema, J. B.; Veliz, OscarNon-linear Error Compensation Technique with Associative Restoration (NECTAR) is a novel approach to the assimilation of fragmentary sensor data to produce a global nowcast of the near-Earth space weather. NECTAR restores missing information by iteratively transforming (“morphing”) an underlying global climatology model into agreement with currently available sensor data. The morphing procedure benefits from analysis of the inherent multiscale diurnal periodicity of the geosystems by processing 24-hr time histories of the differences between measured and climate-expected values at each sensor site. The 24-hr deviation time series are used to compute and then globally interpolate the diurnal deviation harmonics. NECTAR therefore views the geosystem in terms of its periodic planetary-scale basis to associate observed fragments of the activity with the grand-scale weather processes of the matching variability scales. Such approach strengthens the restorative capability of the assimilation, specifically when only a limited number of observatories is available for the weather nowcast. Scenarios where the NECTAR concept works best are common in planetary-scale near-Earth weather applications, especially where sensor instrumentation is complex, expensive, and therefore scarce. To conduct the assimilation process, NECTAR employs a Hopfield feedback recurrent neural network commonly used in the associative memory architectures. Associative memories mimic human capability to restore full information from its initial fragments. When applied to the sparse spatial data, such a neural network becomes a nonlinear multiscale interpolator of missing information. Early tests of the NECTAR morphing reveal its enhanced capability to predict system dynamics over no-data regions (spatial interpolation).Item Restricted Comparative analysis of nocturnal vertical plasma drift velocities inferred from ground-based ionosonde measurements of hmF2 and h′F(Elsevier, 2014-11-22) Adebesin, B. O.; Adeniyi, J. O.; Adimula, I. A.; Oladipo, O. A.; Olawepo, A. O.; Reinisch, B. W.Variations in the evening/nighttime ionosonde vertical plasma drift velocities inferred from the time rate of change of both the base of the F-layer height (Vz(h′F)) and height of the peak electron density (Vz(hmF2)) from an equatorial station were compared for better description of the E×B drifts. For better interpretation, both results were compared with the Incoherent Scatter (IS) radar observations (Vz(ISR)) which is taken to be the most accurate method of measuring drift, and therefore the data of reference level. An equinoctial maximum and June solstice minimum in post-sunset pre-reversal enhancement (PRE) was observed for Vz(hmF2), Vz(ISR), and Vz(h′F). The percentage correlation between VzhmF2 and Vzh′F ranges within 55–70%. While PRE for Vz(hmF2) peaked at 19 LT for all seasons, Vz(h′F) peaked at 18 LT for September equinox and December solstice, and start earlier. The nighttime downward reversal peak magnitudes for Vz(hmF2) and Vz(h′F) are respectively within the range of −4 to −14 and −2 to −14 m/s; whereas Vz(ISR) ranges within −12 and −34 m/s; and the peak time was reached earlier with the ionosonde observations than for the ISR. The PRE peak magnitude for Vz(hmF2), Vz(h′F) and Vz(ISR) varies between 3–14, 2–14, and 4–14 m/s for the entire seasons. Our results revealed higher drift correlation coefficients in both Vz(hmF2) vs. Vz(ISR) (0.983) and Vz(h′F) vs. Vz(ISR) (0.833) relationships during the equinoxes between 16–20 LT, at which time the F-layer altitude is higher than the 300 km threshold value; and lower for solstice period (0.326 and 0.410 in similar order). A better linear relationship between Vz(hmF2) and Vz(h′F2) was observed during the reversal (19–21 LT) phase period. PRE velocity was shown to be seasonal and solar activity dependent. Both VzhmF2 and Vzh′F compares almost equally with the ISR measurement. However, the PRE peak magnitude for the drift inferred using h′F2 is closer to the corresponding ISR magnitude during the equinoxes; whereas the drift inferred from hmF2 best represent the ISR magnitude for solstices. We established that both VzhmF2 and Vzh′F are governed by the same mechanism at nighttime, and as such any of them can be used to infer vertical drift as long as the 300 km threshold value condition is considered, otherwise chemical correction may be required for the F-layer uplift.Item Restricted Comparison between bottomside ionospheric profile parameters retrieved from FORMOSAT3 measurements and ground-based observations collected at Jicamarca(Elsevier, 2011-03-11) Chuo, Y J.; Lee, C. C.; Chen, W. S.; Reinisch, B. W.This study presents the results of a comparison of three ionospheric profile parameters, B0, NmF2, and hmF2, derived from measured ionograms and the FORMOSAT3 radio occultation measurements collected over Jicamarca during the low-solar-activity period from May 2006 to April 2008. The results show that the B0 values are generally in good agreement with those derived from the true electron density profiles. In addition, correlation analysis revealed seasonal and diurnal variation in B0, which is more pronounced during an equinox and daytime (0800–2000), respectively. A comprehensive discussion on the difference between the values of B0, NmF2, and hmF2 derived from two sources is provided in this paper.Item Restricted Comparison of CHAMP and Digisonde plasma frequencies at Jicamarca, Peru(American Geophysical Union, 2007-03-13) McNamara, L. F.; Cooke, D. L.; Valladares, C. E.; Reinisch, B. W.Ionospheric plasma frequencies at the altitude of the CHAMP satellite have been deduced from ionosonde true‐height profiles for Jicamarca, Peru, and have been compared with the in situ measurements made by CHAMP. The differences between the plasma frequencies have been found to be well within the uncertainties associated with the ionosonde profiles, confirming the validity of the CHAMP measurements. For satellite‐ionosonde separations of less than 250 km and for satellite altitudes below the peak of the F2 layer, the average discrepancy between the two plasma frequencies is 0.25 MHz or 4%. For the most reliable ionosonde measurements, the average discrepancies reduce to 0.18 MHz (or 1.7%), with a standard deviation of 0.16 MHz (or 1.5%). Given the validity of the CHAMP plasma frequencies, corresponding ionosonde and CHAMP observations have been used to support the practice of extending the ionosonde profile above hmF2 by assuming a Chapman layer with a constant scale height equal to that of the lower side of the F2 layer peak. The average discrepancy for CHAMP passing above the peak of the F2 layer is 0.22 MHz (or 2.6%), and the standard deviation is 0.8 MHz (or 13.3%).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 Digisonde spread F and GPS phase fluctuations in the equatorial ionosphere during solar maximum(American Geophysical Union, 2006-12-06) Chen, W. S.; Lee, C. C.; Liu, J. Y.; Chu, F. D.; Reinisch, B. W.The Jicamarca (11.95°S, 76.87°W) digisonde and the Arequipa (16.47°S, 71.49°W) GPS receiver observed the equatorial F region irregularities on the western South America from April 1999 to March 2000. The spread F measured by the digisonde were classified into four types, and the GPS phase fluctuations derived from the temporal variation of total electron content were divided into three levels to represent the irregularity strength. The observation shows that the occurrences of all four types of spread F are higher in the D months (January, February, November, and December) than in the E months (March, April, September, and October). For the GPS phase fluctuations, both seasonal and nighttime variations show that the occurrences of strong level irregularities are higher than moderate level irregularities in the E months, but the situation is reversed in the D months. Moreover, the occurrence sequences of four types of spread F and three levels of GPS phase fluctuations all can be explained by the E × B drift variations and the generalized Rayleigh‐Taylor instability. For the comparisons between the GPS phase fluctuations and the digisonde spread F/plasma bubbles, results show that the GPS phase fluctuations can represent the appearances of the digisonde spread F, and the strong level of GPS phase fluctuations are associated with the occurrence of topside plasma bubbles. These results imply that the greater GPS phase fluctuation is related to the larger altitudinal range distribution of irregularities.Item Restricted Electron density profiles in the equatorial ionosphere observed by the FORMOSAT-3/COSMIC and a digisonde at Jicamarca(Springer, 2009-11-26) Liu, J. Y.; Lee, C. C.; Yang, J. Y.; Chen, C. Y.; Reinisch, B. W.We examine for the first time the ionospheric electron density profiles concurrently observed by the GPS occultation experiment (GOX) onboard the FORMOSAT-3/COSMIC (F3/C) and the ground-based digisonde portable sounder DPS-4 at Jicamarca (12°S, 283°W, 1°N geomagnetic) in 2007. Our results show that the F3/C generally underestimates the F2-peak electron density NmF2 and the F2-peak height hmF2. On the other hand, when the equatorial ionization anomaly (EIA) pronouncedly appears during daytime, the total electron content (TEC) derived from the radio occultation of the GPS signal recorded by the F3/C GOX is significantly enhanced. This results in the NmF2 at Jicamarca being overestimated by the Abel inversion on the enhanced TEC during the afternoon period.Item Restricted Equatorial F region evening vertical drift, and peak height, during southern winter months: A comparison of observational data with the IRI descriptions(Elsevier, 2006-05-03) Abdu, M. A.; Batista, I. S.; Reinisch, B. W.; Sobral, J. H. A.; Carrasco, A. J.The equatorial F region evening vertical drift, due to pre-reversal electric field enhancement, is an important condition for the spread F/plasma bubble irregularity generation, that is more frequent during summer-equinoctial months over South America. A comparative study of these vertical drifts with their IRI representations was presented at the Grahamstown IRI 2003 workshop. During southern winter months the post-sunset ESF development is relatively infrequent over South America due to the generally weaker intensity of the sunset zonal electric field, which, however, is critical in determining the equatorial spread F (ESF) development under magnetospherically disturbed conditions. Therefore a detailed understanding of the characteristics of the evening F layer vertical drift, hmF2 and foF2 during southern winter months is important for developing/improving their representations in the IRI scheme. In this paper we have undertaken a study of these parameters over the Brazilian equatorial sites, Sao Luis (2.33S, 44.2W, dip angle: −0.5°, declination angle: 21W°) and the low latitude site, Cachoeira Paulista (22.6°S, 315°E; dip angle: −32°) in comparison with their existing representations in the IRI. The study is made as a function of the solar flux varying from the solar activity minimum to maximum conditions. Some of the results in the Brazilian longitude sector are compared with results from Jicamarca (12°S, 76.9°W; dip latitude: 1°N, declination angle: ∼3°E) in Peru, separated by a large difference in magnetic declination angle. The magnetic equatorial and the low latitude stations analyzed here are all located in the southern geographic hemisphere. Systematic patterns of difference between the observed characteristics of these parameters and their IRI representations are identified for eventual corrections to their existing representations in the IRI model. The study has yielded further important clues towards a better understanding of the possible mechanism for the infrequent ESF occurrence in winter over South America, and especially over Brazil.Item Restricted Equatorial F-layer heights, evening prereversal electric field, and night E-layer density in the American sector: IRI validation with observations(Elsevier, 2004) Abdu, M. A.; Batista, I. S.; Reinisch, B. W.; Carrasco, A. J.The equatorial F-layer height variations resulting from the variabilities in the zonal electric fields and winds and associated variability in ionospheric dynamo strength are important factors in determining the distribution and structuring of the electron density of the equatorial ionization anomaly (EIA) region. Especially, the evening enhancement in the F-layer heights and the associated prereversal enhancement in the zonal electric field due to the F-layer dynamo are believed to provide the most basic precondition for the equatorial spread F plasma bubble irregularity (ESF) generation. A realistic description by the International Reference Ionosphere (IRI) of the quiet time equatorial F-layer heights is therefore of fundamental importance for applications related to the studies of the ESF and EIA variabilities. The existing IRI description scheme (that uses the CCIR coefficients) appears to represent the equatorial F-layer peak density (N m F 2 ƒ o F2) better than the peak height (h m F2) and the heights of specific densities, the largest disagreement with observations being verified during the evening hours. Digisonde data from the three permanent stations in Brazil: Sao Luis (2.33S, 44.2W, dip angle: − . 5); Fortaleza (3.9S, 38.45W, dip angle: − 9); and Cachoeira Paulista (22.6S, 315E; dip angle: − 28) and from Jicamarca (12S, 76.9W; dip latitude: 1N) in Peru have been analysed, to determine the quiet time mean behavior of the key F-layer parameters as a function of local time, season, and solar activity. These are complemented by data from the three conjugate point stations: Boa Vista (02.8N; 60.66W, dip angle: 22.5) in the north and Campo Grande (20.45S; 54.65W, dip angle: − 22.5) in the south, and an equatorial station, Cachimbo (9.47S; 54.83W, dip angle: − 3.9) that were operated during the 2002 COPEX (Conjugate Point Experiment) campaign conducted in Brazil. The data for São Luis and Jicamarca are used to evaluate the longitudinal differences in the prereversal F-layer vertical drift, arising from the large magnetic declination angle difference that characterize these Brazilian and Peruvian longitude sectors. An attempt is made to characterize and quantify any systematic difference that exists between the mean behavioral patterns of the critical parameters as described by the IRI and those observed, with an objective to improve the IRI prediction capability.Item Restricted Formation of an F3 layer in the equatorial ionosphere: A result from strong IMF changes(Elsevier, 2007-07) Paznukhov, V. V.; Reinisch, B. W.; Song, P.; Huang, X.; Bullet, T. W.; Veliz, OscarWe analyzed ionospheric observations made with digisondes in Jicamarca, Ramey, Wallops Island, Ascension Island, and Kwajalein Island during the major magnetic storm of November 9–10, 2004, which was associated with rapid interplanetary magnetic field (IMF) Bz changes. The strongest ionospheric responses to the southward IMF Bz turning were observed at the dip equator at Jicamarca where during the magnetic disturbance a dramatic F2 peak density depletion occurred at around 15:00 local time, accompanied by a fast upward motion of the plasma. In this process, an additional ionospheric layer, the F3 layer, formed with peak densities NmF3 exceeding NmF2. This observation may be considered evidence of an equatorial plasma fountain enhancement caused by the magnetic field disturbance. Responses were observed in a large range of latitudes and local times. The best indicator of the responses appears to be the peak height of the F layer, since competing processes determine the peak densities. The observed responses at low latitude locations in the morning and dusk sectors pose challenges to the simple penetrating electric field model because the upward motion is inconsistent with the E×B drift associated with a dawn–dusk electric field. Clear responses in the Jicamarca local time sector occurred at latitudes as high as 28, at Ramey, Puerto Rico. This latitude range appears to be beyond the range of the flux tube corresponding to the 900 km F3 layer peak height at Jicamarca, indicating a more extended uplifting of flux tubes.Item Restricted Improvement of retrieved FORMOSAT‐3/COSMIC electron densities validated by ionospheric sounder measurements at Jicamarca(American Geophysical Union, 2011-09-01) Aragon‐Angel, A.; Liou, Y. A.; Lee, C. C.; Reinisch, B. W.; Hernández‐Pajares, M.; Juan, M.; Sanz, J.Inversion techniques applied to GPS‐LEO radio occultation data allow the retrieval of accurate and worldwide‐distributed refractivity profiles, which, in the case of the ionosphere, can be converted into electron densities providing information regarding the electron content distribution in this atmospheric region. In order to guarantee the accuracy of the electron density retrievals, two key points should be taken into account: the horizontal gradients of the electronic distribution and the topside electron content above the LEO orbit. The deployment in April 2006 of the satellite Constellation Observing System for Meteorology Ionosphere and Climate (FORMOSAT‐3/COSMIC), carrying GPS receivers on board, provides valuable radio occultation data with global and almost uniform coverage overcoming the sparsity of data from previous LEO missions (for instance, GPS/MET, CHAMP, and SAC‐C). This is also one of the main limitations of other sources providing direct observations, such as ionosondes. In this study, the improved Abel transform inversion is used to analyze derived ionospheric electron density profiles of the whole year 2007 in a scenario with very high electron density gradients: The neighboring area of Jicamarca (76.9°W, 12°S, dip latitude: 1°N), Perú, located at very low latitude and close to the geomagnetic equator, and the influence of the Appleton‐Hartree equatorial anomaly (Davies, 1990). Moreover, different strategies to account for the topside electron content in the occultation data inversion are compared and discussed, taking advantage of the availability of FORMOSAT‐3/COSMIC data sets and manually calibrated measurements from Jicamarca DPS. Statistical results show that for the current scenario the improvements are only about 10%, evidencing that the lack of colocation is one important source of error for the classical Abel inversion. Implications with respect to the plasmaspheric contribution have been derived from this data set analysis, in particular, the necessity to account for it specially when the Total Electron Content (TEC) is small.Item Restricted Modeling the low-latitude thermosphere and ionosphere(Elsevier, 2002-08) Fesen, C. G.; Hysell, D. L.; Meriwether, J. M.; Mendillo, M.; Fejer, B. G.; Roble, R. G.; Reinisch, B. W.; Biondi, M. A.The National Center for Atmospheric Research thermosphere/ionosphere/electrodynamic general circulation model (TIEGCM) is one of the few models that self-consistently solves the coupled equations for the neutral atmosphere and ionosphere. Timely questions are how well the TIEGCM currently simulates the low-latitude ionosphere and what modifications might bring about better predictions. Comparisons between data obtained in and around Jicamarca, Peru, near the magnetic equator, and simulations with the TIEGCM indicate good progress has been made but reveal some serious discrepancies. Good-to-excellent agreement is obtained for electron densities, electron and ion temperatures, and nmax. The agreement is fair to poor for hmax, zonal drifts, the oxygen nightglow, and the horizontal neutral winds. The most important discrepancy is in the simulated neutral temperature, which is at least too cold relative to Fabry–Perot interferometer observations. Increasing the EUV fluxes in the model to improve prediction of the model temperature also improves representation of airglow observations and of the ionosphere, for which the model typically underrepresents the electron densities. The disparity in neutral temperature is also present in comparisons with the empirical model MSIS which represents the largest database of thermospheric temperature measurements. Since the neutral and ionized atmospheres are tightly coupled at low latitudes, simultaneous measurements of neutral and ion parameters, preferably over an extended time period, would be invaluable to further the understanding of the region. Better knowledge of the EUV fluxes and the high altitude O+ fluxes may also help resolve some of the model/data discrepancies.Item Open Access Multistation digisonde observations of equatorial spread F in South America(European Geosciences Union (EGU), 2004-09) Reinisch, B. W.; Abdu, M.; Batista, I.; Sales, G. S.; Khmyrov, G.; Bullett, T. A.; Chau Chong Shing, Jorge Luis; Rios, V.Directional ionogram and F-region drift observations were conducted at seven digisonde stations in South America during the COPEX campaign from October to December 2002. Five stations in Brazil, one in Argentina, and one in Peru, monitored the ionosphere across the continent to study the onset and development of F-region density depletions that cause equatorial spread F (ESF). New ionosonde techniques quantitatively describe the prereversal uplifting of the F layer at the magnetic equator and the eastward motion of the depletions over the stations. Three of the Brazilian stations were located along a field line with a 350-km apex over the equator to investigate the relation of the occurrence of ESF and the presence of sporadic E-layers at the two E-region intersections of the field line. No simple correlation was found.Item Restricted Quiet-condition hmF2, NmF2, and B0 variations at Jicamarca and comparison with IRI-2001during solar maximum(Elsevier, 2006-12) Lee, C. C.; Reinisch, B. W.We use the measurements of the Jicamarca digisonde to examine the variations in F2 layer peak electron density (NmF2), its height (hmF2), and the F2 layer thickness parameter (B0) near the dip equator. The hourly ionograms during geomagnetic quiet-conditions for a 12-month period close to the maximum solar activity, April 1999–March 2000, are used to calculate the monthly averages of these parameters, for each month. The averages are compared with the International Reference Ionosphere (IRI)-2001 model values. The results show that the higher hmF2 values during daytime, associated with the upward velocity, are mainly responsible for the greater values of NmF2 and B0; while the nighttime lower hmF2, related to the downward velocity, are responsible for the smaller NmF2 and B0. For daytime, hmF2 and NmF2 are correlated with the solar activity in the equinoctial and summer months. The hmF2 and B0 peaks at sunset with an associated sharp decrease in NmF2 are presented in the equinoctial and summer months, but not in the winter months. Comparison of the measured hmF2 values with the International Radio Consultative Committee (CCIR) maps used in IRI-2001 (IRI-CCIR) reveals an IRI overestimate in hmF2 during daytime. The most significant discrepancy is that the IRI-CCIR does not model the post-sunset peak in hmF2. For the NmF2 comparison, the values obtained from both the CCIR and URSI maps are generally close to the observed values. For the B0 comparison, the highest discrepancy between the observation and the Gulyaeva option (IRI-Gulyaeva) is the location of the annual maximum for the daytime values, also the winter daytime predictions are too low. Additionally, the significant negative difference between the observation and the B0-table option (IRI-B0-table) provides a slightly better prediction, except for 0400–1000 LT when the model significantly overestimates. The post-sunset peak in B0 at some months is predicted by neither the IRI-Gulyaeva nor the IRI-B0-table options.Item Restricted Quiet-condition variations in the scale height at F2-layer peak at Jicamarca during solar minimum and maximum(European Geosciences Union (EGU), 2008-01-02) Lee, C. C.; Reinisch, B. W.This study is the first attempt to examine the quiet-condition variations in scale height (Hm) near the F2-layer peak in the equatorial ionosphere. The data periods of Hm derived from the Jicamarca ionograms are January-December 1996 and April 1999–March 2000. The results show that the greatest and smallest Hm values are generally at 11:00–12:00 LT and 04:00–05:00 LT, respectively. Additionally, the sunrise peak occurs at 06:00 LT only during solar minimum. The post-sunset peaks in the equinoctial and summer months are more obvious during solar maximum. The Hm difference between solar minimum and maximum are significant from afternoon to midnight. On the other hand, the Hm values during 07:00–10:00 LT for solar minimum are close to those for solar maximum. Furthermore, the correlation of Hm with the critical frequency (foF2) of F2-layer is generally low. In contrast, the correlation between Hm and the peak height (hmF2) of F2-layer is high. For Hm and the thickness parameter (B0) of F2-layer, the correlation between these two parameters is almost perfect.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 Open Access Short-term relationship between solar irradiances and equatorial peak electron densities(American Geophysical Union, 2006-06) Wang, X.; Eastes, R.; Reinisch, B. W.; Bailey, S.; Valladares, C. E.; Woods, T.The short-term relationship of the equatorial peak electron density and the solar short-wavelength irradiance is examined using foF2 observations from Jicamarca, Peru and recent solar irradiance measurements from satellites. Solar soft X-ray measurements from both the Student Nitric Oxide Explorer (SNOE) (1998–2000) and Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) (2002–2004) satellites as well as extreme ultraviolet (EUV) measurements from the TIMED satellite are used. Soft X-rays show similar or higher correlation with foF2 at short timescales (27 days or less) than EUV does, although the EUV correlation is higher for longer periods. For the short-term variations, both SNOE and TIMED observations have a higher correlation in the morning (0.46) than in the afternoon (0.1). In the afternoon, SNOE observations have a higher correlation (0.2) with foF2 than the TIMED observations (0.1 correlation), which may be due to differences in the solar cycle. At morning times, foF2 has a 27-day variation, consistent with the solar rotation rate. After noon, but not in the morning, a 13.5-day variation consistently appears in foF2. This 13.5-day variation is attributed to geomagnetic influences.Item Restricted The effects of the pre-reversal drift, the EIA asymmetry, and magnetic activity on the equatorial spread F during solar maximum(European Geosciences Union, 2005-03-30) Lee, C. C.; Liu, J. Y.; Reinisch, B. W.; Chen, W. S.; Chu, F. D.We use a digisonde at Jicamarca and a chain of GPS receivers on the west side of South America to investigate the effects of the pre-reversal enhancement (PRE) in ExB drift, the asymmetry (Ia) of equatorial ionization anomaly (EIA), and the magnetic activity (Kp) on the generation of equatorial spread F (ESF). Results show that the ESF appears frequently in summer (November, December, January, and February) and equinoctial (March, April, September, and October) months, but rarely in winter (May, June, July, and August) months. The seasonal variation in the ESF is associated with those in the PRE ExB drift and Ia. The larger ExB drift (>20m/s) and smaller |Ia| (<0.3) in summer and equinoctial months provide a preferable condition to development the ESF. Conversely, the smaller ExB drift and larger |Ia| are responsible for the lower ESF occurrence in winter months. Regarding the effects of magnetic activity, the ESF occurrence decreases with increasing Kp in the equinoctial and winter months, but not in the summer months. Furthermore, the larger and smaller ExB drifts are presented under the quiet (Kp<3) and disturbed (Kp≥3) conditions, respectively. These results indicate that the suppression in ESF and the decrease in ExB drifts are mainly caused by the decrease in the eastward electric field.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.