Browsing by Author "Lee, C. C."
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Item Restricted Comparative study of the equatorial ionosphere over Jicamarca during recent two solar minima(American Geophysical Union, 2012-01-21) Liu, Libo; Yang, Jun; Le, Huijun; Chen, Yiding; Wan, Weixing; Lee, C. C.It is a critical issue that whether or not the extremely deep solar minimum of solar cycle 23/24 brought serious influences on the Earth's space environment. In this study, we collected and manually scaled the ionograms recorded by a DPS ionosonde at Jicamarca (12.0°S, 283.2°E) to retrieve F layer parameters and electron density (Ne) profiles. A comparative study is performed to evaluate the equatorial ionosphere in solar minima of cycle 22/23 (1996–1997) and 23/24 (2008–2009). The seasonal median values of the critical frequency of F2 layer (foF2) were remarkably reduced in four seasons during the deep solar minimum, compared to those in 1996–1997. It is the first time to find that lower values prevail at most times in 2008–2009 in the F2 layer peak height (hmF2) and Chapman scale height (Hm). The bottomside profile thickness (B0) shows higher values in 2008–2009 than that in 1996–1997 at some daytime intervals, although it also becomes smaller during the rest times. Furthermore, the ionosphere in 2008–2009 is contracted strongly at altitudes above hmF2 and more perceptible in the afternoon hours. The decrease in Ne is strongest in September equinox and weakest in June solstice. The ionospheric responses from solar minimum to minimum are mainly caused by the reduction in solar extreme ultraviolet intensity, and the contribution from dynamical processes competes and is variable. Analysis reveals that semiannual and longer‐scale components are certainly reduced during the deep solar minimum, while shorter scale (e.g., 4 month) components may disrupt the decline picture at some times.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 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 Examine the local linear growth rate of collisional Rayleigh-Taylor instability during solar maximum(American Geophysical Union, 2006-11-09) Lee, C. C.This study is the first attempt to examine the effects of terms in the growth rate formula on the magnitude of growth rate during April 1999 to March 2000 (solar maximum). The estimation of linear growth rate is based on the local linear growth rate of collisional Rayleigh‐Taylor instability (Ossakow et al., 1979). The monthly average values of linear growth rate (γ) and associated terms are calculated from the electron profile of Jicamarca digisonde and the atmospheric quantities of MSISE‐90 model under geomagnetic quiet condition. The result shows that the linear growth rate is an essential factor for the development of equatorial spread F (ESF). Furthermore, during this period, the monthly variation in gradient density scale length is neither correlated nor anticorrelated to that in growth rate. In contrast, the monthly variations in the collision frequency and recombination rate are anticorrelated with that in growth rate during this period. Additionally, the altitude range of γ profile is another important factor for the ESF generation.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 Observed and predicted characteristics of equatorial F1 layer during solar minimum(European Geosciences Union, 2014-05-28) Lee, C. C.This study aims to assess the predictability of IRI-2012 on the equatorial F1 layer during solar minimum. The observed characteristics of F1 layer by the Jicamarca digisonde are compared with the model outputs. The results show that the time range for F1-layer appearance of observation is longer than that of IRI-2012, by at least 1 h in the early morning and later afternoon. In IRI-2012, there are three options for the occurrence probability of F1 layer: IRI-95, Scotto-97 no L, and Scotto-97 with L options. The first option predicts the probability well, but the last two underestimate the probability. The peak density of F1 layer (NmF1) of observation is very close to that of IRI-2012. For the F1 peak height (hmF1), the modeled values are smaller than the observed ones. The observed seasonal variation of hmF1 is not found in the modeled results. Nevertheless, the observed diurnal variation of hmF1 is similar to the modeled results with the B0 choices of Bil-2000 and ABT-2009. Regarding the shape parameter, the values of D1 (the shape parameter of F1 layer in observation) are much greater than the values of C1 (the shape parameter of F1 layer in IRI-2012). The D1 values are 3–6 times the C1 values. The diurnal variation of D1 is similar to that of C1, but the seasonal variation of D1 is not.Item Restricted Occurrence and onset conditions of postsunset equatorial spread F at Jicamarca during solar minimum and maximum(American Geophysical Union, 2010-10-28) Lee, C. C.The data of digisonde and the NRLMSISE‐00 model on the occurrence of postsunset equatorial spread F (ESF) at Jicamarca during solar minimum and maximum years are used in a detailed statistical analysis. The onset conditions chosen in this study are the prereversal enhancement velocity (dhF80/dt), minimum virtual height of F‐layer (h′F), gradient density scale length (L), and local linear growth rate (γ) of collisional Rayleigh‐Taylor instability. The monthly variation in ESF occurrence during solar minimum generally agrees with that during maximum. This agreement indicates that the ESF occurrence does not depend on solar activity. For the onset conditions, the differences between solar minimum and maximum exists in the monthly variations in dhF80/dt, h′F, and L, but not in γ. This consistence in monthly γ variation between two solar epochs mainly account for the solar cycle independence of ESF occurrence. Furthermore, the variations in collision frequency and recombination rate are principally responsible to the consistence in γ.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 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.