Baiqi Ning

Solar activity variations of nighttime ionospheric peak electron density

[1] Monthly median NmF2 (maximum electron density of the F2-layer) data at Okinawa, Yamagawa, Kokubunji, and Wakkanai have been collected to investigate the solar activity dependence of the nighttime ionosphere. The result shows that there are seasonal and latitudinal differences of the solar activity variation of nighttime NmF2. The main seasonal effects are as follows: nighttime NmF2 increases with F107 linearly in equinoctial months (March and September), and it tends to saturate with F107 increasing in summer solstice month (June). What is peculiar is that there is an amplification trend of nighttime NmF2 with F107 in winter solstice month (December). The latitudinal difference is mainly displayed by the evolvement course of the variation trend between NmF2 and F107. Using hmF2 (peak height of the F2-layer) data and the NRLMSISE00 model, we estimated the recombination loss around the F2-peak at different solar activity levels. We found that the solar activity variation of the recombination processes around the F2-peak also shows seasonal dependence, which can explain the variation trends of nighttime NmF2 with F107 qualitatively, and field-aligned plasma influx plays an important role in the equatorial ionization anomaly (EIA) crest region. During the first several hours following sunset in December, there are faster recombination processes around the F2-peak at medium solar activity level in mid-latitude regions. This feature is suggested to be responsible for inducing the amplification trend in winter. In virtue of the calculation of neutral parameters at 300-km altitude and hmF2 data, the variation trend of the recombination processes around the F2-peak with F107 can be explained. It shows that both the solar activity variations of hmF2 and neutral parameters (neutral temperature, density, and vibrational excited N2) are important for the variation trend of nighttime NmF2 with F107. Furthermore, the obvious uplift of hmF2 at low solar activity level following sunset in December is important for the amplification trend.

A study of the Weddell Sea Anomaly observed by FORMOSAT‐3/COSMIC

[1] More than two years of COSMIC electron density profiles at low solar activities are collected to study the evolution of the Weddell Sea Anomaly (WSA), which appears as an evening enhancement in electron density during local summer. Observations show that the change in NmF2 (the F2 peak electron density) is associated with the change in hmF2 (the F2 peak height), while the latter is correlated closely with the components of the geomagnetic field. We find that (1) in the afternoon, hmF2 is more liable to rise drastically in regions with a larger jsin(2I)j value, which would occur early at certain declinations, eastward in the southern hemisphere and westward in the northern hemisphere; (2) subsequently, a larger increment of hmF2 is coincidentally followed by a stronger enhancement of NmF2 and the enhancement ends just around the local sunset; and (3) in midlatitudes, the evolution pattern of hmF2 in the evening of equinoxes and winter is similar to that in summer, albeit without a lasting NmF2 enhancement as that in summer. These features suggest that the NmF2 enhancement and the hmF2 increase could arise from the thermospheric wind effect, and solar photoionization plays a crucial role in the enhancement as well. The general midlatitude F2 layer enhancement in local summer evening is consistent with the WSA on the above features, indicating that the WSA is a manifestation, with a particular geometry of the magnetic field, of the evening enhancement induced by the winds.

Large‐scale traveling ionospheric disturbances observed by GPS total electron content during the magnetic storm of 29–30 October 2003

[1] We have investigated the propagation of large-scale traveling ionospheric disturbances (LSTIDs) during the super magnetic storm of 29 - 30 October 2003. Two-dimensional total electron content (TEC) perturbation maps over North America were built using TEC data provided by the American GPS network and the International GNSS Service. Three LSTID events were observed in the range of 30 degrees N - 50 degrees N, 60 degrees W - 110 degrees W during this period. The first two LSTIDs occurred consecutively during 0620 - 0800 UT on 29 October at the local time of midnight, right after the onset of the big substorm; the third one was found at noon during the expansion phase of another substorm on 30 October. The phase fronts of these LSTIDs passed over the United States and traveled southwestward to the distance of similar to 2000 km with the maximum front width of similar to 4000 km and the duration of less than 2 hours. The maximum amplitude of TEC perturbations attained 3 total electron content units (TECUs). The results differ from the former observation of Afraimovich and Voeykov ( 2004) and Afraimovich et al. ( 2006), who reported a solitary LSTID propagating southwestward over the United States with the amplitudes of up to 14 TECU on 30 October 2003. We have checked the magnetic H component observed at the geomagnetic observatories in North America and found it is most likely that the auroral westward electrojet was the cause of the LSTIDs on 29 October. The source region for these TIDs was likely to be located several hundred kilometers north of 50 degrees N. Cross-spectral analysis was conducted to obtain the global propagation characteristics of LSTIDs during this superstorm. Equatorward LSTIDs were found in all the three sectors of North America, Europe, and Asia, showing high correlation with the occurrence of auroral substorms.

Latitudinal dependence of the ionospheric response to solar eclipses

[1] In this study, we statistically analyze the latitudinal dependence of F2-layer peak electron densities (NmF2) and total electron content (TEC) responses to solar eclipses by using the ionosonde observations during 15 eclipse events from 1973 to 2006 and the GPS TEC observations during six solar eclipse events from 1999 to 2006. We carried out a model study on the latitudinal dependence of eclipse effects on the ionosphere by running a theoretical ionospheric model with the total eclipse occurring at 13 latitudes from 0 Nt o 60N at intervals of 5. Both the observations and simulations show that the NmF2 and TEC responses have the same latitudinal dependence: the eclipse effects on NmF2 and TEC are smaller at low latitudes than at middle latitudes; at the middle latitudes (>40), the eclipse effect decreases with increasing latitude. The simulations show that the smaller NmF2 responses at low latitudes are mainly because of much higher heights of hmF2 at low latitudes and electron density response decreasing rapidly with increasing height. For the eclipse effects at the middle latitudes (>40), the simulations show that the smaller NmF2 or TEC response at higher latitude is mainly ascribed to the larger downward diffusion flux induced by the larger dip angle at this region, which can partly make up for the plasma loss and alleviate the depression of electron density in the F region. The simulated results show that there is an overall decrease in electron temperature throughout the entire height range at the middle latitude, but for the low latitudes the eclipse effect on electron temperature is much smaller at high heights, which is mainly because of the much smaller reduction of photoelectron production rate at its conjugate low heights where only a partial eclipse with small eclipse magnitude occurs.

Solar activity variations of equivalent winds derived from global ionosonde data

equivalentwindsarefoundofnonlinearlydecreaseddiurnalamplitudesinallseasonsatmost stations. This implies that the increase in ion drag more than compensates for pressure gradients and thus restrains the diurnal amplitude at high solar activity. The diurnal phase of the derived equivalent winds generally shifts later at higher solar activity. It is the first time to explicitly report this striking feature that emerged at so many stations. Another pronounced feature is that the diurnal phase has a summer-winter difference. The diurnal phases at most stations in the Northern Hemisphere are later in winter than in summer at higher solar activity. Furthermore, a decrease in the semidiurnal amplitudes of equivalent winds with increasing solar activity is evident in winter over most stations considered and in other seasons at stations with a lower dip, but the decrease trend becomes weak in other seasonsatstationswithalargerdip.However,complicateddependencesonsolaractivitycan be found in the diurnal mean and the semidiurnal phases of equivalent winds at stations considered. INDEX TERMS: 2427 Ionosphere: Ionosphere/atmosphere interactions (0335); 3369 MeteorologyandAtmosphericDynamics:Thermosphericdynamics(0358);3309MeteorologyandAtmospheric Dynamics: Climatology (1620); 2162 Interplanetary Physics: Solar cycle variations (7536); KEYWORDS: ionosphere, climatology, solar activity variation

Traveling ionospheric disturbances associated with the tropospheric vortexes around Qinghai‐Tibet Plateau

To show the relationship between the traveling ionospheric disturbances (TIDs) observed in Central China and the vortexes corresponding to the topography of Qinghai-Tibet Plateau, the wave parameters (frequency, wavenumber and propagation azimuth) and the exciting sources of the TIDs are statistically analyzed under the base of observation of an HF Doppler array. It is found that the observed TIDs propagate mainly in the northeastward (NE) and southeastward (SE) directions. The backward ray tracing shows that the sources of NE and SE TIDs are respectively located in the southeastern and northeastern edges of Qinghai-Tibet Plateau, i.e., in the lee sides of the bulging terrain of the plateau where the vortexes are more probably produced. The and geographical distributions as well as their seasonal variations of the occurrence rate of both the TID sources and the vortexes are very coincident with each other, this leads us to suggest that the observed TIDs be excited by the tropospheric vortexes which in turn are related to the topography of the plateau.

Applying artificial neural network to derive long-term foF2 trends in the Asia/Pacific sector from ionosonde observations

[ 1] An artificial neural network ( ANN) method is first used for deriving long-term trends of the F2-layer critical frequency (foF2) at 19 ionospheric stations in the Asia/Pacific sector. It is found that the ANN method can eliminate the geomagnetic activity effect on foF2 more effectively than usual regression methods. Of the selected 19 stations, there are significant long-term trends corresponding to a confidence level >= 90% at 14 stations and 12 of these stations present negative trends. An average trend of - 0.05% per year in the selected area can be obtained if the 12 stations with significant negative long-term trends be considered. No pronounced diurnal and latitudinal effects in trends and no uniform pattern of seasonal variation in most stations are detected. The long-term trends for low latitude and equatorial stations differ from other stations suggest that some special dynamical processes may take effects in the equatorial anomaly region. Many factors which can influence ionosphere, such as the greenhouse effect, solar and geomagnetic activity, and neutral background gas, might contribute to the trend.

East-west differences in F-region electron density at midlatitude: Evidence from the Far East region

The global configuration of the geomagnetic field shows that the maximum east-west difference in geomagnetic declination of northern middle latitude lies in the US region (similar to 32 degrees), which produces the significant ionospheric east-west coast difference in terms of total electron content first revealed by Zhang et al. (2011). For verification, it is valuable to investigate this feature over the Far East area, which also shows significant geomagnetic declination east-west gradient but smaller (similar to 15 degrees) than that of the US. The current study provides evidence of the longitudinal change supporting the thermospheric zonal wind mechanism by examining the climatology of peak electron density (NmF2), electron density (Ne) of different altitudes in the Far East regions with a longitude separation of up to 40-60 degrees based on ground ionosonde and space-based measurements. Although the east-west difference (R-ew) over the Far East area displays a clear diurnal variation similar to the US feature, that is negative R-ew (West Ne > East Ne) in the noon and positive at evening-night, the observational results reveal more differences including: (1) The noontime negative R-ew is most pronounced in April-June while in the US during February-March. Thus, for the late spring and summer period negative R-ew over the Far East region is more significant than that of the US. (2) The positive R-ew at night is much less evident than in the US, especially without winter enhancement. (3) The magnitude of negative R-ew tends to enhance toward solar maximum while in the US showing anticorrelation with the solar activity. The altitude distribution of pronounced negative difference (300-400 km) moves upward as the solar flux increases and hence produces the different solar activity dependence at different altitude. The result in the paper is not simply a comparison corresponding to the US results but raises some new features that are worth further studying and improve our current understanding of ionospheric longitude difference at midlatitude. Citation: Zhao, B., M. Wang, Y. Wang, Z. Ren, X. Yue, J. Zhu, W. Wan, B. Ning, J. Liu, and B. Xiong (2013), East-west differences in F-region electron density at midlatitude: Evidence from the Far East region, J. Geophys. Res. Space Physics, 118, 542-553, doi:10.1029/2012JA018235.

The low latitude ionospheric effects of the April 2000 magnetic storm near the longitude 120°E

In this paper, we report the responses of the low latitude ionosphere near the longitude 120°E to the April 2000 geomagnetic storm using Digisonde data measured at Chungli (25.0°N, 121.2°E, Mag. 13.8°N), Wuhan (30.6°N, 114.4°E, Mag. 19.3°N), and Kokubunji (35.7°N, 139.5°E, Mag. 25.7°N). At these three stations, the significant ionospheric responses are near-simultaneous height disturbances after the sudden storm commencement (SSC) on April 6, 2000 and wave-like disturbances in the daytime on April 7. The ionospheric height disturbances in the nighttime after the SSC at these stations are suggested to be caused by the storm related perturbed electric fields, and the followed wave-like disturbances may be caused by storm induced atmospheric gravity waves. The vertical effective winds derived from Digisonde measurements imply the existence of significantly large vertical drifts during this storm, which are in agreement with the perturbed zonal electric fields predicted by the model of Fejer and Scherliess (1997) and Scherliess and Fejer (1997). Finally, the storm time derivations of foF2 from its monthly median level at these stations are used to validate the predication ability of the empirical model of Araujo-Pradere et al. (2002), which has included in the International Reference Ionosphere model IRI2000.

Analysis of ionospheric scintillation spectra and TEC in the Chinese low latitude region

GPS L-band scintillations and total electron content (TEC) were recorded at Sanya (18.33°N, 109.52°E) during the period July 2004–July 2005. Automatic recorded raw digital scintillation data are analyzed to obtain the spectral characteristics of irregularities producing ionospheric scintillations and to estimate the correlation between amplitude scintillation and power spectral density. Concurrent measurements of TEC are used to analyze ROTI, defined as the standard deviation of the rate of change of TEC. The statistical results of S4 indices and power spectral indices indicate that the power spectral indices increase with S4 indices for weak scintillation (0.1 ≤ S4 < 0.3), but for moderate and strong scintillation, spectral indices tend to be saturated. In the analyzed data set, the ratio of ROTI/S4 is found to vary between 0.3 and 6, and the variation in estimated zonal drift velocities during geomagnetic quiet days (K p < 3) shows that the motion of the irregularities is highly variable in the initial phase of irregularity development. After about 22:00 LT, the estimated drift velocities tend to follow the same pattern.