武昌低电离层LF观测的季节变化

通过分析武昌低电离层LF观测数据,得到了中低纬度低电离层电子密度剖面的季节变化特征及其与太阳天顶角周年变化的关系,给出LF相位周年振荡幅度的年际变化曲线．     

Collocated ionosonde and dense GPS/GLONASS network measurements of midlatitude MSTIDs

To analyze midlatitude medium-scale travelling ionospheric disturbances (MSTIDs) over Kazan (55.5°N, 49°E), Russia, the sufficiently dense network of GNSS receivers (more than 150 ground-based stations) were used. For the first time, daytime MSTIDs in the form of their main signature (band structure) on high-resolution two-dimensional maps of the total electron content perturbation (TEC maps) are compared with ionosonde data with a high temporal resolution. For a pair of events, a relationship between southwestward TEC perturbations and evolution of F2 layer traces was established. So F2 peak frequency varied in antiphase to TEC perturbations. The ionograms show that during the movement of plasma depletion band (overhead ionosonde) the F2 peak frequency is the highest, and vice versa, for the plasma enhancement band, the F2 peak frequency is the lowest. One possible explanation may be a greater inclination of the radio beam from the vertical during the placement of a plasma enhancement band above the ionosonde, as evidenced by the absence of multiple reflections and the increased occurrence rate of additional cusp trace. Another possible explanation may be the redistribution of the electron content in the topside ionosphere with a small decrease in the F peak concentration of the layer with a small increase in TEC along the line-of-sight. Analysis of F2 peak frequency variation shows that observed peak-to-peak values of TEC perturbation equal to 0.4 and 1 TECU correspond to the values of ΔN/N equal to 13% and 28%. The need for further research is evident.     

Evaluation of the improvement of IRI-2016 over IRI-2012 at the India low-latitude region during the ascending phase of cycle 24

This paper investigated the performance of the latest International Reference Ionosphere model (IRI-2016) over that of IRI-2012 in predicting total electron content (TEC) at three different stations in the Indian region. The data used were Global Positioning System (GPS) data collected during the ascending phase of solar cycle 24 over three low-latitude stations in India namely; Bangalore (13.02°N Geographic latitude, 77.57°E Geographic longitude), Hyderabad (17.25°N Geographic latitude, 78.30°E Geographic longitude) and Surat (21.16°N Geographic latitude, 72.78°E Geographic longitude). Monthly, the seasonal and annual variability of GPS-TEC have been compared with those derived from International Reference Ionosphere IRI-2016 and IRI-2012 with two different options of topside electron density: NeQuick and IRI01-corr. It is observed that both versions of IRI (i.e., IRI-2012 and IRI-2016) predict the GPS-TEC with some deviations, the latest version of IRI (IRI-2016) predicted the TEC similar to those predicted by IRI-2012 for all the seasons at all stations except for morning hours (0500 LT to 1000?LT). This shows that the effect of the updated version is seen only during morning hours and also that there is no change in TEC values by IRI-2016 from those predicted by IRI-2012 for the rest of the time of the day in the Indian low latitude region. The semiannual variations in the daytime maximum values of TEC are clearly observed from both GPS and model-derived TEC values with two peaks around March-April and September-October months of each year. Further, the Correlation of TEC derived by IRI-2016 and IRI-2012 with EUV and F10.7 shows similar results. This shows that the solar input to the IRI-2016 is similar to IRI 2012. There is no significant difference observed in TEC, bottom-side thickness (B0) and shape (B1) parameter predictions by both the versions of the IRI model. However, a clear improvement is visible in hmF2 and NmF2 predictions by IRI-2016 to that by IRI-2012. The SHU-2015 option of the IRI-2016 gives a better prediction of NmF2 for all the months at low latitude station Ahmedabad compare to AMTB 2013.     

Solar activity variations of equatorial spread F occurrence and sustenance during different seasons over Indian longitudes: Empirical model and causative mechanisms

A comprehensive analysis using nearly two decades of ionosonde data is carried out on the seasonal and solar cycle variations of Equatorial Spread F (ESF) irregularities over magnetic equatorial location Trivandrum (8.5°N, 77°E). The corresponding Rayleigh Taylor (RT) instability growth rates (γ) are also estimated. A seasonal pattern of ESF occurrence and the corresponding γ is established for low solar (LSA), medium solar (MSA) and high solar (HSA) activity periods. For LSA, it is seen that the γ maximizes during post sunset time with comparable magnitudes for autumnal equinox (AE), vernal equinox (VE) and winter solstice (WS), while for summer solstice (SS) it maximizes in the post-midnight period. Concurrent responses are seen in the ESF occurrence pattern. For MSA, γ maximizes during post-sunset for VE followed by WS and AE while SS maximises during post-midnight period. The ESF occurrence for MSA is highest for VE (80%), followed by AE (70%), WS (60%) and SS (50%). In case of HSA, maximum γ occurs for VE followed by AE, WS and SS. The concurrent ESF occurrence maximizes for VE and AE (90%), WS and SS at 70%.The solar cycle variation of γ is examined. γ shows a linear variation with F10.7?cm flux. Further, ESF percentage occurrence and duration show an exponential and linear variation respectively with γ. An empirical model on the solar activity dependence of ESF occurrence and sustenance time over Indian longitudes is arrived at using the database spanning two solar cycles for the first time.     

Unusual nighttime impulsive foF2 enhancements at low latitudes: Phenomenology and possible explanations

This paper is focused on unusual nighttime impulsive electron density enhancements that are rarely observed at low latitudes on a wide region of South America, under quiet and medium/high geomagnetic conditions. The phenomenon under investigation is very peculiar because besides being of brief duration, it is characterized by a pronounced compression of the ionosphere. The phenomenon was studied and analyzed using both the F2 layer critical frequency (foF2) and the virtual height of the base of the F region (h′F) values recorded at five ionospheric stations widely distributed in space, namely: Jicamarca (−12.0°, −76.8°, magnetic latitude −2.0°), Peru; Sao Luis (−2.6°, −44.2°, magnetic latitude +6.2°), Cachoeira Paulista (−22.4°, −44.6°, magnetic latitude −13.4°), and São José dos Campos (−23.2°, −45.9°, magnetic latitude −14.1°), Brazil; Tucumán (−26.9°, −65.4°, magnetic latitude −16.8°), Argentina. In a more restricted region over Tucumán, the phenomenon was also investigated by the total electron content (TEC) maps computed by using measurements from 12 GPS receivers. A detailed analysis of isoheight ionosonde plots suggests that traveling ionospheric disturbances (TIDs) caused by gravity wave (GW) propagation could play a significant role in causing the phenomenon both for quiet and for medium/high geomagnetic activity; in the latter case however a recharging of the fountain effect, due to electric fields penetrating from the magnetosphere, joins the TID propagation and plays an as much significant role in causing impulsive electron density enhancements.     

Variation of F-region critical frequency (foF2) over equatorial and low-latitude region of the Indian zone during 19th and 20th solar cycle

The effect of solar cycle and seasons on the daytime and nighttime F-layer ionization has been investigated over the equatorial and low-latitude region during 19th (1954–1964) and 20th (1965–1976) solar cycle. The F-layer critical frequency (foF2) data observed from the three Indian Ionosonde stations has been used for the present study. The dependence of foF2 on solar cycle has been examined by performing regression analysis between the foF2 values and R₁₂ (twelve month running average sunspot number). The result shows that the magnitude of the cycle, seasons and the location of station has considerable effects on foF2. There is a significant nonlinear relationship between the foF2 values and R₁₂ during 19th solar cycle as compared to 20th solar cycle. Further, the nighttime saturation effect is prominently seen during the 19th solar cycle and summer season. It is also observed that the most profound saturation effect appears at the equatorial ionization anomaly crest region. Seasonally, it is seen that all the stations exhibits semiannual anomaly. The phenomenon of winter anomaly decays as we move higher along the latitude and is prominently seen during the intense solar activity.     

Variability of the ionospheric electron density at fixed heights and validation of IRI-2007 profile’s prediction at Ilorin

A study on the variability of the equatorial ionospheric electron density was carried out at fixed heights below the F2 peak using one month data for each of high and low solar activity periods. The data used for this study were obtained from ionograms recorded at Ilorin, Nigeria, and the study covers height range from 100 km to the peak of the F2 layer for the daytime hours and height range from 200 km to the peak of the F2 layer for the nighttime hours. The results showed that the deviation of the electron density variation from simple Chapman variation begins from an altitude of about 200 km for the two months investigated. Daytime minimum variability of between 2.7% and 9.0% was observed at the height range of about 160 and 200 km during low solar activity (January 2006) and between 3.7% and 7.8% at the height range of 210 and 260 km during high solar activity (January 2002). The nighttime maximum variability was observed at the height range of 210 and 240 km at low solar activity and at the height range of 200 and 240 km at high solar activity. A validation of IRI-2007 model electron density profile’s prediction was also carried out. The results showed that B0 option gives a better prediction around the noontime.     

Evidence for the lowering of the centroid of daytime thermospheric O(D) 630.0 nm emission over the magnetic equator: First results

It is shown in this paper for the first time that the intensity of the daytime thermospheric O(¹D) 630.0 nm airglow as measured by the ground-based dayglow photometer over Trivandrum (8.5°N; 77°E; dip lat. 0.5°N), a geomagnetic dip equatorial station, exhibit a direct correlation with the electron density at 180 km. This altitude is about ∼40 km lower than the believed centroid of the O(¹D) 630.0 nm dayglow emission i.e. 220 km. This observation is contrary to the understanding of the behavior of O(¹D) 630.0 nm dayglow over equatorial/low latitudes. Over these latitudes, the variations of the measured intensity of O(¹D) 630.0 nm dayglow are known to be associated with the changes in the electron density at altitudes around 220 km, the centroid of this emission. In this context, the present results indicating the lowering of the peak altitude of O(¹D) 630.0 nm emission from ∼220 to ∼180 km over the dip equator is new. Recent results on solar XUV flux indicate that this could be an important parameter that controls the O(¹D) 630.0 nm dayglow excitation rates through modulations in the neutral and ionic composition in lower thermosphere-ionosphere region. However, the lowering of the centroid of O(¹D) 630.0 nm emission, as shown in this study, has been ascribed primarily to the fountain effect associated with the equatorial ionization anomaly.     

Observations of wave activity in the ionosphere over South Africa in geomagnetically quiet and disturbed periods

The present paper deals with observations of wave activity in the period range 1–60 min at ionospheric heights over the Western Cape, South Africa from May 2010 to July 2010. The study is based on the Doppler type sounding of the ionosphere. The Doppler frequency shift measurements are supplemented with measurements of collocated Digisonde DPS-4D at SANSA Space Sciences, Hermanus. Nine geomagnetically quiet days and nine geomagnetically active days were included in the study. Waves of periods 4–30 min were observed during the daytime independent of the level of geomagnetic activity. Amplitudes of 10–30 min waves always increased between 14:00 and 16:15 UT (16:00–18:15 LT). Secondary maxima were observed between 06:00 and 07:00 UT (08:00–09:00 LT). The maximum wave amplitudes occurred close to the time of passage of the solar terminator in the studied region which is known to act as a source of gravity waves.     

Variation of monthly mean foF2 and hmF2 over a mid latitude station during the period 1997–2006

Ionosonde data of a mid latitude station Novosibirsk (Geog. Lat. 54.6°N, Geog. Long. 83.2°E) has been analyzed for the years 1997–2006 that covers the major part of solar cycle 23. Our results show the presence of winter anomaly in the daytime F2 layer critical frequency during different phases of solar activity. Results also reveal a semiannual variation of foF₂ with two maxima and a minimum that always appears in summer. While the first maximum is in the spring equinox, the second one is found to shift from autumn to winter with the increase of solar activity. The maximum height of F2 layer during the daytime shows variation with the solar activity. It is higher during the higher activity periods and lower during the periods of low activity. Results of ionosonde observations have been compared with those obtained from IRI-2007 model and it is found that model reproduces gross features of foF₂ variation. However, the modeled hmF₂ variations during equinoxes are significantly different from the ones derived using the ionosonde data. The model also underestimates the hmF₂ values.