Trends in foF2 and the 24th solar activity cycle

The trends in foF2 are analyzed based on the data of Juliusruh and Boulder ionospheric stations. It is shown that using the traditional solar activity index F10.7 leads to an impossible trend in foF2 when the data for the 24th solar activity cycle are included into the analysis. It is assumed that the F10.7 index does not describe correctly the solar ultraviolet radiation variations in that cycle. A correction of this index using the Rz (sunspot number) and Ly (intensity of the Lyman-α line in the solar spectrum) is performed, and it is shown that in that case reasonable values of the foF2 trends are obtained.     

Comparison of heliospheric conditions near the earth during four recent solar maxima

Statistical properties of the daily averaged values of the solar activity (sunspot numbers, total solar irradiance and 10.7 cm radio emission indices), the solar wind plasma and the interplanetary magnetic field parameters near the Earth’s orbit are investigated for a period from 1964 to 2002 covering the maxima of four solar cycles from 20th to 23rd. Running half-year averages show significant solar cycle variations in the solar activity indices but only marginal and insignificant changes in comparison with background fluctuations for heliospheric bulk plasma and magnetic field parameters. The current 23rd cycle maximum is weaker than 21st and 22nd maxima, but slightly stronger than 20th cycle in most of solar and heliospheric manifestations.     

The thermospheric composition different responses to geomagnetic storm in the winter and summer hemisphere measured by “SZ” Atmospheric Composition Detectors

Three “SZ” Atmospheric Composition Detectors (ACDs) on board spacecraft “SZ-2”, “SZ-3” and “SZ-4” were launched on 10th January 2001, 26th March 2002 and 31st December 2002 separately. A large quantity of thermospheric composition data at the orbital altitude ranging from 330 to 362 km were collected from the in-situ measurement of ACDs. The spacecrafts’ lifetime was just in the second peak period of the 23rd solar cycle which includes two peaks and the solar activity value F_10.7 was from 89 to 228. During this period, several intense geomagnetic disturbances happened.     

Solar cycle effect on temporal and spatial variation of topside ion density measured by SROSS C2 and ROCSAT 1 over the Indian longitude sector

We investigated the diurnal, seasonal and latitudinal variations of ion density Ni over the Indian low and equatorial topside ionosphere within 17.5°S to 17.5°N magnetic latitudes by combining the data from SROSS C2 and ROCSAT 1 for the 9 year period from 1995 to 2003 during solar cycle 23. The diurnal maximum density is found in the local noon or in the afternoon hours and the minimum occurs in the pre sunrise hours. The density is higher during the equinoxes as compared to that in the June and December solstice. The local time spread of the daytime maximum ion density increases with increase in solar activity. A north south asymmetry with higher ion density over northern hemisphere in the June solstice and over southern hemisphere in December solstice has been observed in moderate and high solar activity years. The crest to crest distance increases with increase in solar flux. Ion density bears a nonlinear relationship with F_10.7 cm solar flux and EUV flux in general. The density increases linearly with solar flux up to ∼150 sfu (1 sfu = 10⁻²²Wm⁻²Hz⁻¹) and EUV flux up to ∼50 units (10⁹ photons cm⁻² s⁻¹). But beyond this the density saturates. Inverse saturation and linear relationship have been observed in some season or latitude also. Inter-comparison of the three solar activity indices F_10.7 cm flux, EUV flux and F_10.7P (= (F_10.7 + F_10.7A)/2, where F_10.7A is the 81 day running average value of F_10.7) shows that the ion density correlates better with F_10.7P and F_10.7 cm fluxes. The annual average daytime total ion density from 1995 to 2003 follows a hysteresis loop as the solar cycle reverses. The ion density at 500 km over the Indian longitude sector as obtained by the international reference ionosphere is in general lower than the measured densities during moderate and high solar activity years. In low solar activity years the model densities are equal or higher than measured densities. The IRI EIA peaks are symmetric (±10°) in equinox while densities are higher at 10°N in June solstice and at 10°S in the December solstice. The model density follows F_10.7 linearly up to about F_10.7 > ∼150 sfu and then saturates.     

Variations of total electron content over high latitude region during the ascending phase of 24th solar cycle

The solar cycle variation and seasonal changes significantly affects the ionization process of earth’s ionosphere and required to be monitored in real time basis for regional level refinement of existing models. In view of this, the present study has been carried out by using the ionospheric Total Electron Content (TEC) data observed with the help of Global Ionospheric Scintillation and TEC monitoring (GISTM) system installed at Indian Antarctic Research Station, “Maitri” [70°46′00″S 11°43′56″E] during the ascending phase of 24th solar cycle. The daily values of solar extreme ultraviolet (EUV) flux (0.1–50?nm wavelength), 10.7?cm radio flux F_10.7 and Sunspot number (SSN) has been taken as a proxy to represent the solar cycle variation to correlate with TEC. The linear regression results revels better correlation of TEC with EUV flux rather than F_10.7 and SSN. Also, the EUV and TEC show better agreement during summer as compared to winter and equinox period. Correlation between TEC and EUV appears significantly noticeable during ten internationally defined quiet days of each month (stable background geophysical condition) as compared to the overall days (2010–2014). Further, saturation effect has been observed on TEC values during the solar maxima year 2014. The saturation effects are more prominent during the night hours of winter and equinox season due to transportation losses manifested by the equator-ward direction of meridional wind.     

Evaluation of the STORM model storm-time corrections for middle latitude

This paper presents results from the Storm-Time Ionospheric Correction Model (STORM) validation for selected Northern and Southern Hemisphere middle latitude locations. The created database incorporated 65 strong-to-severe geomagnetic storms, which occurred within the period 1995–2007. This validation included data from some ionospheric stations (e.g., Pruhonice, El Arenosillo) that were not considered in the development or previous validations of the model. Hourly values of the F2 layer critical frequency, foF2, measured for 5–7 days during the main and recovery phases of each selected storm were compared with the predicted IRI 2007 foF2 with the STORM model option activated. To perform a detailed comparison between observed values, medians and predicted foF2 values the correlation coefficient, the root-mean-square error (RMSE), and the percentage improvement were calculated. Results of the comparative analysis show that the STORM model captures more effectively the negative phases of the summer ionospheric storms, while electron density enhancement during winter storms and the changeover of the different storm phases is reproduced with less accuracy. The STORM model corrections are less efficient for lower-middle latitudes and severe geomagnetic storms.     

Evolutionary features of polar and non-polar coronal holes during solar cycle 24 and the rising phase of cycle 25

This paper presents the results of the analysis of the evolution of coronal holes (CHs) on the Sun during the period May 13, 2010 – March 20, 2022, covering Solar Cycle 24. Our study uses images in the extreme-ultraviolet iron line (Fe XII 193 Å) obtained with the Atmospheric Imager Assembly of the Solar Dynamics Observatory (AIA/SDO). To localize CHs and determine their areas, we used the Heliophysics Event Knowledgebase (HEK). We separate the CHs into polar and non-polar and study the evolutionary features of each group. During this period, an asymmetry between the Northern (N) and Southern (S) Hemispheres (N-S or hemispheric asymmetry) is detected both in the solar activity (SA) indices and in the localization of the maximum areas of the polar and non-polar CHs. It is shown that the hemispheric asymmetry of the areas of polar and non-polar CHs varies significantly over time and that the nature of these changes is clearly related to the SA cycle. We find that for most of the period, the polar CHs were predominated generated in the S- hemisphere while the non-polar CHs were dominant in the N- hemisphere. It is found that the maximum and minimum of the hemispheric imbalance in the areas of non-polar CHs are close in time to the maximum and minimum of the asymmetry of the SA indices (the number and areas of sunspots). The maximum hemispheric imbalance of the polar CH areas is observed at the maximum of Cycle 24, and the minimum imbalance is found at the cycle minimum. These results confirm our assumption that these two types of CHs are of a different nature and that the non-polar CHs, like sunspots, are elements of the general magnetic activity.     

On long-term changes of electron density in the upper mesosphere

Recent review study done jointly by 19 experts of 17 institutes shows zero trend of temperature in the upper mesosphere. In the light of this latest development, we have examined the long-term changes in electron density, [e], in this region. The study has been concentrated at 80 km. At this altitude, electrons are mainly produced by the interaction of nitric oxide, NO, by solar Ly-α. Any long-term change in this flux will affect trend of [e]. Considering this flux proportional to 10.7 cm solar flux, analysis of available 10.7 cm solar flux data from 1948 to 2003 has been made. A decreasing trend up to about 1970 and then an increasing trend are found. The over-all increasing trend of Ly-α flux during the past five decades is ∼0.17% per year. This increase also gives a ∼0.17% increasing trend per year in [e]. This non-anthropogenic increase is much less compared to the observed increase in [e] which is reported to be >0.7% per year. The observed increase in [e] of this magnitude will then, predominantly, be due to the anthropogenic effect. In zero trend in temperature, significant change in electron loss coefficient, α_eff, and [NO] are unlikely to take place to cause a significant change in [e]. The increase in [e] > 0.7% per year then can be explained by considering a decreasing trend in [O₂].     

第23太阳活动周热层大气密度对太阳辐射指数F10.7响应的模拟研究

利用NCAR-TIEGCM计算了第23太阳活动周期间（1996—2008年）400km高度上的大气密度,并统计分析大气密度对太阳辐射指数FF_10.7的响应.结果表明,在第23太阳活动周内,大气密度的变化趋势与太阳辐射指数FF_10.7的变化趋势基本一致,但是大气密度在不同年份、不同月份对太阳辐射指数FF_10.7的响应存在差异.第23太阳活动周内太阳辐射极大值和极小值之比大于4,而大气密度的极大值与极小值之比则大于10.太阳辐射低年的年内大气密度变化不到2倍,而太阳辐射高年的年内大气密度变化可达2倍甚至3倍.大气密度与FF_10.7指数在北半球高纬的相关系数比南半球高纬的相关系数大.在低纬地区,太阳辐射高年大气密度与FF_10.7指数的相关系数比低年的大.不同纬度上,大气密度与太阳辐射指数FF_10.7的27天变化值之间的相关系数都大于其与81天变化值之间的相关系数.      

Empirical models of the thermosphere and requirements for improvements

The different types of variation in the thermosphere are briefly examined and the solar-activity effect is singled out for special attention. To this day, empirical models have made use of the decimetric solar flux F_10.7 as an index of the variable XUV radiation from the sun. To account for the change in the relative intensity of the different types of emissions in the course of the solar cycle, F_10.7 is made to perform double duty: The daily values are used to represent the day-to-day and “27-day” variations, while its averages over several solar rotations are used to represent the variations with the 11-year cycle. The availability of direct solar XUV data should eventually eliminate the need for such a make-shift procedure. Accuracy and continuity requirements of XUV intensity measurements are discussed and a strategy is outlined for sorting out the relevant features from the observational material and putting them to practical use in thermospheric modeling. It is suggested that future models of the diurnal and the geomagnetic variation use as a guide theoretical models which have achieved considerable success in qualitatively representing the observed phenomena.     

Investigation of N–S asymmetry of solar differential rotation by various patterns for solar cycles 20 and 21

For solar cycles 20 and 21 the latitudinal variations of the solar rotation rates are found using data of the H_α filaments and the long-lived magnetic features of negative and positive polarities. Analysis of the data showed that: (a) there is N–S asymmetry in the equatorial rotation of the H_α filaments and the long-lived magnetic features; (b) for both solar cycles the long-lived magnetic features of both polarities have similar behavior; (c) in the solar cycle 20 the long-lived magnetic features of both polarities vary in phase to each other but show some difference during cycle 21. For the long-lived magnetic features of positive polarity the confidence level is lower than for those of negative one.     

Variation of ionospheric total electron content in Taiwan region of the equatorial anomaly from 1994 to 2003

The ionospheric total electron content (TEC) in the northern hemispheric equatorial ionospheric anomaly (EIA) region is studied by analyzing dual-frequency signals of the Global Position System (GPS) acquired from a chain of nine observational sites clustered around Taiwan (21.9–26.2°N, 118.4–112.6°E). In this study, we present results from a statistical study of seasonal and geomagnetic effects on the EIA during solar cycle 23: 1994–2003. It is found that TEC at equatorial anomaly crests yield their maximum values during the vernal and autumnal months and their minimum values during the summer (except 1998). Using monthly averaged I_c (magnitude of TEC at the northern anomaly crest), semi-annual variations is seen clearly with two maxima occurring in both spring and autumn. In addition, I_c is found to be greater in winter than in summer. Statistically monthly values of I_c were poorly correlated with the monthly Dst index (r = −0.22) but were well correlated with the solar emission F_10.7 index (r = 0.87) for the entire database for the period during 1994–2003. In contrast, monthly values of I_c were correlated better with Dst (r ? 0.72) than with F_10.7 (r ? 0.56) in every year during the low solar activity period (1994–1997). It suggests that the effect of solar activity on I_c is a longer term (years), whereas the effect of geomagnetic activity on I_c is a shorter term (months).     

Long-term changes in indices of geomagnetic activity at the auroral station Sodankylä

Here we compare the traditional analog measure of geomagnetic activity, Ak, with the more recent digital indices of IHV and Ah based on hourly mean data, and their derivatives at the auroral station Sodankylä. By this selection of indices we study the effects of (i) analog vs. digital technique, and (ii) full local-time vs. local night-time coverage on quantifying local geomagnetic activity. We find that all other indices are stronger than Ak during the low-activity cycles 15–16 suggesting an excess of very low scalings in Ak at this time. The full-day indices consistently depict stronger correlation with the interplanetary magnetic field strength, while the night-time indices have higher correlation with solar wind speed. The Ak index correlates better with the digital indices of full-day coverage than with any night-time index. However, Ak depicts somewhat higher activity levels than the digital full-day indices in the declining phase of the solar cycle, indicating that, due to their different sampling rates, the latter indices are less sensitive to high-frequency variations driven by the Alfvén waves in high-speed streams. On the other hand, the night-time indices have an even stronger response to solar wind speed than Ak. The results strongly indicate that at auroral latitudes, geomagnetic indices with different local time coverage reflect different current systems, which, by an appropriate choice of indices, allows studying the century-scale dynamics of these currents separately.     

Predicting indices of the ionosphere response to solar activity for the ascending phase of the 25th solar cycle

The international reference ionosphere, IRI, and its extension to plasmasphere, IRI-Plas, models require reliable prediction of solar and ionospheric proxy indices of solar activity for nowcasting and forecasting of the ionosphere parameters. It is shown that IRI prediction errors could increase for the F2 layer critical frequency foF2 and the peak height hmF2 due to erroneous predictions of the ionospheric global IG index and the international sunspot number SSN1 index on which IRI and IRI-Plas models are built. Regression relation is introduced to produce daily SSN1 proxy index from new time series SSN2 index provided from June 2015, after recalibration of sunspots data. To avoid extra errors of the ionosphere model a new solar activity prediction (SAP) model for the ascending part of the solar cycle SC25 is proposed which expresses analytically the SSN1 proxy index and the 10.7-cm radio flux F10.7 index in terms of the phase of the solar cycle, Φ. SAP model is based on monthly indices observed during the descending part of SC24 complemented with forecast of time and amplitude for SC25 peak. The strength of SC25 is predicted to be less than that of SC24 as shown with their amplitudes for eight types of indices driving IRI-Plas model.     

Comparison between observed ionospheric foF2 and IRI-2001 predictions over periods of severe geomagnetic activities at Grahamstown,South Africa

The observed ionospheric F2 critical frequency (foF2) values over a South Africa mid-latitude station, Grahamstown, (geographic coordinates: 33.3°S, 26.5°E), were analysed and compared with International Reference Ionosphere (IRI) model, using the CCIR (Comite´ Consultatif International des Radio communications) and URSI (Union Radio-Scientifique Internationale) coefficients, during four geomagnetically disturbed days in the year 2000. These days are April 5, May 23, August 10 and September 15. The data were analysed for five days around the storm day. Comparisons between the IRI-2001 predicted foF2 values, using both CCIR and URSI coefficients and the observed values are shown with their root-mean-square error (RMSE) and the relative deviation module mean (rdmm) for the various storm periods. The CCIR option performed more accurately than the URSI option.     

第23太阳活动周太阳极紫外辐射与磁暴相关性分析

统计第23个太阳活动周内中等及以上强度（Dst_min<-50nT）的磁暴事件,线性拟合分析磁暴主相DDst_min和达到DDst_min前一个表征太阳极紫外辐射强度的F_10.7之间的相关性.结果表明:随着太阳极紫外辐射增强,DDst_min<-50nT的磁暴出现的总数增多,在弱、中等和强太阳极紫外辐射条件下,其数量分别为56,84和85;随着太阳极紫外辐射增强,强磁暴（-200nT ≤ Dst_min<-100nT）和大磁暴（Dst_min<-200nT）发生的数量和相对发生率呈增长趋势,尤其是大磁暴数目（1,4,12）和相对发生率（1.79%,4.76%,14.12%）明显呈增长趋势;大磁暴（|Dst_min|）与太阳极紫外辐射（F_10.7）之间存在中度正相关关系,其相关系数为0.532,并且主要体现在大磁暴（|Dst_min|）与强太阳极紫外辐射（F_10.7）之间的中度正相关性,其相关系数为0.582.大磁暴与强太阳极紫外辐射之间的相关性可为空间天气预报提供参考依据.      

Solar activity dependence of total electron content derived from GPS observations over Mbarara

Vertical total electron content (VTEC) observed at Mbarara (geographic co-ordinates: 0.60°S, 30.74°E; geomagnetic coordinates: 10.22°S, 102.36°E), Uganda, for the period 2001–2009 have been used to study the diurnal, seasonal and solar activity variations. The daily values of the 10.7 cm radio flux (F_10.7) and sunspot number (R) were used to represent Solar Extreme Ultraviolet Variability (EUV). VTEC is generally higher during high solar activity period for all the seasons and increases from 0600 h LT and reaches its maximum value within 1400 h–1500 h LT. All analysed linear and quadratic fits demonstrate positive VTEC-F_10.7 and positive VTEC-R correlation, with all fits at 0000 h and 1400 h LT being significant with a confidence level of 95% when both linear and quadratic models are used. All the fits at 0600 h LT are insignificant with a confidence level of 95%. Generally, over Mbarara, quadratic fit shows that VTEC saturates during all seasons for F_10.7 more than 200 units and R more than 150 units. The result of this study can be used to improve the International Reference Ionosphere (IRI) prediction of TEC around the equatorial region of the African sector.     

An evaluation of the IRI-2007 storm time model at low latitude stations

This paper discusses the ability of the International Reference Ionosphere IRI-2007 storm time model to predict foF2 ionospheric parameter during geomagnetic storm periods. Experimental data (based on availability) from two low latitude stations: Vanimo (geographic coordinates, 2.7 °S, 141.3 °E, magnetic coordinates, 12.3 °S, 212.50 °E) and Darwin (geographic coordinates, 12.45 °S, 130.95 °E, magnetic coordinates, 22.9 °S, 202.7 °E) during nine storms that occurred in 2000 (Rz₁₂ = 119), 2001(Rz₁₂ = 111) and 2003 (Rz₁₂ = 64) are compared with those obtained by the IRI-2007 storm model. The results obtained show that the percentage deviation between the experimental and IRI predicted foF2 values during these storm periods is as high as 100% during the main and recovery phases. Based on the values of “relative deviation module mean” (RDMM) obtained (i.e. between 0.08 and 0.60), it is observed that there is a reasonable to poor agreement between measured foF2 values and the IRI-storm model prediction values during main and recovery phases of the storms under investigation. As a result, in addition to other studies that have been carried out from different sectors, more studies are required to be carried out. This will enable IRI community to improve on the present performance of the model. In general the IRI-storm model predictions follow normal trend of the foF2 measured values but does not reproduce well the measured values.     

Properties and relationship between solar eruptive flares and Coronal Mass Ejections during rising phase of Solar Cycles 23 and 24

Statistical relationship between major flares and the associated CMEs during rising phases of Solar Cycles 23 and 24 are studied. Totally more than 6000 and 10,000 CMEs were observed by SOHO/LASCO (Solar and Heliospheric Observatory/Large Angle Spectrometric Coronagraph) during 23rd [May 1996–June 2002] and 24th [December 2008–December 2014] solar cycles, respectively. In particular, we studied the relationship between properties of flares and CMEs using the limb events (longitude 70–85°) to avoid projection effects of CMEs and partial occultation of flares that occurred near 90°. After selecting a sample of limb flares, we used certain spatial and temporal constraints to find the flare-CME pairs. Using these constraints, we compiled 129 events in Solar Cycle 23 and 92 events in Solar Cycle 24. We compared the flare-CME relationship in the two solar cycles and no significant differences are found between the two cycles. We only found out that the CME mean width was slightly larger and the CME mean acceleration was slightly higher in cycle 24, and that there was somewhat a better relation between flare flux and CME deceleration in cycle 24 than in cycle 23.     

F3 layer feature under low and medium solar activity observed at a Chinese low latitude station Fuke

Ionogram observations from the ionosonde at Fuke (9.5°N geomagnetic latitude), a Chinese low latitude station, in 2010–2012 are analyzed to present the features of F3 layer under low and moderate solar activity conditions. Structure of the ionogram, displaying the F3 layer, was more distinct and clear during MSA than LSA periods especially during spring to summer. Start time of occurrence of the F3 layer is about at 0830–0900 LT and is approximately the same for LSA and MSA conditions. The average duration time of the F3 layer occurrence was 181 min per day under F_10.7 = 75 condition, 263 min in F_10.7 = 99 and 358 min in F_10.7 = 125, respectively. The differences of h′F2 and h′F3 exhibited obvious semiannual variation observed at Fuke from March 2010 to June 2012 and increased with increasing solar activity. The difference of foF2 and foF3 in the months February, March, September, October and November is less evident in the middle solar activity period 2011–2012 than the low solar activity 2010 and in the other period it shows a slight increase (0.5 MHz) or keeps constant. The results show that the solar activity dependence of the F3 layer occurrence at low latitude away from the magnetic equator is different from that at near the magnetic equator.