Essential features of long-term changes of areas and diameters of sunspot groups in solar activity cycles 12–24

We analyze the Greenwich catalog data on areas of sunspot groups of last thirteen solar cycles. Various parameters of sunspots are considered, namely: average monthly smoothed areas, maximum area for each year and equivalent diameters of groups of sunspots. The first parameter shows an exceptional power of the 19th cycle of solar activity, which appears here more contrastively than in the numbers of spots (that is, in Wolf’s numbers). It was found that in the maximum areas of sunspot groups for a year there is a unique phenomenon: a short and high jump in the 18th cycle (in 1946–1947) that has no analogues in other cycles. We also studied the integral distributions for equivalent diameters and found the following: (a) the average value of the index of power-law approximation is 5.4 for the last 13 cycles and (b) there is reliable evidence of Hale's double cycle (about 44?years). Since this indicator reflects the dispersion of sunspot group diameters, the results obtained show that the convective zone of the Sun generates embryos of active regions in different statistical regimes which change with a cycle of about 44?years.     

Assessment of improvement of the IRI model over Ethiopia for the modeling of the variability of TEC during the period 2013–2016

This paper discusses the monthly and seasonal variation of the total electron content (TEC) and the improvement of performance of the IRI model in estimating TEC over Ethiopia during the solar maximum (2013–2016) phase employing as reference the GPS derived TEC data inferred from four GPS receivers installed in different regions of Ethiopia; Assosa (geog 10.05°N, 34.55°E, Geom. 7.01°N), Ambo (8.97°N, 37.86°E, Geom. 5.42°N), Nazret (8.57°N, 39.29°E, Geom. 4.81°N) and Arba Minch (6.06°N, 37.56°E, Geom. 2.62°N). The results reveal that, in the years 2013–2016, the highest peak GPS-derived diurnal VTEC is observed in the March equinox in 2015 over Arba Minch station. Moreover, both the arithmetic mean GPS-derived and modelled VTEC values, generally, show maximum and minimum values in the equinoctial and June solstice months, respectively in 2014–2015. However, in 2013, the minimum and maximum arithmetic mean GPS-derived values are observed in the March equinox and December solstice, respectively. The results also show that, even though overestimation of the modelled VTEC has been observed on most of the hours, all versions of the model are generally good to estimate both the monthly and seasonal diurnal hourly VTEC values, especially in the early morning hours (00:00–03:00?UT or 03:00–06:00?LT). However, it has also been shown that the IRI 2007 and IRI 2012 versions generally perform best in matching the diurnal GPS derived TEC values as compared to that of the IRI 2016 version. In addition, the IRI 2012 version with IRI2001 option for the topside electron density shows the highest overestimation of the VTEC as compared to the other options. None of the versions of the IRI model are proved to be able to capture the effects of geomagnetic storms.     

Storm time spatial variations in TEC during moderate geomagnetic storms in extremely low solar activity conditions (2007–2009) over Indian region

The total electron content (TEC) measurements from a network of GPS receivers were analyzed to investigate the storm time spatial response of ionosphere over the Indian longitude sector. The GPS receivers from the GPS Aided Geo Augmented Navigation (GAGAN) network which are uniquely located around the ∼77°E longitude are used in the present study so as to get the complete latitudinal coverage from the magnetic equator to low mid-latitude region. We have selected the most intense storms but of moderate intensity (−100 nT < Dst < −50 nT) which occurred during the unusually extremely low solar activity conditions in 2007–2009. Though the storms are of moderate intensity, their effects on equatorial to low mid-latitude ionosphere are found to be very severe as TEC deviations are more than 100% during all the storms studied. Interesting results in terms of spatial distribution of positive/negative effects during the main/early recovery phase of storms are noticed. The maximum effect was observed at crest region during two storms whereas another two storms had maximum effect near the low mid-latitude region. The associated mechanisms like equatorial electrodynamics and neutral dynamics are segregated and explained using the TIMED/GUVI and EEJ data during these storms. The TEC maps are generated to investigate the storm time development/inhibition of equatorial ionization anomaly (EIA).     

Comparison of GPS TEC variations with IRI-2007 TEC prediction at equatorial latitudes during a low solar activity (2009–2011) phase over the Kenyan region

This work presents an analysis of the Total Electron Content (TEC) derived from the International GNSS Service (IGS) receivers at Malindi (mal2: 2.9^oS, 40.1^oE, dip −26.813^o), Kasarani (rcmn: 36.89^oE, 1.2^oS, dip −23.970^o), Eldoret (moiu: 35.3^oE, 0.3^oN, dip −21.037^o) and GPS-SCINDA (36.8^oE, 1.3^oS, dip −24.117^o) receiver located in Nairobi for the period 2009–2011. The diurnal, monthly and seasonal variations of the GPS derived TEC (GPS-TEC) and effects of space weather on TEC are compared with TEC from the 2007 International Reference Ionosphere model (IRI-TEC) using the NeQuick option for the topside electron density. The diurnal peaks in GPS-TEC is maximum during equinoctial months (March, April, October) and in December and minimum in June solstice months (May, June, July). The variability in GPS-TEC is minimal in all seasons between 0:00 and 04:00 UT and maximum near noon between 10:00 and 14:00 UT. Significant variability in TEC at post sunset hours after 16:00 UT (19:00 LT) has been noted in all the seasons except in June solstice. The TEC variability of the post sunset hours is associated with the occurrence of the ionization anomaly crest which enhances nighttime TEC over this region. A comparison between the GPS-TEC and IRI-TEC indicates that both the model and observation depicts a similar trend in the monthly and seasonal variations. However seasonal averages show that IRI-TEC values are higher than the GPS-TEC. The IRI-TEC also depicts a double peak in diurnal values unlike the GPS-TEC. This overestimation which is primarily during daytime hours could be due to the model overestimation of the equatorial anomaly effect on levels of ionospheric ionization over the low latitude regions. The IRI-TEC also does not show any response to geomagnetic activity, despite the STORM option being selected in the model; the IRI model generally remains smooth and underestimates TEC during a storm. The GPS-TEC variability indicated by standard deviation seasonal averages has been presented as a basis for extending the IRI-model to accommodate TEC-variability.     

α–ω Effect and peculiarities of the 27-day variations of the galactic cosmic rays intensity,solar wind and solar activity parameters

We show that the amplitudes of the 27-day variations of galactic cosmic ray (GCR) intensity, solar wind and solar activity parameters have a periodicity with duration of three to four Carrington rotation periods (3–4 CRP). We assume that the general reason for this phenomenon may be related to similar cyclicity of topological structure of the solar magnetic field lines created owing to the asymmetry of turbulent solar dynamo and solar differential rotation transforming the Sun’s poloidal magnetic field to the toroidal (α–ω effect), and vice versa.     

Influence of the 27-day solar flux variations on the ionosphere parameters measured at Irkutsk in 2003–2005

We present an investigation of the influence of the 27-day solar flux variations, caused by solar rotation, on the ionosphere parameters such as the F2 layer critical frequency (foF2) and the total electron content (TEC). Our observational data were obtained with the Irkutsk Digisonde (DPS-4) located at 52.3 North and 104.3 East during the period from 2003 to 2005. In addition, we use TEC data from the Global Ionosphere Maps (GIM) based on Global Positioning System (GPS) satellites. The solar radiation flux at a wavelength of 10.7 cm (F10.7 index) is used as an index characterizing the solar activity level. A good correlation between observed ionosphere parameters and solar activity variations is found especially in autumn-to-winter season. We estimate the impact of the 27-day solar flux variations on the day-to-day variability and determine the time delay of the ionosphere response.     

Characterization of GNSS scintillations over Lagos,Nigeria during the minimum and ascending phases (2009–2011) of solar cycle 24

This study characterizes equatorial scintillations at L-band frequency over Lagos, Nigeria during the minimum and ascending phases of solar cycle 24. Three years (2009–2011) of amplitude scintillation data were used for the investigation. The data were grouped on daily, monthly, seasonal, and yearly scales at three levels of scintillation (weak (0.3 ? S₄ < 0.4), moderate (0.4 ? S₄ < 0.7), and intense (S₄ ? 0.7)). To ensure reliable statistical inferences, three data cut-off criteria were adopted. Scintillations were observed to have a daily trend of occurrence during the hours of 1900–0200 LT, and higher levels of scintillations were localized within the hours of 2000–2300 LT. On monthly basis, September and October recorded the highest occurrences of scintillation, while January recorded the least. Scintillations were recorded during all the months of 2011, except January. Surprisingly, pockets of scintillation events (weak levels) were also observed during the summer months (May, June, and July). Seasonally, equinoxes recorded the highest occurrences of scintillation, while June solstice recorded the least occurrences. Scintillation activity also increases with solar and geomagnetic activity. On a scintillation active day, the number of satellites available to the receiver’s view reduces as the duration of observation reduces. These results may support the development of future models that could provide real-time predictability of African equatorial scintillations, with a view to supporting the implementation of GNSS-based navigation for aviation applications in Africa.     

Investigation of ionospheric response to two moderate geomagnetic storms using GPS–TEC measurements in the South American and African sectors during the ascending phase of solar cycle 24

The responses of the ionospheric F region using GPS–TEC measurements during two moderate geomagnetic storms at equatorial, low-, and mid-latitude regions over the South American and African sectors in May 2010, during the ascending phase of solar cycle 24, are investigated. The first moderate geomagnetic storm studied reached a minimum Dst value of −64 nT at 1500 UT on 02 May 2010 and the second moderate geomagnetic storm reached a minimum Dst value of −85 nT at 1400 UT on 29 May 2010. In this paper, we present vertical total electron content (VTEC) and phase fluctuations (in TECU/min) from Global Positioning System (GPS) observations from the equatorial to mid-latitude regions in the South American and African sectors. Our results obtained during these two moderate geomagnetic storms from both sectors show significant positive ionospheric storms during daytime hours at the equatorial, low-, and mid-latitude regions during the main and recovery phases of the storms. The thermospheric wind circulation change towards the equator is a strong indicator that suggests an important mechanism is responsible for these positive phases at these regions. A pre-storm event that was observed in the African sector from low- to the mid-latitude regions on 01 May 2010 was absent in the South American sector. This study also showed that there was no generation or suppression of ionospheric irregularities by storm events. Therefore, knowledge about the suppression and generation of ionospheric irregularities during moderate geomagnetic storms is still unclear.     

Seasonal variability of GPS-VTEC and model during low solar activity period (2006–2007) near the equatorial ionization anomaly crest location in Chinese zone

Variability of vertical TEC recorded at Fuzhou (26.1°N, 119.3°E, geomagnetic latitude 14.4°N), Xiamen (24.5°N, 118.1°E, geomagnetic latitude 13.2°N), Nanning (22.8°N, 108.3°E, geomagnetic latitude 11.4°N), China, during the low solar activity in 2006–2007 have been analyzed and discussed. Remarkable seasonal anomaly was found over three stations with the highest value during spring and the lowest value during summer. The relative standard deviation of VTEC is over 20% all the time, with steady and smooth variation during daytime while it has a large fluctuation during nighttime. The biggest correlation coefficient was found in the VTEC-sunspot pair with a value of over 0.5. It seems that solar activity has a better correlation ship than geomagnetic activity with the variation of VTEC and better correlations are found with more long-term data when comparing our previous study. The results of comparing observation with model prediction in three sites reveal again that the SPIM model overestimates the measured VTEC in the low latitude area.     

Comparative analysis of TEC disturbances over tropical cyclone zones in the North–West Pacific Ocean

In this paper, we study ionospheric total electron content (TEC) disturbances associated with tropical cyclones (TCs). The study relies on the statistical analysis of six cyclones of different intensity which occurred in the North–West Pacific Ocean in September–November 2005. We have used TEC data from the international network of two-frequency ground-based GPS receivers and NCEP/NCAR meteorological archive. TEC variations of different period ranges (02–20 and 20–60 min) are shown to be more intense during TC peaks under quiet geomagnetic conditions. The highest TEC variation amplitudes are registered when the wind speed in the cyclone and the TC area are maximum. The intensification of TEC disturbances is more pronounced when several cyclones occur simultaneously. We have revealed that the ionospheric response to TC can be observed only after the cyclone has reached typhoon intensity. The ionospheric response is more pronounced at low satellite elevation angles.     

Statistical analysis of the occurrence rate of geomagnetic storms during solar cycles 20–24

This study examines the occurrences rate of geomagnetic storms during the solar cycles (SCs) 20–24. It also investigates the solar sources at SCs 23 and 24. The Disturbed storm time (Dst) and Sunspot Number (SSN) data were used in the study. The study establishes that the magnitude of the rate of occurrences of geomagnetic storms is higher (lower) at the descending phases (minimum phases) of solar cycle. It as well reveals that severe and extreme geomagnetic storms (Dst < -250 nT) seldom occur at low solar activity but at very high solar activity and are mostly associated with coronal mass ejections (CMEs) when occurred. Storms caused by CME + CH-HSSW are more prominent during the descending phase than any other phase of the solar cycle. Solar minimum features more CH-HSSW- associated storms than any other phase. It was also revealed that all high intensity geomagnetic storms (strong, severe and extreme) are mostly associated with CMEs. However, CH-HSSW can occasionally generate strong storms during solar minimum. The results have proven that CMEs are the leading cause of geomagnetic storms at the ascending, maximum and the descending phases of the cycles 23 and 24 followed by CME + CH-HSSW. The results from this study indicate that the rate of occurrence of geomagnetic storms could be predicted in SC phases.     

Ionization effect of solar protons in the Earth atmosphere – Case study of the 20 January 2005 SEP event

The cosmic ray ground level enhancement on January 20, 2005 is among the largest recorded events in the history of cosmic ray measurements. The solar protons of MeV energies cause an excess of ionization in the atmosphere, specifically over polar caps following major solar disturbances. The ionization effect in the Earth atmosphere is obtained for various latitudes on the basis of solar proton energy spectra, reconstructed from GOES 11 measurements and subsequent full Monte Carlo simulation of cosmic ray induced atmospheric cascade. The estimation of ionization rates is based on a numerical model for cosmic ray induced ionization. The evolution of atmospheric cascade is performed with the CORSIKA 6.52 code using FLUKA 2006b and QGSJET II hadron interaction models. The atmospheric ion rate ionization is explicitly obtained for 40°N, 60°N and 80°N latitudes. The time evolution of obtained ion rates is presented. It is demonstrated that ionization effect is negative for 40°N and small for 60°N, because of accompanying Forbush decrease. The ionization effect is significant only in sub-polar and polar atmosphere during the major ground level enhancement of 20 January 2005.     

Global structure,seasonal and interannual variability of the eastward propagating tides seen in the SABER/TIMED temperatures (2002–2007)

The present paper is focused on the global spatial (altitude and latitude) structure, seasonal and interannual variability of the most stable in amplitude and phase eastward propagating diurnal and semidiurnal tides with zonal wavenumbers 2 and 3 derived from the SABER/TIMED temperatures for full 6 years (January 2002–December 2007). The tidal results are obtained by an analysis method where the tides (migrating and nonmigrating) and the planetary waves (zonally travelling, zonally symmetric and stationary) are simultaneously extracted from the satellite data. It has been found that the structures of the eastward propagating diurnal tides with zonal wavenumbers 3 and 2 change from antisymmetric with respect to the equator below ∼85 km height, to more symmetric above ∼95 km. The seasonal behavior of the DE3 is dominated by annual variation with maximum in August–September reaching average (2002–2007) amplitude of ∼15 K, while that of the DE2 by semiannual variation with solstice maxima and with average amplitude of ∼8 K. These tides revealed some interannual variability with a period of quasi-2 years. The seasonal behavior of the eastward propagating semidiurnal tide with zonal wavenumber 2 in the southern hemisphere (SH) is dominated by annual variation with maximum in the austral summer (November–January) while that in the northern hemisphere (NH) by semiannual variation with equinoctial maxima. The SE2 maximizes near 115 km height and at latitude of ∼30° reaching an average amplitude of ∼6 K. The seasonal behavior of the eastward propagating semidiurnal tide with zonal wavenumber 3 in both hemispheres indicates a main maximum during June solstice and a secondary one during December solstice. The tide maximizes near 110–115 km height and at a latitude of ∼30° reaching an average amplitude of ∼4.8 K in the SH and ∼4 K in the NH. The tidal structures of the two eastward propagating semidiurnal tides are predominantly antisymmetric about the equator.     

Comparison of GPS-TEC measurements with NeQuick2 and IRI model predictions in the low latitude East African region during varying solar activity period (1998 and 2008–2015)

This paper examines the performances of NeQuick2, the latest available IRI-2016, IRI-2012 and IRI-2007 models in describing the monthly and seasonal mean total electron content (TEC) over the East African region. This is to gain insight into the success of the various model types and versions at characterizing the ionosphere within the equatorial ionization anomaly. TEC derived from five Global Positioning System (GPS) receivers installed at Addis Ababa (ADD, 5.33°N, 111.99°E Geog.), Asab (ASAB, 8.67°N, 116.44°E Geog.), Ambo (ABOO, 5.43°N, 111.05°E Geog.), Nairobi (RCMN, ?4.48°N, 108.46°E Geog.) and Nazret (NAZR, 4.78°N, 112.43°E Geog.), are compared with the corresponding values computed using those models during varying solar activity period (1998 and 2008–2015). We found that different models describe the equatorial and anomaly region ionosphere best depending on solar cycle, season and geomagnetic activity levels. Our results show that IRI-2016 is the best model (compared to others in terms of discrepancy range) in estimating the monthly mean GPS-TEC at NAZR, ADD and RCMN stations except at ADD during 2008 and 2012. It is also found that IRI-2012 is the best model in estimating the monthly mean TEC at ABOO station in 2014. IRI show better agreement with observations during June solstice for all the years studied at ADD except in 2012 where NeQuick2 better performs. At NAZR, NeQuick2 better performs in estimating seasonal mean GPS-TEC during 2011, while IRI models are best during 2008–2009. Both NeQuick2 and IRI models underestimate measured TEC for all the seasons at ADD in 2010 but overestimate at NAZR in 2009 and RCMN in 2008. The periodic variations of experimental and modeled TEC have been compared with solar and geomagnetic indices at ABOO and ASAB in 2014 and results indicate that the F10.7 and sunspot number as indices of solar activity seriously affects the TEC variations with periods of 16–32?days followed by the geomagnetic activity on shorter timescales (roughly periods of less than 16?days). In this case, NeQuick2 derived TEC shows better agreement with a long term period variations of GPS-TEC, while IRI-2016 and IRI-2007 show better agreement with observations during short term periodic variations. This indicates that the dependence of NeQuick2 derived TEC on F10.7 is seasonal. Hence, we suggest that representation of geomagnetic activity indices is required for better performance over the low latitude region.     

Multi-year behaviour of the midnight OH1 temperature according to observations at Zvenigorod over 2000–2016

Using spectral measurements of the hydroxyl airglow at the Zvenigorod station (56° N, 37° E), Moscow region, over 2000–2016, we obtained the long-term set of data comprising 1822 midnight values of the OH¹ temperature in the mesopause region. These data revealed a 17-year series of its mean annual values, as well as amplitudes and phases of the first two harmonics of its annual variation. The obtained parameters were analyzed to determine statistically relevant characteristics of their long-term variations. As a result, we found that the long-term behaviour of the mean annual OH¹ temperature features a small negative linear trend (?0.07 ± 0.03 K/year) over the addressed period. Besides, its dependence on solar activity is shown to be 4.1 ± 0.5 K/100 SFU. Regarding the long-term behaviour of the mean annual OH¹ temperature, we revealed the existence of two oscillations with 3-year (the amplitude being 1.3 ± 0.2 K) and 4.1-year (the amplitude being 0.6 ± 0.2 K) periods. We obtained empirical relations describing year-to-year variations in the amplitudes and phases of the annual and semi-annual harmonics.     

Study of the influence of solar variability on a regional (Indian) climate: 1901–2007

We use Indian temperature data of more than 100 years to study the influence of solar activity on climate. We study the Sun–climate relationship by averaging solar and climate data at various time scales; decadal, solar activity and solar magnetic cycles. We also consider the minimum and maximum values of sunspot number (SSN) during each solar cycle. This parameter SSN is correlated better with Indian temperature when these data are averaged over solar magnetic polarity epochs (SSN maximum to maximum). Our results indicate that the solar variability may still be contributing to ongoing climate change and suggest for more investigations.     

Advances in the Researches of the Middle and Upper Atmosphere in China in 2012–2014

In this report we summarize the research results by Chinese scientists in 2012–2014. The focuses are placed on the researches of the middle and upper atmosphere, specifically the researches related to ground-based observation capability development, dynamical processes, the property of circulation and chemistry-climate coupling of the middle atmospheric layers.