24th Solar Cycle Research Articles

Trends in hmF2 and the 24th solar activity cycle

The measurements of the F2-layer height, hmF2, at Juliusruh station during 1958–2018 are analyzed by the method developed by the authors earlier and used to determine long-term trends in parameters of the F2 layer. It is found that, in the same way as for the behavior of the critical frequency described by the authors in the previous publication, peculiarities in the hmF2 behavior are observed after 2002–2003. If the solar activity index F10.7 is used, an increase in the ΔhmF2 value is observed and provides a positive trend in hmF2 that contradicts to the available ideas on the continuing process of cooling and contraction of the upper atmosphere. If the F10.7 index corrected by other SA indices (the sunspot number Rz or the Lyman-alpha line intensity) is used, as it has been done for the foF2 trends in the previous publications by the authors, the picture of changes in the hmF2 trends is obtained similarly to the changes in the foF2 trends. A well pronounced negative trend in hmF2 is observed to 2002–2003, and then it is substituted by a chaotic change in ΔhmF2 during the “vague” period, whereas during the last analyzed years it becomes negative again. Analysis of the slope s of the foF2(hmF2) dependence shows that there is a systematic decrease in s with time. That confirms the conclusion of the authors on a decrease in the oxygen amount in the thermosphere.

GPS loss of lock statistics over Brazil during the 24th solar cycle

A statistical analysis of Loss of Lock (LoL) over Brazil throughout the 24th solar cycle is performed. Four geodetic GPS dual-frequency (L1, L2) receivers, deployed at different geographic latitudes ranging from about 25° to 2° South in the eastern part of the country, are used to investigate the LoL dependence on time of the day, season, solar and geomagnetic activity. The results of the analysis show that LoL is most likely in the post-sunset hours during summer and equinox, especially within the southern crest of the Equatorial Ionospheric Anomaly (EIA), in a region between about 10°S and 25°S of geographic latitude, matching the typical behaviour of scintillation over Brazil. This is confirmed by the correlation found between the relative occurrence of LoL (LoL (%)) and the Rate Of TEC Index (ROTI), used as a proxy of scintillation index and calculated for each receivers along the entire period of investigation. The LoL (%) for given solar and geomagnetic indices show some correlation with increasing the severity of the index. This correlation is strongest in the area of the southern crest of the EIA, while there is little to no apparent impact closer to the equator, depending on the index. LoL (%) increases with increasing geomagnetic disturbances, varying between ~1% and ~10% for AE ranged between 400 and 1200 nT, and exceeding 3% when Dst is around −100 nT, both related to moderate-severely disturbed conditions.

The Comprehensive Study of Low Thermospheric Sodium Layers during the 24th Solar Cycle

The low thermospheric sodium layer (LTSL) is the separate sodium atom layer above 105 km. Based on 11,607 h of lidar observations from Yanqing (40.5° N, 116.0° E) from 2010 to 2016, we found 38 LTSLs wherein the peak densities were more than five percent above those of the main sodium layers. This work presents the peak altitudes, peak local times and peak densities of the LTSLs as well as the long-term characteristics of the seasonal and inter-annual variations of LTSLs. We analyzed the correlation between the LTSL and sporadic E layer (Es). The seasonal variation trends of the occurrences of LTSL and Es are similar, and the results showed that 95% of the LTSLs were accompanied by Es. We also found that 69% of the LTSL cases exhibited apparent downward phase progressions, while the descending rates of the LTSLs are consistent with the phase speeds of the tide.

Ionospheric response to the June 2015 geomagnetic storm in the South American region

The ionospheric dynamics in the South America (SA) sector during geomagnetic disturbed period from 21 to 24 June 2015 is investigated through ground ionosonde stations and Global Navigation Satellite System (GNSS) receivers, supported by Very Low Frequency (VLF) and magnetometer data. These disturbances were caused by 3 interplanetary shocks (IS) derived from 3 consecutives coronal mass ejections (CME) from the same solar active region; the first two CME were caused by filament eruptions, and the third was a much larger full halo CME, associated with a M2.6 solar flare. The first 2 shocks were compressive and did not cause an immediate response to the ionosphere in the analyzed region, while the third shock increased considerably the electron density from low to high-latitudes, triggering the second strongest geomagnetic storm of the 24th solar cycle. It was possible to observe the expansion of the crest of equatorial ionospheric anomaly (EIA) at midlatitudes and high-latitudes mainly due to prompt penetration electric field (PPEF) during the main phase and the recovery phase of the geomagnetic storm during the day.

Extreme Field-Aligned Currents during Magnetic Storms of the 24th Solar Cycle: March 2015 and September 2017

Characteristics of field-aligned currents (FACs) obtained from the observations of SWARM satellites during two strong magnetic storms in March 16–19, 2015 and September 6–9, 2017 are presented. The storms were accompanied by high-intensity substorms. Satellites crossed the dusk, noon, pre- and after midnight MLT sectors. It was shown that variations in FAC density and the latitudinal position of the equatorial FAC boundaries are controlled by both the development of the storm and, to a considerable degree, substorm activity. The average densities of downward and upward FACs reach 3–4 μA/m2 at a substorm’s peak, while the undisturbed level is about 0.2 μA/m2. The minimum latitude of equatorial FAC boundaries is limited to 50° MLat. Large-scale FACs consist of small-scale filamentary structures with high current density, which are always present in SWARM observations. Small-scale peak currents with a density of 50–100 µA/m2 appear during periods of a general increase in density during activation of substorms. Local increases in concentration and temperature of electrons show that the current structure can be associated with a mesoscale discrete auroral arc.

Coronal Holes during the Period of Maximum Asymmetry in the 24th Solar Activity Cycle

The current 24th solar activity cycle differs substantially from previous cycles in a number of parameters, especially in terms of a large asymmetry in the number of sunspots (Sp) during the second peak of its maximum. From March 2013 through December 2015. a significant predominance was observed in the number of sunspots in the southern hemisphere. The main purpose of this paper is to clarify the behavior of coronal holes (CH) during this period. This study is based on an analysis of data recorded by the 19.3 nm channel of the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO). Two methods of detecting CH (simplified visual and the Spatial Possibilistic Clustering Algorithm (SPoCA)) are used to obtain time series of daily total CH areas for the Sun’s northern and southern hemispheres. The two methods agree on the estimated areas of the CH. A comparison of the observed variations in the areas of the CH with the numbers and areas of sunspots showed that in this period, activity predominates in the S-hemisphere both in terms of sunspots and in terms of the total CH area. Here a rise in the area of the CH follows a rise in sunspot activity by roughly half a year. It is suggested that the CH and spots are associated elements of the overall magnetic activity of the Sun, in qualitative agreement with studies of activity complexes. A dipole poloidal field in the form of open CH fields and a toroidal field in the form of active regions are related on time scales substantially shorter than the solar cycle.

Long-term observation of magnetic pulsations through the ELF Hylaty station located in the Bieszczady Mountains (south–eastern Poland)

Ground-based measurements of ultra- and extremely low-frequency waves (ULF/ELF) carried out in 2005–2016 (the 23rd and 24th solar cycle) at the ELF Hylaty station in Bieszczady Mountains (south–eastern Poland) were used to identify the days (360 days) in which magnetic pulsation events (MPEs) occurred. To reveal sources of MPEs at the Sun we considered their correlation with the basic indices describing solar activity. Our analysis, like earlier studies, did not reveal a significant positive correlation between the MPE detection rate and the sunspot numbers (SSN). On the other hand, we showed that MPEs are strongly correlated (correlation coefficient ≈0.70) with moderate (Dst < −70 nT) and intense (Dst < −100 nT) geomagnetic disturbances expressed by the Disturbance Storm Index (Dst). We recognized all sources of these geomagnetic storms associated with the considered MPEs. Only 44% of the MPEs were associated with storms caused by CMEs listed in the CDAW LASCO CME catalog. 56% of the MPEs were associated with storms caused by other phenomena including corotating interaction regions (CIRs), slow solar wind or CMEs not detected by LASCO. We also demonstrated that the CMEs associated with the MPEs were very energetic, i.e. they were extremely wide (partial and halo events) and fast (with the average speed above 1100 km s−1). CMEs and CIRs generally appear in different phases of solar cycles. Because MPEs are strongly related to both of these phenomena they cannot be associated with any phase of a solar cycle or with any indicator characterizing a 11-year solar activity. We also suggested that the low number of MPEs associated with CMEs is due to the anomalous 24 solar cycle. During this cycle, due to low density of the interplanetary medium, CMEs could easily eject and expand, but they were not geoeffective.

Modelling the solar, geomagnetic and periodic variations of the ionosphere over Ethiopia

A careful and continuous ionospheric modelling can significantly influence the performance of activities such as Positioning, Navigation and Timing services related with the Global Navigation Satellite System applications as well as the Earth Observations System, satellite communication and Space weather forecasting applications. In this paper, the linear time-series modelling that consists of the solar, geomagnetic and periodic components has been carried out on the daily ionospheric vTEC at two different Ethiopian GPS locations, at Arbaminch, ARMI (geographic 6.06ºN, 37.56ºE) and Bahir Dar, BDMT (geographic 11.60ºN, 37.38ºE), for the year 2012, 2014 and 2016 in the 24th solar cycle. The variations of vTEC due to the solar activities, geomagnetic activities and periodic oscillations have been explicitly investigated. The results confirmed that the correlation coefficient of the linear model based estimated vTEC and the observed GPS-vTEC is around 80% in the year 2014. Besides, solar activity is identified as the key component for the 27 days period variations of vTEC whereas geomagnetic activity is identified as the key component that influences the short-period variations of the daily average vTEC. In addition to the correlation analysis, the accuracy of the model has been assessed by comparing the International Reference Ionosphere (IRI 2016) model based vTEC and GPS-vTEC measurements as well as with the quadratic model based vTEC. Consequently, the linear model formulated with the solar, geomagnetic and periodic components significantly captured the variations (78-80%) of the observed vTEC compared with both the IRI 2016 and the quadratic models during the years 2012, 2014 and 2016. The comparison of the observed and predicted vTEC variations has also been examined using the continuous wavelet transform. The decomposed waves from the wavelet analysis have revealed that the predicted and observed vTEC have had simultaneous periods of variations specifically with the period of 27 days whereas the IRI 2016 could capture the short-period variations of vTEC. Moreover, the analysis from the transformed data in the year 2014 over both Arbaminch and Bahir Dar has indicated that the linear model based vTEC and the observed GPS-vTEC have had common pattern of variations with the period of 27 days that had lasted for 150 days (from day of the year 100 to 250).

Solar Activity Indices for Ionospheric Parameters in the 23rd and 24th Cycles

Characteristics of changes in the solar activity indices (the flux of the solar radio emission at a wavelength of 10.7 cm F and the new version of the relative sunspot number Ri) and the ionospheric index of this activity T in the 23rd and 24th solar cycles, which were low by the amplitude of solar and geomagnetic activity, are analyzed. The running mean over 12 months and smoothed (by a 24-month filter) values of these indices are considered in the analysis. It is found that the relation between the T and F indices was on average stable for these cycles and did not differ from the previous solar cycles. The relation between the T and Ri indices changed with time in the 23rd and 24th cycles and was different from the previous cycles. Moreover, a distinct hysteresis effect in the dependence of the smoothed values of T on Ri was observed in the 24th cycle when different values of T corresponded to a fixed value of Ri in the growing and declining phases of the solar cycle. This effect was absent in the dependence of T on F. Thus, it is confirmed that the F index is a more exact indicator of solar activity for the ionosphere than Ri.

Response of IAR frequency scale to solar and geomagnetic activity in solar cycle 24

The ionospheric Alfven resonator (IAR) is an integral element of the entire ionosphere-magnetosphere system. It plays an essential role in energy exchange and interaction between the magnetosphere and ionosphere. The parameters of this resonator reflect the state of the ionosphere. The purpose of this study was to study three types of IAR frequency modulation (daily, seasonal and solar-cyclic) and to identify their relationship with solar and magnetic activity. We used the results of magnetic observations of the IAR emission at the Mondy mid-latitude observatory for the 24th solar activity cycle from 2009 to 2019. The dependence of the difference in the neighboring harmonics frequencies (i.e., the emission frequency scale) on solar activity (the sunspot number) and on the magnetic indices Kp, Dst and AE was studied. The correlation between the frequency scale and the indices of solar and magnetic activity was investigated at different time scales by comparing the variations in daily, monthly, and annual averages. The dependence of the IAR frequency scale on solar activity turns out to be much closer than the same dependence of any of the three magnetic indices. The closest relationship is exhibited between the annual average values of the frequency scale, on the one hand, and the sunspot number and the Dst index, on the other. This result is interpreted as a demonstration of the cumulative effect of solar activity on the state of the ionosphere and, first of all, on the electron concentration in the F2 region of the ionosphere.

Single-Frequency Time-Step Ionospheric Delay Gradient Estimation at Low-Latitude Stations

The irregularity of the local-area ionospheric delay is a primary impediment for Ground-Based Augmentation System (GBAS) services. Excessive ionospheric delay gradients may degrade aircraft positioning for high precision landing systems. Therefore, the spatial gradients of the nominal background ionosphere must be studied as their statistics will be sent to the approaching aircraft. For the well-known station-pair method, ionospheric delay gradient estimation requires at least 2 Global Navigation Satellite System (GNSS) reference stations. This method can be applied to both single or dual-frequency GNSS receivers. However, when the GNSS stations are far apart, it is not suitable for estimating the ionospheric delay gradients at short baselines, and the time-step method is an attractive alternative. In this work, we propose a single-frequency time-step method for ionospheric delay gradient estimation. Careful baseline length selection is needed, due to ionospheric piercing point movements. We applied our method to GNSS data in 2014, at the peak of the 24th solar cycle, and showed that the standard deviations of the vertical ionospheric delay gradients were comparable to those derived from the dual-frequency time-step method. The standard deviations of vertical ionospheric gradients, $\sigma _{\mathrm {VIG}}$ , ranged between 4 and 6 mm/km. The $\sigma _{\mathrm {VIG}}$ values around the equinoxes were ~1.5 mm/km greater than at other times.

Performance evaluation of neural network TEC forecasting models over equatorial low-latitude Indian GNSS station

Global Positioning System (GPS) services could be improved through prediction of ionospheric delays for satellite-based radio signals. With respect to latitude, longitude, local time, season, solar cycle and geomagnetic activity the Total Electron Content (TEC) have significant variations in both time and space. These temporal and spatial TEC variations driven by interplanetary space weather conditions such as solar and geomagnetic activities can degrade the communication and navigation links of GPS. Hence, in this paper, performance of TEC forecasting models based on Neural Networks (NN) have been evaluated to forecast (1-h ahead) ionospheric TEC over equatorial low latitude Bengaluru (12.97∘N,77.59∘E), Global Navigation Satellite System (GNSS) station, India. The VTEC data is collected for 2009–2016 (8 years) during current 24th solar cycle. The input space for the NN models comprise the solar Extreme UV flux, F10.7 proxy, a geomagnetic planetary A index (AP) index, sunspot number (SSN), disturbance storm time (DST) index, solar wind speed (Vsw), solar wind proton density (Np), Interplanetary Magnetic Field (IMF Bz). The performance of NN based TEC forecast models and International Reference Ionosphere, IRI-2016 global TEC model has evaluated during testing period, 2016. The NN based model driven by all the inputs, which is a NN unified model (NNunq) has shown better accuracy with Mean Absolute Error (MAE) of 3.15 TECU, Mean Square Deviation (MSD) of 16.8 and Mean Absolute Percentage Error (MAPE) of 19.8% and is 1–25% more accurate than the other NN based TEC forecast models (NN1, NN2 and NN3) and IRI-2016 model. NNunq model has less Root Mean Square Error (RMSE) value 3.8 TECU and highest goodness-of-fit (R2) with 0.85. The experimental results imply that NNunq/NN1 model forecasts ionospheric TEC accurately across equatorial low-latitude GNSS station and IRI-2016 model performance is necessarily improved as its forecast accuracy is limited to 69–70%.

Oscillations of the Microwave Emission of Solar Active Region 12673 before Flares

We present a study of quasi-periodic pulsations in the microwave emission from solar active region (AR) NOAA 12673, which produced the strongest flares of the 24th solar activity cycle. The data from daily observations of the Sun at the Nobeyama Radioheliograph at a frequency of 17 GHz were used. The microwave emission of the AR was analyzed for September 3–4, 2017, i.e., before the first M-class flare, which occurred on September 4. Long-period pulsations in the microwave emission with periods of approximately 100 min appeared at least 7 h before the first M-class flare, which was followed by strong flare activity in NOAA 12673. The effect is similar to the previously discovered preflare oscillatory phenomena with different periods manifested in different ranges.

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.

Temperature and pressure variability in mid-latitude low atmosphere and stratosphere-ionosphere coupling

This study presents the continuation of our previous analysis of variations of atmospheric and space weather parameters above Iberian Peninsula along two years near the 24th solar cycle maximum. In the previous paper (Morozova et al., 2017) we mainly discussed the first mode of principal component analysis of tropospheric and lower stratospheric temperature and pressure fields, which was shown to be correlated with lower stratospheric ozone and anti-correlated with cosmic ray flux. Now we extend the investigation to the second mode, which suggests a coupling between the stratosphere and the ionosphere.This second mode, located in the low and middle stratosphere (and explaining ~7% of temperature and ~3% of geopotential height variations), showed to be statistically significantly correlated with variations of the middle stratosphere ozone content and anti-correlated with variations of ionospheric total electron content. Similar co-variability of these stratospheric and ionospheric parameters was also obtained with the wavelet cross-coherence analysis.To investigate the role of atmospheric circulation dynamics and the causal nature of the found correlations, we applied the convergent cross mapping (CCM) analysis to our series. Strong evidence for the stratosphere-ionosphere coupling were obtained for the winter 2012–2013 that is characterized by the easterly QBO phase (quasi-biennial oscillations of the direction of the stratospheric zonal winds) and a strong SSW (sudden stratospheric warming event). Further analysis (for the three-year time interval 2012–2015) hint that SSWs events play main role in emphasizing the stratosphere-ionosphere coupling.

The Magnetic Storm of August 25–26, 2018: Dayside High Latitude Geomagnetic Variations and Pulsations

The features of daytime high latitude geomagnetic disturbances and geomagnetic pulsations during the recent strong magnetic storm on August 25–26, 2018, which occurred at the end of the decline phase of the 24th solar activity cycle with a very low level of solar flare activity, are considered. As a rule, during this phase of the solar activity cycle, magnetic storms are caused by high-speed solar wind flows from coronal holes; however, the magnetic storms during the decay of the 24th cycle were caused by coronal mass ejections (CMEs). It was shown that, despite very weak disturbances on the Sun and a low solar wind speed, a rather strong magnetic storm occurred in the Earth’s magnetosphere in August 2018 (Dst = –171 nT). The storm SC with a slight (~25 nT) jump in the Dst index caused quite intense daytime geomagnetic pulsations ipcl at the latitudes of the possible position of the daytime polar cusp. A feature of the recovery phase of this storm was the development of a magnetosphere substorm on a global scale, i.e., the appearance of a negative magnetic bay recorded simultaneously in the auroral night sector and in the polar latitudes of the day sector. A possible interpretation is considered.

Systematics of $K$ index in the geomagnetic field

In this paper we study the systematics of $K$ index in fourteen Chinese stations in the 2009–2017 time interval. We find that the frequency distribution of $K$ index ($K=0\mbox{--}9$) follows a Gaussian function with $K$ values, while its rank-ordered frequency distribution follows a modified power function of the rank. We investigate empirical correlation between the annual frequency of $K \geq 4$ (denoted by $M$) and that of geomagnetic storm (denoted by $m$), and predict ($M$, $m$) values in 2018 and 2019, respectively. We find that $M$ follows a mixture of several Gaussian functions of year, and predict the 24th solar cycle length to be 11.4 years, which is consistent with the prediction of the auto-regressive processes (AR) model.

Lyman Decrements of Neutral Hydrogen Lines in the Spectrum of the Sun Based on SDO/EVE Observations. Variations During the 24-th Solar Cycle and in Individual Flares of Classes M and X

Data from satellite observations of fluxes in Lyman series lines and their relative variations, both during the 24-th cycle and in individual class M and X flares, are analyzed. The decrement D1 (D1 = F(Lα)/F(Lβ)) has varied over the 24-th activity cycle by roughly 25% from its maximum values to minimum values during 2014, when there was a secondary maximum in the 24-th cycle according to the main indices. Decrement D2 (D2 = F(Lγ)/F(Lβ)) varies by roughly 10% during the cycle, and decrement D3 (D3 = F(Lδ)/F(Lβ)), by roughly 5% during the cycle. D1, D2, and D3 also vary during the cycle at smaller amplitudes with periods analogous to the periods of the quasi-biennial variations of the main activity indices. In flares of x-ray classes from M to X the variations of the Lyman decrements range from 2 to 20%, with the variations in the decrements being larger on the average for flares with higher energies in the soft x-ray range.

On the radiation belt location during the 23rd and 24th solar cycles

Abstract. Within the last two solar cycles (from 2001 to 2018), the location of the outer radiation belt (ORB) was determined using NOAA/Polar-orbiting Operational Environmental Satellite (POES) observations of energetic electrons with energies above 30 keV. It was found that the ORB was shifted a little (∼1∘) in the European and North American sectors, while in the Siberian sector the ORB was displaced equatorward by more than 3∘. The displacements corresponded qualitatively to the change in the geomagnetic field predicted by the IGRF-12 model. However, in the Siberian sector, the model has a tendency to underestimate the equatorward shift of the ORB. The shift became prominent after 2012, which might have been related to a geomagnetic “jerk” that occurred in 2012–2013. The displacement of the ORB to lower latitudes in the Siberian sector can contribute to an increase in the occurrence rate of midlatitude auroras observed in the Eastern Hemisphere.

Suzaku observation of Jupiter’s X-rays around solar maximum

Abstract We report on results of imaging and spectral studies of X-ray emission from Jupiter observed by Suzaku. In 2006, Suzaku found diffuse X-ray emission in 1–5 keV associated with Jovian inner radiation belts. It has been suggested that the emission is caused by the inverse-Compton scattering by ultra-relativistic electrons (∼50 MeV) in Jupiter’s magnetosphere. To confirm the existence of this emission and to understand its relation to the solar activity, we conducted an additional Suzaku observation in 2014 around the maximum of the 24th solar cycle. As a result, we successfully found the diffuse emission around Jupiter in 1–5 keV again, and also found point-like emission in 0.4–1 keV. The luminosity of the point-like emission, which was probably composed of solar X-ray scattering, charge exchange, or auroral bremsstrahlung emission, increased by a factor of ∼5 with respect to the findings from 2006, most likely due to an increase of the solar activity. The diffuse emission spectrum in the 1–5 keV band was well-fitted with a flat power-law function (Γ = 1.4 ± 0.1) as in the past observation, which supported the inverse-Compton scattering hypothesis. However, its spatial distribution changed from ∼12 × 4 Jovian radius (Rj) to ∼20 × 7 Rj. The luminosity of the diffuse emission increased by the smaller factor of ∼3. This indicates that the diffuse emission is not simply responding to the solar activity, which is also known to cause little effect on the distribution of high-energy electrons around Jupiter. Further sensitive study of the spatial and spectral distributions of the diffuse hard X-ray emission is important to understand how high-energy particles are accelerated in Jupiter’s magnetosphere.