Solar Minimum Years Research Articles

Migrating and non-migrating tidal influences on the high occurrence of post-midnight spread F over Ascension Island during solar minimum

The digisonde observations over Ascension Island (8°S, 14°W, dip latitude: 16°S) reveal that the occurrence of post-midnight spread F (or more appropriately delayed spread F) is more when compared to that of the post-sunset spread F during solar minimum years. While the monthly variations of the percentage of occurrence (PO) of post-sunset spread F (18:00–22:00) do not now show any consistent seasonal variation, the PO of the post-midnight spread F reveals a seasonal variation with a high PO during austral summer (November-February) and minimum PO during austral winter (May-August) and it shows a large interannual variability. The high PO during September 2019 may be associated with the austral sudden stratospheric warming (SSW). In the year 2020, the PO is higher (>60%) in November-February. When the PO of the post-midnight spread F is larger, there is a strong semi-diurnal variation of peak electron density height of F-layer (hmF2) with a maximum height at noon and midnight hours, suggesting a possible semi-diurnal tidal influence. It is also found that the occurrence of spread F just after sunset hours is less over Ascension Island and it does not show any clear seasonal variation. The space-time spectral analysis of SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) of TIMED (Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics) satellite reveals the dominant presence of migrating semi-diurnal tide (SW2) at 10°N during January-February 2020. During November-December 2020, though the SW2 tidal amplitudes are weaker, there is a dominant presence of the eastward propagating non-migrating diurnal (DE3) tide. The thermospheric winds measured by MIGHTI (Michelson Interferometer for Global High-resolution Thermospheric Imaging) instrument of ICON (Ionospheric Connection Explorer) mission show equatorward winds in the late evening hours during November-February 2020. The migrating diurnal tide (DW1) dominates in MIGHTI zonal winds at F-region heights, whereas SW2, as well as the eastward propagating semi-diurnal tide with zonal wavenumber 2 (SE2), tidal amplitudes are comparable to DW1 in the meridional wind. It is suggested that the semi-diurnal variation in the meridional winds turning the winds equatorward during late evening hours can cause eastward electric field that can lift the F layer to higher heights and thereby favouring the Rayleigh-Taylor Instability (RTI) to act, which can cause the post-midnight spread F. It is also shown that the growth rate can be sufficiently positive around midnight hours to cause the RTI to act.

On the high percentage of occurrence of type-B 150-km echoes during the year 2019 and its relationship with mesospheric semi-diurnal tide and stratospheric ozone

• Anomalously high occurrence of 150-km radar echoes over Kototabang during September 2019. • In the years 2017 and 2019, SW2 dominates DW1 due to the larger stratospheric ozone, when compared to other years. • Dominant SW2 along with solar minimum conditions may lead to large echo occurrence through interchange instability. The Kototabang (0.20°S, 100.3°E, 10.36°S dip latitude) Equatorial Atmosphere Radar (EAR) observations of valley region field-aligned irregularities (FAI) reveal maximum percentage of occurrence (PO) of high signal to noise ratio (greater than 3 dB) type-B 150-km echoes during boreal summer (June-August) and winter (December-January) of solar moderate to minimum years 2016–2019. The PO is anomalously high in the solar minimum year 2019 and particularly during the September equinox when a major austral sudden stratospheric warming (SSW) event has occurred. Its possible relation with mesospheric tides and stratospheric ozone is investigated. The space–time spectral analysis of upper mesospheric temperature information obtained from the spaceborne radiometer instrument Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) reveals that migrating semi-diurnal tide (SW2) is dominant during June-August, when the PO of the 150-km echoes maximizes, whereas migrating diurnal tide (DW1) is dominant during equinox months. It also reveals quasi-biennial variability of the DW1 tide probably due to similar variability of the stratospheric ozone. It is suggested that the DW1 tides generated due to solar radiation absorption by stratospheric ozone and tropospheric water vapour can have phases opposite to each other, leading to the suppression of DW1 tide during September 2017 and 2019 resulting in the relative dominance of SW2 tide over DW1 tide. The meridional wind shear associated with the relatively dominant SW2 tide results in an interchange instability developed on the gradient of daytime descending ion layer along with solar minimum conditions leads to plasma irregularities responsible for these echoes. The large eastward propagating diurnal tide with zonal wavenumber 3 (DE3) observed during June-September 2019 is unlikely to cause the instability, due to its weak meridional component.

Lunar Tidal Effect on Equatorial Ionization Anomaly Region in China Low Latitude

AbstractThe equatorial ionization anomaly (EIA) crest derived from GPS observations, F2‐layer peak height (hmF2), critical frequency (foF2), and equatorial electrojet (EEJ) in China low latitude are used to study the lunar tidal effect on EIA region for the years from 2006 to 2015. A predominant 14.76‐day periodic component in EIA crest, hmF2, and EEJ concurrently appears in some seasonal intervals, which coincides with the half of the lunar revolution period (29.53 days) and the lunar phase, indicating that the 14.76‐day periodic oscillation in EIA region is modulated by the lunar tide. This 14.76‐day periodic oscillation occurs in all Northern Hemisphere (NH) winter and corresponds to their respective sudden stratospheric warming (SSW) period from 2006 to 2015. In addition, this 14.76‐day periodic oscillation also occurs in other seasons for some years, for example, May and August, in which wave power is sometimes comparable to that in SSW period. The occurrence of the 14.76‐day periodic oscillation and its wave power during non‐SSW seasons in the solar maximum years (2012–2015) is more frequent and higher than that in the solar minimum years (2008–2010). Our results suggest that, besides the NH SSW condition, the solar activity and some other unclear factors are also responsible for the lunar tide enhancement in EIA region.

Anomalous Increase in the Occurrence of Post‐Midnight FAI Radar Echoes in September 2019 and Its Relation With the Austral Sudden Stratospheric Warming

AbstractThe F‐region observations of field aligned irregularities (FAI) by the Equatorial Atmosphere Radar (EAR) at Kototabang (0.2°S, 100.3°E, 10.36°S dip latitude) show maximum percentage of occurrence (PO) of post‐midnight echoes due to spread F during boreal summer of solar minimum years. However, the PO is observed to be anomalously high during September 2019, when compared to the similar solar minimum year 2018 even after removing geomagnetically disturbed days indicating the lower atmospheric dynamical forcing. Coincidently, an austral polar sudden stratospheric warming (SSW) event has occurred during September 2019, which precedes large planetary wave activity. Due to the circulation changes associated with the SSW, though the accumulation of ozone over the equatorial stratosphere is inferred from the space‐borne Sounding of the Atmosphere by Broadband Emission Radiometry (SABER) measurements, the expected enhancement in the amplitude of the upper mesospheric migrating semi‐diurnal tide (SW2) is not observed in SABER temperature. But there is a drastic decrease in the amplitude of the migrating diurnal tide (DW1). The enhancement of stationary diurnal tide (DS0) suggests that the DW1 may have interacted with the planetary wave of zonal wavenumber 1 to generate DS0 at the expense of the DW1 tide. It is suggested that the relative increase of SW2 over DW1 leads to the dominant semi‐diurnal variation of the zonal electric field, which can become eastward at a later time as observed in the ionosonde measurements and lift the F‐layer to higher heights creating favorable conditions for the growth of Rayleigh‐Taylor instability to generate spread F around midnight.

Ionospheric variability during quiet and disturb geomagnetic conditions for low and high solar activity year

Present Paper describes analysis of GPS-data recorded at low latitude station Hyderabad (Geographic latitude 17 $$^\circ $$ , 25′ N, longitude 78 $$^\circ $$ , 33′ E), India to study the effect of magnetic activity on ionospheric TEC. The total electron content (TEC) data were considered for the ionosphere during five most quiet and most disturbed days for each month of the solar minimum year 2009 and the high solar activity year 2013. The results show that GPS-TEC is found to be more during disturbed days than that during quiet days with a maximum difference during the equinox season. In addition, the difference in TEC during quiet and disturbed days is found to be higher for solar maximum than that for solar minimum year. Moreover the GPS-TEC variations depend on the season with the maximum in equinox season and minimum in summer and winter season. Generally daytime TEC is found to be larger than that in the nighttime which is maximized around the local noontime hour but the peak formation delayed for the solar minimum year. The EIA development process during the solar minimum is quite slow and occurs in late afternoon as compared to that during the high solar activity year. In addition, this delay is larger for summer and winter season than that for equinox months. The formation of post sunset anomaly in the post-sunset sector (1900–2300 LT) is prominent during high solar activity year 2013 whereas same is insignificant for solar minimum year 2009. The post-sunset anomaly could be attributed to pre-reversal enhancement (PRE) of zonal electric field.

A comparative study of the northern and southern equatorial ionization anomaly crests in the East-Asian sector during 2006–2015

The GPS observation data measured at GXUN (22.84°N, 108.33°E, MLAT ~ 12.14°N) and BAKO (6.49°S, 106.84°E, MLAT ~ 17.18°S) stations from 2006 to 2015 are utilized to make a comparative study of the northern and southern equatorial ionization anomaly (EIA) crests in the East-Asian sector. The intensity of northern EIA crest is similar to that of southern EIA crest during the solar minimum years, while during the solar maximum years, the intensity of northern EIA crest is a little stronger than that of southern EIA crest except in summer season of northern hemisphere. The geomagnetic latitude (MLAT) of northern EIA crest shows a weak semiannual variation with the highest in equinoctial months and the lowest in solistical months, but the MLAT of southern EIA crest shows a clear annual variation with the highest in summer season of southern hemisphere and the lowest in winter season of southern hemisphere, which is particularly obvious during the solar minimum years. The intensities of northern and southern EIA crests have good correlation, which is independent of the solar activity, but the correlation coefficients of MLAT between northern and southern EIA crests increase with the solar activity. A comprehensive analysis of the previous studies and our results indicate that the intensity, MLAT, and interhemispheric asymmetry of EIA crest are mainly dominated by photochemical process, equatorial fountain effect, and neutral wind, respectively.

Seasonal analysis of Klobuchar and NeQuick G single-frequency ionospheric models performance in 2018

For single frequency positioning, the ionospheric effects can be minimized by using an ionospheric model, e.g., the Klobuchar or the NeQuick G. These models, respectively associated with the Global Positioning System (GPS) and Galileo systems, are based on a set of transmitted coefficients that describe the background ionospheric behavior and can be used to estimate the ionospheric delay at any time and location on the globe. We evaluate the accuracy and seasonal behavior of the Klobuchar and the NeQuick G models at different latitudes during the solar minimum year of 2018. Calibrated ionospheric Slant Total Electron Content (STEC) values from 33 MGEX (The Multi-GNSS Experiment) stations tracking GPS and Galileo satellites are used as a reference for comparison. The evaluation results show that, for low solar activity conditions (Winter Solstice), the Klobuchar model usually overestimates the ionospheric delay measured on the L1 frequency, with a mean bias of 0.93 m, and performs better than NeQuick G in regions and periods of high ionospheric activity (Spring and Autumn Equinoxes). NeQuick G usually underestimates the delay, with a mean bias of −0.14 m, performing better than Klobuchar in regions and periods of low ionospheric activity. Overall, Klobuchar and NeQuick G presented a modelling error RMS of 1.54 m and 1.19 m respectively.

Modeling Australian TEC Maps Using Long-Term Observations of Australian Regional GPS Network by Artificial Neural Network-Aided Spherical Cap Harmonic Analysis Approach

The global ionosphere map (GIM) is not capable of serving precise positioning and navigation for single frequency receivers in Australia due to sparse International GNSS Service (IGS) stations located in the vast land. This study proposes an approach to represent Australian total electron content (TEC) using the spherical cap harmonic analysis (SCHA) and artificial neural network (ANN). The new Australian TEC maps are released with an interval of 15 min for longitude and latitude in 0.5° × 0.5°. The validation results show that the Australian Ionospheric Maps (AIMs) well represent the hourly and seasonally ionospheric electrodynamic features over the Australian continent; the accuracy of the AIMs improves remarkably compared to the GIM and the model built only by the SCHA. The residual of the AIM is inversely proportional to the level of solar radiation. During the equinoxes and solstices in a solar minimum year, the residuals are 2.16, 1.57, 1.68, and 1.98 total electron content units (TECUs, 1 TECU = 1016 electron/m2), respectively. Furthermore, the AIM has a strong capability in capturing the adequate electrodynamic evolutions of the traveling ionospheric disturbances under severe geomagnetic storms. The results demonstrate that the ANN-aided SCHA method is an effective approach for mapping and investigating the TEC maps over Australia.

An investigation of mid-latitude ionospheric peak in TEC using the TIEGCM

A local maximum value of ionospheric total electron content (TEC) in midlatitude regions, named midlatitude ionospheric peak in this study, has been frequently observed during both the daytime and nighttime. We have carried out a modeling study of midlatitude ionospheric peaks in TEC using the Thermosphere Ionosphere Electrodynamic General Circulation Model (TIEGCM) to investigate possible mechanisms driving these peaks. Comparisons between model results and Global Position System (GPS) TEC observations in the solar minimum year of 2009 show that TIEGCM can reproduce the nighttime and daytime midlatitude ionospheric peaks. Model results indicate that midlatitude ionospheric peaks in the daytime and nighttime are likely caused by different mechanisms. Nighttime midlatitude ionospheric peaks are attributed to the downwards-moving plasma ambipolar diffusive flux from the plasmasphere. However, the daytime upwards-moving plasma ambipolar diffusive flux and poleward meridional winds, which can cause ionospheric depletion, might play dominated roles in producing midlatitude ionospheric peaks in the early morning in the winter season.

Role of Bottom‐Side Density Gradient in the Development of Equatorial Plasma Bubble/Spread F Irregularities: Solar Minimum and Maximum Conditions

AbstractFrom the analysis of Digisonde data over Brazilian equatorial and low‐latitude sites, we investigate the relative importance of the different parameters driving the generation of rising bubble‐type and bottom‐type spread F (SF) irregularities. Data for the complete month of October 2001, a solar maximum epoch (F10.7 = 210), and that of October 2008, an extended solar minimum period (F10.7 = 70), are analyzed to examine the SF intensity and occurrence rate as a function of the evening prereversal vertical drift velocity and the corresponding F layer heights and the bottom‐side density gradient. While the SF at the equatorial site is indicative of both the bottom‐side irregularities and rising bubbles, the SF at the low latitude represents exclusively the latter. Comparison of the results, from the two epochs, reveals a large decrease in the intensity and occurrence rate of plasma bubbles, with a decrease in solar flux. But a notable increase in these characteristics is observed in the case of bottom‐side SF. It is found that a larger (steeper) density gradient of the F layer bottom side that exists in the low solar flux condition is responsible for an enhanced Raleigh‐Taylor instability growth, counterbalancing a reduction in this rate that may arise from a smaller prereversal vertical drift and lower layer height that also characterize the low solar flux condition. Thus, the role of the bottom‐side density gradient in the ESF instability growth has been identified for the first time in terms of its ability to explain the contrasting irregularity features as observed during solar flux maximum and minimum years.

New observations of the total electron content and ionospheric scintillations over Ho Chi Minh City

In January 2018, a Trimble NetR9 GNSS receiver was installed at International University - Vietnam National University (IU-VNU), which is located at 10°52'N, 106°48'E in Ho Chi Minh City (HCMC). The GNSS signals recorded from the receiver are useful for studying the ionospheric variations over this station as well as the magnetosphere-ionosphere coupling effects, therefore, we aim to preliminarily process and evaluate data recorded from this new station. Based on the data obtained with this GNSS receiver, we first estimated the total electron content (TEC) using the carrier-phase method which is a combination of code and phase measurements. We then calculated the rate of change of TEC index (ROTI) with respect to time and investigated its day-to-day variations. Our results show some typical features in the diurnal and seasonal TEC and ionospheric scintillation variations during 2018-2019. The distributions of ROTI over these two years of solar minimum show significant occurrences of scintillation, which are caused by small-scale ionospheric irregularities in the equatorial ionosphere. In addition, we found a significant increase of TEC in the latest strong geomagnetic storm in August 2018. The disturbance dynamo appears to have suppressed plasma bubbles after sunset and enhanced their formation at midnight. Thus, the disturbance dynamo effectively caused a delay of ionospheric scintillations. The TEC observed in HCMC also contributes to the data of ground-based observational receiver systems along 105o E longitude for studying ionospheric variations in low-latitude and equatorial regions. Our preliminary results indicate that the GNSS data collected at IU-VNU station is a valuable reference dataset for further research of the ionosphere.

Semidiurnal Tidal Influence on the Occurrence of Postmidnight F Region FAI Radar Echoes

AbstractThe Equatorial Atmosphere Radar observations at Kototabang (0.2°S, 100.3°E) are used to study the possible semidiurnal tidal influence on the occurrence of postmidnight echoes from the field‐aligned irregularities (FAIs) due to spread F. It is found that the postmidnight FAI echoes show high percentage of occurrence (PO) during June–July and low PO in December–January of low solar activity years. As solar activity approaches minimum, the PO increase is extended to May, August, and September. The space‐time Fourier analysis on the temperature information obtained from the Sounding of Atmosphere by Broadband Emission Radiometry instrument onboard Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite reveals that at the low‐latitude upper mesospheric heights, though the migrating diurnal tide propagating westward with zonal wavenumber (k) 1 (DW1) dominates the migrating semidiurnal tide propagating westward with k = 2 (SW2) particularly during the equinox months, the SW2 tidal amplitudes are larger than DW1 during June–July. The postmidnight FAI echoes appear to be more frequent during the sudden stratospheric warming of the Solar Minimum Years 2018–2019 and 2008–2009 but not in the Solar Maximum Year 2013. The dominance of SW2 over DW1 is also noted during these years. This indicates that the SW2 plays a major role in the occurrence of postmidnight spread F. It is suggested that the dominant semidiurnal variation of zonal electric field can become eastward and lift the F layer to higher heights during midnight hours, which can favor the growth of the Rayleigh‐Taylor instability to cause delayed spread F around midnight.

A Simulation of the Influence of DE3 Tide on Nitric Oxide Infrared Cooling

AbstractUsing GCITEM‐IGGCAS model, we simulate the influence of the eastward propagating nonmigrating diurnal tide with Zonal Wave Number 3 (DE3) on nitric oxide (NO) infrared cooling rate. We find that the DE3 tide can drive a DE3 signal in lower thermospheric NO cooling rate, and the simulated altitudinal and seasonal variations are according with that of DE3 signal in equatorial lower thermospheric NO cooling rate observed by Oberheide et al. (2013, https://doi.org/10.1002/2013JA019278), which is based on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry observations during the solar minimum year 2008. This signal mainly shows an annual variation, which is stronger between June and September and weaker near November. The maximum of the absolute signal, whose value is about 0.35 * 10−9 W/m3, occurs near the height of 130 km, but the relative signal mainly shows its peak with a value of 40% near the height of 100 km. Due to the difference of the driving mechanism, the distribution of NO signals in different latitudinal regions shows obvious difference. The middle‐ and low‐latitude NO signals show smooth variation, while the high‐latitude signal is discontinuous. The DE3 signal in NO cooling rate is mainly controlled by DE3 temperature tide and DE3 NO tide; meanwhile, the influences of DE3 neutral density tide on the DE3 signal can be ignored. The relative contributions of the DE3 NO tide and of the DE3 temperature tide vary with geographic latitude. The DE3 cooling rates in middle and low latitudes and in high latitude are, respectively, mainly driven by the DE3 temperature tide DE3 NO tide. DE3 tide may not only drive the DE3 signal but also affect the lower thermospheric zonal mean NO cooling rate. The maximum of the absolute influence, whose value is about 0.12 * 10−9 W/m3, occurs above the height of 140 km, but the relative influence mainly shows its peak with a value of 10% near the height of 100 km.

Characterization of ionospheric total electron content data using wavelet-based multifractal formalism

Abstract Understanding of the spatio-temporal behaviour of nonlinear geophysical signals, such as ionospheric total electron content (TEC) by multifractal analysis brings out the chaotic and intermittent nature of the signal under consideration. Wavelet-based multifractal analysis was performed on TEC data and the horizontal component of the Earth’s magnetic field (henceforth referred to as H-component) data recorded during geomagnetic storm events at a few sites in equatorial, mid-latitude and high latitude regions (30oS to 80oN), confined to a narrow longitude band ( 35 o W − 80 o W ) (geographic coordinates) during the solar minimum (2008) and solar maximum (2014) years. The study was done using the magnitude cumulant analysis of the wavelet transform. The advantage of this technique, over the well known wavelet transform modulus maxima (WTMM) method in studying the multifractal behaviour of nonlinear signals is explained. Results show that during the major geomagnetic storm events (Dst. Index ≤ − 50 nT) both TEC and the H-component data exhibit strong multifractal behavior and that the degree of multifractality (representative of the width of the multifractal spectrum) for the H-component data is more than that of TEC for all latitudes regardless of solar conditions. A nonlinear P-model, representative of multiplicative cascades for the above data sets, also supports the above observation. It has been observed that these observations hold good when multifractal behaviour of TEC data, with and without its dominant diurnal component, is compared with that of H-component data. A statistical hypothesis testing of the above results obtained using bootstrapping technique also establishes the significance level of the multifractal behaviour of the data.

Long-term trends in the ionospheric response to solar extreme-ultraviolet variations

Abstract. The thermosphere–ionosphere system shows high complexity due to its interaction with the continuously varying solar radiation flux. We investigate the temporal and spatial response of the ionosphere to solar activity using 18 years (1999–2017) of total electron content (TEC) maps provided by the international global navigation satellite systems service and 12 solar proxies (F10.7, F1.8, F3.2, F8, F15, F30, He II, Mg II index, Ly-α, Ca II K, daily sunspot area (SSA), and sunspot number (SSN)). Cross-wavelet and Lomb–Scargle periodogram (LSP) analyses are used to evaluate the different solar proxies with respect to their impact on the global mean TEC (GTEC), which is important for improved ionosphere modeling and forecasts. A 16 to 32 d periodicity in all the solar proxies and GTEC has been identified. The maximum correlation at this timescale is observed between the He II, Mg II, and F30 indices and GTEC, with an effective time delay of about 1 d. The LSP analysis shows that the most dominant period is 27 d, which is owing to the mean solar rotation, followed by a 45 d periodicity. In addition, a semi-annual and an annual variation were observed in GTEC, with the strongest correlation near the equatorial region where a time delay of about 1–2 d exists. The wavelet variance estimation method is used to find the variance of GTEC and F10.7 during the maxima of the solar cycles SC 23 and SC 24. Wavelet variance estimation suggests that the GTEC variance is highest for the seasonal timescale (32 to 64 d period) followed by the 16 to 32 d period, similar to the F10.7 index. The variance during SC 23 is larger than during SC 24. The most suitable proxy to represent solar activity at the timescales of 16 to 32 d and 32 to 64 d is He II. The Mg II index, Ly-α, and F30 may be placed second as these indices show the strongest correlation with GTEC, but there are some differences in correlation during solar maximum and minimum years, as the behavior of proxies is not always the same. The indices F1.8 and daily SSA are of limited use to represent the solar impact on GTEC. The empirical orthogonal function (EOF) analysis of the TEC data shows that the first EOF component captures more than 86 % of the variance, and the first three EOF components explain 99 % of the total variance. EOF analysis suggests that the first component is associated with the solar flux and the third EOF component captures the geomagnetic activity as well as the remaining part of EOF1. The EOF2 captures 11 % of the total variability and demonstrates the hemispheric asymmetry.

A solar electron event model in near-Earth space

A new solar electron event model is developed based on Virtual Timeline Method (VTM). We study events individually by analyzing the 17-year data of 3DP instrument on WIND spacecraft. This model is established in different solar cycle phases and is based on statistics of duration, fluence, and waiting time of solar electron events. The fluences follow a log-normal distribution and logarithmic durations fit well with logarithmic fluences linearly. We prove that waiting times of events significantly deviates from the Poisson process by investigating the stationary and event independence property of Poisson distribution. After a comparison study on waiting times, we choose the Lévy distribution in solar minimum and maximum years. During solar minimum, the event frequency is much lower than that of solar maximum, but the event magnitude is independent of solar cycle period. Large events also happen in solar minimum years. In different solar cycle phases, this model can output a spectrum with confidence level and mission duration by generating many series of virtual timelines composed of many pseudo-events based on Monte Carlo method. On the other hand, spectra in solar minimum years are softer than that in solar maximum years. The fluences in solar maximum years are about one order of magnitude higher than that in solar minimum years in a given mission period. We also compare this model with Interplanetary Electron Model (IEM) quantitatively and prove that this model is advanced.

The effect of solar and geomagnetic parameters on total electron content over Ankara, Turkey

In this study, the relationship between total electron content (TEC) and solar and geomagnetic parameters for Ankara station (39.7 N, 32.76 E), Turkey located in the mid-latitude ionosphere is investigated. In this context, F10.7 solar flux and Interplanetary Magnetic Fields (IMF) from solar parameters and Kp and Dst indices from geomagnetic parameters affecting on TEC are considered. The relationship between the variables is investigated by means of the statistical multiple regression model at the universal time (UT) (Local Time = UT + 2 h) 1200 and 2400 in the years when the 24th solar cycle was minimum (2007–2009) and maximum (2015). As a result, it is found that explainable rates by solar and geomagnetic parameters of TEC changes in 2007–2009 are lower than in 2015 at daytime, while the explainable rates in the solar minimum years are higher than those the maximum year at nighttime. To be higher than the solar maximum of explainable rate in the solar minimum years at nighttime may be related to the fact that the dynamics of the ionosphere is significantly different than expected in this deep minimum period. As expected in 2015, the relationship between TEC and independent parameters is greater at daytime than at nighttime.

Solar Cycle Variation of the Heliospheric Plasma Sheet Thickness

Past independent studies of the heliospheric plasma sheet (HPS) have shown that the thickness is highly variable, ranging from \(\approx 3.8 \times 10^{5}\) to \(8.9 \times 10^{6}\) km. Here we conduct a survey of the previous results and find a solar cycle dependence – where the HPS tends to be wider during solar-minimum years and narrower during solar-maximum years. The HPS is thicker near solar minimum than near solar maximum by a factor of 1.6 (in Solar Cycle 23) and 8 (in Solar Cycle 24). We also found that the average HPS thickness in 2007 (near the minimum of Solar Cycle 23/24) was almost ten times larger than that in 1995 (near minimum of Solar Cycle 22/23), and it was associated with a weak polar magnetic field in 2007. Based on the solar-surface-field measurements, we found that the average solar magnetic-field strength [\(| \boldsymbol{B}|\)] at 2.5 solar radii [R⊙] was \(\approx 40\)% larger in 1995 than in 2007 (0.22 gauss versus 0.16 gauss). We also found a larger (\(\approx 27 \)%) magnetic pressure-gradient force in 1995 than in 2007. Because this magnetic gradient force points toward the Equator in the corona (which is probably also true farther out), a wider HPS is expected to occur in 2007 than in 1995, at least close to the Sun. This result supports the so-called heliospheric plasma-sheet inflation hypothesis, i.e. the HPS is wider if the Sun’s polar field is weaker and narrower if the Sun’s polar field is stronger.

Electron density in the F1 layer over Norilsk in 2007–2014

We report the results of the analysis of annual variations in daily electron density (N) for various solar activity conditions — minimum, rise, and maximum (2007–2014) — obtained from digisonde measurements at the ionospheric station Norilsk (69.4° N, 88.1° E). New coefficients of the known semi-empirical model (SEM) describing the connection between N and thermosphere characteristics are calculated to identify regularities of these variations exactly at Norilsk station. The height changes of annual variations in the noon electron density N are obtained in the F1 region (120–200 km). The experimental data approximation describes N quite satisfactorily at these heights in the daytime of different seasons under different solar activity conditions. It is shown that in the years of solar minimum at all heights of the F1 layer the tendency remains for maximum N in summer and for minimum N in winter. In later years and in the year of maximum solar activity, a characteristic feature of the behavior of N is the change in the phase of the annual variation by 180° in the range of heights from 170 to180 km: maximum N is observed in winter; and minimum, in summer.

Электронная концентрация на высотах ионосферного слоя F1 в период 2007–2014 гг. над Норильском

We report the results of the analysis of annual variations in daily electron density (N) for various solar activity conditions — minimum, rise, and maximum (2007–2014) — obtained from digisonde measurements at the ionospheric station Norilsk (69.4° N, 88.1° E). New coefficients of the known semi-empirical model (SEM) describing the connection between N and thermosphere characteristics are calculated to identify regularities of these variations exactly at Norilsk station. The height changes of annual variations in the noon electron density N are obtained in the F1 region (120–200 km). The experimental data approximation describes N quite satisfactorily at these heights in the daytime of different seasons under different solar activity conditions. It is shown that in the years of solar minimum at all heights of the F1 layer the tendency remains for maximum N in summer and for minimum N in winter. In later years and in the year of maximum solar activity, a characteristic feature of the behavior of N is the change in the phase of the annual variation by 180° in the range of heights from 170 to180 km: maximum N is observed in winter; and minimum, in summer.