Different Phases Of Solar Cycle Research Articles

Geomagnetic field variations due to solar tides at the Indian Observatories

This study investigates the response of solar (S) tidal signatures on the horizontal component of the geomagnetic field at two observatories in India during 1980–2002 over solar cycles (SC) 21–23: Hyderabad (HYB), located in the low-latitude region, and Ettaiyapuram (ETT), situated at the magnetic equator. HYB represents the characteristics of solar quiet (Sq), while ETT is under the equatorial electrojet (EEJ) effect. Our results show the additional information about ter (S3), and quarta-diurnal (S4) tidal signatures of Sq and EEJ, along with diurnal (S1) and semi-diurnal (S2) at both observatories. In Sq solar tide, the average amplitude of S1 tide is consistently higher than that of EEJ tide by ~ 10%. During the same period, the S2, S3, and S4 tidal signatures of Sq are weaker than EEJ by ~ 2%, 5%, and 2.5%, respectively. During solar cycle maxima, the amplitude of the Sq tide is higher in SC-21 than in SC-22 and SC-23 by ~ 13% and 16%, while SC-22 has higher EEJ tidal amplitudes than other SCs by ~ 9%. We observe that the tidal signatures of Sq and EEJ closely follow the trend of solar radio flux (F10.7\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sqrt{F10.7}$$\\end{document}), except for S4 of Sq. The Pearson correlation coefficients (P) between F10.7\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sqrt{F10.7}$$\\end{document} and Sq/EEJ tidal amplitudes exhibit negative to positive correlation coefficients during different phases of SCs. The solar tidal amplitudes of Sq/EEJ (S1-S4) with F10.7\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sqrt{F10.7}$$\\end{document} during D, E, and J seasons have varying correlation coefficients, indicating that each tide has a distinct response on the geomagnetic field.Graphical

Variability of the Magnetospheric Convection Electric Field (MCEF) Under Shock Activity During the Solar Cycle 24

In the present work, we have analyzed the statistical variability of the MCEF under shock activity during solar cycle 24 as a function of the different phases of solar cycle 24 and as a function of the seasons. We compared the MCEF under shock periods with that under quiet periods. Solar cycle 24 recorded an average MCEF value of 0.28816888 mV/m, i.e. an average value of 0.26399935 mV/m for the ascending phase, 0.2961038 mV/m for the maximum phase and 0.28652747 mV/m for the descending phase. The minimum quantitative deviation from quiet activity observed is 59%. Seasonally, the highest MCEF values were recorded in Summer and Winter, and the lowest in Autumn and Spring. The corresponding mean values are 0.32707785 mV/m, 0.30966472 mV/m, 0.27061512 mV/m and 0.27020401 mV/m respectively. The minimum quantitative deviation from quiet activity observed is 66 %. This study showed that in most cases magnetic reconnection occurs on the night side between 1800 UT and 2400 UT.

Variation in the Flaring Potential of Different Sunspot Groups During Different Phases of Solar Cycles 23 and 24

In this present study, we have analyzed different types of X-ray solar flares (C, M, and X classes) coming out from different classes of sunspot groups (SSGs). The data which we have taken under this study cover the duration of 24 yr from 1996 to 2019. During this, we observed a total of 15015 flares (8417 in SC-23 and 6598 in SC-24) emitted from a total of 33780 active regions (21746 in SC-23 and 12034 in SC-24) with sunspot only. We defined the flaring potential or flare-production potential as the ratio of the total number of flares produced from a particular type of SSG to the total number of the same-class SSGs observed on the solar surface. Here we studied yearly changes in the flaring potential of different McIntosh class groups of sunspots in different phases of SC-23 and 24. In addition, we investigated yearly variations in the potential of producing flares by different SSGs (A, B, C, D, E, F, and H) during different phases (ascending, maximum, descending, and minimum) of SC-23 and 24. These are our findings: (1) D, E, and F SSGs have the potential of producing flares ≥8 times greater than A, B, C and H SSGs; (2) The larger and more complex D, E, and F SSGs produced nearly 80% of flares in SC-23 and 24; (3) The A, B, C and H SSGs, which are smaller and simpler, produced only 20% of flares in SC-23 and 24; (4) The biggest and most complex SSGs of F-class have flaring potential 1.996 and 3.443 per SSG in SC-23 and 24, respectively. (5) The potential for producing flares in each SSG is higher in SC-24 than in SC-23, although SC-24 is a weaker cycle than SC-23. (6) The alterations in the number of flares (C+M+X) show different time profiles than the alterations in sunspot numbers during SC-23 and 24, with several peaks. (7) The SSGs of C, D, E, and H-class have the highest flaring potential in the descending phase of both SC-23 and 24. (8) F-class SSGs have the highest flaring potential in the descending phase of SC-23 but also in the maximum phase of SC-24.

Modeling the Solar Wind during Different Phases of the Last Solar Cycle

We describe our first attempt to systematically simulate the solar wind during different phases of the last solar cycle with the Alfvén Wave Solar atmosphere Model (AWSoM) developed at the University of Michigan. Key to this study is the determination of the optimal values of one of the most important input parameters of the model, the Poynting flux parameter, which prescribes the energy flux passing through the chromospheric boundary of the model in the form of Alfvén wave turbulence. It is found that the optimal value of the Poynting flux parameter is correlated with the area of the open magnetic field regions with the Spearman’s correlation coefficient of 0.96 and anticorrelated with the average unsigned radial component of the magnetic field with the Spearman’s correlation coefficient of −0.91. Moreover, the Poynting flux in the open field regions is approximately constant in the last solar cycle, which needs to be validated with observations and can shed light on how Alfvén wave turbulence accelerates the solar wind during different phases of the solar cycle. Our results can also be used to set the Poynting flux parameter for real-time solar wind simulations with AWSoM.

Comparative study on the effects of CME and CIR-induced geomagnetic storms on the ionosphere of northern and southern hemispheric regions during the different phases of solar cycle 24

The ionospheric response to CME and CIR-induced geomagnetic storms has been studied using TEC data from northern and southern hemispheric low-latitude stations. During the main phase of geomagnetic storm, opposite effects are observed for almost all the CME and CIR-driven storms at the two stations. During the recovery phase, alternately positive and negative storm effects are observed for almost all CIR-driven storms, while no such fluctuations are observed for CME-driven storms. Recovery periods of CIR-driven storms are longer than that of CME-driven storms. Storm effects are larger at the southern hemispheric station compared to the northern hemispheric station. The observations are explained by the contribution of storm-time prompt penetration electric fields (PPEFs), disturbed dynamo electric fields (DDEFs), and disturbed meridional (equatorward) winds, as well as the neutral compositional changes over low latitudes.

Solar wind Alfvénicity during solar cycle 23 and 24

Context.Alfvénic fluctuations are ubiquitous features observed in solar wind, especially in the inner heliosphere. However, strong Alfvénic fluctuations are recovered in the near-Earth solar wind too, mainly in fast streams, but also in some cases in slow wind intervals, as highlighted in recent studies.Aims.The present study focuses on a statistical comparison between different phases of solar cycles 23 and 24 with regard to the Alfvénic content of solar wind fluctuations. Particular attention is devoted to the Alfvénic slow solar wind, in relation to the solar wind composition and other parameters.Methods.Two-dimensional histograms of the solar wind speed versus the normalized cross-helicity have been used to feature the Alfvénic character of solar wind turbulence on each phase of the solar cycles considered. Moreover, we characterize the different phases of solar cycles by also using composition data. Finally, case studies are discussed to better highlight the similarities and differences between the two solar maxima, which more clearly show a predominance of Alfvénic slow solar wind.Results.The statistical analysis highlights similarities between two solar cycles and confirms that the Alfvénic slow wind is more frequently observed during the maximum of solar activity. The two representative time intervals, containing samples of this solar wind regime, show similar characteristics, with a particular reference to the spectral analysis.Conclusions.This study has important implications for future observations by Parker Solar Probe and Solar Orbiter, devoted to the study of the inner heliosphere inside Mercury’s orbit. In fact, both missions will operate up to the maximum of solar cycle 25 which is fast approaching. These unprecedented measurements will then provide insights into the origin and evolution of the Alfvénic solar wind close to the region where it is generated and accelerated.

Clustering of Fast Coronal Mass Ejections during Solar Cycles 23 and 24 and the Implications for CME–CME Interactions

Abstract We study the clustering properties of fast coronal mass ejections (CMEs) that occurred during solar cycles 23 and 24. We apply two methods: the Max Spectrum method can detect the predominant clusters, and the declustering threshold time method provides details on the typical clustering properties and timescales. Our analysis shows that during the different phases of solar cycles 23 and 24, CMEs with speeds ≥1000 km s−1 preferentially occur as isolated events and in clusters with, on average, two members. However, clusters with more members appear, particularly during the maximum phases of the solar cycles. Over the total period and in the maximum phases of solar cycles 23 and 24, about 50% are isolated events, 18% (12%) occur in clusters with two (three) members, and another 20% in larger clusters ≥4, whereas in a solar minimum, fast CMEs tend to occur more frequently as isolated events (62%). During different solar cycle phases, the typical declustering timescales of fast CMEs are τ c = 28–32 hr, irrespective of the very different occurrence frequencies of CMEs during a solar minimum and maximum. These findings suggest that τ c for extreme events may reflect the characteristic energy build-up time for large flare and CME-prolific active regions. Statistically associating the clustering properties of fast CMEs with the disturbance storm time index at Earth suggests that fast CMEs occurring in clusters tend to produce larger geomagnetic storms than isolated fast CMEs. This may be related to CME–CME interaction producing a more complex and stronger interaction with Earth’s magnetosphere.

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.

Comparison of IRI-2016 model-predictions of F2-layer peak density height options with the ionosonde-derived hmF2 at the equatorial station during different phases of solar cycle

We have investigated the performance of 2016 International Reference Ionosphere (IRI) at equatorial Ouagadougou (12.4°N, 358.5°E; dip latitude: 1.5°N), Burkina Faso station, using 4 years 5 months of hourly monthly median M(3000)F2 values from January to May 1986 and from January 1987 to December 1990 using Shimazaki (1955) analysis technique. The deduced values of hmF2 are compared with the predictions of the three global IRI-2016 hmF2 model options for four different levels of solar activity with yearly averaged solar flux intensity F10.7 value of 85, 140, 214 and 190, corresponding to a year of 1987 solar minimum, 1988 moderate solar activity, 1989 very high solar activity and 1990 solar maximum, respectively. We find that the three IRI-hmF2 models: SHU-2015, AMTB-2013, and BSE-1979 reproduce the diurnal features of ionosonde-based hmF2 fairly well, showing daytime maximum and the prominent evening peak, except for solar cycle minimum where such feature is absent in Bilitza et al. (1979) and weakly expressed in Shubin (2015) hmF2 models. The magnitudes of the evening peaks in hmF2 for both data and models essentially correlate with solar activity but the occurrence times are independent of the phases of the solar cycle for all models. On average, the dayside maximum occurs between about noon and 1400 LT for both ionosonde-derived hmF2 and IRI models with typical values of 430 and 420 km, while the prominent evening peak in hmF2 occurs between about 1800–1900 LT with characteristic values of 390–450 km. We observed SHU-2015 persistently occur at about 1800 LT with a lower magnitude compared to ionosonde-based hmF2, the digisonde-based hmF2 model (AMTB), and Bilitza et al. (1979) model. We also show that SHU-2015 is consistently and significantly lower in values that the ionosonde hmF2, AMTB-2013 and BSE-1979 models. SHU-2015 deviates from data by about 2 times during low and median solar activity years but diverges by about 3–4 times during very and high solar maximum. Quantitative analyses indicate that the average absolute discrepancy between ionosonde-derived hmF2 and IRI model is 15%, 5.2% and 5.8% for SHU-2015, AMTB-2013, and BSE-1979, respectively. SHU-2015 model root-mean-squared error (RMSE) varies strongly with increasing solar activity with magnitudes roughly within the range ∼41–74 km, whereas AMBT-2013 and BSE-1979 RMSE vary between ∼19–36 km and ∼25–27 km, correspondingly. BSE-1979 RMSE does not exhibit any dependence on solar activity, while AMBT-2013 RMSE appears to decrease with an increase in solar activity. In contrast, SHU-2015 mean-relative-deviation (MRD) increases dramatically with increasing solar activity with values between ∼10–16%. AMTB-2013 and BSE-1979 both indicate a slight decrease with increasing solar activity, with values ranging between ∼3.3–8.9% and ∼4.8–6.7%, respectively. On the basis of evidence provided in this work, we conclude that AMTB-2013 and BSE-1979 have higher accuracy than SHU-2015. Although the hmF2 calculated from ionograms and from empirical formula of Shimazaki could differ rather substantially from the real hmF2 values, especially in the equatorial region. The results of this study show that the AMTB-2013 and BSE-1979 models describes better the hmF2 obtained from the M(3000)F2 values, consequently, we recommend AMTB-2013 and BSE-1979 for equatorial region over West African longitude sector.

Bihelical Spectrum of Solar Magnetic Helicity and Its Evolution

Abstract Using a recently developed two-scale formalism to determine the magnetic helicity spectrum, we analyze synoptic vector magnetograms built with data from the Vector Spectromagnetograph instrument on the Synoptic Optical Long-term Investigations of the Sun telescope during 2010 January–2016 July. In contrast to an earlier study using only three Carrington rotations (CRs), our analysis includes 74 synoptic CR maps. We recover here bihelical spectra at different phases of solar cycle 24, where the net magnetic helicity in the majority of the data is consistent with a large-scale dynamo with helical turbulence operating in the Sun. More than 20% of the analyzed maps, however, show violations of the expected sign rule.

Slow solar wind in different phases of solar cycle as observed by Ulysses

Slow solar wind in different phases of solar cycle as observed by Ulysses

Characteristics of Solar Wind Density Depletions During Solar Cycles 23 and 24

Solar wind density depletions are phenomena that solar wind density is rapidly decreased and keep the state. They are generally believed to be caused by the interplanetary (IP) shocks. However, there are other cases that are hardly associated with IP shocks. We set up a hypothesis for this phenomenon and analyze this study. We have collected the solar wind parameters such as density, speed and interplanetary magnetic field (IMF) data related to the solar wind density depletion events during the period from 1996 to 2013 that are obtained with the advanced composition explorer (ACE) and the Wind satellite. We also calculate two pressures (magnetic, dynamic) and analyze the relation with density depletion. As a result, we found total 53 events and the most these phenomena’s sources caused by IP shock are interplanetary coronal mass ejection (ICME). We also found that solar wind density depletions are scarcely related with IP shock’s parameters. The solar wind density is correlated with solar wind dynamic pressure within density depletion. However, the solar wind density has an little anti-correlation with IMF strength during all events of solar wind density depletion, regardless of the presence of IP shocks. Additionally, In 47 events of IP shocks, we find 6 events that show a feature of blast wave. The quantities of IP shocks are weaker than blast wave from the Sun, they are declined in a short time after increasing rapidly. We thus argue that IMF strength or dynamic pressure are an important factor in understanding the nature of solar wind density depletion. Since IMF strength and solar wind speed varies with solar cycle, we will also investigate the characteristics of solar wind density depletion events in different phases of solar cycle as an additional clue to their physical nature.

Investigation of the Geoeffectiveness of Disk-Centre Full-Halo Coronal Mass Ejections

We studied the occurrence and characteristics of geomagnetic storms associated with disk-centre full-halo coronal mass ejections (DC-FH-CMEs). Such coronal mass ejections (CMEs) can be considered as the most plausible cause of geomagnetic storms. We selected front-side full-halo coronal mass ejections detected by the Large Angle and Spectrometric Coronagraph onboard the Solar and Heliospheric Observatory (SOHO/LASCO) from the beginning of 1996 till the end of 2015 with source locations between solar longitudes E10 and W10 and latitudes N20 and S20. The number of selected CMEs was 66 of which 33 (50%) were deduced to be the cause of 30 geomagnetic storms with $\mathrm{Dst} \leq- 50~\mbox{nT}$ . Of the 30 geomagnetic storms, 26 were associated with single disk-centre full-halo CMEs, while four storms were associated, in addition to at least one disk-centre full-halo CME, also with other halo or wide CMEs from the same active region. Thirteen of the 66 CMEs (20%) were associated with 13 storms with $-100~\mbox{nT} < \mbox{Dst} \leq- 50~\mbox{nT}$ , and 20 (30%) were associated with 17 storms with $\mbox{Dst}\leq- 100~\mbox{nT}$ . We investigated the distributions and average values of parameters describing the DC-FH-CMEs and their interplanetary counterparts encountering Earth. These parameters included the CME sky-plane speed and direction parameter, associated solar soft X-ray flux, interplanetary magnetic field strength, $B_{t}$ , southward component of the interplanetary magnetic field, $B_{s}$ , solar wind speed, $V_{sw}$ , and the $y$ -component of the solar wind electric field, $E_{y}$ . We found only a weak correlation between the Dst of the geomagnetic storms associated with DC-FH-CMEs and the CME sky-plane speed and the CME direction parameter, while the correlation was strong between the Dst and all the solar wind parameters ( $B_{t}$ , $B_{s}$ , $V_{sw}$ , $E_{y}$ ) measured at 1 AU. We investigated the dependences of the properties of DC-FH-CMEs and the associated geomagnetic storms on different phases of solar cycles and the differences between Solar Cycles 23 and 24. In the rise phase of Solar Cycle 23 (SC23), five out of eight DC-FH-CMEs were geoeffective ( $\mbox{Dst} \leq- 50~\mbox{nT}$ ). In the corresponding phase of SC24, only four DC-FH-CMEs were observed, three of which were nongeoeffective ( $\mbox{Dst} > - 50~\mbox{nT}$ ). The largest number of DC-FH-CMEs occurred at the maximum phases of the cycles (21 and 17, respectively). Most of the storms with $\mbox{Dst}\leq- 100~\mbox{nT}$ occurred at or close to the maximum phases of the cycles. When comparing the storms during epochs of corresponding lengths in Solar Cycles 23 and 24, we found that during the first 85 months of Cycle 23 the geoeffectiveness rate of the disk-centre full-halo CMEs was 58% with an average minimum value of the Dst index of $- 146~\mbox{nT}$ . During the corresponding epoch of Cycle 24, only 35% of the disk-centre full-halo CMEs were geoeffective with an average value of Dst of $- 97~\mbox{nT}$ .

Longitudinal Variation of EEJ Current during Different Phases of Solar Cycle

The equatorial electrojet (EEJ) is an eastward current flow around the dip equator in E-region of the ionosphere within the range of 90 to 120 km altitude. The longitudinal variation of EEJ was studied using the ground-based measurement from six different longitudinal sectors. The ground magnetometer data were provided by magnetometer networks such as those from Magnetic Data Acquisition System (MAGDAS) /Circum-pan Pacific Magnetometer Network (CPMN), Indian Institute of Geomagnetism (IIG) and International Real-time Magnetic Observatory Network (INTERMAGNET). The EUEL index used in this study was calculated from geomagnetic northward H component. The H-component was normalized to the dip equator using the CM4 model. This study present results of the longitudinal variation of EEJ during the three phases of solar cycle 24: solar minimum (2008), inclination phase (2011) and solar maximum (2014). Results show that EEJ current is higher in the American sector and lowest between African and Indian sector in 2008 and 2011. On the other hand, during the year of solar maximum, this current component is comparable in American and Southeast Asian sector.

Magnetic field inversions at 1 AU: Comparisons between mapping predictions and observations

AbstractLarge‐scale magnetic field configurations are important for the transport of solar wind strahl electrons, which are suprathermal and directed along the field outward from the Sun. Strahl electrons are routinely used to infer not only the field configurations between the Sun and Earth but also local field structures, i.e., field inversions, where the magnetic field is locally folded back or inverted. Using solar wind data from ACE observations and a 2‐D data‐driven solar wind model with nonzero azimuthal magnetic field at the solar wind source surface, magnetic field lines are mapped between the Sun and Earth and beyond, in the solar equatorial plane. Standard verification metrics are used to assess, for five solar rotations at different phases of solar cycle 23, the performance of the mapping predictions for observed inversions, which are inferred from solar wind suprathermal electrons and magnetic fields measured by ACE. The probability of detection is consistently ≈0.70 across the different phases. The success ratio, the Hanssen‐Kuipers skill score, and the Heidke skill score are ≈0.55–0.70 for the four rotations in the rising, solar maximum, and declining phases, but ≈0.35–0.60 for the rotation near solar minimum, during which almost half of the samples have undetermined field configurations. Our analyses confirm the persistence of inversions throughout solar cycle 23, suggest for most observed inversions a solar/coronal origin at the wind's source surface or below, and predict that inversions should be less common for larger heliocentric distance r ∼&gt; 3 AU than for smaller r.

ON THE ORIGIN OF HIGHLY ALFVÉNIC SLOW SOLAR WIND

Alfvénic fluctuations are a common feature in the solar wind and are found especially in the trailing edges of fast wind streams. The slow wind usually has a lower degree of Alfvénicity, being more strongly intermixed with structures of non-Alfvénic nature. In the present paper we show the first evidence in the interplanetary space of two different kinds of slow solar wind: one coming from coronal streamers or active regions and characterized by non-Alfvénic structures and the other one being highly Alfvénic and originating from the boundary of coronal holes. The Alfvénic character of fluctuations, either outward or inward, can be studied by means of the normalized cross-helicity, , which is an indicator of the alignment. The evolution of toward lower values with increasing radial distance is interpreted both as a decrease of the presence of the outward modes and as a continuous production of inward modes within those regions such as stream shears where some plasma instability is active. On the other hand, the decrease of is often related also to magnetic field and/or density enhancements which specifically act on the destruction of the alignment. In the present analysis we study the role of compressibility presenting both case studies and a statistical analysis over different phases of solar cycle 23. Our findings indicate that the presence of regions of magnetic field compression generally play a major role in the depletion of and thus in the destruction of the alignment.

Transient variations of vertical total electron content over some African stations from 2002 to 2012

This paper presents the vertical total electron content vTEC variations for three African stations, located at mid-low and equatorial latitudes, and operating since more than 10years. The vTEC of the middle latitude GPS station in Alexandria, Egypt (31.2167°N; 29.9667°E, geographic) is compared to the vTEC of two others GPS stations: the first one in Rabat/Morocco (33.9981°N; 353.1457°E, geographic), and the second in Libreville/Gabon (0.3539°N; 9.6721°E, geographic). Our results discussed the diurnal, seasonal, and solar cycle dependences of vTEC at the local ionospheric conditions, during different phases of solar cycle in the light of the classification of Legrand and Simon. The vTEC over Alexandria exhibits the well-known equinoctial asymmetry which changes with the phases of the solar cycle; the spring vTEC is larger than that of autumn during the maximum, decreasing and minimum phases of solar cycle 23. During the increasing phase of solar cycle 24, it is the contrary. The diurnal variation of the vTEC presents multiple maxima during the equinox from 2005 to 2008 and during the summer solstice from 2006 to 2012. A nighttime vTEC enhancement and winter anomaly are also observed. During the deep solar minimum (2006–2009) the diurnal variation of the vTEC observed over Alexandria is similar to the diurnal variation observed during quiet magnetic period at equatorial latitudes. We observed also that the amplitude of vTEC at Libreville is larger than the amplitude of vTEC observed at Alexandria and Rabat, indeed Libreville is near the southern crest of the Equatorial Ionization anomaly. Finally, the correlation coefficient between vTEC and the sunspot number Rz is high and changes with solar cycle phases.

Formation of coronal mass ejections at different phases of solar activity

This paper investigates the dependence of the observed coronal mass ejections and their parameters on evolutionary changes in the global solar magnetic field at different phases of solar cycles 23–24. Four periods in the evolution of the solar cycle are identified, depending on the dominance ratio of the sectoral and zonal magnetic field structures. The parameters of coronal mass ejections observed during these periods are analyzed. The evolving structure in the global magnetic field is identified, and its influence on coronal mass ejections is examined.

EVALUATING POTENTIAL FOR DATA ASSIMILATION IN A FLUX-TRANSPORT DYNAMO MODEL BY ASSESSING SENSITIVITY AND RESPONSE TO MERIDIONAL FLOW VARIATION

We estimate here a flux-transport dynamo model's response time to changes in meridional flow speed. Time-variation in meridional flow primarily determines the shape of a cycle in this class of dynamo models. In order to simultaneously predict the shape, amplitude and timing of a solar cycle by implementing an Ensemble Kalman Filter in the framework of Data Assimilation Research Testbed (DART), it is important to know the model's sensitivity to flow variation. Guided by observations we consider a smooth increase or decrease in meridional flow speed for a specified time (a few months to a few years), after which the flow speed comes back to the steady speed, and implement that time-varying meridional flow at different phases of solar cycle. We find that the model's response time to change in flow speed peaks at four to six months if the flow change lasts for one year. The longer the changed flow lasts, the longer the model takes to respond. Magnetic diffusivity has no influence in model's response to flow variation as long as the dynamo operates in the advection-dominated regime. Experiments with more complex flow variations indicate that the shape and amplitude of flow-perturbation have no influence in the estimate of model's response time.

Influence of interplanetary solar wind sector polarity on the ionosphere

Knowledge of solar sector polarity effects on the ionosphere may provide some clues in understanding of the ionospheric day‐to‐day variability and “hysteresis” effect on foF2. Ionospheric response to changes in solar sector polarity has not been fully documented previously, partly due to the limitation of observations. In this study, a solar‐terrestrial connection ranging from solar sector boundary (SB) crossings, geomagnetic disturbances and ionospheric perturbations has been demonstrated. The increases in interplanetary solar wind speed within three days are seen after SB crossings, while the decreases in solar wind dynamic pressure and magnetic field intensity immediately after SB crossings are confirmed by the superposed epoch analysis results. Furthermore, the interplanetary magnetic field (IMF) Bz component turns from northward to southward in March equinox and June solstice as the Earth passes from a solar sector of outward to inward directed magnetic fields, whereas the reverse situation occurs for the transition from toward to away sectors. The IMF Bz component for the same solar sector polarity has opposite signs between March equinox and September equinox, and also between June solstice and December solstice. In order to know how the ionosphere reacts to the interplanetary solar wind variations linkage of SB crossings, the F2 region critical frequency (foF2) covering about four solar cycles and total electron content (TEC) during 1998–2011 are utilized to extract the related information, revealing that they are not modified significantly and vary within the range of ±15% on average. The responses of the ionospheric TEC to SB crossings exhibit complex temporal and spatial variations and have strong dependencies on season, latitude, and solar cycle. This effect is more appreciable in equinoctial months than in solstitial months, which is mainly caused by larger southwardBzcomponents in equinox. In September equinox, latitudinal profile of relative variations of foF2 at noon is featured by depressions at high latitudes and enhancements in low‐equatorial latitudes during IMF away sectors. The negative phase of foF2 is delayed at solar minimum relative to it during other parts of solar cycle, which might be associated with the difference in longevity of major interplanetary solar wind drivers perturbing the Earth's environment in different phases of solar cycle.