Phase Of Solar Activity Research Articles

A neural network forecasting of relativistic electron flux at geostationary orbit: solar activity phase dependence

Geostationary-satellite anomalies are mainly caused by extreme space environment such as flux enhancements of relativistic electrons. A prediction of the relativistic electrons at geostationary orbit (GEO) is crucial to alert of the flux enhancements before it reaches a harmful condition. This work investigates contribution of solar activities at various phases in the daily electron flux (E > 2 MeV) at GEO measured by GOES satellites during the solar cycles 23-24. An alternative method based on the solar activity and feed forward neural network (NN) scheme with back-propagation learning in Waikato Environment for Knowledge Analysis is adopted to predict the electron fluxes in GEO by sorting the solar activity into maximum, descending, and ascending phases. As the model inputs, the historical daily sum K p and the > 2 MeV electron fluxes are used. Results indicate that the prediction capability of the NN model is dependent on the solar activity in relation to the sum K p. Crossing the test over the similar solar phases gives a better prediction result than crossing over different phases. The NN model based on the lagged sum K p and log-flux input are more suitable for the forecasting during the descending and ascending phases, while the lagged log-flux input is more suitable during the maximum phase.

The Extreme Space Weather Event in 1903 October/November: An Outburst from the Quiet Sun

Abstract While the Sun is generally more eruptive during its maximum and declining phases, observational evidence shows certain cases of powerful solar eruptions during the quiet phase of solar activity. Occurring in the weak Solar Cycle 14 just after its minimum, the extreme space weather event in 1903 October–November is one of these cases. Here, we reconstruct the time series of geomagnetic activity based on contemporary observational records. With the mid-latitude magnetograms, the 1903 magnetic storm is thought to be caused by a fast coronal mass ejection (≈1500 km s−1) and is regarded as a superstorm with an estimated minimum of the equivalent disturbance storm time index (Dst’) of ≈−531 nT. The reconstructed time series has been compared with the equatorward extension of auroral oval (≈44.°1 in invariant latitude) and the time series of telegraphic disturbances. This case study shows that potential threats posed by extreme space weather events exist even during weak solar cycles or near their minima.

North-South asymmetry of equatorial ionospheric anomaly computed from the IRI model

Using the electron density from the IRI-2016 model and processed results of the in-situ measurements (GRACE and CHAMP) in Xiong et al. (2013), the morphological features of the equatorial ionization anomaly (EIA) such as its magnitude and inter-hemispheric asymmetry have been studied during 2001-2009. The effect of solar activity on ability of the IRI-2016 model to predict the EIA parameters is studied and the results are compared with in-situ measurements from GRACE and CHAMP during high and low solar activity phases. The IRI-2016 generally follows the same latitudinal asymmetry (ACL - Asymmetry of Crest Latitude) at 400 km (the same trend of ACL as for CHAMP) while it tends to be asymmetric towards the North at 480 km (GRACE) (Positive ACL). In addition, during June solstice the IRI-2016 model shows larger departure from observation after 19:00 UT with a larger difference during high solar activity than that during the low solar activity. Also the IRI-2016 model failed to predict significantly North-South asymmetry (southern crest disappears) in the electron density for high solar activity period. This is probably the reason for larger discrepancy of observations and the IRI-2016 model particularly during the summer solstice of the high solar activity period. This suggests that data input in the IRI model particularly in the equatorial- and low-latitude regions are required so that it could better predict the location of EIA crests.

Transmission Lines in Poland and Space Weather Effects

We analyze the solar influence of space weather events on Polish energy infrastructure via geomagnetic disturbances. We consider two time intervals of very different levels of solar activity (SA) in the course of the solar activity cycle 24: 2010—an early rising phase of SA, near the solar minimum and 2014—solar maximum. From the total number of electrical grid failures, we extract almost five thousand failures in 2010 and more than ten thousand in 2014, which could be related to space weather effects. We analyze data of failures having unidentified causes, failures coupled with the aging of transmission network elements and electronic devices, which appeared during the time intervals of enhanced geomagnetic activity. We show that around the times of increasing geomagnetic activity level, the amount of transmission lines failures in the groups mentioned above grows, but mostly with some delay. It might be connected to some cumulative effect due to the result of transient states and their propagation in the distribution network. Although we do not exclude that the other factors may play a role, the solar-originating disturbances strengthen these effects.

Solar activity phase dependence of the magnetospheric processes and relativistic electron flux at geostationary orbit

This study aims to investigate the effects of the solar activity phases on the magnetospheric processes in relation to daily relativistic electron ($E>2$ MeV) dynamics at geosynchronous orbit (GEO) during Solar Cycles 23–24. GOES observations indicate that in the descending phase the electron fluxes are seasonally dependent with largest flux in equinoctial periods, relatively high, and 27-day recurrent, while in the maximum phases they are relatively low and nonrecurrent. The electron fluxes are relatively low and partially recurrent in the ascending phases. The cross correlation coefficients (c.c.s) of daily $K_{\textrm{p}}$–$V_{\textrm{sw}}$, $K_{\textrm{p}}$–$AE$, $K_{\textrm{p}}$–$AL$ evolve in the similar trend for both solar cycles: highest during the descending phases, lower in the ascending phases, and lowest in the maximum and minimum phases. The correlation of $K_{\textrm{p}}$–viscous term is strongest during descending phases and weakest around the maximum phases. The correlation of $K_{\textrm{p}}$–merging term slightly varies in a chaotic way from one solar phase to another, but drops to its lowest value in the solar minimum. The correlation analysis results signify the role of the solar activity in controlling the solar wind–magnetosphere couplings. Two case studies during maximum (2000) and descending (2017) phases indicate that the solar activity dependence of the magnetospheric processes and relativistic electrons is characterized by substorm activity and solar wind drivers. Most of the substorms induced by coronal mass ejections in the maximum phase exhibit strong but short and non-repetitive features that associate with low electron fluxes at GEO. In contrast, high intensity, prolonged, and repetitive substorm activities induced by high-speed solar winds are the main cause of the relativistic electron enhancements during the descending phases. The enhancements strongly depend on $V_{\textrm{sw}}$ and $\Sigma K_{\textrm{p}}$ in which only appropriate $V_{\textrm{sw}}$ and $\Sigma K_{\textrm{p}}$ are required. In the descending phase, the requirements for the flux enhancements to $\geq 10^{3}$ cm−2 sr−1 s−1 are the simultaneity of $V_{\textrm{sw}}>500$ km/s and the prolongation of $\Sigma K_{\textrm{p}}>220$ and to $\geq 10^{4}$ cm−2 sr−1 s−1 when $V_{\textrm{sw}}\geq 630$ km/s and $\Sigma K_{\textrm{p}}>227$. Furthermore, the correlations of $\Sigma K_{\textrm{p}}$ and log-electron fluxes are stronger in the descending/ascending phases than in the maximum phase, while the time lag between them is longer in the maximum phase. The remarkable correlations of $V_{\textrm{sw}}$–$K_{\textrm{p}}$ and in the descending phase indicate appropriate magnetospheric convection in association with the repetitive substorms that can effectively trigger the stochastic mechanisms of electron acceleration. The results are extensively discussed in the light of observations and current theories of radiation belt dynamics.

Long-Term Evolution of the Solar Corona Using PROBA2 Data

We use The Sun Watcher with Active Pixel System detector and Image Processing (SWAP) imager onboard the Project for Onboard Autonomy 2 (PROBA2) mission to study the evolution of large-scale EUV structures in the solar corona observed throughout Solar Cycle 24 (from 2010 to 2019). We discuss the evolution of the on-disk coronal features and at different heights above the solar surface based on EUV intensity changes. We also look at the evolution of the corona in equatorial and polar regions and compare them at different phases of the solar cycle, as well as with sunspot number evolution and with the PROBA2/Lyman-Alpha Radiometer (LYRA) signal. The main results are as follows: The three time series (SWAP on-disk average brightness, sunspot number and LYRA irradiance) are very well correlated, with correlation coefficients around 0.9. The average rotation rate of bright features at latitudes of +15, 0, and -15 degrees was around 15 degree/day throughout the period studied. A secondary peak in EUV averaged intensity at the Poles was observed on the descending phase of SC24. These peaks (at North and South poles respectively) seem to be associated with the start of the development of the (polar) coronal holes. Large-scale off-limb structures were visible from around March 2010 to around March 2016, meaning that they were absent at the minimum phase of solar activity. A fan at the North pole persisted for more than 11 Carrington rotations (February 2014 to March 2015), and it could be seen up to altitudes of 1.6 Rs.

GLOBAL DISTURBANCE OF EARTH’S MAGNETOSPHERE AND ITS CONNECTION WITH SPACE WEATHER

This paper deals with extreme conditions of the global magnetosphere disturbance: very quiet and greatly disturbed, and their relationship with interplanetary medium parameters. The degree of global magnetosphere disturbance was estimated from the monthly and annual number of magnetically quiet and magnetically disturbed days. The cyclic and seasonal distributions of magnetically quiet and disturbed days were compared, and their relationship with interplanetary medium parameters in solar cycles 20–24 was analyzed. Magnetically quiet days are shown to be mainly observed at the ascending phase of solar activity and during the winter solstice season. Magnetically disturbed days dominate at the descending phase of the solar cycle and during the equinox season. An anomalously large increase in the number of quiet days was found in solar cycle 24 as compared to previous cycles. It has been established that the cyclic variation in the annual number of quiet and disturbed days is determined by the behavior and magnitude of the speed, temperature, dynamic pressure of solar wind plasma and the interplanetary magnetic field modulus. The detected burst in the number of quiet days during the ascending phase of solar cycle 24 is assumed to reflect internal processes on the Sun and related changes in interplanetary medium parameters. The patterns of cyclical and seasonal variation in the number of magnetically quiet and disturbed days and their relationship with the solar wind parameters can be used to predict space weather.

Глобальная возмущенность магнитосферы Земли и ее связь с космической погодой

This paper deals with extreme conditions of the global magnetosphere disturbance: very quiet and greatly disturbed, and their relationship with interplanetary medium parameters. The degree of global magnetosphere disturbance was estimated from the monthly and annual number of magnetically quiet and magnetically disturbed days. The cyclic and seasonal distributions of magnetically quiet and disturbed days were compared, and their relationship with interplanetary medium parameters in solar cycles 20–24 was analyzed. Magnetically quiet days are shown to be mainly observed at the ascending phase of solar activity and during the winter solstice season. Magnetically disturbed days dominate at the descending phase of the solar cycle and during the equinox season. An anomalously large increase in the number of quiet days was found in solar cycle 24 as compared to previous cycles. It has been established that the cyclic variation in the annual number of quiet and disturbed days is determined by the behavior and magnitude of the speed, temperature, dynamic pressure of solar wind plasma and the interplanetary magnetic field modulus. The detected burst in the number of quiet days during the ascending phase of solar cycle 24 is assumed to reflect internal processes on the Sun and related changes in interplanetary medium parameters. The patterns of cyclical and seasonal variation in the number of magnetically quiet and disturbed days and their relationship with the solar wind parameters can be used to predict space weather.

Occurrence characteristics of electromagnetic ion cyclotron waves at sub-auroral Antarctic station Maitri during solar cycle 24

We present a statistical study of electromagnetic ion cyclotron (EMIC) waves observed at Antarctic station (geographic 70.7{^circ },hbox {S}, 11.8{^circ },hbox {E}, L=5) on quiet and disturbed days during 2011–2017. The data span a fairly good period of both ascending and descending phases of the solar cycle 24, which has witnessed extremely low activity. We noted EMIC wave occurrence by examining wave power in different frequency ranges in the spectrogram. EMIC wave occurrence during ascending and descending phases of solar cycle 24, its local time, seasonal dependence and durations have been examined. There are total 2367 days for which data are available. Overall, EMIC waves are observed for 3166.5 h (approx 5.57% of total duration) which has contributions from 1263 days. We find a significantly higher EMIC wave occurrence during the descending phase (approx 6.83%) as compared to the ascending phase (approx 4.08%) of the solar cycle, which implies nearly a twofold increase in EMIC wave occurrence. This feature is attributed to the higher solar wind dynamic pressure during descending phase of solar activity. There is no evident difference in the percentage occurrence of EMIC waves on magnetically disturbed and quiet days. On ground, EMIC waves show marginally higher occurrence during winter as compared to summer. This seasonal tendency is attributed to lower electron densities and conductivities in the ionosphere, which can affect the propagation of EMIC waves through ionospheric ducts. In local time, the probability distribution function of EMIC wave occurrence shows enhancement during 11.7–20.7 LT (i.e., afternoon–dusk sector). Daily durations of EMIC waves are in the range of 5–1015 min and it is noted that the longer duration (240–1015 min) events are prevalent on quiet days and are mostly seen during the descending phase of solar cycle.

Testing and validating IRI-2016 model over Ethiopian ionosphere

This paper focuses on the validation of the latest version of the International Reference Ionosphere (IRI-2016) model in predicting the vertical total electron content (VTEC) variation over Ethiopian ionospheric region during a high solar activity (2012-2016) phase. The diurnal, monthly and seasonal VTEC variations were analyzed from dual-frequency global positioning system (GPS) and IRI-2016 model at the Debark station. The diurnal, monthly and seasonal maximum TEC values measured between around 9:00 and 18:00 UT (12:00 and 21:00 LT) hours and the minimum TEC values measured before 9:00 (12:00 LT) and after 18:00 UT (21:00 LT). The overall results show that IRI-2016 model generally overestimates the diurnal, monthly and seasonal mean VTEC values during the high solar activity (2012-2016) phase. The model prediction generally follows that the monthly and seasonal variations of the measured VTEC values in equinoctial months are higher than solstice months. The monthly mean and seasonal mean values of IRI-TEC and GPS-TEC mostly correlates before early morning and after evening hours and also slightly IRI-TEC underestimates starting from morning hours up to evening hours. The magnetic storm effects on GPS-TEC are associated with fluctuations. However, the IRI-TEC values show smooth pattern during both in storm on and off options. Consequently, the IRI-2016 model does not correctly respond to the effects of the resulting storm disturbance.

Performance evaluation of IRI-2016 with GPS-derived TEC at the meridian of 110oE in China of 2014

The latest version of the International Reference Ionosphere (IRI-2016) is evaluated for ionospheric total electron content (TEC) over China during a high solar activity phase of 2014. TEC is derived from observations by the GNSS receiver network of Crustal Movement Observation Network of China (CMONOC). IRI-2016 uses the NeQuick option for the topside electron density profile. The comparisons shows that IRI-2016 underestimates GPS-TEC slightly in spring and winter, but overestimates slightly in summer and autumn at the mid-latitudes of 35oN-45oN. The deviation of TEC estimation is about 5 TECU at 35oN-45oN. As a whole, IRI-2016 underestimates GPS-TEC at the lower latitudes of 15oN-35oN, and the deviations of TEC estimation are about 4 TECU, 10 TECU, 26 TECU and 33 TECU in summer, autumn, winter and spring, respectively. The largest difference between IRI-TEC and GPS-TEC is about 50 TECU at the northern crest of equatorial ionization anomaly (EIA) in spring. There is an obvious winter anomaly for GPS-TEC, while it is not predicted by IRI-2016. Due to the influence of nighttime enhancement of ionosphere, the extreme latitudinal gradient of GPS-TEC appears after sunset in spring and winter, while it is not shown on that of IRI-TEC. Because of the equatorial fountain effect, the maximum time rate of GPS-TEC appears at the beginning of EIA (∼10:00–11:00 LT) in equinoxes, while it is not shown on that of IRI-TEC. The most rapid decrease of IRI-TEC appears after sunset (∼19:00 LT) in spring and winter, while GPS-TEC quickly decreases at ∼22:00 LT.

Spatial and Temporal Variations of Polar Ionospheric Total Electron Content over Nearly Thirteen Years.

It is of great significance for the global navigation satellite system (GNSS) service to detect the polar ionospheric total electron content (TEC) and its variations, particularly under disturbed ionosphere conditions, including different phases of solar activity, the polar day and night alternation, the Weddell Sea anomaly (WSA) as well as geomagnetic storms. In this paper, four different models are utilized to map the ionospheric TEC over the Arctic and Antarctic for about one solar cycle: the polynomial (POLY) model, the generalized trigonometric series function (GTSF) model, the spherical harmonic (SH) model, and the spherical cap harmonic (SCH) model. Compared to other models, the SCH model has the best performance with ±0.8 TECU of residual mean value and 1.5–3.5 TECU of root mean square error. The spatiotemporal distributions and variations of the polar ionospheric TEC are investigated and compared under different ionosphere conditions in the Arctic and Antarctic. The results show that the solar activity significantly affects the TEC variations. During polar days, the ionospheric TEC is more active than it is during polar nights. In polar days over the Antarctic, the maximum value of TEC always appears at night in the Antarctic Peninsula and Weddell Sea area affected by the WSA. In the same year, the ionospheric TEC of the Antarctic has a larger amplitude of annual variation than that of the TEC in the Arctic. In addition, the evolution of the ionization patch during a geomagnetic storm over the Antarctic can be clearly tracked employing the SCH model, which appears to be adequate for mapping the polar TEC, and provides a sound basis for further automatic identification of ionization patches.

Science Opportunities from Observations of the Interstellar Neutral Gas with Adjustable Boresight Direction

The interstellar neutral (ISN) gas enters the heliosphere and is detected at a few au from the Sun, as demonstrated by Ulysses and the nterstellar Boundary Explorer (IBEX) missions. Ulysses observed ISN gas from different vantage points in a polar orbit from 1994 to 2007, while IBEX has been observing in an Earth orbit in a fixed direction relative to the Sun from 2009. McComas et al. 2018 reported about an IMAP-Lo detector on board the Interstellar Mapping and Acceleration Probe (IMAP), with an ability to track the ISN flux in the sky. We present observation geometries for ISN gas for a detector with the capability to adjust the boresight direction along the Earth orbit over a year within a multichoice ISN observation scheme. We study science opportunities from the observations as a function of time during a year and the phase of solar activity. We identify observation geometries and determine the observation seasons separately for various ISN species and populations. We find that using an adjustable viewing direction allows for ISN gas observations in the upwind hemisphere, where the signal is not distorted by gravitational focusing, in addition to the viewing of ISN species throughout the entire year. Moreover, we demonstrate that with appropriately adjusted observation geometries, primary and secondary populations can be fully separated. Additionally, we show that atoms of ISN gas on indirect trajectories are accessible for detection, and we present their impact on the study of the ionization rates for ISN species.

Low-latitude ionospheric response from GPS, IRI and TIE-GCM TEC to Solar Cycle 24

In the present study inter-comparison of Total Electron Content (TEC) derived from the Global Positioning System, the International Reference Ionosphere (IRI-2016) model and the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) during Solar Cycle 24 has been carried out. The short- and long-term variabilities in TEC have been assessed by using spectral, regression and statistical analysis. To ascertain the quiet-time climatology, TEC data have been used after filtering out the solar flares and geomagnetic storms effects. The present analysis exhibits a double-hump structure and clockwise hysteresis in TEC as regards Solar Cycle 24. The solar flux and TEC trend are found to be irregularly and slow during the rise and smooth and quick during the fall period of Solar Cycle 24. Seasonally, the semiannual anomaly is found to be a consistent feature for all phases of the solar cycle, while the winter anomaly seems to be facilitated with a level of solar activity during solstices. Another purpose of the present work is to investigate the performance of IRI-2016 and the TIE-GCM 2.0 models in comparison with GPS-TEC during Solar Cycle 24. Almost perfect agreement is found between observed and modeled TEC, delineating similar trends of the solar cycle, and of semiannual and seasonal variations. Nevertheless, significant biases are apparent between the observed and modeled TEC in terms of local time, seasons and phases of solar activity. Our results show the error in the model estimations in noontime TEC, which is as high as 100% in the IRI model and as low as 60% in the TIE-GCM model. Thus, in general, the IRI model overestimates the noon time TEC values, while TIE-GCM underestimates them.

Thermal Properties of Coronal Cavities

We have analyzed 33 cavities observed between 2012 and 2018, from solar activity maximum to minimum. For each cavity we applied a differential emission measure method to obtain both a temperature distribution and a value of the average temperature. We find that cavities are filled with material hotter than the surrounding streamer, with temperatures in the range of 1.67 – 2.15 MK. Differences between temperatures of cavities and surrounding streamers are in the range of 0.11 – 0.32 MK with an average value of 0.21 MK. We found that temperatures of both, cavities and streamers, vary as a function of different phases of solar activity. During solar maximum the structures are slightly hotter than those observed during solar minimum (1.85 – 2.15 MK vs. 1.67 – 1.88 MK for cavities and streamers, respectively).

Maximum Proton Trapping Energy in the Earth Radiation Belt

During the PAMELA satellite experiment started in 2006 onboard the Resurs-DK1 spacecraft (350–600 km, an inclination of 70°), cosmic ray charged particle fluxes in low near-Earth orbits were measured to the beginning of 2016. The data obtained during the experiment on high-energy proton (0.1–10 GeV) fluxes in the radiation belt made it possible to measure maximum proton trapping energies in various solar activity phases. The measurement results showed that the maximum proton trapping energy at L shells of 1.16–1.22 in the periods of both solar activity minimum (2009) and maximum (2014) was ∼2 GeV.

Performance of NeQuick-2, IRI-Plas 2017 and GIM models over Ethiopia during varying solar activity periods

This paper investigates the performance of NeQuick-2, the latest version of International Reference Ionosphere IRI extended to the Plasmasphere (IRI-Plas 2017), and Global Ionospheric Map (GIM) models over four stations deployed in Ethiopia during medium, high, and low solar activity periods. The monthly and seasonal mean diurnal variations of Vertical Total Electron Content (VTEC) derived from NeQuick-2, IRI-Plas 2017 and GIM models were compared with the GPS-VTEC values in terms of Root-Mean-Square Errors (RMSE) and correlation coefficients (r). It was found that the monthly and seasonal mean diurnal VTEC variations agree fairly well with the GPS-VTEC at all stations although with some offsets. For the entire years of the study, the highest monthly mean VTEC variation is observed during equinox months, whereas minimum in the solstice months. The NeQuick-2 model monthly and seasonal mean VTEC output were generally better than IRI-Plas 2017 and GIM models output in most of the stations during medium solar activity year of 2012. For all models, the significant discrepancy (i.e, overestimations or underestimations) is observed during daytimes and seasons of high solar activities. In some cases, the IRI-Plas 2017 model displayed poor performance during medium and low solar activity periods, while it showed a better agreement for some stations (e.g. asos, armi, and bdmt) during high solar activity phase (especially in March equinox and December solstice). The performance of GIM model was good for most of the stations during low-solar activity year of 2016 and during the study years for adis, while it has poor performance during the periods of high solar activity year of 2014. All models correlated well, although GIM model was highly correlated with the GPS-VTEC value (r ≥0.99). The IRI-Plas 2017 model correlated better with the GPS-VTEC than NeQuick-2 model during the study years. Moreover, the performance of IRI-Plas 2017 model was better during high than low and medium periods of solar activity years. The GIM model has better performance for most stations (e.g. adis, asos, and bdmt) over the study region during low-solar activity period.

Variations of Energetic Electron Fluxes in the Ionosphere during Periods of Solar Cycles

Energetic electrons (with energies of 10 to 1000 keV) from the Earth’s radiation belt (ERB) play an important role in the processes associated with the ionization of the upper atmosphere and ionosphere from the equator to high latitudes. The main sources and properties of energetic electrons at the altitudes of the ionosphere of up to 1000 km are considered. At high latitudes (more than 50°), precipitations from the outer belt are the sources of such electrons. At low latitudes, intensive fluxes of electrons appear as a result of their injection from the inner ERB. The analysis of the solar-cyclic variation of energetic electron fluxes during two solar cycles in the period from 1998 to 2018 is performed. It is shown that the maximum electron fluxes are most often observed at the declining phases of solar activity (SA), which is associated with the contribution from the recurrent geomagnetic activity. The lowest fluxes are found in the deep minimum of the SA of 2009. As a result, the energetic electron fluxes during the current, 24th, solar cycle are found to be weaker than those in the previous cycle. In addition, an anomalous shift of the position of the maximum of the outer ERB to the equator by 4° (which is equivalent to a distance of 400 km) which is not predicted by the standard model is observed over Siberia. Apparently, this is associated with an acceleration of the change in the geomagnetic field in this region.

Occurrence characteristics of equatorial plasma bubbles and total electron content during solar cycle peak 23rd to peak 24th over Bangalore (13.02∘ N, 77.57∘ E)

We present the monthly, seasonal and annual variation in the ionospheric total electron content (TEC) and % occurrence rate of Equatorial plasma bubble (EPBs) during the lowest to highest solar activity phase for the period of 2002–2013. The Total Electron Content (TEC) is computed using Global Positing System (GPS) from Bangalore (13.02∘ N, 77.57∘ E) IGS station for the period 2002 to 2013. The total 4383 days during the period 2002–2013, out of which 4229 days GPS data were analyzed i.e. 96.48%. Total 1175 days data shows signature of EPBs. The average occurrence rates of EPBs were 5.93% during the disturbed period and 47.07% during quiet period. This shows that the EPBs occurrence rate is higher in quiet period than that of disturbed period. We also found that both the average GPS-TEC and % occurrence rate of plasma bubbles are positively correlated with solar flux for the entire 12 year period. This study investigates the causal linkage between EPBs and TEC using their statistics during the solar minimum and maximum period. The studies on dynamics of EPBs are essential because they affects the satellite communication system, mainly due to depletions in the Total Electron Content (TEC) which encounters most of the effects on GPS signals.

Seasonal variation of plasma bubbles during solar cycle 23–24 over the Brazilian equatorial region

In this study, OI 630.0 nm nightglow image data obtained from an all-sky imaging station located at Sao Joao do Cariri (7.4°S, 36.5°W), have been used to study the occurrence of equatorial plasma bubbles (EPBs) over the Brazilian equatorial region. The observations, which took place from September 2000 to December 2010 (almost eleven years), covered a significant part of solar cycle 23 (descending phase of SC23) and the very beginning of solar cycle 24 (ascending phase of SC24). There were 1337 nights of observations with 666 nights with EPB events, which translates to an occurrence rate of ∼ 49.8%. Of these nights, 1290 were considered quiet (Dst ≥ -50 nT) with ∼ 50% plasma bubbles occurrence rate. The occurrence of EPBs was studied under three solar activity phases – high solar activity phase (HSA, September 2000 – March 2004), moderate solar activity phase (MSA or descending phase, March 2004 – October 2006) and low solar activity phase (LSA, October 2006 – December 2010). The low solar activity phase is part of the peculiar solar cycle 24, considered the weakest in over a century with most sunspotless days (2008–2009). The maximum occurrence of bubbles, equal to 54.2%, was found during the HSA phase, with percentages characterizing MSA and LSA being respectively 52.4% and 45.8%. The analysis also showed clear seasonal variation in the EPB occurrence with maximum rates in summer, spring, autumn, and the minimum rates in winter for all solar activity phases. Overall, there was observed solar cycle variation in each season with maximum occurrence in HSA followed by MSA and then LSA except in autumn where higher occurrence rate was observed in LSA phase than in MSA phase.