Search

This work presents the contribution of solar atmospheric tides (diurnal, semidiurnal, and terdiurnal modes) to the variability of the parameters critical frequency (foF2) and peak height of the F2-layer (hmF2) in the American sector during the transition of solar cycles #23 and #24, a period considered one of the lowest solar activities of the modern era. The Digisonde data available in the GIRO data center were analyzed (12 stations), and the solar tide modes were evaluated regarding their amplitude, latitude, and seasonal dependence. The results showed that the hmF2 and foF2 strongly depend on latitude and seasonality, being more intense in the stations located in the south hemisphere. The same behavior is seen for the tidal amplitude fitted in these parameters, except for hmF2 diurnal tide, which is more intense at latitudes farther from the equator. Moreover, the seasonal variability of the amplitude of hmF2 in most cases presented an annual and semiannual component. A terannual component was also observed in 8 h tide mode in the height and frequency parameters. Likewise, what was observed in foF2, the variability in the mean amplitude and different modes of tides of hmF2 are higher over the sectors located in the southern hemisphere. [ABSTRACT FROM AUTHOR]

• Solar and ionospheric proxies have been evaluated for foF2 using IRI-Plas 2020. • Mg-II appeared to be the best index for foF2 estimation over the stations under study. • The findings may be helpful for operational HF communication support. The present study evaluates the ionospheric and solar proxy indices for the International Reference Ionosphere extended to the Plasmasphere (IRI-Plas 2020) model in predicting the critical frequency of F2-layer ( f o F 2 ) over Pakistan and Japan longitudinal sectors during different solar activity epochs. The f o F 2 values are obtained from the ionosondes installed at Wakkanai (45.39° N, 141.69° E), Kokubunji (35.72° N, 139.48° E), Islamabad (33.71° N, 73.06° E), Okinawa (26.68° N, 128.15° E) & Karachi (24.95° N, 67.14° E) during 1993, 2000 & 2007 representing moderate, high & low solar activity, MSA, HSA and LSA, respectively. The IRI-Plas 2020 estimated f o F 2 values being reported were computed using F10.7, IG, Mg-II and Lyman- α with CCIR coefficients selecting "no extra input" option. The analysis showed that the model performed the best with the solar proxy Mg-II among all other solar and ionospheric indices on the basis of the Root Mean Square Error (RMSE) between the ionosonde and IRI-Plas estimated f o F 2 values. Such performance evaluation of a global model is important for space weather community to validate ionosonde data by selecting the appropriate index to model f o F 2 in the considered longitudinal sectors. The findings of the study may be used for operation sky wave communication support in the regions under study. [ABSTRACT FROM AUTHOR]

• IRI 2016, IRI Plas 2017 and SAMI2 overestimate the TEC during low solar activities. • The highest overestimation is observed for IRI Plas. • The assimilation of TEC into the IRI Plas model improves the model performance. • For low solar activity, the drift model in SAMI2 needs to be replaced or updated. We have examined the performance of the International Reference Ionosphere (IRI 2016), International Reference Ionosphere extended to Plasmasphere (IRI-Plas 2017), and Sami2 is Another Model of the Ionosphere (SAMI2) models for long and deep solar minimum quiet time period around 100°E by comparing the models with the total electron content (TEC) obtained from a chain of Global Navigation Satellite Systems receivers. Results of the study reveal that; all the models overestimate the observed TEC for all stations and seasons, with the highest overestimation shown by IRI Plas. The IRI 2016 model performed better for all stations and seasons than IRI Plas. IRI Plas 2017 model is also simulated by assimilating the TEC derived from GPS (GPS-TEC) and Global Ionospheric Map (GIM-TEC) into the model code. The "no input" option of the model is used as a reference to study the effect of data assimilation on the model's efficacy. It is observed that the assimilation of GPS TEC and GIM-TEC into the model code reproduces TEC quite well for all the quiet days considered during both solar minima. IRI-Plas and IRI 2016 simulation is also compared with SAMI2 simulations for the same space–time configuration. SAMI2 also overestimates the quiet time GNSS TEC during daytime peak hours in all northern and southern hemisphere. Changes are made to the SAMI2 model inputs based on previously reported changes during deep solar minimum. By changing the ionizing EUV, neutral thermospheric composition, and neutral temperature, a good agreement between the simulated and measured data is obtained. However, due to strong tidal effects during low solar activity, additional modifications may be needed to the neutral density, temperature, and EUV. The drift model used in SAMI2 should also be replaced or modified for the low solar activity periods. [ABSTRACT FROM AUTHOR]

Plasma blobs are night-time ionospheric irregularities whose generation mechanism is still under investigation. A large number of observations highlighted several aspects of their morphology and dynamics. However, the plasma blobs have not been attributed convincingly to a known mechanism. We analyzed the OI 630.0 nm emission images during March and October of 2019 and 2020 (minimum solar activity) using the ground-based all-sky imager at ZF-2 (2.58° S, 60.22° W) in the Amazon region of Brazil. The novelties of the present study are the rarely reported observation of both plasma blob merging and bifurcation. We studied the evolutional dynamics of plasma blobs and observed that blobs are distinct phenomena with unique properties. We attribute the merging of plasma blobs to the "wind reversion effect" (WRE) mechanism caused by a change in the direction of the zonal thermospheric wind from east to west. In some cases, the slower-drifting plasma blobs may merge with the faster ones. Moreover, blobs were observed initially bifurcating at the topside and later divided into two. The activity of the polarized electric field inside the plasma bubble mapping along the magnetic field lines is possibly responsible for the blob's bifurcation. Subjecting the two features of ionospheric plasma blobs to simulation may reveal further the physics of blobs' merging and bifurcation. [ABSTRACT FROM AUTHOR]

We report the preliminary inter-satellite comparisons of the in situ ion density measurements by the ion velocity meter (IVM) onboard FORMOSAT-7/COSMIC-2 (F7/C2) and Ionospheric Connection Explorer (ICON) missions, during the solar minimum period of December 2019 to November 2020. The initial comparisons reveal identical diurnal, seasonal, and latitude/longitude variations in the two ion-density measurements, with F7/C2 consistently yielding stronger values than ICON, which could partly result from the difference in their orbit altitudes. The diurnal variation in the equatorial region did not show any effect of pre-reversal enhancement (PRE) during 2019–2020. The daytime plasma distributions show larger ion densities over a narrow latitudinal belt around the geomagnetic equator in all seasons, and the low-latitude densities reveal signatures of hemispherical asymmetry, with larger values occurring in the summer hemisphere. The observations also reveal distinct wavenumber-4 longitudinal modulation, which is most prominent in equinox and becomes less distinguishable during December solstice months. The simultaneous observations from F7/C2 IVM and ICON IVM also provide opportunities to study the spatial configuration and time evolution of ionospheric irregularities in the equatorial and low latitude regions. The F7/C2 and ICON simultaneously observed the equatorial plasma bubbles (EPBs) occurring around Taiwan on 18 October 2020, and the observations are consistent with each other. The EPBs were also observed by an all-sky imager located in Taiwan, comparing the satellite observations. Key points: F7/C2 IVM shows similar patterns of diurnal, seasonal and latitude/longitude variations of ion density to ICON IVM but with stronger magnitudes. Distinct latitudinal and longitudinal variations of plasma distributions along seasons were observed during 2019-2020. Simultaneous observations by the multi-satellite constellation of F7/C2 and ICON and all-sky imager provide opportunity to monitor evolutions of EPBs. [ABSTRACT FROM AUTHOR]

Equinoctial asymmetry (EA) of F2-layer critical frequency ( f o F 2) during minimum years of solar cycle 22–23 (1996) and 23–24 (2008) has been studied for low and mid latitude stations ranges from 20°N - 55° N. Year 2008 provided an opportunity to study this phenomenon in extreme low EUV conditions. Pronounced/reasonable increase in EA strength at low/mid latitudes from 1996 to 2008 indicates role of EUV in EA. In addition to an increase in strength of EA from 1996 to 2008, variation in its strength and sign (positive if Mar Equinox f o F 2 means > Sep Equinox f o F 2 means and vice versa) with latitude and longitude are also observed during both years. Similar to previous studies, EA is observed in daytime during both years. However at low latitude stations, two peaks of f o F 2 are also observed at night: the first peak in summer and the second peak in the beginning of the September equinox which shows superposition of semi-annual and annual components. Hourly analysis showed asymmetry (Sep Equinox f o F 2 means > Mar Equinox f o F 2 means ) from sunset to mid-night at low mid to mid latitude. It appears that the Mid latitude Summer Nighttime Anomaly (MSNA) is extending to September equinox with decreasing magnitudes from Summer to Sep equinox. IRI-16 showed EA during daytime only with over- and under- estimation as compared to observed data. Serious discrepancy between observed and IRI-2016 model values especially during deep minimum suggests modification of model for extreme low EUV condition as well as incorporation of parameters related to global wind pattern in the model. [ABSTRACT FROM AUTHOR]

Magnetic reconnection can be used for studying the geoeffective processes in the coupled Sun--Solar wind -- Magnetosphere dynamics leading to geomagnetic disturbance. In this study, 1-hour resolution solar wind plasma parameters from OMNIweb were used to investigate the relationship between moderate southward interplanetary magnetic field, IMF-Bz (i.e., Bz > -10 nT) and geomagnetic storm time disturbance, Dst, during the ascending, maximum and descending phases of solar cycle 23. Occurrences of different classes of geomagnetic storms during moderate southward Bz are reported. The occurrence of weak and moderate geomagnetic storms is more predominant during maximum solar activity than intense and super intense storms. It was found that 10.11 % (181) of all the classes of the storm were intense, and 0.17 % (3) were super intense storms. Furthermore, it was found that 4 (2.2 %) out of the 181 intense storms were caused by southward Bz > -10 nT which were associated with the complex structure due to the high-speed solar wind stream and corotating interacting region. In such a complex structure and Bz > -10nT, we observed that an intense geomagnetic storm rarely occurs and if it does, would be predominant around solar maximum. It was found that longduration (Δt > 6 hrs) of southward Bz (i.e., -10 nT < Bz ≤ -3:6 nT) can also lead to an intense geomagnetic storm during the solar maximum and descending phase (moderate solar activity) of a solar cycle. The complex structure of intense geomagnetic storms associated with the Bz > -10nT is rare and possesses a special configuration of magnetic field and solar wind parameters structures which are CIR manifestations. [ABSTRACT FROM AUTHOR]

In situ measurements by the Low Earth Orbital (LEO) satellites, such as CHAMP, GRACE, and C/NOFS satellites have shown that the International Reference Ionosphere (IRI) model has shortcomings in describing the solar activity variation of the topside electron density. In particular IRI overestimates the measured densities in the topside ionosphere during the very low solar activity reached during the last solar minimum (2008–2009). We have used Alouette and ISIS topside sounder data and CHAMP, GRACE, as well as Swarm in situ measurements to deduce a correction term for the IRI electron density topside model that more accurately describes the variation with solar activity. We have used a linear variation with the solar index PF10.7 and described the latitudinal and altitudinal variation of the regression parameters A0 and A1 (intercept and slope). We find good agreement between the regression parameters deduced from the topside sounder and from the CHAMP and GRACE observations. Swarm results show the same latitudinal structure as the other data sets, however, a scaling factor is needed to obtain agreement of the absolute values. The new model was evaluated with Alouette and ISIS topside sounder, as well as LEO satellites in situ data showing a significant improvement over the current IRI model. [ABSTRACT FROM AUTHOR]

The SDO/AIA 193 Å and ACE/SWEPAM data acquired in 2019 are used to compare the effects of polar and equatorial coronal holes (CHs) on solar wind (SW) characteristics under conditions of low solar activity. As expected, most geomagnetic storms in this period were caused by high-speed SW streams ( km s ) originating from equatorial CHs. At the same time, it has been shown that at a deep solar activity minimum polar CHs can exert a noticeable influence on SW characteristics at the Earth's orbit. A correlation with a correlation coefficient of 0.8 has been found for the integrated polar CH area and the SW speed in the investigated period. The southern polar CH, for which a correlation with the SW speed with a correlation coefficient of 0.82 was found in the spring of 2019 (in the period when the south solar pole was maximally tilted to the Earth), exerted a particularly significant influence on the SW speed. The northern polar CH had virtually no effect on the SW speed. An anticorrelation of the polar CH area with the SW speed at the Earth's orbit was found in the fall of 2019, in the period when the north solar pole was tilted to the Earth. We discuss a possible mechanism for the influence of polar CHs on SW characteristics and propose an interpretation of the results obtained. [ABSTRACT FROM AUTHOR]

To study synodic period and its subharmonics, we have utilized hourly average data of solar wind plasma field, geomagnetic index, and galactic cosmic ray intensity along with 5 min average data of selected solar wind field parameters during the recent minimum of Solar Cycle 24. Wavelet analyses of hourly average data reveal 28.1, 13.1, and 6.8 day of sigma in interplanetary magnetic field; 27.1, 13.1, and 9.0 day of plasma speed; 27.1, 13.6, 3.9, and 2.2 day of interplanetary electric field; 27.1, 13.6, 3.8, and 2.2 day of southward component of magnetic field; 26.2, 13.6, 8.6, and 3.0 day of geomagnetic Ap index; and 28.1, 11.0, 9.3, 7.0, and 1.6 day periods of galactic cosmic ray intensity. In addition to these prominent short‐term periods, we have significantly observed 6.6, 3.3, and 1.7 day periods of sigma in interplanetary magnetic field (σB); 5.6, 4.2, and 2.2 day of electric field (Ey) and 5.6, 3.3, and 2.4 day of southward component of magnetic field (Bz) in the 5 min average data. During the recent solar minimum, we have observed one solar rotational period and its second, third, fourth, fifth, sixth, seventh, eighth, tenth, eleventh, and twelfth subharmonics. The observed subharmonics of solar rotational period make the current solar minimum very special and noteworthy to study. Key Points: Fourth subharmonics of variance of IMF is stronger than synodic periodSignature of 3.8, 2.2 days periods in the hourly average Ey and Bz time seriesSignificant presence of fifth, sixth, eighth, eleventh, and twelfth subharmonics in high‐resolution data [ABSTRACT FROM AUTHOR]

• Observation of ionospheric irregularities over common ionospheric volume by optical and radio measurements. • Ionospheric irregularities observed beyond northern crest of EIA during solar minimum. • Ionospheric irregularities observed at L-band during late evening and midnight hours during solar minimum. The present paper reports coordinated ionospheric irregularity measurements at optical as well as GPS wavelengths. Optical measurements were obtained from Tiny Ionospheric Photometer (TIP) sensors installed onboard the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites. GPS radio signals were obtained from a dual frequency GPS receiver operational at Calcutta (22.58°N, 88.38°E geographic; geomagnetic dip: 32.96°; 13.00°N, 161.63°E geomagnetic) under the SCIntillation Network Decision Aid (SCINDA) program. Calcutta is located near the northern crest of Equatorial Ionization Anomaly (EIA) in the Indian longitude sector. The observations were conducted during the unusually low and prolonged solar minima period of 2008–2010. During this period, four cases of post-sunset GPS scintillation were observed from Calcutta. Among those cases, simultaneous fluctuations in GPS Carrier-to-Noise ratios (C/N o) and measured radiances from TIP over a common ionospheric volume were observed only on February 2, 2008 and September 25, 2008. Fluctuations observed in measured radiances (maximum 0.95 Rayleigh) from TIP due to ionospheric irregularities were found to correspond well with C/N 0 fluctuations on the GPS links observed from Calcutta, such effects being noted even during late evening hours of 21:00–22:00 LT from locations around 40° magnetic dip. These measurements indicate the existence of electron density irregularities of scale sizes varying over several decades from 135.6 nm to 300–400 m well beyond the northern crest of the EIA in the Indian longitude sector during late evening hours even in the unusually low solar activity conditions. [ABSTRACT FROM AUTHOR]

The variability of NmF2 during solar minima of solar cycle 21–22, 22–23 and 23–24 has been studied for two low latitude stations, Karachi (Pakistan) and Okinawa (Japan), as both stations are situated at the Equatorial Ionization Anomaly (EIA) crest region. Diurnal, seasonal and annual behaviour of variability has been investigated using relative standard deviation approach. At both the stations, a direct relation of variability (CV) with Smoothed Sunspot Numbers (SSN) is noted which is opposite to the results found by many authors who have worked on different solar activity periods. It is suggested that the E-layer is responsible for this direct relation as an increase in the E-layer ionization with decrease in SSN for solar minimum has also been noted for Karachi in our previous work (Talha et al., 2019). The interaction of tides and waves with the E-layer might be responsible for the increase in ionization of the E-layer and the decrease in variability in the F-layer. Also, the night-time variability is observed to be higher than the daytime variability during minima as expected due to the low mean NmF2 or large deviation during night-time. Very prominent post-midnight peaks and not very well shaped post-sunset peaks are also noticed at both stations. Besides the mentioned peaks, a small increase in variability during mid-day is noticed which might be due to the flow of fountain plasma from the equator to the EIA crest region. Seasonal comparison of variability showed high variability during daytime in winter and during night-time in equinox while lowest variability is noted in summer. Lowest variability at post-midnight is noted in winter instead of summer which might be due to the complex interaction of waves and tides with the lower ionosphere. Small difference in variability between both stations is also noticed due to difference in their longitude. Geomagnetic field configuration at both stations is different which in turn causes change in the electron density and also difference in variability at both stations. [ABSTRACT FROM AUTHOR]

The rapid increase in the comparative studies between the Global Positioning System (GPS) measured Total Electron Content (TEC) and model-predicted TEC on Earth helps in mitigating the biases in the ionospheric delays of radio waves. In this paper, an evaluation on the performance of the latest International Reference Ionosphere (IRI-2016) modelled TEC is conducted with GPS TEC measurements in different regions of Pakistan during the descending phase of solar cycle 24 (2015–2017). We have analyzed the diurnal, monthly and seasonal variabilities in the measured TEC and compared with IRI-2016 modelled TEC. The diurnal variability of measured TEC is found to be maximum during 06:00–12:00 UT (11:00–17:00 LT) and minimum during 21:00–24:00 UT (02:00–05:00 LT). Moreover, maximal and minimal monthly measured TEC for each station and model are also observed in April and December, respectively. In the summer solstice months at Islamabad station during 2015–2017, the measured TEC are larger than the corresponding modelled TEC by about 19%, 22% and 24%, respectively for each year. On the other hand, the difference between the measured and modelled TEC in summer solstice at Multan station during 2015 is 25% of the normal distribution and at Quetta station during 2016 the difference is about 29% in the summer solstice. The results show that the TEC measurements from GPS coincide with TEC from IRI-2016 during the selected days by a correlation of more than 0.86, while a very high correlation coefficient (0.91–0.99) is shown during each month and season over all the stations for the years 2015–2017. IRI-2016 is a good climate model that can provide accurate estimates of ionospheric delays of radio signals propagated over Pakistan. [ABSTRACT FROM AUTHOR]

The ionospheric electron density peak ( Nm F2) is analyzed for the recent minima of solar activity for two mid-latitude stations, Rome (41.8°N, 12.5°E, geomagnetic latitude 41.7°N, Italy) and Gibilmanna (37.9°N, 14.0°E, geomagnetic latitude 37.6°N, Italy), and for the low-latitude station of Tucumán (26.9°S, 294.6°E, geomagnetic latitude 17.2°S, Argentina), located in the south ridge of the equatorial ionization anomaly. An inter-minima comparison reveals that from an ionospheric point of view the last minimum of solar activity (minimum 23/24) was peculiar, with values of Nm F2 lower than those recorded during the previous minima for all the stations and all the hours of the day. A more pronounced decrease is observed at Tucumán than at Rome and Gibilmanna. The study of the winter and semi-annual anomaly shows that at mid-latitude stations the winter anomaly is not visible only for the years 2008 and 2009, which represent the deeper part of the prolonged and anomalous last solar minimum. The same is for the semi-annual anomaly. A comparison with the version 2012 of the International Reference Ionosphere model (IRI) is also carried out. The results reveal that for low solar activity the model works better at mid latitudes than at low latitudes, confirming the problems of IRI in correctly representing the low-latitude ionosphere. Nevertheless, using as input updated values of the solar and geomagnetic indices, no loss of accuracy is detected in the IRI performances for the last solar minimum with respect to the previous ones, both at mid and low latitudes. [ABSTRACT FROM AUTHOR]

During the years of high and moderate activity, the energetic flux of electrons at geostationary region increases compensated by the losses and as a result, the overall level of the population persists. The transitions from high to low intensity and back by 3–4 orders of magnitude are fast within hours and caused by the magnetic disturbances, storms and substorms. During the years of the low activity, the balance of increases and losses is violated; the whole flux of electrons is reduced. Particularly noticeable was the devastation of the outer belt during the minimum of 2009–2010 years. The slow type of decreases became most active. The transition from the fast type to the slow type of decreases was created by the prolonged periods of the low substorm activity. The slow type of intensity decreases is mostly probable caused by the losses on the magnetopause. [ABSTRACT FROM AUTHOR]

To achieve deeper understanding of climate variability during the last millennium in SE Europe, we report new sedimentological and paleoecological data from Etoliko Lagoon, Western Greece. The record represents the southernmost annually laminated (i.e., varved) archive from the Balkan Peninsula spanning the Little Ice Age, allowing insights into critical time intervals of climate instability such as during the Maunder and Dalton solar minima. After developing a continuous, ca. 500-year-long varve chronology, high-resolution μ –XRF counts, stable-isotope data measured on ostracod shells, palynological (including pollen and dinoflagellate cysts), and diatom data are used to decipher the season-specific climate and ecosystem evolution at Etoliko Lagoon since 1450 AD. Our results show that the Etoliko varve record became more sensitive to climate change from 1740 AD onwards. We attribute this shift to the enhancement of primary productivity within the lagoon, which is documented by an up to threefold increase in varve thickness. This marked change in the lagoon's ecosystem was caused by: (i) increased terrestrial input of nutrients, (ii) a closer connection to the sea and human eutrophication particularly from 1850 AD onwards, and (iii) increasing summer temperatures. Integration of our data with those of previously published paleolake sediment records, tree-ring-based precipitation reconstructions, simulations of atmospheric circulation and instrumental precipitation data suggests that wet conditions in winter prevailed during 1740–1790 AD, whereas dry winters marked the periods 1790–1830 AD (Dalton Minimum) and 1830–1930 AD, the latter being sporadically interrupted by wet winters. This variability in precipitation can be explained by shifts in the large-scale atmospheric circulation patterns over the European continent that affected the Balkan Peninsula (e.g., North Atlantic Oscillation). The transition between dry and wet phases at Etoliko points to longitudinal shifts of the precipitation pattern in the Balkan Peninsula during the Little Ice Age. [ABSTRACT FROM AUTHOR]

The performance of NeQuick 2 model in computing the vertical total electron content (VTEC) over a wide region placed at middle latitudes of North America during the deep solar minimum that occurred in 2008 has been checked. The long term relationship between EUV irradiance and F10.7 solar flux has changed markedly during the cycle 23/24 minimum with EUV levels decreasing more than expected from F10.7 proxy. A decrease of ionization in the ionosphere could have occurred. Thus, it could be expected that the models overestimate the value of ionospheric parameters for that deep solar minimum. For this study a high density VTEC data grid that covers the Continental United States (CONUS) has been compared with monthly median maps constructed with NeQuick 2. The results show that NeQuick 2 generally gives good predictions for the region which lies between 35°N to 50°N suggesting that nothing exceptional was happening during the 2008 minimum in terms of VTEC NeQuick 2's predictive capabilities. Taking into account that the modeled value is obtained by integration in height of the electron density profile, NeQuick2 would be assuming an inadequate profile for the few highest deviations observed, between 30°N and 35°N. Overall, the model does not give significant overestimation of VTEC as could be expected. [ABSTRACT FROM AUTHOR]

The Mars' ionospheric behavior during two consecutive solar minima (23/24 and 24/25) is investigated with the same dataset. In particular, we use the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on board Mars Express to investigate the total electron content behavior of the whole atmosphere in relation to the solar irradiance (EUV and X-ray fluxes), the solar zenith angle and the heliocentric distance. The topside variability of the electron density profiles is also investigated through variations in the peak density and neutral scale height. Moreover, the equations of the NeMars empirical model of the Martian ionosphere for low solar activity are tested for both minima. We have found that the topside ionosphere of Mars behaved similarly at both solar minima. However, when considering the bottomside, a pronounced reduction in ionization in particular cases is suggested. In addition, larger TEC values are found during the solar minimum 24/25 in the nightside sector that may indicate possible larger plasma transport than during the minimum 23/24. Finally, this study confirms that the ionospheric empirical NeMars model equations derived by Sanchez-Cano et al. (2016) for the low solar activity period during the solar minimum 23/24 are also valid and accurate for the solar minimum 24/25. The long duration of Mars Express is a critical factor for determining the long-term Martian ionospheric variability, which in turn, is essential for understanding the global evolution of the planet's atmosphere. • Mars' ionosphere topside behaves similarly at both solar minima. • The bottomside ionosphere may be the most affected region during low solar activity. • NeMars empirical model equations are valid for solar minima 23/24 and 24/25. [ABSTRACT FROM AUTHOR]

The 2008–2009 solar minimum period was unprecedentedly deep and extended. We compare the IRI-2012 with global TEC data from JASON-1 satellite and with electron density profiles observed from incoherent scatter radars (ISRs) at middle and high latitudes for this solar minimum period. Global daily mean TECs are calculated from JASON-1 TECs to compare with the corresponding IRI TECs during the 2008–2009 period. It is found that IRI underestimates the global daily mean TEC by about 20–50%. The comparison of global TEC maps further reveals that IRI overall underestimates TEC for the whole globe except for the low-latitude region around the equatorial anomaly, regardless of season. The underestimation is particularly strong in the nighttime winter hemisphere where the ionosphere seems to almost disappear in IRI. In the daytime equatorial region, however, the overestimation of IRI is mainly due to the misrepresentation of the equatorial anomaly in IRI. Further comparison with ISR electron density profiles confirms the significant underestimation of IRI at night in the winter hemisphere. [ABSTRACT FROM AUTHOR]

The ionospheric slab thickness is a measure of the shape of the ionospheric profile and can be obtained from the ratio of total electron content (TEC) to the F-region peak electron density (N m F 2 ). In this study the ionospheric slab thickness was obtained during one year of extremely low solar activity (from March 2009 to February 2010). The period analyzed were separated in three seasonal groups: equinoxes, June solstice (winter) and December solstice (summer) months. The ionospheric slab thickness was studied at two stations located around the southern crest of the equatorial ionization anomaly (EIA). One station located in the inner edge of the anomaly region, named Palmas (10.12° S, 48.21° O, 7.73° S dip lat), and another station located under the southern crest of the anomaly, situated at São José dos Campos (23.07° S, 45.52° O, 19.61° S dip lat). The TEC data have been obtained from dual-frequency Global Navigation Satellite System (GNSS) receivers and the N m F 2 data were calculated from the f o F 2 parameter scaled from ionograms recorded by simultaneous measurements of digital ionosondes. The diurnal and seasonal variations of the ionospheric slab thickness at the EIA region were then analyzed. The minimum values of TEC and N m F 2 were observed in both stations during the early morning, and the maximum values during afternoon hours. During the summer and equinoctial periods the slab thickness reached their highest values. The nearly equatorial site of Palmas showed values of daytime slab thickness larger than those observed at the low latitude station of São José dos Campos, except during the June solstice months. At São José dos Campos, a pronounced pre-dusk increase in the equivalent slab thickness was observed during the winter solstice months. The atmospheric neutral temperature ( T n ) was also estimated from the correlation analysis between the MSIS modeled T n and the calculated ionospheric slab thickness over the two observations sites. Other relevant aspects of the ionospheric slab thickness behavior and its comparison with International Reference Ionosphere (IRI) model results will be presented and discussed. [ABSTRACT FROM AUTHOR]

The recent, unusual solar cycle and solar minima between Cycles 23 and 24 have been studied extensively. Wavelet analysis of hourly cosmic-ray intensity during the last five solar minima reveals a number of short-term variations with few temporal shifts in the periods. This study suggests that deviations with a one-solar-rotation period and its harmonics are small during the recent minimum. However, deviations are considerably larger during the other minima. Analysis also demonstrates that the behavior of active regions is nearly the same during the minima of Cycles 19, 20, 21, and 22. The results also suggest that regions outside the streamer stalk are significantly larger in the recent solar minimum as opposed to the other minima. [ABSTRACT FROM AUTHOR]

This research investigates anomalous nighttime ion density behaviour over the Kharkiv, Ukraine incoherent scatter radar (49.6° N, 36.3° E, 45.3° inv) during the equinoxes of 2006–2010. The observations show that the altitude of the transition from O + to lighter ions was much lower than empirical and physical models predict. The standard physical model produces very good agreement for the O + densities but underestimates the H + densities by a factor of 2 in March 2006 and a factor of 3 in March 2009. The anomalously low transition height is a result of similar lowering of the ionospheric peak height and also of significantly increased H + density. The lower ionospheric peak height may be caused by weaker nighttime neutral winds. The calculations indicate that the higher measured topside ionosphere H + densities are most likely due to higher neutral hydrogen densities. Both factors could be the result of weaker than usual magnetic activity, which would reduce the energy input to high latitudes. Prolonged low activity periods could cause a global redistribution of hydrogen and also allow more neutral hydrogen to settle down from the exosphere into the mid-latitude ionosphere. The finding of the need for higher H densities agrees well with recent H-alpha airglow measurements and it is important for accurate modelling of plasmasphere refilling rates and night-time N m F 2 values. [ABSTRACT FROM AUTHOR]

We report on in situ measurements of equatorial plasma depletions in the dawn sector from the DMSP F13 satellite during magnetic disturbances which occurred within the solar minimum period of 1995–1996. Examples are given when DMSP F13 observed prominent pre-sunrise depletions associated with weak magnetic disturbances, while the most severe magnetic storms of the period showed only small amplitude depletions of plasma density. Our results indicate that the depletions were observed within about two third of the magnetically perturbed days with the negative maximum excursions of SYM-H less than −40 nT at any season and at various longitudes. Commonly, the depletions appear following the negative SYM-H peaks in the recovery phase of the magnetic disturbances. We discuss the roles of corotating evening depletions and near-dawn effects of the prompt penetration overshielding and the disturbance dynamo eastward electric fields as sources for the reported depletions. [ABSTRACT FROM AUTHOR]

Solar activity during 2008 - 2009 was extremely low among several recent solar cycles, raising an interesting question regarding the temporal and spatial distribution of the plasma parameters in ionosphere during this time period. This study analyzes the electron density (Ne) data recorded by the DEMETER satellite at the height of 670 km and compares it with the International Reference Ionosphere (IRI) 2007 model with special emphasis on the solar minimum year of 2008. The results show that in local nighttime around 22:30 the global distribution of Ne by the DEMETER satellite exhibited similar behavior as that by IRI model, but Ne is overestimated above 100% by the IRI in equatorial and beyond geomagnetic latitudes of 50°, while underestimated by -70% at 20° - 40° in the north or south hemisphere during different seasons. In local daytime around 10:30, Ne values are mostly overestimated by the IRI model especially in the equatorial area where double crests were exhibited clearly at ±10° in the IRI-NeQuick model, but only one crest around 0° - 10°N shown by DEMETER. Combined with other satellite data double crests of Ne over the equatorial area may gradually evolve into a single peak near LT 10:30 at 600 km height. The DEMETER comparison with three options in IRI 2007 revealed that only the IRI-2001 option gave a single crest as with DEMETER, and three IRI options all largely overestimated the crest Ne values from DEMETER in local daytime. The results in this paper provide new information for improving the IRI model in the future, helping to understand the ionosphere and upper atmosphere physics, with applications in communications, navigation and spacecraft orbit determination. [ABSTRACT FROM AUTHOR]

Abstract: The paper is focused on ionospheric response to occasional magnetic disturbances above selected ionospheric stations located at middle latitudes of the Northern and Southern Hemisphere under extremely low solar activity conditions of 2007–2009. We analyzed changes in the F2 layer critical frequency foF2 and the F2 layer peak height hmF2 against 27-days running mean obtained for different longitudinal sectors of both hemispheres for the initial, main and recovery phases of selected magnetic disturbances. Our analysis showed that the effects on the middle latitude ionosphere of weak-to-moderate CIR-related magnetic storms, which mostly occur around solar minimum period, could be comparable with the effects of strong magnetic storms. In general, both positive and negative deviations of foF2 and hmF2 have been observed independent on season and location. However positive effects on foF2 prevailed and were more significant. Observations of stormy ionosphere also showed large departures from the climatology within storm recovery phase, which are comparable with those usually observed during the storm main phase. The IRI STORM model gave no reliable corrections of foF2 for analyzed events. [Copyright &y& Elsevier]

Sixty days of Doppler images from the Solar and Heliospheric Observatory (SOHO) / Michelson Doppler Imager (MDI) investigation during the 1996 and 2008 solar minima have been analyzed to show that certain supergranule characteristics (size, size range, and horizontal velocity) exhibit fluctuations of three to five days. Cross-correlating parameters showed a good, positive correlation between supergranulation size and size range, and a moderate, negative correlation between size range and velocity. The size and velocity do exhibit a moderate, negative correlation, but with a small time lag (less than 12 hours). Supergranule sizes during five days of co-temporal data from MDI and the Solar Dynamics Observatory (SDO) / Helioseismic Magnetic Imager (HMI) exhibit similar fluctuations with a high level of correlation between them. This verifies the solar origin of the fluctuations, which cannot be caused by instrumental artifacts according to these observations. Similar fluctuations are also observed in data simulations that model the evolution of the MDI Doppler pattern over a 60-day period. Correlations between the supergranule size and size range time-series derived from the simulated data are similar to those seen in MDI data. A simple toy-model using cumulative, uncorrelated exponential growth and decay patterns at random emergence times produces a time-series similar to the data simulations. The qualitative similarities between the simulated and the observed time-series suggest that the fluctuations arise from stochastic processes occurring within the solar convection zone. This behavior, propagating to surface manifestations of supergranulation, may assist our understanding of magnetic-field-line advection, evolution, and interaction. [ABSTRACT FROM AUTHOR]

Solar activity during 2007 – 2009 was very low, causing anomalously low thermospheric density. A comparison of solar extreme ultraviolet (EUV) irradiance in the He  ii spectral band (26 to 34 nm) from the Solar Extreme ultraviolet Monitor (SEM), one of instruments on the Charge Element and Isotope Analysis System (CELIAS) on board the Solar and Heliospheric Observatory (SOHO) for the two latest solar minima showed a decrease of the absolute irradiance of about 15±6 % during the solar minimum between Cycles 23 and 24 compared with the Cycle 22/23 minimum when a yearly running-mean filter was used. We found that some local, shorter-term minima including those with the same absolute EUV flux in the SEM spectral band show a higher concentration of spatial power in the global network structure from the 30.4 nm SOHO/ Extreme ultraviolet Imaging Telescope (EIT) images for the local minimum of 1996 compared with the minima of 2008 – 2011. We interpret this higher concentration of spatial power in the transition region’s global network structure as a larger number of larger-area features on the solar disk. These changes in the global network structure during solar minima may characterize, in part, the geo-effectiveness of the solar He  ii EUV irradiance in addition to the estimations based on its absolute levels. [ABSTRACT FROM AUTHOR]

Abstract: The recent availability of new data sets during the recent extreme solar minimum provides an opportunity for testing the performance of the International Reference Ionosphere in historically undersampled regions. This study presents averages and variability of topside ionospheric densities over Africa as a function of season, local time, altitude, and magnetic dip latitude as measured by the Communication/Navigation Outage Forecast System (C/NOFS) satellite. The results are compared to the three topside model options available in IRI-2007. Overall, the NeQuick model is found to have the best performance, though during the deepest part of the solar minimum all three options significantly overestimate density. [Copyright &y& Elsevier]

Highlights: [•] Galactic Cosmic Ray (GCR) exposure during the deep solar minimum in 2009 calculated. [•] Assessment for near-Earth interplanetary space & ISS orbit using GCR model by Matthia et al., 2013a. [•] Highest absorbed dose & dose equivalent rates estimated in late 2009 since 1970. [•] Peak dose rates in 2009 compared with estimates from other GCR models. [•] Badhwar-O’Neill2010, CREME96 & CREME2009 GCR models produce lower dose values. [Copyright &y& Elsevier]

Abstract: The end of the last solar cycle was at least 3 years late, and to date, the new solar cycle has seen mainly weaker activity since the onset of the rising phase toward the new solar maximum. The newspapers now even report when auroras are seen in Norway. This paper is an update of our review paper written during the deepest part of the last solar minimum [1]. We update the records of solar activity and its consequent effects on the interplanetary fields and solar wind density. The arrival of solar minimum allows us to use two techniques that predict sunspot maximum from readings obtained at solar minimum. It is clear that the Sun is still behaving strangely compared to the last few solar minima even though we are well beyond the minimum phase of the cycle 23–24 transition. [Copyright &y& Elsevier]

Abstract: During 2008, the solar activity is extremely low. The satellite observations show that the ionospheric height and electron density is much lower than the predictions by the international reference ionosphere (IRI) model. In this paper, we compared the slant total electron content (TEC) observed by the COSMIC satellites during 2008 with the IRI model results. It is found that the IRI model with IRI2001 and IRI2001 Cor. topside options will always overestimate the electron density in both lower and higher altitudes. But the rest two topside options (NeQuick, and TTS) tend to overestimate the electron density in the F layer and underestimate it in the topside altitudes. The switch altitude between overestimation and underestimation and the latitude-local time distribution of the model deviation depend on the topside option. The current investigation might be useful for the model improvement as well as data assimilation work based on the IRI model and the LEO TEC data. [Copyright &y& Elsevier]

The generally low interplanetary magnetic field magnitude around the minimum between Solar Cycles 23 and 24 (SC 23/24 minimum) allows us to identify weak and small solar wind structures. We use observations from near-Earth and twin STEREO spacecraft to study solar wind conditions from January 2007 through December 2010. In addition to 84 clear interplanetary coronal mass ejections (ICMEs), we identified 58 ICME-like transients, which exhibit some classical ICME signatures but have weak magnetic fields (<7 nT) and/or short durations (<10 hours). The number of ICME-like transients peaked during the SC 23/24 minimum, while the ICME rate increased with increasing solar activity. The magnetic structures of flux rope type ICMEs and transients show similar solar cycle variation trends, suggesting that ICMEs and transients originate from similar polarity regions at the Sun. We observed a gradual transition from ICME-like structures to ICMEs. The identified events display continuous distributions in duration and magnetic field magnitude ranging from a few hours to several days and from a few nanoteslas to a few tens of nanoteslas, respectively. Our ICME-like transient rate (less than one event/month) is considerably smaller than that suggested by solar observations of narrow CMEs. This implies that the majority of small coronal ejections are merged as a part of the solar wind by the time they reach 1 AU. We found that ICME-like transients generally occur closer to stream interaction regions (SIRs) than ICMEs, and the majority of the events we identified in declining parts of fast solar wind streams were ICME-like structures. This suggests that ICME-like transients tend to arise close to coronal hole boundaries and thus may have an important role in coronal hole dynamics. Diverse solar wind transients presumably manifest the variation of solar eruptions from small-scale blobs to wide CMEs. [ABSTRACT FROM AUTHOR]

In this work the galactic cosmic ray modulation in relation to solar activity indices and heliospheric parameters during the years 1996 - 2010 covering solar cycle 23 and the solar minimum between cycles 23 and 24 is studied. A new perspective of this contribution is that cosmic ray data with a rigidity of 10 GV at the top of the atmosphere obtained from many ground-based neutron monitors were used. The proposed empirical relation gave much better results than those in previous works concerning the hysteresis effect. The proposed models obtained from a combination of solar activity indices and heliospheric parameters give a standard deviation < 10 % for all the cases. The correlation coefficient between the cosmic ray variations of 10 GV and the sunspot number reached a value of r=−0.89 with a time lag of 13.6±0.4 months. The best reproduction of the cosmic ray intensity is obtained by taking into account solar and interplanetary indices such as sunspot number, interplanetary magnetic field, CME index, and heliospheric current sheet tilt. The standard deviation between the observed and calculated values is about 7.15 % for all of solar cycle 23; it also works very well during the different phases of the cycle. Moreover, the use of the cosmic ray intensity of 10 GV during the long minimum period between cycles 23 and 24 is of special interest and is discussed in terms of cosmic ray intensity modulation. [ABSTRACT FROM AUTHOR]

Abstract: Cycle 23 was exceptional in that it lasted almost two years longer than its predecessors and in that it ended in an extended minimum period that proved all predictions wrong. Comparisons of the International Reference Ionosphere (IRI) with CHAMP and GRACE in-situ measurements of electron density during the minimum have revealed significant discrepancies at 400–500km altitude (. Our study investigates the causes for these discrepancies with the help of ionosonde and Planar Langmuir Probe (PLP) data from the Communications/Navigation Outage Forecasting System (C/NOFS) satellite. Our C/NOFS comparisons confirm the earlier CHAMP and GRACE results. But the ionosonde measurements of the F-peak plasma frequency (foF2) show generally good agreement throughout the whole solar cycle. At mid-latitude stations yearly averages of the data-model difference are within 10% and at low latitudes stations within 20%. The 60–70% differences found at 400–500km altitude are not seen at the F peak. We will discuss how these seemingly contradicting results from the ionosonde and insitu data-model comparisons can be explained and which parameters need to be corrected in the IRI model. [Copyright &y& Elsevier]

We study solar modulation of galactic cosmic rays (GCRs) during the deep solar minimum, including the declining phase, of solar cycle 23 and compare the results of this unusual period with the results obtained during similar phases of the previous solar cycles 20, 21, and 22. These periods consist of two epochs each of negative and positive polarities of the heliospheric magnetic field from the north polar region of the Sun. In addition to cosmic-ray data, we utilize simultaneous solar and interplanetary plasma/field data including the tilt angle of the heliospheric current sheet. We study the relation between simultaneous variations in cosmic ray intensity and solar/interplanetary parameters during the declining and the minimum phases of cycle 23. We compare these relations with those obtained for the same phases in the three previous solar cycles. We observe certain peculiar features in cosmic ray modulation during the minimum of solar cycle 23 including the record high GCR intensity. We find, during this unusual minimum, that the correlation of GCR intensity is poor with sunspot number (correlation coefficient R=−0.41), better with interplanetary magnetic field ( R=−0.66), still better with solar wind velocity ( R=−0.80) and much better with the tilt angle of the heliospheric current sheet ( R=−0.92). In our view, it is not the diffusion or the drift alone, but the solar wind convection that is the most likely additional effect responsible for the record high GCR intensity observed during the deep minimum of solar cycle 23. [ABSTRACT FROM AUTHOR]

Abstract: The plasmaspheric electron content (PEC) was estimated by comparison of GPS TEC observations and FORMOSAT-3/COSMIC radio occultation measurements at the extended solar minimum of cycle 23/24. Results are retrieved for different seasons (equinoxes and solstices) of the year 2009. COSMIC-derived electron density profiles were integrated up to the height of 700km in order to retrieve estimates of ionospheric electron content (IEC). Global maps of monthly median values of COSMIC IEC were constructed by use of spherical harmonics expansion. The comparison between two independent measurements was performed by analysis of the global distribution of electron content estimates, as well as by selection specific points corresponded to mid-latitudes of Northern America, Europe, Asia and the Southern Hemisphere. The analysis found that both kinds of observations show rather similar diurnal behavior during all seasons, certainly with GPS TEC estimates larger than corresponded COSMIC IEC values. It was shown that during daytime both GPS TEC and COSMIC IEC values were generally lower at winter than in summer solstice practically over all specific points. The estimates of PEC (h >700km) were obtained as a difference between GPS TEC and COSMIC IEC values. Results of comparative study revealed that for mid-latitudinal points PEC estimates varied weakly with the time of a day and reached the value of several TECU for the condition of solar minimum. Percentage contribution of PEC to GPS TEC indicated the clear dependence from the time with maximal values (more than 50–60%) during night-time and lesser values (25–45%) during day-time. [Copyright &y& Elsevier]

The observations both near the Sun and in the heliosphere during the activity minimum between solar cycles 23 and 24 exhibit different phenomena from those typical of the previous solar minima. In this paper, we have chosen Carrington rotation 2070 in 2008 to investigate the properties of the background solar wind by using the three-dimensional (3D) Solar-InterPlanetary Conservation Element/Solution Element Magnetohydrodynamic (MHD) model. We also study the effects of polar magnetic fields on the characteristics of the solar corona and the solar wind by conducting simulations with an axisymmetric polar flux added to the observed magnetic field. The numerical results are compared with the observations from multiple satellites, such as the Solar and Heliospheric Observatory (SOHO), Ulysses, Solar Terrestrial Relations Observatory (STEREO), Wind and the Advanced Composition Explorer (ACE). The comparison demonstrates that the first simulation with the observed magnetic fields reproduces some observed peculiarities near the Sun, such as relatively small polar coronal holes, the presence of mid- and low-latitude holes, a tilted and warped current sheet, and the broad multiple streamers. The numerical results also capture the inconsistency between the locus of the minimum wind speed and the location of the heliospheric current sheet, and predict slightly slower and cooler polar streams with a relatively smaller latitudinal width, broad low-latitude intermediate-speed streams, and globally weak magnetic field and low density in the heliosphere. The second simulation with strengthened polar fields indicates that the weak polar fields in the current minimum play a crucial role in determining the states of the corona and the solar wind. [ABSTRACT FROM AUTHOR]

Supergranulation is a component of solar convection that manifests itself on the photosphere as a cellular network of around 35 Mm across, with a turnover lifetime of 1 - 2 days. It is strongly linked to the structure of the magnetic field. The horizontal, divergent flows within supergranule cells carry local field lines to the cell boundaries, while the rotational properties of supergranule upflows may contribute to the restoration of the poloidal field as part of the dynamo mechanism, which controls the solar cycle. The solar minimum at the transition from cycle 23 to 24 was notable for its low level of activity and its extended length. It is of interest to study whether the convective phenomena that influence the solar magnetic field during this time differed in character from periods of previous minima. This study investigates three characteristics (velocity components, sizes and lifetimes) of solar supergranulation. Comparisons of these characteristics are made between the minima of cycles 22/23 and 23/24 using MDI Doppler data from 1996 and 2008, respectively. It is found that whereas the lifetimes are equal during both epochs (around 18 h), the sizes are larger in 1996 (35.9 ± 0.3 Mm) than in 2008 (35.0 ± 0.3 Mm), while the dominant horizontal velocity flows are weaker (139 ± 1 m s in 1996; 141 ± 1 m s in 2008). Although numerical differences are seen, they are not conclusive proof of the most recent minimum being inherently unusual. [ABSTRACT FROM AUTHOR]

Abstract: Observations of ionospheric vertical total electron content (vTEC) from European ground-based Global Navigation Satellite Systems (GNSS) receivers during the period January 2008–January 2010 are used to investigate, for the first time, vTEC sensitivity to weak geomagnetic disturbances under extreme solar minimum conditions. This study shows a significant number of events for the period in question, all of which exhibited some form of exceptionally large values of vTEC during small-magnitude geomagnetic disturbances. To illustrate our point on the importance of vTEC enhancements during the extreme solar minimum and its relevance for the current GNSS and future Galileo applications, we present in this paper the results associated with two significant events that both occurred in equinoctial months. The 10–12 October 2009 event of anomalous TEC enhancement at two distant mid-latitude locations HERS (0.3 E; 50.9 N) and NICO (33.4 E, 35.1 N) is discussed in the context of strong vTEC variations during the well established ionospheric storm on 11 October 2008. We conclude with a short summary of the new findings and their consequences on ionospheric monitoring and modelling for operational communication and navigation systems. [Copyright &y& Elsevier]

New perspectives regarding the possible cooling of the Earth's climate as a result of solar changes are reviewed in this paper. The major findings include: (1) solar activity is weakening to its very low level, which is comparable with the level in the early 20th century; (2) the current grand solar maximum has already lasted for eight 11-year solar cycles and might end in the coming one/two 11-year cycles; (3) a grand solar minimum might prevail in the next 100-200 years; and (4) the number of sunspots in the coming solar maximum (M)-year, around 2013, is an important indicator that needs to be closely monitored. [ABSTRACT FROM AUTHOR]

Abstract: The electron density profiles retrieved from the Constellation Observing System for Meteorology, Ionosphere & Climate (COSMIC) satellite Radio Occultation (RO) observations during 2008 are used to derive ionospheric upper transition height, where the density of O+ is equal to that of light ions (mainly H+ and He+). It is found that the ionosphere upper transition height is very low, with significant local time, latitude and seasonal variations, during the extremely low solar minimum of 2008. The transition height is higher in the daytime than at night, except over middle latitude region of winter hemisphere, where the transition height has minimum in the morning. There is a pronounced peak over equator for all seasons. The transition height is higher in summer than in winter hemisphere. Our results have comparability with C/NOFS satellite observations around the equatorial region during June–August of 2008. However, the IRI model gives much higher transition height than those from COSMIC and cannot reproduce its latitude and season variations well during 2008. [ABSTRACT FROM AUTHOR]

Abstract: Radio wave scintillation due to the presence of ionospheric irregularities can cause fading and phase variation of L-band navigation signals such as those used by the Global Positioning System (GPS). The high-latitude regions experience increased scintillation events under geomagnetically disturbed conditions, particularly during solar maximum. This paper presents phase scintillation measurements observed at the South African Antarctic polar research station during solar minimum conditions. In addition, a multi-instrument approach is shown in which the scintillation events are associated with auroral electron precipitation. Furthermore, it is shown that external energy injection into the ionosphere is an important factor in the development of irregularities producing scintillation. [Copyright &y& Elsevier]

Abstract: In this study, magnetic field measurements obtained by the Venus Express spacecraft are used to determine the bow shock position at solar minimum. The best fit of bow shock location from solar zenith angle 20–120° gives a terminator bow shock location of 2.14 R V (1 R V=6052km) which is 1600km closer to Venus than the 2.40 R V determined during solar maximum conditions, a clear indication of the solar cycle variation of the Venus bow shock location. The best fit to the subsolar bow shock is 1.32 R V, with the bow shock completely detached. Finally, a global bow shock model at solar minimum is constructed based on our best-fit empirical bow shock in the sunlit hemisphere and an asymptotic limit of the distant bow shock which is a Mach cone under typical Mach number of 5.5 at solar minimum. We also describe our approach to making the measurements and processing the data in a challenging magnetic cleanliness environment. An initial evaluation of the accuracy of measurements shows that the data are of a quality comparable to magnetic field measurements made onboard magnetically clean spacecraft. [Copyright &y& Elsevier]

Abstract: A tree-ring thickness time series from Passo Fundo (Southern Brazil) for the interval 1741–2004 was studied by spectral, wavelet and cross-wavelet analyses in order to identify the non-stationary characteristics in tree-ring and sunspot number data. Periods corresponding to the solar cycles of 11, 22, and 80yr were found. The 11yr solar cycle is detected in tree-ring data only during maximum solar activity interval from 1764 to 1804, 1824 to 1884, and 1924 to 1984. The Hale 22yr solar cycle was observed in tree-ring wavelet map for the 1764–1864 and 1904–2004 intervals. The Gleissberg solar cycle was also observed in tree-ring wavelet map for the 1844–1904 interval. [Copyright &y& Elsevier]

Abstract: We have investigated the solar activity signal in tree ring data from two locations in Chile. The tree ring time series extended over a period of ∼400yr. Spectral and wavelet analysis techniques were employed. We have found evidence for the presence of the solar activity Schwabe (∼11yr), Hale (∼22yr), fourth-harmonic of the 208-yr Suess cycle (∼52yr) and Gleissberg (∼80yr) cycles. The Gleissberg cycle of tree ring data is in anti-phase with solar activity. Wavelet and cross-wavelet techniques revealed that the periods found are intermittent, possibly because solar activity signals observed in tree rings are mostly due to solar influence on local climate (rainfall, temperature, and cloud cover) where trees grow up. Further, cross-wavelet analysis between sunspot and tree ring time series showed that the cross power around the 11yr solar cycle is more significant during periods of high solar activity (grand maximum) than during periods of low solar activity (grand minimum). As Glaciar Pio XI is practically at the Pacific Ocean level, the tree-ring response may be stronger due to the heating of the Pacific Ocean water following an increase of the solar radiation incidence rather than at the higher altitudes of Osorno region. [Copyright &y& Elsevier]

Abstract: Plasma and magnetic field parameter variations through fast forward interplanetary shocks were correlated with the peak geomagnetic activity index Dst in a period from 0 to 3 days after the shock, during solar maximum (2000) and solar minimum (1995–1996). Solar wind speed (V) and total magnetic field (B t) were the parameters with higher correlations with peak Dst index. The correlation coefficients were higher during solar minimum (r 2 =56% for V and 39% for B t) than during solar maximum (r 2 =15% for V and 12% for B t). A statistical distribution of geomagnetic activity levels following interplanetary shocks was obtained. It was observed that during solar maximum, 36% and 28% of interplanetary shocks were followed by intense (Dst⩽−100nT) and moderate (−50⩽Dst<−100nT) geomagnetic activity, whereas during solar minimum 13% and 33% of the shocks were followed by intense and moderate geomagnetic activity. It can be concluded that the upstream/downstream variations of V and B t through the shocks were the parameters better correlated with geomagnetic activity level, and during solar maximum a higher relative number of interplanetary shocks can be followed by intense geomagnetic activity than during solar minimum. One can extrapolate, for forecasting goals, that during a whole solar cycle a shock has a probability of around 50% to be followed by intense/moderate geomagnetic activity. [Copyright &y& Elsevier]

Abstract: In this work a study is performed on the correlation between fast forward interplanetary shock parameters at 1 Astronomical Unit and sudden impulse (SI) amplitudes in the H-component of the geomagnetic field, for periods of solar activity maximum (year 2000) and minimum (year 1995–1996). Solar wind temperature, density and speed, and total magnetic field, were taken to calculate the static pressures (thermal and magnetic) both in the upstream and downstream sides of the shocks. The variations of the solar wind parameters and pressures were then correlated with SI amplitudes. The solar wind speed variations presented good correlations with sudden impulses, with correlation coefficients larger than 0.70 both in solar maximum and solar minimum, whereas the solar wind density presented very low correlation. The parameter better correlated with SI was the square root dynamic pressure variation, showing a larger correlation during solar maximum (r =0.82) than during solar minimum (r =0.77). The correlations of SI with square root thermal and magnetic pressure were smaller than with the dynamic pressure, but they also present a good correlation, with r >0.70 during both solar maximum and minimum. Multiple linear correlation analysis of SI in terms of the three pressure terms have shown that 78% and 85% of the variance in SI during solar maximum and minimum, respectively, are explained by the three pressure variations. Average sudden impulse amplitude was 25 nT during solar maximum and 21 nT during solar minimum, while average square root dynamic pressure variation is 1.20 and 0.86 nPa1/2 during solar maximum and minimum, respectively. Thus on average, fast forward interplanetary shocks are 33% stronger during solar maximum than during solar minimum, and the magnetospheric SI response has amplitude 20% higher during solar maximum than during solar minimum. A comparison with theoretical predictions (Tsyganenko’s model corrected by Earth’s induced currents) of the coefficient of sudden impulse change with solar wind dynamic pressure variation showed excellent agreement, with values around 17 nT/nPa1/2. [Copyright &y& Elsevier]

In this work, we investigate the Brazilian low-latitude ionospheric response to two corotating interaction regions (CIRs) and high-speed streams (HSSs) events during the solar minimum of solar cycle 23, in 2008. The studied intervals are enclosed in the whole heliospheric interval, studied by other authors, for distinct longitudinal sectors. CIRs/HSSs are structures commonly observed during the descending and low solar activity, and they are related to the occurrence of coronal holes. These events cause weak-to-moderate recurrent geomagnetic storms characterized by negative excursions of the interplanetary magnetic field, IMF_Bz, as well as long-duration auroral activity, considered as a favorable scenario for continuous prompt penetration interplanetary electric field (PPEF). In this study, we used the vertical total electron content (VTEC) calculated from GPS receivers database from the Brazilian Continuous Monitoring Network managed by the Brazilian Institute of Geography and Statistics. Moreover, we analyzed the F-layer peak height, hmF2 and the critical plasma frequency, foF2, taken from a Digisonde installed at the southern crest of the equatorial ionization anomaly, in Cachoeira Paulista, CP. It was observed that during the CIRs/HSSs-driven geomagnetic disturbances VTEC increased more than 120% over the quiet times averaged values, which is comparable to intense geomagnetic storms. On the other hand, VTEC decreases were also observed during the recovery phase of the storm. Spectral analysis using gapped wavelet technique (GWT) revealed periodicities of 7, 9, 13.5 days, which are sub-harmonics of the solar rotation period, ~ 27 days. These periods in VTEC are closely associated with those observed in solar and geomagnetic indices such as Vsw, IMF_Bz and AE during CIRs/HSSs intervals. We discuss PPEF associated to IMF_Bz reconnection processes and the auroral activity as the most probable causes for the VTEC variations. These results can be of interest for studies related to space weather monitoring, modeling and forecasting, especially during low solar activity. [ABSTRACT FROM AUTHOR]