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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]

Four extreme ultraviolet (EUV) solar radiation proxies (Magnesium II core-to-wing ratio (MgII), Lyman α flux (Fα), 10.7-cm solar radio flux (F10.7), and sunspot number (Rz)) were analyzed during the last four consecutive solar activity minima to investigate how they differ during minimum periods and how well they represent solar EUV radiation. Their variability within each minimum and between minima was compared by considering monthly means. A comparison was also made of their role in filtering the effect of solar activity from the critical frequency of the ionospheric F2 layer, foF2, which at mid to low latitudes depends mainly on EUV solar radiation. The last two solar cycles showed unusually low EUV radiation levels according to the four proxies. Regarding the connection between the EUV “true” variation and that of solar proxies, according to the foF2 filtering analysis, MgII and Fα behaved in a more stable and suitable way, whereas Rz and F10.7 could be overestimating EUV levels during the last two minima, implying they would both underestimate the inter-minima difference of EUV when compared with the first two minima.

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.

Based on orbit detection data acquired by a positive channel Metal Oxide Semiconductor (PMOS) dose detectors on FY4-A (GEO), BD3-M15 (MEO), and YH1-01A (LEO) between November 2018 and November 2022, investigations reveal variations in total dose and the mechanism of radiation dose increase within the geostationary earth orbit (GEO), medium earth orbit (MEO), and low earth orbit (LEO) during the transition from the 24th to the 25th solar cycles. It provides the radiation dose parameters for the study of the space environment from different altitude orbits, and also provides an important basis for studying the solar minimum activity and dose generation The data indicate directional disparities in radiation doses among the orbital regions, with the hierarchy being FY4-A > YH1-01A > BD3-M15. Furthermore, the results show that the total doses of FY4-A and BD3-M15 were higher than that of YH1-01A by two orders of magnitude, with BD3-M15 > FY4-A > YH1-01A. The monthly radiation dose rates of FY4-A in GEO and BD3-M15 in MEO exhibited positive correlation with their corresponding APs during the solar minimum. Notably, for FY4-A, the monthly radiation dose rate during geomagnetic disturbed periods exceeded that of the dose rate during geomagnetic quiet periods by one order of magnitude. This analysis revealed the substantial impact of geomagnetic storms and space environment disturbances on radiation doses detected by MEO and GEO orbital satellites. These perturbations, attributable to medium- and small-scale high-energy electron storms induced by reproducible coronal holes, emerged as key driving factors of the increase in radiation doses in MEO and GEO environments. [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]

The Whole Heliosphere and Planetary Interactions initiative was established to leverage relatively quiet intervals during solar minimum to better understand the interconnectedness of the various domains in the heliosphere. This study provides an expansive mosaic of observations spanning from the Sun, through interplanetary space, to the magnetospheric response and subsequent effects on the ionosphere‐thermosphere‐mesosphere (ITM) system. To accomplish this, a diverse set of observational datasets are utilized from 2019 July 26 to October 16 (i.e., over three Carrington rotations, CR2220, CR2221, and CR2222) with connections of these observations to the more focused studies submitted to this special issue. Particularly, this study focuses on two long‐lived coronal holes and their varying impact in sculpting the heliosphere and driving of the magnetospheric system. As a result, the evolution of coronal holes, impacts on the inner heliosphere solar wind, glimpses at mesoscale solar wind variability, magnetospheric response to these evolving solar wind drivers, and resulting ITM phenomena are captured to reveal the interconnectedness of this system‐of‐systems. Plain Language Summary: The field of Heliophysics research spans a large range of plasma regimes and disciplines that are all linked through the complex interactions each system has with one another. To better explore the interconnected nature of the heliosphere, this study combines observations from across the sub‐domains of Heliophysics to construct snapshots of the Sun, solar wind, and the magnetospheric and ionospheric systems at Earth over three revolutions of the Sun. This allows for a unique vantage point to put these systems in perspective of one another and understand how they impact one another. Key Points: The evolution and impacts of two coronal holes are investigated over three Carrington rotationsThe evolution of the coronal hole led to changes in the solar wind throughout the inner solar systemSubsequent geospace response differed with each passage due to both changes in the solar wind and preconditioning within the systems [ABSTRACT FROM AUTHOR]

Key points 1. F7/C2 IVM shows similar patterns of diurnal, seasonal and latitude/longitude variations of ion density to ICON IVM but with stronger magnitudes. 2. Distinct latitudinal and longitudinal variations of plasma distributions along seasons were observed during 2019-2020. 3. Simultaneous observations by the multi-satellite constellation of F7/C2 and ICON and all-sky imager provide opportunity to monitor evolutions of EPBs.

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]

Based on orbit detection data acquired by a positive channel Metal Oxide Semiconductor (PMOS) dose detectors on FY4-A (GEO), BD3-M15 (MEO), and YH1-01A (LEO) between November 2018 and November 2022, investigations reveal variations in total dose and the mechanism of radiation dose increase within the geostationary earth orbit (GEO), medium earth orbit (MEO), and low earth orbit (LEO) during the transition from the 24th to the 25th solar cycles. It provides the radiation dose parameters for the study of the space environment from different altitude orbits, and also provides an important basis for studying the solar minimum activity and dose generation The data indicate directional disparities in radiation doses among the orbital regions, with the hierarchy being FY4-A > YH1-01A > BD3-M15. Furthermore, the results show that the total doses of FY4-A and BD3-M15 were higher than that of YH1-01A by two orders of magnitude, with BD3-M15 > FY4-A > YH1-01A. The monthly radiation dose rates of FY4-A in GEO and BD3-M15 in MEO exhibited positive correlation with their corresponding APs during the solar minimum. Notably, for FY4-A, the monthly radiation dose rate during geomagnetic disturbed periods exceeded that of the dose rate during geomagnetic quiet periods by one order of magnitude. This analysis revealed the substantial impact of geomagnetic storms and space environment disturbances on radiation doses detected by MEO and GEO orbital satellites. These perturbations, attributable to medium- and small-scale high-energy electron storms induced by reproducible coronal holes, emerged as key driving factors of the increase in radiation doses in MEO and GEO environments.

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]

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 > -10 nT, we observed that an intense geomagnetic storm rarely occurs and if it does, would be predominant around solar maximum. It was found that long-duration (\Delta t > 6 hrs) of southward Bz (i.e., -10 nT < Bz -10 nT is rare and possesses a special configuration of magnetic field and solar wind parameters structures which are CIR manifestations.

The Floating Potential Measurement Unit onboard the International Space Station includes a Wide sweeping Langmuir Probe that has been operating in the F‐region of the ionosphere at ∼400 km since 2006. While traditional Langmuir probe estimates include critical plasma parameters like electron density and temperature, we have also extracted the O+ percentage from the total ion constituents. This O+ composition data set from the recent minimum in the Solar Cycle 24 reveals orbits with dropouts in O+ to below 80% of the total background ion density at International Space Station orbital altitudes. The observed O+ percentages during these dropouts are much lower than the values predicted by the International Reference Ionosphere 2016 (IRI2016) empirical model. In this paper, we present the climatology of these O+ dropouts with their dependency on season, local time and geographical location. The results show that the lowered O+ percentages are more significant in the winter hemispheres and are routinely observed for orbits in the pre‐sunrise periods. The patterns in O+ dropouts can be explained in part from the lowering of the O+/H+ transition height during solar minimum along with patterns in neutral wind variation. Plain Language Summary: The plasma environment experienced by the International Space Station (ISS) is of key importance to the space physics community. A suite of instruments on the ISS has been operating since 2006 that consist of a spherical Langmuir probe, called the Wide sweeping Langmuir Probe, which is a sensor that periodically collects information about the density and energy of the ambient electrons. Recently, we presented a technique to also infer the composition of O+ ions relative to the total ion density which is an important parameter in understanding the interaction between the terrestrial atmosphere with the exosphere. Results from the most recent period of solar minimum reveal dropouts in O+ densities compared to predictions from empirical models. We show global climatology plots from 2018 to 2019 and observe that these large dropouts in O+ are more prevalent in the post‐midnight region of the winter hemisphere. Past studies have discussed increased presence of lighter H+ ions at higher orbital altitudes, but the fact that O+ dropouts are observed even at ISS altitudes is striking and one of the prominent features of solar minimum. Key Points: O+ percentages often drop below 80% at International Space Station altitudes during solar minimumThe dropouts in O+ abundance are common in post‐midnight sector in winter hemisphereThe climatology of the dropouts is governed by neutral wind variations and lowered O+/H+ transition height [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.

A Radio Finding Detection Network is proposed to detect Solar Electro-Magnetic (EM) Induction effects producing an electromotive force, or voltage, across ancient electrical conducting volcanic rock complexes underlying North America. Electro-Magnetic Pulse (EMP), climate change, hurricanes, tornadoes, lightning, earthquakes, volcanism, and certain types of wildfire outbreaks may be stimulated during a weakening of the solar magnetic field especially during the upcoming solar minimum, increasing Earth's internal inductance power capable of driving much more violent events. This experimental testing is aimed at globally monitoring geophysical EM events to develop new forecasting methods. North American focus is on the New Madrid Fault, Florida hurricanes, and California wildfire and earthquakes, improving the science of natural disaster forecasting, management, investment, and governance, contributing to better resource-related negotiations and policy debates.

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]

A Radio Finding Detection Network is proposed to detect Solar Electro-Magnetic (EM) Induction effects producing an electromotive force, or voltage, across ancient electrical conducting volcanic rock complexes underlying North America. Electro-Magnetic Pulse (EMP), climate change, hurricanes, tornadoes, lightning, earthquakes, volcanism, and certain types of wildfire outbreaks may be stimulated during a weakening of the solar magnetic field especially during the upcoming solar minimum, increasing Earth's internal inductance power capable of driving much more violent events. This experimental testing is aimed at globally monitoring geophysical EM events to develop new forecasting methods. North American focus is on the New Madrid Fault, Florida hurricanes, and California wildfire and earthquakes, improving the science of natural disaster forecasting, management, investment, and governance, contributing to better resource-related negotiations and policy debates.

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.

Abstract 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.

Abstract 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.

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]

Forecasting large earthquakes M ≥ 6.0 with satellite monitoring and Radio Direction Finding (RDF) techniques of Electro-Magnetic (EM) precursors associated with earthquakes are possible. Researches with International Earthquake and Volcano Prediction Center (IEVPC - www.ievpc.org) have uncovered phenomena driving earthquakes that should be considered in a framework of strong global EM coupling between solar corona and the entire Earth system, through EM induction driving ionosphere-air-earth currents. Catastrophic earthquakes have repeatedly stricken the New Madrid Seismic Zone during the last 4 major solar hibernation cycles since 1400 AD. Research suggests it is reasonable to expect another cycle of strong magnitude 6.0 to 8.0 earthquakes in the New Madrid region during the upcoming, solar hibernation or solar minimum period starting around 2020. Possibly it's already begun as evidenced by very little Sunspot activity. Large earthquakes generally occur early in the lower part of the cycle. The 1811-12 earthquakes, occurred in the midst of Dalton Solar Minimum (1793-1830), causing many types of ground failures including lateral spreading and ground subsidence by soil liquefaction across the Mississippi River flood plain and tributaries at least 15,000 square kilometers. The Mississippi River changed course in several places. Studies by the United States Geological Survey (USGS), and damage assessments by Federal Emergency Management Agency (FEMA), estimate damages to infrastructure approaching $600 billion of damage. Understanding the common denominators between seismic precursors associated with a solar EM driver should be of concern. Analyzing data on ionization phenomena in areas under tectonic stress such as: Outgoing Longwave Radiation (OLR); Total Electron Content (TEC); atmospheric effects, such as Jet Stream and other meteorological phenomena related to earthquake clouds and lights is necessary. IEVPC case studies show many M ≥ 6.0 earthquakes seismic locations were identified with Jet Stream precursors that interrupt velocity flow-lines crossing above the earthquake epicenter 1-70 days prior to the event lasting 6-12 hours at -100 km distance from Jet Stream's precursor. Maximum local temperatures within the potential earthquake zones become higher than normal by 5-7°C, gradually increasing over few days. A rise in the range of 7-12°C or more may indicate imminent earthquakes 3 - 4 days beforehand. High regional OLR and TEC values indicate possible earthquakes. Radio broadcasting may go to higher frequencies, while landlines and inflight communications can be disturbed within the epicenter area 3-4 days beforehand, television broadcast within 15 hours of an event. While mobile phones within 30-40 km of an event may become non-functional within 100 minutes of an event. Global seismological archives indicate 3-4 weeks before large M ≥ 8.0 earthquakes, dry wells, rivulets, and brooks may be flooded with oozing ground water. These innovative research and observational techniques for detecting EM and geomagnetic seismic precursors have rarely been implemented, as earthquakes are simplistically considered the result of grinding plate motions, and their true EM nature (solar EM induction triggering lightning from below) has been ignored. Crustal emission of radio waves (detected with RDF) at very low frequency in the band above 20kHz manifests about 20 hours before an earthquake within the epicenter area. By combining RDF information of appropriately spaced antennae array stations (of some tens of km) one can locate the source of EM emission by triangulation discriminating source direction, position, and distance from the station, providing data on the temporal variation of frequency, magnitude, and source intensity. Many of the methods above have been individually verified as valid for earthquake forecasting. [ABSTRACT FROM AUTHOR]

In this study, we documented the midlatitude F2-layer response to five strong geomagnetic storms with minimum Dst < –150 nT that occurred in solar minimum years using hourly values of the F2-layer critical frequency (foF2) from four ionosondes located in different hemispheres. The results were very limited, but they illustrated some peculiarities in the behavior of the F2-layer storm. During equinox, the characteristic ionospheric disturbance patterns over the Japanese station Wakkanai in the Northern Hemisphere and the Australian station Mundaring in the Southern Hemisphere were consistent with the well-known scenario by Prölss (1993); however, during a December solstice magnetic storm, both stations did not observe any noticeable positive ionospheric disturbances. Over the “near-pole” European ionosonde, clear positive ionospheric storms were not observed during the events, but the “far-from-pole” Southern Hemisphere station Port Stanley showed prominent enhancements in F2-layer peak electron density in all magnetic storms except one. No event produced noticeable nighttime enhancements in foF2 over all four ionosondes.

The solar minimum period between solar cycles 23 and 24 has generated extensive study, in part because the changes in the upper atmosphere and ionosphere were so dramatic. Thermospheric density during 2008–2009, at the benchmark 400‐km altitude, was shown to be approximately 30% lower than the previous solar minimum during 1996, in observations and in model simulations. Ionospheric densities were also estimated to be lower, by 10% to 20%. However, earlier analyses using data from the global network of Global Navigation Satellite System receivers did not find any significant reduction in global mean total electron content. Recent work reexamining those data using a limited set of receivers, but with consistent processing, identified a 19% reduction (Emmert et al., 2017, https://doi.org/10.1002/2016JA023680). In this study, we compare model simulations of the thermosphere‐ionosphere system during the 2008–2009 solar minimum to its predecessor and estimate a 16% reduction in global mean total electron content. This is consistent with the neutral thermosphere density change seen in model simulations and in satellite drag observations. In the model simulations, the primary driver is a 10% reduction in solar extreme ultraviolet irradiance, with a smaller contribution from lower geomagnetic activity, and a very small decrement due to increase in atmospheric carbon dioxide. Plain Language Summary: The Sun goes through 11‐year activity cycles, best known with regard to the number of sunspots. At solar minimum, when sunspots mostly vanish, the extreme ultraviolet region of the solar spectrum also diminishes. This causes lower temperature and density in the ionosphere and upper atmosphere above 100 km, where the extreme ultraviolet radiation is absorbed. Past solar minimum periods have seemed basically similar with regard to extreme ultraviolet radiation, and other manifestations of solar activity. However, the solar minimum during 2008–2009 was longer and had fewer sunspots than its predecessor during 1996, or than any minimum period in the past century. The thermosphere was also lower in density, and the ionosphere also appeared to be lower in density, but there were some contradictory measurements. Recent reanalysis of ionospheric measurements found better agreement with the thermospheric measurements. Modeling studies attempting to explain the changes in the ionosphere and thermosphere have suggested that they were caused by a decrease of about 10% in solar extreme ultraviolet radiation. Here we show that the model results and the ionospheric measurements are now in good agreement. This demonstrates that solar minima are not all the same and may have implications for understanding the Sun during extended periods of very low activity. Key Points: Reanalysis of ionospheric electron content by Emmert et al. () found it lower during the 2008‐2009 solar minimum than the 1996 minimumModel simulations showed that both ionospheric electron content and thermospheric neutral density were lower during 2008‐2009 than in 1996Thermosphere and ionosphere reductions during the 2008‐2009 solar minimum were consistent in observations and in model simulations [ABSTRACT FROM AUTHOR]