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Heavy rainfall events causing floods and flash floods are examined in the context of solar wind coupling to the magnetosphere–ionosphere–atmosphere system. The superposed epoch (SPE) analyses of solar wind variables have shown the tendency of severe weather to follow arrivals of high-speed streams from solar coronal holes. Precipitation data sets based on rain gauge and satellite sensor measurements are used to examine the relationship between the solar wind high-speed streams and daily precipitation rates over several midlatitude regions. The SPE analysis results show an increase in the occurrence of high precipitation rates following arrivals of high-speed streams, including recurrence with a solar rotation period of 27 d. The cross-correlation analysis applied to the SPE averages of the green (Fe XIV; 530.3 nm) corona intensity observed by ground-based coronagraphs, solar wind parameters, and daily precipitation rates show correlation peaks at lags spaced by solar rotation period. When the SPE analysis is limited to years around the solar minimum (2008–2009), which was dominated by recurrent coronal holes separated by ∼ 120∘ in heliographic longitude, significant cross-correlation peaks are found at lags spaced by 9 d. These results are further demonstrated by cases of heavy rainfall, floods and flash floods in Europe, Japan, and the USA, highlighting the role of solar wind coupling to the magnetosphere–ionosphere–atmosphere system in severe weather, mediated by aurorally excited atmospheric gravity waves.

The retrodiction and prediction of solar activity are two closely-related problems in dynamo theory. We applied Machine Learning (ML) algorithms and analyses to the World Data Center’s newly constructed annual sunspot time series (1700–2019; Version 2.0). This provides a unique model that gives insights into the various patterns of the Sun’s magnetic dynamo that drives solar activity maxima and minima. We found that the variability in the ~ 11 -year Sunspot Cycle is closely connected with 120-year oscillatory magnetic activity variations. We also identified a previously under-reported 5.5 year periodicity in the sunspot record. This 5.5-year pattern is co-modulated by the 120-year oscillation and appears to influence the shape and energy/power content of individual 11-year cycles. Our ML algorithm was trained to recognize such underlying patterns and provides a convincing hindcast of the full sunspot record from 1700 to 2019. It also suggests the possibility of missing sunspots during Sunspot Cycles −1, 0, and 1 (ca. 1730s-1760s). In addition, our ML model forecasts a new phase of extended solar minima that began prior to Sunspot Cycle 24 (ca. 2008–2019) and will persist until Sunspot Cycle 27 (ca. 2050 or so). Our ML Bayesian model forecasts a peak annual sunspot number (SSN) of 95 with a probable range of 80–115 for Cycle 25 between 2023 and 2025.

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

Detecting and characterizing Total Electron Content (TEC) depletion is important for studying the ionospheric threat due to the Equatorial Plasma Bubble (EPB) when applying the Ground-Based Augmentation System (GBAS) at low latitudes. This paper develops a robust method to automatically identify TEC depletion and derive its parameters. The rolling barrel algorithm is used to automatically identify the TEC depletion candidate and its parameters. Then, the depletion candidates are screened by several improved techniques to distinguish actual depletions from other phenomena such as Traveling Ionospheric Disturbance (TID) or abnormal data. Next, based on the depletion signals from three triangular receivers, the method derives EPB parameters such as velocity, width and gradient. The time lag and front velocity are calculated based on cross-correlation using TEC depletions and the geometrical distribution of three triangular receivers. The width and gradient of slope are then determined by using TEC depletion from a single receiver. By comparison, both the station-pair method and proposed method depend on the assumption that the EPB morphology is frozen during the short time when the plasma bubble moves between the receivers. However, our method relaxes the restriction that the baseline length should be shorter than the width of slope required by the station-pair. This relaxation is favorable for studying small-scale slope of depletions using stations of a longer baseline. In addition, the accuracy of the width and gradient is free of impact from hardware biases and small-scale disturbance, as it is based only on the relative TEC variation. The method is demonstrated by processing Global Positioning System (GPS) and BeiDou Navigation Satellite System (BDS) data on 15 August, 2018, in a solar minimum cycle.

An analysis is performed of a reduction in the intensity of cosmic rays during the minimum of solar cycle 24 in April–May, 2019. It is assumed that the observed slow decline and recovery of the cosmic ray flux consisted of a series of small Forbush decreases.

Utilizing a new version of the sunspot number and group sunspot number dataset available since 2015, we have statistically studied the relationship between solar activity parameters describing solar cycles and the slope of the linear relationship between the monthly sunspot numbers and the monthly number of active days in percentage (AD). As an effort of evaluating possibilities in use of the number of active days to predict solar activity, it is worthwhile to revisit and extend the analysis performed earlier. In calculating the Pearson’s linear correlation coefficient r, the Spearman’s rank-order correlation coefficient rs, and the Kendall’s τ coefficient with the rejection probability, we have calculated the slope for a given solar cycle in three different ways, namely, by counting the spotless day that occurred during the ascending phase and the descending phase of the solar cycle separately, and during the period corresponding to solar minimum ± 2 years as well. We have found that the maximum solar sunspot number of a given solar cycle and the duration of the ascending phase are hardly correlated with the slope of a linear function of the monthly sunspot numbers and AD. On the other hand, the duration of a solar cycle is found to be marginally correlated with the slope with the rejection probabilities less than a couple of percent. We have also attempted to compare the relation of the monthly sunspot numbers with AD for the even and odd solar cycles. It is inconclusive, however, that the slopes of the linear relationship between the monthly group numbers and AD are subject to the even and odd solar cycles.

Using Mars Atmosphere and Volatile EvolutioN (MAVEN) observations from 2014 to 2019, we characterize how the structure and composition of the nightside ionosphere varies on solar cycle and seasonal timescales. At fixed altitudes between 150 and 200 km, plasma densities vary substantially on these timescales in response to variations in fixed thermospheric pressure levels, which cause the ionosphere to rise and fall in altitude. Additionally, solar cycle and seasonal trends in the electron impact ionization (EII) rate affects nightside densities; higher densities are observed near solar maximum and Mars perihelion (when EII rates are largest), and lower densities are observed near solar minimum and Mars aphelion (when EII rates are smallest). Lastly, densities in the high-altitude ( > 200 km) nightside ionosphere vary significantly over the solar cycle: topside O + densities vary by a factor of ∼ 50 and topside O 2 + densities vary by a factor of ∼ 40. Topside ion densities were relatively stable throughout the solar minimum of 2018–2019.

Mars ionospheric temperatures exhibit profound variability based on, for example, changes in solar irradiance, solar local time, season, and latitude. The close coupling between ionospheric temperatures and neutral thermospheric density is of even greater importance. To evaluate these effects and to quantify their contributions to electron temperature, we developed the Temperature of electrons Specification for the Mars Ionosphere (TeSMI). After identifying the strongest correlations between the model inputs and the observed electron temperatures, several additional corrections were examined and implemented. The primary source of climatological variability was related to neutral pressure, local solar time, and 17-22 nm irradiance. In addition to this, changes due to seasonal effects and latitude each required 5–10% of additional corrections. Evaluating the electron temperatures on which TeSMI is based provided an opportunity to study several relevant ionospheric phenomena. One of these is the solar cycle dependence of electron temperature which is found to be relatively small compared with other sources of variability. Solar minimum temperatures are on average 7–12% lower than those at solar maximum during solar cycle 24. The solar cycle dependence also appears stronger at aphelion than at other times. Furthermore, at Mars perihelion, electron temperatures seem to increase in response to a possible source of heating from the lower atmosphere. Furthermore, we identify cold and warm regions with respect to the TeSMI model that occur in specific aerographic locations. These regions of warmer and colder temperatures appear organized with respect to the crustal magnetic field locations at altitudes above 170 km. This is the first time that the effect of the crustal field is identified in the statistical electron temperature data below 200 km. Lastly, at the lowest altitudes (160–180 km) analyzed here, the equatorial ionosphere is colder than the polar regions by 5–15% which is interpreted to be a result of the atmospheric circulation pattern.

The response of Martian nonmigrating tidal perturbations in the thermospheric and ionospheric densities to solar minimum, as contrasted with their behavior in the solar declining phase, is investigated using five years of data collected by the Mars Atmosphere and Volatile EvolutioN (MAVEN) satellite. We find that the average tidal amplitudes, expressed as the percent variation of the neutral and electron densities from their respective average background values, are larger (by a factor of ~1.3 above 170 km) during solar minimum compared to the declining phase of the solar cycle. These larger amplitudes, on the dayside, may be due to a decrease in the atmospheric scale height, which for neutral argon averaged over a Martian year, goes from 12.5 km during declining phase, to 10.5 km for solar minimum. An additional controlling factor is that the average vertical dissipation length scale becomes shorter, going from ~34 km during the declining phase to ~29 km at solar minimum. On the nightside, the amplitude of the neutral atmospheric tides is similar between the declining phase and solar minimum. There are changes in the length scales, with the average night side scale height becoming somewhat greater during solar minimum, as well as an increase in the dissipation length scale. Dayside tidal perturbations in the ionosphere at ~145 km to ~170 km exhibit the expected amplitude relation predicted by simplified photochemistry for the observed neutral perturbation amplitudes as well as strong correlation with the longitudinal neutral density perturbations. Interpretation of the data are faced with the complication of limited coverage between season, latitude, and local times together with long temporal baselines that may mix different tidal modes; discussion on the possible role of the latitude effect on amplitude are discussed. The results of the study are put into context of a hypothesis on how tidal amplitudes might be expected to change in response to solar minimum.

Using Mars Express data from 2007 until 2020 we show how ion outflow from Mars varied over more than a solar cycle, from one solar minimum to another. The data was divided into intervals with a length of one Martian year, starting from 30 April 2007 and ending 13 July 2020. The net escape rate was about 5 × 1 0 24 s − 1 in the first covered minimum, and 2 − 3 × 1 0 24 s − 1 in the most recent minimum. Ion escape peaked at 1 × 1 0 25 s − 1 during the intervening solar maximum. The outflow is a clear function of the solar cycle, in agreement with previous studies which found a clear relationship between solar EUV flux and ion escape at Mars. The outflow during solar maximum is 2.5 to 3 times higher than in the surrounding solar minima. The average solar wind dynamic pressure over a Martian year was investigated, but does not vary much with the solar cycle. The escape rate at solar maximum is in good agreement with some recent MAVEN studies, and dominated by low energy ions at most sampled locations. A simple linear fit to the data gives a prediction of the escape rate for the much stronger solar maximum during the Phobos mission in 1989 that is consistent with observations. The fit also implies a non-linear response of ion escape for low solar EUV, with a lower initial escape response for lower solar EUV levels than those of the studied data set.

Conditions in the ionosphere of Mars vary from solar minimum to solar maximum due to changes in the ionizing solar irradiance. Although changes in the main ionospheric layers, namely the M2 layer and the lower M1 layer, over the solar cycle have been well-characterized by previous work, corresponding changes in the upper regions of the photochemical ionosphere ( ∼ 140–200 km) have not. Here we investigate those changes using ionospheric electron density profiles acquired at solar zenith angles of 70°–90° under solar minimum conditions by the MAVEN Radio Occultation Science Experiment (ROSE) and under solar maximum conditions by a similar radio occultation experiment on Mars Global Surveyor. Due to the increase in ionizing irradiance from solar minimum to solar maximum, we expect that densities at all altitudes will increase from solar minimum to solar maximum. Due to the eccentricity of Mars’s orbit, observed electron density profiles must be adjusted to a common Mars–Sun distance to highlight solar cycle effects. This process involves increasing observed electron density values by a factor proportional to the Mars–Sun distance, which mitigates for variations in incident irradiance, and expressing altitude relative to the main ionospheric peak, which mitigates for variations in heating and expansion of the neutral thermosphere. We find that the scale height of the upper regions of the photochemical ionosphere is remarkably constant between solar minimum and solar maximum, whereas the corresponding neutral temperature increases by 50%. Yet these two quantities are thought to be proportional. Electron density values in the upper regions of the photochemical ionosphere and peak electron density values increase similarly from solar minimum to solar maximum.

The Radio Occultation Science Experiment (ROSE; Withers et al., 2018, 2020) on the Mars Atmosphere and Volatile EvolutioN (MAVEN) satellite has produced over 400 electron density profiles from July 2016 to November 2019. These Ne(h) profiles occurred over a large range of solar zenith angles (54o- 130o) and solar flux conditions (24–54 solar flux units at Mars). One of the goals of the MAVEN mission is to characterize the status of the topside ionosphere at Mars as a reservoir of possible escaping plasma. Here we evaluate how ROSE topside ionospheric measurements, made predominantly under solar minimum conditions, compare with the only empirical model of the topside ionosphere (Němec et al., 2019). To assess congruence between the model and the observations, a deviation factor (DF) is calculated for each predicted versus observed Ne(h) profile. Diurnally, low DFs (and thus higher agreement) occur for occultations with solar zenith angles (SZA)

We investigated the spatio-temporal evolution of disturbed time post mid-night Equatorial Plasma Bubbles (EPBs) using Canadian Advanced Digital Ionosonde (CADI) located at dip equatorial station, Tirunelveli (8.73°N, 77.7°E, 0.23°N Dip. Lat.), an all-sky imager (ASI) observations at low latitude station Panhala (16.48°N, 74.6°E, 11.1°N Dip. Lat.) and Gadanki Ionospheric Radar Interferometer (GIRI) at Gadanki (13.5°N, 79.2°E; 6.5°N Dip. Lat.) which is situated at few degrees towards east and south of Panhala on 02–03 February 2017 night. During this night, IMF Bz showed its periodic variation starting from 16:00 UT to 23:00 UT accompanied by decrease in SYM-H to as low as −35 nT indicating the onset of weak magnetic storm. The analyzed results suggested that cause of post-midnight EPBs could be due to manifestation of fluctuating eastward/westward electric field due to combined under-shielding/over-shielding Electric Fields and disturbance dynamo electric fields that led to rise and fall of the F-layer over dip equator. Interestingly, the EPBs over Panhala showed eastward motion initially that quickly reversed to westward later. Along with westward motion they also started growing until 21:30 UT. However, most of these EPBs disappeared with time except the one that started descending/shrinking towards southern side (i.e. towards equator). The rising and shrinking of EPBs is found to be fairly correlated with the equatorial vertical drifts. The westward drift of EPBs at Panhala and its anti-correlation with vertical drifts has been confirmed from CADI zonal/vertical drifts. Accordingly, the study also investigated the role of storm induced vertical Hall electric field as a possible cause for westward drifts and its anti-correlation with vertical drifts. However, GIRI observations do not show any significant westward drift on this night at Gadanki suggesting that there is a longitudinal gradient in the zonal drift of these EPBs. In addition to longitudinal drift reversal, the latitudinal gradient in zonal drifts also has been noticed. The present work highlights the role of storm induced disturbances in the generation and evolution of post-midnight EPBs which is believed to be triggered by weak magnetic disturbances in the deep low solar minimum.

The F2-layer peak parameters of the International Reference Ionosphere (IRI) model are critical to its subsequent applications because of the vital role of the peak parameters in determining other IRI model outputs. Therefore, we analyzed the statistical deviation and ionospheric climatology of NmF2/hmF2 between the most current version of the IRI model, namely IRI-2016, and ionospheric radio occultation (IRO) data, including Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) and FengYun-3C (FY3C), during the recent solar minimum in 2017–2019. The International Union of Radio Science (URSI) model for F2-layer peak electron density (NmF2) and the Bilitza–Sheikh–Eyfrig (BSE), Altadill-Magdaleno–Torta–Blanch (AMTB), and SHUbin (SHU) models for maximum ionization height (hmF2) in IRI-2016 are compared with the IRO dataset. A statistical analysis shows a systematic offset of less than 10% in NmF2 between IRI-2016 URSI and IRO data, and the SHU option for hmF2 shows a better fit to IRO data than BSE and AMTB. A climatology comparison shows that NmF2 estimated by IRI-2016 URSI is consistent with that of the IRO dataset in terms of the general trend in the typical ionospheric characteristics; except for some discrepancies in the marine area, the IRO data may provide a feasible reference for IRI estimation in marine regions. The hmF2 options of BSE/AMTB/SHU show hemispheric asymmetry like the IRO dataset. Moreover, hmF2 of BSE/AMTB and IRO/SHU show some discrepancies in fine structures, such as the day–night reversal phenomenon.

In this study, Sq(H) field results for Sonmiani geomagnetic observatory (SON), Pakistan are presented first time for solar cycle i.e., Solar Cycle 24. Sonmiani observatory was established in 2008 and was included in the list of INTERMAGNET Magnetic Observatory (IMO) in 2012. The analysis of Sq(H) field is performed to examine diurnal, annual and seasonal behaviour of Sq(H) field at Sonmiani along with Honolulu which is used as reference observatory. Both the observatories lie at the Equatorial Ionisation Anomaly (EIA) crest. In general strong dependency of Sq(H) field on solar cycle has been observed. First peak of SC24 was visible in Sq(H) field at both observatories. However, at HON maximum Sq(H) was observed in 2015 instead of 2014, a year later than the year of solar maximum. Prominent longitudinal difference has been noted between both observatories. At HON, maximum Sq(H) was observed during equinox while at SON it was noted during equinox and summer as a consequence of shift in latitude of Sq focus. Phase shift of Sq(H) at SON followed a general trend, that is maximum Sq(H) shifted to later hours in solar maximum as compared to solar minimum. Whereas, an opposite trend was noted at HON which might be due to coastal effect. In case of seasonal phase shift, at both observatories maximum Sq(H) occurred at later hours in summer than in winter which is opposite to the results obtained by many workers. These points need further investigation and geomagnetic as well as wind, and electric field data of other observatories situated at the EIA crest region is required to interpret the Sq(H) phase shift extensively.