Heavy rainfall, floods, and flash floods in the context of solar wind coupling to the magnetosphere-ionosphere-atmosphere system

. 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 a tendency of severe weather to follow arrivals of high-speed streams from solar coronal holes (Prikryl et al., 2018). Precipitation datasets 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 mid-latitude regions. The SPE analysis results show an increase 15 in occurrence of high precipitation rates following arrivals of high-speed streams, including recurrence with a periodicity of 27 days. 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 20 spaced by 9 days. These results are further demonstrated by cases of heavy rainfall, floods and flash floods in Europe, Japan, and the U.S., highlighting the role of solar wind coupling to the magnetosphere-ionosphere-atmosphere system in severe weather, mediated by aurorally excited atmospheric gravity 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 solar minimum (2008-2009), correlation peaks at lags spaced by 9 days are also revealed, which is a result of high-speed streams from coronal holes spaced in heliographic longitude by approximately 595 120˚. These quantitative results confirm the tendency of an increase in precipitation following the arrivals of high-speed streams, which is further demonstrated by cases of heavy rainfall, floods and flash floods in Europe, Japan, and the U.S. The role of aurorally generated atmospheric gravity waves as the mechanism mediating the influence of the solar wind-magnetosphere-ionosphere-atmosphere coupling on the troposphere is suggested. Down-going gravity waves from sources in the lower thermosphere can over-reflect in the upper troposphere and trigger/release existing moist instabilities, initiating convection and latent heat release, the energy leading to intensification of storms.


Introduction
Extreme rainfall and flash floods have major societal and economic impacts, thus posing significant natural hazards (Gaume 25 et al., 2009) that increasingly require careful flood risk assessment, mitigation strategies and recovery management (National Academies of Sciences, 2019). Although our knowledge and understanding of the multitude of processes that can result in heavy precipitation has advanced (Schumacher, 2019), prediction of extreme precipitation events continues to present difficult challenges in operational forecasting (Fritsch and Carbone, 2004;Villarini et al., 2010;Gourley et al., 2012;Schroeder et al., 2016). Recent climate change has caused an increase in extreme precipitation and flash floods (Groisman et al., 2005;Gutowski 30 et al., 2008;Esposito et al., 2018, Schumacher, 2019. While there are many factors affecting heavy precipitation across a variety of atmospheric scales, the difficulty to predict flash floods stems partly from the fact that they often occur on small 2 spatial scales. Schumacher (2019) summarized some of the aspects of extreme precipitation and research areas aiming for improved understanding and prediction, which, among other things, will require interdisciplinary research collaborations in addition to basic research. 35 While it is well known that the Sun is the major source of energy, primarily via the electromagnetic radiation heating the Earth's atmosphere and driving weather processes, the interaction of the solar wind with the Earth's magnetic field deposit significant energy into the upper portion of the Earth's atmosphere, particularly at high latitudes.
A possible link between solar magnetic sector structure and tropospheric vorticity was shown in the 1970s by Wilcox et al. 40 (1973Wilcox et al. 40 ( , 1974) who used the upper-level tropospheric vorticity area index (VAI), which is a proxy for extratropical storminess.
They observed a statistically significant variation of VAI about the time of Earth crossing the magnetic sector boundary, specifically a minimum in VAI about 1 day later. Sector boundaries, later identified as the heliospheric current sheet (HCS) (Smith et al., 1978;Hoeksema et al., 1983), often precede arrivals of high-speed stream (HSS) interfaces by about one day, unless the two coincide. The HSSs from coronal holes are anchored in the large-scale solar magnetic field structure that is 45 extended into the interplanetary space by solar wind. The observationsis by Wilcox et al.discovery prompted a search for a possible physical mechanisms to explain the "Wilcox effect" and other observations indicating sun-weather links, such as the global electric circuit model and changes in relativistic electron flux (Tinsley, 2000;2008), energetic solar proton events correlated with intensifications of cyclonic activity (Veretenenko and Thejll, 2004), which in turn would affect cloud microphysics (Tinsley, 2012). The dawn-dusk (BY) component of the IMF and the atmospheric electric circuit influences on 50 the ground-level atmospheric pressure have been shown (Burns et al., 2007 and2008;Lam et al., 2013Lam et al., , 2014, and Lam and Tinsley (2016) reviewed the solar wind-atmospheric electricity-cloud microphysics connections to weather and climate.
External factors including galactic cosmic rays (GCRs) varying with heliospheric magnetic field are further discussed by Owens et al. (2014) who considered the polarity of the magnetic field modulating lightning in the UK. Scott et al. (2014) observed solar wind modulation of lightning, an increase in lightning rates and thunder days coinciding with an increased flux 55 of lower energy solar protons following arrival of solar wind high speed streams. For further discussion and references, see, e.g., Prikryl et al. (2019).
HSSs emanating from coronal holes (Krieger et al., 1973) are anchored in the large-scale solar magnetic field structure that is extended into the interplanetary space by the solar wind. HSSs from polar coronal holes have an approximately constant speed 60 of ~750 to 800 km/s (Phillips et al., 1994(Phillips et al., , 1995Tsurutani et al., 2006a). The coronal holes that affect the Earth by HSSs are either extensions of polar coronal holes (Phillips et al., 1994) to low latitudes, or self-contained coronal holes forming at low heliographic latitudes (De Toma (2011). HSSs cause co-rotating interaction regions (CIRs) (Smith and Wolfe, 1976) at the leading edge of HSSs from mid-to low-latitude coronal holes, an interface between the fast and slow solar wind (Richardson, 3 magnetospheric relativistic electrons (Tsurutani et al., 2016), which these authors stated could be relevant to the "Wilcox effect". Tsurutani et al. (2016) also considered importance of the energy deposited in the mesosphere and middle atmosphere possibly driving planetary waves or atmospheric gravity waves. The geo-effectiveness of HSSs/CIRs has been well established (Tsurutani et al., 2006a,b) and associated with high-intensity, long-duration continuous auroral electrojet activity (HILDCAAs) 70 that includes auroral substorms (Tsurutani and Gonzalez, 1987;Tsurutani et al., 1990Tsurutani et al., , 1995. HILDCAAs are caused by trains of solar wind Alfvén waves (Belcher and Davis, 1971) that couple to the magnetosphere-ionosphere system (Dungey, 1961(Dungey, , 1995. For further review of relevant references on solar wind coupling to the magnetosphere-ionosphere-atmosphere (MIA) system, see, e.g., Prikryl et al. (2019). 75 Prikryl et al., (2009a) observedconfirmed the "Wilcox effect" in both the northern and southern hemisphere winters. Severe weather, including severe winter storms, windstorms, snowstorms, and heavy rain events, and explosively developing extratropical cyclones have been examined by Prikryl et al. (2018) in the context of solar wind coupling to the magnetosphereionosphere-atmosphere system. They observed a tendency of significant weather events to follow arrivals of high-speed solar 80 wind. The statistical results (Prikryl et al., 2009a;2018) and a proposed physical mechanism (Prikryl et al., 2009b) were supported by cases of severe weather events, including two flash floods that occurred in Slovakia on July 16 and 24, 2001 (Prikryl et., 2018;their Fig. 10). Both flash floods closely followed arrivals of solar wind high-speed streams from coronal holes, and a series of convective supercells were associated with atmospheric gravity waves (AGWs) from sources in the highlatitude lower thermosphere that could have reached the mid latitude troposphere. The authors suggested that the AGWs played 85 a role in triggering moist instabilities, thus initiating the convection.
In this paper we examine the occurrence of heavy rainfall events leading to floods and flash floods in the geophysical context of the solar wind MIA coupling to the magnetosphere-ionosphere-atmosphere (MIA) system. The goal is to demonstrate the statistical link between the solar wind and the occurrence of heavy rainfall, and to identify the high-latitude sources of AGWs 90 that may play a role in triggering the convection leading to heavy rainfall, floods, and flash floods. Prikryl et al. (2018) have described in detail the solar wind data obtained from the National Space Science Data Center (NSSDC) OMNIWeb http://omniweb.gsfc.nasa.gov (King and Papitashvili, 2005). The data include measurements of solar wind velocity, V, the interplanetary magnetic field (IMF) magnitude, B, the standard deviation, σBz, of the z-component of the 95 IMF, Bz, and proton density, np. upstream from Earth. These solar wind parameters are used to identify HSS/CIRsco-rotating interaction regions (CIRs) at the leading edge of high-speed streams from coronal holes, an interface between the fast and slow 4 solar wind. and the observed HSS velocity maxima, Vmax. It should be noted that this is not the actual maximum velocity of the stream, which cannot be observed at Earth's orbit.

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Measurements of the green coronal emission line (Fe XIV, 530.3 nm) by ground-based coronagraphs from 1939 to 2008 have been merged into a homogeneous coronal dataset (Rybanský, 1975;Rybanský et al., 2001Rybanský et al., , 2005 ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA. The intensity of the emission line at 28.4 nm (Fe XV) observed by the SOHO Extreme ultraviolet Imaging Telescope (EIT) instrument was found to be closely correlated with intensity of the green corona obtained by ground-based coronagraphs (Dorotovič et al., 2014). The coronal intensities are expressed in absolute coronal 105 units (ACU) representing the intensity of the continuous spectrum from the center of the solar disk with a width of 1 Ǻ at the same wavelength as the observational spectral line (1 ACU = 3.89 W m -2 sr -1 at 530.3 nm). The intensity depletions, called coronal holes, are sources of high-speed solar wind streams. While they are most prominent in polar regions (polar coronal holes) they can extend to low heliographic latitudes as depleted corona intensity and are observed at the limb, sometimes even during total solar eclipses (Rušin et al., 2020). The green corona intensity synoptic charts at the solar central meridian are 110 produced by averaging the intensities measured at the east and west limbs 14 days apart.
De-trended GPS total electron content (TEC) maps are used to identify traveling ionospheric disturbances (TIDs) (Tsugawa et 120 al., 2007). The Dense Regional And Worldwide INternational GNSS-TEC observation (DRAWING-TEC) project of the Observation. It funds the project FloodList, a European system for monitoring and reporting floods and flooding news since 130 2013 (http://floodlist.com/). An earlier database of European flash flood data (Gaume et al., 2009) is provided at the Hydrate 5 project webpage (www.Hydrate.tesaf.unipd.it). The rainfall data from SHMU stations across Slovakia and the SHMU annual flood reports (http://www.shmu.sk/sk/?page=128) provided data on significant rainfall leading to floods. The Tropical Rainfall Measuring Mission (TRMM) Multi-Satellite Precipitation Analysis TMPA (3B42) Precipitation (version 7) using a 0.25˚ x 0.25˚ grid for latitudes ±50˚ was produced at the Goddard Earth Sciences Data and Information Services Center (GES DISC) 135 (Huffman et al., 2016) (https://disc.gsfc.nasa.gov/datacollection/TRMM_3B42_Daily_7.html).
2 Superposed epoch analysis of green corona intensity and solar wind variables keyed to onset of significant rainfall leading to floods and flash floods 140 Prikryl et al. (2009a140 Prikryl et al. ( , 2009b140 Prikryl et al. ( , 2016140 Prikryl et al. ( , 2018140 Prikryl et al. ( , 2019 used the superposed epoch (SPE) analysis method to identify a tendency of severe weather, explosive extratropical cyclones and rapid intensification of tropical cyclones to follow arrivals of solar wind disturbances, including corotating interaction regions (HSS/CIRs), generated by solar wind high-speed streams (HSSs) from coronal holes and possibly interplanetary coronal mass ejections (ICMEs) and their upstream sheaths (Burlaga et al., 1981;Tsurutani et al., 1988). In the present paper we extend the analysis to examine the occurrence of heavy rainfall leading to 145 floods and flash floods in the geophysical context.   These results are further supported by a more comprehensive analysis using the SHMU precipitation database and the annual flood reports in Slovakia for a period of 2003-2019. Previously, Prikryl et al. (2018;their Figs. 7d, e, f) showed a tendency of significant weather events, including heavy rainfall, to follow arrivals of HSSs. Here we repeat the SPE analysis of solar wind parameters keyed to significant rainfall events of at least 30 mm/24h recorded at 10 or more stations in 2003-2019 (Fig. 3).
The results show a pattern indicating a tendency of these rainfall events to have occurred about 2 days after arrivals of HSSs 170 from coronal holes.
Heavy rainfall events used in Fig. 3 may or may not have resulted in floods. On the other hand, relatively moderate rainfall rates over a few days can also result in floods. To focus on floods, we compiled start dates of significant rainfall leading to floods as documented in the SHMU annual flood reports. The SPE analysis of the green corona intensity and solar wind 175 parameters keyed to these dates is shown in Figs. 4a and 4b, respectively. The histogram (Fig. 4c) shows a steep increase in the cumulative number of stations where the recorded 24-h rainfall exceeded a given threshold for each epoch day relative to the start of rainfall/flood events, regardless of the location of the SHMU stations. Of note, for some rivers, such as Myjava and Dunaj (Danube) the rainfall that contributed to floods occurred in Czech Republic, Austria and Germany but was not necessarily recorded by SHMU stations. While the available information on rainfall measurements outside Slovakia is used to 180 determine the key dates for some flood events in the SPE analysis (Figs. 4), the statistics of rainfall occurrence relative to the key date ( Fig. 4c) only includes the SMHU stations. The SPE analysis of solar wind parameters ( Fig. 4b) clearly indicates a tendency of the heavy rainfall/flood events to follow arrivals of solar wind HSS/CIRs, which is consistent with the results discussed above (Figs. 1 to 3).

High daily precipitation rates relative to arrivals of major HSS/CIRs 185
The above results of the SPE analysis of solar wind time series keyed by heavy rainfall leading to floods indicate a tendency of these events to follow arrivals of HSS/CIRs. Using the database of daily precipitation rates from rain gauges in Slovakia we now show more direct evidence to support these conclusions. Also, we use the TRMM satellite-based dataset of daily precipitation rates to provide further statistical evidence supporting these results.

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To examine statistically the occurrence of significant rainfall in Slovakia relative to arrivals of HSS/CIRs the key time for the SPE analysis is now defined as the actual arrival time of major HSS/CIRs, and the total cumulative numbers of rain-gauge stations with recorded daily rainfall rate exceeding given thresholds are summed up for each epoch day.
For major HSSs reaching a maximum solar wind velocity of at least 600 km/s in the period of 2003-2019 (same as in Fig. 4), 195 Figs. 5a and 5b show the expected SPE analysis results for the green corona intensity and solar wind parameters, respectively.
The superposition of HSS/CIRs relative to the well-defined interface between the fast and slow solar wind shows peaks in the mean np, B and σBz near the key time while the mean solar wind velocity V is rising from a minimum before to a maximum 7 after the key time (Fig. 5b). In Fig. 5c, for each epoch day relative to HSS/CIR arrivals, the cumulative number of stations with the 24-h rainfall exceeding given thresholds are shown. The number of stations with significant rain increases from a 200 minimum before to a maximum after the HSS/CIR arrival. If more moderate HSS/CIRs (e.g., Vmax > 500 km/s) are included, the increase of significant rainfall occurrence following the key day 0 is still observed (Fig. 5d), although the relative increase is smaller. Conversely, for faster solar wind streams (Figs. 5e and 5f) significant rainfall occurrence show greater relative increases following the HSS/CIR arrivals. Independently of rain-gauge observations, the same relationship between HSSs and the satellite-based daily precipitation rates over Slovakia extracted from the TRMM dataset are shown in Figs. 5g,5i and 5j. 205 This is consistent with the results discussed in Section 3 and confirms the tendency of increased occurrence of heavy rainfall following the HSS/CIR arrivals, which becomes more pronounced for stronger HSS/CIRs. Polar coronal holes and their HSSs have been known to persist for years (Tsurutani et al., 1982;Tsurutani et al., 1995). Coronal holes that extend to low latitudes often persist for manyseveral solar rotations producing recurrent HSSs with a period of about 210 27 days. To show it, the SPE analysis is now extended to ±36 days from key time defined by arrivals of major HSSs reaching a maximum solar wind velocity of at least 600 km/s (Fig. 6). The vertical dotted lines are shown for key time ± 27.28 days (called Carrington, or synodic rotation). The superposition of recurrent coronal holes, which include north and south coronal holes, results in depressions of the mean green corona intensity IGC (Fig. 6a) that are, in this case, centered at about zero heliographic latitude 3-4 epoch days before arrival times of recurrent HSSs. The white dotted line shows the mean IGC at zero 215 latitude from the SPE analysis, with the mean over the period of 72 days subtracted. The standard error bar for one of the deviation minima, and the ordinate scale bar corresponding to the color scale on the right are shown. Fig. 6b shows the superposition of solar wind plasma parameters that includes recurrent HSS/CIRs. As in Fig. 5c, but now for ±36 days from the key time, Fig. 6c shows cumulative numbers of rain-gauge stations in Slovakia with above-threshold daily precipitation rates, N20mm, N30mm, and N40mm. The black bold line shows the precipitation rate averaged over all stations for each epoch day 220 (smoothed by a 3-point running window). The histograms show not only the clear increase in precipitation after the key time that is already shown in Fig. 5c, but also similar increases one solar rotation before and after the key time, i.e., an increase in precipitation following the arrivals of recurrent streams. We now compare variations among six variables (V, IGC, np, B, σBz and the mean daily precipitation rate). The cross-correlation function (CCF) is computed for the mean solar wind velocity V paired with the rest of the variables (Fig 6d; shown in colors corresponding to those in Fig. 6b, except for the green line 225 representing IGC). CCF (IGC, V) has the maximum correlation coefficient at a lag of +1 day with secondary maxima at lags of −26 and +28 days, i.e., +1 day relative to the vertical dotted lines representing recurrent streams. The relative lags for the main and secondary peaks of CCF (B, V) and (σBz, V) are 1 or 2 days, and 2 or 3 days for (np, V). Most importantly, the peaks of the CCF (mean rate, V) are at lags of −27, 0 and +27 days, with correlation coefficients that are comparable with those for the other solar wind variables. These quantitative results further support the qualitative observations discussed above by 230 confirming the tendency of an increase in precipitation following the arrivals of HSS/CIRs, including recurrence with a periodicity of 27 days. The latter is demonstrated once more another way in Fig. 7. It shows the SPE analysis of green corona 8 intensity, solar wind parameters, and cumulative number of rain-gauge stations in Slovakia with above-threshold daily precipitation rates, except the key times are lagged by ±27 days relative to the actual arrival times of HSS/CIRs used in Fig. 5, thus doubling the number of superposed time series. Expectedly, Figs. 7a and 7b show smaller amplitudes, because of 235 averaging "imperfectly" recurrent streams (coronal holes and streams evolve over one solar rotation, shifting in longitude and arrival times, respectively), but the amplitude of the increase in the numbers of stations Slovakia with high precipitation rates doubles, because they are summed up for two returns of recurrent streams separated approximately by 54 days. If the SPE analysis is repeated for key times lagged by ±54 days, the results are very similar, even though now the recurrent streams are separated by approximately 108 days (not shown). 240 Of course, HSS/CIRs and their sources, coronal holes, are more often spaced by less than 27 days, typically by ~9 or 13.5-14 days, depending on how many coronal holes cross the solar meridian in one solar rotation. Because their spacing varies over a period of many years (Fig. 6) shown. On average, the occurrence of heavy rainfall followed the arrivals of major HSSs. This is very similar to the effects on the ionosphere following major HSS/CIRs (Prikryl et al., 2012, their Fig. 7).
The SPE analysis for this period shows quite regularly spaced, predominantly southern coronal holes (Fig. 9a), and 260 corresponding recurrent streams, including CIRs (Fig. 9b). The mean IGC (white dotted line) is now extracted for 10˚S heliographic latitude. Fig. 9c shows the cumulative numbers of stations, N20mm, N30mm, and N40mm, along with the 3-point running average of the mean daily rate, all displaying variations that are very similar to those of solar wind parameters (Fig. 9b). Fig.   9d shows CCFs computed for V paired with IGC, np, B, σBz and the mean precipitation rate, all displaying primary peaks separated by 27 days, and several secondary peaks spaced by 9 days. In particular, the CCFs (B, V), (σBz, V), and (mean rate, 265 V) are almost identical, with maximum correlation coefficients at lag +1 day of 0.71, 0.78 and 0.71, respectively. The CCF (mean rate, B) (not shown) peaks at lag zero with a maximum correlation coefficient of 0.55. Of note, σBz is a measure of solar wind Alfvén wave amplitudes that are largest following arrivals of HSS/CIRs but continue to be relatively high inside the HSS, while B and np tail off faster (Fig. 9b), which is likely the reason why the peak of the CCF (σBz, V) cross-correlation coefficient is highest. 270 These results are similar to the "Wilcox effect" (Prikryl et al., 2009a;their Figures 3 and 4), which is mentioned in the Introduction. The HSSs from coronal holes are anchored in the large-scale solar magnetic field structure that is extended into the interplanetary space by solar wind. The magnetic sector boundaries, known as the heliospheric current sheet (HCS) (Smith et al., 1978;Hoeksema et al., 1983), often precede the HSS interfaces by about one day, unless the two coincide. At the bottom 275 of Figs. 6d and 9d cumulative numbers of HCS crossings show a peak preceding the key time (CIRs) by one day or less, and similar peaks for the recurrent HSS/CIRs at ±27 epoch days, as well as the intermediate HSS/CIRs in Fig. 9b. If the key times in the above SPE analysis isare defined by the magnetic sector boundary crossing (SBCHCS crossings), instead of HSS/CIRs, results that are similar to those in Figs. 6 and 9 are obtained, but the patterns from the minima and maxima of precipitation rates are shifted by about one day (not shown). These results are similar to the "Wilcox effect" (Prikryl et al., 2009a;their 280 Figures 3 and 4) briefly discussed in the Introduction. As already mentioned, the "Wilcox effect" is concerned with the minimum in the VAI following the HCS crossing near epoch day +1. Originally, there was no reference to the maximum in the VAI that followed a few days later. The VAI minimum was linked to a calm solar wind period characterized by a weak magnetic field and absence of Alfvén waves (Prikryl et al., 2009a), referred to as the "calm before the storm" in CIR/magnetosphere interactions (Borovsky and Steinberg, 2006). The minimum in the occurrence of high-rate precipitation 285 appears to correspond to the minimum in "storminess" characterized by the VAI, although it is noted that the latter was limited to winter months. Therefore, the similarity with the present results ends there. Under the influence of long-lasting and intense precipitation, numerous flood waves arise, which may not be adequately 295 mitigated by manipulating hydraulic structures. The water levels in many profiles on watercourses reached historical maxima in the above-mentioned regions. Regular occurrence of floods in eastern Slovakia is given primarily by a total land water regime, which is dominated by the very low absorptive power of heavy clayey soils of the Flysch zone (Azañón et al., 2010), and adverse conditions of forests in eastern Slovakia.

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One locality that has been repeatedly affected by flash floods in Slovakia is the High Tatras mountain range (Tatra National Park). In general, mountainous regions are often associated with flash floods, likely because of orographic enhancement of precipitation and anchoring of convective events, as well as various aspects of topographic relief that promote rapid concentration of streamflow (Borga et al., 2011). The Tatra Mountains are the highest part of the Carpathians and this alpine massif in northern Slovakia has the highest total annual precipitation. Similarly, the northern slopes of the Tatra Mountains in 305 Poland are affected by flash floods that are included in the SPE analysis in Fig. 1.
The most extreme flash flood that affected the southern slopes of the massif occurred on June 28-29, 195828-29, (Pekárová et al., 2011. Several stations recorded daily precipitation rates exceeding 50 mm with the maximum reaching 170 mm at Skalnaté pleso (1778 m a.s.l.) (Šamaj et al., 1985). Although there were no spacecraft monitoring the solar wind at that time, coronal 310 holes, the sources of HSSs, can be identified in synoptic maps of green corona intensity observed at high-altitude observatories.
Furthermore, sector boundary crossings can be estimated from ground-based magnetograms (Svalgaard, 1975). Fig. 10a shows the synoptic map of green corona intensity indicating large southern coronal holes during this period. Here we are concerned with the coronal hole prior to June 27, a source of HSS that impacted the Earth around June 28/29 (SBC HCS is shown by an asterisk in Fig. 10b). There is also a possibility of an ICME from the bright coronal region on the Sun that may have arrived 315 at about the same time, thus amplifying the intensity of the impact on the Earth's magnetosphere and making it more geoeffective. The 3-hourly Kp index of geomagnetic activity (https://www.swpc.noaa.gov/products/planetary-k-index) reached very high values (Fig. 10c) during the strong geomagnetic storm with Dst index (Gonzalez et al., 1994) reaching −180 nT early on June 29, which coincided with the heaviest rainfall. 320 Fig. 11 shows another case of a severe flash flood that occurred on June 29-30, 1973. It followed the arrival of an exceptionally very fast HSS (> 750 km/s; Fig. 11b) from a large northern coronal hole (Fig. 11a). The asterisks show the stream interfaces of two CIRs, both causing a significant increase in geomagnetic activity (Fig. 11d) and triggering moderate geomagnetic storms on June 23/24 and 28/29. Both geomagnetic storms coincided with heavy rainfall events. The second one, with a maximum daily rate of 102.5 mm at Skalnaté pleso (1778 m a.s.l.) on June 30, caused a flash flood in the High Tatras. 325 In July 2008, the maximum precipitation (144.6 mm) was observed in Tatranská Javorina on June 23, and the total between July 20 and 24 reached 260.2 mm, the largest recorded at this station. This was a year of solar minimum, and the green corona intensity was very low. However, a coronal hole can be identified (Fig. 12a), and a small compact coronal hole was observed by SOHO in the EUV images. The arrival of an strong HSS reaching a maximum V of 650 km/s, and a broad/double CIR with 330 two stream interfaces on July 21 and 22 (the first of them marked with asterisk in Fig. 12b) was followed by moderate geomagnetic activity (Fig. 13d). Two spikes of intense rainfall were recorded at many stations.

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The last major flood in Tatra National Park was caused by heavy rainfall from July 17 to 18 (Fig. 13c) with the precipitation maximum of 123 mm on July 18 that closely followed the arrival of HSS/CIRs on July 17 (Fig. 13b) from a large and structured 335 north-south coronal hole (Fig. 13a). While the planetary Kp-index was relatively low (Fig. 13d), the strong CIR on July 16/17 produced by the stream that initially reached Vmax ≈ 440 km/s but a few days later exceeded 500 km/s, triggered intense auroral substorm activity observed by the IMAGE magnetometers on July 16 (https://space.fmi.fi/image/). The CIR was followed by large amplitude ultralow frequency (ULF) fluctuations of the ground magnetic field caused by ionospheric current fluctuations early on July 17. These are the sources of AGWs discussed in the next section. 340 In this section we discussed only some of the most severe flash floods on the southern slopes of the High Tatras mountain range in Slovakia. Two flash floods in July 2001 were already mentioned in the Introduction and have been discussed elsewhere (Prikryl et al., 2018). Flash floods on the Polish side of this mountain range and flash floods mentioned in the SMHU annual reports (2003-2019) also showed a tendency to follow arrivals of HSS/CIRs (Figs. 1c and 1d). 345

Cases of heavy rainfall leading to floods and flash floods in Europe, Japan, and the United States
We now discuss cases of heavy rainfall, floods, and flash floods in the geophysical context of solar wind MIA coupling. The hourly solar wind OMNI data (V, B, np and σBz) show a series of major HSS/CIRs (red asterisks) and ICMEs (orange triangle) 350 in July 2017 and June-July 2018. The negative deflections of the Dst index (green line) show geomagnetic storms that were triggered by HSS/CIRs, the most intense one in conjunction with an ICME on July 16. The black symbols mark the start days/times of heavy rainfall that caused floods in Europe (triangles), Japan (circles) and the USA (squares). Most of these events closely followed the arrivals of HSS/CIRs or ICMEs. Because the actual start times of heavy rainfall events were generally not available, most of the symbols are shown at 12:00 UT, except for cases when the actual start time is known. 355 The orange symbols (◊) in Fig. 14 mark start days/times of significant rainfall that caused floods in Slovakia (taken from the annual reports on floods). The orange dots at the top show daily precipitation rates measured at SHMU stations. The floods on the rivers Bodrog and Ondava that were caused by significant rainfall (July 1 and 10), closely followed arrivals of major HSS/CIRs. The floods on the Hron river tributaries (July 24) occurred in the peak of a strong and structured HSS (Fig. 14a) 360 from a wide and structured coronal hole.
In addition to floods and flash floods, Figs. 14 shows daily precipitation rates measured at SHMU stations in Slovakia (orange dots). Also shown (in purple), are maximum daily rates and number of grid cells with rainfall exceeding 30 mm, from the Deutscher Wetterdienst (DWD) REGNIE dataset. These variables show significant increases following most of the HSS/CIRs. 12 Fig. 15a shows the synoptic map of green corona intensity starting with a large southern coronal hole at the end of June, followed by a northern structured coronal hole that extended toat low heliographic latitudes between July 5 and 10. They were sources ofproduced two major HSSs that impacted the Earth with CIRs arrivals on July 1 and 9, and a minor HSS/CIR on July 6 ( Fig. 15b), each causing an increase in geomagnetic activity (Fig. 15d). The arrivals of the two major HSS/CIRs were 370 associated with maxima in daily precipitation, on June 1 and 10, the latter exceeding 60 mm (Fig. 1c).
The last HSS at the end of July 2017 is presented in Figs. 15e-h. Fig. 15e shows two large north-south coronal holes joined into a wide structured coronal hole in southern latitudes that was a source of a very strong and structured HSS (Fig. 15f). The arrival of this HSS/CIR on July 20 was followed by heavy precipitation in Slovakia that peaked on July 23 exceeding 80 mm, 375 followed by more rain, and floods the next day (Fig. 15g). Furthermore, a flash flood caused by extreme rainfall exceeding 230 mm occurred in Kansas City on July 22, and heavy rainfall exceeding 300 mm in many places in Japan was recorded on July 22-24 (Fig. 14a).
Focusing on floods in Slovakia, a selection of the Meteosat RGB Composites Airmass images (composite based upon data 380 from IR and WV channels) in Fig. 16 show intensifying mesoscale systems on July 1, 10 and 24. Some of the convective cells/bands over Slovakia caused heavy rainfall leading to floods. On July 1, starting at ~02:00 UT a series of cloud/rain bands called striated delta cloud systems (Feren, 1995) developed in the warm frontal zone of an intensifying extratropical cyclone passing over eastern Slovakia and Poland (Fig. 16a).

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We now present relevant geophysical data collected by the SuperDARN radar in Halkasalmi, Finland, with beams pointing poleward over Svalbard measuring line-of-sight velocity Vlos of convection/flows in the ionosphere (Chisham et al., 2007).
After 20:00 UT on June 30, the radar observed a series of pulsed ionospheric flows (PIFs) with periods from 10 to 20 min between N70˚ and 75˚ latitudes (between the northern tip of Scandinavia and Svalbard). Fig. 17 shows the median filtered line-of-sight velocity Vlos for radar beam 11 as a function of geographic latitude. Fig. 18a shows a time series of Vlos for range 390 gate 22 (75˚ latitude). The Fast Fourier Transform (FFT) power spectrum of the detrended time series (dotted line) shows a peak at 1.1 mHz (~15 min). PIFs are sources of atmospheric gravity waves (Prikryl et al., 2005) that were likely launched several hours before the striated delta clouds developed on July 1.
Before we discuss AGWs, their sources in the high-latitude lower thermosphere, and their propagation upward and downward, 395 it should be noted that when the AGW group velocity is downward, the phase velocity is upward, and vice versa, with the two velocity vectors being perpendicular to each other. For clarity, we will refer to downward/upward group (wave energy) propagation as down/up-going AGWs to distinguish from upward/downward AGW phase propagation.

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The equatorward propagating down-going AGWs launched by PIFs (Fig. 17) would reach the troposphere. If ducted, similarly 400 to the case shown in Prikryl et al. (2018;their Figure 15), down-going AGWs could have reached the unstable warm frontal zone of the cyclone over Slovakia and initiated/triggered slantwise convection resulting in a series of cloud bands (Fig. 16a).
Of note, the number of cloud bands in this striated delta cloud was about the same as the number of PIFs (Figs. 17a and 18a).
A tendency of striated delta clouds to develop following the arrivals of HSS/CIR has been shown previously (Prikryl et al., 2018, their Fig. 9), and the present case is consistent with these statistical results and the case study presented there. 405 The increased divergence between 150-300 mb in the warm frontal zone associated with the striated delta cloud (Fig. 16a) suggests an unstable region, and the strong mid-upper level winds with the northward component indicate winds opposing the direction of the incoming/down-going AGWs. These conditions are conducive to over-reflection of AGWs, with a possibility of amplification (Jones, 1968;Cowling et al., 1971;McKenzie, 1972;Eltayeb and McKenzie, 1975). 410 The warm frontal zone is known for moist symmetric instability (MSI) (Schultz and Schumacher, 1999), where a slantwise convection can be readily initiated by even small (infinitesimal) displacements of moist air. It was suggested that MSI can be triggered by over-reflecting down-going AGWs that originate in the lower thermosphere at high (auroral) latitudes. Previously, cloud/rain bands were linked to possible sources of AGWs or observed traveling ionospheric disturbances (TIDs) in a few 415 cases (Prikryl et al., 2009b;Prikryl et al., 2018, including their supplementary material).
On July 10, a string of convective cells developed in the cold frontal zone passing over Slovakia and Poland (Fig. 16b), one of them causing heavy rainfall (Fig. 15c) and floods (Fig. 14a) in Slovakia. We note the similarity with the heavy rainfall/flash flood event that involved a string of supercells (Prikryl et al., 2018, their Fig. 14) that were likely triggered by down-going 420 aurorally generated AGWs reaching the troposphere, while the up-going AGWs were observed in the ionosphere.
Unfortunately, this time, strong HF absorption wiped out most of the ionospheric radar backscatter and no PIFs could be observed at high latitudes. Also, only weak radar ground scatter was observed indicating TIDs between 12:00 and 20:00 UT (not shown). However, AGWs were likely launched by pulsing ionospheric currents observed over Svalbard on July 10, several hours before the string of convective cells developed. Fig. 18b shows the magnetic field X component observed by the IMAGE 425 magnetometer NAL. The FFT spectrum of the detrended time series shows a peak at 0.4 mHz (~40 min). High divergence at the mid-upper level in the cold frontal zone indicates an unstable region where the convection occurred (Fig. 16b). Downgoing AGWs over-reflecting in the unstable region could have triggered/initiated convection. Fig. 16c shows a similar instance of convective cells triggered over Hungary that subsequently moved over Slovakia and 430 caused heavy rainfall and flood on July 24 (Figs. 14 and 15g). Again, PIFs could not be observed by the radar but pulses of ionospheric currents were observed over Svalbard several hours before these convective cells were initiated. Fig. 18c shows the magnetic field X component pulsation observed by the IMAGE magnetometer NAL. The FFT spectrum of the detrended 14 time series shows two prominent peaks at 0.4 mHz (~40 min) and 0.75 mHz (~20 min). These ionospheric currents likely launched down-going AGWs that could have triggered the convection cells in the unstable region of high divergence and 435 strong north-eastward winds at the mid-upper level (Fig. 16c). Fig. 16d shows a string of convective cells in the intensifying mesoscale system Medusa that produced heavy rainfall/flash floods in Greece on July 16, 2017 (Fig. 14a). The overlay CIMSS data products are missing for this day but the next day still shows strong divergence and southerly winds at mid-upper level over Greece (Fig. 16d). Moist instabilities could have been 440 released by AGWs launched by strong ionospheric current pulses that were observed over Svalbard several hours before convection developed on July 16. Fig. 18d shows large-amplitude pulsation of the ground magnetic field X component observed by the IMAGE magnetometer NAL. The FFT spectrum of the detrended time series shows a peak at 0.55 mHz (~30 min). Weak traces of TIDs caused by the up-going AGWs in the thermosphere were observed in the ground scatter by the Hankasalmi radar beam 13 (not shown). 445 The strings of convective cells in Fig. 16 are similar to certain types of mesoscale convective lines (Bluestein and Jain, 1985; their Figure 1). These authors identified four distinct types of mesoscale convective lines of cells that form squall lines, namely, "broken line", "back building", "broken areal", and "embedded areal" types. The squall lines form in a conditionally and convectively unstable atmosphere. The "broken line" forms typically along a cold front with multi-cells appearing at about the 450 same time and transforming "into a solid line as the area of each existing cell expands and new cells develop" (Bluestein and Jain, 1985). This type could represent the cases shown in Figs. 16b and 16d. The "back building" type "consists of the periodic appearance of a new cell upstream, relative to cell motion" and can form along different types of surface boundaries. This type pertains to the case in Fig. 16c, and the cases discussed by Prikryl et. al, 2018, where new cells appeared periodically. The other squall line types may include the warm frontal bands and wide cold frontal bands (for detailed description of their 455 characteristics, see, Bluestein and Jain, 1985). Similarly to striated delta clouds (Prikryl et al., 2018;their Fig. 9), the SPE analysis of solar wind data keyed to dates from the list of cases (Bluestein and Jain, 1985; their Table 1) shows a tendency of these four types of squall lines to follow arrivals of HSS/CIRs/ICMEs (not shown).
The HSS and a broad CIR, combined with an ICME on July 16 (Fig. 14a), caused a moderate geomagnetic storm (Dst = −72 460 nT) starting with the arrival of the interplanetary shock at about 06:00 UT. Fig. 19 shows strong magnetic field perturbations (ionospheric currents) observed by the IMAGE array over a range of latitudes from Fennoscandia to Svalbard. The ionospheric current pulses launched large-amplitude AGWs that produced large-and medium-scale TIDs observed in maps of the GPS Total Electron Content (TEC) over much of Europe as well as the North America on July 15 and 16 ( These events are marked by triangles in Fig. 14a. In the U.S., deadly flash floods struck in Arizona with thunderstorms and heavy rain affecting areas around Payson between 21:00 and 23:00 UT on 15 July: "The National Weather Service issued a flash flood warning for Gila County and estimated that up to 38 mm of rain fell over the area in an hour… A further storm on 16 July caused flooding in the Phoenix area. Litchfield Park recorded 52 mm in 24 hours 16 to 17 July, 2017" 470 (http://floodlist.com/america/usa/flash-floods-arizona-july-2017). The observation here is that large-amplitude TIDs (Fig. 20) were caused by up-going AGWs from sources in the high-latitude lower thermosphere. In turn, the down-going AGWs that can reach the troposphere could have triggered/released moist instabilities initiating convection.
Finally, we briefly discuss the second period in June-July 2018 (Fig. 14b) that shows a series of 6 major HSS/CIRs. On June 475 25, an ICME (orange triangle) triggered a moderate geomagnetic storm that was further intensified with the arrival of a strong HSS/CIR (June 26; red asterisk). As Earth was exiting the HSS , another ICME caused a small solar wind disturbance on June 30. Subsequent HSSs produced broad CIRs with large density and magnetic field fluctuations that resulted in strong MIA coupling.

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In this period, heavy rainfall events caused large number of floods and flash floods in the U.S., Europe, and Japan. All of them followed the arrivals of HSS/CIRs or ICMEs, and some of them occurred on different continents at about the same time. On June 26-29, heavy rain caused floods in Greece, Bulgaria, Romania, and Slovakia. Following the ICME on June 30, extreme rainfall that occurred in Iowa (24-hour rate of 221 mm in Ankeny and 213 mm in Johnston) on June 30 to July 1 caused deadly flash floods. The next major HSS/CIR that arrived on July 5 was preceded by a minor but broad HSS/CIR on July 3-4. Furthermore, daily precipitation rates measured at SHMU stations in Slovakia, as well as the maximum daily rates and the number of REGNIE grid cells with rainfall exceeding 30 mm in Germany (Fig. 14b), show increases following the HSS/CIRs including the HSS/CIR on July 10. The European Severe Weather Database (https://eswd.eu/cgi-bin/eswd.cgi) includes confirmed reports of heavy rain causing floods in several countries from July 10-12, including a large area of Poland that was 495 affected by heavy rain and floods on July 11. Of course, the information about flood events used in this paper is not complete and more data, including the NOAA Storm Events Database events in the U.S. (https://www.ncdc.noaa.gov/stormevents/) will be examined in the future. 500   16 We have examined heavy rainfall leading to floods and flash floods in the context of solar wind, linking the occurrence of such weather events to arrivals of solar wind HSSs, which are dominated by Alfvén waves (Belcher and Davis, 1971) known to cause HILDCAAs; (Tsurutani and Gonzalez, 1987). Solar wind MIA coupling generates medium-to large-scale AGWs globally propagating from sources in the lower thermosphere at high latitudes both upward and downward (Mayr et al., 1984a(Mayr et al., , 505 1984b(Mayr et al., , 1990(Mayr et al., , 2013Hocke and Schlegel, 1996). The coupling is most intense when the HSS/CIRs and/or interplanetary shocks at the leading edges of ICMEs arrive and generate large-amplitude AGWs originating in the lower thermosphere at high latitudes. Pioneer spacecraft in 1970s were the first to observe interaction regions between adjacent solar wind streams (Smith and Wolfe, 1976) and their geo-effectiveness has been well established (Tsurutani et al., 2006). Solar wind magnetohydrodynamic waves, including Alfvén waves, modulate the high-latitude ionospheric currents that can generate globally 510
Aurorally excited AGWs can reach the troposphere and can be ducted in the lower atmosphere over long distances, thus reaching low latitudes. At the reflection point in the troposphere they can trigger moist instabilities to release latent heat, which in turn leads to intensification of extratropical cyclones (Prikryl et al., 2009b;2018) and tropical cyclones (Prikryl et al., 2019). These studies showed that explosive development of extratropical cyclones and rapid intensification of tropical cyclones 515 tend to follow arrivals of solar wind HSSs and/or ICMEs. Prikryl et al. (2009b) suggested that in the extratropical cyclone warm frontal zone, with warm air advection over the cool air mass ahead, the over-reflecting gravity-wave-induced vertical lift may trigger/release moist symmetric instability (MSI) at near-threshold conditions and thus initiate slantwise convection. Latent heat release associated with the mesoscale slantwise convection has been linked to explosive cyclogenesis (e.g. Kuo and Low-Nam, 1990). A commonly raised question about the 520 possibility of strongly attenuated gravity waves, with amplitudes reduced by a factor of 10 3 to10 4 in the troposphere, is why such gravity waves would be considered to trigger convection when there are many other sources of perturbations, including gravity waves generated in the troposphere itself, which have larger amplitudes. One reason is that the release of conditional symmetric instability, particularly in the warm frontal zone, has been known to initiate convection and result in frontal precipitation bands (Bennetts and Hoskins, 1979;Bluestein, 1993;Houze, 1993;Emanuel, 1994). Even an infinitesimally 525 small displacement of the moist air parcel can initiate slantwise convection (Schultz and Schumacher, 1999). The warm frontal cloud bands, sometimes called striated delta clouds (Feren, 1995), in rapidly intensifying extratropical cyclones, were shown to follow arrival of HSSs (Prikryl et al., 2018). When down-going AGWs over-reflect in the warm frontal zone of extratropical cyclones, even the small additional lift they would provide to a moist air parcel that is already rising over the cold air ahead, can initiate slantwise convection, thus forming a precipitation band. As already indicated above, it is also impotant to consider 530 that the over-reflection of gravity waves can result in amplification. We are not aware of any reports of a specific type of tropospheric perturbation that reaches instability region, in this case the warm frontal zone, and trigger banded convection.
Although stochastic fluctuations could trigger convection, they would neither explain the observed link to solar wind HSSs nor the coherent wave structure of the cloud.

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Callaghan and Power (2016) described conditions, including vertical wind structure, that lead to extreme rainfall and major flooding in southeast Australia. They found that most cases exhibited anticyclonic turning wind-direction with increasing height and warm moist air advection. They observed "typically east northeasterly winds at 850 hPa turning anti-cyclonically with height to northerly winds by 500 hPa." The downward propagating AGWs, in this case from the lower thermosphere sources at high southern latitudes, may encounter opposing winds in the upper troposphere, a condition conducive to over-540 reflection, that could release the MSI and trigger convection. Major floods in southeast Australia (Callaghan and Power, 2016) appear to showhave a tendency to occur following HSS/CIRs with results similar to Figs. 1 and 2, which will be discussed in a future publication. Prikryl et al. (2018) discussed two flash floods in Slovakia that closely followed arrivals of two major HSS/CIRs in July 2001. 545 They identified pulsing ionospheric currents as a source of AGWs that were observed in the ionosphere as TIDs. A few hours later a series of convective cells that formed in sequence were observed in infrared satellite images. These observations suggested that down-going AGWs played a role in triggering instabilities and initiating convection in the troposphere. One of the supercells caused heavy rainfall and flash floods in Slovakia. Cases of striated delta clouds were also linked to possible auroral sources of AGWs (Prikryl et al., 2018;supplementary material). 550 Consistent with these previously published results, the statistical results presented in Section 3 show that heavy rainfall events leading to floods and flash floods tend to follow arrivals of HSS/CIRs. The SPE analysis of green corona intensity and solar wind parameters keyed to the onset of heavy rainfall taken from several databases/lists of events in Europe and USA (Figs. 1 and 2) reproducibly show patterns that indicate this tendency, as discussed in Section 3. These conclusions are supported by 555 the analysis of more comprehensive databases of daily precipitation rates and annual flood reports in Slovakia for a period of 2003-2019 using a twofold approach. First, the SPE analysis of green corona intensity and solar wind parameters is keyed to significant rainfall events of at least 30 mm/24h recorded at 10 or more stations (Fig. 3). Focusing on floods in Slovakia, the SPE analysis of the green corona intensity and solar wind parameters keyed to dates/times of significant rainfall leading to floods clearly indicates a tendency of these flood events to follow arrivals of solar wind HSS/CIRs (Fig. 4). Second, with the 560 key times defined by the arrival times of major HSS/CIRs (Section 4), the SPE analysis (Fig. 5) shows an increase in the cumulative number of stations that measured daily precipitation rates above given thresholds. Such a relationship between solar wind HSSs and daily precipitation rates is also found using the satellite-based daily precipitation dataset TRMM. Finally, extending the SPE analysis to ±36 days about the key times defined by arrivals of major HSS/CIR reveals a similar response at epoch days ±27 due to recurrent streams. The cross-correlation between the SPE averages of the green corona intensity as 565 well as solar wind parameters and mean daily precipitation rates quantitatively confirm the observed tendency of high precipitation occurrence following the HSS/CIRs. In Sections 5 and 6, the tendency of heavy rainfall and floods following arrivals of HSS/CIRs is shown in case studies , some supported by satellite images (Fig. 16) of intensifying mesoscale systems displaying striated delta clouds or 570 strings of convective cells. Pulsed ionospheric currents (Figs. 17 to 19) are sources of AGWs that can be observed in the ionosphere (Fig. 20). Ray tracing of gravity waves (Prikryl et al., 2005; and simulations of gravity wave propagation using the Transfer Function Model (Mayr et al., 1984a;2013;Prikryl et al., 2016;2018) showed that AGWs can reach the troposphere and play a role in triggering moist instabilities and initiating convection leading to heavy rainfall and floods.

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Recently, Hagiwara and Tanaka (2020) performed theoretical analysis of propagation of AGWs in the lower atmosphere using an expansion in three-dimensional normal mode functions. They showed that the waves can propagate downward to the troposphere as attenuating gravity waves, with the amplitude reduced by a factor of 10 3 to10 4 in the troposphere. and They found that "the wave propagations and reflections at the surface create an anti-node of geopotential at the bottom of the atmosphere corresponding to the vertical width of the initial state of the impact. On the other hand, standing waves in 580 temperature create a node at the ground surface." They suggested that "due to the standing waves generated in the lower troposphere, the atmospheric stability is altered by the passage of the gravity waves in the meridional direction," and that the change in the stability parameters can affect the development of cyclones. Further theoretical and computational research examining the dynamics of down-going AGWs and their influence in the troposphere is needed to complement the data-driven analysis of this phenomenon. 585

Conclusions
Heavy rainfall causing floods and flash floods tends to follow arrivals of solar wind high-speed streams from coronal holes. 590 The superposed epoch (SPE) analysis results show an increase in precipitation rates following arrivals of high-speed streams, including recurrence with a periodicity of 27 days. The cross-correlation analysis applied to the SPE averages of green corona intensity, 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 solar minimum (2008)(2009), correlation peaks at lags spaced by 9 days are also revealed, which is a result of high-speed streams from coronal holes spaced in heliographic longitude by approximately 595 120˚. These quantitative results confirm the tendency of an increase in precipitation following the arrivals of high-speed streams, which is further demonstrated by cases of heavy rainfall, floods and flash floods in Europe, Japan, and the U.S. The role of aurorally generated atmospheric gravity waves as the mechanism mediating the influence of the solar windmagnetosphere-ionosphere-atmosphere coupling on the troposphere is suggested. Down-going gravity waves from sources in the lower thermosphere can over-reflect in the upper troposphere and trigger/release existing moist instabilities, initiating 600 convection and latent heat release, the energy leading to intensification of storms.       (Svalgaard, 1975).  (https://eumetview.eumetsat.int/static-images/MSG/RGB/AIRMASS/). Overlay data products from the CIMSS data archive of wind analysis (http://tropic.ssec.wisc.edu/archive/) show divergence between 150-300 mb in the middle panels, and mid-985 upper level winds in the bottom panels, for approximately the same times as those of the images in the top panels.