2. Cosmic rays - Venusian atmosphere interactions during different periods of solar activity Cosmic Rays as an agent for space weather at Venus Interactions of the galactic and solar cosmic ray particles with the atmosphere of Venus result in extensive nuclear and electromagnetic cascades that can affect cloud formation and chemistry in deep atmospheric layers. The energy spectrum of these incoming cosmic ray particles, their integrated flux -as well as their direction- can vary in time and space due to solar activity. Such variabilities can result in variations of the cascade properties in the Venusian atmosphere with possible effects in: the local neutral densities particle ionization escape Christina Plainaki[1][2], Pavlos Paschalis[2], Davide Grassi[1], Helen Mavromichalaki[2], Maria Andriopoulou[3] [1]INAF-IAPS, Via del Fosso del Cavaliere, 00133, Rome, Italy; [2]Nuclear and Particle Physics Section, Physics Dpt., National and Kapodistrian University of Athens, Greece; [3]Space Research Institute, Austrian Academy of Sciences, Graz, Austria Primary Cosmic Rays adapted from Universe Today image http://www.universetoday.com/47905/why-is-venus-so-hot/ Cosmic Rays entering the Venusian atmosphere It is of significant importance to understand and quantify the effects of variable space conditions in the Venusian atmosphere in the context of future mission preparation and also data interpretations of previous missions (e.g. VEX). Why study atmosphere – cosmic rays interactions at Venus ? The properties and the evolution characteristics of the secondary particle showers in Venus provide evidence of planetary space weather phenomena which are linked also to atmospheric phenomena in the planet. Moreover, through planetary space weather studies important feedback for understanding better the space weather in the Earth can be provided.

3. Motivation of the current study Method We examine both cases of Galactic Cosmic Rays and Solar Cosmic Rays. In the latter, we use as test cases the event of Sept 1989 and the recent SEP event of 17 May 2012. Galactic cosmic rays (GCRs) are particles that originate from stellar sources and are accelerated to high energies. They consist mainly of protons (~89%), alpha particles (~10%) and a small portion (~1%) of heavier nuclei. The energy spectrum of the cosmic rays is wide, ranging from about 1 GeV to extremely high energies of ~ about 10 21 eV. We apply a Monte Carlo modeling technique namely Dynamic Atmospheric Shower Tracking Interactive Model Application - DYASTIMA that is based on the Geant4 software, previously applied for the Earth case (Paschalis et al., 2014). During periods of intense solar activity, manifested by coronal mass ejections (CMEs) and solar flares, the cosmic ray flux at the vicinity of Venus can be enhanced, as it often happens in the Earth case. Such enhancements are due to the arrival of solar energetic particles (SEPs), also known as Solar Cosmic Rays which reach the planet with a composition similar to that of the galactic cosmic rays and energies from a few keV to a few GeV (Miroshnichenko, 2001). Integral proton flux recorded by EPS/GOES (top) and EPAM/ACE (bottom) during the SEP event of 17 May 2012 (measurements at 1 AU) Plainaki et al., ApJ, 2014 Hillas 2006 Galactic Cosmic Ray energy spectrum the estimation of the ionization rate variability over both long and short term (planetary space weather) the calculation of the ionization rate at Venus due to cosmic rays in order to understand its impact on the atmospheric properties of the planet like electrical conductivity, atmospheric chemistry and charging of cloud particles.

4. Whereas the magnetic field of the Earth provides shielding of the planet from the cosmic ray particles, Venus does not possess a global magnetic field capable of deflecting charged particles, and so even low energy CR primaries have unimpeded access to the planet's atmosphere. At Venus, the absence of an intrinsic magnetic field, allows Solar Cosmic Rays to access the atmosphere every time the geometry of the solar particle event (SEP) is favorable. Coronal mass ejections (CMEs) may result in phenomena similar to those known (for the Earth case) as Forbush decreases (Lockwood, 1971), reducing the local CR flux at Venus The geometrical characteristics of the CME (with respect to the configuration of the interplanetary magnetic field (IMF) and the planet's orbit) are responsible for the extent (in space and in energy) to which the local CR fluxes are reduced. Note that From Reames (1999) DYASTIMA is a new tool for the simulation of cosmic ray showers in a planetary atmosphere, based on the Geant4 toolkit (Agostinelli et al., 2003; Allison et al., 2006). Several key quantities are easily parameterized, in order to adapt to different conditions of the planet’s atmospheric structure and magnetic field and to the primary cosmic ray spectrum (galactic and solar). DYASTIMA outputs can be easily inserted in other applications (e.g. atmospheric models); such an interface is of particular importance for planetary studies since it facilitates the integration of different scientific models in view of an inter- disciplinary scientific output. The DYASTIMA tool SEP event paradigm Blue lines represent positive charge, red lines negative charge and green lines neutral. The shower’s dimension increases with the energy increase. From Paschalis et al., 2014. Shower representation of a vertical proton with 1 GeV (up) and 10 GeV (down) in the Earth case. In view of Planetary Space weather The application of DYASTIMA to Venus is the first step of an effort to apply such codes in different planetary environments (e.g. terrestrial planets with an atmosphere, such as Mars and Venus; satellites of Giant planets, with an atmosphere such as Titan).

5. In this work, we perform a calculation of the atmosphere ionization and ion production rates caused by cosmic rays, as a function of depth in the Venusian atmosphere. We examine the interactions of the planet's atmosphere with: a) galactic cosmic rays, during solar maximum conditions; b) galactic cosmic rays, during solar minimum conditions; c) solar cosmic rays during SEP conditions. The scenario (c) was studied for two paradigm cases: the very energetic SEP/GLE event of Sept. 1989 and the recent less energetic SEP/GLE event of May 2012. SEP Interactions with the Venusian atmosphere For the GLE 71, we considered the SEP properties (integrated flux and spectrum) obtained by the NMBANGLE PPOLA model (Plainaki et al., 2010; 2014) applied previously for the Earth case, scaled to the distance of Venus (i.e. 0.72 AU from the Sun). CME observation by SOHO/LASCO on 17 May 2012 http://www.spaceweather.com/im ages2012/17may12/cme_anim.gif? PHPSESSID=gmt7p9bqv1taqfhivucg nrg640 Average (over latitude and longitude) integral SEP fluxes as derived by the application of the NMBANGLE PPOLA model. The GOES proton flux of particles with energy >100 MeV is also plotted. From Plainaki et al., ApJ, 2014 RESULTS from the application of DYASTIMA Galactic cosmic ray interactions with the Venusian during solar minimum and solar maximum Ionization rate estimated with DYASTIMA as a function of altitude for the solar minimum and solar maximum conditions Our results show an ionization peak of 77 ion pairs cm -3 s -1 at 64 km during Solar Minimum conditions, and an ionization peak of 60 ion pairs cm -3 s -1 at 61 km during Solar Maximum conditions. These results are very close to the results by Nordheim et al. 2015, who estimated an ionization peak of ~ 58 ion pairs cm -3 s -1 at 62.5 km. DYASTIMA gives a slightly higher rate during Solar minimum than during Solar maximum which is consistent with the higher galactic cosmic ray flux in this solar cycle phase. During Solar maximum, Nordheim et al. (2015) obtained the biggest reduction in the ion production rate at the higher altitudes, which is also our case. The results with DYASTIMA are in good agreement also with the results in other studies, although a full comparison is not possible, as these studies do not have the same coverage in altitude.

6. In this work, we perform a calculation of the atmosphere ionization and ion production rates caused by cosmic rays, as a function of depth in the Venusian atmosphere. We examine the interactions of the planet's atmosphere with: a) galactic cosmic rays, during solar maximum conditions; b) galactic cosmic rays, during solar minimum conditions; c) solar cosmic rays during SEP conditions. RESULTS from the application of DYASTIMA for two different SEP events Salactic cosmic ray interactions with the Venusian during solar minimum and solar maximum Upper values of the ionization rate in the Venusian atmosphere as a function of altitude the 29 Oct 1989 and 17 May 2012 SEPs The ion production rate varies by order of magnitude during different SEP events. The altitude where the peak rate is estimated is also different during different events varying from ~ 65 km (17 May 2012 event) to ~ 95 km (October 1989 event) in the test cases considered in the current study. We note that for the October 1989 event the altitude of the peak ionization rate is the same as the one estimated by Nordheim et al., i.e. 95 km. Such variations in the ion production rates due to solar energetic particles events could result in important structural variations of the Venusian atmosphere. However, the extent of their impact should be evaluated considering the respective time-scales of the involved processes. Conclusions A hard rigidity spectrum of accelerated protons was found during the initial phase of GLE 60 and a rather soft spectrum in later phases, i.e. after 14:00 UT (γ ~ -5.5). During the main phase of GLE 71, the rigidity spectrum index γ was estimated to be ~ -2.1; in later phases, i.e. after 02:20 UT, a softer spectrum of accelerated protons (γ ~ -3.8) has been derived. The corresponding values for the energy spectrum are -1.55 and -2.4, respectively. The results for GLE 71 are consistent with the typical range found by Ellison and Ramaty (1985) for shock wave acceleration in case of relativistic SEP events (see also Mewaldt et al., 2012), although a direct flare contribution cannot be excluded (see also Plainaki et al., 2014). The model-results can provide realistic estimation of the SEP fluxes in the energy range where NM increases are registered. For both GLEs, the model seems to overestimate the spatially averaged SEP spectrum in the high rigidity range, where no NM increases are registered. Comparison of the results obtained for the two events is more trustful mainly in the rigidity range 1-3 GV. The spectrum computed in the event main phase results to be harder for GLE 71. This could be due to the different SEP flux spatial distribution with respect to the NM direction of viewing and the different GCR background level (that is reduced during GLE60) The integral SEP fluxes calculated by the NMBANGLE PPOLA model are in good agreement with GOES observations if extrapolated to the lower energy range. Conclusions Through the application of the DYASTIMA code, GCR and SCR cosmic ray ionization profiles for the Venusian atmosphere have been provided. The results on the ion production rates are in good agreement with those of previous studies (Nordheim et al., 2014; Borucki et al., 1982; Dubach et al., 1974). The ionization peak was found to occur at ~64 km, with a value of 77 ion pairs cm - 3 s -1, in absence of solar energetic particle events. Two cases of SEP events were considered in order to calculate the expected increase in the ion production rate in the Venusian atmosphere: the event of Oct 1989 (hard SEP spectrum) and the event of 17 May 2012 (soft SEP spectrum). In the latter case the ionization rate can be up to a factor of 5 bigger than the one expected during normal background conditions. On the contrary, during very extreme events such as the one in 1989, an increase up to a factor of 65 is expected. Such estimations considering both GCRs and SCRs could be useful in the future for evaluating the cosmic ray-induced ionization that may influence cloud formation in the Venusian atmosphere (Aplin 2006).