TP18

Fundamentally, what limits the rate of atmospheric ion escape from non-magnetized planets? Previous orbit-based in situ measurements of escaping heavy ions (O⁺, O₂⁺ and heavier species) at Mars have yielded conflicting estimates of the dependencies on upstream solar wind and solar extreme ultraviolet (EUV) conditions. We compile 7 years (2014-2021) of measured 0.1 eV – 30 keV ion distributions from the STATIC instrument on the MAVEN orbiter to globally map the phase-space ion flux distribution, from which we derive globally integrated outflow, inflow, and net ion fluxes. Through binning the measurements by upstream solar wind (measured simultaneously by the Mars Express orbiter) and EUV conditions, we separately quantify the O⁺ and O₂⁺ escape dependencies on these external drivers. The found trends indicate that ion escape from Mars is a supply/source-limited process under low solar EUV conditions, however, the appearance and increase of gravitationally bound heavy ion return flows under moderate EUV conditions suggests that the escape process can transition to a Venus-like energy-limited state. The findings show that the state of the ion escape process does not only differ between planets, but the state and thus the drivers of ion escape can also differ under varying external conditions. We discuss the implications for ion observations at Mars during the upcoming solar activity maximum, for the evolution of the Martian atmosphere, and for our understanding of atmospheric ion escape as a general process in the solar system and beyond.

How to cite: Ramstad, R., Brain, D., McFadden, J., Mitchell, D., Andersson, L., Espley, J., Halekas, J., Holmström, M., and Curry, S.: MAVEN Observations of a State-Transition in Ion Escape from Mars, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-704, https://doi.org/10.5194/epsc2022-704, 2022.

Venus lacks an intrinsic magnetic filed, as a result, the incoming solar wind interacts directly with its atmosphere, creating an induced magnetosphere. Venus is believed to have once held a significant amount of water, which has since then disappeared from the planet as evidence by the increased, compared to Earth's, Deuterium-Hydrogen ratio measured by Pioneer Venus (Donahue et al. 1982). Its disappearance is mainly contributed to the interaction of the solar wind with the planet's induced magnetosphere. This interaction is the primary cause of atmospheric escape for elements heavier than Hydrogen, such as Oxygen (Futaana et al. 2017). Measurements from Venus Express have shown that the majority of the escape is taking place at the magnetotail, with the ration of Hydrogen to Oxygen escape being approximately equal to two during solar minimum and equal to one during solar maximum (Persson et al. 2018), indicating the escape of water from the nightside. This effect can bring rise to the question: what happens to the atmospheric loss under extreme solar wind conditions, such as under the influence of Interplanetary Coronal Mass Ejections (ICMEs)? Understanding those processes can give us valuable information on the evolution of the planet. Previous works have provided answers by investigating the impact of ICMEs on the Venusian atmospheric escape compared to nominal conditions with observations (Luhmann et al. 2007, McEnulty et al. 2010, Edberg et al. 2011, Collinson et al. 2015) and simulations (Luhmann et al. 2008, Dimmock et al. 2017) to name a few. In this study, we contribute to this subject by using observations taken from Venus Express and simulations to explore the influence of different ICME parameters on the escape.

The simulation used is a global hybrid model, called LatHyS, first developed for Mars (Modolo et al. 2017) and adapted for Venus (Aizawa et al. 2022), that has the advantage of having a self consistently calculated ionosphere. Magnetic field and particle data taken from Venus Express upstream of the bowshock are used as reference for the initial conditions of the simulations. Two ICME events are selected. The first impacted Venus on the 5th of November 2011 and created the highest observed magnetic obstacle at the planet. Upstream of the bowshock it had a temperature of 200 eV and a velocity of 900 km/s. The second event occurred on the 27th of October 2013 and had similar plasma and magnetic field parameters to the first one, but had significantly different temperature of 35 eV and velocity of 600 km/s. Idealized models of the events are simulated and compared. In order to discuss the parameter dependence, for each comparison the simulation inputs of the two events are forced to be the same, imposing either temperature or velocity to be identical, and leaving the studied variable as the only difference. We find an increase of the dayside Oxygen escape for the event with the higher velocity, but a decrease on the nightside outgoing ion flux. Since both of the selected events can be considered as fast solar wind, to better investigate the dependency of the velocity we perform an additional simulation with a much lower solar wind speed of 300 km/s. To study further the effect of the ICME parameters, in a more model driven approach, the second selected event is kept as a reference and additional simulations are performed to examine variables such as the magnetic field strength and cone angle. Finally, the results are compared with statistical data of the mean solar wind conditions on Venus and their corresponding calculated atmospheric escape.

References:

Aizawa et al. 2022, doi: https://doi.org/10.1016/j.pss.2022.105499

Collinson et al. 2015, doi: https://doi.org/10.1002/2014JA020616

Dimmock et al. 2018, doi: https://doi.org/10.1029/2017JA024852

Donahue et al. 1982, doi: https://doi.org/10.1126/science.216.4546.630

Edberg et al. 2011, doi: https://doi.org/10.1029/2011JA016749

Futaana et al. 2017, doi: https://doi.org/10.1007/s11214-017-0362-8

Luhmann et al. 2007, doi: https://doi.org/10.1029/2006JE002820

Luhmann et al. 2008, doi: https://doi.org/10.1029/2008JE003092

McEnulty et al. 2010, doi: https://doi.org/10.1016/j.pss.2010.07.019

Modolo et al. 2016, doi: https://doi.org/10.1002/2015JA022324

Persson et al. 2018, doi: https://doi.org/10.1029/2018GL079454

How to cite: Katrougkalou, M. C., Aizawa, S., Persson, M., André, N., and Modolo, R.: Investigating the impact of different Interplanetary Coronal Mass Ejection parameters on the Venusian atmospheric escape, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-469, https://doi.org/10.5194/epsc2022-469, 2022.

Venus' relatively small induced magnetosphere enables the solar wind to interact directly with the upper atmosphere (Stenberg Wieser et al., 2015), and one could guess that this would yield a larger escape rate compared to a magnetized planet. However, the escape rates reported from Earth are somewhat larger than from Venus. Singly charged oxygen dominates the ion mass outflow from Earth and the average escape rate is estimated to be a few times 10²⁵ s^-1 (André, 2015, and references therein). A strong intrinsic magnetic field creates a huge magnetosphere, which prevents direct solar wind access to the atmosphere, but the big structure instead provides a larger interaction cross section to transfer solar wind energy and momentum into the magnetosphere (e.g., Gunell et al., 2018). This can lead to a larger ion energization and atmospheric escape.

In the absence of a direct interaction between the ionosphere and the solar wind, wave-particle interaction has been identified as a major ion energization process at Earth. Several wave modes at different frequencies are able to heat ions. (André & Yau, 1997). A common type of ion heating is associated with low-frequency broadband electric wavefields (André et al., 1998). The spectral density of such broadband waves does not exhibit a peak at a certain frequency but the wave power available at the ion gyrofrequency may nevertheless efficiently energize the ions (Chang et al., 1986). At Earth this heating mechanism is definitely effective and important (André et al., 1998).

We investigate if ions originating from the Venusian ionosphere can be energized by electric wave power in a similar way as is observed at Earth.

References

André, M. (2015). Previously hidden low-energy ions: a better map of near-earth space and the terrestrial mass balance. Physica Scripta, 90 (12), 128005.

André, M., Norqvist, P., Andersson, L., Eliasson, L., Eriksson, A. I., Blomberg, L. Waldemark, J. (1998). Ion energization mechanisms at 1700 km in the auroral region. Journal of Geophysical Research: Space Physics, 103 (A3), 4199-4222.

André, M., & Yau, A. (1997). Theories and observations of ion energization and outflow in the high latitude magnetosphere. Space Science Reviews, 80 (1), 27-48.

Chang, T., Crew, G. B., Hershkowitz, N., Jasperse, J. R., Retterer, J. M., & Winningham, J. D. (1986). Transverse acceleration of oxygen ions by electromagnetic ion cyclotron resonance with broad band left-hand polarized waves. Geophysical Research Letters, 13 (7), 636-639

Gunell, H., Maggiolo, R., Nilsson, H., Stenberg Wieser, G., Slapak, R., Lindkvist, J., De Keyser, J. (2018). Why an intrinsic magnetic field does not protect a planet against atmospheric escape. A&A, 614.

Stenberg Wieser, G., Ashfaque, M., Nilsson, H., Futaana, Y., Barabash, S., Diéval, C., Zhang, T. L. (2015). Proton and alpha particle precipitation onto the upper atmosphere of venus. Planetary and Space Science, 113-114 , 369-377.

How to cite: Stenberg Wieser, G., André, M., Nilsson, H., and Edberg, N.: Can wave-particle interaction be important for ion heating and escape at Venus?, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-834, https://doi.org/10.5194/epsc2022-834, 2022.

Though ion escape to space is an important mechanism for atmospheric loss on Mars, the processes that accelerate ions to escape velocity have not been identifed and quantified. The lowest altitude where suprathermal planetary ions appear is an important source region for ion escape, where electromagnetic forces and waves begin to energize ions to escape velocity. We have conducted a statistical study of O₂⁺ distribution functions measured by Mars Volatile EvolutioN SupraThermal And Thermal Ion Composition (MAVEN-STATIC) in order to identify this source region for Martian ion escape. We have fit Maxwellians to each measured O₂⁺ distribution function in order to identify distributions containing a suprathermal component. Suprathermal ions appear just above the exobase region at all solar zenith angles, but Maxwellian ion distributions persist to higher altitudes near the terminator than on the dayside or the nightside. We also investigated the effects of crustal magnetism, finding that crustal fields protect planetary plasma from energization on the dayside and enhance energization on the nightside.

How to cite: Hanley, G., Fowler, C., McFadden, J., Mitchell, D., and Curry, S.: MAVEN observations of initial ion acceleration in the Martian ionosphere, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-277, https://doi.org/10.5194/epsc2022-277, 2022.

Interaction of a spacecraft with ambient plasma results in charging of the spacecraft to a potential of the order of the electron temperature.  This is an issue for the ion measurements in the low altitude Martian ionosphere, since the potential energy of the ions in the spacecraft’s electric field is comparable to their thermal energy. However, the unique Mars Express payload makes possible the use of an innovative technique to study this low energy ionospheric population around the spacecraft. It has been discovered that operating the radar MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) onboard Mars Express results in acceleration of the local thermal ions to energies up to 800 eV. Accelerated ions are readily detected by the ion detector of the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3). The analysis of these ASPERA-3 observations showed that that the voltage applied to the MARSIS antenna causes charging of the spacecraft to several hundreds of volts by the electrons of the ambient plasma. Individual observations of accelerated ions consist of almost mono-energetic ion beams that allow measuring densities of the main (O⁺, O₂⁺ and CO₂⁺) and even minor (O₂⁺⁺, C⁺, CO⁺) ionospheric thermal populations. This novel technique of active sounding opens new possibilities to study the Martian ionosphere.

From 2007 to 2016 2,528 Mar Express orbits were found to exhibit sounder accelerated ions (SAI). Created catalogue of the SAI events contains more than 50,000 SAI entries. This allowed to study the mechanisms that causes acceleration of thermal ions in the vicinity of an active transmitter in a plasma with no strong magnetic led. Based on it, a model was developed, that allows to estimate density of cold ionospheric ions from SAI observations.

Figure 1 shows an example of the ion density retrieved using the developed method. Top panels show the ion differential flux measured by IMA. Cold ionospheric plasma is visible as an intense signal at energy close to 10 eV. SAI are present as intense periodic disturbances within energy range between 30 and 800 eV. Bottom panels show comparison of the local electron density estimated using MARSIS ionograms and the ion density estimated from the SAI observations. As one can see, values are in a good agreement and follow similar dependence on altitude and SZA. Both methods show peak of the plasma density close to the periapsis.

Acceleration of the ionospheric plasma above thermal energy facilitates determination of the ion composition by IMA. Figure 2 shows altitude profiles for different ionospheric constituents measured using SAI by MEX. At the altitudes above 300 km main ionospheric species are O₂⁺ and O⁺ . Corresponding densities decrease from 750 pp/cm3 at 300 km to 400pp/cm3 at 500 km. It is interesting to note that at the altitudes 300-350 km density of O⁺ is higher than the density of O₂⁺ while on higher altitudes situation is opposite. Similar results were found by MAVEN.