Global Venus-solar wind coupling and oxygen ion escape

Abstract

The escape of atmospheric particles to space is an important process to understand the evolution of planetary atmospheres. Venus is thought to have lost a large part of its water content to space throughout its history. The escape to space is inherently coupled with the interaction between the planet and the solar wind, and how much energy and momentum that are transferred from the solar wind to the escaping particles. In this study, we determine how much solar wind power is transferred to the ionospheric plasma that escape to space. We also investigate how the transfer coefficient depends on solar wind conditions.

1. Introduction

Today, the Venusian atmosphere is massive and contains only tiny amounts of water. However, measurements indicate that the atmosphere contained a significant amount of water in its early history [e.g.  9]. Several processes are capable of removing water from Venus atmosphere, which can be summarised into two main parts: (1) interaction between surface and atmosphere, and (2) escape of atmospheric constituents to space. In this study, we focus on the second process.

We investigate how non-thermal escape of O⁺ is affected by the upstream solar wind (SW) conditions. As the SW flows past Venus, some of its energy and momentum are transferred to the upper atmospheric particles [e.g. 3, 4]. The additional momentum allows the upper atmospheric O⁺ ions to reach above escape energy (~8 eV) and escape the planet [e.g. see review in 2]. An increase in the available energy in the upstream SW was shown to increase the total O⁺ escape in the magnetotail [5]. However, to fully characterise the interaction between the SW and the ionosphere, the efficiency of the coupling between them is important to determine.

In this study, we compare the available power in the upstream SW that can potentially reach the ionosphere, with the power escaping the atmosphere in the form of O⁺ ions in the magnetotail, to address the efficiency of the energy transfer.

2. Data

We use the full dataset from 2006-2014 of the Ion Mass Analyser (IMA), part of the Analyser of Space Plasmas and Energetic Atoms (ASPERA-4) instrument onboard Venus Express (VEx). The IMA instrument has the capability to measure ions of energy 0.01-36 keV/charge, with a total field-of-view of 90x360˚, where the resolution is 5.6x22.5˚. The moderate mass separation capability of IMA allows to efficiently separate the heavier species from the lighter. For details see [1].

3. Method

We calculate the escape rate by systematically combining O⁺ distribution functions in the magnetotail to create average ion distributions. The distributions are combined with respect to spatial location and upstream SW conditions (solar wind energy flux (SWEF) and solar extreme ultraviolet (EUV) radiation flux).

The upstream SWEF is measured during time periods when VEx was located outside the bow shock [6]. The SWEF is separated into 5 bins, where each is separated into high and low EUV flux. The EUV flux is propagated from measurements at Earth [see details in 5, 6]. The average distributions made for each of the ten upstream conditions are then integrated spatially over the magnetotail to calculate the total average escape rate for each condition [5].

The coupling between the upstream SW and the escape rate is calculated as the ratio of their respective powers. The SW power is calculated as SWEF multiplied by the area over which energy and momentum can be transferred. Here we assumed the size of the interaction area to be the induced magnetosphere boundary at the terminator. The escape power is calculated as an integration of the differential energy flux of the escaping ions. The ratio indicates how efficient the energy transfer is from the SW to the ionosphere and the escaping ions.

4. Results & discussion

The results show that the coupling efficiency decreases as the available power in the SW increases. Even though there is an increase in the number of ions escaping with an increased available energy [5], there is a smaller fraction of the available energy that is transferred.

On average only ~0.008 % of the SW power is transferred to the escaping ions. This indicates that the Venusian plasma environment actively and efficiently screens the ionosphere from the SW. The induced magnetosphere is capable of diverting the majority of the incoming SW energy and momentum, and divert the flow around itself, rather than absorbing it.

We can compare the coupling coefficient at Venus with that at Mars and Earth. The average coupling coefficient at Mars was found as ~0.67 % [7], which is a factor ~100 times higher than that at Venus. This may in part be due to that the induced magnetosphere of Venus is larger than that of Mars, which means that the amount of available energy that can be transferred to the induced magnetosphere is larger. The average escape rates are also comparable with ~2·10²⁴ s^-1 at Mars [6], and (3-6) ·10²⁴ s^-1 at Venus [2]. Therefore, the coupling coefficient is smaller at Venus than at Mars.

At Earth, the coupling was shown to increase as the available power upstream increased, after a threshold was reached [8]. Earth, with its magnetosphere, has a completely different interaction with the SW, compared to the induced magnetospheres of e.g. Venus and Mars. The difference in the coupling between the SW and the escape from Earth and Venus (and Mars) indicates that an intrinsic magnetic field does not provide better protection against SW erosion than an induced magnetosphere.

Acknowledgements

M. Persson acknowledges support of her graduate studies from SNSA (Dnr: 129/14).

References

[1] Barabash, et al. (2007). PSS, 10.1016/j.pss.2007.01.014

[2] Futaana, et al. (2017). Sp. Sci. Rev., 10.1007/s11214-017-0362-8

[3] Lundin, et al. (2011). Icarus, 10.1016/j.icarus.2011.06.034

[4] Perez-de-Tejada, (1986). JGR: 91.A6, pp. 6765–6770

[5] Persson, et al. (2020). JGR, 10.1029/2019JE006336

[6] Ramstad, et al. (2015). JGR, 10.1002/2015JE004816

[7] Ramstad, et al. (2017). JGR, 10.1002/2017JA024306

[8] Schillings, et al. (2019). EPS, 10.1186/s40623-019-1048-0

[9] Way, et al. (2016). GRL. 10.1002/2016GL069790