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Oxygen Loss from Venus and the Influence of Extreme Solar Wind Conditions

  • Author(s): McEnulty, Tess
  • Advisor(s): de Pater, Imke
  • et al.
Abstract

The purpose of this dissertation is to expand our understanding of oxygen ion escape to space from Venus and its dependence on extreme solar wind conditions found during interplanetary coronal mass ejections (ICMEs). This work uses in-situ measurements of ions and magnetic fields from the Venus Express (VEX) spacecraft, which has been orbiting Venus since mid-2006. VEX is in a 24 hour elliptical orbit. The ion instrument operates for ~6 hours near the planet, while the magnetometer is always on. In-situ measurements of the solar wind velocity, density, and magnetic field from Pioneer Venus Orbiter (PVO) are also used for comparison with external conditions during the VEX time period. Coronagraph images from solar monitoring spacecraft are used to identify the solar sources of the extreme solar wind measured in-situ at Venus. For interpretation of planetary ions measured by VEX, a magnetohydrodynamic (MHD) model is utilized.

The solar wind dynamic pressure outside of the Venus bow shock did not exceed ~12 nPa, during 2006-2009, while the solar wind dynamic pressure was higher than this for ~10% of the time during the PVO mission. Oxygen ions escape Venus through multiple regions near the planet. One of these regions is the magnetosheath, where high energy pick-up ions are accelerated by the solar wind convection electric field. High energy (>1 keV) O+ pick-up ions within the Venus magnetosheath reached higher energy at lower altitude when the solar wind was disturbed by ICMEs compared to pick-up ions when the external solar wind was not disturbed, between 2006-2007. However, the count rate of O+ was not obviously affected by the ICMEs during this time period. In addition to high energy pick-up ions, VEX also detects low energy (~10-100 eV) O+ within the ionosphere and wake of Venus. These low energy oxygen ions are difficult to interpret, because the spacecraft's relative velocity and potential can significantly affect the measured energy. If VEX ion data is not corrected for the spacecraft's relative velocity and potential, gravitationally bound O+ could be misinterpreted as escaping. These gravitationally bound oxygen ions can extend on the nightside to ~-2 Venus radii and may even return to the planet after reaching high altitudes in the wake. Gravitationally bound ions will lower the total O+ escape estimated from Venus if total escape is calculated including these ions. However, if the return flux is low compared to the total escaping outflow, this effect is not significant.

An ICME with a dynamic pressure of 17.6 nPa impacted Venus on November 11, 2011. During this ICME, the high energy pick-up O+ and the low energy O+ ions were affected. Oxygen ions in the magnetosheath, ionosphere, and tail had higher energies during the ICME, compared to O+ energies when the external solar wind conditions were undisturbed. High energy ions were escaping within the dayside magnetosheath region when the ICME was passing as well as when the solar wind was undisturbed. However, during the ICME passage, these O+ ions had three orders of magnitude higher counts. The low energy O+ during the undisturbed days was gravitationally bound, while during the ICME a portion of the low energy ions were likely escaping. The most significant difference in O+ during the ICME was high energy pickup ions measured in the wake on the outbound portion of the orbit. These ions had an escape flux of 2.5  108 O+cm-2sec-1, which is higher than the average escape flux in all regions of the wake. In addition, the interplanetary magnetic field (IMF) was in a configuration that may have rotated an even higher escape flux O+ away from the VEX orbit. This needs to be confirmed with sampling of other regions in the wake during large ICMEs. A lower bound on the total O+ escape during this event could be ~2.8  1026 to 6.5  1027 O+/sec, which is 2-3 orders of magnitude higher than the average escape flux measured by VEX. Hence, ICMEs could have played a major role in the total escape of O+ from Venus. Considering that the Sun was likely more active (with more ICMEs) early after solar system formation.

The results presented in this dissertation can be used as a guide for future studies of O+ escape at Venus. As we move into solar maximum, Venus will likely be impacted by more large ICMEs. The ICME from the last study of this dissertation was the largest yet measured by VEX, but its 17.6 nPa dynamic pressure is lower than the largest ICMEs during the PVO time period (~ 80 nPa). The work in this dissertation is also relevant to Mars, since Mars interacts with the solar wind in a similar manner and has analogous ion escape mechanisms. The upcoming MAVEN (Mars Atmosphere and Volatile Evolution) mission will launch at the end of 2013 to study the Martian atmosphere, escape processes, and history of volatiles. This mission will have an in-situ ion instrument and magnetometer similar to those used for the studies in this dissertation, so one could conduct similar studies of the oxygen ion escape from Mars during extreme solar wind conditions.

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