Solar zenith angle‐dependent asymmetries in Venusian bow shock location revealed by Venus Express

Chai, Lihui; Wan, Weixing; Fräenz, M.; Zhang, Tielong; Dubinin, E.; Wei, Yong; Li, Yi; Rong, Zhaojin; Zhong, Jun; Han, Xinyi; Futaana, Yoshifumi

doi:10.1002/2015ja021221

Cited by 14 publications

(27 citation statements)

References 19 publications

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“…The average B y is positive in the +E (+Z) hemisphere and negative in the −E hemisphere (Figures 5b and 5f ). This result that the B y direction correlating with the E hemisphere asymmetry was once obtained by Zhang et al [2010], Rong et al [2014], and Dubinin et al [2014a], and they suggested that dawnward magnetic field over north polar region is related with the E hemisphere asymmetry in the Venusian space environment caused by ion-pickup effect [Phillips et al, 1987;Chai et al, 2015]. Here our results show that besides the biased B y directions, the B z also has biased directions which are not related with the E hemisphere asymmetry.…”

Section: Global Looping Magnetic Field At Venussupporting

confidence: 71%

An induced global magnetic field looping around the magnetotail of Venus

et al. 2016

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Venus serves as the prototype of solar wind interaction with unmagnetized planetary bodies with atmospheres. It has no intrinsic dipole or crustal magnetic field; the only magnetic field is believed to be formed by the draped interplanetary magnetic field (IMF). However, the large‐scale magnetic field observed over the north polar region of Venus has a bias in the dawnward direction and seemingly unresponsive to the IMF's direction. Here we show that besides the draped field, there is a second type of induced global magnetic field at Venus and the dawnward field is only a part of it. This global field has a distribution in a cylindrical shell around the magnetotail and a counterclockwise direction looking from the planetary tail toward the Sun, which demonstrates that there are two currents flowing out and in of the planet along the inner and outer boundaries of the looping field, respectively.

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Section: Global Looping Magnetic Field At Venussupporting

confidence: 71%

An induced global magnetic field looping around the magnetotail of Venus

et al. 2016

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“…An excess of pickup ions increases the size of the obstacle and causes the north-south asymmetry. It was recently demonstrated that the north-south asymmetry is larger for larger solar zenith angles but is observable even in the near-subsolar region (Chai et al 2015). The pole-equator asymmetry is however not observed in the subsolar region.…”

Section: Location and Structure Of The Bow Shockmentioning

confidence: 96%

Solar Wind Interaction and Impact on the Venus Atmosphere

et al. 2017

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Venus has intrigued planetary scientists for decades because of its huge contrasts to Earth, in spite of its nickname of "Earth's Twin". Its invisible upper atmosphere and space environment are also part of the larger story of Venus and its evolution. In 60s to 70s, several missions (Venera and Mariner series) explored Venus-solar wind interaction regions. They identified the basic structure of the near-Venus space environment, for example, existence of the bow shock, magnetotail, ionosphere, as well as the lack of the intrinsic magnetic field. A huge leap in knowledge about the solar wind interaction with Venus was made possible by the 14-year long mission, Pioneer Venus Orbiter (PVO), launched in 1978. More recently, ESA's probe, Venus Express (VEX), was inserted into orbit in 2006, operated for 8 years.Owing to its different orbit from that of PVO, VEX made unique measurements in the polar and terminator regions, and probed the near-Venus tail for the first time. The near-tail hosts dynamic processes that lead to plasma energization. These processes in turn lead to the loss of ionospheric ions to space, slowly eroding the Venusian atmosphere. VEX carried an ion spectrometer with a moderate mass-separation capability and the observed ratio of the escaping hydrogen and oxygen ions in the wake indicates the stoichiometric loss of water from Venus. The structure and dynamics of the induced magnetosphere depends on the prevailing solar wind conditions. VEX studied the response of the magnetospheric system on different time scales. A plethora of waves was identified by the magnetometer on VEX; some of them were not previously observed by PVO. Proton cyclotron waves were seen far upstream of the bow shock, mirror mode waves were observed in magnetosheath and whistler mode waves, possibly generated by lightning discharges were frequently seen. VEX also encouraged renewed numerical modeling efforts, including fluid-type of models and particle-fluid hybrid type of models, describing the plasma interaction on scales ranging from ion gyro radius to the entire induced magnetosphere. In this review article, we review what has been found from space physics measurements around Venus (from the solar wind down to the ionopause), with a particular emphasis on updated results since the Venus Express mission. We conclude the article by a short discussion on the remaining open scientific questions and the future of this field.

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“…However, the effect of pickup ions appears to be asymmetric. Near the terminator, pickup ions appear to play an important role, but in the near‐subsolar (35° < SZA < 75°) region their influence appears less important (Chai et al, ). The shocks observed by Balikhin et al () occur in the near‐subsolar region ( SZA = 67 − 68°), suggesting that pickup ions play a less important role in the observed shocks than would be the case at other locations.…”

Section: Introductionmentioning

confidence: 99%

The First Direct Observational Confirmation of Kinematic Collisionless Relaxation in Very Low Mach Number Shocks Near the Earth

2019

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Collisionless shocks are ubiquitous throughout the known universe. They mainly convert the energy of the directed ion flow into heating. Upon crossing the shock front, the ion distribution becomes nongyrotropic. Relaxation to gyrotropy then occurs mainly via kinematic collisionless gyrophase mixing and interaction with waves. The theory of collisionless relaxation predicts that the downstream pressure of each ion species varies quasi-periodically with the distance from the shock transition layer and the amplitude of the variations gradually decrease. The oscillations due to each species have their own spatial period and damping scale. Pressure balance requires that the variations in the total plasma pressure should cause anticorrelating variations in the magnetic pressure. This process should occur at all Mach numbers, but its observation is difficult at moderate-/high-Mach numbers. In contrast, such magnetic oscillations have been observed at low Mach number cases of the Venusian bow shock and interplanetary shocks. In this paper, simultaneous in situ magnetic field and plasma measurements from the THEMIS-B and THEMIS-C spacecraft are used to study, for the first time, the anticorrelated total ion and magnetic pressure spatial variations at low-Mach number shocks. It is found that kinematic collisionless relaxation is the dominant process in the formation of the downstream ion distribution and in shaping the downstream magnetic profile of the observed shocks, confirming fundamental theoretical results. Comparison with the results from numerical models allows the role of the different ion species to be investigated and confirms the role heavy ions play in forming the downstream magnetic profile.

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Solar zenith angle‐dependent asymmetries in Venusian bow shock location revealed by Venus Express

Cited by 14 publications

References 19 publications

An induced global magnetic field looping around the magnetotail of Venus

An induced global magnetic field looping around the magnetotail of Venus

Solar Wind Interaction and Impact on the Venus Atmosphere

The First Direct Observational Confirmation of Kinematic Collisionless Relaxation in Very Low Mach Number Shocks Near the Earth

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