Abstract
AbstractPrevious missions to Venus have revealed that encounters with plasma irregularities of atmospheric origin outside the atmosphere are not uncommon. A number of mechanisms have been proposed to discuss their origins as well as their roles in the atmospheric evolution of Venus. One such mechanism involves an ionopause with a wavelike appearance. By utilizing the magnetic field and plasma data from Venus Express, we present the first observational statistical analysis of the ionospheric boundary wave phenomena at Venus using data from 2006 to 2014. Results from the minimum variance analysis of all the photoelectron dropout events in the ionosphere reveal that the ionopause of Venus does not always appear to be smooth but often exhibits a wavelike appearance. In the northern polar region of Venus, the normal directions of the rippled ionospheric boundary crossings lie mainly in the terminator plane with the largest component predominantly along the dawn‐dusk (YVSO) direction. The average estimated wavelength of the boundary wave is 212 ± 12 km, and the average estimated velocity difference across the ionopause is 104 ± 6 km/s. The results suggest that the rippled boundary is a result of Kelvin‐Helmholtz instability. Analysis reveals a correlation between the normal directions and the locations of the boundary wave with respect to Venus. This indicates that the draping of magnetic field lines may play a role in enhancing the plasma flow along the dawn‐dusk direction, which could subsequently set up a velocity shear that favors the excitation of ionospheric boundary wave by the Kelvin‐Helmholtz instability along the dawn‐dusk direction.
Highlights
Due to the absence of an intrinsic magnetic field [Russell et al, 1980; Luhmann and Russell, 1997], the solar wind interaction with Venus is highly dynamic
The estimation is made under two criteria: (1) Only two consecutive boundary crossings when αmv, the angle between their respective boundary normal directions, is less than 30◦ or greater than 150◦, are selected to eliminate the crossings of boundary waves that are still in the linear growth phase; i.e. only developed waves are selected, (2) only boundary crossings when βvex, the angle between the spacecraft velocity vector and the boundary normal vector is less than 45◦ are selected to eliminate the boundary crossings of the ‘near-tips’ of the waves as illustrated in Figure 5(e) and to eliminate the boundary which is crossed at a large angle by the Venus Express (VEX)
Further analysis shows that the estimated widths of the ionospheric boundary wave and the estimated velocity difference flow across the boundaries are consistent to the results from previous simulation studies of the Kelvin-Helmholtz Instability
Summary
Due to the absence of an intrinsic magnetic field [Russell et al, 1980; Luhmann and Russell, 1997], the solar wind interaction with Venus is highly dynamic. Even though the KHI is considered the more dominant instability for wave excitation, there are occasions when terms such as the magnetic field stress, gravity, and boundary curvature are more significant, and can give rise to other instabilities, for example the RTI or flute instability [Elphic and Ershkovich, 1984] This wavelike appearance of the ionopause is an important characteristic of Venus and plays a significant role in its atmospheric evolution. Pope et al [2009] suggested that nonlinear vortex-like structures observed in the magnetosheath region using Venus Express (VEX) magnetic field data were associated with the strong shear flow across the ionopause. In contrast to PVO, which was able to sample the Venusian ionosphere in the subsolar region over the period of solar maximum, the high latitude elliptical polar orbit of VEX provides an opportunity to study the dynamics of the Venusian ionosphere in the northern polar region of Venus across nearly a full solar cycle with rather quiet solar activity [Futaana et al, 2017]
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