The solar wind is composed of high energy charged particle which when interact with planetary atmosphere causes degradation to the atmosphere. The Magnetosphere of planet protects the atmosphere of the planet from these solar winds by deflecting charged particles. For example, the planet Mars has no planetary magnetic field to protect its atmosphere from the solar wind which results in continuous loss of atmosphere to space. The planet Venus is much closer to the Sun and has no planetary magnetosphere, then, the question arises, how do Venus protect its atmosphere from the solar wind in the absence of Magnetosphere?
No magnetosphere of Venus
The planet Venus is called the sister planet of Earth because it resembles Earth in many properties like size and structure. But there are lots of differences between these planets like the denser atmosphere of the planet Venus and much higher atmospheric pressure. The planet Venus does not have planetary magnetosphere like Earth. Earth has molten metals inside the core which causes dynamo effect due to rotation of Earth and generates the magnetic field around planet Earth. The planet Venus rotates on its axis very slowly which is not enough to generate the magnetic field around the planet. As a result, the planet does not have any magnetosphere around it.
Detection of Magnetic field around Venus
In 1962, Mariner 2 did not detect any magnetosphere around the planet Venus during flew by at a distance of 6.6 planetary radii. In 1967, mariner 5 detected the deflection in solar wind around the planet Venus indicating the presence of magnetic field. The definitive measurements of the magnetic moment of Venus were obtained in Pioneer Venus orbiter mission in the first year of 1979-81.
Interaction of solar wind with atmosphere
Due to the absence of the atmosphere, the solar wind directly interacts with the atmosphere. On interaction with the molecule of atmosphere, the electric field of charged particles causes the breaking of bonds and ionization in the upper layers of the atmosphere. The ion of atmosphere makes ionosphere of Venus, they align them according to the electric field of solar wind. The alignment of these dipoles creates the magnetic field around the planet which is called induced magnetosphere.
Features of induced magnetosphere
The field and plasma observed are originated from solar wind rather than planetary magnetosphere. The features of induced magnetosphere during solar wind interaction are shown in the figure. The ionospheric obstacle to the solar wind is the surface called ionopause. Ionopause is the surface where equilibrium exists between the dynamic pressure of solar wind outside and the thermal pressure of ion and electron inside. Outside the region of ionopause, the interaction resembles planetary magnetosphere interaction with solar wind. Solar wind also creates bow shock during the interaction. The interesting feature of the bow shock is the changing direction according to the solar wind.
In this way, the induced magnetosphere protects the atmosphere from the solar wind by creating bow shock around the planet. The thick atmosphere of Venus provides better ion density causing this induction more effective. Maybe due to the very thin atmosphere, the planet Mars does not have this type of protection for atmosphere resulting in continuous loss of atmosphere.