Solar wind interaction with magnetosphere. ( Credit: NASA)

Solar wind is composed of high energy charge particle which when interact with planetary atmosphere causes degradation to the atmosphere. The Magnetosphere of planet protects the atmosphere of planet from these solar winds by deflecting charge 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 the space. The planet Venus is much closer to sun and has no planetary magnetosphere then the question arises, how do Venus protect atmosphere from the solar wind in the absence of Magnetosphere?

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 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 magnetic field around planet Earth. The planet Venus rotates on its axis very slowly which is not enough to generate magnetic field around the planet. As a result the planet do not has any magnetosphere around it.

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 magnetic moment of Venus were obtained in Pioneer Venus orbiter mission in the first year of 1979-81.

Due to absence of atmosphere the solar wind directly interacts with the atmosphere. On interaction with the molecule of atmosphere, the electric field of charged particles cause the breaking of bonds and ionization in the upper layers of 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 magnetic field around the planet which is called 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 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 interaction. The interesting feature of 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. May be due to very thin atmosphere the planet Mars does not have this type of protection for atmosphere resulting in continuous loss of atmosphere. 

http://www-ssc.igpp.ucla.edu/personnel/russell/papers/venus_mag/