Venus, unlike Earth, does not produce its own magnetic field. It does, however, have an ionosphere, which is formed when radiation from the Sun ionizes the top layers of the planet’s neutral atmosphere. This ionosphere, because it is made up of charged ions, can interact with the magnetic field embedded in the solar wind blowing past the planet.
As the solar wind encounters Venus’s ionosphere, the magnetic field lines pile up at the front of the planet and become draped around behind it, extending away from the planet in a teardrop-shaped tail. Such a system is known as an induced magnetosphere and contains particles accelerating away from the ionosphere.
Possible signatures of magnetic reconnection—the process in which magnetic field lines with opposite polarities collide and break, then recombine in a new configuration—have been observed around the planet’s induced magnetosphere. When the magnetic field blowing past the planet changes direction, the new field is oriented differently than the field that is built up around the planet, inducing magnetic reconnection.
When reconnection takes place at the front side of the planet, the new fields recombine, so that the field lines are no longer draped around the planet but instead are able to slide around it freely. Scientists theorize that as these field lines slip into the solar wind, they take the part of the tail to which they are connected, and any particles in this field, with them. This loss of the tail is referred to as a disconnection event.
To investigate whether disconnection events occur in Venus’s tail, Vech et al. used measurements from the European Space Agency’s Venus Express, a satellite in orbit around Venus, to examine the magnetic field and escaping particles in the tail.
They identified 117 cases between 2006 and 2010 where the solar wind magnetic field near Venus changed polarity. Studying the ion fluxes in the tail for 1 day before and 3 consecutive days after each event, the scientists found that polarity reversals in the solar wind were associated with decreased ion flux in the Venusian magnetotail shortly after the polarity reversal. The loss of ions in the tail occurring at the same time as a polarity reversal in the solar wind (a time that magnetic reconnection is likely to be occurring at the front side of the planet) suggests that the ions were lost because of a disconnection event.
By the second day, the average ion fluxes had recovered to their nominal values; the tail was repopulated from the Venusian ionosphere below and ready to be stripped away anew should the solar wind magnetic field flip polarity again.
Beyond understanding Venusian magnetospheric dynamics, these possible disconnection events can also be compared to other cases of magnetotail disconnections such as those that occur at Mars, around comets, and even around exoplanets. Studying these types of events can help scientists better understand how even unmagnetized celestial bodies can interact with their environment in complex and unexpected ways. (Journal of Geophysical Research: Space Physics, doi:10.1002/2015JA021995, 2016)
—Aleida K. Higginson, Freelance Writer