Venus is a rarity among planets - a world that does not internally generate a magnetic field. Despite the absence of a large protective magnetosphere, the near-Venus environment does exhibit a number of similarities with planets such as Earth. The latest, surprising, example is the evidence for magnetic reconnection in Venus' induced magnetotail.

Planets which generate magnetic fields in their interiors, such as Earth, Mercury, Jupiter and Saturn, are surrounded by invisible magnetospheres. Their magnetic fields deflect the charged particles of the solar wind (electrons and protons) as they stream away from the Sun. This deflection creates a magnetosphere - a protective "bubble" around the planet - which ends in an elongated magnetotail on the lee side of the magnetosphere.

Since Venus has no intrinsic magnetic field to act as a shield against incoming charged particles, the solar wind sometimes interacts directly with the upper atmosphere. However, Venus is partially protected by an induced magnetic field.

As on Earth, solar ultraviolet radiation removes electrons from the atoms and molecules in the upper atmosphere, creating a region of electrically charged gas known as the ionosphere. This ionised layer interacts with the solar wind and the magnetic field carried by the solar wind.

During the continuous battle with the solar wind, this region of the upper atmosphere is able to slow and divert the flow of particles around the planet, creating a magnetosphere, shaped rather like a comet's tail, on the lee side of the planet

Spacecraft observations over many decades have shown that magnetic reconnection occurs frequently in the magnetospheres of Earth, Mercury, Jupiter and Saturn. This process, which converts magnetic energy into kinetic energy, occurs when oppositely directed magnetic field lines break and reconnect with each other. On Earth, this reconnection is responsible for magnetic storms and polar auroras - the so-called Northern and Southern Lights.

Until now, reconnection was not generally thought to occur on non-magnetised planets. However, Tielong Zhang and an international team of co-authors now report on Science Express, the online version of the journal Science, that they have found the first evidence of magnetic reconnection in Venus' magnetotail.

ESA's Venus Express spacecraft follows a near-polar orbit which is ideal for instruments such as the magnetometer and low-energy particle detector to observe the solar wind - ionosphere - magnetotail interaction. Previous missions, such as Pioneer Venus, have either been in different orbits or been active at different periods of solar activity, so they not been able to detect these reconnection events.

On 15 May 2006, Venus Express was crossing the Venusian magnetotail when it observed a rotational magnetic field structure over a period of about 3 minutes. Calculations based on its duration and speed imply that it was about 3400 km across.

The event, which took place about 1.5 Venus radii (about 9000 km) down the tail, is thought to be evidence of a passing plasmoid - a transient magnetic loop structure which is formed by magnetic reconnection in a planetary magnetotail.

Further studies of the magnetic field data from Venus Express revealed the signatures of many similar observations of energy exchange between the magnetic field and the plasma in the tail.

The data also show that, in many respects, the magnetosphere of Venus is a scaled-down version of Earth's.

Magnetic reconnection occurs in the Earth's magnetotail and plasma sheet at a distance of about 10-30 planetary radii down the magnetotail. Since Earth's magnetosphere is 10 times larger, reconnection at Venus would be expected to occur 1-3 radii down its tail. That is exactly where Venus Express detected the reconnection events.

"Plasmoids are common features in the magnetospheres of planets such as Earth and Jupiter, but they were not expected in the magnetotail of an unmagnetised planet such as Venus," said Tielong Zhang, lead author of the Science paper. Zhang is Principal Investigator for the magnetometer instrument on Venus Express and a Senior Research Scientist at the Space Research Institute in Graz, Austria.

"The reconnection splits the magnetotail, causing most of the plasma in the tail to be ejected into space. It also forms a plasmoid structure which heads towards Venus and channels a fraction of the energy flux of the solar wind into the night-side atmosphere. As a result, the magnetic reconnection causes plasma circulation at Venus, similar to what happens in Earth's magnetotail."

The discovery that plasma is lost from the tail as a result of magnetic reconnections provides a possible new mechanism for explaining how and why gases are lost from Venus's upper atmosphere. This has implications for understanding how Venus lost its water after the planet began to experience a runaway greenhouse effect.

"Although the understanding of atmospheric loss is a key to establishing the evolutionary history of planets, the role of magnetic reconnection is still poorly understood because of the scarcity of in situ observations at planets other than Earth," said Hakan Svedhem, ESA's Venus Express Project Scientist.

"This result confirms that observation of the terrestrial planets by spacecraft such as Venus Express, Mars Express and Cluster is essential if we are to understand the complex evolution of atmospheres and planets in general."

Zhang, T. L., et al., "Magnetic Reconnection in the Near Venusian Magnetotail", Science 1217013 Published online 5 April 2012, doi:10.1126/science.1217013