Press Release 3/2013 - January 29, 2013

The Tail of Venus

When the solar wind nearly breaks off, our neighbor's ionosphere expands into space.

In rare events, the sheath of electrons and ions enveloping Venus in a height of 150 to 300 kilometers can expand into space like a tail. This exceptional deformation occurs on the planet's night side, when the solar wind, the flow of charged particles from the Sun, nearly comes to a stop. Scientists under the lead of the Max Planck Institute for Solar System Research (MPS) in Germany were now for the first time able to study such an event in detail. Their analysis is based on data obtained by instruments on board ESA's spacecraft Venus Express. The results may help to understand, whether particles in our solar system can travel from one planet to another - for example from Venus to Earth.

On 3. and 4. August 2010 the Sun held its breath: After several heavy coronal mass ejections, the solar wind, the continuous flow of electrons and protons from the Sun, almost completely broke off for approximately 18 hours. In this phase, the particle density at Venus sank to 0.2 particles per cubic centimeter. After Mercury, Venus is the second closest planet to the Sun. On normal days, this value is approximately 50 times higher.

"Phases with such weak solar wind are rare, but occur time and again", says Dr. Markus Fränz from the MPS. "However, the event in August 2010 was the first one since the launch of the space probe Venus Express seven years ago", he adds. Due to the spacecraft's pronounced elliptical orbit around Venus, the researchers had the chance to study the processes triggered by the weak solar wind in the planet's atmosphere.

 

Figure 1: A comparison of the ionosphere of Venus under different solar wind conditions. The yellow lines indicate the solar magnetic field lines as they interact with the ionosphere. When the solar wind's dynamic pressure is normal (left image) the ionosphere is confined to a region 150-300 km above the planet's day side. Positively charged particles (ions) travel quite quickly across the day-night terminator to create a similar ionosphere on the planet's night side. When the solar wind pressure drops to a very low level (right image), the ionosphere expands to a higher altitude above the day side of Venus and weaker magnetic fields are created above the terminator. As a result, the region across which ions are able to travel from the day side to the night side becomes larger. This makes it easier for ions to flow across the terminator. Although the weaker pressure of the solar wind reduces the speed at which the ions travel toward the night side, the first effect outweighs the second, enabling the ionosphere to expand in the planet's wake. Observations from Venus Express show that the night-side ionosphere moved outward to at least 15,000 km from Venus' centre over a period of only a few hours, creating a long, comet-like tail.

(Credit : ESA/Wei et al (2012)

 

As is Earth, Venus is surrounded by an ionosphere, a sheath consisting of electrons and ions. Scientists refer to this as a plasma. The charged particles are created on the planets' day sides when ultraviolet light with extremely short wavelengths and X-rays enter the outer layers of the atmospheres. On Earth, the particles are trapped by the strong magnetic field forcing them to rotate together with the Earth (and its magnetic field) around the Earth's axis. In this way, they reach the planet's night side thus forming a complete and spherical layer.

"On Venus, this is completely different", says Dr. Yong Wei from the MPS, the new paper's lead author. "Venus not only lacks a magnetic field of its own. She also rotates much slower", he adds. For one rotation Venus takes a little more than 243 Earth days.

Nevertheless, one can observe an ionosphere on Venus' night side. "Measurements performed by older space probes have shown electrons and ions flowing from the day side to the night side", says Fränz. This flow is driven by the high plasma pressure on the day side. Like a compressed gas escaping from a pressure cylinder, the plasma travels from a region with high pressure to one with lower pressure.

With the help of the magnetometer MAG and the instrument ASPERA-4 (Analyzer of Space Plasmas and Energetic Atoms) on board Venus Express, the researchers were able to take a closer look. They found that in events of weak solar wind Venus' ionosphere is not magnetized. Under normal conditions, these induced magnetic fields bind the charged particles of the ionosphere close to the planet. When the solar wind breaks off, however, the ionosphere in the region between the day and night side can expand. "The charged particles can therefore reach the night side more easily and in greater number", says Fränz. There, a sort of plasma balloon is created that extends into space like a tail. The whole ionosphere thus takes on a tear-shaped form.

The new measurements prove that the night side ionosphere protrudes approximately 15 000 kilometers into space. "But the tail could be much longer. It might measure up to millions of kilometers", says Wei. Since Venus Express' flight route did not lead the spacecraft directly behind the planet however, this question cannot be answered conclusively.

It is also still unclear, whether the ionosphere of Venus can in principle expand far enough to reach Earth. In 1996 researchers from MPS were able to detect plasma from Venus close to our planet. They analyzed data obtained by the spacecraft SoHO that circles the Sun in line with the Earth. Perhaps the processes now observed by their colleagues from MPS offer an explanation for such events. "Possibly phases with tenuous solar wind allow particles to travel from planets close to the Sun to those further away", says Wei.

Original Publication

Y. Wei, M. Fraenz et al.:
A teardrop-shaped ionosphere at Venus in tenuous solar wind
Planetary and Space Science 73 (2012), 254-261

Contact

Dr. Birgit Krummheuer
Press and Public Relations
Max Planck Institute for Solar System Research
Max-Planck-Straße 2
37191 Katlenburg-Lindau
Tel.: 05556 979 462
Fax: 05556 979 240
Mobil: 0173 3958625
Email: krummheuermps.mpg.de

Dr. Markus Fränz
Max Planck Institute for Solar System Research
Max-Planck-Straße 2
37191 Katlenburg-Lindau
Tel.: 05556 979 441
Fax: 05556 979 240
Email: fraenzmps.mpg.de

Dr. Yong Wei
Max Planck Institute for Solar System Research
Max-Planck-Straße 2
37191 Katlenburg-Lindau
Tel.: 05556 979 221
Fax: 05556 979 240
Email: weimps.mpg.de