Cassini reveals magnetic reconnections driven by planetary rapid rotation on Saturn’s dayside
Data captured by the Cassini probe and analysed by an international team of researchers including scientists fronm the University of Liège revealed, for the first time, that a phenomenon called magnetic reconnection, which on Earth can disturb GPS signals and damage electrical networks, can occur on Saturn's dayside well below the boundary that separates interplanetary and planetary plasma populations, which is known as the magnetopause. This discovery is surprising because scientists previously thought that magnetic reconnection on the dayside could only occur on the magnetopause, while planetary rapid rotation could not drive magnetic reconnection when facing the Sun. These results are published in the journal Nature Astronomy(1).
he magnetic field of the Earth - generated by the rotation of the planet’s fused metallic core - affects charged particles in the planet’s environment. As planetary magnetic field interacts with the flow of magnetised particles coming from the Sun (for example during a solar flare) the Earth’s magnetic field lines can temporarily break, and connect with those from the incoming magnetic fields, which change their direction. During this process, interplanetary magnetic fields and charged particles are transferred from the Sun to the planetary magnetosphere, and accumulate in the nightside magnetosphere, known as the magnetotail. As more and more energy accumulates in the nightside magnetotail, magnetic reconnection eventually takes place in the nightside to release the excess energy and energise particles, which may consequently produce magnificent polar aurorae, disturb the GPS signals, damage satellites or even the electrical networks on Earth.
“Magnetic reconnection is a key process that can connect the magnetic field of the Sun, embedded in the solar wind, with the planetary magnetosphere at their interacting boundary, explains Zhonghua Yao, a Marie-Curie COFUND postdoctoral research fellow at LPAP - STAR Institute and co-lead author of the publication. Inside the magnetosphere of Earth, these reconnections are known to happen only on the planet’s side away from the Sun, in the so-called magnetotail, which contains a thin plasma sheet with electrical current flow from dawn to dusk. According to Ampere’s Law, thin current sheet naturally introduce antiparallel magnetic fields, which provides favourable conditions for magnetic reconnection”. Unlike the Earth, the rapid rotation of Saturn’s magnetosphere rapidly rotates expels charged particles away from the planet, and form a plasma disk all around Saturn, while at Earth it is limited to the night side. Due to the compression by the solar wind, the dayside magnetodisc is expected to be thicker than the nightside, and thus not an ideal place for reconnection. A longstanding question is whether or not the compression from the solar wind could fully prohibit the occurrence of rotationally driven magnetospheric reconnection in the dayside magnetodisc. Since, so far, there has been no direct report of a reconnection event in dayside magnetodisc based on the database of Cassin, which explored Saturn’s magnetosphere for more than 10-year, dayside reconnection has rarely been considered to generate particle acceleration. This lead to a big difficulty in explaining the often-observed energetic particles in the dayside magnetosphere, and the dayside auroral pulsation.
Artist’s impression of a magnetic reconnection event detected by the international Cassini mission at Saturn. The orange curve is the bow shock formed by the solar wind (coming from the left side of the frame in this view) as it encounters Saturn’s magnetosphere; the grey curve is the magnetopause, the boundary separating the magnetosphere (on its right) from the surrounding plasma; the blue lines are magnetic field lines in the planet’s magnetosphere. Credit : ESA
Scientists expected the magnetospheres of other planets to behave similarly to Earth until they analysed some data sent by the Cassini space probe that showed, for the first time, evidence of magnetic reconnection occurring on the dayside of a planet facing the Sun, far from the magnetopause. However, locating the event was not easy. "To detect a reconnection event, it is necessary to observe the change in direction of the magnetic field after reconnection, but also the proof of the subsequent acceleration of the particles," explains the young researcher. "Moreover, the region where the reconnection occurs is very small and the instrument must point exactly in this direction. You have to be very lucky."
Yet one question remains... How is it possible that this phenomenon occurs on the dayside of the planet? Saturn's magnetosphere, like the Earth's, is much thicker on the dayside, which should protect it from external interference. “We believe that the magnetic reconnection observed is driven by a totally different mechanism”, explains Denis Grodent, Director of LPAP and STAR Institute. In the Earth's magnetosphere, reconnection is driven solely by interaction with the solar wind. On Saturn, the phenomenon could be caused by the planet's own rotation." Indeed, Saturn's magnetosphere has a structure different from that of the Earth because it rotates as a whole, while for Earth this rotation is limited to a small region, close to the planet.
Although more than 760 times larger than the Earth, Saturn spins in about ten hours, compared to 24 hours for the Earth. As its magnetosphere rotates with the planet, it constantly interacts with the magnetospheres of Saturn's moons and other bodies in the planet's environment. In Saturn's system, the moon Enceladus and its volcanic activity eject a lot of water. “As this water is ionized, it fills up the magnetosphere of Saturn with heavy ions that interact with the rotating magnetic field lines.When these lines change configuration, during a reconnection event for example, this plasma is released and accelerated”, explains Nicolas Altobelli, ESA Cassini project scientist.
In future years, the scientists expect to find more similar events in the treasure trove of data generated by Cassini, which concluded its mission last September with an epic plunge into the atmosphere of Saturn, having studied the planet and its environment for 13 years. (Read : Cassini, clap end !)
"So far, we have only scratched the surface of what we can find with the CAPS instrument. We can now look at the data and see if there are any other examples. We have a very rich data set that will keep us busy for years," concludes co-author Andrew Coates from University College London, UK, who is co-investigator of the Cassini CAPS electron spectrometer used in the study.
Jupiter is the other giant magnetosphere, which is being explored by the NASA JUNO mission. Following the new discovery made by Cassini's data on Saturn, scientists will know directly where and what to look for on the largest planet in our solar system. Quid for Uranus, Neptune and many rapidly rotating exoplanets, which could also have magnetospheres that produce rotational reconnections on the dayside side?
(1) R. L. Guo, Z. H. Yao, et al., "Rotationally driven magnetic reconnection in Saturn’s dayside", Nature Astronomy, June 4 2018.
Zhonghua YAO - firstname.lastname@example.org
Denis GRODENT – email@example.com
Benjamin PALMAERTS – firstname.lastname@example.org