Chasing for Solar Storms : Eruption through the entire solar system

First observation of a solar eruption through the entire solar system

For the first time, astronomers have followed a solar storm on their entire path through the solar system - from the sun to Pluto. For the first time, these observations showed how the speed and magnet properties of such a coronal mass ejection change in the course of its flight.

With a coronary mass ejection on the sun on October 14, 2014, the "chase" began across the solar system.

In fact, the ground team of the ESA Mars Express had been looking forward to observing the comet Siding Spring at its near-by launch on Mars. However, on October 14, 2014, when they prepared everything for this passage, a coronary mass ejection (CME) occurred in the sun: the sun hurled a massive cloud of charged and magnetic plasma into space.

A unique opportunity

For the Earth this solar storm was not a danger, because the plasma cloud was racing towards Mars. However, this gave the astronomers a unique opportunity: as the data from the solar observatories SOHO and STEREO showed, the solar storm moved on a course that would pass eight other space probes.

In the past, solar storms from several spacecraft were observed, explains Olivier Witasse of ESA. But it is extremely unusual that the conditions permit observation of as many positions in the inner and outer solar system as possible.

For the first time, it was possible to trace how such a solar plasma cloud develops on its way through the solar system.

From Venus to Voyager 2

The researchers alerted the ground teams of the spacecraft, who would have a chance to capture the solar storm. Apart from the ESA probe Venus Express, these included three Mars orbiter - Mars Express, MAVEN and Mars Odyssey - and the Marsrover Curiosity. They registered the passage of the solar storm on 17 October 2014. On the outside, the solar storm passed the ESA space probe Rosetta at comet 67P / Churyumov-Gerasimenko on 22 October.

On 12 November 2014 the solar plasma cloud reached the NASA space probe Cassini in the Saturn system. The NASA space probe New Horizons, on its way to distant Pluto, also witnessed the solar storm in February 2015. Finally, at the edge of the solar system, even the - 40 years ago launched - probe Voyager 2 registered subtle signs of the plasma surge at the beginning of March 2016.

The way of the solar storm and the observations of the spacecraft.

Tempo halved

The measurement data from the total of ten probes allowed the reconstruction of the flight of solar storms by the solar system.

So far, it was unclear how the speed of coronary mass ejection changes with increasing distance from the sun, especially in the external solar system, explains Witasse. Thanks to the precise timing of the measurements, we can now better understand this process.

The measurements showed that the solar storm had started at a speed around 1,000 kilometers per second from the sun. Three days later, on Mars, speed had already dropped to 647 kilometers per second. Another five days later, at the height of Rosetta, the solar storm was only 550 kilometers per second fast. On its further path, the speed of the plasma cloud decreased only slightly, at Saturn it was still around 450 to 500 kilometers per second, as the researchers report.

Solar storm displaced cosmic radiation

The space probes registered a further effect during the flyby of the solar storm: during the passage of the plasma cloud the measured values ​​of the probes for cosmic radiation decreased significantly. This so-called Forbush-Decrease occurs because the solar plasma at this moment shields the probe from the cosmic radiation - it forms a kind of protective bubble in which it displaces the particles from the outside.

As a result of this effect, the Mars orbiter recorded a sudden drop in cosmic radiation by 20 percent. This "doldrums" lasted for 35 hours. Rosetta reported a 17% reduction in cosmic radiation, which lasted for 60 hours. Even further outside, on Saturn, the effect was still weakened and scattered.

The closer the solar storm is still to the sun, the steeper, shorter and fiercer is the effect, explain the researchers.

This is due to the fact that in the course of the flight time the characteristics of the plasma clouds change: it becomes slower, its magnetic field becomes weaker and the plasma spreads and is thereby virtually diluted. The measurement data of the space probes indicate that the ejected plasma has expanded and spread outwards at a 116 degree wide angle.

Three chances were missed

This case is an excellent example of how one can follow a coronary mass ejection by at least ten astronomical unit distances by combining several space probes and their observational data, the researchers said.

This could be further optimized: the Venus Express and Dawn probes could have collected more data if their scientific instruments had not been largely switched off - because they were behind the sun or in the flight mode.

At the same time, this example demonstrates the importance of the fact that, if possible, all spacecraft are equipped with magnetometers and other instruments that are important for the observation of the spacecraft, even if this is not their primary purpose. NASA's Spitzer spacecraft telescope was also on its way to the solar storm, but it did not have any measuring devices on board.

Source: Journal of Geophysical Research - Space Physics, 2017; doi: 10.1002 / 2017JA023884
Images: SDO/NASA, SOHO (ESA/ NASA), NASA/GSFC, Witasse