Panoramic view of particles from the Sun
For the first time, joint observations of three space probes make it possible to monitor particle streams from the Sun and its source for several days.
Time and again, the sun hurls energetic charged particles into space in violent eruptions. At the same time, a continuous stream of particles, the solar wind, escapes from its surface. Numerous space probes in the Earth’s vicinity monitor how this matter propagates through space. However, until now this information never showed the whole picture: since the Sun rotates, it was impossible to simultaneously observe the particle stream and its origin on the surface of the star. Scientists led by the Max Planck Institute for Solar System Research (MPS) in Germany have now for the first time combined data from three spacecraft to achieve such a 360 degree view. For several days they were able to track how the sun releases a rare variety of helium.
Most space probes exploring the Sun, orbit our star in close proximity to the Earth. For example, only 1.5 million kilometers separate the solar observatory SoHO from Earth; the Solar Dynamics Observatory (SDO) is an Earth satellite. Such nearby locations in space are favorable, for example for communication between the probe and the ground station. However, they offer one major drawback: only the side of the Sun, that is currently facing the Earth, is in the field of view. The star’s back is always hidden. The twin space probes STEREO A and B have changed this situation fundamentally. Eight years ago, the probes began circling the Sun using almost the same orbit as Earth: one probe precedes our planet, the other lags behind. With each year, the STEREO twins drift farther apart. In early 2011, finally, they were located precisely opposite to each other – and together thus for the first time had an unobstructed view of the entire Sun lying between them.
"Together with probes close to Earth, it is now possible to view all sides of the Sun at the same time”, says Radoslav Bučík from the MPS. “For the first time, we can now monitor long-term processes without interruption”, he adds. Together with colleagues from the Johns Hopkins University (USA) and the University of Alcalá (Spain) the MPS researchers tracked down the rare helium isotope helium 3 in this way.
Helium 3 is a rarity – not only in solar matter, but in the entire Solar System. The heavier isotope helium 4 containing two protons and two neutrons in its core occurs 10 000 times more frequently. The solar wind, the continuous particle stream from the Sun, reflects this ratio. However, for a limited time, regions on the Sun can emit particles with a greatly increased concentration of the rare helium isotope.
"Apparently, there is a mechanism on the Sun that accelerates helium 3 much more effectively into space than other particles”, Bučík explains. However, how this mechanism works is still a mystery. One problem is, that so far it was only possible to track down the particles for one day at the most. Since the Sun rotates, the particles describe a sort of spiral path comparable with the water jet from a rotating lawn sprinkler. After only a few hours, they therefore disappear from the field view of any single spacecraft. The new observational geometry of the STEREO probes offered the hope of gaining deeper insights.
Two such opportunities appeared in July 2011. The MPS researchers have now analyzed the corresponding data. On July 1st, STEREO B registered a significantly increased concentration of helium 3 with its mass spectrometer SIT. Six days later the particles showed up in the measuring field of the particle spectrometer ULEIS onboard the ACE spacecraft located close to Earth. As the source of these particles, the researchers were able to identify a particular active region on the Sun. Such regions are often located in the vicinity of a sunspot and are characterized by high magnetic field strengths. On July 9th and 16th, ACE and STEREO A again recorded elevated levels of helium 3. Again, the researchers were able to trace the rare particles to their origin.
"The fact that helium 3-rich events can persist for several days was a big surprise," said Bučík. Apparently, the necessary conditions are stable for a long time within the solar plasma. This could be an important clue and help to identify the underlying acceleration mechanism. “Understanding this mechanism has wide consequences”, Bučík explains. “It could help to fundamentally learn how the Sun succeeds to hurl particles into space in so-called flares.”