STEREO: A solar storm observed on the far side of the Sun provides important clues on the acceleration of particles to high energies.
The NASA STEREO (Solar Terrestrial Relations Observatory) mission consists of two spacecraft orbiting with the Earth around the Sun, one moving faster ahead of the Earth (STEREO-A) and one moving more slowly behind the Earth (STEREO-B). STEREO is the first solar mission obtaining images of the far side of the Sun from two vantage points.
Figure 1: Three differenced images obtained by STEREO-A imagers at three different times (02 :20UT, 02 :30UT, 02 :38UT). Each composite image combines observations of the solar disk in ultraviolet light through the FeXII 195 Angström line (EUVI instrument) and of the lower corona in white-light by the internal coronagraph (COR-1). The bubble formed by the solar eruption and the associated shock wave surrounding it undergo a strong 3D expansion. The limit of the perturbations associated with the shock wave as observed on the solar disk is marked by a dotted black line.
In a study published this week in the Astrophysical Journal, the eruption of a very energetic solar event, launched on 2011 March 21, on the far side of the Sun (west longitude 135o), could be observed in great detail by the optical instruments on STEREO-A. The shock wave launched by the expelled solar storm was tracked along the base of the corona (about 80Mm above the solar surface) from the launch site situated beyond the west limb, to the visible parts of the solar disk observed from Earth. The particles, thought to be accelerated by the shock wave, were measured in situ by the instruments on STEREO-A and could be compared with similar measurements obtained by the near-Earth orbiting probes.
This study compares, for the first time, the extent of the coronal shock wave in the lower corona, with mutli-point measurements of particle fluxes in the heliosphere during the critical first few minutes that follow the launch of a solar storm. The particles accelerated during this first phase tend to be the most energetic ones for a given storm (often exceeding GeV energies per nucleon), they are a real threat for human activity in space. These particles propagate along magnetic field lines that connect the base of the corona to the interplanetary medium. A delay of approximately 30 minutes was observed between the arrival of particles at STEREO-A and the arrival of particles at Earth. This study shows, for the first time, that this delay corresponds to the time required by the shock wave to propagate from the launch site, magnetically connected to STEREO-A, to the point magnetically connected to the near-Earth environment. This study provides a strong support to the theory of particle acceleration at coronal shocks.It also suggests that warning times of a few dozens of minutes of these high-energy particles are possible with adapted space weather monitoring.
Figure 2: The ecliptic plane viewed from solar north. The eruption and the shock surrounding it are initially confined to a small volume. The particles, that we think are accelerated near the shock, are initially injected along the magnetic field lines connected to STEREO-A. The shock wave undergoes a strong 3-D expansion perturbing an increasingly large volume of the solar corona. This phase lasts 30 minutes until the shock wave intersects magnetic field lines connected to the near-Earth environment. This time of intersection is associated with the onset of particles suddenly measured by probes placed near the Earth.
Contact: Alexis ROUILLARD, Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse (UPS), Toulouse, France
The Astrophysical Journal, 752:44 (20pp), 2012 June 10
THE LONGITUDINAL PROPERTIES OF A SOLAR ENERGETIC PARTICLE EVENT INVESTIGATED
USING MODERN SOLAR IMAGING