The magnetic activity of the Sun follows a typical 11-year cycle, and its previous minimum occurred during the years 2008-2009. During this minimum, the bright "surface" of our star was nearly devoid of dark sunspots, which are the sites from which large flares and eruptions typically originate. The new cycle has entered its rising phase in 2010, with the appearance of new spots. Since then, however, only minor flares had taken place in the solar atmosphere. Solar physicists from the Observatoire de Paris commit themselves to comparing solar observations with their theoretical models for flares and eruptions. Their aim: to better understand - and on the long run to predict - these energetic phenomena, that are of major importance for Solar-Terrestrial relations. Solar eruptions of this magnitude can cause perturbations in the telecommunications from the ground and in space, as well as in electric power distribution systems. The most intense solar flares are triggered in the upper part of the solar atmosphere - the corona - within plasma loops that contain strong magnetic fields and electric currents, and which are rooted lower down in sunspots and their vicinity. The active region NOAA 11158 started to emerge on February 10. During several days, the magnetograph HMI onboard the Solar Dynamics Observatory (SDO) spacecraft followed the growth of several sunspots and their relative displacements (Figure 1). This emergence allowed magnetic fields to deploy in the corona, through which electric currents got amplified as a result of the observed shearing motions. These are evidenced by the slow expansion and the developing writhe of the magnetic loops above the sunspots as revealed by the AIA telescopes onboard SDO, which continuously observe the entire Sun in 8 extreme ultraviolet (EUV) wavelengths simultaneously. During four days, this active region hosted several modest flares and a medium one.
The impact of the CME with Earth’s magnetosphere produced auroras that were visible down to unusually low latitudes. Several observations highlight that this eruption produced a burst of accelerated particles, which propagated towards the Sun’s surface as well as into the interplanetary medium. On the one hand, the precipitation of accelerated electrons in the chromosphere produced not only hard X-ray emitting sources as being identified by the RHESSI satellite, but also elongated EUV brightenings as observed by AIA, which ended up saturating the images in all observed wavelengths . On the other end, an increase of the proton flux was recorded in the vicinity of Earth by the satellite GOES. On February 15 à 1h55 UT, a solar flare of class X2 was triggered inside the active region. A related halo coronal mass ejection (CME) was launched during the peak of the flare. One of SOHO’s telescopes measured its speed to be slightly less that 1000 kilometers/second. The impact of the CME with Earth’s magnetosphere produced auroras that were visible down to unusually low latitudes. Several observations highlight that this eruption produced a burst of accelerated particles, which propagated towards the Sun’s surface as well as into the interplanetary medium. On the one hand, the precipitation of accelerated electrons in the chromosphere produced not only hard X-ray emitting sources as being identified by the RHESSI satellite, but also elongated EUV brightenings as observed by AIA (Figure 1, lower panel), which ended up saturating the images in all observed wavelengths (Figure 2, left panel). On the other end, an increase of the proton flux was recorded in the vicinity of Earth by the satellite GOES. During the impulsive phase of the flare, and before the appearance of the halo CME around the Sun, a fast and oval-shaped front propagating over the whole solar disc was revealed by a data treatment of the SDO images (Figure 2, right panel). These propagation fronts are commonly interpreted as the manifestation of a magneto-sonic wave launched in the corona, either by the impulsive heating from the flare, or by the fast expansion of the related mass ejection.
This exceptional event is already being investigated by a research team from the LESIA* using a model for solar-eruptions. This long-term project is based on 3D magneto-hydrodynamics (MHD) numerical simulations which run on the computers of the "Division Informatique" of Observatoire de Paris. The evolution of the simulated electric currents and magnetic fields presents strong morphological similarities with the event observed with SDO. Within the simulation, the analyzis of the magnetic reconnection mecanism and of the expansion of the twisted magnetic field lines (Figure 3, left panel) relies on numerous studies carried at the Observatoire de Paris over the last ten years. The peculiar geometry of the EUV brightenings can be explained by magnetic reconnection in "quasi-separatrix layers" (Figure 3, middle panel). In addition, the analyzis reveals that the propagation of the perturbation originating from the flare site would not be due to a magneto-sonic wave, but rather to the amplification of electric currents on the edge of the CME, as it pushes away the surrounding coronal magnetic fields.
The last registered solar eruption of similar intensity dates back from December 13, 2006. The solar cycle therefore has clearly restarted, after a two-year break. The SDO satellite, launched on February 11, 2010, reached its nominal observation orbit one year ago and now provides an unprecedented opportunity for researchers to collect outstanding new sets of data, in order to test and refine their models and to stimulate new ideas.
Last update on 21 December 2021