First radio cartography of solar coronal mass ejection For the first time, a coronal mass ejection (or CME) has been mapped at radio-wavelengths, thanks to the radio-heliograph of Nançay : the expanding bubble that it forms around the Sun, was pictured in radio waves (fig 1), bringing a much better understanding of the phenomena associated with these solar storms, which disturb all the interplanetary medium. Until now, the CME were photographed at large distances from the Sun only in the visible and the ultraviolet, mainly from space. This first image in the radio range brings today a new light on these violent matter ejections and their relationships to solar flares, very energetic particles, and radioelectric emissions. The radio astronomers of the Paris-Meudon Observatory, Monique Pick, Alain Kerdraon and Dalmiro Maia, in collaboration with Tim Bastian, of NRAO, Charlottesville, Virginia, USA and Angelos Vourlidas, of the NRL, Washington D.C., studied in detail the powerful CME of April 20, 1998, and their results are in press ( Astrophysical Journal Letters of September 1 ).
Figure 1 : Radio image obtained with Nançay of the 1998, April 20 CME. The constant emission of the solar disc was subtracted (disc indicated). One sees the variable radio emission of a radioelectric noise storm in the North-West, and a large bubble at South-west.
CME Movie Film of the coronal mass ejection produced at Nançay [MPEG 847 KB]
The CME are due to powerful magnetic explosions in the solar corona, which project ionized plasma, and by collision the associated neutral gas, in interplanetary space. The coronal mass ejections transport billion of tons of matter. The expansion, in tenuous interplanetary space, of the plasma initially compressed by the strong pressures existing at the solar surface, increases its volume until sizes comparable with that of the Sun. A gigantic bubble of plasma moves away at supersonic speed from the Sun, with more than 300 000 km/h, by opening the force lines of the solar magnetic field. On its arrival in the vicinity of the Earth after three to four days, the CME interacts with the magnetic fields of the Earth’s atmosphere, produces auroral displays, geomagnetic storms, seriously disturbs the radio communications, the satellites and the electric distribution systems. The CME are observed in the visible with coronographs embarked on satellites. The absence of atmosphere makes it possible to observe the sunlight scattered by the very remote solar corona and the CMEs when they occur. Currently on the satellite SOHO ( Solar and Heliospheric Observatory ), the coronograph LASCO ( Large-Angle Spectrometric Coronograph ) observes the Sun in permanence. The advantage of the radio imagery is that one can observe from the ground, and that it is not necessary to occult the disc of the sun to see the corona : thus one can observe closer to the surface, and detect the CME in an nascent stage. The radio image of the powerful CME of 1998 April 20 (see figure 1) shows a complex system of bubbles or loops, rather similar to those observed in the visible by LASCO (see the Nançay-LASCO comparison in figure 2). According to the radio emission spectrum observed at several frequencies, one can deduce that it is synchrotron emission of relativistic electrons (0.5-5 Mev of energy) spiraling in a magnetic field of 0.1 to a few Gauss.
Figure 2 : (A) LASCO image obtained at 10:04:51 UT, right before the first radio detection of the CME. (b) radio image of the CME, when the flux was maximum at 164 MHz at 10:13:23 UT. The white lines point out the position of the bubble seen by LASCO in a) and c). (c) LASCO image at 10:31:20 UT ; just after that the CME was not detectable any more in radio. (d), (e) and (f) radio images of the CME at 236.6, 327 and 421 MHz, respectively. (g) Distances from the Sun, as a function of time, of the CME bubble at the various wavelengths.
Reference : (article in Astrophysical Journal Letters, 1st September 2001) Contact : Monique Pick (Département DASOP, Observatoire de Paris) Alain Kerdraon (Département DASOP, Observatoire de Paris) Dalmiro Maia (Département DASOP, Observatoire de Paris)
Acknowledgements : The Nancay station for radio astronomy is a facility of the Paris Observatory. The Nancay Radioheliograph is funded by the French Ministry of Education, the Centre National de la Recherche Scientifique, and by the Region Centre. This research has also been supported by the Centre National d’Etudes Spatiales.
Dernière modification le 4 mars 2013