Shock location and CME 3D reconstruction of a solar type II radio burst with LOFAR
2018
Context. Type II radio bursts are evidence of shocks in the solar atmosphere and inner
heliospherethat emit
radio wavesranging from sub-meter to kilometer lengths. These shocks may be associated with
coronal mass ejections(CMEs) and reach speeds higher than the local magnetosonic speed. Radio imaging of decameter wavelengths (20-90 MHz) is now possible with the Low Frequency Array (
LOFAR), opening a new
radio windowin which to study
coronalshocks that leave the inner solar corona and enter the
interplanetary mediumand to understand their association with CMEs. Aims. To this end, we study a
coronalshock associated with a CME and type II radio burst to determine the locations at which the radio emission is generated, and we investigate the origin of the band-splitting phenomenon. Methods. The type II shock source-positions and spectra were obtained using 91 simultaneous tied-array beams of
LOFAR, and the CME was observed by the
Large Angleand
Spectrometric Coronagraph(LASCO) on board the Solar and
HeliosphericObservatory (SOHO) and by the COR2A
coronagraphof the SECCHI instruments on board the Solar Terrestrial Relation Observatory (STEREO). The 3D structure was inferred using triangulation of the coronographic observations.
Coronalmagnetic fields were obtained from a 3D magnetohydrodynamics (MHD)
polytropicmodel using the photospheric fields measured by the
HeliosphericImager (HMI) on board the Solar Dynamic Observatory (SDO) as lower boundary. Results. The type II radio source of the
coronalshock observed between 50 and 70 MHz was found to be located at the expanding flank of the CME, where the shock geometry is quasi-perpendicular with theta(Bn)similar to 70 degrees. The type II radio burst showed first and second harmonic emission; the second harmonic source was cospatial with the first harmonic source to within the observational uncertainty. This suggests that
radio wavepropagation does not alter the apparent location of the harmonic source. The sources of the two split bands were also found to be cospatial within the observational uncertainty, in agreement with the interpretation that split bands are simultaneous radio emission from upstream and downstream of the shock front. The fast magnetosonic Mach number derived from this interpretation was found to lie in the range 1.3-1.5. The fast magnetosonic Mach numbers derived from modelling the CME and the
coronalmagnetic field around the type II source were found to lie in the range 1.4-1.6.
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