Decametric N burst: a consequence of the interaction of two coronal mass ejections
Pascal Demoulin Submitted: 2006-11-08 04:15
Radio emissions of electron beams in the solar corona and
interplanetary space are tracers of the underlying magnetic
configuration and of its evolution. We analyse radio observations
from the Culgoora and Wind/WAVES spectrographs, in combination with
SOHO/LASCO and SOHO/MDI data, to understand the origin of a type
N burst originating from NOAA AR 10540 on January 20, 2004,
and its relationship with type II and type III emissions.
All bursts are related to the flares and the CME analysed in a
previous paper (Goff et al., 2006).
A very unusual feature of this event was a decametric type N burst,
where a type III-like burst, drifting toward low frequencies (negative
drift), changes drift first to positive, then again to negative. At
metre wavelengths, i.e. heliocentric distances
< 1.5 Rs, these bursts are ascribed to electron beams
bouncing in a closed loop. Neither U nor N bursts are
expected at decametric wavelengths because closed quasi-static loops
are not thought to extend to distances >> 1.5 Rs.
We take the opportunity of the good multi-instrument coverage of this
event to analyse the origin of type N bursts in the high corona.
Reconnection of the expanding ejecta with the magnetic structure of a
previous CME, launched about 8 hours earlier, injects electrons in the
same manner as with type III bursts but into open field lines having a
local dip and apex. The latter shape was created by magnetic
reconnection between the expanding CME and neighbouring (open)
streamer field lines. This particular flux tube shape in the
high corona, between 5-10 Rs, explains the observed
type N burst. Since the required magnetic configuration is only a
transient phenomenon formed by reconnection, severe timing and
topological constraints are present to form the observed decametric
N-burst. They are therefore expected to be rare features.
Authors: Demoulin, P., Klein, K.L., Goff, C.P., van Driel-Gesztelyi, L., Culhane, J.L., Mandrini, C.H., Matthews, S.A., Harra, L.K.
Publication Status: In press, Solar Physics
Last Modified: 2006-11-23 08:56