Issue |
A&A
Volume 396, Number 2, December III 2002
|
|
---|---|---|
Page(s) | 673 - 682 | |
Section | Planets and planetary systems | |
DOI | https://doi.org/10.1051/0004-6361:20021413 | |
Published online | 03 December 2002 |
Band-splitting of coronal and interplanetary type II bursts
II. Coronal magnetic field and Alfvén velocity
1
Hvar Observatory, Faculty of Geodesy, Kačićeva 26, 10000 Zagreb, Croatia
2
INAF 8211, Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34131 Trieste, Italy
3
Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
Corresponding author: B. Vršnak, bvrsnak@geodet.geof.hr
Received:
5
August
2002
Accepted:
20
September
2002
Type II radio bursts recorded in the metric wavelength
range are excited by MHD shocks traveling through the solar
corona. They often expose the fundamental and harmonic emission
band, both frequently being split in two parallel lanes that show
a similar frequency drift and intensity behaviour. Our previous
paper showed that band-splitting of such characteristics is a
consequence of the plasma emission from the upstream and
downstream shock regions. Consequently, the split can be used to
evaluate the density jump at the shock front and to estimate the
shock Mach number, which in combination with the shock speed
inferred from the frequency drift provides an estimate of the
Alfvén velocity and the magnetic field in the ambient plasma. In
this paper such a procedure is applied to 18 metric type II bursts
with the fundamental band starting frequencies up to 270 MHz. The
obtained values show a minimum of the Alfvén velocity at the
heliocentric distance amounting to
400–500 km s-1. It then increases
achieving a local maximum of
450–700 km s-1 at
. The implications regarding the
process of formation and decay of MHD shocks in the corona are
discussed. The coronal magnetic field in the range
decreases as R-3 to R-4, or H-1.5 to H -2 if
expressed as a function of the height. The results are compared
with other estimates of the coronal magnetic field in the range
. Combined data show that below
the magnetic field
is dominated by active region fields, whereas above
it
becomes radial, behaving roughly as
with a
plausible value of
nT at 1 a.u.
Key words: Sun: radio radiation / MHD / shock waves / Sun: corona
© ESO, 2002
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