Naslov
Band-splitting of coronal and interplanetary type II bursts. II. Coronal magnetic field and alfvén velocity

Autori
Vršnak, Bojan ; Magdalenić, Jasmina ; Aurass, H. ; Mann, G.

Izvornik
Astronomy and Astrophysics (0004-6361) 396 (2002), 2; 673-682

Vrsta, podvrsta i kategorija rada
Radovi u časopisima, članak, znanstveni

Ključne riječi
Sun : radio radiation. Mhd. Shock waves. Sun : corona.

Sažetak
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 Alfven 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 Alfven velocity at the heliocentric distance Rapproximate to2 amounting to v(A)approximate to400-500 km s(-1). It then increases achieving a local maximum of v(A)approximate to450-700 km s(-1) at Rapproximate to2.5. The implications regarding the process of formation and decay of MHD shocks in the corona are discussed. The coronal magnetic field in the range 1.3<R<3 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 1<R<10. Combined data show that below H<0.3 the magnetic field is dominated by active region fields, whereas above H=1 it becomes radial, behaving roughly as B=2xR(-2) with a plausible value of B&AP ; 5 nT at 1 a.u.

Izvorni jezik
Engleski

Znanstvena područja
Fizika

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