Aa. Norton et al., Characteristics of magnetohydrodynamic oscillations observed with the Michelson Doppler Imager, ASTROPHYS J, 518(2), 1999, pp. L123-L126
We report on the spatial distribution of the magnetogram oscillatory power
and the phase angles between velocity and magnetogram signals as observed w
ith the Michelson Doppler Imager. The data set is 151." 25 x 151." 25, cont
aining the sunspot from 1997 December 2 with a temporal sampling interval o
f 60 s and spatial sampling of 0." 605. The simultaneously observed continu
um intensity and surface velocity accompany the magnetic information, We fo
cus on three frequency regimes: 0.5-1.0, 3.0-3.5, and 5.5-6.0 mHz, correspo
nding roughly to timescales of magnetic evolution, p-modes, and the 3 minut
e resonant sunspot oscillation. Significant low-frequency magnetogram power
is found in lower flux pixels, 100-300 G, in a striking ring with filament
ary structure surrounding the sunspot. The 5 minute magnetogram power peaks
in extended regions of flux that measure 600-800 G. The 3 minute oscillati
on is observed in the sunspot umbra in pixels whose flux measures 1300-1500
G, Phase angles of approximately -90 degrees between velocity and magnetic
flux in the 3.0-3.5 and 5.5-6.0 mHz regimes are found in regions of signif
icant cross amplitude.