Center-to-Limb Angle Dependence of Phases (v, δ|B|) Observed with the Michelson Doppler Imager

Details Center-to-Limb Angle Dependence of Phases (v, δ|B|) Observed with the Michelson Doppler Imager Center-to-Limb Angle Dependence of Phases (v, δ|B|) Observed with the Michelson Doppler Imager

Nov 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001apj...561..435n&link_type=abstract

The Astrophysical Journal, Volume 561, Issue 1, pp. 435-443.

Astronomy and Astrophysics

Astronomy

8

Magnetohydrodynamics: Mhd, Sun: Oscillations, Sun: Photosphere

Scientific paper

We test the hypothesis that phases between line-of-sight magnetic and velocity signals depend upon center-to-limb angle if the magnetic response to photospheric velocity perturbations is both magneto-acoustic and Alfvénic. The Michelson Doppler Imager observes two bipolar active regions. Sunspots exhibit the predicted shift toward phase angles of 0° at the limb. Phases for the first sunspot group are -100°, -55°, and -31° for data at center-to-limb angles of 14°, 41°, and 60°. Phases for the second sunspot group are -107°, -66°, and -14° for data at center-to-limb angles of 16°, 29°, and 57°. The systematic march from ~-90° phase angles at disk center toward 0° at the limb could be interpreted that Alfvén waves are more easily observed at the limb. Unexpectedly, leading sunspot phases split into positive and negative values as the far side sunspot fields turn away from the observer. This cannot be explained mathematically by the change of positive to negative field because phase angles are calculated using δ|B|. Consistent wave propagation direction, or motion guided by field lines, is implied since matter moving away from an observer along negative lines must simultaneously move toward the observer on positive lines to produce this result. Plage behavior is consistent with the observed splitting of phases in the leading sunspot. All plage phases are ~-90° at disk center but split into positive and negative groups near the limb. Limbward plage phases are 17° for negative polarity and -95° for positive polarity. Field configurations calculated from the potential field model show phases differ between open loops, where outgoing waves could exist, and closed loops whose finite length may trap waves.

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