Magnetic Field Elements at High Latitude: Lifetime and Rotation Rate
Yang Liu Submitted: 2009-12-17 15:50
Using 1-minute cadence time-series full disk magnetograms taken by the SOHO/MDI, we
have studied the magnetic field elements at high latitude (poleward of 65circ in latitude).
It is found that an average lifetime of the magnetic field elements is 16.5 hours during solar
minimum, much longer than that during solar maximum (7.3 hours). During solar minimum,
number of the magnetic field elements with the dominant polarity is about 3 times as that of
the opposite polarity elements. Their lifetime is 21.0 hours on average, longer
than that of the opposite polarity elements (2.3 hours). It is also found that
the lifetime of the magnetic field elements is related with their size, consistent with the magnetic
field elements in the quiet sun at low latitude found by Hagenaar (1999). During solar
maximum, the polar regions are equally occupied by magnetic field elements with both polarities,
and their lifetimes are roughly the same on average. No evidence shows there is a correlation
between the lifetime and size of the magnetic field elements. Using an image cross-correlation method,
we also measure the solar rotation rate at high latitude, up to 85 degree in latitude. The rate is
( omega = 2.914 - 0.342 imes sin^2phi - 0.482 imes sin^4phi ) murad s-1 sidereal.
It agrees with previous studies using the spectroscopic and image cross-correlation methods,
and also agrees with the results using the element tracking method when the sample
of the tracked magnetic field elements is large. The consistency of those results strongly
suggests that this rate at high latitude is reliable.
Authors: Yang Liu, Junwei Zhao
Publication Status: Published in Solar Physics
Last Modified: 2009-12-21 12:28