The Mark III K-Coronameter (MK3) is a 23 cm objective coronameter-polarimeter located on Mauna Loa, in Hawaii. It is used to observe the outer atmosphere of the sun, the solar corona, in polarized optical light. With the MK3, we are able to observe the corona from 1.15 - 2.45 solar radii.
The corona tells about the extended magnetic field of the sun. It is in the corona where the magnitude and extent of Coronal Mass Ejections (CMEs) are best observed. Large CMEs can carry a million tons of material out into interplanetary space at a million miles an hour. (10^15 grams at 500 km/sec). Some of that material can reach the earth and disrupt communications satellites, power grids, shortwave radio communications, and cause aurora.
The MK3 uses a rotation half-wave retarder to modulate linear polarization. A polarizing beam splitter projects beams onto a pair of linear diode arrays (each containing 128 diodes). By reading these arrays rapidly compared to the rotation of the modulator, the magnitude and orientation of the polarization is measured. The two arrays scan azimuthally around the center of the sun, mapping approximately every 1/2 degree in solar azimuth. An entire image is taken every three minutes. Roughly six hours of observing are completed each day, weather permitting. A calibration procedure is used to isolate the K-corona which is produced by electron scattering tangent to the solar limb.
The MK3 has been operational since February 4, 1980. Its data are often compared with Halpha and Helium I data from the dPmon/PICS and CHIP instruments at MLSO, coronal data from the Solar Maximum Mission (SMM) satellite, and xray data from YOHKOH. Data are primarily used to monitor the rotation of the corona and to detect transients or coronal mass ejections (CME's) from the sun.
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Last revision: 8 August 2000 - A. Stanger