A unit of area = one millionth of half the surface of the sun. Used to describe the size of active regions on the sun. Symbol, MH. Often, and always in the past, called a “millionth”.
Such active regions are measured in millionths of a solar hemisphere, where 1 micro-hemisphere, or MH, is about 600,000 square miles. This region [AR12192] topped out at 2,750 MH, making it the 33rd largest region out of approximately 32,000 active regions that have been tracked and measured since 1874. It is the largest sunspot seen since AR 6368, which measured 3,080 MH on Nov. 18, 1990.
www.nasa.gov/content/goddard/tracking-a-gigantic-sunspot-across-the-sun/ Accessed 2 Nov 2014.
Sunspots vary by size from tiny 4.5 millionths of the solar hemisphere (MSH) in area (which corresponds to a circular feature of about 4200 km in diameter) to behemoth “Great Sunspot of 1947” with area exceeding 6100 MSH.
Andrey G. Tlatov and Alexei A. Pevtsov.
Bimodal Distribution of Magnetic Fields and Areas of Sunspots.
Posted on 27 January 2015, retrieved 10 June 2015.
[footnote to “Maximum Area” column heading in a table]* Millionths of a solar hemisphere measured at Greenwich Observatory. To express the area in units of a million square miles multiply these figures by 1.17.
Edison R. Hoge.
The Great Sunspot Group of March and April 1947.
Publications of the Astronomical Society of the Pacific, vol. 59, no. 348, pages 109-111 (June 1947).
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Last revised: 10 June 2015.