Orwell Astronomical Society (Ipswich)

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Solar Active Region AR9169, 25 September 2000

It's official - the Sun has reached solar maximum, the peak of its 11-year cycle of activity! Solar activity is of interest to everyone on Earth because it affects the Earth's weather (can we blame it for all the rain in late summer 2000?) and because there is an increased chance of geomagnetic storms at solar maximum. A geomagnetic storm occurs when a burst of charged particles (plasma) ejected from the Sun interferes with the Earth's magnetic field; it can disrupt radio communications and electricity supplies and produce spectacular auroral displays.

Of the several measures of solar activity, the most readily observable by amateur astronomers is the number of sunspots. Sunspots are caused by the rotation of the Sun. The Sun's equatorial regions rotate in about 25 days while its polar regions take more than 35 days. The difference in rotation periods contorts the Sun's magnetic field so that lines of magnetic flux snap and re-form, creating sunspots and other phenomena.

Ironically, at about the time that the Sun appeared to reach solar maximum in 2000, its disc appeared devoid of sunspots. Then, around the limb of the Sun came the largest sunspot group seen since 1991, shortly after the last peak of solar activity. As the Sun rotated it carried the sunspot group - designated Active Region 9169 - to the middle of the Sun's disc around the third/fourth week of September. The group was easily seen by projection and several members of OASI observed and photographed it. OASI member Nick Sullivan captured the images below on 25 September 2000, using a Meade ETX90 with Thousand Oaks Type 2+ solar filter.

20000925_AR9169_1_NABS.jpg 26 mm Plossl eyepiece.

20000925_AR9169_2_NABS.jpg 12.4 mm Plossl eyepiece.

AR9169 covered an area of the Sun about 13 times larger than the surface area of the Earth! The size of sunspots and sunspot groups is conventionally measured in millionths of the surface area of one hemisphere of the Sun. At its largest, AR9169 covered 2140 millionths of the solar hemisphere. The largest ever recorded sunspot group was seen in 1947 and measured over 6132 millionths.

The easiest way to observe sunspots is to use a small telescope or binoculars to project an image of the Sun on to a sheet of white card. Top tips for solar projection are:

Pete Richards & Nick Sullivan