Astronomical Society of Coonabarabran

Solar Activity: Ephemeral Active Regions (July 2008)

by Harry Roberts

The writer's first experience of a solar minimum is proving to be both exciting and challenging. The excitement comes from searching for and often finding small sunspot groups (Active Regions AR) on a solar disc that at first looks blank. The detail visible in such small AR is very seeing dependant, and mapping the details is a challenge. As spots now seem to disappear as quickly as they arise, another challenge is to establish their positions before they vanish.

Two telescopes are in regular use: a Celestron 8 stopped down to four inches is used for both white light and hydrogen alpha observations, and can track the Sun for hours if H-alpha transients are likely. Alternatively a 4" F10 maksutov is employed for white light observation when time allows only a quick look at the Sun to monitor activity. The maksutov is hand guided on a portable alt-az tripod.

As noted elsewhere solar activity is presently (2007 July) dominated by events on the Sun's southern hemisphere, and that the normal "current" or "conveyor" that moves new AR progressively closer to the equator appears to have stopped. The result has been a huge surplus of southern sunspot activity over northern activity. Why is this so, and does it predict future activity levels?

The small AR referred to are probably invisible in projected images, and the writer finds "Helio" freeware useful for determining their positions when viewed in an eyepiece. With a cross-hair eyepiece a transit timing is made (drive OFF) through the solar disc at its widest point, and a second transit is made through the sunspot from limb to limb, noting the time when the cross-hair passes the spot. When this data is entered into the "Helio" programme the solar latitude and longitude of the spot (AR) is generated. The accuracy is impressive, and the method works for the C8 as well as the alt-az 'scope. "Helio" will also yield the solar coordinates of prominences seen in H-alpha, and assist in mapping filaments on the disc. Filaments are very faint lately, presumably because solar fields (and confining magnetic shears) have greatly weakened.

Two recent groups (AR) are examples of ephemeral sunspots and are shown in Figs 1 and 2. Firstly, AR965 was sighted on 2007 July 28 03:07 UT at lat.11ºS long.196º as a single umbral spot (Fig 1a). The Mt Wilson digital magnetogram showed large areas of strong polarity at the site, and tangles of faint filaments were seen in H-alpha. Bright plage spread south of the spot, then curved to the NW. A thin dark active region filament (ARF) radiated southwards from the spot and at 04:45 UT a small blue-tuned surge erupted along this line. A patch of plage brightened almost to subflare level at 04:36 nearby, but GOES X-ray Flux showed hardly any activity.

The white light view on July 29 03:18 UT (Fig 1b) showed that a small penumbra now enclosed the leading two spots and a cluster of spots and pores trailed 3º behind. With a total of six nuclei the R number was 16.

In white light on July 30 at 03:06 UT (Fig 1c) the AR had faded to a single spot within an elongated penumbra and two small spotless patches of penumbrae with a spot trailing 3º behind well north of the previous day's trailing spots (Fig 1c). The next day's view showed only a single tiny spot or pore at the site. AR 965 had a short life of about three days. It is interesting to contrast this AR with the group that arose on the Sun on the same day (the 30th) at 9ºS and 23º west of 965 at longitude 220º (Fig 2).

This AR was not given an active region number by NOAA. It comprised an obvious pair of spots in a single penumbra (preceding), with a small spot following 2º behind, and some bright plage in H-alpha on the south side. This group was recorded by Monty Leventhal some hours before my own observation. It was also recorded on the Mt Wilson 150' solar tower drawing for the same date, and gained a mention on the Spaceweather website too. The group was visible on the 31st as a single small spot in very poor seeing. Nothing was seen at the site on the next day, so the group lasted perhaps only 36 hours.

Both small groups, though short-lived, were about 10º below the solar equator. Very few northern groups were seen over the last six months, and for the period 2007 February to July (inclusive) four northern groups appeared compared to 28 southern groups, a huge southern surplus (preliminary data from Monty Leventhal). Most of the southern groups were well away from the solar equator, although large AR 963 in July lay only ~2º south.

Figure 1

Figure 2