ASE logo The Astronomical Society of Edinburgh

Journal

No 53 - July 2007

Solar observing

I recently had the use of the Society's Coronado NearStar solar telescope and, during an unusually good spell of weather, took the opportunity of observing more or less daily. Despite this being an intense period of minimum solar activity (see http://www.spaceweather.com, April 28th, 2007 archive and later in this article), I was fortunate in being able to follow active area 953 as it developed. This is not intended as a complete guide to solar observing - I'm just beginning myself - merely a taster based on personal experience which may hopefully encourage some of you to make use of the summer months to explore the Sun in some detail.

Before I proceed, however, lets get the necessary out of the way:

WARNING - Never look directly at the Sun with the unaided eye or through any optical instrument without the use of specialist filters expressly for that purpose, ensuring that any such filter is undamaged in any way. Blindness or permanent damage will result. The only safe way of observing the Sun is by projection. Never leave any instrument pointed at the Sun unattended. Always supervise children and the general public when they are near solar observing equipment.

There are various options for solar observing nowadays, but some people will always be uncomfortable with direct optical viewing, regardless of the filtering employed. Eyepiece projection onto a piece of white card is a safe method and if the card is secured in a box to provide some shade, drawings may be made directly onto templates. Of course, with such incredible Internet resources, you don't need to do any direct observing. You could follow the ever-changing Sun online - regardless of the British weather. Various web sites have been mentioned in this article and I have also collected these for reference at the end.

I first started following sunspot 953 on the 27th April, 2007 and made sketches in my observing notebook.

notebook pages
Notebook pages.
drawing 20/4/07 drawing 1/5/07
Drawings for 30/4/07 (left) and 1/5/07 (right).

It was fascinating to see the active area develop over the space of a few days. Sketching at the eyepiece is a great aid to improving your observing skills. If you are able to put down any detail on paper, it means that you have really studied the object in question - not just glanced casually through the eyepiece, or at a photograph. Like any form of sketching, drawing at the telescope takes practice, but anyone can do it. These sketches were done in a few minutes with the dual purpose of recording the changes from day to day and having a record to look back at.

Gary Palmer's image
Hα image taken by Gary Palmer.

Active area 953 was also captured in amazing detail by Gary Palmer of Los Angeles, California, using a Coronado double stack 90 Hydrogen-alpha scope and an SBIG ST-2000XM camera. Many thanks to Gary for allowing the use of this image. Hopefully, my sketch on the same day bears some resemblance to this image! It is also well worth having a look at his impressive solar movies at http://www.thesuninmotion.com.

The Society's NearStar is filtered at the objective end with an energy rejection filter which cuts out all the potentially harmful radiation in the infrared and ultraviolet. It is further filtered at the eyepiece end with a narrow bandpass filter to let through hydrogen alpha - Hα. Hα radiation has a wavelength of 656.3 nm and corresponds to a temperature of around 10,000 K which occurs in the chromosphere - the layer above the visible 'surface' of the Sun known as the photosphere. As this wavelength is within the visible part of the spectrum, it allows us to see activity in the chromosphere as well as several features in the photoshere below. It is also possible to fine-tune the wavelength via the small thumb-screw on the diagonal which helps bring out details of particular features. Various features can be observed in Hα. Prominences - vast arches and sheets of plasma being thrown outward from the edges of the solar disc. Filaments - dark lines on the face of the Sun - are actually prominences seen face-on against the bright solar disc. Plages are bright areas usually seen near sunspots (such as 953) and may actually be seen in areas of potential sunspot formation. It is also possible to see through to photosphere features such as sunspots and granulation - lozenge-shaped tops of convection cells which are constantly reforming over periods of around ten minutes.

whole disc image
Whole disc image of the Sun. Courtesy of SOHO/MDI.

I also followed its progress on the excellent web site http://www.spaceweather.com. A quick glance at the home page will show you whether or not there are any active areas on the Sun. The Spaceweather main page uses a whole disc image like this one from the SOHO/MDI consortium. SOHO or 'Solar and Heliospheric Observatory' is a 3-axis stabilised spacecraft that constantly faces the Sun. It orbits around the 1st Lagrangian point (L1) at a distance of around 1.5 million kilometres where the combined gravity of Sun and Earth keep SOHO in an orbit locked to the Sun-Earth line. It carries 12 instruments which observe at various wavelengths. The results can be seen (almost real-time) at their website http://sohowww.nascom.nasa.gov. Spaceweather also indicates the likelihood of solar flares and resultant auroral activity. If there is a possibility of auroral activity - check out the Aurorawatch website at http://www.dcs.lancs.ac.uk/iono/aurorawatch.

The '953' refers to an 'active area' and this may contain more than one sunspot. The numbering was started on January 5th, 1972. However, as it is limited to 4 digits, when 10,000 was reached in June, 2002, it rolled over to start again at 0000. So although this active region is referred to as 953, it is actually 10,953 since records began. This is not to be confused with the daily 'sunspot number', which is a calculated value indicating the current level of solar activity.

Because the Sun is not a solid body, it exhibits differential rotation about its axis. The period of rotation at the equator is approximately 27 days, but this increases to around 40 days at the poles. This differential motion causes a distortion in the Sun's magnetic field. This twisting, together with the effect of convection currents, produces tubes of magnetic field lines arching in and out of the photosphere - the visible 'surface' of the Sun. These regions prevent the flow of heat by convection from the lower levels and so a cooler, darker area is seen as a sunspot. The darkest central area, or 'umbra' is the coolest part - as much as 2000 K cooler than the surrounding photosphere. The lines of magnetic flux are also at their most concentrated here. In the lighter 'penumbra', the flux lines are becoming weaker and more horizontal. The longer term effects of this differential motion also apparently accounts for the solar cycle, which has been observed and recorded for around 200 years. This equates to a periodic increase and decrease in solar activity over approximately 11 years. When the Sun is at its most active, there are a greater number of sunspots. The last maximum was in 2000, so we should have passed the minimum by now. However, it has been agreed by a panel of experts that the predicted minimum will be March 2008, give or take 6 months. There was also some uncertainty as to when the next solar maximum would occur. Some believe there will be an intense maximum in 2011; others favour a less intense maximum in 2012. See the Spaceweather archive for the 28th April at http://www.spaceweather.com.

Basic discs labelled with NSEW can be used for drawing observed solar detail or for drawing directly over a projected image. You do, however, have to take into account the apparent tilt of the Sun's axis as the day progresses (unless you are using a correctly set up equatorial mount). At midday, the celestial North-South axis is vertical; but at the start and end of the day, it will appear tilted from the vertical as the Sun proceeds around the curve of the celestial equator. A drift method can be employed to determine the East-West axis. With the telescope fixed and the drive (if you have one) switched off, allow the spot to drift across the eyepiece or across the template if you are projecting. Rotate the image until the spot travels along the East-West line. Remember to check the orientation regularly if you are observing over a period of time - it doesn't take long for the Sun to travel across the sky. Just as the Earth's axis is inclined to the ecliptic by just over 23°, the Sun's equator is inclined to the orbital plane of the Earth by just over 7°. The combined effect of these inclinations causes an apparent wobble of the solar sphere over the space of a year, as we complete an orbit around it. This is explained in more detail on Peter Meadows' website http://www.petermeadows.com together with an excellent animation on the 'Sun from Earth' page. You can also download his freeware program 'Helio' which provides an ephemeris for your location, together with aids for drawing and recording sunspot activity. For recording more detailed positional information of sunspot groups, you can use Stonyhurst discs or Zürich grids. These are templates with lines of latitude and longitude which take into account the foreshortening effect near the limbs. It is quite easy to forget - especially in a telescopic view - that you are looking at a sphere rather than a flat two-dimensional disc. Stonyhurst discs in various sizes and formats can be downloaded from Peter's website.

P.-M. Hedén's image
Image taken by P.-M. Hedén.

One interesting phenomenon, and new to me, which cropped up during the study of sunspot 953 was that of the so-called Ellerman Bombs. Examples in active region 953 can be seen arrowed in this image by P.-M. Hedén of Sweden. It was taken using a Canon Digital Rebel XT through an Orion 80ED with SolarMax60 filter. Many thanks to him for allowing its inclusion in this article. Please also take the time to visit his website at http://www.clearskies.se where you will find an excellent collection of images. Of particular interest to a number of our members will be some stunning noctilucent clouds and aurorae.

In 1904, Ferdinand Ellerman joined George Ellery Hale (with whom he had previously worked at Yerkes Observatory) on Mount Wilson. They began a program of observing using a 12" solar telescope previously used on a Yerkes eclipse expedition in 1900. In 1915, whilst observing in the Hα band, Ellerman reported a phenomenon 'so extraordinary it hardly seemed real'. It appeared to be something in the nature of an explosion, lasting only a few minutes, and he adopted the name 'hydrogen bomb'. See http://adsabs.harvard.edu for his original article - Solar Hydrogen "bombs". They are now commonly known as Ellerman Bombs (Ebs), but may also be referred to as micro-flares or moustaches - due to their spectral profile extending either side of the Hα line. They are most likely to occur in areas of flux emergence and strong magnetic fields, for example in the vicinity of the penumbra around isolated sunspots - such as 953. The average life-span is thought to be around 10-14 minutes and if you should see one - keep watching, because they are likely to recur in the same area.

As I said earlier, this is not intended as a complete guide to solar observing - I've merely scratched the surface of a very complex and fascinating subject. So, whatever your preferred method, get out there and do some active observing of our closest star.

Websites and books:

Ken Thomas


Contents

Cover page

Solar observing

More on cooling the Earth

Minor planet (7170) Livesey

Recent observations

Moonrise

Two BAA meetings in Scotland

Glen Lyon weekend 16-18 March 2007

Forthcoming events

About the ASE Journal


This journal as a single web page

This journal as PDF file (690 kByte)


Valid XHTML 1.0 Strict Valid CSS!