Measuring R number for the Sun

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    Duncan Hale-Sutton

    I made an observation of the sun yesterday (see attachment) at 10:02 UT by projecting the image onto paper. I didn’t have time to get the east-west drift line and so compared my image to an SDO/HMI image taken at 13:45 UT (see attached also) to get the correct orientation (I hope). I then looked at to assign active regions (AR numbers). I am new to all this but I have read quite a bit of the BAA’s Observing Guide on the sun. I would like to add to the basic information by measuring the R number but I have some questions about this.

    Firstly, I understand that you count the number of individual groups of spots (say G) and then the number of spots (say S) themselves. An individual group is defined if it is “more than 10 degrees apart on the solar surface” from any other group. Then R=10G+S. However, if you look at my observation AR3107 and AR3105 are pretty close together. How would you estimate how far apart these two groups are? Do people have a feeling for this or do they go through some more rigorous approach?

    Secondly, counting the spots themselves. The spots have a dark umbra and then a lighter penumbra. Do you count the umbrae areas as separate spots or can a spot be several umbrae that can be linked by a common penumbra? Some of the spots are extremely small or very faint (but just noticeable). How far down the chain of size do you keep counting spots?

    Finally, is the R number something that is dependent on the equipment and the subjectivity of the observer (for example you can see another group of spots on the SDO/HMI image at the extreme west that my image missed) or is it robust enough to provide a measure of the activity of the sun. Would two different observers come up with similar R numbers if observing at the same time?

    Peter Meadows

    Hi Duncan,

    Thanks for your questions about how to determine the sunspot number R from your observations. The general guidance about counting the number of groups is, as you say, based on the longitude and latitude separation of sunspots: 10 deg in longitude and about 5 deg in latitude. In the case of AR3105 and 3107, the center of these groups in longitude are more than 10deg apart but less in latitude. Certainly using the annotations on SDO HMI image from can be useful to gain some experience of group separation but with experience this can be done oneself.

    For sunspots with penumbra, it is the number of umbrae that should be counted towards the number of sunspots S. So if there is a penumbral sunspot with 3 umbrae, then this sunspot will contribute 3 to S. Individual sunspots, however small, also count towards S.

    Yes, the observing equipment & conditions, the observer all contribute to each observers R value. It will also depend on whether an observers uses a low eyepiece for the full disk and then a high magnification eyepiece for the smaller sunspots. The key is to be consistent with your observing procedure from observation to observation, otherwise your daily R values will differ depending on how you observe. Each observer will have their own R values which will be different from other observers even from the same day – this is the advantage of combining observations from many observers as is done within the solar section.

    You are welcome to add your daily observations to the BAA Solar Database which are collated monthly and published in the monthly newsletter (see attachment) and in the Journal. See for more details.

    BAA Solar Section Assistant Director

    Bill Barton

    The full version of the formula can be expressed as

    R = k(10g + s)

    Where R, g & s are as above. k is an observing constant based on personal experience and instrumentation.

    Back in the 1990’s when I used to observe the Sun I found my k was 0.8.

    Deciding what was one or two active areas is very much a matter of experience (feeling), but a Stonyhurst Disc could be used as a somewhat crude guide (if you observe by whole disc drawing).

    Duncan Hale-Sutton

    Thanks very much Peter, that clarifies things a lot. I hadn’t thought that it was the center of the groups that was used to determine their distance apart on the solar disk, I had just assumed that it was how close their edges were. So that helps. I also didn’t realise it was just the umbrae that counted towards the spot number, either. I think I might try and add some observations to the database that you mentioned once I get the hang of things a bit better but I notice from your newsletter that we can email images to you. Do you have any preference about whether the image is annotated in any way (e.g. with AR numbers?). I note using the time, date and initials in the file name and copying them to Lyn. Any preferred format for the file name? many thanks.

    Duncan Hale-Sutton

    Thanks Bill, that kind of makes sense that there is some kind of multiplying factor that is observer and instrumentation dependent. I was wondering how this all worked because I could see that the better the instrumentation and resolution you had, the more sunspots you might count. I haven’t tried using a Stonyhurst disc yet, that was something I was going to try and grapple with next. I think for the moment I will continue with my approach of projecting the image first and getting a handle on the basics before moving on to solar filters etc.

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