Solar Section        

 
 
    OBSERVING SOLAR WHITE LIGHT FLARES

    by
    Richard Hill
    A.L.P.O. Solar Section Coordinator(1985-2000)

    Solar activity is not just gauged by the number of sunspots
    observed. There are many other manifestations of solar activity
    also quantified that indicate well the level of activity. Flare
    production and strength are two such parameters. Flares are
    sudden discharges of energy and subatomic particles that take
    place in and around large sunspot groups as magnetic fields
    change above the groups. Flares release prodigious amounts of
    energy across most of the electromagnetic spectrum and are thus
    observable by a number of techniques. Larger flares can emit as
    much as a thousandth the energy of the sun during the duration of
    that flare. Subatomic particles are shot out at various speeds as
    well. These releases take different times to traverse the space
    between Earth and Sun but eventually impact the Earth’s
    atmosphere causing changes in propagation of radio waves and the
    beautiful aurorae seen at temperate and polar latitudes.

    Typically, flares last a few minutes to as much as four hours
    though most are from ten to twenty minutes in duration. More
    energetic flares tend to be of longer duration, especially when
    observed in shorter wavelengths. In visible spectrum observations
    done by amateurs the relationship is not quite as good. Flares
    are best seen in monochromatic light such as H-alpha or the H and
    K lines of calcium where only light of one absorption line is
    allowed to enter the telescope. Since flares are in emission in
    these lines, whereas the rest of the disk of the Sun is generally
    in absorption, they appear quite bright against the disk. In some
    cases flares can be so energetic that they will even be seen in
    the light of the continuum of the spectrum (between the dark
    absorption lines) as viewed in the amateur’s telescope. These
    White Light Flares or WLFs were once thought to be relatively
    rare.

    Not all sunspot groups produce flares. In 1938, M. Waldmeier
    devised the Zurich Sunspot Classification of these groups. It
    consists of nine steps or classes (A through J, omitting I) that
    delineate characteristic evolutionary stages of sunspot groups,
    though not all groups go through all classes. Most groups go only
    part way through the sequence and then either rapidly go
    backwards through the classes or decay to the final class. In
    general, the greater the area of a group the more asymmetrical
    will be its growth curve. So a large group will rise rapidly from
    class A to E and decay more slowly as it goes from E to J. Groups
    of classes D, E, and F are the big flare producers. But not all
    such groups produce big flares. This was a problem for flare
    forecasters on whose work various broadcast and space industries
    depended. Even with the most active class, F, a forecaster had a
    marginal chance of predicting flare probability in any given 24
    hour period. In order for flares to be studied, a reliable system
    for identifying flare producing sunspot groups was needed. An
    observer would have to spend much time at the telescope observing
    every well developed group in hopes of seeing these elusive
    events. It would have been highly advantageous, on the basis of a
    few parameters, to weed out many less productive groups.

    In 1966, Patrick McIntosh of the Space Enviroment Services Center
    of the National Oceanic and Atmospheric Administration,
    introduced a sunspot classification system that improved the
    older Zurich system. The new classifications consist of three
    letters. First is the Modified Zurich Class. It basically retains
    the old Zurich Class but G and J were removed as being redundant.
    A Modified Zurich Class was used rather than a totally new system
    to be an making it easier for observers that might be reluctant
    to switch to the new system. The Second letter represents an
    assessment of the Largest Spot of the group. This is not
    necessarily the leading spot, but rather the LARGEST. The third
    letter represents an assessment of the Spot Distribution within
    the group. It takes only slightly longer than the old system to
    classify all the groups on the sun for a given day using the
    McIntosh System, but the information returned and usefulness of
    the new system makes it worth the slightly added effort.

    In order to understand the McIntosh Classification system better,
    two terms have to be defined:

    Unipolar Sunspot Group

    This is a single spot or compact cluster of spots with the
    greatest separation between spots being less than 3 heliographic
    degrees (degrees on the Sun’s surface). With a Class H group the
    separation is taken to be the distance between the outer border
    of the main sunspot penumbra and the most distant attendant
    umbra.

    Bipolar Sunspot Group

    This is two or more spots forming and elongated cluster with a
    length of 3 or more heliographic degrees. If there is a large
    principal spot then the cluster should be greater than 5 degrees
    in extent.

    Now that we have these defined, below are the descriptive text
    that define the various classes in the McIntosh System. (All
    degrees are heliographic.)

    For the Modified Zurich Classes:

    A – A unipolar group with no penumbra. This can be either the
    early or final stage in the evolution of the group.

    B – A bipolar group with no penumbrae on any spots.

    C – A bipolar group with penumbra on one end of the group,
    usually surrounding the largest leader umbra.

    D – A bipolar group with penumbrae on spots at both ends of the
    group and a length of less than 10 degrees.

    E – A bipolar group with penumbrae on spots at both ends of the
    group with a length of 10-15 degrees.

    F – A bipolar group with penumbrae on spots at both ends of the
    group and a length greater than 15 degrees.

    H – A unipolar group with penumbra, usually the remains of a
    bipolar group

    For the Largest Spot:

    x – No penumbra (for groups with classes A & B)

    r – Rudimentary penumbra that usually only partially surrounds
    the largest spot. Such a penumbra will likely be granular rather
    than filamentary, making it appear brighter than a mature
    penumbra. The width of the penumbra will only be a couple to a
    few granules (of the photospheric granulation) and may be either
    forming or dissolving.

    s – Small, symmetric spot (similar to Zurich Class J) and the
    spot will have a mature, dark, filamentary penumbra of circular
    or elliptical shape with a clan sharp border. If there are
    several umbrae in the penumbra they will form a tight cluster
    mimicking the symmetry of the penumbra with a north-south
    diameter of 2.5 degrees or less.

    a – Small, asymmetric spot with irregular surrounding penumbra
    and the umbrae within separated. North-south diameter of 2.5
    degrees or less.

    h – A large symmetric spot. Like type “s” but the north-south
    diameter is greater than 2.5 degrees.

    k – A large asymmetric spot. Like type “a” but the north-south
    diameter is greater than 2.5 degrees.

    (north-south diameters are used since they suffer no
    foreshortening during rotation.)

    For Sunspot Distribution:

    x – Unipolar group of Modified Zurich Classes A or H (i.e. a
    solitary spot).

    o – Open distribution with a leader and follower spot and few or
    none between. Any spots between should be very small umbral
    spots.

    i – Intermediate distribution where numerous umbral spots lie
    between the leader and follower spots.

    c – Compact distribution where the area between the leader and
    follower spots contains many spots with at least one having
    penumbra. In extreme cases the whole group may be enveloped into
    one complex penumbra.

    This system has proven a more accurate predictor of flares in the
    thirty years of its use. Indeed, it has helped solar astronomers
    understand better the relationship between flares and sunspots.
    Sunspot groups that produce flares are relatively rare. Because
    of this it has taken several solar cycles of observations to
    demonstrate the effectiveness of the new system.
    Using the old Zurich system it was found that groups of class F
    were most likely to produce flares. But only a 40% flare
    probability in a 24 hour period could be predicted using this
    parameter alone. With the McIntosh System, using Modified Zurich
    Class F, the probability improved to 60%. Using just the Largest
    Spot class of “k” the probability in 24 hours was 40-50%. If just
    Spot Distribution category “c” were used, flare probability went
    up to about 70%. But, when all three dimensions of this system
    were used, classes Fsi, Fki and Fkc, showed a probability of up
    to 100% for production of M flares in a 24 hour period and the
    McIntosh Class of Fkc had a further probability of up to 50% in X
    flares (x-ray) production! This surpasses any former method of
    flare prediction used, including sunspot area.

    In optical regions of the spectrum flares are classified by size:
    (All degrees are heliographic.)

    s – subflare of less than 2 degrees area.

    1 – “Importance 1″ flares, greater than 2 degrees but less than
    5.1 degrees in area.

    2 – “Importance 2″ flares, greater than 5.1 degrees but less than
    12.4 degrees area.

    3 – “Importance 3″ flares, greater than 12.4 degrees but less
    than 24.7 degrees in area.

    4 – “Importance 4″ flares, greater than 24.7 degrees in area.

    and by their brightness:

    F – faint or barely noticeable

    N – Normal or noticeable

    B – Bright or obvious

    This means that a 2B flare is one that was bright and between 5.1
    and 12.4 square heliographic degrees area. An SF would be a faint
    subflare, the most common type. For other parts of the
    electromagnetic spectrum, like x-ray, there are other
    classification systems. But since amateur solar astronomers and
    most particularly the White Light Flare Patrol, or WoLF Patrol,
    will be observing in the visible spectrum these will not be
    discussed here.

    Flares occur in places where magnetic change is taking place or
    where the neutral line between areas of different polarity lies.
    There are some precursors to solar flares in these places.
    Filaments near the flare site may go into rapid motion or may
    change brightness due to such motion in the line of sight if you
    are observing in relatively monochromatic light (doppler
    shifting). In larger flares the first sign is a pulse in hard x-
    ray and a slower pulsing in soft x-rays. Following this there may
    be flashes seen in longer wavelengths including optical. (This is
    again in rather monochromatic visible light and I know of no case
    where this has been reported in broadband white light.) The
    pulses are caused by electrons being shot through the corona at
    nearly half the speed of light causing oscillations in coronal
    plasma above the site which generates the radio bursts (called
    type III bursts) at frequencies from about 10 to 800 Mhz (note
    its the FM Band). Receive these emissions and you will be
    forewarned of the optical flare.

    Lacking a magnetograph, amateur astronomers must look for flares
    (in monochromatic light, continuum or broadband white light) min
    the most common places where they occur. The priority list of
    McIntosh classes to be watched are: Fkc, Fki, Ekc, Eki, Dkc, Dai,
    Dso and Hsx. These are the most flare productive groups of the 64
    classes in order of productivity. In the latter class, flares
    usually occur just beyond the outer penumbral boundary. Patrick
    McIntosh once advised me to also watch groups that suddenly
    arrange their spots in a line. He call this a “linear
    accelerator” and a good bet as a site for flares.

    Within these groups one should watch:

    -penumbrae that are chaotic, disturbed, or detached

    -great clusters of smaller spots and penumbral bits between the
    main spots

    -thin light bridges or light bridges caused by detachment of
    penumbrae

    -sunspots with or without penumbrae, that are breaking apart
    without reducing in area

    -and rapidly moving spots in a group.

    Observing WLFs requires some special equipment and precautions.
    The goal here is to maximize contrast between the flare and its
    surroundings. Thus all optics should be very clean since
    scattering from dust and other contaminants on your optics will
    scatter light and reduce contrast. The telescope f/ratio should
    be long, f/20 or longer is good and helps to reduce apparent
    defocussing from optical heating and normal, daytime seeing.
    According to studies by Bray & Loughhead (1963), daytime seeing
    is 1 second of arc only about 1% of the time at a good site. So
    reducing the aperture of your telescope, especially those big
    light buckets, to 4-6 inches will result in little or no loss of
    resolution and will yield an improvement in image steadiness (and
    contrast by producing and unobstructed aperture. This will more
    than make up for any perceived losses.

    You can increase your chance of seeing WLFs by increasing the
    contrast between the flare and bright photosphere. This can be
    done through wideband filtration, unlike the narrow band
    filtration of only a fraction of an Angstrom (Ang.) used in H-
    alpha observations. A good region to filter around is at 4300
    Ang. or 430 nm called the G-Band. There are a number of
    absorption lines clustered here that go into emission in flares.
    When these normally dark lines become bright the difference in
    brightness is greater than if you were looking at a bright region
    of the spectrum. With a narrow band filter the contrast is much
    greater. Since broad band filtration will take in a fair portion
    of continuum it is still considered “white light”.

    Projection techniques will only detect the very brightest WLFs,
    explaining the paucity of them in the historical records. We have
    never received a report of a WLF by anyone using the projection
    method. The most simple method of observation in searching for
    WLFs, is to just use a mylar-type filter. The blue image of these
    filters, normally detested by amateurs, is quite close to our
    target wavelength. This is also the safest way to look for these
    flares. These filters are also very good and showing faculae
    associated with these complex groups, well in towards the center
    of the disk. Only the highest quality filter should be used.
    There are some filters on the market and some homemade out of
    lesser quality materials. These show a good deal of sky
    brightness just off the limb of the Sun. Such filters will
    probably scatter enough light to obliterate all but the brightest
    flares. The author has seen several using the so-called “neutral
    density” filters but it would have been impossible without the
    prior experience with more selective filtration.

    Beyond this one can obtain filters of about 100 Ang. bandpass to
    use at the eyepiece WITH A MYLAR-TYPE PREFILTER!! It may be
    necessary to reduce the density of the prefilter but such
    experimentation should be done with great caution. Do not risk
    your eyes at any time and NEVER USE EYEPIECE FILTRATION ALONE! A
    prefilter is a must unless you have a specially designed solar
    telescope. If you are going to do this kind of observing you
    might do well to build a specialized telescope for the task, but
    that will not be discussed here. Some experimentation will likely
    be necessary if you want to go to narrower broadband filters but
    novices and the unsure are strongly encouraged to just stick with
    the commercial mylar-type filters. If you don’t know, or aren’t
    sure, don’t do it!

    When observing, first make a sketch of the region to be watched
    on the A.L.P.O. Solar Section WoLF Patrol standardized observing
    form. (See instructions and form.) Let the spot group fill the
    box. Once this is done observing may begin. Attempt to observe
    the region at least once every ten to twenty minutes, the average
    lifetime of a flare. To check less often would risk missing one.
    Be patient! It may be quite a while before you bag the first one.
    If you observe the McIntosh E and F groups cited in our priority
    list, you will be more likely to see one sooner. Do not sit
    constantly at the telescope. You will not see change. It is too
    gradual and your eye needs the rest between observations.

    Note ANY changes on the form. Not all flares behave the same way
    and not all precursors are well known. Record time to the nearest
    second, start and stop. Surges have been photographed in white
    light but have only been viewed at the limb (see Sky & Telescope,
    Dec., 1961, p.330). Some of the rapid changes reported in
    penumbrae in history may well have been observations of such
    surges. The only way to be sure of such observations is to build
    a larger data base of observations from which patterns may
    emerge.

    Professional observations indicate that WLFs begin as a bright
    point of probably granule size in one of the sites noted earlier.
    My own observations during cycle 22 in AR 5060 and 5062 (June,
    1988) tend to support this. Other points will pop up near the
    first in only a couple minutes, or the single one may be seen to
    enlarge rapidly. These early stages are about all that happens in
    the smaller flares, and in sub-flares the single point,
    substantially brighter than the photosphere, may be the full
    extent lasting only a few minutes. The human eye can detect only
    changes that are in excess of about 10% against such a background
    so be aware of any brightening. Dr. Don Neidig, of Sacramento
    Peak Solar Observatory, in New Mexico, once expressed the
    suspicion that more such faint flares are visible in white light
    but because of the low contrast, short lifetime and smallness, go
    unnoticed.

    In larger flares the points will grow in brightness, merge and
    become a bright area or, if you are very lucky, a bright ribbon.
    If you are so lucky, watch for a double ribbon (running on either
    side of the neutral line of magnetic polarity) or a shaded,
    penumbral-like area near the flare which could be a surge.

    Observing these, the most energetic events in our solar system is
    an exciting, and taxing business. It’s taxing in the long wait
    for the flares and exciting in the activity when it happens. It
    does not require long term commitment of daily observations but
    is a type of solar observing that can be done on the odd day when
    you have a couple hours to spare or while puttering about the
    yard. You just go to the telescope every ten minutes or so and
    make your observation. The data is of value to solar astronomy
    and will be reported by the ALPO Solar Section in both the
    Rotation Report and our regular reports in the Strolling
    Astronomer, the Journal of the ALPO.

    At the risk of stating the obvious, the observations will not
    make themselves. So make your observations and report them in a
    fashion where they can be useful, with the WoLF Patrol. Thus, you
    will by pursuing your hobby, see the most energetic events in our
    solar system and be contributing to science. Remember, only a
    couple of hours on a Saturday afternoon may repay you with a view
    of more energy than has been collectively used by humans in our
    2.5 million years of existence!

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