OBSERVING SOLAR WHITE LIGHT FLARES
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
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
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
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
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
o – Open distribution with a leader and follower spot and few or
none between. Any spots between should be very small umbral
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
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
-thin light bridges or light bridges caused by detachment of
-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
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
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!