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Here is
an explanation of some of the terms used in these pages.
-
Carrington
Rotation Numbers. Solar
rotations are numbered using a
series devised by Carrington that began on 9th November 1853. The mean
length of a solar rotation is 27.2573 days.
-
Faculae.
Bright patches seen on the photosphere. These are usually near the limb
of the Sun due to limb darkening and are associated with sunspot groups.
-
Heliographic
coordinates. The position
of any feature on the
photosphere of the Sun can be measured in terms of heliographic
latitude and longitude. The latitude and longitude of a particular
point on the solar disk changes with time because of four factors:-
- solar rotation,
- the rotation axis of
the Earth being inclined by 23.4°
to the Earth's orbital plan, the ecliptic,
- the rotation axis of
the Sun being inclined by 7.25°
to ecliptic and
- changes in the
Earth to Sun distance.
Four parameters used
in the calculate the
heliographic coordinates of a feature on the solar disk:-
- Lo - the
heliographic longitude of the centre of
the disk - this decreases by an average value of 13.2° per day,
- P - the position
angle of the north end of the axis of rotation, measured +ve if east of
the north point of the disk,
- Bo - the heliographic
latitude of the centre of the disk and
- the apparent
diameter of the Sun.
-
McIntosh
Sunspot Group Classification.
Sunspot groups are
classified by a three letter code. The first code describes the group
type (based on a modified version of the Zurich sunspot group
classification), the second code describes the penumbra of the largest
spot of the group while the third code describes the compactness of the
spots in the immediate part of a group. These code and their meanings
are now given together with the possible combinations of the three
codes. Also given is an illustration of the codes (from the
paper The
Classification of Sunspot Groups
by Patrick McIntosh).
Group
Type
- A: Unipolar group
without penumbra,
- B: Bipolar group
without penumbra on any spots,
- C: Bipolar group
with penumbra on one end of
group, usually surrounding largest of leading umbra,
- D: Bipolar group
with penumbrae on spots at both
ends of group and with longitudinal extent less than 10°,
- E: Bipolar group
with penumbrae on spots at both
ends of group and with longitudinal length between 10° and
15°.
- F: Bipolar group
with penumbrae on spots at both
ends of group and with longitudinal length more than 15°.
- H: Unipolar group
with penumbra.
Penumbra
of Largest Spot
- x: No penumbra
(class A or B),
- r: Rudimentary
penumbra partly surrounds largest
spot,
- s: Small,
symmetric penumbra, elliptical or
circular and N-S size smaller than 2.5°,
- a: Small,
asymmetric penumbra, irregular in
outline and N-S size smaller than 2.5°,
- h: Large,
symmetric penumbra, N-S size larger
than 2.5°,
- k: Large,
asymmetric penumbra, N-S size larger
than 2.5.
Spot
Compactness
- x: Assigned to (but
undefined for) unipolar groups (types A and H),
- o: Open - few, if
any, spots between leader and
follower,
- i: Intermediate -
numerous spots between leader
and follower, all without mature penumbra,
- c: Compact - many
large spots between leader and
follower, with at least one mature penumbra.
Allowed
Groups
|
Group
Type
|
Penumbra
of
Largest Spot
|
Spot
Compactness
|
Number
of Types
|
|
A
|
x
|
x
|
1
|
|
B
|
x
|
o
i
|
2
|
|
C
|
r
s a h k
|
o
i
|
10
|
|
D
E F
|
r
|
o
i
|
6
|
|
D
E F
|
s
a h k
|
o
i c
|
36
|
|
H
|
r
s a h k
|
x
|
5
|
| Total
number of allowed
groups: 60 |
- Illustration
of the Codes
- MDF.
Mean daily frequency - the average of a measurement made once per day
during a month.
-
Photosphere.
This is the visible surface of the Sun which has an effective
temperature of 5800 K. The photosphere has a mottled appearance due to
the convection (granulation). The edge of the visible surface is darker
than the centre due to cooler material being seen towards the limb
compared to the centre (limb darkening).
-
Solar
Cycle. Activity on the Sun
varies with a period, on
average, of 11 years. The solar cycle is most evident through the
number of sunspots present on the solar disk. At the start of new cycle
there will be no or very few spots. Those that are present will be at
high heliographic latitudes. As the cycle progresses, the number of
spots increase and the latitude at which these spots appear reduces.
The solar maximum will occur typically 3 to 5 years after solar
minimum. The decline to minimum is longer than the rise to maximum.
Sunspots with the largest areas tend to occur in the two years after
solar maximum. At the end of the cycle, sunspots can be found close to
the solar equator (together with high latitude spots from the new
cycle).
-
Solar
Rotation. The mean sidereal
rotation period is 25.38
days (the Carrington period). The corresponding synodic rotation period
is 27.2753 days. The sidereal period is the rotation period as viewed
from a fixed point in space whereas the synodic period is the rotation
period as viewed from Earth (and thus is used when calculating
heliographic coordinates).
- Sunspots.
These appear as dark patches on the photosphere. The larger spots have
a darker central region (the umbra) surrounded by a lighter outer
region (the penumbra). Sunspots are cooler than the photosphere with
the umbral regions being about 2000°C cooler and the penumbra
about 500°C cooler. Individual spots can range in diameter from
1000 km to 100,000 km (the Earth has a diameter of 12750 km). Sunspots
are linked to the surface magnetic field of the Sun.
- Sunspot
Area. The area of a sunspot
is measured in millionths of
the Sun's visible hemisphere. For a standard disk drawing size of 152mm
(6 inches), a spot area of 1mm by 1mm corresponds to an area of 28.29
millionths. The area of sunspots are distorted by the spherical shape
of the Sun. Corrected for this foreshortening are made by dividing the
measured area by cos(spot latitude) and then by cos (spot longitude).
- Sunspot
Groups. Sunspots usually
appear in groups. Normally it
is fairly easy to count the number of groups as they are spread out
across the disk and in both hemispheres. Difficulties can occur when
sunspots appear close together. Then experience of different types of
sunspot groups can be used to determine the number of groups present.
- Sunspot
Number (R). This is
calculated from the number of
sunspot groups and the number of sunspots using the equation R = k.(10g
+ f). k is an observers factor, g is the number of sunspot groups and f
is the number of sunspots (k is usually set to 1 from an individual
oberver - it is only when observations from different observers are
compared is k set to a value other than 1).
- UT.
Universal Time. The timing of astronomical observations are made in UT
and is synonymous with Greenwich Mean Time (GMT).
Last
updated on
22 April 2002.
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