Repeated Structures Found After the Solar Maximum in the
Butterfly Diagrams of Coronal Holes
M.Y. Hofer and M. Storini†
Research and Scientific Support Dept. of ESA, ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands.
†
IFSI/CNR, Via del Fosso del Cavaliere 100, 00133 Roma, Italy.
Abstract. The influence of the solar cycle evolution on the coronal hole space-time distribution is well known, for polar as
well as for equatorial isolated sources of high speed solar wind. Among them the long-lived coronal holes occurrence from the
sunspot cycle 21 on is investigated, using the coronal hole catalogue based on HeI (1083 nm) observations (Sanchez-Ibarra
and Barraza-Paredes). In at least these two solar cycles (n. 21 and n. 22) a similar structure in the latitude-time diagram of
coronal holes is found. The area occurs shortly after the solar maximum at around 35Æ heliolatitude and consists of over
several Carrington Rotations stable coronal holes (>5 Carr. Rot.s). The diagonal disappears 2-3 years later at the helioequator.
Furthermore, the analysis results in a close relation between long-lived isolated coronal holes and the soft X-class flares.
INTRODUCTION
A good knowledge of coronal hole (CH) evolution in
time and in the different heliographic latitudinal belts is
relevant for the understanding of the decay and/or the
stability of large-scale solar magnetic fields (MFs) and
their extensions in the heliosphere. The first investigations of these uni-polar and low density areas in the solar atmosphere were most probably made by Waldmeier
[1956]. Coronal holes are the source of the high speed
solar wind. Moreover, the coronal temperature and density are low in such regions.
The largest areas with uni-polar fields are the southern and northern polar CHs. The polar CHs change their
shape during the solar activity cycle. They expand towards the heliographic equator during the decreasing
phase of the solar activity cycle, shrink back to the poles
in the ascending one, and disappear during the maximum activity phase for a certain time. The new polar
CHs appear with inverted magnetic polarity. In addition,
uni-polar field regions, isolated from the polar CHs, are
observed at any heliographic belt below 60 Æ latitude.
In a previous analysis peculiar features in CH occurrence were underlined [Hofer and Storini, 2002a]. Indications for a north/south asymmetry in the number distribution of the polar coronal holes and a 22-year periodicity in the longitudinal width of the extensions of the polar
CHs, i.e. MF regions observed below 60 Æ latitude being
well connected to the polar coronal holes, were found.
Using the CH catalogue, compiled mainly from the
HeI absorption line (1083 nm) measurements ([SanchezIbarra and Barraza-Paredes, 1992], and updated from the
NOAA-Boulder Web Pages), we here investigate the latitudinal distributions of long-lived CH occurrence (ages
>5 Carr.Rot.s) mainly during the sunspot cycles 21 and
22.
USED DATA CATALOGUE
Data for the CH occurrence [Sanchez-Ibarra and
Barraza-Paredes, 1992] were taken from the NOAA
(Boulder) Web pages to analyze the variation of the
CH heliographic coordinates from 1970.1 to 1995.4.
These data were mainly obtained by observing the HeI
absorption line (1083 nm) by the National Solar Observatory/Kitt Peak. The catalogue provides the central
location of the CHs in heliographic coordinates and the
longitudinal and latitudinal extensions (widths) of the
CH area. The CHs were even identified over several
Carr. Rot.s. using other data sources, such as the CH
contours from the H α synoptic charts.
The catalogue mainly lists two types of CHs:
i) The first class: EP-CHs (Extended Polar Coronal
Holes), consists of uni-polar MF regions observed below
the 60Æ heliographic latitude; they are well connected
with the polar CHs (i.e. they are extended polar CHs).
ii) The second class: I-CHs (Isolated Coronal Holes),
are found at any heliographic latitudinal belt below the
60Æ heliographic latitude and are therefore clearly
isolated from the polar coronal holes.
CP679, Solar Wind Ten: Proceedings of the Tenth International Solar Wind Conference,
edited by M. Velli, R. Bruno, and F. Malara
© 2003 American Institute of Physics 0-7354-0148-9/03/$20.00
234
BUTTERFLY DIAGRAM OF THE
CORONAL HOLES
of the EP-CHs do not seem to change so dramatically
from one to the next solar cycle as it can be seen in the
left panel of Figure 1.
The butterfly diagrams of extensions of the polar (EPCHs) and isolated (I-CHs) coronal holes are shown in
Figure 1. The dashed long vertical lines mark the solar
minima occurrence. The solar equator is shown by a
horizontal dashed line.
In the left panel of Figure 1, the butterfly diagramm
of the extension of the polar coronal holes (EP-CHs) is
shown. The horizontal width of the strings corresponds
to the observation time of the EP-CHs. The labeled short
dashed verticals mark the CHs that extend to the solar
equator in 1973, in 1984 and in 1994. The EP-CH reach
the equatorial regions about four years after the maximum, two to three years before the minimum. There are
no EP-CH during the maximum activity phase in 1980
and in 1990. There is a tendency that long-term EP-CHs
are observed below 50 Æ heliographic latitude.
In the right panel of Figure 1, the strings report the
central heliographic latitude of the I-CHs, as a function
of time (the horizontal length of the strings gives the total
observation time of the CHs). The dotted curve on top
of the distribution points out the external contour of the
I-CH distribution. The ellipses show two repeated large
regions without I-CHs. The dashed diagonals mark:
a) a selected region with long-lived I-CHs, appearing
after 1979 (sunspot cycle 21);
b) an example for an area without any I-CHs (sunspot
cycle 22).
From the external contour we see that it tends to follow
the solar activity level. Shortly after the solar maxima,
the widths are maximum as well. Furthermore, the solar
minima are in vicinity of the minimum widths of the
latitude-time distribution of the I-CHs. In other words,
an 11-year cycle characterizes such distribution.
In Figure 2, the latitudinal distribution of EP-CHs are
shown using a grey color. The I-CHs are represented by
dark strings (left: long-term I-CHs > 3 Carr. Rot.s; right:
long-term I-CHs >5 Carr.Rot.). The vertical dashed lines
mark the times of the solar minima occurrence. During
the time intervals 1980-1983 (south) and 1990-1992
(north) two diagonals of long-living I-CHs, starting at
around 35 Æ heliolatitude, shortly after the solar maximum, and ending close to the equator, two to three years
later, can be found in Figure 2. Each line consists of more
than six I-CHs with ages of more than 5 Carr.Rot.s.
In Figure 3, the latitudinal distribution of the solar optical flares associated with soft X-ray flares of X-class are
shown. The slopes of long-lived isolated coronal holes on
the selected diagonal (as shown in the panels of Figure
2) and the soft X-class flares are both steeper in the second time interval. They are therefore somehow related,
whereas the equatorward boundaries of the distribution
235
DISCUSSION AND SUMMARY
We investigated the coronal holes distribution from the
sunspot cycle 21 on, using the coronal hole catalogue
based on HeI (1083 nm) observations (Sanchez-Ibarra
and Barraza-Paredes, 1992).
The influence of the solar cycle evolution on the
envelop of the coronal hole space-time distribution is
clearly visible, for the polar as well as for the low-latitude
isolated sources of high speed solar wind.
The disappearance of the extensions of the polar coronal holes for a time period of about one year is expected
because even the corresponding polar coronal hole is not
formed, and consists for while of several regions with
different polarities (see, for instance Sanderson et al.
[2001] and Fox et al. [1998]). During the same time period the region around 55 Æ does not seem to be governed by quiet MF as is can be seen in the left panel of
Figure 1. Furthermore, few isolated CHs are observed at
high heliolatitude around solar activity maximum as it
can be seen in right panel of Figure 1. Regarding Figure 3, active X-ray flares are found to occur up to 45 Æ
heliographic latitude. Therefore, the region between 45 Æ
and 60Æ heliographic latitude during the maximum active
phase does not contain large quiet MF regions and not
even active areas. It looks like a belt between the two extremes. It would be interesting to analyse this region with
respect to the well-known Gnevyshev gap (e.g. Feminella
and Storini [1997] and references therein), during which
a reduction of the solar activity effects is found also in
the heliosphere (e.g. Storini and Felici [1994], Storini
[1995], Storini and Pase [1995], for early works; Storini
and Hofer [2001], Storini et al. [2002] for reviews).
Concluding, we remark that in at least two solar cycles
(n. 21 and n. 22), we identified a similar structure in the
latitude-time diagram of the isolated long-lived coronal
holes. The edge of the area starts at around 35 Æ heliolatitude, shortly after the solar maximum, and ends close
to the equator, two to three years later. The structure occurs after and below the above mentioned latitudinal belt
separating the two extreme areas. It is a diagonal region
consisting of over several Carrington Rotations stable
isolated coronal holes. More precisely, during the time
intervals 1980-1984 (south) and 1990-1992 (north) two
diagonals of long-living I-CHs emerged, being each one
characterized by more than six I-CHs (each with an age
of more than five Carr.Rot.s).
The found structure does not evolve symmetrically on
the northern and the southern solar hemispheres. The
Extension of the Polar Coronal Hole.
Isolated Coronal Holes.
90
1994
1984
1973
90
Central Heliographic Latitude
Central Heliographic Latitude
60
30
0
-30
-60
-90
75
80
85
90
60
30
0
-30
-60
-90
95
73 75
80
85
90
95
Year (after 1900)
Year (after 1900)
FIGURE 1. Butterfly diagrams of extensions of the polar coronal holes (left) and isolated coronal holes (right). The dashed
vertical lines in both panels mark the times of the solar minima occurrences. The short labeled line represent the CHs that extend
into the equatorial regions in 1973, 1984, 1994. The diagonals and the ellipses point out selected areas in the distributions. Both
figures were adapted from Hofer and Storini [2000].
Polar and Long-term(>5CR) Equatorial Coronal Holes
90
90
60
60
Central Heliographic Latitude
Central Heliographic Latitude
Polar and Long-term (>3CR) Equatorial Coronal Holes
30
0
-30
-60
-90
30
0
-30
-60
-90
1975
1980
1985
1990
1995
1975
Time
1980
1985
1990
1995
Time
FIGURE 2. Extensions of the polar coronal holes and long-term isolated coronal holes. a) left: long-term I-CHs > 3 Carr. Rot.;
b) right: long-term I-CHs >5 Carr.Rot. The I-CHs are represented by dark horizontal strings. The central latitude of the EP-CHs is
shown with filled grey small triangles. The dashed vertical lines mark the times of the solar minima occurrences.
fact that the structure is found first in the South than in
the North motivates two interpretations:
i) a reasonable 22-year periodicity (Hale cycle) in the
coronal hole occurrence;
ii) the found structure is closely related to the time
evolution of the nearby magnetic fields.
236
A north-south asymmetry is also found for the latitudinal extension of the I-CHs as reported by Hofer and
Storini [2002a], Storini and Hofer [1999]. The second interpretation deserves special attention because, from our
study, it results in a close relationship between long-lived
isolated coronal holes and soft X-class flares. In a parallel work we show that the extensions of the polar coronal hole and the sunspot areas are related in a similar
way, when their latitude-time distributions are consid-
FIGURE 3. Latitudinal distribution of the solar optical flares associated with soft X-ray flares of X-class. The figure is adapted
from Storini and Hofer [1999].
ered [Hofer and Storini, 2002b].
We conclude that the latitudinal distribution of the
equatorial and polar coronal holes should be regarded
more frequently for the understanding of the solar and
heliospheric evolution in time.
ACKNOWLEDGMENTS
Part of this work was supported by the Swiss National
Science Foundation (fellow grant 81BE-57318) and the
National Antarctic Research Program (PNRA) of Italy
in the frame of Science for Solar-Terrestrial Relations.
MYH also thanks ESA for the present research fellowship.
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