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Evolution of the ‘gorgeous’ coronal hole
EVOLUTION
OF THE 'GORGEOUS'
CORONAL
HOLE
A. Takeda 1'2 and S. Kubo 2
1Solar Physics Research Corporation, ~720 Calle Desecada Tucson, AZ 85718, USA 2Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa, 229-8510, Japan
ABSTRACT A distinct (or 'gorgeous') coronal hole was observed from October to December, 2000, at around 180 ~ in Carrington longitude. Its formation, evolution, and rotation rates are discussed based on an analysis of Yohkoh Soft X-ray Telescope (SXT) images and synoptic maps of the Kitt Peak magnetic field.
INTRODUCTION Extensive studies on coronal holes have been done since the mid-1970s, after the advent of Skylab. The famous boot-shaped coronal hole, 'CHI', observed during the Skylab mission ( J u n e - September, 1973), was analyzed in detail by Timothy et al. (1975), later explained by the model of Wang & Sheeley (1990, 1993), and is still referred to as a prototype for coronal holes. Compared with the 1970s when CH1 was observed, we now obtain images with much better temporal and spatial resolutions and with a greater variety of wavelengths. It is therefore worthwhile to revisit detailed analysis of an individual coronal hole to verify the current understanding of coronal holes. As a first step, in this short paper we will present some results from a morphological study of a coronal hole observed from October to December, 2000. This coronal hole had an elongated shape in the north-south direction across the equator and maintained its outline for at least three Carrington Rotations (C.R.) (1968-1970, see Figure 1). One may describe it as 'gorgeous' in the sense that it took a boot-like shape, reminiscent of CH1, during its October rotation, and transformed into a chevron-like shape in its December rotation. We generated a series of synoptic maps of the soft X-ray intensities obtained with the Yohkoh SXT (for C.R. 1964-1972) and closely compared with the corresponding maps of the photospheric magnetic fields obtained at Kitt Peak. RESULTS AND DISCUSSION Comparison of SXT and Kitt Peak synoptic maps during the same rotation shows that the coronal hole area defined in SXT was located roughly over the central part of a large negative unipolar region in the magnetic field image. Looking over consecutive magnetic field maps for a few rotations before and after the period shown in Figure 1, it turns out that this large unipolar region was comprised of three parts which originated from different groups of active regions which emerged during C.R. 1964-1965. The first group of active regions were those which emerged in the range of 180~ ~ in Carrington longitude and 0~176 in heliographic latitude. The negative polarity part of their remnants were merged together, moved westward with a velocity near to differential rotation rate at their latitude, and came to form the equatorial part of the coronal hole in C.R. 1968. The two other groups of active regions were located at latitudes 25~ and 15~ and were in the range 90~ ~ in longitude. The latter two remnants moved eastward on synoptic maps due to their differential rotation and formed the high-latittlde parts of the coronal hole in each hemisphere. - 343 -
A. Takeda and S. Kubo
Fig. 1. Left column : Yohkoh SXT images of the 'gorgeous' coronal hole for three consecutive central meridian passages. Middle column : SXT synoptic maps for C.R. 1968-1970 (from top to bottom). Dashed lines show the approximate axis of the coronal hole as of C.R. 1968, and the solid curves represent the predicted location of the axis after one or two Carrington rotation periods assuming the differential rates given in Astrophysical Quantities (Allen 1973). Right column : synoptic maps of the Kitt Peak magnetic field intensities. Since C.R. 1968, when the outline of the coronal hole first became clear in the SXT images, the coronal hole rotated with velocities particular to their latitude and finally broke into three pieces, disappearing in early January, 2001. The rotation rate of the SXT boundaries at low latitudes ( < 15 ~ was near to the differential rate, in accordance with the equatorial part of the underlying magnetic unipolar region. However, at higher latitudes, the rotation rate was significantly faster than the differential rate, as indicated in the SXT synoptic maps shown in Figure 1. It is known that CH1 rotated quasi-rigidly, i.e., its rotation rate at high latitude was faster than the differential rate (Timothy et al. 1975). According to Wang & Sheeley (1993), this property is typical for a coronal hole, called 'polar extension', and is often observed in the declining phase of the solar cycle. However, the hole analyzed here appeared around solar maximum, when positive polarities were still maintained at the north pole. Thus it has a different origin from CH1. The same rotational property observed here may require a different mechanism from the quasi-rigid rotation of coronal holes. In the case of the 'gorgeous' hole, the western side of the underlying unipolar region appeared to be more effectively reinforced by the nearby active region remnants, and this seemed to cause a drift of the coronal hole boundary away from the unipolar regions above which the hole had initially been located. REFERENCES Allen, C.W., Astrophysical Quantities, 3rd ed, Athlone, London, (1973). Timothy, A.R., Krieger, A.S., & Vaiana, G.S., Solar Physics 42, 135 (1975). Wang, Y.-M., & Sheeley, N.R.Jr., Astrophysical Journal 365, 372 (1990). Wang, Y.-M., & Sheeley, N.R.Jr., Astrophysical Journal 414, 916 (1993).
- 344 -
OF THE 'GORGEOUS'
CORONAL
HOLE
A. Takeda 1'2 and S. Kubo 2
1Solar Physics Research Corporation, ~720 Calle Desecada Tucson, AZ 85718, USA 2Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa, 229-8510, Japan
ABSTRACT A distinct (or 'gorgeous') coronal hole was observed from October to December, 2000, at around 180 ~ in Carrington longitude. Its formation, evolution, and rotation rates are discussed based on an analysis of Yohkoh Soft X-ray Telescope (SXT) images and synoptic maps of the Kitt Peak magnetic field.
INTRODUCTION Extensive studies on coronal holes have been done since the mid-1970s, after the advent of Skylab. The famous boot-shaped coronal hole, 'CHI', observed during the Skylab mission ( J u n e - September, 1973), was analyzed in detail by Timothy et al. (1975), later explained by the model of Wang & Sheeley (1990, 1993), and is still referred to as a prototype for coronal holes. Compared with the 1970s when CH1 was observed, we now obtain images with much better temporal and spatial resolutions and with a greater variety of wavelengths. It is therefore worthwhile to revisit detailed analysis of an individual coronal hole to verify the current understanding of coronal holes. As a first step, in this short paper we will present some results from a morphological study of a coronal hole observed from October to December, 2000. This coronal hole had an elongated shape in the north-south direction across the equator and maintained its outline for at least three Carrington Rotations (C.R.) (1968-1970, see Figure 1). One may describe it as 'gorgeous' in the sense that it took a boot-like shape, reminiscent of CH1, during its October rotation, and transformed into a chevron-like shape in its December rotation. We generated a series of synoptic maps of the soft X-ray intensities obtained with the Yohkoh SXT (for C.R. 1964-1972) and closely compared with the corresponding maps of the photospheric magnetic fields obtained at Kitt Peak. RESULTS AND DISCUSSION Comparison of SXT and Kitt Peak synoptic maps during the same rotation shows that the coronal hole area defined in SXT was located roughly over the central part of a large negative unipolar region in the magnetic field image. Looking over consecutive magnetic field maps for a few rotations before and after the period shown in Figure 1, it turns out that this large unipolar region was comprised of three parts which originated from different groups of active regions which emerged during C.R. 1964-1965. The first group of active regions were those which emerged in the range of 180~ ~ in Carrington longitude and 0~176 in heliographic latitude. The negative polarity part of their remnants were merged together, moved westward with a velocity near to differential rotation rate at their latitude, and came to form the equatorial part of the coronal hole in C.R. 1968. The two other groups of active regions were located at latitudes 25~ and 15~ and were in the range 90~ ~ in longitude. The latter two remnants moved eastward on synoptic maps due to their differential rotation and formed the high-latittlde parts of the coronal hole in each hemisphere. - 343 -
A. Takeda and S. Kubo
Fig. 1. Left column : Yohkoh SXT images of the 'gorgeous' coronal hole for three consecutive central meridian passages. Middle column : SXT synoptic maps for C.R. 1968-1970 (from top to bottom). Dashed lines show the approximate axis of the coronal hole as of C.R. 1968, and the solid curves represent the predicted location of the axis after one or two Carrington rotation periods assuming the differential rates given in Astrophysical Quantities (Allen 1973). Right column : synoptic maps of the Kitt Peak magnetic field intensities. Since C.R. 1968, when the outline of the coronal hole first became clear in the SXT images, the coronal hole rotated with velocities particular to their latitude and finally broke into three pieces, disappearing in early January, 2001. The rotation rate of the SXT boundaries at low latitudes ( < 15 ~ was near to the differential rate, in accordance with the equatorial part of the underlying magnetic unipolar region. However, at higher latitudes, the rotation rate was significantly faster than the differential rate, as indicated in the SXT synoptic maps shown in Figure 1. It is known that CH1 rotated quasi-rigidly, i.e., its rotation rate at high latitude was faster than the differential rate (Timothy et al. 1975). According to Wang & Sheeley (1993), this property is typical for a coronal hole, called 'polar extension', and is often observed in the declining phase of the solar cycle. However, the hole analyzed here appeared around solar maximum, when positive polarities were still maintained at the north pole. Thus it has a different origin from CH1. The same rotational property observed here may require a different mechanism from the quasi-rigid rotation of coronal holes. In the case of the 'gorgeous' hole, the western side of the underlying unipolar region appeared to be more effectively reinforced by the nearby active region remnants, and this seemed to cause a drift of the coronal hole boundary away from the unipolar regions above which the hole had initially been located. REFERENCES Allen, C.W., Astrophysical Quantities, 3rd ed, Athlone, London, (1973). Timothy, A.R., Krieger, A.S., & Vaiana, G.S., Solar Physics 42, 135 (1975). Wang, Y.-M., & Sheeley, N.R.Jr., Astrophysical Journal 365, 372 (1990). Wang, Y.-M., & Sheeley, N.R.Jr., Astrophysical Journal 414, 916 (1993).
- 344 -