Solar Coronal Activity and Evolution of the Magnetic Field

SOLAR CORONAL ACTIVITY AND EVOLUTION MAGNETIC FIELD

OF T H E

E. E. Benevolenskaya 1,2, A. G. Kosovichev 2 and P. H. Scherrer 2

1Pulkovo Astronomical Observatory, St. Petersburg, Russia 2 W. W.Hansen Experimental Physics Laboratory, Stanford University, Stanford

ABSTRACT We discuss the role of the solar magnetic field in forming of stable activity structures during the current solar cycle 23. These structures are clustered in long-lived complexes of activity in the photosphere and are visible in the transition region and corona in an extreme ultraviolet irradiance as bright loops. We have represented the EUV data from SOHO/EIT in four wavelengths (171/~, 195.~, 284/~ and 304/~) in the form of coronal synoptic maps for 1996-2000. The distributions of intensity of the coronal lines are related to magnetic activity and show non-uniform distributions as a function of Carrington longitude and latitude. In the axisymmetrical case we have detected two waves of coronal activity in both hemispheres. The first, equatorward wave, is connected with complexes of sunspot activity. The second, polarward wave, reflects the giant loops structures which connect regions of polar magnetic field with the following parts of complexes of solar activity, and, probably, plays important role in the topological evolution of the solar magnetic field. INTRODUCTION Having observed sunspots Carrington (1863) suspected that they did not form randomly over solar longitudes. There are some longitudinal zones in which solar activity greater than in others. For description of this phenomenon term 'Active longitudes' is commonly used. It defines zones, where large sunspots (greater than 500 mph) reappear during long periods, more than several years (Vitinskii, 1982). There are also other terms for describing non-uniform longitudinal distributions of solar activity. Bumba (1987) introduced 'Magnetic active longitudes': zones of magnetic field concentration, 20 ~ - 40 ~ wide. Castenmiller et al. (1986) defined sunspot groups which show strong tendency in compact clusters as 'Sunspot nests'. 'Sunspot nests' are the same structures as 'Sonnenfleckenherd' discovered by Becker (1955). Bai (1990) suggested term 'Hot spots' because the Northern and Southern hemispheres may differ in the spatial organization of solar activity. To emphasize the close relation between the complexes of activity and regions of weaker fields Feyman and Hundhausen (1988) suggested term 'Evolving magnetic structures'. The differences in the described terms can be explained by different methods of investigation and analysis. Investigations of rotation of reappearing complexes of solar activity show that the average rotation rate is approximately equal to Carrington rotation rate (with the synodic period of 27.28 days) (Bumba and Howard, 1969; Gaizauskas et al., 1983; Benevolenskaya et al., 1999). Therefore, it is preferable to consider of the evolution of solar activity in the Carrington coordinate system and use terms 'Active longitudes' or 'Active longitudinal zones'. EIT SYNOPTIC MAPS For studying the coronal structure and its evolution with the solar cycle we have constructed coronal synoptic maps using the Extreme Ultraviolet Imaging Telescope (EIT) data obtained with 4 EUV filters. The FeIX,X

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Fig. 1. Synoptic maps of the solar magnetic field (left panel) and the EIT 195A/171/~, line ratio (right panel) for Carrington Rotations 1911 to 1936 during the activity minimum between solar cycles 22 and 23.

images (171A) display unresolved emission which is present over the most quiet Sun, including coronal holes. The Fe XII images (195A) are dominated by closed magnetic field regions. All the hotest active region loops are visible in this wavelength but not as clear as in Fe XV (284/~) line. The Fe XV images are dominated by the hot loops. The HeII images (304A) are dominated by the transition region network structure and prominences on the solar limb. Synoptic maps of EUV images in three lines Fe and HeII (171/~, 195/~, 284/~ and 304/~) are represented by values of the line intensity at the central meridian (Ill). We have constructed the synoptic maps for Carrington rotations from CR1911 to CR1967 for Fe and HeII lines, and also for the ratio of Fe XII (195 /~) and Fe IX,X (171 /~) lines. The resolution of these maps is 1~ in both longitude from 0~ to 360 ~ and latitude from-83 ~ to 83 ~ The EIT images provide estimates of coronal temperatures from 0.6 to 2 MK (Moses et al., 1997). The EIT estimates of coronal temperature are based on the ratio of the intensities in different Fe lines. Generally, the temperature diagnostics from the fluxes observed in the three channels is an ill-posed mathematical problem. However, the ratio of the Fe XII and Fe IX,X fluxes only weakly depends on plasma density because both emission lines come from ionized states of Fe. Therefore, the flux ratio provides the information about the temperature distribution in the corona. The ratio of the Fe XII (195 /~) and Fe IX,X (171 ~) fluxes is predominantly sensitive to coronal structures with the plasma temperature from 1 to 2 MK. The ratio maps generally outline closed field regions. Active regions are seen as the hottest structures in the ratio images. Coronal holes are clearly defined as cold, open magnetic field regions in these images. Both the magnetic (from Kitt Peak Observatory) and coronal (I195/I171) synoptic maps during the transition period from cycle 22 to cycle 23 and beginning of cycle 23 are shown in Figure 1. The coronal synoptic maps (right panel) clearly show the same active longitudes as the magnetic maps. The large-scale stable

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Solar Coronal Activity and Evolution of the Magnetic FieM longitudinal patterns are better seen in the coronal maps than in the magnetic maps. During the investigated period the magnetic structure of the Sun was determined by the interaction between the 'old' and 'new' magnetic fluxes: the 'old' flux was concentrated in two active longitudinal zones, and most of the initial 'new' flux emerged in the same zones (Benevolenskaya et al., 1999). It should be emphasized that the distribution of I195/I171 during the period CR1911-CR1936 characterizes the evolution of coronal structures heated up to 1-2 MK (Figure 1, right panel). The brightest structures in the synoptic maps correspond to the hottest loops. They coincide with sunspot complexes of activity at the photospheric level. The sunspot complexes typically display bipolar magnetic field structures which are correspond to '~-loops' of the toroidal component of the magnetic field. Thus, the d i s t r i b u t i o n of t h e coronal t e m p e r a t u r e is n o n - a x i s y m m e t r i c a l a n d this is a consequence of t h e significant n o n - u n i f o r m longitudinal d i s t r i b u t i o n of solar m a g n e t i c activity or t h e existence of 'Active l o n g i t u d e s ' . CORONAL ACTIVITY WAVES Secchi (1877) discovered migration of the zone of polar prominences poleward during the rising phase of the solar cycle. Mr. and Mrs. L. d'Azambuja (1945) have made a general study of evolution of prominences and their movements using Ha synoptic maps of the Meudon Observatory (1919-1937). They identified motion of the filaments toward the poles and found that Sp6rer's law applies to the "starting-points" of the prominences. Waldmeier (1957) confirmed the existence of poleward waves of activity. He observed them in coronal emission of the green line (5303/~) using data from Arosa Observatory. He obtained the first coronal synoptic maps above the solar limb (since 1938) as a function of latitude (from-90 ~ to 90 ~ and time, and found that the green line emission is related to sunspots and faculae. Leroy and Trellis (1974) have drawn synoptic maps for three solar cycles since 1943 for the red (6374/~) and green coronal lines as a function of latitude (from-60 ~ to 60~) and time, and compared with the sunspot activity. Using observations made at Lomnincky Stit over 1967-1986, Rusin, Rybansky and Minarovjech (1990) confirmed the existence of the equatorward and poleward coronal activity waves. Altrock (1997) obtained axisymmetrical distributions of intensity of the green line at a height of 0.15 solar radii for the period 1984-1996, and found only the equatorward wave of activity. However, in the distribution of temperature he found enhanced coronal heating migrating poleward (Altrock, 1998). These observations inspired suggestions that the polar waves of activity might be associated with another dynamo wave in the convection zone and that, in fact, the poloidal field is reversed by the alpha-effect rather than by the surface diffusion (Makarov et al. 1987; Belvedere et al. 1991). It has also been found that both low- and high-latitude waves of solar activity may last longer the l 1-year sunspot cycle, forming so-called "extended solar cycle" (e.g. Altrock, 1997). DETECTION OF THE CORONAL ACTIVITY WAVES USING EIT/SOHO DATA For the detection of the coronal waves we used the coronal synoptic maps for Carrington rotations from CR1911 to CR1967, during June 28, 1996 - September 30, 2000 (Benevolenskaya et al., 2001). To obtain the latitudinal distribution we averaged the synoptic maps over longitude, and plotted as a function of latitude (0) and time (t). An example of the latitudinal distribution of the intensity in line 195/~ EUV is shown in Fig. 2(a). This map shows the evolution of the axisymmetrical component, and correspond to the well-known "butterfly" sunspot diagram. For comparison, we plotted the azimuthally averaged distributions of the unsigned (absolute) magnetic flux, IBlll (Fig. 2b) obtained from the Kitt Peak Observatory synoptic magnetic maps. The dashed curves show the location of the zonal magnetic neutral lines separating magnetic polarities at high latitudes. These neutral lines reflect position of Bil(t, 0 ) = 0 for magnetic field averaged over longitude and separate the polar magnetic field formed during the previous solar cycle from the magnetic field emerging during the current cycle. When the neutral lines reach the poles (usually around the sunspot maxima) this results in magnetic polarity reversal at the poles. The IBlll map, which basically shows the location of sunspots and active regions, reveals the familiar butterfly diagram: the sunspot zones of the current solar cycle start at about 30~ latitude in mid 1997, and then gradually migrate towards the equator as the cycle progresses.

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Fig. 2. The azimuthally averaged intensity of the solar corona as a function of latitude and time in the 195/~, EUV line b) IBII I. The dashed curves show the high-latitude magnetic neutral lines.

In the coronal EUV maps, we see in each hemisphere two sets of migrating structures: low-latitude structures which migrate towards the equator following IBIII, and high-latitude structures which migrate towards the poles parallel to the magnetic neutral lines. The coronal structures associated with the high-latitude waves are easily identified on the EUV synoptic charts as longitudinally extended bright structures at 50 ~ - 70~ latitude. The coronal structures associated with the high-latitude waves are easily identified on the EUV synoptic charts as longitudinally extended bright structures at 50 ~ - 70 ~ latitude. On the solar limb these structures are seen as giant loops connecting the following parts of active regions with the polar regions. These limb loops are better seen in 284/~ line ( Benevolenskaya et al.,2001). We conclude t h a t t h e bright coronal s t r u c t u r e s d e t e c t e d in t h e E U V d a t a from S O H O / E I T , which m i g r a t e d to t h e poles d u r i n g t h e rising phase of t h e solar cycle, were f o r m e d by density e n h a n c e m e n t s in t h e p o l e w a r d footpoints of m a g n e t i c field lines c o n n e c t i n g the m a g n e t i c fields of the following p a r t s of active regions with t h e polar field. T h e s e giant m a g n e t i c loops connect the toroidal field of t h e new solar cycle with t h e polar poloidal field formed d u r i n g the previous cycle (Benevolenskaya et al., 2001). This work was partly supported by JURRISS Program NASA NRA 98-OSS-08 and the SOI/SOHO NASA contract NAG5-8878 to Stanford University, and by the Russian Federal Programme 'Astronomy', Grant 1.5.3.4. REFERENCES Altrock, R.C. 1997, Sol. Phys., 170, 411 Altrock, R.C. 1998, in: Synoptic Solar Physics, eds. K.S. Balasubramaniam, J.W. Harvey, & D.M. Rabin, ASP Conf. Set., v. 140, 339 Bai, T., ApJ, 364, L17-L20, 1990. Becker, U., Z. Astrophysik, 37, 47-66, 1955. Belvedere, G., Lanzafame, G. & Proctor, M.R.E. 1991, Nature, 350, 481 Benevolenskaya, E. E., Hoeksema, J.T., Kosovichev, A.G., and Scherrer, P.H., ApJ, 517, L163-L166, 1999. Benevolenskaya, E.E., Kosovichev, A.G., Scherrer, P.H., 2001, ApJ, 554, L107 Bumba, V., Bull. Astron. Inst. Czechosl., 38, No 2, 92-101, 1987. Bumba, V., Howard, R., Solar Phys., 7, 28, 1969. Carrington, R.C., Spots on the sun, London, 1863. Castenmiller, M.J.M., Zwaan, C., and van der Zalm, E.B.J., Solar Phys., 105, 237-255, 1986. d'Azambuja, L.,1945, ApJ, 101,260 Feynman, J. and Hundhausen, A.J., J. Geophys. Res., 99, 8451-8464, 1994. Gaizauskas, V, Harvey, K.L., Harvey, J.W., and Zwaan, C., ApJ, 265, 1056-1065, 1983. Leroy, J.L., Trellis, M., 1974, Astron. Astrophys., 35, 283 Makarov, V.I., Ruzmaikin, A.A., & Starchenko, S.V. 1987, Sol. Phys., 111,267

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