NEW RESULTS OF SOLAR ACTIVITY AND MAGNTIC FIELD ON THE SUN (REVIEW)

NEW RESULTS OF SOLAR ACTIVITY AND MAGNTIC FIELD ON THE SUN (REVIEW) Elena E. Benevolenskaya 1, 2 1 Stanford University, W.W. Hansen Experimental Physics Laboratory, Stanford, CA 945, USA, e-mail:elena@sun.stanford.edu; 2 Pulkovo Astronomical Observatory, St. Petersburg, 19614, Russia Abstract. For the last decade the solar space missions (e.g. Yohkoh, Coronas, Ulysses, SOHO) make a progress in the investigations of the solar activity in Photosphere, Corona and solar wind. The Hinode and Stereo space labs continue the progress of previous missions and extend our knowledge about the solar activity, an evolution of vector magnetic field, a structure of photosphere, chromosphere and corona. Upcoming Solar Dynamics Observatory will investigate the vector magnetic field, corona and solar irradiance with a purpose to understand the nature of the solar cycle on the base of the highresolution images. In this paper, I review the recent and most important results of the investigation of the solar activity from the interior to the corona and their relationship to the dynamo theory. Introduction The last decade and present of observations of the Sun from Space are called The golden Ages of Solar Physics. And it is not surprised due to the new data and results. If we go to the ADS abstract search system and type, for example, keyword SOHO (Solar and Heliospheric Laboratory) in abstract words, we get more than 5 references. Investigations of the Sun from the Space enable see our Sun in the invisible wavelength which is no observable from the ground Observatories, such as Extreme Ultraviolet (EUV) and X-ray. Here is a brief list (not the whole) of the most important space mission contributed to solar cycle studies: YOHKOH (31.8.199-14.12.21), CORONAS-F (31.7. 21-6.12.25), SOHO started on 2 December 1995, ULYSSES (1.6.199-3.3.28), TRACE (April 1998-current). And, it is new space laboratories: STEREO, which launched on October 25, 26 and HINODE (Solar-b) has been observing the Sun since 22 September 26. Each mission has its own signature. Yohkoh has measured the X-ray Solar corona by Soft X-ray Telescope (SXT) with two filters: Al 5-12 A and Al/Mg/Mn (AlMg) 6-13 A. Examples of images for filter AlMg (Tsuneta et al. 1991). Figure1. Left panel: Solar X-ray coronal image from YOHKOH shows the plasma heated up to the 3MK (bright features). Middle panel: EUV corona from SOHO/EIT telescope. Right panel: Line-of-sight component of the magnetic field measured by MDI (Michelson Doppler Imager) on the board SOHO (white color shows a positive polarity, a negative polarity is marked by black). Large-scale hottest features were detected and observed several times in the solar corona in the hightemperature Mg XII line (T= 5 2 MK, Tmax = 1 MK) with the soft X-ray telescope of the SPIRIT 34

instrumentation complex onboard the CORONAS-F spacecraft. Zhitnik et al. (23) phenomenologically described such features as long-living plasma bodies with bright orbed cores, localized at a height of.1.3 solar radii, and darker legs, probably giant loops, connecting the cores with active regions. Ulysses spacecraft flied over the poles of the Sun, climbed its maximum latitude of 8.2 degrees North on 31 July 1995 with the last passing on 14 January 28. Figure 2 shows the decreasing of the temperature distribution above the solar Polar Regions. Figure 2. The temperature of the Sun s polar coronal holes as measured by the SWICS instrument on board Ulysses (Credit to Ulysses). Among the numerous results of HINODE let s look at the X-ray jets which are observed in the solar polar atmosphere (e.g. Kulhane et al. 27; Filippov, Golub and Koutchmi 27; Savcheva 27). Recent results (Shimojo et al. 27; Shimojo 28) reveal that over 7% of jets occur in mixed polarity regions and X-ray jets in the polar coronal hole are not always associated with the kg-patches (kilo Gauss magnetic field). Some X-ray jets are associated with very weak magnetic field. And, the jets are strongly associated with the emerging/cancelling magnetic flux. The series of wonderful EUV images from two satellites with fine resolution of A (head) and B (behind) of the Solar TErrestrial RElations Observatory (STEREO) provide 3D reconstruction of the coronal loops and corona (Aschwanden et al. 28a, 28b). SOHO (Solar and Heliospheric observatory) is the leader in the list of the space observatories not only due to the numerous publications; SOHO demonstrates a successful international collaboration inside the SOHO teams and collaborations with other teams among the solar and the heliospheric society. Birth of the Solar Cycle "On January 4, 28, a reversed-polarity sunspot appeared and this signals the start of Solar Cycle 24" (David Hathaway of the Marshall Space Flight Center, HASA deadlines) Figure 3. Left image: Sun in continuum. Right image: MDI (Michelson Doppler Imager) magnetogram. White color shows a positive polarity, black is marked a negative polarity. 35

The sunspot, 1 NOAA 981 at the latitude N o and at the carrington longitude (L) equals 246 o, has emerged in the same longitudinal region as sunspot NOAA 98 of the old polarity (S6 o, L239 o ). O n March 28, 28 the cycle 23 returned: three big sunspots ( 1 NOAA 987, NOAA 988, and NOAA 989) appeared and they were all old cycle spots. After that, the Sun was practically blank. Usually, the beginning of the new cycle corresponds to the appearance of sunspots with a new polarity at the latitude 25 o -35 o, and the all sunspots of the old polarity, that are located close to the equator, are disappeared. But, there is a period of the overlapping of two cycles when both sunspots with the old and the new polarity coexist on the Sun. On October 5, 28 the sunspot (NOAA 13) of the new cycle appeared in the southern hemisphere at 23 o latitude and at 222 o carrington longitude (L). It has disappeared rapidly, and next several days we observe plages instead of sunspot in the same place. On 11 October 28, small sunspot (NOAA 14) emerged at S8 and L188 and one more (NOAA 15) at higher latitude in the North (N26, L116). During the next day two plages 13 NOAA (S23, L222) and NOAA 14 (S8, L188) have coexisted with the sunspot (NOAA 15) in the North which coordinates are N26 o and L116 o. The tendency of the solar cycle to appear at the preferred longitudes was found by Benevolenskaya, Hoeksema, Kosovichev and Scherrer (1999) and Bumba, Garcia, and Klvana (2). Figure 4. Left panel: Synoptic maps of the solar magnetic field for Carrington rotations (CR) 1911 1934 derived from the SOHO/MDI magnetograms during the activity minimum between cycles 22 and 23. Values of the line-of-sight component of the magnetic field are represented in light and dark colors for positive and negative polarities, respectively. The gray scale shows magnetic field in the range from -1 to 1 G. Right panel: More detailed synoptic magnetic maps for Carrington rotations 1916 1923. The plots indicate the NOAA sunspot number for selected active regions. Bins are 1 o square and extend to latitude ±65 o. (Benevolenskaya et al., 1999). The transition from old (cycle 22) to new flux (cycle 23 ) is largely concentrated in the interval CR 1916-1923, which is shown in more detail in Figure 4 (right panel). One major zone occurred at longitudes 24 o 28 o and lived over a year. This longitude was active from CR 1911 to CR 1917 (region 1 NOAA 86) 1 NOAA sunspot region number reached 9999 and rolled over to on 16 June 22. 36

in the southern hemisphere and from CR 1916 to CR 1918 (NOAA 7997) in the northern hemisphere. This active zone of old flux, which was gradually decaying and migrating westward, reactivated in CR 1923, when a new-cycle complex of solar activity (NOAA 846) emerged in the southern hemisphere at longitude ~28 o. Another interesting strong active zone developed at 16 o 2 o and drifted slowly westward. This zone of old-cycle flux first appeared in the southern hemisphere in CR 1916 (NOAA 7999) and persisted in CR 1917 (87A); then in CR 1918 new cycle flux emerged in this zone at latitude 2 o S. During CR 192 one of the last regions of the old cycle appeared at longitudes 2 o 24 o in the northern hemisphere (NOAA 82) and decayed over the next two rotations; then, in CR 1923 a new cycle region appeared in the same hemisphere but at higher latitudes. In CR 192 and 1921 both new (821 and 827) and old (82 and 829A) fluxes existed at the same longitude of.21 o, but in different hemispheres. We see a similar coexistence of new (86) and old (85) regions in the southern hemisphere in CR 1917. In both active longitude zones the old-cycle magnetic flux was replaced by new cycle flux. Solar magnetic flux emerging There are several types of the solar magnetic flux emerging. The first is the bipolar emerging which associated with the activity complexes. It is observed in ephemerical regions (Harvey and Zwaan 1993; Hagenaar 2), and in small magnetic loop structure in the quiet Sun, according the Hinode data (Centeno, et al. 27). The next, it is a unipolar magnetic flux emerging which related to polar faculae. Mechanism of forming of the unipolar magnetic flux emerging suggested by Lamb, DeForest, Hagenaar, Parnell and Welsh (28) is represented in Figure 5. Figure 5. Two possible scenarios for detecting negative (black) flux while hiding the positive (white) component. (a) The cross section of the positive end of the flux tube is larger, so the weaker average field does not exceed the tracking detection threshold. (b) Each end of the flux tube is not detectable by itself, but if two or more likepolarity ends come together, the average field strength can exceed detection limitations for that polarity only. In this case the tubes are not newly emerging, so the positive ends could be arbitrarily far away at the time of detection (Figure 4, Lamb et al. 28). Helioseismology points out on the multiple fluxes emerging. However, the magnetic flux rate reveals two or three peaks of intensive flux emergence; each was about one day long. It appears that the active region was formed by multiple magnetic flux emergence events. (Kosovichev, Duvall 28). Figure 6. Model of the Emerging Flux Region. White and dark ovals represent the footpoints observed with Stokes V. The solid lines stand for magnetic tubes above the photosphere and the dashed lines for those below the Photosphere. An emerging flux region (EFR) is a young active region (AR) where magnetic flux loops emerge from underneath the photosphere. (Otsuji et al., 27). Figure 6 (i.e. Figure 9 in Otsuji et al., 27) shows a model of this EFR. The temporal evolution of the observed flux emergence will be as follows: (1) A magnetic flux tube emerges from the intergranular lane (17:54 UT). (2) The flux tube splits along its axis into two parts (flux tubes 1 and 2). (3) After the emergence of flux tubes 1 and 2, newly emerged flux tubes appear (flux tubes 3 6). These emergences of flux will be these observed by Pariat et al. (24) and simulated by Isobe et al. (27). One remarkable new finding from Hinode is the discovery of ubiquitous horizontal magnetic fields in the quiet internetwork regions (Lites et al. 27). The stronger horizontal fields occur separately from the 37

vertical fields. The vertical fields occur mainly in the intergranular lanes. The horizontal fields occur over the bright granules, but avoid the brightest portions of the granules. They most commonly occur at the outer edges of the granules. Horizontal fields are not associated with the stronger network elements; they are a phenomenon of the internetwork only. Polar magnetic field and dynamo theory The polar magnetic fields on t he Sun have been an attractive subject for solar researches since Horace and Harold Babcocks measured them in solar cycle 19 ( Babcock and Babcock 1955). One of the remarkable features of the polar magnetic fields is their reversal during the maxima of 11-year sunspot cycles (Babcock and Livingston 1958; Babcock 1959). To understand the origin of the polar magnetic field reversals many investigators employed the mean-field dynamo theory (e.g. Dikpati et al, 24). But, now, the question is raised. What is a new we know about the polar magnetic field? latitude (deg) Proceedings of the 7th International Conference "Problems of Geocosmos" (St. Petersburg, Russia, 26 May 28) 6 4A -6 a) 171A 6-6 6-6 b) 195A Figure 7. Azimuthally averaged intensity of the solar corona as a function of latitude and time in the EUV lines: (a) 4 A, (b) 171 A, (c) 195 A, and (d) 284 A, and the corresponding line-of-sight photospheric magnetic field values: (e) B and (f) B (red shows the field of the positive polarity, and blue the negative polarity). The dashed curves show the high-latitude magnetic neutral lines. Study of the EUV from SOHO/EIT and the X-ray from YOHKOH data revealed a large scale connectivity in the corona between polar regions and the following parts of complexes of solar activity in the rising phase of the solar cycle (Benevolenskaya, Kosovichev, Scherrer 21; Benevolenskaya, Kosovichev, Lemen, Slater, Scherrer 22). In the longitudinally averaged coronal EUV maps (Figure 7), we see in each hemisphere two sets of migrating structures: low- latitude structures that migrate toward the equator following B c) and high-latitude structures that migrate toward the poles parallel 284A 6 to the magnetic neutral lines. However, these coronal structures are located 15 o 2 o higher in latitude than the neutral line. In the EUV data, the polar branches of coronal activity started in 1997 almost simultaneously with the equatorial branch and reached the -6 d) lower and upper boundaries of our synoptic maps (±83 o ) in early 2. The polar branches are easily identified in 4, 171, and 195 B 6 A maps. The bright coronal structures detected in the EUV data from SOHO/EIT, which migrated to the poles during the rising phase of the solar cycle, were formed by density enhancements in the poleward footpoints of magnetic field lines connecting the -6 e) magnetic fields of the following parts of active regions with the B polar field (Figure 8). It was suggested that giant coronal loops 6 together with meridional circulation and turbulent diffusion play very important role in polar magnetic field reversals. Part of the magnetic flux from mid-latitude goes to plasma heating inside these loops (Figure 8). After the polar magnetic field reversals, the -6 f) situation is changed. The transequatorial loops connected the following parts of the activity complexes or sunspot complexes are 1997 1998 1999 2 year prevail (Figure 9). Gopalswamy, Lara, Yashiro, Howard (23) proposed that with these closed configurations of the magnetic field connecting coronal mass ejections (CMIs) associated the following parts of complexes of solar activity with the open magnetic flux of polar regions. And, CMIs may be really an important mechanism of the magnetic field decay in the polar reversals. Moreover, according to Von Steiger, Zurbuchen, Kilclenmann (26), the number of CME displays latitude dependence: polar CME s are greater on the rising phase of solar cycle. 38

Coronal topology before the polar magnetic field reversals Figure 8. Left panel: Soft X-ray image from Yohkoh spacecr aft. Right panel: correspondent topology of the magnetic field. Lc- is a Carrington longitude of the central meridian of the Sun. Figure 9. Left panel: Topology of the magnetic field. Right panels: EUV image of Fe IX, X (171 A o ) from /SOHO/EIT (upper) and MDI/SOHO (bottom) images. Fisk and Schwadron (21) suggested that the polar magnetic field reversals occurred because of the diffusion of open magnetic field lines on the solar surface (due to transport and decay) that were reconnected with closed loops. Cohen, Fisk, Roussev, Toth and Gombosi (26) have considered the two-dimensional transport model of open magnetic flux on the surface of the Sun. The diffusion process represents: 39

1) diffusion of the filed line footpoints and 2) diffusion due to reconnection of open field lines with closed loops. They demonstrated that the rate of emergence of flux on the photosphere can control the magnitude of meridional flow. But, they found that the effect of diffusion due to magnetic reconnection is significant for the case of structured magnetic configuration (solar minimum conditions) and it small for the case of unstructured magnetic configuration (solar maximum conditions). Abramenko, Fisk, Yurchishin (26) found that the coronal hole which forms after the polar magnetic field reversals (22-23) displays the local minimum for the rate of emergence of new magnetic flux. Dipole emergence rate in quiet sun exceeds twice that in Coronal holes. However, Hagenaar, Schrijver, and DeRosa (28) have found that the emergence frequency of ephemeral regions does not depend on the presence of coronal holes. Instead, the frequency of ephemeral regions is found to depend on the degree of flux imbalance in the photosphere. This explains the observations by Abramenko, Fisk, & Yurchyshyn (26) that fewer ephemeral regions emerge in quiet Sun inside coronal holes, than outside coronal holes. The surface-diffusion or transport models explain the polar magnetic field reversals as a result of turbulent diffusion, meridional circulation and differential rotation (e.g. Wang, Sheeley and Nash, 1991; Schrijver and Title 21). Schrijver, De Rosa, Title (22) also point out that the transport process leads to a transport of closed connections from equator to pole even as open solar flux is transported from the high latitudes to the equator. Fox, McIntosh and Wilson (1997) described the evolution of the large-scale fields and their association with polar coronal holes. Their question was whether the polar fields resulted from the local polar dynamo or not. There is no a certain answer to this question. But, Durrant, Turner and Wilson (22) have observed that high-latitude flux emergence can affect the evolution of individual high-latitude plumes, but this flux does not seriously affect the whole reversal times of the polar magnetic field. However, the polar magnetic elements involve in the supergranular motion, solar rotation and reflect the subsurface gradient of angular velocity (Benevolenskaya, 27). Conclusion The current and future high- (low-) resolution solar observations enable to investigate physical processes at the all levels on the Sun (convection zone, photosphe re, chromosphere, and corona), simultaneously. SOHO and others missions show how our Sun is variable. In this paper I reviewed the present results with focus on the solar cycle studies. Because of the next solar space laboratory, Solar Dynamics Laboratory (SDO), is coming to replace SOHO with a purpose to understand the nature of the solar cycle. References Abramenko, V. I., Fisk, L. A. and Yurchyshyn V. B. (26), The rate of emergence of magnetic dipoles in Coronal holes and adjacent quiet-sun regions, ApJ, 641, L65-L68 Aschwanden, M. J., Wuelser, L.-P, Nitta, N. V., Lemen, J.R. (28a), First 3D Reconstructions of Coronal loops with the STEREO A and B Spacecraft.I. Geometry, ApJ, 679, 827-842 Aschwanden, M. J., Nitta, N. V., Wuelser, L.-P., Lemen, J.R. (28b), First 3D Reconstructions of Coronal loops with the STEREO A+B Spacecraft. II.Electron Density and Temperature Measurements, ApJ, 68 (2), 1477-1495 Babcock, H. W., and Babcock, H. D. (1955), The Sun s magnetic field, 1952-1954, ApJ, 121, 349-366 Babcock, H. W., Livingston W. C. (1958), Changes in the Sun s polar magnetic field, Science, 127, 158 Babcock, H. D. (1959), The Sun s polar magnetic field, ApJ, 1, 364-365 Benevolenskaya, E. E., Hoeksema, J. T., Kosovichev, A. G. and Scherrer, P. H. (1999), The interaction of new and old magnetic fluxes at the beginning of solar cycle 23, ApJ, 517, L163-L166 Benevolenskaya, E. E., Kosovichev, A. G. and Scherrer, P. H. (21), Detection of high-latitude waves of solar coronal activity in extreme-ultraviolet data from the Solar and heliospheric observatory EUV imaging telescope, ApJ, 554, L17-L11 Benevolenskaya, E. E., Kosovichev, A. G., Lemen, J. R., Scherrer, P. H., Slater, G. L. (22), Large-scale solar coronal structures in soft X-ray and their relationship to the magnetic flux, ApJ, 571, L181-L185 Benevolenskaya, E.E., (27), Rotation of the magnetic elements in Polar Regions on the Sun, Astron. Nachr., 328 (1), 116-119 4

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