dynamics

CONNECTION BETWEEN PHOTOSPHERIC M A G N E T I C FIELDS A N D C O R O N A L STRUCTURE/DYNAMICS T. Shimizu

National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan

ABSTRACT

Yohkoh has provided new observations of the X-ray corona for over 10 years and has made several advances in understanding magnetic origins of the heating and energy releases in the corona. We have, especially, learned much about weak transient activities, such as jets and transient brightenings (microflares) from Yohkoh observations and also from EUV observations by the SOHO and TRA CE spacecraft. Some observations have been published, clearly showing a key role of magnetic flux just after newly emerging from below the surface. These observations can be explained by an emerging flux model. INTRODUCTION For more than 10 years while the Yohkoh satellite was in operation on orbit, a large number of observations of photospheric magnetic fields that were simultaneous with Yohkoh X-ray observations were conducted at various ground-based observatories and by the Michelson Doppler Imager (MDI) on SOHO. These observarious have provided new views for understanding magnetic origins of the heating of transient and steady structures in the corona. It has been well known that the location and strength of identifiable X-ray features in the corona are associated with strong magnetic fields at the surface. Bright features seen in soft X-rays are located above sunspot groups observed in visible light. Yohkoh observations have quantitatively confirmed that the thermal properties of the "active-region" corona are well correlated to the integrated and averaged magnetic properties derived from magnetogram observations. The total thermal energy involved in an active region is well related to its total magnetic flux from a tiny active region (~ 3 • 1020 Mx) to a large active region (,-~ 7 • 1022 Mx) (Fisher et al. 1998). The coronal gas pressure averaged over an active region is also correlated to the average of the magnetic flux density (Yashiro & Shibata 2001) from a diffuse active region (,,~ 40 gauss) to a well-confined active region (,,~ 300 gauss). The active-region corona consists of a variety of coronal loops, which trace out magnetic field structures filled with hot plasma in the corona (Figure 1). Bright coronal loops connect the leading sunspot area to the following sunspot area, and these loops appear to be rooted beside and/or in the penumbra of sunspots. It is interesting that the corona is in general dark above the umbra of sunspots, although sunspots are the cross-section of strong magnetic field bundles at the surface. Therefore, for better understanding of the heating of the active-region corona, it is important to look more deeply into how the thermal properties of coronal structures in active regions are related to magnetic fields at the sites where the coronal structures are rooted. -29-

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Fig. i. An active region observed in soft X-rays (left) and visible light (right) by Yohkoh on 29 March, 1992

One of the remarkable discoveries made by Yohkoh is that the corona is more dynamic than thought before. This view is brought to us by the soft X-ray telescope (SXT) onboard Yohkoh, which provides continuous sequences of soft X-ray coronal (> 3 MK plasma) images with high temporal/spatial resolution and high sensitivity, making it possible to detect weak transient activities that have not been well observed so far, such as X-ray jets and active-region transient brightenings (e.g. Shibata et al. 1992; Shimizu et al. 1992). Shimizu (1993) showed that X-ray transient brightenings (microflares) have preferred locations in active regions for their occurrence (Figure 2). Transient brightenings are well observed around the outer boundary of the penumbra of well-developed spots in emerging flux regions. Magnetic fields around the penumbra are one of the key features for understanding magnetic origins of weak transient activities. Also, since bright coronal loops are rooted beside and/or in the penumbrae, magnetic fields around the penumbrae are important for understanding the heating of the active-region corona.

Fig. 2. Spatial distribution of X-ray transient brightenings (microflares) in active region NOAA 7260 (Shimizu 1993). This map shows 639 transient brightenings observed from 15 through 20 August, 1992.

This paper first reviews what kinds of evolution of magnetic fields are well observed in active regions at the surface, and their possible associations with the heating of transient and steady coronal structures. Coronal observations with simultaneous magnetogram observations have been obtained to investigate how magnetic fields at the surface are responsible for weak transient activities. The next section briefly reviews weak transient activities and also some observational examples of jets and microflares that clearly show magnetic connection between the coronal and photospheric magnetic fields. Using unique simultaneous observations by Y o h k o h / S X T and La Palma, magnetic origins of point-like transient brightenings (microflares) were studied by Shimizu et al. (2002). Some observations presented in Shimizu et al. (2002) are reviewed and discussed.

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Connection between Photospheric Magnetic Fields and Coronal Structure~Dynamics

Fig. 3. Magnetic field and coronal evolution of active region NOAA 9231 from 16 November to 23 November 2000. Longitudinal magnetograms are taken with 50HO MDI, and coronal images are obtained with Yohkoh SXT. Contours in coronal images are +/-200 gauss levels in magnetograms.

MAGNETIC EVOLUTION IN ACTIVE REGIONS Figure 3 shows the day-by-day evolution of magnetic fields at the photosphere and magnetic structures in the corona of an active region. Typical magnetic activities have been well observed in series of magnetograms: newly emerging magnetic flux, small patches around well-developed sunspots, merging of the same polarity flux, flux canceling with opposite polarity flux, disappearing magnetic flux maybe due to fragmentation and diffusion, shearing magnetic bipoles, and so on. The large-scale emergences of magnetic flux from below the photosphere are observed as a magnetic bipole labeled D and E and another magnetic bipole labeled F and G. As magnetic flux successively emerges, the bundle of bright loops is newly developed above the newly emerging magnetic flux and frequent occurrence of transient brightenings (microflares) and X-ray jets is observed (Kawai et al. 1992, Yoshimura & Kurokawa 1999). The positive-polarity patches labeled D and F approach negative-polarity patches labeled A, B, and I after 19 November, and it appears that they are partially canceled. Associated with the cancellation, a new loop system connecting the positive-polarity area (D and F) to the negative-polarity area (A and B) is observed after 20 November. A magnetic bipole labeled B and C shows shearing motion; the negative patch B slowly separates from the positive patch C, and the direction of the line across B and C slightly rotates counterclockwise. The loops connecting B to C maintain their brightness during the observation. In Figure 3, compact X-ray sources and faint loops extending from the compact X-ray sources are seen around the leading large sunspot, where a large number of X-ray transient brightenings are observed with -31 -

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Fig. 4. Satellite spots, enclosed by squares, and moving magnetic features, marked by arrows. Longitudinal magnetograms are taken with 50HO MDI on 17 November 2000.

Yohkoh. They are associated with the formation of satellite spots, the polarity of which is opposite to the leading spot (examples enclosed by squares in Figure 4, Leka et al. 1994, Shimizu 1993, Shimojo et al. 1998). Moreover, numerous moving magnetic features (MMFs) are commonly observed around well-developed spots (examples shown by arrows in Figure 4). The MMFs are small magnetic bipoles which are born at the outer edge of the penumbra of well-developed spots and then go outward in the radial direction from the spot. It appears that compact X-ray sources are not associated with MMFs.

WEAK TRANSIENT ACTIVITIES Variety of Weak Transient Activities Yohkoh SXT has revealed that X-ray transient brightenings (microflares) occur in the bright corona (Shimizu et al. 1992, 1994). SXT also discovered X-ray jets as transitory X-ray enhancements with apparent collimated motion (Shibata et al. 1992, Shimojo et al. 1996). They are associated with small flares in XBPs, transient

brightenings or small flares in active regions or emerging flux regions. Since then, several kinds of weak transient activities have been reported from the Yohkoh, S O H O , and T R A CE observations. They are named using different terminology, because some differences can be found from Weak transient activities previously reported, or observations are made with different instruments. In Figure 5, weak transient activities observed in the corona are summarized as a function of involved energy in the vertical axis and the location of occurrences in the horizontal axis. Note that weak transient activities found in transition-region EUV lines, such as blinkers (Harrison 1997), explosive events and EUV jets (Brueckner & Bartoe 1983, Innes et al. 1997) are not included in this figure. Newly observed coronal weak activities are distributed between 1029 and 1024 ergs. Weak transient activities are more easily found in quiet regions because of the low quasi-steady X-ray background level. No significant differences except for the involved energy and occurrence location may be found among X-ray transient brightenings, XBP flares (Strong et al. 1992, Kundu et al. 1994), network flares (Krucker et al. 1997), and EUV transient brightenings (Berghmans et al. 1998, Krucker & Benz 1998, Benz & Krucker 1998). The durations of these activities are all less than roughly 10 min, and they show soft X-ray light curves with a sudden increase at the beginning and a slow decrease in the late phase, which are temporal behaviors similar to those of standard flares. The coronal loops showing these activities are compact, and smaller energy activities appear to be confined into more compact loops. Coronal jets are observed in a wide range of weak transient activities, as illustrated by a hatched region in Figure 5. Because of a lack of observations, it is currently uncertain whether small variations in quasi-steady long loops are similar to the other weak transient activities. However, they appear to be small variations at limited parts within coronal loops, whereas the other weak activities are X-ray brightenings of entire compact loops (Shimizu & Tsuneta 1997, Katsukawa & Tsuneta 2001). -32-

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Fig. 5. Coronal weak transient activities as a function of energy in vertical axis and occurrence location in horizontal axis (Shimizu !999). The abbreviation ARTB means X-ray transient brightenings (microflares).

Implication for Heating the Corona One of the attractive concepts for the heating of the corona is that numerous small energy releases (microflaxes, nanoflares, or picoflaxes) may be a possible source for heating the corona (Parker 1988). The frequency distribution of energy releases by weak transient activities as a function of the magnitude has been well studied to examine the concept of heating by microflaxes and nanoflaxes. Shimizu (1995) studied the frequency distribution in the microflare energy range with Yohkoh observations, and then Krucker & Benz (1998), Parnell & Jupp (2000) and Aschwanden et al. (2000) estimated the frequency distribution in nanoflaxe energy range with EUV observations by S O H O / E I T and TRACE. They are all well represented by a power-law function with the slope similar to that of standard flares (e.g. Crosby, Aschwanden, & Dennis 1993), meaning that the flare power-law distribution is maintained over almost eight orders of magnitude in energy (1024 ,,~ 1032 ergs). The total thermal energy supplied is estimated to be at most a factor of 5 smaller than the heating rate required for the active-region corona (Shimizu 1995, Benz & Krucker 2002), and the total energy released by weak transient activities observed is not sufficient to explain the entire heating of the corona. The energy released by weak transient activities, however, plays a key role in generating > 5 MK hot plasma in the corona (Watanabe et al. 1995, Yoshida & Tsuneta 1996). Photosphere-Corona Connection A lot of observational studies have shown that major solar flares frequently occur in sheared magnetic regions (e.g. Sakurai et al. 1992), in emerging flux regions (e.g. Hanaoka 1996), and with complicated magnetic topology in active regions. However, since the photospheric magnetic field configurations of major flaxes are generally too complicated to completely understand, small-scale activities in the corona as seen in X-ray and EUV wavelengths can provide a better opportunity to understand fundamental physical mechanisms of energy build up and triggering. It is expected that the photospheric magnetic field dynamics responsible for small-scale activities is smaller than for major flares, but recent visible light data with high spatial and temporal resolution make the detailed study of small-scale activities in the corona possible and reliable. -33-

T. Shimizu

By comparing coronal data with magnetograms, some studies were recently made to understand the energy build up and trigger of these small-scale activities. Shimojo et al. (1998) provided a statistical result on photospheric magnetic-field patterns favorable to the occurrence of X-ray jets by studying longitudinal magnetograms at the footpoints of X-ray jets. Zhang et al. (2000) observed simultaneous occurrence of an X-ray jet and a surge in Hfl at the site where the pre-existing magnetic flux was "canceled" by newly emerging flux of opposite polarity. Chae et al. (1999) found several EUV jets that repeatedly occurred where pre-existing magnetic flux was "canceled" by newly emerging flux of opposite polarity. Yoshimura et al. (2002) reported that surge activities were observed in Ha where the pre-existing magnetic flux was "canceled" by newly emerging flux of opposite polarity, although no enhanced X-ray emissions were found. On the other hand, Tang et al. (2000) found a soft X-ray microflare for which the impulsive enhancement of the emerging flux in magnetograms occurred about 20 minutes before the the peaks of the soft X-ray brightening. Shimizu et al. (2002) have found several X-ray transient brightenings (microflares) showing close relationships with emergence of magnetic flux, as described in detail in the next section. These observations indicate that newly emerging flux and/or magnetic cancellation with newly emerging flux play a vital role in causing transient energy releases in the upper solar atmosphere. YOHKOH/SXT-LA PALMA OBSERVATIONS By combining Yohkoh soft X-ray images with high resolution magnetograms simultaneously obtained at La Palma, Shimizu et al. (2002) studied photospheric magnetic signatures responsible for soft X-ray transient brightenings (microflares).

16 Point-like Transient Brightenings

~

5-30min Before the Onset )

Identification of Associated Magnetic Activities In order to have a reliable correspondence between the photosphere and the corona, 16 point-like transient brightenings with X-ray source size less than 10 arcsec occurring during periods when the seeing is excellent at La Palma have been studied, although a lot of transient brightenings are in the form of multiple or single loop structures. In half of the studied events, smallscale emergences of magnetic flux loops are found in the vicinity of the transient brightenings (Figure 6). Six events of the half show that a small-scale flux emergence occurs 5 ~ 30 minutes prior to the onset of the X-ray brightening (Figure 7). In the other half of the studied events, no apparent evolutionary change of magnetic flux elements is found associated with the transient brightenings. Many of these events are found in rather strong magnetic fields, such as sunspots and pores, implying that small-scale changes of magnetic flux are obscured or suppressed by strong magnetic fields.

Fig. 6. Summary of photospheric magnetic activities associated with transient brightenings (Shimizu et aL 2002). The outer circle shows what kinds of magnetic activities are observed in the vicinity of 16 pointlike transient brightenings. The inner circle indicates whether transient brightenings are observed in strong magnetic regions or weak magnetic regions.

Detailed Spatial Relationship The spatial relationship among newly emerging fux, soft X-ray source, and tiny brightenings observed in Ha provides information on the configuration of magnetic fields involved in the energy release. The location of a -34-

Connection between Photospheric Magnetic Fields and Coronal Structure~Dynamics @ Flux Birth in Magnetograms

Onset of Transient Brightenings

La Palma Observing Period

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(min) Event Date/Time (1992) 20 May 12:59i13:02

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Fig. 7. Temporal relationships between the first appearance of small-scale magnetic flux in magnetograms and the onset of transient brightenings (Shimizu et al. 2002). For 8 pointlike transient brightenings associated with the emergence of small-scale magnetic flux, each line shows the timing of flux emergence relative to the onset of transient brightening, with the period of magnetogram observation at La Palma.

small emerging magnetic bipole detected in magnetograms tells where the newly emerging flux appears at the photosphere from the interior. The soft X-ray source is the signature of the energy release, giving the place where magnetic fields involved in the energy release are located in the corona. Tiny brightenings observed in Ha can be used to infer the location of footpoints of the heated soft X-ray loops, because Ha brightenings are probably heated by thermal conduction from the soft X-ray source. Since the La Palma images are co-aligned with SXT soft X-ray images with an accuracy limited by the SXT pixel size (2.46 arcsec), the spatial distribution with larger than this accuracy gives meaningful information on the spatial relationship. Figure 8 shows the spatial relationship among emerging flux, soft X-ray source, and Ha brightenings for the 6 transient brightenings in which newly emerging flux is detected in the magnetograms, showing that the center position of the soft X-ray source core is not spatially coincident with the newly emerging magnetic flux. In 5 of the 6 events, one of the tiny brightenings in Ha is observed at one end of an emerging magnetic bipole and the other brightenings are located apart from the emerging bipole. DISCUSSION

Observed Temporal Delay The driving force for the emergence of magnetic loops is the enhanced magnetic buoyancy of flux tubes. The first appearance of emerging flux in the photosphere is an anomalously dark intergranular lane observed in white light granules. The dark intergranular structures last about 10 minutes. The dark lanes indicate an emerging magnetic flux loop crossing the photospheric layer (Strous et al. 1996). At this time, the signature of the emerging flux may not be observable in longitudinal magnetograms because the loop is nearly horizontal (Lites, Skumanich, & Martinez Pillet 1998). Then, at the ends of the elongated dark structures, bright elements appear in G-band images and the elements separate from each other. At this time, the signature of the emerging flux is observable in longitudinal magnetograms, because the ends of the emerging loop are no longer horizontal at the photospheric level. In successive emergences of magnetic flux in ephemeral active regions, the rate of expansion is the order of 5 km s -1 in the first few minutes after -35-

T. Shimizu

Fig. 8. Spatial relationship among newly emerging flux, soft X-ray source, and tiny brightenings in Ha for 6 transient brightenings. The distance between the center position of the soft X-ray source core and the newly emerging flux is given at the lower right corner in each frame.

the emergence, then drops to values between 1.3 and 0.7 km s -1 during the next several hours (Harvey & Martin 1973). We have found that the speed of moving magnetic elements is 2.8 km s -1 in the 9:48 UT 21 June 1992 transient brightening, which is in good agreement with previous observations. In this transient brightening, the first appearance of the new magnetic flux in magnetograms is about 10 minutes prior to the onset of the X-ray brightening. Assuming the vertical speed of the emergence to be approximately equal to the measured horizontal speed, the emerging magnetic flux loop would reach ~ 1700 km height above the photospheric level, which may be at the mid-upper chromosphere. After the emergence, the chromospheric response to emerging flux is observed in H c~. An arch filament system is observed to form, connecting the plages of opposite polarity. The rise velocity of filaments is 10 ,-~ 15 km s -1. The rise velocity of emerging flux loops is accelerated due to magnetic buoyancy from less than a few km s -1 at the photospheric level to 10 ,-~ 15 km s -1 at the chromospheric level, although there is lack of observational information on the acceleration. However, the dynamical behavior of emerging flux loops is well demonstrated by numerical simulations. The time scale for the emergence of magnetic flux from the photosphere to the coronal level is about 20 minutes (Shibata et al. 1989). This is comparable to the observed time difference between the first appearance of the flux emergences in magnetograms and the onset of coronal X-ray transient brightenings. This observation suggests that high spatial observations with temporal resolution of less than a few minutes are essential for investigating in detail the dynamical response of the coronal magnetic fields to magnetic emergences. Spatial Relationship and Emerging Flux Model The observations mentioned in this paper show that flux emergence is involved in the occurrence of X-ray transient brightenings (microflares) and X-ray/EUV jets, strongly suggesting that the magnetic fields just emerged from below the photosphere play a key role in the transient release of magnetic energy in the -36-

Connection between Photospheric Magnetic Fields and Coronal Structure~Dynamics corona. The emerging flux model (Heywaerts et al. 1977, Yokoyama & Shibata 1996) has been considered as an important process for converting magnetic energy into thermal and kinetic energy in the corona (Figure 9). In this model, a new magnetic flux loop rises and collides with pre-existing magnetic fields, creating a current sheet between them. Recent numerical simulations show that X-ray emitting hot plasma can be created by a magnetic reconnection in a neutral sheet between emerging and pre-existing coronal magnetic fields, and the hot plasma is ejected upwards with a compact micro-flaring loop (X-ray jet). The emerging flux model explains the observed spatial and temporal relationships. No X-ray jet is, however, observed in the events examined in the previous section, although the model predicts the existence of an X-ray jet. Instead, the observations show tiny ejections from the brightening site in Ha in 3 cases. Whether an X-ray jet exists or not may depend on the pre-existing magnetic field environment. When the pre-existing field is rather strong, strong magnetic pressure may force the reconnection site to the lower atmosphere. In this case, an X-ray jet may be produced with a micro-flaring compact loop. It may be also observed at the photospheric level that one polarity of the emerging magnetic bipole is canceled with the pre-existing magnetic flux, because of the magnetic reconnection in the lower atmosphere. When the pre-existing field is rather weak, weak magnetic pressure may force the reconnection site to the higher atmosphere. In this case, a micro-flaring loop may be produced with no apparent appearance of X-ray plasma ejection, because of low plasma density and low magnetic tension.

Fig. 9. An emerging flux model for explaining the simultaneous occurrence of X-ray jet and microflare. Hc~ brightenings are added to the picture from Yokoyama (1996).

ACKNOWLEDGEMENTS The author would like to express his thanks to the Scientific Organizing Comittee members of the symposium. Yohkoh observations have made it possible for the author to study magnetic connection between the corona and the photosphere for soft X-ray transient brightenings, and the author thanks the Yohkoh project personnel and all the people who have made contributions to the Yohkoh observations. The author also thanks M. Kubo for providing Figure 3 and Figure 4 from his master thesis. REFERENCES

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