1. 2008/12/10Solar Cycle 24 @ Napa1 Magnetic field activities at the photosphere for causing microflares in the corona Toshifumi SHIMIZU (ISAS/JAXA) shimizu.toshifumi@isas.jaxa.jp

2. 2008/12/10Solar Cycle 24 @ Napa2 1. Introduction Magnetic topology of coronal dynamics 1990’s~ X-ray/EUV coronal observations by Yohkoh/SoHO/TRACE Hard X-ray by RHESSI Frequency Event energy (ergs) (Aschwanden et al. 2000) Yohkoh (shimizu 1995) EUV by SOHO/TRACE Flares (HXR) 10(24)10(32) Transient energy release events with various size/energy scale Flares, micro-flares, nano- flares, … Physical process in transient conversion of magnetic energy to thermal energy “magnetic reconnection” limb Cusp flare (Soft X-rays)

3. 2008/12/10Solar Cycle 24 @ Napa3 Solar Microflares: Active-Region Transient Brightenings (ARTBs) Numerous numbers of small-scale explosive energy releases are observed anywhere in the solar corona, especially active-region corona. ARTBs –Soft X-ray observations of active regions (Shimizu et al. 1992) –10 25 -10 28 erg (Shimizu 1995) cf. Solar flares: 10 29 -10 33 erg –Morphologically full of variety: Point-like brightening Single & multiple-loop brightening (Shimizu et al. 1994) May be accompanied by a jet (Shibata, Shimojo 1992) –Fine structures inside brightening loops and their dynamic evolution, revealed with Hinode XRT high spatial/cadence observations (Kano et al. 2008) Hinode XRT’s initial observation of an active region

4. 2008/12/10Solar Cycle 24 @ Napa4 Microflares: Coronal morphology (Yohkoh) Multiple coronal loops suddenly brighten simultaneously Compact brightening (Shimizu et al. 1994) Coronal loops  selective coronal magnetic fields

5. 2008/12/10Solar Cycle 24 @ Napa5 Magnetic reconnection as a base mechanism Magnetic Reconnection Soft X-ray loop morphology Point-like Single loop Multiple loops Emerging flux Flux canceling features Signatures at the photospheric magnetic field Unknown Related to complicated magnetic fields. Coalescence of current loops?

6. 2008/12/10Solar Cycle 24 @ Napa6 Models: magnetic-field configuration for multiple-loop interaction Magnetic reconnection as a basic mechanism e.g., Heyvaerts et al. (1977) e.g., Parker (1983) e.g., Gold & Hoyle (1960) Coalescence of current loops

7. 2008/12/10Solar Cycle 24 @ Napa7 Soft X-Ray (Yohkoh) Magnetogram (La Palma, SST+SOUP) 10arcsec 5arcsec Emerging flux (8-16x10 17 Mx) exists! 2~2.8km/s 10min (Shimizu et al. 2002) 1. Flux emergence driven microflares It was identified with high-resolution magnetogram observation that a small- scale flux emergence is observed 10-30 min before the onset of transient brightenings. (Shimizu et al. 2002)

8. 2008/12/10 Solar Cycle 24 @ Napa 8 (Kano et al. 2008 Boulder) event-(e) 11:13:51.472 11:05:41-5911:10:41-5911:25:41-5811:15:42-5911:20:41-58 11:25:3411:05:3511:10:3511:15:3511:20:34 SOT -FG Magnetogram SOT -FG Ca-II H XRT Al/Poly. 4x10 18 Mx CaII-H Emerging Reconnection Submerging 5” 5000 km flux emergence

9. 2008/12/10Solar Cycle 24 @ Napa9 2. Canceling flux driven microflares “Moving magnetic features” (MMFs) well observed around the sunspot This is one of major drivers for X-ray bright points (XBPs) in quiet Sun

10. 2008/12/10 Solar Cycle 24 @ Napa 10 (Kano et al. 2008 Boulder) event-(g) 11:14:58.493 11:05:41-5911:10:41-5911:25:41-5811:15:42-5911:20:41-58 11:25:3411:05:3511:10:3511:15:3511:20:34 SOT -FG Magnetogram SOT -FG Ca-II H XRT Al/Poly. 4x10 18 Mx CaII-H 5” 5000 km MMF Reconnection Submerging penumbra

11. 2008/12/10Solar Cycle 24 @ Napa11 3. Unknown drivers for multiple-loop brightenings “Multiple or single” loop-type brightenings –They are more frequently observed than point-like brightenings. –They are extremely lack of photospheric magnetic field observations in high spatial resolution. “Hinode” observations would improve our observational knowledge. –SOT observations coordinated with XRT observations Identify the exact position of roots of brightening loops at photosphere Understand magnetic-field properties at the footpoints

12. 2008/12/10Solar Cycle 24 @ Napa12 3. Events of interest (1): Multiple-loop-type brightenings XRT Movie: 2 May 2007 11:20-17UT Ti_poly, 263”x263” Zoom in on SOT field of view (112”x112”) Transient loop brightenings occurred at two locations in steady coronal structure: 1)Multiple-loop brightenings at the center at least two times, 2)Brightenings at the upper area (evolving from short loops to longer loops)

13. 2008/12/10Solar Cycle 24 @ Napa13 Magnetic activities at footpoints NFI Fe6302 LOS magnetogram （Ｖ /I): 1min cadence No clear magnetic activities, such as emerging and canceling, are seen for multiple-loop brightenings at the center. Magnetically single pole region at each end of brightening loops. No small bi-pole field exists near the footpoints. Successive emergence of magnetic flux observed at the upper area.

14. 2008/12/10Solar Cycle 24 @ Napa14 Magnetic field properties of footpoints in umbra Electric current is enhanced at and beside the area surrounded by the flare ribbons. The magnetic field and current directions are same in multiple brightening loops. SOT Spectro-Polarimeter: fast map, 14:00-14:32 Stokes Inversion (Millen-Eddington, Yokoyama et al., available at NAOJ Hinode Science Center) Azimuth 180 deg ambiguity was resolved using Dr. Kubo’s sot_azam_beta.pro, which is based on HAO AZAM program. Magnetic field inclination on local coordinate (deg) Magnetic field azimuth on local coordinate (deg) Vertical current (mA/m 2 ) CaIIH footpoints of 13:40UT brightening (also contours)

15. 2008/12/10Solar Cycle 24 @ Napa15 Twisted magnetic fields What does the observed current enhancement mean? Twisted magnetic fields are formed. The energy is stored in the twisted magnetic fields in the corona. Observed parameters of twisted fields –Observed current density j z = 35 mA/m 2 –Cross section S= 0.5x0.5arcsec (CaH kernel) S= 0.5 x 15 arcsec (flare ribbon) –Total current J= j z S = 5 x 10 9 (kernel) – 2 x 10 11 (ribbon) A –Field strength B= 2000-2300 G Linear force-free field parameter –α ～ 0.25 [1/Mm] –Loop length L ＝ 60” ~ 45 Mm –Number of pitch along the loop N pitch = 0.9 The brightening loops may have twisted magnetic field which has almost one turn along the loop.

16. 2008/12/10Solar Cycle 24 @ Napa16 Magnetic properties of footpoints in plage Multiple # of footpoints in a single polarity region Most of the footpoints is not located in magnetic flux concentrations. –Deviated from G-band bright point distribution. –The gas convection can work more actively in non-concentrated area. Magnetic field –Weakly polarized signal (but > 0.4%) –1kG field, but low filling factor below 0.25. –Total flux involved is in order of 10 17-18 Maxwell. Magnetic field inclination on local coordinate (deg) G-band image at 13:40 (FOV 22”x22”) CaIIH image at 13:40Magnetic flux (SP)

17. 2008/12/10Solar Cycle 24 @ Napa17 4. Electric currents at loop footpoints (another examples) Event 5 (9:17:57UT) Event 6 (9:27:57UT) SXR CaIIH GG Jz B B One of loop footpoints is located in the inner part of sunspot penumbra. Electric currents on order of 20-30mA/m^2 exist not only near the loop footpoints but also in other areas.

18. 2008/12/10Solar Cycle 24 @ Napa18 Summary Magnetic-field properties of the observed multiple-loop-type brightenings: –One side of the footpoints is located deeply inside the spot umbra. –Vertical electrical current enhancement is clearly observed there, meaning the twisted magnetic field in the corona. –No apparent magnetic activities, such as emerging and canceling, are observed at the footpoints. –Footpoints at plage are located in non-concentrated magnetic flux Different from emerging and canceling process, another triggering process should exist. –Energy is stored in twisted magnetic fields in the corona. –Gas convection may effectively work for triggering the energy release. –Alternately, coalescence of current loops may take place (e.g., Gold and Hoyle 1960)

19. 2008/12/10Solar Cycle 24 @ Napa19

20. 2008/12/10Solar Cycle 24 @ Napa20 Compact microflares Easy to identify the footpoint location Too compact to resolve detailed coronal internal structure Yohkoh SXT An X-ray jet is ejected from near brightening site in some microflares. (e.g., Shimojo et al. 1998) Jet emerging flux model Emerging bipole SXR HH (e.g., Shibata et al. 1992, Yokoyama 1996) TRACE

21. 2008/12/10Solar Cycle 24 @ Napa21 Footpoints identified with Ca II H Morphological evolution of X- ray emission is rather complicated. Similar to flare ribbon, tiny two ribbon brightenings can be observed in chromospheric Ca IIH line images, giving the exact position of the footpoints of brightening loops. One end is located well inside the sunspot umbra. The ribbon consists of two groups (loop-loop interaction). CaIIH XRT CaIIH intensity increase from 13:33UT

22. 2008/12/10Solar Cycle 24 @ Napa22 GOES ~B1 class Brightenings at the upper area Multiple-loop brightening at the center XRT/SOT sequence examined C9 flare occurred at the south of the active region

23. 2008/12/10Solar Cycle 24 @ Napa23 4. Enhanced currents in sunspots April 30, 2 days before the May 2 events examined. Magnetic field inclination on local coordinate (deg) Magnetic field azimuth on local coordinate (deg) Vertical current (mA/m 2 ) Continuum image

24. 2008/12/10Solar Cycle 24 @ Napa24 4. Enhanced currents in sunspots April 30, 2 days before the May 2 events examined. Magnetic field inclination on local coordinate (deg) Magnetic field azimuth on local coordinate (deg) Vertical current (mA/m 2 ) Continuum image