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2008/12/10 Solar Cycle 24 @ Napa 1
Magnetic field activities at the photosphere for causing microflares in the c
orona
Toshifumi SHIMIZU (ISAS/JAXA)
2008/12/10 Solar Cycle 24 @ Napa 2
1. Introduction Magnetic topology of coronal dynamics 1990’s~
X-ray/EUV coronal observations by Yohkoh/SoHO/TRACE Hard X-ray by RHESSI
Fre
quen
cy
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-flare
s, … Physical process in transient
conversion of magnetic energy
to thermal energy
“magnetic reconnection”
limb
Cusp flare(Soft X-rays)
2008/12/10 Solar Cycle 24 @ Napa 3
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 re
gions (Shimizu et al. 1992)
– 1025-1028 erg (Shimizu 1995) cf. Solar flares: 1029-1033 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
2008/12/10 Solar Cycle 24 @ Napa 4
Microflares: Coronal morphology (Yohkoh)
Multiple coronal loops suddenly brighten simultaneously
Compact brightening
(Shimizu et al. 1994)
Coronal loops ≒ selective coronal magnetic fields
2008/12/10 Solar Cycle 24 @ Napa 5
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?
2008/12/10 Solar Cycle 24 @ Napa 6
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
2008/12/10 Solar Cycle 24 @ Napa 7
Soft X-Ray(Yohkoh)
Magnetogram(La Palma, SST+SOUP)
10arcsec
5arcsec
Emerging flux (8-16x1017Mx) exists!
2~2.8km/s
10min
(Shimizu et al. 2002)
1. Flux emergence driven microflaresIt 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)
2008/12/10 Solar Cycle 24 @ Napa 8
(Kano et al. 2008 Boulder)
event-(e)event-(e)
11:13:51.472
11:05:41-59 11:10:41-59 11:25:41-5811:15:42-59 11:20:41-58
11:25:3411:05:35 11:10:35 11:15:35 11:20:34
SOT-
FG
Ma
gn
eto
gra
mSO
T-F
GC
a-I
I H
XR
TA
l/Po
ly.
4x1018 Mx
CaII-H
Emerging
Reconnection
Submerging
5” 5
000
km
fluxemergence
2008/12/10 Solar Cycle 24 @ Napa 9
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
2008/12/10 Solar Cycle 24 @ Napa 10
(Kano et al. 2008 Boulder)
event-(g)event-(g)
11:14:58.493
11:05:41-59 11:10:41-59 11:25:41-5811:15:42-59 11:20:41-58
11:25:3411:05:35 11:10:35 11:15:35 11:20:34
SOT-
FG
Ma
gn
eto
gra
mSO
T-F
GC
a-I
I H
XR
TA
l/Po
ly.
4x1018 Mx
CaII-H
5” 5
000
km
MMF
MMF
Reconnection
Submergingpenumbra
2008/12/10 Solar Cycle 24 @ Napa 11
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
2008/12/10 Solar Cycle 24 @ Napa 12
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)
2008/12/10 Solar Cycle 24 @ Napa 13
Magnetic activities at footpointsNFI 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.
2008/12/10 Solar Cycle 24 @ Napa 14
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/m2)CaIIH footpoints of 13:40UT brightening(also contours)
2008/12/10 Solar Cycle 24 @ Napa 15
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 jz = 35 mA/m2
– Cross section S= 0.5x0.5arcsec (CaH kernel) S= 0.5 x 15 arcsec (flare ribbon)
– Total current J= jz S = 5 x 109 (kernel) – 2 x 1011 (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 Npitch= 0.9
The brightening loops may have twisted magnetic field which has almost one turn along the loop.
zhoriz jB 0
z
z
z
horiz
B
j
B
B 0
0)( BBBJ
4/LN pitch
2008/12/10 Solar Cycle 24 @ Napa 16
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 bel
ow 0.25. – Total flux involved is in order of 1
017-18 Maxwell.
Magnetic field inclination on local coordinate (deg)
G-band image at 13:40(FOV 22”x22”)
CaIIH image at 13:40 Magnetic flux (SP)
2008/12/10 Solar Cycle 24 @ Napa 17
4. Electric currents at loop footpoints (another examples)
Event 5 (9:17:57UT) Event 6 (9:27:57UT)
SXR SXRCaIIH CaIIH GG
JzJz BB
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.
2008/12/10 Solar Cycle 24 @ Napa 18
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 fi
eld 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)
2008/12/10 Solar Cycle 24 @ Napa 20
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
(e.g., Shibata et al. 1992, Yokoyama 1996)
TRACE
2008/12/10 Solar Cycle 24 @ Napa 21
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
2008/12/10 Solar Cycle 24 @ Napa 22
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
2008/12/10 Solar Cycle 24 @ Napa 23
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/m2)
Continuum image