2008/12/10Solar Cycle 24 @ Napa1 Magnetic field activities at the photosphere for causing...

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)

[email protected]


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)


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


Microflares: Coronal morphology (Yohkoh)

Multiple coronal loops suddenly brighten simultaneously

Compact brightening

(Shimizu et al. 1994)

Coronal loops ≒ selective coronal magnetic fields


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?


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


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)


(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


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


(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


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


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)


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.


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)


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


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.


G-band image at 13:40(FOV 22”x22”)

CaIIH image at 13:40 Magnetic flux (SP)


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.


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)


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


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


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


4. Enhanced currents in sunspots April 30, 2 days before the May 2 events examined.


Magnetic field azimuth on local coordinate (deg) Vertical current (mA/m2)

Continuum image

2008/12/10Solar Cycle 24 @ Napa1 Magnetic field activities at the photosphere for causing...

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