Astrophysics and Space Science Proceedings Phần 9 ppt

Presumably, the magnetic field is pre-dominantly concentrated at the network boundaries and, within coronal holes, thefootpoints of coronal funnels emanate from these network boundaries.

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434 G.R Gupta et al.have measured oscillations in coronal holes in the polar off-limb regions of the Sun.All these studies point to the presence of compressional waves thought to be slowmagnetoacoustic waves as found by Deforest and Gurman (1998);O’Shea et al.(2006,2007) Recently, Gupta et al (2009) have reported the detection of thesewaves in the disk part of the polar coronal hole (hereafter PCH) They also find adifference in nature of the compressional waves between bright (network) and dark(internetwork) regions in the PCH In this contribution, we extend such analysis toanother dataset More detail is given inGupta et al.(2009).

2 Observations and Data Analysis

The data used in this analysis were taken on 25 February 1997, during 00:00–13:59 UT with the 1 30000 slit of SUMER and an exposure time of 60 s in the

N IV 765 ˚A and Ne VIII 770 ˚A lines in a southern PCH Details of the data tion are given inGupta et al.(2009)

reduc-The chromosphere and transition region show enhanced-intensity networkboundaries and darker internetwork cells Presumably, the magnetic field is pre-dominantly concentrated at the network boundaries and, within coronal holes, thefootpoints of coronal funnels emanate from these network boundaries As the ob-serving duration of this dataset is very long, the locations of bright and dark pixelsalong the slit change with time For this reason, we have analyzed the whole datasetpixel by pixel and timeframe by timeframe

For example, for one given moment, we first determined the average intensityalong the slit All pixels having an intensity higher than 1.25 times this averageintensity were chosen as bright pixels If such pixels are bright for at least 60 min (or

60 timeframes), then these are considered to be a bright network location over thattime interval The bright pixel identification is done only for the low-temperature

N IV line; the network pixels obtained from it are assumed to be the same in thehigher-temperature Ne VIII line

3 Results and Discussion

Figure1 shows a representative example of the oscillations measured in a brightregion of the PCH We use wavelet analysis to provide information on the temporalsignal variation (Torrence and Compo 1998) Further details on this wavelet anal-ysis are found inGupta et al.(2009);O’Shea et al.(2001) and references therein.Figure1shows oscillations of about 18 min periodicity in both lines at the same lo-cation This suggests that these two layers are linked by a propagating wave passingfrom one layer to the other To test this hypothesis and to ascertain the nature of thepropagating waves, we measured phase delays in intensity and in Dopplershift be-tween the two lines at each of the measurable pixels along the slit for a full frequency

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Network Loop Oscillations with EIS/HinodeA.K Srivastava, D Kuridze, T.V Zaqarashvili, B.N Dwivedi, and B Rani

Abstract We analyze a time sequence of He II 256.32 ˚A images obtained withEIS/Hinode, sampling a small magnetic loop in magnetic network Wavelet anal-ysis indicates 11-min periodicity close to the loop apex We interpret this oscillation

as forcing through upward leakage by the fundamental acoustic eigenmode of theunderlying field-free cavity The observed loop length corresponds to the value pre-dicted from this mechanism

1 Introduction

Field-free cavities under bipolar magnetic canopies (Centeno et al 2007) in thevicinity of magnetic network are likely to serve as resonators for fast magnetoacous-tic waves (Kuridze et al 2007) Srivastava et al (2008) have studied the properties

of the fundamental fast magnetoacoustic mode in brightened magnetic network

It leaks through the magnetic network into the upper solar atmosphere Recently,Martin´ez Gonz´alez et al (2007) found evidence for low-lying loops in magneticinternetwork In EIS/Hinode observations of bright magnetic network, we found asmall loop located near the south pole We search for magnetoacoustic oscillations

in this loop through wavelet analysis

2 Observations

The observations were acquired on 11 March 2007 during 19:04–19:54 UT inthe study HPW005 QS Slot 60m The slot-center position was X D 11800;

Y D 97300/, with a 4000 51200 field of view (Fig.1) The data were binned

A.K Srivastava ( ) and B Rani

Aryabhatta Research Institute of Observational Sciences, Manora Peak, Nainital, India

D Kuridze and T.V Zaqarashvili

Abastumani Astrophysical Observatory, Tbilisi, Georgia

B.N Dwivedi

Department of Applied Physics, Institute of Technology, Banaras Hindu University, Varanasi, India

S.S Hasan and R.J Rutten (eds.), Magnetic Coupling between the Interior

and Atmosphere of the Sun, Astrophysics and Space Science Proceedings,

DOI 10.1007/978-3-642-02859-5 50, c Springer-Verlag Berlin Heidelberg 2010

437

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Dynamical Evolution of X-Ray Bright Points with Hinode/XRT

R Kariyappa, B.A Varghese, E.E DeLuca, and A.A van Ballegooijen

Abstract We analyzed a 7-h long time sequence of soft X-ray images obtained

on 14 April 2007 from a quiet region using the X-Ray Telescope (XRT) onboardHinode The aim was to observe intensity oscillations in coronal XBPs of differ-ent brightness and to study differences, if any, in the periodicity of the intensityvariations and the heating mechanism during their dynamical evolution We havecompared the XRT images with GONG magnetograms using Coronal ModelingSoftware (CMS), and found that some of the XBPs are located at magnetic bipoles.The coronal XBPs are highly dynamic and oscillatory in nature, showing a widevariety of time scales in their intensity variations

1 Introduction

Coronal X-ray bright points (XBPs) were discovered using a soft X-ray telescope

on a sounding rocket in the late 1960s (Vaiana et al 1978) Their nature remainedenigmatic Later, using Skylab and Yohkoh/SXT X-ray images, XBPs were studied

in detail (Golub et al 1974;Longcope et al 2001;Hara and Nakakubo-Morimoto2003) The number of XBPs that is daily present on the visible hemisphere of theSun varies from several hundred to a few thousand (Golub et al 1974), with 800 onthe entire solar surface at any given time (Zhang et al 2001) The number of coronalbright points varies inversely with the solar activity cycle (Sattarov et al 2002;Haraand Nakakubo-Morimoto2003) The XBP diameters are about 10–2000(Golub et al.1974) Their lifetime ranges from a few hours to a few days (Zhang et al 2001;Kariyappa and Varghese 2008)

In this contribution, we report the analysis of XBPs on soft X-ray images tained from Hinode/XRT and on magnetograms from GONG We briefly discussthe dynamical evolution of the XBPs in relation to the magnetic field

ob-R Kariyappa ( ) and B.A Varghese

Indian Institute of Astrophysics, Bangalore, India

E.E DeLuca and A.A van Ballegooijen

Harvard-Smithsonian Center for Astrophysics, Cambridge, USA

440

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Dynamical Evolution of X-Ray Bright Points with Hinode/XRT 441

We use a 7-h (17:00–24:00 UT) time sequence of soft X-ray images obtained on 14April 2007 with the X-Ray Telescope (XRT) onboard the Hinode mission, using the

Ti poly filter for a quiet region near the center of the solar disk We selected 14 XBPsfor analysis, marking them as XBP1, XBP2, , XBP14, and two background, verydark coronal comparison regions as XBP15 and XBP16 The XRT images havebeen calibrated using the SSW subroutine xrt prep.pro (Kariyappa and Varghese2008) We also obtained the full-disk magnetograms obtained with GONG duringthe XRT observing period These magnetograms have been co-registered with theXRT images using the Coronal Modeling Software (CMS) developed by the fourthauthor

The XBPs, defined as the sites where intense brightness enhancement is seen,are highly dynamic in nature We derived light curves for the XBPs by summingtheir brightness over small square image cut-outs covering the selected XBPs

Fig 1 GONG magnetogram overlayed on an XRT image using CMS modeling The magnetic field lines are computed from a potential-field extrapolation of the magnetogram

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Helicity at Photospheric and Chromospheric Heights

S.K Tiwari, P Venkatakrishnan, and K Sankarasubramanian

Abstract In the solar atmosphere, the twist parameter ˛ has the same sign as

magnetic helicity It has been observed using photospheric vector magnetogramsthat negative/positive helicity is dominant in the northern/southern hemisphere ofthe Sun Chromospheric features show dextral/sinistral dominance in the north-ern/southern hemisphere and sigmoids observed in X-rays also have a dominantsense of reverse-S/forward-S in the northern/southern hemisphere It is of interestwhether individual features have one-to-one correspondence in terms of helicity atdifferent atmospheric heights We use UBF H˛ images from the Dunn Solar Tele-scope (DST) and other H˛ data from Udaipur Solar Observatory and Big Bear SolarObservatory Near-simultaneous vector magnetograms from the DST are used to es-tablish one-to-one correspondence of helicity at photospheric and chromosphericheights We plan to extend this investigation with more data including coronalintensities

1 Introduction

Helicity is a physical quantity that measures the degree of linkage and twistedness

in the field (Berger and Field 1984) It is derived from a volume integral over the

scalar product of the magnetic field B and its vector potential A Direct calculation

of helicity is not possible due to the nonuniqueness of the vector potential A and

the limited availability of data sampling different layers of the solar atmosphere.The force-free parameter ˛ estimates one component of helicity, that is, twist, theother component being writhe, which cannot be derived from the available data.This ˛ is a measure of degree of twist per unit axial length It has the same sign

as magnetic helicity (Pevtsov et al 2008,Pevtsov 2008) It is now well knownthat negative/positive helicity dominates in the northern/southern hemisphere For

S.K Tiwari ( ) and P Venkatakrishnan

Udaipur Solar Observatory, Physical Research Laboratory, Udaipur, India

K Sankarasubramanian

Space Astron & Instrument Div., ISRO Satellite Center, Bangalore, India

443

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444 S.K Tiwari et al.active regions the hemispheric helicity rule holds in the photosphere, seeHagino andSakurai (2005) and references therein Similarly for the chromospheric and coronalhelicity rules, seeBernasconi et al.(2005) and references therein, andPevtsov et al.(2001) and references therein The topology of chromospheric and coronal featuresdecide the sign of the associated helicity Chirality is the term used for the sign ofthe helicity in these features Thus, helicity is a physical measure of chirality Thechirality of active region features shows correspondence with the sign of the helic-ity in the associated lower/upper atmospheric features For example, the chirality

of X-ray features with S (inverse-S) shapes are associated with sinistral (dextral)filaments (Martin 2003,Rust 2003).Chae(2000) reported for a few cases that ac-tive filaments showing dextral/sinistral chirality are related with negative/positivemagnetic helicity.Pevtsov et al.(2001) demonstrated correspondence between pho-tospheric and coronal chirality for a few active regions However, this needs to beconfirmed We have reported similar helicity signs at photospheric, chromospheric,and coronal heights for a few active regions (Tiwari et al 2008)

Comparison between magnetic helicity signs at different heights in the solar mosphere may be a useful tool to predict solar eruptions leading to interplanetaryevents Also, it may help to constrain modeling chromospheric and coronal fea-tures taking the photosphere as boundary condition However, the data required

at-to do this are not directly available and are often nonconclusive Vecat-tor magneticfields are not available as routinely as is necessary to derive photospheric twist val-ues Chromospheric H˛ images may be available most of the time by combiningdata from different telescopes, but are not always conclusive due to lack of angularresolution Analysis of coronal loop observations is required to determine coronalhelicity signs, but these are also not available routinely Above all, it is hard to finddata taken simultaneously at different heights in the solar atmosphere In this work

we combine photospheric and chromospheric data from multiple solar observatoriesand telescopes They were often not taken at precisely the same time We thereforeassume that the sign of the magnetic helicity does not change within a few hours

2 Sign of Magnetic Helicity

The sign of helicity in the photosphere is usually found from the force-free eter ˛, for example, ˛best(Pevtsov et al 1995), averaged ˛, for example,< ˛z > =

param-< J z =B z > (Pevtsov et al 1994) with current density Jz D r B z , where B zisthe vertical component of the magnetic field Some authors have used the currenthelicity density Hc D B z J z(Bao and Zhang 1998;Hagino and Sakurai 2005)

A good correlation was found between ˛bestandh˛zi byBurnette et al.(2004) andLeka et al.(1996) The force-free parameter ˛ has the same sign as magnetic helicity(Pevtsov et al 2008) Also, the current helicity (which is not a conserved quantitylike magnetic helicity) has the same sign as that of magnetic helicity (Seehafer 1990;Hagyard and Pevtsov 1999;Pevtsov 2008;Sokoloff et al 2008) In this study, weuse the sign of the global ˛ parameter as the sign of magnetic helicity, giving thetwist present in the active region

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Helicity at Photospheric and Chromospheric Heights 445Numerical measurement of the sign of the chromospheric magnetic helicity

is not possible due to non-availability of vector magnetic field observations atthese heights However, the twist present in morphological intensity features werereported already long ago (Hale 1925;Richardson 1941) to tend to follow the hemi-spheric helicity rule, independent of the solar cycle Later, many researchers studiedthe chirality of different chromospheric features such as filaments, fibrils, filamentchannels etc (Martin 1998,2003) We use the chirality of these chromospheric fea-tures, mostly whirls observed in H˛, to derive its association with the photosphericsign of magnetic helicity

3 Data

Apart from a few data sets, most are obtained from different solar observatoriesand telescopes due to the unavailability of all required data from the same place.The vector magnetic field data were obtained from the Advanced Stokes Polarime-ter (ASP,Elmore et al 1992) as well as the Diffraction Limited Spectropolarimeter(DLSP,Sankarasubramanian et al 2004,2006), both mounted at the DST Near-simultaneous H˛ images from the Universal Bi-refringent Filter (UBF) at the DSTare used whenever obtained along with the ASP and DLSP For the vector fieldobservations that do not have corresponding UBF data, H˛ images from UdaipurSolar Observatory (USO) and Big Bear Solar Observatory (BBSO) were used Wemade sure that in these cases the H˛ images were collected within less than a day.Standard and well-established processing was done to derive vector fields The pro-cedure is described in the references given above

Table 1 shows how the sign of helicity at the photospheric level and of the chirality inassociated features at chromospheric heights are related with each other Figure1a, bclearly show that the H˛ whirls follow the transverse magnetic field vectors mea-sured at photospheric heights The positive/negative helicity derived from the globaltwist in this sunspot in the photospheric vector data is directly associated with thesinistral/dextral chirality derived from the chromospheric H˛ data

In this preliminary analysis, we thus conclude that the sign of helicity tive/negative) derived from global twist present around sunspots in the photospherehas one-to-one correspondence with the (sinistral/dextral) sense of chirality ob-served in the associated chromospheric data We mostly use the chirality of chromo-spheric whirls to derive the chromospheric helicity sign It is known (Martin 1998,2003) that filaments, filament channels, etc have the same sense of chirality as thewhirls above the associated active regions The chirality of filaments associated with

(posi-an active region c(posi-an therefore be used to determine the chromospheric sense of rality when high resolution H˛ data are not available

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chi-Evolution of Coronal Helicity in a Twisted

Emerging Active Region

B Ravindra and D.W Longcope

Abstract Active-region magnetic fields are believed to be generated near the shear

layer of the convection zone by dynamo processes These magnetic fields are centrated into fluxtubes, which rise, due to buoyancy, through the convection zone

con-to appear in the form of bipoles at the phocon-tosphere Thin-fluxtube simulations gest that active regions require twist to emerge All regions are observed to emergewith some twist; some of them show larger twist than others A theoretical model[Longcope and Welsch 2000, ApJ, 545, 1089] predicts that an emerging fluxtube in-jects helicity into the corona for one or two days after its initial emergence throughrotation of its footpoints driven by magnetic torque There have been very few ob-servational studies of helicity injection into the corona by emerging flux This paperpresents a study of helicity during the emergence of active region NOAA 8578 Thetime history of spin helicity injection, related to footpoint rotation, suggests that anAlfv´en wave packet crossed the apex of the emerging fluxtube

sug-1 Introduction

Active regions often emerge as bipolar regions on the solar surface resembling

˝ shaped fluxtubes These newly emerged active regions appear as sunspots,concentrated with intense magnetic field strength.Longcope and Welsch(2000) pre-dicted, through a simplified dynamical model, that a fraction of the current generated

in the twisted emerging fluxtube will enter the corona Any initial current mismatchbetween the photosphere and the corona results in a magnetic torque that drivesrotation of the photospheric footpoints This rotation is part of a torsional Alfv´enwave propagating downward along the fluxtube and injecting helicity upward intothe corona

B Ravindra ( )

Indian Institute of Astrophysics, Bangalore, India

D.W Longcope

Department of Physics, Montana State University, Bozeman, USA

448

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Evolution of Coronal Helicity in a Twisted Emerging Active Region 449

By decomposing the photospheric helicity flux into spin and braiding nents, it is possible to interpret an observed emergence in terms of the model ofLongcope and Welsch(2000) In this paper we perform such a decomposition onhigh-resolution MDI magnetograms (0.600pixel size) taken at 1-min cadence duringthe emerging of active region NOAA 8578

compo-2 Results

Co-aligned portions of MDI magnetograms were corrected for magnetic field estimation (Berger and Lites 2003) and then 5-min boxcar-averaged to reduce noise.The horizontal velocities were computed and partitioning of the magnetogram se-quence was performed as inLongcope et al.(2007)

under-The emergence of AR 8578 is shown as a magnetogram time sequence in Fig.1.The active region started to emerge at 07:00 UT on 08 June 1999 at a rapid rate

of 1020Mx h1 As the active region emerged, the bipoles moved away from eachother at a rate of 292 m s1 The active region emerged with a tilt away from theEast-West direction, and after 40 h of emergence aligned itself along the East-Westdirection as can be seen in Fig.1

The two largest regions resulting from the partition, P01 and N01, are shown inthe left-hand panel of Fig.2 The right-hand part of Fig.2shows the rotation rate ofthe regions P01 and N01, computed with the method ofLongcope et al.(2007) Assoon as the active region started emerging, region P01 rotated anticlockwise, but itgradually changed its direction of rotation to clockwise The region then changed

Fig 1 Emergence of a bipole in active region NOAA 8578 on the solar surface close to disk center

at latitude 19ı The date and time of the magnetogram snapshots are specified on each image

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Power-Law Nanoflare Heating

L Prasad and V.K Joshi

Abstract Nanoflares are small impulsive events in the corona that dissipate

magnetic energy in the range 5 1023 to 1026ergs We model their istics through assuming a power-law event distribution

character-1 Introduction

The nanoflare hypothesis of Parker (1988) is that the corona contains a largeassembly of high-temperature elemental magnetic filaments or loops, created to-gether with the coronal magnetic field through randomly distributed impulsiveheating events They heat the loop plasma to 5 106K (Shimizu 1995) and suppos-edly are one of the main agents generating the high coronal temperature (Yoshidaand Tsuneta 1996;Watanabe 1995) Nanoflare coronal heating in terms of power-law distributions has been discussed by Dennis(1985).Kopp and Poletto (1993)extended this work to power-law indices ˛ > 2

be-REmax

E min.dN=dE/ dED A=.˛ 1/ E1˛

min

L Prasad ( ) and V.K Joshi

Kumaun University, Nainital, India

452

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Spectroscopic Diagnostics of Polar Coronal

Plumes

K Wilhelm, B.N Dwivedi, and W Curdt

Abstract Polar coronal plumes seen during solar eclipses can now be studied with

space-borne telescopes and spectrometers We briefly discuss such observationsfrom space with a view to understanding their plasma characteristics Using these

observations, especially from SUMER/SOHO, but also from EUVI/STEREO, we

deduce densities, temperatures, and abundance anomalies in plumes and inter-plumeregions, and discuss their implications for better understanding of these structures

in the Sun’s atmosphere

1 Introduction

Polar coronal plumes are ray-like structures aligned along open magnetic field lines

in polar coronal holes A total eclipse of the Sun shows these rays in white light,depicting the magnetic configuration of the Sun in a coronal hole Many studieshave been carried out to relate these rays to the coronal magnetic field inferred bycurrent-free photospheric magnetic field extrapolation The coronal plumes and theinter-plume regions seem to play a rˆole in the acceleration mechanism of the fastsolar wind They have been extensively observed from space across the electro-magnetic spectrum Investigations have been made to unravel the appearance anddisappearance of these plumes The fact remains that we know little about them,probably because we have no direct knowledge of the coronal magnetic field Theidentification of the sources that produce coronal plumes and their contribution tothe fast solar wind is still a matter of investigation (DeForest et al 1997;Wang

et al.1997;Wilhelm et al 1998;Gabriel et al 2003;Teriaca et al 2003;Antonucci

et al.2004;Curdt et al 2008) To understand the processes of plume formation, weneed to know the physical conditions in plumes and the surrounding inter-plume

K Wilhelm ( ) and W Curdt

Max-Planck-Institut f¨ur Sonnensystemforschung, Katlenburg-Lindau, Germany

B.N Dwivedi

Max-Planck-Institut f¨ur Sonnensystemforschung, Katlenburg-Lindau, Germany

and

Department of Applied Physics, Institute of Technology, BHU, Varanasi, India

454

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Spectroscopic Diagnostics of Polar Coronal Plumes 455environment, such as electron densities, ne, and electron temperatures, Te, the effec-tive ion temperatures and non-thermal motions, the plume cross-section relative tothe size of the coronal hole, and the plasma bulk speeds.

In this paper, we briefly discuss the observations of polar coronal plumes fromspace with a view for understanding their plasma characteristics Using these

measurements, especially from SUMER/SOHO, we deduce electron densities and

temperatures as well as abundance anomalies in plumes This will improve the derstanding of these structures in the Sun’s atmosphere, which are the subject of anInternational Team Study at ISSI, Bern.1

un-2 Spectroscopic Observations of Coronal Plumes

In the framework of a Hinode/STEREO/SOHO cooperation, observations of

coro-nal plumes in a corocoro-nal hole were performed in April 2007, using spectrographs

and imagers (cf., EUVI/STEREO) aboard these spacecraft (cf.,Curdt et al 2008).SUMER performed a scan in the southern coronal hole of the Sun from 7 April 2007(01:01 UTC) to 8 April 2007 (12:19 UTC) Emission was observed from the OVI,

NeVIII, MgVIII, MgIX, SiVIII, SiIX, AlIX, and NaIX lines The spectral lineswere recorded almost simultaneously at each location Contribution functions andthe FIP (First Ionization Potential) values of the corresponding elements are shown

in Fig.1 Density-dependent SiVIII and temperature-dependent MgIX line ratios

Fig 1 Contribution functions of the observed lines and the FIP values of the corresponding ements, based on ionic fractions from Mazzotta et al ( 1998 ) Neon and oxygen have high FIP values, while the other elements have low values <10 eV

el-1 http://www.issibern.ch/teams/solarcoronal

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456 K Wilhelm et al.were observed to produce neand Temaps Line-width studies allowed us to monitorthe ion temperatures, which are much higher than the electron temperatures.

Figure2shows a large raster above the southern coronal hole, obtained in severalVUV emission lines All maps are noisy above a height of 150 Mm The densityand temperature maps are, therefore, averaged over larger height ranges along theline of sight A detailed analysis of similar observations obtained in 2005 has shownthat the plume density is about five times higher than that of the environment in thisaltitude range (Wilhelm 2006) The electron temperature, Te, in plumes is lowerthan that in the interplume regions (cf.,Wilhelm et al 1998) This is confirmed bythe MgIXTe-sensitive line pair in the present data The insensitivity of the SiVIII

ratio to scattered radiation is discussed byWilhelm et al.(1998) It is caused by thelines being barely visible on the disk (Curdt et al 2001) so that the stray-light issubtracted by the standard background correction for coronal observations.Electron density and temperature maps are shown in Fig.3a, b, respectively,

in which gray represents cooler plasma conditions All radiance maps (except for

AlIX) show the plume structures It is still to be investigated why the AlIXradiancemap does not show plume structure The line ratio NeVIII/MgVIII can monitorthe abundance variations between high-FIP and low-FIP elements However, thedifferent temperatures in plume and inter-plume regions should be taken into ac-count in view of the high-temperature tail of the lithium-like Ne7C The contributionfunctions of NeVIIIand MgVIIIoverlap considerably in the temperature range justbelow 1 MK The contribution functions of NeVIIIand MgIX are more similar at

Fig 2 A large raster above the southern coronal hole was obtained in several VUV emission lines.

It took 36 h starting on 7 April 2007 at 01:01 UTC from West to East The photon radiance of O VI

1032 is shown here Radial dashed-dotted lines are shown at˙12 ıoff the pole

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A Flaring Polar Filament

C Dumitrache

Abstract A special, huge polar filament producing multiple CMEs is analyzed.

Around this double-S shaped filament a two-ribbon flare occurred which rose into

a CME

1 Introduction

This work is part of a series of papers concerning studies of a special sigmoid polarfilament It was observed on the solar disk between 11 and 19 August 2001 At first,two parallel filaments appeared, but on 13 August a double S-shape linked thesetwo into a single huge feature (Dumitrache 2008) On 15 August, multiple CMEswere registered (Dumitrache submitted) A double-S polar filament rose above thepolarity inversion line between the remnants of a complex of activity (a cluster ofactive regions that had developed during the previous solar rotation) and the unipolarmagnetic field surrounding it In the same rotation (CR 1979), another active region(AR9571) was located towards the South-West but too far away to influence thepolar filament

In this paper, we analyze an unusual event that occurred on 14 August 2001 inthe polar filament: the occurrence of a spotless flare It produced a CME registered

by LASCO at UT 16:30 as an LDE (long decay event) Eruptive flares tend to lead

to such long-decay events (MacCombie and Rust 1979) and to be more stronglyassociated with coronal mass ejections They are cooler and fainter than active-region flaring events A two-ribbon flare occurred in the main body of the complexdouble-S filament, representing only one event in the long and rich history of itsevolution Unfortunately, lack of data during the flare development inhibits detailedanalysis of the flare onset The two ribbons of the flare were situated on the filamentsides, while the post-flare loops covered all of the filament channel in the directionwhere the filament had minimum curvature

C Dumitrache ( )

Astronomical Institute of Romanian Academy, Bucharest, Romania

459

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460 C DumitracheThe Stanford whole-sun magnetograms indicate that large-scale magneticreconnections started on 13 August 2001 (Dumitrache 2008) On 15 August,the double-S filament gave repeated CMEs in a squall while part of it disappearedtemporary to reform during the next rotation We suggest that these large-scalereconnections caused the filament destabilization and the explosive events.

2 Observations

The association of S-shaped soft X-ray features with eruptivity is well established

An X-ray C2.3 GOES type flare was registered on 14 August between UT 10:56and 14:04, with its maximum at UT 12:42 The soft X-ray telescope of the Yohkohmission observed this flare between UT 10:47 and 16:42 with a mass ejection start-ing at UT 12:14 (Fig.1) A H˛ image (Fig.2) displays a two-ribbon flare with thefilament in the middle of the ribbons, but actually the latter are the foot points ofthe bright post-flare arcade The maximum of the flare was accompanied by a smallwave visible in EIT images and more evident in soft X-rays This wave stopped

in the bright plage near the filament South end (marked with a circle) The flareonset was situated very close to this region The flare development as seen in EITobservations is displayed in Fig.3

A halo CME was registered by SOHO at UT 16:01 at 618 km s1 speed On 14August 2001, deceleration of the differential rotation of the filament occurred as es-timated from the d’Azambuja law Actually, the foots of the filament end approachedeach other so that the curve of the filament’s tilt angle had an inflexion point On

Fig 1 Soft X-ray observations

A good correlation was found between ˛bestandh˛zi byBurnette et al.(2004) andLeka et al.( 199 6) The... Interior

DOI 10.1007 /97 8-3-642-028 59- 5 53, c Springer-Verlag Berlin... 106K (Shimizu 199 5) and suppos-edly are one of the main agents generating the high coronal temperature (Yoshidaand Tsuneta 199 6;Watanabe 199 5) Nanoflare coronal heating in

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