Active Region Evolution and the Removal of Magnetic Helicity by CMEs Len Culhane Mullard Space Science Laboratory University College London

12 th July, 2001Helicity Removal by CMEs2 SUMMARY Results of two investigations are presented: I.Long-term Evolution of Sigmoidal Structures and Related CME Activity (Glover, Harra, Matthews, Hori, Culhane, 2001, In Press, A&A) Morphological study of an AR complex over three rotations from 9-Mar-00 to 11-May-00 II.Magnetic Evolution of a Long-lived AR: the Source of Magnetic Helicity (Mandrini, Demoulin, van Driel-Gesztelyi, Plunkett, Thompson, Kovari, Aulanier, 2001, Submitted, A&A) Quantitative study of the magnetic helicity budget of AR 7978 over seven rotations from 6-Jul-96 to 16-Dec-96

12 th July, 2001Helicity Removal by CMEs3 I. Evolution of the AR 8906 Activity Complex - Sigmoids and CMEs (Glover et al., 2001, Astron. & Astrophys., In Press) Recent studies suggest high probability of eruption and associated CMEs (Hudson et al., 1998, GRL., Canfield et al., 1999, GRL., Glover et al., 2000, GRL.) Sigmoids – possible prediction of CMEs? (Sterling and Hudson 1997, Ap.J. Lett.)

12 th July, 2001Helicity Removal by CMEs4 Activity evolution is followed for three solar rotations (Glover et al.) SXT and MDI full disk observations are for each central meridian passage: 13-Mar-2000 (a, b), 10-Apr-2000 (c, d) 7-May-2000 (e, f). Arrows indicate a small emerging flux region – designated AR 8984 (e, f). A filament channel is seen in frames d and f. Evolution of the Activity Complex AR 8906

12 th July, 2001Helicity Removal by CMEs5 Time Distribution of GOES Events (> B1.0) and CME Onsets Numbers of X-ray events and CME launches that originated in the AR 8906 activity complex during three solar rotations in the interval 9 March to 12 May Central meridian passages are: 13 Mar, 10 Apr and 7 May.

12 th July, 2001Helicity Removal by CMEs6 Formation of a Sigmoidal Feature on 10-Apr-00 Full disk SXT (a,b,c) and MDI (d) images on 10-Apr-00 (Rotation 2) Flux emergence is seen at 19:28 U.T. along with the formation of a sigmoidal feature stretching from the emerging flux region to the filament channel The sigmoid is fully developed by 22:15 U.T. and remains visible for ~ 12 hours. The most CME-active period during the second rotation seems associated with the appearance of the sigmoid. Did newly emerging twisted flux increase the helicity content which was then shed by CMEs?

12 th July, 2001Helicity Removal by CMEs7 Yohkoh SXT full-disk image of a sigmoid on 8-May-00 at 09:45 UT Structure links remnants of earlier ARs associated with AR 8906 Inset shows the same region prior to eruption at 04:36 UT Sigmoid Structure formed on Rotation 3 between the Dispersed Remnants of Earlier Active Regions 8-May-00 04:36 UT 8-May-00 09:45 UT

12 th July, 2001Helicity Removal by CMEs8 Activity during Rotation 3 on 8 May-00 SXT EIT 195 HH Images taken before (a, c, e) and after (b, d, f) the eruption on 8 May. In the SXT images (a, b), several diffuse S-shaped loops are gradually replaced by a single twisted flux-rope Arrows in c and d show position of post-flare loops in the Southern half of the filament channel Arrows in e and f show the S-shaped filament underlying the soft X-ray Arcade. Events on 8 May are: B6.8 flare with peak at 06:23 U.T. Partial Halo CME Partial filament eruption

12 th July, 2001Helicity Removal by CMEs9 SXT image shows the sigmoid formed on 8 May (inset) while the full-disk image shows the situation on 10 May when a less sheared arcade replaced the sigmoid Events on 10 May are: Unclassified GOES flare with peak at 22:16 U.T. Expanding loop CME Filament eruption Formation of the less sheared structure coincides with an eruption and the launch of the last CME – are we again seeing helicity removal? Sigmoid replaced by less sheared Arcade following a CME 8-May-00 09:45 UT 10-May-00 12:28 UT

12 th July, 2001Helicity Removal by CMEs10 SXT Observations of Activity Evolution on 10-May-01 SXT images showing the region pre- and post eruption on 8 May. Pre-eruption (a), a highly sheared arcade is indicated by the arrow. Gradual rise and expansion of the arcade is shown in (b) and (c). Following a filament eruption, a much less sheared arcade is seen in (d) A loop-like CME is observed by LASCO C2 at 19:26 U.T. The height-time plot is consistent with with the gradual rise of the SXT arcade. Big Bear H  data shows a filament eruption. MDI data are consistent with helical field erupting with the filament material.

12 th July, 2001Helicity Removal by CMEs11 II. Source of Magnetic Helicity Shed by CMEs Demoulin, Mandrini, Van Driel-Gesztelyi, Plunkett et al. (2001) studied the Helicity budget of AR 7978 for 7 rotations from Jul-1996 Procedure:  Extrapolate (LFF) photospheric field (MDI) into corona/derive   Compute coronal AR helicity for each rotation  Calculate helicity due to differential rotation  Identify all AR 7978 CMEs and from average magnetic cloud properties, estimate the helicity shed by the CMEs  Compare the three helicity values to check if differential rotation could be the helicity source For the field extrapolation,  xB =  B so:  Compute field as f(  ) and match structures with observed loops  Repeat for different values of  to find best global fit

12 th July, 2001Helicity Removal by CMEs12 Magnetic Field Extrapolations N-S shear gradient usually requires two  values  = Mm -1    Mm -1 Parameter  determined by iteration Coronal field is computed for a given  and a match sought between field lines and SXT loops Repeated for a range of  values to find the best one for a global fit

12 th July, 2001Helicity Removal by CMEs13 Magnetic Helicity Magnetic Helicity is a globally conserved quantity: - Convection zone → Corona → Interplanetary Medium Helicity H = ∫ V A.B dV where A is the vector potential with B =  xA Requirement for V to be arbitrarily large posed a problem until: –Berger and Field (1984) defined relative magnetic helicity or where H r is gauge invariant and is independent of B and B 0 outside V

12 th July, 2001Helicity Removal by CMEs14 Production and Removal of Magnetic Helicity 1. Coronal Helicity is given by where B is the Fourier amplitude of the harmonic (n x,n y ), k x =  n x /L, k y = 2  n y /L, l = (k 2 x + k 2 y –  2 ) 1/2 and L represents the horizontal extent of the computational box The size and complexity of the AR require a large number of harmonics or N x = N y = 256

12 th July, 2001Helicity Removal by CMEs15 Production and Removal of Magnetic Helicity 2. Helicity generated by Differential Rotation has two terms or: where twist relates to the rotation of each magnetic polarity and writhe involves the relative rotation of positive and negative polarities. However the writhe term has always the opposite sign from the twist term for solar differential rotation so the two contributions oppose Helicity generation is affected by: Latitude of the AR Degree of magnetic field dispersion Tilt of the bipole

12 th July, 2001Helicity Removal by CMEs16 Production and Removal of Magnetic Helicity 3. Helicity in Ejected Magnetic Clouds Magnetic clouds are twisted flux tubes expanding as they move out from the Sun in a CME so assume a one-to-one association. Following Berger (1999), the relative helicity per unit length for a twisted flux tube is of the form where B axial is the cloud axial field and R the cloud radius. Following Lepping et al. (1990), B axial ~ G and R ~ cm For L ~ 0.5 AU (assumes disconnection from Sun, DeVore, 2000), H r ~ Mx 2 /cloud or per CME For L ~ 2 AU (assumes connection to Sun, Richardson, 1997) H r ~ Mx 2 /cloud or per CME

12 th July, 2001Helicity Removal by CMEs17 Evolution of AR 7987 Flares and CMEs in AR 7978 Rot. Date of CMP Flares CMEs no. X M C B Helicity Budget of AR 7978 Coronal Diff. Rot. Mag. Clouds (10 42 Mx 2 ) ( Mx 2 ) ( Mx 2 ) 17-July Aug Aug Sep Oct Nov [16, 64] [ 5, 11] 3 [10, 40] [17, 23] 3 [ 2, 8] [ 9, 12] 1 [14, 56] [ 4, 6] 0.8 [ 6, 24] [ 8, 32] Differential rotation generally fails to provide sufficient helicity for either the coronal field or the CMEs Ejected helicity is equivalent to that of a flux tube with the same flux as AR 7978 and having between 0.6 and 2.4 turns OUTCOME

12 th July, 2001Helicity Removal by CMEs18 CONCLUSIONS For the AR 8906 activity complex, S-shaped structures appear to form due to increasing shear leading to a build-up of helicity This build-up could be due to differential rotation or to the emergence of already sheared flux tubes S-like morphology is reduced by continued CME activity and is finally removed by CMEs in the third rotation when flare activity has almost ceased Main source of helicity is probably due to the emergence of twisted magnetic flux tubes that form the active region Following the work of Berger and others, relative magnetic helicity is defined and calculated for finite volumes related to AR 7968 After field extrapolations and calculation of the helicity i) stored in the corona and ii) injected by differential rotation, it is found that CMEs shed more helicity than is provided by i) and ii)

12 th July, 2001Helicity Removal by CMEs19