Helicitydriven sigmoid evolution and its role in CME initiation

1
Helicity-driven sigmoid evolution and its role in
CME initiation
David Alexander, Rice University
Evolution and eruption of a soft X-ray sigmoid
associated with an instability arising from the
continuous injection of helicity by the rotation
of the underlying sunspot (from Alexander et al.,
2005)
SOHO/MDI Magnetograms showing the evolution of a
long-lived active region spanning 4 months note
the counter-clockwise rotation of the main spot
group (from Tian et al., 2005)
2
Helicity-driven sigmoid evolution and its role in
CME initiation
David Alexander, Rice University
The proposed effort builds upon recent
observations of rotating sunspots in TRACE white
light images and their apparent association with
soft X-ray sigmoids evolution and flare
production. The role played by the rotating
spots, and the associated injection of helicity,
in the generation of flares and CMEs is not yet
fully understood although there have been a
number of intriguing studies by various groups.
Despite the delays caused by the transition of
the original grant to Rice University, following
the PIs move there, and the setting up of a
contract with Lockheed Martin co-investigator,
Dr. Rich Nightingale, significant progress has
been made in our study of rotating sunspots
resulting in several presentations at scientific
meetings and the production of a number of
relevant papers (see below). As part of the
setting up process the PI hired post-doc Dr.
Lirong Tian at Rice University in June of 2004.
The number of rotating sunspot events observed
with TRACE has significantly increased and these
are currently being prepared for a detailed
velocity analysis. Preliminary studies of some of
the largest events have shown a clear association
between sunspot rotation and the production of
X-class flares (Nightingale and Metcalf, 2005).
This has been augmented by a SOHO/MDI study of
104 active regions exhibiting a magnetic
d-configuration and utilizing the apparent
rotation of the sunspot and the tilt angle to the
equator to determine the relationship between
twist and writhe helicity (Tian et al., 2005a).
In a complementary study we explored the behavior
of the magnetic field and associated coronal
activity of a single long-lived (over 4 solar
rotations) active region to show that the sense
of helicity and the sense of sunspot rotation are
consistent with an emergence of a twisted
W-fluxtube which has been
subjected to the kink instability in the
sub-photosphere. This has been written up for
Solar Physics and is currently under review (Tian
et al., 2005b). One of the primary aims of the
original proposal was examine the role of the
coronal helicity injection implicit in the
sunspot rotation as the proximate cause for the
sigmoid evolution. An initial study of two
sunspot rotation cases and using a stackplot
velocity analysis has indicated that the
evolution of the sunspot rotation, i.e. the
duration over which it injects helicity into the
corona, can determine whether a coronal sigmoid
reaches an instability limit and therefore erupts
or not (Alexander et al., 2005). This study is
to be augmented by detailed local correlation
tracking techniques and vector magnetic field
measurements to better determine the helicity
injected into individual coronal structures.
Publications 1. Magnetic twist and writhe of
delta active regions, L.Tian, D. Alexander, Y.
Liu J.Yang, Solar Phys., 2005a, in press. 2.
Formation of a twisted and kink-driven fluxtube
for a long-lived active region AR 9632, L.
Tian, Y. Liu, J.Yang D. Alexander, Solar Phys.,
2005b, in review. 3. The role of sunspot
rotation in driving sigmoid eruptions, D.
Alexander, R. Nightingale, T. R. Metcalf, D. S.
Brown, 2005, in preparation. 4. TRACE
Observations of Many Active Regions with X-flares
and Rotating Sunspots in the Current Solar Cycle,
R. Nightingale T. R. Metcalf, 2005, in
preparation. 5. Sunspot and sunspot group
rotation of NOAA AR 9684, L. Tian, D. Alexander,
Y. Liu J. Yang, 2005c, in preparation.