10 research outputs found
On the Absence of Photospheric Net Currents in Vector Magnetograms of Sunspots Obtained From Hinode (SOT/SP)
Various theoretical and observational results have been reported regarding
the presence/absence of net electric currents in the sunspots. The limited
spatial resolution of the earlier observations perhaps obscured the
conclusions. We have analyzed 12 sunspots observed from Hinode (SOT/SP) to
clarify the issue. The azimuthal and radial components of magnetic fields and
currents have been derived. The azimuthal component of the magnetic field of
sunspots is found to vary in sign with azimuth. The radial component of the
field also varies in magnitude with azimuth. While the latter pattern is a
confirmation of the interlocking combed structure of penumbral filaments, the
former pattern shows that the penumbra is made up of a "curly interlocking
combed" magnetic field. The azimuthally averaged azimuthal component is seen to
decline much faster than 1/ in the penumbra, after an initial increase
in the umbra, for all the spots studied. This confirms the confinement of
magnetic fields and absence of a net current for sunspots as postulated by
\cite{parker96}. The existence of a global twist for a sunspot even in the
absence of a net current is consistent with a fibril-bundle structure of the
sunspot magnetic fields.Comment: 15 pages, 4 figures, 1 table; accepted for publication in the ApJ
Letter
Magnetic Non-Potentiality of Solar Active Regions and Peak X-Ray Flux of the Associated Flares
Predicting the severity of the solar eruptive phenomena like flares and
Coronal Mass Ejections (CMEs) remains a great challenge despite concerted
efforts for several decades. The advent of high quality vector magnetograms
obtained from Hinode (SOT/SP) has increased the possibility of meeting this
challenge. In particular, the Spatially Averaged Signed Shear Angle (SASSA)
seems to be an unique parameter to quantify the non-potentiality of the active
regions. We demonstrate the usefulness of SASSA for predicting the flare
severity. For this purpose we present case studies of the evolution of magnetic
non-potentiality using 115 vector magnetograms of four active regions namely
ARs NOAA 10930, 10960, 10961 and 10963 during December 08-15, 2006, June 03-10,
2007, June 28-July 5, 2007 and July 10-17, 2007 respectively. The NOAA ARs
10930 and 10960 were very active and produced X and M class flares
respectively, along with many smaller X-ray flares. On the other hand, the NOAA
ARs 10961 and 10963 were relatively less active and produced only very small
(mostly A and B-class) flares. For this study we have used a large number of
high resolution vector magnetograms obtained from Hinode (SOT/SP). The analysis
shows that the peak X-ray flux of the most intense solar flare emanating from
the active regions depends on the magnitude of the SASSA at the time of the
flare. This finding of the existence of a lower limit of SASSA for a given
class of X-ray flare will be very useful for space weather forecasting. We have
also studied another non-potentiality parameter called mean weighted shear
angle (MWSA) of the vector magnetograms along with SASSA. We find that the MWSA
does not show such distinction as the SASSA for upper limits of GOES X-Ray flux
of solar flares, however both the quantities show similar trends during the
evolution of all active regions studied.Comment: 25 pages, 5 figures, accepted for publication in the Astrophysical
Journa
Global Twist of Sunspot Magnetic Fields Obtained from High Resolution Vector Magnetograms
The presence of fine structures in the sunspot vector magnetic fields has
been confirmed from Hinode as well as other earlier observations. We studied 43
sunspots based on the data sets taken from ASP/DLSP, Hinode (SOT/SP) and SVM
(USO). In this \emph{Letter}, (i) We introduce the concept of signed shear
angle (SSA) for sunspots and establish its importance for non force-free
fields. (ii) We find that the sign of global (force-free parameter) is
well correlated with the global SSA and the photospheric chirality of sunspots.
(iii) Local patches of opposite signs are present in the umbra of each
sunspot. The amplitude of the spatial variation of local in the umbra
is typically of the order of the global of the sunspot. (iv) We find
that the local is distributed as alternately positive and negative
filaments in the penumbra. The amplitude of azimuthal variation of the local
in the penumbra is approximately an order of magnitude larger than
that in the umbra. The contributions of the local positive and negative
currents and in the penumbra cancel each other giving almost no
contribution for their global values for whole sunspot. (v) Arc-like structures
(partial rings) with a sign opposite to that of the dominant sign of
of the umbral region are seen at the umbral-penumbral boundaries of some
sunspots. (vi) Most of the sunspots studied, belong to the minimum epoch of the
23 solar cycle and do not follow the so-called hemispheric helicity
rule.Comment: 15 pages, 3 figures, 1 table; Accepted for publication in the ApJ
Letter
Effect of Polarimetric Noise on the Estimation of Twist and Magnetic Energy of Force-Free Fields
The force-free parameter , also known as helicity parameter or twist
parameter, bears the same sign as the magnetic helicity under some restrictive
conditions. The single global value of for a whole active region gives
the degree of twist per unit axial length. We investigate the effect of
polarimetric noise on the calculation of global value and magnetic
energy of an analytical bipole. The analytical bipole has been generated using
the force-free field approximation with a known value of constant and
magnetic energy. The magnetic parameters obtained from the analytical bipole
are used to generate Stokes profiles from the Unno-Rachkovsky solutions for
polarized radiative transfer equations. Then we add random noise of the order
of 10 of the continuum intensity (I) in these profiles to simulate
the real profiles obtained by modern spectropolarimeters like Hinode (SOT/SP),
SVM (USO), ASP, DLSP, POLIS, SOLIS etc. These noisy profiles are then inverted
using a Milne-Eddington inversion code to retrieve the magnetic parameters.
Hundred realizations of this process of adding random noise and polarimetric
inversion is repeated to study the distribution of error in global and
magnetic energy values. The results show that : (1). the sign of is
not influenced by polarimetric noise and very accurate values of global twist
can be calculated, and (2). accurate estimation of magnetic energy with
uncertainty as low as 0.5% is possible under the force-free condition.Comment: 33 pages, 6 figures, 1 table; Accepted for publication in the Ap
Nonlinear force-free modelling: influence of inaccuracies in the measured magnetic vector
Context: Solar magnetic fields are regularly extrapolated into the corona
starting from photospheric magnetic measurements that can suffer from
significant uncertainties. Aims: Here we study how inaccuracies introduced into
the maps of the photospheric magnetic vector from the inversion of ideal and
noisy Stokes parameters influence the extrapolation of nonlinear force-free
magnetic fields. Methods: We compute nonlinear force-free magnetic fields based
on simulated vector magnetograms, which have been produced by the inversion of
Stokes profiles, computed froma 3-D radiation MHD simulation snapshot. These
extrapolations are compared with extrapolations starting directly from the
field in the MHD simulations, which is our reference. We investigate how line
formation and instrumental effects such as noise, limited spatial resolution
and the effect of employing a filter instrument influence the resulting
magnetic field structure. The comparison is done qualitatively by visual
inspection of the magnetic field distribution and quantitatively by different
metrics. Results: The reconstructed field is most accurate if ideal Stokes data
are inverted and becomes less accurate if instrumental effects and noise are
included. The results demonstrate that the non-linear force-free field
extrapolation method tested here is relatively insensitive to the effects of
noise in measured polarization spectra at levels consistent with present-day
instruments. Conclusions heading: Our results show that we can reconstruct the
coronal magnetic field as a nonlinear force-free field from realistic
photospheric measurements with an accuracy of a few percent, at least in the
absence of sunspots.Comment: A&A, accepted, 9 Pages, 4 Figure
Magnetic Tension of Sunspot Fine Structures
The equilibrium structure of sunspots depends critically on its magnetic
topology and is dominated by magnetic forces. Tension force is one component of
the Lorentz force which balances the gradient of magnetic pressure in
force-free configurations. We employ the tension term of the Lorentz force to
clarify the structure of sunspot features like penumbral filaments, umbral
light bridges and outer penumbral fine structures. We compute vertical
component of tension term of Lorentz force over two active regions namely NOAA
AR 10933 and NOAA AR 10930 observed on 05 January 2007 and 12 December 2006
respectively. The former is a simple while latter is a complex active region
with highly sheared polarity inversion line (PIL). The vector magnetograms used
are obtained from Hinode(SOT/SP). We find an inhomogeneous distribution of
tension with both positive and negative signs in various features of the
sunspots. The existence of positive tension at locations of lower field
strength and higher inclination is compatible with the uncombed model of the
penumbral structure. Positive tension is also seen in umbral light bridges
which could be indication of uncombed structure of the light bridge. Likewise,
the upward directed tension associated with bipolar regions in the penumbra
could be a direct confirmation of the sea serpent model of penumbral
structures. Upward directed tension at the PIL of AR 10930 seems to be related
to flux emergence. The magnitude of the tension force is greater than the force
of gravity in some places, implying a nearly force-free configuration for these
sunspot features. From our study, magnetic tension emerges as a useful
diagnostic of the local equilibrium of the sunspot fine structures.Comment: 06 pages, 6 figures; Accepted for publication in the Astronomy &
Astrophysics as a "Letter to the Editor
On the Force-Freeness of the Photospheric Sunspot Magnetic Fields as Observed from Hinode (SOT/SP)
A magnetic field is force-free if there is no interaction between the
magnetic field and plasma in surrounding atmosphere i.e., electric currents are
aligned with the magnetic field, giving rise to zero Lorentz force. Computation
of various magnetic parameters such as magnetic energy, gradient of twist of
sunspot fields and any kind of extrapolations, heavily hinge on the force-free
approximation of the photospheric sunspot magnetic fields. Thus it is important
to inspect the force-freeness of sunspot fields. The force-freeness of sunspot
magnetic fields has been examined earlier by some researchers ending with
incoherent results. Accurate photospheric vector field measurements with high
spatial resolution are required to inspect the force-free nature of sunspots.
We use several such vector magnetograms obtained from the Solar Optical
Telescope/Spectro-Polarimeter aboard the Hinode. Both necessary and sufficient
conditions for force-freeness are examined by checking global and local nature
of magnetic forces over sunspots. We find that the sunspot magnetic fields are
not much away from force-free configuration, although they are not completely
force-free on the photosphere. The umbral and inner penumbral fields are more
force-free than the middle and the outer penumbral fields. During their
evolution, sunspot magnetic fields are found to maintain their proximity to
force-free behaviour. Although a dependence of net Lorentz force components is
seen on the evolutionary stages of the sunspots, we don't find a systematic
relationship between the nature of sunspot fields and associated flare
activity. Further, we examine whether the fields at photosphere follow linear
or non-linear force free conditions. After examining this in various complex
and simple sunspots we conclude that,in either case,the photospheric sunspot
fields are closer to satisfy non linear force-free field approximation.Comment: 34 pages, 4 figures, 2 tables; accepted for publication in the
Astrophysical Journa