22 research outputs found
Nonlinear force-free magnetic field extrapolations: comparison of the Grad-Rubin and Wheatland-Sturrock-Roumeliotis algorithm
We compare the performance of two alternative algorithms which aim to
construct a force-free magnetic field given suitable boundary conditions. For
this comparison, we have implemented both algorithms on the same finite element
grid which uses Whitney forms to describe the fields within the grid cells. The
additional use of conjugate gradient and multigrid iterations result in quite
effective codes. The Grad-Rubin and Wheatland-Sturrock-Roumeliotis algorithms
both perform well for the reconstruction of a known analytic force-free field.
For more arbitrary boundary conditions the Wheatland-Sturrock-Roumeliotis
approach has some difficulties because it requires overdetermined boundary
information which may include inconsistencies. The Grad-Rubin code on the other
hand loses convergence for strong current densities. For the example we have
investigated, however, the maximum possible current density seems to be not far
from the limit beyond which a force free field cannot exist anymore for a given
normal magnetic field intensity on the boundary.Comment: 21 pages, 13 figure
How to optimize nonlinear force-free coronal magnetic field extrapolations from SDO/HMI vector magnetograms?
The SDO/HMI instruments provide photospheric vector magnetograms with a high
spatial and temporal resolution. Our intention is to model the coronal magnetic
field above active regions with the help of a nonlinear force-free
extrapolation code. Our code is based on an optimization principle and has been
tested extensively with semi-analytic and numeric equilibria and been applied
before to vector magnetograms from Hinode and ground based observations.
Recently we implemented a new version which takes measurement errors in
photospheric vector magnetograms into account. Photospheric field measurements
are often due to measurement errors and finite nonmagnetic forces inconsistent
as a boundary for a force-free field in the corona. In order to deal with these
uncertainties, we developed two improvements: 1.) Preprocessing of the surface
measurements in order to make them compatible with a force-free field 2.) The
new code keeps a balance between the force-free constraint and deviation from
the photospheric field measurements. Both methods contain free parameters,
which have to be optimized for use with data from SDO/HMI. Within this work we
describe the corresponding analysis method and evaluate the force-free
equilibria by means of how well force-freeness and solenoidal conditions are
fulfilled, the angle between magnetic field and electric current and by
comparing projections of magnetic field lines with coronal images from SDO/AIA.
We also compute the available free magnetic energy and discuss the potential
influence of control parameters.Comment: 17 Pages, 6 Figures, Sol. Phys., accepte
Can We Improve the Preprocessing of Photospheric Vector Magnetograms by the Inclusion of Chromospheric Observations?
The solar magnetic field is key to understanding the physical processes in
the solar atmosphere. Nonlinear force-free codes have been shown to be useful
in extrapolating the coronal field upward from underlying vector boundary data.
However, we can only measure the magnetic field vector routinely with high
accuracy in the photosphere, and unfortunately these data do not fulfill the
force-free condition. We must therefore apply some transformations to these
data before nonlinear force-free extrapolation codes can be self-consistently
applied. To this end, we have developed a minimization procedure that yields a
more chromosphere-like field, using the measured photospheric field vectors as
input. The procedure includes force-free consistency integrals, spatial
smoothing, and -- newly included in the version presented here -- an improved
match to the field direction as inferred from fibrils as can be observed in,
e.g., chromospheric H images. We test the procedure using a model
active-region field that included buoyancy forces at the photospheric level.
The proposed preprocessing method allows us to approximate the chromospheric
vector field to within a few degrees and the free energy in the coronal field
to within one percent.Comment: 22 pages, 6 Figur
Magnetic Connectivity between Active Regions 10987, 10988, and 10989 by Means of Nonlinear Force-Free Field Extrapolation
Extrapolation codes for modelling the magnetic field in the corona in
cartesian geometry do not take the curvature of the Sun's surface into account
and can only be applied to relatively small areas, \textit{e.g.}, a single
active region. We apply a method for nonlinear force-free coronal magnetic
field modelling of photospheric vector magnetograms in spherical geometry which
allows us to study the connectivity between multi-active regions. We use vector
magnetograph data from the Synoptic Optical Long-term Investigations of the Sun
survey (SOLIS)/Vector Spectromagnetograph(VSM) to model the coronal magnetic
field, where we study three neighbouring magnetically connected active regions
(ARs: 10987, 10988, 10989) observed on 28, 29, and 30 March 2008, respectively.
We compare the magnetic field topologies and the magnetic energy densities and
study the connectivities between the active regions(ARs). We have studied the
time evolution of magnetic field over the period of three days and found no
major changes in topologies as there was no major eruption event. From this
study we have concluded that active regions are much more connected
magnetically than the electric current.Comment: Solar Physic
Coronal Magnetic Field Structure and Evolution for Flaring AR 11117 and its Surroundings
In this study, photospheric vector magnetograms obtained with the Synoptic
Optical Long-term Investigations of the Sun survey (SOLIS), are used as
boundary conditions to model the three-dimensional nonlinear force-free (NLFF)
coronal magnetic fields as a sequence of nonlinear force-free equilibria in
spherical geometry. We study the coronal magnetic field structure inside active
regions and its temporal evolution. We compare the magnetic field configuration
obtained from NLFF extrapolation before and after flaring event in active
region (AR) 11117 and its surroundings observed on 27 October 2010. We compare
the magnetic field topologies and the magnetic energy densities and study the
connectivities between AR 11117 and its surroundings. During the investigated
time period, we estimate the change in free magnetic energy from before to
after the flare to be 1.74x10^{32}erg which represents about 13.5% of nonlinear
force-free magnetic energy before the flare. In this study, we find that
electric currents from AR 11117 to its surroundings were disrupted after the
flare.Comment: 14 pages, 14 figures, Accepted by Solar Physics Journa
PROFIL WISATAWAN MUSEUM RADYA PUSTAKA SURAKARTA
Anggit Margaret, C9407031 2011. Profil Wisatawan Museum
Radya Pustaka Surakarta. Program Studi Diploma III Usaha Perjalanan
Wisata Fakultas Sastra Dan Seni Rupa Universitas Sebelas Maret Surakarta.
Penelitian tugas akhir ini mengkaji tentang Profil Wisatawan di
Museum Radya Pustaka Surakarta. Tujuan dari penelitian ini adalah untuk
mengetahui dari daerah mana saja wisatawan yang berkunjung ke Museum
Radya Pustaka, bagaimana ciri-ciri wisatawan yang berkunjung ke Museum
Radya Pustaka serta harapan-harapan yang diinginkan wisatawan terhadap
Museum Radya Pustaka.
Penelitian dilakukan dengan metode kualitatif. Pengumpulan data
dilakukan melalui wawancara dengan narasumber wisatawan yang berkujung
di Museum Radya Pustaka Surakarta tempat penulis melakukan penelitian,
serta studi pustaka dan studi dokumen guna menambah sumber data.
Hasil penelitian menunjukkan bahwa (1) Sebagian besar wisatawan
yang datang berasal dari Semarang sebesar 32%. (2) Mayoritas wisatawan
yang berkunjung ke Museum Radya Pustaka berusia antara 17-25 tahun dan
kebanyakan dari mereka adalah pelajar atau mahasiswa dengan prosentase
52%. (3) Sebagian besar wisatawan yang datang ke Museum Radya Pustaka
adalah bertujuan untuk melakukan penelitian yaitu sebesar 34%. (4) Harapan
wisatawan yang berkunjung terhadap kelangsungan Museum Radya Pustaka
sebagian besar adalah agar ditingkatkan lagi pengelolaan dan keamanan
museum, agar kejadian hilangnya benda-benda koleksi museum tidak terulang
lagi dikemudian hari.
Kesimpulan dari hasil penelitian ini bahwa wisatawan yang berkujung
ke Museum Radya Pustaka Surakarta mayoritas berasal dari Semarang,
mayoritas berusia 17-25 tahun dan kebanyakan dari mereka adalah berprofesi
sebagai pelajar dan mahasiswa. Kebanyakan wisatawan yang datang bertujuan
untuk melakukan penelitian, serta harapan wisatawan terhadap Museum
Radya Pustaka adalah supaya lebih ditingkatkan lagi pengelolaan dan
keamanan museum