84 research outputs found
Latitude of Ephemeral Regions as Indicator of Strength of Solar Cycles
Digitized images of full disk CaK spectroheliograms from two solar
observatories were used to study cycle variation of ephemeral regions (ERs)
over ten solar cycles 14-23. We calculate monthly averaged unsigned latitude of
ERs and compare it with annual sunspot number. We find that average latitude of
ERs can be used as a predictor for strength of solar cycle. For a short-term
prediction (dT about 1-2 years), maximum latitude of ephemeral regions (in
current cycle) defines the amplitude of that cycle (higher is the latitude of
ERs, larger are the amplitudes of sunspot cycle). For a long-term prediction
(dT about 1.5 solar cycles), latitude of ERs at declining phase of n-th cycle
determines the amplitude of (n+2)-th sunspot cycle (lower is the latitude of
ERs, stronger is the cycle). Using this latter dependency, we forecast the
amplitude of sunspot cycle 24 at W=92 +/- 13 (in units of annual sunspot
number).Comment: Memorie della Societ\`a Astronomica Italiana, in pres
Bimodal Distribution of Magnetic Fields and Areas of Sunspots
We applied automatic identification of sunspot umbrae and penumbrae to daily
observations from the Helioseismic Magnetic Imager (HMI) on board the Solar
Dynamics Observatory (SDO) to study their magnetic flux (B) and area (A). The
results confirm a previously known logarithmic relationship between the area of
sunspots and their maximum flux density. In addition, we find that the relation
between average magnetic flux (Bavg) and sunspot area shows a bimodal
distribution: for small sunspots and pores (A < 20 millionth of solar
hemisphere, MSH), Bavg = 800 G (gauss), and for large sunspots (A > 100 MSH),
Bavg is about 600 G. For intermediate sunspots, average flux density linearly
decreases from about 800 G to 600 G. A similar bimodal distribution was found
in several other integral parameters of sunspots. We show that this bimodality
can be related to different stages of sunspot penumbra formation and can be
explained by the difference in average inclination of magnetic fields at the
periphery of small and large sunspots.Comment: 13 pages, 5 figures, accepted for publication in Solar Physic
Signature of Differential Rotation in Sun-as-a-Star Ca II K Measurements
The characterization of solar surface differential rotation (SDR) from
disk-integrated chromospheric measurements has important implications for the
study of differential rotation and dynamo processes in other stars. Some
chromospheric lines, such as Ca II K, are very sensitive to the presence of
activity on the disk and are an ideal choice for investigating SDR in Sun-as-a
star observations. Past studies indicate that when the activity is low, the
determination of Sun's differential rotation from integrated-sunlight
measurements becomes uncertain. However, our study shows that using the proper
technique, SDR can be detected from these type of measurements even during
periods of extended solar minima. This paper describes results from the
analysis of the temporal variations of Ca II K line profiles observed by the
Integrated Sunlight Spectrometer (ISS) during the declining phase of Cycle 23
and the rising phase of Cycle 24, and discusses the signature of SDR in the
power spectra computed from time series of parameters derived from these
profiles. The described methodology is quite general, and could be applied to
photometric time series of other Main-Sequence stars for detecting differential
rotation.Comment: Accepted for publication in The Astrophysical Journal. 38 pages, 10
figure
Search for a Signature of Twist-Removal in the Magnetic Field of Sunspots in Relation with Major Flares
We investigate the restructuring of the magnetic field in sunspots associated
with two flares: the X6.5 flare on 6 December 2006 and the X2.2 flare on 15
February 2011. The observed changes were evaluated with respect to the
so-called twist-removal model, in which helicity (twist) is removed from the
corona as the result of an eruption. Since no vector magnetograms were
available for the X6.5 flare, we applied the azimuthal symmetry approach to
line-of-sight magnetograms to reconstruct the pseudo-vector magnetic field and
investigate the changes in average twist and inclination of magnetic field in
the sunspot around the time of the flare. For the X2.2 flare, results from the
full vector magnetograms were compared with the pseudo-vector field data. For
both flares, the data show changes consistent with the twist-removal scenario.
We also evaluate the validity of the azimuthal symmetry approach on simple
isolated round sunspots. In general, the derivations based on the azimuthal
symmetry approach agree with true-vector field data though we find that even
for symmetric sunspots the distribution of the magnetic field may deviate from
an axially symmetric distribution.Comment: accepted for publication in the Ap
Camera Gap Removal in SOLIS/VSM Images
The Vector Spectromagnetograph (VSM) instrument on the Synoptic Optical
Longterm Investigations of the Sun (SOLIS) telescope is capable of obtaining
spectropolarimetry for the full Sun (or a select latitudinal range) with one
arcsecond spatial resolution and 0.05 Angstrom spectral resolution. This is
achieved by scanning the Sun in declination and building up spectral cubes for
multiple polarization states, utilizing a beamsplitter and two separate 2k x 2k
CCD cameras. As a result, the eastern and western hemispheres of the Sun are
separated in preliminary VSM images by a vertical gap with soft edges and
variable position and width. Prior to the comprehensive analysis presented in
this document, a trial-and-error approach to removing the gap had yielded an
algorithm that was inconsistent, undocumented, and responsible for incorrectly
eliminating too many image columns. Here we describe, in detail, the basis for
a new, streamlined, and properly calibrated prescription for locating and
removing the gap that is correct to within approximately one arcsecond (one
column)
Correlation Between Sunspot Number and ca II K Emission Index
Long-term synoptic observations in the resonance line of Ca II K constitute a
fundamental database for a variety of retrospective analyses of the state of
the solar magnetism. Synoptic Ca II K observations began in late 1904 at the
Kodaikanal Observatory, in India. In early 1970s, the National Solar
Observatory (NSO) at Sacramento Peak (USA) started a new program of daily
Sun-as-a-star observations in the Ca II K line. Today the NSO is continuing
these observations through its Synoptic Optical Long-term Investigations of the
Sun (SOLIS) facility. These different data sets can be combined into a single
disk-integrated Ca II K index time series that describes the average properties
of the chromospheric emission over several solar cycles. We present such a Ca
II K composite and discuss its correlation with the new entirely revised
sunspot number data series. For this preliminary investigation, the scaling
factor between pairs of time series was determined assuming a simple linear
model for the relationship between the monthly mean values during the duration
of overlapping observations.Comment: 16 pages, 8 figures, accepted for publication in Solar Physic
Evolution of Magnetic Helicity in Solar Cycle 24
We propose a novel approach to reconstruct the surface magnetic helicity
density on the Sun or sun-like stars. The magnetic vector potential is
determined via decomposition of vector magnetic field measurements into
toroidal and poloidal components. The method is verified using data from a
non-axisymmetric dynamo model. We apply the method to vector field synoptic
maps from Helioseismic and Magnetic Imager (HMI) onboard of Solar Dynamics
Observatory (SDO) to study evolution of the magnetic helicity density during
solar cycle 24. It is found that the mean helicity density of the
non-axisymmetric magnetic field of the Sun evolves in a way which is similar to
that reported for the current helicity density of the solar active regions. It
has predominantly the negative sign in the northern hemisphere, and it is
positive in the southern hemisphere. Also, the hemispheric helicity rule for
the non-axisymmetric magnetic field showed the sign inversion at the end of
cycle 24. Evolution of magnetic helicity density of large-scale axisymmetric
magnetic field is different from that expected in dynamo theory. On one hand,
the mean large- and small-scale components of magnetic helicity density display
the hemispheric helicity rule of opposite sign at the beginning of cycle 24.
However, later in the cycle, the two helicities exhibit the same sign in
contrast with the theoretical expectations.Comment: 8 pages 5 figure
First use of synoptic vector magnetograms for global nonlinear force free coronal magnetic field models
The magnetic field permeating the solar atmosphere is generally thought to
provide the energy for much of the activity seen in the solar corona, such as
flares, coronal mass ejections (CMEs), etc. To overcome the unavailability of
coronal magnetic field measurements, photospheric magnetic field vector data
can be used to reconstruct the coronal field. Currently there are several
modelling techniques being used to calculate three-dimension of the field lines
into the solar atmosphere. For the first time, synoptic maps of photospheric
vector magnetic field synthesized from Vector Spectromagnetograph (VSM) on
Synoptic Optical Long-term Investigations of the Sun (SOLIS) are used to model
the coronal magnetic field and estimate free magnetic energy in the global
scale. The free energy (i.e., the energy in excess of the potential field
energy) is one of the main indicators used in space weather forecasts to
predict the eruptivity of active regions. We solve the nonlinear force-free
field equations using optimization principle in spherical geometry. The
resulting three-dimensional magnetic fields are used to estimate the magnetic
free energy content E_{free}=E_{nlfff}-E_{pot}, i.e., the difference of the
magnetic energies between the nonpotential field and the potential field in the
global solar corona. For comparison, we overlay the extrapolated magnetic field
lines with the extreme ultraviolet (EUV) observations by the Atmospheric
Imaging Assembly on board SDO. For a single Carrington rotation 2121, we find
that the global NLFFF magnetic energy density is 10.3% higher than the
potential one. Most of this free energy is located in active regions.Comment: Submitted to Astronomy and Astrophysics Journa
Zeemanfit: Use and Development of the solis_vms_zeemanfit code
The purpose of the SOLIS Zeemanfit Code is to provide a straight-forward,
easily checked measure of the total magnetic-field strength in the
high-strength umbral regions of the solar disk. In the highest-strength
regions, the Zeeman splitting of the 6302-angstrom Fe line becomes wide enough
for the triplet nature of the line to be visible by eye in non-polarized light.
Therefore, a three-line fit to the spectra should, in principle, provide a
fairly robust measure of the total magnetic-field strength.
The code uses the Level-1.5 spec-cube data of the SOLIS VSM 6302-vector
observations (specifically the Stokes-I and Stokes-V components) to fit the
line profiles at each appropriate pixel and calculate the
magnetic-field-strength from the line-center separation of the two fit 6302.5
sigma-components. The 6301.5-angstrom Fe line is also present and fit in the
VSM 6302-vector data, but it is an anomalous-Zeeman line with a weaker response
to magnetic fields. Therefore, no magnetic- field measure is derived from this
portion of the spectral fit.Comment: 39 pages containing 30 figures. Represents the primary documentation
for the new SOLIS VSM Zeemanfit data product; corrected confusing variable
units listed below Equation
The reversal of the Sun's magnetic field in cycle 24
Analysis of synoptic data from the Vector Stokes Magnetograph (VSM) of the
Synoptic Optical Long-term Investigations of the Sun (SOLIS) and the NASA/NSO
Spectromagnetograph (SPM) at the NSO/Kitt Peak Vacuum Telescope facility shows
that the reversals of solar polar magnetic fields exhibit elements of a
stochastic process, which may include the development of specific patterns of
emerging magnetic flux, and the asymmetry in activity between northern and
southern hemispheres. The presence of such irregularities makes the modeling
and prediction of polar field reversals extremely hard if possible. In a
classical model of solar activity cycle, the unipolar magnetic regions (UMRs)
of predominantly following polarity fields are transported polewards due to
meridional flows and diffusion. The UMRs gradually cancel out the polar
magnetic field of the previous cycle, and re-build the polar field of opposite
polarity setting the stage for the next cycle. We show, however, that this
deterministic picture can be easily altered by the developing of a strong
center of activity, or by the emergence of an extremely large active region, or
by a "strategically placed" coronal hole. We demonstrate that the activity
occurring during the current cycle 24 may be the result of this randomness in
the evolution of the solar surface magnetic field.Comment: 19 pages, 5 figures, 1 table, accepted for publication in "The
Journal Solar-Terrestrial Physics
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