466 research outputs found
The Vorticity of Solar Photospheric Flows on the Scale of Granulation
We employ time sequences of images observed with a G-band filter (4305{\AA})
by the Solar Optical Telescope (SOT) on board of Hinode spacecraft at different
latitude along solar central me-ridian to study vorticity of granular flows in
quiet Sun areas during deep minimum of solar activity. Using a feature
correlation tracking (FCT) technique, we calculate the vorticity of
granular-scale flows. Assuming the known pattern of vertical flows (upward in
granules and downward in inter-granular lanes), we infer the sign of kinetic
helicity of these flows. We show that the kinetic helicity of granular flows
and intergranular vortices exhibits a weak hemispheric preference, which is in
agreement with the action of the Coriolis force. This slight hemispheric sign
asymmetry, however, is not statistically significant given large scatter in the
average vorticity. The sign of the current he-licity density of network
magnetic fields computed using full disk vector magnetograms from the Synoptic
Optical Long-term Investigations of the Sun (SOLIS) does not show any
hemispheric preference. The combination of these two findings suggests that the
photospheric dynamo operating on the scale of granular flows is non-helical in
nature.Comment: 11 pages, 5 figures, in press Geomagnetism and Aeronom
Magnetic helicity of global field in cycles 23 and 24
For the first time we reconstruct the magnetic helicity density of global
axisymmetric field of the Sun using method proposed by Brandenburg et al.
(2003) and Pipin et al. (2013). To determine the components of the vector
potential, we apply the gauge which is typically employed in mean-field dynamo
models. This allows for a direct comparison of reconstructed helicity with the
predictions from the mean-field dynamo models. We apply the method to two
different data sets: the synoptic maps of line-of-sight (LOS) magnetic field
from the Michelson Doppler Imager (MDI) on board of Solar and Heliospheric
Observatory (SOHO) and vector magnetic field measurements from Vector
Spectromagnetograph (VSM) on Synoptic Optical Long-term Investigations of the
Sun (SOLIS) system. Based on the analysis of MDI/SOHO data, we find that in
solar cycle 23 the global magnetic field had positive (negative) magnetic
helicity in the northern (southern) hemisphere. This hemispheric sign asymmetry
is opposite to helicity of solar active regions, but it is in agreement with
the predictions of mean-field dynamo models. The data also suggest that the
hemispheric helicity rule may have reversed its sign in early and late phases
of cycle 23. Furthermore, the data indicate an imbalance in magnetic helicity
between the northern and southern hemispheres. This imbalance seem to correlate
with the total level of activity in each hemisphere in cycle 23. Magnetic
helicity for rising phase of cycle 24 is derived from SOLIS/VSM data, and
qualitatively, its latitudinal pattern is similar to the pattern derived from
SOHO/MDI data for cycle 23.Comment: 25 pages, 9 figures, accepted for Ap
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
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
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
Comparison of Ground- and Space-based Longitudinal Magnetograms
We compare photospheric line-of-sight magnetograms from the Synoptic
Long-term Investigations of the Sun (SOLIS) vector spectromagnetograph (VSM)
instrument with observations from the 150-foot Solar Tower at Mt. Wilson (MWO),
Helioseismic and Magnetic Imager (HMI) on Solar Dynamics Observatory (SDO), and
Michelson Doppler Imager (MDI) on Solar and Heliospheric Observatory (SOHO). We
find very good agreement between VSM and the other data sources for both
disk-averaged flux densities and pixel-by-pixel measurements. We show that the
VSM mean flux density time series is of consistently high signal-to-noise with
no significant zero-offsets. We discuss in detail some of the factors -spatial
resolution, flux dependence and position on the solar disk- affecting the
determination of scaling between VSM and SOHO/MDI or SDO/HMI magnetograms. The
VSM flux densities agree well with spatially smoothed data from MDI and HMI,
although the scaling factors show clear dependence on flux density. The factor
to convert VSM to HMI increases with increasing flux density (from 1
to 1.5). The nonlinearity is smaller for the VSM vs. ~SOHO/MDI scaling
factor (from 1 to 1.2).Comment: Accepted for publication in Solar Physic
First Synoptic Maps of Photospheric Vector Magnetic Field from SOLIS/VSM: Non-Radial Magnetic Fields and Hemispheric Pattern of Helicity
We use daily full-disk vector magnetograms from Vector Spectromagnetograph
(VSM) on Synoptic Optical Long-term Investigations of the Sun (SOLIS) system to
synthesize the first Carrington maps of the photospheric vector magnetic field.
We describe these maps and make a comparison of observed radial field with the
radial field estimate from LOS magnetograms. Further, we employ these maps to
study the hemispheric pattern of current helicity density, Hc, during the
rising phase of the solar cycle 24. Longitudinal average over the 23
consecutive solar rotations shows a clear signature of the hemispheric helicity
rule, i.e. Hc is predominantly negative in the North and positive in South.
Although our data include the early phase of cycle 24, there appears no
evidence for a possible (systematic) reversal of the hemispheric helicity rule
at the beginning of cycle as predicted by some dynamo models. Further, we
compute the hemispheric pattern in active region latitudes (-30 deg \le \theta
\le 30 deg) separately for weak (1001000 G)
radial magnetic fields. We find that while the current helicity of strong
fields follows the well-known hemispheric rule (i.e., \theta . Hc < 0), H_c of
weak fields exhibits an inverse hemispheric behavior (i.e., \theta . Hc > 0)
albeit with large statistical scatter. We discuss two plausible scenarios to
explain the opposite hemispheric trend of helicity in weak and strong field
region.Comment: 15 pages, 7 figures, to appear in Astrophysical Journa
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
Uncertainties in Solar Synoptic Magnetic Flux Maps
Magnetic flux synoptic charts are critical for reliable modeling of the
corona and heliosphere. Until now, however, these charts were provided without
any estimate of uncertainties. The uncertainties are due to instrumental noise
in the measurements and to the spatial variance of the magnetic flux
distribution that contributes to each bin in the synoptic chart. We describe
here a simple method to compute synoptic magnetic flux maps and their
corresponding magnetic flux spatial variance charts that can be used to
estimate the uncertainty in the results of coronal models. We have tested this
approach by computing a potential-field-source-surface model of the coronal
field for a Monte Carlo simulation of Carrington synoptic magnetic flux maps
generated from the variance map. We show that these uncertainties affect both
the locations of source-surface neutral lines and the distributions of coronal
holes in the models.Comment: 15 pages, 7 figures. Accepted for publication in "Solar Physics
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)
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