The observed photospheric magnetic field is a crucial parameter for understanding a range of fundamental solar and heliospheric phenomena. Synoptic maps, in particular, which are derived from the ...observed line-of-sight photospheric magnetic field and built up over a period of 27 days, are the main driver for global numerical models of the solar corona and inner heliosphere. Yet, in spite of 60 years of measurements, quantitative estimates remain elusive. In this study, we compare maps from seven solar observatories (Stanford/WSO, NSO/KPVT, NSO/SOLIS, NSO/GONG, SOHO/MDI, UCLA/MWO, and SDO /HMI) to identify consistencies and differences among them. We find that while there is a general qualitative consensus, there are also some significant differences. We compute conversion factors that relate measurements made by one observatory to another using both synoptic map pixel-by-pixel and histogram-equating techniques, and we also estimate the correlation between datasets. For example, Wilcox Solar Observatory (WSO) synoptic maps must be multiplied by a factor of 3 – 4 to match Mount Wilson Observatory (MWO) estimates. Additionally, we find no evidence that the MWO saturation correction factor should be applied to WSO data, as has been done in previous studies. Finally, we explore the relationship between these datasets over more than a solar cycle, demonstrating that, with a few notable exceptions, the conversion factors remain relatively constant. While our study was able to quantitatively describe the relationship between the datasets, it did not uncover any obvious “ground truth.” We offer several suggestions for how this may be addressed in the future.
We have detected several periodicities in the solar equatorial rotation rate of sunspot groups in the catalog Greenwich Photoheliographic Results (GPR) during the period 1931 – 1976, the
Solar ...Optical Observing Network
(SOON) during the period 1977 – 2014, and the Debrecen Photoheliographic Data (DPD) during the period 1974 – 2014. We have compared the results from the fast Fourier transform (FFT), the maximum entropy method (MEM), and the Morlet wavelet power-spectra of the equatorial rotation rates determined from SOON and DPD sunspot-group data during the period 1986 – 2007 with those of the Mount Wilson Doppler-velocity data during the same period determined by Javaraiah
et al.
(
Solar Phys.
257
, 61,
2009
). We have also compared the power-spectra computed from the DPD and the combined GPR and SOON sunspot-group data during the period 1974 – 2014 to those from the GPR sunspot-group data during the period 1931 – 1973. Our results suggest a ∼ 250-day period in the equatorial rotation rate determined from both the Mt. Wilson Doppler-velocity data and the sunspot-group data during 1986 – 2007. However, a wavelet analysis reveals that this periodicity appears mostly around 1991 in the velocity data, while it is present in most of the solar cycles covered by the sunspot-group data, mainly near the minimum epochs of the solar cycles. We also found the signature of a period of ∼ 1.4 years in the velocity data during 1990 – 1995, and in the equatorial rotation rate of sunspot groups mostly around the year 1956. The equatorial rotation rate of sunspot groups reveals a strong ∼ 1.6-year periodicity around 1933 and 1955, a weaker one around 1976, and a strong ∼ 1.8-year periodicity around 1943. Our analysis also suggests periodicities of ∼ 5 years, ∼ 7 years, and ∼ 17 years, as well as some other short-term periodicities. However, short-term periodicities are mostly present at the time of solar minima. Hence, short-term periodicities cannot be confirmed because of the larger uncertainty in the data.
The solar wind prediction is based on the Wang and Sheeley (
Astrophys. J.
365
, 372,
1990
) empirical relationship between the solar wind speed observed at 1 AU and the rate of magnetic flux tube ...expansion (FTE) between the photosphere and the inner corona, where the FTE is computed using coronal models (e.g. the current sheet source surface (CSSS) and the potential field source surface models). These models take the photospheric flux density synoptic maps as their inner boundary conditions to extrapolate the photospheric magnetic fields and to deduce the coronal and the heliospheric magnetic field configuration. These synoptic maps are among the most widely-used of all solar magnetic data products and therefore, the uncertainties in the model predictions that are caused by the uncertainties in the synoptic maps are worthy of study. However, an estimate of the uncertainties in the construction of these synoptic maps was not available until recently when Bertello et al. (
Solar Phys.
289
, 2419,
2014
) obtained the spatial standard deviation synoptic maps. For each photospheric synoptic map, they obtained 98 Monte Carlo realizations of the spatial standard deviation maps.
In this article, we present an estimate of uncertainties in the solar wind speed predicted at 1 AU by the CSSS model due to the uncertainties in the photospheric flux density synoptic maps. We also present a comparison of the coronal hole locations predicted by the models with the EUV synoptic maps obtained by the Sun Earth Connection Coronal and Heliospheric Investigation instruments on board the Solar Terrestrial Relations Observatory. For the present study, we used the Heliospheric and Magnetic Imager vector and longitudinal photospheric synoptic maps and the corresponding spatial standard deviation maps. In order to quantify the extent of the uncertainties involved, we compared the predicted speeds with the OMNI solar wind data during the same period (taking the solar wind transit time into account) and obtained the root mean square error between them. To illustrate the significance of the uncertainty estimate in the solar wind prediction, we carried out the analysis for three Carrington rotations at three different phases of the Solar Cycle 24, CR 2102 (3 – 30 October 2010), CR 2137 (14 May – 11 June 2013) and CR 2160 (1 – 28 February 2015), which fall within the extended minimum, the late-ascending, and the early-descending phases, respectively, of Solar Cycle 24.
Flow-like landslides in clayey soils represent serious threats for populations and infrastructures and have been the subject of numerous studies in the past decade. However, despite the rising need ...for landslide mitigation with growing urbanization, the transient mechanisms involved in the solid-fluid transition are still poorly understood. One way of characterizing the solid-fluid transition is to carry out rheometrical tests on clayey soil samples to assess the evolution of viscosity with the shear stress. In this study, we carried out geotechnical and rheometrical tests on clayey samples collected from six flow-like landslides in order to assess if these clayey soils exhibit similar characteristics when they fluidize (solid-fluid transition). The results show that (1) all tested soils except one exhibit a yield-stress fluid behavior that can be associated with a bifurcation in viscosity (described by the critical shear rate
γ
c
̇
) and in shear modulus
G
; (2) the larger the amplitude of the viscosity bifurcation, the larger the associated drop in
G
; and (3) the water content (
w
) deviation from the Atterberg liquid limit (
LL
) seem a key parameter controlling a common mechanical behavior of these soils at the solid-fluid transition. We propose exponential laws describing the evolution of the critical shear stress τ
c
, the critical shear rate
γ
c
̇
, and the shear modulus
G
as a function of the deviation
w-LL
.
Full disk vector magnetic fields are used widely for developing better understanding of large-scale structure, morphology, and patterns of the solar magnetic field. The data are also important for ...modeling various solar phenomena. However, observations of vector magnetic fields have one important limitation that may affect the determination of the true magnetic field orientation. This limitation stems from our ability to interpret the differing character of the Zeeman polarization signals which arise from the photospheric line-of-sight vs. the transverse components of the solar vector magnetic field, and is likely exacerbated by unresolved structure (non-unity fill fraction) as well as the disambiguation of the 180° degeneracy in the transverse-field azimuth. Here we provide a description of this phenomenon, and discuss issues, which require additional investigation.
It is well established that both total and spectral solar irradiance are modulated by variable magnetic activity on the solar surface. However, there is still disagreement about the contribution of ...individual solar features for changes in the solar output, in particular over decadal time scales. Ionized Ca
ii
K line spectroheliograms are one of the major resources for these long-term trend studies, mainly because such measurements have been available now for more than 100 years. In this paper we introduce a new Ca
ii
K plage and active network index time series derived from the digitization of almost 40 000 photographic solar images that were obtained at the 60-foot solar tower, between 1915 and 1985, as a part of the monitoring program of the Mount Wilson Observatory. We describe here the procedure we applied to calibrate the images and the properties of our new defined index, which is strongly correlated to the average fractional area of the visible solar disk occupied by plages and active network. We show that the long-term variation of this index is in an excellent agreement with the 11-year solar-cycle trend determined from the annual international sunspot numbers series. Our time series agrees also very well with similar indicators derived from a different reduction of the same data base and other Ca
ii
K spectroheliograms long-term synoptic programs, such as those at Kodaikanal Observatory (India), and at the National Solar Observatory at Sacramento Peak (USA). Finally, we show that using appropriate proxies it is possible to extend this time series up to date, making this data set one of the longest Ca
ii
K index series currently available.
We compare photospheric line-of-sight magnetograms from the
Synoptic Optical Long-term Investigations of the Sun
(SOLIS) Vector Spectro-Magnetograph (VSM) instrument with observations from the ...150-foot Solar Tower at Mt. Wilson Observatory (MWO), the
Helioseismic and Magnetic Imager
(HMI) on the
Solar Dynamics Observatory
(SDO), and the
Michelson Doppler Imager
(MDI) on the
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 ratio 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 a 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).
Magnetic flux synoptic charts are critical for a reliable modeling of the corona and heliosphere. Until now, however, these charts were provided without uncertainty estimates. 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.
We study the variability of solar activity using new photospheric proxies originally developed for the analysis of stellar magnetism with the CoRoT and Kepler photometric observations. These proxies ...were obtained by tracking the temporal modulations in the observations associated with the spots and magnetic features as the Sun rotates. We analyzed 21 yr of observations, spanning solar cycles 23 and 24, collected by the space-based photometric VIRGO and radial velocity GOLF instruments on board the SoHO satellite. We then calculated the photospheric activity proxy Sph is for each of the three VIRGO photometers and the associated Svel proxy from the radial velocity GOLF observations. Comparisons with several standard solar activity proxies sensitive to different layers of the Sun demonstrate that these new activity proxies, Sph and Svel, provide a new manner to monitor solar activity. We show that both the long- and short-term magnetic variabilities respectively associated with the 11-yr cycle and the quasi-biennial oscillation are well monitored, and that the magnetic field interaction between the subsurface, photosphere, and chromosphere of the Sun was modified between Cycle 24 and Cycle 23. Furthermore, the photometric proxies show a wavelength dependence of the response function of the solar photosphere among the three channels of the VIRGO photometers, providing inputs for the study of the stellar magnetism of Sun-like stars.
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 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 methodology described is quite general, and could be applied to photometric time series of other main-sequence stars for detecting differential rotation.
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