Solar Magnetic Field Measurements Research Articles

Probing the variations in the timing of the Sun’s polar magnetic field reversals through observations and surface flux transport simulations

ABSTRACT The polar field reversal is a crucial process in the cyclic evolution of the large-scale magnetic field of the Sun. Various important characteristics of a solar cycle, such as its duration and strength, and also the cycle predictability, are determined by the polar field reversal time. While the regular measurements of solar magnetic field have been accumulated for more than half a century, there is no consensus in the heliophysics community concerning the interpretation of the Sun’s polar field measurements and especially the determination of polar field reversal time. There exists a severe problem of non-reproducibility in the reported results even from studies of the same observational data set, and this causes an obstacle to make more accurate forecasts of solar cycle. Here, we analyze the solar magnetograms from four instruments for the last four cycles, to provide a more correct interpretation of the polar field observations and to find more accurate time of the reversals. We show the absence of triple (multiple) reversals in Cycles 21–24, significant variations in the time interval between reversals in the hemispheres and in the time interval between a reversal and a cycle beginning. In order to understand the origin of the reversal time variation, we perform Surface Flux Transport (SFT) simulations and find out that the presence of the ‘anomalous’ bipolar magnetic regions (BMRs) in different phases of a cycle can cause cycle-to-cycle variations of the reversal time within the similar range found in observations.

Application of the tunable narrowband filter in measurements of solar magnetic fields

磁场测量是太阳科学观测中最重要的一部分，围绕磁场展开的相关研究是太阳物理的研究热点。磁像仪是一种可以进行高分辨磁场观测的精密测量器件，在地基和空间太阳观测中得到广泛的应用。窄带可调谐滤光器是磁像仪获得窄带光的核心光学部件，其特性会对磁像仪的性能产生影响。滤光器分为两大类，具体可包括四种结构，具有不同的性能特征，可根据磁像仪的需求进行选择。本文调研了国内外著名地基、球载、空间太阳磁场望远镜，总结了太阳磁像仪中常用的各种窄带可调谐滤光系统的实现方式，滤光器的结构、原理、轮廓特性以及在磁像仪中的应用场景，归纳了不同太阳磁像仪滤光系统的设计特点和关键技术。本研究能够对未来太阳磁像仪滤光系统的研制提供参考。

Assessing the Influence of Input Magnetic Maps on Global Modeling of the Solar Wind and CME‐Driven Shock in the 2013 April 11 Event

AbstractIn the past decade, significant efforts have been made in developing physics‐based solar wind and coronal mass ejection (CME) models, which have been or are being transferred to national centers (e.g., SWPC, Community Coordinated Modeling Center) to enable space weather predictive capability. However, the input data coverage for space weather forecasting is extremely limited. One major limitation is the solar magnetic field measurements, which are used to specify the inner boundary conditions of the global magnetohydrodynamic (MHD) models. In this study, using the Alfvén wave solar model, we quantitatively assess the influence of the magnetic field map input (synoptic/diachronic vs. synchronic magnetic maps) on the global modeling of the solar wind and the CME‐driven shock in the 11 April 2013 solar energetic particle event. Our study shows that due to the inhomogeneous background solar wind and dynamical evolution of the CME, the CME‐driven shock parameters change significantly both spatially and temporally as the CME propagates through the heliosphere. The input magnetic map has a great impact on the shock connectivity and shock properties in the global MHD simulation. Therefore this study illustrates the importance of taking into account the model uncertainty due to the imperfect magnetic field measurements when using the model to provide space weather predictions.

МАГНІТОГРАФІЧНІ ТА CПЕКТРАЛЬНІ ДОСЛІДЖЕННЯ СОНЯЧНИХ МАГНІТНИХ ПОЛІВ В АСТРОНОМІЧНІЙ ОБСЕРВАТОРІЇ КИЇВСЬКОГО НАЦІОНАЛЬНОГО УНІВЕРСИТЕТУ ІМЕНІ ТАРАСА ШЕВЧЕНКА В 1975–1980 рр.

In 1975, at the Astronomical Observatory of the Taras Shevchenko National University of Kyiv, a magnetograph of the full vector of the SIBIZMIR design was installed and began to operate, and this initiated the creation of a new promising scientific direction at the observatory – the measurement of solar magnetic fields by both magnetographic and spectral-polarization methods. This article provides a brief chronology of the events in the first 5 years of the existence of the new direction, as well as the most important scientific results during this period. Вased on the experience of observations obtained at the Kyiv observatory, it was concluded that this magnetograph in the Kyiv’s astroclimate can not provide such scientifically valuable data as those magnetographs used in other observatories far outside large cities. In particular, although in Kyiv the sensitivity of measurements in the longitudinal magnetic field channel was quite high – about 5 G in the longitudinal field channel and 75 G in the transverse field channel, “noise” in the radial velocity channel reached 0.3 km/sec, which is about an order of magnitude higher than in the Crimea magnetograph. Another reason for the limited use of the magnetograph in the Kyiv observatory was that in the late 1970s significant problems with the calibration of magnetographic measurements became apparent, as a result of which such measurements in the areas of sunspots could have errors of 200–300%. However, in parallel with magnetographic research, spectral-polarization studies were also developed, and they allowed to obtain the most important results. In particular, the new data were obtained on the existence of magnetic fields with a strength of » 10 kG in active regions on the Sun, on the discreteness of the strengths in subtelescopic structures with very strong fields, as well as data on the significant suppression of turbulent motions in them. The corresponding conclusions were confirmed and detailed by new research in the following years.

A High-Efficiency and High-Accuracy Polarimeter for Solar Magnetic Field Measurements

Solar activity is dominated by the magnetic field. Nowadays, a polarimeter is a mandatory tool to measure solar magnetic fields, which are generally faint and correspond to a polarization of an order of $10^{-2}$ – $10^{-4}$ . As such, polarization measurements of high efficiency with a high accuracy are crucial to investigate faint magnetic fields. Here we propose a high-efficiency and high-accuracy polarimeter, which is based on a pair of nematic liquid crystal variable retarders (LCVRs) and a Wollaston prism (WP). It uses a dedicated Stokes modulation strategy to achieve high efficiency. A calibration unit (CU) is developed to measure the polarimeter response matrix, which provides a high-precision calibration to correct possible systematic errors. Compared with other traditional polarimeters, the modulation scheme of our polarimeter is flexible. In addition to be able to measure all the three Stokes polarization components ( $Q$ , $U$ , or $V$ ) simultaneously, it can also measure one or two of these polarization components alone, with high polarization efficiency. Dedicated alignment and calibration techniques optimized for our polarimeter are developed and high measurement accuracy is achieved. In our laboratory experimental test, our two-image based polarization measurement delivers an overall measurement accuracy of the order of $10^{-4}$ , which is about 10 times better, compared with our previous polarimeters that use the traditional four-image polarization modulation. This work provides a new option for high-efficiency and high-accuracy polarization measurement for future solar synoptic observations.

Observing the Sun with the Atacama Large Millimeter/submillimeter Array – from continuum to magnetic fields

AbstractThe Atacama Large Millimeter/submillimeter Array offers regular observations of our Sun since 2016. After an extended period of further developing and optimizing the post-processing procedures, first scientific results are now produced. While the first observing cycles mostly provided mosaics and time series of continuum brightness temperature maps with a cadence of 1-2s, additional receiver bands and polarization capabilities will be offered in the future. Currently, polarization capabilities are offered for selected receiver bands but not yet for solar observing. An overview of the recent development, first scientific results and potential of solar magnetic field measurements with ALMA will be presented.

From polarimetry to helicity: Studies of solar magnetic fields at the Huairou Solar Observing Station

In this paper, we briefly introduce the basic questions in the measurements of solar magnetic fields and the possible error sources due to the approximation of the theory of radiation transfer of spectral lines in the solar atmosphere. We introduce some basic research progress in magnetic field measurement at Huairou Solar Observing Station of National Astronomical Observatories of the Chinese Academy of Sciences, especially concerning the non-potentiality in solar active regions, such as the magnetic shear, current and helicity. We also discuss some basic questions for the measurements of the magnetic fields and corresponding challenges for the future studies.

Solar cycle 24: An unusual polar field reversal

Context. It is well known that the polarity of the Sun’s magnetic field reverses or flips around the maximum of each 11 year solar cycle. This is commonly known as polar field reversal and plays a key role in deciding the polar field strength at the end of a cycle, which is crucial for the prediction of the upcoming cycle. Aims. We aim to investigate solar polar fields during cycle 24, using measurements of solar magnetic fields in the latitude range 55°–90° and 78°–90°, to report a prolonged and unusual hemispheric asymmetry in the polar field reversal pattern in solar cycle 24. Methods. This study was carried out using medium resolution line-of-sight synoptic magnetograms from the magnetic database of the National Solar Observatory at Kitt Peak (NSO/KP), USA for the period between February 1975 and October 2017, covering solar cycles 21–24 and high-resolution line-of-sight synoptic magnetograms from the Michaelson Doppler Imager instrument onboard the Solar Heliospheric Observatory. Synoptic magnetograms using radial measurements from the Heliospheric Magnetic Imager instrument onboard the Solar Dynamics Observatory, covering solar cycle 23 and 24, were also used. Results. We show that the southern solar hemisphere unambiguously reversed polarity in mid-2013 while the reversal in the field in the northern solar hemisphere started as early as June 2012, was followed by a sustained period of near-zero field strength lasting until the end of 2014, after which the field began to show a clear rise from its near-zero value. While this study compliments a similar study carried out using microwave brightness measurements which claimed that the field reversal process in cycle 24 was completed by the end of 2015, our results show that the field reversal in cycle 24 was completed earlier that is, in late 2014. Signatures of this unusual field reversal pattern were also clearly identifiable in the solar wind, using our observations of interplanetary scintillation at 327 MHz which supported our magnetic field observations and confirmed that the field reversal process was completed at the end of 2014.

Design and calibration of a high-sensitivity and high-accuracy polarimeter based on liquid crystal variable retarders

Polarimetry plays an important role in the measurement of solar magnetic fields. We developed a high-sensitivity and high-accuracy polarimeter (HHP) based on nematic liquid crystal variable retarders (LCVRs), which has a compact setup and no mechanical moving parts. The system design and calibration methods are discussed in detail. The azimuth error of the transmission axis of the polarizer as well as the fast axes of the two LCVRs and the quarter-wave plate were determined using dedicated procedures. Linearly and circularly polarized light were employed to evaluate the performance of the HHP. The experimental results indicate that a polarimetric sensitivity of better than can be achieved by using a single short-exposure image, while an accuracy on the order of 10−5 can be reached by using a large number of short-exposure images. This makes the HHP a high-performance system that can be used with a ground-based solar telescope for high-precision solar magnetic field investigations.

Variations in Ratio and Correlation of Solar Magnetic Fields in the Fe i 525.02 nm and Na i 589.59 nm Lines According to Mount Wilson Measurements During 2000 – 2012

Variations in the solar magnetic-field ratio over 13 years are analyzed, relying on the comparison of simultaneous measurements in two spectral lines at the Mount Wilson Observatory (MWO). The ratio and correlation coefficient are calculated over the general working range of measured magnetic-field values and in various ranges of the field magnitudes. Variations in both parameters are considered. We found the following tendencies: i) the parameters show changes with the cycle of solar activity in the general case; ii) their dependence on magnetic-field magnitude is a nonlinear function of time, and this is especially pronounced in the ratio behavior; iii) several separate ranges of the field magnitudes can be distinguished based on the behavioral patterns of the ratio variations. Correspondences between these ranges and the known structural objects of the solar atmosphere are discussed. This permits us to reach the conclusion that the dependence of parameters considered on the magnetic-field magnitude and time is connected with the variety of magnetic structural components and their cyclic rearrangements. The results represented may be useful for solving interpretation problems of solar magnetic-field measurements and for the cross-calibration of applicable instruments. They can also be of interest for tasks related to the creation of a uniform long temporal series of solar magnetic-field data from various sources.

On the Origin of Differences in Helicity Parameters Derived from Data of Two Solar Magnetographs

We analyzed how sensitivity and accuracy in solar magnetic field measurements may affect the values of mean current helicity density $h_{\mathrm{c}}$ and twist parameter $\alpha_{\mathrm{av}}$ by comparing these values obtained from two magnetographs (SMFT at Beijing and SFT at Mitaka, Tokyo). When we computed the helicity parameters from the SFT data, we replaced the values of the longitudinal field component, transverse field strength, and transverse field azimuth angle with those from the SMFT data and examined the differences. The results show that the correlation coefficient and the fraction of the data that agree in signs of $h_{\mathrm{c}}$ or $\alpha_{\mathrm{av}}$ increase when an SFT parameter is substituted by the corresponding SMFT parameter because one source of discrepancy is removed. The increase in correlation coefficient is largest when the azimuthal angles and transverse field strengths are set identical in the two instruments; the correlation coefficient of $h_{\mathrm{c}}$ ( $\alpha_{\mathrm{av}}$ ) increases from 0.74 (0.56) to 0.86 (0.78), respectively, indicating that the differences in the transverse field strength and its azimuthal angle are the largest source of discrepancy in the values of $h_{\mathrm{c}}$ or $\alpha_{\mathrm {av}}$ . We found a nonlinear relationship in the components of the magnetic field between the two instruments for some data samples; we conclude that this is due to the discrepancy in the calibration procedure between the two instruments. This nonlinearity can be another source of difference in determining helical parameters between the two instruments.

Decay of activity complexes and the formation of coronal holes

Analysis of long-term measurements of solar magnetic fields and the flux of UV radiation from the Sun indicates a cause-effect relationship between activity complexs, their residual magnetic fields, and coronal holes. A comparison of the background magnetic fields of the Sun and the evolution of former activity complexes reveals unipolar magnetic regions that form after the decay of these complexes. The latitude and time evolution of unipolar magnetic regions in solar cycles 21–24 is studied. A North-South asymmetry in solar activity is manifest in the distribution of unipolar regions migrating toward higher latitudes. It is shown that, when residual magnetic fields of the opposite polarity reach the polar regions, this leads to a sign change of the polar magnetic field and a decrease in the area of polar coronal holes, or even their complete disappearance. These interactions can explain the triple sign change of the polar magnetic field of the Sun in cycle 21 and the short-term polarity reversals observed in 2010 and 2011.

Resolving Azimuth Ambiguity Using Vertical Nature of Solar Quiet-Sun Magnetic Fields

The measurement of solar magnetic fields using the Zeeman effect diagnostics has a fundamental 180 degree ambiguity in the determination of the azimuth angle of the transverse field component. There are several methods that are used in the community and each one has its merits and demerits. Here we present a disambiguation idea that is based on the assumption that most of the magnetic field on the sun is predominantly vertical. While the method is not applicable to penumbra or other features harboring predominantly horizontal fields like the sheared neutral lines, it is useful for regions where fields are predominantly vertical like network and plage areas. The method is tested with the full-disk solar vector magnetograms observed by the VSM/SOLIS instrument. We find that statistically about 60-85 % of the pixels in a typical full-disk magnetogram has field inclination in the range of 0-30 degrees with respect to the local solar normal, and thus can be successfully disambiguated by the proposed method. Due to its non-iterative nature, the present method is extremely fast and therefore can be used as a good initial guess for iterative schemes like nonpotential field computation (NPFC) method. Furthermore, the method is insensitive to noisy pixels as it does not depend upon the neighboring pixels or derivatives.

Enigmas in measurements of solar magnetic field

The mean magnetic field (MMF) of the photosphere of the Sun as a star was measured in 2001–2010 at the Crimean Astrophysical Observatory using two Fe I absorption lines with λ = 524.7 nm and λ = 525.0 nm. The regression coefficient b for 1054 pairs of daily values measured simultaneously on both lines equals 0.82 (a correlation coefficient is 0.94; magnetic field strengths determined by the line with λ = 525.0 nm are lower than those for the line with λ = 524.7 nm). However, the b value varied significantly along with phases of the 11-year cycle from 0.88 in 2003 to 0.49 in 2009. It is difficult to ascribe these variations to purely instrumental or solar causes. Moreover, the semiannual value of b decreased with the decrease in the absolute strength of the MMF, which contradicts the model of thin magnetic flux ropes of the photosphere. Similar behavior of b was also observed in the comparison of MMF measured at the Crimean Astrophysical Observatory and Stanford by the line with λ = 525.0 nm. The inconsistency of the results obtained by these two iron lines on different instruments has been noted. It has been concluded that the variance in and odd behavior of b are predetermined not only by the instrument and the Sun (by the so-called fine structure of the photosphere field), but also by the act of measuring. When recording solar (and stellar) magnetic fields and modeling atmospheric processes, quantum effects have to be taken into account, such as nonlocality, indistinguishability, and the entanglement of photons, as well as that a photon only acquires its properties at the exact moment of its detection. The best approximation to reality can be achieved by averaging the MMF measurements carried out with different magnetographs and in different spectral lines.

Are Solar Active Regions with Major Flares More Fractal, Multifractal, or Turbulent Than Others?

Multiple recent investigations of solar magnetic field measurements have raised claims that the scale-free (fractal) or multiscale (multifractal) parameters inferred from the studied magnetograms may help assess the eruptive potential of solar active regions, or may even help predict major flaring activity stemming from these regions. We investigate these claims here, by testing three widely used scale-free and multiscale parameters, namely, the fractal dimension, the multifractal structure function and its inertial-range exponent, and the turbulent power spectrum and its power-law index, on a comprehensive data set of 370 timeseries of active-region magnetograms (17,733 magnetograms in total) observed by SOHO's Michelson Doppler Imager (MDI) over the entire Solar Cycle 23. We find that both flaring and non-flaring active regions exhibit significant fractality, multifractality, and non-Kolmogorov turbulence but none of the three tested parameters manages to distinguish active regions with major flares from flare-quiet ones. We also find that the multiscale parameters, but not the scale-free fractal dimension, depend sensitively on the spatial resolution and perhaps the observational characteristics of the studied magnetograms. Extending previous works, we attribute the flare-forecasting inability of fractal and multifractal parameters to i) a widespread multiscale complexity caused by a possible underlying self-organization in turbulent solar magnetic structures, flaring and non-flaring alike, and ii) a lack of correlation between the fractal properties of the photosphere and overlying layers, where solar eruptions occur. However useful for understanding solar magnetism, therefore, scale-free and multiscale measures may not be optimal tools for active-region characterization in terms of eruptive ability or, ultimately,for major solar-flare prediction.

Two-dimensional solar spectropolarimetry with the KIS/IAA Visible Imaging Polarimeter

Spectropolarimetry at high spatial and spectral resolution is a basic tool to characterize the magnetic properties of the solar atmosphere. We introduce the KIS/IAA Visible Imaging Polarimeter (VIP), a new post-focus instrument that upgrades the TESOS spectrometer at the German VTT into a full vector polarimeter. VIP is a collaboration between the KIS and the IAA. We describe the optical setup of VIP, the data acquisition procedure, and the calibration of the spectropolarimetric measurements. We show examples of data taken between 2005 and 2008 to illustrate the potential of the instrument. VIP is capable of measuring the four Stokes profiles of spectral lines in the range from 420 to 700 nm with a spatial resolution better than 0.5". Lines can be sampled at 40 wavelength positions in 60 s, achieving a noise level of about 2 x 10E-3 with exposure times of 300 ms and pixel sizes of 0.17" x 0.17" (2 x 2 binning). The polarization modulation is stable over periods of a few days, ensuring high polarimetric accuracy. The excellent spectral resolution of TESOS allows the use of sophisticated data analysis techniques such as Stokes inversions. One of the first scientific results of VIP presented here is that the ribbon-like magnetic structures of the network are associated with a distinct pattern of net circular polarization away from disk center. VIP performs spectropolarimetric measurements of solar magnetic fields at a spatial resolution that is only slightly worse than that of the Hinode spectropolarimeter, while providing a 2D field field of view and the possibility to observe up to four spectral regions sequentially with high cadence. VIP can be used as a stand-alone instrument or in combination with other spectropolarimeters and imaging systems of the VTT for extended wavelength coverage.

Helioseismic Network Provides Valuable Data

[1] I read with interest the editorial in Space Weather entitled “The need for long-term solar-terrestrial data series” (7, S05006, doi:10.1029/2009SW000496). As discussed in the editorial, it can be difficult to convince funding agencies of the value of long-term observational programs with consistent instrumentation over decades. I would like to point out that one such program, the Global Oscillation Network Group (GONG), which operates a set of six sites around the world to obtain helioseismic data of the Sun's interior, has been providing solar data since 1995. GONG has achieved an average 24-hour observing fraction of 85–90% since its beginning. Continual solar magnetic field measurements are also being provided by GONG, and data on the hydrogen alpha emission line will be provided to the U.S. Air Force starting in summer of 2010.

Fe I line pairs with different magnetic sensitivity

We present a list of Fe I line pairs with different magnetic sensitivity which are suitable for measurement of solar magnetic fields using the Stokes V amplitude ratio. The list contains the spectral solar line data in the wavelength range λλ = 303–996 nm given by Gurtovenko and Kostyk.

Plasma Polarization Spectroscopy. Measurement of Solar Magnetic Fields by Means of the Spectro-Polarimetory.

Magnetic fields play a fundamental role in driving various dynamical phenomena of the solar atmosphere, and the precise measurement of photospheric magnetic fields is of vital importance in solar physics research. A measurement of the spectral line polarizations produced by the Zeeman effect provides the most powerful means for investigating solar magnetic fields. The principle of measurements and some aspects of recent observational results are presented.

Ellipsometry of coelostat coatings using a Babinet compensator: Simulation of the experimental accuracy

The precise measurement of solar magnetic fields requires an accurate measurement of the Muller matrix of the optical components in the path of the light beam, which again requires a careful measurement of the optical constants of the reflecting surfaces in the case of a 3-mirror coelostat system. Here we present a method to measure the optical constants (the real and imaginary part of the refractive index) to an accuracy of the order of 1% for bulk aluminium. This work is directed towards the measurement of instrumental polarisation at the Kodaikanal solar tower telescope, although it can be used for any metallic coated optics.