Solar Magnetic Field Observations Research Articles

A Strong-flare Prediction Model Developed Using a Machine-learning Algorithm Based on the Video Data Sets of the Solar Magnetic Field of Active Regions

It is well accepted that the physical properties obtained from the solar magnetic field observations of active regions (ARs) are related to solar eruptions. These properties consist of temporal features that might reflect the evolution process of ARs, and spatial features that might reflect the graphic properties of ARs. In this study, we generated video data sets with timescales of 1 day and image data sets of the SHARP radial magnetic field of the ARs from 2010 May to 2020 December. For the ARs that evolved from “quiet” to “active” and erupted the first strong flares in 4 days, we extract and investigate both the temporal and spatial features of ARs from videos, aiming to capture the evolution properties of their magnetic field structures during their transition process from “quiet” (non–strong flaring) to “active” (strong flaring). We then conduct a comparative analysis of the model performance by video input and single-image input, as well as of the effect of the model performance variation with the prediction window up to 3 days. We find that for those ARs that erupted the first strong flares in 4 days, the temporal features that reflect their evolution from “quiet” to “active” before the first strong flares can be recognized and extracted from the video data sets by our network. These features turn out to be important predictors that can effectively improve strong-flare prediction, especially by reducing the false alarms in a nearly 2 day prediction window.

A Multi‐Purpose Heliophysics L4 Mission

AbstractThe Earth‐Sun Lagrangian point 4 is a meta‐stable location at 1 AU from the Sun, 60° ahead of Earth's orbit. It has an uninterrupted view of the solar photosphere centered on W60, the Earth's nominal magnetic field connection to the Sun. Such a mission on its own would serve as a solar remote sensing observatory that would oversee the entire solar radiation hemisphere with significant relevance for protecting Moon and Mars explorers from radiation exposure. In combination with appropriately planned observatories at L1 and L5, the three spacecraft would provide 300° longitude coverage of photospheric magnetic field structure, and allow continuous viewing of both solar poles, with >3.6° elevation. Ideally, the L4 and L5 missions would orbit the Sun with a 7.2° inclination out of the heliographic equator, 14.5° out of the ecliptic plane. We discuss the impact of extending solar magnetic field observations in both longitude and latitude to improve global solar wind modeling and, with the development of local helioseismology, the potential for long‐term solar activity forecasting. Such a mission would provide a unique opportunity for interplanetary and interstellar dust science. It would significantly add to reliability of operational observations on fast coronal mass ejections directed at Earth and for human Mars explorers on their round‐trip journey. The L4 mission concept is technically feasible, and is scientifically compelling.

Does Nonaxisymmetric Dynamo Operate in the Sun?

Abstract We explore effects of random nonaxisymmetric perturbations of kinetic helicity (the α effect) and diffusive decay of bipolar magnetic regions on generation and evolution of large-scale nonaxisymmetric magnetic fields on the Sun. Using a reduced 2D nonlinear mean-field dynamo model and assuming that bipolar regions emerge due to magnetic buoyancy in situ of the large-scale dynamo action, we show that fluctuations of the α effect can maintain the nonaxisymmetric magnetic fields through a solar-type α 2Ω dynamo process. It is found that diffusive decay of bipolar active regions is likely to be the primary source of nonaxisymmetric magnetic fields observed on the Sun. Our results show that nonaxisymmetric dynamo models with stochastic perturbations of the α effect can explain periods of extremely high activity (“super-cycle” events) as well as periods of deep decline of magnetic activity. We compare the models with synoptic observations of solar magnetic fields for the last four activity cycles and discuss implications of our results for interpretation of observations of stellar magnetic activity.

Contribution to the Solar Mean Magnetic Field from Different Solar Regions

Seven-year long seeing-free observations of solar magnetic fields with the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) were used to study the sources of the solar mean magnetic field, SMMF, defined as the net line-of-sight magnetic flux divided over the solar disk area. To evaluate the contribution of different regions to the SMMF, we separated all the pixels of each SDO/HMI magnetogram into three subsets: weak (B_W), intermediate (B_I), and strong (B_S) fields. The B_W component represents areas with magnetic flux densities below the chosen threshold; the B_I component is mainly represented by network fields, remains of decayed active regions (ARs), and ephemeral regions. The B_S component consists of magnetic elements in ARs. To derive the contribution of a subset to the total SMMF, the linear regression coefficients between the corresponding component and the SMMF were calculated. We found that: i) when the threshold level of 30 Mx cm^-2 is applied, the B_I and B_S components together contribute from 65% to 95% of the SMMF, while the fraction of the occupied area varies in a range of 2-6% of the disk area; ii) as the threshold magnitude is lowered to 6 Mx cm^-2, the contribution from B_I+B_S grows to 98%, and the fraction of the occupied area reaches the value of about 40% of the solar disk. In summary, we found that regardless of the threshold level, only a small part of the solar disk area contributes to the SMMF. This means that the photospheric magnetic structure is an intermittent, inherently porous medium, resembling a percolation cluster. These findings suggest that the long-standing concept that continuous vast unipolar areas on the solar surface are the source of the SMMF may need to be reconsidered.

Possibilities and problems of solar magnetic field observations for space weather forecast

An essential part of the space weather problem, important in the last decades, is the forecast of near-Earth space parameters, ionospheric and geomagnetic conditions on the basis of observations of various phenomena on the Sun. Of particular importance are measurements of magnetic fields as they determine the spatial structure of outer layers of the solar atmosphere and, to a large extent, solar wind parameters. Due to lack of opportunities to observe magnetic fields directly in the corona, the almost only source of various models for quantitative calculation of heliospheric parameters are daily magnetograms measured in photospheric lines and synoptic maps derived from these magnetograms. It turns out that results of the forecast, in particular of the solar wind velocity in Earth’s orbit and the position of the heliospheric current sheet, greatly depend not only on the chosen calculation model, but also on the original material because magnetograms from different instruments (and often observations in different lines at the same), although being morphologically similar, may differ significantly in a detailed quantitative analysis. A considerable part of this paper focuses on a detailed analysis of this particular aspect of the problem of space weather forecast.

Modelling coronal electron density and temperature profiles based on solar magnetic field observations

AbstractThe density and temperature profiles in the solar corona are complex to describe, the observational diagnostics is not easy. Here we present a physics-based model to reconstruct the evolution of the electron density and temperature in the solar corona based on the configuration of the magnetic field imprinted on the solar surface. The structure of the coronal magnetic field is estimated from Potential Field Source Surface (PFSS) based on magnetic field from both observational synoptic charts and a magnetic flux transport model. We use an emission model based on the ionization equilibrium and coronal abundances from CHIANTI atomic database 8.0. The preliminary results are discussed in details.

Modelling short-term Solar Spectral Irradiance (SSI) using coronal electron density and temperature profiles based on solar magnetic field observations

AbstractSome key physical processes that impact the evolution of Earth's atmosphere on time-scale from days to millennia, such as the EUV emissions, are determined by the solar magnetic field. However, observations of the solar spectral irradiance are restricted to the last few solar cycles and are subject to large uncertainties. We present a physics-based model to reconstruct short-term solar spectral irradiance (SSI) variability. The coronal magnetic field is estimated to employ the Potential Field Source Surface extrapolation (PFSS) based on observational synoptic charts and magnetic flux transport model. The emission is estimated to employ the CHIANTI atomic database 8.0. The performance of the model is compared to the emission observed by TIMED/SORCE.

On the peculiarities of manifestation of kG magnetic elements in observations of the Sun with low spatial resolution

AbstractOn the agenda of modern astrophysics is the exploration of not only disk-integrated stellar magnetic fields but surface mapping of them. However, it is hardly possible to expect that spatial resolution better than some dozens or hundreds pixels over stellar disk will be achieved for this goal in the foreseeable future. Among other reasons this fact makes very important observations of the average and large-scale magnetic fields of the Sun, which can be naturally used for testing polarimetric measurements on other stars, especially on solar-type stars. In this study we explore different aspects of observations of solar magnetic fields (SMF) with low spatial resolution, including Sun-as-a-star observations, which are characterized by extremely low magnetic flux densities. Comparison of disk-integrated and spatially resolved Stokes observations of the Sun allow us to demonstrate how Stokes V profiles depend on the distribution of large-scale magnetic fields in the disk center. It is shown that center-to-limb variations of magnetic strength ratios (MSR) and area asymetries, most likely could be interpreted as the manifestation of kG magnetic flux tubes. We have made cross-calibration of the full-disk magnetograms obtained by space-borned SDO/HMI and by the ground-based STOP telescope, and pretty good agreement is found. Finally, the absence of significant systematic time variations of MSRs with solar cycle is demonstrated.

How reliable are observations of solar magnetic fields? Comparison of full-disk measurements in different spectral lines and calibration issues of space missions SOHO, Hinode, and SDO

AbstractAn urgent problem in modern solar physics, which is not completely solved up to now, is to obtain realistic magnetic field strength values from parameters measured magnetographs or Stokes-meter instruments. One of the important tools on this way is a comparison of observations made in different spectral lines with the same or with the different telescopes. This issue is an actual task in the analysis of the new data sets provided by the space missions SOHO and Hinode, which measurements are available for several years already, and SDO, which data appeared recently. The main aim of this study is a cross-comparison of magnetic field observations made in different spectral lines used on the above mentioned space observatories: Ni i λ676.77 nm (SOHO/MDI), Fe i λ630.152 nm and Fe i λ630.25 nm (Hinode/SP), and Fe i λ617.33 nm (SDO/HMI). Full-disk high-precision Stokes-meter measurements with the STOP telescope at the Sayan observatory in these lines are used basically, as well as some observations in other spectral lines having a great diagnostic impact, such as Fe i λ525.02 nm, Fe i λ523.29 nm and Fe i λ532.42 nm. The difference between one-instrument (STOP) simultaneous or quasi-simultaneous observations in different spectral lines do not exceed the factor of 2-3 depending on the combination of spectral lines and the position on the solar disk. This is significantly less than in some other studies devoted to cross-comparison of different data sets. Importance and consequences of the obtained results are discussed.

Observations of the magnetic field and plasma in the heliosheath by Voyager 2 from 2007.7 to 2009.4

The density and temperature profiles of the plasma measured by Voyager 2 (V2) behind the termination shock changed abruptly near 2008.6 from relatively large average values and large fluctuations during 2007.7 to 2008.6 (interval A) to relatively low average values and very small‐amplitude fluctuations during 2008.6 to 2009.4 (interval B). This paper shows that the change in the magnetic field strength B(t) was less abrupt than the plasma changes, and the fluctuations of the magnetic field strength in interval B were of moderate amplitude, with indications of a quasiperiodic structure in part of the interval. The magnetic field was directed away from the sun (positive polarity) ∼ 78% ± 5% of the time in both interval A and interval B, changing in an irregular way from positive to negative polarities throughout the interval. The polarity distribution indicates that the minimum latitudinal extent of the heliospheric current sheet (HCS) was near V2 throughout the interval, consistent with the extrapolated minimum latitudes of the HCS computed from solar magnetic field observations. Thus, V2 was observing magnetic fields from the southern polar coronal hole most of the time. The distribution of B was lognormal in interval A and Gaussian interval B.

Spectro-Polarimetric Observations of Solar Magnetic Fields and the SOHO/MDI Calibration Issue

Comparisons of solar magnetic-field measurements made in different spectral lines are very important, especially in those lines in which observations have a long history or (and) specific diagnostic significance. The spectral lines Fe i 523.3 nm and Fe i 525.0 nm belong to this class. Therefore, this study is devoted to a comprehensive analysis using new high-precision Stokes-meter full-disk observations. The disk-averaged magnetic-field strength ratio R=B(523.3)/B(525.0) equals 1.97±0.02. The center-to-limb variation (CLV) is R=1.74−2.43μ+3.43μ 2, where μ is the cosine of the center-to-limb angle. For the disk center, we find R=2.74, and for near-limb areas with μ=0.3, R equals 1.32. There is only a small dependence of R on the spatial resolution. Our results are rather close to those published three decades ago, but differ significantly from recent magnetographic observations. An application of our results to the important SOHO/MDI magnetic data calibration issue is discussed. We conclude that the revision of the SOHO/MDI data, based only on the comparison of magnetic-field measurements in the line pair Fe i 523.3 nm and Fe i 525.0 nm (increasing by a factor of 1.7 or 1.6 on average according to recent publications) is not obvious and new investigations are urgently needed.

Global structure and dynamics of large‐scale fluctuations in the solar wind: Voyager 2 observations during 2005 and 2006

The Voyager 2 (V2) observations of daily averages of the solar wind during 2005 and 2006 from 75.3 AU to 81.6 AU between ≈25.7°S and 27.1°S show both a step‐like trend in the speed V(t) and “large‐scale fluctuations” of the magnetic field strength B, speed V, density N and temperature T. The distribution functions of B, N, and NV2 observed by V2 are lognormal and that of V is approximately Gaussian. We introduce a method for specifying the boundary conditions at all latitudes (except near the poles) on a Sun‐centered surface of radius of 1 AU, based on solar magnetic field observations. This paper uses only the boundary conditions at the latitude of V2 and a 1‐D time‐dependent MHD model to calculate the radial evolution of the large‐scale fluctuations of B(t), V(t) and N(t) at distances between 1 and 90 AU. This model explains the V2 observations of a lognormal distribution of B and the Gaussian distribution of V, but not the observed lognormal distributions of N and NV2. The lognormal distribution of B observed by V2 was produced primarily by dynamical processes beyond 1 AU.

Solar flare forecasting model supported with artificial neural network techniques

Nowadays operational models for solar activity forecasting are still based on the statistical relationship between solar activity and solar magnetic field evolution. In order to set up this relationship, many parameters have been proposed to be the measures. Conventional measures are based on the sunspot group classification which provides limited information from sunspots. For this reason, new measures based on solar magnetic field observations are proposed and a solar flare forecasting model supported with an artificial neural network is introduced. This model is equivalent to a person with a long period of solar flare forecasting experience.

Geoeffective Analysis of CMEs Under Current Sheet Magnetic Coordinates

Using 100 CME–ICME events during 1997.01–2002.11, based on the eruptive source locations of CMEs and solar magnetic field observations at the photosphere, a current sheet magnetic coordinate (CMC) system is established in order to statistically study the characteristics of the CME–ICME events and the corresponding geomagnetic storm intensity. The transit times of CMEs from the Sun to the Earth are also investigated, by taking into account of the angle between the CME eruption normal (defined as the vector from the Sun center to the CME eruption source) and the Sun-Earth line. Our preliminary conclusions are: 1. The distribution of the CME sources in our CMC system is obviously different from that in the ordinary heliographic coordinate system. The sources of CMEs are mainly centralized near the heliospheric current sheet (HCS), and the number of events decreases with the increment of the angular distance from the CME source to the HCS on the solar surface; 2. A large portion of the total events belong to the same–side events (referring to the CME source located on the same side of the HCS as the Earth), while only a small portion belong to the opposite–side events (the CME source located on the opposite side of the HCS as the Earth). 3. The intense geomagnetic storms are usually induced by the same–side events, while the opposite side events are commonly associated with relatively weak geomagnetic storms; 4. The angle between the CME normal and the Sun–Earth line is used to estimate the transit time of the CME in order to reflect the influence of propagation characteristic of the CME along the Sun–Earth direction. With our new prediction method in context of the CMC coordinate, the averaged absolute error for these 100 events is 10.33 hours and the resulting relative error is not larger than 30% for 91% of all the events.

A model of the Alfvén speed in the solar corona

We present an analytic model of the Alfven speed vA in the solar corona. The coronal magnetic field is modeled by a radial component representing the global field and by a dipole representing an active region. The free parameters of the model are constrained by actual observations of solar magnetic fields and coronal electron densities. The coronal magnetic field strength in the quiet Sun is determined by coronal seismology, using EIT waves as proxies for the fast magnetosonic speed vms, and thus for the magnetic field strength. Depending on the orientation of the dipole, we find local minima of vA (and vms )a t the coronal base at distances of 0.2-0.3 solar radii from the center of the modelled active region (AR), as well as above the AR at comparable heights. For all dipole orientations, a global maximum is found at 3.5 solar radii. We apply our model to the study of the formation and propagation of coronal shock waves which are observed as flare waves and as type II radio bursts, using a sample of eight solar events. We find that flare waves are initially highly supermagnetosonic (with magnetosonic Mach numbers of Mms ≈ 2-3). During their propagation, they decelerate until Mms = 1 is reached. This behavior can be explained by a strong shock or large-amplitude simple wave that decays to an ordinary fast magnetosonic wave. The observed starting frequencies and Mach numbers of the associated type II bursts are consistent with the predictions of the model.

Global magnetic field of the Sun and long-term variations of galactic cosmic rays

The paper deals with the relation of long-term variations of 10 GV galactic cosmic rays (GCR) to the global solar magnetic field and solar wind parameters. This study continues previous works, where the tilt of the heliospheric current sheet (HCS) and other solar-heliospheric parameters are successfully used to describe long-term variations of cosmic rays in the past two solar cycles. The novelty of the present work is the use of the HCS tilt and other parameters reconstructed from H α observations of filaments for the period when direct global solar magnetic field observations were unavailable. Thus, we could extend the GCR simulation interval back to 1953. The analysis of data for 1953–1999 revealed a good correlation (the correlation coefficient >0.88) between the solar-heliospheric parameters and GCR in different cycles of solar activity. Moreover, the approach applied makes it possible to describe the behavior of cosmic rays in the epochs of solar maxima, which could not be done before. This indicates both the adequacy of the model and the reliability of the reconstructed global solar magnetic field parameters.

Prediction of daily average solar wind velocity from solar magnetic field observations using hybrid intelligent systems

A hybrid intelligent system, combining theory driven and data driven models, is used to predict the daily solar wind velocity at 1 AU from solar magnetic field observations. The Potential Field Model (theory driven) is used to calculate the coronal magnetic field up to the source surface placed at 2.5 R ⊙. The Earth's position is projected onto the source surface using a time-lag τ and the corresponding magnetic field strength B s is calculated. The field line is then mapped down to the photosphere and the magnetic field strength B ⊙ is determined. The daily full disc RMS photospheric magnetic field strength B RMS is also determined. An Artificial Neural Network (ANN) (data driven) is trained using B ⊙( t) and B s ( t) as input and the solar wind velocity V( t+ τ) as output. The ANN is trained on data from solar cycle 21 and tested on data from solar cycle 22. The function realised in the ANN corresponds very well to the mapping 1 f s ↦ V , where f s = ( R ⊙ R s ) 2 B ⊙ B s is the flux tube expansion factor. There seems to be no explicit dependence on B s . Also including B RMS ( t) to the input improves the results slightly because now there is some information about the daily magnetic field. Extending the input over a time series t − 2, t − 1, t (days) improves the results further. The predicted velocity is now 3 days ahead ( V( t+3)). In this case the ANN also models the dynamic behaviour of the solar wind. This also means that the inverse mapping problem of structures from 1 AU to the Sun is partially avoided. The final model gives an error of 90 km/s and a correlation of 0.58 on the test data set.

Magnetic fields in the distant heliosphere approaching solar minimum: Voyager 1 and 2 observations during 1994

During 1994, Voyager 2 (V2) was at a heliocentric distance 〈R〉 = 43.4 AU and latitude 〈δs/c〉 = 11.9°S, while Voyager 1 (Vl) was at 〈R〉 = 56.3 AU and 〈δs/c〉 = 32.5°N. The mean magnetic field strength observed by Vl during 1994 was 0.05 nT, the weakest and most distant magnetic field ever measured in situ. A bimodal distribution of azimuthal magnetic field directions was observed by V2, and a single‐peaked distribution was observed by Vl. Thus V2 was in the “sector zone” (the range of latitudes that contains the heliospheric current sheet), and Vl was in the unipolar region above it, consistent with the latitudes of the heliospheric current sheet obtained from an extrapolation of the neutral line computed from solar magnetic field observations. A lognormal distribution of the magnetic field strength (hourly averages) was observed for 95% of the Vl data and most of the V2 data. An approximately lognormal distribution of B is a general property of the heliospheric magnetic field, observed out to 58.1 AU and up to 32.6°S latitude. During 1994 an exponential tail in the distribution of the magnetic field strength was observed by both Vl and V2 for fields stronger than average. An exponential tail in the magnetic field strength distribution was observed by Vl and V2 from 1983 to 1994 at latitudes from 11°S to 33°N and at distances from 13 to 58 AU. Thus an exponential tail of the distribution of magnetic field strengths is a basic property pf the heliosphere beyond 13 AU. There was no significant latitudinal gradient in the exponential tail observed by V2 in the sector zone and observed by Vl above the sector zone during 1994. Multifractal structure in the large‐scale fluctuations of the magnetic field strength was observed by Vl for periods from 8 hours to 256 hours equaling 10.7 days for the magnetic fields stronger than average. A multiffactal spectrum is a general property of the large‐scale magnetic field strength fluctuations, from 1983 through 1994 between 13 AU and 58 AU. There remains a need for a statistical theory of the solar wind that can account for these observations and for cosmic ray models that include the effect of multifractal magnetic field strength fluctuations.

Imaging Stokes polarimetry with piezoelastic modulators and charge-coupled-device image sensors

A new type of 2-D polarimeter is developed for use in high-resolution observations of solar magnetic fields. Two piezoelastic polarization modulators are used in combination with charge-coupled-device (CCD) image sensors to simultaneously record all four Stokes parameters. Demodulation of the fast 50 and 100 kHz intensity modulations produced by the piezoelastic modulators is achieved by CCD sensors used as synchronous integrators sensitive to a single frequency. The temporary buffer storage needed to separate the charges generated during the two modulation half periods is obtained by covering every second row of the CCD sensors with an opaque mask. The charges are shifted back and forth between the photosensitive uncovered and the adjacent storage rows in phase with the modulation. The polarization signal is calculated from the difference between the charges accumulated in two adjacent rows. A separate CCD sensor is needed for each normalized Stokes parameter <i>Q/I</i>, <i>U/I</i>, or <i>V/I</i>. Because the high modulation frequency lies well above the seeing frequencies occurring in solar observations, precision polarimetry becomes possible. We have demonstrated the capability of this new type of instrument to achieve a polarimetric sensitivity below 10^-3 in a single frame. By frame averaging the noise level in the fractional polarization can be reduced to the order of 10^-5.

The global structure of the solar wind in June 1991

A numerical simulation of the global solar wind structure for Carrington rotation 1843 (31 May–28 June, 1991) is performed based on a fully three-dimensional, steady-state MHD model of the solar wind (Usmanov, 1993b). A self-consistent solution for 3-D MHD equations is constructed for the spherical shell extending from the solar photosphere up to 10 AU. Solar magnetic field observations are used to prescribe boundary conditions. The computed distribution of the magnetic field is compared with coronal hole observations and with the IMF measurements made by IMP-8 spacecraft at the Earth's orbit.