Measuring the magnetic origins of solar flares, CMEs and Space Weather
TL;DR: In this article, the authors identify a region in the near UV part of the spectrum as optimal for studying the development of magnetic free energy over active regions using data from SORCE, Hubble Space Telescope, and SKYLAB, along with 1D computations of the near-UV (NUV) spectrum and numerical experiments based on the MURaM radiation-MHD and HanleRT radiative transfer codes.
Abstract: We take a broad look at the problem of identifying the magnetic solar causes of space weather. With the lackluster performance of extrapolations based upon magnetic field measurements in the photosphere, we identify a region in the near UV part of the spectrum as optimal for studying the development of magnetic free energy over active regions. Using data from SORCE, Hubble Space Telescope, and SKYLAB, along with 1D computations of the near-UV (NUV) spectrum and numerical experiments based on the MURaM radiation-MHD and HanleRT radiative transfer codes, we address multiple challenges. These challenges are best met through a combination of near UV lines of bright \ion{Mg}{2}, and lines of \ion{Fe}{2} and \ion{Fe}{1} (mostly within the $4s-4p$ transition array) which form in the chromosphere up to $2\times10^4$ K. Both Hanle and Zeeman effects can in principle be used to derive vector magnetic fields. However, for any given spectral line the $\tau=1$ surfaces are generally geometrically corrugated owing to fine structure such as fibrils and spicules. By using multiple spectral lines spanning different optical depths, magnetic fields across nearly-horizontal surfaces can be inferred in regions of low plasma $\beta$, from which free energies, magnetic topology and other quantities can be derived.
Based upon the recently-reported successful suborbital space measurements of magnetic fields with the CLASP2 instrument,
we argue that a modest space-borne telescope will be able to make significant advances in the attempts to predict solar eruptions. Difficulties associated with blended lines are shown to be minor in an Appendix.
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TL;DR: In this article, an overview of observing facilities for solar research is presented, which are planned or will come into operation in the near future, focusing on facilities which harbor specific potential for solar magnetometry.
Abstract: In this review we present an overview of observing facilities for solar research, which are planned or will come to operation in near future. We concentrate on facilities, which harbor specific potential for solar magnetometry. We describe the challenges and science goals of future magnetic measurements, the status of magnetic field measurements at different major solar observatories, and provide an outlook on possible upgrades of future instrumentation.
16 citations
16 Dec 2014
TL;DR: In this article, a support vector machine (SVM) was used to forecast M-and X-class solar flares using four years of data from the Solar Dynamics Observatory's Helioseismic and Magnetic Imager.
Abstract: We attempt to forecast M- and X-class solar flares using a machine-learning algorithm, called support vector machine (SVM), and four years of data from the Solar Dynamics Observatory's Helioseismic and Magnetic Imager, the first instrument to continuously map the full-disk photospheric vector magnetic field from space. Most flare forecasting efforts described in the literature use either line-of-sight magnetograms or a relatively small number of ground-based vector magnetograms. This is the first time a large data set of vector magnetograms has been used to forecast solar flares. We build a catalog of flaring and non-flaring active regions sampled from a database of 2071 active regions, comprised of 1.5 million active region patches of vector magnetic field data, and characterize each active region by 25 parameters. We then train and test the machine-learning algorithm and we estimate its performances using forecast verification metrics with an emphasis on the true skill statistic (TSS). We obtain relatively high TSS scores and overall predictive abilities. We surmise that this is partly due to fine-tuning the SVM for this purpose and also to an advantageous set of features that can only be calculated from vector magnetic field data. We also apply a feature selection algorithm to determine which of our 25 features are useful for discriminating between flaring and non-flaring active regions and conclude that only a handful are needed for good predictive abilities.
11 citations
TL;DR: The magnetic field is the main driver of the activity in the solar upper atmosphere, but its measurement is notoriously difficult as discussed by the authors , which is why a number of recent advances have activated the development of this research field.
Abstract: The magnetic field is the main driver of the activity in the solar upper atmosphere, but its measurement is notoriously difficult. In order to determine the magnetic field in the chromosphere, transition region, and corona, we need to measure and interpret the polarization signals that the scattering of anisotropic radiation and the Hanle and Zeeman effects introduce in the emitted spectral line radiation. A number of recent advances have activated the development of this research field. ▪ The quantum theory of the generation and transfer of polarized radiation explains allows us to explain the polarization signals observed in chromospheric and coronal lines and to make successful predictions in unexplored spectral regions. ▪ The development of diagnostic techniques for the solar upper atmosphere has served to improve our empirical knowledge of the magnetic field in a variety of plasma structures, as well as to pave the way for their application to the unprecedented data that the new generation of solar telescopes are expected to provide. However, further improvements are required. ▪ The CLASP suborbital experiments have opened a new diagnostic window, namely ultraviolet (UV) spectropolarimetry as a tool for probing the magnetism and geometry of the upper chromosphere and transition region. A space telescope equipped with a UV spectropolarimeter would lead to major advances in our empirical understanding of solar magnetism. Expected final online publication date for the Annual Review of Astronomy and Astrophysics Volume 60 is August 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
11 citations
TL;DR: In this paper , an artificial intelligence-generated solar farside magnetogram using data sets from the Solar Terrestrial Relations Observatory (STEREO) and Solar Dynamics Observatory (SDO) is presented.
Abstract: Here we greatly improve artificial intelligence (AI)–generated solar farside magnetograms using data sets from the Solar Terrestrial Relations Observatory (STEREO) and Solar Dynamics Observatory (SDO). We modify our previous deep-learning model and configuration of input data sets to generate more realistic magnetograms than before. First, our model, which is called Pix2PixCC, uses updated objective functions, which include correlation coefficients (CCs) between the real and generated data. Second, we construct input data sets of our model: solar farside STEREO extreme-ultraviolet (EUV) observations together with nearest frontside SDO data pairs of EUV observations and magnetograms. We expect that the frontside data pairs provide historic information on magnetic field polarity distributions. We demonstrate that magnetic field distributions generated by our model are more consistent with the real ones than previously, in consideration of several metrics. The averaged pixel-to-pixel CC for full disk, active regions, and quiet regions between real and AI-generated magnetograms with 8 × 8 binning are 0.88, 0.91, and 0.70, respectively. Total unsigned magnetic flux and net magnetic flux of the AI-generated magnetograms are consistent with those of real ones for the test data sets. It is interesting to note that our farside magnetograms produce polar field strengths and magnetic field polarities consistent with those of nearby frontside magnetograms for solar cycles 24 and 25. Now we can monitor the temporal evolution of active regions using solar farside magnetograms by the model together with the frontside ones. Our AI-generated solar farside magnetograms are now publicly available at the Korean Data Center for SDO (http://sdo.kasi.re.kr).
6 citations
TL;DR: In this article , the effects of spectral resolution on the magnetic field values retrieved through the weak-field approximation (WFA) from the cores of the Mg ii h and k lines were studied.
Abstract: We study the effects of finite spectral resolution on the magnetic field values retrieved through the weak-field approximation (WFA) from the cores of the Mg ii h and k lines. The retrieval of the line-of-sight component of the magnetic field, B LOS, from synthetic spectra generated in a uniformly magnetized FAL-C atmosphere is accurate when restricted to the inner lobes of Stokes V. As we degrade the spectral resolution, partial redistribution effects, which more prominently affect the outer lobes of Stokes V, are brought into the line core through spectral smearing, degrading the accuracy of the WFA and resulting in an inference bias, which is more pronounced as the resolution becomes poorer. When applied to a diverse set of spectra emerging from a sunspot simulation, we find good accuracy in the retrieved B LOS when comparing it to the model value at the height where the optical depth in the line core is unity. The accuracy is preserved up to field strengths of B ∼ 1500 G. Limited spectral resolution results in a small bias toward weaker retrieved fields. The WFA for the transverse component of the magnetic field is also evaluated. Reduced spectral resolution degrades the accuracy of the inferences, because spectral mixing results in the line effectively probing deeper layers of the atmosphere.
2 citations
References
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TL;DR: In this article, an extensive grid of spherically-symmetric models (supplemented with plane-parallel ones for the highest surface gravities), built on up-to-date atomic and molecular data, is presented.
Abstract: Context. In analyses of stellar spectra and colours, and for the analysis of integrated light from galaxies, a homogeneous grid of model atmospheres of late-type stars and corresponding flux spectra is needed. Aims. We construct an extensive grid of spherically-symmetric models (supplemented with plane-parallel ones for the highest surface gravities), built on up-to-date atomic and molecular data, and make it available for public use. Methods. The most recent version of the MARCS program is used. Results. We present a grid of about 104 model atmospheres for stars with 2500K <= T-eff <= 8000 K, -1 <= log g = log (GM/R-2) <= 5 (cgs) with various masses and radii, -5 <= [Me/H] <= + 1, with [alpha/Fe] = 0.0 and 0.4 and different choices of C and N abundances. This includes "CN-cycled" models with C/N=4.07 (solar), 1.5 and 0.5, C/O ranging from 0.09 to (normally) 5.0 to also represent stars of spectral types R, S and N, and with 1.0 <= xi(t) = 5km s(-1). We also list thermodynamic quantities (T, P-g, P-e, rho, partial pressures of molecules, etc.) and provide them on the World Wide Web, as well as calculated fluxes in approximately 108 000 wavelength points. Underlying assumptions in addition to 1D stratification (spherical or plane-parallel) include hydrostatic equilibrium, mixing-length convection and local thermodynamic equilibrium. We discuss a number of general properties of the models, in particular in relation to the effects of changing abundances, of blanketing, and of sphericity. We illustrate positive and negative feedbacks between sphericity and molecular blanketing. We compare the models with those of other available grids and find excellent agreement with planeparallel models of Castelli & Kurucz (if convection is treated consistently) within the overlapping parameter range. Although there are considerable departures from the spherically-symmetric NextGen models, the agreement with more recent PHOENIX models is gratifying. Conclusions. The models of the grid show considerable regularities, but some interesting departures from general patterns occur for the coolest models due to the molecular opacities. We have tested a number of approximate "rules of thumb" concerning effects of blanketing and sphericity and often found them to be astonishingly accurate. Some interesting new phenomena have been discovered and explored, such as the intricate coupling between blanketing and sphericity, and the strong effects of carbon enhancement on metal-poor models. We give further details of line absorption data for molecules, as well as details of models and comparisons with observations in subsequent papers.
2,411 citations
TL;DR: In this article, the authors examined the theoretical consequences to be expected during the extended relaxation phase which must follow such events, characterized by a gradual reconnection of the outward-distended field lines.
Abstract: Many classes of transient solar phenomena, such as flares, flare sprays, and eruptive prominences, cause major disruptions in the magnetic geometry of the overlying corona. Typically, the results from Skylab indicate that pre-existing closed magnetic loops in the corona are torn open by the force of the disruption. We examine here some of the theoretical consequences to be expected during the extended relaxation phase which must follow such events. This phase is characterized by a gradual reconnection of the outward-distended field lines. In particular, the enhanced coronal expansion which occurs on open field lines just before they reconnect appears adequate to supply the large downward mass fluxes observed in Ha loop prominence systems that form during the post-transient relaxation. In addition, this enhanced flow may produce nonrecurrent high speed streams in the solar wind after such events. Calculations of the relaxation phase for representative field geometries and the resulting flow configurations are described.
1,467 citations
TL;DR: In this paper, a theoretical model of Ha, EUV, soft X-ray and flare-associated solar wind is presented to explain the observed quantities in Ha and EUV.
Abstract: A theoretical model of flare which explains observed quantities in Ha, EUV, soft X-ray and flare-associated solar wind is presented. It is assumed that large mass observed in the soft X-ray flare and the solar wind comes from the chromosphere by the process like evaporation while flare is in progress. From mass and pressure balance in the chromosphere and the corona, the high temperature in the soft X-ray flare is shown to be attained by the larger mass loss to the solar wind compared with the mass remained in the corona, in accord with observations. The total energy of 10 a2 erg, the electron density of l013'5 cm -a in Hc~ flare, the temperature of the X-ray flare of 107.3K and the time to attain maximum Ha brightness (600 s) are derived consistent with observations. It is shown that the top height of the Ha flare is located about 1000 km lower than that of the active chromosphere because of evaporation. So-called limb flares are assigned to either post-flare loops, surges or rising prominences. The observed small thickness of the Ha flare is interpreted by free streaming and/or heat conduction. Applications are suggested to explain the maximum temperature of a coronal condensation and the formation of quiescent prominences.
1,074 citations
"Measuring the magnetic origins of s..." refers background in this paper
...We now have a “standard model” of flares (Carmichael 1964; Sturrock 1966; Hirayama 1974; Kopp & Pneuman 1976), involving a loss of equilibrium of arcades of closed magnetic flux systems, particle acceleration and, frequently, ejection of plasmoids....
[...]
932 citations
"Measuring the magnetic origins of s..." refers background in this paper
...We now have a “standard model” of flares (Carmichael 1964; Sturrock 1966; Hirayama 1974; Kopp & Pneuman 1976), involving a loss of equilibrium of arcades of closed magnetic flux systems, particle acceleration and, frequently, ejection of plasmoids....
[...]
Johns Hopkins University Applied Physics Laboratory1, California Institute of Technology2, University of California, Berkeley3, United States Naval Research Laboratory4, University of Michigan5, Harvard University6, Southwest Research Institute7, University of Texas at San Antonio8, Goddard Space Flight Center9
TL;DR: The first spacecraft to fly into the low solar corona is the Solar Probe Plus (SPP) as discussed by the authors, which is scheduled for launch in mid-2018 and will perform 24 orbits over a 7-year nominal mission duration.
Abstract: Solar Probe Plus (SPP) will be the first spacecraft to fly into the low solar corona. SPP’s main science goal is to determine the structure and dynamics of the Sun’s coronal magnetic field, understand how the solar corona and wind are heated and accelerated, and determine what processes accelerate energetic particles. Understanding these fundamental phenomena has been a top-priority science goal for over five decades, dating back to the 1958 Simpson Committee Report. The scale and concept of such a mission has been revised at intervals since that time, yet the core has always been a close encounter with the Sun. The mission design and the technology and engineering developments enable SPP to meet its science objectives to: (1) Trace the flow of energy that heats and accelerates the solar corona and solar wind; (2) Determine the structure and dynamics of the plasma and magnetic fields at the sources of the solar wind; and (3) Explore mechanisms that accelerate and transport energetic particles. The SPP mission was confirmed in March 2014 and is under development as a part of NASA’s Living with a Star (LWS) Program. SPP is scheduled for launch in mid-2018, and will perform 24 orbits over a 7-year nominal mission duration. Seven Venus gravity assists gradually reduce SPP’s perihelion from 35 solar radii (
$R_{S}$
) for the first orbit to ${<}10~R_{S}$
for the final three orbits. In this paper we present the science, mission concept and the baseline vehicle for SPP, and examine how the mission will address the key science questions
906 citations