arXiv subjects

Mark C. M. Cheung

Publications and source records attributed to Mark C. M. Cheung.

At least 37 records · Page 2Linked to original sources

arXiv2021

Global Earth Magnetic Field Modeling and Forecasting with Spherical Harmonics Decomposition

Modeling and forecasting the solar wind-driven global magnetic field perturbations is an open challenge. Current approaches depend on simulations of computationally demanding models like the Magnetohydrodynamics (MHD) model or sampling spatially and temporally through sparse ground-based stations (SuperMAG). In this paper, we develop a Deep Learning model that forecasts in Spherical Harmonics space 2, replacing reliance on MHD models and providing global coverage at one minute cadence, improving over the current state-of-the-art which relies on feature engineering. We evaluate the performance in SuperMAG dataset (improved by 14.53%) and MHD simulations (improved by 24.35%). Additionally, we evaluate the extrapolation performance of the spherical harmonics reconstruction based on sparse ground-based stations (SuperMAG), showing that spherical harmonics can reliably reconstruct the global magnetic field as evaluated on MHD simulation.

physics.geo-ph↗

arXiv2021

Multi-Channel Auto-Calibration for the Atmospheric Imaging Assembly using Machine Learning

Solar activity plays a quintessential role in influencing the interplanetary medium and space-weather around the Earth. Remote sensing instruments onboard heliophysics space missions provide a pool of information about the Sun's activity via the measurement of its magnetic field and the emission of light from the multi-layered, multi-thermal, and dynamic solar atmosphere. Extreme UV (EUV) wavelength observations from space help in understanding the subtleties of the outer layers of the Sun, namely the chromosphere and the corona. Unfortunately, such instruments, like the Atmospheric Imaging Assembly (AIA) onboard NASA's Solar Dynamics Observatory (SDO), suffer from time-dependent degradation, reducing their sensitivity. Current state-of-the-art calibration techniques rely on periodic sounding rockets, which can be infrequent and rather unfeasible for deep-space missions. We present an alternative calibration approach based on convolutional neural networks (CNNs). We use SDO-AIA data for our analysis. Our results show that CNN-based models could comprehensively reproduce the sounding rocket experiments' outcomes within a reasonable degree of accuracy, indicating that it performs equally well compared with the current techniques. Furthermore, a comparison with a standard "astronomer's technique" baseline model reveals that the CNN approach significantly outperforms this baseline. Our approach establishes the framework for a novel technique to calibrate EUV instruments and advance our understanding of the cross-channel relation between different EUV channels.

astro-ph.SR↗

arXiv2020

Non-Neutralized Electric Current of Active Regions Explained as a Projection Effect

Active regions (ARs) often possess an observed net electric current in a single magnetic polarity. We show that such "non-neutralized" currents can arise from a geometric projection effect when a twisted flux tube obliquely intersects the photosphere. To this end, we emulate surface maps of an emerging AR by sampling horizontal slices of a semi-torus flux tube at various heights. Although the tube has no net toroidal current, its poloidal current, when projected along the vertical direction, amounts to a significant non-neutralized component on the surface. If the tube emerges only partially as in realistic settings, the non-neutralized current will 1) develop as double ribbons near the sheared polarity inversion line, (2) positively correlate with the twist, and 3) reach its maximum before the magnetic flux. The projection effect may be important to the photospheric current distribution, in particular during the early stages of flux emergence.

astro-ph.SR↗

arXiv2020

The Coronal Global Evolutionary Model: Using HMI Vector Magnetogram and Doppler Data to Determine Coronal Magnetic Field Evolution

The Coronal Global Evolutionary Model (CGEM) provides data-driven simulations of the magnetic field in the solar corona to better understand the build-up of magnetic energy that leads to eruptive events. The CGEM project has developed six capabilities. CGEM modules (1) prepare time series of full-disk vector magnetic field observations to (2) derive the changing electric field in the solar photosphere over active-region scales. This local electric field is (3) incorporated into a surface flux transport model that reconstructs a global electric field that evolves magnetic flux in a consistent way. These electric fields drive a (4) 3D spherical magneto-frictional (SMF) model, either at high-resolution over a restricted range of solid angle or at lower resolution over a global domain, to determine the magnetic field and current density in the low corona. An SMF-generated initial field above an active region and the evolving electric field at the photosphere are used to drive (5) detailed magneto-hydrodynamic (MHD) simulations of active regions in the low corona. SMF or MHD solutions are then used to compute emissivity proxies that can be compared with coronal observations. Finally, a lower-resolution SMF magnetic field is used to initialize (6) a global MHD model that is driven by an SMF electric-field time series to simulate the outer corona and heliosphere, ultimately connecting Sun to Earth. As a demonstration, this report features results of CGEM applied to observations of the evolution of NOAA Active Region 11158 in February 2011.

astro-ph.SR↗

arXiv2020

Sun-as-a-star Spectral Irradiance Observations of Transiting Active Regions

Major solar flares are prone to occur in active region atmospheres associated with large, complex, dynamically-evolving sunspots. This points to the importance of monitoring the evolution of starspots, not only in visible but also in ultra violet (UV) and X-rays, in understanding the origin and occurrence of stellar flares. To this end, we perform spectral irradiance analysis on different types of transiting solar active regions by using a variety of full-disk synoptic observations. The target events are an isolated sunspot, spotless plage, and emerging flux in prolonged quiet-Sun conditions selected from the past decade. We find that the visible continuum and total solar irradiance become darkened when the spot is at the central meridian, whereas it is bright near the solar limb; UV bands sensitive to the chromosphere correlate well with the variation of total unsigned magnetic flux in the photosphere; amplitudes of EUV and soft X-ray increase with the characteristic temperature, whose light curves are flat-topped due to their sensitivity to the optically thin corona; the transiting spotless plage does not show the darkening in the visible irradiance, while the emerging flux produces an asymmetry in all light curves about the central meridian. The multi-wavelength sun-as-a-star study described here indicates that such time lags between the coronal and photospheric light curves have the potential to probe the extent of coronal magnetic fields above the starspots. In addition, EUV wavelengths that are sensitive to the transition-region temperature sometimes show anti-phased variations, which may be used for diagnosing plasmas around starspots.

astro-ph.SR↗

arXiv2020

The PDFI_SS Electric Field Inversion Software

We describe the PDFI_SS software library, which is designed to find the electric field at the Sun's photosphere from a sequence of vector magnetogram and Doppler velocity measurements, and estimates of horizontal velocities obtained from local correlation tracking using the recently upgraded FLCT code. The library, a collection of Fortran subroutines, uses the "PDFI" technique described by Kazachenko et al. (2014), but modified for use in spherical, Plate-Carrée geometry on a staggered grid. The domain over which solutions are found is a subset of the global spherical surface, defined by user-specified limits of colatitude and longitude. Our staggered-grid approach, based on that of Yee (1966), is more conservative and self-consistent compared to the centered, Cartesian grid used by Kazachenko et al. (2014). The library can be used to compute an end-to-end solution for electric fields from data taken by the HMI instrument aboard NASA's SDO Mission. This capability has been incorporated into the HMI pipeline processing system operating at SDO's JSOC. The library is written in a general and modular way so that the calculations can be customized to modify or delete electric field contributions, or used with other data sets. Other applications include "nudging" numerical models of the solar atmosphere to facilitate assimilative simulations. The library includes an ability to compute "global" (whole-Sun) electric field solutions. The library also includes an ability to compute Potential Magnetic Field solutions in spherical coordinates. This distribution includes a number of test programs which allow the user to test the software.

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arXiv2020

Coronal Dimming as a Proxy for Stellar Coronal Mass Ejections

Solar coronal dimmings have been observed extensively in the past two decades and are believed to have close association with coronal mass ejections (CMEs). Recent study found that coronal dimming is the only signature that could differentiate powerful ares that have CMEs from those that do not. Therefore, dimming might be one of the best candidates to observe the stellar CMEs on distant Sun-like stars. In this study, we investigate the possibility of using coronal dimming as a proxy to diagnose stellar CMEs. By simulating a realistic solar CME event and corresponding coronal dimming using a global magnetohydrodynamics model (AWSoM: Alfven-wave Solar Model), we first demonstrate the capability of the model to reproduce solar observations. We then extend the model for simulating stellar CMEs by modifying the input magnetic flux density as well as the initial magnetic energy of the CME flux rope. Our result suggests that with improved instrument sensitivity, it is possible to detect the coronal dimming signals induced by the stellar CMEs.

astro-ph.SR↗

arXiv2020

Comparative Study of Data-driven Solar Coronal Field Models Using a Flux Emergence Simulation as a Ground-truth Data Set

For a better understanding of magnetic field in the solar corona and dynamic activities such as flares and coronal mass ejections, it is crucial to measure the time-evolving coronal field and accurately estimate the magnetic energy. Recently, a new modeling technique called the data-driven coronal field model, in which the time evolution of magnetic field is driven by a sequence of photospheric magnetic and velocity field maps, has been developed and revealed the dynamics of flare-productive active regions. Here we report on the first qualitative and quantitative assessment of different data-driven models using a magnetic flux emergence simulation as a ground-truth (GT) data set. We compare the GT field with those reconstructed from the GT photospheric field by four data-driven algorithms. It is found that, at least, the flux rope structure is reproduced in all coronal field models. Quantitatively, however, the results show a certain degree of model dependence. In most cases, the magnetic energies and relative magnetic helicity are comparable to or at most twice of the GT values. The reproduced flux ropes have a sigmoidal shape (consistent with GT) of various sizes, a vertically-standing magnetic torus, or a packed structure. The observed discrepancies can be attributed to the highly non-force-free input photospheric field, from which the coronal field is reconstructed, and to the modeling constraints such as the treatment of background atmosphere, the bottom boundary setting, and the spatial resolution.

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arXiv2019

The Multi-slit Approach to Coronal Spectroscopy with the Multi-slit Solar Explorer (MUSE)

The Multi-slit Solar Explorer (MUSE) is a proposed mission aimed at understanding the physical mechanisms driving the heating of the solar corona and the eruptions that are at the foundation of space weather. MUSE contains two instruments, a multi-slit EUV spectrograph and a context imager. It will simultaneously obtain EUV spectra (along 37 slits) and context images with the highest resolution in space (0.33-0.4 arcsec) and time (1-4 s) ever achieved for the transition region and corona. The MUSE science investigation will exploit major advances in numerical modeling, and observe at the spatial and temporal scales on which competing models make testable and distinguishable predictions, thereby leading to a breakthrough in our understanding of coronal heating and the drivers of space weather. By obtaining spectra in 4 bright EUV lines (Fe IX 171A, Fe XV 284A, Fe XIX-XXI 108A) covering a wide range of transition region and coronal temperatures along 37 slits simultaneously, MUSE will be able to "freeze" the evolution of the dynamic coronal plasma. We describe MUSE's multi-slit approach and show that the optimization of the design minimizes the impact of spectral lines from neighboring slits, generally allowing line parameters to be accurately determined. We also describe a Spectral Disambiguation Code to resolve multi-slit ambiguity in locations where secondary lines are bright. We use simulations of the corona and eruptions to perform validation tests and show that the multi-slit disambiguation approach allows accurate determination of MUSE observables in locations where significant multi-slit contamination occurs.

astro-ph.IM↗

arXiv2019

Three-dimensional modeling of chromospheric spectral lines in a simulated active region

Because of the complex physics that governs the formation of chromospheric lines, interpretation of solar chromospheric observations is difficult. The origin and characteristics of many chromospheric features are, because of this, unresolved. We focus here on studying two prominent features: long fibrils and flare ribbons. To model them, we use a 3D MHD simulation of an active region which self-consistently reproduces both of them. We model the H$α$, Mg II k, Ca II K, and Ca II 8542 Å lines using the 3D non-LTE radiative transfer code Multi3D. This simulation reproduces long fibrils that span between the opposite-polarity sunspots and go up to 4 Mm in height. They can be traced in all lines due to density corrugation. Opposite to previous studies, H$α$, Mg II h&k, and Ca II H&K, are formed at similar height in this model. Magnetic field lines are aligned with the H$α$ fibrils, but the latter holds to a lesser extent for the Ca II 8542 Å line. The simulation shows structures in the H$α$ line core that look like flare ribbons. The emission in the ribbons is caused by a dense chromosphere and a transition region at high column mass. The ribbons are visible in all chromospheric lines, but least prominent in Ca II 8542 Å line. In some pixels, broad asymmetric profiles with a single emission peak are produced, similar to the profiles observed in flare ribbons. They are caused by a deep onset of the chromospheric temperature rise and large velocity gradients. The simulation produces long fibrils similar to what is seen in observations. It also produces structures similar to flare ribbons despite the lack of non-thermal electrons in the simulation. The latter suggests that thermal conduction might be a significant agent in transporting flare energy to the chromosphere in addition to non-thermal electrons.

astro-ph.SR↗

arXiv2019

Machine learning reveals systematic accumulation of electric current in lead-up to solar flares

Solar flares - bursts of high-energy radiation responsible for severe space-weather effects - are a consequence of the occasional destabilization of magnetic fields rooted in active regions (ARs). The complexity of AR evolution is a barrier to a comprehensive understanding of flaring processes and accurate prediction. Though machine learning (ML) has been used to improve flare predictions, the potential for revealing precursors and associated physics has been underexploited. Here, we train ML algorithms to classify between vector-magnetic-field observations from flaring ARs, producing at least one M-/X-class flare, and non-flaring ARs. Analysis of magnetic-field observations accurately classified by the machine presents statistical evidence for (1) ARs persisting in flare-productive states --- characterized by AR area --- for days, before and after M- and X-class flare events, (2) systematic pre-flare build-up of free energy in the form of electric currents, suggesting that associated subsurface magnetic field is twisted, (3) intensification of Maxwell stresses in the corona above newly emerging ARs, days before first flares. These results provide new insights into flare physics and improving flare forecasting.

astro-ph.SR↗

arXiv2019

A Machine Learning Dataset Prepared From the NASA Solar Dynamics Observatory Mission

In this paper we present a curated dataset from the NASA Solar Dynamics Observatory (SDO) mission in a format suitable for machine learning research. Beginning from level 1 scientific products we have processed various instrumental corrections, downsampled to manageable spatial and temporal resolutions, and synchronized observations spatially and temporally. We illustrate the use of this dataset with two example applications: forecasting future EVE irradiance from present EVE irradiance and translating HMI observations into AIA observations. For each application we provide metrics and baselines for future model comparison. We anticipate this curated dataset will facilitate machine learning research in heliophysics and the physical sciences generally, increasing the scientific return of the SDO mission. This work is a direct result of the 2018 NASA Frontier Development Laboratory Program. Please see the appendix for access to the dataset.

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arXiv2019

Multi-component Decomposition of Astronomical Spectra by Compressed Sensing

The signal measured by an astronomical spectrometer may be due to radiation from a multi-component mixture of plasmas with a range of physical properties (e.g. temperature, Doppler velocity). Confusion between multiple components may be exacerbated if the spectrometer sensor is illuminated by overlapping spectra dispersed from different slits, with each slit being exposed to radiation from a different portion of an extended astrophysical object. We use a compressed sensing method to robustly retrieve the different components. This method can be adopted for a variety of spectrometer configurations, including single-slit, multi-slit (e.g., the proposed MUlti-slit Solar Explorer mission; MUSE) and slot spectrometers (which produce overlappograms).

astro-ph.SR↗

arXiv2019

The Origin of Major Solar Activity - Collisional Shearing Between Nonconjugated Polarities of Multiple Bipoles Emerging Within Active Regions

Active Regions (ARs) that exhibit compact Polarity Inversion Lines (PILs) are known to be very flare-productive. However, the physical mechanisms behind this statistical inference have not been demonstrated conclusively. We show that such PILs can occur due to the collision between two emerging flux tubes nested within the same AR. In such multipolar ARs, the flux tubes may emerge simultaneously or sequentially, each initially producing a bipolar magnetic region (BMR) at the surface. During each flux tube's emergence phase, the magnetic polarities can migrate such that opposite polarities belonging to different BMRs collide, resulting in shearing and cancellation of magnetic flux. We name this process 'collisional shearing' to emphasize that the shearing and flux cancellation develops due to the collision. Collisional shearing is a process different from the known concept of flux cancellation occurring between polarities of a single bipole, a process that has been commonly used in many numerical models. High spatial and temporal resolution observations from the Solar Dynamics Observatory for two emerging ARs, AR11158 and AR12017, show the continuous cancellation of up to 40% of the unsigned magnetic flux of the smallest BMR, which occurs at the collisional PIL for as long as the collision persists. The flux cancellation is accompanied by a succession of solar flares and CMEs, products of magnetic reconnection along the collisional PIL. Our results suggest that the quantification of magnetic cancellation driven by collisional shearing needs to be taken into consideration in order to improve the prediction of solar energetic events and space weather.

astro-ph.SR↗

arXiv2018

Solar ultraviolet bursts

The term "ultraviolet (UV) burst" is introduced to describe small, intense, transient brightenings in ultraviolet images of solar active regions. We inventorize their properties and provide a definition based on image sequences in transition-region lines. Coronal signatures are rare, and most bursts are associated with small-scale, canceling opposite-polarity fields in the photosphere that occur in emerging flux regions, moving magnetic features in sunspot moats, and sunspot light bridges. We also compare UV bursts with similar transition-region phenomena found previously in solar ultraviolet spectrometry and with similar phenomena at optical wavelengths, in particular Ellerman bombs. Akin to the latter, UV bursts are probably small-scale magnetic reconnection events occurring in the low atmosphere, at photospheric and/or chromospheric heights. Their intense emission in lines with optically thin formation gives unique diagnostic opportunities for studying the physics of magnetic reconnection in the low solar atmosphere. This paper is a review report from an International Space Science Institute team that met in 2016-2017.

astro-ph.SR↗

arXiv2018

A Truly Global Extreme Ultraviolet Wave from the SOL2017-09-10 X8.2+ Solar Flare-Coronal Mass Ejection

We report SDO/AIA observations of an extraordinary global extreme ultraviolet (EUV) wave triggered by the X8.2+ flare-CME eruption on 2017 September 10. This was one of the best EUV waves ever observed with modern instruments, yet it was likely the last one of such magnitudes of Solar Cycle 24 as the Sun heads toward the minimum. Its remarkable characteristics include the following. (1) The wave was observed, for the first time, to traverse the full-Sun corona over the entire visible solar disk and off-limb circumference, manifesting a truly global nature, owing to its exceptionally large amplitude, e.g., with EUV enhancements by up to 300% at 1.1 Rsun from the eruption. (2) This leads to strong transmissions (in addition to commonly observed reflections) in and out of both polar coronal holes, which are usually devoid of EUV waves. It has elevated wave speeds >2000 km/s within them, consistent with the expected higher fast-mode magnetosonic wave speeds. The coronal holes essentially serve as new "radiation centers" for the waves being refracted out of them, which then travel toward the equator and collide head-on, causing additional EUV enhancements. (3) The wave produces significant compressional heating to local plasma upon its impact, indicated by long-lasting EUV intensity changes and differential emission measure increases at higher temperatures (e.g., log T=6.2) accompanied by decreases at lower temperatures (e.g., log T=6.0). These characteristics signify the potential of such EUV waves for novel magnetic and thermal diagnostics of the solar corona {\it on global scales}.

astro-ph.SR↗

arXiv2018

An Observationally-Constrained Model of a Flux Rope that Formed in the Solar Corona

Coronal mass ejections (CMEs) are large-scale eruptions of plasma from the coronae of stars. Understanding the plasma processes involved in CME initiation has applications to space weather forecasting and laboratory plasma experiments. James et al. (Sol. Phys. 292, 71, 2017) used EUV observations to conclude that a magnetic flux rope formed in the solar corona above NOAA Active Region 11504 before it erupted on 14 June 2012 (SOL2012-06-14). In this work, we use data from the Solar Dynamics Observatory to model the coronal magnetic field of the active region one hour prior to eruption using a nonlinear force-free field extrapolation, and find a flux rope reaching a maximum height of 150 Mm above the photosphere. Estimations of the average twist of the strongly asymmetric extrapolated flux rope are between 1.35 and 1.88 turns, depending on the choice of axis, although the erupting structure was not observed to kink. The decay index near the apex of the axis of the extrapolated flux rope is comparable to typical critical values required for the onset of the torus instability, so we suggest that the torus instability drove the eruption.

astro-ph.SR↗

arXiv2017

Witnessing a Large-scale Slipping Magnetic Reconnection along a Dimming Channel during a Solar Flare

We report the intriguing large-scale dynamic phenomena associated with the M6.5 flare~(SOL2015-06-22T18:23) in NOAA active region 12371, observed by RHESSI, Fermi, and the Atmospheric Image Assembly (AIA) and Magnetic Imager (HMI) on the Solar Dynamic Observatory (SDO). The most interesting feature of this event is a third ribbon (R3) arising in the decay phase, propagating along a dimming channel (seen in EUV passbands) towards a neighboring sunspot. The propagation of R3 occurs in the presence of hard X-ray footpoint emission, and is broadly visible at temperatures from 0.6~MK to over 10~MK through the Differential Emission Measure (DEM) analysis. The coronal loops then undergo an apparent slipping motion following the same path of R3, after a $\sim$ 80~min delay. To understand the underlying physics, we investigate the magnetic configuration and the thermal structure of the flaring region. Our results are in favor of a slipping-type reconnection followed by the thermodynamic evolution of coronal loops. In comparison with those previously reported slipping reconnection events, this one proceeds across a particularly long distance ($\sim$60 Mm) over a long period of time ($\sim$50 min), and shows two clearly distinguished phases: the propagation of the footpoint brightening driven by nonthermal particle injection and the apparent slippage of loops governed by plasma heating and subsequent cooling.

astro-ph.SR↗