Global Oscillation Network Group Research Articles

A Compact Full-disk Solar Magnetograph Based on Miniaturization of the GONG Instrument* *Submitted January XX, 2023.

The design of compact instruments is crucial for the scientific exploration by smaller spacecraft such as CubeSats and deep space missions, as these missions require minimal instrument mass. In this proof-of-concept study, we demonstrate the miniaturization of the Global Oscillation Network Group (GONG) instrument. GONG instruments routinely produce full-disk maps of the Sun’s photosphere using the Ni i 676 nm absorption line, measuring Doppler shifts and magnetic fields. To miniaturize the GONG optical design, we propose replacing the bulky Lyot filter with a narrow-band interference filter. We validate this concept through numerical modeling and proof-of-concept observations. Finally, we propose a simple optical design for building a compact version of GONG.

Formation of an Intermediate Filament Driven by Small-scale Magnetic Reconnection

We present the formation process of a filament in NOAA active region 12765 from 2020 June 5 to 8, using observations from the New Vacuum Solar Telescope, the Solar Dynamics Observatory, and the Global Oscillation Network Group. We found that intermittent small-scale magnetic reconnection occurs at the northern part of the filament, and the small-scale magnetic reconnection shows the characteristics of the oscillatory reconnections. During the magnetic reconnection process, a large amount of material is continuously injected into the filament channel. Furthermore, there are bidirectional inflow and outflow, current sheets, and bright cusp-shaped structures. The velocities of the material injections range from 17 to 183 km s−1 with an average velocity of about 57 km s−1. A total of 53 material injections were found from 03:10 UT on 2020 June 5 to 00:10 UT on June 8. The total mass carried by the injection events is about 7.39 × 1014 g, and the total kinetic energy released through magnetic reconnection is approximately 3.09 × 1021 J. The projection area of the filament increased from less than 1 × 102 Mm2 to around 7 × 102 Mm2. We conclude that the filament is formed by direct material injection into the filament channel due to the small-scale magnetic reconnections.

Precision and systematic errors in global helioseismology mode fitting and inversions: Leveraging some 25 years of nearly uninterrupted observations

We have on hand some 25 years of nearly uninterrupted high-quality and high-cadence global helioseismic data. The Global Oscillations Network Group (GONG) project has been producing science quality data since 1995, the Michelson Doppler Imager (MDI) started in 1996, and the Helioseismic and Magnetic Imager (HMI) took over in 2010. Fundamental new constraints have been imposed by helioseismic inferences, yet global helioseismology data processing seems somewhat frozen in time for some of its methodologies. I review and discuss some specific aspects of global helioseismology data analysis, with an emphasis on the issues and challenges presented by mode fitting and inversion techniques. I compare and contrast results derived by different fitting methods, whether using different techniques, different lengths of time series, or different fitting parameters, like leakage matrices or the inclusion or omission of the mode profile asymmetry, leading to our current best handle on the residual systematic errors.

Radial gradient of the solar rotation rate in the near-surface shear layer of the Sun

We study the radial gradient of the solar rotation rate in the near-surface shear layer (NSSL) from about .950 R⊙ to the solar surface and its variation during Solar Cycles 23 and 24 with ring-diagram analysis applied to Global Oscillation Network Group (GONG) and Helioseismic and Magnetic Imager (HMI) Dopplergrams. The average radial gradient is ∂ log Ω/∂ log r = − 1.0 ± .1 at .990 R⊙ in agreement with previous studies. The average radial gradient is ∂ log Ω/∂ log r = − .11 ± .01 at the base of the NSSL at .950R⊙, while it is steeper than −1 closer to the surface between .990R⊙ and .997R⊙. The average radial gradient is rather flat within ±40° latitude from about .970 R⊙ to the solar surface. The radial gradient of the solar rotation rate varies with the solar cycle. At locations of high magnetic activity, the radial gradient is more negative than average from about .970 R⊙ to .990 R⊙, while in quiet regions the radial gradient is less negative than average at these depths. Close to the surface at .997 R⊙, this relationship appears to be reversed. Prominent features of the solar-cycle variation of large-scale flows, such as poleward branches or precursor flows, are not obviously present. The variation of the radial gradient thus more likely indicates the presence or absence of magnetic flux above a certain threshold. The temporal variations derived from the different HMI and GONG data sets agree within one error bar at most depths and latitudes, while their amplitudes might be different.

Update on Global Helioseismic Observations of the Solar Torsional Oscillation

We present an up-to-date latitude–time map of the solar zonal flows at 0.99 R ⊙ from global helioseismology using data from the Global Oscillation Network Group, the Michelson Doppler Imager, and the Helioseismic and Magnetic Imager.

Deep-learning Reconstruction of Sunspot Vector Magnetic Fields for Forecasting Solar Storms

Solar magnetic activity produces extreme solar flares and coronal mass ejections, which pose grave threats to electronic infrastructure and can significantly disrupt economic activity. It is therefore important to appreciate the triggers of explosive solar activity and develop reliable space weather forecasting. Photospheric vector magnetic field data capture sunspot magnetic field complexity and can therefore improve the quality of space weather prediction. However, state-of-the-art vector field observations are consistently only available from Solar Dynamics Observatory/Helioseismic and Magnetic Imager (HMI) since 2010, with most other current and past missions and observational facilities, such as Global Oscillations Network Group (GONG), only recording line-of-sight (LOS) fields. Here, using an inception-based convolutional neural network (CNN), we reconstruct HMI sunspot vector field features from LOS magnetograms of HMI and GONG with high fidelity (∼90% correlation) and sustained flare forecasting accuracy. We rebuild vector field features during the 2003 Halloween storms, for which only LOS field observations are available, and the CNN-estimated electric current helicity accurately captures the observed rotation of the associated sunspot prior to the extreme flares, showing a striking increase. Our study thus paves the way for reconstructing three solar cycles worth of vector field data from past LOS measurements, which are of great utility in improving space weather forecasting models and gaining new insights about solar activity.

Comparison of Two Methods for Deriving the Magnetic Field in a Filament Channel

Understanding the magnetic structure of filament channels is difficult but essential for identifying the mechanism (s) responsible for solar eruptions. In this paper we characterize the magnetic field in a well-observed filament channel with two independent methods, prominence seismology and magnetohydrodynamics flux-rope modeling, and compare the results. In 2014 May and June, active region 12076 exhibited a complex of filaments undergoing repeated oscillations over the course of 12 days. We measure the oscillation periods in the region with both Global Oscillation Network Group Hα and Solar Dynamics Observatory (SDO) Advanced Imaging Assembly EUV images, and then utilize the pendulum model of large-amplitude longitudinal oscillations to calculate the radius of curvature of the fields supporting the oscillating plasma from the derived periods. We also employ the regularized Biot–Savart laws formalism to construct a flux-rope model of the field of the central filament in the region based on an SDO Helioseismic and Magnetic Imager magnetogram. We compare the estimated radius of curvature, location, and angle of the magnetic field in the plane of the sky derived from the observed oscillations with the corresponding magnetic-field properties extracted from the flux-rope model. We find that the two models are broadly consistent, but detailed comparisons of the model and specific oscillations often differ. Model observation comparisons such as these are important for advancing our understanding of the structure of filament channels.

Sub-surface plasma flows and the flare productivity of solar active regions

The extreme space weather conditions resulting from high energetic events likes solar flares and Coronal Mass Ejections demand for reliable space weather forecasting. The magnetic flux tubes while rising through the convection zone gets twisted by the turbulent plasma flows, energizing the system and resulting in flares. We investigate the relationship between the subsurface plasma flows associated with flaring active regions and their surface magnetic flux and current helicity. The near-surface horizontal velocities derived from the ring-diagram analysis of active region patches using Global Oscillation Network Group Doppler velocity measurements are used to compute the fluid dynamics descriptors like vertical divergence, vorticity and kinetic helicity used in this work. The flaring active regions are observed to have large value of vertical vorticity and kinetic helicity. Also, the horizontal flow divergence, vorticity, flux, kinetic and current helicities are observed to be significantly correlated and evolve in phase with each other. We observe that the integrated values of the above flow and magnetic parameters observed 1 day prior to the flare are significantly correlated with the integrated flare intensity of the active region. Hence, we show that strong vorticity/kinetic helicities lead to larger active region twisting, presumably generating high-intensity flares.

Cycle dependence of a quasi-biennial variability in the solar interior

ABSTRACT We investigated the solar cycle dependence on the presence and periodicity of the Quasi-Biennial Oscillation (QBO). Using helioseismic techniques, we used solar oscillation frequencies from the Global Oscillations Network Group (GONG), Michelson Doppler Imager (MDI), and Helioseismic and Magnetic Imager (HMI) in the intermediate-degree range to investigate the frequency shifts over Cycles 23 and 24. We also examined two solar activity proxies, the F10.7 index and the Mg ii index, for the last four solar cycles to study the associated QBO. The analyses were performed using Empirical Mode Decomposition (EMD) and the Fast Fourier Transform (FFT). We found that the EMD analysis method is susceptible to detecting statistically significant Intrinsic Mode Functions (IMFs) with periodicities that are overtones of the length of the data set under examination. Statistically significant periodicities, which were not due to overtones, were detected in the QBO range. We see a reduced presence of the QBO in Cycle 24 compared to Cycle 23. The presence of the QBO was not sensitive to the depth to which the p-mode travelled, nor the average frequency of the p-mode. The analysis further suggested that the magnetic field responsible for producing the QBO in frequency shifts of p-modes is anchored above approximately 0.95 R⊙.

A Double-decker Filament Formation Driven by Sunspot Rotation and Magnetic Reconnection

In this paper, through analyzing data from the Solar Dynamics Observatory (SDO) and the Global Oscillation Network Group (GONG), we present a study on the formation of a double-decker filament in NOAA Active Region 12665 from 2017 July 8 to 14. We find that magnetic reconnection occurs between two smaller filaments to form a longer filament. According to the evolution of the leading sunspot, it is obvious that the sunspot experiences a continuous rotation around its umbra. During the period from 03:00 UT on July 11 to 10:00 UT on July 14, the average speed of sunspot rotation is about 3.°7 hr–1. The continuous rotation of sunspot stretches the filament and results in the formation of a reversed S-shaped filament. Due to the motion of the magnetic field and internal magnetic reconnection, the filament splits into two branches and forms a double-decker filament structure. In the process of filament separation, internal magnetic reconnection can also accelerate the filament separation. Nonlinear force-free field extrapolation indicates that there are two magnetic flux ropes, which are consistent with the observed results. Eventually, the upper filament erupts and produces an M-class flare and a halo coronal mass ejection.

Sympathetic Quiet and Active Region Filament Eruptions

We present the observations of three sympathetic filament eruptions occurring on 19 July 2015 namely F1, F2, and F3. The events were observed in UV/EUV wavelengths by Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory and by Global Oscillation Network Group telescope in H{\alpha} line. As filament F1 starts to erupt, a part of it falls close to the location of the F2 and F3 filaments. This causes the eruption of F2 and F3 during which the two filaments merge together and trigger a medium-class CME and a long-duration GOES C2.1 class flare. We discuss the dynamics and kinematics of these three filament eruptions and related phenomena.

Improving the Alfvén Wave Solar Atmosphere Model Based on Parker Solar Probe Data

Abstract In van der Holst et al. (2019), we modeled the solar corona and inner heliosphere of the first encounter of NASA’s Parker Solar Probe (PSP) using the Alfvén Wave Solar atmosphere Model (AWSoM) with Air Force Data Assimilative Photospheric flux Transport–Global Oscillation Network Group magnetograms, and made predictions of the state of the solar wind plasma for the first encounter. AWSoM uses low-frequency Alfvén wave turbulence to address the coronal heating and acceleration. Here, we revise our simulations, by introducing improvements in the energy partitioning of the wave dissipation to the electron and anisotropic proton heating and using a better grid design. We compare the new AWSoM results with the PSP data and find improved agreement with the magnetic field, turbulence level, and parallel proton plasma beta. To deduce the sources of the solar wind observed by PSP, we use the AWSoM model to determine the field line connectivity between PSP locations near the perihelion at 2018 November 6 UT 03:27 and the solar surface. Close to the perihelion, the field lines trace back to a negative-polarity region about the equator.

What Seismic Minimum Reveals about Solar Magnetism below the Surface

Abstract The Sun’s magnetic field varies on multiple timescales. Observations show that the minimum between cycles 24 and 25 was the second consecutive minimum that was deeper and wider than several earlier minima. Since the active regions observed at the Sun’s surface are manifestations of the magnetic field generated in the interior, it is crucial to investigate/understand the dynamics below the surface. In this context, we report by probing the solar interior with helioseismic techniques applied to long-term oscillations data from the Global Oscillation Network Group, that the seismic minima in deeper layers have been occurring about a year earlier than that at the surface for the last two consecutive solar cycles. Our findings also demonstrate a decrease in strong magnetic fields at the base of the convection zone, the primary driver of the surface magnetic activity. We conclude that the magnetic fields located in the core and near-surface shear layers, in addition to the tachocline fields, play an important role in modifying the oscillation frequencies. This further strengthens the existence of a relic magnetic field in the Sun’s core.

Continuous Solar Observations from the Ground—Assessing Duty Cycle from GONG Observations

Continuous observations play an important role in the studies of solar variability. While such observations can be achieved from space with an almost 100% duty cycle, it is difficult to accomplish a very high duty cycle from the ground. In this context, we assess the duty cycle that has been achieved from the ground by analyzing the observations of a six station network of identical instruments, the Global Oscillation Network Group (GONG). We provide a detailed analysis of the duty cycle using GONG observations spanning over 18 yr. We also discuss the duty cycle of individual sites and point out various factors that may impact individual site or network duty cycles. The mean duty cycle of the network is 93%, however it reduces by about 5% after all images pass through the stringent quality-control checks. The standard deviations in monthly and yearly duty cycle values are found to be 1.9% and 2.2%, respectively. These results provide a baseline that can be used in the planning of future ground-based networks.

Evidence of Solar-cycle-related Structural Changes in the Solar Convection Zone

Abstract While it has been relatively easy to determine solar-cycle related changes in solar dynamics, determining changes in structure in the deeper layers of the Sun has proved to be difficult. By using helioseismic data obtained over two solar cycles, and sacrificing resolution in favor of lower uncertainties, we show that there are significant changes in the solar convection zone, and perhaps even below it. Using Michelson Doppler Imager (MDI) data, we find a relative squared sound-speed difference of (2.56 ± 0.71) × 10−5 at the convection-zone base between the maximum of solar Cycle 23 and the minimum between Cycles 23 and 24. The squared sound-speed difference for the maximum of Cycle 24 obtained with Helioseismic and Magnetic Imager (HMI) data is (1.95 ± 0.69) × 10−5. Global Oscillation Network Group (GONG) data support these results. We also find that the sound speed in the solar convection zone decreases compared to the sound speed in the layers below it as the Sun becomes more active. We find evidence of changes in the radial derivative of the sound-speed difference between the solar minimum and other epochs at the base of the convection zone, implying possible small changes in the position of the convection-zone base; however, the results are too noisy to make any definitive estimates of the change.

Obtaining space-based SDO/AIA solar UV and EUV images from ground-based Hα observations by deep learning

In this work, we explore the mappings from solar images taken in Hα (6563 Å) by the Global Oscillation Network Group (GONG) on the ground to those observed in eight different wavelengths (94, 131, 171, 193, 211, 304, 335 and 1600 Å) by SDO/AIA in space. Eight mappings are built by training the conditional Generative Adversarial Networks (cGANs) on datasets with 500 paired images, which are [Hα, AIA94], [Hα, AIA131], [Hα, AIA171], [Hα, AIA193], [Hα, AIA211], [Hα, AIA304], [Hα, AIA335] and [Hα, AIA1600]. We evaluate the eight trained cGANs models on validation and test datasets with 154-pair images and 327-pair images, respectively. The model generated fake AIA images match the corresponding observed AIA images well on large-scale structures such as large active regions and prominences. But the small-scale flare loops and filament threads are difficult to reconstruct. Four quantitative comparisons are carried out on the validation and test datasets to score the mappings. We find that the model-generated images in 304 and 1600 Å match the corresponding observed images best. This exploration suggests that the cGANs are promising methods for mappings between ground-based Hα and space-based EUV/UV images, while some improvements are necessary.

Implementing the MULTI-VP coronal model in EUHFORIA: Test case results and comparisons with the WSA coronal model

Context.In this study, we focus on improving EUHFORIA (European Heliospheric Forecasting Information Asset), a recently developed 3D magnetohydrodynamics space weather prediction tool. The EUHFORIA model consists of two parts covering two spatial domains: the solar corona and the inner heliosphere. For the first part, the semiempirical Wang-Sheeley-Arge (WSA) model is used by default; this model employs the potential field source surface and Schatten current sheet models to provide the necessary solar wind plasma and magnetic conditions above the solar surface, at 0.1 AU, which serve as boundary conditions for the inner heliospheric part. Herein, we present the first results of the implementation of an alternative coronal model in EUHFORIA, the so-called MULTI-VP model.Aims.After we replace the default EUHFORIA coronal setup with the MULTI-VP model, we compare their outputs both at 0.1 AU and 1 AU, for test cases involving high speed wind streams (HSSs). We select two distinct cases in which the standard EUHFORIA setup failed to reproduce the HSS plasma and magnetic signatures at Earth to test the performance of MULTI-VP coupled with EUHFORIA-heliosphere.Methods.To understand the quality of modeling with MULTI-VP in comparison with the default coronal model in EUHFORIA, we considered one HSS case during a period of low solar activity and another one during a period of high solar activity. Moreover, the modeling of the two HSSs was performed by employing magnetograms from different providers: one from the Global Oscillation Network Group (GONG) and the second from theWilcoxSpace Observatory (WSO). This way, we were able to distinguish differences arising not only because of the different models but also because of different magnetograms.Results.The results indicate that when employing a GONG magnetogram, the combination MULTI-VP+EUHFORIA-heliosphere reproduces the majority of HSS plasma and magnetic signatures measured at L1. On the contrary, the standard WSA+EUHFORIA-heliosphere combination does not capture the arrival of the HSS cases at L1. When employing WSO magnetograms, MULTI-VP+EUHFORIA-heliosphere reproduces the HSS that occurred during the period of high solar activity. However, it is unclear if it models the HSS during the period of low solar activity. For the same magnetogram and periods of time, WSA+EUHFORIA-heliosphere is not able to capture the HSSs of interest.Conclusions.The results show that the accuracy of the simulation output at Earth depends on the choice of both the coronal model and input magnetogram. Nevertheless, a more extensive statistical analysis is necessary to determine how precisely these choices affect the quality of the solar wind predictions.

Divergence and Vorticity of Subsurface Flows During Solar Cycles 23 and 24

We study the solar-cycle variation of the divergence and vorticity of subsurface horizontal flows from the surface to a depth of 16 Mm. The flows were derived with ring-diagram analysis applied to Michelson Doppler Imager (MDI) Dynamics Program, Global Oscillation Network Group (GONG), and Helioseismic and Magnetic Imager (HMI) Dopplergrams. We study their variation for the complete data set and for two subsets representing active and quiet regions. All three data sets show alternating bands of diverging and converging flows and bands of cyclonic and anticyclonic flows moving from mid-latitudes toward the equator during a solar cycle. For Solar Cycle 24, these bands are precursors of the magnetic activity appearing several years before magnetic activity is present at a given latitude even leading the fast bands of the flows. The amplitude differences between the cyclonic and anticyclonic and the converging and diverging bands during a solar cycle agree within the error bars between the complete data set and the two subsets. For Solar Cycle 24, the amplitude differences are $6.0 \pm 0.7 \;10^{-8}~\text{s}^{-1}$ for the bands of vorticity and $-4.9 \pm 0.6\;10^{-8}~\text{s}^{-1}$ for those of divergence averaged over 2.0 – 11.6 Mm using the complete data set. The amplitude differences of Solar Cycle 23 are $26 \pm 3$ % smaller than those of Solar Cycle 24. The flows of the active-region subset are more converging and cyclonic than those of the quiet-region subset with an extra vorticity of $1.3 \pm 0.1 \;10^{-8}~\text{s}^{-1}$ and an extra divergence of $-6.7 \pm 0.3\;10^{-8}~\text{s}^{-1}$ averaged over 7.5∘ – 30∘ and all depths and epochs. The amplitude of the extra divergence of active regions is about a factor of 1.3 larger at depths shallower than 6 Mm and decreases with increasing depth, while the extra vorticity is nearly constant with depth.

Comparison of Synoptic Maps and PFSS Solutions for The Declining Phase of Solar Cycle 24

AbstractThe global distributions of photospheric magnetic field are routinely provided by synoptic maps, which often serve as fundamental observational input for coronal and heliospheric models. In this paper, we compare synoptic maps and PFSS‐derived magnetic field results obtained during 2017/12/06–2018/12/06, a period at the declining phase of solar cycle 24. We use four kinds of synoptic maps, the Helioseismic and Magnetic Imager (HMI) maps, the Air Force Data Assimilative Photospheric Flux Transport (ADAPT) maps, the Global Oscillation Network Group (GONG) maps with their zero‐point uncertainty corrected (GONGz maps) or uncorrected (GONGb maps). Qualitatively similarity among the four maps are found in the low‐latitude region, but the polar fields have notable differences. While the polar fields of HMI, ADAPT and GONGz maps are unipolar and relative stable in large scale, those of the GONGb maps are variable. The PFSS results of HMI, ADAPT, and GONGz compare reasonably with coronal remote observations and near‐Earth in situ data, and the HMI maps perform slightly better. There is generally no significant difference among results of the 12 ADAPT ensemble maps, but exceptions are also found. The PFSS results of GONGb maps deviate from observation significantly. The deviations are attributed to the problematic polar field, and the source of the problem may be the zero‐point uncertainty of the magnetograms. As GONGb maps perform well in the declining phase of solar cycle 23, results in this study highlight the importance of continuous assessment of synoptic map data quality.

Measuring three-dimensional shapes of stable solar prominences using stereoscopic observations from SDO and STEREO

Aims. Although the real shapes and trajectories of erupting solar prominences in three dimensions have been intensively studied, the three-dimensional (3D) shapes of stable prominences before eruptions have not been measured accurately so far. We intend to make such a measurement to constrain 3D prominence models and to extend our knowledge of prominences. Methods. Using multiperspective observations from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory (SDO) and the Extreme Ultraviolet Imager on board the Solar Terrestrial Relations Observatory (STEREO), we reconstructed 3D coordinates of three stable prominences: a quiescent, an intermediate, and a mixed type. Based on the 3D coordinates, we measured the height, length, and inclination angle of the legs of these prominences. To study the spatial relationship between the footpoints of prominences and photospheric magnetic structures, we also used the Global Oscillation Network Group Hα images and magnetograms from the Helioseismic and Magnetic Imager on board the SDO. Results. In three stable prominences, we find that the axes of the prominence legs are inclined by 68 ± 6° on average to the solar surface. Legs at different locations along a prominence axis have different heights with a two- to threefold difference. Our investigation suggests that over 96% of prominence footpoints in a sample of 70 footpoints are located at supergranular boundaries. The widths of two legs have similar values measured in two orthogonal lines of sight. We also find that a prominence leg above the solar limb showed horizontal oscillations with larger amplitudes at higher locations. Conclusions. With a limited image resolution and number of cases, our measurement suggests that the legs of prominences may have various orientations and do not always stand vertically on the surface of the sun. Moreover, the locations of prominence legs are closely related to supergranules.