Extreme Ultraviolet Intensity Research Articles

Broadening the Canonical Picture of EUV-driven Photoevaporation of Accretion Disks

Photoevaporation driven by hydrogen-ionizing extreme-ultraviolet (EUV) radiation profoundly shapes the lives of diverse astrophysical objects. We construct an analytical model accounting for the finite timescales of photoheating and photoionization and apply it to the dispersal of protoplanetary disks. The model yields improved estimates for the ionization, temperature, and velocity versus distance from the central source when compared to the classical picture of fully ionized and isothermal winds with temperatures ≈104 K and speeds ≈10 km s−1. In contrast to the classical picture, the photoevaporative winds take on several distinct hydrodynamical and thermochemical states depending on the central star’s EUV emission rate and spectral hardness: T Tauri stars with EUV luminosities around 1030 erg s−1 drive nonisothermal ionized disk winds at lower temperatures than the classical value if the spectrum is soft, with an average deposited energy per photoionization less than about 3.7 eV. If, however, the spectrum is hard, the winds tend to be atomic and isothermal at most disk radii. For lower EUV intensities, even with soft spectra, atomic winds can emerge beyond ∼10 au through advection. We show that these predictions are in general agreement with detailed radiation hydrodynamics calculations. The model furthermore illustrates how the energy efficiency of photoevaporation varies with the intensity and spectral hardness of the EUV illumination, as well as addressing discrepancies in the literature around the effectiveness of X-ray photoevaporation. The findings highlight the importance of the photoheating and photoionization timescales both for modeling and for understanding winds’ observed behavior.

Solar Type J Radio Bursts and the Associated Coronal Loop

The solar type J radio burst is a variant of type III bursts, which are a probe for understanding solar energetic electrons and local electron density. This study investigates a type J burst event on 2017 September 9. We have combined the data from the extreme-ultraviolet (EUV) imaging and the EUV Imaging Spectrometer (EIS) to analyze the event. Within 4 minutes several type J bursts with similar morphology occur. Two of them, with clear fundamental and second harmonic bands, are studied in detail. We find a delay of 2 ± 0.5 s between their different harmonic bands. During type J bursts, only one coronal loop brightens significantly at its northern footpoint, in correlation with the continuous injection of erupting jets into the loop. The EUV intensity of the brightening footpoint is correlated with the radio flux at 245 and 410 MHz, with correlation coefficients of 0.2 and 0.4, respectively. These observations suggest that the type J bursts should originate from this coronal loop. By analyzing the electron number density distribution along the coronal loop diagnosed from the EIS data and the time evolution of the plasma frequency calculated from the type J burst, we determine that the velocities of the energetic electrons exciting the two type Js are 0.10 ± 0.02c and 0.12 ± 0.02c. Our results confirm previous studies on type J bursts.

Solar extreme ultraviolet variability as a proxy for nanoflare heating diagnostics

Aims. We aim to improve the existing techniques to probe the nanoflare hypothesis for the coronal heating problem. For this purpose, we propose using the solar extreme ultraviolet (EUV) emission variability registered with modern space-based imagers. Methods. We followed a novel model-based approach. As a starting point, we used the EBTEL 0d hydrodynamic model. We integrated the arising system of stochastic differential equations to calculate the covariance matrix for plasma parameters. We then employed a Taylor expansion technique to relate model parameters with observable EUV intensity variation statistics. Results. We found that in the high-frequency approximation, the variability of the EUV emission is defined by the dimensionless factor ϖ, which is inversely proportional to the frequency. We calculated the factor ϖ throughout the solar disk and found that it does not exceed 0.01, except for the finite number of compact regions. The distribution of ϖ follows the power law with an index of ≈ − 2.6. To validate our approach, we used it to probe the temperature of the coronal plasma. We show that the line-of-sight temperature distribution is close to homogeneous with a mode of ≈1.25 MK, which is in perfect agreement with the results of the spectroscopic diagnostics.

Textrm{O}^+ density distribution in the nightside ionosphere reconstructed from ISS-IMAP/EUVI data

The textrm{O}^+ density distribution in the nightside ionosphere has been reconstructed from extreme ultraviolet (EUV) images taken by the EUVI-B imager of the International Space Station Ionosphere, Mesosphere, upper Atmosphere, and Plasmasphere mapping (ISS-IMAP) cameras. The EUVI-B imager covers the wavelength range from about 70 nm to 110 nm and mainly observes the 91.1 nm emission from the recombination of textrm{O}^+ ions and electrons. Assuming that the electron density is equal to the textrm{O}^+ density in the F-region where the imager observes, the EUV intensity observed by EUVI-B is approximately proportional to the line-of-sight integral of the square of the textrm{O}^+ density. This enables us to estimate the textrm{O}^+ density distribution in the F-region from a sequence of EUVI-B data in each International Space Station (ISS) orbit with a Bayesian method. We demonstrate the reconstruction of the textrm{O}^+ distribution. In particular, the textrm{O}^+ density structure of the equatorial ionization anomaly (EIA) in the vicinity of an ISS orbit is obtained.Graphical

Recurrently Propagating Intensity Disturbances along Polar Plumes Observed in White Light and Extreme Ultraviolet

We study properties of intensity disturbances along polar coronal rays that are associated with plumes below. For this, we draw azimuth–time images of extreme ultraviolet (EUV) emission of 171 Å band observed by the SDO/AIA and white light (WL) observed by the SOHO/LASCO C2 in 2020 July. From the azimuth–time image, we define two tracks in which the EUV intensities were recurrently enhanced during two weeks. The two EUV tracks are rooted at 78.°8 and 81.°4 latitudes, but their projected azimuth angles are changed with time as the Sun rotates. Coherent WL tracks at different altitudes are determined by scaling the azimuth angles of the EUV tracks, accounting for the effect of inclination of coronal rays. From this, we construct time–distance images of WL intensities along WL tracks, whose projected azimuth angle changes along time and altitude, but the intensities are correlated with the EUV intensities measured below. The time–distance images of WL show repeated and inclined intensity features. The propagation speeds in the altitude range 2.3–6 solar radii are calculated to be 159 ± 8 km s−1 and 300 ± 24 km s−1. The EUV and WL intensities are found to be coherent at 1–2 day periods. It is also found that dynamic burst events along the EUV track are responsible for the enhanced emission. We conclude that the variation of the WL intensity along the polar coronal rays is related with the evolution of the EUV intensity below.

Coronal voids and their magnetic nature

Context. Extreme ultraviolet (EUV) observations of the quiet solar atmosphere reveal extended regions of weak emission compared to the ambient quiescent corona. The magnetic nature of these coronal features is not well understood. Aims. We study the magnetic properties of the weakly emitting extended regions, which we name coronal voids. In particular, we aim to understand whether these voids result from a reduced heat input into the corona or if they are associated with mainly unipolar and possibly open magnetic fields, similar to coronal holes. Methods. We defined the coronal voids via an intensity threshold of 75% of the mean quiet-Sun (QS) EUV intensity observed by the high-resolution EUV channel (HRIEUV) of the Extreme Ultraviolet Imager on Solar Orbiter. The line-of-sight magnetograms of the same solar region recorded by the High Resolution Telescope of the Polarimetric and Helioseismic Imager allowed us to compare the photospheric magnetic field beneath the coronal voids with that in other parts of the QS. Results. The coronal voids studied here range in size from a few granules to a few supergranules and on average exhibit a reduced intensity of 67% of the mean value of the entire field of view. The magnetic flux density in the photosphere below the voids is 76% (or more) lower than in the surrounding QS. Specifically, the coronal voids show much weaker or no network structures. The detected flux imbalances fall in the range of imbalances found in QS areas of the same size. Conclusions. We conclude that coronal voids form because of locally reduced heating of the corona due to reduced magnetic flux density in the photosphere. This makes them a distinct class of (dark) structure, different from coronal holes.

Robust construction of differential emission measure profiles using a regularized maximum likelihood method

Context.Extreme-ultraviolet (EUV) observations provide considerable insight into evolving physical conditions in the active solar atmosphere. For a prescribed density and temperature structure, it is straightforward to construct the corresponding differential emission measure profileξ(Τ), such thatξ(Τ) dTis proportional to the emissivity from plasma in the temperature range [T,T+ dT]. Here we study the inverse problem of obtaining a validξ(T) profile from a set of EUV spectral line intensities observed at a pixel within a solar image.Aims.Our goal is to introduce and develop a regularized maximum likelihood (RML) algorithm designed to address the mathematically ill-posed problem of constructing differential emission measure profiles from a discrete set of EUV intensities in specified wavelength bands, specifically those observed by the Atmospheric Imaging Assembly (AIA) on the NASA Solar Dynamics Observatory.Methods.The RML method combines features of maximum likelihood and regularized approaches used by other authors. It is also guaranteed to produce a positive definite differential emission measure profile.Results.We evaluate and compare the effectiveness of the method against other published algorithms, using both simulated data generated from parametric differential emission profile forms, and AIA data from a solar eruptive event on 2010 November 3. Similarities and differences between the differential emission measure profiles and maps reconstructed by the various algorithms are discussed.Conclusions.The RML inversion method is mathematically rigorous, computationally efficient, and produces acceptable measures of performance in the following three key areas: fidelity to the data, accuracy in the reconstruction, and robustness in the presence of data noise. As such, it shows considerable promise for computing differential emission measure profiles from datasets of discrete spectral lines.

Enhanced extreme ultraviolet lighting using carbon nanotube-based cold cathode electron beam irradiation

Laser-produced tin (Sn) plasma (LPP) is used to produce high-performance semiconductors using extreme ultraviolet (EUV), but LPP generates debris that limits the lifespan of the optical system, so research on debris mitigation is being conducted. When electrons are directly irradiated to a solid target, occurrence of debris can be minimized, and compact and low-cost EUV lighting can be manufactured. Here, we demonstrated enhanced EUV generation by optimizing the irradiation of Sn with electrons emitted from carbon nanotube-based cold cathode electron beam (C-beam). To minimize debris, a usable zone in which Sn does not melt was identified. It was demonstrated that using two C-beams in the usable zone increased the EUV intensity by about two times compared to using one C-beam. The multiple C-beam irradiation technique enables high-output EUV lighting by overlapping EUV light, which should be led by the development of advanced lithography and inspection technology.

Extreme Ultraviolet Lighting Using Carbon Nanotube-Based Cold Cathode Electron Beam.

Laser-based plasma studies that apply photons to extreme ultraviolet (EUV) generation are actively being conducted, and studies by direct electron irradiation on Sn for EUV lighting have rarely been attempted. Here, we demonstrate a novel method of EUV generation by irradiating Sn with electrons emitted from a carbon nanotube (CNT)-based cold cathode electron beam (C-beam). Unlike a single laser source, electrons emitted from about 12,700 CNT emitters irradiated the Sn surface to generate EUV and control its intensity. EUV light generated by direct irradiation of electrons was verified using a photodiode equipped with a 150 nm thick Zr filter and patterning of polymethyl methacrylate (PMMA) photoresist. EUV generated with an input power of 6 W is sufficient to react the PMMA with exposure of 30 s. EUV intensity changes according to the anode voltage, current, and electron incident angle. The area reaching the Sn and penetration depth of electrons are easily adjusted. This method could be the cornerstone for advanced lithography for semiconductor fabrication and high-resolution photonics.

A DEFT Way to Forecast Solar Flares

Abstract Solar flares have been linked to some of the most significant space weather hazards at Earth. These hazards, including radio blackouts and energetic particle events, can start just minutes after the flare onset. Therefore, it is of great importance to identify and predict flare events. In this paper we introduce the Detection and EUV Flare Tracking (DEFT) tool, which allows us to identify flare signatures and their precursors using high spatial and temporal resolution extreme-ultraviolet (EUV) solar observations. The unique advantage of DEFT is its ability to identify small but significant EUV intensity changes that may lead to solar eruptions. Furthermore, the tool can identify the location of the disturbances and distinguish events occurring at the same time in multiple locations. The algorithm analyzes high temporal cadence observations obtained from the Solar Ultraviolet Imager instrument aboard the GOES-R satellite. In a study of 61 flares of various magnitudes observed in 2017, the “main” EUV flare signatures (those closest in time to the X-ray start time) were identified on average 6 minutes early. The “precursor” EUV signatures (second-closest EUV signatures to the X-ray start time) appeared on average 14 minutes early. Our next goal is to develop an operational version of DEFT and to simulate and test its real-time use. A fully operational DEFT has the potential to significantly improve space weather forecast times.

Nanoscale Transient Magnetization Gratings Created and Probed by Femtosecond Extreme Ultraviolet Pulses.

We utilize coherent femtosecond extreme ultraviolet (EUV) pulses from a free electron laser (FEL) to generate transient periodic magnetization patterns with periods as short as 44 nm. Combining spatially periodic excitation with resonant probing at the M-edge of cobalt allows us to create and probe transient gratings of electronic and magnetic excitations in a CoGd alloy. In a demagnetized sample, we observe an electronic excitation with a rise time close to the FEL pulse duration and ∼0.5 ps decay time indicative of electron-phonon relaxation. When the sample is magnetized to saturation in an external field, we observe a magnetization grating, which appears on a subpicosecond time scale as the sample is demagnetized at the maxima of the EUV intensity and then decays on the time scale of tens of picoseconds via thermal diffusion. The described approach opens multiple avenues for studying dynamics of ultrafast magnetic phenomena on nanometer length scales.

Further development of the Xe laser plasma 11-nm radiation source – new data on laser energy absorption and spectroscopy

Absorption of the laser pulse energy in the plasma has been shown to change from 8.5% at irradiation of a gas-puff target with a beam narrow-focused onto a near-central area of the jet up to 65% at the wide defocused irradiation. An analysis of this phenomenon is based on a hypothesis that plasma density decays significantly during the 10ns laser pulse due to hydrodynamic expansion of the hot plasma. A similarity of the EUV (Extreme UltraViolet) intensity and the laser light absorption as functions of the laser beam diameter suggests a revision of the conventional idea of a strong EUV radiation self-absorption in a cold peripheral shell of the laser plasma whereas the high absorptivity of the laser radiation by the plasma looks like a major feature to gain high efficiency of an EUV source.

Coronal Dimmings Associated with Coronal Mass Ejections on the Solar Limb

Abstract We present a statistical analysis of 43 coronal dimming events associated with Earth-directed coronal mass ejections (CMEs) that occurred during the period of quasi-quadrature of the Solar Dynamics Observatory (SDO) and Solar Terrestrial Relations Observatory (STEREO) satellites. We studied coronal dimmings that were observed above the limb by STEREO Extreme Ultraviolet Imager and compared their properties with the mass and speed of the associated CMEs. The unique position of the satellites allowed us to compare our findings with the results from Dissauer et al., who studied the same events observed against the solar disk by the SDO Atmospheric Imaging Assembly. Such statistics is done for the first time and confirms the relation of coronal dimmings and CME parameters for the off-limb viewpoint. The observations of dimming regions from different lines of sight reveal a similar decrease in the total extreme ultraviolet intensity (c = 0.60 ± 0.14). We find that the (projected) dimming areas are typically larger for off-limb observations (mean value of 1.24 ± 1.23 × 1011 km2 against 3.51 ± 0.71 × 1010 km2 for on-disk), with a correlation of c = 0.63 ± 0.10. This systematic difference can be explained by the (weaker) contributions to the dimming regions higher up in the corona that cannot be detected in the on-disk observations. The off-limb dimming areas and brightnesses show very strong correlations with the CME mass (c = 0.82 ± 0.06 and 0.75 ± 0.08), whereas the dimming area and brightness change rate correlate with the CME speed (c ∼ 0.6). Our findings suggest that coronal dimmings have the potential to provide early estimates of the mass and speed of Earth-directed CMEs, relevant for space weather forecasts, for satellite locations at both L1 and L5.

Comparison of the Scaling Properties of EUV Intensity Fluctuations in Coronal Hole and Quiet-Sun Regions

Abstract Using detrended fluctuation analysis and rescaled range analysis, we investigate the scaling properties of extreme ultraviolet (EUV) intensity fluctuations of low-latitude coronal holes (CHs) and neighboring quiet-Sun (QS) regions in signals obtained with the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly instrument. Contemporaneous line-of-sight SDO/Helioseismic and Magnetic Imager magnetic fields provide a context for the physical environment. We find that the intensity fluctuations in the time series of EUV images present at each spatial point a scaling symmetry over the range ∼20 minute to ∼1 hr. Thus we are able to calculate a generalized Hurst exponent and produce image maps, not of physical quantities like intensity or temperature, but of a single dynamical parameter that sums up the statistical nature of the intensity fluctuations at each pixel. In QS regions and in CHs with magnetic bipoles, the scaling exponent (1.0 < α ≤ 1.5) corresponds to anticorrelated turbulent-like processes. In CHs, and in QS regions primarily associated with (open) magnetic field of dominant polarity, the generalized exponent (0.5 < α < 1) corresponds to positively correlated (persistent) processes. We identify a tendency for α ∼ 1 near CH boundaries and in other regions in which open and closed magnetic fields are in proximity. This is a signature of an underlying 1/f type process that is characteristic for self-organized criticality and shot-noise models.

Influence of Extreme Ultraviolet Irradiance Variations on the Precipitating Ion Flux From MAVEN Observations

AbstractWe study the influence of the solar extreme ultraviolet (EUV) flux intensity on the precipitating ion fluxes as seen by the Solar Wind Ion Analyzer, an energy and angular ion spectrometer aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. We defined three periods with significantly different EUV flux intensity (1.6 and 3.2 times the lowest EUV intensity) and compare the precipitating ion flux measured by MAVEN/Solar Wind Ion Analyzer during each period. At low energy [30–650] eV, we find that the median (average) precipitating ion flux during the medium and low EUV periods are, respectively, 1.7 (2.1) and 3 (3.5) times more intense than the flux during the high EUV period. At high energy [650–25,000] eV, a similar trend in the intensity of the precipitating ion flux is observed but with an increase by 50% (46%) and 70% (79%), respectively. A larger EUV flux does therefore not seem to favor heavy ion precipitation into Mars's atmosphere, contrary to modeling prediction and overall expectations.

Predicting radiation-induced carbon contamination of EUV optics

Predictions are made for the radiation-induced carbon contamination threat to ruthenium-coated extreme ultraviolet (EUV) optics for a range of incident EUV intensities, exposure pressures and types of hydrocarbon. A calculational philosophy is developed that acknowledges the ruthenium capping layer may have adsorbed oxygen on it and that the carbon contamination film is partially hydrogenated. The calculations incorporate the Nitta Multisite Adsorption framework, which accounts for the configurational adsorption difficulty encountered by the adsorption of large molecules on surfaces. Contributions from “out-of-band” radiation are included, both in the direct photon-induced dissociation of hydrocarbon molecules and in the out-of-band production of secondary electrons. For the hydrocarbon molecules, n-tetradecane, n-dodecane, n-decane, and benzene, for a range of EUV powers and hydrocarbon pressures, predictions are made for carbon thicknesses, the overall carbon deposition rates, and the relative amounts of contamination produced by primary photon excitation, secondary electrons, and out-of-band radiation. The comparison is made to relevant prior experiments. The model, with no adjustable parameters, provides a good account of prior experiments on n-tetradecane, n-decane, and benzene over the pressure ranges examined by the experiments (∼1 × 10−10 to ∼1 × 10−7 Torr) and over the EUV intensity range 0.001–100 mW/mm2. The level of agreement is within a factor of ∼4 or better, which is consistent with expectations based on the experimental uncertainties. Comparison with prior data for n-decane indicates that the carbon deposit produced by the EUV-induced dissociation of hydrocarbons is substantially hydrogenated. Out-of-band radiation accounts for ∼9%–12% of the overall optic contamination. Secondary electrons account for ∼2% of the overall optic contamination. The results show that the dominant mechanistic cause of the EUV carbon contamination is primary photon absorption by the adsorbed hydrocarbon molecule. The removal of carbon or hydrogen by electron stimulated desorption due to secondary electrons or photon stimulated desorption by primary EUV absorption can be safely ignored as negligible compared to the EUV-induced carbon deposition rate. The results allow comparison with past experiments, provide a framework for conducting future experiments, and predict contamination threats relevant for practical EUV lithography tool operation. The calculations also clarify the underlying physical phenomena at work in the EUV carbon contamination problem.

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

Abstract We report Solar Dynamics Observatory/Atmospheric Imaging Assembly (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 from the eruption. (2) This leads to strong transmissions (in addition to commonly observed reflections) in and out of both polar coronal holes (CHs), which are usually devoid of EUV waves. It has elevated wave speeds >2000 within the CHs, consistent with the expected higher fast-mode magnetosonic wave speeds. The CHs 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., ) accompanied by decreases at lower temperatures (e.g., ). These characteristics signify the potential of such EUV waves for novel magnetic and thermal diagnostics of the solar corona on global scales.

Three-phase Evolution of a Coronal Hole. I. 360° Remote Sensing and In Situ Observations

Abstract We investigate the evolution of a well-observed, long-lived, low-latitude coronal hole (CH) over 10 solar rotations in the year 2012. By combining extreme ultraviolet (EUV) imagery from the Solar TErrestrial RElations Observatories (STEREO-A/B) and the Solar Dynamics Observatory (SDO), we are able to track and study the entire evolution of the CH having a continuous 360° coverage of the Sun. The remote sensing data are investigated together with in situ solar wind plasma and magnetic field measurements from STEREO-A/B, the Advanced Composition Explorer (ACE), and WIND. From this, we obtain how different evolutionary states of the CH as observed in the solar atmosphere (changes in EUV intensity and area) affect the properties of the associated high-speed stream measured at 1 au. Most distinctly pronounced for the CH area, three development phases are derived: (a) growing, (b) maximum, and (c) decaying phase. During these phases the CH area (a) increases over a duration of around three months from about 1 · 1010 km2 to 6 · 1010 km2, (b) keeps a rather constant area for about one month of >9 · 1010 km2, and (c) finally decreases in the following three months below 1 · 1010 km2 until the CH cannot be identified anymore. The three phases manifest themselves also in the EUV intensity and in in situ measured solar wind proton bulk velocity. Interestingly, the three phases are related to a different range in solar wind speed variations, and we find for the growing phase a range of 460–600 km s−1, for the maximum phase 600–720 km s−1, and for the decaying phase a more irregular behavior connected to slow and fast solar wind speeds of 350–550 km s−1.

Segmentation of photospheric magnetic elements corresponding to coronal features to understand the EUV and UV irradiance variability

Context. The magnetic field plays a dominant role in the solar irradiance variability. Determining the contribution of various magnetic features to this variability is important in the context of heliospheric studies and Sun-Earth connection. Aims. We studied the solar irradiance variability and its association with the underlying magnetic field for a period of five years (January 2011–January 2016). We used observations from the Large Yield Radiometer (LYRA), the Sun Watcher with Active Pixel System detector and Image Processing (SWAP) on board PROBA2, the Atmospheric Imaging Assembly (AIA), and the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). Methods. The Spatial Possibilistic Clustering Algorithm (SPoCA) is applied to the extreme ultraviolet (EUV) observations obtained from the AIA to segregate coronal features by creating segmentation maps of active regions (ARs), coronal holes (CHs) and the quiet sun (QS). Further, these maps are applied to the full-disk SWAP intensity images and the full-disk (FD) HMI line-of-sight (LOS) magnetograms to isolate the SWAP coronal features and photospheric magnetic counterparts, respectively. We then computed full-disk and feature-wise averages of EUV intensity and line of sight (LOS) magnetic flux density over ARs/CHs/QS/FD. The variability in these quantities is compared with that of LYRA irradiance values. Results. Variations in the quantities resulting from the segmentation, namely the integrated intensity and the total magnetic flux density of ARs/CHs/QS/FD regions, are compared with the LYRA irradiance variations. We find that the EUV intensity over ARs/CHs/QS/FD is well correlated with the underlying magnetic field. In addition, variations in the full-disk integrated intensity and magnetic flux density values are correlated with the LYRA irradiance variations. Conclusions. Using the segmented coronal features observed in the EUV wavelengths as proxies to isolate the underlying magnetic structures is demonstrated in this study. Sophisticated feature identification and segmentation tools are important in providing more insights into the role of various magnetic features in both the short- and long-term changes in the solar irradiance.

Solar Open Flux Migration from Pole to Pole: Magnetic Field Reversal

Coronal holes are solar regions with low soft X-ray or low extreme ultraviolet intensities. The magnetic fields from coronal holes extend far away from the Sun, and thus they are identified as regions with open magnetic field lines. Coronal holes are concentrated in the polar regions during the sunspot minimum phase, and spread to lower latitude during the rising phase of solar activity. In this work, we identify coronal holes with outward and inward open magnetic fluxes being in the opposite poles during solar quiet period. We find that during the sunspot rising phase, the outward and inward open fluxes perform pole-to-pole trans-equatorial migrations in opposite directions. The migration of the open fluxes consists of three parts: open flux areas migrating across the equator, new open flux areas generated in the low latitude and migrating poleward, and new open flux areas locally generated in the polar region. All three components contribute to the reversal of magnetic polarity. The percentage of contribution from each component is different for different solar cycle. Our results also show that the sunspot number is positively correlated with the lower-latitude open magnetic flux area, but negatively correlated with the total open flux area.