Extreme Ultraviolet Emission Research Articles

Sunquake with a second bounce, other sunquakes, and emission associated with the X9.3 flare of 6 September 2017

In this paper we present the interpretation of the observations of the flare from 6 September 2017 reported in Paper I. These include gamma-ray (GR), hard X-ray (HXR), soft X-rays, Lyαline, extreme ultraviolet (EUV), Hα, and white light (WL) emission, which were recorded during the two flaring events 1 (FE1) and 2 (FE2) that occurred at 11:55:37 UT (FE1) and 12:06:40 UT (FE2). Paper I also reported the first detection of the sunquake with first and second bounces of seismic waves combined with four other sunquakes in different locations supported with the observations of HXR, GR, EUV, Hα, and WL emission with strongly varying spatial resolution and temporal coverage. In the current paper, we propose some likely scenarios for heating of flaring atmospheres in the footpoints with sunquakes which were supported with EUV and Hαemission. We used a range of parameters derived from the HXR, EUV, and Hαline observations to generate hydrodynamic models, which can account for the blueshifts derived from the EUV emission and the redshifts observed with the EUV Imaging Spectrometer in the He II line and by the CRisp Imaging Spectro-Polarimeter in the Swedish Solar Telescope in Hαline emission. The parameters of hydrodynamic shocks produced by different beams in flaring atmospheres were used as the initial conditions for another type of hydrodynamic models that were developed for acoustic wave propagation in the solar interior. These models simulate the sets of acoustic waves produced in the interior by the hydrodynamic shocks from atmospheres above deposited in different footpoints of magnetic loops. The Hαline profiles with large redshifts in three kernels (two in FE1 and one in FE2) were interpreted with the full non-local thermodynamic equilibrium radiative simulations in all optically thick transitions (Lyman lines and continuum Hα, Hβ, and Pα) applied for flaring atmospheres with fast downward motions while considering thermal and non-thermal excitation and ionisation of hydrogen atoms by energetic power-law electron beams. The observed Hαline profiles in three kernels were fit with the simulate blue wing emission of the Hαline profiles shifted significantly (by 4–6 Å) towards the line red wings, because of strong downward motions with velocities about 300 km s−1by the shocks generated in flaring atmospheres by powerful beams. The flaring atmosphere associated with the largest sunquake (seismic source 2 in FE1) is found consistent with being induced by a strong hydrodynamic shock produced by a mixed beam deposited at an angle of −30° from the local vertical. We explain the occurrence of a second bounce in the largest sunquake by a stronger momentum delivered by the shock generated in the flaring atmosphere by a mixed beam and deeper depths of the interior where this shock was deposited. Indeed, the shock with mixed beam parameters is found deposited deeply into the interior beneath the flaring atmosphere under the angle to the local vertical that would allow the acoustic waves generated in the direction closer to the surface to conserve enough energy for the second bounces from the interior layers and from the photosphere. The wave characteristics of seismic sources 1 and 3 (in FE1) were consistent with those produced by the shocks generated by similar mixed beams deposited at the angles −(0 − 10)° (seismic source 1) and +30° (seismic source 3) to the local vertical. The differences of seismic signatures produced in the flares of 6 September 2011 and 2017 are also discussed.

Sunquake with a second bounce, other sunquakes, and emission associated with the X9.3 flare of 6 September 2017

Aims.The 6 September 2017 X9.3 solar flare produced very unique observations of magnetic field transients and a few seismic responses, or sunquakes, detected by the Helioseismic and Magnetic Imager (HMI) instrument aboard Solar Dynamic Observatory (SDO) spacecraft, including the strongest sunquake ever reported. This flare was one of a few flares occurring within a few days or hours in the same active region. Despite numerous reports of the fast variations of magnetic field, and seismic and white light emission, no attempts were made to interpret the flare features using multi-wavelength observations. In this study, we attempt to produce the summary of available observations of the most powerful flare of the 6 September 2017 obtained using instruments with different spatial resolutions (this paper) and to provide possible interpretation of the flaring events, which occurred in the locations of some seismic sources (a companion Paper II).Methods.We employed non-linear force-free field extrapolations followed by magnetohydrodynamic simulations in order to identify the presence of several magnetic flux ropes prior to the initiation of this X9.3 flare. Sunquakes were observed using the directional holography and time–distance diagram detection techniques. The high-resolution method to detect the Hαline kernels in the CRISP instrument at the diffraction level limit was also applied.Results.We explore the availableγ-ray (GR), hard X-ray (HXR), Lyman-α, and extreme ultra-violet (EUV) emission for this flare comprising two flaring events observed by space- and ground-based instruments with different spatial resolutions. For each flaring event we detect a few seismic sources, or sunquakes, using Dopplergrams from the HMI/SDO instrument coinciding with the kernels of Hαline emission with strong redshifts and white light sources. The properties of sunquakes were explored simultaneously with the observations of HXR (with KONUS/WIND and the Reuven Ramaty High Energy Solar Spectroscopic Imager payload), EUV (with the Atmospheric Imaging Assembly (AIA/SDO and the EUV Imaging Spectrometer aboard Hinode payload), Hαline emission (with the CRisp Imaging Spectro-Polarimeter (CRISP) in the Swedish Solar Telescope), and white light emission (with HMI/SDO). The locations of sunquake and Hαkernels are associated with the footpoints of magnetic flux ropes formed immediately before the X9.3 flare onset.Conclusions.For the first time we present the detection of the largest sunquake ever recorded with the first and second bounces of acoustic waves generated in the solar interior, the ripples of which appear at a short distance of 5–8 Mm from the initial flare location. Four other sunquakes were also detected, one of which is likely to have occurred 10 min later in the same location as the largest sunquake. Possible parameters of flaring atmospheres in the locations with sunquakes are discussed using available temporal and spatial coverage of hard X-ray, GR, EUV, hydrogen Hα-line, and white light emission in preparation for their use in an interpretation to be given in Paper II.

Amplified spontaneous emission in the extreme ultraviolet by expanding xenon clusters

Extreme-ultraviolet emission at 65-nm wavelength from xenon nanoclusters irradiated by an intense pulse of soft x rays from a free-electron laser is investigated in a joint experimental-theoretical study. Clear indications of a long-lived population inversion, and subsequent amplified spontaneous emission, are identified.

Soft X-ray and EUV emission spectra of beryllium plasma produced by neodymium-glass laser radiation with broad frequency and angular spectra

We present the results of an experimental study of soft X-ray (SXR) and extreme ultraviolet emission spectra of the plasma produced by exposing a plane solid beryllium target to laser radiation with broad frequency and angular spectra. SXR lines up to 1s – 9p of Be IV as well as the plasma continuum are recorded for a laser focal-spot intensity of 5.3 × 1013 W cm−2. To model the SXR beryllium plasma spectra, simulations are carried out using the INDHAUS programme and the FLYCHK code in the framework of local thermodynamic equilibrium model, which agree nicely with experimentally obtained results.

Delay measurement of attosecond emission in solids

We characterize an attosecond pulse without using a secondary laser pulse. Instead, we use the carrier-envelope-phase dependence of the extreme ultraviolet (XUV) emission to measure the dipole current from a solid material. To confirm, we apply our approach to data derived by solving the time-dependent Schrödinger equation with a periodic potential. Finally, using MgO, we measure the delay shift at the strong XUV absorption feature and explain it as the quantum interference between the continuum and bound states that participate in the generation process.

HAZMAT VI: The Evolution of Extreme Ultraviolet Radiation Emitted from Early M Stars

Abstract Quantifying the evolution of stellar extreme ultraviolet (EUV, 100–1000 Å) emission is critical for assessing the evolution of planetary atmospheres and the habitability of M dwarf systems. Previous studies from the HAbitable Zones and M dwarf Activity across Time (HAZMAT) program showed the far- and near-UV (FUV, NUV) emission from M stars at various stages of a stellar lifetime through photometric measurements from the Galaxy Evolution Explorer (GALEX). The results revealed increased levels of short-wavelength emission that remain elevated for hundreds of millions of years. The trend for EUV flux as a function of age could not be determined empirically because absorption by the interstellar medium prevents access to the EUV wavelengths for the vast majority of stars. In this paper, we model the evolution of EUV flux from early M stars to address this observational gap. We present synthetic spectra spanning EUV to infrared wavelengths of 0.4 ± 0.05 M ☉ stars at five distinct ages between 10 and 5000 Myr, computed with the PHOENIX atmosphere code and guided by the GALEX photometry. We model a range of EUV fluxes spanning two orders of magnitude, consistent with the observed spread in X-ray, FUV, and NUV flux at each epoch. Our results show that the stellar EUV emission from young M stars is 100 times stronger than field age M stars, and decreases as t −1 after remaining constant for a few hundred million years. This decline stems from changes in the chromospheric temperature structure, which steadily shifts outward with time. Our models reconstruct the full spectrally and temporally resolved history of an M star’s UV radiation, including the unobservable EUV radiation, which drives planetary atmospheric escape, directly impacting a planet’s potential for habitability.

Investigation on the charge-state distribution diagnosis oflaser-produced Al plasmas

Laser-produced plasmas (LPP) have been known to possess great application potential under extreme ultraviolet (EUV) lithography, soft X-ray micrography, and heavy ion accelerator, highly charged ion optical frequency standards, pulsed laser deposition, astrophysics and nuclear fusion. The applications in these fields require a clearer understanding of the evolutionary behavior of the LPP. However, LPP is neither homogeneous nor static during their expanding and cooling process, which involves complex physical processes. For a deep understanding of the LPP, many diagnostic methods and techniques have been developed, such as spectroscopy, fast imaging, time-of-flight (TOF), laser interferometry and Thomson scattering. Among these methods, spectroscopy has been the most widely used diagnostic method, which can be used to obtain relatively more state information. However, the diagnosis results of charge-state distribution obtained by spatio-temporally resolved spectroscopy are local and transient, and cannot therefore obtain the overall distribution. In order to have a more comprehensive understanding of charge-state distribution and its evolutionary behavior of the LPP, other diagnostic methods are needed. The TOF method is used for the measurement of highly charged ions of the LPP, which can be used to obtain the overall charge-state distribution information. Therefore, in this work, spectrometry and TOF method were used for the diagnosis of charge-state distribution of the LPP in a vacuum. The EUV emission spectra of laser-produced Al plasmas were measured by using spatio-temporally resolved laser-produced plasmas spectroscopy technique. The charge-state distributions with different delay times at the distance of 2.0 mm from target surface were diagnosed by spectral simulation and their evolution behavior were also analyzed. The TOF spectra of laser-produced Al plasmas were measured by the TOF method, and the overall charge-state distribution of ions in laser-produced Al plasmas was obtained. The results revealed that spectroscopy can diagnose the local and transient charge-state distribution of ions in the laser-produced plasmas, and the TOF method can be used to diagnose its overall distribution, and the two diagnostic methods therefore complement each other. This work aims to clarify the complementarity of the two methods, and to provide a method support for the comprehensive and accurate understanding of the charge-state distribution and evolution evolutionary behavior during the expansion process of the LPP, and also provides a reference for further optimization and modification of the radiation hydrodynamics model of the LPP.

Quasi-periodic Pulsation Detected in Lyα Emission During Solar Flares

Abstract We investigated the quasi-periodic pulsation (QPP) in Lyα, X-ray, and extreme-ultraviolet (EUV) emissions during two solar flares, i.e., an X-class (SOL2012-01-27T) and a C-class (SOL2016-02-08T). The full-disk Lyα and X-ray flux during these solar flares were recorded by the EUV Sensor and X-Ray Sensor on board the Geostationary Operational Environmental Satellite. The flare regions were located from the EUV images measured by the Atmospheric Imaging Assembly. The QPP could be identified as a series of regular and periodic peaks in the light curves, and its quasi-periodicity was determined from the global wavelet and Fourier power spectra. A quasi-periodicity at about 3 minutes is detected during the impulsive phase of the X-class flare, which could be explained as the acoustic wave in the chromosphere. Interestingly, a quasi-periodicity at roughly 1 minute is discovered during the entire evolutionary phases of solar flares, including the precursor, impulsive, and gradual phases. This is the first report of 1 minute QPP in the Lyα emission during solar flares, in particular during the flare precursor. It may be interpreted as a self-oscillatory regime of the magnetic reconnection, such as magnetic dripping.

Asymmetries and Broadenings of Spectral Lines in Strongly Charged Iron Produced during Solar Flares

Abstract Spatially resolved spectroscopic observations show wing enhancements and broadening in extreme ultraviolet emission lines, particularly in hot iron lines. Various explanations ranging from plasma turbulence and magnetic perturbations, to nonthermal ion populations and multiple flows in unresolved structures have been proposed. In this work, we revisit the role of single loop plasma dynamics in spectral line shape by reproducing the wing enhancements of Fe xxiii and xxiv observed during a C-class solar flare using a single loop hydrodynamic model. We also run simulations with different loop lengths and the same beam parameters to investigate the role of loop length in line broadening and asymmetry. We find that the single loop model successfully reproduces line asymmetries and the loop length plays an important role in explaining some of the key observations such as the positive correlation between the Doppler shifts and line width, and broad but symmetric hot Fe lines.

Synthesising solar radio images from Atmospheric Imaging Assembly extreme-ultraviolet data

During non-flaring times, the radio flux of the Sun at wavelengths of a few centimeters to several tens of centimeters mostly originates from thermal bremsstrahlung emission, very similar to extremeultraviolet (EUV) radiation. Owing to such a proximity, it is feasible to investigate the relationship between the EUV emission and radio emission in a quantitative way. In this paper, we reconstruct the radio images of the Sun through the differential emission measure obtained from multi-wavelength EUV images of the Atmospheric Imaging Assembly on board Solar Dynamics Observatory (SDO). Through comparing the synthetic radio images at 6GHz with those observed by the Siberian Radioheliograph, we find that the predicted radio flux is qualitatively consistent with the observed value, confirming thermal origin of the coronal radio emission during non-flaring times. The results further show that the predicted radio flux is closer to the observations in the case that includes the contribution of plasma with temperatures above 3MK than in the case of only involving low temperature plasma, as was usually done in the pre-SDO era. We also discuss applications of the method and uncertainties of the results.

Incoherent and coherent extreme ultraviolet emission from boron plasma

The search for reliable sources of incoherent and coherent extreme ultraviolet (XUV) radiation in different spectral ranges is an important task in laser physics. Here we report on the incoherent emission of XUV radiation in the region of 6 nm from boron plasma. The optimization of target ablation allowed the generation of XUV emission mainly originated from a single B IV transition (λ = 6.03 nm). We also demonstrate the generation of the 806 nm radiation harmonics up to the 55th order (λ = 14.6 nm) and of tunable near infrared radiation (1280–1400 nm) up to the 35th order (λ=38 nm), during the propagation of femtosecond pulses through boron plasma. The incoherent and coherent emissions from this target were compared and analyzed considering the influence of ionic transitions on the B IV emission and harmonic efficiency. We present different schemes of laser-plasma interaction (two-color pump of B plasma, generation of tunable harmonics in XUV range, formation of structured extended plasma, ablation of boron nanoparticles, laser-induced breakdown spectroscopy, overheating of boron targets, quasi-phase matching of plasma harmonics, etc.) during the ablation of boron-containing targets.

Influence of Pre-Ionized Plasma on the Dynamics of a Tin Laser-Triggered Discharge-Plasma

The effect of laser-current delay on extreme ultraviolet emission by laser-triggered discharge-plasma has been investigated. Typical waveforms for current, voltage, laser signals, and X-ray signals have been compared. Theoretical tin spectra were simulated among the electron temperature ranges from 10 to 50 eV to compare with the experimental results. The results show that longer laser-current delay time is propitious to increase the steady-state time of plasma at high temperatures, and it increases the intensity and spectral purity of 13.5 nm emission in 2% band. The 13.5 nm radiation intensity increases about 120% with the delay increasing from 0.7 to 5 μs, and the extreme ultraviolet (EUV) emission conversion efficiency (CE) increases from 0.5% to 1.1%.

Time-resolved studies of low-temperature, EUV-induced plasmas: EUV emission in selected spectral ranges

AbstractInteraction of extreme ultraviolet (EUV) pulses of high intensity with gases results in the creation of non-thermalized plasmas. Energies of the driving photons and photoelectrons are sufficient for creation of excited states, followed by emission of the EUV photons. In most cases, decay times of these states are short comparing to the driving EUV pulse. It means that just after stopping of the driving pulse, the EUV emission corresponding to the excited states should also stop. From our earlier measurements in the optical range, however, it can be concluded that lifetimes of such plasmas exceed a time duration of the driving pulse even two orders of magnitude. Hence, it can be expected that the time duration of the EUV emission can be also significantly longer than the irradiation time. In this work, EUV-induced, low-temperature helium (He), krypton, and xenon plasmas were investigated. EUV emission from these plasmas was studied, using a specially prepared detection system, allowing for time-resolved measurements, in selected spectral ranges. The detection system was based on a paraboloidal collector and a semiconductor photodiode, sensitive for the EUV photons. For spectral selection, the corresponding filters or multilayer mirrors were employed. In most cases, the time duration of the EUV emission was significantly longer than the driving EUV pulse. In case of He plasmas, the emission corresponding to excited atoms was detected even hundreds of nanoseconds after the irradiation. It was also shown that the corresponding time profiles depended on densities of gases to be ionized.

Rapid Escape of Ultra-hot Exoplanet Atmospheres Driven by Hydrogen Balmer Absorption

Abstract Atmospheric escape is key to explaining the long-term evolution of planets in our solar system and beyond, and in the interpretation of atmospheric measurements. Hydrodynamic escape is generally thought to be driven by the flux of extreme-ultraviolet photons that the planet receives from its host star. Here, we show that the escape from planets orbiting hot stars proceeds through a different yet complementary process: drawing its energy from the intense near-ultraviolet emission of the star that is deposited within an optically thin, high-altitude atmospheric layer of hydrogen excited into the lower state of the Balmer series. The ultra-hot exoplanet KELT-9b likely represents the first known instance of this Balmer-driven escape. In this regime of hydrodynamic escape, the near-ultraviolet emission from the star is more important at determining the planet mass loss than the extreme-ultraviolet emission, and uncertainties in the latter become less critical. Further, we predict that gas exoplanets around hot stars may experience catastrophic mass loss when they are less massive than 1–2 Jupiter masses and closer in than KELT-9b, thereby challenging the paradigm that all large exoplanets are stable to atmospheric escape. We argue that extreme escape will affect the demographics of close-in exoplanets orbiting hot stars.

Tunable high-order frequency mixing for XUV transient absorption and photoelectron spectroscopies.

We generate tunable extreme ultraviolet emission through high-order frequency mixing between a strong near-infrared field and a weak shortwave-infrared pulse whose wavelength can be adjusted. In this two-color driving scheme, new harmonics appear in between the single-color harmonics at energies which are linear combinations of photons from the two pulses. We demonstrate the utility of tunable two-color harmonics by employing them for XUV transient absorption spectroscopy and time-resolved photoelectron spectroscopy. We show that the two-color harmonics can be used to address the dynamics associated with excited states in Helium and Oxygen which are inaccessible using single-color harmonics. Specifically, we show the ability to switch between excitation of 3p and 4p states in Helium, control of transitions to the light induced states, observe new four-wave-mixing emissions, and selectively address different principal and vibrational quantum numbers associated with Oxygen Rydberg states.

Coronal Density and Temperature Profiles Calculated by Forward Modeling EUV Emission Observed by SDO/AIA

Abstract We present a model for the intensity of optically thin extreme ultraviolet (EUV) emission for a plasma atmosphere. We apply our model to the solar corona as observed using the six optically thin EUV channels of the Solar Dynamics Observatory/Atmospheric Imaging Assembly instrument. The emissivity of the plasma is calculated from the density and temperature using CHIANTI tables and the intensity is then determined by integration along the line of sight. We consider several different profiles for the radial density and temperature profiles, each of which are constrained by the observational data alone with no further physical assumptions. We demonstrate the method first by applying it to a quiet region of the corona, and then use it as the background component of a model including coronal holes, allowing the plasma densities and temperatures inside and outside the hole to be estimated. We compare our results with differential emission measure inversions. More accurate estimates for the coronal density and temperature profiles have the potential to help constrain plasma properties such as the magnetic field strength when used in combination with methods such as seismology.

Synthetic Extreme-ultraviolet Emissions Modulated by Leaky Fast Sausage Modes in Solar Active Region Loops

We study the extreme-ultraviolet (EUV) emissions modulated by leaky fast sausage modes (FSMs) in solar active region loops and examine their observational signatures via spectrometers like EIS. After computing fluid variables of leaky FSMs with MHD simulations, we forward-model the intensity and spectral properties of the Fe X 185~\AA~and Fe XII 195~\AA~lines by incorporating non-equilibrium ionization (NEI) in the computations of the relevant ionic fractions. The damping times derived from the intensity variations are then compared with the wave values, namely the damping times directly found from our MHD simulations. Our results show that in the equilibrium ionization cases, the density variations and the intensity variations can be either in phase or in anti-phase, depending on the loop temperature. NEI considerably impacts the intensity variations but has only marginal effects on the derived Doppler velocity or Doppler width. We find that the damping time derived from the intensity can largely reflect the wave damping time if the loop temperature is not drastically different from the nominal formation temperature of the corresponding emission line. These results are helpful for understanding the modulations to the EUV emissions by leaky FSMs and hence helpful for identifying FSMs in solar active region loops.

Target materials for efficient plasma-based extreme ultraviolet sources in the range of 6 to 8 nm

In a comparative study the extreme ultraviolet (XUV) emission of laser-produced plasmas (LPP) of gadolinium (Gd), terbium (Tb), aluminum (Al), magnesium (Mg) and a magnesium–copper–gadolinium alloy (Mg65–Cu25–Gd10) targets is investigated in order to benchmark the emission potential at a wavelength of around 6.7 nm. Compared to the Gd/Tb targets, the advantage of the alloy is a reduced melting point of Tm < 500 °C which leads to the option of a liquid or droplet target system as is usually used for a regenerative target concept. Theoretical considerations are presented to compare the emission potential of the four targets in direct comparison. Experimental results of the LPP are discussed with respect to the 6.x nm inband energy, which are consistent with the theoretical considerations. It is shown that the line radiators Al and Mg seem to be suitable emitters with comparable brightness to Gd- or Tb-based sources. The measured conversion efficiency of the Mg65Cu25Gd10 target reaches 80% of that of the pure Gd LPP although the Gd fraction is only 2% (atom percent).

Opacity effects on laser-produced plasma radiation sources

The escape of spectral line radiation from laser-produced plasma radiation sources with moderate opacity is examined using a simple model of emission with a planar geometry, constant source function, and an empirically determined optical depth. The model is applied to determine the radiation produced by laser irradiation of tin targets used as the source of radiation in extreme ultraviolet (EUV) lithography. Variations in emission relative to optically thin plasmas in agreement with previous experimental measurements of both the angular variation of the emission of EUV light at 13.5 nm and the effect of plasma opacity in reducing EUV emission at laser intensities above 1011 W cm−2 are found. The model is extended to predict optimum conditions for future lithography radiation sources at ≈6.7 nm.

Tabletop extreme ultraviolet time-of-flight spectrometry for trace analysis of high ionization energy samples.

Species with ionization energies beyond what is accessible using state-of-the-art lab sources are affected by poor detection limits in ordinary mass spectrometry setups, whose throughput is also often limited. Extreme ultraviolet (XUV) photoionization mass spectrometry, in combination with linear time-of-flight (TOF), is necessary for the sensitive detection of high ionization energy compounds at trace level. XUV photoionization is available at beamlines, although with limited access. A tabletop setup may fill such a gap. A self-developed tabletop system, based on a plasma discharge with extreme ultraviolet emission (λ = 5-50 nm) coupled to a TOF mass spectrometer, was used in this study. Simultaneous validation measurements with a reference electron ionization quadrupole mass filter were carried out. An in-house developed hollow toroidal coil (HTC) induction detector was used for concomitant photoelectron detection. Straightforward XUV mass spectra without fragmentation, thanks to the single-photon ionization, were acquired. The measurements with the reference quadrupole were in agreement with the spectra acquired by XUV-TOF. The resolution obtained for N2 was at least factor of 2 higher than that measured with the reference quadrupole. Initial energy distributions of photoelectrons were retrieved by cross-correlation that gave access to the photoionization distribution. The system allows XUV single-photon ionization of elements and molecules with IE >10 eV that are of fundamental interest e.g. for water splitting and catalysis research. The demonstrated performance is now suitable for a prototype platform.