Soft X-ray Profiles Research Articles

Candidate explanation for the mild core oscillations in dominant electron heating scenario on experimental advanced superconducting tokamak

The interchange-like transport is observed between two resonant surfaces (q = 1 and q = 4/3, where q is the safety factor) in a finite small positive magnetic shear regime with mild core oscillations in the experimental advanced superconducting tokamak strong on-axis electron heating H-mode plasmas. It is synchronized with the increasing gradient of the soft X-ray profile and the intensifying electron density fluctuations in the core. The analysis of two-fluid simulations combined with experimental measurements indicates the destabilization of collective resistive interchange modes with several toroidal mode numbers. The overall effect of modes leads to strong perturbations at the two resonant surfaces in contrast to that between them where the anomalous electron flux is low. Their radial displacement is beyond the resistive layer width which satisfies the condition for the nonlinear destabilization of tearing modes [L. Comisso et al., Phys. Plasmas 23, 100702 (2016)]. Evidence and analysis shown in this paper tend to understand the mechanism of mild oscillations in the core.

A curious case of the accretion-powered X-ray pulsar GX 1+4

We present detailed spectral and timing studies using a NuSTAR observation of GX 1+4 in October 2015 during an intermediate intensity state. The measured spin period of 176.778 s is found to be one of the highest values since its discovery. In contrast to a broad sinusoidal-like pulse profile, a peculiar sharp peak is observed in profiles below ~25 keV. The profiles at higher energies are found to be significantly phase-shifted compared to the soft X-ray profiles. Broadband energy spectra of GX 1+4, obtained from NuSTAR and Swift observations, are described with various continuum models. Among these, a two component model consisting of a bremsstrahlung and a blackbody component is found to best-fit the phase-averaged and phase-resolved spectra. Physical models are also used to investigate the emission mechanism in the pulsar, which allows us to estimate the magnetic field strength to be in $\sim$(5-10)$\times$10$^{12}$ G range. Phase-resolved spectroscopy of NuSTAR observation shows a strong blackbody emission component in a narrow pulse phase range. This component is interpreted as the origin of the peculiar peak in the pulse profiles below $\le$25 keV. The size of emitting region is calculated to be $\sim$400 m. The bremsstrahlung component is found to dominate in hard X-rays and explains the nature of simple profiles at high energies.

A DEEP X-RAY VIEW OF THE BARE AGN ARK 120. I. REVEALING THE SOFT X-RAY LINE EMISSION

ABSTRACT The Seyfert 1 galaxy Ark 120 is a prototype example of the so-called class of bare nucleus active galactic nuclei (AGNs), whereby there is no known evidence for the presence of ionized gas along the direct line of sight. Here deep (>400 ks exposure), high-resolution X-ray spectroscopy of Ark 120 is presented from XMM-Newton observations that were carried out in 2014 March, together with simultaneous Chandra/High Energy Transmission Grating exposures. The high-resolution spectra confirmed the lack of intrinsic absorbing gas associated with Ark 120, with the only X-ray absorption present originating from the interstellar medium (ISM) of our own Galaxy, with a possible slight enhancement of the oxygen abundance required with respect to the expected ISM values in the solar neighborhood. However, the presence of several soft X-ray emission lines are revealed for the first time in the XMM-Newton RGS spectrum, associated with the AGN and arising from the He- and H-like ions of N, O, Ne, and Mg. The He-like line profiles of N, O, and Ne appear velocity broadened, with typical FWHMs of ∼5000 km s−1, whereas the H-like profiles are unresolved. From the clean measurement of the He-like triplets, we deduce that the broad lines arise from a gas of density n e ∼ 1011 cm−3, while the photoionization calculations infer that the emitting gas covers at least 10% of 4π steradian. Thus the broad soft X-ray profiles appear coincident with an X-ray component of the optical–UV broad-line region on sub-parsec scales, whereas the narrow profiles originate on larger parsec scales, perhaps coincident with the AGN narrow-line region. The observations show that Ark 120 is not intrinsically bare and substantial X-ray-emitting gas exists out of our direct line of sight toward this AGN.

ACCELERATION PHASES OF A SOLAR FILAMENT DURING ITS ERUPTION

Filament eruptions often lead to coronal mass ejections (CMEs), which can affect critical technological systems in space and on the ground when they interact with the geo-magnetosphere in high speeds. Therefore, it is an important issue to investigate the acceleration mechanisms of CMEs in solar/space physics. Based on observations and simulations, the resistive magnetic reconnection and the ideal instability of magnetic flux rope have been proposed to accelerate CMEs. However, it remains elusive whether both of them play a comparable role during a particular eruption. It has been extremely difficult to separate their contributions as they often work in a close time sequence during one fast acceleration phase. Here we report an intriguing filament eruption event, which shows two apparently separated fast acceleration phases and provides us an excellent opportunity to address the issue. Through analyzing the correlations between velocity (acceleration) and soft (hard) X-ray profiles, we suggest that the instability and magnetic reconnection make a major contribution during the first and second fast acceleration phases, respectively. Further, we find that both processes have a comparable contribution to accelerate the filament in this event.

Bayesian soft X-ray tomography using non-stationary Gaussian Processes

In this study, a Bayesian based non-stationary Gaussian Process (GP) method for the inference of soft X-ray emissivity distribution along with its associated uncertainties has been developed. For the investigation of equilibrium condition and fast magnetohydrodynamic behaviors in nuclear fusion plasmas, it is of importance to infer, especially in the plasma center, spatially resolved soft X-ray profiles from a limited number of noisy line integral measurements. For this ill-posed inversion problem, Bayesian probability theory can provide a posterior probability distribution over all possible solutions under given model assumptions. Specifically, the use of a non-stationary GP to model the emission allows the model to adapt to the varying length scales of the underlying diffusion process. In contrast to other conventional methods, the prior regularization is realized in a probability form which enhances the capability of uncertainty analysis, in consequence, scientists who concern the reliability of their results will benefit from it. Under the assumption of normally distributed noise, the posterior distribution evaluated at a discrete number of points becomes a multivariate normal distribution whose mean and covariance are analytically available, making inversions and calculation of uncertainty fast. Additionally, the hyper-parameters embedded in the model assumption can be optimized through a Bayesian Occam's Razor formalism and thereby automatically adjust the model complexity. This method is shown to produce convincing reconstructions and good agreements with independently calculated results from the Maximum Entropy and Equilibrium-Based Iterative Tomography Algorithm methods.

Prototype high resolution multienergy soft x-ray array for NSTX

A novel diagnostic design seeks to enhance the capability of multienergy soft x-ray (SXR) detection by using an image intensifier to amplify the signals from a larger set of filtered x-ray profiles. The increased number of profiles and simplified detection system provides a compact diagnostic device for measuring T(e) in addition to contributions from density and impurities. A single-energy prototype system has been implemented on NSTX, comprised of a filtered x-ray pinhole camera, which converts the x-rays to visible light using a CsI:Tl phosphor. SXR profiles have been measured in high performance plasmas at frame rates of up to 10 kHz, and comparisons to the toroidally displaced tangential multi-energy SXR have been made.

Shafranov shift measurements by a soft x-ray CCD camera for internal diffusion barrier discharges in the Large Helical Device

Magnetic axis shift (Shafranov shift) in a high density operational regime with an internal diffusion barrier (IDB) formation in Large Helical Device experiments has been studied using a two-dimensional imaging system composed of a soft x-ray CCD camera and beryllium filters. Magnetic axis positions are derived from the fitting of pre-calculated equilibria to measured (line-integrated) soft x-ray profiles assuming that the emissivity profile could be expressed by Fourier–Bessel series expansion. It has been clearly observed that the Shafranov shift in a plasma with IDB is much larger than that without IDB at a similar average beta value measured by a diamagnetic loop because of the strongly peaked pressure profile in this regime. According to systematic analyses of the Shafranov shift for various high density plasmas in this regime, the dependence of the Shafranov shift on the central beta value is summarized and compared with that in discharges without IDB.

EVOLUTION OF SOLAR MAGNETIC FIELD AND ASSOCIATED MULTIWAVELENGTH PHENOMENA: FLARE EVENTS ON 2003 NOVEMBER 20

We analyze H-alpha images, soft X-ray profiles, magnetograms, extreme ultra-violet images and radio observations of two homologous flare events (M1.4/1N and M9.6/2B) on 20 November 2003 in the active region NOAA 10501 and study properties of reconnection between twisted filament systems, energy release and associated launch of coronal mass ejections (CMEs). During both events twisted filaments observed in H-alpha approached each other and initiated the flare processes. However, the second event showed the formation of cusp as the filaments interacted. The rotation of sunspots of opposite polarities, inferred from magnetograms likely powered the twisted filaments and injection of helicity. Along the current sheet between these two opposite polarity sunspots, the shear was maximum, which could have caused the twist in the filament. At the time of interaction between filaments, the reconnection took place and flare emission in thermal and non-thermal energy ranges attained the maximum. The radio signatures revealed the opening of field lines resulting from reconnection. The H-alpha images and radio data provide the inflow speed leading to reconnection and the scale size of particle acceleration region. The first event produced a narrow and slow CME, whereas the later one was associated with a fast full halo CME. The halo CME signatures observed between Sun and Earth using white-light and scintillation images and in-situ measurements indicated the magnetic energy utilized in the expansion and propagation. The magnetic cloud signature at the Earth confirmed the flux rope ejected at the time of filament interaction and reconnection.

STEREO OBSERVATIONS OF ENERGETIC NEUTRAL HYDROGEN ATOMS DURING THE 2006 DECEMBER 5 SOLAR FLARE

We report the discovery of energetic neutral hydrogen atoms (ENAs) emitted during the X9 solar event of 2006 December 5. Beginning ~1 hr following the onset of this E79 flare, the Low Energy Telescopes (LETs) on both the STEREO A and B spacecraft observed a sudden burst of 1.6-15 MeV protons beginning hours before the onset of the main solar energetic particle event at Earth. More than 70% of these particles arrived from a longitude within ±10° of the Sun, consistent with the measurement resolution. The derived emission profile at the Sun had onset and peak times remarkably similar to the GOES soft X-ray profile and continued for more than an hour. The observed arrival directions and energy spectrum argue strongly that the particle events < 5 MeV were due to ENAs. To our knowledge, this is the first reported observation of ENA emission from a solar flare/coronal mass ejection. Possible origins for the production of ENAs in a large solar event are considered. We conclude that the observed ENAs were most likely produced in the high corona and that charge-transfer reactions between accelerated protons and partially stripped coronal ions are an important source of ENAs in solar events.

Energy resolved soft x-ray imaging using a charge coupled device camera for long pulse discharges in the Large Helical Device

Energy resolved soft x-ray imaging system using a charge coupled device camera and a multifilter disk has recently been installed to the Large Helical Device for the measurements in the long pulse discharges. Eight images with different cutoff energies are measured sequentially during a single discharge by rotating a filter disk mounting eight beryllium filters with different thicknesses. A tangential line-integrated profile for a specific photon energy range can be obtained by taking intensity difference between two images measured with a filter pair of adjacent thicknesses. The typical photon energy corresponding to each difference ranges from 1.9 to 4.8 keV with the bandwidth of 2-3 keV. In the initial results, the difference in the line-integrated soft x-ray profile by the energy range has been clearly observed. This diagnostic method can possibly be applied to the observation of the dependence of two dimensional soft x-ray profile on photon energy range especially if local non-Maxwellian component appears in electron energy distribution function.

A complicated solar eruption event on 2003 October 26

We present multi-wavelength observations of a complicated solar eruption event to associate with an X1.2 flare and a Coronal Mass Ejection (CME) on 2003 October 26. The soft X-ray profile shows a possibility for occurrence of two flares with peaks around 06:20 and 07:00 UT. According to our observations, there are many evidences to show that they are corresponding to two energy releases. The first one produces type II, type III, moving type IV continua, a decimetric burst (DCIM) and strong emissions at Hα, 195 and 1600 A; While the second energy release only produces a group of RS-III bursts, DCIM and Hα emissions. It appears that the first energy release is associated with a CME, while the second CME is quiet. Such observational difference between two energy releases is found indicating two magnetic reconnection processes occurrence with different plasma situation.

Soft X-ray Emission Profile and Mode Structure During MHD Events in the TST-2 Spherical Tokamak

Mode structures during MHD events in the TST-2 spherical tokamak are revealed by means of singular value decomposition. Soft X-ray and magnetic coil signals show growths of modes with toroidal mode numbers n = 1 and 2. Slow evolution of the soft X-ray profile indicates beginning of localized crash. Occurrence of these events is correlated with the spatial gradient of the soft X-ray profile, suggesting that these events are driven by the pressure gradient. Increases in impurity ion temperatures (CIII and OV) are observed. These are positively correlated with the increment of the plasma current during these events.

The toroidicity-induced Alfvén eigenmode structure in DIII-D: Implications of soft x-ray and beam-ion loss data

The internal structure of the toroidicity-induced Alfvén eigenmode (TAE) is studied by comparing soft x-ray profile and beam ion loss data taken during TAE activity in the DIII-D tokamak [W. W. Heidbrink et al., Nucl. Fusion 37, 1411 (1997)] with predictions from theories based on ideal magnetohydrodynamic (MHD), gyrofluid, and gyrokinetic models. The soft x-ray measurements indicate a centrally peaked eigenfunction, a feature which is closest to the gyrokinetic model’s prediction. The beam ion losses are simulated using a guiding center code. In the simulations, the TAE eigenfunction calculated using the ideal MHD model acts as a perturbation to the equilibrium field. The predicted beam ion losses are an order of magnitude less than the observed ∼6%–8% losses at the peak experimental amplitude of δBr/B0≃2–5×10−4.

A Study of Microwave‐selected Coronal Transient Brightenings

We present the results of a search for radio-selected transient brightenings (TBs) in the solar atmosphere as a complement to the more common X-ray-selected surveys. The Sun was generally quiet during the observations, making these data sensitive to weak TBs both in and outside active regions. Five small impulsive events were identified in a set of VLA observations at 4.5, 1.5, and 0.33 GHz and compared with soft X-ray images from Yohkoh and EUV images from SOHO/EIT. Four of the events were located at the edges of an active region, but one was located 100'' away in a quiet region of the atmosphere. Possible emission mechanisms for these brightenings are investigated. The time profiles of the radio TBs show impulsive peaks, while the corresponding soft X-ray profiles are gradual. The impulsive radio peaks were up to 35% polarized. Our data favor an interpretation in terms of gyrosynchrotron radiation from mildly relativistic electrons. A small number of nonthermal electrons with spectral index 3 can explain the observed properties of the TBs. Thus, nonthermal TBs can be found away from active regions. Two of the microwave TBs also show evidence for type III radio emission at 327 MHz.

Interaction of ELMs with the core transport barrier in CH-mode in PBX-M

An addition of ion Bernstein wave power of only a tenth of the neutral beam power into an H-mode causes the formation of a core transport barrier. At the location of the barrier, reduced ELM losses in the soft x-ray profile are observed. During the IBW-modified ELM, one can observe enhanced fluctuations in the soft x-ray fluctuation profile over the whole of the observed frequency range. In the region of the core transport barrier, the enhancement of fluctuations seems to be strongly reduced. The ELM loss propagates to the core with a velocity greater than that which would be consistent with a normal energy diffusion process.

The effect of divertor plate biasing on impurity transport and levels in the central plasma of TdeV

Impurity concentration and transport are studied by interpreting line emissions in the VUV and soft X-ray emission, using the MIST impurity code. Standard tokamak discharges have been used with modest divertor plate bias voltages Vb (-300 to +300 V). The intrinsic impurities studied are oxygen and carbon. Neon, an extrinsic impurity, is introduced using hydrogen gas fuelling with a 0.3% trace of neon. Aluminium is introduced by laser ablation. The radial profiles of C IV emissivity show an increasing diffusivity from negative to positive biasing, while to obtain compatible changes between the VUV emissions (C IV, O VII and Ne IX) and the soft X-ray profiles, the inward convection velocity and the impurity concentrations must be increased as Vb increases from -300 V to +300 V. The changes in the convection predicted by six different models are compared with the experimentally deduced convection. These models are based on ion mobility, ion orbit losses, anomalous viscosity caused by turbulence, the skin size electromagnetic drift mode, neoclassical theory in the presence of turbulence and mobility related to E*B turbulent diffusivity. This comparison allows the changes in the convection velocity to be related qualitatively to the ambipolar radial electric field Eamb, which may be different from the purely neoclassical value Eneo, owing to turbulence which is usually intrinsically non-ambipolar; the turbulence can also cause an anomalous viscosity, affecting the rotational velocities and hence Eamb

The Neupert effect: What can it tell us about the impulsive and gradual phases of solar flares?

The Neupert Effect is the name given to the correlation obsrved in many flares between the impulsive time profile of the microwave and hard X-ray emissions and the time derivative of the soft X-ray profile. We have used data collected between 1980 and 1989 from the Hard X-Ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM) and the soft X-ray detector on GOES to determine which events show this correlation and which do not. We have found that of 66 HXRBS events observed in 1980 with a peak rate of >1000 counts/s, 58 (80%) showed good correlation with peaks in the GOES time derivative plot corresponding to peaks in the HXR plots to within ±20 s. The more gradually varying X-ray events that are commonly referred to as Type C flares and that are often accompanied by eruptive flares, tend to show poorer correlation between the SXR time derivative and the HXR time profile. In seve al cases studied, the later, more gradually varying, peaks either did not register at all in the SXR time derivative plots or resulted in very broad peaks that, in one ase on 1981 April 26, was delayed by 13 minutes.

A statistical analysis of the soft X-ray profiles of solar flares

We undertake a statistical analysis of the soft X-ray (3.5 – 5.5 keV) profiles of solar flares as observed with the Hard X-ray Imaging Spectrometer on the SMM. The durations, maximum intensities and intensity profiles of the flares are examined. The properties of the “typical” solar flare are discussed. The distributions of the measured parameters with respect to one another reveal some interesting results. In common with past studies, we conclude that there is no evidence to suggest that more than one type of event is being viewed, despite a desire evident in the literature to place events into distinct groups. We also conclude that commonly held views about the relatonships between flare duration and intensity, profile asymmetries and intensity etc.. are in error. For more details of the flare events, the selection of data and the method of analysis, the reader is referred to Pearce and Harrison (1988).

JET soft x-ray diode array diagnostic

The arrays of silicon diodes recently installed on the Joint European Torus (JET) measure the plasma soft x-ray emission with high temporal and spatial resolution, allowing magnetohydrodynamic (MHD) effects to be investigated. The diagnostic consists of two ‘‘pinhole’’ cameras which view the same poloidal cross section in orthogonal directions. A data-acquisition system based upon three cascaded digital filters can convert the 200-kHz detector signal analog-to-digital converter (ADC) output to lower sampling rates of 10, 1, and 0.1 kHz. Initially, studies have concentrated on the sawtooth collapse. By means of a trigger system data on the sawtooth crash have regularly been recorded at a sampling frequency of 200 kHz. Using tomographic reconstruction techniques, the soft x-ray emissivity cross section during the crash has been reconstructed. First results during ion cyclotron radio frequency (ICRF) heating show no precursor activity prior to the sawtooth collapse, while during the crash, the hot central plasma is displaced off axis by about 30 cm on a time scale of 50–200 μs, after which a hollow soft x-ray profile forms.

On the Thermalisation of Flare-Time Energetic Electrons Observed at Radio and X-Ray Wavelengths

An interesting microwave event at 2800 MHz was recorded at Ahmedabad on September 19, 1977 at 1026 UT at the same time as the H-Alpha solar flare of importance 3B. The microwave burst was of impulsive nature, with as many as twenty impulses in seventy minutes with a quasi-periodicity of 1 to 5 minutes. An X-ray burst recorded by GOES Satellite in 1-8A band showed at the same time a smooth soft X-ray profile with apparently no sign of hard X-ray bursts. This indicates that the acceleration of discrete electron streams which produced impulsive microwave bursts was not sufficient to produce the hard X-ray component but got thermalised to produce soft X-ray emission, with a gradual rise and a slow decay covering a long duration of more than 2 1/2 hours.