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[en] New calculations of the energy levels, radiative transition rates, and collisional excitation rates of Fe IX have been carried out using the Flexible Atomic Code, paying close attention to experimentally identified levels and extending existing calculations to higher energy levels. For lower levels, R-matrix collisional excitation rates from earlier work have been used. Significant emission is predicted by these calculations in the 5f-3d transitions, which will impact analysis of Solar Dynamics Observatory Atmospheric Imaging Assembly observations using the 94 A filter.
[en] We present an analysis of soft X-ray (SXR) and extreme-ultraviolet (EUV) observations of solar flares with an approximate C8 Geostationary Operational Environmental Satellite (GOES) class. Our constraint on peak GOES SXR flux allows for the investigation of correlations between various flare parameters. We show that the duration of the decay phase of a flare is proportional to the duration of its rise phase. Additionally, we show significant correlations between the radiation emitted in the flare rise and decay phases. These results suggest that the total radiated energy of a given flare is proportional to the energy radiated during the rise phase alone. This partitioning of radiated energy between the rise and decay phases is observed in both SXR and EUV wavelengths. Though observations from the EUV Variability Experiment show significant variation in the behavior of individual EUV spectral lines during different C8 events, this work suggests that broadband EUV emission is well constrained. Furthermore, GOES and Atmospheric Imaging Assembly data allow us to determine several thermal parameters (e.g., temperature, volume, density, and emission measure) for the flares within our sample. Analysis of these parameters demonstrate that, within this constrained GOES class, the longer duration solar flares are cooler events with larger volumes capable of emitting vast amounts of radiation. The shortest C8 flares are typically the hottest events, smaller in physical size, and have lower associated total energies. These relationships are directly comparable with several scaling laws and flare loop models.
[en] We report on observations of the first diffuse coronal wave detected by Hinode/XRT. The event occurred near the west solar limb on 2007 May 23, originating from active region (AR) 10956 and was associated with a coronal mass ejection (CME) and coronal dimmings. The bright emission forming the coronal wave expanded predominantly to the east and south of the AR. We use X-Ray Telescope (XRT) and STEREO Behind (B) data combined with a potential magnetic field extrapolation to derive an understanding of the global magnetic field connectivity. We attribute the brightening to the east of the AR to compression and channeling of the plasma along large-scale loops. The brightening to the south of the AR expands across the quiet Sun, making the southern component a likely candidate for identification as a diffuse coronal wave. We analyze the bright front in STEREO/EUVI (B) 171, 195, and 284 A images, as well as in XRT data, finding the strongest components to be largely cospatial in all bandpasses. We also exploit the near-limb location of this event by combining STEREO/COR1 and Extreme Ultra-Violet Imaging Telescope (EUVI) data. Using all the data, we derive a full picture of the low-coronal development of the eruption. The COR1 data show that the southernmost outer edge of the CME is progressively displaced southward during the expansion. EUVI data below the COR1 occulting disk show that the CME is significantly distorted in the low corona as a result of the associated filament eruption. The core coronal dimmings map to the core of the CME; the secondary coronal dimmings map to the CME cavity; and the diffuse coronal wave maps to the outermost edge of the expanding CME shell. The analysis of this near-limb event has important implications for understanding earlier eruptions originating from the same AR on 2007 May 16, 19, and 20.
[en] We use coronal imaging observations with the Solar Dynamics Observatory/Atmospheric Imaging Assembly (AIA), and Hinode/Extreme-ultraviolet Imaging Spectrometer (EIS) spectral data to explore the potential of narrowband EUV imaging data for diagnosing the presence of hot (T ∼> 5 MK) coronal plasma in active regions. We analyze observations of two active regions (AR 11281, AR 11289) with simultaneous AIA imaging and EIS spectral data, including the Ca XVII line (at 192.8 Å), which is one of the few lines in the EIS spectral bands sensitive to hot coronal plasma even outside flares. After careful co-alignment of the imaging and spectral data, we compare the morphology in a three-color image combining the 171, 335, and 94 Å AIA spectral bands, with the image obtained for Ca XVII emission from the analysis of EIS spectra. We find that in the selected active regions the Ca XVII emission is strong only in very limited areas, showing striking similarities with the features bright in the 94 Å (and 335 Å) AIA channels and weak in the 171 Å band. We conclude that AIA imaging observations of the solar corona can be used to track hot plasma (6-8 MK), and so to study its spatial variability and temporal evolution at high spatial and temporal resolution.
[en] One scenario proposed to explain the million degree solar corona is a finely stranded corona where each strand is heated by a rapid pulse. However, such fine structure has neither been resolved through direct imaging observations nor conclusively shown through indirect observations of extended superhot plasma. Recently, it has been shown that the observed difference in the appearance of cool and warm coronal loops (∼1 MK and ∼2-3 MK, respectively)-warm loops appearing 'fuzzier' than cool loops-can be explained by models of loops composed of subarcsecond strands, which are impulsively heated up to ∼10 MK. That work predicts that images of hot coronal loops (∼> 6 MK) should again show fine structure. Here we show that the predicted effect is indeed widely observed in an active region with the Solar Dynamics Observatory, thus supporting a scenario where impulsive heating of fine loop strands plays an important role in powering the active corona.
[en] We investigate X-ray emission properties of the peculiar X-ray source θ2 Ori A in the Orion Trapezium region using more than 500 ks of HETGS spectral data in the quiescent state. The amount of exposure provides tight constraints on several important diagnostics involving O, Ne, Mg, and Si line flux ratios from He-like ion triplets, resonance line ratios of the H- and He-like lines, and line widths. Accounting for the influence of the strong UV radiation field of the O9.5V star, we can now place the He-like line origin well within two stellar radii of the O-star's surface. The lines are resolved with average line widths of 341 ± 38 km s-1. In the framework of standard wind models, this likely implies a rather weak wind with moderate post-shock velocities. The emission measure distribution of the X-ray spectrum, as reported previously, includes very high temperature components which are not easily explained in this framework. The X-ray properties are also not consistent with coronal emissions from an unseen low-mass companion nor with typical signatures from colliding wind interactions. The properties are more consistent with X-ray signatures observed in the massive Trapezium star θ1 Ori C which has recently been successfully modeled with a magnetically confined wind model.
[en] The Chandra HETG Orion Legacy Project (HOLP) is the first comprehensive set of observations of a very young massive stellar cluster that provides high-resolution X-ray spectra of very young stars over a wide mass range (0.7–2.3 ). In this paper, we focus on the six brightest X-ray sources with T Tauri stellar counterparts that are well-characterized at optical and infrared wavelengths. All stars show column densities which are substantially smaller than expected from optical extinction, indicating that the sources are located on the near side of the cluster with respect to the observer as well as that these stars are embedded in more dusty environments. Stellar X-ray luminosities are well above 1031 erg s−1, in some cases exceeding 1032 erg s−1 for a substantial amount of time. The stars during these observations show no flares but are persistently bright. The spectra can be well fit with two temperature plasma components of 10 MK and 40 MK, of which the latter dominates the flux by a ratio 6:1 on average. The total emission measures range between 3–8 × 1054 cm−3 and are comparable to active coronal sources. The fits to the Ne ix He-Like K-shell lines indicate forbidden to inter-combination line ratios consistent with the low-density limit. Observed abundances compare well with active coronal sources underlying the coronal nature of these sources. The surface flux in this sample of 0.6–2.3 classical T Tauri stars shows that coronal activity increases significantly between ages 0.1 and 10 Myr. The results demonstrate the power of X-ray line diagnostics to study coronal properties of T Tauri stars in young stellar clusters.
[en] θ1 Ori E is a young, moderate mass binary system, a rarely observed case of spectral-type G-giants of about 3 solar masses, which are still collapsing toward the main sequence, where they presumably become X-ray faint. We have obtained high-resolution X-ray spectra with Chandra and find that the system is very active and similar to coronal sources, having emission typical of magnetically confined plasma. It has a broad temperature distribution with a hot component and significant high energy continuum; narrow emission lines from H- and He-like ions, as well as a range of Fe ions, and relative luminosity, Lx /Lbol = 10-3, at the saturation limit. Density, while poorly constrained, is consistent with the low density limits, our upper limits being ne < 1013 cm-3 for Mg XI and ne < 1012 cm-3 for Ne IX. Coronal elemental abundances are sub-solar, with Ne being the highest at about 0.4 times solar. We find a possible trend in Trapezium hot plasmas toward low relative abundances of Fe, O, and Ne, which is hard to explain in terms of the dust depletion scenarios of low-mass young stars. Variability was unusually low during our observations relative to other coronally active stars. Qualitatively, the emission is similar to post-main-sequence G-stars. Coronal structures could be compact or comparable to the dimensions of the stellar radii. From comparison to X-ray emission from similar mass stars at various evolutionary epochs, we conclude that the X-rays in θ1 Ori E are generated by a convective dynamo, present during contraction, but which will vanish during the main-sequence epoch possibly to be resurrected during post-main-sequence evolution.
[en] The Atmospheric Imaging Assembly (AIA) and the Extreme-ultraviolet Variability Experiment (EVE) on board the Solar Dynamics Observatory (SDO) include spectral windows in the X-ray/EUV band. Accuracy and completeness of the atomic data in this wavelength range is essential for interpretation of the spectrum and irradiance of the solar corona, and of SDO observations made with the AIA and EVE instruments. Here, we test the X-ray/EUV data in the CHIANTI database to assess their completeness and accuracy in the SDO bands, with particular focus on the 94 Å and 131 Å AIA passbands. Given the paucity of solar observations adequate for this purpose, we use high-resolution X-ray spectra of the low-activity solar-like corona of Procyon obtained with the Chandra Low Energy Transmission Grating Spectrometer (LETGS). We find that while spectral models overall can reproduce quite well the observed spectra in the soft X-ray range λ ∼< 50 Å, and at the EUV wavelengths λ ∼> 130 Å, they significantly underestimate the observed flux in the 50-130 Å wavelength range. The model underestimates the observed flux by a variable factor ranging from ≈1.5, at short wavelengths below ∼50 Å, up to ≈5-7 in the ∼70-125 Å range. In the AIA bands covered by LETGS, i.e., 94 Å and 131 Å, we find that the observed flux can be underestimated by large factors (∼3 and ∼1.9, respectively, for the case of Procyon presented here). We discuss the consequences for analysis of AIA data and possible empirical corrections to the AIA responses to model more realistically the coronal emission in these passbands.
[en] It is typically assumed that emission in the passbands of the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) is dominated by single or several strong lines from ions that under equilibrium conditions are formed in a narrow range of temperatures. However, most SDO/AIA channels also contain contributions from lines of ions that have formation temperatures that are significantly different from the 'dominant' ion(s). We investigate the importance of these lines by forward modeling the emission in the SDO/AIA channels with three-dimensional radiative MHD simulations of a model that spans the upper layer of the convection zone to the low corona. The model is highly dynamic. In addition, we pump a steadily increasing magnetic flux into the corona, in order to increase the coronal temperature through the dissipation of magnetic stresses. As a consequence, the model covers different ranges of coronal temperatures as time progresses. The model covers coronal temperatures that are representative of plasma conditions in coronal holes and quiet Sun. The 131, 171, and 304 Å AIA passbands are found to be the least influenced by the so-called non-dominant ions, and the emission observed in these channels comes mostly from plasma at temperatures near the formation temperature of the dominant ion(s). On the other hand, the other channels are strongly influenced by the non-dominant ions, and therefore significant emission in these channels comes from plasma at temperatures that are different from the 'canonical' values. We have also studied the influence of non-dominant ions on the AIA passbands when different element abundances are assumed (photospheric and coronal), and when the effects of the electron density on the contribution function are taken into account.