arXiv subjects

G. Del Zanna

Publications and source records attributed to G. Del Zanna.

At least 55 records · Page 3Linked to original sources

arXiv2016

Density diagnostics derived from the OIV and SIV intercombination lines observed by IRIS

The intensity of the \oiv~2s$^{2}$ 2p $^{2}$P-2s2p$^{2}$ $^{4}$P and \siv~3 s$^{2}$ 3p $^{2}$P- 3s 3p$^{2}$ $^{4}$ P intercombination lines around 1400~Å~observed with the \textit{Interface Region Imaging Spectrograph} (IRIS) provide a useful tool to diagnose the electron number density ($N_\textrm{e}$) in the solar transition region plasma. We measure the electron number density in a variety of solar features observed by IRIS, including an active region (AR) loop, plage and brightening, and the ribbon of the 22-June-2015 M 6.5 class flare. By using the emissivity ratios of \oiv\ and \siv\ lines, we find that our observations are consistent with the emitting plasma being near isothermal (log$T$[K] $\approx$ 5) and iso-density ($N_\textrm{e}$ $\approx$~10$^{10.6}$ cm$^{-3}$) in the AR loop. Moreover, high electron number densities ($N_\textrm{e}$ $\approx$~10$^{13}$ cm$^{-3}$) are obtained during the impulsive phase of the flare by using the \siv\ line ratio. We note that the \siv\ lines provide a higher range of density sensitivity than the \oiv\ lines. Finally, we investigate the effects of high densities ($N_\textrm{e}$ $\gtrsim$ 10$^{11}$ cm$^{-3}$) on the ionization balance. In particular, the fractional ion abundances are found to be shifted towards lower temperatures for high densities compared to the low density case. We also explored the effects of a non-Maxwellian electron distribution on our diagnostic method.

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arXiv2016

Scaling of collision strengths for highly-excited states of ions of the H- and He-like sequences

Emission lines from highly-excited states (n >= 5) of H- and He-like ions have been detected in astrophysical sources and fusion plasmas. For such excited states, R-matrix or distorted wave calculations for electron-impact excitation are very limited, due to the large size of the atomic basis set needed to describe them. Calculations for n >= 6 are also not generally available. We study the behaviour of the electron-impact excitation collision strengths and effective collision strengths for the most important transitions used to model electron collision dominated astrophysical plasmas, solar, for example. We investigate the dependence on the relevant parameters: the principal quantum number n or the nuclear charge Z. We also estimate the importance of coupling to highly-excited states and the continuum by comparing the results of different sized calculations. We provide analytic formulae to calculate the electron-impact excitation collision strengths and effective collision strengths to highly-excited states (n >= 8) of H- and He-like ions. These extrapolated effective collision strengths can be used to interpret astrophysical and fusion plasma via collisional-radiative modelling.

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arXiv2015

Simultaneous IRIS and Hinode/EIS observations and modelling of the 27 October 2014 X 2.0 class flare

We present the study of the X2-class flare which occurred on the 27 October 2014 and was observed with the Interface Region Imaging Spectrograph (IRIS) and the EUV Imaging Spectrometer (EIS) on board the Hinode satellite. Thanks to the high cadence and spatial resolution of the IRIS and EIS instruments, we are able to compare simultaneous observations of the \xxi~1354.08~Å~and \xxiii~263.77~Å~high temperature emission ($\gtrsim$ 10~MK) in the flare ribbon during the chromospheric evaporation phase. We find that IRIS observes completely blue-shifted \xxi~line profiles, up to 200 km s$^{-1}$ during the rise phase of the flare, indicating that the site of the plasma upflows is resolved by IRIS. In contrast, the \xxiii~line is often asymmetric, which we interpret as being due to the lower spatial resolution of EIS. Temperature estimates from SDO/AIA and Hinode/XRT show that hot emission (log($T$)[K] $>$ 7.2) is first concentrated at the footpoints before filling the loops. Density sensitive lines from IRIS and EIS give electron number density estimates of $\gtrsim$~10$^{12}$~cm$^{-3}$ in the transition region lines and 10$^{10}$~cm$^{-3}$ in the coronal lines during the impulsive phase. In order to compare the observational results against theoretical predictions, we have run a simulation of a flare loop undergoing heating using the HYDRAD 1D hydro code. We find that the simulated plasma parameters are close to the observed values which are obtained with IRIS, Hinode and AIA. These results support an electron beam heating model rather than a purely thermal conduction model as the driving mechanism for this flare.

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arXiv2015

CHIANTI - An atomic database for Emission Lines. Version 8

We present version 8 of the CHIANTI database. This version includes a large amount of new data and ions, which represent a significant improvement in the soft X-ray, EUV and UV spectral regions, which several space missions currently cover. New data for neutrals and low charge states are also added. The data are assessed, but to improve the modelling of low-temperature plasma the effective collision strengths for most of the new datasets are not spline-fitted as previously, but are retained as calculated. This required a change of the format of the CHIANTI electron excitation files. The format of the energy files has also been changed. Excitation rates between all the levels are retained for most of the new datasets, so the data can in principle be used to model high-density plasma. In addition, the method for computing the differential emission measure used in the CHIANTI software has been changed.

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arXiv2015

Validity of the ICFT R-matrix method: Be-like Al$^{9+}$ a case study

We have carried-out 98-level configuration-interaction / close-coupling (CI/CC) intermediate coupling frame transformation (ICFT) and Breit-Pauli R-matrix calculations for the electron-impact excitation of Be-like Al$^{9+}$. The close agreement that we find between the two sets of effective collision strengths demonstrates the continued robustness of the ICFT method. On the other hand, a comparison of this data with previous 238-level CI/CC ICFT effective collision strengths shows that the results for excitation up to n=4 levels are systematically and increasingly underestimated over a wide range of temperatures by R-matrix calculations whose close-coupling expansion extends only to n=4 (98-levels). Thus, we find to be false a recent conjecture that the ICFT approach may not be completely robust. The conjecture was based upon a comparison of 98-level CI/CC Dirac R-matrix effective collision strengths for Al$^{9+}$ with those from the 238-level CI/CC ICFT R-matrix calculations. The disagreement found recently is due to a lack of convergence of the close-coupling expansion in the 98-level CI/CC Dirac work. The earlier 238-level CI/CC ICFT work has a superior target to the 98-level CI/CC Dirac one and provides more accurate atomic data. Similar considerations need to be made for other Be-like ions and for other sequences.

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arXiv2014

Signatures of the non-Maxwellian $κ$-distributions in optically thin line spectra \subtitle{I. Theory and synthetic Fe IX--XIII spectra}

We investigate the possibility of diagnosing the degree of departure from the Maxwellian distribution using single-ion spectra originating in astrophysical plasmas in collisional ionization equilibrium. New atomic data for excitation of Fe IX-XIII are integrated under the assumption of a kappa-distribution of electron energies. Diagnostic methods using lines of a single ion formed at any wavelength are explored. Such methods minimize uncertainties from the ionization and recombination rates, as well as the possible presence of non-equilibrium ionization. Approximations to the collision strengths are also investigated. The calculated intensities of most of the Fe IX-XIII EUV lines show consistent behaviour with kappa at constant temperature. Intensities of these lines decrease with kappa, with the vast majority of ratios of strong lines showing little or no sensitivity to kappa. Several of the line ratios, especially involving temperature-sensitive lines, show a sensitivity to kappa that is of the order of several tens of per cent, or, in the case of Fe IX, up to a factor of two. Forbidden lines in the near-ultraviolet, visible, or infrared parts of the spectrum are an exception, with smaller intensity changes or even a reverse behaviour with kappa. The most conspicuous example is the Fe X 6378A red line, whose intensity incerases with kappa. This line is a potentially strong indicator of departures from the Maxwellian distribution. We find it possible to perform density diagnostics independently of kappa, with many Fe XI-XIII line ratios showing strong density-sensitivity and negligible sensitivity to kappa and temperature. We also tested different averaging of the collision strengths. It is found that averaging over 0.01 interval in log(E/Ryd) is sufficient to produce accurate distribution-averaged collision strengths at temperatures of the ion formation in ionization equilibrium.

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arXiv2014

Thermal structure of hot non-flaring corona from Hinode/EIS

In previous studies a very hot plasma component has been diagnosed in solar active regions through the images in three different narrow-band channels of SDO/AIA. This diagnostic from EUV imaging data has also been supported by the matching morphology of the emission in the hot Ca XVII line, as observed with Hinode/EIS. This evidence is debated because of unknown distribution of the emission measure along the line of sight. Here we investigate in detail the thermal distribution of one of such regions using EUV spectroscopic data. In an active region observed with SDO/AIA, Hinode/EIS and XRT, we select a subregion with a very hot plasma component and another cooler one for comparison. The average spectrum is extracted for both, and 14 intense lines are selected for analysis, that probe the 5.5 < log T < 7 temperature range uniformly. From these lines the emission measure distributions are reconstructed with the MCMC method. Results are cross-checked with comparison of the two subregions, with a different inversion method, with the morphology of the images, and with the addition of fluxes measured with from narrow and broad-band imagers. We find that, whereas the cool region has a flat and featureless distribution that drops at temperature log T >= 6.3, the distribution of the hot region shows a well-defined peak at log T = 6.6 and gradually decreasing trends on both sides, thus supporting the very hot nature of the hot component diagnosed with imagers. The other cross-checks are consistent with this result. This study provides a completion of the analysis of active region components, and the resulting scenario supports the presence of a minor very hot plasma component in the core, with temperatures log T > 6.6.

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arXiv2014

The EUV spectrum of the Sun: SOHO CDS NIS radiances during solar cycle 23

For the first time, we present and discuss EUV radiances of the solar transition region (TR) and corona obtained during a solar cycle. The measurements were obtained with the SOHO/coronal diagnostic spectrometer (CDS) during the period from 1996 to 2010. We find that limb-brightening significantly affects any characterisation of the solar radiances. We present the limb-brightening function for the main lines and find that it does not change measurably during the cycle. We confirm earlier findings that the radiance histogram of the cooler lines have a well defined, log-normal quiet-Sun component, although our results differ from previous ones. The width of the lowest-radiance log-normal distribution is constant along the cycle. Both the analysis of the centre-to-limb variation and of the radiance statistical distribution point to a constant QS emission along solar cycle 23. Lines formed above 1 MK are dramatically affected by the presence of active regions, and indeed, no "quiet Sun" region can be defined during periods of maximum activity. Much of the irradiance variability in lines formed below 1.5 MK is due to a change in the emitting area. For hotter lines, the emitting area saturates to almost 100% of full solar disk at the maximum of activity, while simultaneously the emission due to active regions increases by more than an order of magnitude. We show that structures around active regions, sometimes referred to as dark halos or dark canopies, are common and discuss their similarities and differences with coronal holes. In particular, we show how they are well visible in TR lines, contrary to coronal holes.

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arXiv2013

A revised radiometric calibration for the Hinode/EIS instrument

A preliminary assessment of the in-flight radiometric calibration of the Hinode EUV Imaging Spectrometer (EIS) is presented. This is done with the line ratio technique applied to a wide range of observations of the quiet Sun, active regions and flares from 2006 until 2012. The best diagnostic lines and the relevant atomic data are discussed in detail. Radiances over the quiet Sun are also considered, with comparisons with previous measurements. Some departures in the shapes of the ground calibration responsivities are found at the start of the mission. These shapes do not change significantly over time, with the exception of the shorter wavelengths of the EIS short-wavelength (SW) channel, which shows some degradation. The sensitivity of the SW channel at longer wavelengths does not show significant degradation, while that of the long-wavelength (LW) channel shows a significant degradation with time. By the beginning of 2010 the responsivity of the LW channel was already a factor of two or more lower than the values measured on the ground. A first-order correction is proposed. With this correction, the main ratios of lines in the two channels become constant to within a relative 20%, and the He II 256 A radiances over the quiet Sun also become constant over time. This correction removes long-standing discrepancies for a number of lines and ions, in particular those involving the strongest Fe X, Fe XIII, Fe XIV, Fe XVII, and Fe XXIV lines, where discrepancies of factors of more than two were found. These results have important implications for various EIS science analyses, in particular for measurements of temperatures, emission measures and elemental abundances.

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arXiv2013

On-Orbit Degradation of Solar Instruments

We present the lessons learned about the degradation observed in several space solar missions, based on contributions at the Workshop about On-Orbit Degradation of Solar and Space Weather Instruments that took place at the Solar Terrestrial Centre of Excellence (Royal Observatory of Belgium) in Brussels on 3 May 2012. The aim of this workshop was to open discussions related to the degradation observed in Sun-observing instruments exposed to the effects of the space environment. This article summarizes the various lessons learned and offers recommendations to reduce or correct expected degradation with the goal of increasing the useful lifespan of future and ongoing space missions.

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arXiv2012

Benchmarking atomic data for astrophysics: a first look at the soft X-ray lines

A collection of the best solar and laboratory spectra in the soft X-rays is used here to perform a preliminary benchmark in this wavelength region, by comparing observed vs. predicted wavelengths and calibrated solar irradiances. The benchmark focuses on the Fe IX - Fe XIV ions,for which we have recently calculated the relevant atomic data, however a few other ions have also been benchmarked. The iron ions are dominating the soft X-rays, however a large fraction of the strongest soft X-ray lines due to n=4 -> n=3 transitions were previously unidentified. The strongest transitions are all identified here, in particular the decays from the core-excited levels (3s 3p^l 4s, l=$ 5,4,3,2,1 for Fe X, Fe XI, Fe XII, Fe XIII, and Fe XIV respectively) which are the strongest soft X-ray transitions from these ions. Many new identifications are proposed, some only tentatively. Good agreement in terms of solar irradiances between the soft X-ray and EUV (n=3 -> n=3) transitions is found, confirming the reliability of the new large-scale calculations. Some of the new atomic data and identifications are particularly important for the Solar Dynamic Observatory (SDO) Atmospheric Imaging Assembly (AIA) 94 A band.

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arXiv2012

Propagating Disturbances in Coronal Loops: A Detailed Analysis of Propagation Speeds

Quasi-periodic disturbances have been observed in the outer solar atmosphere for many years now. Although first interpreted as upflows (Schrijver et al. (1999)), they have been widely regarded as slow magnetoacoustic waves, due to observed velocities and periods. However, recent observations have questioned this interpretation, as periodic disturbances in Doppler velocity, line width and profile asymmetry were found to be in phase with the intensity oscillations (De Pontieu et al. (2010),Tian1 et al. (2011))}, suggesting the disturbances could be quasi-periodic upflows. Here we conduct a detailed analysis of the velocities of these disturbances across several wavelengths using the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). We analysed 41 examples, including both sunspot and non sunspot regions of the Sun. We found that the velocities of propagating disturbances (PDs) located at sunspots are more likely to be temperature dependent, whereas the velocities of PDs at non sunspot locations do not show a clear temperature dependence. We also considered on what scale the underlying driver is affecting the properties of the PDs. Finally, we found that removing the contribution due to the cooler ions in the 193 A wavelength suggests that a substantial part of the 193 emission of sunspot PDs can be contributed to the cool component of 193Å.

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arXiv2012

Coronal Diagnostics from Narrowband Images around 30.4 nm

Images taken in the band centered at 30.4 nm are routinely used to map the radiance of the He II Ly alpha line on the solar disk. That line is one of the strongest, if not the strongest, line in the EUV observed in the solar spectrum, and one of the few lines in that wavelength range providing information on the upper chromosphere or lower transition region. However, when observing the off-limb corona the contribution from the nearby Si XI 30.3 nm line can become significant. In this work we aim at estimating the relative contribution of those two lines in the solar corona around the minimum of solar activity. We combine measurements from CDS taken in August 2008 with temperature and density profiles from semiempirical models of the corona to compute the radiances of the two lines, and of other representative coronal lines (e.g., Mg X 62.5 nm, Si XII 52.1 nm). Considering both diagnosed quantities from line ratios (temperatures and densities) and line radiances in absolute units, we obtain a good overall match between observations and models. We find that the Si XI line dominates the He II line from just above the limb up to ~2 R_Sun in streamers, while its contribution to narrowband imaging in the 30.4 nm band is expected to become smaller, even negligible in the corona beyond ~2 - 3 R_Sun, the precise value being strongly dependent on the coronal temperature profile.

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arXiv2010

Science Objectives for an X-Ray Microcalorimeter Observing the Sun

We present the science case for a broadband X-ray imager with high-resolution spectroscopy, including simulations of X-ray spectral diagnostics of both active regions and solar flares. This is part of a trilogy of white papers discussing science, instrument (Bandler et al. 2010), and missions (Bookbinder et al. 2010) to exploit major advances recently made in transition-edge sensor (TES) detector technology that enable resolution better than 2 eV in an array that can handle high count rates. Combined with a modest X-ray mirror, this instrument would combine arcsecondscale imaging with high-resolution spectra over a field of view sufficiently large for the study of active regions and flares, enabling a wide range of studies such as the detection of microheating in active regions, ion-resolved velocity flows, and the presence of non-thermal electrons in hot plasmas. It would also enable more direct comparisons between solar and stellar soft X-ray spectra, a waveband in which (unusually) we currently have much better stellar data than we do of the Sun.

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arXiv2010

Active Region Moss: Basic physical parameters and their temporal variation

We present a study of the physical plasma parameters such as electron temperature, electron density, column depth and filling factors in the moss regions and their variability over a short (an hour) and a long period (5 consecutive days) of time. Primarily, we have analyzed the spectroscopic observations recorded by the Extreme-ultraviolet Imaging Spectrometer (EIS) aboard Hinode. In addition we have used supplementary observations taken from TRACE and the X-Ray Telescope (XRT). We find that the moss emission is strongest in the Fe xii and Fe xiii lines. Based on analyses using line ratios and emission measure we found that the moss region has a characteristic temperature of log T = 6.2. The electron densities measured at different locations in the moss regions using Fe xii ratios are about 1-3\times1010 cm(-3) and about 2-4\times10^9 cm^(-3) using Fe xiii and Fe xiv. The electron density substantially increases (by a factor of about 3-4 or even more in some cases) when a background subtraction was performed. The density and temperature show very small variation over time. The filling factor of the moss plasma can vary between 0.1-1 and the path length along which the emission originates is from a few 100 to a few 1000 kms long. By combining the observations recorded by TRACE, EIS and XRT, we find that the moss regions correspond to the foot-points of both hot and warm loops.

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arXiv2009

Are Coronal Loops Isothermal or Multithermal? Yes!

Surprisingly few solar coronal loops have been observed simultaneously with TRACE and SOHO/CDS, and even fewer analyses of these loops have been conducted and published. The SOHO Joint Observing Program 146 was designed in part to provide the simultaneous observations required for in-depth temperature analysis of active region loops and determine whether these loops are isothermal or multithermal. The data analyzed in this paper were taken on 2003 January 17 of AR 10250. We used TRACE filter ratios, emission measure loci, and two methods of differential emission measure analysis to examine the temperature structure of three different loops. TRACE and CDS observations agree that Loop 1 is isothermal with Log T $=$ 5.85, both along the line of sight as well as along the length of the loop leg that is visible in the CDS field of view. Loop 2 is hotter than Loop 1. It is multithermal along the line of sight, with significant emission between 6.2 $<$ Log T $<$ 6.4, but the loop apex region is out of the CDS field of view so it is not possible to determine the temperature distribution as a function of loop height. Loop 3 also appears to be multithermal, but a blended loop that is just barely resolved with CDS may be adding cool emission to the Loop 3 intensities and complicating our results. So, are coronal loops isothermal or multithermal? The answer appears to be yes!

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arXiv2008

On active region loops: Hinode/EIS observations

Coronal loops are fundamental building blocks of the solar active regions and the corona. Therefore, a clear understanding of the physics of coronal loops will help us understand the physics of active region heating in particular and coronal heating in general. This requires a precise measurement of physical quantities such as electron densities and filling factors, temperatures, and flows in coronal loops. In this paper we have carried out an investigation of a spatially well resolved coronal loop using the EIS onboard Hinode to measure the above mentioned physical quantities. Based on this study we find that a nano-flare model could explain most of the observed characteristics of this loop.

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arXiv2008

EIT and TRACE responses to flare plasma

Aims: To understand the contribution of active region and flare plasmas to the $λ$195 channels of SOHO/EIT (Extreme-ultraviolet Imaging Telescope) and TRACE (Transition Region and Coronal Explorer). Methods: We have analysed an M8 flare simultaneously observed by the Coronal Diagnostic Spectrometer (CDS), EIT, TRACE and RHESSI. We obtained synthetic spectra for the flaring region and an outer region using the differential emission measures (DEM) of emitting plasma based on CDS and RHESSI observations and the CHIANTI atomic database. We then predicted the EIT and TRACE count rates. Results: For the flaring region, both EIT and TRACE images taken through the $λ$195 filter are dominated by Fe ${\rm XXIV}$ (formed at about 20 MK). However, in the outer region, the emission was primarily due to the Fe${\rm XII}$, with substantial contributions from other lines. The average count rate for the outer region was within 25% the observed value for EIT, while for TRACE it was a factor of two higher. For the flare region, the predicted count rate was a factor of two (in case of EIT) and a factor of three (in case of TRACE) higher than the actual count rate. Conclusions: During a solar flare, both TRACE and EIT $λ$195 channels are found to be dominated by Fe ${\rm XXIV}$ emission. Reasonable agreement between predictions and observations is found, however some discrepancies need to be further investigated.

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