[en] The CHIANTI spectral code consists of an atomic database and a suite of computer programs to calculate the optically thin spectrum of astrophysical objects and carry out spectroscopic plasma diagnostics. The database includes atomic energy levels, wavelengths, radiative transition probabilities, collision excitation rate coefficients, ionization and recombination rate coefficients, as well as data to calculate free-free, free-bound and two-photon continuum emission. All CHIANTI data are critically evaluated in two different ways: by comparing individual atomic data and rates with other calculations and laboratory measurements, and by comparing predicted spectral line intensities with observed spectra from the laboratory (when available) and from astrophysical sources. In this talk, I will introduce the main features of CHIANTI, discuss its current status in light of current and future atomic data needs, describe the past and ongoing activities to benchmark and validate CHIANTI atomic data and rates, and outline future developments of the database. (author)

[en] In the present work we review the thermal structure of the solar atmosphere. We first discuss the main diagnostic techniques used to measure it, outlining their pitfalls and limitations. Then, we review the recent measurements of the thermal structure of the solar atmosphere carried out with the SOHO spectrometers CDS and SUMER. The review shows that the solar upper atmosphere is made by an ensemble of few, nearly isothermal plasmas with fixed temperatures, disconnected from the colder, thermally continuous lower atmosphere.

[en] In the early 1940s it was at last accepted that the temperature of the solar corona is at least 1 MK and varies considerably from region to region throughout the solar activity cycle. It was recognized that during solar minimum periods the electron temperatures of plasmas in polar regions do not exceed 1 MK, but during solar maximum periods the plasma temperatures of highly active regions could be as high as 3 MK. Nevertheless, until recently the consensus among the solar physics community was that coronal temperatures vary among the different regions in a continuous manner. In the present paper we review the evidence showing that solar coronal plasmas (T_e>0.7 MK) are isothermal and their temperature can have only a small set of fixed values

[en] The nonthermal broadening of spectral lines formed in the solar corona is often used to seek evidence of Alfvén waves propagating in the corona. To have a better understanding of the variation of line widths at different altitudes, we measured the line widths of the strong Fe xii 192.4, 193.5, and 195.1 Å and Fe xiii 202.0 Å in an off-limb southern coronal hole up to 1.5 R _⊙ observed by the Extreme Ultraviolet Spectrometer on board the Hinode satellite. We compared our measurements to the predictions from the Alfvén Wave Solar Model (AWSoM) and the SPECTRUM module. We found that the Fe xii and Fe xiii line widths first increase monotonically below 1.1 R _⊙ and then keep fluctuating between 1.1 and 1.5 R _⊙. The synthetic line widths of Fe xii and Fe xiii below 1.3 R _⊙ are notably lower than the observed ones. We found that the emission from a streamer in the line of sight significantly contaminates the coronal hole line profiles even up to 1.5 R _⊙ both in observations and simulations. We suggest that either the discrepancy between the observations and simulations is caused by insufficient nonthermal broadening at the streamer in the AWSoM simulation or the observations are less affected by the streamer. Our results emphasize the importance of identifying the origin of the coronal EUV emission in off-limb observations.

[en] The Arcetri Spectral Code evaluates lines and continuum emission of thin thermal plasmas in the temperature range from 10⁴ K to 10⁸ K and for electron number density lower than 10¹² cm^-3, in the spectral range 1-2000 Aa. For each ion of the most common elements the ionization balance is evaluated and the population level is computed assuming statistical equilibrium between excitation and decay processes. A sampling of the code, that is in the way of upgrading, is available on Mosaic(http://www.arcetri.astro.it)

[en] Solar wind measurements in the heliosphere predominantly comprise protons, alphas, and minor elements in a highly ionized state. The majority of low-charge states, such as He⁺, measured in situ are often attributed to pick-up ions of nonsolar origin. However, through inspection of the velocity distribution functions of near-Earth measurements, we find a small but significant population of He⁺ ions in the normal solar wind whose properties indicate that it originated from the Sun and has evolved as part of the normal solar wind. Current ionization models, largely governed by electron impact and radiative ionization and recombination processes, underestimate this population by several orders of magnitude. Therefore, to reconcile the singly ionized He observed, we investigate the recombination of solar He²⁺ through charge exchange with neutrals from circumsolar dust as a possible formation mechanism of solar He⁺. We present an empirical profile of neutrals necessary for charge exchange to become an effective vehicle to recombine He²⁺ to He⁺ such that it meets observational He⁺ values. We find that the formation of He⁺ is not only sensitive to the density of neutrals but also to the inner boundary of the neutral distribution encountered along the solar wind path. However, further observational constraints are necessary to confirm that the interaction between solar α particles and dust neutrals is the primary source of the He⁺ observations.

[en] The origin and acceleration of the solar wind are still debated. In this paper, we search for signatures of the source region and acceleration mechanism of the solar wind in the plasma properties measured in situ by the Advanced Composition Explorer spacecraft. Using the elemental abundances as a proxy for the source region and the differential velocity and ion temperature ratios as a proxy for the acceleration mechanism, we are able to identify signatures pointing toward possible source regions and acceleration mechanisms. We find that the fast solar wind in the ecliptic plane is the same as that observed from the polar regions and is consistent with wave acceleration and coronal-hole origin. We also find that the slow wind is composed of two components: one similar to the fast solar wind (with slower velocity) and the other likely originating from closed magnetic loops. Both components of the slow solar wind show signatures of wave acceleration. From these findings, we draw a scenario that envisions two types of wind, with different source regions and release mechanisms, but the same wave acceleration mechanism.

[en] We analyze the Cartwheel coronal mass ejection's (CME; 2008 April 9) trajectory in the low corona with the ForeCAT model. This complex event presented a significant rotation in the low corona and a reversal in its original latitude direction. We successfully reproduce the observed CME’s trajectory (latitude and longitude deflection) and speed. Through a ${\mathrm{\chi <!--\; ??\; -->}}^2$ test, we are able to constrain the CME’s mass to (2.3−3.0) × 10¹⁴ g and the CME’s initial shape. We are able to constrain the expansion of the CME as well: the angular width linearly increases until 2.1 $R_{\odot <!--\; ???\; -->}$, and is constant afterward. In order to match the observed latitude, we include a non-radial initial speed of −42 km s⁻¹. Despite allowing the CME to rotate in the model, the magnetic forces of the solar background are not able to reproduce the observed rotation. We suggest that the complex reversal in latitude and the significant rotation of the Cartwheel CME can be justified with an asymmetrical reconnection event that ejected the CME non-radially and also initiated its rotation.

[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] The solar atmosphere contains thermal plasma at a wide range of temperatures. This plasma is often quantified, in both observations and models, by a differential emission measure (DEM). The DEM is a distribution of the thermal electron density squared over temperature. In observations, the DEM is computed along a line of sight, while in the modeling it is over an elementary volume element (voxel). This description of the multithermal plasma is convenient and widely used in the analysis and modeling of extreme ultraviolet emission, which has an optically thin character. However, there is no corresponding treatment in the radio domain, where the optical depth of emission can be large, more than one emission mechanism is involved, and plasma effects are important. Here, we extend the theory of thermal gyroresonance and free–free radio emissions in the classical single-temperature Maxwellian plasma to the case of a multitemperature plasma. The free–free component is computed using the DEM and temperature-dependent ionization states of coronal ions, contributions from collisions of electrons with neutral atoms, the exact Gaunt factor, and the magnetic field effect. For the gyroresonant component, another measure of the multitemperature plasma is used, which describes the distribution of the thermal electron density over temperature. We give representative examples demonstrating important changes in the emission intensity and polarization due to the effects considered. The theory is implemented in available computer code.