Paola Testa

The Interface Region Imaging Spectrograph (IRIS)

The Interface Region Imaging Spectrograph (IRIS) small explorer spacecraft provides simultaneous spectra and images of the photosphere, chromosphere, transition region, and corona with 0.33u2009–u20090.4xa0arcsec spatial resolution, two-second temporal resolution, and 1xa0kmu2009s−1 velocity resolution over a field-of-view of up to 175xa0arcsec × 175xa0arcsec. IRIS was launched into a Sun-synchronous orbit on 27 June 2013 using a Pegasus-XL rocket and consists of a 19-cm UV telescope that feeds a slit-based dual-bandpass imaging spectrograph. IRIS obtains spectra in passbands from 1332u2009–u20091358xa0Å, 1389u2009–u20091407xa0Å, and 2783u2009–u20092834xa0Å, including bright spectral lines formed in the chromosphere (Mgxa0ii h 2803xa0Å and Mgxa0ii k 2796xa0Å) and transition region (Cxa0ii 1334/1335xa0Å and Sixa0iv 1394/1403xa0Å). Slit-jaw images in four different passbands (Cxa0ii 1330, Sixa0iv 1400, Mgxa0ii k 2796, and Mgxa0ii wing 2830xa0Å) can be taken simultaneously with spectral rasters that sample regions up to 130xa0arcsec × 175xa0arcsec at a variety of spatial samplings (from 0.33 arcsec and up). IRIS is sensitive to emission from plasma at temperatures between 5000 K and 10 MK and will advance our understanding of the flow of mass and energy through an interface region, formed by the chromosphere and transition region, between the photosphere and corona. This highly structured and dynamic region not only acts as the conduit of all mass and energy feeding into the corona and solar wind, it also requires an order of magnitude more energy to heat than the corona and solar wind combined. The IRIS investigation includes a strong numerical modeling component based on advanced radiative–MHD codes to facilitate interpretation of observations of this complex region. Approximately eight Gbytes of data (after compression) are acquired by IRIS each day and made available for unrestricted use within a few days of the observation.

OBSERVING CORONAL NANOFLARES IN ACTIVE REGION MOSS

The High-resolution Coronal Imager (Hi-C) has provided Fe XII?193? images of the upper transition region moss at an unprecedented spatial (~0.3-0.4) and temporal (5.5?s) resolution. The Hi-C observations show in some moss regions variability on timescales down to ~15 s, significantly shorter than the minute-scale variability typically found in previous observations of moss, therefore challenging the conclusion of moss being heated in a mostly steady manner. These rapid variability moss regions are located at the footpoints of bright hot coronal loops observed by the Solar Dynamics Observatory/Atmospheric Imaging Assembly in the 94 ? channel, and by the Hinode/X-Ray Telescope. The configuration of these loops is highly dynamic, and suggestive of slipping reconnection. We interpret these events as signatures of heating events associated with reconnection occurring in the overlying hot coronal loops, i.e., coronal nanoflares. We estimate the order of magnitude of the energy in these events to be of at least a few 1023 erg, also supporting the nanoflare scenario. These Hi-C observations suggest that future observations at comparable high spatial and temporal resolution, with more extensive temperature coverage, are required to determine the exact characteristics of the heating mechanism(s).

TEMPERATURE DISTRIBUTION OF A NON-FLARING ACTIVE REGION FROM SIMULTANEOUS HINODE XRT AND EIS OBSERVATIONS

We analyze coordinated Hinode X-ray Telescope (XRT) and Extreme Ultraviolet Imaging Spectrometer (EIS) observations of a non-flaring active region to investigate the thermal properties of coronal plasma taking advantage of the complementary diagnostics provided by the two instruments. In particular, we want to explore the presence of hot plasma in non-flaring regions. Independent temperature analyses from the XRT multi-filter data set, and the EIS spectra, including the instrument entire wavelength range, provide a cross-check of the different temperature diagnostics techniques applicable to broadband and spectral data, respectively, and insights into cross-calibration of the two instruments. The emission measure distributions, (EM(T)), we derive from the two data sets have similar width and peak temperature, but show a systematic shift of the absolute values, the EIS (EM(T)) being smaller than the XRT (EM(T)) by approximately a factor two. We explore possible causes of this discrepancy, and we discuss the influence of the assumptions for the plasma element abundances. Specifically, we find that the disagreement between the results from the two instruments is significantly mitigated by assuming chemical composition closer to the solar photospheric composition rather than the often adopted coronal composition. We find that the data do not provide conclusive evidence on the high temperature (log T(K) 6.5) tail of the plasma temperature distribution, however, suggesting its presence to a level in agreement with recent findings for other non-flaring regions.

Hinode/EIS Spectroscopic Validation of Very Hot Plasma Imaged with the Solar Dynamics Observatory in Non-flaring Active Region Cores

We use coronal imaging observations with SDO/AIA, and Hinode/EIS spectral data, to explore the potential of narrow band EUV imaging data for diagnosing the presence of hot (T >~5MK) 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 CaXVII line (at 192.8A) which is one of the few lines in the EIS spectral bands sensitive to hot coronal plasma even outside flares. After careful coalignment of the imaging and spectral data, we compare the morphology in a 3 color image combining the 171, 335, and 94A AIA spectral bands, with the image obtained for CaXVII emission from the analysis of EIS spectra. We find that in the selected active regions the CaXVII emission is strong only in very limited areas, showing striking similarities with the features bright in the 94A (and 335A) AIA channels and weak in the 171A band. We conclude that AIA imaging observations of the solar corona can be used to track hot plasma (6-8MK), and so to study its spatial variability and temporal evolution at high spatial and temporal resolution.

TESTING EUV/X-RAY ATOMIC DATA FOR THE SOLAR DYNAMICS OBSERVATORY

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 A and 131 A 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 {lambda} {approx} 130 A, they significantly underestimate the observed flux in the 50-130 A wavelength range. The model underestimates the observed flux by a variable factor ranging from Almost-Equal-To 1.5, at short wavelengths below {approx}50 A, up to Almost-Equal-To 5-7 in the {approx}70-125 A range. In the AIAmorexa0» bands covered by LETGS, i.e., 94 A and 131 A, we find that the observed flux can be underestimated by large factors ({approx}3 and {approx}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.«xa0less

INVESTIGATING THE RELIABILITY OF CORONAL EMISSION MEASURE DISTRIBUTION DIAGNOSTICS USING THREE-DIMENSIONAL RADIATIVE MAGNETOHYDRODYNAMIC SIMULATIONS

Determining the temperature distribution of coronal plasmas can provide stringent constraints on coronal heating. Current observations with the Extreme ultraviolet Imaging Spectrograph (EIS) on board Hinode and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory provide diagnostics of the emission measure distribution (EMD) of the coronal plasma. Here we test the reliability of temperature diagnostics using three-dimensional radiative MHD simulations. We produce synthetic observables from the models and apply the Monte Carlo Markov chain EMD diagnostic. By comparing the derived EMDs with the true distributions from the model, we assess the limitations of the diagnostics as a function of the plasma parameters and the signal-to-noise ratio of the data. We find that EMDs derived from EIS synthetic data reproduce some general characteristics of the true distributions, but usually show differences from the true EMDs that are much larger than the estimated uncertainties suggest, especially when structures with significantly different density overlap along the line of sight. When using AIA synthetic data the derived EMDs reproduce the true EMDs much less accurately, especially for broad EMDs. The differences between the two instruments are due to the: (1) smaller number of constraints provided by AIA data and (2) broad temperature response function of the AIA channels which provide looser constraints to the temperature distribution. Our results suggest that EMDs derived from current observatories may often show significant discrepancies from the true EMDs, rendering their interpretation fraught with uncertainty. These inherent limitations to the method should be carefully considered when using these distributions to constrain coronal heating.

Fe IX CALCULATIONS FOR THE SOLAR DYNAMICS OBSERVATORY

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.

Monte Carlo Markov chain DEM reconstruction of isothermal plasmas

Smithsonian Astrophysical Observatory, 60 Garden St., Cambridge MA 02138, USAPreprint online version: December 14, 2011ABSTRACTContext.Recent studies carried out with SOHO and Hinode high-resolution spectrometers have shown that the plasma in the off-disksolar corona is close to isothermal. If confirmed, these findi ngs may have significant consequences for theoretical model s of coronalheating. However, these studies have been carried out with diagnostic techniques whose ability to reconstruct the plasma distributionwith temperature has not been thoroughly tested.Aims.In this paper, we carry out tests on the Monte Carlo Markov Chain (MCMC) technique with the aim of determining: 1) itsability to retrieve isothermal plasmas from a set of spectral line intensities, with and without random noise; 2) to what extent can itdiscriminate between an isothermal solution and a narrow multithermal distribution; and 3) how well it can detect multiple isothermalcomponents along the line of sight. We also test the effects of 4) atomic data uncertainties on the results, and 5) the number of ionswhose lines are available for the DEM reconstruction.Methods.We first use the CHIANTI database to calculate synthetic spec tra from different thermal distributions: single isothermalplasmas, multithermal plasmas made of multiple isothermal components, and multithermal plasmas with a Gaussian DEM distributionwith variable width. We then apply the MCMC technique on each of these synthetic spectra, so that the ability of the MCMC techniqueat reconstructing the original thermal distribution can be evaluated. Next, we add a random noise to the synthetic spectra, and repeatthe exercise, in order to determine the effects of random errors on the results. We also we repeat the exercise using a different set ofatomic data from those used to calculate synthetic line intensities, to understand the robustness of the results against atomic physicsuncertainties. The size of the temperature bin of the MCMC reconstruction is varied in all cases, in order to determine the optimalwidth.Results.We find that the MCMC technique is unable to retrieve isotherm al plasmas to better than ∆logT ≃ 0.05. Also, the DEMcurves obtained using lines calculated with an isothermal plasma and with a Gaussian distribution with FWHM of logT ≃ 0.05 arevery similar. Two near-isothermal components can be resolved if their temperature separation is ∆logT = 0.2 or larger. Thus, DEMdiagnostics has an intrinsic resolving power of logT = 0.05. Atomic data uncertainties may significantly a ffect both temperature andpeak DEM values, but do not alter our conclusions. The availability of small sets of lines also does not worsen the performance of theMCMC technique, provided these lines are formed in a wide temperature range.Conclusions.Our analysis shows the present limitations in our ability to identify the presence of strictly isothermal plasmas in stellarand solar coronal spectra.Key words. Methods: data analysis — Techniques: spectroscopic — Sun: c orona — Sun: UV radiation

Neon and oxygen abundances and abundance ratio in the solar corona

In this work we determine the Ne/O abundance ratio from Solar and Heliospheric Observatory (SOHO)/Solar Ultraviolet Measurement of Emitted Radiation (SUMER) off-disk observations of quiescent streamers over the 1996-2008 period. We find that the Ne/O ratio is approximately constant over solar cycle 23 from 1996 to 2005, at a value of 0.099 ± 0.017; this value is lower than the transition region determinations from the quiet Sun used to infer the neon photospheric abundance from the oxygen photospheric abundance. Also, the Ne/O ratio we determined from SUMER is in excellent agreement with in situ determinations from ACE/SWICS. In 2005-2008, the Ne/O abundance ratio increased with time and reached 0.25 ± 0.05, following the same trend found in the slowest wind analyzed by ACE/SWICS. Further, we measure the absolute abundance in the corona for both oxygen and neon from the data set of 1996 November 22, obtaining A o = 8.99 ± 0.04 and A Ne = 7.92 ± 0.03, and we find that both elements are affected by the first ionization potential (FIP) effect, with oxygen being enhanced by a factor of 1.4-2.1 over its photospheric abundance, and neon being changed by a factor of 0.75-1.20. We conclude that the Ne/O ratio is not constant in the solar atmosphere, both in time and at different heights, and that it cannot be reliably used to infer the neon abundance in the photosphere. Also, we argue that the FIP effect was less effective during the minimum of solar cycle 24, and that the Ne/O = 0.25 ± 0.05 value measured at that time is closer to the true photospheric value, leading to a neon photospheric abundance larger than assumed by ≈40%. We discuss the implications of these results for the solar abundance problem, for the FIP effect, and for the identification of the source regions of the solar wind.

The Temperature of Quiescent Streamers during Solar Cycles 23 and 24

Recent in-situ determinations of the temporal evolution of the charge state distribution in the fast and slow solar wind have shown a general decrease in the degree of ionization of all the elements in the solar wind along solar cycles 23 and 24. Such a decrease has been interpreted as a cooling of the solar corona which occurred during the decline and minimum phase of solar cycle 23 from 2000 to 2010. In the present work, we investigate whether spectroscopic determinations of the temperature of the quiescent streamers show signatures of coronal plasma cooling during cycles 23 and 24. We measure the coronal electron density and thermal structure at the base of 60 quiescent streamers observed from 1996 to 2013 by SOHO/SUMER and Hinode/EIS and find that both quantities do now show any significant dependence on the solar cycle. We argue that if the slow solar wind is accelerated from the solar photosphere or chromosphere, the measured decrease in the in-situ wind charge state distribution might be due to an increased efficiency in the wind acceleration mechanism at low altitudes. If the slow wind originates from the corona, a combination of density and wind acceleration changes may be responsible for the in-situ results.