John T. Mariska

High-precision density measurements in the solar corona I. Analysis methods and results for Fe XII and Fe XIII

Aims. The EUV Imaging Spectrometer (EIS) instrument on board the Hinode satellite has access to some of the best coronal density diagnostics, and the high sensitivity of the instrument now allows electron number density, Ne, measurements to an unprecedented precision of up to ±5% in active regions. This paper gives a thorough overview of data analysis issues for the best diagnostics of Fe xii and Fe xiii and assesses the accuracy of the measurements. Methods. Two density diagnostics each from Fe xii (λ186.88/λ195.12 and λ196.64/λ195.12) and Fe xiii (λ196.54/λ202.04 and λ203.82/λ202.04) are analysed in two active region datasets from 2007 May 3 and 6 that yield densities in the range 8.5 ≤ log (Ne/cm −3 ) ≤ 11.0. The densities are derived using v5.2 of the CHIANTI atomic database. Blending, line fitting, and instrumental issues are discussed, and line fit parameters presented. Results. The Fe xii and Fe xiii diagnostics show broadly the same trend in density across the active region, consistent with their similar temperatures of formation. However, the high precision of the EIS measurements demonstrates significant discrepancies of up to 0.5 dex in derived log Ne values, with Fe xii always giving higher densities than Fe xiii. The discrepancies may partly be due to real physical differences between the emitting regions of the two plasmas, but the dominant factor lies in the atomic models of the two ions. Two specific problems are identified for Fe xii λ196.64 and Fe xiii λ203.82: the former is found to be underestimated in strength by the CHIANTI atomic model, while the high-density limit of the λ203.82/λ202.04 ratio appears to be inaccurate in the CHIANTI atomic model. The small grating tilt of the EIS instrument is found to be very significant when deriving densities from emission lines separated by more than a few angstroms. Revised wavelengths of 196.518 ± 0.003 A and 196.647 ± 0.003 A are suggested for the Fe xiii λ196.54 and Fe xii λ196.64 lines, respectively.

Flows and Nonthermal Velocities in Solar Active Regions Observed with the EUV Imaging Spectrometer on Hinode: A Tracer of Active Region Sources of Heliospheric Magnetic Fields?

From Doppler velocity maps of active regions constructed from spectra obtained by the EUV Imaging Spectrometer (EIS) on the Hinode spacecraft we observe large areas of outflow (20-50 km s -->−1) that can persist for at least a day. These outflows occur in areas of active regions that are faint in coronal spectral lines formed at typical quiet-Sun and active region temperatures. The outflows are positively correlated with nonthermal velocities in coronal plasmas. The bulk mass motions and nonthermal velocities are derived from spectral line centroids and line widths, mostly from a strong line of Fe XII at 195.12 A. The electron temperature of the outflow regions estimated from an Fe XIII to Fe XII line intensity ratio is about -->(1.2–1.4) × 106 K. The electron density of the outflow regions derived from a density-sensitive intensity ratio of Fe XII lines is rather low for an active region. Most regions average around -->7 × 108 cm -->−3, but there are variations on pixel spatial scales of about a factor of 4. We discuss results in detail for two active regions observed by EIS. Images of active regions in line intensity, line width, and line centroid are obtained by rastering the regions. We also discuss data from the active regions obtained from other orbiting spacecraft that support the conclusions obtained from analysis of the EIS spectra. The locations of the flows in the active regions with respect to the longitudinal photospheric magnetic fields suggest that these regions might be tracers of long loops and/or open magnetic fields that extend into the heliosphere, and thus the flows could possibly contribute significantly to the solar wind.

Evolving Active Region Loops Observed with the Transition Region and Coronal explorer. II. Time-dependent Hydrodynamic Simulations

Observations with the Transition Region and Coronal Explorer (TRACE) have revealed a new class of active region loops. These loops have relatively flat filter ratios, suggesting approximately constant temperatures near 1 MK along much of the loop length. The observed apex intensities are also higher than static, uniformly heated loop models predict. These loops appear to persist for much longer than a characteristic cooling time. Recent analysis has indicated that these loops first appear in the hotter Fe XV 284 A or Fe XII 195 A filters before they appear in the Fe IX/Fe X 171 A filter. The delay between the appearance of the loops in the different filters suggests that the loops are impulsively heated and are cooling when they are imaged with TRACE. In this paper we present time-dependent hydrodynamic modeling of an evolving active region loop observed with TRACE. We find that by modeling the loop as a set of small-scale, impulsively heated filaments we can generally reproduce the spatial and temporal properties of the observed loop. These results suggest that both dynamics and filamentation are crucial to understanding the observed properties of active region loops observed with TRACE.

Coronal Plasma Motions near Footpoints of Active Region Loops Revealed from Spectroscopic Observations with Hinode EIS

The solar active region 10938 has been observed from the disk center to the west limb with the Hinode EUV Imaging Spectrometer. In the disk-center observation, subsonic upflow motions of tens of km s -->−1 and enhanced nonthermal velocities have been found near the footpoints of the active region loops assuming a single Gaussian approximation for the emission-line profiles. When the same part of the active region is observed near the limb, both upflows and enhanced nonthermal velocities essentially decrease. There is a strong correlation between Doppler velocity and nonthermal velocity. Significant deviations from a single Gaussian profile are found in the blue wing of the line profiles for the upflows. These suggest that there are unresolved high-speed upflows. We discuss the implications for coronal heating mechanisms.

Transition Region and Coronal Explorer and Soft X-Ray Telescope Active Region Loop Observations: Comparisons with Static Solutions of the Hydrodynamic Equations

Active region coronal loop observations with broadband X-ray instruments have often been found to be consistent with the predictions of static loop models. Recent observations in the EUV, however, have discovered a class of active region loops that are difficult to reconcile with static loop models. In this paper, we take a comprehensive look at how coronal loops compare with static models. We select 67 loops with a large range of apex temperatures and half-lengths observed with either the Transition Region and Coronal Explorer or the Soft X-Ray Telescope. We compare these observations to static loop models using both uniform and nonuniform heating. We find that only 2 of the 67 loops are fully consistent with static solutions with uniform heating and a filling factor of unity. We further find that long, cool ( 3 MK) loops are as much as 63 times underdense when compared to the static solutions with uniform heating. We then consider the possibility that the disparity in the density could be due to steady, nonuniform heating along the loop and find that footpoint heating can increase densities only by a factor of 3 over density solutions with uniform heating while loop-top heating results in density solutions that are, at most, a factor of 2.5 smaller than the density solutions with uniform heating. Only 19 of the 67 loops in this data set could be fully consistent with hydrodynamic solutions with steady heating. Hence, we conclude that static loop models are poor representations of most active region loops.

The Bragg Crystal Spectrometer for SOLAR-A

The Bragg Crystal Spectrometer (BCS) is one of the instruments which makes up the scientific payload of the SOLAR-A mission. The spectrometer employs four bent germanium crystals, views the whole Sun and observes the resonance line complexes of H-like Fexxvi and He-like Fexxv, Caxix, and Sxv in four narrow wavelength ranges with a resolving power (λ/Δλ) of between 3000 and 6000. The spectrometer has approaching ten times better sensitivity than that of previous instruments thus permitting a time resolution of better than 1 s to be achieved. The principal aim is the measurement of the properties of the 10 to 50 million K plasma created in solar flares with special emphasis on the heating and dynamics of the plasma during the impulsive phase. This paper summarizes the scientific objectives of the BCS and describes the design, characteristics, and performance of the spectrometers.

Numerical simulations of impulsively heated solar flares

The response of a model solar atmosphere to heating by an electron beam has been studied for electron beam flux spectra which are power laws with low-energy knees (rising linearly with time to a peak at 30 s and then falling linearly to 0 at 60 s) ranging from 10 to 20 keV. The results indicate that high peak electron beam fluxes, low-energy knees, and larger spectral indices all move the atmospheric response toward greater enhancements of the parameters in the coronal regions of the atmosphere. Coronal responses can thus be used as a diagnostic of the parameters of the electron beam. 23 refs.

A new model of solar EUV irradiance variability: 1. Model formulation

We present a new model of solar irradiance variability at extreme ultraviolet wavelengths (EUV, 50–1200 A). In this model, quiet Sun, coronal hole, and active region intensities for optically thin emission lines are computed from emission measure distributions determined from spectrally and spatially resolved observations. For optically thick emission lines and continua, empirical values are used. The contribution of various solar features to the spectral irradiance variability is determined from a simple model of limb-brightening and full-disk solar images taken at the Big Bear Solar Observatory and by the Soft X-Ray Telescope on Yohkoh. To extend our irradiance model beyond the time period covered by the available images, we use correlations with proxies for solar activity. Comparisons with the available irradiance data from the Atmospheric Explorer E (AE-E) spacecraft show that our model is capable of reproducing the rotational modulation of the EUV irradiance near solar maximum. The AE-E data, however, show systematically more solar cycle variability than our model estimates.

Nonthermal velocities in solar active regions observed with the extreme-ultraviolet imaging spectrometer on Hinode

We discuss nonthermal velocities in an active region as revealed by the Extreme-Ultraviolet Imaging Spectrometer (EIS) on the Hinode spacecraft. The velocities are derived from spectral line profiles in the extreme-ultraviolet (EUV) from a strong line of Fe XII at 195.12 A by fitting each line profile to a Gaussian function. We compare maps of the full width at half-maximum values, the Fe XII spectral line intensity, the Fe XII Doppler shift, the electron temperature, and electron density. We find that the largest widths in the active region do not occur in the most intense regions, but seem to concentrate in less intense regions, some of which are directly adjacent to coronal loops, and some of which concentrate in regions which also exhibit relative Doppler outflows. The increased widths can also occur over extended parts of the active region.

Optics and mechanisms for the Extreme-Ultraviolet Imaging Spectrometer on the Solar-B satellite.

The Extreme-Ultraviolet Imaging Spectrometer (EIS) is the first of a new generation of normal-incidence, two-optical-element spectroscopic instruments developed for space solar extreme-ultraviolet astronomy. The instrument is currently mounted on the Solar-B satellite for a planned launch in late 2006. The instrument observes in two spectral bands, 170-210 A and 250-290 A. The spectrograph geometry and grating prescription were optimized to obtain excellent imaging while still maintaining readily achievable physical and fabrication tolerances. A refined technique using low ruling density surrogate gratings and optical metrology was developed to align the instrument with visible light. Slit rasters of the solar surface are obtained by mechanically tilting the mirror. A slit exchange mechanism allows selection among four slits at the telescope focal plane. Each slit is precisely located at the focal plane. The spectrograph imaging performance was optically characterized in the laboratory. The resolution was measured using the Mg iii and Ne iii lines in the range of 171-200 A. The He ii line at 256 A and Ne iii lines were used in the range of 251-284 A. The measurements demonstrate an equivalent resolution of ~2 arc sec? on the solar surface, in good agreement with the predicted performance. We describe the EIS optics, mechanisms, and measured performance.