Resik: A Bent Crystal X-ray Spectrometer for Studies of Solar Coronal Plasma Composition

We describe the RESIK (REntgenovsky Spektrometr s Izognutymi Kristalami) instrument, consisting of two double-channel X-ray spectrometers, designed to observe solar active region and flare plasmas. RESIK is one of the instruments making up the scientific payload of the Russian CORONAS-F solar mission. The uncollimated spectrometer uses two silicon and two quartz bent crystals observing flare, active region and coronal spectra in four wavelength bands with a resolving power (/ ) of 1000. The wavelength coverage, 3.3–6.1 Å, includes emission lines of Si, S, Cl, Ar, and K and in the third diffraction order, the wavelength range includes He-like Fe lines (1.85 Å) and Ni lines (1.55 Å) with dielectronic satellites, emitted during intense, hot flares. The instrument is believed to be the best calibrated space-borne crystal spectrometer flown to date. The spectrometer dynamically adjusts the data gathering intervals from 1 s to 5 minutes, depending on the level of solar X-ray emission at the time of observation. The principal aims of RESIK are the measurements of relative and absolute element abundances in the emitting plasma and the temperature distribution of plasma (differential emission measure) over the temperature interval 3 and 50 MK. This paper summarizes the scientific objectives of RESIK and describes the design, characteristics, and performance of the instrument.     

Solar gravitational redshift

Wavelengths of solar spectrum lines should be shifted toward the red by the Sun's gravitational field as predicted by metric theories of gravity according to the principle of equivalence. Photographic wavelengths of 738 solar Fe i lines and their corresponding laboratory wavelengths have been studied. The measured solar wavelength minus the laboratory wavelength (_observed) averaged for the strong lines agrees well with the theoretically predicted shift (_theoretical). Studies show that the departures depend on line strength. No dependence of the departures on wavelength was found within the existing data.By studying strong lines over a wide spectral range, velocity shifts caused by the complex motions in the solar atmosphere seem to affect the results in a minimal fashion.     

Observing the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping

The Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope has commenced science observations of the Sun starting in late 2016. Since the Sun is much larger than the field of view of individual ALMA dishes, the ALMA interferometer is unable to measure the background level of solar emission when observing the solar disk. The absolute temperature scale is a critical measurement for much of ALMA solar science, including the understanding of energy transfer through the solar atmosphere, the properties of prominences, and the study of shock heating in the chromosphere. In order to provide an absolute temperature scale, ALMA solar observing will take advantage of the remarkable fast-scanning capabilities of the ALMA 12 m dishes to make single-dish maps of the full Sun. This article reports on the results of an extensive commissioning effort to optimize the mapping procedure, and it describes the nature of the resulting data. Amplitude calibration is discussed in detail: a path that uses the two loads in the ALMA calibration system as well as sky measurements is described and applied to commissioning data. Inspection of a large number of single-dish datasets shows significant variation in the resulting temperatures, and based on the temperature distributions, we derive quiet-Sun values at disk center of 7300 K at \(\lambda = 3~\mbox{mm}\) and 5900 K at \(\lambda = 1.3~\mbox{mm}\). These values have statistical uncertainties of about 100 K, but systematic uncertainties in the temperature scale that may be significantly larger. Example images are presented from two periods with very different levels of solar activity. At a resolution of about \(25''\), the 1.3 mm wavelength images show temperatures on the disk that vary over about a 2000 K range. Active regions and plages are among the hotter features, while a large sunspot umbra shows up as a depression, and filament channels are relatively cool. Prominences above the solar limb are a common feature of the single-dish images.     

Microscopic filamentation due to electrothermal instability and plasma heating in time-dependent solar transition layer

We have investigated nonlinear equilibrium states of a microscopic current filamentation (electrothermal instability) in solar atmosphere. The microscopic filamentation instability will develop for transition zone ion temperature plasmas, provided T _e/T_i > 1, where T _e and T _i are the electron and ion temperatures, respectively. Since the temperature radio for a steady-state solar atmosphere is approximately unity, the electrothermal instability will develop only in a time-dependent solar atmosphere. Indeed, such a condition is provided by time-dependent currents, which seem to exist in many magnetic loops as recent analysis by Porter et al. (1987) indicates. When the onset condition for the electrothermal instability is satisfied, the instability drives a current filamentation to a nonlinear equilibrium state with a spatially periodic electron temperature variation with the wavelength comparable to several ion-Larmor radii. The amplitude of the periodic temperature variation may be so large that the transition layer temperature and coronal temperature plasmas may exist within several Larmor radii — coexistence of the transition zone and corona within the same macro-volume.     

Analysis of the high-resolution x-ray spectra obtained aboard the intercosmos 16 satellite

The Bragg-type, flat ADP crystal spectrometer, launched on board the INTERCOSMOS 16 satellite has been used for measurements of the X-ray spectra emitted from solar active region plasmas. During the period of the instrument operation (August–September, 1976) only a few active regions were present on the Sun (minimum of the solar activity). About 60 spectra have been registered. In the present paper using a spectrum averaged over 20 scans, we measured the wavelengths corresponding to the statistically significant spectral features seen in this spectrum in the wavelength range 9.14–9.33 Å. By comparison with the calculated line wavelengths and intensities predicted in the framework of the thermal model of the average active region, we performed the identification of these features. Besides rather prominent resonance, intercombination, and forbidden lines of the He-like ion Mg xi, it was possible to identify the satellite lines which correspond to 1s ² nl - 1s2p nl transitions from the states with n = 2, 3, and 4. The present paper is the first in a series dealing with the INTERCOSMOS 16 Mg xi spectra.     

Electron Excitation Rates in the Solar Corona for non-Maxwellian Electron Distributions

The influence of electron non-Maxwellian distributions (power and -distribution) on the electron excitation rate in the solar corona is demonstrated. It is shown that the deviations in electron excitation rate are sufficient to affect intensities of spectral lines. As an example the diagnostics of a power-law distribution are demonstrated for a simplified calculation of the resonance lines of Fexxiv, Fexxv and Fexxvi. The results can be used in diagnosing solar flare plasmas, where the deviations of the electron distribution from a Maxwellian distribution can be large.     

Wavelength Shifts in the Solar Photospheric Spectrum

Wavelength shifts converted to velocities between solar lines observed at disc center and laboratory wavelengths of Fei, Feii, Tii, Nii, and Fei lines in the near infrared are plotted as a function of the logarithm of their solar equivalent width in milliångstroms. The need for wavelengths based on the wavelength standards is stressed. A comparison of photographic Fei solar wavelength is shown to agree, on the average, with Fourier Transform Spectrometer solar wavelengths within less than 0.5 milliångstroms. Using Balthasar's limb effect tables we convert the disc center velocities to limb velocities and find, though the scatter is large, that there is little evidence for a super-gravitational red shift.     

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?α 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 _⊙ 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 _⊙, the precise value being strongly dependent on the coronal temperature profile.     

A solar spectrum for PFS data analysis

A measured calibrated solar radiance in the range 1.2-, with the spectral sampling of does not exist. When studying the measured Planetary Fourier Spectrometer (PFS) spectra of the Earth's or Mars's atmosphere we discover that the most used solar spectrum contains several important errors. Here we present a “calibrated” solar radiance in the wavelength range 1.2-, with the spectral resolution of PFS , which we are going to use for studying Martian spectra. This spectrum has been assembled using measurements from Kitt Peak and from ATMOS Spacelab experiment (uncalibrated high resolution) and theoretical results, together with low resolution calibrated continuum. This is the best we can have in this moment to be used with PFS, while waiting to have good solar calibrated radiances. Examples of solar lines at Mars are given.     

Spectra of solar flares from 8.5 Å to 16 Å

X-ray spectra of solar flares in the spectral range from 8.5 Å to 16 Å have been obtained from a Naval Research Laboratory crystal spectrometer flown on the sixth Orbiting Solar Observatory (OSO-6). A list of emission features is presented and tentative identifications of some of the features are suggested. The time-behavior of the emission lines during flares is discussed, and the possibility of determining electron densities in flare plasmas using density sensitive lines of highly ionized iron is considered. Approximate calculations are performed for a density sensitive line of Fexxii.     

Linear MHD Wave Propagation in Time-Dependent Flux Tube

MHD waves and oscillations in sharply structured magnetic plasmas have been studied for static and steady systems in the thin tube approximation over many years. This work will generalize these studies by introducing a slowly varying background density in time, in order to determine the changes to the wave parameters introduced by this temporally varying equilibrium, i.e. to investigate the amplitude, frequency, and wavenumber for the kink and higher order propagating fast magnetohydrodynamic wave in the leading order approximation to the WKB approach in a zero-β plasma representing the upper solar atmosphere. To progress, the thin tube and over-dense loop approximations are used, restricting the results found here to the duration of a number of multiples of the characteristic density change timescale. Using such approximations it is shown that the amplitude of the kink wave is enhanced in a manner proportional to the square of the Alfvén speed, $V_{\mathrm{A}}^{2}$ . The frequency of the wave solution tends to the driving frequency of the system as time progresses; however, the wavenumber approaches zero after a large multiple of the characteristic density change timescale, indicating an ever increasing wavelength. For the higher order fluting modes the changes in amplitude are dependent upon the wave mode; for the m=2 mode the wave is amplified to a constant level; however, for all m≥3 the fast MHD wave is damped within a relatively small multiple of the characteristic density change timescale. Understanding MHD wave behavior in time-dependent plasmas is an important step towards a more complete model of the solar atmosphere and has a key role to play in solar magneto-seismological applications.     

Some considerations from the direct comparison between the observations and the theory of solar disk polarization

Simple considerations of observed variation with wavelength of polarization on the solar disc, computed continuum polarization, and estimated line polarization have led to the following simple conclusions (more qualitative than quantitative); (a) The metal abundances in the photosphere are five times larger than the classical values adopted in the BCA model of the solar photosphere. (b) The depolarization factor k in lines is an increasing function of wavelength. (c) Assuming that k varies as ², an additional polarization (which can be either solar or instrumental) has to be taken into account.     

Spike observations in flares in China

Variations on short time-scales have been found in solar flares at different wavelengths. Millisecond scale radio spikes are a quickly developing area of solar radio astronomy. The solar radio astronomy group of Beijing Astronomical Observatory (BAO) has found fine structures of microwave bursts with millisecond time-scale at 2840 MHz. In this paper, we briefly summarize the observations. A joint-observation network for observing solar radio bursts with high time resolution has also been established. The equipment in the network covers a frequency domain of more than 10:1, including 1.3, 2.0, 6, 10, 15, 20 cm, and meter wavelengths. In particular, a multi-channel polarimeter with super-fast sampling (10 s) at 2600 MHz, an intensity interferometer with 1 ms sampling rate at 6 cm wavelength, and an auto-correlation radio spectrograph with 8 ms time constant at 21 cm wavelength are being established. We pay close attention to research on the spike emission features over wide bands, and their relationship to special characteristics in other spectral ranges.     

Atmospheric refraction of solar neutrons during the event of 24 May 1990

We revise the published neutron monitor raw data for the increase caused by the solar neutron event of the 24 May 1990. With these data we calculate the attenuation length, , of solar neutrons in the Earth's atmosphere assuming either a minimum path as given by the spread of elastically scattered neutrons, or using the minimum mass path estimated by Smart, Shea, and O'Bren (1995) due to an atmospheric refraction effect. In both cases reduces to a value around 100 g cm^–2, which is more in accordance with data on neutron cross-sections (Shibata, 1994). These two phenomenological calculations suggest that solar neutrons do not propagate in straight lines in the atmosphere. The previous estimate of the attenuation length, =208 g cm^–2, was calculated assuming straight-ahead transport (Smart, Shea, and O'Bren, 1995). Dorman, Valdes-Galicia, and Dorman (1999) performed a numerical simulation and an analytical approximation to the problem of solar neutron scattering and attenuation in the Earth's atmosphere. These solutions incorporate the refraction effect as a natural consequence of the greater absorption experienced by neutrons scattered to large zenith angles. They are able to reproduce the normalised observed counting rates of neutron monitors for this event.     

Line-intensities in the photosphere-chromosphere transition region

We present results from eclipse spectra in the wavelength region 4588–4682 Å, taken during the eclipse of February 15, 1961 at Bra, Yugoslavia. Absolute line intensities have been determined in the photosphere-chromosphere transition region. The method of reduction is discussed in Section 2 and the observed variation of the total intensity as a function of the height is shown for a number of lines in Figures 5–8. The results for the Fe i lines are compared with computations (as described in Paper I: van Dessel, 1970) for various solar model atmospheres (Section 4). The model, which combines Holweger's (1967) temperature distribution for the excitation temperature with the HSRA model (Gingerich et al., 1971) for the electron temperature, yields a better agreement than all pure LTE-models.     

太阳大气中动力学阿尔文波的产生与加热研究进展

动力学阿尔文波是垂直波长接近离子回旋半径或者电子惯性长度的色散阿尔文波.由于波的尺度接近粒子的动力学尺度,动力学阿尔文波在太阳和空间等离子体加热、加速等能化现象中起重要作用.因此,动力学阿尔文波通常被认为是日冕加热的候选者.本研究工作深入、系统地调研了太阳大气中动力学阿尔文波的激发和耗散机制.基于日冕等离子体环境,介绍了几种常见的动力学阿尔文波激发机制:温度各向异性不稳定性、场向电流不稳定性、电子束流不稳定性、密度非均匀不稳定性以及共振模式转换.还介绍了太阳大气中动力学阿尔文波的耗散机制,并讨论了这些耗散机制对黑子加热、冕环加热以及冕羽加热的影响.不仅为认识太阳大气中动力学阿尔文波的驱动机制、动力学演化特征以及波粒相互作用提供合理的理论依据,而且有助于揭示日冕等离子体中能量储存和释放、粒子加热等能化现象的微观物理机制.     

Determination of solar Doppler widths by Goldberg's method

The so-called Goldberg or profile intercomparison method of determining Doppler widths from spectral line profiles was used to analyze a group of high quality solar profiles which were observed with the McMath solar telescope at the Kitt Peak National Observatory. The observations consisted of profiles of eight lines of calcium and chromium, made at either four or five positions on the solar disk.A depth dependence of solar microturbulence could not be derived by applying the Goldberg method to a single pair of lines at a single disk position. When the depth dependence was determined from line pairs at different disk positions, a depth dependent turbulence model was obtained. The final model was anisotropic with radial and tangential components increasing with depth in the atmosphere.Now at Teledyne Brown Engineering, Huntsville, Alabama.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Review of optical observations of solar flares

The recent observations of solar flares, made with a Lyot filter and a spectrograph in H, HeD3, higher Balmer lines, metallic lines, and continuum, are discussed. It is important to study the energy supply of non thermal particle/ conduction/ irradiation into the lower atmosphere from the optical observations with high temporal and spatial resolutions. Simultaneous observations from ground-based observatories and instruments on board satellites are necessary for understanding flare plasma of low and high energy.     

Thermal and density structure of polar plumes

Normal incidence multilayer coated EUV/XUV optical systems provide a powerful technique for the study of the structure of the solar corona. Such systems permit the imaging of the full solar disk and corona with high angular resolution in narrow wavelength bands that are dominated by a single line or a line multiplet excited over a well defined range of temperatures. We have photometrically analysed, and derived temperature and density information from, images of polar plumes obtained with a multilayer Cassegrain telescope operating in the wavelength interval = 171 to 175 , which is dominated by FeIX and FeX emission. This observation was obtained in October 1987, and is the first high resolution observation of an astronomical object obtained with normal incidence multilayer optics techniques. We find that photometric data taken from this observation, applied to a simple, semi-empirical model of supersonic solar wind flow, are consistent with the idea that polar plumes are a source of the solar wind. However, we are not able to uniquely trace high speed streams to polar plumes. The temperatures that we observed are typically 1 500 000 K for both the plumes and the interplume regions, with the plume temperatures slightly higher than those of the surrounding atmosphere. Typical electron densities of the plume and interplume regions, respectively, are 5 × 10⁹ cm^–3 and 1 × 10⁸ cm^–3 at the limb of the Sun.     

FORMATION OF Ca II LINES IN SOLAR PROMINENCES

We compute the profiles of the resonance lines and infrared triplet of ionized calcium emitted by some representative models of prominences. These models consist of plane-parallel slabs of different temperatures, pressures and thicknesses, standing vertically above the solar surface. These slabs are assumed to be observed at the limb, and to be perpendicular to the line of sight. They are irradiated by the Sun on both sides, and the incoming intensities, at every relevant wavelength, are taken from observations. The model atom includes 3 stages of ionization (Cai, Caii and Caiii) with 5 discrete levels for Caii. We study the relations between emitted intensities and physical properties of the slabs, and compare the intensities emitted in Caii and hydrogen lines for the same set of models. As a result of ionization, the intensity decreases more rapidly with temperature in Caii lines than in Hi lines, so that the ratio of Caii to Hi line intensities may be used as a temperature indicator. The intensity ratio between resonance and infrared lines of Caii depends principally on the optical thickness of the structure. At high pressure and low temperature, the ratio Caii 8542Å/H is found to increase with pressure. This behaviour, which is due to the saturation of H, is opposite to that found by Heasley and Milkey (1978) for low pressures (optically thin structures).