The NRL SOLRAD X-ray detectors: A summary of the observations and a comparison with the SMS/GOES detectors

The Naval Research Laboratory flew solar X-ray ionization chamber detectors on a series of Solar Radiation (SOLRAD) satellites from 1960 through 1979. The flare responses of the SOLRAD 11 detectors are compared with those of the similar NOAA SMS/GOES detectors during two periods of common observations. The nominal GOES fluxes exceed those of SOLRAD 11 by a factor of 1.5–2 in the 0.5–4 Å band, but fall below those of SOLRAD by a factor of 2–4 in the 1–8 Å band. Significant passband differences account for these relationships between the detector responses. Since the X-ray detectors are standardized among the various SOLRAD satellites, and all detectors are closely matched among the various SMS/GOES satellites, these conversion factors allow the SOLRAD flare observations to serve as proxies for GOES X-ray observations prior to the GOES era. We summarize the detector characteristics and data sources of the 0.5–3 Å and 1–8 Å detectors for the SOLRAD series.     

Updated Expressions for Determining Temperatures and Emission Measures from Goes Soft X-Ray Measurements

{We investigate the conversion of the 0.5–4 and 1–8 Å soft X-ray flux measurements made by detectors on the Geostationary Operational Environmental Satellites (GOES) into temperature and emission measures of coronal plasma using modern spectral models and modern understanding of coronal abundances. In particular, the original analysis by Thomas, Starr and Crannell (1985) is updated to take into account the realization that coronal abundances may be quite different from photospheric abundances. An important result of this analysis is that the derived temperatures and emission measures depend strongly on the assumed abundances even at high temperatures where continuum rather than spectral lines dominates the Sun’s X-ray spectrum. This occurs because the higher coronal abundances mean that most of the continuum is due to free–bound emission processes, not free–free emission, and thus is abundance-dependent. We find significant differences between modern calculations of the temperature response of the flux measurements and the versions currently in use: for a typical flare, emission measures may be up to a factor of 4 smaller than the current software suggests. Derived temperatures are similar for both photospheric and coronal abundances for cool flares (e.g., 15 MK), but for hot flares (e.g., 35 MK) coronal abundances can lead to significantly (~25%) lower temperatures being derived.     

Localization of the source of flare X-ray emission during the eclipse of 7 March, 1970

An occultation of X-ray emission from a solar flare occurred during the eclipse of 7 March, 1970 and was observed by an NRL instrument aboard the OSO-5 satellite. Ionization chamber photometers covering the wavelength ranges 0.5–3 Å, 1–8 Å, and 8–16 Å provided flux measurements once every 15 s providing a spatial resolution of 20 arc sec at the solar surface. Within this limitation the X-ray flare was observed to be confined within a region 136 000 km in one dimension.However, the measurements indicate the existence of a denser core 54 000 km wide in the direction of advance of the Moon's limb. Comparison of these results with X-ray photographs of flare regions are made and a model for the development of the soft X-ray flare is proposed.     

Solar soft X-ray flare spectra from OSO-4

Solar flare spectral data, covering the wavelength range 0.7–8.5 Å, are derived from the NRL Bragg crystal spectrometers aboard OSO-4. A detailed analysis of the soft X-ray spectra for the 3b flare of 16 November 1967 (2140 UT) is presented, and it is found that electron temperatures derived from continua and emission lines are compatible with a two or more component plasma, differing in temperature by 6–10 × 10⁶K.     

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.     

Simultaneous measurements of EUV and soft X-ray solar flare emission

Broadband sensors aboard the Naval Research Laboratory's SOLRAD 11 satellites measured solar emission in the 0.5 to 3 Å, 1 to 8 Å, 8 to 20 Å, 100 to 500 Å, 500 to 800 Å, and 700 to 1030 Å bands between March 1976 and October 1979. Measurements of EUV and soft X-ray emission from a large number of solar flares were obtained. Although solar flare measurements in the soft X-ray bands are continuously made and used as a standard of a flare's geophysical significance, direct measurements of flare EUV emission are quite rare. We present measurements of the X-ray and EUV emission from several flares with special emphasis on the relative EUV response associated with flares in different categories determined by 1 to 8 Å soft X-ray flux. An example of a flare exhibiting an impulsive (nonthermal) phase is included.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 Semptember 1980, Scheveningen, The Netherlands.     

X-ray and extreme ultraviolet (1–400 Å) spectroscopy of the sun,from OSO-III

X-ray spectra of the sun have been obtained during solar flares. New emission lines are observed in the spectral range from 1.3 Å to 3 Å, and 8 Å–20 Å, the most intense of the new emission features being tentatively attributed to optical transitions in high stages of ionization of iron (Fexxv through Fexx). Studies of the variability of these lines during flares provide new information of the development of a hot plasma in the initial stages of the flare event.     

FIRST RESULTS OF THE SUMER TELESCOPE AND SPECTROMETER ON SOHO – I. Spectra and Spectroradiometry

SUMER – the Solar Ultraviolet Measurements of the Emitted Radiation instrument on the Solar and Heliospheric Observatory (SOHO) – observed its first light on January 24, 1996, and subsequently obtained a detailed spectrum with detector B in the wavelength range from 660 to 1490 Å (in first order) inside and above the limb in the north polar coronal hole. Using detector A of the instrument, this range was later extended to 1610 Å. The second-order spectra of detectors A and B cover 330 to 805 Å and are superimposed on the first-order spectra. Many more features and areas of the Sun and their spectra have been observed since, including coronal holes, polar plumes and active regions. The atoms and ions emitting this radiation exist at temperatures below 2 × 10⁶ K and are thus ideally suited to investigate the solar transition region where the temperature increases from chromospheric to coronal values. SUMER can also be operated in a manner such that it makes images or spectroheliograms of different sizes in selected spectral lines. A detailed line profile with spectral resolution elements between 22 and 45 mÅ is produced for each line at each spatial location along the slit. From the line width, intensity and wavelength position we are able to deduce temperature, density, and velocity of the emitting atoms and ions for each emission line and spatial element in the spectroheliogram. Because of the high spectral resolution and low noise of SUMER, we have been able to detect faint lines not previously observed and, in addition, to determine their spectral profiles. SUMER has already recorded over 2000 extreme ultraviolet emission lines and many identifications have been made on the disk and in the corona.     

Extreme ultraviolet spectrum of the solar flare of 2114 UT March 27, 1967

Scanning spectrometer measurements in the range 1310–270 Å, observed from the satellite OSO 3, are reported for the solar flare of 2114 UT March 27, 1967. This flare was a long lasting sequence of bursts with EUV spectra consisting of enhanced lines and recombination continua normally emitted from the chromosphere and chromosphere-corona transition region, with unusually small increases in lines normally emited from the corona. An EUV flare spectrum is presented and suggested as one example for interpreting broadband observations of EUV bursts. Any broadband continuum other than known recombination continua contributed less than 6 % of the meassured line and hydrogen recombination continua in the range 270–1310 Å. The ratio of photon flux of Ciii 1176 Å to that of Ciii 977 Å was 0.86, which suggests an ambient density in the region of emission greater than 10¹² cm^-3 at temperatures near 60000 K.     

Cross-Calibration of SMART-1 XSM with GOES and RHESSI

A number of X-ray instruments have been active in observing the solar coronal X-ray radiation this decade. We have compared XSM observations with simultaneous GOES and RHESSI observations. We present flux calibrations for all instruments and compare XSM and GOES total emission measures (TEM) and temperatures (T).     

Measurement of the X-ray emission of the solar atmosphere during a period of low activity

A large-area high-sensitivity X-ray spectrometer has been constructed and used to measure the 1.8–5.3 Å X-ray emission of the Sun under quiescent conditions. The instrument utilizes Bragg reflection from mosaic graphite crystals. The data indicate that the X-ray emission can best be accounted for by a multitemperature model of the solar atmosphere in which both the over-all corona and active regions contribute to the X-ray spectrum. Theoretical calculations of the X-ray flux of a hot, optically thin plasma have been used to estimate the solar conditions at the time when the measurements were made.     

Soft X-ray and microwave observations of hot regions in solar flares

Hot regions in solar flares produce X-radiation and microwaves by thermal processes. Recent X-ray data make it possible to specify the temperature and emission measure of the soft X-ray source, by using, for instance, a combination of the 1–8 Å (peak response at about 2 keV) and the 0.5–3 Å (peak response at about 5 keV) broad-band photometers. The temperatures and emission measures thus derived satisfactorily explain the radio fluxes, within systematic errors of about a factor of 3. Comparison of 15 events with differing parameters shows that a hot solar flare region has an approximately isothermal temperature distribution. The time evolution of the correlation in a single event shows that the hot material originates in the chromosphere, rather than the corona. The density must lie between 10¹⁰ and 2 × 10¹¹ cm^–3. For an Importance 1 flare, this implies a stored energy of roughly 2 x 10³⁰-10²⁹ ergs. A refinement of the data will enable us to choose between conductive and radiative cooling models.     

Determination of the energy or pressure of a solar X-ray structure using X-ray filtergrams from a single filter

While the X-ray fluxes measured through two different broadband filters can be used to derive the temperatures and emission measures of solar plasmas, it is generally assumed that no plasma parameters can be measured quantitatively with a single filtergram alone. We show that the thermal energy density (or pressure) of a solar plasma can be measured with an ideal single filtergram. Calculations have been done to compare the properties of the Skylab S-054 X-ray telescope filters to those of the ideal filter.     

UV Observations with the Transition Region and Coronal Explorer

The Transition Region and Coronal Explorer is a space-borne solar telescope featuring high spatial and temporal resolution. TRACE images emission from solar plasmas in three extreme-ultraviolet (EUV) wavelengths and several ultraviolet (UV) wavelengths, covering selected ion temperatures from 6000 K to 1 MK. The TRACE UV channel employs special optics to collect high-resolution solar images of the H i L line at 1216 Å, the C iv resonance doublet at 1548 and 1550 Å, the UV continuum near 1550 Å, and also a white-light image covering the spectrum from 2000–8000 Å.We present an analytical technique for creating photometrically accurate images of the C iv resonance lines from the data products collected by the TRACE UV channel. We use solar spectra from several space-borne instruments to represent a variety of solar conditions ranging from quiet Sun to active regions to derive a method, using a linear combination of filtered UV images, to generate an image of solar C iv 1550 Å emission. Systematic and statistical error estimates are also presented. This work indicates that C iv measurements will be reliable for intensities greater than 1014 photons s–1 cm–2 sr–1. This suggests that C iv 1550 Å images will be feasible with statistical error below 20% in the magnetic network, bright points, active regions, flares and other features bright in C iv. Below this intensity the derived image is dominated by systematic error and read noise from the CCD.     

Two-component temperature analysis of OSO-5 X-ray flare data

Using data covering the 2.6–10 Å wavelength range from the OSO-5 satellite a four-parameter model of the emitting region in a flare process is derived. The thermal emission spectrum of Landini and Fossi is used to calculate the plasma parameters, electron temperature and emission measure. The X-ray flare data is explained by a model which treats the source volume as two time-varying temperature regions.     

Comparison of mm-wave and X-ray diagnostics of flare plasma

To compare mm-wave and X-ray diagnostics of solar flare plasma, five flares observed in 1980–1991 in Metsähovi at 22 and 37 GHz and with GOES, SMM, and GRO are studied. The first impulsive peak of the mm-wave bursts under investigation coincides in time with hard X-ray emission. The second gradual component in mm-wave emission coincides with the maximum of the soft X-ray emission measure. The bremsstrahlung mm-wave radiation from hot chromospheric plasma and gyrosynchrotron radiation driven by common population of superthermal electrons are calculated. It is shown that for mm-wave events with the first peak intensity 100 s.f.u., the thermal bremsstrahlung is more important than the gyrosynchrotron emission. The total energy of fast electrons deduced from the first peak of mm-wave bursts is one to two orders of magnitude less than that determined from the hard X-ray emission in the approximation of a thick-target nonthermal model. That can testify in favour of the hybrid thermal/nonthermal model proposed by Holman and Benka (1992). The emission measure and the energy of evaporated plasma using both mm-wave and soft X-ray data are also determined. For events investigated here the energy of evaporated chromospheric plasma is larger than the total energy of fast electron beams. We have concluded that, for evaporation, additional energy release in the chromosphere is needed. The possibility of such energy release in the framework of an advanced circuit model for solar flares is discussed.     

Estimates of the neutron emission during large solar flares in the rising and maximum period of solar cycle 24

We searched for solar neutrons using the data collected by six detectors from the International Network of Solar Neutron Telescopes and one Neutron Monitor between January 2010 and December 2014. We considered the peak time of the X-ray intensity of thirty five ≥ X1.0 class flares detected by GOES satellite as the most probable production time of solar neutrons. We prepared a light-curve of the solar neutron telescopes and the neutron monitor for each flare, spanning ± 3 h from the peak time of GOES. Based on these light curves, we performed a statistical analysis for each flare. Setting a significance level at greater than 3σ, we report that no statistically significant signals due to solar neutrons were found. Therefore, upper limits are determined by the background level and solar angle of these thirty five solar flares. Our calculation assumed a power-law neutron energy spectrum and an impulsive emission profile at the Sun. The estimated upper limits of the neutron emission are consistent within the order of magnitude of the successful detections of solar neutrons made in solar cycle 23.     

Discrete subresolution structures in the solar transition zone

During operations on the Spacelab-2 Shuttle mission, the NRL High Resolution Telescope and Spectrograph (HRTS) recorded spectra of a variety of solar features in the 1200–1700 Å wavelength region which contains spectral lines and continua well suited for investigating the temperature minimum, the chromosphere and transition zone. These data show that, at the highest spatial resolution, the transition zone spectra are broken up from a continuous intensity distribution along the slit into discrete emission elements. The average dimensions of these discrete transition zone structures is 2400 km along the slit, but an analysis of their emission measures and densities shows that the dimensions of the actual emitting volume is conciderably less. If these structures are modelled as an ensemble of subresolution filaments, we find that these filaments have typical radii of from 3 to 30 km and that the cross-sectional fill factor is in the range from 10^–5 to 10^–2. The transport of mass and energy through these transition zone structures is reduced by this same factor of 10^–5 to 10^–2 which has significant consequences for our understanding of the dynamics of the solar atmosphere. Because the HRTS transition zone line profiles are not broadened by resolved large-spatial-scale solar velocity fields, the line widths of the Civ lines have been analyzed. The average line width is 0.195 Å (FWHM) and requires an average nonthermal velocity of 16 km s^–1 (most-probable) or 19 km s^–1 (root-mean-square) which is lower than previously observed values.     

The emission of solar X-rays in the 0.5–3 Å wavelength range and its relation to the magnetic configuration of active centers

The slowly varying component of solar X-rays in the 0.5–3 Å wavelength range has been studied using data obtained by the satellite Explorer 30 (Solrad 8). The intensity of these X-rays is poorly correlated with the centimeter radio flux, contrary to the good correlation found in the spectral bands 1–8, 8–16 and 44–60 Å. On the other hand the 0.5–3 Å X-ray intensity is often connected to the development of a specific magnetic configuration in the sun spot group which may thus be associated with the X-ray producing active center.