R. W. Kreplin

EIT: Extreme-UltraViolet Imaging Telescope for the SOHO Mission

The Extreme-ultraviolet Imaging Telescope (EIT) will provide wide-field images of the corona and transition region on the solar disc and up to 1.5 R⊙ above the solar limb. Its normal incidence multilayer-coated optics will select spectral emission lines from Fe IX (171 Å), Fe XII (195 Å), Fe XV (284 Å), and He II (304 Å) to provide sensitive temperature diagnostics in the range from 6 × 104 K to 3 × 106 K. The telescope has a 45 x 45 arcmin field of view and 2.6 arcsec pixels which will provide approximately 5-arcsec spatial resolution. The EIT will probe the coronal plasma on a global scale, as well as the underlying cooler and turbulent atmosphere, providing the basis for comparative analyses with observations from both the ground and other SOHO instruments. This paper presents details of the EIT instrumentation, its performance and operating modes.

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.

The spectral dependence of solar soft X-ray flux values obtained by SOLRAD 9

Abstract The determination of solar energy flux values from ionization chamber experiments is discussed. Procedures and calculations necessary for the correction of SOLRAD 9 0.5–3 A and 1–8 A data and for the determination of the flare plasma temperature and emission measure are presented. These calculations are based on the continuum spectrum of Culhane (1969), the line spectrum of Tucker and Koren (1971), and experimental values of the detector efficiencies.

X-ray line and continuum spectra of solar flares from 0.5 to 8.5 angstroms.

Two crystal spectrometers aboard the orbiting solar observatory OSO-4 cover the wavelength ranges 0.5 to 3.9 angstroms and 1.0 to 8.5 angstroms. Within this range, there appear emission lines from hydrogen-like and helium-like states of calcium, sulfur, silicon, magnesium, and aluminum. The Mg XII Lyman-α is present strongly in all x-ray flares. The most intense flares (such as class 3) produce strong Si XIV Lyman-α and often S XVI Lyman-α. Emission, in the form of Ka lines of highly ionized states of calcium, iron, aluminum, and silicon is usually present. The continuum from 1 to 10 angstroms always dominates the line emission by more than an order of magnitude. Electron temperatures derived from the slope of the continuum spectrum are in the range of 107 to 108 �K, considerably higher than theoretical ionization equilibrium temperatures.

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.

Acceleration of electrons in absence of detectable optical flares deduced from type III radio bursts, H alpha activity and soft X-ray emission

The relationship between Hα absorption features, type III radio bursts and soft X-ray emission has been examined in order to determine the characteristics of the particle acceleration process operating when a Hα-flare may not be detectable. It is found that transient Hα activity observed in the absence of reported flares is associated with production of relatively weak type III radio and soft X-ray emission. Since such optical phenomena are much more frequent than flares themselves, it is concluded that instabilities generating fast particles may be produced in the corona in a quasi-continuous way with coincident perturbations in the lower solar atmosphere.The soft X-ray component, which is similar to the precursor in flares, is not necessarily the direct product of fast particles, but is probably associated with some type of heating since both the soft X-ray emission and the Hα features exhibit a similar evolution, the type III bursts occurring near the maximum of this perturbation. The observations are consistent with a model in which the electron acceleration region is located at an altitude where the ion density is ∼ 109 cm−3 and most of the accelerated electrons(≳ 20 keV) are confined to coronal altitudes where the ion density is ≲ 1010 cm−3.

A multithermal analysis of solar x-ray emission

The NRL SOLRAD 10 satellite carries six ionization chambers to measure solar X-radiation in the 0.5 to 60 Å wavelength band. The X-ray emission spectrum in this range is determined by the derivative of the coronal emission measure (∫ Ne2dV) with respect to temperature when the thermal processes of bremsstrahlung, radiative recombination and line radiation are considered. If a simple model for this differential emission measure is used and detector responses to the calculated spectra are fitted to the SOLRAD data by a least squares method, the differential emission measure can be obtained for temperatures between 2 × 106K and 64 × 106K. Data during quiet and flaring periods are analyzed and the general behavior of the differential emission measure during flares is presented. This analysis is based on experimental measurements of the efficiencies of the SOLRAD detectors.

Direct measurements of the gradual extreme ultraviolet emission from large solar flares

Broadband sensors aboard the Naval Research Laboratorys 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. Data from sixteen large flares show that the EUV emission is dominated by gradual emission which parallels the soft X-ray emission in duration and magnitude. The data are consistent with the separation of EUV and X-ray flare emission into two distinct components. A persistent component is made up of gradual EUV and gradual soft X-ray emissions. A brief component consists of hard X-rays, impulsive soft X-rays, and impulsive EUV emission.

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 Moons 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.

Optical Aspect System for Rockets

A simple electro‐optical aspect system for use in upper air research rockets is described. Observations of the earths albedo or night air glow along with observations of celestial bodies provide data from which the rockets aspect is determined. A mechanical computer is described which greatly simplified aspect data reduction.