Solar Extreme Ultraviolet Spectrum Research Articles

Ionization Efficiency in the Dayside Ionosphere of Mars: Structure and Variability

AbstractIonization efficiency, the ratio between photoelectron impact ionization and primary photoionization, is an important quantity from the perspective of ionospheric energy balance and as a quantity to be parameterized for use in global simulations. We investigate ionization efficiency using MAVEN in situ measurements of solar extreme ultraviolet (EUV) flux, suprathermal electron flux, and neutral density. Its behavior with respect to pressure and solar zenith angle (SZA) is explained by ionization cross‐sections and photoabsorption, plus photoelectron transport near the terminator. We find similar ionization efficiencies between the species CO2, O, N2, and CO, with Argon ∼40% higher, explained largely by its higher ionization potential. Efficiency depends positively on solar activity, increasing by ∼50–100% (depending on season) from low to moderate solar EUV levels, before flattening at higher EUV levels. EUV spectral hardening appears to be responsible for only ∼15% of this increase, the remainder best explained by variability in neutral composition. We find a negligible dependence of ionization efficiency on the strength and direction of crustal magnetic fields. Efficiencies agree poorly with the theoretical model of Nicholson et al. (2009),https://doi.org/10.1111/j.1365-2966.2009.15463.x, possibly reflecting differences in model versus measured solar EUV spectrum and neutral densities. We fit the data to the empirical parameterization of Mendillo et al. (2011),https://doi.org/10.1029/2011ja016865but find that a single fit cannot capture ionization efficiency behavior across all dayside SZA values. We believe that an empirical model of Mars ionization efficiency will require MAVEN data and validated transport modeling to reach to sufficiently high pressures to capture Mars' photochemical peaks M1 and M2.

A Model of Fluxes of Solar Extreme Ultraviolet Irradiance

We develop an approach to create a model of the solar extreme ultraviolet spectrum in the wavelength range responsible for dissociation of molecular oxygen (115−242 nm). The model is based on the concept of the linear dependence of irradiance fluxes in intervals 1 nm in width on the intensity in the Lyman-alpha hydrogen line (this line is supposed to be measured by photometers of the extreme ultraviolet irradiance onboard space probes). The linear dependence coefficients were obtained for each of these intervals. Comparison of the model calculation results with observations showed that the model inaccuracy does not exceed 1−2%, which is sufficient for calculations of the thermospheric state.

Reconstruction of the solar EUV irradiance from 1996 to 2010 based on SOHO/EIT images

The solar Extreme UltraViolet (EUV) spectrum has important effects on the Earth’s upper atmosphere. For a detailed investigation of these effects it is important to have a consistent data series of the EUV spectral irradiance available. We present a reconstruction of the solar EUV irradiance based on SOHO/EIT images, along with synthetic spectra calculated using different coronal features which represent the brightness variation of the solar atmosphere. The EIT images are segmented with the SPoCA2 tool which separates the features based on a fixed brightness classification scheme. With the SOLMOD code we then calculate intensity spectra for the 10–100 nm wavelength range and each of the coronal features. Weighting the intensity spectra with the area covered by each of the features yields the temporal variation of the EUV spectrum. The reconstructed spectrum is then validated against the spectral irradiance as observed with SOHO/SEM. Our approach leads to good agreement between the reconstructed and the observed spectral irradiance. This study is an important step toward understanding variations in the solar EUV spectrum and ultimately its effect on the Earth’s upper atmosphere.

Properties of the Lower Transition Region: The Widths of Optically Allowed and Intersystem Spectral Lines

The widths of spectral lines in the ultraviolet (UV) and extreme ultraviolet (EUV) spectral regions that are formed in the solar transition region and corona are usually greater than the optically thin widths due to thermal Doppler broadening calculated under the assumption of ionization equilibrium. Although opacity can explain the widths of some lines, there are a host of optically thin lines for which the excess widths are attributed to nonthermal motions. Interest in these motions for coronal heating theories has led to the measurement and comparison of spectral line profiles/widths throughout the solar UV and EUV spectrum. We find that for the quiet Sun the widths of some optically allowed lower transition region lines, deduced from spectra obtained by the Solar Ultraviolet Measurements of Ultraviolet Radiation (SUMER) spectrometer on the Solar and Heliospheric Observatory (SOHO) spacecraft, are considerably larger than predicted from simply scaling previously measured wavelengths of other lines from the same ion. For example, the O III lines of the multiplet near 834 A are considerably wider than predicted from the previously measured (from Skylab) width of the optically thin O III 1666.15 A intersystem line. The excess widths are not due to nonthermal motions, as these are already included in the width of the 1666.15 A line. In this paper, we analyze the widths of some prominent optically allowed lines and discuss possible causes for discrepancies with previous measurements of intersystem lines.

Spectral emission of fractional quantum energy levels of atomic hydrogen from a helium–hydrogen plasma and the implications for dark matter

From a solution of a Schrödinger-type wave equation with a nonradiative boundary condition based on Maxwell's equations, Mills predicts that atomic hydrogen may undergo a catalytic reaction with certain atomized elements and ions which singly or multiply ionize at integer multiples of the potential energy of atomic hydrogen, m27.2 eV wherein m is an integer. The reaction involves a nonradiative energy transfer to form a hydrogen atom that is lower in energy than unreacted atomic hydrogen that corresponds to a fractional principal quantum number ( n=1/ p=1/integer replaces the well-known parameter n=integer in the Rydberg equation for hydrogen excited states). One such atomic catalytic system involves helium ions. The second ionization energy of helium is 54.4 eV ; thus, the ionization reaction of He + to He 2+ has a net enthalpy of reaction of 54.4 eV which is equivalent to 2×27.2 eV . Since the products of the catalysis reaction have binding energies of m27.2 eV , they may further serve as catalysts. Extreme ultraviolet (EUV) spectroscopy was recorded on microwave and glow discharges of helium with 2% hydrogen. Novel emission lines were observed with energies of q13.6 eV where q=1,2,3,4,6,7,8,9, or 11 or these lines inelastically scattered by helium atoms wherein 21.2 eV was absorbed in the excitation of He (1 s 2) to He (1 s 12 p 1) . These lines were identified as hydrogen transitions to electronic energy levels below the “ground” state corresponding to fractional quantum numbers. Furthermore, astrophysical data was reviewed and such transitions were found to match certain spectral lines of the extreme ultraviolet background of interstellar space. They may resolve the paradox of the identity of dark matter and account for many celestial observations such as: diffuse H α emission is ubiquitous throughout the Galaxy and widespread sources of flux shortward of 912 A ̊ are required. Fractional hydrogen transitions were also assigned to unidentified lines in the solar EUV spectrum which may resolve the solar neutrino problem, the mystery of the cause of sunspots and other solar activity, and why the Sun emits X-rays.

Energy levels, lifetimes and branch fractions for Fe XI

We present transition energies, probabilities and branching ratios for the electric dipole allowed (E1) and forbidden (M1, E2, M2) lines of the 3s23p4, 3s3p5 and 3s23p33d configurations of Fe xi. Large-scale multiconfiguration Dirac–Fock wavefunctions are applied to include the effects of relativity, correlation and rearrangement of the electron density consistently within the same (computational) model. From the transition probabilities, lifetimes of the 46 lowest-lying excited levels are derived, and the prospects for experimental verification are discussed. An M2 transition is identified in the solar extreme ultraviolet spectrum.

Energy levels, lifetimes and branch fractions for Fe xi

We present transition energies, probabilities and branching ratios for the electric dipole allowed (E1) and forbidden (M1, E2, M2) lines of the 3s23p4, 3s3p5 and 3s23p33d configurations of Fe xi. Large-scale multiconfiguration Dirac-Fock wavefunctions are applied to include the effects of relativity, correlation and rearrangement of the electron density consistently within the same (computational) model. From the transition probabilities, lifetimes of the 46 lowest-lying excited levels are derived, and the prospects for experimental verification are discussed. An M2 transition is identified in the solar extreme ultraviolet spectrum.

Gas ionization solar spectral monitor

We are currently developing an instrument free from optical components to measure the full-disk solar spectrum in the extreme-ultraviolet (EUV) regime covering wavelengths from 75 to 400Å. The instrument consists of a windowless noble gas ionization cell followed by a toroidal electrostatic analyzer to spatially disperse photoelectrons as a function of their energies. A microchannel plate based position sensitive detector is used to detect individual electrons, indirectly returning the solar EUV spectrum. The instrument was launched aboard a NASA Black Brant sounding rocket on October 27, 1992. Power to this instrument was lost early in the flight, however, and a reflight is planned for October 1993. Calibration results for the He II 304-Å line are presented.

A position sensitive detector for EUV remote sensing

The authors describe a photon-counting extreme ultraviolet (EUV) detector system used in a rocket-borne spectroscopic instrument for remote sensing of upper atmospheric composition and temperature. The detector uses a KBr coated microchannel plate (MCP) Z stack in combination with a wedge-and-strip image readout system. Three separate detector fields of view are used to sense the Earth dayglow spectrum (980 AA to 1040 AA, and 1300 AA to 1360 AA) and the solar EUV spectrum (250 AA to 1400 AA). The authors demonstrate high gain (2*10/sup 7/), tight pulse-height distribution (35% FWHM), and a spatial resolution of approximately 35 mu m FWHM (full width at half maximum), which is the highest resolution for a wedge-and-strip anode MCP detector flown to date. The background, image linearity, and flat-field performance are discussed. Raw spectra from the rocket flight are also presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Classification of iron VIII to XII and XIV lines in the solar extreme ultra-violet spectrum and their isoelectronic sequences from argon to nickel

Many of the stronger members of the group of solar emission lines between 167 A and 220 A have been classified as transitions of the type 3pn-3pn-13d in Fe IX to Fe XIV and 3p63d-3p53d2 in Fe VIII. The method is based upon following the isoelectronic sequences from nickel down, where possible, to known spectra. The sources used include Zeta and vacuum sparks. It was necessary to take account in several cases of the interaction between 3d and 4s orbitals at some point in the sequence. Failure to do so in the past has led to wrong classification of lines from 3pn4s terms in the literature. New wavelengths are given for several sequences from argon or potassium to nickel, and many of the new nickel lines can also be found in the solar spectrum between 144 A and 165 A.

Intensity variations in the solar extreme ultraviolet spectrum observed by OSO-I

This paper reviews solar extreme ultraviolet observations made in the spectral region λ170 — λ400 by the first Orbiting Solar Observatory during March, April, and May, 1962. Application of the data to the identification of emission lines in this spectral region is discussed. The observations are compared with concurrent 2800 MHz radio emission and other measurements of solar activity.