Solar Lyman Alpha Radiation Research Articles

Global structure of the heliosphere: 3D kinetic-MHD model and the interpretation of spacecraft data

This paper is a brief overview of research into the interaction between the solar wind and local interstellar medium. This interaction determines the global structure of the heliosphere (the region occupied by the solar wind) and has a complex multicomponent character. We describe the three-dimensional kinetic-magnetohydrodynamic model of the interaction, which includes plasma and neutral components of the interstellar medium and solar wind, the heliospheric and interstellar magnetic fields, and the latitudinal and temporal variations of solar wind parameters. In describing the results, magnetic-field-related effects are given special attention, in particular, the magnetic-field-driven plasma depletion in the vicinity of the heliopause. The model explains a sufficiently large body of data from Voyager-1 and Voyager-2, the Interstellar Boundary Explorer (IBEX), the Solar and Heliospheric Observatory (SOHO), the Hubble Space Telescope (HST), and other spacecraft. Based on the experimental data and using the model, the parameters of the interstellar medium and of the interstellar magnetic field are determined. It is shown, however, that a single model using a single set of boundary conditions cannot self-consistently explain all the data available. The Voyager-1 data on the scattered solar Lyman-alpha radiation is taken as an example to illustrate the difficulties that arise.

Remote diagnostic of the hydrogen wall through measurements of the backscattered solar Lyman alpha radiation by Voyager 1/UVS in 1993–2003

AbstractWe perform a new analysis of the Lyman alpha data obtained by Voyager 1 during the spatial scans in 1993–2003 while Voyager 1 was at 53–88 AU from the Sun. These data are the important source of information on the hydrogen distribution in the outer heliosphere. A sophisticated global kinetic‐MHD model of the heliospheric interface and a radiative transfer model are used for the analysis. It is shown for the first time that the ratio of the Lyman alpha intensities detected in the downwind and upwind lines of sight in the outer heliosphere is sensitive to the configuration (peak value and location) of the hydrogen wall. The hydrogen wall is a source of Doppler‐shifted backscattered Lyman alpha photons, so it can be seen from inside the heliosphere. Therefore, Voyager 1/ultraviolet spectrometer (UVS) Lyman alpha data can be used for remote sensing of the hydrogen wall. We show that our current global model of the outer heliosphere, which is consistent with many other measurements including Lyman alpha data from both Voyager 1 and 2 in 1980–1993, provides a systematically larger downwind to upwind intensity ratio compared with the UVS data in 1993–2003. In order to decrease the ratio, a higher and/or closer hydrogen wall is needed.

Atmospheric Changes Observed in Antarctica Related to the Sun-Earth Interactions

Here we present the ionosphere behavior obtained from measurements done with the ionosonde operating at Comandante Ferraz Brazilian Antarctic Station (EACF) in 2011. We also discuss its long-term behavior obtained from 2004 to 2011 using very low frequency radio signals and GPS measurements. During quiet periods the ionosphere is controlled by the solar Lyman alpha radiation, presenting variations closely associated with the 11-year sunspot number. But it is strongly affected by the excess of X-ray emission produced during the solar flares. The long-term studies of ionosphere behavior have also shown it presents strong variations during local wintertime, which were found to be closely related to the waves of neutral atmospheric origin. The studies of ionosphere behavior have been improving our understanding of its response to natural phenomena, and about its coupling with the other atmospheric layers. The energy exchange among atmospheric layers might be an important factor in the climate conditions/changes, which affect the terrestrial and marine environment, especially in the polar region.

O2 density and temperature profiles retrieving from direct solar Lyman-alpha radiation measurements

The resonance transition 2P-2S of the atomic hydrogen (Lyman-alpha emission) is the strongest and most conspicuous feature in the solar EUV spectrum. The Lyman-alpha radiation transfer depends on the resonance scattering from the hydrogen atoms in the atmosphere and on the O2 absorption. Since the Lyman-alpha extinction in the atmosphere is a measure for the column density of the oxygen molecules, the atmospheric O2 density and temperature profiles can be calculated thereof. A detector of solar Lyman-alpha radiation was manufactured in the Stara Zagora Department of the Solar-Terrestrial Influences Laboratory (STIL). Its basic part is an ionization camera, filled in with NO. A 60 V power supply is applied to the chamber. The produced photoelectric current from the sensor is fed to a two-channel amplifier, providing analog signal. The characteristics of the Lyman-alpha detector were studied. It passed successfully all tests and the results showed that the so-designed instrument could be used in rocket experiments to measure the Lymanalpha flux. From the measurements of the detector, the Lyman-alpha vertical profile can be obtained. Programs are created to compute the O2 density, atmospheric power and temperature profiles based on Lymanalpha data. The detector design appertained to ASLAF project (Attenuation of the Solar Lyman-Alpha Flux), a scientific cooperation between STIL—Bul.Acad.Sci., Stara Zagora Department and the Atmospheric Physics Group at the Department of Meteorology (MISU), Stockholm University, Sweden. The joint project was part of the rocket experiment HotPay I, in the ALOMAR eARI Project, EU’s 6th Framework Programme, Andøya Rocket Range, Andenes, Norway. The project is partly financed by the Bulgarian Ministry of Science and Education.

Radiation transport of heliospheric Lyman-α from combined Cassini and Voyager data sets

Heliospheric neutral hydrogen scatters solar Lyman-alpha radiation from the Sun with '27-day' intensity modulations observed near Earth due to the Sun's rotation combined with Earth's orbital motion. These modulations are increasingly damped in amplitude at larger distances from the Sun due to multiple scattering in the heliosphere, providing a diagnostic of the interplanetary neutral hydrogen density independent of instrument calibration. This paper presents Cassini data from 2003-2004 obtained downwind near Saturn at approximately 10 AU that at times show undamped '27-day' waves in good agreement with the single-scattering models of Pryor et al., 1992. Simultaneous Voyager 1 data from 2003- 2004 obtained upwind at a distance of 88.8-92.6 AU from the Sun show waves damped by a factor of -0.21. The observed degree of damping is interpreted in terms of Monte Carlo multiple-scattering calculations (e.g., Keller et al., 1981) applied to two heliospheric hydrogen two-shock density distributions (discussed in Gangopadhyay et al., 2006) calculated in the frame of the Baranov-Malama model of the solar wind interaction with the two-component (neutral hydrogen and plasma) interstellar wind (Baranov and Malama 1993, Izmodenov et al., 2001, Baranov and Izmodenov, 2006). We conclude that multiple scattering is definitely occurring in the outer heliosphere. Both models compare favorably to the data, using heliospheric neutral H densities at the termination shock of 0.085 cm(exp -3) and 0.095 cm(exp -3). This work generally agrees with earlier discussions of Voyager data in Quemerais et al., 1996 showing the importance of multiple scattering but is based on Voyager data obtained at larger distances from the Sun (with larger damping) simultaneously with Cassini data obtained closer to the Sun.

Aeronomy, a 20th Century emergent science: the role of solar Lyman series

Abstract. Aeronomy is, by definition, a multidisciplinary science which can be used to study the terrestrial atmosphere, as well as any planetary atmosphere and even the interplanetary space. It was officially recognized in 1954 by the International Union of Geodesy and Geophysics. The major objective of the present paper is to show how aeronomy developed since its infancy. The subject is so large that a guide-line has been chosen to see how aeronomy affects our atmospheric knowledge. This guideline is the solar Lyman alpha radiation which has different effects in the solar system. After a short description of the origins of aeronomy the first observations of this line are summarized since the beginning of the space age. Then the consequences of these observations are analyzed for the physics and chemistry of the neutral terrestrial atmosphere. New chemical processes had to be introduced, as well as new transport phenomena. Solar Lyman alpha also influences the structure of the Earth’s ionosphere, particularly the D-region. In the terrestrial exosphere, solar Lyman alpha scattered resonantly by atomic hydrogen is at present the only way to estimate this constituent in an almost collisionless medium. Since planetary atmospheres also contain atomic hydrogen, the Lyman alpha line has been used to deduce the abundance of this constituent. The same is true for the interplanetary space where Lyman alpha observations can be a good tool to determine the concentration. The last section of the paper presents a question which is intended to stimulate further research in aeronomy.Key words. Atmospheric composition and structure (middle atmosphere – composition and chemistry; thermosphere – composition and chemistry) – history of geophysics (atmospheric sciences)

Simulation and methods for the data analysis of the Hydrogen-Deuterium-Absorption-Cell-experiment on Cassini

The Hydrogen-Deuterium-Absorption-Cell (HDAC) will be flown as part of the UltraViolet Imaging Spectrograph on the Cassini mission with the objective to determine the deuterium-to-hydrogen (D/H) ratio in the exosphere of Titan. The HDAC is a photometer designed to measure separately the H and D resonant scattered solar Lyman alpha radiation. A non-parametric reconstruction of the Lyman alpha line profiles by means of local linear filters has been developed for the data analysis. An apriori line profile has been statistically described. Applying a minimum variance criterion leads to systems of linear equations, whose solutions yield the filter coefficients. The quality of the line profiles reconstructed with the proposed algorithm in realistic conditions of observations shows that is workable for the real mission.

Effect of local interstellar medium hydrogen fractional ionization on the distant solar wind and interface region

The paper investigates the influence of the local interstellar medium (LISM) proton number density on the gasdynamics of LISM / solar wind interaction. It uses the self‐consistent axisymmetric model of Baranov and Malama (1993) for a constant value of the LISM neutral hydrogen number density (nH∞ = 0.14 cm−3). It is shown that the flow structure is asymmetric relative to the upwind and downwind directions. Numerical results are presented for four values of the proton number density in the LISM: np∞ = 0.005; 0.025; 0.1 and 0.28 cm−3. The results are given along the axis of symmetry only (in upwind and downwind directions). Velocity, number density, and temperature distributions of the plasma component as well as of the H atoms of the LISM are presented. It is shown that the influence of H atoms, penetrating from the LISM into the solar system on the solar wind may be strong enough to be observed by the “Voyager” spacecraft. The results show that the number density of the LISM's plasma component as well as of the solar wind has very large gradients in the interface region (between the bow and termination shocks). This effect may be very important for the interpretation of the kHz radiation detected by “Voyager”. Moreover, the asymmetric (in upwind and downwind directions) distributions of H atom number density, bulk velocity, and temperature may be observed on the basis of the scattered solar Lyman alpha radiation data.

On the role of solar Lyman alpha radiation in radio-wave absorption in theD-region

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Iue high‐resolution observation of the Venus dayglow spectrum 1280‐1380 Å

The spectrum of the Venus dayglow between 1280 and 1380 A has been measured in high‐resolution with the International Ultraviolet Explorer. Seven exposures which were made with the short wavelength camera in the high‐dispersion mode using the large aperture were combined to give a total exposure time of 309 min. The atomic oxygen lines at 1302.2, 1304.9, 1306.0, and 1355.6 A are present. In addition, the (14,3) and (14,4) bands of the carbon monoxide fourth positive system at 1317 and 1354 A respectively are identified. These bands are compared with synthetic spectra, showing the excitation mechanism to be fluorescent scattering of solar Lyman alpha radiation.

Volatiles in the cometary nuclei

The argument for the similarity of the composition of cometary volatiles to that of interstellar molecules has been strengthened by the analysis of CO+ and CO 2 + emission of the comet West. The strong 6300 A emission of oxygen atoms can be interpreted in terms of photodissociation of OH by the solar Lyman-alpha radiation, and not as being due to photo-dissociation of CO2 of speculatively large amount.

Molecular oxygen concentrations in the equatorial mesosphere

A nitric oxide filled ion chamber with lithium fluoride window was used to measure the absorption profile of solar Lyman alpha radiation in the Earth's atmosphere on a rocket flight from Thumba during relatively undisturbed conditions on the Sun. The results were used to derive molecular oxygen concentrations in the 70–95 km region. The estimates show good agreement with the recent atmospheric model of Groves (1970) for heights above about 83 km but exhibit systematic departures from the model below this height.

Lyman-alpha measurements of neutral hydrogen in the outer geocorona and in interplanetary space

Results of hydrogen Lyman-alpha (1216 A) measurements made on a continuous basis by a two-channel photometer on Ogo 5 from March 1968 to June 1971. The highly elliptical orbit provided measurements of both the outer geocorona and of the 1216-A sky background emission, since geocoronal scattering is minimal at the apogee distance of 150,000 km. Selected data (through 1970) are presented, as well as an interpretation of the three principal discoveries to date - namely, (1) a pronounced antisolar enhancement of the geocoronal scattering beyond 70,000 km, which is regarded as evidence for a hydrogen 'geotail' produced by solar Lyman-alpha radiation pressure; (2) a clear correlation of periodic variations in the sky background emission with solar activity associated with solar rotation; and (3) an annual variation of the 1216-A sky background emission, caused by the earth's orbital motion within the cavity created by the solar wind in the nearby interstellar hydrogen.

Interpretation of Extraterrestrial Lyman-alpha Observations

The solar Lyman-alpha radiation pressure affects the orbits and the velocities of the interstellar particles entering the solar system. This leads to enhanced particle losses in the heliosphere, since particles spend a longer time crossing it. This causes a stronger decrease of the density with decreasing distances from the sun than had been calculated without accounting for the radiation pressure. Furthermore, the emission pattern of the solar Lyman-alpha radiation is anisotropic and rotates with the sun in a 27-day period. This causes a temporal change in the location of the intensity extrema. At the same time it produces hydrogen density anisotropies with extrema deviating in their directions from those which had been calculated without consideration of the radiation pressure.

Theory of some Quiet Magnetospheric [Phenomena related to the Geomagnetic Tail

THIS is an outline of a theory which correlates four quiet aeronomic phenomena—(1) the ionized plasma in the geomagnetic field, particularly the tail; (2) the maintenance of the polar F region in winter; (3) type A red polar cap aurora; and (4) some aspects of the scattering of solar Lyman alpha radiation (La) into the night sky.

DIURNAL AND SEASONAL VARIATIONS IN D-REGION ELECTRON DENSITIES DERIVED FROM OBSERVATIONS OF CROSS MODULATION

Pulsed cross modulation, arising in the region from 60 to 80 km, has been observed during undisturbed days in the spring, summer, and fall of 1960. The manner in which these observations were obtained and the uncertainties associated with the electron-density profiles determined from them are discussed.Average electron-density profiles for each hour of quiet spring days have been deduced. These show that the most rapid changes in electron density occur within one hour of ground sunrise and sunset. Around sunrise a rather uniform layer of about 100 electrons/cc is created almost simultaneously throughout the entire region from 60 to 80 km. As the day progresses, the average electron density between 70 and 80 km changes by a factor of about 10, while the average density between 60 and 70 km changes by only a factor of 2.These features of the D region are discussed in the light of theoretical predictions which assume that cosmic rays and solar Lyman-alpha radiation account for the normal ionization in this region.