Star Scintillations Research Articles

Debussy, Discourse, Time: Listening for “la modernité” in the Nocturnes

O night! O refreshing darkness! You are for me the signal for an inner festival, you are the deliverance from my anguish! Within the solitude of the plains, within the stony labyrinths of a capital city, scintillation of stars, explosion of street-lamps, you are the fireworks of the goddess Liberty! You will hear and possess the three Nocturnes, which have given me more trouble, in their three parts, than the five acts of Pelléas. I hope that this will be music of the open sky, which will shiver in the breeze from the mighty wingbeat of Liberty (Good lord! such fine phrases!) Do the famous musings on “la modernité” in Charles Baudelaire’s essay “The Painter of Modern Life” offer a new way of hearing Claude Debussy’s last nineteenth-century composition, the three orchestral Nocturnes (1897–99)?1 Given that this hard-won, laboriously revised triptych has become deeply blanketed by clichés, the attempt by René Descartes to secure the grounds of certain knowledge in his Discourse on the Method offers an excellent model for an exercise of skepticism about certain accepted conventional interpretations.

SCIDAR: an optical turbulence profiler for Dome A

AbstractThis paper introduces a plan to detect turbulence profiles at Dome A with a Single Star Scidar (SSS), to enhance our understanding of the characteristics of the site. The development of a portable monitor for profiling vertical atmospheric optical turbulence and wind speed is presented. By analyzing the spatial auto and cross-correlation functions of very short exposure images of single star scintillation patterns, the SSS can provide the vertical profiles of turbulence intensity C2n(h) and wind speed V(h). A SSS prototype is already operational at Ali in Tibet which will be improved in order to become fully robotic and adapted to extreme weather conditions that prevail at Dome A in Antarctica.

How achromatic is the stellar scintillation on large telescopes?

The atmospheric scintillation of stars is the main reason why the ground-based photometry of astronomical objects has limited accuracy. This becomes particularly noticeable for a variability study with amplitudes of the order of thousandths of stellar magnitude or less. We examine the problem of colour scintillation (i.e. fluctuations of the difference between light intensities measured simultaneously in two different photometric bands). We derive the relations between the colour scintillation power (index) and the atmospheric turbulence, telescope diameter and the characteristics of the photometric channels. Asymptotic dependences for large telescopes (1–10 m) are obtained, which allow us to predict the value of the colour scintillation for a particular telescope and detector. It is shown that the colour scintillation index is ∝ D−3 for measurements with both short (milliseconds) and long (seconds) exposures. We estimate the impact of the atmospheric dispersion, which amplifies colour scintillation away from the zenith. We show that colour scintillation in the long-exposure regime depends strongly on the wind direction in the upper atmosphere.

Rainbows, water droplets, and seeing—slow motion analysis of experiments in atmospheric optics

Many physics processes underlying phenomena in atmospheric optics happen on a rather short time scale such that neither the human eye nor video cameras are able to analyze the details. We report applications of high-speed imaging of laboratory experiments in atmospheric optics with subsequent slow motion analysis. The potential to study respective transient effects is investigated in general and for a few phenomena in detail, in particular for rainbow scattering due to single oscillating droplets during free fall, and for light propagation effects through atmospheric paths with turbulences, leading, e.g., to scintillation of stars or shimmering of mirage images.

Technical note: Scintillations of the double star α Cru observed by GOMOS/Envisat

Abstract. In this paper, we discuss scintillation time-spectra of the double star α Cru, which were measured by the GOMOS/Envisat photometer. The components of α Cru are not resolved by the angular field of view of the detector. The double structure of the light source reveals itself in the modulation of the observed scintillation spectra; this modulation is caused by anisotropic irregularities of the stratospheric air density. We present a qualitative and quantitative explanation of the properties of the double-star scintillation spectra. Possibilities of using double star scintillations for studying atmospheric air density irregularities are also discussed in the paper.

Spread F – an old equatorial aeronomy problem finally resolved?

Abstract. One of the oldest scientific topics in Equatorial Aeronomy is related to Spread-F. It includes all our efforts to understand the physical mechanisms responsible for the existence of ionospheric F-region irregularities, the spread of the traces in a night-time equatorial ionogram – hence its name – and all other manifestations of the same. It was observed for the first time as an abnormal ionogram in Huancayo, about 70 years ago. But only recently are we coming to understand the physical mechanisms responsible for its occurrence and its capricious day to day variability. Several additional techniques have been used to reveal the spatial and temporal characteristics of the F-region irregularities responsible for the phenomenon. Among them we have, in chronological order, radio star scintillations, trans-equatorial radio propagation, satellite scintillations, radar backscatter, satellite and rocket in situ measurements, airglow, total electron content techniques using the propagation of satellite radio signals and, recently, radar imaging techniques. Theoretical efforts are as old as the observations. Nevertheless, 32 years after their discovery, Jicamarca radar observations showed that none of the theories that had been put forward could explain them completely. The observations showed that irregularities were detected at altitudes that were stable according to the mechanisms proposed. A breakthrough came a few years later, again from Jicamarca, by showing that some of the "stable" regions had become unstable by the non-linear propagation of the irregularities from the unstable to the stable region of the ionosphere in the form of bubbles of low density plasma. A problem remained, however; the primary instability mechanism proposed, an extended (generalized) Rayleigh-Taylor instability, was too slow to explain the rapid development seen by the observations. Gravity waves in the neutral background have been proposed as a seeding mechanism to form irregularities from which the instability would grow, but the former are difficult to observe as a controlling parameter. Their actual role still needs to be determined. More recently, radar observations again have shown the existence of horizontal plasma drift velocities counter streaming the neutral wind at the steep bottom of the F-region which produces a fast growing instability from which a generalized Rayleigh-Taylor instability can grow. The mechanisms proposed would explain the rapid development of the large and medium scale irregularities that have been observed, including some seen only by radars. Nevertheless, a proper quantitative theoretical mechanism that would explain how these irregularities break into the very important meter scale ones, responsible for the radar echoes, needs to be developed. This paper makes a selective historical review of the observations and proposed theories since the phenomenon was discovered to our current understanding.

Single star scidar: atmospheric parameters profiling using the simulated annealing algorithm

In this paper, we present a new method to estimate, for each turbulent layer labelled i, the horizontal wind speed v(h i ), the standard deviation of the horizontal wind speed fluctuations σv(hi) and the integrated value of C 2 over the thicknesshi of the turbulent layer C 2 (hi)� hi, where hi is the altitude of the turbulent layer. These parameters are extracted from single star scintillation spatiotemporal cross-correlation functions of atmospheric speckles obtained within the generalized mode. This method is based on the simulated annealing algorithm to find the optimal solution required to solve the problem. Astrophysics parameters for adaptive optics are also calculated using C 2 (hi) and v (hi) values. The results of other techniques support this new method.

Study of stratospheric air density irregularities based on two‐wavelength observation of stellar scintillation by Global Ozone Monitoring by Occultation of Stars (GOMOS) on Envisat

The scintillations of stars observed through the Earth's atmosphere are generated by random irregularities of air density. We propose the qualitative theory for description of coherency and correlations of optical scintillations measured at two wavelengths. It is based on a two‐component model of air density irregularities: One of the components corresponds to anisotropic irregularities, while the second one is generated by locally isotropic turbulence. The main conclusion of the developed theory is that chromatic aberration results in low coherency of isotropic scintillations. The scintillations measured by GOMOS fast photometers (FP) on board the Envisat satellite have confirmed the theoretical conclusions. The coherency of scintillation measurements at wavelength 672 and 499 nm visualize the regions of high coherency where the anisotropic irregularities dominate. Observations have allowed also the detection of layers with low coherence. They are located generally between altitudes of 30 and 40 km. The thickness of the layers and their altitude distribution depend on observation location. It is expected that the locally isotropic turbulence is strongly developed within these layers. We show that the low values of the cross‐correlation coefficient of two‐wavelength scintillations can be used as a qualitative indicator for the presence of layers with prevailing isotropic turbulence. The obtained results showed that the analysis of two‐wavelength coherency and cross‐correlation functions is a sensitive approach which will allow visualizing IGW breakdown in the stratosphere.

Atmospheric Intensity Scintillation of Stars. III. Effects for Different Telescope Apertures

ABSTRACTStellar intensity scintillation in the optical was extensively studied at the astronomical observatory on La Palma (Canary Islands). Atmospheric turbulence causes “flying shadows” on the ground, and intensity fluctuations occur both because this pattern is carried by winds and is intrinsically changing. Temporal statistics and time changes were treated in Paper I, and the dependence on optical wavelength in Paper II. This paper discusses the structure of these flying shadows and analyzes the scintillation signals recorded in telescopes of different size and with different (secondary‐mirror) obscurations. Using scintillation theory, a sequence of power spectra measured for smaller apertures is extrapolated up to very large (8 m) telescopes. Apodized apertures (with a gradual transmission falloff near the edges) are experimentally tested and modeled for suppressing the most rapid scintillation components. Double apertures determine the speed and direction of the flying shadows. Challenging photometry tasks (e.g., stellar microvariability) require methods for decreasing the scintillation “noise.” The true source intensity I(λ) may be segregated from the scintillation component ΔI(t,λ,x,y) in postdetection computation, using physical modeling of the temporal, chromatic, and spatial properties of scintillation, rather than treating it as mere “noise.” Such a scheme ideally requires multicolor high‐speed (≲10 ms) photometry on the flying shadows over the spatially resolved (≲10 cm) telescope entrance pupil. Adaptive correction in real time of the two‐dimensional intensity excursions across the telescope pupil also appears feasible, but would probably not offer photometric precision. However, such “second‐order” adaptive optics, correcting not only the wavefront phase but also scintillation effects, is required for other critical tasks such as the direct imaging of extrasolar planets with large ground‐based telescopes.

ATMOSPHERIC INTENSITY SCINTILLATION OF STARS. II. DEPENDENCE ON OPTICAL WAVELENGTH

ABSTRACT Atmospheric intensity scintillation of stars on milli- and microsecond time scales was extensively measured at the astronomical observatory on La Palma (Canary Island). Scintillation statistics and temporal changes were discussed in Paper I, while this paper shows how scintillation depends on optical wavelength. Such effects originate from the changing refractive index of air, and from wavelength-dependent diffraction in atmospheric inhomogeneities. The intensity variance σ2/I increases for shorter wavelengths, at small zenith distances approximately consistent with a theoretical λ -7/6 slope, but with a tendency for a somewhat weaker dependence. Scintillation in the blue is more rapid than in the red. The increase with wavelength of autocorrelation time scales (roughly proportional to sqrt{λ} is most pronounced in very small apertures, but was measured up to \\o 20 cm. Scintillation at different wavelengths is not simultaneous: atmospheric chromatic dispersion stretches the atmospherically induced 'flying shadows' into 'flying spectra' on the ground. As the 'shadows' fly past the telescope aperture, a time delay appears between fluctuations at different wavelengths whenever the turbulence-carrying winds have components parallel to the direction of dispersion. These effects increase with zenith distance (reaching \\approx 100 ms cross-correlation delay between blue and red at Z = 60°), and also with increased wavelength difference. This time delay between scintillation in different colors is a property of the atmospheric flying shadows, and thus a property that remains unchanged even in very large telescopes. However, the wavelength dependence of scintillation amplitude and time scale is 'fully' developed only in small telescope apertures (less than about 5 cm), the scales where the 'flying shadows' on the Earth's surface become resolved. Although these dependences rapidly vanish after averaging in larger apertures, an understanding of chromatic effects may still be needed for the most accurate photometric measurements. These will probably require a sampling of the [stellar] signal with full spatial, temporal and chromatic resolution to segregate the scintillation signatures from those of astrophysical variability.

Using stars for remote sensing of the Earth’s stratosphere

A new UV-visible spectrometer system that measures the absorption of light from stars and planets by constituents in the Earth's atmosphere is described. Because it can be used to make measurements at night, the system has a significant advantage for measuring polar constituents in winter, when conditions that might give rise to ozone loss are initiated. Other advantages arise from the use of a cooled two-dimensional CCD array as the detector: an array detector avoids spectral noise resulting from scintillation of stars or from clouds passing overhead and allows for the possibility of measuring several constituents simultaneously; its second dimension permits auroral light from the atmosphere adjacent to the star to be measured simultaneously and subtracted from the stellar light, and a modern low-noise CCD allows us to use a telescope of modest diameter. The few previous measurements of constituents made by the use of stellar absorption did not have these advantages. The instrument was configured for simplicity and ease of use in field measurements and was deployed outside in winter in Northern Sweden in 1991. Examples of ozone measurements are shown.

Middle stratosphere temperature spectra observed with stellar scintillation and in situ techniques

Abstract The vertical temperature spectra within the middle stratosphere (20–30 km) was measured using two independent techniques. The first technique involves orbital station observations of star scintillation during their occultation by the Earth's atmosphere. The time spectra of star brightness is then converted into vertical temperature spectra. The second technique consists of in situ measurements of temperature on a ballon-borne gondola. The spectra obtained by these two methods are consistant with each other both on the spectral slope, close to −3, and on the spectral level for scale range between 50 m and 1 km. However, for scales smaller than 50 m, the spectra obtained by scintillation technique display a sharp cut-off that is not observed with balloon measurements. A tentative interpretation of this discrepancy is presented.

Sound and light propagation in a weakly inhomogeneous atmosphere†

Abstract Small-scale inhomogeneities caused by atmospheric turbulence have a considerable effect on sound and light propagation, producing the fluctuations of these wave fields. V A Krasilnikov [1, 2] performed experiments on phase and amplitude variations of a sound wave propagating through the atmosphere. Fluctuations of light-wave parameters occur, for example, in the well known phenomenon of star scintillation, apparently strongly connected with turbulent irregularities of the atmospheric temperature field [3, 4]‡ ‡ The theoretical analysis of turbulent fluctuations of the temperature field and some corresponding experimental data can be found in [5–7]. . Some calculations of phase (arrival angle) and amplitude fluctuations for a wave propagating through a turbulent medium are described by Krasilnikov [1, 3, 8]. All these and similar calculations are based on the geometrical optics (acoustics) approximation, which may be the reason for disagreement between calculation and experimental data in some cases. Thus, for example, amplitude fluctuations in the geometrical approximation turn out to be proportional to the distance of propagation through a turbulent medium to the power of 3/2. However, observations usually show much slower fluctuation growth. This paper represents an attempt to consider the problem of amplitude and phase variations for a scalar wave field in terms of more general equations including some diffraction effects. Incidentally, the range of validity of geometrical optics theory becomes clear.

Anisotropy of spatial structures in the middle atmosphere

In 1988–1989 a series of observations of star scintillations during star settings was carried out from the orbital station “Mir”. These observations allow us to investigate the fine spatial structure of temperature and density in the middle atmosphere. It was shown that the temperature and density inhomogeneities are strongly anisotropic. From measured spectra of star scintillations the vertical spectra of temperature fluctuations can be extracted. The horizontal structure of star scintillations was also studied. From these data an evaluation of the anisotropic coefficient of the temperature fluctuations of about 150 was obtained, utilizing the results of the derived vertical spectra. Strong anisotropy is typical for heights of 12 – 45 km. The analysis of 18 scintillation observations in the latitudes 32°N – 70°N and the longitudes 85°E – 154°E gives an estimation of temperature and density spectra variability. Aerosol distribution anisotropy was also studied by analyzing the Earth's limb photographs during sunset with an MKF-6 camera from onboard the orbital station “Salyut-6”. The horizontal correlation scale of brightness fluctuations of light scattered by aerosols is about 100 times larger than the corresponding vertical one.

Thin stratified structure of the middle atmosphere

Numerous observations from space towards the Earth's horizon have been performed to study the thin structure of the middle atmosphere. The following layers under 1 km thick have been observed: aerosol (altitudes, 8–16 km), ozone (20–30 km), noctilucent clouds (80–85 km) and emission in the E-region of ionosphere (95–105 km). The length of the layers has been found to be 10–100 times as much as their thickness. In terms of atmospheric refractive distortions of the Sun and Moon's limbs, it has been concluded that there are anomalous gradients of air density with the same characteristic dimensions at the altitudes of 8–25 km. The relation has been found between the abnormal temperature gradients and aerosol layers and between the aerosol layers and ozone layers. The space observations of scintillations of stars, when setting beyond the atmosphere, have permitted the air density spectrum to be studied within the vertical scale range of 10 m to 1 km at the altitudes of 20–40 km. The results of observations and MST radar monitoring make it possible to conclude that the thin structure with the vertical scale of about 1 km or less exists far and wide in the middle atmosphere and lower ionosphere. A preliminary hypothesis that there is a possible mechanism generating the structure of a stable stratified atmosphere has been proposed.

Self-action effects associated with the generation of plasma irregularities during ionospheric modification experiments

Five poster papers presented by various authors at the URSI Symposium on Active Experiments in Space Plasmas (Florence, August 1984) are previewed. New results from the heating facilities at both Arecibo and Ramfjordmoen are reported. The observations include: 1. (a) heater generated intermediate scale plasma irregularities diagnosed by UHF radio star scintillations, UHF radar incoherent scatter and HF signal fading techniques; 2. (b) UHF plasma line enhancements; 3. (c) anomalous absorption of both heater and HF diagnostic waves. These experimental results are discussed in terms of various self-action processes, such as self-focussing and pump depletion. The results of some recent theoretical investigations into anomalous absorption and reflection are also presented.

First experimental evidence of HF produced electron density irregularities in the polar ionosphere; Diagnosed by UHF radio star scintillations

HF‐produced electron density irregularities with scale sizes of several hundred meters were observed in the polar ionosphere by means of UHF‐scintillations using the new facilities at Tromsø. HF‐power densities as low as 20 µW/m² excited irregularities during over‐dense (HF < penetration frequency) modification.

Coordinated study of subkilometer and 3‐m irregularities in the F region generated by high‐power HF heating at Arecibo

High‐power high‐frequency transmitters near Arecibo were used to generate artificial ionospheric irregularities in the F region. Radio star scintillation observations at 430 MHz were performed at Arecibo Ionospheric Observatory with the 305‐m antenna, and radar backscatter measurements at 50 MHz were simultaneously made from Guadeloupe Island to probe the subkilometer and 3‐m irregularities in the heated volume. Scintillation studies indicate a low‐frequency modulation of the faster intensity fluctuation structure. By the use of plasma drift data this low‐frequency temporal structure translates to spatial dimensions of 1–2 km. The frequency of the modulation envelope is found to be controlled by the heater power and is related to the dominant irregularity wavelength generated by the self‐focusing instability. Scintillation spectra imply a steep power law index of −5 in the scale length range of about 300 m to 150 m and a shallow index of −2 at less than 150 m. The steep power law index may arise from an aperture averaging effect of the large 305‐m antenna at Arecibo. Simultaneous measurements of 430‐MHz scintillations and 50‐MHz radar backscatter from field‐aligned striations were performed to show that subkilometer irregularities can be generated by both O and X mode heating whereas the 3‐m irregularities are excited only by the O mode heating, as is predicted by the theories of self‐focusing and parametric instability. The width of the 50‐MHz echo Doppler spectra is observed to be very narrow, only ∼2–3 Hz, and independent of the background plasma drift, implying that the frequency bandwidth of the scattered signal is probably controlled by the instability process.

HF produced ionospheric electron density irregularities diagnosed by UHF radio star scintillations

High frequency waves incident on an overdense ionosphere (i.e. HF < penetration frequency) are known to produce large-scale irregularities with scale sizes of several hundred meters in the F-region of the ionosphere. Three observations of radio star intensity fluctuations at UHF are reported for HF ionospheric modification experiments performed at the Arecibo Observatory. Two observations at 430 MHz and one observation at 1400 MHz indicate that the thin phase screen theory is a good approximation to the observed power spectra. However, the theory has to be extended to include antenna filtering. Such filtering is important for UHF radio star scintillations since the antenna usually has a narrow beam width. HF power densities of less than 37 μW m −2 incident on the ionosphere produce electron density irregularities larger than 13% of the ambient density (at 260 km) having scale sizes of ~510 m perpendicular to the geomagnetic field. The irregularities form within 20–25 s after the HF power is turned on. From the observed power spectra driftvelocities of the irregularities can be estimated.

Calculations of equatorial scintillations at VHF and gigahertz frequencies based on a new model of the disturbed equatorial ionosphere

The weak scattering, thin screen theory of scintillation of radio stars and satellites has been applied, in a mathematically rigorous way, to the observations of the plasma density structure deduced from rocket and radar measurements during equatorial spread F conditions. It is shown that bottomside spread F is capable of producing moderate scintillation at VHF, but not at gigahertz frequencies. Upwelling regions of depleted plasma density however, such as those described in several recent publications, are shown to have properties that could cause intense VHF scintillations as well as scintillations in the gigahertz range.