Brightness Modulation Research Articles

Image Processing Strategies Dedicated to Visual Cortical Stimulators: A Survey

Multi-electrode devices are constantly evolving toward a state where complexity and reliability are adequate for providing a breakthrough in visual cortical stimulation allowing the blind to recover partial vision. Yet few research teams have focused on the development of the front-end subsystem that transforms an input image from a camera into stimulation commands. This article collects state-of-the-art knowledge about the appearance and organization of phosphenes, and previous work in image processing dedicated to visual cortical stimulation. Observations and hypothesis about important issues are highlighted, and six image processing strategies that could be used in such a subsystem are presented, from the most optimistic that use brightness modulation to emulate grayscale to the most conservative that use only on/off phosphene evocation.

Bidirectional Texture Contrast Function

Three dimensional surface corrugations on globally smooth surfaces give rise to brightness modulations of global shading patterns. We study systematic variations of such 3D image texture as a function of illumination and viewing geometry. The 3D texture is especially noticeable near the shadow terminator (for collimated illumination) or near the dark pole (for hemispherical diffuse illumination). We find that a simple micro-facet model, assuming locally Lambertian scattering, suffices to robustly describe texture contrast gradients of a large variety (measured and rendered textures; laboratory and field conditions) in a semi-quantitative manner. Robust statistical measures of the texture allows one to draw inferences concerning the nature of the light field (collimated to diffuse) and of surface roughness parameters, which can be used as input to the simplest BRDF models.

Voluntary attention modulates the brightness of overlapping transparent surfaces

A new class of brightness illusions is introduced that cannot be entirely accounted for by bottom-up models of neuronal processing. In these new illusions, brightness can be modulated by the location of voluntary attention in the absence of eye movements. These effects may arise from top-down or mid-level mechanisms that determine how 3D surfaces and transparent layers are constructed, which in turn influence perceived brightness. Attention is not the only factor that influences perceived brightness in overlapping transparent surfaces. For example, grouping procedures may favor the minimal number of transparent layers necessary to account for the geometry of the stimulus, causing surfaces on a common layer to change brightness together. Attentional modulation of brightness places constraints on possible future models of filling-in, transparent surface formation, brightness perception, and attentional processing.

Manifestations of sudden commencement of a severe magnetic storm of November 20, 2003, in generation of an SC pulse, PSC pulsations, and spatial modulation of the auroral brightness

The results of investigation of the geomagnetic and auroral response to the commencement of a severe magnetic storm of November 20, 2003, are presented. It is established that the onset of SC led to the brightening of the auroral arc in the dusk sector for 2–3 min with its extent to the east with a velocity of 10–20 km/s and to displacement poleward with a velocity of ∼1.0 km/s. Furtheron, the fast auroral expansions of short duration (∼5 min) to the pole up to ∼2–4°were observed, repeating every 5–10 min during ∼40 min, which led to the spatial modulation of the brightness of the glow and to generation of PSC pulsations with similar periods of oscillations. The broadening of aurora poleward had a steplike character, with generation of new arcs poleward of previous ones ∼5 arcs per ∼1.5 min. The modulation of brightness of the glow and PSC were observed against the background of intensification of a two-cell DP2 type current system and were accompanied by multiple turnings of the IMF Bz from south to north and back. It is assumed that the source of PSC pulsations was a modulation of the intensity of the DP2 ionosphere currents as a result of variations of the magnetosphere convection level.

Rotational Brightness Modulation and Starspots on the RS CVn-type Stars IN Com, IL Com, UX Ari, and V711 Tau

UBVRI photometric observations and models of spotting are presented for four noneclipsing RS CVn systems: IN Com (G5III/IV), IL Com (F8V+F8V), UX Ari (K01V+G5V), and V711 Tau (K1IV+G5V). A low amplitude variability caused by cold (ΔT=1700K)spots which can occupy up to 19% of the star's surface is confirmed for the little-studied star IL Com. Long-term light curves are constructed and the stellar magnitudes and color indices of the unspotted photospheres are estimated for IN com, UX Ari, and V711 Tau. It is shown that UX Ari becomes bluer with decreasing brightness, so its variability cannot be fully explained in terms of cold spots. Models of spotting on In Com and V711 Tau are constructed from the full set of available photometric observations. The spots on both of these variables lie at middle latitudes and occupy up to 22% (In Com) and 33% (V711 Tau) of the stars' surfaces. Both stars manifest a tendency for the width of the spots to decrease as their area increases. This is a crude analog of the Maunder butterflies. These stars experience cyclical spot activity and have a differential rotation of the type found on the sun.

Disk wind in young binaries and the origin of the cyclic activity of young stars

We present the results of our numerical simulations of the cyclic brightness modulation in young binary systems with eccentric orbits and low-mass secondary components. We suggest that the binary components accrete matter from the remnants of the protostellar cloud, with the main accretor (according to current models) being the low-mass component. The brightness variations of the primary are attributable to the periodic extinction variations on the line of sight caused by the disk wind from the secondary and by the common envelope produced by this wind. The distribution of matter in the envelope was calculated in the ballistic approximation. When calculating the optical effects produced by the dust component of the disk wind, we adopted the dust-to-gas mass ratio of 1:100 characteristic of the interstellar medium and the optical parameters of the circumstellar dust typical of young stars. Our calculations show that the theoretical light curves for binaries with elliptical orbits exhibit a wider variety of shapes than those for binaries with circular orbits. In this case, the parameters of the photometric minima (their depth, duration, and shape of the light curve) depend not only on the disk-wind parameters and the orbital inclination of the binary to the line of sight, but also on the longitude of the periastron. We investigate the modulation of the scattered radiation from the common envelope with orbital phase in the single-scattering approximation. The modulation amplitude is shown to be at a maximum when the system is seen edge-on and to be also nonzero in binaries seen pole-on. We discuss possible applications of the theory to young stellar objects. In particular, several model light curves have been found to be similar to those of candidate FU Orionis stars (FUORs).

Rapid Oscillations in Cataclysmic Variables

ABSTRACTI give an overview of the rich phenomenology of dwarf nova oscillations (DNOs) and quasi‐periodic oscillations (QPOs) observed in cataclysmic variable stars (CVs). The favored interpretation of these rapid brightness modulations (3 to >1000 s timescales) is that they are magnetic in nature—magnetically channeled accretion from the inner accretion disk for DNOs and possible magnetically excited traveling waves in the disk for QPOs. There is increasing evidence for the magnetic aspects, which extend to lower fields the well‐known properties of strong field (polar) and intermediate strength field (intermediate polar) CVs. The result is that almost all CVs show the presence of magnetic fields on their white dwarf primaries, although for many the intrinsic field may be locally enhanced by the accretion process itself. There are many behaviors that parallel the QPOs seen in X‐ray binaries, with high‐ and low‐frequency X‐ray QPOs resembling, respectively, the DNOs and QPOs in CVs.

Dynamics of the solar chromosphere IV. Evidence for atmospheric gravity waves from TRACE

We study the low-frequency brightness modulation of internetwork regions in the low solar chromosphere using simultaneous ultraviolet and white-light image sequences from the Transition Region and Coronal Explorer(TRACE). The ultraviolet sequences exhibit a slowly varying brightness pattern in internetwork regions on which the more familiar acoustic three-minute oscillation is superimposed, with about half of the peak brightness reached in internetwork grains contributed by the low-frequency background. We address the nature of the latter, applying two-dimensional Fourier filtering to isolate it from the acoustic modulation. Spatio-temporal comparisons and selective time-delay scatter correlations between the ultraviolet and white-light low-frequency sequences establish that reversed granulation constitutes at most a minor part of the ultraviolet background. Fourier analysis shows that the meso-scale contribution dominates and consists of atmospheric gravity waves.

The nucleus of Deep Impact target Comet 9P/Tempel 1

On UT 2000 August 21 we obtained simultaneous visible and mid-infrared observations of Comet 9P/Tempel 1, the target of the upcoming NASA Discovery Program mission Deep Impact. The comet was still quite active while 2.55 AU from the Sun (post-perihelion). Two independent analyses of our data, one parameterizing the coma morphology and the other modeling infrared spectrophotometry, show that the nucleus's cross section at the time the data were taken corresponds to an effective radius of 3.0±0.2 km. Based on visible-wavelength photometry of the comet taken during this observing run and others in the summer of 2000, all of which show the rotational modulation of the nucleus's brightness, we find that the infrared data were obtained near the maximum of the light curve. If we assume that the nucleus's light curve had a peak-to-valley range of 0.6±0.2 mag, then the mean effective radius is 2.6±0.2 km. Visible-wavelength photometry of the nucleus, including data published by other groups, lets us constrain the nucleus's R-band geometric albedo: 0.072±0.016. The nucleus's flux contributed about 85% of the light in the mid-infrared images.

Butterfly diagrams of strongly spotted late type stars

At present, long-term (over 30 years) multicolor photometric observations give the possibility to determine general properties of spotted areas on late-type stars. Star-spot modeling from broadband photometric data has been carried out by Alekseev and Gershberg since 1996 under the assumption that spots are situated in two latitudinal zones. Here we propose a new analysis of their results for several G and K dwarf stars with high irregular activity. On these stars, EK Dra, VY Ari, V775 Her, and V833 Tau, two spot belts exist separately and do not merge into a single equatorial active region, as occurs on cooler red-dwarf stars. The zonal spottedness models allow us to fit simultaneously both rotational modulation and long-term variability of stellar brightness. These models give evidence for an equatorward drift of the lower latitude boundary of the spotted region, φ0, during the rising phase of activity, beyond any possible errors concerned with our methodology. In order to evaluate the drift rate we introduce the concept of `effective' spot belt, whose width is independent of longitude. This permits us to construct butterfly diagrams for stellar spots. The equatorward drift rates of the lower boundary of the spotted region D=dφlow/dt are (− 1)–(− 2) deg year−1 in the years of increasing spottedness. These values are less than the analogous solar one D≈−4 deg year−1 for the rising phase of the cycle. Thus, cyclic activity can be revealed from butterfly diagrams and derived drifts of starspots prior to a possible detection from the spectral analysis of photometric variability. Finally, we briefly discuss a possible explanation of high-latitude activity and surface drifts of starspots in the framework of the current state of dynamo theory.

SN 1986J VLBI: The Evolution and Deceleration of the Complex Source and a Search for a Pulsar Nebula

We report on VLBI observations of supernova 1986J in the spiral galaxy NGC 891 at two new epochs, 1990 July and 1999 February, t=7.4 and 15.9 yr after the explosion, and on a comprehensive analysis of these and earlier observations from t~4 yr after the explosion date, which we estimate to be 1983.2 +/- 1.1. The source is a shell or composite, and continues to show a complex morphology with large brightness modulations along the ridge and with protrusions. The supernova is moderately to strongly decelerated. The average outer radius expands as t^(0.71 +/- 0.11), and the expansion velocity has slowed to 6000 km/s at t=15.9 yr from an extrapolated 20,000 km/s at t=0.25 yr. The structure changes significantly with time, showing that the evolution is not self-similar. The shell structure is best visible at the latest epoch, when the protrusions have diminished somewhat in prominence and a new, compact component has appeared. The radio spectrum shows a clear inversion above 10 GHz. This might be related to a pulsar nebula becoming visible through the debris of the explosion. The radio flux density between 1.5 and 23 GHz decreases strongly with time, with the flux density proportional to t^(-2.94 +/- 0.24) between t~15 to 19 yr. This decrease is much more rapid than that found in earlier measurements up to t~6 yr.

Cyclical period changes in Z Chamaeleontis

We report the identification of cyclical changes in the orbital period of the eclipsing dwarf nova Z Cha. We used times of mid-eclipse collected from the literature and our new eclipse timings to construct an observed-minus-calculated diagram covering 30 years of observations (1972-2002). The data present cyclical variations that can be fitted by a linear plus sinusoidal function with period 28+/-2 yr and amplitude 1.0+/-0.2 minute. The statistical significance of this period by an F-test is larger than 99.9%. The derived fractional period change, Delta P/P= 4.4 x 10^{-7}, is comparable to that of other short-period cataclysmic variables (CVs), but is one order of magnitude smaller than those of the long-period CVs. Separate fits to the first and second half of the data lead to ephemerides with quite different cycle periods and amplitudes, indicating that the variation is not sinusoidal or, most probably, is not strictly periodic. The observed cyclical period change is possibly caused by a solar-type magnetic activity cycle in the secondary star. An incremental variation in the Roche lobe of the secondary star of Delta R_{L2}/R_{L2} = 1.7 x 10^{-4} is required in order to explain both the observed period change and the modulation of the quiescent brightness previously reported by Ak, Ozkan & Mattei.

Quasi-Periodic Oscillations in Accretion Discs

Quasi-Periodic Oscillations (QPOs) in Cataclysmic Variable stars (CVs) with periods in the range 100 – 2000 s have been known for 25 years (see Chapter 8 of Warner, 1995a). We distinguish here between QPOs and Dwarf Nova Oscillations (DNOs) – the latter being of shorter period (5 – 50 s) and greater coherence. QPOs appear as brightness modulations and typically have quite short coherence lengths (a few cycles).QPOs are largely associated with discs transferring mass at a high rate – i.e., dwarf novae during outburst or nova-like variables; but the QPOs are not always present and many systems have not shown QPOs at all.Generally it is thought that QPOs result from non-radial oscillations of the accretion discs surrounding the white dwarf component in CVs (Carroll et al., 1985; Collins et al., 2000). They could, e.g., be quite coherent oscillations of individual annuli in a disc, each with a different period. Alternatively, a particular oscillation mode may be excited for only a few cycles and then be re-excited at an arbitrary phase. Most theoretical proposals have sought to make the QPOs self-luminous pulsations of the disc, in analogy with the kappa or epsilon processes of stellar pulsation.

High-speed photometry of faint cataclysmic variables - I. V359 Cen, XZ Eri, HY Lup, V351 Pup, V630 Sgr, YY Tel, CQ Vel and CE-315

The first results of a photometric survey of faint Cataclysmic Variables are presented. V359 Cen is an SU UMa star with a period of 112 min. Even though observed at quiescence, the mass transfer rate in this old nova may be sufficiently high that in such a short period system (with its implied small mass ratio) the disc may be excited into an elliptical shape with the result that the observed brightness modulation gives a superhump period rather than an orbital period. XZ Eri is an eclipsing dwarf nova with an orbital period (P(orb)) of 88.1 min. HY Lup has only slight variability. V351 Pup, the remnant of Nova Puppis 1991, has P(orb) = 2.837 h and a light curve that strongly resembles that of the magnetic Nova Cyg 1975. V630 Sgr is the first nova remnant that has both positive superhumps (P(sh) = 2.980 h) and eclipses (P(orb) = 2.831 h). The YY Tel identification is somewhat uncertain. The correct identification for CQ Vel is provided from discovery of its flickering activity. The light curve of CE-315, a recently discovered AM CVn star, shows similarities to that of GP Com, with no apparent orbital modulation.

The dwarf nova WX Cet: a clone of WZ Sge or a pretender?

We present charge-coupled device (CCD) photometry of WX Cet in quiescence. Apart from the flickering which is characteristic to cataclysmic binaries, our data also reveal the periodic modulation of the brightness of WX Cet with a period of 0.058 27±0.000 02, with further restrictions on it. This period is derived from our data alone, but it agrees, within errors, with the spectroscopic period of Thorstensen et al. Hence the most likely spectroscopic and photometric periods are identical and correspond to the orbital motion. Our data were obtained during two observational seasons in 1990 and 1998. In the former season we observed what appears to be the ordinary orbital hump. However, in 1998 we observed both single- and double-hump orbital modulation. Several authors have noted the similarities between WX Cet and WZ Sge: the occurrence of rare, large-amplitude, long-lasting superoutbursts with superhump modulation, and the slow rate of decline. Both stars have similar, extremely short orbital periods. We recorded further similarities: the orbital modulation of brightness, with switching between single- and double-hump light curves. Patterson noticed that superhump excesses in WX Cet and WZ Sge are different in that they may fall on different evolutionary branches (pre-period minimum versus post-period minimum). We demonstrate that the masses of their white dwarfs differ by a factor of two.

Atomic displacements in the modulated structure of Bi2Sr2(Ca(1-x)Pr(x))Cu2O(8+delta) and their effect on HRTEM reverse contrast image

The amplitude of transverse atomic displacement wave along the direction of incident electron beam exerts a significant effect on the brightness of spots in a high-resolution transmission electron microscope (HRTEM) reverse contrast image. The bright spot is associated with the projection of the modulated atomic chain along the beam direction. In general, the spot brightness is roughly inversely proportional to the amplitude of the displacement wave in an appropriate region of defocus, specimen thickness and amplitude. Monotonic decrease in the spot brightness with increasing amplitude can be attributed to the decrease in the degree of interference between scattered electrons. A complicated domain configuration has been found by the analysis of the spot brightness modulation observed in a [0 0 1] HRTEM reverse contrast image.

Freja and ground‐based analysis of inverted‐V events

During two campaigns, ground‐based auroral observations were performed in coordination with Freja. The high temporal and spatial resolution of the satellite instruments as well as the real‐time recording with a stereoscopic camera system from the ground enabled detailed comparison of small‐ and large‐scale optical phenomena with particle and field data measured by the satellite. Three passes of the satellite over inverted‐V auroral arcs and over precipitation regions with strong field‐aligned electron spectra are investigated. Brightness modulations within auroral arcs coincide with modulations of primary electron fluxes. The dynamics of small‐scale structures within arcs as well as the proper motion of arcs are analyzed and compared with electric fields measured by the satellite and with BARS radar measurements. Energy fluxes independently determined from the ground and from the satellite are used to calculate the field‐aligned conductance. The results agree with predictions of the kinetic theory of the mirror force, if we allow for variations of the density and thermal energy of the electrons in the source region of the magnetosphere. Detailed comparison of electron spectra and electric and magnetic field perturbations provide evidence of different acceleration mechanisms for the electrons, electrostatic acceleration inside inverted‐V's, and wave acceleration in transient regions.

Millisecond Pulsations from a Low‐Mass X‐Ray Binary in the Galactic Center Region

We report the detection with the proportional counter array (PCA) onboard the Rossi X-Ray Timing Explorer (RXTE) of 589 Hz oscillations during three type I X-ray bursts from a low-mass X-ray binary located in the Galactic center region. The bursts, which occurred on 1996 May 15.814, June 4.612, and June 19.414 UT, were observed serendipitously during routine monitoring observations of GRO J1744-28 by RXTE. The strongest pulsations were observed from the May 15 burst, reaching an amplitude of 18% of the >8 keV flux in this burst. The pulsation amplitudes in each burst are a strong function of photon energy since no signals were detected in the 1-8 keV band, but there were strong detections above 8 keV. The evolution of the X-ray spectrum through each burst is consistent with modest radius expansion. The pulsations are confined to the postcontraction portion of each burst profile, consistent with the oscillations recently reported in a burst from KS 1731-26 and also some bursts from 4U 1728-34. The location of the burster in the 1° (FWHM) PCA field of view is constrained by comparing the count rates in the five independent detectors of the PCA. This analysis strongly excludes GRO J1744-28 as the source of these bursts but does not yet allow a definitive identification of the source with any known burster in the field of view. However, the derived position strongly overlaps that of MXB 1743-29, identifying this source as a strong candidate for the 589 Hz burster. We argue that the observed oscillations are consistent with rotational modulation of the X-ray brightness.

Ultraviolet Jets and Bright Points in the Solar Chromosphere. II. Statistical Correlations

We use HRTSVI rocket observations of the solar chromosphere to search for relationships between high-Dopplershift ii jets ˇˇ observed in the C I lines near j 156 nm and internetwork ii bright points ˇˇ observed in the j 160 nm continuum, in sequel to the analysis by Cook et al. which failed to —nd a direct connection between these phenomena. We now use the same data to establish statistical corre- lations between C I Dopplershift and 160 nm brightness modulation in internetwork areas. These mean relations emerge only after extensive spatial averaging and have small amplitude, but are de—nitely sig- ni—cant. They show that both C I Dopplershift and 160 nm brightness participate in oscillatory behavior with 3 minute periodicity and mesoscale (8 Mm wavelength) as well as small-scale (1.4 Mm wavelength) spatial patterning. We —nd spatial and temporal phase relations between Dopplershift and brightness that con—rm that jets and bright points should not be interpreted as isolated entities. Rather, they are chromospheric manifestations, with much pattern interference, of the oscillatory acoustic shock dynamics in the internetwork which also cause Ca II grains. Additional small-scale modulation is present K 2V which we attribute to waves with f-mode character. Subject headings: Sun: chromosphereSun: oscillationsSun: UV radiation

Arm width and brightness modulation of spontaneous behaviour of two strains of mice tested in the elevated plus-maze

Amongst the nonpharmacological variables that may influence spontaneous behaviour in the elevated plus-maze, at least two (namely, the width and brightness level of the maze arms) have not been evaluated for possible consequences on baseline activity. We therefore investigated the effect of width (5,7, or 9 cm) and brightness level (grey vs. white) of the maze arms on the activity patterns of NMRI and C57/BL 10j mice in a plus-maze based on that described by Lister. Results indicated a clear strain difference in spontaneous behaviour in the open/closed arms with C57/BL mice making no visits on the open arms of the maze. In the NMRI strain, enlarging the arms resulted in significantly increased running in open arms but brightness level did not affect behavioural parameters. In the C57/BL mice, no significant behavioural changes were detected after increasing the arm width; however, the maze with a white floor significantly increased the latency to move and decreased ambulation. Finally, employing a maze with white closed arms and open grey arms resulted in ambulation by C57/BL mice into the open arms, a behaviour not normally observed in this strain using a uniformly bright maze. The results are discussed in terms of their putative effects in pharmacological testing.