Direction Of Observation Research Articles

Venus' cloud top wind study: Coordinated Akatsuki/UVI with cloud tracking and TNG/HARPS-N with Doppler velocimetry observations

We present new wind velocities and its variability at Venus cloud-tops (≈70 km) based on coordinated observations with two complementary techniques: (1) space-based cloud-tracking using JAXA's Akatsuki/UVI imaging and (2) ground-based Doppler velocimetry using the TNG/HARPS-N (Telescopio Nazionale Galileo). This was the first use of HARPS-N to observe a Solar system planetary atmosphere. The measurement of both zonal and meridional wind velocities and the constraints of temporal and spatial wind variability at cloud top is essential to our understanding of the mechanisms which yield and maintain the dynamics of Venus' atmospheric superrotational circulation.From space, clouds features were tracked on images obtained by the Akatsuki instrument (UVI) operating in the ultraviolet range (365 nm filter), acquired in orbit #39, between 26 and 31 of January 2017. From the ground, data acquired on 28–29 January 2017 using TNG/HARPS-N, Doppler velocimetry measurements are based on high-resolution spectra (resolution of ∼115,000) of Fraunhofer lines in the visible range (383–690 nm) using incoming solar radiation scattered by cloud top particles in the observer's direction (Widemann et al., 2008; Machado et al., 2014; Machado et al., 2017).The analysis and results of this new coordinated dataset indicate (1) cross-validation and complementarity of cloud-tracking and Doppler velocimetry measurements; (2) a significant North-South asymmetry of zonal wind circulation, of the order of 10 ms−1 higher in the South hemisphere, in Akatsuki/UVI observations (in accordance with previous detection by Horinouchi et al. (2018)); (3) we present a high precision cloud-top meridional wind flow, obtained with HAPRS-N; a poleward meridional flow, symmetrical with respect to equator, clearly measured in both hemispheres; it peaks around 40° N and S with an amplitude of v¯m = 30±5 ms−1; (4) spatial and temporal variability of Akatsuki's zonal and meridional wind flow is also observed, with a significant increase of wind flow near evening terminator.

Magnetic absorption of VHE photons in the magnetosphere of the Crab pulsar

The detection of the pulsed $\sim 1 $~TeV gamma-ray emission from the Crab pulsar reported by MAGIC and VERITAS collaborations demands a substantial revision of existing models of particle acceleration in the pulsar magnetosphere. In this regard model independent restrictions on the possible production site of the VHE photons become an important issue. In this paper, we consider limitations imposed by the process of conversion of VHE gamma rays into $e^{\pm}$ pairs in the magnetic field of the pulsar magnetosphere. Photons with energies exceeding 1~TeV are effectively absorbed even at large distances from the surface of the neutron star. Our calculations of magnetic absorption in the force-free magnetosphere show that the twisting of the magnetic field due to the pulsar rotation makes the magnetosphere more transparent compared to the dipole magnetosphere. The gamma-ray absorption appears stronger for photons emitted in the direction of rotation than in the opposite direction. There is a small angular cone inside which the magnetosphere is relatively transparent and photons with energy $1.5$~TeV can escape from distances beyond $0.1$~light cylinder radius ($R_{\rm{lc}}$). The emission surface from where photons can be emitted in the observer's direction further restricts the sites of VHE gamma-ray production. For the observation angle $57^{\circ}$ relative to the Crab pulsar axis of rotation and the orthogonal rotation, the emission surface in the open field line region is located as close as $0.4\,R_{\rm{lc}}$ from the stellar surface for a dipole magnetic field, and $0.1\,R_{\rm{lc}}$ for a force-free magnetic field.

Interpreting the time variable RM observed in the core region of the TeV blazar Mrk 421

In this work we interpret and discuss the time variable rotation measure (RM) found, for the first time over a 1-yr period, in the core region of a blazar. These results are based on a one-year, multi-frequency (15, 24, and 43 GHz) Very Long Baseline Array (VLBA) monitoring of the TeV blazar Markarian 421 (Mrk 421). We investigate the Faraday screen properties and its location with respect to the jet emitting region. Given that the 43 GHz radio core flux density and the RM time evolution suggest a similar trend, we explore the possible connection between the RM and the accretion rate. Among the various scenarios that we explore, the jet sheath is the most promising candidate for being the main source of Faraday rotation. During the one-year observing period the RM trend shows two sign reversals, which may be qualitatively interpreted within the context of the magnetic tower models. We invoke the presence of two nested helical magnetic fields in the relativistic jet with opposite helicities, whose relative contribution produce the observed RM values. The inner helical field has the poloidal component ($B_{\rm p}$) oriented in the observer's direction and produces a positive RM, while the outer helical field, with $B_{\rm p}$ in the opposite direction, produces a negative RM. We assume that the external helical field dominates the contribution to the observed RM, while the internal helical field dominates when a jet perturbation arises during the second observing epoch. Being the intrinsic polarization angle parallel to the jet axis, a pitch angle of the helical magnetic field $\phi\gtrsim 70^\circ$ is required. Additional scenarios are also considered to explain the observed RM sign reversals.

HIGH-RESOLUTION SATELLITE IMAGING OF THE 2004 TRANSIT OF VENUS AND ASYMMETRIES IN THE CYTHEREAN ATMOSPHERE

This paper presents the only space-borne optical-imaging observations of the 2004 June 8 transit of Venus, the first such transit visible from Earth since AD 1882. The high-resolution, high-cadence satellite images we arranged from NASA's Transition Region and Coronal Explorer (TRACE) reveal the onset of visibility of Venus's atmosphere and give further information about the black-drop effect, whose causes we previously demonstrated from TRACE observations of a transit of Mercury. The atmosphere is gradually revealed before second contact and after third contact, resulting from the changing depth of atmospheric layers refracting the photospheric surface into the observer's direction. We use Venus Express observations to relate the atmospheric arcs seen during the transit to the atmospheric structure of Venus. Finally, we relate the transit images to current and future exoplanet observations, providing a sort of ground truth showing an analog in our solar system to effects observable only with light curves in other solar systems with the Kepler and CoRoT missions and ground-based exoplanet-transit observations.

The entrance pupil of the human eye: a three-dimensional model as a function of viewing angle

Precise peripheral ocular measurements have become important in vision research. These measurements are influenced by the shape and position of the peripherally observed entrance pupil. A long-held assumption is that its apparent shape is elliptical and is optically centered in its position. Our three-dimensional model shows that as viewing angle increases, the entrance pupil moves forward, tilts and curves towards the observer’s direction. Moreover, the tangential pupil size narrows and exhibits asymmetric distortions. Consequently, its shape is non-elliptical and its geometric mid-point departs from the optical center. These findings may have implications on the accuracy of peripheral ocular measurements.

Venus Doppler winds at cloud tops observed with ESPaDOnS at CFHT

We present new wind measurements in Venus’ lower mesosphere from visible spectroscopy during the 2007 worldwide coordinated ground campaign in support of ESA's Venus Express mission. These observations consisted of high-resolution spectra of Fraunhofer lines in the entire visible range (0.37–1.05 μm) to measure the winds near 68 km using the Doppler shift of solar radiation scattered by clouds toward the observer's direction. The observations included various points of the dayside hemisphere at a phase angle of ∼109°. We took advantage of two symmetrical elongations in July and September 2007 at Canada–France–Hawaii's 3.6-m telescope. Kinematical fits to the Doppler winds provide a mean equatorial velocity of (104±10) m s −1 for the zonal retrograde flow. This velocity agrees quite well with the mean value obtained by tracking the UV markings from several spacecraft.

Chapter 16 Neural analysis of visual information during locomotion

For most vertebrate species, vision is critical during locomotion. In this review, we survey what is known about neural mechanisms that might be involved in this visual analysis. Problems that such mechanisms are likely to solve include: (1) determination of heading (the discrepancy between an observer's direction of motion and direction of gaze); (2) detection of course changes; (3) setting of courses using goals and landmarks; (4) obstacle avoidance; (5) accurate foot placement. To understand how the visual system handles these tasks, we must first consider what neurons 'see' during locomotion: this is determined both by an observer's motion and by his gaze. We then review response properties of neurons in the cortical 'motion pathways' of the monkey and cat as they relate to problems encountered during locomotion. The most studied areas are the medial superior temporal area in the monkey, which has been linked to heading determination, and the lateral suprasylvian area in the cat, where many cells are sensitive to motion in depth, and some are selective for optic flow patterns generated during locomotion. A few subcortical populations have also been linked to visual analysis during locomotion. Most notable are cells in the pigeon's nucleus rotundus: these respond selectively to looming stimuli, some firing at a specific time before the stimulus collides with the bird. Another intriguing population is in the cat's visual pontine nucleus, where cells respond to large displays suggestive of optic flow during locomotion.

Optic flow illusion and single neuron behaviour reconciled by a population model.

Radial patterns of optic flow contain a centre of expansion that indicates the observer's direction of self-movement. When the radial pattern is viewed with transparently overlapping unidirectional motion, the centre of expansion appears to shift in the direction of the unidirectional motion [Duffy, C.J. & Wurtz, R.H. (1993) Vision Res., 33, 1481-1490]. Neurons in the medial superior temporal (MST) area of monkey cerebral cortex are thought to mediate optic flow analysis, but they do not shift their responses to parallel the illusion created by transparent overlap. The population-based model of optic flow analysis proposed by Lappe and Rauschecker replicates the illusory shift observed in perceptual studies [Lappe, M. & Rauschecker, J.P. (1995) Vision Res., 35, 1619-1631]. We analysed the behaviour of constituent neurons in the model, to gain insight into neuronal mechanisms underlying the illusion. Single model neurons did not show the illusory shift but rather graded variations of their response specificity. The shift required the aggregate response of the population. We compared the model's predictions about the behaviour of single neurons with the responses recorded from area MST. The predicted distribution of overlap effects agreed with that observed in area MST. The success of the population-based model in predicting the illusion and the neuronal behaviour suggests that area MST uses the graded responses of single neurons to create a population response that supports optic flow perception.

The effect of gap depth on the perception of whether a gap is crossable.

Four experiments were performed in order to examine the effect of gap depth on human observers' perception of whether or not a gap is crossable. Experiments 1 and 2 showed that as the gap's depth increased, observers tended to increasingly underestimate the maximum width of a gap they could step across. Experiments 3 and 4 clarified this finding: The observed covariation of perceived gap crossability and gap depth depended on the observer's direction of gaze, rather than on the physical depth of the gap. The optical relations to which observers might be attending are discussed, as well as the possibility that cognitive-affective processes might have contributed to observers' underestimation of their actual capabilities.

A Determination of Pulsar Emission Geometry from Polarization Observations

AbstractOne of the important problems in pulsar studies is to determine the magnetic inclination angle α, the intrinsic width of the radiation beam (2ρ) and the angle (α + β) between the observer's direction and the rotation axis. In this paper we solve this problem for individual pulses by using the observed pulse width (2Δ𝜙), the swing of polarization angle (2Δψ), and its central gradient (dψ/d𝜙)max.From the polar cap model we establish three basic geometrical relations, a complete set of equations from which explicit solutions can be obtained using the observed data. This is the first time that the orientation of pulsar emission is solved analytically solely on the basis of a geometrical model. However, the results are shown to be sensitively connected to the polarization-angle swing (2Δψ), which is not well measured for most pulsars. So the number of pulsars to which our method can be applied is limited. The importance of the measurement of Δψ is seen from our method. To ensure the credibility of our results, we have discussed the conditions to be satisfied by all reliable pulsar measurements. Our method is shown to be more favorable for pulsars with large pulse width 2Δ𝜙, large central gradient (dψ/d𝜙)max and large magnetic inclination angle α. Out of 120 pulsars (from Lyne and Manchester 1988), 40 are solvable, and 7 are believed to be reliable. We discuss our method for the determination of pulsar geometry in comparison with other methods.

Relevance of Variable Negative Radial Velocities to Models of Sco X-1

Westphal, Sandage and Kristian1 reported that the emission line radial velocities of Sco X-1 are variable on a time scale of days or even hours, with an amplitude of perhaps ±50 km s−1, or occasionally somewhat greater. I wish to point out that this has a bearing on possible models for Sco X-1. Naturally, this rapid velocity variation requires the source gas to be in close association with the central physical object and not simply a detached cloud outside its immediate gravitational influence. It must be, then, that the generally negative velocities (mean value ≃ −100 km s−1) are due either to (a) a true approach velocity for the central object, or (b) a preferential direction of motion in the observer's direction for the gas streams causing the lines. I shall now discuss the implication of points (a) and (b).