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In social interactions, our sense of when we have eye contact with another person relies on the distribution of luminance across their eye region, reflecting the position of the darker iris within the lighter sclera of the human eye. This distribution of luminance can be distorted by the lighting conditions, consistent with the fundamental challenge that the visual system faces in distinguishing the nature of a surface from the pattern of light falling upon it. Here we perform a set of psychophysics experiments in human observers to investigate how illumination impacts on the perception of eye contact. First, we find that simple changes in the direction of illumination can produce systematic biases in our sense of when we have eye contact with another person. Second, we find that the visual system uses information about the lighting conditions to partially discount or ‘explain away’ the effects of illumination in this context, leading to a significantly more robust sense of when we have eye contact with another person. Third, we find that perceived eye contact is affected by specular reflections from the eye surface in addition to shading patterns, implicating eye glint as a potential cue to gaze direction. Overall, this illustrates how our interpretation of social signals relies on visual mechanisms that both compensate for the effects of illumination on retinal input and potentially exploit novel cues that illumination can produce.

A stable auroral red (SAR) arc is an aurora with a dominant 630 nm emission at subauroral latitudes. SAR arcs have been considered to occur due to the spatial overlap between the plasmasphere and the ring‐current ions. In the overlap region, plasmaspheric electrons are heated by ring‐current ions or plasma waves, and their energy is then transferred down to the ionosphere where it causes oxygen red emission. However, there have been no study conducted so far that quantitatively examined plasma and electromagnetic fields in the magnetosphere associated with SAR arc. In this paper, we report the first quantitative evaluation of conjugate measurements of a SAR arc observed at 2204 UT on 28 March 2017 and investigate its source region using an all‐sky imager at Nyrölä (magnetic latitude: 59.4°N), Finland, and the Arase satellite. The Arase observation shows that the SAR arc appeared in the overlap region between a plasmaspheric plume and the ring‐current ions and that electromagnetic ion cyclotron waves and kinetic Alfven waves were not observed above the SAR arc. The SAR arc was located at the ionospheric trough minimum identified from a total electron content map obtained by the GNSS receiver network. The Swarm satellite flying in the ionosphere also passed the SAR arc at ~2320 UT and observed a decrease in electron density and an increase in electron temperature during the SAR‐arc crossing. These observations suggest that the heating of plasmaspheric electrons via Coulomb collision with ring‐current ions is the most plausible mechanism for the SAR‐arc generation.

We present a comprehensive investigation on the propagation characteristics of duskside medium‐scale traveling ionospheric disturbances (MSTIDs) using 630.0‐nm airglow emissions over Tromsø (69.6°N, 19.2°E; magnetic latitude: 66.7°N). The unique points of our observation are (1) duskside MSTIDs primarily exhibited eastward motion under quiet conditions but turned to the westward direction associated with geomagnetic disturbances, (2) the westward moving MSTIDs again turned to the eastward direction when the geomagnetic disturbance ceased, (3) the turning of MSTIDs to the westward direction was invariably associated with an increase of the northward component of the magnetic field observed by the local ground‐based magnetometers and with the equatorward expansion of the auroral oval, and (4) the Super Dual Auroral Radar Network convection maps revealed that the location of Tromsø was inside (outside) the duskside convection cell during the time of appearance of westward (eastward) moving MSTIDs. The average eastward and westward velocities of MSTIDs were ~25–80 and ~40–140 m/s, respectively. The Doppler shift measurement of the 630‐nm airglow by a Fabry‐Perot interferometer at Tromsø showed that northeastward winds were predominant during the appearance of eastward moving MSTIDs. These experimental evidences suggest that the oscillatory motion of MSTIDs over high latitudes is driven by the convection electric field. The MSTIDs tend to move eastward under geomagnetically quiet conditions but show westward motion under the influence of convection electric field associated with auroral activities in the duskside of two‐cell convection pattern.

Chorus waves, among the most intense electromagnetic emissions in the Earth’s magnetosphere, magnetized planets, and laboratory plasmas, play an important role in the acceleration and loss of energetic electrons in the plasma universe through resonant interactions with electrons. However, the spatial evolution of the electron resonant interactions with electromagnetic waves remains poorly understood owing to imaging difficulties. Here we provide a compelling visualization of chorus element wave–particle interactions in the Earth’s magnetosphere. Through in-situ measurements of chorus waveforms with the Arase satellite and transient auroral flashes from electron precipitation events as detected by 100-Hz video sampling from the ground, Earth’s aurora becomes a display for the resonant interactions. Our observations capture an asymmetric spatial development, correlated strongly with the amplitude variation of discrete chorus elements. This finding is not theoretically predicted but helps in understanding the rapid scattering processes of energetic electrons near the Earth and other magnetized planets.

Energy input from the magnetosphere during substorms can strongly affect the high-latitude thermosphere. The ionospheric current caused by thermospheric wind variations may also provide a feedback to the magnetosphere. In this study, we investigate the characteristics of high-latitude thermospheric wind variations at local substorm onsets at Tromsø, Norway, as well as the possibility of such feedback mechanism. A Fabry–Perot interferometer (FPI) at Tromsø provided wind measurements estimated from the Doppler shift of red-line emission (630.0 nm) of aurora and airglow. We analyzed wind data in 2009 with a time resolution of ~ 13 min. We first carefully identified the onset times of isolated local substorms at Tromsø and extracted four wind measurements from red-line emission. All these events showed increases of eastward components at local substorm onsets. For northward components, these events showed decreases except for those at midnight. The observed wind variations at local substorm onsets were less than 49 m/s. These values are much smaller than the typical plasma convection speed in the auroral zone. We speculate that the ionospheric current caused by thermospheric wind variations at local substorm onsets does not provide strong feedback to the development of substorm expansion phase in the magnetotail. We discuss the possible causes of these wind variations in the context of plasma convection, diurnal tides, and arc-associated electric field.