Sky Camera Research Articles

All Sky Camera and Fabry-Perot Interferometer Observations in the Northern Polar Cap

We report all sky camera and Fabry-Perot interferometer (FPI) observations of mesospheric gravity waves and a 12-hour wave at Resolute <TEX>$(75^{\circ}N)$</TEX> and a joint observation of 10-hour wave with Eureka <TEX>$(80^{\circ}N)$</TEX>. All sky camera observations showed a low occurrence of mesosphere gravity waves during equinoxes, which is similar to the mid-latitude region. A slightly higher occurrence near solstice appears to indicate that gravity waves are not filtered out by the neutral wind in the winter. The FPI observation of a 12-hour wave showed amplitude variations from day to day. The phase of the wave is mostly stable and consistent with the GSWM prediction in the winter. The phase shifts with season as predicted by the GSWM. Four events of the 12-hour wave were found in spring with amplitudes larger than the GSW predictions. The FPls at Resolute and Eureka also observed a wave with period close to 10 hours. The 10-hour wave maybe the result of the non-linear interaction between the semi-diurnal tide and the quasi-two day wave. Further studies are under way. Overall, the combined Resolute and Eureka observation have revealed some new fractures about the mesospheric gravity wave, tidal wave, and other oscillations.

Effect of cloud on UVA and exposure to humans.

The daily autumn and winter ultraviolet-A (320-400 nm) (UVA) exposures and 6 min UVA irradiance data for a southern hemisphere subtropical site (Toowoomba, Australia, 27.6 degrees S, 151.9 degrees E) are presented. This data is used to quantify the effect of cloud on UVA using an integrated sky camera and radiation system. Additionally, an estimate of the effect of enhanced UVA exposure on humans is made. The measurement system consisted of broad-band visible-infrared and UVA sensors together with a sun tracking, wide-angle video camera. The mean daily June exposure was found to be 409 kJ m-2. Under the constraints of the uncertainty of both the UVA measurement system and clear-sky model, one case of enhanced UVA irradiance was found. Three cases of cloud enhancement of daily UVA exposure, approaching clear-sky levels, were also determined using a calculated clear-sky envelope. It was also determined that for a fulltime outdoor worker the additional UVA exposure could approach approximately that of one third of a full winter's day. For indoor workers with an outside lunch break of 12:00-1:00 P.M. the additional UVA exposure was on an average 6.9 kJ m-2 over three cloud-enhanced days. To the authors' knowledge this is the first paper to present some evidence of cloud-enhanced UVA human exposure.

Evaluation of a sky/cloud formula for estimating UV‐B irradiance under cloudy skies

A new sky/cloud formula is evaluated using results collected over 1 year from an integrated sky camera and irradiance measurement system. Sky properties were recorded every 6 min, simultaneously alongside solar radiation measurements at Toowoomba, Australia (27.6°S, 151.9°E), over the solar zenith angle range of 4.2° to 64.3°. This is the first sky formula that incorporates a complement of parameters based on sky properties around the Sun. The sky properties included cloud cover, cloud brokenness, cloud brightness variation, the angle of maximum cloud cover and aureole brightness. The sky formula was developed by correlating the ratio of the measured UV‐B irradiance to the modeled clear sky UV‐B irradiance for the same atmospheric conditions, with each of the five sky properties listed above. Variation of the UV‐B ratio for each individual sky property is examined. This sky formula, used in conjunction with a clear sky semiempirical UV‐B model, was compared to three published cloud formulas based on cloud cover alone. Using a data set restricted to cases of cloud obscuring the solar disk, the new formula was found to have the highest r2 value of 0.82. Further comparison with a fourth published formula using a subset of data containing predominately cirrus cloud type, produced an explained variance of 0.91 between the measured and the modeled data.

The effect of clouds on enhancing UVB irradiance at the Earth's surface: A one year study

The most extensive study of frequency, magnitude and sky properties associated with UVB enhancement is reported. The first integrated automatic sky camera and irradiance measurement system was used for simultaneous data collection over the solar zenith angle range of 4.2 to 64.3°. It is reported that 3% of UVB irradiance measurements collected at intervals of 6 minutes, over a one‐year period, were cloud enhanced. It was found that 85% of possible UVB enhancements occurred for solar zenith angles ranging from 40 to 63°. The study was undertaken during September 1997 to August 1998 at Toowoomba, Australia (27.6°S, 151.9°E). The maximum UVB enhancement was found to have an irradiance of 8% greater than that of an equivalent clear‐sky. A mechanism for UVB enhancement for cases of solar obstruction by cirrus cloud is postulated. Analysis showed that 86% of enhancement occurred under hazy/cirrus skies.

Evaluation of a Ground-Based Sky Camera System for Use inSurface Irradiance Measurement

This paper describes the evaluation of a ground-based sky camera system for studying the effect of clouds on the level of the ambient ultraviolet radiation. The system has been developed for research in the characterization of the effect of clouds around the sun. It is the first sky camera system to be used for the assessment of cloud conditions in the vicinity of the sun, rather than a whole-sky assessment. The system features a sun-tracking sky camera with an integrated measurement of horizontally received radiation at the same location. The imageprocessing algorithm uses solar radiation readings to reduce reflections from the sun on the camera system being mistaken for cloud in the images. Cloud amount was estimated in an angular region of between 12.58 and 37.58 around the sun. The algorithm also estimates the amount of solar obstruction by cloud (sun not covered, partially or totally covered). The system was evaluated during September 1997 at Toowoomba, Australia (27.68S latitude). Compared to manual assessment of 592 images, 76.5% were identified correctly by the algorithm for the degree of solar obstruction and 81.9% for cloud amount. The behavior of ultraviolet radiation levels with cloud conditions is discussed.

雷観測のための広帯域及び狭帯域干渉計の比較

The location of lightning radiation sources observed by our narrow band interferometer and the lightning channel image by an all sky camera are compared with that of our broad band interferometer. The broad band interferometer system is described by Ushio and Kawasaki et al. [1997] and measures the two dimen-sional direction of the radiation sources from 25 to 250 MHz with sub-microsecond time resolution, using 500 MHz digitizer which has 8 Megabyte memory. These three observations show the excellent correspondence, and we can conclude the potential capability of the newly designed wide band interferometer. By analyzing simultaneously observed data by narrow band and wide band interferometers for of one flash, the advantages and disadvantages of the both type of interferometers are discussed. The analysis shows that the broad band system can also locate two radiation sources, which propagate simultaneously like branching, in time and space. And we conclude this is a new technology.

The high latitude convection response to an interval of substorm activity

Abstract. On 17 March 1991, five clear substorm onsets/intensifications took place within a three hour interval. During this interval ground-based data from the EISCAT incoherent scatter radar, a digital CCD all sky camera, and an extensive array of magnetometers were available, in addition to data from the CRRES and DMSP spacecraft, whose footprints passed over Scandinavia very close to most of the ground-based instrumentation. This interval of substorm activity has been interpreted as being in support of a near-Earth current disruption model of substorm onset. In the present study the ionospheric convection response, observed some four hours to the west in MLT by the Halley HF radar in Antarctica, is related to the growth, expansion and recovery phases of two of the substorm onsets/expansions observed in the Northern Hemisphere. Bursts of ionospheric flow and motion of the convection reversal boundary (CRB) are observed at Halley in response to the substorm activity and changes in the IMF. The delay between the substorm expansion phase onset and the response in the CRB location is dependent on the local time separation from, and latitude of, the initial substorm onset region. These results are interpreted in terms of a synthesis of the very near-Earth current disruption model and the near-Earth neutral line model of substorm onset.

Simultaneous Satellite and Ground Based Observations of Polar Cap Phenomena

In order to understand the processes of formation and motion of sun-aligned polar cap arcs as well as patches, we have analyzed simultaneous observations from the Akebono (EXOS-D) satellite and ground based instruments at Qaanaaq. Results demonstrated the correspondence between the localized electron precipitation signatures observed from Akebono and multiple sun-aligned arcs observed using an all sky camera (ASC) at Qaanaaq and have confirmed that sun-aligned arcs correspond to divE < 0. We also confirmed the time delay ofappearance and disappearance of sun-aligned arcs in response to IMF change. As for patches, most of the patches drifted anti-sunward, but some ceased to drift within field of view of the all sky camera. Results of comparison between patch movement and the convection demonstrated a general consistency between them.

The high latitude convection response to an interval of substorm activity

On 17 March 1991, five clear substorm onsets/intensifications took place within a three hour interval. During this interval ground-based data from the EISCAT incoherent scatter radar, a digital CCD all sky camera, and an extensive array of magnetometers were available, in addition to data from the CRRES and DMSP spacecraft, whose footprints passed over Scandinavia very close to most of the ground-based instrumentation. This interval of substorm activity has been interpreted as being in support of a near-Earth current disruption model of substorm onset. In the present study the ionospheric convection response, observed some four hours to the west in MLT by the Halley HF radar in Antarctica, is related to the growth, expansion and recovery phases of two of the substorm onsets/expansions observed in the Northern Hemisphere. Bursts of ionospheric flow and motion of the convection reversal boundary (CRB) are observed at Halley in response to the substorm activity and changes in the IMF. The delay between the substorm expansion phase onset and the response in the CRB location is dependent on the local time separation from, and latitude of, the initial substorm onset region. These results are interpreted in terms of a synthesis of the very near-Earth current disruption model and the near-Earth neutral line model of substorm onset.

CRRES/Ground-based multi-instrument observations of an interval of substorm activity

Abstract. Observations are presented of data taken during a 3-h interval in which five clear substorm onsets/intensifications took place. During this interval ground-based data from the EISCAT incoherent scatter radar, a digital CCD all sky camera, and an extensive array of magnetometers were recorded. In addition data from the CRRES and DMSP spacecraft, whose footprints passed over Scandinavia very close to most of the ground-based instrumentation, are available. The locations and movements of the substorm current system in latitude and longitude, determined from ground and spacecraft magnetic field data, have been correlated with the locations and propagation of increased particle precipitation in the E-region at EISCAT, increased particle fluxes measured by CRRES and DMSP, with auroral luminosity and with ionospheric convection velocities. The onsets and propagation of the injection of magnetospheric particle populations and auroral luminosity have been compared. CRRES was within or very close to the substorm expansion phase onset sector during the interval. The onset region was observed at low latitudes on the ground, and has been confirmed to map back to within L=7 in the magnetotail. The active region was then observed to propagate tailward and poleward. Delays between the magnetic signature of the substorm field aligned currents and field dipolarisation have been measured. The observations support a near-Earth plasma instability mechanism for substorm expansion phase onset.

CRRES/Ground-based multi-instrument observations of an interval of substorm activity

Abstract. Observations are presented of data taken during a 3-h interval in which five clear substorm onsets/intensifications took place. During this interval ground-based data from the EISCAT incoherent scatter radar, a digital CCD all sky camera, and an extensive array of magnetometers were recorded. In addition data from the CRRES and DMSP spacecraft, whose footprints passed over Scandinavia very close to most of the ground-based instrumentation, are available. The locations and movements of the substorm current system in latitude and longitude, determined from ground and spacecraft magnetic field data, have been correlated with the locations and propagation of increased particle precipitation in the E-region at EISCAT, increased particle fluxes measured by CRRES and DMSP, with auroral luminosity and with ionospheric convection velocities. The onsets and propagation of the injection of magnetospheric particle populations and auroral luminosity have been compared. CRRES was within or very close to the substorm expansion phase onset sector during the interval. The onset region was observed at low latitudes on the ground, and has been confirmed to map back to within L =7 in the magnetotail. The active region was then observed to propagate tailward and poleward. Delays between the magnetic signature of the substorm field aligned currents and field dipolarisation have been measured. The observations support a near-Earth plasma instability mechanism for substorm expansion phase onset.

Magnetic impulses and associated optical signatures in the dayside aurora

We present five magnetic impulse events observed at South Pole and in the magnetically conjugate area at Iqaluit. The magnetic signatures recorded during these events appear to be similar to those discussed by several authors as possible ionospheric signatures of sporadic dayside reconnection via the flux transfer process. Coordinated ground‐based image‐intensified all sky camera data were acquired at South Pole during the events. The optical data show that prior to the onset of each of the events the dayside cusp aurora is several degrees poleward of the station, signifying event initiation on closed field lines. Overhead aurora appears at the onset of the magnetic event and precipitation seems to expand over a large region equatorward of the quiescent cusp aurora. As the event decays the aurora dies away and the quiescent dayside cusp aurora, in four out of five cases, returns poleward of the station. This morphology is inconsistent with the model in which the ground based signature of an FTE is caused by a field aligned current configuration moving over a ground station in the antisunward direction. The observations show temporal events which begin at or inside the boundary of closed field lines. These are consistent with sudden precipitation activity on flux tubes which were previously equatorward of the cusp/cleft boundary and which became temporarily disconnected (non‐dipolar).

Polar cap F‐layer auroras

Polar cap auroras were measured at Thule Air Base, Greenland in December 1979, using an all sky imaging photometer. These images combined with digital ionosonde measurements, show the orientation, structure, and drift motion of sun‐aligned, F layer auroras. Drifts in both the dawn‐to‐dusk and dusk‐to‐dawn direction were observed. These auroras were not visible on the all sky camera films; however, coincident Defense Meteorological Satellite Program (DMSP) precipitating electron measurements show that these arcs are produced by fluxes of low energy (E&lt;500 ev) electrons. Simultaneous satellite‐to‐ground signal amplitude measurements show that these auroras are accompanied by km scale size ionospheric irregularities, which cause strong amplitude fluctuations (scintillation) on the received satellite signal.

Sealed Position Sensitive Hard X-Ray Detector Having Large Drift Region for All Sky Camera with High Angular Resolution

A sealed position sensitive proportional counter filled with two atmospheres of 95% xenon and 5% methane, and containing a drift region of 24 atm cm, has operated in a stable manner for many months. The detector contains G-10 frames to support the anode and cathode wires. The detector was sealed successfully by a combination of vacuum baking the G-10 frames at 150°C for two weeks followed by assembly into the detector in an environment of dry nitrogen, and the use of passive internal getters. The counter is intended for use with a circumferential cylindrical collimator containing many random slit openings. Together they provide a very broad field of view detection system with the ability to locate cosmic hard X-ray and soft gamma ray sources to an angular precision of a minute of arc. A set of instruments based on this principle have been proposed for satellites to detect and precisely locate cosmic gamma ray bursts.

A spherical harmonic analysis of the austral auroral oval

Computerized spherical harmonic analysis is applied to the morphology of the southern auroral oval. Records from 23 All Sky Camera stations together with visual observation reports for the period 1957–1959 constitute the raw data set. The mode of the derived auroral occurrence distribution function F( K p , θ, φ) is regarded as the maximum probability contour and yields a set of auroral ovals. These 10 contours, one for each K p level, are expressed in the invariant magnetic co-ordinates of Bond (1968).

Total electron content measurements during visible auroras

The ionospheric total electron content was measured continuously from Macquarie Island (54.5°S, 150.0°E geographic) by monitoring the Faraday rotation of the polarization angle of the radio beacon from the geostationary satellite INTELSAT 2F-2. The ray path from the satellite often passes through the auroral oval during nighttime, and an association has been found between total electron content increases and auroral particle precipitation as determined from all sky camera photographs. The results from 31 March 1970 are examined. Simultaneous all sky camera photographs indicate a total electron content increase caused by a multiple quiet arc structure. The average electron density was calculated from electron content data in conjunction with the auroral dimensions determined from the photographs. The average electron density over a height range up to about 170 km for a multiple quiet arc structure was 3.0 × 10 5 cm −3. The subsequent breakup and poleward expansive phase of the substorm produced a further electron content increase.

Two substorm studies of relations between westward electric fields in the outer plasmasphere, auroral activity, and geomagnetic perturbations

Temporal variations of the westward component of the magnetospheric convection electric field in the outer plasmasphere were compared to auroral activity near L = 7, and to variations in the geomagnetic field at middle and high latitudes. The substorms occurred on July 29, 1965 near 0530 UT and on August 20, 1965 near 0730 UT. The results on westward electric field E(w) were obtained by the whistler method using data from Eights, Antarctica (L is approximately 4). All sky camera records were obtained from Byrd, Antarctica, (L is approximately 7), located within about 1 hour of Eights in magnetic local time. It was found that E(w) within the outer plasmasphere increased rapidly to substorm levels about the time of auroral expansion at nearby longitudes. This behavior is shown to differ from results on E(w) from balloons, which show E(w) reaching enhanced levels prior to the expansion. A close temporal relation was found between the rapid, substorm associated increases in E(w) and a well known type of nightside geomagnetic perturbation. Particularly well defined was the correlation of E(w) rise and a large deviation of the D component at middle latitudes.

Relationship between magnetospheric electric fields and the motion of auroral forms

Balloon-measured magnetospheric electric fields comparison with all sky camera pictures of large scale visible auroral form motions, noting relationship

Artificial red aurora—I: Phenomenology and electronic excitation

A theoretical analysis of the red glow created in the southern conjugate region by the atmospheric nuclear burst of 9 July 1962 is described. This investigation is based on photometer measurements, all sky camera measurements, plus other material published previously. Heavy charged particles from the north are assumed to be the carriers of energy which initiated the red glow. A mechanism by which this ion source was coupled to the red glow in the 300–500 km altitude regime is assumed: energy transfer to ambient atoms and free electrons and subsequent electronic excitation of the 5577 Å and 6300 Å lines of atomic oxygen. This mechanism is investigated and it is concluded on the basis of a comparison of the theoretical calculations with the data that electronic excitation was likely the dominating glow contributing source. This result is then checked for consistency against the background of other known information on nuclear effects.

On the orientation of auroral arcs

Information on the direction of auroral arcs, mostly all sky camera data during IGY, has been collected and organized in a geomagnetic coordinate system, corrected by the higher order spherical harmonic terms.By means of interpolation and integration between the measured values it was possible to obtain curves in a polar plot representing auroral arcs over the polar region in the latitude range 64–90 degrees.The ovals of auroral arcs do not follow the corrected geomagnetic latitudes. They are furthest from the pole on the night-side e.g. 70.5° at 24, 79.9° at 06, 79.2° at 12 and 75.5° of latitude at 18 hours. A small dawn-dusk asymmetry has also been observed, the curves being closer to the pole at 06 hours than at 18 hours.In the polar region between latitudes 80–85° for a time interval around 1600 hours it has been found that the auroral arcs have an angle relative to the solar direction of about 20 degrees.The shape of the auroral orientation curves corresponds (in the auroral zone region) with the auroral oval based on the occurrence of aurora.