24 research outputs found
Combined EISCAT radar and optical multispectral and tomographic observations of black aurora
Black auroras are recognized as spatially well-defined regions within a uniform diffuse auroral background where the optical emission is significantly reduced. Black auroras typically appear post-magnetic midnight and during the substorm recovery phase, but not exclusively so. We report on the first combined multimonochromatic optical imaging, bistatic white-light TV recordings and incoherent scatter radar observations of black aurora by EISCAT of the phenomenon. From the relatively larger reduction in luminosity at 4278 Å than at 8446 Å we show that nonsheared black auroras are most probably not caused by downward directed electrical fields at low altitude. From the observations, we determine this by relating the height and intensity of the black aurora to precipitating particle energy within the surrounding background diffuse aurora. The observations are more consistent with an energy selective loss cone. Hence the mechanism causing black aurora is most probably active in the magnetosphere rather than close to Earth
Results from the intercalibration of optical low light calibration sources 2011
Following the 38th Annual European Meeting on Atmospheric Studies by Optical Methods in Siuntio in Finland, an intercalibration workshop for optical low light calibration sources was held in Sodankylä, Finland. The main purpose of this workshop was to provide a comparable scale for absolute measurements of aurora and airglow. All sources brought to the intercalibration workshop were compared to the Fritz Peak reference source using the Lindau Calibration Photometer built by Wilhelm Barke and Hans Lauche in 1984. The results were compared to several earlier intercalibration workshops. It was found that most sources were fairly stable over time, with errors in the range of 5–25%. To further validate the results, two sources were also intercalibrated at UNIS, Longyearbyen, Svalbard. Preliminary analysis indicates agreement with the intercalibration in Sodankylä within about 15–25%
The electron energy distribution during HF pumping, a picture painted with all colors
The shape of the electron energy distribution has long been a central question in the field of high-frequency radio-induced optical emission experiments. This report presents estimates of the electron energy distribution function, <i>f<sub>e</sub></i>(E), from 0 to 60 eV, based on optical multi-wavelength (6300, 5577, 8446, 4278Å) data and 930-MHz incoherent scatter radar measurements of ion temperature, electron temperature and electron concentration. According to our estimate, the electron energy distribution has a depression at around 2 eV, probably caused by electron excitation of vibrational states in N<sub>2</sub>, and a high energy tail that is clearly supra-thermal. The temporal evolution of the emissions indicates that the electron temperature still plays an important role in providing electrons with energies close to 2 eV. At the higher energies the electron energy distribution has a non-thermal tail.<p> <b>Keywords.</b> Active experiments; Ionosphere atmosphere interaction; Ionospheric physic
Enhanced EISCAT UHF backscatter during high-energy auroral electron precipitation
Natural enhancements in the backscattered power
of incoherent scatter radars up to 5 orders of magnitudes
above the thermal backscatter are sometimes observed at
high latitudes. Recently observations of enhancements in the
backscattered power including a feature at zero Doppler shift
have been reported. These enhancements are limited in altitude
to tens of kilometers. The zero Doppler shift feature has
been interpreted as a signature of electron density cavitation.
Enhanced plasma lines during these observations have also
been reported. We report on the first EISCAT UHF observations
of enhanced backscattered radar power including a zero
Doppler shift feature. The enhancements originated from two
distinct and intermittent layers at about 200 km altitude. The
altitude extent of the enhancements, observed during auroral
high-energy electron precipitation, was < 2 km
A study of fine structure of diffuse aurora with ALIS-FAST measurements
We present results of an investigation of the fine structure of the night sector diffuse auroral zone, observed simultaneously with optical instruments (ALIS) from the ground and the FAST electron spectrometer from space 16 February 1997. Both the optical and particle data show that the diffuse auroral zone consisted of two regions. The equatorward part of the diffuse aurora was occupied by a pattern of regular, parallel auroral stripes. The auroral stripes were significantly brighter than the background luminosity, had widths of approximately 5 km and moved southward with a velocity of about 100 m/s. The second region, located between the region with auroral stripes and the discrete auroral arcs to the north, was filled with weak and almost homogeneous luminosity, against which short-lived auroral rays and small patches appeared chaotically. From analysis of the electron differential fluxes corresponding to the different regions of the diffuse aurora and based on existing theories of the scattering process we conclude the following: Strong pitch angle diffusion by electron cyclotron harmonic waves (ECH) of plasma sheet electrons in the energy range from a few hundred eV to 3–4 keV was responsible for the electron precipitation, that produced the background luminosity within the whole diffuse zone. The fine structure, represented by the auroral stripes, was created by precipitation of electrons above 3–4 keV as a result of pitch angle diffusion into the loss cone by whistler mode waves. A so called "internal gravity wave" (Safargaleev and Maltsev, 1986) may explain the formation of the regular spatial pattern formed by the auroral stripes in the equatorward part of the diffuse auroral zone
The © European electron Geosciences energy Union 2005distribution during HF pumping,
picture painted with all color
Auroral all-sky camera calibration
A two-step procedure to calibrate the spectral sensitivity to visible light
of auroral all-sky cameras is outlined. Center pixel response is obtained by
the use of a Lambertian surface and a standard 45 W tungsten lamp. Screen
brightness is regulated by the distance between the lamp and the screen.
All-sky flat-field correction is carried out with a 1 m diameter integrating
sphere. A transparent Lexan dome at the exit port of the sphere is used to
simulate observing conditions at the Kjell Henriksen Observatory (KHO). A
certified portable low brightness source from Keo Scientific Ltd was used
to test the procedure. Transfer lamp certificates in units of Rayleigh per
Ångstrøm (R/Å) are found to be within a relative error of 2%. An all-sky
camera flat-field correction method is presented with only 6 required
coefficients per channel