firstname.lastname@example.org - last update: 11 May 1998, 0800 UT
Despite new observation methods provided by radar and satellite
instrumentation, optical measurements are still
important when studying auroras. These include
- TV- and all-sky-cameras (ASC)
TV- and all-sky-cameras (ASC)
To be written. For the time being, see the information of the
All Sky Camera chain of the Finnish Meteorological Institute .
Photometers measure the absolute luminous intensity of a light source. Because of the monochromatic
nature of the auroral light, photomultiplier tubes with very narrow band filters were used in the
measurements. The typical wavelengths that have been measured are:
From these emissions one can make estimates of the characteristic energy and energy flux of the auroral
particle precipitation, especially when the measurements are made along the field line the particles (in
most cases electrons) are coming down. The photometers are, however, often made to operate in two
possible modes, i.e., field aligned or scanning. In the scanning mode a moving mirror is used
to scan a larger part of the sky, to get a broader view of the auroral distribution. With typical quiet, slowly
drifting east-west aligned auroral arcs this gives good results when the
scanning is done along the magnetic meridian.
For example Doe et al. (1997) have discussed a tomographic method to
utilise this kind of optical data.
- 427.8 nm (blue)
- Molecular nitrogen first negative (rotational) band
- 486.0 nm (blue)
- 557.5 nm (green)
- Atomic oxygen, emission from state O1S
- 630.0 nm (red)
- Atomic oxygen, emission from state O1D
When analysing photometric measurements, some corrections are needed. First of all, background light
should be removed. The only way to know what to subtract is to measure also the background intensities,
either continuously with a dedicated channel, or every now and then during the measurements by slightly
tilting the filters, altering the passband of the filter this way. The spectra of the background emissions
from atmospheric impurities (clouds, ice crystals, etc.) is rather continuous, and measurements just
outside the auroral emission lines are valid for the line itself. Another important correction is the distance
the light has to travel from the generation region to the photometer: especially for scanning photometer
mode this varies considerably from the zenith to the low elevations.
To be written.
- Doe, R. A., J. D. Kelly, J. L. Semeter, and D. P. Steele,
Tomographic reconstruction of 630.0 nm emission structure for a polar
cap arc, Geophys. Res. Lett., 24, 1119-1122, 1997.