Recognizing the blue emission in artificial aurora
Introduction
Artificially induced optical emissions by heating ionospheric electrons with powerful radio waves succeeded for the first time in 1970s at mid-latitudes (Sipler and Biondi, 1972, Gordon and Carlson, 1974, Haslett and Megill, 1974, Adeishvili et al., 1978). That was followed by several attempts at high latitudes by the EISCAT Heating facility near Tromso in Norway (Henriksen et al., 1984, Stubbe et al., 1982). However, there was no convincing success until 1999 (Brändström et al., 1999, Kosch et al., 2000, Pedersen and Carlson, 2001) when atomic oxygen O 1D (630.0 nm auroral red emission line) and O 1S (557.7 nm auroral green emission line) were detected. Since then there have been several successful measurement campaigns to produce artificial aurora using intense radio waves (e.g., Kosch et al., 2002, Rietveld et al., 2003).
During the November 2001 campaign (Kosch et al., 2004) at EISCAT, the blue emission was also observed for the first time. Initially, this was interpreted as 427.8 nm, 1NG(0, 1) as expected for natural aurora. The oxygen O 1D and O 1S states are excited by relatively low energy electrons (1.96 and 4.17 eV, respectively, Vallance Jones, 1974) and they are expected to be seen in the spectrum of artificial aurora (Haslett and Megill, 1974). The 1NG blue emission needs an excitation energy of 18.6 eV (Lofthus and Krupenie, 1977, Itikawa et al., 1986) making it difficult to understand how considerable amounts of nitrogen molecules could be excited to this state.
Section snippets
Instrumentation and experiment
The heating experiment was performed at the EISCAT site (69.58°N, 19.22°E) near Tromso in Norway. The experiment was part of a UK/GE EISCAT measurement campaign, which had as an objective to produce artificially induced auroral emissions using powerful radio waves to input energy into the ionosphere. The radio wave was transmitted in O-mode from the EISCAT site using the Heating facility. Different cycles and antenna directions were used (Kosch et al., 2004) but in this paper only the 2 min on 2
Measurements
The photometer was designed for determining rotational temperatures from the fine structure of the 1NG(0, 1) band. This is a molecular emission band, where the rotational emission lines result from transitions from different rotational energy states. The initial electronic-vibrational state is (ν = 0) and the final state is (ν = 1). The populations of the rotational states follow the rule N(K″) = N0 exp(K″(K″ + 1)Chc/kT), where N0 is total number of molecules in the excited
Results
Table 2 shows that the intensities vary from 5.5 to 11.9 R in the P-peak channel and from 2.8 to 6.6 R in the R-peak channel with averages of 9.0 and 5.4 R, respectively. For the background measurement, the P-peak filter shifts to the background position at a longer wavelength (channel 3b in Table 1) and the R-peak filter to the R-tail position at a shorter wavelength (channel 4b in Table 1) by (un)tilting the relevant filter (Kaila, 2003). This means that during the background measurement the
Conclusions and discussion
The initial aim of the analysis was to determine height of the blue emission in the artificial aurora by using the rotational temperature method. Unfortunately, the blue emission can not be triangulated because there is data from just one station on 12/11/2001 for these wavelengths. However, the intensity ratios R-peak/P-peak were unrealistically high. They were first thought to represent an extremely high temperature of the emitting neutral gas volume, but the measured intensity ratios are in
Acknowledgement
The EISCAT facilities are funded by Research Organisations from Finland, France, Germany, Japan, Norway, Sweden and the United Kingdom.
References (23)
- et al.
Ionospheric modification experiments in northern Scandinavia
J. Atmos. Terr. Phys.
(1982) - et al.
Ionospheric emission caused by an intense radio wave
Sov. J. Plasma Phys.
(1978) - Ashrafi, M., Kosch, M.J., Honary, F. Heater-induced altitude descent of the EISCAT UHF ion line enhancements:...
- et al.
Unambiguous evidence of HF pump-enhanced airglow at auroral latitudes
Geophys. Res. Lett.
(1999) - et al.
Effective rotational temperature (3914 Å) excited by monoenergetic electrons in a crossed beam
J Chim. Phys.
(1997) - et al.
Ionospheric modification at twice the electron cyclotron frequency
Phys. Rev. Lett.
(2005) - et al.
Arecibo heating experiments
Radio Sci.
(1974) Nonlinear phenomena in the ionosphere
(1978)- Gustavsson, B., Sergienko, T., Kosch, M.J., Rietveld, M.T., Steen, A., Brändström, U., Leyser, T.B., Isham, B., Gallop,...
- et al.
A model of the enhanced airglow excited by RF-radiation
Radio Sci.
(1974)