Recognizing the blue emission in artificial aurora

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Abstract

On 12th November 2001, during an EISCAT UK/GE artificial aurora campaign, the optical group from the University of Oulu performed optical measurements at the EISCAT site in Ramfjordmoen, including the first measurement of blue emissions in artificial aurora at high latitudes. Optical instruments, including a photometer, a real speed TV camera and a digital camera, were monitoring the emissions. The emissions that the photometer was designed to measure were 557.7 nm (OI), 630.0 nm (OI) and 427.8 nm (N2+). The energy thresholds of these emissions are approximately 2, 4 and 19 eV, respectively.

In the natural aurora the blue emission at around 427.8 nm is always dominated by the N2+ 1NG(0, 1) band. However, there are two weak emission bands lying under this strong emission, namely, the N2 VK(4, 15) (threshold energy 6 eV) and N2 2P(1, 5) (threshold 11 eV). These excitation energies are lower than the energy needed to excite N2+ 1NG(0, 1) level and therefore could have a stronger intensity compared with N2+ 1NG(0, 1) in the spectrum of artificial aurora than in natural aurora.

The auroral photometer of the University of Oulu has been designed for investigating natural aurora. The photometer was equipped with two channels measuring different wavelength bands around 427.8 nm. These channels were intended to be used to determine rotational temperature from the ratio of the intensities through the channels. However, here we estimate the intensities of the three overlapping emission bands instead.

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, N2+ 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 N2+ 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 N2+ 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 N2+B2Σu+ (ν = 0) and the final state is N2+X1Σg+ (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 N2+ 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)

  • P. Stubbe et al.

    Ionospheric modification experiments in northern Scandinavia

    J. Atmos. Terr. Phys.

    (1982)
  • T.G. Adeishvili 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:...
  • B.U.E. Brändström et al.

    Unambiguous evidence of HF pump-enhanced airglow at auroral latitudes

    Geophys. Res. Lett.

    (1999)
  • G. Culp et al.

    Effective rotational temperature (3914 Å) excited by monoenergetic electrons in a crossed beam

    J Chim. Phys.

    (1997)
  • F.T. Djuth et al.

    Ionospheric modification at twice the electron cyclotron frequency

    Phys. Rev. Lett.

    (2005)
  • W.E. Gordon et al.

    Arecibo heating experiments

    Radio Sci.

    (1974)
  • A.V. Gurevich

    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,...
  • J.C. Haslett et al.

    A model of the enhanced airglow excited by RF-radiation

    Radio Sci.

    (1974)
  • A.E. Hedin

    Extension of the MSIS thermosphere model into the middle and lower atmosphere

    J. Geophys. Res.

    (1991)
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