4.1 MESOSPHERIC TEMPERATURE ESTIMATE
The traditional and most reliable way of monitoring temperatures of the mesosphere is through ground based spectroscopic measurements of the OH airglow layer [Sivjee, 1992; Greet et al., 1998]. It was Meinel  that first identified the molecular band emissions of OH. These emission lines was found to dominate the airglow spectrum from approximately 0.5 to 4 mm. The principal source of the vibration excited OH radicals, the ozone-hydrogen mechanism, was suggested independently by Bates and Nicolet  and Herzberg 
The exothermicity is sufficient to excite the OH molecules up to a vibrational level of n' = 9. The OH airglow layer is, according to in-situ measurements made with rockets, situated around 87 km with a thickness of 8 km regardless of season or latitude [Baker and Stair, 1988].
In order to derive the rotational temperatures from the measured OH band emissions, it is necessary to produce the corresponding synthetic spectra. The theory for calculating these spectra is given by Herzberg  with term values from Krassovsky  and Einstein coefficients from Turnbull and Lowe . In basic, the temperatures can then be derived since the upper populated energy states of the OH band must be distributed according to the Stefan-Boltzmann distribution. The temperature is interpreted as kinetic under the assumption that the excited OH molecules are in thermal equilibrium with the atmosphere. Recent work by Sigernes et al.  explains in detail how to generate synthetic spectra as a function of rotational temperature. In short, the temperature is found by first choosing the optimal fit between the measured background and the synthetic spectrum through iteration, to minimize the least square error. Secondly, when the background level is found, the temperature is derived from the slope of the linear fit to a so called Boltzmann plot (Log-energy term plot). See Sivjee and Hamwey .
Our instrument, The Silver Bullet (See section 2.3), measures on the OH(6,2) band of the airglow. Fig. 4.1 shows the result of a synthetic fit to an averaged spectrum measured prior to the event from 00:30 to 06:45 UT. Fig. 4.2 shows the corresponding Boltzmann plot.
Figure 4.1 The measured - and synthetic spectra of the OH(6,2) band from the Auroral Station in Adventdalen, 06.12.2002. Each line is marked and identified according to quantum state. The curve plotted with the line color red is synthetic and green the measured. The emission line plotted with blue color represents the auroral OI 8446 ┼ emissions. The measured spectrum is averaged over the time period 00:30 to 06:46.
The variance between the linear fit and the P1 values in the Boltzmann plot is 0.003. The corresponding P2 variance is 0.015. Viereck and Deehr  estimated the uncertainty of the above technique / method to be approximately 2 K. Note that the covariance between the spectral fit and the measured spectra is low. This is due to the auroral OI 8846 ┼ emissions.
Figure 4.2 Boltzmann plot for the OH(6,2) P branch of the spectrum in Fig. 4.1. The curve with line color blue is the linear fit using P1 values (red squares). The yellow squares represents P2. A temperature of 192 K is calculated from the slope of the fit. Note that a spread of the P2 values from the linear fit would indicate a departure from thermal equilibrium.
The below Table 4.1 summarizes the temperature calculations for different time periods both prior and after the event. The results are that we do not have any departure from normal temperature conditions in the upper mesosphere [Sigernes et al., 2003].
|Prior||02:56 - 05:42||04:29 - 06:14||00:30 - 06: 45|
|After||14:05 - 16:52||14:16 - 21:23||21:02 - 23:59|
Table 4.1 Synthetic OH(6,2) temperature calculations from the Auroral Station in Adventdalen, 06.12.2002.
The above temperature method have been used to calibrate a meteor radar located about 4 km from The Auroral Station in Adventdalen, further into the valley close to the base of the mine 7 mountain (Hall et al., 2003). The radar is owned by NIPR (National Institute of Polar Research Japan) and operated together by the University of Troms°, Norway. The temperature from the radar is calculated from the echo fading time. While optical methods depend heavily on observing conditions, a meteor radar can easily provide daily measurements, hence providing a temporal coverage hitherto unprecedented at this location. Figure 4.3 shows the obtained temperatures from the radar. Again we see that there is no abnormal low temperatures in December 2002, which tells us that NCL's are not formed in the mesosphere.
Figure 4.3 Neutral air temperatures at 90 km altitude over Svalbard (78░N) deduced from measurements of meteor echo fading times according to the method described by Chilson et al. . The radar is operating in a low-height resolution single-pulse mode. The meteor data is kindly provided by Dr. Aso Takehiko from the National Institute of Polar Research Japan (NIPR).
4.2 STRATOSPHERIC TEMPERATURES FROM ALOMAR (69.75N, 15.75E)
The next interesting question is: What are the temperatures further down into the atmosphere? To answer this question, data from the European Center for Medium Range Weather Forecast (ECMWF) have been applied. Fig. 4.4 shows the 6 hr temperature height profiles at the Arctic LIDAR Observatory for Middle Atmospheric Research (ALOMAR) (69.75N, 15.75E). This site is about 900 km South of Longyearbyen. Nevertheless, there are reports from ALOMAR of high altitude stratospheric cloud formation during the period of our event.
It is well known that below a certain threshold temperature, Polar Stratospheric Clouds can form. These cloud particles, if they are illuminated by the sun, may furthermore provide a surface for heterogeneous reactions that lead to the release of free radicals. The radicals destroy ozone. The process is believed to be the main factor for the springtime stratospheric ozone loss in the polar regions (c.f. Solomon et al., 1999). There are mainly 3 types of PSCs, distinguished by their particle composition. The different types of PSCs are listed in Table 4.2.
|PSC Ia||Solid Nitric acid Trihydrate; Nitric acid Dihydrate (NAT; NAD) particles ~1 mm diameter|
|PSC Ib||Liquid ternary H2O / HNO3 /H2SO4 solution (STS) droplets|
|PSC II||Ice particles ~10 mm diameter|
Table 4.2 Different types of Polar Stratospheric Clouds (PSCs) according to composition.
The threshold temperatures as a function of height, TNAT for PSC Ia, TICE for PSC II and T STS for PSC Ib, are obtained from (Muller et al, 2001). Note that typically 5 ppmv H20 and 9-10 ppbv HNO3 are assumed as trace gas input for the calculations of PSC threshold temperatures (Koop et al., 1997; Tabazadeh et al., 2001).
Figure 4.4 Temperature height profiles from the European Center for Medium Range Weather Forecast (ECMWF). The location is Andenes on And°ya, Norway (69.75N, 15.75E). Date is 06.12.2003. The profiles are color coded according to a 6 hour sample period. Also shown are the height region where PSCs are formed together with the 5 ppmv H2O threshold temperatures for NAT (dotted line), STS (dashed line) and ice (solid line). The data was kindly provided to us by dr. Gerd Baumgarten at ALOMAR.
As seen in Fig. 4.4, the temperature in the Stratosphere was indeed low enough for Polar Stratospheric Clouds (PSC type I) to be formed. The latter observations are also supported by the LIDAR measurements. There is in other words clear evidence of PSC's above Andenes (69.75N, 15.75E) on the 6'th of December 2002.
4.3 STRATOSPHERIC TEMPERATURES FROM NY-┼LESUND (78░55'24''N, 11░55'`53''E)
The promising results from ALOMAR lead us to inquire whether the LIDAR system at the Koldeway Station in Ny-┼lesund operated by the Alfred Wegner Institute (AWI) detected anything unusual during the event. Unfortunately, the weather in Ny-┼lesund was completely overcast on the 6'th of December, 2002. The LIDAR was on the other hand in operation both on the 7'th and the 9'th of December, 2002. The following show results from the AWI group.
First of all, a re-analysis of ECMWF data was conducted by dr. Peter von der Gathen at AWI to include the spatial coverage of temperatures in the stratosphere for 6'th December 2002. The results are shown in Fig. 4.5
Figure 4.5 Northern hemisphere daily temperatures in the stratosphere from the European Center for Medium Range Weather Forecast (ECMWF) as a function of potential temperature. Date is 06.12.2003. The profiles are color coded according to temperature distribution. Also shown are the regions where PSCs are formed (Solid lines between Scandinavia and Svalbard). The data was kindly provided to us by dr. Peter von der Gathen at AWI.
The above plots show the temperature distribution in various height layers, as analyzed by ECMWF. Height is given as potential temperature, values ranging from 400 K to 675 K. Within each plot, a line (contour) which includes the area, where temperature is below the existence temperature of PSC's type I (thin line), or even type II (thick line in panel C of Fig. 4.5). As expected, the main PSC area is exactly between Svalbard and Scandinavia.
Furthermore, the AWI LIDAR data provide us the evidence for the occurrence of PSCs within this area of cold temperatures. The LIDARS were operative on 7'th and 9'th December. Figures 4.6 and 4.7 show the obtained data obtained by the LIDARS..
Figure 4.6 LIDAR measurements from the Koldeway Station in Ny-┼lesund on the 7'th of December, 2002. The signals are time averaged from 16-20 UT and 20-24 UT, respectively. R is the backscatter r ratio as a function of altitude (10-40 km). s is the corresponding Depolarization. The wavelength is 532 nm.
Figure 4.7 LIDAR measurements from the Koldeway Station in Ny-┼lesund on the 9'th of December, 2002. The signals are time averaged from 16-24 UT. R is the backscatter ratio as a function of altitude and wavelength (532 nm VIS (Indigo); 353 nm UV (Green) and 1064 nm IR (Blue)). s is the Depolarization values. The deduced temperature (white line) is plotted together with PSC threshold temperatures for NAT (blue line), STS (indigo line) and ice (yellow line).
The above reveals that a PSC layer was located in the altitude range 23 - 27 km. The PSC contained crystalline particles (seen from the increased Depolarization values) and was a fairly thick cloud, in vertical extension, as well as in the backscatter ratio values. Together with the OSIRIS observations and those from ALOMAR, this is pretty good evidence, that a PSC was available in the right place, time, and altitude for the scattered the light, which was observed in Longyearbyen.