6. DISCUSSION
The data collected during the 6'th December event 2002 from both the ground-based stations (LYR and Ny┼) and the Odin/OSIRIS space borne instrument add the following information.

(A) Ground-based observations
1) The event coincided with the rise and setting of the Sun. Near winter solstice the Sun is at least 14 degrees below the horizon in Longyearbyen. If we assume the event is an emitting layer as a result of the direct and immediate action of sun light, then we can apply the method as described by Chamberlain [1961] (see above calculations) to calculate the altitudes as a function of view angle from zenith. The solar depression angle at 10:00 UT was 11░, and the azimuth angle of observation was close to 21░ South - East. If we include refraction and use a screening height of 12 km, the actual shadow heights ranged from 125 to 35 km for view angles between zenith down to the horizon, respectively. The solid Earth shadow was 31 km at the horizon.

2) No abnormal low temperatures in the upper mesosphere were detected by the spectrometers. The hourly average temperature both prior and after the event was close to 200K. This rules out that the event was caused by sunlit Noctilucent Clouds (NLC) - formed when the temperature is close to the summer minimum (~120 K). The temperatures were calculated using the OH(6-2) band of airglow [cf. Sigernes et al., 2003].

3) It is clear from the spectrometer data that the source of the illumination is the sun. Fraunhofer lines are easily identified in the spectra recorded in zenith from both ground-based sites. Also, since the event is gradually increasing in intensity from weak blue to strong near infra-red spectra, the solar rays must have suffered absorption and scattering by a target above the screening height . It is only the deep red component of the visible solar spectrum that is left. Next, after the initial rays are reflected / scattered, they re-enters the lower atmosphere. Once more the rays suffers scattering. In addition, the ground albedo is high due to the snow covered ground. As a net result, the instruments detect light that has been reflected and scattered several times before it finally enters the narrow 5░ field of view in zenith.

4) The effect of scattering and albedo is also seen in the above image gallery. It is hard to identify any target or structure in the images. It is only the scattered component of the light we detect. The intensities of the MSP are still quite high for view angles greater than 175░, even though the mountains and the low cloud cover blocks the direct line of sight to the event! As a consequence, we believe that what we detect is scattered light from below the horizon.

5) It then follows, since the solid Earth shadow was at most 31 km high as seen towards the horizon, that the projection onto the celestial sphere between the solid Earth shadow line and the stations horizontal line of sight is located about 625 km South - East. This means that the target that is illuminated by the sun is close to or below 75░ North. The target must be located in the Stratosphere.

(B) Space borne observations
6) The OSIRIS spectra have a clear signatures of O2 B-band and water vapor absorptions. These signature are often seen by OSIRIS under twilight conditions and indicates the rays of light have traveled a substantial distance through the troposphere. It is therefore highly unlikely that the source of red light could be anything but the sun. The rays of light have also gone close to the ground.

7) OSIRIS did not detect any signal from above 26 km. This means that we can rule out the possibility of any extended high altitude upper mesospheric source. The lack of signal above 26 km rules out the possibility of a very bright, very localized, high altitude source.

8) The deduced height profile shows that the event is a stratospheric phenomenon. If the red sky was created by a very bright, high altitude scattering source, then it should be expected that the height profile should follow the atmospheric density as the vast majority of scattered signal detected by OSIRIS would be Rayleigh scattered. This is not seen. Instead the height profile resemble a scattering layer with an upper altitude around 20-25 km.

9) Furthermore, The blue and green wavelengths are suffering large amounts of Rayleigh extinction. O3 Chappuis absorption and are not seen. There is so far an undetermined mechanism scattering the solar twilight signal around the Earth up to Longyearbyen. The scattering source may also be preferentially scattering the red wavelengths. The Chappuis absorption is not taking out all of the "Red", indicating low values of ozone in the Stratosphere.

(C) LIDAR observations
10) European Center for Medium Range Weather  Forecast  (ECMWF) analysis concludes that there is an extended area between Scandinavia and Svalbard, consistent with the ground-based observations and the data from OSIRIS, where the temperature in the Stratosphere is low enough for PSC's to be formed. 

11) LIDAR measurements from the Koldeway Station in Ny-┼lesund provide an evidence for the occurrence of PSCs within this area of cold temperatures from both the 7'th and 9'th of December, 2002.

12) The PSC layer was measured to be in the altitude range  23 - 27 km, which is in vertical extension corresponds to a fairly thick cloud.

13) As seen from the increased Depolarization values of the LIDARS, the PCS's contained crystalline particles. The low temperatures from the ECMWF even suggest that PSC type II clouds are formed.

(D) Summary observations 
Based on the above findings, the overall conditions in early December 2002 was indeed in favor for the formation of Polar Stratospheric Clouds (PCS's). The location of the PSC was found to be an area that spanned from about (73░N, 53░E) to (75░N,50░E), which is approximately South East of Longyearbyen - between Svalbard and Scandinavia. The altitude was 20 - 26 km up in the Stratosphere. The low absorption measured by OSIRIS in the O3 Chappuis band explains the red color component of the scattered light and that the ozone content must be low. 

The data analysis by the European Center for Medium Range Weather  Forecast  (ECMWF) confirm that the temperatures in this region of stratosphere was indeed low enough for PSC to be formed. In addition, the LIDARS provided the evidence of PSC existence from the measurements of depolarization and backscatter ratio as a function of wavelength and altitude. The altitude of the PSC was found to be 23 - 27 km, consistent with the OSIRIS measurements.  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.

A PSC at 25 km altitude can see a horizon 5 degrees south of its location. Given that the red sky was observed over Longyearbyen when the sun was at a Solar Zenith Angle (SZA) of ~ 103-105 degrees, then a PSC at an altitude of 25 km directly over Longyearbyen would see a twilight horizon corresponding to a SZA of 98-100 degrees. At these SZA there is still significant solar signal and the cloud will be illuminated by it.  The red sky is therfore most likely caused by a polar stratospheric cloud drifting over Longyearbyen near local noon, illuminated by the twilight.