1. 1st VarSITI General Symposium – June 6-10, 2016, Albena
Dynamical perturbations of the thermosphere inferred from satellite observations of O(1D) nightglow
Marianna G. Shepherd
CRESS, York University, Toronto, Canada

2. Definitions and Main Features
Midnight Temperature Maximum (MTM): a highly variable large scale neutral temperature anomaly with wide spread influence on the nighttime thermosphere at low latitudes, with a northward surge of meridional wind near local midnight. Midnight descent/collapse of the F- region & enhancement of the O(1D) emission and reversal of meridional wind from southward (-) to northward (+) (ground-based observations).
Midnight Density Maximum (Brightness Wave) (MDM/MBW): northeast to southwest O(1D) emission enhancement near local midnight observed from the ground at low latitudes
Observations mostly from the American sector (270°E – 320°E) at low latitudes (20°S – 20°N) [e.g. Jicamarca (12°S) & Arecibo (18°N)].
Seasonal variation of MTM & MDM – suggested due to seasonal variations of the semi-diurnal and terdiurnal tides.
Historically most of the reports on the optical manifestation of the MTM in O(1D) 630 nm airglow come from ground-based observations over the American sector (270°E – 320°E) (Jicamarca (12°S, 283°E), Arequipa (16°S, 289°E), Arecibo (18ºN, 293ºE))
One of the first ground-based observations of MTM was reported by Greenspan (1966), who observed regular occurrences of intensity enhancement symmetrical about local midnight at equatorial latitudes, and centered at ~ 7.5° latitude.

3. WINDII Observations O(1D) Airglow
Wind Imaging Interferometer (WINDII), flown on the UARS satellite - Sept Apr 2003
O(1D) airglow observations: 191 days, 1991 – 1995
Continuous latitudinal coverage: 42°S – 42°N (inclination 57°)
O(1D) measurements - consecutive along the satellite track separated by ~ 3º in latitude
Local time (LT) at a given latitude decreases by 20 min from day to day
For the study: Height profiles of O(1D) nightglow, Doppler Temperature (derived from line width), zonal & meridional winds at 200 km – 300 km.
The Doppler temperatures were obtained from the modulation depth of the Michelson interferometer fringes, what Michelson called their “visibility” I, defined as (Imax – Imin)/(Imax + Imin).

4. at F-region altitudes of 250-350 km.
At night the production of O(1D) 630 nm airglow emission from the O(1D) state – dissociative recombination of O2+:
O2 + O+  O2+ + O
O2+ + e-  O + O(1D)
O(1D)  O+ hν6300
at F-region altitudes of km.
where the O+ has been formed during the day by photoionization) and lost either by emission at 630 nm and nm (O*(1D) → O + hν630nm) or by quenching collisions with molecular nitrogen (O(1D) + N2 → O(3P) + N2), as well as by O2 and electrons

5. Seasonal Variability of the MTM and MERM
DJF
1993 MAM
VER
VER T
The O(1D) VER and Doppler Temps observations at 270 km and 20N-40N, for DJF, MAM, and SON. Comparisons to be made according to season: winter solstice (left) and spring/fall equinox (right four panels). Data are presented only for a one-year season to avoid solar cycle effects. Year-by-year plots and composite plots per season with all available data are also available. This presentation is essentially introduction to the raw data, independently of longitude, but only arranged according to LT and season. The influence of other quantities such as longitude, geophysical disturbance, solar flux and month has not been considered. From a satellite perspective all data for one day have the same LT so that different local times come from different days. That means that two measurements adjacent in LT corresponding to essentially the same local time can come from three months apart and subject to different influences. Different longitudes, however, are all measured on the same day so that they all have the same influences and are entirely consistent. During solstice – MTM, but not a distinct MERM; During Equinox – distinct MERM and MTM. Pre-midnight (spring) and post-midnight (fall) maxima are seen in both fields (light green).
DJF
1992 SON
Seasonal Variability of the MTM and MERM
at 20°N – 40°N, at 270 km

6. Longitudinal Variability
VER U T V
Variability with LT and longitude and relationship to neutral winds.
O(1D) residual VER (zonal mean subtracted) and zonal wind U (left column) and residual DoT (zonal mean subtracted) and meridional wind V (right column) for DJF 1991 – 1992 as a function of longitude and LT at 20°N – 40°N. The VER and DoT observations and their geophysical variance are shown in black; the U and V observations and geophysical variance are shown in blue. The fits to a combination of WN1, WN2, WN3 and WN4 are shown in red for the VER and DoT and in green for the U and V.
Fitted in longitude to a linear regression in least-mean square sense to a combination of sine and cosine model wave functions with wave number (WN) 1, 2, 3 and 4 (denoted also as wave 1, wave 2, wave 3 and wave 4)
For the March/April/May observations, and otherwise the same conditions as above the VER and DoT display strong wave 4 peaks in longitude as do the wind variations, and the same is true for September/October/November but with less well defined peaks. In summary, for the conditions described above, the MTM can be observed at all longitudes, but only as one of the wave 4 peaks. Looking back at the DJF data (conclusion 1), the wave 4 peaks can be very weakly discerned.
MAM 1992
DJF

7. Dependence on Latitude and Local Time
VER T
WINDII O(1D) VERs (upper row) and DoT (lower row) at 5°N − 40°N and 270 km for December/January/February (DJF) 1992 − 1993 and 23 LT (left column), 24 LT (middle column) and 1 LT (right column).
Latitude/longitude maps of VER and DoT show the enhanced VER following the pattern of the well-known tropical O(1D) arcs, also called EIA, with low values at mid-latitudes, while the pattern of the DoT is just the opposite, high values at mid-latitudes. Wave 3 and 4 patterns are evident in these maps.
23 LT
24 LT
1 LT
DJF , 5ºN – 270 km

8. VER MAM 1993 5ºN – 40ºN T SON 1992 5ºN – 40ºN
WINDII O(1D) VERs (upper row) and DoT (lower row) at 5°N − 40°N and 270 km for March/April/May (MAM) 1993 and 23 LT (left column), 24 LT (middle column) and 1 LT (right column).
WINDII O(1D) VERs (upper row) and DoT (lower row) at 5°N − 40°N and 270 km for September/October/November (SON) 1992 and 23 LT (left column), 24 LT (middle column) and 1 LT (right column).
SON 1992
5ºN – 40ºN

9. 23 LT 24 LT 1 LT VER T SON 1993, 5ºN – 40ºN, @ 270 km
The same as in Figure 10, but for September/October/November (SON) 1993.
SON 1993, 5ºN – 270 km

10. Meridional Wind , 20N – 40N , @270 km, MAM 1992 - 1993
Latitude/longitude maps of meridional wind show a rather uniform northward wind over the field except for the American sector, where the wind reverses at all latitudes (20°N – 40°N) to values of about 70 m s-1.

11. Vertical Extent and Day-to-Day Variations
Composite WINDII O(1D) VER (left column) and DoT (right column) at 20°N – 40°N from 210 km to 280 km height for DoY – (March 13 – April 6, 1993).
Altitude versus longitude plots of VER and DoT from 210 km to 280 km show an extremely well-defined wave 4, with rather uniform variation with altitude. However, the peak values for each wave are different, with the one over the American sector being dominant. The VER and DoT are out of phase, as already indicated in the maps of VER and DoT (conclusion 4).
DoY – 93096
(Mar 13 – Apr 6, 1993)
210 km – 280 km

12. Conclusions
The winter (December/January/February) at 20ºN – 40ºN and 270 km height, the O(1D) VER enhancement (MERM) is seen only in the American Sector.
Well defined peak of DoT in longitude around 220°E, does not vary with LT. The meridional wind is near-zero over all longitudes except at the DoT longitude peak - it forms a trough (southward wind) coinciding with the increase in DoT.
The March/April/May observations - the VER and DoT display strong wave 4 peaks in longitude as do the wind variations; the same is true for September/October/November but with less well defined peaks. For the conditions described, the MTM can be observed at all longitudes, but only as one of the wave 4 peaks. Looking back at the DJF data, the wave 4 peaks can be very weakly discerned.
The largest MTM and MERM occur only in the American sector, which is the only longitude range at which the meridional wind reversal is constantly seen at and around midnight. However, this American sector MTM occurs at only one of the wave 4 peaks (the strongest one), which means that weak MERMs and MTMs can be seen at other longitudes.