The Horizontal Wind Model (HWM) has been updated in the thermosphere with new observations and formulation changes. These new data are ground-based 630 nm Fabry-Perot Interferometer (FPI) measurements in the equatorial and polar regions, as well as cross-track winds from the Gravity Field and Steady State Ocean Circulation Explorer (GOCE) satellite. The GOCE wind observations provide valuable wind data in the twilight regions. The ground-based FPI measurements fill latitudinal data gaps in the prior observational database. Construction of this reference model also provides the opportunity to compare these new measurements. The resulting update (HWM14) provides an improved time-dependent, observationally based, global empirical specification of the upper atmospheric general circulation patterns and migrating tides. In basic agreement with existing accepted theoretical knowledge of the thermosphere general circulation, additional calculations indicate that the empirical wind specifications are self-consistent with climatological ionosphere plasma distribution and electric field patterns.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014EA000089

An update to the Horizontal Wind Model (HWM): The quiet time thermosphere

[1] The new Horizontal Wind Model (HWM07) provides a statistical representation of the horizontal wind fields of the Earth's atmosphere from the ground to the exosphere (0–500 km). It represents over 50 years of satellite, rocket, and ground-based wind measurements via a compact Fortran 90 subroutine. The computer model is a function of geographic location, altitude, day of the year, solar local time, and geomagnetic activity. It includes representations of the zonal mean circulation, stationary planetary waves, migrating tides, and the seasonal modulation thereof. HWM07 is composed of two components, a quiet time component for the background state described in this paper and a geomagnetic storm time component (DWM07) described in a companion paper.

/pdf/an-empirical-model-of-the-earth-s-horizontal-wind-fields-vm989vom4t.pdf

An empirical model of the Earth's horizontal wind fields: HWM07

Odin is a 250 kg class satellite built in co-operation between Sweden, Canada, France, and Finland and launched in February 2001. It carries two instruments: a 4-band sub-millimetre radiometer used for both astronomy and atmospheric science and an optical spectrometer and infrared imaging system for purely atmospheric observations. As part of the joint mission Odin will observe the atmospheric limb for 50% of the observation time producing profiles of many species of interest in the middle atmosphere with a vertical resolution of 1-2 km. These species include, among others, ozone, nitrogen dioxide, chlorine monoxide, nitric acid, water vapour, and nitrous oxide. An overview of the mission and the planned measurements is given.

/pdf/an-overview-of-the-odin-atmospheric-mission-2mvh89ncyj.pdf

An overview of the Odin atmospheric mission

Peculiar atmospheric radar echoes from the high-latitude summer mesosphere have spurred much research in recent years. The radar data (taken on frequency bands ranging from 2 to 1290 MHz) have been supplemented by measurements from an increasing arsenal of in situ (rocket borne) and remote sensing (satellites and lidars) instruments. Theories to explain these polar mesosphere summer echoes (PMSEs) have also proliferated. Although each theory is distinct and fundamentally different, they all share the feature of being dependent on the existence of electrically charged aerosols. It is therefore natural to assume that PMSEs are intimately linked to the other fascinating phenomenon of the cold summer mesopause, noctilucent clouds (NLCs), which are simply ice aerosols that are large enough to be seen by the naked eye. In this paper we critically examine both the data collected and the theories proposed, with a special focus on the relationship between PMSEs and NLCs.

/pdf/an-updated-review-of-polar-mesosphere-summer-echoes-2onvh5ac15.pdf

An updated review of polar mesosphere summer echoes: Observation, theory, and their relationship to noctilucent clouds and subvisible aerosols

The transition between the middle atmosphere and the thermosphere is known as the MLT region (for mesosphere and lower thermosphere). This area has some char- acteristics that set it apart from other regions of the atmosphere. Most notably, it is the altitude region with the lowest overall temperature and has the unique characteristic that the temperature is much lower in summer than in winter. The summer-to-winter-tem- perature gradient is the result of adiabatic cooling and warming associated with a vigorous circulation driven primarily by gravity waves. Tides and planetary waves also contribute to the circulation and to the large dynamical variability in the MLT. The past decade has seen much progress in describing and understanding the dynamics of the MLT and the inter- actions of dynamics with chemistry and radiation. This review describes recent observa- tions and numerical modeling as they relate to understanding the dynamical processes that control the MLT and its variability. Results from the Whole Atmosphere Community Climate Model (WACCM), which is a comprehensive high-top general circulation model with interactive chemistry, are used to illustrate the dynamical processes. Selected observations from the Sounding the Atmosphere with Broadband Emission Radiometry (SABER) instrument are shown for comparison. WACCM simulations of MLT dynamics have some differences with observations. These differences and other questions and dis- crepancies described in recent papers point to a number of ongoing uncertainties about the MLT dynamical system.

/pdf/global-dynamics-of-the-mlt-27clum6d5c.pdf

Global Dynamics of the MLT

[1] We present a global empirical disturbance wind model (DWM07) that represents average geospace-storm-induced perturbations of upper thermospheric (200–600 km altitude) neutral winds. DWM07 depends on the following three parameters: magnetic latitude, magnetic local time, and the 3-h Kp geomagnetic activity index. The latitude and local time dependences are represented by vector spherical harmonic functions (up to degree 10 in latitude and order 3 in local time), and the Kp dependence is represented by quadratic B-splines. DWM07 is the storm time thermospheric component of the new Horizontal Wind Model (HWM07), which is described in a companion paper. DWM07 is based on data from the Wind Imaging Interferometer on board the Upper Atmosphere Research Satellite, the Wind and Temperature Spectrometer on board Dynamics Explorer 2, and seven ground-based Fabry-Perot interferometers. The perturbation winds derived from the three data sets are in good mutual agreement under most conditions, and the model captures most of the climatological variations evident in the data.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2008JA013541

DWM07 global empirical model of upper thermospheric storm-induced disturbance winds

Volume emission rate profiles of the O(1D-1S) 5577 A dayglow measured by the WIND imaging interferometer on the Upper Atmosphere Research Satellite are analyzed to examine the O(1S) excitation mechanisms in the sunlit lower thermosphere and upper mesosphere. The observed emission profiles are compared with theoretical profiles calculated using a model which takes into account all of the known daytime sources of O(1S). These include photoelectron impact on atomic oxygen, dissociative recombination of O+2, photodissociation of molecular oxygen, energy transfer from metastable N2(A3 +
 u
 ) and three body recombination of atomic oxygen. Throughout most of the thermosphere the measured and modelled emission rates are in reasonably good agreement, given the limitations of the model, but in the region below 100 km, where the oxygen atom recombination source is likely to dominate, the measured emission rates are considerably larger than those modelled using the MSIS-90 oxygen atom densities. This discrepancy is discussed in terms of possible inadequacies in the MSIS-90 model atmosphere and/or additional sources of O(1S) at low altitude.

The O( 1 S) dayglow emission as observed by the WIND imaging interferometer on the UARS

Variations in the oxygen green line emission rate (at 5577 A) which correspond to a two day wave signature were observed from January 20 to January 30, 1993 by the Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite (UARS). The latitude and height extent of these emission variations are summarized. From the volume emission rate data, fields of oxygen mixing ratio using Mass Spectrometer and Incoherent Scatter (MSIS) model data as the background atmosphere were calculated. Analyses of the mixing ratio fields show the variations of emission rate to be primarily the result (to first order) of vertical motions associated with the quasi two day wave (QTDW), not mixing events associated with wave breaking. The mean peak-to-peak amplitude of the vertical displacement is of the order of 1 km although at times this amplitude is as much as 4 km. The phase of the mixing ratio variations approaches the two day wave zonal wind phase above 100 km as is expected from simple considerations of the phase relationships associated with planetary waves and the lifetime of atomic oxygen.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97GL01022

Two day wave induced variations in the oxygen green line volume emission rate: WINDII observations

Satellite images of polar mesospheric clouds (PMCs) were taken in support of the 1993 ANLC campaign. In July, 1993, they were common at latitudes north of 65°N but increased in abundance poleward to the limit of our viewing horizon of 72°N. The southern boundary edge of the PMCs was observed to move northwards as the season progressed in 1993. Compared with the climatology of other years, the ANLC campaign observations were taken towards the end of a season in a year of modest PMC activity. WINDII, the wind imaging interferometer on the UARS satellite, is a new effective tool in the study of polar mesospheric clouds. Images of the limb give vertical distributions of the light scattered by the molecular and the particle atmosphere. Detection of the PMCs against the Rayleigh background showed that the peak altitude was around 83 km in 1993.

Observation of polar mesospheric clouds in summer, 1993 by the WINDII Instrument on UARS

The mesopause oxygen rotational temperature imager, MORTI, was operated at Bear Lake Observatory (41.9°N, 111.4°W) during the period November, 1991 to May, 1993. Fluctuations of long period in both emission rate and temperature of the O2 Atmospheric (0–1) nightglow layer are evident that seem related to the diurnal and semidiurnal tides. The data show a dominant semidiurnal tidal mode in January, but a dominant diurnal component at the spring equinox. Other long-period fluctuations also appear for which a linear tidal explanation does not seem applicable.

G. G. Shepherd

Papers

DWM07 global empirical model of upper thermospheric storm-induced disturbance winds

The O( 1 S) dayglow emission as observed by the WIND imaging interferometer on the UARS

Two day wave induced variations in the oxygen green line volume emission rate: WINDII observations

Observation of polar mesospheric clouds in summer, 1993 by the WINDII Instrument on UARS

Tides in emission rate and temperature from the O2 nightglow over Bear Lake Observatory