Monitoring the Outer Region of the Neutral Atmosphere
Hydrogen is a primary constituent of the geocorona and is a chemical byproduct of species below such as methane and water vapor, two greenhouse gases. The solar cycle is a dominant source of natural variability in this region and must be accounted for when isolating the effects of coupling processes from below, including that due to potential long-term change in the region. Observations by the Wisconsin H-alpha Mapper Fabry- Perot of geocoronal hydrogen Balmer-alpha emissions over solar cycle 23 have quantified a factor of 1.5 ± 0.15 higher intensities at solar maximum than at solar minimum. These observations are consistent with Fabry-Perot observations from Wisconsin during solar cycle 22. All observations have been consistently calibrated for intensity using the North American Nebula. We used the LYAO_RT radiative transfer code of Bishop to compare the observed Balmer-alpha intensities with intensities calculated using the hydrogen density distribution in the Mass-Spectrometer-Incoherent-Scatter (MSIS) model, a major empirical model used by the middle and upper atmospheric research communities. The MSIS distribution yields a solar maximum to minimum ratio similar to that observed; however, significant differences in magnitude between predicted and observed intensities highlight the need for improved upper atmospheric hydrogen density determinations.
Wind observations by the E-Region Wind Interferometer, ERWIN-2 at Eureka
The E-region wind interferometer, ERWIN, was refurbished and moved from Resolute Bay to the Polar Environment Atmospheric Research Laboratory (PEARL) in Eureka (80N) in the winter of 2008. ERWIN is a field widened Michelson interferometer which measures winds in the E-region using Doppler shifts in hydroxyl, oxygen green line and O2 airglow emssions. The referbished instrument is constructed around the old interferometer and includes imaging capabilities using a CCD detector and an optical system which allows simultaneous viewing in four directions and zenith. The instrument operates by seqentially viewing the three emissions. The observation cadence for the three emissions is ~2 minutes making this the fastest wind measuring instrument in the world for the mesopause region. An overview of the instrument operation and results from the first year of observations (including winds during stratospheric warmings) are presented.
The Michelson Interferometer for Airglow Dynamics Imaging (MIADI)
The Michelson Interferometer for Airglow Dynamics Imaging (MIADI) is a new implementation of the imaging field-widened Michelson interferometer concept which images airglow signatures in the mesopause region and simultaneously records wind and intensity images. The scientific purpose of this instrument is to provide unambiguous information on gravity waves since the background horizontal wind and irradiance variations will be simultaneously obtained. Calibration and characterization of instrument parameters has been completed at a field site in Shigaraki Japan and initial observations have been taken. Co-located alongside MIADI are the MU radar, Na Lidar and several All-Sky Imagers. Observation campaigns are ongoing to acquire simultaneous data sets from these instruments. In this paper, the calibration and characterization results will be summarized. The initial measurements of winds and intensity will be presented and the scientific goals of the current observing campaign outlined.
Meridional Winds Derived From MST Radar Observations Of E-Region Field-Aligned Irregularities And Compared With That Of TIDI Observations And HWM-93 Model
Measurement of neutral winds is important to understand the dynamics and electrodynamics of E-region and the ionospheric variabilities in general. Rockets, Satellites, MF and meteor radars have been used to measure winds in the E-region. Rocket observations have shown winds as high as 170 m s-1. These observations, however, are limited. On the other hand, continuous observations made by MF and meteor radars are comparatively more in number but the wind magnitude is less. Satellite measurement of winds provide average picture and also not continues in time. In this paper we present the meridional neutral winds derived from the field-aligned radar echoes observed using Gadanki MST radar. A large database of the observations has been used to study the variabilities of meridional winds. The derived meridional winds are compared with that of rocket measurements reported previously. They are found to agree with rocket measured values, which are much larger than the MF and meteor radar winds. The derived meridional winds are also compared with that of TIDI observations and horizontal wind model (HWM-93). Local time and seasonal variation of meridional winds are also derived and compared with that of TIDI observations and HWM-93.
SHS Observations of the 3727Å O+ Doublet
We present ground-based observations of the terrestrial O+ doublet (2D -- 4S) emission at 3726 and 3729 Å. These O+ emission lines were detected as part of a Galactic O+ interstellar medium program underway at the University of Wisconsin's Pine Bluff Observatory using a Field-Widened Spatial Heterodyne Spectrometer (FW-SHS). The FW-SHS produces Fizeau fringes by replacing the return mirrors in a Michelson interferometer with diffraction gratings. The FW-SHS combines interferometric and field-widening gains to achieve sensitivities much larger than conventional grating instruments of similar size and resolving power. We present the initial O+ FW-SHS observations, believed to be originating in the F region of the ionosphere. The emission intensities are estimated to be in the range of 0.5-1.5 R. Results from several observation periods ranging from December 2003 through October 2005 are discussed. We include the doublet ratio results for all observation periods as well as an implied seasonal variability in intensity. As this terrestrial O+ emission is a relatively unstudied phenomenon, we outline the methodology we are using to study this doublet and discuss its importance to the F region of the ionosphere.
Occurrence of Neutral Density Enhancement at High Latitudes
The occurrence distribution of large thermospheric neutral density peaks was examined using CHAMP accelerometer data during the four and a half year period from mid-2001 through 2005. Peaks of the total mass density were generally detected around the dayside cusp, suggesting the persistent presence of a density enhancement cell structure in this region. The locations of dayside density peaks tend to move to lower magnetic latitudes with increasing magnetic activity. Surprisingly many density peaks were detected inside the polar cap with > 80 ° magnetic latitudes during periods of moderate geomagnetic activity. To help understand the CHAMP observations, global circulation modeling was conducted using NCAR's Thermosphere Ionosphere Electrodynamics Global Circulation Model (TIEGCM) with input parameters of solar wind plasma and magnetic field measured by ACE satellite. TIEGCM modeling shows enhancement of neutral density in a broad region near the polar cusp where the convective electric field is large. However there is no indication of density enhancement in the polar cap region away from the dayside cusp from the TIEGCM modeling. We speculate that the neutral density peaks observed by CHAMP at > 80 ° magnetic latitudes manifest the deposition of Poynting fluxes and energetic electron precipitation in a small localized region in the polar cap. The physical processes responsible for the localized energy deposition remain to be investigated.
Thermodynamics of homogeneous nucleation of mesospheric ice particles
Although our knowledge of the upper mesospheric region is continuously improving, many aspects of mesospheric dynamics and thermodynamics are still unclear. We address some of these open questions related to the thermodynamics of water at the conditions intrinsic for the polar summer mesosphere. For this we use recently published theoretical and laboratory results on the properties of water at very low temperatures. We present the hypothesis of homogeneous nucleation of ice nano-particles in the polar summer mesosphere. The nucleation of condensed phase is traced back to the first step on the formation pathway, which is assumed to be the transition of water vapor to amorphous cluster. Amorphous clusters then freeze into water ice, likely metastable cubic ice, when they reach the critical size. The estimates based on the equilibrium thermodynamics give the critical size (radius) of amorphous water clusters as about 1.0 nm. The same estimates for the final transition step, that is the transformation of cubic to hexagonal ice, give the critical size of about 15 nm at typical upper mesospheric conditions during the polar summer (about 150K temperature and about 109cm-3 water vapor density).