A. W. Merkel

Polar mesospheric cloud structures observed from the cloud imaging and particle size experiment on the Aeronomy of Ice in the Mesosphere spacecraft: Atmospheric gravity waves as drivers for longitudinal variability in polar mesospheric cloud occurrence

patterns and structures in polar mesospheric clouds (PMCs), around the summertime mesopause region, which are qualitatively similar to structures seen in noctilucent clouds (NLCs) from ground‐based photographs. The structures in PMC are generally considered to be manifestations ofupward propagating atmospheric gravity waves (AGWs). Variability of AGW effects on PMC reported at several lidar sites has led to the notion of longitudinal differences in this relationship. This studycompares thelongitudinal variability in theCIPS‐ observed wave occurrence frequency with CIPS‐measured PMC occurrence frequency and albedo along with mesospheric temperatures measured by the sounding of the atmosphere using broadband emission radiometry instrument on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics spacecraft. Our results for the latitude ranges between 70° and 80° show a distinct anticorrelation of wave structures with cloud occurrence frequency and correlations with temperature perturbations for at least two of the four seasons analyzed, supporting the idea of gravity wave‐induced cloud sublimation. The locations of the observed wave events show regions of high wave activity in both hemispheres. In the Northern Hemisphere, while the longitudinal variability in observed wave structures show changes from the 2007–2008 seasons, there exist regions of both low and high wave activities common to the two seasons. These persistent features may explain some of the observed differences in PMC activity reported by ground‐based lidar instruments distributed at different longitudes. The statistical distribution of horizontal scales increases with wavelength up to at least 250 km. We also discuss the possibility of atmospheric tides, especially the nonmigrating semidiurnal tide, aliasing our observations and affecting the results presented in this analysis.

Numerical simulations of the three-dimensional distribution of polar mesospheric clouds and comparisons with Cloud Imaging and Particle Size (CIPS) experiment and the Solar Occultation For Ice Experiment (SOFIE) observations

[1] Polar mesospheric clouds (PMC) routinely form in the cold summer mesopause region when water vapor condenses to form ice. We use a three‐dimensional chemistry‐climate model based on the Whole‐Atmosphere Community Climate Model (WACCM) with sectional microphysics from the Community Aerosol and Radiation Model for Atmospheres (CARMA) to study the distribution and characteristics of PMCs formed by heterogeneous nucleation of water vapor onto meteoric smoke particles. We find good agreement between these simulations and cloud properties for the Northern Hemisphere in 2007 retrieved from the Solar Occultation for Ice Experiment (SOFIE) and the Cloud Imaging and Particle Size (CIPS) experiment from the Aeronomy of Ice in the Mesosphere (AIM) mission. The main discrepancy is that simulated ice number densities are less than those retrieved by SOFIE. This discrepancy may indicate an underprediction of nucleation rates in the model, the lack of small‐scale gravity waves in the model, or a bias in the SOFIE results. The WACCM/CARMA simulations are not very sensitive to large changes in the barrier to heterogeneous nucleation, which suggests that large supersaturations in the model nucleate smaller meteoric smoke particles than are traditionally assumed. Our simulations are very sensitive to the temperature structure of the summer mesopause, which in the model is largely dependent upon vertically propagating gravity waves that reach the mesopause region, break, and deposit momentum. We find that cloud radiative heating is important, with heating rates of up to 8 K/d.

Altitude determination of ultraviolet measurements made by the Student Nitric Oxide Explorer

The spinning motion of the Student Nitric Oxide Explorer spacecraft allows for the measurement of limb profiles of atmospheric radiation. In order to interpret the observations an accurate altitude reading is necessary. Included in the limb profile is Rayleigh-scattered solar radiation that can be used to determine altitude. The Rayleigh profile increases exponentially with decreasing altitude until the absorption due to ozone dominates the shape, producing a peak near 53 km. We use the Rayleigh scattering peak to register the measured profile in altitude for each ultraviolet spectrometer observation. A single-scattering radiance model is developed and used to provide an altitude-dependent shape of the Rayleigh profile. The altitude registration is done by correlating the data to the model for each measured profile. By systematically shifting the measured profile in small increments and fitting the shifted profile to the model the quality of the fit is determined. The shifted profile that best correlates with the model profile determines the correct shift in altitude to 1.5 km precision.

Science Instrumentation for the Student Nitric Oxide Explorer

The student nitric oxide explorer (SNOE) is a small satellite to be designed built and operated at the University of Colorado under the student explorer demonstration initiative from the Universitys Space Research Association (STEDI). The goal of the STEDI program is to demonstrate that low cost satellite missions can be done with large student involvement. The primary science goals of SNOE are to measure thermospheric nitric oxide (NO) and its variability over the lifetime of the mission. SNOE will also monitor the solar irradiance at soft x-ray wavelengths and the auroral energy deposition at high latitudes. Three science instruments are required to achieve the simultaneous measurements: an ultraviolet spectrometer for NO; a solar soft x-ray photometer; and a far ultraviolet photometer for studying the aurora. The instruments are designed to represent a minimum impact on the spacecraft, particularly in terms of data storage and interactions with the command and data handling system. The focus of this paper is the outline of the design of the science instruments. We discuss why these instruments are well suited for smaller, lower cost satellite missions.

New Discoveries From MESSENGER and Insights into Mercury's Exosphere

For most of the orbital phase of the MESSENGER mission, a regular search for weakly emitting or less abundant species in Mercurys exosphere resulted in non-detections. However, during the final Earth year of the mission, emission from multiple lines of manganese, aluminum, and ionized calcium was detected. These observations validate the detection of a single line of ionized calcium during the third MESSENGER Mercury flyby, provide definitive confirmation for weak aluminum detections in ground-based observations, and represent the discovery of manganese in Mercurys exosphere. These detections occurred over a limited range of pre-dawn local times and Mercury true anomaly angles (0o-70o), and each has a distinct spatial distribution. Equally interesting is the absence of detectable emission from oxygen at limits well below the levels reported for Mariner 10.

A cold‐pole enhancement in Mercury's sodium exosphere

The Ultraviolet and Visible Spectrometer (UVVS) component of the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on the MESSENGER spacecraft characterized the local-time distribution of the sodium exosphere over the course of its orbital mission. The observations show that the sodium exosphere is enhanced above Mercurys cold-pole longitudes. Based on previously published sodium exosphere models we infer that these regions act as nightside surface reservoirs, temporary sinks to the exosphere that collect sodium atoms transported anti-sunward. The reservoirs are revealed as exospheric enhancements when they are exposed to sunlight. As in the models the reservoir is depleted as the cold poles rotate from dawn to dusk, but unlike the models the depletion is only partial. The persistence of the reservoir means that it could, over the course of geologically long periods of time, contribute to an increase in the bulk concentration of sodium near the cold-pole longitudes.