SA24A-01 16:00h
Comparisons of PMSE Ice Particle Simulations With Observations From the DROPPS Rocket Campaign
The origin of polar mesospheric summer echoes (PMSEs) and their proposed relationship to dust and aerosol particles and to noctilucent clouds (NLCs) are among the most pressing questions governing the physics of the polar summer mesosphere. Recent studies show increasing evidence for the presence of these particles, and their importance particularly in a charged state. One theory for the formation of the PMSE particles is that they are composed of a rocky core of meteoritic origin, which acts as a seed about which an ice mantle forms. Computer simulations have been conducted to model the flight trajectory and sublimation of spherical ice/rock particles with radius of 0.5 - 6.0 nm through the Particle Impact Detector (PID) charge telescope onboard two rockets that were part of the DROPPS (Distribution and Role of Particles in the Polar Summer Mesosphere) campaign. DROPPS involved two rocket sequences in July, 1999 launched from Andoya, Norway. The first launch sequence was nighttime (July 5-6) during the presence of a strong PMSE and a weak NLC. The second sequence (July 13-14) occurred during a bright NLC, but with no PMSE present. By comparing the PID observations from these two flights with the computer simulations, information about the properties of the PMSE particles, including their core size, ice mantle thickness and distribution, can be deduced.
SA24A-02 16:15h
Rocket-born instrument to detect charged smoke and cloud particles in the mesosphere
An instrument has been developed to detect charged, sub-visible aerosol particles in the upper atmosphere. The instrument is designed to fly on a sounding rocket and has a 30 square centimeter entrance slit. Venting ports are placed lower on the detector in order to let the air out and reduce pressure buildup inside the detector. The air sample flows between four pairs of graphite electrodes biased symmetrically with increasing bias potentials. Electrons, light ions, cluster ions and heavy charged aerosol particles of both polarities are collected mass-selectively on the electrodes that are connected to sensitive electrometers. Direct Simulation Monte Carlo (DSMC) codes have been used to optimize the supersonic airflow around the instrument. The design of the entrance slit reduces the effect of the shock on the motion of the aerosols. A separate code is used to model the motion of charged particles within the detector. The effect of collisions with the residual gas is modeled using Monte Carlo techniques and collection efficiency is calculated. A laboratory prototype of the instrument has been fabricated and is currently under testing using ion beams.
SA24A-03 16:30h
Tides in the Equatorial MLT Region: Results From Simultaneous MF and Meteor Radar Measurements From Indonesian and Indian Sectors
Tides in the equatorial mesosphere and lower thermosphere (MLT) region have continued to be of interest to the researchers because of their variabilities in different temporal and spatial scales that are yet to be modeled and whose causative mechanisms are yet to be understood. Ground based radars in the equatorial region have not been many but those from Christmas Island (2$^{o}$N, 158$^{o}$W), Jakarta (6.4$^{o}$S, 106.7$^{o}$E), Pontianak (0.03$^{o}$N, 109.3$^{o}$E) and Tirunelveli (8.7$^{o}$N, 77.8$^{o}$E) have provided a good amount of data base that has been used in the recent past to characterize many of the observational features of the tides over these sites. Recently, two more radars have been deployed in Indonesia, namely, the meteor radar at Koto Tabang (0.2$^{o}^$S, 100.3$^{o}$E) and the MF radar at Pameungpeuk (7.5$^{o}$S, 107.5$^{o}$E). The present work focuses on the tidal characteristics observed over Indonesia and India at three of those stations mentioned above, namely, Pontianak, Koto Tabang and Tirunelveli. Both short (over a period of a month) and long (over a period of a year) term tidal variabilities are examined in detail and the peculiarities in their behavior over the respective altitudes as well as between the longitudes are highlighted and discussed in this paper.
SA24A-04 16:45h
Space-borne OH rotational temperature measurements with SCIAMACHY
Near-global retrievals of mesopause OH rotational temperatures from satellite-borne Meinel band emission measurements are presented. The nighttime measurements of the OH (3-1) Meinel band near 1.5 micron were performed with the SCIAMACHY instrument on the European Space Agency's environmental satellite Envisat. The derived OH (3-1) rotational temperatures were previously shown to be in reasonable agreement with the CIRA-86 atmosphere temperatures for the seasons and latitudes considered and in good agreement with ground-based OH rotational temperature measurements at different locations. This contribution presents results of the first 2 years of OH temperature data derived from SCIAMACHY. The SCIAMACHY limb nighttime observations provide a unique data set of near-global OH rotational temperature to study seasonal and geographical variations, dynamical processes and possibly long-term temperature trends, if an extended data set becomes available in the future.
SA24A-05 17:00h
The Wind and Temperature Spectrometer (WTS) in the Atmospheric Neutral Density Experiment (ANDE) Satellite
Miniaturization efforts in new spectrometers for ionosphere/thermosphere investigations of the ion-drifts and neutral winds and corresponding temperatures make possible very light (about 250 g) packages consuming less than 0.4 W. Previously described, our approach measures the angular and energy distributions of neutral atoms and molecules (or ions) in two perpendicular planes; using those distributions to determine the full wind vector, the temperature and the relative densities of O and N2, as required for the ANDE mission. The measurements require two separate electron impact ion sources each with its own electron beam cathode. We have developed a low-temperature thermionic emission cathode that delivers 1 mA electron current at 80 mW power, making it possible to operate neutral wind/temperature experiments for the first time with powers less than 0.5 W. Advances in the ion optics of the energy-angle spectrometer enhance the energy resolution-aperture product more than a factor of 3 to enable energy resolutions of a few percent with large apertures. With these technology improvements it is now possible to obtain the full neutral wind vector, temperature and O/N2 density ratio once per second in a 250g/0.4W package with sensitivity up to about 500 km altitude in the thermosphere. We will describe the WTS as deployed in ANDE and show simulated data with the non-linear least squares analysis to illustrate expected performance of the WTS in the estimated errors in the three components of the wind, the temperature, and the relative densities.
SA24A-06 17:15h
Polar Mesospheric Summer Echoes at a Bragg Wavelength of 16 cm
We present measurements of Polar Mesospheric Summer Echoes, PMSE, at the very short Bragg wavelength of 16 cm which is known to occur infrequent. The measurements were carried out with the EISCAT 930 MHz UHF radar. Part of the data was taken under the influence of RF heating. A comprehensive comparison of the measurements with the theory of turbulence including enhancement of the Schmidt number is carried out, but other theories are also considered. Estimates of the energy dissipation rates inferred from the signal spectral widths are used to calculate Batchelor scale lengths as a function of the Schmidt number. Hill's multipolar diffusion model is used to calculate the diffusion rates (which depend on the charge number of ice particles of nanometer size radius) leading to estimates of the Schmidt numbers. It is argued that the use of the slow diffusion coefficient in the calculations should give conservative estimates, i.e., lower bounds, of the necessary ice charge numbers to explain the measurements, the condition for scattering being that the Bragg scale of the radar should be greater than the Batchelor scale. Based on information from other experiments published in the literature, e.g., the charge number of mesospheric ice particles, the main conclusion of this investigation is that none of the most prominent theories can explain satisfactorily the measurements unless charge numbers in excess of 10 electrons for the measured wide spectra and at least in excess of 100s of electrons for the measured narrow spectra. RF heating of the mesosphere appears to have a similar influence on PMSE at a Bragg wavelength of 16 cm as it has at greater wavelengths, namely a weakening of PMSE during the heater on periods and a recovery during the heater off periods. This behaviour is as expected given the rapidity and large amount of electron heating that RF heating produces with the consequent rapid and large enhancement of the electron diffusion rate during heater on periods regardless of the electron diffusion model or reference to any particular model of PMSE.
SA24A-07 17:30h
An earlier lidar observation of a noctilucent cloud above Logan, Utah Center for Atmospheric and Space Sciences, Utah State University
The Atmospheric Lidar Observatory (ALO) Rayleigh-scatter lidar has been operated for 11 years on the Utah State University (USU) campus (41.7$\deg$ N 111.8$\deg$ W). During the morning of 22 June 1995 a noctilucent cloud (NLC) was observed with the lidar, well away from the twilight periods when NLCs are visible. It lasted for approximately one hour. The temperature profiles, calculated at hourly intervals, were at least 20 K cooler than the 11-year June climatological average for ALO near the NLC altitude. These cool temperatures arose, in part, because of a major temperature oscillation. This NLC observation at ALO and a previously reported one from 1999 [{\it Wickwar et al.,} 2002] are very significant because they show the penetration of NLCs equatorward of 50$\deg$, which may have important implications for global change.
SA24A-08 17:45h
Three-parameter size distributions of NLC particles as measured by the ALOMAR Rayleigh/Mie/Raman lidar
The Rayleigh/Mie/Raman lidar of the ALOMAR observatory (located at 69 degrees N) can measure the backscatter coefficient of noctilucent clouds (NLC) and its altitude dependence in full daylight and at any local time throughout the Arctic summer at the wavelength of 532 nm. For medium and strong NLC, it can do so also at the additional wavelengths of 355 nm and 1064 nm. It so happens that in this wavelength range, scatter from the optically active NLC particles significantly deviates from Rayleigh scatter. It is this deviation from Rayleigh scatter which allows the determination of a size distribution through the measurement of backscatter coefficients at three different wavelenghts. Since 1998, the ALOMAR Rayleigh/Mie/Raman lidar has the required capability of daylight observations of NLC in three widely separated wavelengths and we have observed quite a number of NLC events for which particle size distributions could be determined. In our data analysis, we presume the NLC particles to be spherical, to consist of ice and to follow a lognormal size distribution. The latter is described by an absolute particle number density, a median radius and a radius distribution width. We will report on the results of these measurements.
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