Kinetics of ice particles growth in the polar summer mesosphere
The growth kinetics of ice particles in the polar summer mesosphere is discussed. The particle growth time is calculated using the temperature, water vapor density, and ice number density simultaneously measured by the infrared Fourier Transform Spectrometer on the Atmospheric Chemistry Experiment (ACE-FTS) satellite. The formation rate for ice particles is a very strong function of temperature and water vapor concentration. We found the equilibrium radius of ice particles to be in the range from 20 to 70 nm, and the formation time - from about 2 hours at 150 K to about 18 hours at 125 K. Our results imply that in addition to the commonly accepted particle growth during their sedimentation from higher altitudes, in-situ growth to radii of 50-70 nm at mesospheric temperatures near 150 K in two hours or less may also be possible. Our analysis of possible shapes for mesospheric ice particles using the band shape of ice absorption feature measured by ACE-FTS suggests that cubes or compact hexagonal prisms (with an aspect ratio of 1.1) are the best candidates to represent the crystalline ice particles in the polar summer mesosphere.
Multiple Year Variations of the Mesosphere Lower Thermosphere Wind Field and Comparison with Empirical Models
A continuous record of mesosphere lower thermosphere winds collected by satellite instruments exist since late 1991. This data set comprises data from the WINDII and HRDI instruments on the UARS satellite and the TIDI instrument on TIMED. The HRDI and TIDI data overlap by more than two years, permitting the winds to be comprehensively inter-compared and validated and allowing a seamless transition between instruments. Examination of the data sets show many features which are common from year to year, such as an equinox maximum in the diurnal tide with the March equinox typically stronger than September. The data also show significant inter-annual variations, such as an approximately factor of 2 variation in the diurnal tide amplitude. Many of the variations are believed to be driven by the lower atmosphere and will not explicitly appear in an empirical model such as HWM07 which only includes solar drivers. This paper will study the variations in the wind field over time and examine the differences between the observed wind field and that predicted by models to provide an estimate of the systematic limitations of the models.
First Results From OI (6300A) Fringe Measurements Using the TIMED Doppler Interferometer (TIDI)
The TIMED Doppler Interferometer, TIDI, began synoptic observations of the upper thermosphere on 25 September 2008. The routine science mode was adjusted to view the Earth's limb at tangent heights from 160 to 300 km at 20 km increments at 4 seconds per altitude sample. The full mesosphere/thermosphere scan sequence is repeated every 100 seconds. This sequence provides approximately 60 altitude scans per orbit, and there are nearly 15 orbits per day. The precession rate of the TIMED satellite is slow (12 minutes per day) implying that TIDI samples any latitude circle every 24 degrees of longitude at approximately the same dayside Local Solar Time (LST) and the same nightside LST. Finally, TIDI is a multiplex instrument monitoring four tangent points during every integration which allows the measurement of the horizontal neutral wind via Doppler shifted OI (6300A) airglow emission. First look altitude brightness profiles indicate feeble OI (6300A) emission during the current minimum phase of the solar cycle. Brightness profiles peak in the neighbourhood of 220 to 240 km altitude at low latitudes on both the dayside and on the nightside. Brightness registration on global latitude/longitude charts show auroral oval brightening for winter nightside measurements. At the time of this writing, the highest Ap and Kp levels recorded since the initiation of the new TIDI mode occurred on 11 October 2008 (37 and 7 respectively). TIMED beta angle was -71.98 degrees on this date suggesting flight, as well as TIDI sampling, near the dawn/dusk terminator. TIDI cold side telescopes were able to observe nightside OI (6300A) airglow at southern latitudes and did record significant (2 orders of magnitude) enhancement in brightness in the upper thermosphere. There is no question that TIDI samples OI (6300A) thermospheric airglow. The volume emission rate is at extremely low levels during the present solar minimum period. It is anticipated that brightness levels will increase at least four-fold as the solar cycle progresses to maximum in the next few years. Neutral wind results from TIDI will improve as the instruments 'zero-wind' position is determined and constrained. This paper will also describe current 'best-effort' wind results determined from preliminary 'zero-wind' estimates.
Long Term Observations of the Neutral Wind in the Upper Thermosphere Above Carmen Alto Chile
A Fabry Perot interferometer has been installed in Carmen Alto in the Atacama desert of northern Chile for the currently ending solar cycle 23. The instrument is located under very dark skies at a site that is almost always free of cloud cover. Observations provide horizontal neutral wind measurements for the upper thermosphere for an altitude of approximately 230 km. Approximately 100 km south of Carmen Alto, a spaced receiver system has been measuring the UHF and L-band signal from beacons aboard geostationary satellites. If ionospheric structure is present, the unique signature in the radio signal at one element of the receiver system may be correlated with that in the other element providing a measure of the zonal plasma drift. This paper will describe the long term neutral wind data sets observed by the optical system at Carmen Alto. Both the zonal and the meridional components of the neutral wind are measured utilizing the line of sight Doppler shift of OI (6300A) airglow emission. Comparisons between the zonal component of the neutral wind and the zonal plasma drift usually indicate a high degree of coherency between the two independent observations. Future observations with identical systems along the same geomagnetic latitude will provide important information relating to the propagation and the evolution of low latitude ionospheric irregularities.
Direct Comparisons Between Neutral Wind, Temperature, and Airglow Brightness Measurements and Plasma Drift, Temperature, and Density Observations at Sondre Stromfjord Greenland
An NSF facility at Sondre Stromfjord Greenland houses both an incoherent scatter radar and a Fabry Perot interferometer. The two instruments measure characteristics of the ionosphere and the neutral atmosphere at two common altitudes determined by the emission altitudes of the metastable atomic oxygen greenline and redlines. The current solar cycle 23 has experienced a long minimum period providing a unique opportunity to characterize the baseline upper atmosphere. Coincident observations of common volume are infrequent between the two experiments often due to cloud cover obscuring the Doppler shifted line of sight airglow for the Fabry Perot interferometer. A clear night of common measurements began on 15 January 2007 and continued until dawn the next morning. The geomagnetic Kp index was relatively low for the entire period except between 03 and 06 UT when it rose to 5. This paper will describe the common volume coincident measurements for this data set. At approximately 05 UT, both instruments observed representative signatures of geophysical activity above Sondre Stromfjord. In terms of temperature, the trend observed by the radar in ion temperature matched that of the upper thermosphere neutral temperature. The horizontal neutral wind suggested abatement in the meridional component prior to substorm onset followed by the formation of strong equatorward flow in the morning sector. The plasma drift observations at 250 km altitude do not indicate any substantial reduction in the meridional component coincidental with the neutral data, and a clear sunward flow in the morning sector following the substorm.
SABER Mesospheric OH Emission Data Trends Over a Half Solar Cycle
This research is focused on measurements to ascertain trends in the OH infrared airglow column emission rates as observed from SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) aboard the NASA/LaRC TIMED satellite. Particularly, we are examining trends over a seven-year period corresponding to a half solar cycle from 2002 up to 2009. Statistical methods have been employed to calculate averages of global daily emissions in the form of column emission rates. These daily averages are concatenated and then analyzed to reveal periodic trends over the time period in review. We also explore the possible correlation with solar activity as represented both by the Kp index and sunspot counts. Initial analysis suggests a definite downward trend from 2002 through 2008, which parallels the current solar cycle activity. Various periodicities of shorter are seen in the analysis, in addition to the pronounced 10-11 year solar cycle periodic trend. Further analysis of these data should make it possible for us to better understand the mechanics responsible for OH column emission changes with time.