Engineering topics

Aikin, A. C.

Publications and source records attributed to Aikin, A. C..

At least 19 records

NASA NTRS2014

Global Investigation of the Mg Atom and ion Layers using SCIAMACHY/Envisat Observations between 70 km and 150 km Altitude and WACCM-MG Model Results

Mg and Mg+ concentration fields in the upper mesosphere/lower thermosphere (UMLT) region are retrieved from SCIAMACHY/Envisat limb measurements of Mg and Mg+ dayglow emissions using a 2-D tomographic retrieval approach. The time series of monthly means of Mg and Mg+ for number density as well as vertical column density in different latitudinal regions are shown. Data from the limb mesosphere-thermosphere mode of SCIAMACHY/Envisat are used, which covers the 50 km to 150 km altitude region with a vertical sampling of 3.3 km and a highest latitude of 82 deg. The high latitudes are not covered in the winter months, because there is no dayglow emission during polar night. The measurements were performed every 14 days from mid-2008 until April 2012. Mg profiles show a peak at around 90 km altitude with a density between 750 cm(exp−3) and 2000 cm(exp−3). Mg does not show strong seasonal variation at mid-latitudes. The Mg+ peak occurs 5-15 km above the neutral Mg peak at 95-105 km. Furthermore, the ions show a significant seasonal cycle with a summer maximum in both hemispheres at mid- and high-latitudes. The strongest seasonal variations of the ions are observed at mid-latitudes between 20-40 deg and densities at the peak altitude range from 500 cm(exp−3) to 6000 cm(exp−3). The peak altitude of the ions shows a latitudinal dependence with a maximum at mid-latitudes that is up to 10 km higher than the peak altitude at the equator. The SCIAMACHY measurements are compared to other measurements and WACCM model results. In contrast to the SCIAMACHY results, the WACCM results show a strong seasonal variability for Mg with a winter maximum, which is not observable by SCIAMACHY, and globally higher peak densities. Although the peak densities do not agree the vertical column densities agree, since SCIAMACHY results show a wider vertical profile. The agreement of SCIAMACHY and WACCM results is much better for Mg+, showing the same seasonality and similar peak densities. However, there are the following minor differences: there is no latitudinal dependence of the peak altitude for WACCM and the density maximum, passing the equatorial region during equinox conditions, is not reduced as for SCIAMACHY.

SCIAMACHYEnvisat↗

NASA NTRS2001

Summer Antarctic Ozone Behavior and Laminae

The ADEOS/ILAS solar occultation instrument measured the vertical distribution of ozone during the southern polar summer of 1997. A range of latitudes was sampled from 65 to 79 degrees south. Ozone profiles are characterized by laminae, which are confined to the 20 to 50 mb pressure altitude region. For a single day not all longitudes exhibit such structures. Using back trajectory calculations, it is shown that laminae are caused by lower latitude air being transported toward the pole by winds in the 20 to 50 mb region. These winds are not present above this pressure region. The existence of such structures implies that other constituents must also be transported by the winds. Other constituent distributions will be examined to determine the extent to which they are affected by wind transport. The effects on the TOMS total ozone distributions are also examined.

Aikin, A. C.↗

NASA NTRS2001

NRLMSISE-00 Empirical Model of the Atmosphere: Statistical Comparisons and Scientific Issues

The new NRLMSISE-00 model and the associated NRLMSIS database now include the following data: (1) total mass density from satellite accelerometers and from orbit determination, including the Jacchia and Barlier data; (2) temperature from incoherent scatter radar, and; (3) molecular oxygen number density, [O2], from solar ultraviolet occultation aboard the Solar Maximum Mission (SMM). A new component, 'anomalous oxygen,' allows for appreciable O(+) and hot atomic oxygen contributions to the total mass density at high altitudes and applies primarily to drag estimation above 500 km. Extensive tables compare our entire database to the NRLMSISE-00, MSISE-90, and Jacchia-70 models for different altitude bands and levels of geomagnetic activity. We also investigate scientific issues related to the new data sets in the NRLMSIS database. Especially noteworthy is the solar activity dependence of the Jacchia data, with which we investigate a large O(+) contribution to the total mass density under the combination of summer, low solar activity, high latitudes, and high altitudes. Under these conditions, except at very low solar activity, the Jacchia data and the Jacchia-70 model indeed show a significantly higher total mass density than does MSISE-90. However, under the corresponding winter conditions, the MSIS-class models represent a noticeable improvement relative to Jacchia-70 over a wide range of F(sub 10.7). Considering the two regimes together, NRLMSISE-00 achieves an improvement over both MSISE-90 and Jacchia-70 by incorporating advantages of each.

Aikin, A. C.↗

NASA NTRS2000

Isotopic NO as a Chemical Tracer in the Global Stratosphere

Stratospheric NO originates from nitrous oxide reacting with O(1D) and ion-molecule reactions. Most ionic reactions take place in the mesosphere and lower thermosphere. The resulting NO is transported into the stratosphere at high latitudes. Cosmic radiation and tropospheric lightning also produce nitric oxide. This NO originates from ion reactions involving N2. Ionic reactions preserve the N(15)/N(14) ratio present in atmospheric N2. Nitrous oxide has a mass-dependent sink that varies with altitude so that there is an altitude-dependent isotopic distinction in nitrous oxide. This difference will appear in NO formed from N2O. The expected NO isotopic distribution under different conditions will be a combination of NO derived from nitrous oxide with different masses and NO from ion reactions. The expected NO isotopic distribution will be presented talking into account the different processes, including particle events and downward transport in winter.

Aikin, A. C.↗

NASA NTRS2000

The Influence of Particle Charge on Heterogeneous Reaction Rate Coefficients

The effects of particle charge on heterogeneous reaction rates are presented. Many atmospheric particles, whether liquid or solid are charged. This surface charge causes a redistribution of charge within a liquid particle and as a consequence a perturbation in the gaseous uptake coefficient. The amount of perturbation is proportional to the external potential and the square of the ratio of debye length in the liquid to the particle radius. Previous modeling has shown how surface charge affects the uptake coefficient of charged aerosols. This effect is now included in the heterogeneous reaction rate of an aerosol ensemble. Extension of this analysis to ice particles will be discussed and examples presented.

Aikin, A. C.↗

NASA NTRS2000

Satellite Observations of Enhanced Tropospheric Ozone Associated with Biomass Burning in Africa and Madagascar

Tropospheric ozone over Africa and Madagascar is enhanced by 10 to 15 DU in October. This maximum coincides with the time of maximum biomass area burning in Africa and Madagascar. Ozone observations were made from 1979 to 1999 using the TOMS tropospheric ozone convective cloud differential method. As a result of easterly trade winds, ozone originating on Madagascar is transported to the west over the Mozambique Channel. In El Nino years higher level westerly winds descend to transport low level ozone easterly. This results in African continental ozone being transported east of Madagascar. Long range transport of African ozone is observed during El Nino periods.

Aikin, A. C.↗

NASA NTRS1999

Mesospheric Odd Nitrogen Enhancements During Relativistic Electron Precipitation Events

The behavior of mesospheric odd nitrogen species during and following relativistic and diffuse auroral precipitation events is simulated, Below 75 km nitric oxide is enhanced in proportion to the ion pair production function associated with the electron precipitation and the length of the event. Nitrogen dioxide and nitric acid are also enhanced. At 65 km the percentage of odd nitrogen for N is 0.1%, HNO3 is 1.6%, NO2 is 15%, and NO is 83.3%. Between 75 and 85 km NO is depleted during particle events due to the faster destruction of NO by N relative to the production of NO by N reacting with O2. Recovery of NO depends on transport from the lower thermosphere, where NO is produced in abundant amounts during particle events.

Aikin, A. C.↗

NASA NTRS1999

Source of Global Scale Variations in the Midday Vertical Content of Ionospheric Metal Ions

An analysis of long baseline NIMBUS 7 SBUV (Solar Backscatter UV Spectrometer) observations of the latitudinal variation of the noontime vertical Mg' content above approx. 70 km have revealed seasonal, solar activity and magnetic activity dependencies in the Mg+ content. The distributions were categorized in terms of magnetic coordinates partially because transport processes lifting metallic ions from the main meteor ionization layer below 100 km up into the F- region and down again are controlled by electrodynamical processes. Alternatively, the Nimbus Mg+ distributions may simply be a result of ion/neutral chemistry changes resulting from atmospheric changes and not dynamics. In such a case magnetic control would not dominate the distributions. Using in situ satellite measurements of metal ions from the Atmosphere Explorer satellites in the region above the main meteor layer and published sounding rocket measurements of the main metallic ion layers, the effects of the dynamics on the vertical content are delineated. The consequences of atmospheric changes on the vertical content are explored by separating the Nimbus measurements in a geodetic frame of reference.

Joiner, J.↗

NASA NTRS1992

Tropical behavior of mesospheric ozone as observed by SMM

The seasonal behavior of low latitude mesospheric ozone, as observed by the SMM satellite solar occultation experiment, is detailed for the 1985-1989 period. Annual as well as semi-annual waves are observed in the 50-70 km altitude region. In the latitude range of +/- 30 deg the ozone phase and amplitude are functions of temperature and seasonal changes in solar flux. Temperature is the controlling factor for the equatorial region and seasonal changes in solar flux become more dominant at latitudes outside the equatorial zone (greater than +/- 15 deg). There is a hemispheric asymmetry in the ozone annual wave in the 20-30 deg region, with Northern Hemispheric ozone having a larger amplitude than Southern Hemispheric ozone.

Aikin, A. C.↗

NASA NTRS1992

Natural Cycles, Gases

The major gaseous components of the exhaust of stratospheric aircraft are expected to be the products of combustion (CO2 and H2O), odd nitrogen (NO, NO2 HNO3), and products indicating combustion inefficiencies (CO and total unburned hydrocarbons). The species distributions are produced by a balance of photochemical and transport processes. A necessary element in evaluating the impact of aircraft exhaust on the lower stratospheric composition is to place the aircraft emissions in perspective within the natural cycles of stratospheric species. Following are a description of mass transport in the lower stratosphere and a discussion of the natural behavior of the major gaseous components of the stratospheric aircraft exhaust.

Douglass, Anne R.↗

NASA NTRS1991

Temperature trends in the lower mesosphere

The largest atmospheric temperature changes due to the increase of greenhouse gases are expected in the 40 to 60 km altitude region, where enhanced infrared cooling decreases the temperature. Ten-year (1980-1990) temperature trends at 55 km and 0.4 mb, derived using data from the ground-based lidar at Haute Provence, (France), and the SSU-instrument channel 47X on several satellites, are presented. These data show temperature decreases that are as large and in some cases exceed predictions based on current models. At 44 deg N, the ground-based lidar and satellite techniques give a negative trend of -0.10 + or - 0.04 percent per year and -0.14 + or - 0.02 percent per year, respectively. Agreement between these two data sets based on different measurement techniques gives confidence in the detected trends at this latitude. Further analysis of the SSU 47X satellite data between 45 deg S and 45 deg N indicates a maximum decline of 0.16 percent per year near 30 deg N. A minimum trend decrease of 0.07 percent per year is detected between 20 and 30 deg S. Based on NOAA satellite radiance observations, these long-term temperature changes are larger than changes at any of the other stratospheric levels below 55 km monitored during this period.

Aikin, A. C.↗

NASA NTRS1990

A Van de Graaf source mechanism for middle atmospheric vertical electric fields

It is proposed that meteoric and other debris descending through the mesosphere constitute a natural Van de Graaf generator for vertical electric fields within the mesosphere. Dust and aerosol particles falling from above 85 km are charged negatively in the upper D-region. Charge is lost in the region below 70 km. This net charge transport creates a vertical polarization electric field. Calculated fields are in the range of 10 mV/m for the average input of meteoric debris. Observed vertical electric fields are confined to a few occasions when large fields of the order of 4 V/m are observed to maximize at 65 km. Calculated fields from this model also maximize at this altitude, but a special event with increased dust density or another mechanism to increase relative vertical velocity is required to explain the large fields. Such large values are the exception rather than the rule for D-region vertical electric fields.

Aikin, A. C.↗

NASA NTRS1990

SMM mesospheric ozone measurements

The main objective was to understand the secular and seasonal behavior of ozone in the lower mesosphere, 50 to 70 km. This altitude region is important in understanding the factors which determine ozone behavior. A secondary objective is the study of stratospheric ozone in the polar regions. Use is made of results from the SBUV satellite borne instrument. In the Arctic the interaction between chlorine compounds and low molecular weight hydrocarbons is studied. More than 30,000 profiles were obtained using the UVSP instrument on the SMM spacecraft. Several orbits of ozone data per day were obtained allowing study of the current rise in solar activity from the minimum until the present. Analysis of Nimbus 7 SBUV data in Antarctic spring indicates that ozone is depleted within the polar vortex relative to ozone outside the vortex. This depletion confirms the picture of ozone loss at altitudes where polar stratospheric clouds exist. In addition, there is ozone loss above the cloud level indicating that there is another mechanism in addition to ozone loss initiated by heterogeneous chlorine reactions on cloud particles.

Aikin, A. C.↗

NASA NTRS1989

An intercomparison of mesospheric ozone profiles determined by the UVSP and SAGE II solar occultation experiments

A comparison is made of individual UVSP and SAGE II mesospheric ozone profiles between 50 and 70 km altitude as determined by the solar occultation technique. The generally good agreement between the two data sets below about 57 km leads to the conclusion that they may be considered as complementary, thus extending the effective altitude range of both. Comparison of the long-term ozone trend at 55.5 km shows a systematic difference of a few percent between the two measurements.

Aikin, A. C.↗

NASA NTRS1988

The three-dimensional morphology of the Antarctic ozone minimum

The three-dimensional morphology of the Antarctic ozone minimum was obtained by mapping ozone distribution data from Nimbus-7 solar backscatter UV instrument, obtained at different pressure levels, on a south polar projection. These maps show that a clearly defined ozone minimum relative to the local ozone field extends throughout the stratosphere from about 10 km to above 50 km, though the intensity of the ozone decrease becomes less with altitude. The examination of the behavior of the general ozone-hole area suggests the existence of at least three distinct regions: (1) the region below 33 km, where ozone follows the pattern of the ozone hole, exhibiting a strong trend since 1979 and decreasing from late August to a minimum in October; (2) the 33-43 km height region of relatively stable ozone concentration; and (3) the region above 43 km, where ozone is again fluctuating, with a decrease during September to a minimum in October.

Aikin, A. C.↗

NASA NTRS1987

Measurement of methane and other light hydrocarbons in the troposphere and lower stratosphere

The volume mixing ratios of methane, acetylene, ethane, and propane were measured in the troposphere and stratosphere on April 5, 1984, at 33 deg N, over New Mexico, using the technique of grab sampling by evacuated spheres on a balloon platform. Tropospheric volume mixing ratios were CH4, 1.59 ppm; C2H2, 358 ppt (parts per trillion); C2H6, 365 ppt; and C3H8, 1440 ppt. In the stratosphere, acetylene was 60 ppt. For ethane and propane the mixing ratios at 11.6 km were 441 ppt and 84 ppt, respectively.

Aikin, A. C.↗

NASA NTRS1986

Meteoric material and the behavior of upper stratospheric polar zone

Ozone mixing ratios as a function of pressure level and time are presented based on data obtained with the Nimbus-7 SBUV instrument between 1979 and 1984, and implications of this data for the explanations of the spring Antarctic ozone depletion are considered. It is suggested that meteoric atoms react with the atmosphere to bond to OH, and subsequently react with HCl to form salts, resulting in the accumulation of chlorine during polar winter. The sudden release of this chlorine from photodissociation of these salts during spring could account for the loss of ozone in the upper atmosphere.

Aikin, A. C.↗

NASA NTRS1986

Mesospheric ozone changes associated with 27-day solar ultraviolet flux variations

Solar ultraviolet flux changes associated with the 27-day solar rotational period cause corresponding variations in mesospheric ozone near the maximum of the 11-year sunspot cycle. This statement is based on a correlation and spectral analysis of ozone mixing ratios, deduced from Solar Mesospheric explorer satellite-based measurements of 1.27-micron O2 airglow emission and solar flux observations made from the same spacecraft in 1982. With the Lyman-alpha flux taken as an indicator of solar ultraviolet variability, spectral analysis shows a primary period of 27.1 days with a secondary period of 13.5 days. The 27.1-day period is observed in the ozone mixing ratio data together with other periods, including 13.5 days. Both a classical statistical analysis and a time series treatment show that, for 244 days, there is a correlation between ozone and solar flux near 50 km and between 65 and 70 km. Calculations predict a positive correlation over the entire mesosphere if there is no change in temperature accompanying the solar flux. Lack of correlation is temperature induced.

Aikin, A. C.↗