SA23A-0378 1340h
TIMED GUVI and SEE Observations of Solar Irradiance Variations and the Terrestrial Airglow Response
Since the launch of the TIMED mission in 2001, the SEE and GUVI instruments have observed solar radiance changes during numerous solar flares, and measured their short-term impact on the terrestrial airglow, manifested as changes in both resonantly scattered and photoelectron excited emissions. The continuous coverage and higher time resolution of the GUVI airglow observations, in conjunction with the multispectral (5-color) image format, constitute a unique source of information on the time variation of the solar irradiance in different spectral regions. GUVI limb observations provide additional data on heating and composition changes in the thermosphere in response to these energy inputs. We examine changes in the observed airglow between quiet and flare conditions, and attempt to understand the differences between SEE measurements and the radiances inferred from GUVI airglow data.
SA23A-0379 1340h
Accelerometer and Orbital Drag Thermospheric Density Inter-comparisons
Accelerometers on the CHAMP and GRACE satellites provide high-resolution (seconds) thermospheric neutral densities. CHAMP carries the STAR accelerometer and GRACE carries an upgraded SuperSTAR accelerometer. Both instruments are on high-inclination satellites. These data are compared to two types of orbital drag data. One set is from HASDM (High Accuracy Satellite Drag Model). This approach provides spatially and temporally varying density corrections involving tracking of some 100 satellites. A weighted least squares fit to all of the tracking data produces a global density solution with a time resolution of several hours. The second set consists of individual measurements from each of twenty radar-tracked satellites. Densities were derived using newly developed techniques to provide one-day resolution. These data are relatively localized in latitude since all satellites were in highly elliptical orbits. HASDM data are available only during two periods; the first half of 2001 and August-September 2002. Data from each accelerometer is compared to orbital drag results at different periods. CHAMP data are compared to HASDM and individual orbital drag measurements during May-June 2001. Ratios of CHAMP to HASDM, analyzed in latitude-time coordinates show that significant high latitude structures are not observed by HASDM. The ratio of CHAMP absolute density to that from HASDM also increases with latitude. GRACE data are compared to the second set of HASDM data in August 2002. Similar analyses of GRACE data trends vs latitude and of absolute densities are presented. Data sets are analyzed during quiet and disturbed conditions to compare the thermospheric density response to solar and geomagnetic inputs as observed by in situ and orbital drag measurements.
SA23A-0380 1340h
An investigation of thermospheric neutral density vs.O/N$_{2}$ during the Oct-Nov 2003 magnetic storms
Solar flare and magnetic storm activity occurring in October and November of 2003 have been the subjects of intense scrutiny for their extreme effects on the upper atmosphere and ionosphere. During these events, complimentary measurements of the upper atmosphere and ionosphere were made using a broad variety of space-based assets. Important to this initial study of the thermospheric disturbance are in-situ measurements of the neutral density near 400 km from the CHAMP and GRACE satellites. These carry sensitive accelerometers which can detect the slight satellite drag force from which the neutral density is then determined. Global images from the Far-Ultraviolet Spectrographic Imager onboard the NASA-IMAGE satellite provide a measure of the disturbance in thermospheric composition at lower altitudes (130-200 km). Observed from high altitudes, variations in atomic oxygen emissions at 135.6 nm closely follow the column integrated ratio of O/N$_{2}$. In this initial comparison of the two data sets, the correspondence of total density at high altitudes and O/N$_{2}$ throughout the atmospheric column is examined and compared to expected results from empirical atmospheric models (cf. NRL-MSIS 2000). Departures from hydrostatic equilibrium should certainly be observed during the intense magnetic storm. The origin of any observed effects will be studied in the context of drivers including dynamical redistribution or impulsive auroral heating of the high-latitude thermosphere.
SA23A-0381 1340h
Power Spectra and Geophysical Variations in the Auroral Hemispheric Power
Two and a half solar cycles of low energy auroral electron and total hemispheric power indices from 21 NOAA and DMSP satellites were intercalibrated for each hemisphere after addressing contamination and degradation issues. The adjustments ranged within a factor of two. The SEM-2 NOAA satellites were also intercalibrated to provide estimates of the low energy ($<$ 20 keV) auroral ion hemispheric power over half a solar cycle. The winter hemispheric electron power exceeded the summer by 5% to 25% between solar minimum and maximum, while the low energy ion hemispheric power was larger in the summer hemisphere. Hourly and daily median and average composite indices were created. The adjusted and composite indices were compared with various geophysical indices. Spectral power analyses were made to find the long and short time periodicities of the parameters. The best cross-correlations were found to be with the Kp geomagnetic index for the electron hemispheric power and with the Ap geomagnetic index for the ion hemispheric power.
http://cedarweb.hao.ucar.edu/instruments/ehp.html
SA23A-0382 1340h
Direct Comparisons of Energetic Electron Precipitation and Nitric Oxide in the Upper Atmosphere During Geomagnetic Storms
Nitric oxide (NO) densities in the lower thermosphere are in this study directly compared with the energy deposition from precipitating energetic electrons. The comparisons are made for geomagnetic storm time events in 1998. The electron energy is derived from X-ray bremsstrahlung observations from the Polar Ionospheric X-ray Imaging Experiment (PIXIE) on board the Polar spacecraft. Measurements of the NO volume density are performed by the Student Nitric Oxide Explorer (SNOE) on the dayside by measuring airglow spectral features of the NO gamma band. Since a significant part of the electron precipitation takes place during the night, and considering the long lifetime of NO, we have accumulated the X-ray data in geographical boxes. This enables us to follow the total energy deposition over a specific area during the night and morning hours, and we can compare this with the NO density measured by SNOE on the dayside. For the beginning of a storm time event May 2, 1998, we find a clear increase in NO at higher latitudes due to electron precipitation. For this event, at altitude of peak values of both NO density and deposited electron energy, 106 km, over 80 % of the NO density is produced by electron precipitation. Other events from the period 1998 to 2000 will also be presented.
SA23A-0383 1340h
Investigations of the structure of the neutral atmosphere within the dawn and dusk sector diffuse aurora.
The neutral thermosphere within the pre and post midnight substorm recovery phase diffuse aurora is found to show very large horizontal winds, and strong vertical structure. In the dusk side diffuse aurora, rocket measurements indicate neutral winds of up to 400 m/s. Rocket, satellite, and ground based observations during the ARIA (Atmospheric Response in Aurora) campaigns, and earlier dawn side rocket observations, show winds of up to 200 m/s, and a characteristic jet like wind maximum around 110 to 120 km altitude. Strong shears are present in the wind structure on both the dawn and dusk sides, and the regions below these shears sometimes have a Richardson number below 0.25, suggesting that they may be unstable. Instabilities may be associated with waves, turbulence, enhanced diffusivity, and changes in local composition. The enhanced auroral precipitation and electric fields associated with substorms may have significant effects on the E region neutral thermosphere, and observed wind magnitudes are found to have a dependence on geomagnetic activity level. However, recent results suggest that propagating tides play a significant role in generating the strong vertical variations in the neutral winds, and the relative importance of auroral and tidal forcing in generating the observed winds is not well known. Significant differences also exist in the acceleration conditions in the pre and post midnight sectors of the auroral oval, and how these affect the neutral responses on the dawn and dusk sides is not well understood. Investigations have been performed using a three-dimensional high resolution limited area thermosphere model developed at UCLA, to understand the processes which generate the observed neutral structure within the diffuse aurora on both the dawn and dusk sides. Auroral forcing parameters for the model have been estimated using values measured during the dawn and dusk observing campaigns. The CTIP (Coupled Thermosphere Ionosphere Plasmasphere) model has been used to specify global background winds and tides. The response of the neutral atmosphere to changes in auroral forcing, IMF orientation, large scale winds, and propagating tides, including the (2,4), (2,5), and (2,6) semidiurnal modes, has been determined for both the dawn and dusk sectors.
SA23A-0384 1340h
Thermospheric Neutral Temperature Response to Auroral and Solar EUV Energy Input
A Fabry-Perot imager (FPI) has been installed at Syowa Station, Antarctica since 2001 in order to study short-term thermospheric response to auroral activities and long-term variability of the polar-thermospheric circulation. In this paper variations in thermospheric neutral temperature derived from the FPI observation are compared with solar EUV flux and auroral activity. The FPI is equipped with a stabilized Fabry-Perot interferometer with a clear aperture of 150 mm, a special optical system designed for the etalon, and a back-illuminated CCD camera. Spacing of the etalon is periodically checked by a fringe image of frequency-stabilized He-Ne laser emission. The etalon controller and the laser are kept in a thermostatic chamber to avoid temperature drift. The FPI acquired approximately 50,000 images on 71 nights from March 31, 2001 to October 16, 2001. The interference fringe images of OI 557.7 nm and OI 630.0 nm emissions were alternatively obtained every minute with an exposure time of 30 sec. Reference images of the laser and dark images were acquired every 30 min. Horizontal distributions of thermospheric wind and temperature are derived from the fringe images of the auroral emissions. Temporal drift of the etalon spacing is corrected using the laser fringe images. To derive absolute values of wind speed the absolute spacing of the interferometer must be known with an accuracy of 10E-8. The spacing can be determined with the accuracy from the known wavelengths of auroral and laser emissions and the orders of interference determined from differences among average fringe peak positions of the these emissions. Temporal variations of the neutral temperature and wind in the F-region were compared with the solar EUV flux data obtained by SOHO and all-sky auroral image data simultaneously acquired with the FPI data. The daily average temperatures correlate with the solar EUV flux for the time scales of a few days to several months. For faster temperature increases with the time scales of hours to a few days local auroral energy input seems to be a predominant energy source. Acknowledgment: Solar EUV flux data are provided by the CELIAS/SEM experiment onboard SOHO.
SA23A-0385 1340h
Using X-ray Occultation Sounding to Detect Dynamic Atmospheric Features in the Upper Mesosphere and Lower Thermosphere
X-ray Occultation Sounding (XOS) is a new technique for measuring total atmospheric density in the lower thermosphere and mesopause region (70-150 km) . As bright celestial X-ray sources are occulted by the Earth's atmosphere, extinction arising from inner-shell photoionization provides density information which is independent the chemistry, ionization, and thermal states of atmospheric gases. Simplistic density retrievals using XOS have demonstrated densities roughly consistent with the NRLMSISE-00 empirical climatological atmospheric model. We present results from a more detailed retrieval algorithm that enables us to examine the differences between observed density profiles and climatological predictions. An ensemble of X-ray occultations observed with the Pulse Counter Array aboard the Rossi X-ray Timing Explorer satellite are presented. We search for heating effects caused by solar X-ray flares by correlating XOS-based densities against solar X-ray fluxes measured by the GOES satellite. We also consider how other dynamical and space weather effects might account for deviations of measured densities from climatological predictions.
SA23A-0386 1340h
Simultaneous Radar and Optical Observation of E-region Irregularities and F-region Traveling Ionospheric Disturbances
On the night of August 6, 2002, the 46.5-MHz middle and upper atmosphere (MU) radar with 5 beam directions at Shigaraki, Japan, observed typical quasi-periodic (QP) echoes from field-aligned electron density irregularities in the ionospheric E-region. The QP echoes exhibited a wavy structure which had a wavelength of 30 km and propagated southwestward at 100 m/s. During this QP event, an all-sky CCD imager at the MU radar site detected medium-scale traveling ionospheric disturbances (MSTID) in 630 nm airglow images. The MSTID also propagated southwestward at 80 m/s with relative fluctuation amplitudes of 30-50 percent and a wavelength of 300 km. This propagation direction is very similar to the direction of the QP echoes movement. Also, a 630.0 nm Fabry-Perot interferometer at the radar site detected southeastward neutral winds (U) of 106 m/s. U would generate an electric field of 4.5 mV/m through UxB, where B is the geomagnetic field. From these observed values and average F-region electric field of 1.0 mV/m that is derived from previous MU radar observations in summer under high solar activity conditions, we estimate polarization electric fields associated with MSTID are 1.2-2.0 mV/m. When these polarization fields are mapped down along B without attenuation, ExB plasma drift velocities of 28-47 m/s are induced in the E-region. In actual, drift velocities in the QP echoes observed with the MU radar was 42 m/s, which is very consistent with the values (28-47 m/s) estimated above. Moreover, the direction of the ExB drift estimated is almost identical to that of the plasma drift in the QP echoes observed with the MU radar. Thus, the electric fields associated with the F-region MSTID seem to be closely coupled to those that generate QP echoes in the E-region.
SA23A-0387 1340h
Global Distribution of the Thermospheric Total Mass Density Derived from CHAMP
A global distribution of the thermospheric total mass density is derived from the high-accuracy accelerometer on board of the CHAMP satellite with good temporal and spatial coverage. It shows two interesting features. One is the anomalous distribution at low latitudes. Instead of maximizing at the dayside equator, the thermospheric density shows maxima at about 20>|-25>| latitude on both sides of the geomagnetic equator between 11-17 MLT. This distribution resembles fairly well the equatorial ionization anomaly (EIA), thus indicating strong magnetic control of the thermospheric mass density via ionosphere-thermosphere coupling. The thermospheric density shows a secondary maximum at the nightside equator shortly before midnight, in reminiscence of the well known thermospheric Midnight Temperature Maximum (MTM). Another feature to notice is that no well-defined density cells can be identified in high latitudes at the altitude of 400 km, thus showing that the cellular structure recognized by the NCAR TGCM model at 120 - 300 km does not necessarily extend to higher altitudes. A comparison between observations and the MSIS90 model predictions shows that although the model describes the general structure of the observed density reasonably well, it misses the double peaks at low-latitudes completely. This causes an underestimation of the total mass density by 15-20% in the crest region. At high latitudes, an underestimation of 20-30% occurs in the midnight sector and the cusp region. The agreement between observations and predictions is quite good outside these localized areas, with only ~5% difference on average during quiet periods.
SA23A-0388 1340h
Study of the Proton Arc Spreading Effect on Ionization Rates
We investigate the influence of the beam spreading effect in a proton auroral arc on resulting ionization rates, using our three-dimensional Monte Carlo ion precipitation model. A proton arc with a Maxwellian energy spectrum has two dimensions at the top level of 950 km altitude: an infinite extent in the east-west direction and a limited width in the north-south direction. Two types of incident energy flux distribution, a homogeneous and a Gaussian distribution, are imposed. The spreading effect due to charge exchange/electron stripping collisions are numerically simulated in both vertical and tilted magnetic field lines. The results of model calculations show that a single attenuation coefficient introduced in one-dimensional theoretical models as a simplification to the beam spreading effect can not completely solve the problem. In contrast, ionization rates are overestimated at high altitudes but underestimated at low altitudes. These findings can help to better understand calculation results when using simplified 1-D models.
SA23A-0389 1340h
Joule Heating in Small-Scale Aurora: Modeling
The coupling between the magnetosphere and the ionosphere is of major importance for the thermospheric and magnetospheric dynamics. Small-scale large magnitude electric fields associated with aurora can generate considerable Joule heating which is not present on large scale temporal and spatial averages. Presented is a simulation model which considers localized parallel electric fields as a source for discrete auroral dynamics. The evolution produces large electric field in the immediate vicinity of field-aligned current layers. We will discuss properties of the dynamics depending on ionospheric conditions. The Joule heating is determined over several scale averages. It is shown that as the scale averages are increased, a progressively lower value is obtained for the total Joule heating. This represents a mismeasurement.
SA23A-0390 1340h
Estimating Neutral Densities from Energy Sources Using Multiple Linear Regression
Space operations involving satellite tracking requires estimates of the atmosphere's neutral density in order to determine the drag on the satellites. The neutral density models use F10.7 as a proxy for solar EUV input and the ap index as a proxy for the geomagnetic input. These models are typically semi-empirical, models which model the atmosphere as a hydrostatic equilibrium. Some models like the Air Force's High Accuracy Satellite Drag Model (HASDM) try to estimate and predict a dynamically varying high-resolution density field. We present a simple system for predicting neutral density along a satellite track based on solar, Joule, and particle heating. In an earlier study, we used a portion of the CHAMP satellite data set to develop parameters for estimating the neutral density at 450 km, and then used those parameters to estimate the neutral density during a different portion of the CHAMP data set. We developed a skill score to determine how well we were able to predict the neutral density along the CHAMP trajectory. We extend this work to a set of 18 satellites where we know their daily average densities to about 2-3% uncertainty.