SA21B-01
Observations and simulations of the ionospheric and thermospheric response to the December 2006 geomagnetic storm: Initial phase
We have investigated the thermospheric and ionospheric response to the 14-15 December, 2006 geomagnetic storm using a Coupled Magnetosphere Ionosphere Thermosphere (CMIT) 2.0 model simulation. Here we focus on observations and simulations during the initial phase of the storm (about 8 hours), when the shock was driving changes in geospace. The global ionospheric maps of total electron content (TEC), ionosonde data at four stations and Millstone Hill incoherent scatter radar (ISR) observations are compared with the corresponding simulation results from the CMIT model. The observations showed significant positive storm effects occurred in the Atlantic sector after the onset of this storm. The CMIT model is able to capture the temporal and spatial variations of the ionospheric storm effects seen in the GPS TEC observations, although the model slightly underestimates the daytime positive ionospheric storm in the South American sector. The simulations are also in agreement with the ionosonde and ISR ionospheric measurements. Term analysis of the ion continuity equation demonstrates that changes in the electric fields play a dominant role in generating the observed ionospheric positive storm effect in the American sector during the initial phase, although neutral winds and composition changes also contribute. The difference in the strength of the enhancements over North and South America can be explained by the slope of the topside electron density profiles in the two hemispheres. Nighttime changes in electron density in other longitude sectors are also small, because the topside electron densities also decrease slowly with altitude at night.
SA21B-02
Ionospheric Superstorms: Polarization Terminator Effects in the Atlantic Sector
A combination of the stormtime penetration electric fields, the effect of the reduced magnetic field strength in the South Atlantic magnetic anomaly, and the geographic distortion of the magnetic field in the Atlantic sector contribute to the characteristics of the low-latitude polarization electric fields at the sunset terminator. This combination of effects leads to a strong localized enhancement of TEC at low-mid latitudes in the American sector during ionospheric superstorms. At dusk, the low-latitude polarization electric field effects begin on magnetic field lines when the E region at either end goes into darkness. We define the polarization terminator (PT) to be the locus of points at a given altitude for which the E-region shadow height at either end of the magnetic field line equals 100 km. Electric fields associated with the charge build-up in the conductivity-gradient region due to the effects of winds or penetration electric fields are directed perpendicular to the PT and increase in magnitude as the PT is approached from the dayside. The particular configuration of the magnetic field in the Atlantic sector creates a preferred longitude/Universal Time sector (western atlantic/ 21 UT) for the build-up of enhanced TEC on field lines inside the dusk plasmapause. The electric fields associated with the PT sweep up the plasmas of the equatorial anomaly crests and redistribute it into the mid-latitude SAPS channels, forming the high total content storm enhanced density (SED) plumes observed during strong storms in the American sector. This effect is most pronounced for northern hemisphere summer conditions, as experienced during the July 15/16, 2000 superstorm.
SA21B-03
On Extreme Space Weather Conditions in the Equatorial Ionosphere
During the November 2004 Superstorm, unprecedented coverage was available from the radar chain near 75°W. In particular, the Jicamarca Radio Observatory reported the highest upward and downward drifts ever recorded. These huge upward drifts (>120 m/s) were highly correlated (90%) with the dawn-to-dusk component of the interplanetary electric field (the electric field in the frame of reference of the earth) with a 10% efficiency. When a reversal of the IEF abruptly occurred in daytime conditions, the temporal history of the zonal component of the equatorial field was similar but the efficiency was only about 3%. The huge daytime uplift abruptly evacuated the daytime ionosphere but the solar radiation immediately began to create a new ionosphere. With the above-mentioned reversal to downward motion, the high altitude plasma reappeared in the Jicamarca field of view, causing two F layers: one at 300 km and the other near 600 km. Equatorial spread F was suppressed by the northward turning of the IMF, but was initiated with the second episode of upward velocity over 120 m/s. A simple model of the ionospheric effects of these unusual drift patterns can only be reconciled with the data if, in addition to the penetrating electric field, an equatorward wind was also generated. Mid-latitude wind measurements at Millstone Hill and at Arecibo show that, indeed, several equatorward wind pulses were observed during the storm. Evidence for anomalous resistivity in the equatorial electroject will also be presented.
SA21B-04 INVITED
Modeling equatorial spread F: New simulation results
Equatorial spread F (ESF) is a low-latitude ionospheric phenomenon that leads to the development of large scale electron density depletions that adversely affect communications and navigation systems. The development of models to understand and predict the onset and evolution of ESF is therefore critically important to a number of space-based systems. To this end, two new simulation codes have been developed at the Naval Research Laboratory to model equatorial spread F. One code uses an 8th order spatial interpolation scheme and the partial donor cell method as a flux limiter which allow simulation studies with very low numerical diffusion. We present results of the dynamics of equatorial spread F that exhibit multiple bifurcations, secondary structure development, and high speed flows within low density channels. We compare these results to all-sky images and radar data. The second code is based upon the NRL ionosphere model SAMI3 and includes coupling to the E region via field-aligned integrated conducitivities. We show the impact of E region coupling on the evolution of ESF. \medskip Research supported by ONR.
SA21B-05
On Monthly/Seasonal/Longitudinal Variations of Equatorial Irregularity Occurrences and Their Relationship to Post-Sunset Vertical Drift Velocities
Monthly variation of global equatorial density irregularity distribution has been obtained with data taken by ROCSAT-1 at the 600 km topside ionosphere from March 1999 to June 2004 during high to moderate solar activity periods. This global longitudinal distribution of monthly irregularity occurrence variation not only provides the best spatial/temporal distribution existed so far but also fills the gap of irregularity distribution missing over eastern Pacific regions where no ground observation is available. The 5½-year result of the monthly occurrence pattern indicates a smooth variation across the longitudes contrary to some beliefs that a drastic change in irregularity occurrence pattern has occurred across the eastern Pacific longitudes. Excellent agreement is noted for the current results with Aarons' conjectured sketch of global scintillation occurrence distributions published in 1993. Furthermore, the seasonal/longitudinal (s/l) variations of quiettime post-sunset vertical drift velocities are found to track closely with the s/l variations of irregularity occurrences except during the September equinox. Linear regression analysis between the vertical drift velocity and the irregularity occurrence rate indicates that the vertical drift velocities at three different longitude zones of different magnetic declinations have good correlations with the irregularity occurrences for all seasons. This implies that the averaged post-sunset vertical drift velocity is indeed a good indicator for the occurrences of equatorial density irregularities in a longitude region that has same magnetic declination. Smooth variation of monthly/seasonal/longitudinal distributions of the vertical drift velocities that results in smooth variation of the topside density irregularity occurrences imply that global occurrence pattern of either distribution is controlled by the global variation of post-sunset ionospheric condition from the magnetic declination effect and the seasonal variation of the dip equator location with respect to the geographic equator. There seems little need to include the global variation of instability perturbation seed distribution from the atmospheric disturbances for the global distribution of topside irregularity occurrences.
SA21B-06 INVITED
Satellite Studies of Quiet and Disturbed Equatorial Vertical Plasma Drifts
The generation and evolution of equatorial plasma density structures and spread F have been important topics of ionospheric research for over four decades. Recent satellite studies have confirmed the fundamentally important role of the equatorial vertical plasma drift velocity on the generation and longitudinal distribution of equatorial spread F during geomagnetically quiet times. We use vertical plasma drift observations on board of the ROCSAT- 1 satellite during quiet and disturbed conditions to illustrate the strong longitudinal variations of the equatorial evening and nighttime drifts and their strong dependence on the ionospheric conductivity. The evening prereversal velocity enhancements have strongest longitudinal variations in the American sector during December solstice. Our data suggest that the equatorial prompt penetration electric fields resulting from sudden increases in the high latitude convection field are season dependent and have largest magnitudes during June solstice. The disturbance dynamo electric fields generate largest evening downward drift perturbations during equinox; the postmidnight upward disturbance dynamo drifts do change much with season. These empirical perturbation electric field patterns will be compared with results from numerical models.
SA21B-07
Modeling the Climatology of Equatorial Plasma Bubbles Observed by DMSP Satellites
The Defense Meteorological Satellite Program (DMSP) spacecraft, in circular near-polar orbits around 840 km altitude, occasionally observe depletions in plasma density when they cross the geomagnetic equator in the evening sector, at the times and places where low-density plasma plumes associated with equatorial spread F (ESF) radio-scintillation phenomena are expected to occur. Statistics for the frequency of observation of these depletions have been collected over the last eighteen years showing the seasonal and longitudinal variations of their rate of occurrence at various phases of the solar cycle. To better understand these probabilities, we have simulated these observations using first-principle models of the ambient ionosphere and bubble formation, using climatological drivers. The models are those which have been developed in preparation for the AFRL Communication/Navigation Outage Forecasting System (C/NOFS) mission. We will present maps of the frequency of occurrence of bubbles at 840 km altitude as a function of season and longitude calculated with the models and compare the results with the DMSP observations. These maps show the expected peaks in frequency of plasma bubbles near the equinoxes, with the additional winter peak in the American sector and the summer peak in the Pacific sector. The variation of bubble frequency with the phase of solar cycle will also be described.
SA21B-08
Modeling the Effect of Changes in the Terrestrial Magnetic Field on the Climatology of the Mid- and Low-Latitude Ionosphere.
A new model of the global ionosphere has been developed which utilizes a magnetic field structure derived directly from the full International Geomagnetic Reference Field (IGRF) - as opposed to earlier, dipolar approximations to the IGRF. Using this new model, a series of runs has been undertaken to investigate how the ionosphere has been influenced by changes in the Terrestrial magnetic field over the 100 years from 1900 to 2000. For each run, all inputs to the model, such as thermospheric composition and winds, and the Solar EUV flux are kept the same. The only change comes from the magnetic field, with values taken from the IGRF at the relevant year. The results are very interesting: In most regions the changes to the ionosphere since 1900 are fairly insignificant. However within the equatorial region of South America and the South Atlantic it is a different story. Here the changes are very significant and therefore need to be considered in any analysis of long term trends within the ionosphere. Results from the modeling study will be presented and an analysis, in terms of changes in the location of the magnetic equator and the magnetic declination and inclination, will be discussed.