SA11B-01 INVITED 08:00h
Extreme Space Weather: Storm Fronts at the Plasmasphere Boundary Layer
Disturbance electric fields during the major geomagnetic storms of October and November 2003 resulted in the large-scale perturbation and redistribution of Earth's thermal plasma environment. The solar-produced plasmas of the low-latitude ionosphere and plasmasphere erupted poleward in response to storm-time penetrating electric fields, producing greatly-enhanced plasma concentrations at middle latitudes in the North American sector. Sub-auroral electric fields stripped away these regions of enhanced total electron content (TEC) and carried them sunward as intense plumes of storm-enhanced density (SED). Very sharp spatial gradients in mid-latitude TEC developed associated with magnetosphere-ionosphere coupling effects and instability growth at the plasmasphere boundary layer. We use distributed ground-based imagery of total electron content derived from GPS observations to produce high-resolution spatial and temporal maps of the intensity and evolution of these features. The Millstone Hill incoherent scatter radar and overflights with the DMSP satellites are used to investigate the characteristics of the processes involved in the formation of such severe space weather conditions at mid latitudes. During the November 20, 2003 event, TEC over the continental USA approached 300 TECu. The steepest spatial gradients in TEC (in excess of 100 TECu per degree of latitude) occurred as perturbations developed in the sub auroral polarization stream (SAPS) along the plasmasphere boundary layer.
SA11B-02 INVITED 08:20h
CRRES and DMSP Observations of Wave and Plasma Disturbances Associated with the Stormtime Ring Current in the Plasmasphere and Topside Ionosphere
We report on wave and plasma disturbances observed by Combined Release and Radiation Effects (CRRES) and Defense Meteorological Satellite Program (DMSP) satellites during the magnetic storm of June 5, 1991 in the region of ring current/plasmasphere overlap and the conjugate topside ionosphere During three ring current nose encounters near L = 2.4, the plasmasphere was highly-structured. A rich variety of wave phenomena were observed simultaneous with enhanced fluxes of low-energy ($<\sim$ 1 keV) electrons and ions, indicating the wave heating/acceleration source. Earthward of the plasma sheet boundary, which was near L = 5.5, wave-like structures in the dawn-to-dusk electric field with spatial wave-lengths from about 300 to 1000 km and magnitudes of ~1-3 mV/m were apparent. Mapped to ionospheric altitudes, these fields should produce broad irregular SAPS with average sunward velocities ~ 1 km/s. At about the same time DMSP F8, F9, and F10 indeed observed highly-structured SAPS in the topside ionosphere coincident with precipitating ring current ions, enhanced fluxes of suprathermal electrons and ions, elevated electron temperatures, and deep highly-irregular density troughs. Overall, these events represent the so-called strong wave-SAPS phenomenon [Mishin et al., JGR (2003), 108, 1309, 10.1029/2002JA009793]. Their importance for Space Weather is indicated by strong GPS phase and amplitude scintillations observed over the continental US [Basu et al., JGR, 106, 30389, 2001; Ledvina et al., GRL, 29, 10.1029/2002GL014770] coincident with similar events.
SA11B-03 INVITED 08:40h
Steep Electron Density Gradients in the Midlatitude Nighttime Ionosphere: Current Understanding and Future Directions
The midlatitude nighttime ionosphere is generally considered to be a fairly quiet system. Indeed, when compared to the highly active low- and high-latitudes, this seems to be a valid description. However, observations in the 1990s, notably those conducted as part of the multi-instrument Combined Ionospheric Campaigns (CICs) carried out periodically from 1997 to 1999, have shown that disturbances can indeed occur in this region. For example, a relatively common feature seen in allsky images taken at 630.0 nm are bands of depleted intensity aligned from the northwest to southeast that propagate to the southwest (in the northern hemisphere). The general characteristics of these bands are now fairly well documented, as is the basic understanding of their underlying physics. The effects of these medium-scale traveling ionospheric disturbances, or electrobouyancy waves, is to produce gradients in the electron density and F-layer height. During quiet magnetic periods, these gradients are fairly small, of the order of a few TEC units. However, these structures seem to be amplified by high geomagnetic activity at which times the gradients can be of the order of 10 to 20 TEC units over distances as small as several 10s of kilometers. Although much was learned about these structures during the CICs of the late 1990s, there are still several unexplained features that warrant further investigation, notably their genesis region, Kp dependence, and effects (if any) on trans-ionospheric radio signals. We suggest the need for a new set of campaigns in the Caribbean to address these questions, modeled on the CICs but extending their spatial coverage and including additional instrumentation not available during the initial campaigns. It is only through such a comprehensive, multi-technique investigation that we will make further headway on understanding these phenomena.
SA11B-04 INVITED 09:00h
The Impact of Ionospheric Storms on the Wide Area Augmentation System (WAAS)
In 2003, the FAA commissioned the Wide Area Augmentation System (WAAS). This new navigation system is capable of guiding aircraft to within a few hundred feet of the ground. Although the system accuracy is typically better than 2 m (95% in the vertical direction), there exist rare fault modes that can create errors more than ten times as large. Because these rare faults may be unobservable, the system availability is limited to times when they can be guaranteed to be small. The dominant source of these errors is the ionosphere. The WAAS ground segment consists of 25 dual-frequency reference stations that sample the ionosphere at discrete locations throughout the service volume. The users, however, only have access to a single frequency and cannot estimate the ionosphere directly. The ionospheric delay that they experience must be estimated from the ground station measurements. More importantly, the uncertainty from each estimate must be rigidly bounded as these corrections are part of a safety-of-life system. In addition, the users may be anywhere within the service volume; WAAS needs to protect all possible ray paths based on its discrete samples. The vast majority of the time Total Electron Content (TEC) ionospheric delays are smoothly varying functions of time and space. Given even just a few measurements, the surrounding ionosphere can be predicted accurately for tens of minutes. Unfortunately, this is not always the case. During the last solar peak, several major storms were observed that created highly localized disturbances in the ionosphere. These disturbances are not easily modeled by the low bit rate message format used to communicate to the users. Even worse, they may not be sampled at all by the ground stations. Thus, during such storms users may suffer very large correction errors. WAAS therefore includes three protection factors against this threat: a storm detector, and spatial and temporal threat models. The storm detector is an internal consistency check to determine if the observations are consistent with nominal (quiet) or disturbed ionosphere. The spatial, or undersampled, threat model characterizes the magnitude of disturbances that can escape detection given current sampling, while the temporal threat model characterizes the amount it may change over the lifetime of the correction. This paper will describe most significant ionospheric TEC disturbances observed by the WAAS network. It will further explain why these features are so problematic to navigation and what is being done to reduce their impact.
http://waas.stanford.edu/
SA11B-05 09:15h
Two Components of Ionospheric Plasma Structuring at Mid-latitudes during Large Magnetic Storms
Analysis of VHF amplitude scintillation and GPS phase fluctuation data in conjunction with global GPS total electron content (TEC) maps, ground-based measurements of daytime aurora and TIMED GUVI images have indicated that there are two distinct plasma processes that give rise to mid-latitude ionospheric irregularities during large magnetic storms. One is associated with auroral plasma processes and the other with storm enhanced density (SED) features which have been shown to be the ionospheric signatures of plasmaspheric tails (Foster et al., GRL, 2002). The auroral nature of one class of events is observed either during local daytime or nighttime conditions depending on the time of storm commencement and the fast rate of decrease of the SYM-H index which is generally an indicator of the rapid development of the ring current and an equatorward movement of the plasmapause. The second class of events is associated with the ionospheric density gradients seen at the edges of SEDs, which frequently occur in conjunction with large northward and westward plasma convection, and are observed primarily in the afternoon-dusk sector. Examples of both classes of events are provided from the large storms that occurred during October 29-31, 2003 and March 31, 2001. Initial results have been published on the SED associated plasma structuring by an ion temperature gradient convective instability (Keskinen et al., GRL, 2004).
SA11B-06 09:30h
Towards a Model for Midlatitude Stormtime Ionospheric Mesoscale and Small Scale Structure
The response of the midlatitude ionosphere to geomagnetic storms is governed by several factors, e.g., magnetospheric leakage electric fields, dynamo action of thermospheric winds driven equatorward by auroral heating, and the equatorward propagation of fast time scale ionospheric waves and disturbances. Inner magnetospheric electric fields may generate sub-auroral polarization streams (SAPS). Equatorward of SAPS strong ionospheric coupling to plasmaspheric tail-like structures can occur. Several studies have addressed the large scale midlatitude ionospheric and thermospheric response to magnetic storms. However few quantitative models for the mesoscale and small scale structure and dynamics of the midlatitude stormtime ionosphere have been developed. Both ground and space-based observations have shown that the midlatitude stormtime ionosphere can be highly structured down to meter scale sizes. This structure appears to be located in and near the auroral - sub auroral boundary region. Recently, we have shown that SAPS can become structured due to gradient-driven processes in the presence of strong convection. A preliminary composite model for midlatitude stormtime mesoscale and small scale ionospheric structure, which combines auroral and sub-auroral irregularity generation mechanisms, is presented.
SA11B-07 09:45h
On the Source of Steep Gradients at Midlatitud During Geomagnetic Storms
The composition theory indicates a strong seasonal-latitudinal dependence in the ionospheric response to storms. This dependence is clearly visible as the boundary between the positive and negative phases of storms, which frequently is seen in the winter hemisphere. Enhancements of idealized global circulation cells, related to the energy input in the auroral regions during perturbed conditions, can be used to explain the distortion of the latitudinal profile of the mean molecular mass that produces this boundary. Changes in neutral composition explicate the negative phase in summer hemisphere, and the coexistence of positive and negative phases in winter hemisphere. This boundary can migrate across the mid-latitude regions introducing transient sharp gradients in NmF2 and possibly in TEC. Electrodynamic effects on the equatorward side act to further steep the ionospheric gradients. The dynamics of these features produce an increase in variability that can be hard to capture in models.