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Datta-Barua S.,Illinois Institute of Technology | Bust G.S.,JHUAPL | Walter T.,Stanford University | Crowley G.,ASTRA LLC
25th International Technical Meeting of the Satellite Division of the Institute of Navigation 2012, ION GNSS 2012 | Year: 2012

Solar cycle 24 is in progress with significant solar activity beginning in 2011 and ongoing in 2012 prior to the anticipated peak in sunspots in 2013-14. We broadly review the major solar-induced geomagnetic storms of the current cycle that have been geoeffective in the American sector by surveying Kp and Dst geomagnetic indices for these periods. We summarize the WAAS Performance Analysis reports for events that affected WAAS. Focusing on the major October 24-25, 2011 storm, we examine WAAS operational system coverage for CONUS on the 25th. The timing of loss of coverage is coincident with the timing of Dst storm indication, which occurs at local nighttime in the American sector. A second loss of coverage occurs during the local daytime of 25 Oct. We compare this with the WAAS algorithm upgrade Release 3A performance. The upgrade improves on the nighttime loss of coverage slightly, but is much better able to maintain precision approach (PA) service in CONUS through the daytime of 25 Oct. To examine the nighttime ionosphere that led to the CONUS-wide loss of PA, we combine data from multiple networks worldwide including IGS, LISN, and UNAVCO. With the Ionospheric Data Assimilation Four-Dimensional (IDA4D) tool, which uses three-dimensional variational data assimilation to map ionospheric densities globally, we generate total electron content (TEC) maps of the ionosphere on 24-25 Oct 2011. IDA4D is run in a low-resolution (3° x 3°) global mode. We relate these TEC maps to WAAS availability maps for the time periods of interest. IDA4D is also at high-resolution (1° x 1°) for Florida. The IDA4D electron density estimates are then fed to Estimating Model Parameters from Ionospheric Reverse Engineering (EMPIRE). EMPIRE deduces plasma velocities due to ionospheric dynamics, such as neutral winds and electric fields, from the time-evolving 3-dimensional images of ionospheric density. The IDA4D integrated electron densities show a nighttime co-rotating persistent plume extending from Florida west-northwest across central CONUS. Such an effect was observed during some storms of solar cycle 23, most notably in the extreme storm of Oct 2003. Horizontal drifts estimated by EMPIRE are shown to be northwestward, which may partly explain the plume's duration of several hours through the night. We correlate the electron density distribution from IDA4D with ionospheric drivers from EMPIRE, the Dst, and the resulting availability of WAAS precision approach (LPV-200) service. In this way we provide a preliminary end-to-end view of a regionally geoeffective space weather storm from solar maximum 24. Source


Datta-Barua S.,San Jose State University | Bust G.S.,ASTRA LLC
24th International Technical Meeting of the Satellite Division of the Institute of Navigation 2011, ION GNSS 2011 | Year: 2011

In this work, we use Ionospheric Data Assimilation Four-Dimensional (IDA4D) and Estimating Model Parameters from Ionospheric Reverse Engineering (EMPIRE) to resolve the storm-enhanced Density's (SED) three-dimensional time-varying electron density gradients at high resolution and estimate the three-dimensional plasma velocity. These estimated electron densities and plasma velocities are used to construct the ionospheric error that would be observed from a hypothetical test Global Navigation Satellite System (GNSS) user and reference station pair. The differential errors are compared to spatial ionospheric error gradients and electron content speeds that were estimated from observational data in the literature for the Local Area Augmentation System (LAAS) ionospheric model. The IDA4D density model of an extreme ionospheric storm shows slant total electron content (TEC) delays of up to 30 m at L1 frequency, comparable to the delays of ground stations observing at that time. The differential delays and the TEC slopes implied by the density model are 100 mm/km. This slope is the same order of magnitude as those that populate the LAAS ionospheric threat model, but are less than the largest observed, which are around 400 mm/km. The horizontal drifts predicted are northwestward within the SED plume and its boundary, turning northeastward at high latitudes. The vertical drifts are estimated to be high speed upward around 1820 UT, and lower in magnitude afterward above a particular ground station in the region of interest. Source


Datta-Barua S.,San Jose State University | Bust G.S.,ASTRA LLC | Crowley G.,ASTRA LLC
23rd International Technical Meeting of the Satellite Division of the Institute of Navigation 2010, ION GNSS 2010 | Year: 2010

Using GNSS data effectively in the estimation of ionospheric drivers requires consideration and rigorous treatment of the error propagation of the measurements through the model. In this paper, we develop an error propagation model for one such novel algorithm, known as Estimating Model Parameters from Ionospheric Reverse Engineering (EMPIRE). EMPIRE deduces drivers of ionospheric dynamics from time-evolving 3-dimensional images of ionospheric density. In the algorithm, the ion continuity equation is discretized in space-time and formulated as a linear system. The system is solved to estimate the drivers that produced an increase or decrease in the time variation of the density over a region. The EMPIRE error analysis begins with the error covariances of the input densities, which in this case are provided by Ionospheric Data Assimilation 4-Dimensional (IDA4D). We apply error covariances from the IDA4D imaging method and propagate them through the spatial and temporal grid interpolation, as well as the linear system inversion, for covariance estimates on the parameters. For the quantities of interest - drifts parallel to the magnetic field, and perpendicular to the field in the meridional and zonal directions - the errors are at minimum 50-100 m/s, which is, for the high-speed storm-time flows, 30% of the stormtime maximum drifts. The chi-squared statistic is also computed and compared to its probability density function to determine goodness-of-fit. There are three distinct periods identifiable in the stormtime data analyzed. Prior to the storm, the chi-squared value is too low, indicating that the input covariances are too conservative. During the initial period of the storm, the chi-squared value indicates that the measurements are reasonably consistent with the model. After the initial couple of hours of the storm, the chi-squared value indicates that the measurements are no longer consistent with the model, so that the fits and estimates are unreliable. Source


Heelis R.A.,University of Texas at Dallas | Crowley G.,ASTRA LLC | Rodrigues F.,ASTRA LLC | Reynolds A.,ASTRA LLC | And 3 more authors.
Journal of Geophysical Research: Space Physics | Year: 2012

The evolution of the pre-reversal enhancement in the vertical ion drift in the equatorial F region is described via an examination of the current systems determined from a coupled ionosphere thermosphere model. We find that the pre-reversal enhancement is produced by a reversal in the F region zonal wind that results in an additional upward current where the E region Pedersen conductivity is declining across the dusk sector. The continuity of the total current is maintained through an enhancement in the eastward zonal current and an associated upward drift of the ions. © 2012. American Geophysical Union. All Rights Reserved. Source

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