Browsing by Author "Valdez, A."
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Item Restricted Ionospheric Specification and Space Weather Forecasting With an HF Beacon Network in the Peruvian Sector(American Geophysical Union, 2018-08-14) Hysell, D. L.; Baumgarten, Y.; Milla, Marco; Valdez, A.; Kuyeng, K.A network of high‐frequency (HF) transmitters and receivers used for ionospheric specification is being installed in Peru. The HF transmitters employ multiple frequencies and binary phase coding with pseudorandom noise, and the observables provided by the receivers include group delay, Doppler shift, amplitude, bearing (from interferometry), and polarization. Statistical inverse methods are used to estimate F region density in a volume from the data regionally. The method incorporates raytracing based on the principles of Hamiltonian optics in the forward model and involves an ionospheric parametrization in terms of Chapman functions in the vertical and bicubic B‐spline interpolation in the horizontal. Regularization is employed to minimize the global curvature of the reconstructions. HF beacon data for two nights in January 2018 are presented. We use the reconstructions to investigate why plasma irregularities associated with equatorial spread F formed on one occasion and not the other. The data indicate that the background ionospheric flow is not simply frozen in, that is, that longitude and local time variations cannot be equated, even at regional scales. This has ramifications for equatorial spread F forecasting strategies that assume equivalence.Item Restricted Mapping irregularities in the postsunset equatorial ionosphere with an expanded network of HF beacons(American Geophysical Union, 2021-07) Hysell, D. L.; Rojas, E.; Goldberg, H.; Milla, Marco; Kuyeng, K.; Valdez, A.; Morton, Y. T.; Bourne, H.Data from a network of high-frequency (HF) beacons deployed in Peru are used to estimate the regional ionospheric electron density in a volume. Pseudorange, accumulated carrier phase, and signal power measurements for each of the 36 ray paths provided by the network at a 1 min cadence are incorporated in the estimates. Additional data from the Jicamarca incoherent scatter radar, the Jicamarca sounder, and GPS receivers can also be incorporated. The electron density model is estimated as the solution to a global optimization problem that uses ray tracing in the forward model. The electron density is parametrized in terms of B-splines in the horizontal direction and generalized Chapman functions or related functions in the vertical. Variational sensitivity analysis has been added to the method to allow for the utilization of the signal power observable which gives additional information about the morphology of the bottomside F region as well as absorption including absorption in the D and E regions. The goal of the effort is to provide contextual information for improving numerical forecasts of plasma interchange instabilities in the postsunset F region ionosphere associated with equatorial spread F (ESF). Data from two ESF campaigns are presented. In one experiment, the HF data revealed the presence of a large-scale bottomside deformation that seems to have led to instability under otherwise inauspicious conditions. In another experiment, gradual variations in HF signal power were found to be related to the varying shape of the bottomside F layer.