Modeling the low-latitude ionospheric electron density and plasma turbulence in the November 2004 storm period

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Abstract

The storm period of 8–12 November 2004 offers an opportunity for insight into the phenomena of low-latitude ionospheric structure during geomagnetically disturbed times because of the strength of the disturbances, the timing of the storms, and the instrumentation that was operating during the interval. We will take advantage of these factors to model the ambient ionosphere and the plasma turbulence responsible for radio scintillation within it, using the AFRL low-latitude ambient/turbulent ionospheric model and the storm-time model features described in the companion paper [Retterer, J.M., Kelley, M.C., 2009. Solar-wind drivers for low-latitude ionospheric models during geomagnetic storms. J. Atmos. Solar-Terr. Phys., this issue]. The model plasma densities show very good agreement with the densities measured by the Jicamarca ISR as well as with the total electron content (TEC) measured by the Boston College South American chain of GPS receivers. The detection by the radar of coherent returns from plasma turbulence match well the times of predicted ionospheric instability. The predicted geographic extent of the occurrence of equatorial plasma bubbles was matched by DMSP satellite observations and our forecasts of scintillation strength were validated with measurements of S4 at Ancon and Antofagasta by stations of the AFRL SCINDA network.

Introduction

Although even geomagnetically quiet times still pose challenges in modeling low-latitude ionospheric structure and its susceptibility to plasma instabilities, which cause ESF radio-scintillation effects, calculating these properties is even more difficult during disturbed times. The storm period of 8–12 November 2004 offers an opportunity for insight into the phenomena of low-latitude ionospheric structure during geomagnetically disturbed times because of the strength of the disturbances, the timing of the storms, and the instrumentation that was operating during the interval. We will take advantage of these factors when attempting to model the ambient ionosphere and the plasma turbulence responsible for radio scintillation within it, using models (Retterer, 2005) developed at AFRL in preparation for the C/NOFS mission (de La Beaujardiere et al., 2004).

A key parameter for the specification of both the ionospheric structure and its stability is the eastward electric field or vertical ion drift velocity. The Jicamarca ISR radar was operating during a portion of this storm interval and provided us with measurements of this velocity. Interpretation of these data, taken during times of penetration fields as a function of the strength and direction of the interplanetary electric field (IEF) and as measured by the ACE satellite, led to a scaling function (the Kelley model) to provide a low-latitude, eastward penetration electric field in terms of the IEF that can be applied when observations are not available. Another key variable is the enhancement in neutral densities as a result of the energy input into the thermosphere; the CHAMP and GRACE satellites provided neutral densities during this storm interval. These storm-specific model drivers are described in a companion paper (Retterer and Kelley, 2009). This paper presents the results of ionospheric modeling using these storm-time drivers.

Section snippets

Model

The suite of ionospheric models to be used in studying both the ambient ionospheric plasma density and its Rayleigh–Taylor stability and resulting plasma plumes and scintillation was presented by Retterer (2005). The global ambient ionospheric model (Retterer et al., 2005), which will be used to calculate ionospheric structure, is based on the low-latitude ionospheric model of Anderson (1973). It solves the continuity equation for plasma density in terms of the processes of production, loss,

Ionospheric plasma density and total electron content

The single most important factor in the day-to-day variability of the Rayleigh–Taylor instability of the low-latitude ionosphere is the altitude of the post-sunset equatorial ionosphere where this instability originates (Fejer et al., 1999). In turn, this parameter is determined by the time history of the zonal electric field (E), which creates a vertical velocity of the plasma with a magnitude and direction given by E×B/B2 where B is the magnetic field. Fig. 1 shows the history of the vertical

Ionospheric plasma instability, bubbles, and scintillation

The next step for the model is to examine the ionosphere for regions of Rayleigh–Taylor (RT) instability (Retterer, 2005) using the formulation of the growth rate given by Haerendel (1973), and elaborated upon by Sultan (1996). Fig. 8 gives the strength of the Rayleigh–Taylor growth rate for day 314 (9 November), color-coded in the upper panel superimposed on a contour plot of electron density; the lower panel gives the strong (coherent) echoes received by the Jicamarca ISR. Because these

Conclusions

The AFRL low-latitude ionospheric plasma model was run using Jicamarca measurements to specify the plasma drifts. The model plasma densities show good agreement with the densities measured by the Jicamarca ISR as well as with the total electron content (TEC) measured by the BC South American chain of GPS receivers. The times of radar detection of coherent returns from plasma turbulence match well the times of predicted ionospheric instability. Our forecasts of scintillation strength were

Acknowledgments

We thank O. de La Beaujardiere and W. Burke for interesting discussions. This research was partially supported by the Air Force Office of Scientific Research at AFRL and Cornell University (FA955-05-01600), and by AFRL Contracts F19628-04-C-0055 and F19628-02-C0087, AFOSR task 2311AS with Boston College. The Jicamarca Radio Observatory is a facility of the Instituto Geofisico del Peru operated with support from NSF Cooperative Agreement ATM-0432565 through Cornell University. The operation of

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