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Forecasting ionospheric storms at the magnetic equator

Friday, February 22, 2013

enhanced radar backscatter

The postsunset equatorial ionosphere is prone to plasma convective instability related to Rayleigh-Taylor instability in hydrodynamics. During convective “storms,” large scale (10s of km) electron density depletions or ‘bubbles’ can develop and ascend to high altitudes, 1000 km or more at times. Attendant with these large-scale bubbles is a spectrum of electron density irregularities and turbulence
that can extend to wavelengths as short as ~10 cm. The irregularities serve as a broadband diffraction screen through which transionospheric electromagnetic waves pass, causing severe signal degradation. Understanding and modeling these events is important because of their space weather impacts. These include the disruption of satellite-based communication and navigation systems, interference with terrestrial HF communication and over-the-horizon (OTH) radar systems, and the production
of artifacts in synthetic aperture (SAR) and other types of radar imagery. For this reason, convective ionospheric storms are the focus of a number of mitigation efforts at the national level.

A team from EAS including David Hysell and Ph.D. student Ramin Jafari have been investigating convective ionospheric storms using the Jicamarca Radio Observatory near Lima, Peru. Jicamarca is a large radar telescope capable of monitoring state parameters in the ionosphere as well as the onset and evolution of ionospheric turbulence and storms. Figure 1 is an example of enhanced radar backscatter during one such storm, measured at Jicamarca and plotted as a function of altitude and local time. Strong echoes such as these only exist during disturbed periods and are telltale of regions of turbulent,
depleted ionospheric plasma.

While the climatology of convective ionospheric storms is well known, considerable dayto-day variability exists that is analogous to terrestrial weather. Like terrestrial weather, forecasting equatorial convective storms requires numerical simulation. The group in EAS has constructed a
three-dimensional numerical model of the equatorial
ionosphere. This model is an initial-boundary value simulation of the fluid equations for the ionospheric species (electrons plus multiple ions) which are coupled by Maxwell’s equations. Initial conditions for the model along with key ionospheric drivers (the background electric field, neutral winds, etc.) can be measured by Jicamarca and used to drive the model code. The output of the model can then be compared to observations from Jicamarca and elsewhere. More detailed physical processes can then be incorporated in the model until satisfactory model-data agreement is reached.

Throughout a number of experimental campaigns planned for 2013, the EAS group will acquire observations of the convective storms and test the ability of their simulation to recover the most salient features observed. Refinements to the model and the observing procedure should ultimately lead to a forecast capability.

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