Joseph Helmboldt
Remote Sensing Division, Naval Research Laboratory
Mapping Disturbances within the Ionosphere and Plasmasphere with Low-frequency Radio Interferometry
Earth's ionosphere is a dynamic plasma, which is frequently perturbed by disturbances originating both from below (e.g., atmospheric gravity waves) and above (e.g., geomagnetic substorms). This can be problematic for any radio-frequency (RF) system that involves propagation through the ionosphere such as satellite-based navigation and communication and high-frequency (HF) radar/communication systems. There is perhaps no field that is more profoundly impacted by this reality than low frequency (<500 MHz) radio astronomy. Imaging of cosmic sources at these frequencies requires relatively large (several to hundreds of km) interferometric telescopes. Gradients in ion/electron density within the ionosphere on scales comparable to the interferometer can have disastrous effects on image quality and fidelity. Thus, considerable effort has been dedicated to developing methods to counteract these effects, making high angular resolution imaging possible.
A byproduct of the ionospheric calibration techniques developed for low-frequency interferometers is a new method for high-precision remote sensing of ionospheric disturbances. Due to the extremely stable timing required for interferometry and the larger impact of ionospheric fluctuations at lower frequencies, ionospheric phase fluctuations among the array elements can be measured very precisely. When observing a relatively bright cosmic source, the horizontal gradient in the total electron content (TEC) can be specified to a precision as good as 0.00005 TECU/km (1 TEC = 1e16 e/m^2), or the equivalent of ~0.2 ppm. This precision is even good enough to detect and characterize density fluctuations above the ionosphere in the plasmasphere.
Within this seminar, I will discuss efforts to develop new methods to exploit the unique ionospheric remote sensing capabilities of low-frequency radio interferometers. This work relies heavily on data obtained with the low-band system of the Very Large Array (VLA) telescope in New Mexico, especially via the Naval Research Laboratory's VLA Low-band Ionosphere and Transient Experiment (VLITE), which runs continuously on 16 of the 27 VLA antennas. I will describe procedures for extracting the TEC gradient from VLA/VLITE data and methods for analyzing these data to characterize wavelike disturbances that pass through the VLA lines of sight. I will also discuss new techniques for producing tomographic reconstructions of plasmaspheric disturbances from routine VLA/VLITE observations. I will also detail similar tomographic methods for imaging km-scale ionospheric disturbances when the telescopes slew much faster than the sidereal rate in the newly implemented "on-the-fly" mode. Finally, I will discuss the types of phenomena observed and characterized with these methods and what they tell us about coupling between the lower atmosphere and ionosphere, the ionosphere and plasmasphere, and the Earth and its space environment.
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