Radio astronomers are searching the cosmos for new scientific discoveries at increasingly lower radio frequencies and with larger antenna arrays, but their observations of the sky are blurred by the dynamic ionosphere. At the same time, ionospheric scientists are seeking to understand, at increasingly higher spatial and temporal resolutions, the dynamics that drive the ionosphere and its effects on technological systems. Advancements in radio astronomy at the Very Large Array (VLA) are leading to advancements in ionospheric physics and vice versa. We review some of the ionospheric observations made by the VLA at low frequency. Results from a 2003 summer campaign at the VLA are discussed, during which an all‐sky optical camera was used to monitor ionospheric structure during VLA 74‐MHz operations. The camera and additional off‐site sensors, including ionosondes and incoherent scatter radar, were used to identify the dominant, summer nighttime ionospheric phenomena contributing to VLA signal distortion. Knowledge of the specific phenomena, including their spatial and temporal characteristics, can be used to improve low‐frequency, astronomical imaging. Similarly, the VLA observations can be used to investigate ionospheric phenomena in great detail, leading to an improved understanding of ionospheric physics. Key to these findings is the identification of specific ionospheric phenomena using support sensors. Implications for the development of the Long Wavelength Array are discussed.

The SSULI (Special Sensor Ultraviolet Limb Imager) is a low-resolution hyperspectral far and extreme ultraviolet limb-scanning imager designed to monitor ionospheric and thermospheric airglow. SSULI has a spectral range from 80 to 170 nm, and a nominal resolution of 2.1 nm (at 147 nm). The instrument is scheduled to fly aboard all Defense Meteorological Satellite Program (DMSP) Block 5D3 weather satellites. The first SSULI instrument was launched in fall 2003, aboard the DMSP F16 flight, and has been collecting data since December 2003. The second SSULI flight aboard DMSP F17 began in fall 2006. Early in the missions, both instruments began to observe intermittent but significant periods of noise across the entire instrument passband, beyond the expected ion noise associated with sub-auroral latitudes and the South Atlantic Anomaly. The morphology and intensity of the noise correlates strongly with environmental conditions such as spacecraft potential. In order for the ground processing software to extract individual emission features from the measured spectra, the data must be filtered for quality and the noise must be characterized on short time scales and introduced as additional basis functions for use with the Multiple Linear Regression (MLR) feature extraction algorithm. New algorithms, in the form of an Ion Noise Filter, have been developed for use with the MLR. The techniques used in the Ion Noise Filter are discussed and examples of the successful extraction of spectra are demonstrated.

The Atmospheric Neutral Density Experiment (ANDE) is a series of four microsatellites that will study the atmosphere of the Earth from low earth orbit. Each microsatellite is based on a common design; however, each differs in the instrument payloads and the associated science and mission requirements. The primary mission objective is to provide total neutral density along the orbit for improved orbit determination of resident space objects. Each ANDE microsatellite has several secondary goals. It is the unique design of the microsatellites that allows this task to be accomplished.Each microsatellite is a compact, near perfect sphere; this reduces shape and drag errors so that the local density of the atmosphere can be determined by instantaneous tracking variations detected by very high accuracy laser and radar ranging whereby the spacecrafts themselves are the primary sensing instrument. The accuracy of the atmospheric density measurements inferred from the orbital tracking of ANDE rriicrosatellites will be much greater than that achieved by similar experiments in the past or from any currently proposed.Many unique design challenges had to be overcome to achieve the necessary science, mission, and operational requirements as well as severe cost constraints. New methods for parts and assembly fabrication were sought out and implemented. These new methods allowed similar parts to function in each of the microsatellites despite the differences between them. In addition, the command and telemetry links used inexpensive COTS Ham radio transceivers while meeting all the International requirements for operations in the Amateur Satellite Service.

The Ionospheric Mapping and Geocoronal Experiment (IMAGER) is a space-based, multispectral, imaging payload, designed at the U.S. Naval Research Laboratory. The IMAGER's primary science mission is to find, track, and measure ionospheric irregularities as they move across the surface of the Earth and vary with time. IMAGER will observe the ionosphere of the Earth in narrow extreme- and far-ultraviolet passbands centered at 83.4, 130.4, 135.6, and 143.0 nm. These emissions are produced by naturally occurring airglow emission from the nighttime and daytime ionosphere and thermosphere. The IMAGER consists of an imaging telescope with a filter wheel assembly and a pair of microchannel plate-based imaging detectors with cross delay line readouts. The telescope of the instrument consists of a 160 mm diameter, F/4.0 off-axis very fast aplanatic Gregorian telescope. The focal length is 640 mm and the field of view is 1.6° × 1.6° which will cover approximately 1000 × 1000 km2 on the Earth's surface. The modulation transfer function is above 0.90 at 2.8 line pairs-millimeter-1 over the field, which corresponds to a line pair separated by 20 km on the Earth. The spatial resolution is approximately 10 × 10 km2 and is oversampled by a factor of 9 (3 × 3 pixels per resolution element). A system of reflective filters is used to select different wavelengths of interest. The telescope will be gimbaled to provide a field-of-regard encompassing the entire disk and limb of the Earth. The gimbal will also allow the telescope to track the ionospheric irregularities as they move. This paper describes the design of the optical and mechanical systems and their intended performance and includes an overview of the mission and science requirements that defined the aforementioned systems.

The LORAAS instrument aboard the ARGOS satellite observes line‐of‐sight ultraviolet limb intensities from ionosphere and thermosphere airglow. This study uses tomographically reconstructed electron density profiles (EDPs) from the nightside emissions. The ionospheric reconstruction is performed using a two‐dimensional O + 1356Å radiative recombination forward model and discrete inverse theory. The forward model assumes a Chapman layer for the vertical electron density distribution from which h m F 2 , N m F 2 , and topside scale height are derived for every 90 s limb scan, which is equivalent to 5° resolution in latitude. Since ARGOS is in a near Sun‐synchronous orbit, these EDPs form a latitude slice through the equatorial anomaly structures at approximately 0230 LT. These data reflect ongoing ionospheric processes, and it is necessary to assimilate or compare with a model that contains appropriate ionospheric evolution such as the ionospheric forecast model (IFM). This study addresses the reasonableness of both the reconstructed EDPs and the IFM in describing the equatorial anomalies' diurnal and weather variability. The comparison of the LORASS EDPs with those of IFM for October 2000 show that the EDP reconstruction results compare favorably to the IFM EDPs in peak height and topside scale height. Additionally, the sector‐to‐sector climatology of the observed and modeled equatorial anomalies is similar to within the resolution of the instrument and model. The variability observed in each pass of the satellite is much larger than the IFM variability. The LORASS observation variability indicates that careful assessment of the representation error of the observations should be addressed through supplemental observations.