VLA Low-band Ionosphere and Transient Experiment (VLITE) Ionosphere

Ionospheric Analysis

The ionosphere analysis portion of VLITE is dedicated to the study of fine-scale (~1-10 km) ionosphere dynamics and the relationship to larger structures (hundreds of km). The VLA low-band systems have virtually unmatched sensitivity to fluctuations in the ionosphere total electron content (TEC), the integrated density of free electrons along a line of sight. When observing a bright cosmic source, these systems can be used to characterize TEC fluctuations more than two orders of magnitude weaker than those detectable with similar GPS-based methods. Such fluctuations are prevalent on smaller scales, making the VLA an excellent instrument for probing fine-scale ionosphere dynamics. Many continuously operating GPS receivers within New Mexico are also being used to simultaneously study larger-scale fluctuations. The (nearly) continuous data stream delivered by VLITE, when combined with this GPS data, constitutes a singular data set for the study of coupling mechanisms among fine-, medium-, and large-scale ionosphere dynamics. In addition, such a continuous flow of data allows for the characterization of the fine-scale ionosphere response to relatively rare space weather, atmospheric, and/or seismic events such as solar flares (Helmboldt et al. 2015), large storms, earthquakes, and explosions (Huang et al. 2019) that would be missed by proposal-based, low-band observing.

The ionosphere pipeline is optimized to sense fluctuations on small temporal (~seconds), spatial (~few km), and amplitude (~10^-3-10^-4 TECU km^-1) scales. Because the δTEC values represent antenna-based effects that dominate on short time scales (~minutes or less), the general approach to signal processing is as follows:

• Extract good visibility phases from the raw data, while flagging obviously aberrant data.
• Unwrap the phase time series and de-trend to remove slowly varying instrumental and/or source contributions.
• Determine and remove contributions from baseline-based errors.
• Use final δTEC time series to compute TEC gradients.

Plasmaspheric Analysis

Between the ionosphere and the solar wind-driven outer magnetosphere is a region of relatively cold, co-rotating plasma known as the plasmasphere. Magnetic field-aligned irregularities with longitudinal scales of tens of km were discovered with the VLA low-band system in the early 1990s ( Jacobson & Erickson 1992). These were first identified as relatively fast moving/oscillating waves directed toward magnetic east until it was realized that the high speed was due to co-rotation at a relatively large distance (thousands of km). Using archival VLA low-band data and VLITE data, a method was recently developed to produce range/time-resolved images of these structures, sometimes called co-rotating plasmaspheric irregularities (CPIs; Helmboldt et al. 2020). This method spectrally decomposes the time series of TEC gradients measured using the methods described above into several frequency bands. A phase velocity is then estimated for each of the higher-frequency oscillation bands most likely to be CPIs. Because their motions are dominated by co-rotation, the magnitude of this velocity gives the distance to the disturbance. A check is made to confirm that the velocity direction is consistent with what is expected for a CPI. Following this, the data from all bands are recombined to form a range/time image (see an example below). A separate pipeline runs daily to identify any observations of bright calibrators longer than five minutes in duration and performs this imaging analysis on those data.

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