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VLA Low-band Ionosphere and Transient Experiment (VLITE)


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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.

Example of VLITE ionospheric data
Example of antenna-based TEC gradients from November 11, 2014 observation of the Galaxy cluster Abell 2052 (A2052). The upper panel shows the δTEC time series for the V1*V4 antenna baseline (black points) with the values from a polynomial fit to all baselines used to determine the TEC gradients (red). The north-south and east-west components of the gradient are shown in the remaining panels for antennas V1 and V4. Reproduced from Helmboldt et al. 2019).

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.
Please see Helmboldt et al. 2019 for more details on VLITE and its ionospheric pipeline.

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.

Example of VLITE plasmaspheric data
Derived from VLITE observations of a bright calibration source, range/time-resolved images of CPIs (lower panel). The upper panel shows the change in electron density within the plasmasphere as a function of range and time due to a series of planewave electric field disturbances patterned after previous observations of such phenomena with coherent backscatter radars. The middle panel shows a simulated retrieval using the CPI imaging methods describe above, which is qualitatively similar to the actual observations. Reproduced from Helmboldt et al. 2020.

Modified on Friday, 02-Jun-2023 11:04:50 MDT