When the Jicamarca Radio Observatory made its first observations of Earth’s ionosphere in the early 1960s, it was one of the most impressive facilities in the nascent field of space physics. Its massive square array of dipole antennas was laid out in the Peruvian desert east of Lima, nearly 300 meters on each side. The enormous radar facility was designed to probe the ionosphere directly above Earth’s equator; electrons in the ionosphere scatter the radar beams, but a faint return signal gives an indication of their density.
In its initial observing runs, scientists included measurements at very high altitudes, an exercise meant to map out the space surrounding Earth. They also pushed the facility to its limits, requiring powerful radar pulses from all four of its transmitters and many people to operate them. Soon, however, high-altitude operations were canceled; the last runs occurred in 1965.
As the facility began to focus on more popular areas of research, the unpublished high-altitude records languished. Many were lost. The details of the observations and analysis—such as which filtering methods, if any, were used—faded away, limiting the surviving data’s usefulness. Eventually, the capability to reproduce them was lost, as transmitters fell offline and Jicamarca focused on targets closer to the ground.
Today, more than 50 years later, interest in high-altitude observations is on the rise, this time driven by the desire to understand how plasma behaves during geomagnetic storms. Jicamarca remains one of the most important space physics radar facilities, and fortunately, recent upgrades have restored the facility’s ability to carry out high-altitude observations. On 31 May 2016, Jicamarca fired up its transmitters and focused its antennas on high-altitude incoherent scatter in a study conducted by Hysell et al.
In a 24-hour period of observations, the team found that Jicamarca could profile the electron density up to an altitude of roughly 6300 kilometers. That’s high enough to usefully overlap with data from ground-based magnetometers, which can cover a range from roughly 3000 kilometers to 16,000 kilometers. The data analysis also revealed that different filtering methods did not change the results much, which makes it easier to interpret historical data.
The authors used just two of Jicamarca’s four transmitters, all of which have been restored to operational status. Even with only two transmitters, the data quality was similar to the 1965 data, with a slightly better dynamic range. The team notes that future observations using all four transmitters will be more sensitive and should push the observatory’s range occasionally up to 10,000 kilometers. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1002/2016JA023569, 2017)
—Mark Zastrow, Freelance Writer