Polar mesospheric summer echoes
Strong radar echoes from 80–90 km in polar summer mesospheres.
Polar mesospheric summer echoes (PMSE) are anomalous radar echoes observed at altitudes between 80 and 90 km during summer months in polar regions. They are notable for their strong signal power enhancements at VHF radar frequencies, sometimes extending above 1 GHz, and for their association with extremely cold temperatures in the summer mesosphere.
- Altitude range
- 80–90 km
- Arctic season
- May through early August
- Antarctic season
- November through February
- Radar frequency range
- 50 MHz to 250 MHz, at times over 1 GHz
- Peak height
- Slightly below the summer mesopause temperature minimum at 88 km
- Typical observing instrument
- VHF Mesosphere-Stratosphere-Troposphere (MST) radar
Lore & Background
PMSE exhibits dramatic variations in height and intensity, as well as large variations in Doppler shift. Rocket and radar measurements indicate that partial reflection from a multitude of ion layers and constructive interference causes at least some of the echoes. The peak PMSE height is slightly below the summer mesopause temperature minimum at 88 km, and above the noctilucent cloud (NLC) or polar mesospheric cloud (PMC) layer at 83–84 km. PMSE occurs in both the Arctic and Antarctic regions and is sometimes accompanied by noctilucent clouds. A related but much less frequent phenomenon, Mesospheric Summer Echoes (MSE), can be observed at middle latitudes, such as along the Baltic coast. Many years of MSE observations using VHF radars in Northern Germany show that MSE occurs less frequently because the formation mechanism requires the transport of very cold polar air by equatorward mesospheric winds.
Reader's Guide
PMSE is significant as a phenomenon that reveals complex interactions between the neutral atmosphere and ionosphere at high latitudes during summer. The strong radar echoes, observed primarily with VHF MST radars, provide a means to study structural irregularities in electron density between 80 and 90 km. Although the exact cause of PMSE is not yet known, proposed explanations include steep electron density gradients, heavy positive ions, dressed aerosols, gravity waves, and turbulence. The echoes are associated with the extremely cold temperatures that occur above continental Antarctica during summer, and their study has been advanced through radar, LIDAR, and sounding rocket measurements. The existence of the rarer MSE at middle latitudes, dependent on transport of cold polar air, further underscores the role of temperature and dynamics in echo formation. PMSE remains an active area of research, with ongoing observations at sites such as EISCAT and Davis in Antarctica contributing to understanding of mesospheric processes.
From Blueprint to First Echo: The Birth of Jicamarca
The Jicamarca Radio Observatory traces its origins to a 1960–61 construction effort undertaken by the Central Radio Propagation Laboratory, a division of the National Bureau of Standards. The project was spearheaded by Dr. Kenneth L. Bowles, a figure so central to the endeavour that he is widely remembered as the "father of JRO." The site sits roughly thirty minutes by car east of Lima, Peru, about ten kilometres off the Central Highway, at an elevation of 520 metres where the magnetic dip angle hovers near one degree. Remarkably, the facility began producing science before it was even finished: the final dipole was not installed until April 27, 1962, yet the very first incoherent scatter measurements were recorded in early August 1961, using only a fraction of the projected antenna area and a transmitter still missing its final amplification stage. The parent laboratory later passed through the Environmental Science Service Administration before settling into the National Oceanic and Atmospheric Administration, carrying the institutional lineage that would shape decades of support for the observatory.
The World's Largest Incoherent Scatter Antenna
At the heart of Jicamarca stands the largest incoherent scatter radar antenna ever constructed. The structure is a cross-polarized square array of 18,432 half-wavelength dipoles spread across roughly 288 metres by 288 metres. The array is divided into quarters, each quarter further broken into 4×4 modules, giving engineers the flexibility to reconfigure both transmission and reception on the fly. This modularity enables simultaneous multi-beam observations, multi-baseline interferometry, and even radar imaging. The main beam can be nudged up to three degrees off-axis by swapping cables at the module level. On the transmitter side, three units each deliver 1.5 megawatts of peak power, with a fourth under construction to restore the original six-megawatt capability. Because any transmitter can feed any quarter of the array, operators can launch linear, circular, or elliptical polarisation at will. The radar operates at 49.9 MHz and runs in two principal modes: incoherent scatter for bulk plasma parameters and coherent scatter, which captures echoes more than 30 dB stronger from irregularities in the troposphere, stratosphere, mesosphere, equatorial electrojet, and the E and F regions.
A Peruvian Institution: Ownership, Staffing, and Home-Grown Innovation
When the Environmental Science Service Administration handed the observatory to Peru's Instituto Geofísico del Perú in 1969, the transfer was hardly a formality. The IGP had been a close partner during the 1957–58 International Geophysical Year and had been deeply involved in every phase of Jicamarca's construction and early operation. In the years that followed, an informal coalition known as the "Jicamarca Amigos," guided by Prof. William E. Gordon—who had devised the incoherent scatter technique itself in 1958—helped sustain operations. The National Science Fund then stepped in, routing support first through NOAA and, from 1979 onward, through Cornell University via Cooperative Agreements. In 1991 a Peruvian nonprofit called Ciencia Internacional was established to employ most of the observatory's staff and supply goods and services to the IGP. Perhaps most striking is that since 1969, most of the original radar hardware has been swapped out for new systems conceived, designed, and fabricated by Peruvian engineers and technicians, turning Jicamarca into a showcase of local scientific craftsmanship.
Shaping a Field: Scientific Reach and Academic Legacy
Jicamarca sits at the equatorial end of a Western Hemisphere network of incoherent scatter radars that runs from Lima all the way to Søndre Strømfjord in Greenland, and it is widely regarded as the premier global facility for probing the equatorial ionosphere. Its near-zero magnetic dip angle lets the beam be pointed precisely perpendicular to the Earth's magnetic field, yielding the most accurate ionospheric electric-field measurements available. The radar can also determine absolute electron density through Faraday rotation, a capability unique to its geographic location and operating frequency. In incoherent scatter mode it measures electron density, electron and ion temperatures, ion composition, and both vertical and zonal electric fields. The broader research portfolio spans the stable equatorial ionosphere, field-aligned irregularities, neutral-atmosphere dynamics, and meteor physics. The field of equatorial ionospheric science has matured rapidly, in large part because of the steady data stream Jicamarca has produced since the early 1960s. More than sixty doctoral students—many from American universities and fifteen from Peruvian institutions—have conducted research in association with the observatory, cementing its role as a training ground for an entire generation of upper-atmosphere physicists.
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