NARL MST Radar Observations Help Resolve Ionospheric Echoing Riddle

The high power, large aperture Mesosphere-Stratosphere-Troposphere (MST) Radar established at NARL, Gadanki nearly two and half decades ago, was designed to study the middle and upper atmospheric dynamics. This radar has made a major contribution in resolving a 50-year old ionospheric echoing riddle. This riddle is linked with the radar echoes during daytime in the height region of 140-170 km in the earth’s ionosphere where there is no known source of strong current/electric field and density gradient engendering the growth of plasma instability. In a significant way, the NARL MST radar observation has provided the first experimental evidence on the occurrence of the 150-km echoes outside the magnetic equator. These echoes, however, have not been observed at mid latitudes and hence their confinement to low magnetic latitudes indicates the role of near horizontal earth’s magnetic field on the phenomenon. This echoing phenomenon has not only surprised the ionospheric scientists all over the world but remains as the most puzzling and challenging field in the ionospheric plasma physics today. These echoes, however, are used as a reliable means of estimating daytime ionospheric zonal electric field that drives the equatorial plasma fountain and a number of ionospheric phenomena, which are detrimental for satellite based navigation/communication systems.

An example of these echoes observed by the NARL MST radar, presenting the height-time distribution of signal-to-noise ratio of the echoes, shows a spectacular forenoon descent and afternoon ascent of the echoing regions, indicating the direct solar zenith angle control on the echoing process, unlike any other equatorial ionospheric echoing phenomenon.

Height-time Variation of signal-to-noise ratio of the daytime 150-km Echoes

Height-time Variation of signal-to-noise ratio of the daytime 150-km Echoes

 
Surprisingly, the occurrence frequency of these echoes, however, is the lowest during the equinoxes, when the Sun is over the equator, moderate in the winter, and highest in summer, which clearly presents the complex role of the solar radiation and other dynamics on the echoing phenomenon.
 
Seasonal Variations of Daytime 150-Km Echo Occurrence

Seasonal Variations of Daytime 150-Km Echo Occurrence

 

NARL observations have further revealed that these echoes come in two distinct types: in one type (Type-A) echoes come with low Signal-to-Noise-Ratio (SNR) (<5 dB) and spectral width is dependent on SNR, and in the other type (Type-B), echoes come with high SNR (as high as 25 dB) and spectral width is nearly independent of SNR. This finding has later been confirmed by the radar observations from Jicamarca, the most powerful equatorial radar in the world.

The latest large-scale kinetic simulation of photoelectron induced plasma waves, published in Geophysics Research Letters (2016), suggested that the puzzling echoes are possibly linked with energetic photo-electrons, which can drive Langmuir, lower and upper hybrid, and electron Bernstein waves. While this seminal work has begun a new approach to address the five decades long 150-km echoing riddle, two recent new findings made from NARL has raised shortcomings of this theoretical premise. Two important findings from NARL that enforced to modify the existing thinking are the clear connection of the type-B echoes to the unusually deep solar minimum of 2008-2009 and the inverse relationship of the puzzling echoes with the solar EUV radiation, suggesting a clear solar activity dependence of the phenomenon.

Inverse Relationship between the Intensity of the Echoes and Solar EUV Radiation
Inverse Relationship between the Intensity of the Echoes and Solar EUV Radiation
 
These new observational findings, illustrating the complexity and richness of physics in the peak production region of the ionosphere, have been published in the American Geophysical Union journal, Geophysical Research Letters, vol. 43, pp. 11,129-11,136 (2016). The paper has raised important questions, namely, (1) what causes the seasonal, day-to-day and the finer details of the local time variations of the 150 km echoes? and (2) what causes the 150 km echoes including the type-B echoes to occur more during deep solar minimum of 2008-2009 than during relatively high solar condition?, and opened up the challenging unresolved science problems to the diverse scientific community to understand the mystery of nature. As on today, these outstanding questions remain mostly unanswered and call for extensive experimental and theoretical works to understand the mystery of the equatorial ionosphere, especially the solar-terrestrial energetic, and the Sun-Earth linkage in general.

Now the NARL scientists, in coordination with scientists from other ISRO centers, are planning to carry out rocket-borne measurements concurrently with the radar experiments using the newly established active array MST radar and the Gadanki Ionospheric Radar Interferometer (GIRI) and theoretical simulation to resolve the outstanding science questions.