Radar investigations of low‐altitude quasi‐periodic echoes in Chung‐Li

Abstract

[1] Quasi-periodic (QP) radar echoes from the sporadic E-layers are also found at altitudes below 100 km which we call Low-altitude Quasi-Periodic echoes (LQP). We have used the Chung-Li VHF radar to study correct locations of these echoes creating irregularities, which are confined to the height range between 90 km and 100 km, have a thickness of 2 -4 km and are only slowly descending in altitude. They occur with periods of some tens of seconds to maximum three minutes. Since Chung-Li is close to the equatorial region, we can prove that these LQP occur in this temperate region as well. The particular progress reported here is that interferometry allows us to determine the position of the scattering irregularities in three dimensions, which provides new information about the motion of the scatterers. We concur with the assumption that the LQP echoes are created in a strong wind shear below 100 km altitude.

Daytime low‐altitude quasi‐periodic echoes at Gadanki: Understanding of their generation mechanism in the light of their Doppler characteristics

“…Considering the finite beam width and the uncertainty in the time delay measurements, Patra et al [2002] estimated that the height ambiguity of the LQP echoes could be 550 m. In order to determine this more accurately, however, we need to have interferometry measurements, which is not available at this moment. It is worth mentioning that Pan et al [2005] showed the LQP echoes observed from 95–100 km to be localized within ±1° with respect to the magnetic field, indicating their strong aspect sensitive properties. We also rule out the possibility that the low altitude echoes may be due to signal leakage through azimuthal side lobes.…”

“…These echoes are commonly known as low‐altitude quasi‐periodic (LQP) echoes. When plotted in the range/height‐time intensity format, they display slanted striation‐like structures with periods of a few tens of seconds to 4 min [ Urbina et al , 2000; Rao et al , 2000; Patra et al , 2002; Sripathi et al , 2003; Pan et al , 2005; Patra et al , 2006].…”

“…Due to their occurrence in the collision dominated lower‐E region and having periods well below Brunt Vaisala period (∼5 min), some investigators [ Sripathi et al , 2003; Urbina et al , 2004; Pan et al , 2005] invoked the neutral wind shear driven Kelvin‐Helmholtz instability (KHI) as a potential source to account for the observed quasi‐periodicity. Patra et al [2006], however, argued that there is no straightforward way to associate the quasi‐periodic structures with the KHI even if they are correlated.…”

Low‐altitude quasi‐periodic echoes studied using a large database of Gadanki radar observations

“…Notably, Patra et al [2006] showed an LQP event occurring for more than 7 h and the echoes at the end of the run extended to as low as 87 km in altitude. Pan et al [2005], using the Chung‐Li radar (24.9°N, 121.2°E, 13.3°N magnetic latitude) in Taiwan, observed these echoes to occur in the height region of 96–100 km with a period of a few tens of seconds to 3 min.…”

“…Now we know that QP echoes are observed also from altitudes well below 100 km and these echoes have been reported from both middle and low latitudes [ Urbina et al , 2000; Rao et al , 2000; Patra et al , 2002; Pan and Rao , 2002; Sripathi et al , 2003; Pan et al , 2005; Patra et al , 2006]. Because of their occurrence at lower altitudes, they have been stamped as low‐altitude QP (LQP) echoes to distinguish them from the well‐known and much investigated QP echoes occurring above 100 km.…”