Jicamarca mesospheric observations

In explaining the scattering of VHF radar signals from the mesosphere there are two observational facts that must be accounted for. These are; (1) the aspect sensitivity of the scattered signal and that this aspect sensitivity is largest in the lower part of the mesosphere, and (2) the correlating between the scattered power and the signal correlation time. This behavior is similar to that of the scattering from the troposphere/stratosphere region, and it is suggested that the scattering mechanisms are similar in these three regions. Several different experiments are performed. They all show strong indications of aspect sensitivity and changing correlation between scattered power and correlation time. There is no indication of stratified reflecting layers unless these layers are modulated in space and time to a degree that they cannot be distinguished from turbulence in any other way than that they cause somewhat aspect sensitive scattering

Morphology of the scattering targets: Fresnel and turbulent mechanisms, part 2.1A

Refractive index fluctuations cause coherent scattering and reflection of VHF radio waves from the clear air in the altitude region between 0 and approximately 90 km. Similar echoes from the stratosphere/troposphere and the mesosphere are observed at UHF and MF/HF frequencies, respectively. The nature of the refractive index fluctuations has been studied for many years without producing a clear consensus on what mechanism causes them. It is believed that the irregularities can originate from two different mechanisms: turbulent mixing of the gradient of refractive index, and stable horizontally stratified laminae of sharp gradients in the refractive index. In order to explain observations of volume dependence and aspect sensitivity of the echo power in the MST region, a diversity of submechanisms has been proposed. They include isotropic and anisotropic turbulent scattering, Fresnel scattering and reflection, and diffuse reflection. Isotropic turbulent scattering is believed to cause a majority of the clear air echoes observed by MST radars. The mechanism requires active turbulence mixing of a preexisting gradient in the refractive index profile

Spaced antenna drift

It has been suggested that the spaced antenna drift (SAD) technique could be successfully used by VHF radars and that it would be superior to a Doppler-beam-swinging (DBS) technique because it would take advantage of the aspect sensitivity of the scattered signal, and might also benefit from returns from single meteors. It appears, however, that the technique suffers from several limitations. On the basis of one SAD experiment performed at the very large Jicamarca radar, it is concluded that the SAD technique can be compared in accuracy to the DBS technique only if small antenna dimensions are used

Urbana radar systems: Possibilities and limitations

The Aeronomy Laboratory Field Station of the University of Illinois at Urbana contains three different radar systems capable of probing various regions of the atmosphere below about 100 km. These are an mesosphere-stratosphere-troposphere (MST) radar, a VHF meteor radar and an MF partial-reflection radar. All three radars can measure winds and waves in the ionospheric D region. The MST radar is, in addition, capable of probing the lower stratosphere and upper troposphere. A sodium (Na) LIDAR is also located at the Field Station and provides an additional way of studing winds and waves in the mesosphere by observing temporal variations in the sodium density profile

Effects of line-of-sight velocity on spaced-antenna measurements, part 3.5A

Horizontal wind velocities in the upper atmosphere, particularly the mesosphere, have been measured using a multitude of different techniques. Most techniques are based on stated or unstated assumptions about the wind field that may or may not be true. Some problems with the spaced antenna drifts (SAD) technique that usually appear to be overlooked are investigated. These problems are not unique to the SAD technique; very similar considerations apply to measurement of horizontal wind using multiple-beam Doppler radars as well. Simply stated, the SAD technique relies on scattering from multiple scatterers within an antenna beam of fairly large beam width. The combination of signals with random phase gives rise to an interference pattern on the ground. This pattern will drift across the ground with a velocity twice that of the ionospheric irregularities from which the radar signals are scattered. By using spaced receivers and measuring time delays of the signal fading in different antennas, it is possible to estimate the horizontal drift velocities

Radar echoes at 2.66 and 40.92 MHz from the mesosphere, part 2.6A

During recent decades, the ionospheric D region has been scanned extensively by radar in the frequency range from 1 to 60 MHz. Progress has been made in understanding the reflection/scattering of radio waves in that area. Rocket measurement of ion density irregularities were compared to radar echo observations at 2.75 MHz with the conclusion that the received radar signal was due to scattering from isotropic and homogeneous turbulence in the altitude region between 70 and 95 km. However, scattering cross sections at 2 and 6 mHz suggest that the radar echo from the region below 80 km is in part due to partial reflection from stratified layers. The VHF scattering cross section is aspect sensitive in the D region below about 75 km and tends to be isotropic at higher altitudes. Positive correlations between scattered signal power and signal correlation time (P/C) have been observed by VHF radars in the lower mesosphere, with the conclusion that it might be an additional indication of partial reflection from stratified layers. In the upper mesosphere where the P/C correlation is negative, it is generally believed that the scattering is caused by isotropic turbulence. Radar echoes at the 2.16 and 40.92 MHz ranges are compared, assuming that both result from turbulent scatter. Adjusting the radar Bragg wavelength, it was found that both sets are due to scattering from the same layer of turbulence-generated irregularities

Interpretation of radar returns from the mesosphere, part 2.3A

The study of VHF radar signals from the mesosphere has shown that neutral atmosphere turbulence plays a central role in generating the refractive index irregularities that backscatter the radio waves. It follows that an increase in the turbulent energy dissipation rate will result in a decrease in signal correlation time and an increase in scattered signal power. Thus, in turbulence-generated radar echoes a negative correlation between echo power and signal correlation time (P/C) is expected. P/C also changes as a function of altitude, i.e., it is negative in the upper mesosphere but largely positive in the lower, with the latter thought to be a manifestation of partial reflection from stratified layers of refractive index gradient. Partial reflection would also explain the vertical aspect sensitivity of the scattered signal in the lower mesosphere

The Urbana MST radar, capabilities and limitations

The 41-MHz coherent-scatter radar located northeast of the University of Illinois at Urbana is being used for studies of the troposphere, stratosphere and mesosphere regions. The antenna consists of 1008 halfwave dipoles with a physical aperture of 11000 sq m. Transmitted peak power is about 750 kW. Clear-air returns may be received from 6 km to 90 km altitude. Autocorrelation functions of the scattered signal are calculated on-line. From the autocorrelation functions the scattered power, line-of-sight velocity and signal correlation time are calculated. Some aspects of the troposphere/stratosphere and the mesosphere observations are discussed. Capabilities and limitations of the Urbana MST radar are pointed out, and recent and planned improvements to the radar are described

Microsatellite instability has a positive prognostic impact on stage II colorectal cancer after complete resection : results from a large, consecutive Norwegian series

BACKGROUND: Microsatellite instability (MSI) was suggested as a marker for good prognosis in colorectal cancer in 1993 and a systematic review from 2005 and a meta-analysis from 2010 support the initial observation. We here assess the prognostic impact and prevalence of MSI in different stages in a consecutive, population-based series from a single hospital in Oslo, Norway. PATIENTS AND METHODS: Of 1274 patients, 952 underwent major resection of which 805 were included in analyses of MSI prevalence and 613 with complete resection in analyses of outcome. Formalin-fixed tumor tissue was used for PCR-based MSI analyses. RESULTS: The overall prevalence of MSI was 14%, highest in females (19%) and in proximal colon cancer (29%). Five-year relapse-free survival (5-year RFS) was 67% and 55% (P = 0.030) in patients with MSI and MSS tumors, respectively, with the hazard ratio (HR) equal to 1.60 (P = 0.045) in multivariate analysis. The improved outcome was confined to stage II patients who had 5-year RFS of 74% and 56% respectively (P = 0.010), HR = 2.02 (P = 0.040). Examination of 12 or more lymph nodes was significantly associated with proximal tumor location (P < 0.001). CONCLUSIONS: MSI has an independent positive prognostic impact on stage II colorectal cancer patients after complete resection