Polar mesospheric summer echoes
Encyclopedia
Polar mesospheric summer echoes (PMSE) is the phenomenon of anomalous radar echoes found between 80-90 km in altitude from May through early August in the Arctic, and from November through to February in the Antarctic. These strong radar echoes are associated with the extremely cold temperatures that occur above continental Antarctica during the summer. Rocket and radar measurements indicate that a partial reflection from a multitude of ion layers and constructive interference causes at least some of the PMSE.
Generally PMSE exhibits dramatic variations in height and intensity as well as large variations in Doppler shift. PMSE exhibit strong signal power enhancements of scattering cross section at VHF radar frequencies in the range 50 MHz to 250 MHz, at times even to over 1 GHz, that occur in summer at high latitudes. The peak PMSE height is slightly below the summer mesopause temperature minimum at 88 km, and above the noctilucent cloud (NLC) and/or polar mesospheric cloud (PMC) layer at 83–84 km. The usual instrument for observing PMSE is the a VHF Mesosphere
-Stratosphere
-Troposphere
(MST) radar
, although lidar
s and sounding rockets have also been used.
PMSE is believed to be caused by structural irregularities in the ionospheric
electron
density at lower altitudes. The exact cause of PMSE is not yet known, although theorists have proposed steep electron density gradients
, heavy positive ions, dressed aerosols, gravity waves and turbulence as possible explanations.
PMSE occurs in both the Arctic and Antarctic regions, and is sometimes accompanied by noctilucent clouds.
Generally PMSE exhibits dramatic variations in height and intensity as well as large variations in Doppler shift. PMSE exhibit strong signal power enhancements of scattering cross section at VHF radar frequencies in the range 50 MHz to 250 MHz, at times even to over 1 GHz, that occur in summer at high latitudes. The peak PMSE height is slightly below the summer mesopause temperature minimum at 88 km, and above the noctilucent cloud (NLC) and/or polar mesospheric cloud (PMC) layer at 83–84 km. The usual instrument for observing PMSE is the a VHF Mesosphere
Mesosphere
The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring...
-Stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
-Troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....
(MST) radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
, although lidar
LIDAR
LIDAR is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, often using pulses from a laser...
s and sounding rockets have also been used.
PMSE is believed to be caused by structural irregularities in the ionospheric
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
density at lower altitudes. The exact cause of PMSE is not yet known, although theorists have proposed steep electron density gradients
Density Gradient
Density gradient is a spatial variation in density over an area. The term is used in the natural sciences to describe varying density of matter, but can apply to any quantity whose density can be measured...
, heavy positive ions, dressed aerosols, gravity waves and turbulence as possible explanations.
PMSE occurs in both the Arctic and Antarctic regions, and is sometimes accompanied by noctilucent clouds.