Spatial and temporal variations of the atmospheric sodium layer observed with a steerable laser radar
LASER radar observations of resonance scattering from atmospheric sodium during the night have, on occasions, shown evidence of short term changes in the height distribution1–3. It has been suggested that these changes arise from the movement of a horizontally structured sodium layer, but the measurements have all been limited to the vertical direction, and have provided no direct evidence of a horizontal variation. Evidence of horizontal structure at mesospheric heights has been provided by observations of hydroxyl emissions in the airglow, both from the ground4 and from an aircraft5,6. In addition, the observed fluctuations of intensities and infrared rotational temperatures of the hydroxyl bands have been associated with the passage of atmospheric gravity waves7,8. We report here observations intended to examine the short term and small scale horizontal variations of the sodium layer which might also be associated with such waves.
Some observations on the thermal behavior of the mesosphere
Abstract Temperature and wind soundings of the stratosphere and mesosphere using the acoustic grenade technique were made over Barrow, Alaska (71N), during the arctic winter night and during the summer. The winter mesosphere temperatures were generally warmer than at any other location and season and were variable from day to day. Temperatures oscillated with height in wavelike fashion at magnitudes up to 30–40C between 70 and 90 km. These wavelike structures were found to exist also in the winter mesosphere at Churchill (59N) and Wallops Island (38N). However, the magnitude of the oscillations decreased with latitude to a value of 20–25C at Wallops. Conversely, variability In summer armed to decrease with increasing latitude and the wavelike structure was suppressed entirely at Barrow. Temperature variations in summer at Barrow were less than 10C from sounding to sounding at any given altitude from 40 to 85 km. Extremely low mesopause temperatures ranging from 130–140K near 85 km were observed in all of ...
Time and Heritage
As a historian studying what can be considered a form of intellectual history, I have slowly come to adopt Michel de Certeau’s observation made at the end of the 1980s that ‘objectifying the past, for the last three centuries, has undoubtedly left unconsidered time within a discipline that has continued to use it as a taxonomic instrument’. To a certain degree, time has become commonplace for the historian who has preserved or instrumentalized it. It is not considered because it is inconceivable, but because we do not think of it or, more simply, we do not think about it. As a historian attempting to pay attention to the time I’m living in, I have thus, like many others, noticed the swift development of the category of the present until it has become obvious that the present is omnipresent. This is what I refer to here as ‘presentism’.