Seasonal variability and descent of mid-latitude sporadic E layers at Arecibo

Abstract. Sporadic E layers (Es) follow regular daily patterns in variability and altitude descent, which are determined primarily by the vertical tidal wind shears in the lower thermosphere. In the present study a large set of sporadic E layer incoherent scatter radar (ISR) measurements are analyzed. These were made at Arecibo (Geog. Lat. ~18° N; Magnetic Dip ~50°) over many years with ISR runs lasting from several hours to several days, covering evenly all seasons. A new methodology is applied, in which both weak and strong layers are clearly traced by using the vertical electron density gradient as a function of altitude and time. Taking a time base equal to the 24-h local day, statistics were obtained on the seasonal behavior of the diurnal and semidiurnal tidal variability and altitude descent patterns of sporadic E at Arecibo. The diurnal tide, most likely the S(1,1) tide with a vertical wavelength around 25 km, controls fully the formation and descent of the metallic Es layers at low altitudes below 110 km. At higher altitudes, there are two prevailing layers formed presumably by vertical wind shears associated mainly with semidiurnal tides. These include: 1) a daytime layer starting at ~130 km around midday and descending down to 105 km by local midnight, and 2) a less frequent and weaker nighttime layer which starts prior to midnight at ~130 km, descending downwards at somewhat faster rate to reach 110 km by sunrise. The diurnal and semidiurnal-like pattern prevails, with some differences, in all seasons. The differences in occurrence, strength and descending speeds between the daytime and nighttime upper layers are not well understood from the present data alone and require further study.

[1]  C. Tepley,et al.  Seasonal and diurnal variation of electron and iron concentrations at the meteor heights above Arecibo , 2008 .

[2]  John W. MacDougall,et al.  An explanation for the seasonal dependence of midlatitude sporadic E layers , 2007 .

[3]  D. Pancheva,et al.  Terdiurnal tidelike variability in sporadic E layers , 2006 .

[4]  Dora Pancheva,et al.  Ionogram height–time–intensity observations of descending sporadic E layers at mid-latitude , 2006 .

[5]  Y. Morton,et al.  Morphology of nighttime ion, potassium and sodium layers in the meteor zone above Arecibo , 2005 .

[6]  N. Mitchell,et al.  Evidence of a role for modulated atmospheric tides in the dependence of sporadic E layers on planetary waves , 2003 .

[7]  R. Bishop,et al.  Metallic ion transport associated with midlatitude intermediate layer development , 2003 .

[8]  R. Bishop,et al.  Inferred vertical ion velocities associated with intermediate layers , 2002 .

[9]  R. Bishop,et al.  Descending layer variability over Arecibo , 2000 .

[10]  J. M. Forbes,et al.  Global transport and localized layering of metallic ions in the upper atmosphere , 1999 .

[11]  Qihou H. Zhou.  Two-day oscillation of electron concentration in thelower ionosphere , 1998 .

[12]  J. Mathews.  Sporadic E: current views and recent progress , 1998 .

[13]  Y. Morton,et al.  Further evidence for a 6-h tide above Arecibo , 1993 .

[14]  J. Whitehead.  Recent work on mid-latitude and equatorial sporadic-E , 1989 .

[15]  J. Mathews,et al.  Upper atmosphere tides and the vertical motion of ionospheric sporadic layers at Arecibo , 1979 .

[16]  R. Harper.  Tidal winds in the 100‐ to 200‐km region at Arecibo , 1977 .

[17]  R. Lindzen,et al.  Quantitative Theory of Atmospheric Tides and Thermal Tides , 1970 .

[18]  W. Axford,et al.  Vertical movement of temperate‐zone sporadic E layers , 1968 .