Coincident extremely large sporadic sodium and sporadic E layers observed in the lower thermosphere over Colorado and Utah

Abstract

Abstract. On the night of 2 June 2002, the sodium lidar in Fort Collins, CO (40.6 N, 105 W) measured an extremely strong sporadic sodium layer lasting from 03:30 to 05:00 UT with several weaker layers later in the night at 06:00 and 09:00 UT. There is a double layer structure with peaks at 101 and 104 km. The peak sodium density was 21 000 atoms/cm3 with a column abundance of up to twice that of the normal sodium layer. The peak density was 500 times greater than the typical density at that altitude. The sporadic layer abundance and strength factor were higher than any reported in the literature. The two lidar beams, separated by 70 km at this altitude, both measured 0.6 h periodicities in the abundance, but out of phase with each other by 0.3 h. There is also evidence for strong wave activity in the lidar temperatures and winds. The NOAA ionosonde in Boulder, CO (40.0 N, 105 W) measured a critical frequency (foEs) of 14.3 MHz at 03:00 UT on this night, the highest value anytime during 2002. The high values of total ion density inferred means that Na+ fraction must have been only a few percent to explain the neutral Na layer abundances. The Bear Lake, Utah (41.9 N, 111.4 W) dynasonde also measured intense Es between 02:00 and 05:00 UT and again from 06:00 to 08:00 UT about 700 km west of the lidar, with most of the ionograms during these intervals measuring Es up to 12 MHz, the limit of the ionosonde sweep. Other ionosondes around North America on the NGDC database measured normal foEs values that night, so it was a localized event within North America. The peak of Es activity observed in Europe during the summer of 2002 occurred on 4 June. The observations are consistent with the current theories where a combination of wind shears and long period waves form and push downward a concentrated layer of ions, which then chemically react and form a narrow layer of sodium atoms.

Summer sudden Na number density enhancements measured with the ALOMAR Weber Na Lidar

“…3). The sudden enhancements in Na density above ALOMAR around midnight during summer seem to differ from those observed during winter (see below) and those observed during summer at other sites (Williams et al, 2007). For this reason and because of their regular appearance, we avoid the term sudden sodium layer or sporadic sodium layer in this context similar to Friedman et al (2000).…”

A statistical study of sporadic sodium layer observed by Sodium lidar at Hefei (31.8° N, 117.3° E)

“…These observations showed much higher occurrence rates than those observed at mid-latitude before the 1990s. Williams et al (2007) suggested that this possibly indicates a large interannual or decadal variability in the sporadic layer occurrence rate.…”

“…More extensive studies on SSLs by lidars were conducted after the observation by Von Zahn and Hansen (1988) (e.g. Kwon et al, 1988;Batista et al, 1989;Senft et al, 1989;Kane et al, 1991;Gardner et al, 1993;Clemesha, 1995;Clemesha et al, 1998;Qian et al, 1998;Gong et al, 2002;Clemesha et al, 2004;Williams et al, 2006Williams et al, , 2007Fan et al, 2007;Nesse et al, 2008;Vishnu Prasanth et al, 2007;Yi et al, 2007;Heinrich et al, 2008). Sporadic sodium layers have been studied for more than 20 years.…”

“…But subsequent observations showed that the mid-latitude deficit was not thought to be real in recent years. For example, measurements in the Eastern Hemisphere, i.e., Tokyo (35 • N) from November 1991 to October 1993 and Wuhan (31 • N) from February 1996 to August 2001, gave a mean occurrence interval of about 10 h per SSL event (Nagasawa and Abo, 1995;Yi et al, 2002;Gong et al, 2002), and measurements in the Western Hemisphere, i.e., Colorado (40.6 • N) from April to June 2002 and June to August 2003, indicated that 65% of days and 19% of hours had SSLs (Williams et al, 2007). These observations showed much higher occurrence rates than those observed at mid-latitude before the 1990s.…”

Sodium lidar‐observed gravity wave breaking followed by an upward propagation of sporadic sodium layer over Hefei, China

“…Several studies Clemesha et al, 1999;Williams et al, 2007;Dou et al, 2010;Xue et al, 2013] have shown that SSLs are associated with sporadic E (Es) layers. Cox and Plane [1998] reported that Na + can …”