Equatorial bubbles observed at the north and south anomaly crests: Dependence on season, local time, and dip latitude

Abstract

Abstract. Equatorial bubbles, which cause scintillation that disrupts transionospheric RF propagation, are difficult to study because they are unpredictable and extremely variable. This work undertakes to study them by continuous observation throughout a year at solar maximum using ionospheric sounders operating at sites in the western hemisphere in both north and south Appleton anomaly crests. Here direct observation in an extensive case study has found that bubbles can be identified when passing over a sounder near the Appleton anomaly crest by the appearance of range spread F (RSF) that is so severe that it prevents evaluation of foF2 . Using this obscuring spread F as a signature, bubbles are recorded in the hourly tabulations of foF2 on 37 days a total of 104 times by three anomaly sounders at dip latitudes of 16 ø and 20.3 ø in the north and 19.9 ø in the south. At each location, occurrence is maximum near the equinoxes, zero in July with a minimum in December, and maximum between 2200 and 2400 LT. Interpretation of the signature is supported by its consistency in both season and local time with bubbles observed by others. Overall, occurrence decreases by a factor of 0.38 between 16 ø and 20 ø dip latitude, apparently symmetrically about the dip equator. Whereas RSF at the anomaly always results from bubbles, RSF at the equator does not, the most evident period being near December when bubbles have a minimum but equatorial RSF has a maximum, occurring nearly every day following sunset. This nonbubble equatorial RSF apparently results from bottomside spread F.

Concurrent study of bottomside spread F and plasma bubble events in the equatorial ionosphere during solar maximum using digisonde and ROCSAT-1

“…Whalen, 1997Whalen, , 2002Fejer et al, 1999;Huang, 2001;Su et al, 2001). The past results indicate that the occurrences of both BSSF and bubble depend on season, longitude, solar cycle, and geomagnetic activity (Maruyama and Matuura, 1984;Whalen, 1997Whalen, , 2002Fejer et al, 1999;Huang, 2001;Lee at al., 2005). Nevertheless, little work has been published examining the simultaneous occurrence of BSSF and bubble in the equatorial ionosphere (Whalen, 1997(Whalen, , 2002.…”

“…The past results indicate that the occurrences of both BSSF and bubble depend on season, longitude, solar cycle, and geomagnetic activity (Maruyama and Matuura, 1984;Whalen, 1997Whalen, , 2002Fejer et al, 1999;Huang, 2001;Lee at al., 2005). Nevertheless, little work has been published examining the simultaneous occurrence of BSSF and bubble in the equatorial ionosphere (Whalen, 1997(Whalen, , 2002. In his first report, Whalen (1997) compared the monthly occurrence of bubble with that of BSSF during solar maximum, using a chain of ionospheric sounders in the western American sector.…”

“…Therefore, BSSF, which maps via the geomagnetic field to a narrow band of latitude, is observed only by sounders near the dip equator. On the other hand, plasma bubbles can extend to a wider latitudinal coverage, because the flux tubes in a vertical wedge are typical depleted along their north-south extent (Whalen, 1997(Whalen, , 2002.…”

“…Nevertheless, little work has been published examining the simultaneous occurrence of BSSF and bubble in the equatorial ionosphere (Whalen, 1997(Whalen, , 2002. In his first report, Whalen (1997) compared the monthly occurrence of bubble with that of BSSF during solar maximum, using a chain of ionospheric sounders in the western American sector. He found that the BSSF and bubble occurrences were greater in the summer (December) and equinoctial (April and September) months, and had a minimum in the winter (July) months.…”

“…Although Whalen (1997Whalen ( , 2002 simultaneously investigated the BSSF and bubble formations in the equatorial ionosphere, he did not use satellite data or estimated the vertical drift velocity from his data set. The vertical drift velocity has been known to greatly affect the occurrences of the BSSF and plasma bubble (Fejer et al, 1999).…”