The Levels of Equatorial Spread F, From Bottomside Spread F to Equatorial Bubbles: Their Increase With E x B Drift Velocity, and Their Suppression With Geomagnetic Activity

Details The Levels of Equatorial Spread F, From Bottomside Spread F to Equatorial Bubbles: Their Increase With E x B Drift Velocity, and Their Suppression With Geomagnetic Activity The Levels of Equatorial Spread F, From Bottomside Spread F to Equatorial Bubbles: Their Increase With E x B Drift Velocity, and Their Suppression With Geomagnetic Activity

Dec 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufmsa51a0770w&link_type=abstract

American Geophysical Union, Fall Meeting 2001, abstract #SA51A-0770

Mathematics

Probability

2411 Electric Fields (2712), 2415 Equatorial Ionosphere, 2437 Ionospheric Dynamics, 2439 Ionospheric Irregularities, 2447 Modeling And Forecasting

Scientific paper

Because equatorial bubbles occur at random, it is extremely difficult to study their formation. However recent results have thrown light on the conditions that create bubbles and also conditions that suppress them. As observed in the Western American sector during solar maximum, equatorial spread F exists in a series of levels that are a function of maximum pre-reversal vertical F layer drift velocity measured at the equator, hence of maximum eastward pre-reversal electric field. With the increasing of this E x B drift velocity, irregularity increases progressively starting at the bottomside of the F layer from none to weak to strong bottomside spread F (BSSF). Strong BSSF, which occurs at a drift velocity of 40 - 50 m/s, is particularly significant because it is the threshold above which equatorial bubbles can occur and below which they cannot. Bubbles occur sporadically, possibly because they require the timely occurrence of a gravity wave, and increase in height with increasing drift velocity. Geomagnetic activity, expressed as Kp averaged in the 6h preceding sunset, has been found to suppress E x B drift velocity and therefore to suppress equatorial spread F. What is observed directly is the decrease with increasing Kp of equatorial spread F at each observable threshold: weak, strong BSSF observed near the equator and macroscopic bubbles observed in the anomaly during a year at solar maximum. During equinox (Mar, Apr, Sep, Oct) and summer(Nov-Feb) the relative occurrence of each of the measured threshold levels of irregularity decreases as a linear function of this 6 h averaged Kp over the range from 0 to 5. However for the winter months (May-Aug) the levels are independent of Kp. As a result, the probability of occurrence of each level is quantitatively determined for each season as a function of Kp, in particular, of macroscopic bubbles, and of strong BSSF, the necessary condition for the formation of all bubbles. In addition the probability of the absence of strong BSSF yields the probability that bubbles will not occur.

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