The linear dependence of GHz scintillation on electron density observed in the equatorial anomaly

Abstract

The dependence of scintillation caused by equatorial bubbles on the electron density in the post sunset anomaly cannot generally be determined because the irregularities that produce the scintillation obscure the measurement of the electron density. This work addresses this problem with the introduction of a new technique based on the finding that anomaly electron density is so regular that its LT distribution can be fit by a polynomial over its post-sunset extent. By means of this polynomial, gaps in the data caused by the irregularities can be inferred where otherwise not measurable. Observations are of amplitude scintillation at 1.5 GHz from the Marisat satellite and of NmF2 measured by ionospheric soundings at Ascension I. in Mar 2001 during solar maximum. The result is that the well defined maximum of the S4 scintillation index is a linear function of coinciding NmF2. Evidence that supports this result has been found in 3 differing cases: The first finds that both S4 and NmF2 decrease linearly with LT; the second, that both occurrence of scintillation and NmF2 increase linearly with F10.7 cm solar flux; the third finds a linear relation between occurrence of scintillation at 137 MHz and NmF2. The importance of these results is that scintillation is caused by the bubble, but its magnitude is determined by the magnitude of the electron density that the bubble intersects. A further implication is that, given the unpredictability of bubbles, if NmF2 could be predicted, the maximum level of S4 possible could be predicted if a bubble were to occur.