Equatorial plasma bubbles observed by DMSP satellites during a full solar cycle: Toward a global climatology

Abstract

[1] We have examined more than 75,000 latitudinal profiles of plasma densities measured by ion detectors on five Defense Meteorological Satellite Program (DMSP) satellites in the evening local time (LT) sector between 1989 and 2001. This survey established detection frequencies of equatorial bubbles (EPBs) at 840 km over the recent solar cycle. The annual rate of EPB detections decreased by more than an order of magnitude from >1000 during solar maximum to <100 during solar minimum years. EPB data were divided into 24 longitude sectors to determine seasonal and solar cycle variability in rates of encounter by DMSP. During the ascending and descending portions of the solar cycle, each longitude sector showed repeatable seasonal variations. The envelope of seasonally averaged rates of EPB encounters resembles the solar cycle variability for similar averages of the F 10.7 index. On both global and longitude sector scale sizes, annual rates of EPB encounters correlate with the yearly averages of F 10.7 . We also find that throughout the solar cycle the EPB detections were overrepresented during times of high geomagnetic activity signified by Kp ! 5. During solar minimum years, about one third of the EPBs occurred when traces of the Dst index had significant negative slopes (dDst/dt À5 nT/hr). This suggests that electric field penetration of the inner magnetosphere is responsible for driving many EPBs. Comparisons of plasma and neutral density profiles in the evening sector, calculated using the Parameterized Ionospheric Model (PIM) and MSIS-86 Model, indicate that the height of the bottomside of the F layer is >100 km lower during solar minimum than solar maximum. However, the overall effect is to increase the growth rate of the Rayleigh-Taylor instability at solar maximum in the bottomside F layer only by about a factor of 2. We suggest that the variability of electric fields in the postsunset equatorial ionosphere is the source of the observed discrepancy between EPB detections under solar maximum/minimum conditions.

Statistical analysis of C/NOFS planar Langmuir probe data

“…(4) indicates that these effects on the growth rate should be anticorrelated with the longitudinal dependence of the field strength of the magnetic field observed in the lower panel of Fig. 2, for a constant value of the eastward electric field (Huang et al, 2001;Burke et al, 2004a, b). Equation (4) also indicates that the growth rate increases when P E decreases.…”

“…It has been remarked in Sect. 1 that Huang et al (2002) and Burke et al (2004a, b) based their analyses on a different parameter. In principle, differences between corresponding results could be due to the respective measurement periods and the associated solar activities, orbital characteristics of the satellites, instrumentation, processing techniques developed by different authors, etc.…”

“…All these distributions resulted from data recorded in the altitude interval from 350 to 450 km. Huang et al (2002) and Burke et al (2004a) examined more than 75 000 latitudinal ion density profiles measured from 19:30 to 21:30 LT by instruments onboard Defense Meteorological Satellite Program (DMSP) satellites between 1989 and 2001. The extremes of this interval approximately coincide with the maxima of the intense solar cycles 22 and 23, when their early-averaged absolute 10.7 cm solar flux density reached the values 190 and 165 sfu, respectively.…”

“…The occurrence rates of EPBs were determined from the set of ratios between the number of orbits with EPBs and total number of orbits in each bin. Huang et al (2002) classified the results into 24 × 4 (longitude sector, season) bins to determine seasonal and solar-cycle variability of the EPB occurrence rates. They also processed the ion densities measured along 200 orbits with no EPB activity in the evening sector during 1991 and 1996.…”

A comparison of the equatorial spread F derived by the International Reference Ionosphere and the S 4 index observed by FORMOSAT-3/COSMIC during the solar minimum period of 2007–2009

“…There have been many studies on the occurrence characteristics of ionospheric density irregularities using groundbased measurements (Rastogi, 1980;Abdu, 2001), satellitebased in situ measurements (Watanabe and Oya, 1986;Huang et al, 2002;Burke et al, 2004), and topside sounders (Maruyama and Matuura, 1984), and many important features of these have been reported including temporal variations (Sahai et al, 2000;Huang et al, 2002), seasonallongitudinal variability (Burke et al, 2004), several-day variation, and day-to-day variation (Basu et al, 1996) though several enigmatic features of them are yet to be revealed (Thampi et al, 2009). However, studies on continuous and global characteristic features of ionospheric plasma irregularities are very scanty due to the fact that the global coverage is difficult to obtain from ground-based observations.…”