Comparison of topside and bottomside irregularities in equatorial F region ionosphere

Abstract

Abstract. Numerical simulations have been carried out to study and compare the mechanisms of the formations of the topside and bottomside ionospheric irregularities. As is well-known, the topside ionospheric irregularities have always originated from the bottomside ionosphere due to Gravitational Rayleigh-Taylor (GRT) instability. From the penetration process we found that the primary bubble with a horizontal scale of tens of kilometers always has a smooth shape in the bottomside ionosphere and breaks into much smaller-scale structures after penetrating into deep topside ionosphere. Since the plasma bubble is unlikely to break into irregularities in the bottomside ionosphere, the bottomside irregularities probably result from the turbulence in the neutral atmosphere. Examples of simulations are presented to demonstrate that bottomside irregularities may exist in a region with a strong neutral wind jet and seeding gravity wave motion. A strong neutral jet stream always has a strong vertical wind shear, which provides a broad height range of dynamic instability. When the atmospheric gravity wave approaches its critical level, its amplitude will continually grow to break into smaller-scale waves and turbulence. Then all the waves of different scales as well as the turbulence will become the seeds of the bottomside irregularities. IntroductionEquatorial spread F is mainly driven by Gravitational Rayleigh- Taylor is to investigate the cascading process of the bubble from wave structure into irregularities.In contrast to the topside ionospheric irregularities, which are originated in the distant bottomside ionosphere, the bottomside ionospheric irregularities are originated from right where they are, without spreading to the nonperturbed region. Therefore the seeded perturbation is very probably limited to a finite region where the growth of the perturbation is so fast that the perturbation grows in a short time to break into many small-scale structures before it can spread to the nonperturbed region. The exact condition which can confine the bottomside irregularities is still unclear. So another one of the purposes of this study is to find such a possible condition by the method of numerical simulation. In this study we propose that the existence of a strong neutral wind jet superposed by seeding gravity waves in the bottomside ionosphere may generate and confine the irregularities in a limited height range.We will briefly review the numerical method in section 2 and present the simulation results in section 3. The discussions will be given in section 4. Numerical ModelThe model used in this study, which is the same as the model of Zalesak and Ossakow [1980], has been described by Chou and Kuo [1993, 1996]

Testing the thermospheric neutral wind suppression mechanism for day‐to‐day variability of equatorial spread F

“…A multidiagnostic observing campaign conducted via the MISETA consortium was formulated to test a hypothesis concerning the day-to-day control of equatorial spread F. The proposal that strong thermospheric winds flowing across the geomagnetic equator could inhibit or suppress ESF onset had been advanced to explain seasonal-longitude patterns by Maruyama and Matuura [1984] and, potentially, day-to-day variability [Mendillo et al, 1992]. Additional simulation studies by Zalesak and Huba [1991] and Kuo et al [1998] reinforced the role of thermospheric dynamics on the instability growth rate as described in earlier studies [Zalesak et al, 1982;Maruyama, 1988]. Yet little experimental effort was devoted to this topic in the many years following its proposal.…”

“…In time, morphological classification schemes provided some real benefits in guiding physical explanations, and thus today the field is better off than, say, the magnetospheric community's attempt to agree upon what constitutes a substorm. The F region's classic "plasma bubble" encountered by a satellite sensor, the radar "backscatter plume," and the "airglow depletion" captured in an all-sky imager all refer to widely accepted views of an ESF event.The statistical occurrence patterns of ESF are well known [Aarons, 1993], and a theoretical foundation based on the Rayleigh-Taylor gravitational instability mechanism [Haerendel, 1973;Ossakow, 1981;Haerendel et al, 1992] is both widely accepted and used successfully in computer simulations [Zalesak et al, 1982; Maruyarna and Matuura, 1984; Maruyarna, 1988;Kuo et al, 1998]. What remains to be understood is why, during the so-called "ESF season" at a given longitude, ESF does not occur every night.…”

“…The statistical occurrence patterns of ESF are well known [Aarons, 1993], and a theoretical foundation based on the Rayleigh-Taylor gravitational instability mechanism [Haerendel, 1973;Ossakow, 1981;Haerendel et al, 1992] is both widely accepted and used successfully in computer simulations [Zalesak et al, 1982; Maruyarna and Matuura, 1984; Maruyarna, 1988;Kuo et al, 1998]. What remains to be understood is why, during the so-called "ESF season" at a given longitude, ESF does not occur every night.…”

Topside ionospheric irregularities as seen from multisatellite observations

“…As it was stated by Woodman [1993], the topside irregularities are always originated in the linearly unstable bottomside ionosphere. In particular, the cause of the equatorial spread F event, or so called after Kelley et al [2011] convective ionospheric storm, is related to density irregularity structures resulted from a multistep nonlinear plasma process initiated from the large-scale gravitational Rayleigh-Taylor (R-T) instability at the bottomside ionosphere [e.g., Keskinen et al, 1980;Ossakow, 1981;Kuo et al, 1998]; however, it does not require that the bottomside R-T instability to be the ultimate source for all or any kind of the topside irregularities [Fejer and Kelley, 1980]. Next region with the most intense ionospheric irregularities is the high-latitude ionosphere, a highly structured medium containing irregularities that are mostly caused by plasma processes associated with auroral activities, attributed to energetic particle precipitation, and dynamical processes including high-speed plasma convection [e.g., Fejer and Kelley, 1980;Keskinen and Ossakow, 1983].…”

The characteristics of field-aligned currents associated with equatorial plasma bubbles as observed by the CHAMP satellite

“…The growth rate of GRTI can be calculated by either local (Rappaport, 1996), flux-tube-integrated (Sultan, 1996), or ballooning-mode ambient parameters (Basu, 2002), whose merits and demerits are elaborated on in Basu (2002). EPBs are described by linear GRTI at its initial stages, but non-linear dynamics should be introduced at later stages at the topside ionosphere (Kuo et al, 1998). Sekar and Kelley (1998) showed that, as EPBs grow from the bottom to topside ionosphere, vertical shear in the zonal plasma drift leads to EPB westward tilt.…”