The leading role of atomic oxygen in the height of the elves the D-region ledge in nighttime electron density and the OH* Meinel band nightglow layer

Abstract

The Imager of Sprite and Upper Atmospheric Lightning the scientific nighttime payload onboard the Taiwanese FORMOSAT 2 satellite provides observations of TLEs and OH* Meinel nightglow simultaneously in limb view from 2004 to 2016 within the +/-60 degree latitude range of the satellite orbit The physical connection between two optical phenomena occurring at a similar altitude in limb viewing opens another window to study the structure of this critical altitude range that has long been lacking in in situ measurements The ISUAL positioning has now been calibrated by the precise location of stars and the pointing accuracy of ISUAL from 2005 to the end of 2011 is estimated to be 0 05° when viewing the limb region from the satellite ISUAL is the only space-borne instrument equipped for simultaneous observation of elves and OH* nightglow For the limb elves 91% of the 291 events are located within +/- 4 km of the altitude of brightest OH* nightglow emission The Semiannual Oscillation (SAO) at low latitude is a significant feature of the OH* nightglow because of the resulting vertical transport of atomic oxygen affected by tidal motions and migration in the mesosphere The elve heights show the same variation: higher in the solstice seasons and lower in the equinox seasons Based on the observational truth of this collocation the processes and mechanisms which have been questioned or which have not been previously declared are shown and discussed in this thesis: (1) The relation between elves and atomic oxygen: The environment-based electron density profile is adapted as the input to the conventional elve model [Kuo et al [2007] to ascertain the brightest height of elve emission Along with the OH*model from SABER analysis [Xu et al 2012; Smith et al 2012] the model results show clearly that both the height of elves and OH* nightglow are higher when the ledge of the atomic oxygen profile is higher and vice versa (2) The relation between the OH* nightglow and the electron density ledge: The semiannual oscillation driven by the vertical transport of atomic oxygen is primary evidence linking the altitudes of elves OH* nightglow and the VLF waveguide boundary near 87 km [Toledo-Redondo et al 2012] The third question addressed in this work concerns the role of meteoric smoke particles in the electron density ledge when atomic oxygen serves to free the electrons from negative ions: Two measureables of the ED ledge are the height and the scale height and are examined with 102 rocket soundings One third of the soundings show a maximum frequency at 87 km and a mean scale height of 1 2 km The size range for micrometeoroids estimated by the classical model for ablation onset overlaps remarkably well with the fixed height portions of the curves for meteor speeds of 10-20 km/s in the height range 80 to 90 km where the ED ledge elve and OH* nightglow are all located The concentration of MSPs capable of reducing the electron density by scavenging is estimated based on mass conservation This estimate is of the same order as the measured electron density at 86 km so that electron scavenging is plausible Furthermore the meteor velocity observations from the Jicamarca HPLA meteor radar have been used to show a gradual increase in counts (after correction of the velocity distribution according to the ionization probability β(V)) down to the terrestrial escape velocity of 11 km/s where the laboratory measurements of β(V) lack access In short at the altitude where atomic oxygen is present MSPs are required to make the electron density ledge The leading role for atomic oxygen in determining the altitude of elves OH* nightglow and the electron density profile along with MSPs has been verified by both observation and theoretical model The observational truth of the collocation offers a new perspective on the composition the photochemistry and the dynamics of the mesosphere where the transition region between the ionosphere and the neutral atmosphere is difficult to monitor and where new discoveries lie around the corner Key words: Elves OH* nightglow D-region ionosphere electron density atomic oxygen

Date of Award	2017 Jan 24
Original language	English
Supervisor	Rue-Ron Hsu (Supervisor)