FIRI‐2018, an Updated Empirical Model of the Lower Ionosphere

Abstract

The lower ionosphere is difficult to access from the ground, but also, in situ measurements by sounding rockets have their limitations. The semiempirical model of the nonauroral ionosphere FIRI (Faraday‐International Reference Ionosphere) was originally published in 2001 and is based on a simple ion‐chemical model of the lower ionosphere whose output is adjusted by the limited number of reliable in situ rocket measurements, all based on wave propagation methods. Since then some of the data were found to be questionable but could be replaced by about the same number of trustworthy data. In addition, a large number of electron density profiles based on Langmuir probes became available covering the altitude range from 51 to 90 km. These data are used to extend FIRI downward, providing results in a region that would otherwise not be covered at all. Caution was taken that these data do not impair the altitudes covered by wave propagation data which are unaffected by aerodynamic and payload potential effects. The resulting semiempirical model is available as 1980 profiles of electron densities between 60 and 150 km altitude (for 11 solar zenith angles from 0 to 130°, latitudes 0°, 15°, 30°, 45° and 60°, for three solar activities and the middle of each month).

Intercomparison of radar meteor velocity corrections using different ionization coefficients

“…The collective observational evidence serves to sharpen the increase in concentration of the ablation debris with decreasing altitude, without invoking additional modeling complications that are often overparameterized. The rich concentration of meteoric smoke particles noted initially in modeling studies by Hunten et al [] may be a key aspect of the systematic ledge in electron density [ Plane et al , ] and electrical conductivity in the 80–90 km altitude range in the nighttime ionosphere [ Hale , ; Friedrich and Torkar , ]. This concentrated deposition may also be important in explaining the more abrupt disappearance of electrons in meteor trails below 85 km reported earlier by Whipple [] and mentioned in section 1 [ Wu et al , ].…”

“…The first author's interest in the origin of the pronounced nighttime ledge in electron density near 85 km altitude [ Hale , ; Friedrich and Torkar , ] initiated this inquiry into meteor ablation. Whipple 's [] finding of an abrupt increase in the rate of decay of meteor trail ionization below about 85 km was strongly motivational here for investigating meteoric debris/dust as an explanation for the ledge.…”

Statistics and variability of the altitude of elves

“…The energy required for free electrons to excite the positive bands of N 2 can only be reached where electron densities and mean free paths are sufficiently large and where the plasma frequency allows an electric field to penetrate. Studies of nighttime reflection heights using sferics in the extremely and very low frequency range report variations between 82 and 87 km [e.g., Han and Cummer , ; Lay and Shao , ; Maurya et al , ], which roughly correspond to the nocturnal 82–92 km altitude range of the electron density increase from 10 7 to >2 · 10 9 m −3 in profiles taken by rocket probes summarized by Friedrich and Torkar []. The steep electron gradient is often discussed referring to hydrated cluster ions below 85 km, which scavenge free electrons due to their large electron recombination coefficient [e.g., Reid , ; Sugiyama , 1988].…”

VLF Remote Sensing of the D Region Ionosphere Using Neural Networks

“…Some GCPs are along dry ground paths, others are almost all sea paths, and many are a mixture of the two. Han et al (2011) measures sferics emanating from small geographic regions with a receiver in North Carolina during three different days to infer anh ′ and̄curve.h ′ and̄are fit to multiple sounding rocket measurements from (Friedrich et al, 2018, and references therein) at various solar zenith angles and different locations on Earth. These differences are caused by, but not limited to, Earth's seasonal changes and temporal variations in the Sun's behavior.…”

“…McRae and Thomson (2000) use four narrowband VLF transmitters (Omega transmitter in Hawaii, Omega transmitter in Japan, NPM in Hawaii, and NLK in Seattle) and one receiver in Dunedin, New Zealand, to infer a singleh ′ and curve for all four GCPs. Han et al (2011) measures sferics emanating from small geographic regions with a receiver in North Carolina during three different days to infer anh ′ and̄curve.h ′ and̄are fit to multiple sounding rocket measurements from (Friedrich et al, 2018, and references therein) at various solar zenith angles and different locations on Earth. The International Reference Ionosphere 2016 modelh ′ and̄curves are determined at the GCP midpoint location for NML-OX on 4 October 2017 (Bilitza et al, 2016).…”