Abstract
Abstract. A model for the development of electron density height profiles based on space time distributed ionization sources and reaction rates in the lower ionosphere is described. Special attention is payed to the definition of an auroral oval distribution function for energetic electron energy input into the lower ionosphere based on a Maxwellian energy spectrum. The distribution function is controlled by an activity parameter which is defined proportional to radio signal amplitude disturbances of a VLF/LF transmitter. Adjusting the proportionality constant allows to model precipitation caused VLF/LF signal disturbances using radio wave propagation calculations and to scale the distribution function. Field aligned current (FAC) data from the new Swarm satellite mission are used to constrain the spatial extent of the distribution function. As an example electron precipitation bursts during a moderate substorm on the 12 April 2014 (midnight–dawn) are modeled along the subauroral propagation path from the NFR/TFK transmitter (37.5 kHz, Iceland) to a midlatitude site.
Highlights
The lower ionosphere (60–85 km height) is forced from above by extreme UV, especially Lyman α, solar flare Xrays and energetic particle precipitation
On the average Field aligned current (FAC) are negative in the equatorward half of the auroral oval and positive on the poleward half
There are two parameters in the model that are fitted to the data: the proportionality constant cdB (auroral activity per signal amplitude drop) yielding the activity parameter a(t) = cdB(ampundisturbed − ampdisturbed(t)), and the folding energy E0 of the electron energy spectrum (40 keV in our example case). cdB has been adjusted for best fit of the modeled signal amplitude to the disturbed recorded signal amplitude
Summary
The lower ionosphere (60–85 km height) is forced from above by extreme UV, especially Lyman α, solar flare Xrays and energetic particle precipitation (electrons 30 keV, protons 4 MeV, lower energetic particles do not penetrate so deeply). Large scale energetic electron precipitation modify the auroral electrojet and lead to variations of the AL index In this way ground based observations by riometers and magnetometers at proper sites yield important additional information with regard to the identification of energetic electron precipitation along the VLF/LF propagation path. Mapping down electric and magnetic field measurements and derived data (field aligned currents, FACs, Ritter et al, 2013) from the Swarm satellites yields important information for the assessment of the energy input into the ionosphere, with regard to the path and spatial extent of particle precipitation as well as the acceleration mechanisms. 5. In this paper we describe the VLF remote sensing procedure followed by the discussion of our model for the calculation of the electron density profiles, especially with regard to forcing events like electron precipitation. E2 ne me fc (ωp: plasma frequency) we get the classic Wait and Spies (1964) electron density parametrization of the lower ionosphere:
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