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Planning Project 2: D-region Conductance & Multiple substorms

Energetic precipitation has a direct effect on the ionosphere's chemistry, its conductance, and radio wave propagation (See section \ref{section: ionospheric effects}). In this chapter, we focus on the impact of energetic precipitation on ionospheric conductance. Ionospheric conductivity is an essential parameter that determines the amount of current redirected from the magnetosphere. Therefore, it is a crucial parameter that determines the coupling between the magnetospheric and ionospheric plasma.

To model MI coupling, a global conductance model is essential. Several statistical/empirical conductance models have been developed using satellite measurements of particle precipitation \citep{Fuller-Rowell1987, Hardy1987}. These models have been beneficial to the scientific community, and as a result, they are extensively used three decades later. However, they have a few drawbacks. Due to the limitation of particle detectors onboard satellites, conductance estimates are limited to the energy range and resolution of charged particle detectors, therefore not all energy ranges are incorporated in estimating the conductance empirically. As the spatial coverage of satellite precipitation is also limited, these models use large averaging of the data over space and time. Hence, they do not provide a good instantaneous picture of high-latitude dynamics, which is important during magnetically active events such as storms and substorms.

The poor spatial and temporal resolution results in the averaging out of small-scale dynamics. For example, conductance enhancement of discrete auroral arcs is not represented in these global models - even though they couple strongly with the magnetosphere through field-aligned currents. ISR measurements are by their very nature small-scale, and unlike satellite measurements of particle precipitation, they provide a more direct estimate of conductivity through electron density measurements. This technique is at least several decades old. However, by using electronically steerable radars like PFISR, we can develop small-scale maps of Hall and Pedersen conductivities to explore the conductivity structure varying with altitude. We can potentially use such 3-D conductivity estimates to study current closure in the high-latitude ionosphere.

Energetic precipitation ionizes the D-region, resulting in enhancements in the D-region conductivity. Though global conductance models, such as \cite{Fuller-Rowell1987, Hardy1987}, incorporate energetic electrons from POES spacecraft, MI coupling models such as Global Ionosphere Thermosphere Model use self-consistent conductance boundary conditions that are associated with only auroral electron precipitation ($\sim$ 1--30 keV). \cite{Lu2016} showed that the proportion of peak global conductance that electron $\sim>30$ keV contribute is at least twice that of the peak conductance contribution from auroral electrons ($\sim<30$ keV). Furthermore, \cite{Hosokawa2010, Buchert2008} showed that D-region Pedersen conductivity behaves differently from E-region because collisions with the neutrals influence more electrons in the D-region. This results in a Pedersen conductivity that is predominantly contributed by electrons than ions \citep{Hosokawa2010}. Also, electron Pedersen conductivity in the D-region is non-linear at large electric fields, and the Pedersen conductance was $\sim 60$\% higher than when assuming the classical ohms law \citep{Buchert2008}.

Considering the likely importance of D-region ionization's contribution to the total conductance, this chapter is devoted to quantifying the small-scale D-region conductivity structures and its contribution to total ionospheric conductance during different phases of a substorm. We start with the methodology used to estimate D-region conductivity and its limitation. Then we describe observations of D-region conductivity enhancements during the 26 March 2008 substorm growth-phase and expansion phase. Finally, we discuss insights we can gain from the observations. A very conservative estimate during the expansion phase shows that the maximum D-region contribution to the Pedersen conductance is more than $15$\%, and the Hall conductance is more than $60$\%.