Remote sensing of energetic electron precipitation
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Charged particles trapped in Earth's magnetic fields slam or precipitate into the atmosphere during geomagnetic disturbances in the Near-Earth space environment. The particles ionize, excite, and heat the neutral gas, leading to the optical aurora. Below the peak of optical auroral emissions is the ionospheric D-region extending from 70--90 km. Here, in the night time, D-region ionization occurs mainly due to sub-relativistic (100--500 keV) and relativistic (~>500 keV) electron precipitation causing Ultraviolet, X-ray, and faint optical emissions. We can also detect its presence through electron density measurements from Incoherent Scatter Radars. Though the magnetospheric source regions of the precipitation are broadly known, a more constrained estimate of the location and mechanism is needed, especially during magnetic activity. Energetic electron precipitation is also known to cause changes in the upper atmospheric chemistry, increase in ionospheric conductance, and attenuation of radio signals in high-latitude regions. However, a quantitative estimate of these effects has been challenging to obtain due to sparse measurements. This dissertation introduces techniques to measure energetic electron precipitation and its associated auroral forms in the ionosphere, and methods to constrain its sources during magnetically active periods such as substorms. We primarily address the following questions: 1) What are the magnetospheric source regions of energetic electron precipitation observed during substorms? 2) What is the effect of these particles on the atmosphere? By synthesizing measurements from Incoherent Scatter Radars, ground-based optical cameras, and satellites, we identify the two main sources of energetic precipitation during substorms: the near-Earth plasma sheet and the outer radiation belt boundary. The plasma sheet is a thin sheet-like region with a relatively high plasma density, close to the magnetic equatorial plane, between the dipolar field region and the stretched magnetotail. The outer radiation belts are regions of trapped high energy charged particles in the dipolar fields ranging from ~3 to ~10 RE. For the first time, we identified the existence of the outer radiation belt boundary's auroral signature, which is present in at least 40% of strong substorms. These energetic electrons also cause the majority of the high-latitude ionosphere's peak Hall conductance during substorms. The source regions of energetic electron precipitation explored in this dissertation lie in the nightside dipolar transition region - a relatively unexplored part of the magnetosphere. This work will be useful for future explorations of this region, especially for new missions such as the Transition Region Explorer (TREX). The remote sensing effort presented in this dissertation enhances the community's understanding of multi-scale processes that meet the scope of NASA's Heliophysics System observatory. This dissertation provides an extensive background of energetic particle precipitation and its role in the magnetosphere-ionosphere system and a detailed discussion on remote-sensing techniques to constrain precipitation sources using magnetically conjugate measurements.
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