10/16/2023
By Lynne Schaufenbil

Sunip Mukherjee will present his Ph.D. in Physics Thesis Dissertation Proposal Defense on Tuesday, Nov. 7 at 2 p.m. in WAN305.

Title: Multi-Instrument, Multi-Location Observation and Modeling of the Upper Atmosphere-Ionosphere.

Thesis Supervisor: Supriya Chakrabarti

Thesis Committee Members: 

  • Timothy Cook 
  • Anna Yaroslavsky

Abstract:
Photons are emitted from the upper atmosphere through complex interactions in the form of airglow and aurora. By proxy of these emissions, the photochemical and collisional processes in the upper atmosphere, specifically the ionosphere-thermosphere (IT, 90-600 km) region, can be diagnosed. Emissions in airglow and aurora are spectrally discontinuous line emissions. Simultaneous observation of selected emission features allows for the determination of geomagnetic parameters and characteristics of any precipitating particles. Periodic observations can then be used to infer compositional changes in the IT system.

Multispectral imagers have been used at night for ground-based observations of airglow and aurora. However, the emissions cannot be easily observed from the surface of the earth because of contamination by other terrestrial sources without using narrowband (< 2 nm) filters or imaging spectrometers. This dissertation attempts to characterize the densities and temperatures of the electrons and neutral species in the mid-latitude upper atmosphere using a multispectral imager built using readily available parts in a small form factor. The imager was equipped with a mosaic constructed using commercially available filters with 10 nm bandpass, with the spectral bands covering airglow-specific wavelengths or the background continuum. The instrument was flown on a high-altitude balloon and collected data from morning into night. Simultaneous ground-based measurements were performed using an imaging spectrometer to establish the ground truth.
The dissertation presents the techniques developed to analyze the observation data and model the phenomena that result in the observed emissions. For ground-based measurements, the model is shown to match the 630 nm (red line) and the 557.7 nm (green line) emission brightness for nighttime measurements to ±3σ uncertainty. The retrieved vertical total electron content (TEC) from the model is shown to correlate with independent, ground-based measurements.

Additionally, derived height of the ionospheric F2 layer from the ground-based measurement of green line emission correlates with measurements from the Digisonde network to 85% or above, indicating that the morphology of the green line emission is sensitive to the height of the F2 layer for mid-latitude measurements at nighttime in geomagnetically quiet scenarios. The models are also shown to be applicable on data collected at different locations and time of year.

For data collected using the balloon-borne instrument, measurements around dusk indicate the presence of significant scattering of sunlight, originating from a combination of observation geometry and instrument pointing in this time period. The line-of-sight integrated scattering contribution is shown to be brighter than stars present in the field of view of the instrument around this time. The contamination from scattered sunlight, which is a dispersed source similar to the airglow the instrument is trying to measure, is impossible to remove without employing filters with narrower passbands to better sample the airglow emissions and suppress the continuum scattering contribution.

The model predicted red line and green line brightness, also, only qualitatively agrees with the measurements from the imager, which is attributed to imprecise calibration and background subtraction and requires further investigation. The sky brightness in the filter containing the green line emission shows the presence of vertical banding that moves vertically as time passes. Such vertical brightness variations moving with time are also seen in the ground-based observations of the green line.

The single-day observation shows that a spectral imager can be a cost-effective solution capable of performing aeronomy experiments at twilight and nighttime from a high-altitude balloon platform.