Diseño e implementación de un espectrómetro y un polarímetro para la diagnosis de plasmas de aire producidos por eventos luminosos transitorios en la mesofera terrestre

Abstract

In this work of thesis we thoroughly describe the design, development and characterization of a new diagnosis instrument intended to perform systematic campaigns of simultaneous measurement of the spectrum and polarization degree of the light emitted by transient luminous events (TLEs) as a ground support to space missions ASIM (ESA) and TARANIS (CNES) to be launched by early and late 2018, respectively. The GRASSP instrument (GRAnada Sprite Spectrograph and Polarimeter) includes the first medium{high spectrograph specifically designed for the analysis of air plasmas generated by TLEs. All versions of GRASSP have been developed and characterized by our group at the Institute of Astrophysics of Andalusia (IAA, Granada, Spain) and the Institute of Matter Structure (IEM, Madrid, Spain) laboratories, both dependencies of the National Research Council of Spain (CSIC). We have developed four different versions of GRASSP to date. We installed the first (2012 { 2014) and second (2014 { 2015) versions of GRASSP at the Spanish{German Astronomic Center (CAHA) in Calar Alto, Almería, Spain. Both worked in an autonomous way, without the support of an operator. Every sunset, the system powered on automatically, took the calibration images, opened the blinds of the system and aimed the spectrograph to the region of the sky where a TLE was more probable to appear. This could be done thanks to an aiming algorithm we developed that queried the databases of the Spanish Weather Agency in real-time and calculated the coordinates of the closest storm. When the system detected a change of the brightness level in the sky, an audio trigger system launched the simultaneous recording of both field image and spectral image to store them in a data repository. This way we could discern the origin of the recorded spectra. Every sunrise, after the observation night, the algorithm closed blinds and switched off all the GRASSP subsystems. From this location we obtained the first TLE images recorded with GRASSP, we recorded several spectra from lightning optical emissions dispersed on clouds and we had the chance of recording a meteoroid spectrum while it passed in front of GRASSP. Unfortunately, from this location we did not record any TLE spectra because of the remoteness of the storms, that occur most frequently in Spain in the Ebro Delta valley. Hence we decided to relocate our spectrograph in a new and compact version of GRASSP. We installed GRASSP versions three (2015 { 2016) and four (2016 { now) in Castellgallí, Barcelona, Spain. This last compact version is currently located within the stormiest region of north-eastern Spain, with a field of view of almost 360 degrees, and it is manually aimed by an experienced colleague (Oscar Van der Velde, from Polytechnic University of Catalonia). Since the installation of the third version of GRASSP we have recorded up to 44 medium{high resolution TLE spectra, that allowed us to quantify, for the first time, the (rotational) temperature of gas surrounding TLEs. It can be done through the spectral fitting of the recorded spectrum to synthetic spectrum that we have modelled, thanks to the high resolution of our spectrograph (0.235 nm) that allows us to fit the rovibrational bands of the nitrogen molecule. This way we can understand TLEs as natural probes of the air temperature in the Earth mesosphere. It is the first time that systematic campaigns of spectroscopic measurements of TLEs with such high resolution have been developed (the best spectral resolution to date intended to sporadically analyze TLEs spectra is 3 nm), with the goal of feeding a database to statistically characterize the TLEs from a spectroscopic point of view in a near future. The GRASSP polarimeter is currently in calibration stage within our laboratories. It is intended to be operative from summer 2017.