Variability of neutral temperature in the high-latitude upper atmosphere

点击翻译按钮获取中文摘要

The upper atmosphere at high latitudes is a region which is bombarded by electrons and protons, which are the source of the aurora, often seen as spectacular coloured and dynamic lights in the dark sky. The aurora over Svalbard (lat 78.2 N, lon 16.0 E) where our instruments are located, has special properties which make this an ideal place to study the upper atmosphere. The location is particularly important because it is dark during the daytime in the winter months, a special property of this most northerly site. The colour of the aurora, or wavelength of the light emitted, depends on both the energy of the incoming particles and how that energy is lost during the passage of the particles, and on the composition of the atmosphere that the particles travel through. As a result, optical measurements of specific wavelengths can provide detailed information about the atmosphere, and about the energy of the precipitating populations.This project will use an advanced design spectrograph which makes measurements over a range of different wavelengths simultaneously. One emission is from excited oxygen ions O+, which is a signature of low energy electron precipitation (typically electrons with energies of about 100 eV) and has a peak brightness at around 300 km in height. We have discovered recently (Whiter et al Ap.J 2014) that the processes that produce the O+ ion in aurora have some special properties, and as a result the emission can be used to obtain the temperature of the O atoms in the region where they emit. This temperature is known as the neutral temperature, which in the auroral region has not been easy to measure so far; this project provides an exciting new method to quantify the changes that occur during auroral energy input, and to compare these changes to modelling studies and also to existing empirical models, which are known to have large uncertainties. The neutral temperature is an important parameter for studying changes on more global scales, and although our studies are from one specific location, the data we are using has been continuous during the dark hours since 2003.Another emission that we measure is from hydroxyl molecules which are excited by ultra violet radiation. The emission is known as airglow, and is from a region around 85-90 km in height, known as the mesopause. Precise measurements of these emissions can be used to obtain the temperature of the atmosphere at these heights. Consequently, we can add these observations to those described above (from around 300 km) to determine if there are any correlations, and then try to understand what the mechanisms may be. Moving a little higher up in the atmosphere, one of the strongest emissions is from molecular nitrogen, which has a peak emission height of between 100-150 km. We have developed a "synthetic spectrum" of the emission, which is a theoretical solution of the shape of the emission spectrum. This shape is dependent on the temperature of the molecules, and so we can make a best fit of the measured spectrum to the theoretical, in order to estimate the neutral temperature at the height of the emission. In combination we therefore have the possibility of measuring the neutral temperature at three distinct heights, depending on the auroral conditions.Finally we will make use of very high resolution auroral cameras which we operate in the arctic close to the spectrograph. The ASK (Auroral Structure and Kinetics) cameras provide high time and spatial resolution (1/32 s and 10 m) images of the aurora in a frame approximately 5x5 km (at 100 km altitude). ASK consists of three cameras which provide the same image at different wavelengths which, in combination with modelling, are used to find the energy input within the auroral structure. The spatial and temporal variability of precipitating charged particles is at the heart of the physics of the behaviour of the polar upper atmosphere.