Fine-scale auroral structure: Causes and effects
Lead Research Organisation:
British Antarctic Survey
Abstract
The beautiful displays of the Earth's auroras are formed as a result of the interaction between the solar wind, the Earth's magnetic field and the atmosphere. Electrically charged particles are funnelled into the atmosphere, where they hit atoms and molecules in the atmosphere. Some of the energy transferred in these collisions is given off as light, which is what provides the auroral displays.
The aurora is fascinatingly rich in structure (down to horizontal scale sizes of tens of metres), with fast motions and changes of colour. This structure is indicative of rapid changes in the way that energy is deposited into the upper atmosphere. The cause of structuring at such fine scales is unclear, and understanding the cause is one of the objectives of this research. We will investigate the structuring processes in the aurora by using very high resolution auroral cameras which the University of Southampton operates in the Arctic, and comparing these observations with state-of-the-art simulations. The cameras are called ASK (Auroral Structure and Kinetics), and by providing high time and spatial resolution images of the aurora in a frame approximately 5x5 km (at 100 km altitude), we can image the aurora down to a resolution of ~10m. ASK consists of three cameras which provide the same image at different wavelengths. By comparing the images at different wavelengths, we can glean much information about the energy of the precipitating particles which are stimulating the aurora, and calculate information about the local fine-scale ionospheric flows. These parameters are then used as input to theoretical models, which allow us to determine the processes which give rise to auroral structuring.
In the second half of the project we will also exploit the first data from a new state-of-the-art radar system called EISCAT_3D, which provides observations of the radar analogues of the auroral structure we will examine on a spatial scale approaching that of ASK itself. EISCAT_3D is a major new European facility which is currently being built; its 'first light' will occur in 2022.
The structuring of the aurora indicates underlying structuring of electric fields and currents which have a number of important consequences, such as causing changes to the rate at which the upper atmosphere is heated, which in turn has implications for modelling of climate change and prediction of satellite orbit decay. Therefore, the spatial and temporal variability of precipitating charged particles is at the heart of the physics of the behaviour of the upper atmosphere.
The aurora is fascinatingly rich in structure (down to horizontal scale sizes of tens of metres), with fast motions and changes of colour. This structure is indicative of rapid changes in the way that energy is deposited into the upper atmosphere. The cause of structuring at such fine scales is unclear, and understanding the cause is one of the objectives of this research. We will investigate the structuring processes in the aurora by using very high resolution auroral cameras which the University of Southampton operates in the Arctic, and comparing these observations with state-of-the-art simulations. The cameras are called ASK (Auroral Structure and Kinetics), and by providing high time and spatial resolution images of the aurora in a frame approximately 5x5 km (at 100 km altitude), we can image the aurora down to a resolution of ~10m. ASK consists of three cameras which provide the same image at different wavelengths. By comparing the images at different wavelengths, we can glean much information about the energy of the precipitating particles which are stimulating the aurora, and calculate information about the local fine-scale ionospheric flows. These parameters are then used as input to theoretical models, which allow us to determine the processes which give rise to auroral structuring.
In the second half of the project we will also exploit the first data from a new state-of-the-art radar system called EISCAT_3D, which provides observations of the radar analogues of the auroral structure we will examine on a spatial scale approaching that of ASK itself. EISCAT_3D is a major new European facility which is currently being built; its 'first light' will occur in 2022.
The structuring of the aurora indicates underlying structuring of electric fields and currents which have a number of important consequences, such as causing changes to the rate at which the upper atmosphere is heated, which in turn has implications for modelling of climate change and prediction of satellite orbit decay. Therefore, the spatial and temporal variability of precipitating charged particles is at the heart of the physics of the behaviour of the upper atmosphere.
Organisations
People |
ORCID iD |
| Mervyn Freeman (Principal Investigator) |
| Description | The two main research results are: 1. The structure of fine-scale auroral emissions measured by the ASK camera in the Arctic were analysed statistically. The statistical distribution of image brightness was found generally to be composed of two or more overlapping distributions, which provides a new technique for objectively separating images into different component structures for analysis of their morphologies and dynamics. The distributions were found to be of normal or lognormal form and physical reasons for these distributions were proposed and examined, including field-aligned electron acceleration and turbulence. 2. A new method of classifying auroral images was devised and analysed based on simply the compression ratio of the zipped image file, which is a measure of the information entropy or complexity of the image. By examining the statistical distribution of compression ratios, it was found that the method could be used to readily separate out the relatively rare physically interesting images without directly previewing lots of uninteresting images. The objective information entropy classification was also compared with different classes of auroral morphology that had been previously subjectively identified by eye in a citizen science project. These results are being prepared for two separate publications. |
| Exploitation Route | The information entropy method may be suitable for classifying a wide range of scientific data, which may help identify and understand different physical regimes and make the selection of data for scientific analysis more efficient. |
| Sectors | Environment |