IMAGE Auroral Boundary Data

The objective of this project was to investigate whether magnetic reconnection in the space environment has a characteristic scale in space and time by characterising statistically the spatial and temporal structure of the footprint of reconnection as observed in the Earth’s ionosphere. This knowledge has the potential to provide constraints for multi-scale reconnection models. In this project the reconnection character was inferred from auroral boundary motion in conjunction with a reconnection model.

The most important achievements of the project were:

1. The development of an improved, more general method for accurately identifying ionospheric auroral boundaries in images of the polar ionosphere taken by satellite ultra-violet imagers (in particular those from the IMAGE spacecraft). In particular, the identification of the poleward auroral luminosity boundary that is a proxy for the open-closed magnetic field line boundary (the location of the ionospheric footprint of magnetic reconnection). The method was calibrated by a statistical comparison with established proxies for the boundary in particle precipitation data measured by low-altitude spacecraft (DMSP) which themselves sample the boundary only sparsely and infrequently. See Longden et al. (2010) for full details.
2. The production of a large temporal database of auroral boundary locations, including boundaries approximately every 2 minutes for about 2 years of IMAGE spacecraft data. This database allows the estimation of the total net reconnection rate in the magnetosphere from the rate of change of area enclosed by the open-closed field line boundary. This database and its associated metadata are publicly available through this site.
3. The characterisation of the poleward auroral boundary motion, across the full range of magnetic local times, on timescales from 2 minutes to several hours. A structure function analysis was used to investigate the scaling of the boundary motion (and hence, of the reconnection process; see (4) below). A fractal regime of boundary motion was identified up to a time scale of 90 minutes suggesting that the reconnection process is scale-free on time scales less than the substorm time scale and that its character varies with magnetic local time. See Longden et al. (2014) for full details.
4. The development of a theory, based on stationary solutions of the Ornstein-Uhlenbeck process, that explains the observed scaling variation with magnetic local time as well as the observed distributions of boundary motion for a full range of time scales. The theory is a mathematical and statistical development of the well-established expanding-contracting polar cap model which describes how the region of open magnetic flux in the polar cap responds to variations in both dayside and nightside magnetic reconnection.

References

Longden, N., Chisham, G., Freeman, M.P., Abel, G.A., and Sotirelis, T. (2010), Estimating the location of the open-closed magnetic field line boundary from auroral images, Ann. Geophys., 28, 1659-1678, doi:10.5194/angeo-28-1659-2010.
Longden, N., Chisham, G., and Freeman, M.P. (2014), Magnetic local time variation and scaling of poleward auroral boundary dynamics, J. Geophys Res., 119, 10006-10022, doi:10.1002/2014JA020430.