We present a statistical study of substorm energy growth and release in the Earth's tail. During the growth phase of a substorm, energy is transferred from the solar wind to the magnetosphere and is then accumulated in the magnetotail lobes; this energy is then released during the substorm expansion phase. Understanding the processes of storage and release is important in order to inform studies of the total energy released in a substorm. In order to identify when a spacecraft is in the lobes we use data from the European Cluster Assimilation Technology (ECLAT) project, alongside a new identification technique which examines the strength of and fluctuations in the magnetic field in order to identify the lobes from Cluster magnetometer data.

Using this database of lobe intervals, we estimate the amount of energy contained in the lobes, with the aim of illustrating how energy is built up during the growth phase and released during the expansion phase. This analysis is performed on a statistical basis, providing an estimate against which other measures of substorm energy processes can be compared in the future.