Multiple intensifications inside the auroral bulge and their association with plasma sheet activities
Authors: Keiling A. , Angelopoulos V. , Larson D. , McFadden J. , Carlson C. , Fillingim M. , Parks G. , Frey S. , Glassmeier K.‐H. , Auster H. U. , Magnes W. , Liu W. , Li X. 
Published: December 2008

In this coordinated ground and space study, we report multiprobe measurements from Time History of Events and Macroscale Interactions during Substorms (THEMIS), LANL‐97A, Polar, and ground observatories for a substorm that occurred on 23 March 2007. The THEMIS fleet and LANL‐97A were located in the premidnight, near‐Earth plasma sheet in the radial range from 6.6 to 13 RE, placing the spacecraft into different plasma environments which were subject to different activities. Simultaneous global Polar Ultraviolet Imager images of the aurora revealed a fine structure in the auroral bulge in the form of several time‐delayed regions of brightening. We demonstrate a correspondence between this fine structure and the spatially separated plasma sheet activities (substorm injections with energies >100 keV) by showing that both executed periodic (100–150 s) one‐to‐one correlated modulations. Additionally, the different auroral brightening regions were modulated approximately out of phase to one another, as were the separated plasma sheet activities. The periodic plasma sheet and optical modulations were also one‐to‐one correlated with large‐amplitude (δH ∼ 150 nT) ground Pi2 pulsations. In contrast to the most energetic ions (>100 keV), the lower‐energetic plasma sheet ions executed separate oscillations during the development of the substorm, including the preintensification phase, and showed the following properties. (1) The oscillation periods were different at different spacecraft locations and had a tendency to increase during the evolution of the substorm. During the preintensification phase, multiple (possibly harmonic) spectral components existed. (2) The oscillations were coupled to westward moving perturbations of an energized plasma boundary. The boundary perturbations were likely conjugate to azimuthally spaced auroral forms (“beads”) observed by Polar‐UVI during the preintensification phase and could play a role in the onset of the substorm intensification. (3) The oscillations of the lower‐energetic ions were also one‐to‐one correlated with smaller‐amplitude ground Pi2 pulsations (<15 nT). In conclusion, the combination of these observations allowed us to construct a 3‐D picture of low‐frequency, near‐Earth plasma sheet phenomena associated with a substorm and their connection to aurora and the ground. It appeared that not only one substorm current wedge, but additional current structures existed which started at different times, pulsated out of phase, and mapped from different active regions into the ionosphere. The active space regions appeared to be coupled and transferring energy from one region to the other while pulsating. We propose that the wave‐like structures in the plasma sheet, observed before and during the substorm/intensification phase, and their demonstrated properties support a wave phenomenon (such as a ballooning‐type mode) for the onset and development of the substorm/intensification, rather than directly driven periodic bursty bulk flow activations.