Observational Evidence on a Class of Azimuthal Dipolarizations in the Dipole-Tail Transition Region (5-12 Re) of the Earth's Magnetosphere during Geomagnetic Storms
Abstract
Dipolarizations within geosynchronous orbit received renewed interest after the launch of the Van Allen Probes (RBSP). Recently, based on two-spacecraft timing using the RBSP and GOES spacecraft, Ohtani et al (2018) showed in a statistical study that the phase speeds of the dipolarization in this region are 50-60 km/s and azimuthally away from the midnight. These results motivate further questions about the macroscopic properties of the dipolarizations. For example, how long do individual dipolarizations persist, and how far in magnetic local time (MLT) do they propagate while maintaining structural coherence? What is the 2-dimensional shape of the dipolarization, and how are the propagation and evolution of dipolarization related to Poynting fluxes and auroral activities? To answer these questions, arrays of up to 6 spacecraft are used to track the motion of dipolarizations in the dipole-tail transition region (5-12 Re) of the earth's magnetotail. Three-spacecraft timing is used to unambiguously determine the dipolarization normal and its normal speed. In this report, we show events when dipolarizations were observed over 6 hr of MLT away from the midnight during the course of 0.5 hr. A quantitative analysis on the time lags reveals evidence for a class of dipolarizations that propagate azimuthally from the local midnight through either dusk or dawn and extend from 5 to 11 Re in radial distances. The shape of the dipolarizations are finger-like: the main gradients are 1-4 Re wide in azimuth and 6 Re long in radius. The motion of the dipolarization along the normal is primarily azimuthal, 1-5 deg/min in terms of the angular speed, which corresponds to 10-50 km/s at the geosynchronous orbit. The dipolarizations are associated with strong earthward Poynting flux flowing along magnetic field lines conjugate with auroral expansion. The dipolarizations are also associated with strong gradients of the magnetic field, which provides an important candidate mechanism in transporting keV-MeV particles radially through the gradient B drift.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMSM13D3322T
- Keywords:
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- 2431 Ionosphere/magnetosphere interactions;
- IONOSPHERE;
- 2740 Magnetospheric configuration and dynamics;
- MAGNETOSPHERIC PHYSICS;
- 2756 Planetary magnetospheres;
- MAGNETOSPHERIC PHYSICS;
- 7524 Magnetic fields;
- SOLAR PHYSICS;
- ASTROPHYSICS;
- AND ASTRONOMY