Physics
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufmsm52b..01k&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #SM52B-01
Physics
2788 Storms And Substorms
Scientific paper
Pi2 pulsations (T 40-150 s) are irregular magnetic field oscillations observed to occur near substorm onset. In this talk we present the results of studies exploring the generation of Pi2 pulsations by transient magnetotail flow bursts. We find that waveforms of low-latitude Pi2 match the variations in flow velocity measured in the middle magnetotail with a 1-3 minute delay. This correlation had not been previously reported and identifies the intrinsic periodicity in flow bursts as the source of low-latitude Pi2. Pi2 generation is related to the types of impulses generated when a flow reaches the transition region between stretched tail and dipolar field lines. The field-aligned, Alfvénic component partially reflects at the ionosphere and leads to the generation of what we refer to as transient response (TR) Pi2. The periods of these Pi2 are intrinsic to the local flux tubes and are unrelated to the periodicity of flow enhancements. Transient currents caused by flow deceleration are also generated and we term the pulsations that result inertial current (IC) Pi2. An impulsive, cross-field, compressional wave is also produced by the braking of the flow. How this signal generates low-latitude Pi2 was unknown prior to this work. We offer compelling evidence that the impulsive signal travels sunward and perturbs field lines launching Alfvén waves towards the ionosphere. The Pi2 perturbations observed on the ground are directly-driven, and the waveforms of these waves match the flow variations.
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