Characteristic time constants and velocities of mid-latitude Pi 2's

Details Characteristic time constants and velocities of mid-latitude Pi 2's Characteristic time constants and velocities of mid-latitude Pi 2's

Apr 1985

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985jgr....90.3448s&link_type=abstract

Journal of Geophysical Research (ISSN 0148-0227), vol. 90, April 1, 1985, p. 3448-3456. Research supported by the Natural Scienc

Physics

Geophysics

7

Auroral Electrojets, Geomagnetic Micropulsations, Midlatitude Atmosphere, Phase Velocity, Polar Substorms, Time Constant, Field Aligned Currents, Magnetic Signatures, Magnetic Storms, Magnetohydrodynamic Waves, Polarization (Waves)

Scientific paper

This study presents measurements of group delays, rise times, fall times, frequencies, and effective phase velocities of mid-latitude (39 deg-55 deg geomagnetic latitude) Pi 2's recorded by the Air Force Geophysics Laboratory array of flux gate magnetometers. Measured group delays over the array are sometimes less than 5-10 s, indicating that in some instances Pi 2's can be used to give excellent timing of the onset of substorm expansive phases. Rise and fall times for the envelopes of Pi 2 oscillations are typically 60-100 s, bringing into doubt resonance theories for these types of waves. Most phase velocities indicate westward propagation, particularly near the center of the substorm current wedge, although there is some indication of eastward velocities far to the east of the center. It is argued that these observations indicate that the magnetic fields associated with mid-latitude Pi 2's are due to transient changes in the field-aligned currents feeding the auroral electrojets at the beginning of the expansive phase of the substorm.

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