Some properties of Alfvén waves: Observations in the tail lobes and the plasma sheet boundary layer

Abstract

We report properties of substorm-related, globally excited Alfvén waves on a temporal scale of 6 to 300 s (3.3 to 167 mHz) at geocentric distances between 5 and 6 RE. The waves were observed in the tail lobes and the plasma sheet boundary layer (PSBL) by the Polar satellite. In each region we made the following observations: (1) The tail lobe Alfvén waves started at substorm onset as determined from ground magnetometer data. Hence these ULF lobe waves can possibly be used as a new substorm indicator. Although on open field lines, they often showed local standing wave signatures with a large perpendicular scale size and a near-zero net Poynting flux. We do not classify those waves as FLR but interpret them as the superposition of incident and reflected waves. The same oscillations were simultaneously recorded in ground magnetometer data. Immediately poleward of the PSBL, the lobe Alfvén waves traveled earthward (no reflection), suggesting their dissipation in the ionosphere. The lobe waves were superimposed on the signature of a field-aligned current (FAC). The onset of this FAC was simultaneous to the onset of the magnetic substorm bay. (2) The substorm-related PSBL Alfvén waves carried two to three orders of magnitude larger Poynting flux (∼1 erg cm -2 s-1) than the lobe Alfvén waves. These PSBL waves were a mixture of standing and traveling Alfvén waves for different frequency ranges. Most Poynting flux was carried in large-scale earthward traveling waves (40-300 s). For one event, we also measured large standing wave components (>0.5 erg cm-2 s-1), but such events are rare. In the intermediate range (40-67 s), which overlaps with the Pi2 range, some waves showed clear standing wave signatures. At smaller periods (6-24 s), noninterfering earthward and tailward traveling waves were present with small Poynting fluxes (<0.05 erg cm-2 s-1). A trend for increasing E to B ratios with increasing wave frequency was observed. The PSBL waves were left-hand elliptically polarized. The wave vector was within 35° of the background magnetic field direction, suggesting that the waves were phase-mixed. The large-amplitude, substorm-related PSBL Alfvén wave events (∼1 erg cm-2 s-1) were found in regions of upward currents. Copyright 2005 by the American Geophysical Union.