Propagation Properties of Electromagnetic Ion Cyclotron Waves near the Plasmaspause

It is generally considered that electromagnetic ion cyclotron (EMIC) waves seen on the ground as Pc1 ultra low frequency (ULF) waves (0.2-5Hz) originate in the equatorial region of the magnetosphere with many showing a repetitive periodicity, relating to wave packets bouncing between conjugate ionospheres; the so-called “pearl pulsations”. This mechanism has recently been questioned and there are now results that suggest the Pc1 repetitive periods may due to the generating ion cyclotron instability being modulated by long period ULF wave in Pc3-5 bands. To investigate this and other wave properties, three typical EMIC wave events with frequencies in the range 1.5-2.7 Hz have been selected from Cluster four satellite data when the satellites were near the equatorial plasmapause, passing through perigee (L~4.3) and moving northward closely aligned with the geomagnetic field, on 2 November 2001, 30 March 2002 and 22 November 2003. The events were observed roughly symmetrically around the magnetic equatorial plane within a magnetic latitude range ~ ±15° at magnetic local times 0800, 2200 and 0730 hr respectively. The wave events were respectively associated with a moderate storm (Dst=-106nT), a reasonably quiet time and a large storm (Dst=-422nT). A wave packet fine structure is seen in the dynamic spectra of the wave activity which is mostly at frequencies above the local helium gyrofrequency, with the exception of the 30 March 2002 event which also shows wave activity in the band between the local helium and oxygen gyrofrequencies. Using magnetic and electric field wave data from the four satellites shows the waves are highly polarized with the degree of polarization larger than 0.6, and mainly left-hand or linear, Only at a few specific times during the 2 November 2001 event is right-hand polarization seen. Poynting flux vectors dominantly point away from the equatorial region, roughly along field lines at angles smaller than 30° with respect to the geomagnetic field, while wave normal angles calculated by the minimum variance method show similar low angles. Additional analysis concentrates on the similarities and differences between the wave properties and propagation characteristics seen at the four satellites in relation to their spatial separation and the local plasma environment, particularly with respect to the plasmapause.