SpaceWeb@SkyNet.oulu.fi - last update: 25 July 1997, 1200 UT (RR)
Compressional Pc 3 are daytime, relatively low amplitude (a few nT) fluctuations of field amplitude with typical periods of 20-30 s (ULF range). They are polarized predominantly along the field lines, and strongest magnetospheric waves are seen in an island near the average plasmapause in the late morning. There is close correspondence between waves at geostationary orbit and on the ground (both at low and high latitudes). Most importantly, these waves correlate with the interplanetary magnetic field (IMF) . The correlations with the IMF cone angle suggest that the source population of waves is associated with the quasi-parallel region of the bow shock (the magnetopause is exposed to quasi-parallel region of the shock only for sufficiently radial IMF). The correlation of compressional Pc 3 frequency with IMF magnitude (f[Hz]=0.006*B [nT]) results from particle-wave interactions at local cyclotron frequencies in the solar wind as the protons are back-streaming from the quasi-parallel shock into the upstream solar wind . Compressional Pc 3 waves are thus most likely a direct result of compressional energy derived from wave-particle interaction in the foreshock and shock, impinging on the magnetopause and launching compressional fast mode waves to propagate through magnetospheric cavity. See, for example, Anderson (1994).
On the dayside magnetosphere, azimuthally polarized toroidal multi-harmonic pulsations occur commonly with up to 6 harmonics simultanously present over a range of L shells. Because the most prominent harmonics occur in 20-40 s period range, they are called toroidal Pc 3 (however, lower frequencies at Pc 4 range are also present). At geostationary orbit they are the most commonly observed coherent pulsations, and their frequency depends on mass density on the field line and the L-shell. For example, the effect of the plasmapause can be seen, the period being longer inside. In occurrence, they show many similarities with the compressional Pc 3 waves, and are thus most likely also associated with a quasi-parallel shock geometry. The two types of Pc 3 may represent different steps in the same energy flow, or they may reflect different flow pathways (Anderson, 1994). Note, however, that lately the upstream source has been questioned for some lower frequency events that resemble Pc 5 pulsations (Ziesolleck et al., 1997).
The toroidal Pc 3's can be attributed to coupling between compressional Pc 3 and field line resonances, since continuum of toroidal resonances are readily excited by broadband compressional power. It is also possible that an ionospheric link exists (Engebretson, 1991). Pc 3 frequency power can gain access to the ionosphere in the cusp region, where also Pc 3 range fluctuations in the auroral electron precipitation have been seen. These fluctuations, attributed to the field-aligned currents associated with sheath turbulence typical of quasi-parallel shock geometry, drive multi-harmonic pulsations through ionospheric coupling of region 1 and 2 current systems.