ULF waves (geomagnetic pulsations)

spaceweb@oulu.fi - last update: 5 January 1999, 1500 UT (RR)

Introduction

Geomagnetic pulsations, i.e., ultra-low-frequency (ULF) waves cover roughly the frequency range from 1 mHz to 1 Hz, i.e., from the lowest the magnetospheric cavity can support up to the various ion gyrofrequencies. Pulsation frequency is considered to be "ultra" low when it is lower than the natural frequencies of the plasma, like plasma frequency and the ion gyrofrequency. Geomagnetic pulsations were first observed in the ground-based measurements of the 1859 Great Aurora events (Stewart, 1861).

Typical classification scheme for the ULF waves is according to the period of the pulsation (Jacobs et al., 1964):

Pulsation classes
	Continuous pulsations					Irregular pulsations
	Pc 1	Pc 2	Pc 3	Pc 4	Pc 5	Pi 1	Pi 2
T [s]	0.2-5	5-10	10-45	45-150	150-600	1-40	40-150
f	0.2-5 Hz	0.1-0.2 Hz	22-100 mHz	7-22 mHz	2-7 mHz	0.025-1 Hz	2-25 mHz

Pulsations are studied with in situ observations in space of both magnetic and electric fields, and with ground-based magnetometers. In order to study the generation mechanisms of these waves, several points should be studied, including

frequency characteristics, incl. harmonic structures
spatial distribution, incl. possible propagation of the waves
polarization characteristics
correlation with IMF/solar wind parameters
correlation with geomagnetic activity, e.g., phases of storms and substorms
correlation with in situ particle data

On ground, an important additional data comes from the optical auroral measurements.

Source of the higher frequency ULF waves

Continuous Pc 1-2 pulsations are locally generated at equatorial magnetosphere by EMIC instability. Some of the irregular pulsations are actually seen only on ground, as they are nothing more than fluctuations of the ionospheric current system above the station (they cannot travel into the magnetosphere, or even duct across magnetic field lines within the ionospheric waveguide). Many other irregular pulsations are cavity modes, either within the ionosphere (see IAR) or inner magnetosphere (see cavity modes). They can even be propagating ("proper") magnetospheric waves.

Source of the lower frequency ULF waves

At lower frequencies continuous pulsations sharing the same frequency range may differ, e.g., in their polarization, harmonic structure, or occurrence region, and have thus different origin. They can be divided according to whether they result from a local wave-particle instability or from coupling of wave energy propagating through the magnetophere and produced either in the solar wind / magnetosheath or at the magnetopause / boundary layer (all referred to as "upstream" below). Latter type relate to resonances of field lines and magnetospheric cavity. One can categorize these waves in the following way (Anderson, 1993, 1994):

Category	Location	Source	Note
Compressional Pc 3	dayside	upstream	relate to wave-particle interaction in the foreshock and shock
Toroidal Pc 3 or multi-harmonics	dayside	upstream	field line resonance harmonics in Pc 3 / Pc 4 range, compressional Pc3 as a driver (coupling with the fundamental toroidal mode)
Poloidal Pc 4	afternoon evening	local	related to injections of energetic plasma and subsequent low activity or convection electric field; occurring at the second harmonic field line resonance frequency
Compressional Pc 5	nightside dawn and dusk	local	related to of high beta plasma (ion injections)
Toroidal Pc 5	dawn and dusk flanks	upstream	fundamental mode field line resonances; source in the flanks
Incoherent noise	everywhere	unknown	increases with magnetic activity

For upstream generated waves, one important task is to establish linkage between waves external and internal to the magnetosphere. The question is, what are the sites of transfer of external to internal wave energy. One model would have compressional oscillations in the foreshock (see bow shock ) propagate directly through the shock, sheath, and subsolar magnetopause into the lower magnetosphere, while the other would have waves entering along cusp /cleft/boundary layer field lines, transferring to the interior dayside magnetosphere via an ionospheric process.

Note that actually all pulsations, ultimately, derive their energy from the solar wind. This is because the particles taking part in the local instabilities are energized via the convection electric field imposed by the solar wind. Similarly the precipitating particles and impinging electric field in the ionosphere are solar wind powered.

References

Anderson, B. J., Statistical studies of Pc 3-5 pulsations and their relevance for possible source mechanisms of ULF waves, Ann. Geophysicae, 11, 128-143, 1993.
Anderson, B. J., An overview of spacecraft observations of 10 s to 600 s period magnetic pulsations in the Earth's magnetosphere, in Solar Wind Sources of Magnetospheric Ultra-Low-Frequency Waves, eds. M. J. Engebretson, K. Takahashi, and M. Scholer, AGU Geophysical Monograph 81, 25-43, 1994.
Jacobs, J. A., Y. Kato, S. Matsushita, and V. A. Troitskaya, Classification of geomagnetic micropulsations, J. Geophys. Res., 69, 180-, 1964.
Stewart, B, On the great magnetic disturbance which extended from August 28 to September 7, 1859, as recorded by photography at the Kew Observatory, Philos. Trans. R. Soc. London, 423, 1861.