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Magnetic Pulsations

Geomagnetic pulsations are magnetohydrodynamic waves  within the Earth’s magnetosphere. They can be of two types: the magnetosonic wave and the Alfven wave. Magnetosonic waves propagate in any direction and generate compressions and rarefactions of both the magnetic field and the plasma; Alfven waves propagate along the direction of the ambient magnetic field and produce magnetic perturbations transversal to the field lines. Compressive MHD waves propagating in the magnetosphere, if their frequency matches the field line eigenfrequency, can drive a resonance mechanism. In this situation, standing oscillations of the geomagnetic field lines, which behave as strings with the ends fixed in the ionosphere, occur (Figure 1). 

Figure 1 Schematic plot in the meridian plane of compressive MHD waves within the magnetosphere together with transverse standing waves along geomagnetic field lines.

The period of geomagnetic pulsations ranges from tenths of seconds to few minutes. As shown in the Table, pulsations are classified, according to their morphological properties, into continuous pulsations (Pc) and irregular pulsations (Pi); within each of these two groups they are further divided according to their period. This classification was proposed by IAGA (International Association for Geomagnetism and Aeronomy) in 1964; it is important however to underline that the frequency values in the different pulsation classes don’t reflect a particular physical meaning, then such a division can be considered just a useful classification scheme. 

IAGA classification of geomagnetic pulsations
Continuous pulsations Irregolar pulsations
Notation Period(s) Frequency(mHz) Notation Period(s) Frequency(mHz)
Pc1 0.2 - 5 200 - 5000 Pi1  1 - 40 25 - 1000
Pc2 5 - 10 100 - 200 Pi2  40 - 150 7 - 25
Pc3 10 - 45 22 - 100      
Pc4 45 - 150 7 - 22      
Pc5 150 - 600 2 - 7      

On the ground the pulsation amplitude ranges from tenths of nT to hundreds of nT (i.e. can reach few thousandths of the ambient magnetic field, whose intensity is around 30.000 nT at the equator and 60.000 at the poles), and generally increases for increasing period (Figure 2) and magnetic latitude. 

Figure 2 Sketch of the amplitude spectrum of geomagnetic pulsations as a function of frequency.
Amplitudes depicted are typical values observed at mid latitudes, during periods of moderate geomagnetic activity.

Low frequency geomagnetic pulsations are of external origin, in that they are due to the interaction between the solar wind and the magnetosphere. Generation mechanisms of Pc3-Pc5 pulsations can be basically related to the solar wind flow, which can trigger surface waves along the flanks of the magnetopause (Kelvin-Helmholtz instability), to solar wind pressure pulses impacting the magnetopause, which can generate magnetospheric standing oscillations, and to the penetration into the magnetosphere of fluctuations generated in the foreshock region (upstream waves). Lastly, the occurrence of magnetic substorms can trigger, in the nightside magnetosphere, irregular pulsations (Pi2), interpreted at high latitude as stading Alfven waves and at low latitude as oscillation modes of the plasmaphere.
Geomagnetic pulsations can be considered a useful tool for a diagnosis of the magnetosphere and of its interaction with the solar wind. For instance, Pi2 pulsations recorded at low latitudes can indicate the occurrence of magnetic substorms, while the study of Pc3 pulsations can be used as a diagnostic for determining plasma density along geomagnetic field lines as well as plasmapause temporal variations.

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