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Upstream waves

Diurnal pulsations in the Pc3-Pc4 frequency range can be originated from the penetration into the magnetosphere of fluctuations (known as upstream waves) generated in the foreshock region by wave-particle interaction phenomena by solar wind protons, reflected by the bow shock and backward propagating along the interplanetary magnetic field lines (Figure 1). 

Figure 1 Schematic view, in the equatorial plane, of upstream waves generated along interplanetary magnetic field lines, indicated by the 45° inclined lines (from Formisano, in Correlated Interplanetary and Magnetospheric Observations, edited by D.E. Page, D. Reidel, 1974).

Waves in the foreshock region can be amplified by a cyclotron resonant mechanism, which takes place if the wave frequency matches the proton cyclotron frequency, so there is linear relation between the wave frequency f and the interplanetary magnetic field strength B, such as f (mHz) ≈ 6 B (nT) (Figure 2).


Figure 2 Dependence of the frequency of Pc3 pulsations observed at the Italian geomagnetic observatory at Terra Nova Bay (Antarctica, TNB)  on the interplanetary magnetic field strength.

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