Geomagnetic pulsations or micropulsations (1 mHz to 5 Hz)
The ULF waves of Magnetospheric science
Magnetospheric science defines ULF (Ultra Low Frequency) waves differently from ITU: waves with a frequency that is lower than that of the plasma (ion and electron plasma frequency)
Geomagnetic pulsations are distinguished in continuous pulsations (Pc) and irregular pulsations (Pi)
They are modelled with Magnetohydrodynamic (MHD) equations
OBSERVATIONS OF Pi2 PULSATIONS AT LOW LATITUDES
"The pulsations most commonly observed during local nighttime are Pi2 pulsations, which are impulsive, damped oscillations of the geomagnetic field in the frequency range 5-30 mHz and with
amplitudes in the range 0.25-2.5 nT. The braking of high-speed ion flows in the near-Earth central plasma sheet, at the boundary between regions of dipolar and tail-like field, produce the substorm
current wedge and compressional pulses, which lead to Pi2 pulsations at high and low latitudes respectively (*)."
"At high latitudes, Pi2 pulsations are shear Alfvèn waves associated with the “switch on” of the substorm current wedge (*) and are observed only close to local midnight. At low latitudes Pi2
pulsations are due to cavity mode resonances (*). At low latitude ground stations, they are observed at all local times at night and also often observed during local daytime (*)."
OBSERVATIONS OF Pc3 PULSATION FIELD LINE RESONANCES
"The geomagnetic pulsations most commonly observed at low to middle latitude stations, such as Hermanus, during local daytime are Pc3 and Pc4 quasi-sinusoidal continuous pulsations. The
frequency of oscillation is generally in the range 25-100 mHz, and amplitudes typically range from 0.1-1.0 nT."
"The dominant characteristics of these pulsations are consistent with those expected of field line resonances (FLRs), which are transverse standing Alfvén waves along geomagnetic field lines,
that is, equivalent to the concept of a vibrating field line fixed between the ionospheres in opposite hemispheres."
"Baransky et al. (1985) initially proposed a method for the direct measurement of the eigenfrequency of magnetic field lines using ground-based magnetometer data. They demonstrated that either
the difference or ratio of Pc3-4 pulsation amplitude spectra observed at two closely spaced meridianal ground stations can be used to determine the eigenfrequency associated with the field lines
between the two stations."
"Waters et al. (1991) proposed a more reliable technique of determining the presence of a field line resonance (FLR) by the use of the cross-phase spectrum. With this method, the peak in the
phase difference of the H-components from two closely spaced stations identifies the resonant frequency."
Geomagnetic pulsation monitors operated by the British Geological Survey
The British Geological Survey operates:
▪️ a broadband pulsation monitor (Figure)
"Synchronization of Human Autonomic Nervous System Rhythms with Geomagnetic Activity in Human Subjects"
Similar more recent paper from the same group of authors indicating that daily Autonomic Nervous System (ANS) activity responds to changes in geomagnetic and solar activity:
HRV is used as an indicator of ANS function and dynamics
Note: Mood changes and fatigue may be associated to geomagnetic pulsations