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 categorized as follows:

Oscillations with quasisinusoidal waveform are called pulsations continuous (Pc). Those with waveforms that are more irregular are called pulsations irregular (Pi).

They are modelled with Magnetohydrodynamic (MHD) equations.

Excerpts from McPherron et al 2014

"Magnetic micropulsation is the name originally given to low frequency oscillations of the Earth’s magnetic field. The name represents the manner in which these oscillations were discovered through observation of the end of a very long compass needle with a microscope."

"How are ULF waves created? ULF waves are created by a variety of processes in a magnetized plasma. Plasma is the name given to highly ionized gases threaded by a magnetic field. When the collision frequency in a plasma is sufficiently low, the charged particles simply gyrate around the magnetic field and travel along it. Any force that moves the charged particles also moves the magnetic field and vice versa. In this situation the field and plasma are "frozen together". This concept enabled Hannes Alfvén to describe a simple process that creates a low frequency wave that propagates along a magnetic field line. He received the Nobel Prize for this discovery and the wave he described is now called the Alfvén wave. Alfvén’s model for wave generation is summarized in Figure 3."

"Gyro resonance (or cyclotron resonance) applies to the situation where a circularly polarized wave and a charged particle rotate (gyrate) about the magnetic field at the same frequency. Because the electric field of the wave and the particle velocity maintain a constant angle with respect to each other, the field is able to exert a force on the charge for a long interval. This allows energy transfer to occur (Brice, 1964). In which direction the energy flows depends on the relative angle. In the magnetosphere cyclotron resonance between a left circularly polarized wave and protons is the mechanism that creates Pc 1 magnetic pulsations (0.5–5 Hz). This physical process is briefly described in the following paragraphs."

Excerpts from "Studies of Geomagnetic Pulsations Using Magnetometer Data from the Champ Low-Earth-Orbit Satellite and Ground-Based Stations": https://datascience.codata.org/articles/abstract/10.2481/dsj.IAGA-03/

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)
▪️ a Pi1 monitor
▪️ a Pi2 monitor

## "Synchronization of Human Autonomic Nervous System Rhythms with Geomagnetic Activity in Human Subjects"

https://www.mdpi.com/1660-4601/14/7/770/htm

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:
https://www.nature.com/articles/s41598-018-20932-x

HRV is used as an indicator of ANS function and dynamics

Note: Mood changes and fatigue may be associated to geomagnetic pulsations