ULF Magnetic Pulsations Research Articles

Plasma and Electromagnetic Effects Caused by the Seismic-Related Disturbances of Electric Current in the Global Circuit

This paper is devoted to the use of an electrodynamical model for lithosphere – atmosphere – ionosphere (LAI) coupling to explain plasma and electromagnetic earthquake (EQ) precursors. Our consideration is based on the calculation results of electromagnetic perturbations and ionospheric irregularities accompanying the electric field and electric current occurring in the global atmosphere – ionosphere electric circuit. Our theoretical results are confirmed by satellite- and ground-based experimental data of plasma and electromagnetic perturbations obtained for several days before an EQ. It is shown that the growth of current in the global circuit might result in the AGW (acoustic gravity wave) instability in the ionosphere, the formation of field-aligned current and plasma irregularities, magnetic field ULF oscillations and electromagnetic ELF radiation, spectral broadening of VLF transmitter signals registered by satellites, depressions of ULF magnetic pulsations, VHF radio emissions generated in the troposphere and propagation of the signals of a VHF transmitter behind the horizon. Moreover, the generation of electric current in the global circuit is accompanied with the modification of D, E and F ionospheric layers. All of these phenomena are shown to be attributed just to a single cause; namely, the variation of conducting electric current in the global circuit by the injection of charged aerosols into the atmosphere during seismic activity.

Quasiperiodic emissions observed by the Cluster spacecraft and their association with ULF magnetic pulsations

Quasiperiodic (QP) emissions are electromagnetic waves at frequencies of about 0.5–4 kHz characterized by a periodic time modulation of the wave intensity, with a typical modulation period on the order of minutes. We present results of a survey of QP emissions observed by the Wide‐Band Data (WBD) instruments on board the Cluster spacecraft. All WBD data measured in the appropriate frequency range during the first 10 years of operation (2001–2010) at radial distances lower than 10 RE were visually inspected for the presence of QP emissions, resulting in 21 positively identified events. These are systematically analyzed, and their frequency ranges and modulation periods are determined. Moreover, a detailed wave analysis has been done for the events that were strong enough to be seen in low‐resolution Spatio‐Temporal Analysis of Field Fluctuations‐Spectrum Analyzer data. Wave vectors are found to be nearly field‐aligned in the equatorial region, but they become oblique at larger geomagnetic latitudes. This is consistent with a hypothesis of unducted propagation. ULF magnetic field pulsations were detected at the same time as QP emissions in 4 out of the 21 events. They were polarized in the plane perpendicular to the ambient magnetic field, and their frequencies roughly corresponded to the modulation period of the QP events.

Depression of the ULF geomagnetic pulsation related to ionospheric irregularities

We consider a depression in intensity of ULF magnetic pulsations, which is observed on the ground surface due to appearance of the irregularities in the ionosphere. It is supposed that oblique Alfven waves in the ULF frequency range are downgoing from the magnetosphere and the horizontal irregularities of ionospheric conductivity are created by upgoing atmospheric gravity waves from seismic source. Unlike the companion paper by Molchanov et al. (2003), we used a simple model of the ionospheric layer but took into consideration the lateral inhomogeneity of the perturbation region in the ionosphere. It is shown that ULF intensity could be essentially decreased for frequencies f = 0.001-0.1 Hz at nighttime but the change is negligible at daytime in coincidence with observational results.

Variations of phase velocity and gradient values of ULF geomagnetic disturbances connected with the Izu strong earthquakes

Abstract. Results of study of anomaly behavior of amplitudes, phase velocities and gradients of ULF electromagnetic disturbances (F = 0.002 – 0.5 Hz) before and during a seismic active period are presented. Investigations were carried out in Japan (Izu and Chiba peninsulas) by two groups of magnetic stations spaced apart at a distance ~ 140 km. Every group (magnetic gradientometer) consists of three 3-component high sensitive magnetic stations arranged in a triangle and spaced apart at distance 4–7 km. Kakioka magnetic station (>200 km to the North from Izu) was used as a reference point. Available data (only night-time intervals 00:00–07:00 LT) were preliminary filtrated by narrow pass-band filters (16 frequency bands – periods T = 2–512 s). The amplitude, gradient and phase velocity values and probabilities of directions of gradient and phase velocity vectors were constructed for the every frequency band. Apparent resistivities of the Earth’s crust in the every frequency band were calculated using the phase velocity values. It was found that Z component amplitudes of the ULF magnetic disturbances increased at Izu peninsula 2–4 weeks before the seismic active period and 2–4 days before the strongest seismic shocks (M > 6). Ratio of Z component amplitudes of Kamo (Izu) and Kakioka data (Zk/Zkk) increased during 2–4 weeks before the seismic activity start (27 June 2000) and reached a maximum just before a moment of the strongest seismic shock (EQ with M = 6.4). The gradient and the phase velocity values had an anomaly behavior during the same 2–4 weeks before the start of seismic active period. The gradient vectors of the total horizontal component of the ULF magnetic pulsations were probably directed to the regions with increased conductivity. New additional direction of the gradient vectors appeared 2–3 weeks before the seismic activity start – the direction to the seismic active area which appeared due to a magma rising.

Dynamics of long-period magnetic activity and energetic particle precipitation during the May 15, 1997 storm

The large-scale behavior of quasi-steady magnetic activity and energetic particle precipitation and their variations in the nominal Pc5-6/Pi3 band (1.7– 10 mHz ) during the main phase of the intense magnetic storm on May 15, 1997, is studied using data from an array of magnetic and riometer stations in the Northern Hemisphere. The global azimuthal structure of magnetic and cosmic noise absorption disturbances at ∼64° geomagnetic latitude is revealed with the help of MLT-UT diagrams. The ionospheric westward electrojet intensifies in the morning sector and the eastward electrojet intensifies in the evening sector. Intense storm-related ULF magnetic activity is evident in two regions: in the early morning hours and in the evening sector. In the dawn sector, the westward electrojet intensification and ULF magnetic pulsations are accompanied by concurrent variations of cosmic noise absorption. In the dusk sector, where observations at two magnetic stations with small separations (<10°) are available, the data indicate small-scale structure of storm-related Pi3 pulsations with longitudinally sunward propagation, as might be expected according to the injected proton drift. The small-scale meridional pattern of ULF-modulated electron precipitation during the substorm interval as determined from the IRIS riometer in Scandinavia indicate poleward propagation in the dawn sector. The results suggest that the physical mechanism of long-period magnetic disturbances observed during the main phase of a magnetic storm is different from common dawn Pc5 pulsations and is related to the injection of energetic electrons and protons into the early morning and dusk sectors of magnetosphere, correspondingly. This difference is important for the modeling of relativistic electron energization by ULF waves.

Ionospheric Manifestation of Sc-Associated Magnetic Pulsations in the HF Doppler Frequency Shifts

Ionospheric manifestation of ssc-associated ULF magnetic pulsations is investigated by analyzing the simultaneous data of HF Doppler frequency shift and magnetic H component on the ground. The theoretical Doppler velocities associated with magnetic pulsations are also calculated using the method of Sutcliffe and Poole (1989). Comparison of the observed phase difference between the ionospheric and magnetic pulsations with the one by theory reveals that in several of the pulsation events the ionospheric response can be controlled by the advection and/or compression mechanisms attributable to the pulsation EM fields.

Mechanism of parallel current variations. Theory of ULF magnetic pulsations at balloon altitude during substorms

A mechanism of parrallel current variations is based on a model of transverse Alfvèn wave packets which close the parallel current variations by polarising the magnetospheric plasma. When an incident Alfvèn wave reflects off the ionosphere, it gives rise to a reflected wave packet and ionospheric Hall currents. The reflected Alfvèn wave packet closes a parallel current variation which adds to the current variation closed by the incident wave packet. Balloon observations are expected to be advantageous because of the small ionospheric screening of the ionospheric Hall magnetic signatures at the balloon altitude.

Magnetometer array for cusp and cleft studies observations of the spatial extent of broadband ULF magnetic pulsations at cusp/cleft latitudes

We have used magnetometer data from 10 locations in Arctic Canada and Greenland, covering over 5 hours in magnetic local time at magnetic latitudes from 75° to 79°, to characterize the dayside patterns of enhanced long‐period ULF (10‐ to 600‐s period) wave power at cusp/cleft latitudes. We conclude the following: (1) In agreement with earlier single‐station studies, we find that the most common wave type is broadband noise (Pi 1‐2). Distinct Pc 3‐4 activity and more sustained monochromatic Pc 5 activity are most apparent when this broadband noise is weak. (2) Multistation observations also make clear that strong, broadband Pi 1‐2 signals are both temporally and spatially structured: Although their amplitude is somewhat larger near local noon and near nominal cusp latitudes, they often occur simultaneously (to within a few minutes) at all stations. They are thus not local signals, and cannot be interpreted as evidence of passage of an auroral region or boundary over an individual magnetic observatory. In particular, we have found no evidence for a distinctive “cusp” signature in broadband ULF waves in this frequency range. (3) The occurrence of strong broadband Pi 1‐2 signals at these latitudes appears to be controlled largely by solar wind velocity. We found good correlations between the occurrence of strong Pi 1‐2 signals and high solar wind velocity, and we also noted some dependence on the cone angle of the interplanetary magnetic field for moderate to low solar wind velocities. We speculate that there may be an additional dependence on enhanced levels of trapped plasma in regions topologically connected to the very high latitude dayside ionosphere, such as the entry layer, high‐latitude dayside field minimum regions, or plasma mantle. Available satellite data on the level of trapped energetic electron fluxes at geosynchronous orbit showed that broadband power levels appeared to correlate with enhanced flux levels on the time scale of days, but not on shorter time scales, suggesting that any such dependence is not directly related to substorm injections.

Pc3 magnetic pulsations at Davis, Antarctica

A brief morphological study of Pc3 (10–45 s) ULF magnetic pulsations observed at the polar cusp latitude station, Davis, Antarctica (Λ ∼ 74.5°) is presented. A significant observation of these deeply amplitude-modulated near monochromatic Pc3 magnetic pulsations is their large amplitudes, i.e. ⩽ 32 nT, which may provide a ground diagnostic for the location of the last closed magnetic field lines, adjacent to the dayside polar cusp. Observational analyses include both diurnal and seasonal amplitude and occurrence distributions, power spectral analyses, dynamic sonagrams and polarization studies.

Greater than 0.1‐Hz ULF magnetic pulsations measured at South Pole, Antarctica

South Pole pulsation activity at frequencies greater than 0.1‐Hz is analyzed in this study and found to consist predominantly of large‐amplitude, irregular pulsations (Pi 1) closely correlated with overhead auroral activity. These pulsations therefore have two diurnal maxima related to the substorm polar expansion nightside aurora and cusp or dayside aurora. The dependence of this magnetic nightside Pi activity at South Pole on Kp and interplanetary magnetic field parameters is consistent with a strong substorm correlation. As found at lower latitudes, when pulsating aurora is evident in the photometric data there can be a good detailed correlation between the auroral luminosity fluctuations and the magnetic pulsations, suggesting that the pulsations are locally produced by overhead currents driven by the particle precipitation. Nightside Pi activity is often of larger‐amplitude than the dayside Pi and appears in bursts of pulsations having 5 to 20 minsduration. The burst‐like character arises because of the rapid poleward expansion of discrete auroral forms from the nightside auroral oval, as is evident in the South Pole photometric data. South Pole nightside auroral magnetic pulsations generally commence well after the onset of similar auroral magnetic pulsations recorded at Siple, Antarctica, a subauroral location. One third of the hours during which structured, band‐limited pulsations (Pc 1) are observed at the low‐latitude, Siple, Antarctica, location also contain Pc 1 observed at South Pole. When it is possible to identify similar structures in Pc 1 between South Pole and Siple they are in phase, in contrast to out of phase structures for opposite hemisphere stations. This supports horizontal ducting of Pc 1 from lower‐latitudes to South Pole. However, there often appears much more structure at South Pole than recorded at Siple, suggesting many paths of horizontal propagation from multiple lower‐latitude sources if the signals are not generated on South Pole field lines. The pronounced inverse correlation of South Pole Pc 1 with magnetic activity means that the magnetic field topology and/or ionospheric conditions associated with substorm activity preclude the possible propagation of Pc 1 to South Pole from lower‐latitudes when the magnetic activity is high.

Ionospheric ion heating by ULF Pc 5 magnetic pulsations

Frictional heating of the ions resulting from dc ionospheric electric fields is experimentally and theoretically well known. We extend these results to ion heating due to ULF magnetic pulsations of periods as low as 3 min. Ion temperature fluctuations as measured by the European incoherent scatter facility are very well correlated to magnetic Pc 5 pulsations. We present a method which estimates these ion temperature enhancements from ion velocity measurements.