Simultaneous Observations of EMIC Waves,ELF/VLF Waves,and Energetic Particle Precipitation during Multiple Compressions of the Magnetosphere

Geomagnetism and Aeronomy - Simultaneous observations of ELF/VLF and EMIC waves from Van Allen Probe satellites in the daytime Earth’s magnetosphere and on the ground during multiple...     

Quasi-periodic very low frequency emissions, very low frequency chorus, and geomagnetic Pc4 pulsations (Event on April 3, 2011)

The results of an analysis of ground-based observations of very low frequency (VLF) emissions in Scandinavia (L ∼ 5) in April 2011 are discussed. A detailed study is conducted of an non-typical event (April 3, 2011) of simultaneous generation of VLF chorus at frequencies below 3 kHz and quasi-periodic VLF emissions (QP) in the band of 4–6 kHz, which were not discrete emissions but consisted of separate short (about 20 s) bursts of hiss. It is shown that these emissions were mainly characterized by right-hand polarization, which indicates the location of the exit point of waves from the ionosphere near the point of ground observations. Based on an analysis of the spectral characteristics of emissions, it is concluded that the generation regions of chorus and QP emissions were located at different L shells. The appearance of QP emissions coincided with the excitation of resonance geomagnetic pulsations of the Pc4 range in the magnetosphere with a period that was close to the quasi-period of repetition of spectral forms in QP emissions. However, based on the available data, it is not possible to conclude that these geomagnetic pulsations caused the quasi-periodic generation of bursts of VLF hiss. The time shift between the peaks of QP and geomagnetic pulsations was inconsistent and varied from one burst of hiss to another. It is suggested that the discussed QP emissions were a result of the development of self-oscillations in the Earth’s radiation belts.     

Seismomagnetic signals from the strong Sumatra earthquake

Electromagnetic oscillations generated by surface Love waves in the Earth’s crust were detected by the spectral polarization method at the Mondy and Sodankylä observatories, located at distances of 5390 and 8798 km from the epicenter of the strong Sumatra earthquake of December 26, 2004. Detailed analysis of this event is instructive because it indicates that the difficulty in detecting the coseismic magnetic oscillations predicted by theory is only due to the presence of strong noise. Observations of this type are shown to be significant for experimental studies of mechanoelectromagnetic conversions in the crust.     

Spatial structure of standing wave electromagnetic fields at the lower harmonics of the ionospheric Alfvén resonator

During the multiband wave Pc1 event on March 7, 2001 the EISCAT UHF and VHF incoherent scatter radars operated simultaneously covering an exceptionally wide altitude range of the ionosphere ～90—2000 km. This made possible to test the ionospheric Alfvén resonator (IAR) model over a large altitude range. The three lowest IAR eigenfrequencies, where the most of the Pc1 pulsation signal bands occur, were selected for the spatial analysis of the standing wave electromagnetic fields, applying the full-wave numerical simulation method. The altitude spread of amplitude maxima and nodes together with polarization characteristics of oscillation maxima in the horizontal plane are presented. The comparison of the standing wave oscillations on the altitude profile with the signal amplitude observed on the ground is also presented.     

Polarization of Pc1 Geomagnetic Pulsations as an Indicator of the Position of Their Source

Geomagnetism and Aeronomy - The article considers the scenario of the propagation of geomagnetic pulsations Pc1 from the region of their generation in the magnetosphere to a ground-based receiver,...     

High-latitude geomagnetic disturbances during the initial phase of a recurrent magnetic storm (from February 27 to March 2, 2008)

A complex of geophysical phenomena (geomagnetic pulsations in different frequency ranges, VLF emissions, riometer absorption, and auroras) during the initial phase of a small recurrent magnetic storm that occurred on February 27–March 2, 2008, at a solar activity minimum has been analyzed. The difference between this storm and other typical magnetic storms consisted in that its initial phase developed under a prolonged period of negative IMF B _z values, and the most intense wave-like disturbances during the storm initial phase were observed in the dusk and nighttime magnetospheric sectors rather than in the daytime sector as is observed in the majority of cases. The passage of a dense transient (with N _p reaching 30 cm⁻³) in the solar wind under the southward IMF in the sheath region of the high-speed solar wind stream responsible for the discussed storm caused a great (the AE index is ∼1250 nT) magnetospheric substorm. The appearance of VLF chorus, accompanied by riometer absorption bursts and Pc5 pulsations, in a very long longitudinal interval of auroral latitudes (L ∼ 5) from premidnight to dawn MLT hours has been detected. It has been concluded that a sharp increase in the solar wind dynamic pressure under prolonged negative values of IMF B _z resulted in the global (in longitude) development of electron cyclotron instability in the Earth’s magnetosphere.     

A comparative Pc1 case study applying two modes of ionospheric Alfvén resonator modeling

The case study of four Pc1 subauroral pulsation events from Finland has been carried out on the basis of the full-wave numerical method. This method has been applied to simultaneous Scandinavian EISCAT radar measurements of the ionospheric plasma parameters, and their vertical (altitude) profiles have been utilized. Two alternative plasma profiles with different ion composition displays have been put to the test. A comparison between both types of the modeled ionospheric Alfvén resonator (IAR) ground signal frequency response and the frequency range of the Pc1 signal records has been studied. The results of the applied method can illustrate possible quiescent or disturbance changes in the upper ionosphere above the dense F2 layer. The ionospheric region up to ∼ 2000 km has been taken into account for this comparative analysis.