Electrostatic electron cyclotron harmonic waves observed by the Akebono satellite near the equatorial region of the plasmasphere

Abstract

Analysis of the plasma wave observation data provided by the plasma waves and sounder experiment (PWS) on board the Akebono satellite frequently reveals the presence of electrostatic electron cyclotron harmonic (ESCH) waves in the low-latitude region (MLAT < 45• ) of the plasmasphere within an altitude range from about 3000 km to the apogee of the satellite (initial apogee was 10,500 km). Even at moderate or low geomagnetic activity, intense ESCH waves often appear near the equatorial region of the plasmasphere above the upper hybrid resonance (UHR) frequency at the lowest harmonic number branch of the f Qn ESCH waves. We identified these plasma waves as the equatorial plasmasphere f Qn waves (EP-f Qn ). The spectra of the EP-f Qn waves are characterized by a narrow band structure and by a strong nature, with a wave intensity that ranges from 3.46 × 10 −8 to 3.31 × 10 −4 V/m. The maximum intensity is nearly coincident with the upper limit of the PWS receiver in the low-gain mode. Statistical analysis results reveal that the EP-f Qn waves are observable in all the local time sectors; however, the occurrence probability shows a clear enhancement in the early morning sector of 01-03 MLT in the plasmasphere. The EP-f Qn wave activities are suppressed within a period of strong magnetic disturbances as well as solar minimum phase. The linear dispersion relation analysis using a two-component plasma model reveals that supra-thermal plasma with the energy of about 750 eV and with a large temperature anisotropy (A = T-perp/T-parallel − 1 > 40) must be present in order to realize an appearance of a positive growth rate at the observed frequency and propagation angle of the ESCH waves. Since the hot plasma with such a high anisotropy has not been detected, the validity of the present two-component plasma model remains an open question. The occurrence feature of the ESCH waves showed that there is a constant activation or a constant flow-in of free energy to generate the strong plasma instability of ESCH waves near the post-midnight sector of the plasmasphere. The existence of ESCH waves revealed that the nature of the plasmaspheric plasma is more turbulent and active than has been believed.

Detection of UHR Frequencies by a Convolutional Neural Network From Arase/PWE Data

“…One is the contamination of ECH waves around the geomagnetic equator. Large‐intensity ECH waves are frequently observed around the geomagnetic equator (Shinbori et al, 2007), and the frequency range of the ECH wave is almost the same as that of the typical UHR frequency along the orbit of Arase. Figures 2c and 2f show an example of intensely ECH‐contaminated event.…”

Observation of wake‐induced plasma waves around an ionospheric sounding rocket

“…Kennel et al [1970] first reported ESCH waves in the Earth's magnetosphere, which was based on results from the OGO-5 satellite. Many subsequent studies have revealed ESCH wave activities in the magnetosphere beyond the plasmapause [e.g., Ni et al, 2011] and inside the plasmasphere [e.g., Shinbori et al, 2007]. Some of the suggested generation mechanisms are, for example, loss cone instabilities [e.g., Horne et al, 2003] and a two-component plasma with temperature anisotropy [Shinbori et al, 2007].…”

“…Many subsequent studies have revealed ESCH wave activities in the magnetosphere beyond the plasmapause [e.g., Ni et al ., ] and inside the plasmasphere [e.g., Shinbori et al ., ]. Some of the suggested generation mechanisms are, for example, loss cone instabilities [e.g., Horne et al ., ] and a two‐component plasma with temperature anisotropy [ Shinbori et al ., ]. However, not much attention has been given to ESCH waves in ionospheric plasma, especially in the low‐latitude and midlatitude ionosphere, because ionospheric plasma is considered to have fewer energy sources to modulate the Maxwellian form and more collision processes than magnetospheric plasma.…”

Numerical study of electrostatic electron cyclotron harmonic waves due to Maxwellian ring velocity distributions

“…Electrostatic electron cyclotron harmonic (ECH) waves are observed in the near-Earth equatorial region (e.g., Kennel et al, 1970;Rönnmark et al, 1978;Kurth et al, 1979aKurth et al, , b, 1980Rönnmark and Christiansen, 1981;Horne et al, 1981Horne et al, , 1987Koons and Fennel, 1984;Matsumoto and Usui, 1997;Usui et al, 1997Usui et al, , 1999aShinbori et al, 2007). The ECH wave emissions were observed at frequencies f ∼ (n + 1 2 ) f ce , where n is a positive integer and f ce denotes the electron cyclotron frequency.…”