The wave normals of magnetospheric chorus emissions observed on board GEOS 2
Authors: Hayakawa M. , Yamanaka Y. , Parrot M. , Lefeuvre F. 
Published: February 2008

The wave normal directions of magnetospheric chorus emissions have been analyzed by using data from the geostationary GEOS 2 satellite which is located in the equatorial region at L = 6.6. As the result of analyses for a specific day of December 19, 1979, chorus is found to be generated in conjunction with substorms and at local times from around midnight to afternoon. Two types of chorus are observed: impulsive burstlike and rising tone. The chorus during midnight to late morning hours is impulsive, and at local times from noon to dusk we notice an increase in the occurrence of rising tone chorus. The chorus spectra have shown the importance of the characteristic frequency of fH/2 (fH is the electron gyrofrequency). We notice the impulsive chorus only in the lower band (f<fH/2; f is the emission frequency) at local times from ∼0300 to ∼0700, and then at LT = ∼0800 to ∼1600 the predominant frequency of the emissions, whether impulsive or rising tone, lies in the upper band (f>fH/2) with infrequent occurrence in the lower band, and there is obviously no correlation between the elements in both bands. However, there are several intense tones which seem to be correlated in both bands for the strong two‐banded impulsive chorus emissions observed only at LT ≃ 0800. This correlation might be related with a notch in chorus risers at f ≃ fH/2 caused by Landau damping, as previously suggested. The wave normals are determined by using only the magnetic field data. The direction finding studies have been made for impulsive and rising tone chorus, separately, which have yielded different results. The lower band is discussed first. The wave normals of the rising tones having values of df/dt as normally observed are found to take very small angles (5°–20°). Then it is found that the rising tones with increased values of df/dt have shown larger wave normal angles of 30°–45°. These relatively small wave normal angles for the lower band give further support to the loss cone instability as the generation mechanism. The impulsive chorus is found to take slightly larger wave normal angles (about 5°) than most rising tones. Further, it may be suggested that different types of chorus are associated with different wave normals. Next we are concerned with the upper band chorus, whose generation mechanism is not well agreed upon. It is shown that the impulsive chorus in the upper band is generated with its wave normal close to the oblique resonance cone. This suggests that the upper band chorus is quasi‐electrostatic, and it is furthermore confirmed by the estimated ratio, c|B|/|E| (|E| and |B| are the total electric and magnetic field intensities; c is the light velocity). The rising tones in the upper band, however, have indicated a difference such that the closeness of their wave normals to the oblique resonance cone is much less distinct. An intensive comparison is made between the direction finding results for the upper band and the theoretical mechanisms so far proposed in order to elucidate the possible generation mechanism of the upper band chorus.