Observations of electron conies by the Viking satellite

Electron angular distributions peaked at oblique angles to the magnetic field, electron conies, are frequently found in the Viking data at all magnetic local times, but with a maximum in the dusk sector. Several types of electron conies have been observed by Viking as well as by other satellites. One type is frequently seen below the acceleration region. It is caused by adiabatic motion of electrons with a field‐aligned distribution that has a broader angular width than the loss cone. The atmospheric loss of particles will result in a distribution peaked at pitch angles close to the loss cone. Another type is due to heating or acceleration processes at low altitudes and is seen above the acceleration region. Particle and wave characteristics and possible generation mechanisms for this type of electron conic are discussed. The ion arid electron angular distributions observed by Viking indicate that a parallel electric field is present below Viking during this type of event. Ion observations give important information on the processes that produce the electron conies. Almost 200 electron conic events have been detected in the Viking data, that is, in about every third orbit that was studied. Their frequency of occurrence in altitude maximizes in the upper part of the acceleration region, between 10,000 and 11,000 km. The number of events are found to decrease at higher altitudes. This might indicate that diffusion processes, associated with wave generation, remove the anisotropy in the electron distribution function in and above the acceleration region. Wave observations show the presence of both low‐frequency waves and waves close to the electron gyro frequency. Acceleration in a fluctuating (approximately 1 Hz) parallel electric field is suggested as a likely mechanism to create the electron conies observed by Viking. The period of the fluctuations is comparable to the travel time, below the acceleration region, of the electrons forming the conic distribution.