Pulsating auroras are a visible manifestation of modulation in the fluxes of magnetospheric electrons precipitating into the Earth's ionosphere. They are usually observed at the morning hours during a substorm recovery phase. This auroral phenomenon has been studied for a long time but the question of its generation is still more or less open. It is widely believed that whistler waves play an important role in the generation of pulsations. Various theories of pulsating auroras are based on the wave-particle interaction between VLF waves and electrons in the equatorial region of the magnetosphere.
Pulsating precipitation of electrons into the ionosphere give also rise to other pulsating quantities, like X-rays and enhanced ionization. One commonly observed phenomenon on the ground is the correlation of auroral pulsations with magnetic field variations, see Pi C pulsations.
By pulsating auroras we mean the repetitive intensity modulation in the auroral luminosity. The period of pulsation is typically of the order of 1 - 30 s. The intensity variations can be repetitive, quasi-periodic or occasionally periodic (Royrvik and Davis, 1977) and variations can last sometimes only for a few pulses but sometimes even for hours. Pulsating auroras are usually fairly faint phenomena; the intensity of 427.8 nm emission is only a few hundred Rayleigh (R) to a few kR and newer exceedind 10 kR (Royrvik and Davis, 1977). Although pulsations are related to the diffuse auroras, they anticorrelate with proton emissions.
Pulsating behaviour can occur in auroral arcs, arc segments and in patches. Pulsating auroral forms are found drift in the evening sector of the auroral oval westward and in the morning sector eastward with speeds up to 1 km/s (Royrvik and Davis, 1977). The sizes of the forms are very variable. Here is shown an example of auroral pulsations measured by low-light-level-TV camera.
When oppositely moving VLF waves and energetic electrons interact, so called cyclotron resonance interaction (CRI, see the figure) is possible. In these interactions VLF waves are amplified by the transition of energy from the spiraling electrons to the waves. As a result the pitch angle of the electrons is reduced and they scatter into the loss cone and subsequently into the ionosphere.
Coroniti and Kennel (1970) proposed in their classical paper that hydromagnetic (HM) waves modulate the electron cyclotron instability and thus the excitation rate of VLF waves in the equatorial region of the magnetosphere. The HM waves could also propagate into the ionosphere and to the Earth and be recorded as geomagnetic pulsations. If this is true the time delay between auroral pulsations and correlating magnetic pulsations would be of the order of a few tens of seconds because of different propagation times. The observed time delay was however, found to be usually only about 0-2 seconds (e.g. Oguti , 1982; Arnoldy et al., 1982). Therefore magnetic pulsations during auroral pulsations cannot be explained by the Coroniti and Kennel (1970) theory. Also satellite measurements in the equatorial region of the magnetosphere during times of pulsating auroras do not show any evidence for the simultaneous presence of HM waves (Oguti et al., 1986).
Davidson and Chiu (1991 and references therein) have proposed so called relaxation oscillation mechanism for the generation of auroral pulsations. In their theory VLF waves are generated in the equatorial region by the instabilities in trapped particle populations. Electrons interact with these waves in CRI process and after this electrons can diffuse into the ionosphere. Decrese in anisotrophy reduces the wave growth.
Tagirov el al. (1986), Trakhtengerts et al. (1986) and Demekhov and Trakhtengerts (1994) developed the flowing cyclotron maser (FCM) theory for auroral pulsations. FCM theory includes a flux tube of cold plasma as a resonator for VLF waves. New electrons drift into the tube and after CRI electrons flow into the ionosphere. In this theory the drift of new particles into the tube is important. Can explain a wide range of pulsation periods and common features of pulsating patches.
Sometimes a correlation between VLF emissions and pulsating auroras is possible to observe at simultaneous ground-based VLF and auroral measurements. For example Helliwell et al. (1980), Tsuruda et al. (1981), Johnstone (1983), Scourfield et al. (1984), Hansen et al. (1988), Hansen and Scourfield (1989) and Manninen et al. (1992) report of the correlating VLF emissions and pulsating auroral forms. Rosenberg et al. (1971) has found also a one-to-one correlation between bursts of >30 keV X-rays and discrete VLF emissions at balloon altitudes. All experimental results have been explained by the interaction between VLF waves and energetic electrons near the equatorial region of the magnetosphere. Here is shown an example of the correlating VLF emissions and auroral pulsations.
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