F. Z. Feygin

Non-stationary Alfvén resonator: new results on Pc1 pearls and IPDP events

Abstract We analyse a Pc1 pearl event observed by the Finnish search-coil magnetometer network on 15 December 1984, which subsequently developed into a structured IPDP after a substorm onset. The EISCAT radar was simultaneously monitoring the mid- to high-latitude ionosphere. We have calculated the ionospheric resonator properties during the different phases of the event using EISCAT observations. Contrary to the earlier results, we find that the Pc1/IPDP (Interval of Pulsations of Diminishing Period) frequency observed on the ground corresponds to the maximum of the transmission coefficient rather than that of the reflection coefficient. This casts strong doubts on the bouncing wave packet model of Pc1 pearls. Instead, we present evidence for an alternative model of pearl formation in which long-period ULF waves modulate the Pc1 growth rate. Moreover, we propose a new model for IPDP formation, whereby the ionosphere acts as an active agent in forming the IPDP signal on the ground. The model calculations show that the ionospheric resonator properties can be modified during the event so that the resonator eigenfrequency increases according to the observed frequency increase during the IPDP phase. We suggest that the IPDP signal on the ground is a combined effect of the frequency increase in the magnetospheric wave source and the simultaneous increase of the resonator eigenfrequency. The need for such a complicated matching of the two factors explains the rarity of IPDPs on the ground despite the ubiquitous occurrence of EMIC waves in the magnetosphere and the continuous substorm cycle.

Non-stationary Alfvén resonator: vertical profiles of wave characteristics

Abstract A Pc1/IPDP event recorded by the Finnish search coil magnetometers on 15 December 1984 was analyzed in a companion paper (Mursula et al., 2000. Non-stationary Alfven resonator: new results on Pc1 pearls and IPDP events. J. Atmos. Solar-Terr. Phys. 62(4), 299–309) using numerical simulations of the ionospheric Alfven resonator (IAR). EISCAT incoherent scatter radar data were used to determine the vertical profiles of ionospheric plasma parameters. In this paper, the detailed altitude profiles of several wave characteristics at the IAR eigenfrequency are computed up to 1000 km height, including, e.g., the real normalized amplitude of the magnetic wave field component, ellipticity and orientation of the polarization ellipse in the horizontal plane. We also calculate the altitude profile of the energy flux density (Poynting vector). These features illustrate in detail the ionospheric effects on the wave spectral structure in a non-stationary IAR, and their significance in the formation of the Pc1/IPDP signal on the ground.

Excitation of helium cyclotron harmonic waves during quiet magnetic conditions

A general approach to the generation of ion cyclotron harmonic waves observed on board the Akebono satellite in the deep plasmasphere is presented. It is shown that during quiet magnetic conditions the development of the hydrodynamic cyclotron instability with growth rate γ ∝ ni1/2, where ni is the number density of the hot heavy ions, is suppressed by the field-aligned inhomogeneity of the dipole magnetic field. The instability is, in this case, controlled by the weak resonant interaction of the waves and the trapped particles with growth rate γ ∝ ni. The waves are generated by a kinetic instability involving hot helium ions with a ring-like distribution. Such ions are present in the magnetosphere during quiet magnetic conditions. A simple analytical model of this instability accounting for the inhomogeneity of the ambient magnetic field is used. It is shown that the ULF wave observations during quiet times on board the Akebono satellite are in a reasonable agreement with the present theoretical approach.

Ionospheric Alfvén Resonator Control over the Frequency-Variable Pc1 Event in Finland on May 14, 1997

The ionospheric Alfvén resonator (IAR) control mechanism over the EMIC wave transmission to the ground is demonstrated on a selected long-term frequency-variable subauroral Pcl event. The proper ionospheric plasma data obtained from EISCAT were accessible in a wide altitude range. Applying the numerical method of simulation of a realistic inhomogeneous IAR, the problem of appearance and disappearance of the ground Pc1 signal record was clarified on the basis of coincidence between the EMIC wave frequency spectrum and the IAR fundamental frequency peak (the frequency window). A shift of the signal source field line to lower latitudes during the development of the disturbance was noticed, and the signal frequency variation on the ground was modelled in the nonstationary IAR. Variation of the IAR altitude structure in the fundamental frequency was illustrated on altitude profiles of the normalized wave magnetic field amplitude in the horizontal and vertical components. Particular conditions of L‖- and R‖-wave mode incidence were assumed. The electron density vertical profile of IAR determines the effective resonator dimensions. In this way the IAR fundamental frequency window controls the signal within the ionosphere and on the ground.

Exo‐plasmaspheric refilling due to ponderomotive forces induced by geomagnetic pulsations

The effect of exo-plasmaspheric refilling due to ponderomotive forces induced by geomagnetic pulsations is considered. It is shown that two maxima of high-density cold plasma can appear on field lines near the day side magnetospheric boundary, located symmetrically with respect to the equator. When moving away from the noon meridional plane the plasma density distribution along the field line undergoes a smooth transition from two off-equatorial maxima to one maximum at the equator. We calculate the plasma condensation due to the ponderomotive forces in this region. On the other hand, the satellite data have shown that exoplasmaspheric Pc1 pulsations have a maximum in the noon-dusk sector. These facts bring us to a conclusion that the formation of high-density cold plasma outside plasmasphere in the magnetospheric trough in the noon-dusk sector may be accelerated by ponderomotive effects of Pc1 pulsations.

Effect of heavy ions on ponderomotive forces due to ion cyclotron waves

We study ponderomotive effects induced by the electromagnetic ion cyclotron (EMIC) waves in the Pel frequency band (0.2 to 5.0 Hz) in a two-ion (H+ and one heavy ion, e.g., He+) plasma. Near the dayside boundary of the magnetosphere, the ponderomotive forces lead to a noticeable accumulation of cold plasma along the field line in two regions of minimum magnetic field intensity located symmetrically around the equator. In the inner magnetosphere, one maximum of cold plasma at the equator is found. At frequencies less than the heavy ion gyrofrequency, the accumulation of cold plasma increases with increasing heavy ion concentration. At frequencies above the heavy ion gyrofrequency, the ponderomotive forces due to EMIC waves are enhanced because of a resonance at the stop band frequencies. We investigate the stop band structure of EMIC waves in a nondipolar magnetosphere and discuss the properties of wave propagation along the field line for different heavy ion plasma concentrations.

Fluctuations of the repetition period of Pc1 Pearl pulsations

In this paper we investigate fluctuations of the repetition period of geomagnetic Pc1 pearl pulsations. Starting from calculated repetition period we present a general formula for the deviation of the repetition period as a function of wave frequency and stochastic parameters of the medium along the ray trace. We then apply this formula for a dipole magnetic field with a simple plasma distribution, and show that a linear correlation between repetition period and its deviation is predicted. This correlation and the frequency dependence of fluctuations are then compared with experimental values measured from selected Pc1 pearl events observed in Finland.

NUMERICAL SIMULATION OF THE HIGH-LATITUDE NON- STATIONARY IONOSPHERIC ALFVÉN RESONATOR DURING AN IPDP EVENT

The ionospheric Alfvén resonator (IAR) was numerically simulated under non-stationary ionospheric and magnetospheric conditions of the IPDP event of December 4, 1986. The full numerical wave method was applied using height profiles of the ionospheric plasma parameters obtained from the Scandinavian EISCAT radar measurements close to the Ivalo latitude. An attempt to model the inverse problem of numerical simulation—prolongation of the electron density profiles at altitudes above the ionospheric F layer—was made on the basis of the IAR simulation in correlation with the IPDP frequency increase. The change of the IAR wave characteristics during the substorm was illustrated by height profiles of the total wave amplitude and various polarization characteristics, taking into consideration the ordinary L-mode and the extraordinary R-mode waves for parallel and non-parallel incidence with respect to the magnetic field line.