R. Rasinkangas

Dispersive Pc1 bursts observed by Freja

Electric field measurements by the Freja double probe sensor are used to study equatorially generated ion cyclotron waves, also called Pc1 pulsations. We have examined the global occurrence and spectral properties of these waves in the upper ionosphere during 12-hour period on Nov. 18, 1992, when a long chain of structured Pc1 waves (pearls) was observed on ground. In agreement with ground observations, Pc1 waves were found to occur as short bursts of 10–25 s in early morning to postnoon MLT sector. Most Pc1 activity was detected within a small latitudinal range, extending from 60° CGMlat at dawn to 63° CGMlat at noon. The latitudinal width of the source was only about 0.5° CGMlat. Observations give evidence for a plasmapause connected source region that was several hours wide in MLT and active during many hours. One burst displayed a fully developed classical dispersive Pc1 pearl, now detected for the first time above the ionosphere. In all studied Pc1 events, two spectral maxima (bands) were observed. The longer Pc1 wave bursts showed evidence for a small time delay between the lower and upper frequency bands, unveiling a new dispersive phenomenon.

Nonbouncing Pc 1 wave bursts

On April 11, 1986, at about 0600 UT a long Pc 1 wave event of the hydromagnetic chorus type started on the ground, as registered by the Finnish pulsation magnetometer network. The main pulsation band at about 0.3 Hz was observed for several hours. Soon after start, this band smoothly extended to higher frequencies, forming another separate wave band which finally reached up to 0.5 Hz. During the event the Viking satellite was on its southbound pass over Scandinavia, close to the MLT sector of the ground network. From 0650 until 0657 UT, Viking observed a chain of Pc 1 bursts with increasing frequency. The strongest bursts could be grouped into two separate wave regions whose properties differed slightly. The higher-latitude region had a frequency of 0.3 Hz, well in agreement with the main Pc 1 band on the ground. The lower-latitude region contained the highest frequencies observed on the ground at about 0.5 Hz. The latitudinal extent of both wave regions was about 0.5°. They had slightly different normalized frequencies, Alfven velocities, and repetition periods. Most interestingly, the repetition periods of both wave sources were too short for the bursts to be due to a wave packet bouncing between the two hemispheres. The results give new information about the high-latitude Pc 1 waves, showing that they can consist of separate repetitive but nonbouncing bursts. We suggest that the long-held bouncing wave packet hypothesis is generally incorrect and discuss two alternative models where the burst structure is formed at the equatorial source region of the waves.

Modulation of magnetospheric EMIC waves by Pc 3 pulsations of upstream origin

We recently [Mursula et al., 1997] studied a series of bursts of electromagnetic ion cyclotron (EMIC) waves observed by the Viking satellite and on ground, and showed that these bursts can not be explained by the bouncing wave packet model. Here we show that the burst structure is due to simultaneous Pc 3 waves of upstream origin.

A modulated multiband Pc1 event observed by POLAR/EFI around the plasmapause

Abstract We study an electromagnetic ion cyclotron (EMIC) wave event observed simultaneously by the Electric Field Instrument (EFI) on board the POLAR spacecraft and by the Finnish pulsation magnetometer chain on April 25, 1997, when the two were in a good conjunction. EFI recorded waves at two frequency bands from L = 4.3 (in the outer plasmasphere) to L=6.2 (just outside the plasmapause). Both bands were observed in several conjugate stations on ground. The waves showed repetitive variations in amplitude, corresponding to classical Pc1 pearls. The repetition period was the same on ground and in space. Moreover, the repetition period of Pc1 pearls coincided with the period of simultaneous Pc4 waves observed by POLAR and on ground. The observations suggest that Pc1 pearls (EMIC waves in general) are modulated by Pc4 waves rather than result from the bouncing of a wave packet from one hemisphere to another.

An EISCAT study of a pulsating auroral arc: simultaneous ionospheric electron density, auroral luminosity and magnetic field pulsations

Abstract We present a case study of a pulsating auroral are using EISCAT incoherent scatter radar measurements of energetic electron precipitation ∼ 5–30 keV) combined with ground-based observations of auroral luminosity and magnetic pulsations. The event under consideration occurred during a magnetically quiet period on 1 February 1987 between 0:00 and 0:30 UT. Pronounced pulsations with a period of about 1 min were present in all measured quantities. The magnetic pulsations of this period exhibited in phase oscillations over the spatial scale of the EISCAT Magnetometer Cross (some 250 km in longitude and 1000 km in latitude). Spectral analysis revealed also variations of a shorter time scale of about 10 s in all measured quantities except for the flux of precipitating electrons having energies below 10 keV (above 10 keV the electron flux exhibited 10 s variations). In the framework of the cyclotron resonant interaction of electrons with whistler waves, the long period pulsations are attributed to temporal modulation of the energetic electron source. The simultaneously observed pulsations with a period of about 10 s are explained within the self-oscillating regime of the whistler cyclotron instability in the magnetosphere. We present computational results from a self-consistent instability model taking all the conjectured effects into account.

Coordinated photometer and incoherent scatter radar measurement of pulsating arcs with high time resolution

Abstract Coordinated photometer and incoherent scatter radar measurements with high time resolution (0.1 s) were made on 1 February 1987 in order to investigate auroral pulsations. Also a low light level TV-camera operated in real speed mode during the experiment. The total energy flux and characteristic energy of Maxwellian energy distribution were measured with a field-aligned multichannel photometer. The radar beam crossed the photometer beam at an altitude of 110 km, where the electron density was modelled successfully using a characteristic energy of ~ 2 keV. With the time resolution of 1.0 s also electron density pulsations were seen, showing correlation with the optical pulsations. Furthermore, during the observed pulsating arcs enhanced electron densities were found in the altitude range of 85–100 km. This indicates a hardening of the precipitating electrons. This change has been modelled by double-Maxwellian distribution with characteristic energies of about 1.3 and 7 keV.

Comparison of the lower border of aurorae determined by two optical emission ratio models

Abstract Two models for the electron density profile calculations, assuming either a Maxwellian or a monoenergetic energy distribution for the precipitating auroral electrons, have been compared with electron density measurements by the EISCAT incoherent scatter radar. In both models, the column integrated emission ratio I (630.0)/ I (427.8) is used to deduce the energy parameter for the precipitation, and the time dependent electron density profiles are calculated using the continuity equation. The emission ratio models used in the Maxwellian and monoenergetic models are by Rees and Luckey (1974, J. geophys. Res. 79 , 5181) and Vallance Jones (1975, Can. J. Phys. 53 , 2267), respectively. The ionization rate profiles needed are calculated by using the energy deposition functions by Rees (1963, Planet. Space Sci. 11 , 1209). In the case of two auroral arcs the monoenergetic model is shown to give more reliable information about the lower border altitude of aurorae than the Maxwellian model. On the other hand, in the case of a discrete auroral patch the electron spectrum is of a Maxwellian type, and the electron density profile deduced from the Maxwellian model fits the measured profile very well.

High resolution measurements of pulsating aurora by EISCAT, optical instruments and pulsation magnetometers

Abstract On Jan 30. – Feb 1. 1987 a 18 hours long Finnish EISCAT experiment was made with optical and magnetic measurements. The aim of this experiment was to study pulsating auroras with high time resolution. The calculated electron densities obtained from photometer data correlate well with measured electron densities at 110 km. The electron density pulsations are observed during auroral pulsations. Also enhanced electron densities in altitude range of 85 – 100 km are observed during pulsations. This may be caused by two different Maxwellian distributions with characteristic energies of around 2 keV and 10 keV.