T. Bösinger

Simultaneous ground‐satellite observations of structured Pc 1 pulsations

Structured Pc 1 pulsations are investigated using simultaneous multipoint ground-satellite observations recorded on September 10, 1986, during Viking orbit 1103. The multipoint Pc 1 observations were acquired using the Viking magnetic field experiment at 13,550 km altitude from L = 5.1 to 5.5 and three Finnish ground stations at Rovaniemi, Ivalo, and Kilpisjarvi. These stations are all located within 30 min magnetic local time and 1° in latitude of the ionospheric field line footprint of the Pc 1 source field line. Structured Pc 1 pulsations between 0.5 and 1.0 Hz were observed both on the ground and in space at similar frequencies, with similar frequency dispersion, and with a similar repetition period. The wave source region, based on Viking Langmuir probe observations, was just inside the plasmapause. The wave transit time between Viking and the ground was 12 ± 2 s, where Viking leads the ground. This implies that the waves propagate downward from Viking to the ground, consistent with previous downward Poynting flux estimates.

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.

Generation of artificial magnetic pulsations in the Pc1 frequency range by periodic heating of the Earth's ionosphere: indications of Alfvén resonator effects

Abstract A series of six experiments with the EISCAT HF heater device assisted by the EISCAT (European-Incoherent-Scatter) radar were carried out with the purpose of producing artificial magnetic pulsations in the 0.1–3 Hz frequency range. In only 3 of the 30 h of experiment time under a variety of ionospheric conditions was an artificial magnetic signal detected by ground-based magnetometers. A numerical model was used to explain the sporadic nature of the artificial signal in terms of ionospheric parameters. For several experiments the EISCAT radar provided an in situ electric field and/or electron density values; otherwise standard neutral atmosphere and ionosphere models were used. The PGI model was only partially successful. It could produce the right order of magnitude for the artificial signal when it was observed and it could demonstrate the different efficiencies when using either the O- or the X-mode of the HF wave, but it could not explain why the artificial signal was observed at a particular time and not at others. This is only partially due to the uncertainty in one or more input parameters. When the artificial signal was observed its spectrum usually exhibited spectral resonance structures of the Ionospheric Alfven Resonator (IAR), indicating that a “DC approach” is insufficient and that the generation of oscillating field-aligned currents, and thereby shear Alfven waves, has to be taken into account. We believe, however, that even with the introduction of the IAR into the model it will not be possible to resolve the sporadic character of the heating-induced artificial magnetic signal entirely. A more realistic way of D-region modeling will without doubt also be an important factor in resolving the puzzle.

Dayside high latitude magnetic impulsive events: their characteristics and relationship to sudden impulses

Abstract On 17 December 1990 a series magnetic impulsive events (MIEs) were observed at high latitudes near local noon. EISCAT, situated some 5 hours of MLT away from the noon sector, detected simultaneous impulsive electron density enhancements at heights between 90 and 120 km. The MIEs at noon were also associated with riometer absorption spikes. The correlated EISCAT and riometer observations indicate that there was an elongated electron precipitation region some 3000 km wide stretching from local noon to morning. In close association with the impulsive electron precipitation, VLF emissions were observed by groundbased stations in the morning side. We interpret the large scale electron precipitation and VLF emissions as signatures of a global compression of the Earths magnetosphere. This is confirmed by the specific type of magnetic variations simultaneously recorded at the worldwide network of magnetometers. We conclude that the small scale MIEs with their drifting ionospheric current vortex structures can (but do not necessarily have to) occur in conjunction with large scale SIs. Moreover, MIEs and SIs have a common origin: the interaction of solar wind inhomogeneities with the Earths magnetosphere. They do, however, represent different effects of the same primary agent.

Pi1B type magnetic pulsations simultaneously observed at mid and high latitudes

Abstract Properties of simultaneously observed Pi1B s are derived from data from one mid latitude and five high latitude stations. We show that the high and mid latitude observations exhibit a high degree of correlation in the onset, duration and temporal fine structure, but not in the amplitude behaviour. In the Pi1B power spectrum, an enhancement at 0.08–0.25 Hz is frequently observed at all stations in both horizontal components. In the approximately geographic north-south direction, the attenuation of the Pi1B amplitude at mid and high latitudes at the southern flank of the amplitude maximum is of the order of 10 and 40 dB/1000 km, respectively. In the frequency range where the power enhancement is observed, a right-handed sense of polarization dominates at high latitudes and a left-handed one at mid latitudes. The implications of these observations on the generation and propagation mechanisms of the Pi1B signal are discussed.

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.

Nighttime patterns of ionospheric convection, conductance, horizontal and field-aligned currents during a steady magnetospheric convection event

Abstract On 21 April 1988 the EISCAT radar carried out measurements in the CP-3 mode (large meridional scan) in the dusk to midnight MLT sector during a Steady Magnetospheric Convection (SMC) event lasting for 8 hours. This rare occasion made it possible to study for the first time the 2-dimensional distribution of convection velocities, conductances, electric potential, equivalent and real hoizontal currents of the auroral ionosphere during SMC conditions. In spite of a modest level of magnetic activity (AE-index between 200 and 250 nT) many characteristics resembled those of high magnetic activity. The auroral oval was at the nightside about 10° of latitude wide and the cross polar cap potential reached values up to 80 kV. Dominating features of the nightside convection pattern were the formation of a convection throat (with 15kV potential difference per 1 h of LT) centered at 23 MLT and the absence of the Harang Discontinuity (HD). The HD is traditionally understood as the overlapping region between two electrojets generated by a partial intrusion of the west-ward electrojet into the northern flank of the eastward electrojet. Intense ∼ 1pμA m −2 ) field-aligned currents were generated at the large north-south conductance gradients in the convection throat region mainly due to the divergence of Hall currents. We suggest that the so generated upward flowing current in the middle part of the auroral zone can provide the ionospheric closure of the! field-aligned current generated in the magnetosphere due to the dawn-dusk asymmetry of the cross-tail current and it also offers an explanation for the absence of the HD during SMC events.

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.