Kari U. Kaila

A substorm observed by EISCAT and other ground-based instruments — evidence for near-Earth substorm initiation

Abstract We present observations of the development of a substorm in the ionosphere made by the EISCAT radar, optical and magnetic instruments. A typical growth phase started 30 min before the substorm onset with equatorward drifting arcs. The arc which crossed the EISCAT beam drifted equatorward with approximately the same velocity as the ambient plasma and it approached the most equatorward discrete arc. The most equatorward discrete arc (the breakup arc) stopped its drift at L = 5.3. Arcs about 1° of latitude poleward of the breakup arc continued their equatorward motion with no significant changes in intensity after the onset of the substorm, which was timed on the basis of the explosive intensification in the breakup arc. Similarly, no dramatic changes in electric fields or other plasma parameters measured by EISCAT were observed poleward of the breakup arc. The observations indicate that the instability that triggered the substorm onset was localized in the near-Earth magnetotail. A westward travelling surge (WTS) developed in the breakup arc and moved westward with a very high velocity, 13 ± 3 km s−1. Extremely high conductances were measured by EISCAT from the poleward boundary of the poleward expanding bulge with a maximum value of ΣH = 214 S obtained with a time resolution of 0.2 s. The westward electrojet (WEJ) was observed to be latitudinally very inhomogenous and concentrated near conductivity enhancements, especially close to the head of the WTS. The localization of the WEJ close to discrete arcs and consequent motions with the arcs gave the charasteristic spiky appearance of the magnetic X-component in the pre-midnight sector.

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.

An iterative method for calculating the altitudes and positions of auroras along the arc

Abstract A method for three-dimensional calculation of discrete auroral forms has been developed. The auroral altitudes and positions can be calculated along the discrete auroral form using two or more all-sky camera pictures. The calculations will be made iteratively and thus no corresponding points are needed. The accuracy of the determination of the altitude and position is optimally ± 1 km. The only assumption in this method is the thin sheet approximation, i.e. the auroral forms are thin sheets of luminosity with negligible thickness. Four discrete arcs have been chosen as examples. From these the altitudes and positions were calculated using six camera pairs from four different all-sky stations in Northern Finland. In two of the cases there was also a second arc far in the North. The calculations were also made for these arcs, but using only two stations. The calculated altitudes were 94–112 km and three of the arcs had constant altitude within ± 2 km. One arc was clearly inclined and the two arcs far in the North were very probably also inclined.

Determination of the energy of auroral electrons by the measurements of the emission ratio and altitude of aurorae

Abstract The energy of auroral electrons has been determined by two independent methods. First, the altitude of discrete auroral forms has been estimated from two all-sky camera pictures using an iterative method. The altitudes have been converted to energies using the theories of electron penetration into the atmosphere. Secondly, the intensity ratio of 630.0 and 427.8 nm emissions has been measured from the same auroral forms. The emission ratio energy has been derived from theoretical emission ratio calculations. Using these methods the two energies have been compared in about 140 cases. The emission ratio energies calculated on the basis of the monoenergetic electron flux theory by Vallance Jones (1975, Can. J. Phys . 53 , 2267) are in the best agreement with the electron penetration theory by Banks et al . (1974, J. geophys. Res . 79 , 1459). By using Maxwellian electron flux the emission ratio model by Rees and Luckey (1974, J. geophys. Res . 79 , 5181) gives too high characteristic energies as compared with electron penetration theories. The measured emission ratios and altitude energies of the same auroral arcs have also been fitted to cubic spline function. A new nomogram has been drawn where the altitude energy of auroral electrons in discrete arcs can be determined as a function of 427.8 nm intensity and 630.0 nm/427.8 nm intensity ratio.

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.

Tomographic inversion of auroral camera of photometer observations

Abstract A tomographic inversion programme for auroral tomography is introduced and samples of inversion results are shown. The programme is based on stochastic inversion, which gives the most probable values of the volume emission rate. Although it was originally designed for satellite tomography, it has later been used for auroral tomography as well. In this paper a new version modified for the special needs of auroral tomography is presented, which is more suitable for studying field-aligned structures. The essential difference is that, instead of the conventional rectangular mesh shape, the present version uses parallelograms oriented along the direction of the local magnetic field. Then it is possible to feed a priori information to the solver, which is expected to make the inversion favour field-aligned structures instead of horizontal or vertical ones.

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.