Projects (publications) of Annika Olsson and Pekka Janhunen for understanding the generation mechanisms of auroral arcs, first stable, in later projects also disturbed. Brief comments are given below for each paper to help you get a quick idea what is inside the paper (but in most cases, there are many more important results in the papers than what the brief comments describe).
Project 1: How does the U-shaped potential close above the acceleration region? A study using Polar data (Janhunen, Olsson, Mozer, Laakso). Annales Geophysicae, 17, 1276-1283, 1999. We found that U-shaped potentials often do not close above 4 R_E radial distance. The first paper where a closed potential structure is suggested. A much improved study is Project 7 below.
Project 2a: A study of inverted-V auroral acceleration mechanisms using Polar/FAST conjunctions (Janhunen, Olsson, Peterson, Laakso, Pickett, Pulkkinen, Russell). J. Geophys. Res., 106, 18995-19011, 2001. A conjunction event study, somewhat similar to what Reiff et al. (1988) made for DE-1/DE-2 pair.
Project 2b: New model for auroral acceleration: O-shaped potential structure cooperating with waves (Janhunen and Olsson). Ann. Geophysicae 18, 596-607, 2000. The first paper where a self-consistent closed potential structure is discussed. Includes test particle simulation. Project 8 below demonstrates using simulation that the ideas presented here indeed work (however, electron dynamics is still not included in project 8).
Project 4: A statistical study of nightside inverted-V events using Freja electron data: Implications for the current-voltage relationship (Olsson and Janhunen). JASTP, 62, 81-92, 2000. Statistically, we do not find a linear or even a monotonic current-voltage relationship. Rather, the current and voltage are largely independent of each other, when all events are put together.
Project 5: Difference in the current-voltage relationships between dawn and duskside inverted-V events (Olsson and Janhunen). J. Geophys. Res., 105, 5373-5380, 2000. An improved version of project 4 where the size of the database is doubled and the data are binned in MLT. We find a clear difference in the C/V characteristics of eveningside versus morningside inverted-V events. This tells something about the nature of electron acceleration. Theoretically, the question is still open.
Project 6: Characteristics of a stable isolated arc using FAST and MIRACLE (Janhunen, Olsson, Amm and Kauristie). Ann. Geophysicae 18, 152-160, 2000. Classical case study of an evening sector arc using satellite and ground-based instruments.
Alfven-conference proceedings: Auroral potential structures and current-voltage relationship: Summary of recent results (Janhunen and Olsson). Physics and Chemistry of the Earth, 26, 107-111, 2001. No new results, but summarises earlier results in readable form.
COSPAR-2000 proceedings: Altitude extension of auroral potential structures by event-based and statistical studies (Janhunen, Olsson and Laakso). Adv. Space Res., 28, 1575-1580, 2001. Another easy-to-read summary paper.
Project 7: The occurrence frequency of auroral potential structures and electric fields as a function of altitude using Polar/EFI data (Janhunen, Olsson and Laakso). Ann. Geophysicae, in press, 2003. Large, comprehensive statistical study of negative potential structures as a function of altitude, MLT and Kp. Includes also statistics of raw electric fields, i.e. fields that are not necessarily associated with a potential structure. Since the results are similar, the implication is that potential structures dominate also the raw field statistics.
Project 8: A hybrid simulation model for a stable auroral arc (Janhunen and Olsson). Ann. Geophysicae 20, 1603-1616, 2002. Self-consistent electrostatic quasineutral simulation of the whole auroral arc driven by wave activity. What is still missing is self-consistent electron dynamics. Includes an innovative monopole solver that enables one to take about 1000 times longer timesteps than in traditional methods. Shows that both O- and U-shaped potentials are possible solutions. O-potential forms if the energetic driver is parallel wave-induced electron energisation, while a classical U-potential forms if the driver is explicit shear flow in the magnetospheric end of the simulation box.
Project 9: The occurrence frequency of upward ion beams in the auroral zone as a function of altitude using Polar/TIMAS and DE-1/EICS data (Janhunen, Olsson and Peterson). Ann. Geophysicae, 21, 2059-2072, 2003. Ion beam occurrence frequency as a kink at about 3 R_E radial distance. This is expected in the O-shaped potential model because ions speed up when moving inside the negative potential structure.
Project 10a: Middle-energy electron anisotropies in the auroral region (Janhunen, Olsson, Laakso and Vaivads). Ann. Geophysicae, in press, 2003. We show that parallel energised electrons in the 100-1000 eV range are common in the auroral region at all altitudes. Together with the cigar-shaped middle-energy electron population, an isotropic hot population usually exists, thus the distribution function as a whole cannot be described by a simple anisotropic Maxwellian.
Project 10b: Statistical study of parallel-dominated broadband 1-10 Hz waves in the auroral region (Olsson, Janhunen, Laakso and Vaivads). J. Atmos. Solar Terr. Phys., submitted, 2003. Parallel-dominated waves have an occurrence frequency peak near 3 R_E radial distance.
Project 11: Ion shell distributions as free energy source for plasma waves on auroral field lines mapping to plasma sheet boundary layer (Olsson, Janhunen and Peterson). Ann. Geophysicae, submitted, 2003. Previously, ion shell distributions have been studied in connection with pickup ions, but here we show that they are also common in the auroral magnetosphere. Some of them are related to VDIS (velocity-dispersed ion signature) events.
Project 12: Altitude dependence of plasma density in the auroral zone (Janhunen, Olsson and Laakso). Ann. Geophysicae 20, 1743-1750, 2002. Auroral density depletions are most common at intermediate altitudes, near 3 R_E radial distance. We use thresholding of Polar spacecraft potential to track relative density changes.
Project 14: Generation of Bernstein waves by ion shell distributions in the auroral energization region (Janhunen, Olsson, Vaivads and Peterson). Ann. Geophysicae, 21, 881-891, 2003. A traditional 2-D electrostatic PIC simulation is used to show that ion shell distributions can drive ion Bernstein waves which in turn may energise the electrons in the parallel direction.
Project 19a: Observational study of generation mechanism of substorm-associated low-frequency AKR emissions (Olsson, Janhunen, Hanasz, Mogilevsky, Perraut and Menietti). Ann. Geophysicae, submitted, 2003. We propose that the so-called dot-AKR emission is caused by transient Alfven waves interacting with a pre-existing auroral cavity. The dot-AKR emission has a frequency around 50 MHz, telling that it originates from the same 3 R_E radial distance where many other interesting phenomena studied above (projects 7,9,10a,10b,12) also occur.
Project 19b: Different Alfven wave acceleration processes of electrons in substorms at 4-5 R_E and 2-3 R_E radial distance (Janhunen, Olsson, Hanasz, Russell, Laakso and Samson). Ann. Geophysicae, submitted, 2003. We develop the dot-AKR idea further by doing some more quantitative calculations. We also introduce the "Alfven Resonosphere" (ARS), a layer at 4-5 R_E radial distance where the Alfven speed is close to electron speed, thus enabling a Landau resonance between them. Because the Alfven speed depends on the radial distance R roughly as 1/R^3, the ARS is a surprisingly narrow and well-defined layer. We propose that the "island" of cavities and electric fields at this altitude (seen in projects 7 and 12) is due to Alfven waves energising electrons there. The process works only during disturbed conditions when powerful enough transient Alfven waves are present.
Project 21: Some recent developments in understanding auroral electron acceleration processes (Olsson and Janhunen). IEEE Trans. Plasma Sci., in press, 2003. The most recent summary paper, covers everything up to project 14. Read this first if you want to learn about out work.