AURORAL OVAL IN THE NIGHTSIDE IONOSPHERE

Transcription

1 DYNAMICS OF THE POLAR CAP BOUNDARY AND THE AURORAL OVAL IN THE NIGHTSIDE IONOSPHERE TIMO PITKÄNEN Department of Physics University of Oulu Finland Academic dissertation to be presented, with the permission of the Faculty of Science of the University of Oulu, for public discussion in the Auditorium L10, Linnanmaa, on 10 June, 2011, at 12 o clock noon. REPORT SERIES IN PHYSICAL SCIENCES Report No. 67 OULU 2011 UNIVERSITY OF OULU

2 Opponent Dr. Stephan Buchert, Swedish Institute of Space Physics, Uppsala, Sweden Reviewers Dr. Stephan Buchert, Swedish Institute of Space Physics, Uppsala, Sweden Prof. Eija Tanskanen, Finnish Meteorological Institute, Helsinki, Finland, and University of Bergen, Norway Custos Doc. Anita Aikio, University of Oulu, Finland ISBN ISBN (PDF) ISSN Uniprint Oulu Oulu 2011

3 Pitkänen, Timo, Dynamics of the polar cap boundary and the auroral oval in the nightside ionosphere Department of Physics, University of Oulu, Finland. Report No. 67 (2011) Abstract The high-latitude polar ionosphere is characterized by two regions, the polar cap and the auroral oval. In the polar cap, the geomagnetic field lines are open and connect to the solar wind, whereas the field lines in the auroral oval are closed and map to the plasma sheet and the plasma sheet boundary layer in the magnetosphere. The two substantially different magnetic and plasma domains are separated by a separatrix, the polar cap boundary (PCB), which is an ionospheric projection of the open-closed field line boundary (OCB) in the magnetosphere. In this thesis, a new method to determine the location of the PCB in the nightside ionosphere based on electron temperature measurements by EISCAT incoherent scatter radars is introduced. Comparisons with other PCB proxies like poleward boundary of the auroral emissions, poleward edge of the auroral electrojets and poleward boundary of energetic particle precipitation show general agreement. By applying the method to several events together with other supporting ground-based and space-borne observations, dynamic processes and phenomena in the vicinity of the PCB and inside the auroral oval are studied. The main results include the following. During substorm expansion, the PCB moves poleward in a burstlike manner with individual bursts separated by 2 10 min, indicating impulsive reconnection in the magnetotail. In one event, a possible signature of the highaltitude counterpart of the Earthward flowing field-aligned current of the Hall current system at the magnetotail reconnection site is observed. Investigation of the relation between the auroral activity and the local reconnection rate estimated from the EISCAT measurements reveals direct association between individual auroral poleward boundary intensifications (PBIs) and intensifications in the ionospheric reconnection electric field within the same MLT sector. The result confirms earlier suggestions of positive correlation between PBIs and enhanced flux closure in the magnetotail. In another event, quiet-time bursty bulk flows (BBFs) and their ionospheric signatures are studied. The BBFs are found to be consistent with the so called bubble model with Earthward fast flows inside the region of depleted plasma density (bubble). The tailward return flows show an interesting asymmetry in plasma density. Whereas the duskside return flows show signatures of a depleted wake, consistent with a recent suggestion, no similar feature is seen for the dawnside return flows, but rather increase in density. The BBFs are associated with auroral streamers in the conjugate ionosphere, consistently with previous findings. The related ionospheric plasma flow patterns are interpreted as ionospheric counterpart of the BBF flows, excluding the dawnside return flows which could not be identified in the ionosphere. The BBFs and streamers are found to appear during an enhanced reconnection electric field in the magnetotail. Keywords: Polar ionosphere, Magnetosphere-ionosphere interactions, Auroral phenomena

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5 Preface This work has been carried out at the Department of Physics of the University of Oulu, Finland. I wish to express my deepest gratitude to my principal supervisor, Doc. Anita Aikio for guiding me through this work. The enormous amount of advice and support from her during the work is something I sincerely appreciate. I am also grateful to Prof. Tuomo Nygrén for all support as a co-supervisor. Plenty of thanks go to my principal co-authors, Dr. Alexander Kozlovsky from the Sodankylä Geophysical Observatory (SGO)/Oulu, Doc. Olaf Amm and Dr. Kirsti Kauristie from the Finnish Meteorological Institute (FMI), as well as Dr. Benoit Hubert from Univ. of Liège, Belgium, for all support and fruitful collaboration. The work by other co-authors not mentioned here by name, is fully acknowledged too. I also would like to thank Dr. Reijo Rasinkangas for helping in computer issues, Mrs. Ritva Kuula for assistance in data analysis and Mrs. Leena Kalliopuska for secretarial help. The Space Physics Group at Oulu has provided a nice and dynamic environment to carry out the work. I would like to thank Prof. Kalevi Mursula, Virtanen brothers, just to mention few, and all other present and former members of the group. I also would like to thank colleagues and friends both in the north (SGO) and in the south (FMI) for the Finnish space physics community. The major part of this work has been funded by the Department of Physics, where I have been working as a teaching assistant. I am very grateful to the Department for this possibility, since besides working on my PhD, I have been able to improve my teaching skills. Other financial support has been given by the Finnish Graduate School in Astronomy and Space Physics, and Faculty of Science. Special thanks go to the Vilho, Yrjö and Kalle Väisälä Foundation, which made it possible to finalize the thesis. Finally, I would like to thank my parents and my sister s family for all support and love they have given me during the work, and during my life. Oulu, May 16, 2011 Timo Pitkänen

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7 Original publications This thesis consists of an introduction and the following five original papers. I II III IV V A. T. Aikio, T. Pitkänen, A. Kozlovsky, and O. Amm, Method to locate the polar cap boundary in the nightside ionosphere and application to a substorm event, Ann. Geophys., vol. 24, pp , 2006, A. T. Aikio, T. Pitkänen, D. Fontaine, I. Dandouras, O. Amm, A. Kozlovsky, A. Vaivdas, and A. Fazakerley, EISCAT and Cluster observations in the vicinity of the dynamical polar cap boundary, Ann. Geophys., vol. 26, pp , 2008, a) T. Pitkänen, A. T. Aikio, A. Kozlovsky, and O. Amm, Reconnection electric field estimates and dynamics of high-latitude boundaries during a substorm, Ann. Geophys., vol. 27, pp , 2009a, b) T. Pitkänen, A. T. Aikio, A. Kozlovsky, and O. Amm, Corrigendum to Reconnection electric field estimates and dynamics of high-latitude boundaries during a substorm published in Ann. Geophys., vol. 27, pp , 2009, Ann. Geophys., vol. 27, pp , 2009b, /2009/ B. Hubert, A. T. Aikio, O. Amm, T. Pitkänen, K. Kauristie, S. E. Milan, S. W. H. Cowley, and J.-C. Gérard, Comparison of the open-closed field line boundary location inferred using IMAGE-FUV SI12 images and EISCAT radar observations, Ann. Geophys., vol. 28, pp , 2010, T. Pitkänen, A. T. Aikio, O. Amm, K. Kauristie, H. Nilsson, and K. U. Kaila, EISCAT-Cluster observations of quiet-time near-earth magnetotail fast flows and their signatures in the ionosphere, Ann. Geophys., vol. 29, pp , 2011,

8 In the text, the original papers are referred to using Roman numerals I-V. Paper I introduces a new method to determine the location of the polar cap boundary in the nightside ionosphere by the EISCAT incoherent scatter radars. The method is applied to a substorm event. Paper II continues applying the method. The dynamics of the polar cap boundary and auroral oval are studied during late expansion and recovery of a substorm. Paper III examines dynamics of several high-latitude ionospheric boundaries during a substorm period. In addition, the relation between the appearance of auroral poleward boundary intensifications (PBIs) and the nightside reconnection rate is studied. Paper IV compares the location of the polar cap boundary inferred from the EISCAT data by the method described in Paper I with optical data from the IMAGE satellite. Paper V studies a sequence of quiet-time Earthward bursty bulk flows (BBFs) and their counterparts in the conjugate ionosphere. Also the relation of the BBFs and their ionospheric signatures to the nightside reconnection rate is examined. In Papers I and III, the author has performed the EISCAT and Polar UVI data analysis. The Polar UVI data (intensity values) were received from the PI of the instrument (G. Parks and M. Fillingim). In Papers II and IV, the author has made the EISCAT data analysis, and in Paper V the EISCAT and Cluster satellite data analysis. Cluster data for Paper V were obtained from CDAWeb and from one of the co-authors (H. Nilsson). The author has contributed to the science interpretation in Papers I and II, and significantly in Papers III and V. In Papers III and V the author is the principal writer of the paper. The idea for the study published in Paper V came from the author.

9 Abbreviations AACGM ACE AE ASC BBELF BBF CD CHAMP CIS CODIF DMSP DNL EESA EEJ EFW EISCAT ESR FAC FAST FGM FUV GOES HD HEEA HIA HLBL IESA IMAGE Altitude Adjusted Corrected GeoMagnetic Advanced Composition Explorer Auroral Electrojet All-Sky Camera BroadBand Extra Low Frequency Bursty Bulk Flow Current Disruption CHAllenging Minisatellite Payload Cluster Ion spectrometry COmposition DIstribution Function Defence Meteorological Satellite Program Distant Neutral Line Electron ElectroStatic Analyzer Eastward ElectroJet Electric Field and Wave European Incoherent SCATter Eiscat Svalbard Radar Field-Aligned Current Fast Auroral SnapshoT FluxGate Magnetometer Far UltraViolet Geostationary Operational Environmental Satellites Harang Discontinuity High Energy Electron Analyzer Hot Ion Analyzer High-Latitude Boundary Layer Ion ElectroStatic Analyzer International Monitor for Auroral Geomagnetic Effects or Imager for Magnetopause-to-Aurora Global Exploration

10 IGRF/DGRF International/Definitive Geomagnetic Reference Field IMF Interplanetary Magnetic Field INTERMAGNET INTErnational Real-time MAGnetic observatory NETwork IS Incoherent Scatter ISR Incoherent Scatter Radar KEV, KIL Kevo, Kilpisjärvi LEEA Low Energy Electron Analyzer LHB, LHBl Lyman-Birge-Hopfield, Lyman-Birge-Hopfield long LLBL Low-Latitude Boundary Layer LYR Longyearbyen MAG MAGnetic field experiment MCRB Magnetic Convection Reversal Boundary MFI Magnetic Field Investigation MHD MagnetoHydroDynamics MIRACLE Magnetometers - Ionospheric Radars - All-sky Cameras Large Experiment MLAT, MLON Magnetic LATitude, Magnetic LONgitude MLT Magnetic Local Time MSP Meridian Scanning Photometer NENL Near-Earth Neutral Line OCB Open-Closed field line Boundary PAE Poleward Auroral Emission PBI Poleward Boundary Intensification PEACE Plasma Electron And Current Experiment PCB Polar Cap Boundary PSBL Plasma Sheet Boundary Layer R1 Region 1 R2 Region 2 SAMNET Sub-Auroral Magnetometer Network SCW Substorm Current Wedge SI12 Spectrographic Imager 12 SuperDARN Super Dual Auroral Radar Network SWB Spectral Width Boundary SWE Solar Wind Experiment SWEPAM Solar Wind Electron Proton Alpha Monitor T96 Tsyganenko 96 magnetic field model TRO Tromsø UHF Ultra High Frequency UV UltraViolet UVI UltraViolet Imager VHF Very High Frequency WEJ Westward ElectroJet WIC Wideband Imaging Camera WTS Westward Traveling Surge XY GSM plane XY plane in Geocentric Solar Magnetic coordinate system 1-D, 2-D or 3-D 1-Dimensional, 2-Dimensional or 3-Dimensional

11 Symbols A pc B B x, B y, B z B I E E r E 0r Φ B dl R E T e t V day V night v b v p Polar cap area Magnetic field Magnetic field components Magnetic field magnitude in the ionosphere Electric field Ionospheric reconnection electric field Magnetospheric reconnection electric field Magnetic flux Unit length vector Earth radius Electron temperature Time Dayside reconnection voltage Nightside reconnection voltage Polar cap boundary velocity normal to the boundary Plasma flow velocity normal to the polar cap boundary

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13 Contents Abstract Preface Original publications Abbreviations Symbols Contents 1. Introduction Magnetosphere General structure Plasma regions and current systems Ionosphere General structure Polar ionosphere and aurorae Solar wind-magnetosphere-ionosphere coupling Plasma convection Polar ionospheric currents Instrumentation EISCAT incoherent scatter radars Other ground-based instruments Instruments on board satellites Determining the location of the polar cap boundary Methods to estimate the location of the polar cap boundary Method to locate the PCB in the nightside by EISCAT measurements Dynamic phenomena linking the magnetosphere and the auroral ionosphere in the nightside Magnetic reconnection and the reconnection electric field Magnetic reconnection Reconnection electric field Substorms Bursty bulk flows and their signatures in the ionosphere Summary and conclusions References

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15 1. Introduction Life on the planet Earth is enabled and sustained by light and warmth provided by electromagnetic radiation emitted by the Sun, our nearest star. However, the Earth is affected by the Sun also in another way. The solar outer atmosphere, the corona expands through the solar system as a continuous flow of ionized plasma, the solar wind, carrying the solar magnetic field along as an interplanetary magnetic field. The Earth with its dipolar magnetic field appears as an obstacle for the solar wind flow and a magnetic cavity, the magnetosphere, is formed by the terrestrial geomagnetic field. The solar wind interacts with the magnetosphere, which allow particles and energy from the solar wind enter the cavity and cause disturbances that, eventually, lead to a spectacular show of aurorae. Aurorae appear in the weakly ionized part of the upper atmosphere, the ionosphere, which connects to the magnetosphere. The above is a short description of the solar-terrestrial interaction, a long chain of coupled phenomena starting from the Sun, acting in the near-earth space and ending in the skies of polar regions. In this thesis, we study the magnetosphereionosphere interaction part of the chain. A new technique to determine the location of one of the key boundaries in the near-earth space, the polar cap boundary (PCB) in the nightside ionosphere, by the EISCAT incoherent scatter radars is introduced. The method is applied to several events, and dynamic processes and phenomena in the vicinity of the PCB and inside the auroral oval are studied. The introductory part of the thesis is organized as follows: In this chapter, a short introduction to the near-earth space environment is given. In Chapter 2, the instrumentation used in this work is briefly described. In Chapter 3, different methods to locate the PCB are reviewed and the new technique introduced in this thesis is presented. In Chapter 4, some processes and phenomena linking the magnetosphere and the auroral nightside ionosphere are explained and discussed together with the results of this thesis. Finally, in Chapter 5 a summary and conclusions of the results are given. The original Papers I V are enclosed in the end.

16 Magnetosphere General structure Fig Schematic illustration of the magnetosphere in the noon-midnight meridional plane drawn to scale. The Sun is on the left. The unit of the axes is the Earth radius (R E = 6371 km) [Hughes, 1995]. The Earth s magnetosphere (Fig. 1.1) is formed by the interaction between the solar wind and the geomagnetic field. On the dayside, the dipolar closed geomagnetic field lines are compressed by the solar wind flow pressure and in the nightside the field is stretched by the solar wind flow to a long elongated tail. Due to the supersonic solar wind flow, a shock front called bow shock is formed in front of the magnetosphere at a distance of about R E from the Earth [e.g. Slavin and Holzer, 1981]. At the bow shock, the supersonic solar wind deccelerates to subsonic and both plasma and magnetic field (interplanetary magnetic field, IMF) are compressed. In addition, a substantial fraction of the kinetic energy of the solar wind is converted into thermal energy. Behind the bow shock the compressed, thermalized plasma continues to drift tailwards across a turbulent region, the magnetosheath [e.g. Lucek et al., 2005] before it is deflected around the magnetopause, which is the boundary that separates the magnetosphere from the solar wind [Chapman and Ferraro, 1931; Cahill and Amazeen, 1963]. On the dayside, the nose of the magnetopause is typically located at a distance of about 10 R E upstream from the Earth. In the nightside, the magnetotail extends past 220 R E downstream. The tail flares slightly, the radius of the tail lobes increases from 15 near the Earth to 30 R E farther tailwards [e.g. Slavin et al., 1983; 1985, and references therein]. Beyond 150 R E downstream the radius of the tail is estimated to be constant [Coroniti and Kennel, 1972].

17 15 The field lines of the tail lobes are connected to the solar wind. The outer boundary layer of the lobes is called the plasma mantle, which may have thickness up to a few R E [Rosenbauer et al., 1975]. Between the outer mantle and dayside closed dipolar field lines are the cleft regions known as cusps, which provide an open access to the solar wind plasma to enter the magnetosphere from the magnetosheath [e.g. Smith and Lockwood, 1996, and references therein]. The tail lobes are separated by the long elongated closed field line region, the plasma sheet [Bame et al., 1967]. During normal conditions, the last closed field line may extend past 100 R E and 200 R E downstream in the central parts and on the edges of the magnetotail, respectively [McPherron, 1995]. The last closed field line corresponds to a separatrix, which separates the plasma sheet from the tail lobes and defines the open-closed field line boundary, OCB (see also Section 1.2.2). Open field lines of the tail lobes are closed by magnetic reconnection at the X line (see Sections and 4.1). Magnetic reconnection takes place also on the dayside magnetopause and it is controlled by the IMF (Section 1.3.1) Plasma regions and current systems Figure 1.2 shows the main plasma regions and current systems in the magnetosphere. The plasma mantle contains plasma originating from the magnetosheath [Rosenbauer et al., 1975]. Plasma density and temperature in the mantle are about cm 3 and 100 ev, respectively. The tail lobes contain lower energy plasma of low density, number densities are < 0.1 cm 3. In addition, ions of ionospheric origin (H +, He +, and O + with energies 10 ev 1 kev) are often detected in the lobes [e.g. Sharp et al., 1981; Engwall et al., 2009] (Section 1.2.2). The plasma sheet contains hotter ( kev) and denser plasma than the surrounding tail lobes. The inner plasma sheet is separated from the tail lobes by the 1 2 R E thick plasma sheet boundary layer (PSBL), in which densities are typically of the order of 0.1 cm 3 [Eastman et al., 1984; Baumjohann et al., 1988]. The PSBL is characterized by frequently observed field-aligned unidirectional or counterstreaming ion beams probably generated by reconnection [e.g. Grigorenko et al., 2007, and references therein]. In the inner or central plasma sheet, plasma density varies typically from 0.1 to 1 cm 3 [Eastman et al., 1984; Baumjohann et al., 1989] and it is where transient fast bulk plasma flows are often seen (Section 4.3). The inner edge of the plasma sheet is located typically at about 7 R E tailward in the midnight sector. The plasma beta, which is the ratio between the thermal pressure and the magnetic pressure is above 0.3 in the central plasma sheet whereas values < 0.3 are measured in the PSBL [Baumjohann et al., 1988; 1989]. Earthward of the inner edge of the plasma sheet the geomagnetic field lines are dipolar and energetic ( 100 kev 100 MeV) particles are effectively trapped. These toroidial regions are called radiation belts [e.g. Van Allen and Frank, 1959; Li and Temerin, 2001]. The innermost magnetospheric region, the co-rotating

18 16 Fig Schematic figure of the magnetosphere showing the main plasma regions and current systems [Russell and Luhmann, 1997]. plasmasphere, extends on a distance 3 5 R E from the Earth, overlapping with the radiation belts [Carpenter, 1963; 1966; Nishida, 1966]. Plasmasphere contains dense ( 10 3 cm 3 ) low-energy ( 1 ev) plasma and it is basically a magnetospheric extension of the ionosphere. The eastward flowing magnetopause current on the dayside magnetopause is induced by the interaction between the solar wind particles and the dayside magnetosphere. In the nightside, the tail lobes are separated by the cross-tail current, or neutral sheet current, which flows from dawn to dusk in the neutral sheet of the central plasma sheet. The neutral sheet current closes via magnetopause currents, or tail currents, flowing around the tail lobes. In the inner magnetosphere, the ring current is built up by the radiation belt particles. The westward flowing ring current is carried mainly by ions of energies from 1 kev to a few hundreds of kev [e.g. Daglis et al., 1999] and it intensifies significantly during geomagnetic storms. The field-aligned currents connect the magnetospheric currents to currents flowing in the ionosphere (see Section 1.3.2).

19 Ionosphere General structure The ionosphere is produced by solar photoionization on ultraviolet and X-ray wavelengths, minor contribution coming from ionization by cosmic rays. In addition, at high latitudes, sporadic auroral particle precipitation from the magnetosphere may significantly contribute to the ionization. The ionization is balanced by recombination processes in which ions are neutralized. The ionosphere is divided into regions based on the altitude variations in electron density (Fig. 1.3) [e.g. Brekke, 1997]. The lowest part of the ionosphere is the D region, which covers an altitude range of about km. It is weakly ionized and mainly composed by O + 2, N+ 2 and NO+ ions with some negative ions present. Nominally, electron density increases from less than 10 7 m 3 below 60 km to m 3 at 90 km, and the region disappears for the night. The E region covers the altitudes km, the main constituents being O + 2 and NO +. The E region weakens for the night, but at high latitudes auroral precipitation can cause significant temporal rise in the local electron density. The F region lies above the E region and extends up to magnetosphere. The most abundant ion is O +, but in the topside ionosphere H + becomes the dominating species. On the average, electron density maximizes at an altitude of about 300 km. The values of electron density can reach up to m 3. Generally, electron density in the ionosphere is subject to temporal and spatial variations e.g. owing to time of the day, solar cycle and the level of the geomagnetic activity Polar ionosphere and aurorae The high-latitude polar ionosphere is characterized by two regions, the polar cap and the auroral oval (Fig. 1.4). The polar cap is defined as a region around the geomagnetic pole where the geomagnetic field lines are open and connected to the solar wind. The region equatorward of the polar cap is the auroral oval, which is on closed field lines and where aurorae appear. The polar cap is separated from the auroral oval by the polar cap boundary (PCB), which is an ionospheric projection of the open-closed field line boundary (OCB) in the magnetosphere. In the nightside, the PCB separates the open field lines extending to the tail lobe from the closed field lines mapping to the plasma sheet in the magnetotail. On the dayside, the PCB maps to the magnetopause. The auroral oval and the polar cap are asymmetric in the noon-midnight direction. On the average, the central auroral oval appears at 78 and 67 geomagnetic latitude on the day and nightside, respectively. The shape and width of the oval, as well as the location of the PCB and hence the size of the polar cap, depend on the geomagnetic activity [Feldstein and Starkov, 1967]. As the geomagnetic activity increases, the oval widens and expands to lower latitudes. During moder-

20 18 + O O + 2, NO + + O 2+, N 2, NO + Fig Typical night and day electron density profiles in opposite phases of the solar cycle at midlatitudes. The different ionospheric regions together with the most abundant ion species are also shown [after Richmond, 1987]. ate activity level (Kp-index = 3), the widths of the day- and nightside ovals are typically about 3 and 10 MLAT, respectively [Carbary et al., 2005]. In the polar ionosphere, the almost vertical geomagnetic field lines provide an open access for the plasma to move between the ionosphere and magnetosphere (Fig. 1.4). On and near the polar cap open field lines, ambipolar outflow of suprathermal (energies less than a few ev) light ions (H +, He +, and O + ) and electrons along the field lines forms continuous particle flow to the magnetosphere, the polar wind [Axford, 1968; Ganguli, 1996]. In the auroral oval, the bulk ion outflow is dominated by O +. However, most ions cannot escape without further energization to energies of >10 ev at the topside ionosphere. The energization processes involved are considered to include e.g. topside frictional heating, acceleration by parallel potential drops along the field lines and perpendicular energization by various plasma wave modes at great altitudes [e.g. André and Yau, 1997; Yau and A n d r é, 1997, and references therein]. The plasma field-aligned inflow to the ionosphere on the polar cap is dominated by precipitation of low-energy ( 100 ev) suprathermal electrons from the solar wind via open field lines, which is known as the polar rain [Winningham and Heikkila, 1974; Baker et al., 1987]. In the auroral oval, the field-aligned inflow is

21 19 Tail lobe Polar wind Polar rain Ion outflow Auroral precipitation Polar cap PCB Auroral oval Plasma sheet Fig Schematic figure of the polar ionosphere showing the main regions and their connection to the magnetosphere. In addition, principal field-aligned particle inflows and outflows are marked [after Townsend, 1982]. characterized by auroral precipitation. Nightside aurorae are caused by precipitation from the plasma sheet and the PSBL as a result of energization processes in the neutral sheet, by wave-particle interactions [e.g. Lyons et al., 1999a] and by acceleration processes at high altitudes [e.g. Marklund et al., 2011, and references therein]. Dayside aurorae originate from the dayside plasma sheet and the solar wind in the cusp/mantle region. Typically, auroral electrons in the nightside aurorae have characteristic energies of 1 10 kev, whereas on the dayside the energies are lower, about ev [Hultqvist, 2007]. The nightside oval consists of a discrete part in which structured auroral forms appear, and diffuse oval, which lies mainly equatorward of the discrete part. The diffuse oval is caused by precipitation of low-energetic electrons scattered into the loss cone by wave-particle interactions in the plasma sheet [Thorne et al., 2010]. Intense discrete forms, such as auroral arcs, are associated with parallel electrostatic potential drops along the field line, which further accelerate electrons downward within the arc [e.g. Marklund et al., 2011]. In addition, electrons accelerated by Alfvén waves may cause aurorae, most commonly near the polar cap boundary [e.g. Keiling, 2009]. The aurorae appear at about km altitudes, where emissions below 90 km are caused by the most energetic precipitation (>100 kev). The most common emissions observed are the green nm and red nm lines from atomic oxygen. The approximate peak altitudes for these emissions are 110 and 250 km, respectively [Vallance Jones, 1974]. Auroral H β emission at wavelength of nm is produced by precipitating protons that have captured an electron, thus becoming a fast excited hydrogen atom. The H β emissions are typically produced

22 20 by protons with energies of 1 30 kev. In addition, the proton Lyman-α line at nm and Lyman-Birge-Hopfield (LHB) emission bands around 150 and 170 nm from N 2 molecules on ultraviolet wavelengths are important sources of aurora and are used in global observations of aurora by space-borne instruments Solar wind-magnetosphere-ionosphere coupling Plasma convection The solar wind interacts with the magnetosphere, which allows the solar wind energy, momentum and plasma to be transferred to the magnetosphere-ionosphere system. The interaction is effectively controlled by magnetic reconnection (Section 4.1) between the IMF and geomagnetic field lines. Figure 1.5 shows a simplified schematic illustration of the situation during the southward IMF conditions, which gives a rise to the global circulation pattern of plasma and magnetic flux tubes, magnetospheric convection, as originally suggested by Dungey [1961].When the IMF has a southward component (B z < 0), the IMF field lines (1 ) are reconnected with the closed geomagnetic field lines (1) in the X line formed at low latitudes on the dayside magnetopause. The newly created open field lines (2 5) are then transported antisunward across the polar cap to the magnetotail by the solar wind flow. There, somewhere at a distance of R E downtail, the field lines are reconnected again in the tail X line (6). Finally, magnetic tension of the newly closed field lines forces them to convect Earthward and via dawn or dusk back to the dayside (7 9) where they are ready to reconnect with the IMF again. Along the highly conducting field lines the electric potential is nearly constant and the electric fields associated with the magnetospheric convection map to the ionosphere. Under influence of these electric fields, F region ionospheric plasma convects by the E B drift velocity forming two convection cells, in which plasma flows antisunward from noon to midnight over the polar cap and returns back to the dayside at lower latitudes roughly within the dawn and dusk auroral oval (Fig. 1.5). The non-zero IMF B y component displaces the reconnection region towards postnoon (B y > 0) or pre-noon (B y < 0) sectors in the northern hemisphere (oppositely in the southern hemisphere) leading asymmetry in the convection cells [e.g. Cowley and Lockwood, 1992]. When the IMF has a northward component (B z > 0), the dayside reconnection occurs at high latitudes with the open tail lobe field lines [Maezawa, 1976]. Reconnection may occur on both or only on one hemisphere. In the former case, a closed field line is created and reverse Sunward convection may appear in the central polar cap. In the latter case, the open field line causes only lobe stirring in the tail lobe and the amount of open flux is not changed [e.g. Cowley and Lockwood, 1992]. However, during northward IMF with non-zero B y component fulfilling the condition B y > B z, field lines equatorward of the cusps may reconnect [Nishida et al., 1998].

23 21 Fig Plasma convection in the magnetosphere and ionosphere [Hughes, 1995]. In addition to the reconnection-driven convection described above, also viscous interaction between the solar wind flow and the closed field lines of the low-latitude boundary layer on the magnetospheric flanks drives convection. In the ionosphere, this is seen as antisunward convection on the closed field lines poleward of the convection reversal boundary [e.g. Sotirelis et al., 2005, and references therein]. The overall contribution of the viscous interaction to the plasma convection has been estimated to be about % [Hughes, 1995] Polar ionospheric currents In the ionospheric D region the collisions dominate and the dynamics of the ions and electrons are controlled by the neutral atmosphere. In the E region, ions are still collisional, but the electron-neutral collision frequency is much smaller than the electron gyrofrequency [e.g. Brekke, 1997]. Hence, while ion motion is

24 22 controlled by neutrals, electrons perform E B drift motion and follow the magnetospheric convection. The different flow velocity of positive ions and negative electrons produces horizontal electric currents in the E region. In the F region, above 160 km, also ions are decoupled from neutrals and move together with the electrons by the same convection velocity, and no net horizontal currents flow. The ionospheric currents can be divided into horizontal E region Pedersen currents (parallel to the electric field, but perpendicular to the magnetic field) and Hall currents (perpendicular to both the electric and magnetic fields), and fieldaligned currents. Collisional ions contribute both to the Pedersen and Hall current, but electrons carry only the Hall component, which flows in the opposite direction of the electron convection. Left panel in Fig. 1.6 shows the main currents flowing in the nightside polar ionosphere. The Sunward return convection of electrons creates Hall currents called auroral east- and westward electrojets (EEJ and WEJ) that are confined within the auroral oval. The oval is embedded also by a large-scale field-aligned current (FAC) system, in which upward and downward FACs flow in the poleward and equatorward part of the FAC region in the dusk side, respectively [Iijima and Potemra, 1976a;b]. In the dawn side the order of the current distribution is opposite. The poleward and equatorward FACs are called the Region 1 (R1) and Region 2 (R2) currents, respectively. The Region 1 currents flow near the polar cap boundary and it is believed that they close to the flanks of the magnetosphere. The Region 2 currents map to the inner magnetosphere and are considered to close via ring current (see also Fig. 1.2). In the ionosphere, the field-aligned currents are closed by Pedersen currents across the oval by the convection electric field, which points duskward across the polar cap, and poleward and equatorward in the dusk and dawn oval, respectively. The dashed line in Fig. 1.6 indicates the region where the evening sector eastward electrojet turns westward in association with the convection reversal. The region was named as Harang discontinuity (HD) by Heppner [1972], after the pioneering work by Harang [1946] a few decades earlier. The HD region is associated with upward FACs flowing at the flow reversal, connecting the evening and morning side R1 and R2 upward FAC regions. The electric HD, or the convection reversal, is typically located 1 2 MLAT poleward of the magnetic HD i.e. the electrojet reversal (or the magnetic convection reversal) derived from the ground magnetic signatures [e.g. Kamide and Vickrey, 1983; Fontaine and Peymirat, 1996]. The difference may arise e.g. from the integration of currents by magnetometers in which the relative strength of the electrojets contributes to the shift, or the effects caused by the HD-associated upward FACs [Koskinen and Pulkkinen, 1995, and references therein].

25 23 Fig Left: Schematic picture showing the principal currents flowing in the northern nightside polar ionosphere. The open arrows mark the flow directions of the eastward (EEJ) and westward (WEJ) electrojets. Circles with dots or crosses inside indicate the upward and downward field-aligned currents, respectively. The dashed line marks the Harang discontinuity (HD). R1 and R2 refer to the Region 1 and Region 2 current patterns. Right: The plasma convection pattern corresponding to the left panel. Shaded arrows indicate the plasma flow direction [Koskinen and Pulkkinen, 1995].

26 2. Instrumentation 2.1. EISCAT incoherent scatter radars The basis of the data used in this thesis is formed by measurements of the EIS- CAT incoherent scatter radars operated by the international EISCAT Scientific Association in the northern Fennoscandia and on Svalbard (Fig. 2.1). The VHF radar with a parabolic cylindrical antenna (40 m x 120 m) [Folkestad et al., 1983] is located on the mainland near Tromsø, Norway, and operates at the frequency of 224 MHz. The EISCAT Svalbard Radar (ESR) [Wannberg et al., 1997] consists of two antennas, a fully steerable 32-m (diameter) parabolic dish and another 42-m parabolic dish, which is fixed to point to the direction of the local magnetic field line. The ESR radar facility is located near Longyearbyen on Svalbard archipelago, and operates at the frequency of 500 MHz. The oldest UHF radar [Folkestad et al., 1983] operating at the frequency of 931 MHz with a fully steerable 32-m parabolic dish antenna is located at the Tromsø site. The UHF radar includes two remote receivers, one in Kiruna, Sweden, and another one in Sodankylä, Finland (data from the remotes are not used in this thesis). The incoherent scatter, predicted by Gordon [1958] and observed first by Bowles [1958], measured by the radars arises from scattering of the transmitted signal by individual electrons (Thomson scattering) in ionospheric plasma. However, electrons effectively follow much slower and more massive ions, and consequently, most of the measured power in the spectrum of the received backscattered signal is concentrated to so called ion lines, the shape of which is determined by ion motion. The ion lines appear at the frequency range of a few khz around the transmission frequency and result from Landau-damped ion-acoustic waves induced by random thermal fluctuations in ionospheric plasma. Minor contribution to the incoherent scatter spectrum comes from electro-acoustic waves at higher frequency range of a few MHz. From the spectrum, several plasma parameters, e.g. electron density, electron and ion temperatures and ion line-of-sight velocity can be inferred [e.g. Evans, 1969, and references therein]. Since the backscattered signal is weak due to the small cross-section of the Thomson scattering ( m 2 ), powerful radars are needed. The peak powers of the EISCAT VHF, UHF and ESR radars are about

27 25 2, 2 and 1 MW, respectively. Typical time and spatial resolutions obtained vary from fractions of seconds to few seconds and few hundreds of meters to few tens of kilometers, respectively, depending the radar modulation used. However, often post-integration to longer periods and larger spatial scales is needed to increase the signal-to-noise ratio Other ground-based instruments The supporting ground-based data were provided by the Magnetometers - Ionospheric Radars - All-sky Cameras Large Experiment (MIRACLE) [Syrjäsuo et al., 1998]. The 2-D MIRACLE network consists of instruments placed around the northern Fennoscandia and Svalbard for monitoring mesoscale variations in the auroral electrodynamics. The instrument network is operated as an international collaboration under the leadership of the Finnish Meteorological Institute. In this thesis, we use geomagnetic field data from the MIRACLE International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer stations [Viljanen and Häkkinen, 1997] (black open circles in Fig. 2.1). The magnetometers record the geographical components of the geomagnetic field (X (north), Y (east) and Z (downwards)) by a resolution of 1 nt or better at a 10-s sampling interval. The geomagnetic data is used as input in calculations of the ionospheric 1-D eastwest [Papers I V] and 2-D equivalent currents [Paper V] by methods developed by Vanhamäki et al. [2003] and Amm [1997]; Amm and Viljanen [1999], respectively. In addition, data from MIRACLE all-sky cameras (ASCs) are used. The cameras operate continuously during the dark season and in the usual mode provide a nm image at every 20 s, and nm images once a minute by exposure times of 1, 2 and 2 s, respectively. The nm data are used in Papers II and V, nm data in Paper IV. As an example, the field-of-view of Kevo (KEV) ASC mapped to an altitude of 110 km is shown as a large black-edged circle in Fig Besides the instruments introduced above, a meridian-scanning photometer by University of Oulu [Kaila, 2003] at Kilpisjärvi [Paper V, measurement range marked by black dashed line centered at Kilpisjärvi in Fig. 2.1], coherent Super- DARN radar network [Greenwald et al., 1995] [Papers II and IV V], SAMNET and INTERMAGNET magnetometer chains [Papers I III] contributed as sources of data. The AE index [Davis and Sugiura, 1966] data characterizing global auroral electrojet activity in the northern auroral zone was used in Papers II and IV V and obtained from the World Data Center for Geomagnetism, Kyoto Instruments on board satellites The space-borne data used in this thesis come from variety of instruments on board several spacecraft including ACE, WIND, DMSP, Polar, IMAGE and Cluster. The

28 26 solar wind was measured by the Magnetic Field Experiment (MAG) [Smith et al., 1998] and the Solar Wind Electron Proton Alpha Monitor (SWEPAM) [McComas et al., 1998] on the ACE satellite near the Lagrangean L 1 point at a distance of about 220 R E Sunward from the Earth [Papers II III]. Alternatively, observations by the Magnetic Field Investigation (MFI) [Lepping et al., 1995] and the Solar Wind Experiment (SWE) [Ogilvie et al., 1995] on the WIND spacecraft ( 200 R E upstream) were used [Paper V]. Global optical images of the northern auroral oval were provided by the UV imagers on the Polar and IMAGE satellites. The images by the Polar Ultraviolet Imager (UVI) [Torr et al., 1995] taken through the Lyman-Birge-Hopfield long (LBHl, nm) filter are used in Papers I and III. The LBHl emissions originate from N 2 molecules and the emission luminosity depends primarily on the total energy flux of precipitating electrons. The images were taken using an integration time of 37 s and latitudinal resolutions of the images varied on the range of MLAT and MLAT in Papers I and III, respectively. In Paper IV, global observations by the two IMAGE Far Ultraviolet (FUV) imagers [Mende et al., 2000a] were used. The FUV Spectral Imager SI12 detects Lyman-α (121.8-nm) proton aurorae and practically is not sensitive to the dayglow. The Wideband Imaging Camera (WIC) is sensitive for N 2 LBH bands ( nm) including the same LBHl emission wavelengths as the used Polar UVI data described above. The SI12 images were taken using an integration time of 5 s, where the WIC image integration time was 10 s. The latitudinal resolution of the FUV data used was 1 MLAT. Details of the IMAGE FUV SI12 and WIC instruments can be found in Mende et al. [2000b] and Mende et al. [2000c], respectively. Magnetospheric data used in Papers II and V were obtained from the Cluster satellite measurements. Cluster is formed by four spacecraft with identical set of eleven instruments [Escoubet et al., 1997]. In this thesis, data from four instruments out of the eleven are used. The Cluster Magnetic Field Investigation (Fluxgate Magnetometer, FGM) experiment [Balogh et al., 2001] is built up of two triaxial fluxgate magnetometers, which provide the three components of the magnetic field vector by sampling rates of vectors/s (Normal mode) or vectors/s (Burst mode). Depending on the operative range currently in use, the magnetic field accuracy varies from 8 pt to 0.5 nt for ± 64 nt and ± 4096 nt ranges, respectively. However, normally spacecraft spin-averaged ( 4 s) data are used. The Cluster Ion Spectrometry (CIS) experiment [Rème et al., 2001] consists of two distinct detectors, the COmposition DIstribution Function (CODIF) and the Hot Ion Analyzer (HIA). Both detectors provide a measurement of full 3-D ion distributions every spacecraft spin period ( 4 s). The CODIF mass spectrometer is able to separate the major ion species (proton, He ++, He +, and O + ) from the thermal energies (< 15 ev/e) up to 38 kev/e. The HIA detector records incoming ion distributions from the energy range of 5 ev/e 32 kev/e without mass resolution. From the distribution functions, higher moments like particle densities, the three components of the velocity vector and temperatures are computed. The Plasma Electron And Current Experiment (PEACE) [Johnstone et al.,

29 ] has two sensors, the Low Energy Electron Analyzer (LEEA) and High Energy Electron Analyzer (HEEA). Both sensors can measure complete 3-D distributions of electrons on the energy range from 0.59 ev to 26.4 kev in the 4-s spin period. However, the LEEA is designed to cover the very lowest electron energies ( ev) whereas the HEEA sensor is specialized to measure the higher-energy range of the electron energy spectrum. The Electric Field and Wave (EFW) experiment [Gustafsson et al., 1997; 2001] consists of four spherical probes at the ends of orthogonal 44-m long wire booms in the spin plane of the spacecraft. The potential difference measured between the opposite probes separated by 88 m is used to obtain the components of the electric field in the spin plane. Additionally, from the potential difference between a probe and the spacecraft, electron density and temperature can be derived. Normally, the electric field is measured with a sampling rate of 25 s 1 (Normal mode, 40 ms time resolution) or 450 s 1 (Burst mode, 2.2 ms). However, sampling rates up to s 1 can be used over short periods of time using data buffer memory. The measurement range of the quasi-static and oscillating electric fields cover mvm 1 and 10 mvm 1 1 µvm 1, respectively. FGM and CIS CODIF (and HIA) data are used both in Paper II (and V). Data from the PEACE HEEA and EFW instruments are used only in Paper II. To support the optical observations, particle precipitation data from low-altitude orbiting satellites like DMSP [Papers I, III, and IV] or FAST [Paper II] were used whenever available. The Defence Meteorological Satellite Program (DMSP) satellites have about 100-min almost circular Sun-synchronous polar orbits at altitudes of 830 km. The DMSP SSJ/4 instrument [Hardy et al., 1984] has detectors that measure fluxes of precipitating electrons and ions on the energy range from 32 ev to 30 kev. The FAST data are from the electron (EESA, 6 ev 30 kev) and ion (IESA, 5 ev 24 kev) spectrometers [Carlson et al., 2001] on board the spacecraft.

30 28 75 º ESR 42m LYR ESR 32m 70 º VHF 65 º TRO KIR SOD 130 º 60 º 120 º 55 º 90 º 100 º 110 º Fig Map of ground-based instrumentation used in this thesis in AACGM coordinates. Black dots indicate the EISCAT radar locations. The VHF radar is located near Tromsø (TRO, 66.6 MLAT, MLON) and ESR 32m and 42m antennas of the ESR system close to the Longyearbyen (LYR, 75.2 MLAT, MLON) on Svalbard. The UHF transmitter-receiver is located at the Tromsø site and the two remote receivers in Kiruna (KIR, 64.9 MLAT, MLON) and Sodankylä (SOD, 64.1 MLAT, MLON). Black lines are the ground-level projections of the VHF and ESR 32m radar beams pointing to opposite directions, as during the measurements studied in Paper V. The black small open circles mark stations of the MIRACLE magnetometer network, and the large circle the field of view of the KEV (66.3 MLAT, MLON) ASC. Black dashed line centered at Kilpisjärvi indicates the measurement range of the KIL MSP used in Paper V.

31 3. Determining the location of the polar cap boundary The ability to determine the location of polar cap boundary (PCB) has a great importance in studies of energy and plasma transfer processes, like substorms and reconnection events in general (Chapter 4). However, the PCB is not directly observable, but has to be identified by using proxies arising from differences between the plasma properties in the open polar cap and closed auroral field line regions Methods to estimate the location of the polar cap boundary Currently, the most precise technique in identifying the PCB is considered to be particle precipitation measurements by low-altitude polar orbiting satellites, like DMSP satellites. From the energy spectra of particles, different precipitation regions, e.g. energetic precipitation on closed field lines and low-energy polar rain on open field lines, can be distinguished [e.g. Newell et al., 1991a;b; 1996a;b; Sotirelis et al., 1999]. However, the point measurements on single longitudes and sparse 100 min orbital periods do not allow global or continuous detection of the boundary. Nevertheless, precipitation data have an important role in calibrating the other proxies. E.g. Blanchard et al. [1995; 1996; 1997a] introduced a method based on ground-based auroral observations of 630-nm emissions by a meridianscanning photometer, which provides continuous, though local estimate for the PCB in the nightside. They identified the boundary as a jump in the emission intensity in transition from the polar cap to closed field lines. In their comparison, deviation of the 630-nm boundary from the DMSP PCB estimates was found to be about ±1 MLAT. Space-borne optical imaging by satellites provides the possibility to observe the PCB globally. The polar cap boundary is identified as the poleward auroral emission (PAE) boundary from the global auroral images. Either a fixed threshold value of the auroral luminosity [e.g. Frank and Craven, 1988; Brittnacher et al.,