| 1. |
- Arvelius, Johan, 1973-
(författare)
-
Calibration and quality assessment of DESCARTES : grabsampler for stratospheric tracers
- 2005
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- DESCARTES is a light-weight, balloon-borne grab sampler for stratospheric long-lived tracers developed at the University of Cambridge. 33 flights have been performed with two versions of the instrument at northern latitudes by the DESCARTES team at the Swedish Institute of Space Physics (IRF) in Kiruna during the years 1997-2000.The general interest in long-lived stratospheric tracers is to study the general global circulation of air in the stratosphere and the exchange between the stratosphere and troposphere. In the study of chemical ozone depletion in the stratosphere, long-lived tracers serve as an important reference to distinguish between the variations in ozone of dynamical and chemical origin.This thesis focuses on calibrations and quality assessment of the measurements made with the third version of the DESCARTES instrument based at IRF. Two different general approaches to make calibrations are discussed. Uncertainty estimations for both of these methods are made and the results are tested by laboratory methods and by comparisons to other instruments, including comparisons between two versions of DESCARTES. Analyzed and calibrated flight data for all successful flights are presented.The basic principle of the instrument is to chemically adsorb a number of tracers (in practice only CFC-11 is measured) in an adsorption bed of Carboxen in a micro trap through which the sampled air is driven by a pump. After recovery the adsorbed species in the trap is desorbed by electrical heating of the trap and analysed by gas chromatography.The resulting estimated mixing ratios from the instrument are directly dependent on the adsorption of the sampled species being quantitative in the traps. Laboratory experiments are described using two traps in series, where the performance of the first is tested by sampling the breakthrough by the second. A model is developed to recreate these tests in order to be able to compensate for breakthrough during flights. The model showed that the adsorption in the traps is not explained by simple chromatographic theory and the results allow us only to give an estimation of the uncertainty due to breakthrough.
|
|
| 2. |
- Arvelius, Sachiko, 1970-
(författare)
-
Energization and Acceleration of Dayside Polar Outflowing Oxygen
- 2005
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with energetic oxygen ions (i.e. single-charged atomic oxygen ions, O+) at altitudes higher than 5 Earth radii (RE) and at latitudes above 75 (toward 90) degrees invariant latitude (deg ILAT) in the dayside polar magnetosphere observed by Cluster. The instrument used in this study is CIS (Cluster Ion Spectrometry experiment) / CODIF (a time-of-flight ion COmposition and DIstribution Function analyser), which covers an energy range from »10 eV up to 38 keV. Cluster detected O+ with energies more than 1 keV (hereafter termed “keV O+”), indicating that energization and/or acceleration process(es) take place in the dayside high-altitude (inside magnetopause) and high-latitude region. These O+ are outflowing (precisely, upward-going along the geomagnetic field lines), and these outflowing keV O+ show a heated (or energized) signature in the velocity distribution as well.First, outflowing O+ are observed at the poleward cusp and/or the mantle formed a partial shell-like configuration seen in the velocity distribution. Second, the latitudinal distribution of outflowing O+ (most of them have energies less than 1 keV statistically) observed below 7 RE is consistent with velocity filter effect by the polar convection, while the latitudinal distribution of outflowing keV O+ observed above 7 RE cannot be explained by velocity filter effect only, i.e. this indicates that additional energization and/or acceleration takes place at higher altitudes in the dayside polar region. Thirdly, a tendency to observe outflowing keV O+ for during different geomagnetic conditions is studied. The keV O+ above 9 RE is more often for K p¸5 rather than for K p•3. However the energy of O+ is not dependent on ASY /SYM indices.Finally, the dependence on the solar wind conditions is also studied. The energization and/or acceleration of outflowing O+ is controlled by both solar wind moments (except solar wind electric field) and strong southward interplanetary magnetic field (IMF) at the time scale of tens of minutes at only higher altitudes. Further examination shows that solar wind dependence is different at three regions: one is the poleward cusp, another is the low-altitude polar cap, and finally the high-altitude polar cap, combining all the results. There is (a) new energization and/or acceleration process(es) at the high-altitude polar cap. On the other hand, flux enhancement of O+ observed above 5 RE is also controlled by solar wind moments (e.g. solar wind electric field) and strong southward IMF, however the ionospheric changes play a more important role on the flux enhancement of O+.
|
|
| 3. |
- Barabash, Victoria, 1965-
(författare)
-
Investigation of Polar Mesosphere Summer Echoes in Northern Scandinavia
- 2003
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This PhD thesis deals with phenomena which are closely related to the unique thermal structure of the polar summer mesosphere, namely Polar Mesosphere Summer Echoes (PMSE). PMSE are strong radar echoes commonly observed by VHF MST radars from thin layers in the 80-90 km altitude interval at high latitudes during summer. They follow a seasonal pattern of abrupt appearance in late May and a gradual disappearance in mid-August. This period corresponds roughly to the time between the completion of the summer time cooling of the polar mesopause to the time of reversal of the mesospheric circulation to autumn condition. In this connection, PMSE are associated with the extremely low temperatures, i.e. below 140 K, which are unique to the polar summer mesopause. Traditional theories of radar (partial) reflection and scattering have been unable to explain the PMSE and the exact mechanism for their occurrence remains unclear despite the steadily increasing interest in them over the past 20 years. Currently accepted theories regarding the mechanism giving rise to PMSE agree that one of the conditions needed for enhanced radar echoes is the presence of low-mobility charge carries such as large cluster ions and ice aerosols which capture the ambient electrons. It has been established that the PMSE are in some way associated with noctilucent clouds (NLC), layers of ice crystals, which constitute the highest observed clouds in the earth’s atmosphere. PMSE occurrence and dynamics are also found to be closely connected with the planetary and gravity waves.Observations of PMSE presented in this thesis have been carried out by the Esrange MST radar (ESRAD) located at Esrange (67°56’N, 21°04’E) just outside Kiruna in northernmost Sweden. The radar operates at 52 MHz with 72 kW peak power and a maximum duty cycle of 5%. The antenna consists of 12x12 array of 5-element Yagis with a 0.7l spacing. During the PMSE measurements the radar used a 16-bit complementary code having a baud length of 1mS. This corresponds to height resolution of 150 m. The sampling frequency was set at 1450 Hz. The covered height range was 80-90 km. The presence of PMSE was determined on the basis of the radar SNR (signal-to-noise ratio). The PMSE measurements have been made during May-August each year since 1997.PMSE seasonal and diurnal occurrence rates as well as dynamics have been studied in connection with tidal winds, planetary waves, temperature and water vapor content in the mesosphere (Papers I, IV and VI). Simultaneous and common-volume observations of PMSE and noctilucent clouds have been performed by radar, lidar and CCD camera (Paper V). Correlation between variations in PMSE and variations in extra ionization added by precipitating energetic electrons or high-energy particles from the Sun has been examined (Papers II and III). Possible influence of transport effects due to the electric field on PMSE appearance has been studied during a solar proton event (Paper III).
|
|
| 4. |
- Belova, Alla, 1973-
(författare)
-
Studies of planetary waves in ozone and temperature fields as observed by the Odin satellite in 2002-2007
- 2008
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The results presented in this PhD thesis are mainly based on measurements collected by the advanced sub-mm radiometer (SMR) aboard the Odin satellite in 2002-2007. The primary data are series of temperature and ozone profiles in the middle atmosphere up to 68 km. These data are used to estimate global properties of planetary wave propagation in both horizontal and vertical directions. As good-quality retrievals from Odin are not available above 68 km, additional data sources have been considered in order to extend coverage of planetary wave properties to higher levels. These sources are temperature observations at 85-90 km obtained by the ground-based meteor radars located in the polar region in the Northern Hemisphere in Scandinavia at Esrange and at Andenes, and in Canada at Resolute Bay and at Yellowknife. Also, the series of ozone profiles from the ground-based Kiruna mm-wave radiometer, KIMRA, are used in order to compare the wave properties in ozone fields measured globally by Odin and locally by KIMRA.The main task of this PhD thesis is to study the 5-day planetary wave characteristics in the Earth’s atmosphere. The influence of waves on the atmospheric circulation causes, for example, substantial local departures from radiative equilibrium, observed in the winter stratosphere and close to the summer mesopause. Seasonal variations of the 5-day planetary wave properties and physical phenomena related to these variations are also studied in this thesis.During winter, planetary waves propagate freely in the vertical direction, and maximal wave amplitudes are found in the extratropical stratosphere. The Northern Hemisphere (NH) winter periods of 2002-2003 and 2005 have been examined and a comparison has been carried out between the planetary wave properties in temperature and ozone variations. In general, the results show an expected in-phase behavior between the temperature and ozone fields in the lower stratosphere (due to dynamic effects) and an out-of-phase pattern in the upper stratosphere (which is expected as a result of photochemical effects).Earlier theoretical and experimental studies have shown that, despite unfavourable summertime wind conditions, 5-day planetary waves can be registered not only in the stratosphere but also at higher altitudes in the mesosphere. The NH summers of 2003-2005 and 2007 have been considered and results have confirmed the existence of 5-day planetary waves up to the mesopause level (85-90 km). The results demonstrate that, for different periods, the possible source of the observed waves could be located at lower altitudes in both hemispheres with successive propagation into the summer mesosphere, or the waves could be generated in-situ as a result of the baroclinic instability of summer easterly jet.
|
|
| 5. |
- Bergman, Sofia, 1991-
(författare)
-
The effect of spacecraft charging on low-energy ion measurements around comet 67P/Churyumov-Gerasimenko
- 2020
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A spacecraft in space interacts with the surrounding environment and aqcuires an electrostatic potential. Charged particles are constantly bombarding the surface of the spacecraft, and at the same time solar EUV radiation induces photoemission, causing electrons to be emitted from the surface. The result is a transfer of charge between the environment and the spacecraft surface, and the surface charges to a positive or negative potential. The charged surface can cause interferences with scientific instruments on board. In this thesis, we investigate how spacecraft charging affects low-energy ion measurements. The Rosetta spacecraft visited comet 67P/Churyumov-Gerasimenko between the years 2014-2016. On board the spacecraft, the Ion Composition Analyzer (ICA) was measuring positive ions in the environment around the comet with the aim of investigating the interaction between cometary particles and the solar wind. Important for this interaction is ions with a low energy. Measuring these ions is, however, difficult due to the charged spacecraft surface. Rosetta was commonly charged to a negative potential, and consequently the measured positive ions were accelerated toward the surface before detection, affecting both their energy and travel direction. In this thesis, we study how the changed travel directions affected the effective field of view (FOV) of the instrument. We use the Spacecraft Plasma Interaction Software (SPIS) to simulate the spacecraft plasma interactions and the ion trajectories around the spacecraft. The results show that the FOV of ICA is severely distorted at low ion energies, but the distortion varies between different viewing directions of the instrument and is dependent on the properties of the surrounding plasma.
|
|
| 6. |
- Brinkfeldt, Klas
(författare)
-
Instrumentation for energetic Neutral atom measurements at Mars, Venus and The Earth
- 2005
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with the development and calibrations of sensors to measure energetic neutral atoms (ENAs) at Mars, Venus, and the Earth. ENAs are formed in charge exchange processes between energetic, singly--charged ions and a cold neutral gas. Since ENAs can travel in long straight trajectories, unaffected by electric or magnetic fields, they can be used to remotely image plasma interactions with neutral atmospheres. ENA instrument techniques have matured over the last decade and ENA images of the Earth's ring current for example, have successfully been analyzed to extract ion distributions and characterize plasma flows and currents in the inner magnetosphere.Three different ENA sensors have been developed to image ENAs at Mars, Venus, and the Earth. Two of them, the nearly identical Neutral Particle imagers (NPIs) are on-board the Mars Express and Venus Express spacecraft as a part of the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3 and 4) instruments. The third is the Neutral Atom Detector Unit, NUADU, aboard the TC-2 spacecraft of the Double Star mission. The NPI design is based on a surface reflection technique to measure low energy (~0.3-60 keV) ENAs, while the NUADU instrument is based on a simple design with large geometrical factor and solid state detectors to measure high energy ENAs (~20-300 keV).The calibration approach of both NPI sensors were to define the detailed response, including properties such as the angular response function and efficiency of one reference sensor direction then find the relative response of the other sensor directions. Because of the simple geometry of the NUADU instrument, the calibration strategy involved simulations to find the cutoff energy, geometrical factor and angular response. The NUADU sensor head was then calibrated to find the response to particles of different mass and energy. The NPI sensor for the Mars Express mission revealed a so-called priority effect in the sensor that lowers the angular resolution at high detector bias. During the calibration of the Venus Express NPI sensor tests were made which showed that the priority effect is a result of low amplitude (noise) pulses generated in the detector system. The conclusion is that the effect is caused by capacitive couplings between different anode sectors of the sensor. The thresholds on the preamplifiers were set higher on the Venus Express NPI, which removed the priority effect.Two of the three ENA experiments, the Double Star NUADU instrument and the Mars Express NPI sensor, have successfully measured ENAs that are briefly described in the thesis. The first ENA measurements at Mars were performed with Mars Express. Initial results from the NPI include measurements of ENAs formed in the Martian magnetosheath and solar wind ENAs penetrating to the nightside of Mars. The first results from NUADU in Earth orbit show the expected ENA emissions from a storm time ring current. Also, together with the HENA instrument on the IMAGE spacecraft, NUADU have produced the first multi-point ENA image of the ring current.
|
|
| 7. |
- Brändström, Urban, 1965-
(författare)
-
The Auroral Large Imaging System : design, operation and scientific results
- 2003
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Auroral Large Imaging System (ALIS) was proposed in 1989 by Åke Steen as a joint Scandinavian ground-based nework of automated auroral imaging stations. The primary scientic objective was in the field of auroral physics, but it was soon realised that ALIS could be used in other fields, for example, studies of Polar Stratospheric Clouds (PSC), meteors, as well as other atmospheric phenomena.This report describes the design, operation and scientic results from a Swedish prototype of ALIS consisting of six unmanned remote-controlled stations located in a grid of about 50 km in northern Sweden. Each station is equipped with a sensitive high-resolution (1024 x 1024 pixels) unintensified monochromatic CCDimager. A six-position filter-wheel for narrow-band interference filters facilitates absolute spectroscopic measurements of, for example, auroral and airglow emissions. Overlapping fields-of-view resulting from the station baseline of about 50 km combined with the station field-of-view of 50° to 60°, enable triangulation as well as tomographic methods to be employed for obtaining altitude information of the observed phenomena.ALIS was probably one of the first instruments to take advantage of unintensi- fied (i.e. no image-intensifier) scientific-grade CCDs as detectors for spectroscopic imaging studies with multiple stations of faint phenomena such as aurora, airglow, etc. This makes absolute calibration a task that is as important as it is dificult.Although ALIS was primarily designed for auroral studies, the majority of the scientific results so far have, quite unexpectedly, been obtained from observations of HF pump-enhanced airglow (recently renamed Radio-Induced Aurora). ALIS made the first unambiguous observation of this phenomena at high-latitudes and the first tomography-like inversion of height profiles of the airglow regions. The scientific results so far include tomographic estimates of the auroral electron spectra, coordinated observations with satellite and radar, as well as studies of polar stratospheric clouds. An ALIS imager also participated in a joint project that produced the first ground-based daytime auroral images. Recently ALIS made spectroscopic observations of a Leonid meteor-trail and preliminary analysis indicates the possible detection of water in the Leonid.
|
|
| 8. |
- Carozzi, Tobia
(författare)
-
Radio waves in the ionosphere : Propagation, generation and detection
- 2000
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- We discuss various topics concerning the propagation, generation, and detec-tionof high-frequency (HF) radio waves in the Earth's ionosphere. With re-gardsto propagation, we derive a full wave Hamiltonian and a polarization evo-lutionequation for electromagnetic waves in a cold, stratified magnetoplasma.With regards to generation, we will be concerned with three experiments con-ducted at the ionosphere- radio wave interaction research facilities at Sura, Rus-siaand Tromsø, Norway. These facilities operate high power HF transmittersthat can inject large amplitude electromagnetic waves into the ionosphere andexcite numerous nonlinear processes. In an experiment conducted at the Surafacility, we were able to measure the full state of polarization of stimulatedelectromagnetic emissions for the first time. It is expected that by using thetechnique developed in this experiment it will be possible to study nonlinearpolarization effects on powerful HF pump waves in magnetoplasmas in the fu-ture.In another experiment conducted at the Sura facility, the pump frequencywas swept automatically allowing rapid, high-resolution measurements of SEEdependence on pump frequency with minimal variations in ionospheric condi-tions.At the Tromsø facility we discovered by chance a highly variable, pumpinduced, HF emission that most probably emanated from pump excited spo-radicE. Regarding detection, we have proposed a set of Stokes parametersgeneralized to three dimension space; and we have used these parameters in aninvention to detect the incoming direction of electromagnetic waves of multiplefrequencies from a single point measurement.
|
|
| 9. |
- Ekeberg, Jonas, 1978-
(författare)
-
Solitary waves and enhanced incoherent scatter ion lines
- 2011
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis addresses solitary waves and their significance for auroral particle acceleration, coronal heating and incoherent scatter radar spectra. Solitary waves are formed due to a balance of nonlinear and dispersive effects. There are several nonlinearities present in ideal magnetohydrodynamics (MHD) and dispersion can be introduced by including theHall termin the generalised Ohm’s law. The resulting system of equations comprise the classical ideal MHD waves, whistlers, drift waves and solitarywave solutions. The latter reside in distinct regions of the phase space spanned by the speed and the angle (to the magnetic field) of the propagating wave. Within each region, qualitatively similar solitary structures are found. In the limit of neglected electron intertia, the solitary wave solutions are confined to two regions of slow and fast waves, respectively. The slow (fast) structures are associated with density compressions (rarefactions) and positive (negative) electric potentials. Such negative potentials are shown to accelerate electrons in the auroral region (solar corona) to tens (hundreds) of keV. The positive electric potentials could accelerate solar wind ions to velocities of 300–800 km/s. The structure widths perpendicular to themagnetic field are in the Earth’s magnetosphere (solar corona) of the order of 1–100 km (m). This thesis also addresses a type of incoherent scatter radar spectra, where the ion line exhibits a spectrally uniform power enhancement with the up- and downshifted shoulder and the spectral region in between enhanced simultaneously and equally. The power enhancements are one order of magnitude above the thermal level and are often localised to an altitude range of less than 20 km at or close to the ionospheric F region peak. The observations are well-described by a model of ion-acoustic solitary waves propagating transversely across the radar beam. Two cases of localised ion line enhancements are shown to occur in conjunction with auroral arcs drifting through the radar beam. The arc passages are associated with large gradients in ion temperature, which are shown to generate sufficiently high velocity shears to give rise to growing Kelvin-Helmholtz (K-H) instabilities. The observed ion line enhancements are interpreted in the light of the low-frequency turbulence associated with these instabilities.
|
|
| 10. |
- Ekenbäck, Andreas, 1975-
(författare)
-
Numerical modelling of ENAs from stellar wind interactions
- 2008
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Energetic neutral atoms (ENAs) are produced whenever a stellar wind encounters a neutral atmosphere. If a stellar wind proton comes sufficiently close to a neutral a charge-exchange reaction may take place, transforming the proton into an ENA. Unaffected by magnetic and electric fields, ENAs provide an opportunity for global imaging of stellar wind interactions.This thesis presents methods and results of numerical modelling of stellar wind interactions. In particular it treats in depth production of ENAs at comets, Mars and the extrasolar planet HD 209458b.Sufficiently accurate numerical models of stellar wind interactions require extensive computations. Parallel computing has therefore been used throughout the work, both for fluid and particle simulations of space plasmas. This thesis describes the use of a general simulation tool, providing parallel computing for space plasma simulations.The thesis presents estimations of the magnitude and morphology of the ENA production at comets and HD 209458b. It compares the results obtained with observations and analyzes them in the light of ENA production at similar objects. Also, simulated ENA images for Mars were produced and compared to observations.
|
|
| 11. |
- Grigoriev, Alexander, 1975-
(författare)
-
The Neutral Particle Detector on the Mars and Venus Express missions
- 2007
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Neutral Particle Detector (NPD) is a new type of instrumentation for energetic neutral atom (ENA) diagnostics. This thesis deals with development of the NPD sensor designed as a part of the plasma and neutral particle packages ASPERA-3 and ASPERA-4 on board Mars Express and Venus Express, the European Space Agency (ESA) satellites to Mars and Venus, respectively. It describes how the NPD sensors were designed, developed, tested and calibrated. It also presents the first scientific results obtained with NPD during its operation at Mars. The NPD package consists of two identical detectors, NPD1 and NPD2. Each detector has a 9o x 90o intrinsic field-of-view divided into three sectors. The ENA detection principle is based on the surface interaction technique. NPD detects ENA differential fluxes within the energy range of 100 eV to 10 keV and is capable of resolving hydrogen and oxygen atoms by time-of-flight (TOF) measurements or pulse height analysis.During the calibration process the detailed response of the sensor was defined, including properties such as an angular response function and energy dependent efficiency of each of the sensor sectors for different ENA species. Based on the NPD measurements at Mars the main scientific results reported so far are:- observation of the Martian H-ENA jet / cone and its dynamics, - observations of ENA emissions from the Martian upper atmosphere, - measurements of the hydrogen exosphere density profile at Mars, - observations of the response of the Martian plasma environment to an interplanetary shock, - observations of the H-ENA fluxes in the interplanetary medium.
|
|
| 12. |
- Kastinen, Daniel
(författare)
-
From meteors to space safety : dynamical models and radar measurements of space objects
- 2022
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Every day the Earth's atmosphere is bombarded by 10-200 metric tons of dust-sized particles and larger pieces of material from space called meteoroids. Dust and meteoroids come from parent bodies such as comets and asteroids, which are remnants from the formation of the solar system. In addition to natural objects, geospace contains artificial satellites and space debris that needs to be monitored to reduce the risk of collisions. Studies of all these kinds of space objects form a cross-disciplinary research field that stretches from meteors to space safety. The primary goal of this thesis has been to rigorously connect measurements and their uncertainties with high-level analysis and dynamical simulations of distributions.An automated radar data analysis algorithm was developed for meteor head echo measurements. The analysis algorithm is able to produce realistic uncertainties for each individual meteor event, including the meteoroid orbit. Many of the resulting probability distributions are non-Gaussian, which needs to be accounted for. The analysis algorithm was applied to interferometric high-power large-aperture MU radar data in a case study on high altitude meteors. The study found that 74 out of 106,000 meteors appeared higher than 130 km and a few confirmed detections reached up to 150 km altitude.Comet 21P/Giacobini–Zinner is the parent body of the meteoroid stream giving rise to the October Draconid meteor shower. The meteoroid stream was simulated accounting for parent body orbital uncertainties to estimate meteor shower parameters. The simulation was able to model the unexpected mass distribution observed in the 2011 and 2012 October Draconids. It also successfully predicted a meteor outburst in 2018. Further, methods to reduce the computation time of meteoroid stream simulations using importance sampling were derived and implemented on a test model.EISCAT radar measurements were performed to study space debris from the Kosmos-1408 satellite, which had been destroyed and fragmented in orbit on 15 November, 2021. A novel method to estimate the size distribution of debris objects was developed. Data from two EISCAT radars were used to demonstrate a new initial orbit determination technique, yielding good agreement with known catalogue orbits. Finally, the detectability of near-Earth objects (NEOs) with the EISCAT~3D radar currently under construction was simulated. It was predicted that as many as seven temporarily captured NEOs, i.e. minimoons, could be discovered per year depending on the amount of allocated observation time. The predictions also show that hundreds of NEOs could be tracked yearly to improve their orbits.
|
|
| 13. |
- Kero, Johan, 1978-
(författare)
-
High-resolution meteor exploration with tristatic radar methods
- 2008
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A meteor observed with the naked eye is colloquially called a shooting star. The streak of light is generated by an extra-terrestrial particle, a meteoroid, entering the Earth’s atmosphere. The term meteor includes both luminosity detectable by optical means and ionization detectable by radar. The radar targets of meteor head echoes have the same motion as the meteoroids on their atmospheric flight and are relatively independent of aspect angle. They appear to be compact regions of plasma created at around 100 km altitude and have no appreciable duration. This thesis reviews the meteor head echo observations carried out with the tristatic 930 MHz EISCAT UHF radar system during four 24h runs between 2002 and 2005, and a 6h run in 2003 with the monostatic 224 MHz EISCAT VHF radar. It contains the first strong observational evidence of a submillimeter-sized meteoroid breaking apart into two distinct fragments. This discovery promises to be useful in the further understanding of the interaction processes of meteoroids with the Earth’s atmosphere and thus also the properties of interplanetary/interstellar dust. The tristatic capability of the EISCAT UHF system makes it a unique tool for investigating the physical properties of meteoroids and the meteor head echo scattering process. The thesis presents a method for determining the position of a compact radar target in the common volume of the antenna beams and demonstrates its applicability for meteor studies. The inferred positions of the meteor targets are used to estimate their velocities, decelerations, directions of arrival and radar cross sections (RCS) with unprecedented accuracy. The head echoes are detected at virtually all possible aspect angles all the way out to 130° from the meteoroid trajectory, limited by the antenna pointing directions. The RCS of individual meteors simultaneously observed with the three receivers are equal within the accuracy of the measurements with a very slight trend suggesting that the RCS decreases with increasing aspect angle. A statistical evaluation of the measurement technique shows that the determined Doppler velocity agrees with the target range rate. This demonstrates that no contribution from slipping plasma is detected and that the Doppler velocities are unbiased within the measurement accuracy. The velocities of the detected meteoroids are in the range of 19-70 km/s, but with very few detections at velocities below 30 km/s. The thesis compares observations with a numerical single-body ablation model, which simulates the physical processes during meteoroid flight through the atmosphere. The estimated meteoroid masses are in the range of 10-9 - 10-5.5 kg.
|
|
| 14. |
|
|
| 15. |
- Nikulin, Grigory, 1973-
(författare)
-
Impact of Rossby waves on ozone distribution and dynamics of the stratosphere and troposphere
- 2005
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Several physical mechanisms concerning the impact of Rossby waves on ozone distribution and circulation in the stratosphere and troposphere are studied in the thesis.Summertime total ozone variability over Middle Asia and Northern Scandinavia shows similar wave-like behaviour with typical periods of 10-20 days and amplitudes of 20-50 Dobson units. These variations are caused by eastward travelling Rossby waves in the lower stratosphere. The same mechanism plays the primary role in the formation of an intense low ozone episode over Scandinavia in August 2003. A strong anticyclone was formed in the troposphere over Europe as a part of a Rossby wave train. The anticyclone coincides with a displaced Artic pool of low-ozone air in the stratosphere aloft of the anticyclone. A combination of the two above-mentioned processes results in the total ozone minimum over Northern Europe for summer 2003.Interannual variability of the atmospheric circulation and total ozone during winter is strongly controlled by the diabatic (Brewer-Dobson) circulation which is driven by upward propagating waves from the troposphere. In the Northern Hemisphere midlatitudes, wintertime total ozone shows antiphase behaviour with the Arctic Oscillation (AO) index on interannual and decadal time-scales. Weaker (stronger) wave activity leads to less (more) northward ozone transport and to a stronger (weaker) AO.Rossby wave activity occurs as episodic wave events and this wave forcing is not uniform during winter. The November-December stratospheric eddy heat flux is strongly anticorrelated with the January-February eddy heat flux in the midlatitude stratosphere and troposphere. Weaker upward wave fluxes in early winter lead to stronger upward wave fluxes from the troposphere as well as to a stronger polar night jet during midwinter and vice versa. Hence upward wave activity fluxes in early winter define, to a considerable extent, the subsequent evolution of the midwinter circulation in the stratosphere and troposphere.
|
|
| 16. |
- Persson, Moa, 1991-
(författare)
-
Escape to space or return to venus : ion flows measured by venus express
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The present-day Venusian atmosphere is crushingly dense, extremely hot and arid. Yet, in its early history, Venus presumably had a massive amount of water, which, if spread evenly over the surface, provided a water depth of 10s to 100s of meters. Therefore, over the course of the atmospheric evolution, the water must have been removed from Venus. The main processes responsible for water loss can be catagorised into either diffusion into the surface materials or escape to space, where the focus of this thesis is the latter. Determining the contribution on the atmospheric evolution from each of these processes can help us understand how planetary atmospheres evolve, both here in our Solar System and in extra-solar systems, and tell us why Venus became so dry.The water escape to space is determined by several processes, where the main processes are a consequence of the interaction between the Venusian atmosphere and the solar wind. As Venus does not have an intrinsic magnetic field, its atmosphere interacts directly with the solar wind, and creates a, so called, induced magnetosphere. The interaction causes part of the solar wind energy and momentum to be transferred to the upper atmospheric particles. The additional momentum may allow the ions to reach above escape energy and escape the planet. Therefore, the interaction between the atmosphere and the solar wind is important to study to determine the rate of escape of atmospheric constituents to space.In this thesis, the escape of atmospheric constituents to space is investigated through measurements of the H+ and O+ ion flows. These ion flows were measured by the Ion Mass Analyser (IMA) on board the Venus Express spacecraft, which orbited Venus during 2006-2014. Using IMA measurements near the North Pole ionosphere, the ionospheric ion flows were shown to have a strong dusk-to-dawn component along the terminator, inside the collisional region of the atmosphere. From ion flow measurements in the magnetotail, the rate of escape of atmospheric H+ and O+ ions were shown to be affected by the solar cycle, with an average escape rate ratio near two, the stoichiometric ratio of water. The change is mainly attributed to the decrease in the net escape rates of H+, which is a result of the increase in return flows, i.e. ions that flow back towards Venus in the magnetotail. Furthermore, the O+ net escape rate increases as the amount of energy available in the upstream solar wind increases. The increase indicates, as expected, that a portion of the available energy in the upstream solar wind is transferred to the escaping ions. However, the total portion of energy transferred from the solar wind to the escaping ions decreases as the available upstream energy increases. Using the simple relation between the O+ escape rate and the upstream solar wind energy flux, the total atmospheric escape was extrapolated backwards in time, by accounting for the evolution of the solar wind parameters. The resulting total escape over the past 3.9 Ga can be translated into a global equivalent water depth of 0.02-0.6 m. This result cannot explain the massive historical water content on Venus.
|
|
| 17. |
- Szasz, Csilla, 1979-
(författare)
-
Radio meteors above the Arctic Circle : radiants, orbits and estimated magnitudes
- 2008
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents results based on data collected with the 930 MHz EISCAT UHF radar system and three SKiYMet specular meteor radars. It describes in detail a method for meteoroid orbit calculation. The EISCAT UHF system comprises three identical 32 m parabolic antennae: one high-power transmitter/receiver and two remote receivers. Precise meteoroid deceleration and radar cross section are determined from 410 meteor head echoes simultaneously observed with all three receivers between 2002 and 2005, during four 24h runs at the summer/winter solstice and the vernal/autumnal equinox. The observations are used to calculate meteoroid orbits and estimate meteor visual magnitudes. None of the observed meteors appear to be of extrasolar or asteroidal origin; comets, particularly short period (<200 years) ones, may be the dominant source for the particles observed. About 40% of the radiants are associated with the north apex sporadic meteor source and 58% of the orbits are retrograde. The geocentric velocity distribution is bimodal with a prograde population centred around 38 km/s and a retrograde population peaking at 59 km/s. The absolute visual magnitudes of meteors are estimated to be in the range of +9 to +5 using a single-object numerical ablation model. They are thus observable using intensified CCD cameras with telephoto lenses. The thesis also investigates diurnal meteor rate differences and sporadic meteor radiant distributions at different latitudes using specular meteor trail radar measurements from 68°N, from 55°N and from 8°S. The largest difference in amplitude of the diurnal flux variation is at equatorial latitudes, the lowest variation is found at high latitudes. The largest seasonal variation of the diurnal flux is observed with the high-latitude meteor radar. The investigations show a variation in the sources with both latitude and time of day. The EISCAT UHF system and the high-latitude meteor radar are located close to the Arctic Circle. Such a geographical position means that zenith points towards the North Ecliptic Pole (NEP) once every day all year round. This particular geometry allows the meteoroid influx from the north ecliptic hemisphere to be compared throughout the year as the ecliptic plane coincides with the local horizon. Considering only the hour when NEP is closest to zenith, the EISCAT UHF head echo rate is about a factor of three higher at summer solstice than during the other seasons, a finding which is consistent with the high-latitude meteor radar measurements.
|
|
| 18. |
- Waara, Martin, 1980-
(författare)
-
High altitude ion heating observed by the Cluster spacecraft
- 2011
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with heating of outowing oxygen ions at high altitude above the polar cap using data from the Cluster spacecraft. oInospheric plasma may flow up from the ionosphere but at velocities which are low enough that the ions are still gravitationally bound. For the ions to overcome gravity, further acceleration is needed. The cusp/polar cap is an important source of outowing oxygen ions. In the cusp/polar cap, transverse heating is more common than eld-aligned acceleration through a magnetic eld-aligned electric eld. It is thus believed that transverse heating of ions is important for ion outow and one of the probable explanations for transverse heating is wave-particle interaction. A general conclusion from our work on high altitude oxygen ion energization is that ion energization and outow occur in the high altitude cusp and mantle. The particles are often heated perpendicularly to the geomagnetic eld and resonant heating at the gyrofrequency is most of the time intense enough to explain the observed O+ energies measured in the high altitude (8 { 15 Earth radii, RE ) cusp/mantle region of the terrestrial magnetosphere. The observed average waves can explain the observed average O+ energies. At lower altitude only a few percent of the observed spectral density around the oxygen gyrofrequency needs to be in resonance with the ions to obtain the measured O+ energies. A difference as compared to low altitude measurements is that we must assume that almost all wave activity is due to waves which can interact with the ions, and of these we assume 50 % to be left-hand polarized. We also have shown a clear correlation between temperature and wave intensity at the gyrofrequency at each measurement point. We have described the average wave intensity and corresponding velocity diffusion oeffcients as a function of altitude in a format convenient for modelers. Furthermore we have shown that the wave activity observed in this high altitude region is consistent with Alfven waves, and inconsistent with static structures drifting past the spacecraft. We have also shown how large the variability of the observed spectral densities is, and how sporadic the waves typically are. Based on three cases we have found that the regions with enhanced wave activity and increased ion temperature are typically many ion gyro radii in perpendicular extent.
|
|
