Browsing by Author "Alho, Markku"
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Item The atmosphere of comet 67P/Churyumov-Gerasimenko diagnosed by charge-exchanged solar wind alpha particles(2016-03-01) Simon Wedlund, C.; Kallio, E.; Alho, Markku; Nilsson, H.; Stenberg Wieser, G.; Gunell, H.; Behar, Etienne; Pusa, Janne; Gronoff, G.; Department of Radio Science and Engineering; Esa Kallio Group; Luleå University of Technology; Swedish Institute of Space Physics; Royal Belgian Institute for Space Aeronomy; SSAIContext. The ESA/Rosetta mission has been orbiting comet 67P/Churyumov-Gerasimenko since August 2014, measuring its dayside plasma environment. The ion spectrometer onboard Rosetta has detected two ion populations, one energetic with a solar wind origin (H+, He2+, He+), the other at lower energies with a cometary origin (water group ions such as H2O+). He+ ions arise mainly from charge-exchange between solar wind alpha particles and cometary neutrals such as H2O. Aims. The He+ and He2+ ion fluxes measured by the Rosetta Plasma Consortium Ion Composition Analyser (RPC-ICA) give insight into the composition of the dayside neutral coma, into the importance of charge-exchange processes between the solar wind and cometary neutrals, and into the way these evolve when the comet draws closer to the Sun. Methods. We combine observations by the ion spectrometer RPC-ICA onboard Rosetta with calculations from an analytical model based on a collisionless neutral Haser atmosphere and nearly undisturbed solar wind conditions. Results. Equivalent neutral outgassing rates Q can be derived using the observed RPC-ICA He+/He2+ particle flux ratios as input into the analytical model in inverse mode. A revised dependence of Q on heliocentric distance Rh in AU is found to be Rh -7.06Rh-7.06 between 1.8 and 3.3 AU, suggesting that the activity in 2015 differed from that of the 2008 perihelion passage. Conversely, using an outgassing rate determined from optical remote sensing measurements from Earth, the forward analytical model results are in relatively good agreement with the measured RPC-ICA flux ratios. Modelled ratios in a 2D spherically-symmetric plane are also presented, showing that charge exchange is most efficient with solar wind protons. Detailed cometocentric profiles of these ratios are also presented. Conclusions. In conclusion, we show that, with the help of a simple analytical model of charge-exchange processes, a mass-capable ion spectrometer such as RPC-ICA can be used as a "remote-sensing" instrument for the neutral cometary atmosphere.Item Emission of hydrogen energetic neutral atoms from the Martian subsolar magnetosheath(2016-01-01) Wang, X. D.; Alho, Markku; Jarvinen, R.; Kallio, E.; Barabash, S.; Futaana, Y.; Swedish Institute of Space Physics; Department of Radio Science and Engineering; Finnish Meteorological InstituteWe have simulated the hydrogen energetic neutral atom (ENA) emissions from the subsolar magnetosheath of Mars using a hybrid model of the proton plasma charge exchanging with the Martian exosphere to study statistical features revealed from the observations of the Neutral Particle Detectors on Mars Express. The simulations reproduce well the observed enhancement of the hydrogen ENA emissions from the dayside magnetosheath in directions perpendicular to the Sun-Mars line. Our results show that the neutralized protons from the shocked solar wind are the dominant ENA population rather than those originating from the pickup planetary ions. The simulation also suggests that the observed stronger ENA emissions in the direction opposite to the solar wind convective electric field result from a stronger proton flux in the same direction at the lower magnetosheath; i.e., the proton fluxes in the magnetosheath are not cylindrically symmetric. We also confirm the observed increasing of the ENA fluxes with the solar winddynamical pressure in the simulations. This feature is associated with a low altitude of the induced magnetic boundary when the dynamic pressure is high and the magnetosheath protons can reach to a denser exosphere, and thus, the charge exchange rate becomes higher. Overall, the analysis suggests that kinetic effects play an important and pronounced role in the morphology of the hydrogen ENA distribution and the plasma environment at Mars, in general.Item Evolution of the ion environment of comet 67P during the Rosetta mission as seen by RPC-ICA(2017) Nilsson, Hans; Wieser, Gabriella Stenberg; Behar, Etienne; Gunell, Herbert; Wieser, Martin; Galand, Marina; Simon Wedlund, Cyril; Alho, Markku; Goetz, Charlotte; Yamauchi, Masatoshi; Henri, Pierre; Odelstad, Elias; Vigren, Erik; Department of Electronics and Nanoengineering; Swedish Institute of Space Physics; Luleå University of Technology; Royal Belgian Institute for Space Aeronomy; Imperial College London; Technical University of Braunschweig; Laboratoire de Physique et Chimie de l’Environnement et de l’Espace; University of OsloRosetta has followed comet 67P from low activity at more than 3.6 au heliocentric distance to high activity at perihelion (1.24 au) and then out again. We provide a general overview of the evolution of the dynamic ion environment using data from the RPC-ICA ion spectrometer. We discuss where Rosetta was located within the evolving comet magnetosphere. For the initial observations, the solar wind permeated all of the coma. In 2015 mid-April, the solar wind started to disappear from the observation region, to re-appear again in 2015 December. Low-energy cometary ions were seen at first when Rosetta was about 100 km from the nucleus at 3.6 au, and soon after consistently throughout the mission except during the excursions to farther distances from the comet. The observed flux of low-energy ions was relatively constant due to Rosetta's orbit changing with comet activity. Accelerated cometary ions, moving mainly in the antisunward direction gradually became more common as comet activity increased. These accelerated cometary ions kept being observed also after the solar wind disappeared from the location of Rosetta, with somewhat higher fluxes further away from the nucleus. Around perihelion, when Rosetta was relatively deep within the comet magnetosphere, the fluxes of accelerated cometary ions decreased, as did their maximum energy. The disappearance of more energetic cometary ions at close distance during high activity is suggested to be due to a flow pattern where these ions flow around the obstacle of the denser coma or due to charge exchange losses.Item How to use multitasking environment in bank's workstations?(1995) Alho, Markku; Talvitie, Kari; Tietotekniikan osasto; Teknillinen korkeakoulu; Helsinki University of Technology; Sulonen, ReijoItem Hybrid modeling of cometary plasma environments: II. Remote-sensing of a cometary bow shock(EDP SCIENCES S A, 2019-10-01) Alho, Markku; Simon Wedlund, Cyril; Nilsson, Hans; Kallio, Esa; Järvinen, Riku; Pulkkinen, Tuija; Department of Electronics and Nanoengineering; Esa Kallio Group; Tuija Pulkkinen Group; Swedish Institute of Space Physics; University of OsloContext. The ESA Rosetta probe has not seen direct evidence of a fully formed bow shock at comet 67P/Churyumov-Gerasimenko (67P). Ion spectrometer measurements of cometary pickup ions measured in the vicinity of the nucleus of 67P are available and may contain signatures of the large-scale plasma environment. Aims. The aim is to investigate the possibility of using pickup ion signatures to infer the existence or nonexistence of a bow shock-like structure and possibly other large-scale plasma environment features. Methods. A numerical plasma model in the hybrid plasma description was used to model the plasma environment of a comet. Simulated pickup ion spectra were generated for different interplanetary magnetic field conditions. The results were interpreted through test particle tracing in the hybrid simulation solutions. Results. Features of the observed pickup ion energy spectrum were reproduced, and the model was used to interpret the observation to be consistent with a shock-like structure. We identify (1) a spectral break related to the bow shock, (2) a mechanism for generating the spectral break, and (3) a dependency of the energy of the spectral break on the interplanetary magnetic field magnitude and bow shock standoff distance.Item Oxygen Ion Escape from Venus is Modulated by Ultra-Low-Frequency Waves(AMERICAN GEOPHYSICAL UNION, 2020-06-16) Järvinen, Riku; Alho, Markku; Kallio, Esa; Pulkkinen, Tuija; Department of Electronics and Nanoengineering; Tuija Pulkkinen Group; Esa Kallio GroupWe study the solar wind-driven, nonthermal escape of O+ ions from Venus in a global hybrid simulation. In the model, a well-developed ion foreshock forms ahead of the Venusian quasi-parallel bow shock under nominal upstream conditions. Large-scale magnetosonic ultra-low frequency (ULF) waves at 20- to 30-s period are excited and convect downstream along the foreshock with the solar wind. We show that the foreshock ULF waves transmit through the bow shock in the downstream region and interact with the planetary ion acceleration, causing 25% peak-to-peak fluctuations in the O+ escape rate. These results demonstrate the importance of upstream plasma waves on the energization and escape of heavy ions from the planetary atmospheres.Item Parametrisyyden liittäminen kvadraattiseen optimointiin Wolfen menetelmää soveltaen(1974) Alho, Markku; HKKK. Kans. Pro gradu; Kauppakorkeakoulu; School of BusinessItem Remote sensing of cometary bow shocks: Modelled asymmetric outgassing and pickup ion observations(OXFORD UNIV PRESS INC, 2021-10) Alho, Markku; Järvinen, Riku; Simon Wedlund, Cyril; Nilsson, Hans; Kallio, Esa; Pulkkinen, Tuija; Department of Electronics and Nanoengineering; Esa Kallio Group; Swedish Institute of Space Physics; Austrian Academy of SciencesDespite the long escort by the ESA Rosetta mission, direct observations of a fully developed bow shock around 67P/Churyumov–Gerasimenko have not been reported. Expanding on our previous work on indirect observations of a shock, we model the large-scale features in cometary pickup ions, and compare the results with the ESA Rosetta Plasma Consortium Ion Composition Analyser ion spectrometer measurements over the pre-perihelion portion of the escort phase. Using our hybrid plasma simulation, an empirical, asymmetric outgassing model for 67P, and varied interplanetary magnetic field (IMF) clock angles, we model the evolution of the large-scale plasma environment. We find that the subsolar bow shock standoff distance is enhanced by asymmetric outgassing with a factor of 2 to 3, reaching up to 18000km approaching perihelion. We find that distinct spectral features in simulated pickup ion distributions are present for simulations with shock-like structures, with the details of the spectral features depending on shock standoff distance, heliocentric distance, and IMF configuration. Asymmetric outgassing along with IMF clock angle is found to have a strong effect on the location of the spectral features, while the IMF clock angle causes no significant effect on the bow shock standoff distance. These dependences further complicate the interpretation of the ion observations made by Rosetta. Our data-model comparison shows that the large-scale cometary plasma environment can be probed by remote sensing the pickup ions, at least when the comet’s activity is comparable to that of 67P, and the solar wind parameters are known.Item Simulation of Mercury's magnetosheath with a combined hybrid-paraboloid model(2017-08-01) Parunakian, David; Dyadechkin, Sergey; Alexeev, Igor; Belenkaya, Elena; Khodachenko, Maxim; Kallio, Esa; Alho, Markku; Department of Electronics and Nanoengineering; Esa Kallio Group; Lomonosov Moscow State UniversityIn this paper we introduce a novel approach for modeling planetary magnetospheres that involves a combination of the hybrid model and the paraboloid magnetosphere model (PMM); we further refer to it as the combined hybrid model. While both of these individual models have been successfully applied in the past, their combination enables us both to overcome the traditional difficulties of hybrid models to develop a self-consistent magnetic field and to compensate the lack of plasma simulation in the PMM. We then use this combined model to simulate Mercury's magnetosphere and investigate the geometry and configuration of Mercury's magnetosheath controlled by various conditions in the interplanetary medium. The developed approach provides a unique comprehensive view of Mercury's magnetospheric environment for the first time. Using this setup, we compare the locations of the bow shock and the magnetopause as determined by simulations with the locations predicted by stand-alone PMM runs and also verify the magnetic and dynamic pressure balance at the magnetopause. We also compare the results produced by these simulations with observational data obtained by the magnetometer on board the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft along a dusk-dawn orbit and discuss the signatures of the magnetospheric features that appear in these simulations. Overall, our analysis suggests that combining the semiempirical PMM with a self-consistent global kinetic model creates new modeling possibilities which individual models cannot provide on their own.Item Solar wind charge exchange in cometary atmospheres. I. Charge-changing and ionization cross sections for He and H particles in H2O(EDP SCIENCES S A, 2019-10-01) Wedlund, Cyril Simon; Bodewits, Dennis; Alho, Markku; Hoekstra, Ronnie; Behar, Etienne; Gronoff, Guillaume; Gunell, Herbert; Nilsson, Hans; Kallio, Esa; Beth, Arnaud; Department of Electronics and Nanoengineering; Esa Kallio Group; Auburn University; University of Groningen; Swedish Institute of Space Physics; NASA Langley Research Center; Royal Belgian Institute for Space Aeronomy; Imperial College London; University of OsloContext. Solar wind charge-changing reactions are of paramount importance to the physico-chemistry of the atmosphere of a comet, mass-loading the solar wind through an effective conversion of fast light solar wind ions into slow heavy cometary ions. Aims. To understand these processes and place them in the context of a solar wind plasma interacting with a neutral atmosphere, numerical or analytical models are necessary. Inputs of these models, such as collision cross sections and chemistry, are crucial. Methods. Book-keeping and fitting of experimentally measured charge-changing and ionization cross sections of hydrogen and helium particles in a water gas are discussed, with emphasis on the low-energy/low-velocity range that is characteristic of solar wind bulk speeds (<20 keV u-1/2000 km s-1). Results. We provide polynomial fits for cross sections of charge-changing and ionization reactions, and list the experimental needs for future studies. To take into account the energy distribution of the solar wind, we calculated Maxwellian-averaged cross sections and fitted them with bivariate polynomials for solar wind temperatures ranging from 105 to 106 K (12-130 eV). Conclusions. Single- and double-electron captures by He2+ dominate at typical solar wind speeds. Correspondingly, single-electron capture by H+ and single-electron loss by H- dominate at these speeds, resulting in the production of energetic neutral atoms (ENAs). Ionization cross sections all peak at energies above 20 keV and are expected to play a moderate role in the total ion production. However, the effect of solar wind Maxwellian temperatures is found to be maximum for cross sections peaking at higher energies, suggesting that local heating at shock structures in cometary and planetary environments may favor processes previously thought to be negligible. This study is the first part in a series of three on charge exchange and ionization processes at comets, with a specific application to comet 67P/Churyumov-Gerasimenko and the Rosetta mission.Item Solar wind charge exchange in cometary atmospheres: III. Results from the Rosetta mission to comet 67P/Churyumov-Gerasimenko(EDP SCIENCES S A, 2019-10-01) Wedlund, Cyril Simon; Behar, Etienne; Nilsson, Hans; Alho, Markku; Kallio, Esa; Gunell, Herbert; Bodewits, Dennis; Heritier, Kevin; Galand, Marina; Beth, Arnaud; Rubin, Martin; Altwegg, Kathrin; Volwerk, Martin; Gronoff, Guillaume; Hoekstra, Ronnie; Department of Electronics and Nanoengineering; Esa Kallio Group; Swedish Institute of Space Physics; Royal Belgian Institute for Space Aeronomy; Auburn University; Imperial College London; University of Bern; Austrian Academy of Sciences; NASA Langley Research Center; University of Groningen; University of OsloContext. Solar wind charge-changing reactions are of paramount importance to the physico-chemistry of the atmosphere of a comet. The ESA/Rosetta mission to comet 67P/Churyumov-Gerasimenko (67P) provides a unique opportunity to study charge-changing processes in situ. Aims. To understand the role of these reactions in the evolution of the solar wind plasma and interpret the complex in situ measurements made by Rosetta, numerical or analytical models are necessary. Methods. We used an extended analytical formalism describing solar wind charge-changing processes at comets along solar wind streamlines. The model is driven by solar wind ion measurements from the Rosetta Plasma Consortium-Ion Composition Analyser (RPC-ICA) and neutral density observations from the Rosetta Spectrometer for Ion and Neutral Analysis-Comet Pressure Sensor (ROSINA-COPS), as well as by charge-changing cross sections of hydrogen and helium particles in a water gas. Results. A mission-wide overview of charge-changing efficiencies at comet 67P is presented. Electron capture cross sections dominate and favor the production of He and H energetic neutral atoms (ENAs), with fluxes expected to rival those of H+ and He2+ ions. Conclusions. Neutral outgassing rates are retrieved from local RPC-ICA flux measurements and match ROSINA estimates very well throughout the mission. From the model, we find that solar wind charge exchange is unable to fully explain the magnitude of the sharp drop in solar wind ion fluxes observed by Rosetta for heliocentric distances below 2.5 AU. This is likely because the model does not take the relative ion dynamics into account and to a lesser extent because it ignores the formation of bow-shock-like structures upstream of the nucleus. This work also shows that the ionization by solar extreme-ultraviolet radiation and energetic electrons dominates the source of cometary ions, although solar wind contributions may be significant during isolated events.Item Space weather effects on the bow shock, the magnetic barrier, and the ion composition boundary at Venus(2015) Vech, Daniel; Szego, K.; Opitz, A.; Kajdic, P.; Fraenz, M.; Kallio, Esa; Alho, Markku; Esa Kallio Group; Department of Radio Science and EngineeringWe present a statistical study on the interaction between interplanetary coronal mass ejections (ICMEs) and the induced magnetosphere of Venus when the peak magnetic field of the magnetic barrier was anomalously large (>65nT). Based on the entire available Venus Express data set from April 2006 to October 2014, we selected 42 events and analyzed the solar wind parameters, the position of the bow shock, the size and plasma properties of the magnetic barrier, and the position of the ion composition boundary (ICB). It was found that the investigated ICMEs can be characterized with interplanetary shocks and unusually large tangential magnetic fields with respect to the Venus-Sun line. In most of the cases the position of the bow shock was not affected by the ICME. In a few cases the interaction between magnetic clouds and the induced magnetosphere of Venus was observed. During these events the small magnetosonic Mach numbers inside magnetic clouds caused the bow shock to appear at anomalously large distances fromthe planet. The positions of the upper and lower boundaries of the magnetic barrier were not affected by the ICMEs. The position of the ICB on the nightside was found closer to the planet during ICME passages which is attributed to the increased solar wind dynamic pressure. Key Points Statistical study of the ICME-Venus interaction Analysis of solar wind and magnetic barrier conditions during ICME passages Decreased altitude of the nightside ionosphere during ICME passages ©2015. American Geophysical Union. All Rights Reserved.Item Ultra-low frequency waves in the ion foreshock of Mercury: A global hybrid modeling study(OXFORD UNIV PRESS INC, 2020-01-01) Järvinen, Riku; Alho, Markku; Kallio, Esa; Pulkkinen, Tuija; Department of Electronics and Nanoengineering; Tuija Pulkkinen Group; Esa Kallio GroupWe study the solar wind interaction with Mercury using a global three-dimensional hybrid model. In the analysed simulation run, we find a well-developed, dynamic Hermean ion foreshock ahead of the quasi-parallel bow shock under upstream solar wind and interplanetary magnetic field (IMF) conditions corresponding to the orbital perihelion of the planet. A portion of the incident solar wind ion flux is scattered back upstream near the quasi-parallel bow shock including both major solar wind ion species, protons and alphas. The scattered particles form the Hermean suprathermal foreshock ion population. A significant part of the suprathermal population is backstreaming with a velocity component towards the Sun in the near-foreshock at the planetocentric distance of few planetary radii in the plane of the IMF. The ion foreshock is associated with large-scale, oblique fast magnetosonic waves in the ultra-low-frequency (ULF) range convecting downstream with the solar wind. The ULF wave period is about 5 s in the analysed upstream condition case at Mercury, which corresponds to the 30-s foreshock waves at Earth when scaled by the IMF magnitude.
