Browsing by Subject "MAGNETIC-FIELD"

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  • Kilpua, Emilia; Juusola, Liisa; Grandin, Maxime; Kero, Antti; Dubyagin, Stepan; Partamies, Noora; Osmane, Adnane; George, Harriet; Kalliokoski, Milla; Raita, Tero; Asikainen, Timo; Palmroth, Minna (2020)
    We study here energetic-electron (E > 30 keV) precipitation using cosmic noise absorption (CNA) during the sheath and ejecta structures of 61 interplanetary coronal mass ejections (ICMEs) observed in the near-Earth solar wind between 1997 and 2012. The data come from the Finnish riometer (relative ionospheric opacity meter) chain from stations extending from auroral (IVA, 65.2 degrees N geomagnetic latitude; MLAT) to subauroral (JYV, 59.0 degrees N MLAT) latitudes. We find that sheaths and ejecta lead frequently to enhanced CNA (> 0.5 dB) both at auroral and subauroral latitudes, although the CNA magnitudes stay relatively low (medians around 1 dB). Due to their longer duration, ejecta typically lead to more sustained enhanced CNA periods (on average 6-7 h), but the sheaths and ejecta were found to be equally effective in inducing enhanced CNA when relative-occurrence frequency and CNA magnitude were considered. Only at the lowest-MLAT station, JYV, ejecta were more effective in causing enhanced CNA. Some clear trends of magnetic local time (MLT) and differences between the ejecta and sheaths were found. The occurrence frequency and magnitude of CNA activity was lowest close to midnight, while it peaked for the sheaths in the morning and afternoon/evening sectors and for the ejecta in the morning and noon sectors. These differences may reflect differences in typical MLT distributions of wave modes that precipitate substorm-injected and trapped radiation belt electrons during the sheaths and ejecta. Our study also emphasizes the importance of substorms and magnetospheric ultra-low-frequency (ULF) waves for enhanced CNA.
  • Juvela, Mika; Guillet, Vincent; Liu, Tie; Ristorcelli, Isabelle; Pelkonen, Veli-Matti; Alina, Dana; Bronfman, Leonardo; Eden, David J.; Kim, Kee Tae; Koch, Patrick M.; Kwon, Woojin; Lee, Chang Won; Malinen, Johanna; Micelotta, Elisabetta; Montillaud, Julien; Rawlings, Mark G.; Sanhueza, Patricio; Soam, Archana; Traficante, Alessio; Ysard, Nathalie; Zhang, Chuan-Peng (2018)
    Context. The sub-millimetre polarisation of dust emission from star-forming clouds carries information on grain properties and on the effects that magnetic fields have on cloud evolution. Aims. Using observations of a dense filamentary cloud G035.39-00.33, we aim to characterise the dust emission properties and the variations of the polarisation fraction. Methods. JCMT SCUBA-2/POL-2 observations at 850 mu m were combined with Planck 850 mu m (353 GHz) data to map polarisation fraction at small and large scales. With previous total intensity SCUBA-2 observations (450 and 850 mu m) and Herschel data, the column densities were determined via modified black-body fits and via radiative transfer modelling. Models were constructed to examine how the observed polarisation angles and fractions depend on potential magnetic field geometries and grain alignment processes. Results. POL-2 data show clear changes in the magnetic field orientation. These are not in contradiction with the uniform orientation and almost constant polarisation fraction seen by Planck, because of the difference in the beam sizes and the POL-2 data being affected by spatial filtering. The filament has a peak column density of N(H-2) similar to 7 x 10(22) cm(-2), a minimum dust temperature of T similar to 12 K, and a mass of similar to 4300 M-circle dot for the area N(H-2) > 5 x 10(21) cm(-2). The estimated average value of the dust opacity spectral index is beta similar to 1.9. The ratio of sub-millimetre and J-band optical depths is tau (250 mu m)/tau(J) similar to 2.5 x 10(-3), more than four times the typical values for diffuse medium. The polarisation fraction decreases as a function of column density to p similar to 1% in the central filament. Because of noise, the observed decrease of p(N) is significant only at N(H-2) > 2 x 10(22) cm(-2). The observations suggest that the grain alignment is not constant. Although the data can be explained with a complete loss of alignment at densities above similar to 10(4) cm(-3) or using the predictions of radiative torques alignment, the uncertainty of the field geometry and the spatial filtering of the SCUBA-2 data prevent strong conclusions. Conclusions. The G035.39-00.33 filament shows strong signs of dust evolution and the low polarisation fraction is suggestive of a loss of polarised emission from its densest parts.
  • Pomoell, Jens; Poedts, Stefaan (2018)
    The implementation and first results of the new space weather forecasting-targeted inner heliosphere model "European heliospheric forecasting information asset" (EUHFORIA) are presented. EUHFORIA consists of two major components: a coronal model and a heliosphere model including coronal mass ejections. The coronal model provides data-driven solar wind plasma parameters at 0.1AU by constructing a model of the coronal large-scale magnetic field and employing empirical relations to determine the plasma state such as the solar wind speed and mass density. These are then used as boundary conditions to drive a three-dimensional time-dependent magnetohydrodynamics model of the inner heliosphere up to 2 AU. CMEs are injected into the ambient solar wind modeled using the cone model, with their parameters obtained from fits to imaging observations. In addition to detailing the modeling methodology, an initial validation run is presented. The results feature a highly dynamic heliosphere that the model is able to capture in good agreement with in situ observations. Finally, future horizons for the model are outlined.
  • Moestl, C.; Amerstorfer, T.; Palmerio, E.; Isavnin, A.; Farrugia, C. J.; Lowder, C.; Winslow, R. M.; Donnerer, J. M.; Kilpua, E. K. J.; Boakes, P. D. (2018)
    Forecasting the geomagnetic effects of solar storms, known as coronal mass ejections (CMEs), is currently severely limited by our inability to predict the magnetic field configuration in the CME magnetic core and by observational effects of a single spacecraft trajectory through its 3-D structure. CME magnetic flux ropes can lead to continuous forcing of the energy input to the Earth's magnetosphere by strong and steady southward-pointing magnetic fields. Here we demonstrate in a proof-of-concept way a new approach to predict the southward field B-z in a CME flux rope. It combines a novel semiempirical model of CME flux rope magnetic fields (Three-Dimensional Coronal ROpe Ejection) with solar observations and in situ magnetic field data from along the Sun-Earth line. These are provided here by the MESSENGER spacecraft for a CME event on 9-13 July 2013. Three-Dimensional Coronal ROpe Ejection is the first such model that contains the interplanetary propagation and evolution of a 3-D flux rope magnetic field, the observation by a synthetic spacecraft, and the prediction of an index of geomagnetic activity. A counterclockwise rotation of the left-handed erupting CME flux rope in the corona of 30 degrees and a deflection angle of 20 degrees is evident from comparison of solar and coronal observations. The calculated Dst matches reasonably the observed Dst minimum and its time evolution, but the results are highly sensitive to the CME axis orientation. We discuss assumptions and limitations of the method prototype and its potential for real time space weather forecasting and heliospheric data interpretation.
  • Boi, Simone; Mazzino, Andrea; Muratore-Ginanneschi, Paolo; Olivieri, Stefano (2018)
    One of the cornerstones of turbulent dispersion is the celebrated Taylor's formula. This formula expresses the rate of transport (i.e., the eddy diffusivity) of a tracer as a time integral of the fluid velocity autocorrelation function evaluated along the fluid particle trajectories. Here, we review the hypotheses which permit us to extend Taylor's formula to particles of any inertia. The hypotheses are independent of the details of the inertial particle model. We also show by explicit calculation that the hypotheses encompass cases when memory terms such as Basset's and Faxén's corrections are taken into account in the modeling of inertial particle dynamics.
  • Moschou, Sofia-Paraskevi; Pierrard, Viviane; Keppens, Rony; Pomoell, Jens (2017)
    An exospheric kinetic solar wind model is interfaced with an observation-driven single-fluid magnetohydrodynamic (MHD) model. Initially, a photospheric magnetogram serves as observational input in the fluid approach to extrapolate the heliospheric magnetic field. Then semi-empirical coronal models are used for estimating the plasma characteristics up to a heliocentric distance of 0.1 AU. From there on, a full MHD model that computes the three-dimensional time-dependent evolution of the solar wind macroscopic variables up to the orbit of Earth is used. After interfacing the density and velocity at the inner MHD boundary, we compare our results with those of a kinetic exospheric solar wind model based on the assumption of Maxwell and Kappa velocity distribution functions for protons and electrons, respectively, as well as with in situ observations at 1 AU. This provides insight into more physically detailed processes, such as coronal heating and solar wind acceleration, which naturally arise from including suprathermal electrons in the model. We are interested in the profile of the solar wind speed and density at 1 AU, in characterizing the slow and fast source regions of the wind, and in comparing MHD with exospheric models in similar conditions. We calculate the energetics of both models from low to high heliocentric distances.
  • Milillo, A.; Fujimoto, M.; Murakami, G.; Benkhoff, J.; Zender, J.; Aizawa, S.; Dosa, M.; Griton, L.; Heyner, D.; Ho, G.; Imber, S. M.; Jia, Yan; Karlsson, T.; Killen, R. M.; Laurenza, M.; Lindsay, S. T.; McKenna-Lawlor, S.; Mura, A.; Raines, J. M.; Rothery, D. A.; Andre, N.; Baumjohann, W.; Berezhnoy, A.; Bourdin, P. A.; Bunce, E. J.; Califano, F.; Deca, J.; de la Fuente, S.; Dong, C.; Grava, C.; Fatemi, S.; Henri, P.; Ivanovski, S. L.; Jackson, B. V.; James, M.; Kallio, E.; Kasaba, Y.; Kilpua, E.; Kobayashi, M.; Langlais, B.; Leblanc, F.; Lhotka, C.; Mangano, V.; Martindale, A.; Massetti, S.; Masters, A.; Morooka, M.; Narita, Y.; Oliveira, J. S.; Odstrcil, D.; Orsini, S.; Pelizzo, M. G.; Plainaki, C.; Plaschke, F.; Sahraoui, Afaf; Seki, K.; Slavin, J. A.; Vainio, R.; Wurz, P.; Barabash, S.; Carr, C. M.; Delcourt, D.; Glassmeier, K. -H.; Grande, M.; Hirahara, M.; Huovelin, J.; Korablev, O.; Kojima, H.; Lichtenegger, H.; Livi, S.; Matsuoka, A.; Moissl, R.; Moncuquet, M.; Muinonen, K.; Quemerais, E.; Saito, Y.; Yagitani, S.; Yoshikawa, I.; Wahlund, J. -E. (2020)
    The ESA-JAXA BepiColombo mission will provide simultaneous measurements from two spacecraft, offering an unprecedented opportunity to investigate magnetospheric and exospheric dynamics at Mercury as well as their interactions with the solar wind, radiation, and interplanetary dust. Many scientific instruments onboard the two spacecraft will be completely, or partially devoted to study the near-space environment of Mercury as well as the complex processes that govern it. Many issues remain unsolved even after the MESSENGER mission that ended in 2015. The specific orbits of the two spacecraft, MPO and Mio, and the comprehensive scientific payload allow a wider range of scientific questions to be addressed than those that could be achieved by the individual instruments acting alone, or by previous missions. These joint observations are of key importance because many phenomena in Mercury's environment are highly temporally and spatially variable. Examples of possible coordinated observations are described in this article, analysing the required geometrical conditions, pointing, resolutions and operation timing of different BepiColombo instruments sensors.
  • Grandin, Maxime; Palmroth, Minna; Whipps, Graeme; Kalliokoski, Milla; Ferrier, Mark; Paxton, Larry J.; Mlynczak, Martin G.; Hilska, Jukka; Holmseth, Knut; Vinorum, Kjetil; Whenman, Barry (2021)
    Recently, citizen scientist photographs led to the discovery of a new auroral form called "the dune aurora" which exhibits parallel stripes of brighter emission in the green diffuse aurora at about 100 km altitude. This discovery raised several questions, such as (i) whether the dunes are associated with particle precipitation, (ii) whether their structure arises from spatial inhomogeneities in the precipitating fluxes or in the underlying neutral atmosphere, and (iii) whether they are the auroral manifestation of an atmospheric wave called a mesospheric bore. This study investigates a large-scale dune aurora event on 20 January 2016 above Northern Europe. The dunes were observed from Finland to Scotland, spanning over 1,500 km for at least 4 h. Spacecraft observations indicate that the dunes are associated with particle precipitation and reveal the presence of a temperature inversion layer below the mesopause during the event, creating suitable conditions for mesospheric bore formation. The analysis of a time lapse of pictures by a citizen scientist from Scotland leads to the estimate that, during this event, the dunes propagate toward the west-southwest direction at about 200 m s(-1), presumably indicating strong horizontal winds near the mesopause. These results show that citizen science and dune aurora studies can fill observational gaps and be powerful tools to investigate the least-known region of near-Earth space at altitudes near 100 km.
  • Myllys, M.; Partamies, N.; Juusola, L. (2015)
    To validate the usage of global indices in studies of geomagnetic activity, we have examined the latitude dependence of geomagnetic variations in Fennoscandia and Svalbard from 1994 to 2010. Daily standard deviation (SD) values of the horizontal magnetic field have been used as a measure of the ground magnetic disturbance level. We found that the timing of the geomagnetic minimum depends on the latitude region: corresponding to the minimum of sunspot cycle 22 (in 1996), the geomagnetic minimum occurred between the geomagnetic latitudes 57-61 degrees in 1996 and at the latitudes 64-67 degrees in 1997, which are the average auroral oval latitudes. During sunspot cycle 23, all latitude regions experienced the minimum in 2009, a year after the sunspot minimum. These timing differences are due to the latitude dependence of the 10 s daily SD on the different solar wind drivers. In the latitude region of 64-67 degrees, the impact of the high-speed solar wind streams (HSSs) on the geomagnetic activity is the most pronounced compared to the other latitude groups, while in the latitude region of 57-61 degrees, the importance of the coronal mass ejections (CMEs) dominates. The geomagnetic activity maxima during ascending solar cycle phases are typically caused by CME activity and occur especially in the oval and sub-auroral regions. The strongest geomagnetic activity occurs during the descending solar cycle phases due to a mixture of CME and HSS activity. Closer to the solar minimum, less severe geomagnetic activity is driven by HSSs and mainly visible in the poleward part of the auroral region. According to our study, however, the timing of the geomagnetic activity minima (and maxima) in different latitude bands is different, due to the relative importance of different solar wind drivers at different latitudes.
  • Willamo, T.; Hackman, T.; Lehtinen, J. J.; Käpylä, M. J.; Ilyin, I.; Henry, G. W.; Jetsu, L.; Kochukhov, O.; Piskunov, N. (2019)
    Context. Starspots are important manifestations of stellar magnetic activity. By studying their behaviour in young solar analogues, we can unravel the properties of their magnetic cycles. This gives crucial information of the underlying dynamo process. Comparisons with the solar cycle enable us to infer knowledge about how the solar dynamo has evolved during the Sun's lifetime. Aims. Here we study the correlation between photometric brightness variations, spottedness, and mean temperature in V889 Her, a young solar analogue. Our data covers 18 years of spectroscopic and 25 years of photometric observations. Methods. We use Doppler imaging to derive temperature maps from high-resolution spectra. We use the Continuous Period Search method to retrieve mean V-magnitudes from photometric data. Results. Our Doppler imaging maps show a persistent polar spot structure varying in strength. This structure is centred slightly off the rotational pole. The mean temperature derived from the maps shows an overall decreasing trend, as does the photometric mean brightness, until it reaches its minimum around 2017. The filling factor of cool spots, however, shows only a weak tendency to anti-correlate with the decreasing mean brightness. Conclusions. We interpret V889 Her to have entered into a grand maximum in its activity. The clear relation between the mean temperature of the Doppler imaging surface maps and the mean magnitude supports the reliability of the Doppler images. The lack of correlation between the mean magnitude and the spottedness may indicate that bright features in the Doppler images are real.
  • Pulkkinen, T. I.; Dimmock, A. P.; Lakka, A.; Osmane, A.; Kilpua, E.; Myllys, M.; Tanskanen, E. I.; Viljanen, A. (2016)
    We examine the role of the magnetosheath in solar wind-magnetosphere-ionosphere coupling using the Time History of Events and Macroscale Interactions during Substorms plasma and magnetic field observations in the magnetosheath together with OMNI solar wind data and auroral electrojet recordings from the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer chain. We demonstrate that the electric field and Poynting flux reaching the magnetopause are not linear functions of the electric field and Poynting flux observed in the solar wind: the electric field and Poynting flux at the magnetopause during higher driving conditions are lower than those predicted from a linear function. We also show that the Poynting flux normal to the magnetopause is linearly correlated with the directly driven part of the auroral electrojets in the ionosphere. This indicates that the energy entering the magnetosphere in the form of the Poynting flux is directly responsible for driving the electrojets. Furthermore, we argue that the polar cap potential saturation discussed in the literature is associated with the way solar wind plasma gets processed during the bow shock crossing and motion within the magnetosheath.
  • Palmroth, Minna; Hietala, Heli; Plaschke, Ferdinand; Archer, Martin; Karlsson, Tomas; Blanco-Cano, Xochitl; Sibeck, David; Kajdic, Primoz; Ganse, Urs; Pfau-Kempf, Yann; Battarbee, Markus; Turc, Lucile (2018)
    We use a global hybrid-Vlasov simulation for the magnetosphere, Vlasiator, to investigate magnetosheath high-speed jets. Unlike many other hybrid-kinetic simulations, Vlasiator includes an unscaled geomagnetic dipole, indicating that the simulation spatial and temporal dimensions can be given in SI units without scaling. Thus, for the first time, this allows investigating the magnetosheath jet properties and comparing them directly with the observed jets within the Earth's magnetosheath. In the run shown in this paper, the interplanetary magnetic field (IMF) cone angle is 30 degrees, and a foreshock develops upstream of the quasi-parallel magnetosheath. We visually detect a structure with high dynamic pressure propagating from the bow shock through the magnetosheath. The structure is confirmed as a jet using three different criteria, which have been adopted in previous observational studies. We compare these criteria against the simulation results. We find that the magnetosheath jet is an elongated structure extending earthward from the bow shock by similar to 2.6 R-E, while its size perpendicular to the direction of propagation is similar to 0.5 R-E. We also investigate the jet evolution and find that the jet originates due to the interaction of the bow shock with a high-dynamic-pressure structure that reproduces observational features associated with a short, large-amplitude magnetic structure (SLAMS). The simulation shows that magnetosheath jets can develop also under steady IMF, as inferred by observational studies. To our knowledge, this paper therefore shows the first global kinetic simulation of a magnetosheath jet, which is in accordance with three observational jet criteria and is caused by a SLAMS advecting towards the bow shock.
  • Möstl, C.; Isavnin, A.; Boakes, P. D.; Kilpua, E. K. J.; Davies, J. A.; Harrison, R. A.; Barnes, D.; Krupar, V.; Eastwood, J. P.; Good, S. W.; Forsyth, R. J.; Bothmer, V.; Reiss, M. A.; Amerstorfer, T.; Winslow, R. M.; Anderson, B. J.; Philpott, L. C.; Rodriguez, L.; Rouillard, A. P.; Gallagher, P.; Nieves-Chinchilla, T.; Zhang, T. L. (2017)
    We present an advance toward accurately predicting the arrivals of coronal mass ejections (CMEs) at the terrestrial planets, including Earth. For the first time, we are able to assess a CME prediction model using data over two thirds of a solar cycle of observations with the Heliophysics System Observatory. We validate modeling results of 1337 CMEs observed with the Solar Terrestrial Relations Observatory (STEREO) heliospheric imagers (HI) (science data) from 8 years of observations by five in situ observing spacecraft. We use the self-similar expansion model for CME fronts assuming 60 degrees longitudinal width, constant speed, and constant propagation direction. With these assumptions we find that 23%-35% of all CMEs that were predicted to hit a certain spacecraft lead to clear in situ signatures, so that for one correct prediction, two to three false alarms would have been issued. In addition, we find that the prediction accuracy does not degrade with the HI longitudinal separation from Earth. Predicted arrival times are on average within 2.6 +/- 16.6 h difference of the in situ arrival time, similar to analytical and numerical modeling, and a true skill statistic of 0.21. We also discuss various factors that may improve the accuracy of space weather forecasting using wide-angle heliospheric imager observations. These results form a first-order approximated baseline of the prediction accuracy that is possible with HI and other methods used for data by an operational space weather mission at the Sun-Earth L5 point. Plain Language Summary Solar storms are formed by incredibly powerful explosions on the Sun and travel as clouds of plasma threaded by magnetic fields through the solar system. Depending on their propagation direction, they may impact planets such as Earth, where they elicit colorful aurorae or, in very seldom cases, can lead to power failures with potentially tremendous economical and societal effects, thus posing a serious natural hazard. In this work, we have shown how well the solar storm impact can be forecasted when using a special type of instrument that can actually image the solar storms as they propagate toward the planets and even as they sweep over them. Our analysis includes two thirds of a solar cycle with 8 years of data, and spacecraft at Mercury, Venus, Earth, and in the solar wind to check on the correctness of our predictions. We could forecast the arrival time within +/- 16 h, and for one correct impact there are two to three false alarms. This forms a new baseline for the science of space weather prediction. Clearly, the modeling should be further improved to be used on a daily basis for a space weather mission to the Sun-Earth L5 point.
  • Honkonen, I.; Palmroth, M.; Pulkkinen, T. I.; Janhunen, P.; Aikio, A. (2011)
  • Kalliokoski, Milla M. H.; Kilpua, Emilia K. J.; Osmane, Adnane; Turner, Drew L.; Jaynes, Allison N.; Turc, Lucile; George, Harriet; Palmroth, Minna (2020)
    The energetic electron content in the Van Allen radiation belts surrounding the Earth can vary dramatically at several timescales, and these strong electron fluxes present a hazard for spacecraft traversing the belts. The belt response to solar wind driving is, however, largely unpredictable, and the direct response to specific large-scale heliospheric structures has not been considered previously. We investigate the immediate response of electron fluxes in the outer belt that are driven by sheath regions preceding interplanetary coronal mass ejections and the associated wave activity in the inner magnetosphere. We consider the events recorded from 2012 to 2018 in the Van Allen Probes era to utilise the energy- and radial-distance-resolved electron flux observations of the twin spacecraft mission. We perform a statistical study of the events by using the superposed epoch analysis in which the sheaths are superposed separately from the ejecta and resampled to the same average duration. Our results show that the wave power of ultra-low frequency Pc5 and electromagnetic ion cyclotron waves, as measured by a Geostationary Operational Environmental Satellite (GOES), is higher during the sheath than during the ejecta. However, the level of chorus wave power, as measured by the Van Allen Probes, remains approximately the same due to similar substorm activity during the sheath and ejecta. Electron flux enhancements are common at low energies (<1 MeV) throughout the outer belt (L = 3-6), whereas depletion predominantly occurs at high energies for high radial distances (L > 4). It is distinctive that the depletion extends to lower energies at larger distances. We suggest that this L-shell and energy-dependent depletion results from the magnetopause shadowing that dominates the losses at large distances, while the wave-particle interactions dominate closer to the Earth. We also show that non-geoeffective sheaths cause significant changes in the outer belt electron fluxes.
  • Planck Collaboration; Aghanim, N.; Keihanen, E.; Kiiveri, K.; Kurki-Suonio, H.; Lindholm, V.; Savelainen, M.; Suur-Uski, A. -S.; Valiviita, J. (2020)
    Observations of the submillimetre emission from Galactic dust, in both total intensity I and polarization, have received tremendous interest thanks to the Planck full-sky maps. In this paper we make use of such full-sky maps of dust polarized emission produced from the third public release of Planck data. As the basis for expanding on astrophysical studies of the polarized thermal emission from Galactic dust, we present full-sky maps of the dust polarization fraction p, polarization angle psi, and dispersion function of polarization angles ?. The joint distribution (one-point statistics) of p and N-H confirms that the mean and maximum polarization fractions decrease with increasing N-H. The uncertainty on the maximum observed polarization fraction, (max) = 22.0(-1.4)(+3.5) p max = 22 . 0 - 1.4 + 3.5 % at 353 GHz and 80 ' resolution, is dominated by the uncertainty on the Galactic emission zero level in total intensity, in particular towards diffuse lines of sight at high Galactic latitudes. Furthermore, the inverse behaviour between p and ? found earlier is seen to be present at high latitudes. This follows the ?proportional to p(-1) relationship expected from models of the polarized sky (including numerical simulations of magnetohydrodynamical turbulence) that include effects from only the topology of the turbulent magnetic field, but otherwise have uniform alignment and dust properties. Thus, the statistical properties of p, psi, and ? for the most part reflect the structure of the Galactic magnetic field. Nevertheless, we search for potential signatures of varying grain alignment and dust properties. First, we analyse the product map ?xp, looking for residual trends. While the polarization fraction p decreases by a factor of 3-4 between N-H=10(20) cm(-2) and N-H=2x10(22) cm(-2), out of the Galactic plane, this product ?xp only decreases by about 25%. Because ? is independent of the grain alignment efficiency, this demonstrates that the systematic decrease in p with N-H is determined mostly by the magnetic-field structure and not by a drop in grain alignment. This systematic trend is observed both in the diffuse interstellar medium (ISM) and in molecular clouds of the Gould Belt. Second, we look for a dependence of polarization properties on the dust temperature, as we would expect from the radiative alignment torque (RAT) theory. We find no systematic trend of ?xp with the dust temperature T-d, whether in the diffuse ISM or in the molecular clouds of the Gould Belt. In the diffuse ISM, lines of sight with high polarization fraction p and low polarization angle dispersion ? tend, on the contrary, to have colder dust than lines of sight with low p and high ?. We also compare the Planck thermal dust polarization with starlight polarization data in the visible at high Galactic latitudes. The agreement in polarization angles is remarkable, and is consistent with what we expect from the noise and the observed dispersion of polarization angles in the visible on the scale of the Planck beam. The two polarization emission-to-extinction ratios, R-P/p and R-S/V, which primarily characterize dust optical properties, have only a weak dependence on the column density, and converge towards the values previously determined for translucent lines of sight. We also determine an upper limit for the polarization fraction in extinction, p(V)/E(B-V), of 13% at high Galactic latitude, compatible with the polarization fraction p approximate to 20% observed at 353 GHz. Taken together, these results provide strong constraints for models of Galactic dust in diffuse gas.
  • Ade, P. A. R.; Juvela, M.; Keihanen, E.; Kurki-Suonio, H.; Poutanen, T.; Suur-Uski, A. -S.; Planck Collaboration (2013)
  • Asvestari, E.; Heinemann, S. G.; Temmer, M.; Pomoell, J.; Kilpua, E.; Magdalenic, J.; Poedts, S. (2019)
    The adopted Wang-Sheeley-Arge (WSA) model embedded in EUHFORIA (EUropean Heliospheric FORecasting Information Asset) is compared to EUV observations. According to the standard paradigm, coronal holes are sources of open flux; thus, we use remote sensing EUV observations and CATCH (Collection of Analysis Tools for Coronal Holes) to extract CH areas and compare them to the open flux areas modeled by EUHFORIA. From the adopted WSA model we employ only the Potential Field Source Surface (PFSS) model for the inner corona and the Schatten Current Sheet (SCS) model for the outer (PFSS+SCS). The height, R-ss, of the outer boundary of the PFSS, known as the source surface, and the height, R-i, of the inner boundary of the SCS are important parameters affecting the modeled CH areas. We investigate the impact the two model parameters can have in the modeled results. We vary R-ss within the interval [1.4, 3.2]R-circle dot with a step of 0.1R(circle dot), and R-i within the interval [1.3, 2.8]R-circle dot with the same step, and the condition that R-i
  • Stepanek, Petr; Coriani, Sonia; Sundholm, Dage; Ovchinnikov, Vasily A.; Vaara, Juha (2017)
    The recently theoretically described nuclear spin-induced circular dichroism (NSCD) is a promising method for the optical detection of nuclear magnetization. NSCD involves both optical excitations of the molecule and hyperfine interactions and, thus, it offers a means to realize a spectroscopy with spatially localized, high-resolution information. To survey the factors relating the molecular and electronic structure to the NSCD signal, we theoretically investigate NSCD of twenty structures of the four most common nucleic acid bases (adenine, guanine, thymine, cytosine). The NSCD signal correlates with the spatial distribution of the excited states and couplings between them, reflecting changes in molecular structure and conformation. This constitutes a marked difference to the nuclear magnetic resonance (NMR) chemical shift, which only reflects the local molecular structure in the ground electronic state. The calculated NSCD spectra are rationalized by means of changes in the electronic density and by a sum-over-states approach, which allows to identify the contributions of the individual excited states. Two separate contributions to NSCD are identified and their physical origins and relative magnitudes are discussed. The results underline NSCD spectroscopy as a plausible tool with a power for the identification of not only different molecules, but their specific structures as well.
  • Dubart, Maxime; Ganse, Urs; Osmane, Adnane; Johlander, Andreas; Battarbee, Markus; Grandin, Maxime; Pfau-Kempf, Yann; Turc, Lucile; Palmroth, Minna (2020)
    Kinetically driven plasma waves are fundamental for a description of the thermodynamical properties of the Earth's magnetosheath. The most commonly observed ion-scale instabilities are generated by temperature anisotropy of the ions, such as the mirror and proton cyclotron instabilities. We investigate here the spatial resolution dependence of the mirror and proton cyclotron instabilities in a global hybrid-Vlasov simulation using the Vlasiator model; we do this in order to find optimal resolutions and help future global hybrid-Vlasov simulations to save resources when investigating those instabilities in the magnetosheath. We compare the proton velocity distribution functions, power spectra and growth rates of the instabilities in a set of simulations with three different spatial resolutions but otherwise identical setup. We find that the proton cyclotron instability is absent at the lowest resolution and that only the mirror instability remains, which leads to an increased temperature anisotropy in the simulation. We conclude that the proton cyclotron instability, its saturation and the reduction of the anisotropy to marginal levels are resolved at the highest spatial resolution. A further increase in resolution does not lead to a better description of the instability to an extent that would justify this increase at the cost of numerical resources in future simulations. We also find that spatial resolutions between 1.32 and 2.64 times the inertial length in the solar wind present acceptable limits for the resolution within which the velocity distribution functions resulting from the proton cyclotron instability are still bi-Maxwellian and reach marginal stability levels. Our results allow us to determine a range of spatial resolutions suitable for the modelling of the proton cyclotron and mirror instabilities and should be taken into consideration regarding the optimal grid spacing for the modelling of these two instabilities, within available computational resources.