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Gunell, Herbert
Publications (10 of 11) Show all publications
Maggiolo, R., Gibbons, A., Cessateur, G., De Keyser, J., Dhooghe, F., Gunell, H., . . . Vaeck, N. (2019). Effect of the Surface Roughness of Icy Grains on Molecular Oxygen Chemistry in Molecular Clouds. Astrophysical Journal, 882(2), Article ID 131.
Open this publication in new window or tab >>Effect of the Surface Roughness of Icy Grains on Molecular Oxygen Chemistry in Molecular Clouds
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2019 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 882, no 2, article id 131Article in journal (Refereed) Published
Abstract [en]

Molecular cloud and protosolar nebula chemistry involves a strong interaction between the gas phase and the surface of icy grains. The exchanges between the gas phase and the solid phase depend not only on the adsorption and desorption rates but also on the geometry of the surface of the grains. Indeed, for sufficient levels of surface roughness, atoms and molecules have a significant probability to collide with the grain icy mantle several times before being potentially captured. In consequence, their net sticking probability may differ from their sticking probability for a single collision with the grain surface. We estimate the effectiveness of the recapture on uneven surfaces for the various desorption processes at play in astrophysical environments. We show that surface roughness has a significant effect on the desorption rates. We focus in particular on the production of O-2 since unexpectedly large amounts of it, probably incorporated in the comet when it formed, have been detected in the coma of comet 67P by the Rosetta probe. Our results suggest that the higher escape probability of hydrogen compared to heavier species on rough surfaces can contribute to enhancing the production of O-2 in the icy mantles of grains while keeping its abundance low in the gas phase and may significantly decrease the desorption probability of molecules involved in the O-2 chemical network.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2019
Keywords
astrochemistry, comets: general, comets: individual (67P/Churyumov-Gerasimenko), ISM: abundances, ISM: clouds, ISM: molecules
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:umu:diva-164410 (URN)10.3847/1538-4357/ab3400 (DOI)000485693400011 ()
Available from: 2019-10-22 Created: 2019-10-22 Last updated: 2019-10-22Bibliographically approved
De Spiegeleer, A., Hamrin, M., Gunell, H., Volwerk, M., Andersson, L., Karlsson, T., . . . Kistler, L. M. (2019). Oscillatory Flows in the Magnetotail Plasma Sheet: Cluster Observations of the Distribution Function. Journal of Geophysical Research - Space Physics, 124(4), 2736-2754
Open this publication in new window or tab >>Oscillatory Flows in the Magnetotail Plasma Sheet: Cluster Observations of the Distribution Function
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2019 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, no 4, p. 2736-2754Article in journal (Refereed) Published
Abstract [en]

Plasma dynamics in Earth's magnetotail is often studied using moments of the distribution function, which results in losing information on the kinetic properties of the plasma. To better understand oscillatory flows observed in the midtail plasma sheet, we investigate two events, one in each hemisphere, in the transition region between the central plasma sheet and the lobes using the 2-D ion distribution function from the Cluster 4 spacecraft. In this case study, the oscillatory flows are a manifestation of repeated ion flux enhancements with pitch angle changing from 0 degrees to 180 degrees in the Northern Hemisphere and from 180 degrees to 0 degrees in the Southern Hemisphere. Similar pitch angle signatures are observed seven times in about 80 min for the Southern Hemisphere event and three times in about 80 min for the Northern Hemisphere event. The ion flux enhancements observed for both events are slightly shifted in time between different energy channels, indicating a possible time-of-flight effect from which we estimate that the source of particle is located similar to 5-25R(E) and similar to 40-107R(E) tailward of the spacecraft for the Southern and Northern Hemisphere event, respectively. Using a test particle simulation, we obtain similar to 21-46 R-E for the Southern Hemisphere event and tailward of X similar to - 65R(E) (outside the validity region of the model) for the Northern Hemisphere event. We discuss possible sources that could cause the enhancements of ion flux.

Place, publisher, year, edition, pages
AMER GEOPHYSICAL UNION, 2019
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-162349 (URN)10.1029/2018JA026116 (DOI)000477707800027 ()
Available from: 2019-08-27 Created: 2019-08-27 Last updated: 2019-08-27Bibliographically approved
Hamrin, M., Gunell, H., Lindkvist, J., Lindqvist, P.-A., Ergun, R. E. & Giles, B. L. (2018). Bow shock generator current systems: MMS observations of possible current closure. Journal of Geophysical Research - Space Physics, 123, 242-258
Open this publication in new window or tab >>Bow shock generator current systems: MMS observations of possible current closure
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2018 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 123, p. 242-258Article in journal (Refereed) Published
Abstract [en]

We use data from the first two dayside seasons of the Magnetospheric Multiscale (MMS) mission to study current systems associated with quasi‐perpendicular bow shocks of generator type. We have analyzed 154 MMS bow shock crossings near the equatorial plane. We compute the current density during the crossings and conclude that the component perpendicular to the shock normal (J⊥) is consistent with a pileup of the interplanetary magnetic field (IMF) inside the magnetosheath. For predominantly southward IMF, we observe a component Jn parallel (antiparallel) to the normal for GSM Y> 0 (<0), and oppositely directed for northward IMF. This indicates current closure across the equatorial magnetosheath, and it is observed for IMF clock angles near 0∘ and 180∘. To our knowledge, these are the first observational evidence for bow shock current closure across the magnetosheath. Since we observe no clear signatures of |J⊥| decreasing toward large |Y| we suggest that the main region of current closure is further tailward, outside MMS probing region. For IMF clock angles near 90∘, we find indications of the current system being tilted toward the north‐south direction, obtaining a significant Jz component, and we suggest that the current closes off the equatorial plane at higher latitudes where the spacecraft are not probing. The observations are complicated for several reasons. For example, variations in the solar wind and the magnetospheric currents and loads affect the closure, and Jn is distributed over large regions, making it difficult to resolve inside the magnetosheath proper.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-143055 (URN)10.1002/2017JA024826 (DOI)000425637600018 ()
Available from: 2017-12-14 Created: 2017-12-14 Last updated: 2018-06-09Bibliographically approved
Hajra, R., Henri, P., Vallières, X., More, J., Gilet, N., Wattieaux, G., . . . Rubin, M. (2018). Dynamic unmagnetized plasma in the diamagnetic cavity around comet 67P/Churyumov-Gerasimenko. Monthly notices of the Royal Astronomical Society, 475(3), 4140-4147
Open this publication in new window or tab >>Dynamic unmagnetized plasma in the diamagnetic cavity around comet 67P/Churyumov-Gerasimenko
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2018 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 475, no 3, p. 4140-4147Article in journal (Refereed) Published
Abstract [en]

The Rosetta orbiter witnessed several hundred diamagnetic cavity crossings (unmagnetized regions) around comet 67P/Churyumov-Gerasimenko during its two year survey of the comet. The characteristics of the plasma environment inside these diamagnetic regions are studied using in situ measurements by the Rosetta Plasma Consortium instruments. Although the unmagnetized plasma density has been observed to exhibit little dynamics compared to the very dynamical magnetized cometary plasma, we detected several localized dynamic plasma structures inside those diamagnetic regions. These plasma structures are not related to the direct ionization of local cometary neutrals. The structures are found to be steepened, asymmetric plasma enhancements with typical rising-to-descending slope ratio of similar to 2.8 (+/- 1.9), skewness similar to 0.43 (+/- 0.36), mean duration of similar to 2.7 (+/- 0.9) min and relative density variation Delta N/N of similar to 0.5 (+/- 0.2), observed close to the electron exobase. Similar steepened plasma density enhancements were detected at the magnetized boundaries of the diamagnetic cavity as well as outside the diamagnetic region. The plausible scalelength and propagation direction of the structures are estimated from simple plasma dynamics considerations. It is suggested that they are large-scale unmagnetized plasma enhancements, transmitted from the very dynamical outer magnetized region to the inner magnetic field-free cavity region.

Place, publisher, year, edition, pages
Oxford University Press, 2018
Keywords
methods: data analysis, methods: observational, comets: general, comets: individual: P/Churyumov-Gerasimenko
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-146551 (URN)10.1093/mnras/sty094 (DOI)000427141900087 ()
Available from: 2018-05-15 Created: 2018-05-15 Last updated: 2018-06-09Bibliographically approved
Lindkvist, J., Hamrin, M., Gunell, H., Nilsson, H., Simon Wedlund, C., Kallio, E., . . . Karlsson, T. (2018). Energy conversion in cometary atmospheres: Hybrid modeling of 67P/Churyumov-Gerasimenko. Astronomy and Astrophysics, 616, Article ID A81.
Open this publication in new window or tab >>Energy conversion in cometary atmospheres: Hybrid modeling of 67P/Churyumov-Gerasimenko
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 616, article id A81Article in journal (Refereed) Published
Abstract [en]

Aims. We wish to investigate the energy conversion between particles and electromagnetic fields and determine the location where it occurs in the plasma environment of comets.

Methods. We used a hybrid plasma model that included photoionization, and we considered two cases of the solar extreme ultraviolet flux. Other parameters corresponded to the conditions of comet 67P/Churyumov-Gerasimenko at a heliocentric distance of 1.5 AU.

Results. We find that a shock-like structure is formed upstream of the comet and acts as an electromagnetic generator, similar to the bow shock at Earth that slows down the solar wind. The Poynting flux transports electromagnetic energy toward the inner coma, where newly born cometary ions are accelerated. Upstream of the shock-like structure, we find local energy transfer from solar wind ions to cometary ions. We show that mass loading can be a local process with a direct transfer of energy, but also part of a dynamo system with electromagnetic generators and loads.

Conclusions. The energization of cometary ions is governed by a dynamo system for weak ionization, but changes into a large conversion region with local transfer of energy directly from solar wind protons for high ionization.

Place, publisher, year, edition, pages
EDP Sciences, 2018
Keywords
comets: individual: 67P/Churyumov-Gerasimenko, Sun: UV radiation, solar wind, methods: numerical, plasmas, acceleration of particles
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-148207 (URN)10.1051/0004-6361/201732353 (DOI)000442541100001 ()
Funder
Swedish National Space Board, 201/15Swedish National Space Board, 112/13Swedish Research Council, 2015-04187
Available from: 2018-05-30 Created: 2018-05-30 Last updated: 2018-09-10Bibliographically approved
Madsen, B., Wedlund, C. S., Eriksson, A., Goetz, C., Karlsson, T., Gunell, H., . . . Miloch, W. J. (2018). Extremely Low-Frequency Waves Inside the Diamagnetic Cavity of Comet 67P/Churyumov-Gerasimenko. Geophysical Research Letters, 45(9), 3854-3864
Open this publication in new window or tab >>Extremely Low-Frequency Waves Inside the Diamagnetic Cavity of Comet 67P/Churyumov-Gerasimenko
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2018 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 45, no 9, p. 3854-3864Article in journal (Refereed) Published
Abstract [en]

The European Space Agency/Rosetta mission to comet 67P/Churyumov-Gerasimenko has provided several hundred observations of the cometary diamagnetic cavity induced by the interaction between outgassed cometary particles, cometary ions, and the solar wind magnetic field. Here we present the first electric field measurements of four preperihelion and postperihelion cavity crossings on 28 May 2015 and 17 February 2016, using the dual-probe electric field mode of the Langmuir probe (LAP) instrument of the Rosetta Plasma Consortium. We find that on large scales, variations in the electric field fluctuations capture the cavity and boundary regions observed in the already well-studied magnetic field, suggesting the electric field mode of the LAP instrument as a reliable tool to image cavity crossings. In addition, the LAP electric field mode unravels for the first time extremely low-frequency waves within two cavities. These low-frequency electrostatic waves are likely triggered by lower-hybrid waves observed in the surrounding magnetized plasma. Plain Language Summary As sunlight heats a comet nucleus, frozen volatile gases sublimate are ionized and interact with the solar wind and its embedded magnetic field, inducing a dynamical plasma environment around the comet. With the cornerstone European mission Rosetta and its 2years of near-continuous orbiting of comet 67P/Churyumov-Gerasimenko, the origin, structure, and evolution of this environment are only starting to be unveiled. Exciting are the numerous crossings of the diamagnetic cavity, the innermost plasma region from which the solar wind magnetic field is excluded. Whilst the magnetic field structure of the cavity crossings is well studied, the related electric field activity remains until now unexplored. Studying the electric field with the Langmuir probes onboard Rosetta, we find that whereas the large-scale electric field structure agrees well with the observed magnetic field behavior during cavity crossings, unexpected short-lived low-frequency electric field signals manifest themselves within the cavity. We interpret these as electrostatic waves triggered by a modulating of the cavity boundary caused by observed electrostatic waves at the same frequency in the surrounding magnetized plasma. This unravels a new aspect of the electromagnetic activity in the inner cometary environment, which is crucial for our understanding of the comet-solar wind-induced plasma environment.

Place, publisher, year, edition, pages
AMER GEOPHYSICAL UNION, 2018
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-150695 (URN)10.1029/2017GL076415 (DOI)000434111700012 ()
Available from: 2018-09-05 Created: 2018-09-05 Last updated: 2018-09-05Bibliographically approved
Nilsson, H., Gunell, H., Karlsson, T., Brenning, N., Henri, P., Goetz, C., . . . Vallieres, X. (2018). Size of a plasma cloud matters: The polarisation electric field of a small-scale comet ionosphere. Astronomy and Astrophysics, 616, Article ID A50.
Open this publication in new window or tab >>Size of a plasma cloud matters: The polarisation electric field of a small-scale comet ionosphere
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 616, article id A50Article in journal (Refereed) Published
Abstract [en]

Context. The cometary ionosphere is immersed in fast flowing solar wind. A polarisation electric field may arise for comets much smaller than the gyroradius of pickup ions because ions and electrons respond differently to the solar wind electric field.

Aims. A situation similar to that found at a low activity comet has been modelled for barium releases in the Earth's ionosphere. We aim to use such a model and apply it to the case of comet 67P Churyumov-Gerasimenko, the target of the Rosetta mission. We aim to explain the significant tailward acceleration of cometary ions through the modelled electric field.

Methods. We obtained analytical solutions for the polarisation electric field of the comet ionosphere using a simplified geometry. This geometry is applicable to the comet in the inner part of the coma as the plasma density integrated along the magnetic field line remains rather constant. We studied the range of parameters for which a significant tailward electric field is obtained and compare this with the parameter range observed.

Results. Observations of the local plasma density and magnetic field strength show that the parameter range of the observations agree very well with a significant polarisation electric field shielding the inner part of the coma from the solar wind electric field.

Conclusions. The same process gives rise to a tailward directed electric field with a strength of the order of 10% of the solar wind electric field. Using a simple cloud model we have shown that the polarisation electric field, which arises because of the small size of the comet ionosphere as compared to the pick up ion gyroradius, can explain the observed significant tailward acceleration of cometary ions and is consistent with the observed lack of influence of the solar wind electric field in the inner coma.

Place, publisher, year, edition, pages
EDP Sciences, 2018
Keywords
plasmas, acceleration of particles, comets: individual: 67P/Churyumov-Gerasimenko
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:umu:diva-151393 (URN)10.1051/0004-6361/201833199 (DOI)000441817100003 ()
Funder
Swedish National Space Board, 108/12Swedish National Space Board, 112/13Swedish National Space Board, 96/15Swedish National Space Board, 94/11Swedish Research Council, 2015-04187
Available from: 2018-09-05 Created: 2018-09-05 Last updated: 2018-09-07Bibliographically approved
Gunell, H., Goetz, C., Wedlund, C. S., Lindkvist, J., Hamrin, M., Nilsson, H., . . . Holmström, M. (2018). The infant bow shock: a new frontier at a weak activity comet [Letter to the editor]. Astronomy and Astrophysics, 619, Article ID L2.
Open this publication in new window or tab >>The infant bow shock: a new frontier at a weak activity comet
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 619, article id L2Article in journal, Letter (Refereed) Published
Abstract [en]

The bow shock is the first boundary the solar wind encounters as it approaches planets or comets. The Rosetta spacecraft was able to observe the formation of a bow shock by following comet 67P/Churyumov-Gerasimenko toward the Sun, through perihelion, and back outward again. The spacecraft crossed the newly formed bow shock several times during two periods a few months before and after perihelion; it observed an increase in magnetic field magnitude and oscillation amplitude, electron and proton heating at the shock, and the diminution of the solar wind further downstream. Rosetta observed a cometary bow shock in its infancy, a stage in its development not previously accessible to in situ measurements at comets and planets.

Place, publisher, year, edition, pages
EDP Sciences, 2018
Keywords
comets: general, comets: individual: 67P/Churyumov-Gerasimenko, plasmas, shock waves
National Category
Astronomy, Astrophysics and Cosmology Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-153545 (URN)10.1051/0004-6361/201834225 (DOI)000449276800001 ()
Funder
Swedish National Space Board, 201/15
Available from: 2018-11-22 Created: 2018-11-22 Last updated: 2018-11-22Bibliographically approved
Nilsson, H., Wieser, G. S., Behar, E., Gunell, H., Wieser, M., Galand, M., . . . Vigren, E. (2017). Evolution of the ion environment of comet 67P during the Rosetta mission as seen by RPC-ICA. Paper presented at International Conference on Cometary Science - Comets - A New Vision after Rosetta and Philae, NOV 14-18, 2016, Toulouse, FRANCE. Monthly notices of the Royal Astronomical Society, 469, S252-S261
Open this publication in new window or tab >>Evolution of the ion environment of comet 67P during the Rosetta mission as seen by RPC-ICA
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2017 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 469, p. S252-S261Article in journal (Refereed) Published
Abstract [en]

Rosetta 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.

Place, publisher, year, edition, pages
Oxford University Press, 2017
Keywords
plasmas, methods: data analysis, comets: individual: 67P
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:umu:diva-153853 (URN)10.1093/mnras/stx1491 (DOI)000443940500024 ()
Conference
International Conference on Cometary Science - Comets - A New Vision after Rosetta and Philae, NOV 14-18, 2016, Toulouse, FRANCE
Funder
Swedish National Space Board, 108/12Swedish National Space Board, 112/13Swedish National Space Board, 96/15Swedish National Space Board, 94/11Swedish Research Council, 2015-04187
Available from: 2018-12-05 Created: 2018-12-05 Last updated: 2018-12-05Bibliographically approved
Wieser, G. S., Odelstad, E., Wieser, M., Nilsson, H., Goetz, C., Karlsson, T., . . . Gunell, H. (2017). Investigating short-time-scale variations in cometary ions around comet 67P. Paper presented at International Conference on Cometary Science - Comets - A New Vision after Rosetta and Philae, NOV 14-18, 2016, Toulouse, FRANCE. Monthly notices of the Royal Astronomical Society, 469, S522-S534
Open this publication in new window or tab >>Investigating short-time-scale variations in cometary ions around comet 67P
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2017 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 469, p. S522-S534Article in journal (Refereed) Published
Abstract [en]

The highly varying plasma environment around comet 67P/Churyumov-Gerasimenko inspired an upgrade of the ion mass spectrometer (Rosetta Plasma Consortium Ion Composition Analyzer) with new operation modes, to enable high time resolution measurements of cometary ions. Two modes were implemented, one having a 4 s time resolution in the energy range 0.3-82 eV/q and the other featuring a 1 s time resolution in the energy range 13-50 eV/q. Comparing measurements made with the two modes, it was concluded that 4 s time resolution is enough to capture most of the fast changes of the cometary ion environment. The 1462 h of observations done with the 4 s mode were divided into hour-long sequences. It is possible to sort 84 per cent of these sequences into one of five categories, depending on their appearance in an energy-time spectrogram. The ion environment is generally highly dynamic, and variations in ion fluxes and energies are seen on time-scales of 10 s to several minutes.

Place, publisher, year, edition, pages
Oxford University Press, 2017
Keywords
plasmas, instrumentation: detectors, methods: data analysis, methods: statistical, space vehicles: instruments, comets: individual: 67P
National Category
Fusion, Plasma and Space Physics Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:umu:diva-153854 (URN)10.1093/mnras/stx2133 (DOI)000443940500049 ()
Conference
International Conference on Cometary Science - Comets - A New Vision after Rosetta and Philae, NOV 14-18, 2016, Toulouse, FRANCE
Funder
Swedish National Space Board, 96/15Swedish National Space Board, 101/15Swedish National Space Board, 95/15
Available from: 2018-12-05 Created: 2018-12-05 Last updated: 2018-12-05Bibliographically approved
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