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Solar Wind Dynamics within The Atmosphere of comet 67P/Churyumov-Gerasimenko
Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.ORCID-id: 0000-0003-0177-510x
2018 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
Abstract [en]

In this thesis, we explore the dynamics of the solar wind as it perme-ates and flows through a tenuous cometary atmosphere, with a focuson the interaction observed at comet 67P/Churyumov–Gerasimenko.

Seven comets had already been visited by nine different probes when the European spacecraft Rosetta reached comet Churyumov–Gerasimenko in August 2014. The mission was however the first to orbit its host comet, which it did for a total duration of more than two years, corre-sponding to a large part of the comet’s orbit around the Sun. This en-abled to study how the dynamics of the plasma environment evolvedas the comet itself was transformed from one of the smallest obstaclesto the solar wind in the Solar System when far away from the Sun, toa well-established magnetosphere at perihelion.

Most of our efforts tackle the early part of this transformation, when the creation of new-born cometary ions starts to induce significant disturbances to the incident flow. During this stage, a kinetic descrip-tion of the interaction is necessary, as the system of interest cannot be reduced to a hydrodynamic problem. This contrasts with the situation closer to the Sun, where a fluid treatment can be used, at Churyumov–Gerasimenko as well as at previously visited comets.

Rosetta was not a mission dedicated to plasma studies, however. It directly translates into a limited spatial coverage of the cometary plasma environment, which by its nature extends over several spatial scales. An approach solely based on the analysis of in-situ data cannot properly address the major questions on the nature and physics of the plasma environment of Churyumov–Gerasimenko. This thesis there-fore largely exploits the experimental–analytical–numerical triad of approaches. In Chapters 3 and 4 we propose simple models of the ion dynamics and of the cometary plasma environment, and these are tested against experimental and numerical data. Used together,they give a global description of the solar wind ion dynamics through the cometary atmosphere, that we explore in the 2-dimensional and 3-dimensional cases (Chapter 5). In Chapter 6, we propose a view onthe interaction and its fluid aspects when closer to the Sun.

Abstract [sv]

I  denna  avhandling  undersöks  solvindens  dynamik  när  den  flödar genom  en  tunn  kometatmosfär,  med  fokus  på  den  interaktion  somobserverats vid kometen 67P/Churyumov-Gerasimenko.

När  Rosetta  nådde  from  till  komet  67P/Churyumov–Gerasimenko  i augusti 2014 hade redan sju olika rymdfarkoster besökt nio olika ko- meter.  Rosetta  var  dock  den  första  missionen  som  cirklade  runt  en komet och följde den sedan i dess bana i totalt mer än två år. Detta motsvarade en signifikant del av hela kometens bana runt solen. Det- ta gjorde det möjligt att studera plasmats dynamiska utveckling un- der en period när kometen utvecklades från ett av de minsta hindren solvinden möter i solsystemet, när kometen är långt från solen, till enfullt utvecklad magnetosfär nära perihelion.

Avhandlingen behandlar främst den tidiga delen av denna utveckling, när nyligen joniserade molekyler från kometatmosfären börjar ge en signifikant störning i solvindens flöde. Under dessa förhållanden är en kinetisk beskrivning av växelverkan mellan solvinden och kome- tatmosfären  nödvändig.  Systemet  vi  studerar  kan  inte  reduceras  till ett hydrodynamiskt problem. Detta till skillnad från situationen när kometen är närmare solen, då växelverkan kan beskrivas med fluid- teori, för både Churyumov–Gerasimenko och de tidigare undersöktakometerna.

Rosetta var inte en mission främst ägnad åt plasma-studier. Detta med- förde begränsad rumslig täckning av de plasma-processer som äger rum, vilka äger rum över flera olika rumsskalor. Att förstå solvindens växleverkan  med  kometatmosfären  enbart  med  enpunktsmätningar låter sig därför inte göras. I detta arbete används därför en kombina- tion av experimentella data, analytiska beskrivningar och numeriska beräkningar. I kapitel 3 och 4 föreslår vi enkla modeller för att beskri- va jondynamiken och kometens plasmamiljö. Dessa testas sedan mot observationer och numeriska modeller. Tillsammans ger de en global beskrivning  av  solvindens  dynamik  när  den  flödar  genom  kometat- mosfären,  vilket  utforskas  i  både  2 och  3 dimensioner  (kapitel  5).  I kapitel 6 utforskar vi interaktionen som sker när kometen är närmaresolen, och hur den nu i högre grad kan beskrivas som en fluid.

Ort, förlag, år, upplaga, sidor
Luleå: Luleå University of Technology, 2018.
Serie
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Nationell ämneskategori
Fusion, plasma och rymdfysik Rymd- och flygteknik
Forskningsämne
Rymdteknik
Identifikatorer
URN: urn:nbn:se:ltu:diva-68785ISBN: 978-91-7790-156-3 (tryckt)ISBN: 978-91-7790-157-0 (digital)OAI: oai:DiVA.org:ltu-68785DiVA, id: diva2:1206755
Disputation
2018-10-12, Aulan, IRF, Rymdcampus, Kiruna, 09:00 (Engelska)
Opponent
Handledare
Tillgänglig från: 2018-05-21 Skapad: 2018-05-18 Senast uppdaterad: 2018-10-10Bibliografiskt granskad
Delarbeten
1. Mass loading at 67P/Churyumov-Gerasimenko: A case study
Öppna denna publikation i ny flik eller fönster >>Mass loading at 67P/Churyumov-Gerasimenko: A case study
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2016 (Engelska)Ingår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 43, nr 4, s. 1411-1418Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

We study the dynamics of the interaction between the solar wind ions and a partially ionized atmosphere around a comet, at a distance of 2.88 AU from the Sun during a period of low nucleus activity. Comparing particle data and magnetic field data for a case study, we highlight the prime role of the solar wind electric field in the cometary ion dynamics. Cometary ion and solar wind proton flow directions evolve in a correlated manner, as expected from the theory of mass loading. We find that the main component of the accelerated cometary ion flow direction is along the antisunward direction and not along the convective electric field direction. This is interpreted as the effect of an antisunward polarization electric field adding up to the solar wind convective electric field.

Nationell ämneskategori
Rymd- och flygteknik
Forskningsämne
Atmosfärsvetenskap
Identifikatorer
urn:nbn:se:ltu:diva-8286 (URN)10.1002/2015GL067436 (DOI)000373109000001 ()2-s2.0-84975743627 (Scopus ID)6c6ac809-0b7a-4666-8ba3-4712cf4ebf06 (Lokalt ID)6c6ac809-0b7a-4666-8ba3-4712cf4ebf06 (Arkivnummer)6c6ac809-0b7a-4666-8ba3-4712cf4ebf06 (OAI)
Anmärkning

Validerad; 2016; Nivå 2; 20160219 (andbra)

Tillgänglig från: 2016-09-29 Skapad: 2016-09-29 Senast uppdaterad: 2018-07-10Bibliografiskt granskad
2. Mass-loading of the solar wind at 67P/Churyumov-Gerasimenko: Observations and modelling
Öppna denna publikation i ny flik eller fönster >>Mass-loading of the solar wind at 67P/Churyumov-Gerasimenko: Observations and modelling
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2016 (Engelska)Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 596, artikel-id A42Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Context. The first long-term in-situ observation of the plasma environment in the vicinity of a comet, as provided by the European Rosetta spacecraft. Aims. Here we offer characterisation of the solar wind flow near 67P/Churyumov-Gerasimenko (67P) and its long term evolution during low nucleus activity. We also aim to quantify and interpret the deflection and deceleration of the flow expected from ionization of neutral cometary particles within the undisturbed solar wind. Methods. We have analysed in situ ion and magnetic field data and combined this with hybrid modeling of the interaction between the solar wind and the comet atmosphere. Results. The solar wind deflection is increasing with decreasing heliocentric distances, and exhibits very little deceleration. This is seen both in observations and in modeled solar wind protons. According to our model, energy and momentum are transferred from the solar wind to the coma in a single region, centered on the nucleus, with a size in the order of 1000 km. This interaction affects, over larger scales, the downstream modeled solar wind flow. The energy gained by the cometary ions is a small fraction of the energy available in the solar wind. Conclusions. The deflection of the solar wind is the strongest and clearest signature of the mass-loading for a small, low-activity comet, whereas there is little deceleration of the solar wind

Nationell ämneskategori
Rymd- och flygteknik
Forskningsämne
Atmosfärsvetenskap
Identifikatorer
urn:nbn:se:ltu:diva-61146 (URN)10.1051/0004-6361/201628797 (DOI)000390797900062 ()2-s2.0-85000443624 (Scopus ID)
Anmärkning

Validerad; 2017; Nivå 2; 2016-12-19 (andbra)

Tillgänglig från: 2016-12-19 Skapad: 2016-12-19 Senast uppdaterad: 2018-07-10Bibliografiskt granskad
3. Cometary ion dynamics observed in the close vicinity of comet 67P/Churyumov-Gerasimenko during the intermediate activity period
Öppna denna publikation i ny flik eller fönster >>Cometary ion dynamics observed in the close vicinity of comet 67P/Churyumov-Gerasimenko during the intermediate activity period
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2018 (Engelska)Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 613, s. 1-8Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Aims.Cometary ions are constantly produced in the coma, and once produced they are accelerated and eventually escape the coma.We describe and interpret the dynamics of the cometary ion flow, of an intermediate active comet, very close to the nucleus and in theterminator plane.Methods.We analysed in situ ion and magnetic field measurements, and characterise the velocity distribution functions (mostly usingplasma moments). We propose a statistical approach over a period of one month.Results.On average, two populations were observed, separated in phase space. The motion of the first is governed by its interactionwith the solar wind farther upstream, while the second one is accelerated in the inner coma and displays characteristics compatiblewith an ambipolar electric field. Both populations display a consistent anti-sunward velocity component.Conclusions.Cometary ions born in different regions of the coma are seen close to the nucleus of comet 67P/Churyumov–Gerasimenko with distinct motions governed in one case by the solar wind electric field and in the other case by the position relative tothe nucleus. A consistent anti-sunward component is observed for all cometary ions. An asymmetry is found in the average cometaryion density in a solar wind electric field reference frame, with higher density in the negative (south) electric field hemisphere. Thereis no corresponding signature in the average magnetic field strengt

Ort, förlag, år, upplaga, sidor
EDP Sciences, 2018
Nationell ämneskategori
Fusion, plasma och rymdfysik Rymd- och flygteknik
Forskningsämne
Atmosfärsvetenskap
Identifikatorer
urn:nbn:se:ltu:diva-68784 (URN)10.1051/0004-6361/201732082 (DOI)000433880200003 ()
Anmärkning

Validerad;2018;Nivå 2;2018-06-14 (andbra)

Tillgänglig från: 2018-05-18 Skapad: 2018-05-18 Senast uppdaterad: 2018-06-21Bibliografiskt granskad
4. Solar wind dynamics around a comet: A 2D semi-analytical kinetic model
Öppna denna publikation i ny flik eller fönster >>Solar wind dynamics around a comet: A 2D semi-analytical kinetic model
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2018 (Engelska)Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 620, artikel-id A35Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Aims.We aim at analytically modelling the solar wind proton trajectories during their interaction with a partially ionised cometaryatmosphere, not in terms of bulk properties of the flow but in terms of single particle dynamics.Methods.We first derive a generalised gyromotion, in which the electric field is reduced to its motional component. Steady-stateis assumed, and simplified models of the cometary density and of the electron fluid are used to express the force experienced byindividual solar wind protons during the interaction.Results.A three-dimensional (3D) analytical expression of the gyration of two interacting plasma beams is obtained. Applying it to acomet case, the force on protons is always perpendicular to their velocity and has an amplitude proportional to 1/r2. The solar winddeflection is obtained at any point in space. The resulting picture presents a caustic of intersecting trajectories, and a circular regionis found that is completely free of particles. The particles do not lose any kinetic energy and this absence of deceleration, togetherwith the solar wind deflection pattern and the presence of a solar wind ion cavity, is in good agreement with the general results of theRosettamission.Conclusions.The qualitative match between the model and thein situdata highlights how dominant the motional electric field isthroughout most of the interaction region for the solar wind proton dynamics. The model provides a simple general kinetic descriptionof how momentum is transferred between these two collisionless plasmas. It also shows the potential of this semi-analytical modelfor a systematic quantitative comparison to the data.

Ort, förlag, år, upplaga, sidor
EDP Sciences, 2018
Nationell ämneskategori
Fusion, plasma och rymdfysik Rymd- och flygteknik
Forskningsämne
Atmosfärsvetenskap
Identifikatorer
urn:nbn:se:ltu:diva-68781 (URN)10.1051/0004-6361/201832736 (DOI)000451249600003 ()2-s2.0-85051991586 (Scopus ID)
Anmärkning

Validerad;2018;Nivå 2;2018-12-05 (svasva)

Tillgänglig från: 2018-05-18 Skapad: 2018-05-18 Senast uppdaterad: 2019-01-10Bibliografiskt granskad
5. Dawn-dusk asymmetry induced by the Parker spiral angle in the plasma dynamics around comet 67P/Churyumov-Gerasimenko
Öppna denna publikation i ny flik eller fönster >>Dawn-dusk asymmetry induced by the Parker spiral angle in the plasma dynamics around comet 67P/Churyumov-Gerasimenko
2018 (Engelska)Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 478, nr 2, s. 1570-1575Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

When interacting, the solar wind and the ionised atmosphere of a comet exchange energy and momentum. Our aim is to understand the influence of the average Parker spiral configuration of the solar wind magnetic field on this interaction. We compare the theoretical expectations of an analytical generalised gyromotion with Rosetta observations at comet 67P/Churyumov-Gerasimenko. A statistical approach allows one to overcome the lack of upstream solar wind measurement. We find that additionally to their acceleration along (for cometary pick-up ions) or against (for solar wind ions) the upstream electric field orientation and sense, the cometary pick-up ions are drifting towards the dawn side of the coma, while the solar wind ions are drifting towards the dusk side of the coma, independent of the heliocentric distance. The dynamics of the interaction is not taking place in a plane, as often assumed in previous works.

Ort, förlag, år, upplaga, sidor
Royal Astronomical Society, 2018
Nationell ämneskategori
Fusion, plasma och rymdfysik Rymd- och flygteknik
Forskningsämne
Atmosfärsvetenskap
Identifikatorer
urn:nbn:se:ltu:diva-68782 (URN)10.1093/mnras/sty1111 (DOI)000439547400010 ()2-s2.0-85048857464 (Scopus ID)
Anmärkning

Validerad;2018;Nivå 2;2018-06-19 (andbra)

Tillgänglig från: 2018-05-18 Skapad: 2018-05-18 Senast uppdaterad: 2018-08-09Bibliografiskt granskad
6. The root of a comet tail: Rosetta ion observations at comet 67P/Churyumov–Gerasimenko
Öppna denna publikation i ny flik eller fönster >>The root of a comet tail: Rosetta ion observations at comet 67P/Churyumov–Gerasimenko
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2018 (Engelska)Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 616, artikel-id A21Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Context.The first 1000 km of the ion tail of comet 67P/Churyumov–Gerasimenko were explored by the EuropeanRosettaspacecraft,2.7 au away from the Sun.Aims.We characterised the dynamics of both the solar wind and the cometary ions on the night-side of the comet’s atmosphere.Methods.We analysed in situ ion and magnetic field measurements and compared the data to a semi-analytical model.Results.The cometary ions are observed flowing close to radially away from the nucleus during the entire excursion. The solar windis deflected by its interaction with the new-born cometary ions. Two concentric regions appear, an inner region dominated by theexpanding cometary ions and an outer region dominated by the solar wind particles.Conclusions.The single night-side excursion operated byRosettarevealed that the near radial flow of the cometary ions can beexplained by the combined action of three different electric field components, resulting from the ion motion, the electron pressuregradients, and the magnetic field draping. The observed solar wind deflection is governed mostly by the motional electric field−uion×B.

Ort, förlag, år, upplaga, sidor
EDP Sciences, 2018
Nationell ämneskategori
Fusion, plasma och rymdfysik Rymd- och flygteknik
Forskningsämne
Atmosfärsvetenskap
Identifikatorer
urn:nbn:se:ltu:diva-68780 (URN)10.1051/0004-6361/201832842 (DOI)000440884600004 ()2-s2.0-85051990837 (Scopus ID)
Anmärkning

Validerad;2018;Nivå 2;2018-08-17 (andbra)

Tillgänglig från: 2018-05-18 Skapad: 2018-05-18 Senast uppdaterad: 2018-09-03Bibliografiskt granskad
7. The birth and growth of a solar wind cavity around a comet: Rosetta observations
Öppna denna publikation i ny flik eller fönster >>The birth and growth of a solar wind cavity around a comet: Rosetta observations
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2017 (Engelska)Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 469, nr Suppl. 2, s. S369-S403Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

The Rosetta mission provided detailed observations of the growth of a cavity in the solar wind around comet 67P/Churyumov–Gerasimenko. As the comet approached the Sun, the plasma of cometary origin grew enough in density and size to present an obstacle to the solar wind. Our results demonstrate how the initial slight perturbations of the solar wind prefigure the formation of a solar wind cavity, with a particular interest placed on the discontinuity (solar wind cavity boundary) passing over the spacecraft. The slowing down and heating of the solar wind can be followed and understood in terms of single particle motion. We propose a simple geometric illustration that accounts for the observations, and shows how a cometary magnetosphere is seeded from the gradual steepening of an initially slight solar wind perturbation. A perspective is given concerning the difference between the diamagnetic cavity and the solar wind cavity.

Ort, förlag, år, upplaga, sidor
Oxford University Press, 2017
Nyckelord
acceleration of particles, plasmas, methods: data analysis, techniques: imaging spectroscopy, comets: individual: 67P/Churyumov–Gerasimenko
Nationell ämneskategori
Fusion, plasma och rymdfysik Rymd- och flygteknik
Forskningsämne
Atmosfärsvetenskap
Identifikatorer
urn:nbn:se:ltu:diva-68779 (URN)10.1093/mnras/stx1871 (DOI)000443940500039 ()2-s2.0-85045874053 (Scopus ID)
Anmärkning

Validerad;2018;Nivå 2;2018-12-07 (marisr)

Tillgänglig från: 2018-05-18 Skapad: 2018-05-18 Senast uppdaterad: 2018-12-07Bibliografiskt granskad

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