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Solar Wind Proton Interactions with Lunar Magnetic Anomalies and Regolith
Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Solvindsprotoners växelverkan med månens magnetiska anomalier och yta (Swedish)
Abstract [en]

The lunar space environment is shaped by the interaction between the Moon and the solar wind. In the present thesis, we investigate two aspects of this interaction, namely the interaction between solar wind protons and lunar crustal magnetic anomalies, and the interaction between solar wind protons and lunar regolith. We use particle sensors that were carried onboard the Chandrayaan-1 lunar orbiter to analyze solar wind protons that reflect from the Moon, including protons that capture an electron from the lunar regolith and reflect as energetic neutral atoms of hydrogen. We also employ computer simulations and use a hybrid plasma solver to expand on the results from the satellite measurements.

The observations from Chandrayaan-1 reveal that the reflection of solar wind protons from magnetic anomalies is a common phenomenon on the Moon, occurring even at relatively small anomalies that have a lateral extent of less than 100 km. At the largest magnetic anomaly cluster (with a diameter of 1000 km), an average of ~10% of the incoming solar wind protons are reflected to space. Our computer simulations show that these reflected proton streams significantly modify the global lunar plasma environment. The reflected protons can enter the lunar wake and impact the lunar nightside surface. They can also reach far upstream of the Moon and disturb the solar wind flow. In the local environment at a 200 km-scale magnetic anomaly, our simulations show a heated and deflected plasma flow and the formation of regions with reduced or increased proton precipitation.

We also observe solar wind protons reflected from the lunar regolith. These proton fluxes are generally lower than those from the magnetic anomalies. We find that the proton reflection efficiency from the regolith varies between ~0.01% and ~1%, in correlation with changes in the solar wind speed. We link this to a velocity dependent charge-exchange process occurring when the particles leave the lunar regolith. Further, we investigate how the properties of the reflected neutral hydrogen atoms depend on the solar wind temperature. We develop a model to describe this dependence, and use this model to study the plasma precipitation on the Moon when it is in the terrestrial magnetosheath. We then use the results from these and other studies, to model solar wind reflection from the surface of the planet Mercury.

Abstract [sv]

Rymdmiljön runt månen formas av den växelverkan som sker mellan månen och solvinden. I den föreliggande avhandlingen undersöker vi två aspekter av denna växerverkan, nämligen växelverkan mellan solvindsprotoner och magnetiserade områden i månskorpan, och växelverkan mellan solvindsprotoner och månens ytdamm. Vi använder oss av partikelsensorer på månsatelliten Chandrayaan-1 för att analysera solvindsprotoner som reflekteras från månen, även de protoner som fångar upp en elektron från ytan och reflekteras som neutrala väteatomer. Vi använder oss också av datorsimuleringar för att bygga vidare på de uppmätta resultaten.

Observationerna från Chandrayaan-1 visar att reflektion av solvindsprotoner från magnetiserade områden är ett vanligt förekommande fenomen på månen, som inträffar även vid magnetiseringar som är utbredda över mindre än 100 km. Vid det största magnetiserade området på månen (1000 km i diameter), reflekteras i genomsnitt ~10% av de infallande solvindsprotonerna. Våra datorsimuleringar visar att dessa protonflöden har globala effekter på månens plasmamiljö. De reflekterade protonerna kan nå månens nattsida. De kan också nå långt uppströms om månen och störa solvindsflödet. I den lokala plasmamiljön vid ett magnetiserat område av storleken 200 km visar våra simuleringar ett förändrat solvindsflöde, där det skapas områden som delvis skyddas från solvinden, likväl som områden som utsätts för mer solvind.

Vi observerar även solvindsprotoner som reflekterats från ytdammet på månen. Dessa protonflöden är lägre än de från de magnetiska fälten. Reflektionen från ytan varierar mellan ~0.01% och 1% av solvindsflödet, i samband med förändringar i solvindshastigheten. Vi förklarar detta med att partiklarnas laddning bestäms av den hastighet de har när de lämnar måndammet. Vidare undersöker vi hur egenskaperna hos de reflekterade neutrala väteatomerna beror på solvindstemperaturen. Vi skapar en modell för att beskriva sambandet och använder sedan denna modell för att studera hur solvinden faller in mot månens yta när den befinner sig i jordens magnetoskikt, där jordens magnetfält orsakar en upphettning av solvindsflödet. Resultaten från dessa och andra studier använder vi sedan för att modellera solvindsreflektion från planeten Merkurius yta, för jämförelse med framtida observationer.

Place, publisher, year, edition, pages
Umeå: Umeå universitet , 2015. , 180 p.
Series
IRF Scientific Report, ISSN 0284-1703 ; 306
Keyword [en]
the Moon, solar wind, magnetic anomalies, regolith, space physics, plasma physics, particle-surface interactions, mini-magnetospheres, energetic neutral atoms
National Category
Fusion, Plasma and Space Physics
Research subject
Space Physics
Identifiers
URN: urn:nbn:se:umu:diva-111254ISBN: 978-91-982951-0-8 (print)OAI: oai:DiVA.org:umu-111254DiVA: diva2:868749
Public defence
2015-12-04, Aulan vid Institutet för rymdfysik, Rymdcampus 1, Kiruna, 14:23 (English)
Opponent
Supervisors
Funder
Swedish National Space Board, 97/11
Available from: 2015-11-13 Created: 2015-11-11 Last updated: 2016-02-03Bibliographically approved
List of papers
1. Strong influence of lunar crustal fields on the solar wind flow
Open this publication in new window or tab >>Strong influence of lunar crustal fields on the solar wind flow
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2011 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 38, L03202Article in journal (Refereed) Published
Abstract [en]

We discuss the influence of lunar magnetic anomalies on the solar wind and on the lunar surface, based on maps of solar wind proton fluxes deflected by the magnetic anomalies. The maps are produced using data from the Solar WInd Monitor (SWIM) onboard the Chandrayaan-1 spacecraft. We find a high deflection efficiency (average ∼10%, locally ∼50%) over the large-scale (>1000 km) regions of magnetic anomalies. Deflections are also detected over weak (<3 nT at 30 km altitude) and small-scale (<100 km) magnetic anomalies, which might be explained by charge separation and the resulting electric potential. Strong deflection from a wide area implies that the magnetic anomalies act as a magnetosphere-like obstacle, affecting the upstream solar wind. It also reduces the implantation rate of the solar wind protons to the lunar surface, which may affect space weathering near the magnetic anomalies.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-111251 (URN)10.1029/2010GL046215 (DOI)000287038300004 ()
Available from: 2015-11-11 Created: 2015-11-11 Last updated: 2017-12-01Bibliographically approved
2. Effects of protons reflected by lunar crustal magnetic fields on the global lunar plasma environment
Open this publication in new window or tab >>Effects of protons reflected by lunar crustal magnetic fields on the global lunar plasma environment
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2014 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 119, no 8, 6095-6105 p.Article in journal (Refereed) Published
Abstract [en]

Solar wind plasma interaction with lunar crustal magnetic fields is different than that of magnetized bodies like the Earth. Lunar crustal fields are, for typical solar wind conditions, not strong enough to form a (bow) shock upstream but rather deflect and perturb plasma and fields. Here we study the global effects of protons reflected from lunar crustal magnetic fields on the lunar plasma environment when the Moon is in the unperturbed solar wind. We employ a three-dimensional hybrid model of plasma and an observed map of reflected protons from lunar magnetic anomalies over the lunar farside. We observe that magnetic fields and plasma upstream over the lunar crustal fields compress to nearly 120% and 160% of the solar wind, respectively. We find that these disturbances convect downstream in the vicinity of the lunar wake, while their relative magnitudes decrease. In addition, solar wind protons are disturbed and heated at compression regions and their velocity distribution changes from Maxwellian to a non-Maxwellian. Finally, we show that these features persists, independent of the details of the ion reflection by the magnetic fields.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 2014
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:umu:diva-97267 (URN)10.1002/2014JA019900 (DOI)000344809600006 ()
Funder
Swedish National Space BoardSwedish National Infrastructure for Computing (SNIC)
Available from: 2014-12-12 Created: 2014-12-12 Last updated: 2017-12-05Bibliographically approved
3. Solar wind plasma interaction with Gerasimovich lunar magnetic anomaly
Open this publication in new window or tab >>Solar wind plasma interaction with Gerasimovich lunar magnetic anomaly
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2015 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, no 6, 4719-4735 p.Article in journal (Refereed) Published
Abstract [en]

We present the results of the first local hybrid simulations (particle ions and fluid electrons) for the solar wind plasma interaction with realistic lunar crustal fields. We use a three-dimensional hybrid model of plasma and an empirical model of the Gerasimovich magnetic anomaly based on Lunar Prospector observations. We examine the effects of low and high solar wind dynamic pressures on this interaction when the Gerasimovich magnetic anomaly is located at nearly 20 degrees solar zenith angle. We find that for low solar wind dynamic pressure, the crustal fields mostly deflect the solar wind plasma, form a plasma void at very close distances to the Moon (below 20km above the surface), and reflect nearly 5% of the solar wind in charged form. In contrast, during high solar wind dynamic pressure, the crustal fields are more compressed, the solar wind is less deflected, and the lunar surface is less shielded from impinging solar wind flux, but the solar wind ion reflection is more locally intensified (up to 25%) compared to low dynamic pressures. The difference is associated with an electrostatic potential that forms over the Gerasimovich magnetic anomaly as well as the effects of solar wind plasma on the crustal fields during low and high dynamic pressures. Finally, we show that an antimoonward Hall electric field is the dominant electric field for similar to 3km altitude and higher, and an ambipolar electric field has a noticeable contribution to the electric field at close distances (<3km) to the Moon.

Keyword
Interaction with lunar crustal fields, Hybrid modeling of plasma
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:umu:diva-107180 (URN)10.1002/2015JA021027 (DOI)000358199100048 ()
Available from: 2015-08-24 Created: 2015-08-19 Last updated: 2017-12-04Bibliographically approved
4. Chandrayaan-1 observations of backscattered solar wind protons from the lunar regolith: Dependence on the solar wind speed
Open this publication in new window or tab >>Chandrayaan-1 observations of backscattered solar wind protons from the lunar regolith: Dependence on the solar wind speed
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2014 (English)In: Journal of Geophysical Research - Planets, ISSN 2169-9097, E-ISSN 2169-9100, Vol. 119, no 5, 968-975 p.Article in journal (Refereed) Published
Abstract [en]

We study the backscattering of solar wind protons from the lunar regolith using the Solar Wind Monitor of the Sub-keV Atom Reflecting Analyzer on Chandrayaan-1. Our study focuses on the component of the backscattered particles that leaves the regolith with a positive charge. We find that the fraction of the incident solar wind protons that backscatter as protons, i.e., the proton-backscattering efficiency, has an exponential dependence on the solar wind speed that varies from similar to 0.01% to similar to 1% for solar wind speeds of 250km/s to 550km/s. We also study the speed distribution of the backscattered protons in the fast (similar to 550km/s) solar wind case and find both a peak speed at similar to 80% of the solar wind speed and a spread of similar to 85km/s. The observed flux variations and speed distribution of the backscattered protons can be explained by a speed-dependent charge state of the backscattered particles.

Keyword
Moon, proton, backscatter, regolith
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-91225 (URN)10.1002/2013JE004582 (DOI)000337693900004 ()
Available from: 2014-07-21 Created: 2014-07-21 Last updated: 2017-12-05Bibliographically approved
5. Scattering characteristics and imaging of energetic neutral atoms from the Moon in the terrestrial magnetosheath
Open this publication in new window or tab >>Scattering characteristics and imaging of energetic neutral atoms from the Moon in the terrestrial magnetosheath
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(English)Manuscript (preprint) (Other academic)
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-111247 (URN)
Funder
Swedish National Space Board, 97/11
Available from: 2015-11-11 Created: 2015-11-11 Last updated: 2015-11-12
6. Solar wind scattering from the surface of Mercury: Lessons from the Moon
Open this publication in new window or tab >>Solar wind scattering from the surface of Mercury: Lessons from the Moon
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2017 (English)In: Icarus, ISSN 0019-1035, Vol. 296, 39-48 p.Article in journal (Refereed) Published
Abstract [en]

We discuss the surface-scattering of solar wind protons at Mercury based on observed scattering characteristics from lunar regolith. The properties of the impinging plasma are expected to be different between different regions on Mercury, and between Mercury and the Moon. Here, we review the expected Hermean plasma conditions and lunar empirical scattering models. We present observed and modeled energy spectra for scattered protons and hydrogen energetic neutral atoms (ENAs) for three cases of very different plasma conditions at the Moon. Then, we simulate scattering from the Hermean surface by applying the empirical models to four different scenarios of plasma precipitation on Mercury. The results suggest that surface-scattering is a strong source of ENAs at Mercury (up to similar to 10(8) cm(-2) s(-1)), which can be very useful for remote-sensing of the plasma conditions at the surface. Protons scattered from the surface back into space are also expected with high fluxes up to similar to 10(7) cm(-2) s(-1), and may be important for wave generation and the filling in of the loss cone of mirroring and quasi-trapped populations. Scattered protons at the cusp region (of similar to 10(6) cm(-2) s(-1)) can potentially be detected by orbiters as outflowing protons within the loss cone.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-111248 (URN)10.1016/j.icarus.2017.05.019 (DOI)000406989200004 ()
Available from: 2015-11-11 Created: 2015-11-11 Last updated: 2017-09-29Bibliographically approved

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