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Solar wind- and EUV-dependent models for the shapes of the Martian plasma boundaries based on Mars Express measurements
Swedish Institute of Space Physics, Kiruna.ORCID iD: 0000-0003-0458-4050
Swedish Institute of Space Physics, Kiruna.
Swedish Institute of Space Physics, Kiruna.ORCID iD: 0000-0002-7056-3517
Swedish Institute of Space Physics, Kiruna.ORCID iD: 0000-0001-5494-5374
2017 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, no 7, p. 7279-7290Article in journal (Refereed) Published
Abstract [en]

The long operational life (2003-present) of Mars Express (MEX) has allowed the spacecraft tomake plasma measurements in the Martian environment over a wide range of upstream conditions. Wehave analyzed ∼7000 MEX orbits, covering three orders of magnitude in solar wind dynamic pressure, withdata from the on board Analyzer of Space Plasmas and Energetic Particles (ASPERA-3) package, mappingthe locations where MEX crosses the main plasma boundaries, induced magnetosphere boundary (IMB), ionosphere boundary (IB), and bow shock (BS). A coincidence scheme was employed, where data fromthe Ion Mass Analyzer (IMA) and the Electron Spectrometer (ELS) had to agree for a positive boundaryidentification, which resulted in crossings from 1083 orbit segments that were used to create dynamictwo-parameter (solar wind density, nsw, and velocity vsw) dependent global dynamic models for the IMB, IB,and BS. The modeled response is found to be individual to each boundary. The IMB scales mainly dependenton solar wind dynamic pressure and EUV intensity. The BS location closely follows the location of the IMB atthe subsolar point, though under extremely low nsw and vsw the BS assumes a more oblique shape. The IBclosely follows the IMB on the dayside and changes its nightside morphology with different trends for nswand vsw. We also investigate the influence of extreme ultraviolet (EUV) radiation on the IMB and BS, findingthat increased EUV intensity expands both boundaries.

Place, publisher, year, edition, pages
2017. Vol. 122, no 7, p. 7279-7290
National Category
Fusion, Plasma and Space Physics
Identifiers
URN: urn:nbn:se:umu:diva-141929DOI: 10.1002/2017JA024098ISI: 000407627100025OAI: oai:DiVA.org:umu-141929DiVA, id: diva2:1157316
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2018-06-09
In thesis
1. Ion escape from Mars: measurements in the present to understand the past
Open this publication in new window or tab >>Ion escape from Mars: measurements in the present to understand the past
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Present-day Mars is a cold and dry planet with a thin CO2-dominated atmosphere comprising only a few ­­­mbar pressure at low altitudes. However, the Martian surface is marked with valley networks, hydrated mineral clays, carbonates and the remains of deltas and meandering rivers, i.e. traces of an active hydrological cycle present early in the planet's geological history. A strong greenhouse effect, and thus a thicker atmosphere, would have been required to sustain a sufficiently warm environment, particularly under the weaker luminosity of the early Sun. The fate of this early atmosphere is currently unknown.

While several mechanisms can remove atmospheric mass over time, a prominent hypothesis suggests that the lack of an intrinsic Earth-like global magnetic dipole has allowed the solar wind to erode the early Martian atmosphere by imparting energy to the planet's ionosphere which subsequently flows out as ion escape, over time depleting the greenhouse gasses and collapsing the ancient hydrological cycle. Previous studies have found insignificant ion escape rates under present-day conditions, however, the young Sun emitted significantly stronger solar wind and photoionizing radiation flux compared to the present. The geological record establishes the time of collapse of the hydrological cycle, estimated to have occurred in the mid-late Hesperian period (~3.3 billion years ago) at latest. To constrain the amount of atmosphere lost through ion escape since, we use the extensive database of ion flux measurements from the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) particles package on the Mars Express orbiter (2004-present) to quantify the ion escape rate dependence on upstream solar wind and solar radiation conditions.

The Martian ion escape rate is shown to be insensitive to solar wind parameters with a weak inverse dependence on solar wind dynamic pressure, and linearly dependent on solar ionizing photon flux, indicating efficient screening of the bulk ionosphere by the induced magnetic fields. The impact of an extreme coronal mass ejection is studied and found to have no significant effect on the ion escape rate. Instead, intense solar wind is shown to only increase the escaping energy flux, i.e. total power of escaping ions, without increasing the rate by accelerating already escaping ions. The orientation of the strongest magnetized crustal fields are shown to modulate the ion escape rate, though to have no significant time-averaged effect. We also study the influence of solar wind and solar radiation on the major Martian plasma boundaries and discuss factors that might limit the ion escape rate, including solar wind-ion escape coupling, which is found to be ≲1% and decreasing with increased solar wind dynamic pressure. The significant escape rate dependencies found are extrapolated back in time, considering the evolution of solar wind and ionizing radiation, and shown to account for only 4.8 ± 1.1 mbar equivalent surface pressure loss since the time of collapse of the Martian hydrosphere in the Hesperian, with ~6 mbar as an upper estimate. Extended to the late Noachian period (3.9 billion years ago), the found dependencies can only account for ≲10 mbar removed through ion escape, an insignificant amount compared to the ≳1 bar surface pressure required to sustain a warm climate on early Mars.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2017. p. 66
Series
IRF Scientific Report, ISSN 0284-1703 ; 309
Keywords
Mars, escape, solar wind, evolution, CME, coupling, plasma, atmosphere
National Category
Fusion, Plasma and Space Physics
Research subject
Space Physics
Identifiers
urn:nbn:se:umu:diva-141892 (URN)978-91-982951-3-9 (ISBN)978-91-7601-806-4 (ISBN)
Public defence
2017-12-08, Aulan, Rymdcampus 1, Kiruna, 09:00 (English)
Opponent
Supervisors
Funder
Swedish National Space Board, 172/12
Available from: 2017-11-17 Created: 2017-11-15 Last updated: 2018-06-09Bibliographically approved

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Ramstad, RobinFutaana, YoshifumiHolmström, Mats

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