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The Martian atmospheric ion escape rate dependence on solar wind and solar EUV conditions: 1. Seven years of Mars Express observations
Swedish Institute of Space Physics, Kiruna.ORCID iD: 0000-0003-0458-4050
Swedish Institute of Space Physics, Kiruna.ORCID iD: 0000-0002-7056-3517
Swedish Institute of Space Physics, Kiruna.ORCID iD: 0000-0002-7787-2160
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2015 (English)In: Journal of Geophysical Research - Planets, ISSN 2169-9097, E-ISSN 2169-9100, Vol. 120, no 7, p. 1298-1309Article in journal (Refereed) Published
Abstract [en]

More than 7 years of ion flux measurements in the energy range 10 eV–15 keV have allowed the ASPERA-3/IMA (Analyzer of Space Plasmas and Energetic Ions/Ion Mass Analyzer) instrument on Mars Express to collect a large database of ion measurements in the Mars environment, over a wide range of upstream solar wind (density and velocity) and radiation (solar EUV intensity) conditions. We investigate the influence of these parameters on the Martian atmospheric ion escape rate by integrating IMA heavy ionflux measurements taken in the Martian tail at similar (binned) solar wind density (nsw), velocity (vsw), and solar EUV intensity (IEUV) conditions. For the same solar wind velocity and EUV intensity ranges (vsw and IEUV constrained), we find a statistically significant decrease of up to a factor of 3 in the atmospheric ion escape rate with increased average solar wind density (5.6 × 1024 s−1 to 1.9 × 1024 s−1 for 0.4 cm−3 and 1.4 cm−3, respectively). For low solar wind density (0.1–0.5 cm−3) and low EUV intensity, the escape rate increaseswith increasing solar wind velocity from 2.4 × 1024 s−1 to 5.6 × 1024 s−1. During high solar EUV intensities the escape fluxes are highly variable, leading to large uncertainties in the estimated escape rates; however, a statistically significant increase in the escape rate is found between low/high EUV for similar solar wind conditions. Empirical-analytical models for atmospheric escape are developed by fitting calculated escape rates to all sufficiently sampled upstream conditions.

Place, publisher, year, edition, pages
2015. Vol. 120, no 7, p. 1298-1309
National Category
Fusion, Plasma and Space Physics
Identifiers
URN: urn:nbn:se:umu:diva-141934DOI: 10.1002/2015JE004816ISI: 000359903800005OAI: oai:DiVA.org:umu-141934DiVA, id: diva2:1157326
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2022-03-08
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, YoshifumiNilsson, HansHolmström, Mats

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