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Low-Energy Ion Escape from the Terrestrial Polar Regions
2009 (English)Doktorsavhandling, sammanläggning (Other academic)
Abstract [en]

The contemporary terrestrial atmosphere loses matter at a rate of around 100,000 tons per year. A major fraction of the net mass loss is constituted by ions, mainly H+ and O+, which escape from the Earth’s ionosphere in the polar regions. Previously, the outflow has only been measured at low altitudes, but to understand what fraction actually escapes and does not return, the measurements should be conducted far from the Earth. However, at large geocentric distances the outflowing ions are difficult to detect with conventional ion instruments on spacecraft, since the spacecraft electrostatic potential normally exceeds the equivalent energy of the ions. This also means that little is known about the ion outflow properties and distribution in space far from the Earth.

In this thesis, we present a new method to measure the outflowing low-energy ions in those regions where they previously have been invisible. The method is based on the detection by electric field instruments of the large wake created behind a spacecraft in a flowing, low-energy plasma. Since ions with low energy will create a larger wake, the method is more sensitive to light ions, and our measured outflow is essentially the proton outflow.

Applying this new method on data from the Cluster spacecraft, we have been able to make an extensive statistical study of ion outflows from 5 to 19 Earth radii in the magnetotail lobes. We show that cold proton outflows dominate in these large regions of the magnetosphere in both flux and density. Our outflow values of low-energy protons are close to those measured at low altitudes, which confirms that the ionospheric outflows continue far back in the tail and contribute significantly to the magnetospheric content. We also conclude that most of the ions are escaping and not returning, which improves previous estimates of the global outflow. The total loss of protons due to high-latitude escape is found to be on the order of 1026 protons/s.

Place, publisher, year, pages
Uppsala: Acta Universitatis Upsaliensis, 2009. 91 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 640
Keyword [en]
space physics, ion outflow, polar wind, auroral upflows, atmospheric escape, magnetotail lobes, spacecraft wake, electric field measurements
National Category
Fusion, Plasma and Space Physics
Research subject
Space and Plasma Physics
Identifiers
urn:nbn:se:uu:diva-100650 (URN)978-91-554-7512-3 (ISBN)oai:DiVA.org:uu-100650 (OAI)
Public defence
2009-05-20, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from2009-04-28 Created:2009-04-04 Last updated:2009-04-30Bibliographically approved
List of papers
1. Wake formation behind positively charged spacecraft in flowing tenuous plasmas
Open this publication in new window or tab >>Wake formation behind positively charged spacecraft in flowing tenuous plasmas
2006 (English)In: Physics of Plasmas, ISSN 1070-664X, Vol. 13, no 6, 062904-1-062904-10Artikel i tidskrift (Refereed) Published
Abstract [en]

Spacecraft in tenuous plasmas become positively charged because of photoelectron emission. If the plasma is supersonically drifting with respect to the spacecraft, a wake forms behind it. When the kinetic energy of the positive ions in the plasma is not sufficient to overcome the electrostatic barrier of the spacecraft potential, they scatter on the potential structure from the spacecraft rather than get absorbed or scattered by the spacecraft body. For tenuous plasmas with Debye lengths much exceeding the spacecraft size, the potential structure extends far from the spacecraft, and consequently in this case the wake is of transverse dimensions much larger than the spacecraft. This enhanced wake formation process is demonstrated by theoretical analysis and computer simulations. Comparison to observations from the Cluster satellites shows good agreement.

Melville, USA: American Institute of Physics, 2006
Keyword
spacecraft potential, spacecraft wake, Debye length, PIC simulation, double-probe instrument, electric field instrument, tenuous plasma
National Category
Fusion, Plasma and Space Physics Fusion, Plasma and Space Physics
Research subject
Space and Plasma Physics
Identifiers
urn:nbn:se:uu:diva-100644 (URN)10.1063/1.2199207 (DOI)
Available from2009-04-04 Created:2009-04-04 Last updated:2009-10-08Bibliographically approved
2. Double-Probe Measurements in Cold Tenuous Space Plasma Flows
Open this publication in new window or tab >>Double-Probe Measurements in Cold Tenuous Space Plasma Flows
2006 (English)In: IEEE Transactions on Plasma Science, ISSN 0093-3813, Vol. 34, no 5:2, 2071-2077Artikel i tidskrift (Refereed) Published
Abstract [en]

Cold flowing tenuous plasmas are common in the terrestrial magnetosphere, particularly in the polar cap and tail lobe regions, which are filled by the supersonic plasma flow known as the polar wind. Electric field measurements with double-probe instruments in these regions suffer mainly from two error sources: 1) an apparent sunward electric field due to photoemission asymmetries in the probe-boom system and 2) an enhanced negatively charged wake forming behind the spacecraft, which will affect the probe measurements. The authors investigate these effects experimentally by Fourier analysis of the spin signature from the double-probe instrument Electric Fields and Waves (EFW) on the Cluster spacecraft. They show that while the signature due to photoemission asymmetry is very close to sinusoidal, the wake effect is characterized by a spectrum of spin harmonics. The Fourier decomposition can therefore be used for identifying wake effects in the data. As a spin-off, the analysis has also given information on the cold flowing ion population.

Nuclear & Plasma Sciences Society, 2006
Keyword
Cluster spacecraft, electric field measurements, photoelectron emission, space plasma flows, spacecraft wakes
National Category
Fusion, Plasma and Space Physics
Research subject
Space and Plasma Physics
Identifiers
urn:nbn:se:uu:diva-100645 (URN)10.1109/TPS.2006.883375 (DOI)
Available from2009-04-04 Created:2009-04-04 Last updated:2009-10-08Bibliographically approved
3. Low-energy (order 10 eV) ion flow in the magnetotail lobes inferred from spacecraft wake observations
Open this publication in new window or tab >>Low-energy (order 10 eV) ion flow in the magnetotail lobes inferred from spacecraft wake observations
Show others...
2006 (English)In: Geophysical Research Letters, ISSN 0094-8276, Vol. 33, L06110-1-L06110-4Artikel i tidskrift (Refereed) Published
Abstract [en]

Cold ionospheric ions with eV energies are common inthe magnetosphere and can travel far out in the magnetotail.However, they are difficult to measure with conventional ionspectrometers mounted on spacecraft, since the potential of asunlit spacecraft often reaches several tens of volts. In thispaper we present two alternative methods of measuring thecold-ion flow with the Cluster spacecraft and apply them onone case in the magnetotail at 18 RE: 1. Ion spectrometer incombination with artificial spacecraft potential control;2. Deriving ion flow velocity (both perpendicular andparallel) from electric field instruments. The secondmethod takes advantage of the effect on the doubleprobeinstrument of the wake formed behind a spacecraftin a plasma flow. The results from the two methods showgood agreement and are also consistent with polar windmodels and previous measurements at lower altitudes,confirming the continuation of low-energy ion outflows.

American Geophysical Union, 2006
Keyword
low-energy ions, plasma flow, polar wind, spacecraft wake, Cluster satellites, measurement technique, ion outflow
National Category
Fusion, Plasma and Space Physics
Research subject
Space and Plasma Physics
Identifiers
urn:nbn:se:uu:diva-100646 (URN)10.1029/2005GL025179 (DOI)
Note
Corrected in Geophysical Research Letters, 33, 14102 (2006).Available from2009-04-04 Created:2009-04-04 Last updated:2009-10-08Bibliographically approved
4. Earth’s ionospheric outflow dominated by hidden cold plasma
Open this publication in new window or tab >>Earth’s ionospheric outflow dominated by hidden cold plasma
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2009 (English)In: Nature Geoscience, ISSN 1752-0894, Vol. 2, no 1, 24-27Artikel i tidskrift (Refereed) Published
Abstract [en]

The Earth constantly loses matter, mostly in the form of H+and O+ ions, through various outflow processes from the upper atmosphere and ionosphere. Most of these ions are cold (below 1 eV in thermal energy), but can still escape and travel farther out along the magnetic field lines into the magnetospheric tail lobes. The outflow has previously beenmeasured close to the Earth. To understand what fraction does not return but instead escapes, the measurements should be conducted at larger geocentric distances. However, at high altitudes the cold ions are normally invisible to spacecraft measurements, because the potential of a sunlit spacecraft exceeds the equivalent energy of the ions. Here we show that cold ions dominate in both flux and density in the distant magnetotail lobes, using a new measurement technique on the Cluster spacecraft. The total loss of cold hydrogen ions from the planet is inferred to be of the order of 1026 s−1, which is larger than the previously observed more energetic outflow. Quantification and insight of the loss processes of the Earth’s atmosphere and ionosphere are also important for understanding the evolution of atmospheres on other celestial bodies.

Nature Publishing Group, 2009
Keyword
Cluster spacecraft, ion outflow, low-energy ions, polar wind, lobal wind, electric field measurements, atmospheric loss, magnetotail lobe
National Category
Fusion, Plasma and Space Physics
Research subject
Space and Plasma Physics
Identifiers
urn:nbn:se:uu:diva-100647 (URN)10.1038/NGEO387 (DOI)000262638500015 (ISI)
Available from2009-04-04 Created:2009-04-04 Last updated:2010-12-14Bibliographically approved
5. Survey of cold ionospheric outflows in the magnetotail
Open this publication in new window or tab >>Survey of cold ionospheric outflows in the magnetotail
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2009 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 27, no 8, 3185-3201Artikel i tidskrift (Refereed) Published
Abstract [en]

Low-energy ions escape from the ionosphere and constitute a large part of the magnetospheric content, especially in the geomagnetic tail lobes. However, they are normally invisible to spacecraft measurements, since the potential of a sunlit spacecraft in a tenuous plasma in many cases exceeds the energy-per-charge of the ions, and little is therefore known about their outflow properties far from the Earth. Here we present an extensive statistical study of cold ion outflows (0-60 eV) in the geomagnetic tail at geocentric distances from 5 to 19 R-E using the Cluster spacecraft during the period from 2001 to 2005. Our results were obtained by a new method, relying on the detection of a wake behind the spacecraft. We show that the cold ions dominate in both flux and density in large regions of the magnetosphere. Most of the cold ions are found to escape from the Earth, which improves previous estimates of the global outflow. The local outflow in the magnetotail corresponds to a global outflow of the order of 10(26) ions s(-1). The size of the outflow depends on different solar and magnetic activity levels.

Keyword
Cluster spacecraft, ion outflow, low-energy ions, polar wind, lobal wind, electric field measurements, atmospheric loss, magnetotail lobe
National Category
Fusion, Plasma and Space Physics
Research subject
Space and Plasma Physics
Identifiers
urn:nbn:se:uu:diva-100649 (URN)10.5194/angeo-27-3185-2009 (DOI)000269440700021 (ISI)
Available from2009-04-04 Created:2009-04-04 Last updated:2014-01-21Bibliographically approved

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