Swedish Institute of Space Physics

Uppsala, Sweden

Swedish Institute of Space Physics

Uppsala, Sweden

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Fu H.S.,Swedish Institute of Space Physics | Fu H.S.,Beihang University | Khotyaintsev Y.V.,Swedish Institute of Space Physics | Vaivads A.,Swedish Institute of Space Physics | And 2 more authors.
Nature Physics | Year: 2013

The mechanism that produces energetic electrons during magnetic reconnection is poorly understood. This is a fundamental process responsible for stellar flares, substorms, and disruptions in fusion experiments. Observations in the solar chromosphere an. The Earth's magnetosphere indicate significant electron acceleration during reconnection, whereas in the solar wind, energetic electrons are absent. Here we show that energetic electron acceleration is caused by unsteady reconnection. I. The Earth's magnetosphere an. The solar chromosphere, reconnection is unsteady, so energetic electrons are produced; in the solar wind, reconnection is steady, so energetic electrons are absent. The acceleration mechanism is quasi-adiabatic: betatron and Fermi acceleration in outflow jets are two processes contributing to electron energization during unsteady reconnection. The localized betatron acceleration in the outflow is responsible for at least half of the energy gain fo. The peak observed fluxes. © 2013 Macmillan Publishers Limited. All rights reserved.


News Article | September 19, 2016
Site: www.rdmag.com

Graphene is the stuff of the future. For years, researchers and technologists have been predicting the utility of the one-atom-thick sheets of pure carbon in everything from advanced touch screens and semiconductors to long-lasting batteries and next-generation solar cells. But graphene's unique intrinsic properties – supreme electrical and thermal conductivities and remarkable electron mobility, to name just a few – can only be fully realized if it is grown free from defects that disrupt the honeycomb pattern of the bound carbon atoms. A team led by Materials Scientist Anirudha Sumant with the U.S. Department of Energy's (DOE) Argonne National Laboratory's Center for Nanoscale Materials (CNM) and Materials Science Division, along with collaborators at the University of California-Riverside, has developed a method to grow graphene that contains relatively few impurities and costs less to make, in a shorter time and at lower temperatures compared to the processes widely used to make graphene today. Theoretical work led by Argonne nanoscientist Subramanian Sankaranarayanan at the CNM helped researchers understand the molecular-level processes underlying the graphene growth. "I'd been dealing with all these different techniques of growing graphene, and you never see such a uniform, smooth surface." The new technology taps ultrananocrystalline diamond (UNCD), a synthetic type of diamond that Argonne researchers have pioneered through years of research. UNCD serves as a physical substrate, or surface on which the graphene grows, and the source for the carbon atoms that make up a rapidly produced graphene sheet. "When I first looked at the [scanning electron micrograph] and saw this nice uniform, very complete layer, it was amazing," said Diana Berman, the first author of the study and former postdoctoral research associate who worked with Sumant and is now an Assistant Professor at the University of North Texas. "I'd been dealing with all these different techniques of growing graphene, and you never see such a uniform, smooth surface." Current graphene fabrication protocols introduce impurities during the etching process itself, which involves adding acid and extra polymers, and when they are transferred to a different substrate for use in electronics. "The impurities introduced during this etching and the transferring step negatively affect the electronic properties of the graphene," Sumant said. "So you do not get the intrinsic properties of the graphene when you actually do this transfer." The team found that the single-layer, single-domain graphene can be grown over micron-size holes laterally, making them completely free-standing (that is, detached from the underlying substrate). This makes it possible to exploit the intrinsic properties of graphene by fabricating devices directly over free-standing graphene. The new process is also much more cost-effective than conventional methods based on using silicon carbide as a substrate. Sumant says that the 3- to 4-inch silicon carbide wafers used in these types of growth methods cost about $1,200, while UNCD films on silicon wafers cost less than $500 to make. The diamond method also takes less than a minute to grow a sheet of graphene, where the conventional method takes on the order of hours. The high quality of graphene was confirmed by the UC Riverside co-authors Zhong Yan and Alexander Balandin by fabricating top-gate field-effect transistors from this material and measuring its electron mobility and charge carrier concentration. "It is well known that certain metals, such as nickel and iron, dissolve diamond at elevated temperatures, and the same process has been used for many years to polish diamond," said Sumant. He and his team used this property to employ nickel in converting the top layer of diamond into amorphous carbon, but it was not clear how these freed carbon atoms converted instantly into high-quality graphene. After Sumant's and Berman's initial breakthrough of growing graphene directly on UNCD, Sankaranarayanan and his postdocs Badri Narayanan and Sanket Deshmukh, computational material scientists at the CNM used resources at the Argonne Leadership Computing Facility (ALCF) to help the team better understand the mechanism of the growth process underlying this interesting phenomenon using reactive molecular dynamic simulations. Computer simulations developed by Narayanan, Deshmukh and Sankaranarayanan showed that certain crystallographic orientation of nickel-111 highly favor nucleation, and subsequent rapid growth of graphene; this was then confirmed experimentally. These large-scale simulations also showed how graphene forms. The nickel atoms diffuse into the diamond and destroy its crystalline order, while carbon atoms from this amorphous solid move to the nickel surface and rapidly form honeycomb-like structures, resulting in mostly defect-free graphene. The nickel then percolated through the fine crystalline grains of the UNCD, sinking out of the way and removing the need for acid to dissolve away excess metal atoms from the top surface. "It is like meeting a good Samaritan at an unknown place who helps you, does his job and leaves quietly without a trace," said Sumant. "The proven predictive power of our simulations places us in a position of advantage to enable rapid discovery of new catalytic alloys that mediate growth of high-quality graphene on dielectrics and move away on their own when the growth is completed," added Narayanan. In addition to the utility in making minimally defective, application-ready graphene for things like low-frequency vibration sensors, radio frequency transistors and better electrodes for water purification, Berman and Sumant say that the Argonne team has already secured three patents arising from their new graphene growth method. The researchers have already struck a collaboration with Swedish Institute of Space Physics involving the European Space Agency for their Jupiter Icy Moons Explorer (JUICE) program to develop graphene-coated probes that may help exploratory vehicles sense the properties of plasma surrounding the moons of Jupiter. Closer to home, the team has also crafted diamond and graphene needles for researchers at North Carolina University to use in biosensing applications. The Argonne researchers are now fine-tuning the process – tweaking the temperature used to catalyze the reaction and adjusting the thickness of the diamond substrate and the composition of the metal film that facilitates the graphene growth – to both optimize the reaction and to better study the physics at the graphene-diamond interface. "We're trying to tune this more carefully to have a better understanding of which conditions lead to what quality of graphene we're seeing," Berman said. Other Argonne authors involved in the study were Alexander Zinovev and Daniel Rosenmann. The paper, "Metal-induced rapid transformation of diamond into single and multilayer graphene on wafer scale," is published inNature Communications.


News Article | September 19, 2016
Site: www.cemag.us

Graphene is the stuff of the future. For years, researchers and technologists have been predicting the utility of the one-atom-thick sheets of pure carbon in everything from advanced touch screens and semiconductors to long-lasting batteries and next-generation solar cells. But graphene’s unique intrinsic properties — supreme electrical and thermal conductivities and remarkable electron mobility, to name just a few — can only be fully realized if it is grown free from defects that disrupt the honeycomb pattern of the bound carbon atoms. A team led by Materials Scientist Anirudha Sumant with the U.S. Department of Energy’s (DOE) Argonne National Laboratory’s Center for Nanoscale Materials (CNM) and Materials Science Division, along with collaborators at the University of California-Riverside, has developed a method to grow graphene that contains relatively few impurities and costs less to make, in a shorter time and at lower temperatures compared to the processes widely used to make graphene today. Theoretical work led by Argonne nanoscientist Subramanian Sankaranarayanan at the CNM helped researchers understand the molecular-level processes underlying the graphene growth. The new technology taps ultrananocrystalline diamond (UNCD), a synthetic type of diamond that Argonne researchers have pioneered through years of research. UNCD serves as a physical substrate, or surface on which the graphene grows, and the source for the carbon atoms that make up a rapidly produced graphene sheet. “When I first looked at the [scanning electron micrograph] and saw this nice uniform, very complete layer, it was amazing,” says Diana Berman, the first author of the study and former postdoctoral research associate who worked with Sumant and is now an Assistant Professor at the University of North Texas. “I’d been dealing with all these different techniques of growing graphene, and you never see such a uniform, smooth surface.” Current graphene fabrication protocols introduce impurities during the etching process itself, which involves adding acid and extra polymers, and when they are transferred to a different substrate for use in electronics. “The impurities introduced during this etching and the transferring step negatively affect the electronic properties of the graphene,” Sumant says. “So you do not get the intrinsic properties of the graphene when you actually do this transfer.” The team found that the single-layer, single-domain graphene can be grown over micron-size holes laterally, making them completely free-standing (that is, detached from the underlying substrate). This makes it possible to exploit the intrinsic properties of graphene by fabricating devices directly over free-standing graphene. The new process is also much more cost-effective than conventional methods based on using silicon carbide as a substrate. Sumant says that the 3- to 4-inch silicon carbide wafers used in these types of growth methods cost about $1,200, while UNCD films on silicon wafers cost less than $500 to make. The diamond method also takes less than a minute to grow a sheet of graphene, where the conventional method takes on the order of hours. The high quality of graphene was confirmed by the UC Riverside co-authors Zhong Yan and Alexander Balandin by fabricating top-gate field-effect transistors from this material and measuring its electron mobility and charge carrier concentration. “It is well known that certain metals, such as nickel and iron, dissolve diamond at elevated temperatures, and the same process has been used for many years to polish diamond,” says Sumant. He and his team used this property to employ nickel in converting the top layer of diamond into amorphous carbon, but it was not clear how these freed carbon atoms converted instantly into high-quality graphene. After Sumant’s and Berman’s initial breakthrough of growing graphene directly on UNCD, Sankaranarayanan and his postdocs Badri Narayanan and Sanket Deshmukh, computational material scientists at the CNM used resources at the Argonne Leadership Computing Facility (ALCF) to help the team better understand the mechanism of the growth process underlying this interesting phenomenon using reactive molecular dynamic simulations. Computer simulations developed by Narayanan, Deshmukh and Sankaranarayanan showed that certain crystallographic orientation of nickel-111 highly favor nucleation, and subsequent rapid growth of graphene; this was then confirmed experimentally. These large-scale simulations also showed how graphene forms. The nickel atoms diffuse into the diamond and destroy its crystalline order, while carbon atoms from this amorphous solid move to the nickel surface and rapidly form honeycomb-like structures, resulting in mostly defect-free graphene. The nickel then percolated through the fine crystalline grains of the UNCD, sinking out of the way and removing the need for acid to dissolve away excess metal atoms from the top surface. “It is like meeting a good Samaritan at an unknown place who helps you, does his job and leaves quietly without a trace,” says Sumant. “The proven predictive power of our simulations places us in a position of advantage to enable rapid discovery of new catalytic alloys that mediate growth of high-quality graphene on dielectrics and move away on their own when the growth is completed,” adds Narayanan. In addition to the utility in making minimally defective, application-ready graphene for things like low-frequency vibration sensors, radio frequency transistors and better electrodes for water purification, Berman and Sumant say that the Argonne team has already secured three patents arising from their new graphene growth method. The researchers have already struck a collaboration with Swedish Institute of Space Physics involving the European Space Agency for their Jupiter Icy Moons Explorer (JUICE) program to develop graphene-coated probes that may help exploratory vehicles sense the properties of plasma surrounding the moons of Jupiter. Closer to home, the team has also crafted diamond and graphene needles for researchers at North Carolina University to use in biosensing applications. The Argonne researchers are now fine-tuning the process — tweaking the temperature used to catalyze the reaction and adjusting the thickness of the diamond substrate and the composition of the metal film that facilitates the graphene growth — to both optimize the reaction and to better study the physics at the graphene-diamond interface. “We’re trying to tune this more carefully to have a better understanding of which conditions lead to what quality of graphene we’re seeing,” Berman says. Other Argonne authors involved in the study were Alexander Zinovev and Daniel Rosenmann. The paper, “Metal-induced rapid transformation of diamond into single and multilayer graphene on wafer scale,” is published in Nature Communications. The study used resources of the CNM and the ALCF as well as the National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory, all DOE Office of Science User Facilities. Additional support was provided by the U.S. Department of Energy’s Office of Science.


Tamburini F.,University of Padua | Thide B.,Swedish Institute of Space Physics | Molina-Terriza G.,Macquarie University | Anzolin G.,ICFO - Institute of Photonic Sciences
Nature Physics | Year: 2011

Kerr black holes are among the most intriguing predictions of Einstein's general relativity theory1,2. These rotating massive astrophysical objects drag and intermix their surrounding space and time, deflecting and phase-modifying light emitted near them. We have found that this leads to a new relativistic effect that imprints orbital angular momentum on such light. Numerical experiments, based on the integration of the null geodesic equations of light from orbiting point-like sources in the Kerr black hole equatorial plane to an asymptotic observer3, indeed identify the phase change and wavefront warping and predict the associated light-beam orbital angular momentum spectra4. Setting up the best existing telescopes properly, it should be possible to detect and measure this twisted light, thus allowing a direct observational demonstration of the existence of rotating black holes. As non-rotating objects are more an exception than a rule in the Universe, our findings are of fundamental importance. © 2011 Macmillan Publishers Limited. All rights reserved.


News Article | November 30, 2015
Site: phys.org

Charles Lue holds a lunar globe showing the reflection of solar wind from magnetic fields of the lunar crust. The strongest reflection takes place in the areas marked in red on the lunar globe. Credit: Hans Huybrighs. Illustration: Charles Lue The lunar space environment is much more active than previously assumed. The solar wind is reflected from the surface and crustal magnetic fields of the moon which has effects on for instance lunar water levels. This according to a dissertation by Charles Lue at the Swedish Institute of Space Physics and Umeå University in Sweden. The Swedish space instrument SARA has measured a strong and varied interaction between the moon and solar wind. The solar wind is a continuous flow of plasma from the Sun which affects the planets in the Solar System and contributes to causing aurora on Earth. The lunar atmosphere, on the other hand, is too thin to show the same phenomenon and the moon also lacks a global magnetic field to regulate the solar wind. It has, therefore, long been believed that the moon passively absorbs solar wind without noticeably affecting its surroundings. Now, however, evidence shows that the surface of the moon, and also local magnetic fields of the lunar crust, reflect some of the solar wind. "This knowledge is of great importance to the lunar space environment which is affected both on the lunar dayside and nightside surfaces," says Charles Lue. The reflected solar wind ions move in spiralling tracks that can take them from the lunar dayside, where the solar wind strikes first, to the nightside of the moon. In local areas with strong magnetism, the solar wind flow is restricted on the surface at the same time as adjacent areas receive an increased flow. In the long term, this has effects on the surface of the moon and can, for instance, have an effect on the water levels in the lunar crust. "The effects can even be seen in the form of visible light – like bright swirls imprinted on the surface of the moon," says Charles Lue. The particle instrument SARA (Sub-keV Atoms Reflecting Analyzer) that was developed at the Swedish Institute of Space Physics travelled to the moon on board the Indian satellite Chandrayaan-1. SARA studied the solar wind interaction with the moon in 2009, and the observations made by the instrument have since been analysed by researchers, among them Charles Lue. "The observations help us map and understand the variations in the lunar space environment. They also give us clues about the physical processes involved and the long-term effects they have on the lunar surface," he explains. Explore further: How the Moon produces its own water


News Article | December 6, 2015
Site: www.techtimes.com

Waves of plasma from the sun known as solar wind have been found to affect the levels of water on the moon as well as other lunar features, according to new a research conducted by scientists in Sweden. Researcher Charles Lue and his colleagues at the Umea University and the Swedish Institute of Space Physics (IRF) detected a considerable amount of activity between solar wind and the moon using the space instrument known as the Sub-keV Atom Reflecting Analyzer (SARA). This observation contradicts earlier assumptions by scientists that the Earth's natural satellite only passively absorbs plasma waves from the sun without affecting its environment. "This knowledge is of great importance to the lunar space environment which is affected both on the lunar dayside and nightside surfaces," Lue said. The researchers observed that ions of solar wind reflected by the moon travel through a spiraling motion. This takes them from the moon's lunar dayside, where the solar wind initially hits, to its nightside. In lunar regions with high levels of magnetism, the flow of solar wind is mostly limited on the moon's surface while adjacent areas tend to receive a significant amount of the plasma flow. Lue pointed out that the impact of solar wind on the moon can be observed as visible light such as bright swirling markings on the lunar surface. It is believed that the occurrence affects the lunar surface in the long run as well as impacts the levels of water found in the moon's crust. "The observations help us map and understand the variations in the lunar space environment," Lue said. "They also give us clues about the physical processes involved and the long-term effects they have on the lunar surface." The IRF's SARA space instrument was transported to the moon aboard the Indian lunar spacecraft Chandrayaan-1. The research device has collected data on the interaction between the moon and solar wind in 2009, which scientists, including Lue, have since analyzed.


News Article | November 7, 2016
Site: phys.org

Earth's magnetic field is dominated by a single, strong source: the dynamo deep below the planet's surface. However, the same cannot be said for Mars. Rather than possessing a single source of magnetic field, Mars has many. The Red Planet has numerous pockets of strong magnetism locked up within its crust, remnants from its earliest days. Modern-day Mars may be known for its relative lack of magnetism but young Mars was likely a different world; it was probably warmer and wetter, with a denser atmosphere and a hotter core. Scientists believe the young planet also had a sizeable magnetic field, driven by the circulating motion of molten material within its core (known as a planetary dynamo). This global field switched off long ago – likely as the core cooled and solidified, freezing the dynamo in place – but the planet still boasts anomalous patches of strong remnant magnetism spread across its surface, known as 'crustal fields'. Parts of Mars' crust and rock remain magnetised today due to a phenomenon known as 'ferro-magnetism', which lasts even when the external magnetic field is no longer present (as is the case with Mars). Mars' crust cooled to below a specific temperature – known as the Curie temperature – when the planet's core dynamo, and thus its magnetic field, was still active and present, causing residual magnetism to become permanently locked within ferrous (iron-containing) material in the crust. Similar crustal magnetic fields are also found on the Earth and the Moon. These fields can later be removed by reheating material to above the Curie temperature – via large impacts, for example – and then allowing it to cool again in the absence of a magnetic field. Magnetism is thought to have been wiped out from sizeable patches of the martian crust in this way, but large portions of the southern, and smaller parts of the northern, hemisphere of Mars remain magnetised to some degree, with pockets scattered planet-wide. These crustal fields are strong enough to drive features in Mars' upper atmosphere akin to the aurorae seen on Earth – such features have been seen by ESA's Mars Express). "They may be weak in terms of absolute strength – hundreds of nanotesla in the upper atmosphere on average, or between 0.1 and 1 per cent of the field strength produced by the Earth's dynamo at the equivalent altitude – but Mars' crustal fields are significantly stronger than those found on the Earth or the Moon," says Markus Fraenz of the Max Planck Institute for Solar System Research in Göttingen, Germany. "This indicates that Mars' dynamo field was once at least as strong as Earth's – but in order to produce such strong patches of remnant crustal magnetisation, it was probably stronger than our planet's has ever been." Unfortunately no lander or rover has yet reached these sites of strong magnetisation, but comprehensive observations from long-lived orbiters such as NASA's Mars Global Surveyor and ESA's Mars Express have helped scientists to characterise Mars' magnetic environment. Mars Express has been in orbit around Mars since 2003, and has completed numerous studies using its MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) and ASPERA-3 (Analyzer of Space Plasmas and Energetic Atoms) instruments to explore the effect these crustal fields have on Mars' ionosphere. "Mars' crustal fields appear to strongly control the plasma in the planet's upper atmosphere," says David Andrews of the Swedish Institute of Space Physics in Uppsala. More specifically, they affect a layer of weakly ionised gas known as the ionosphere, which sits sandwiched between the bulk of Mars' neutral atmosphere and the intense radiation of outer space (including the solar wind, a stream of charged particles – protons and electrons – emanating from the Sun). Mars' ionosphere is quite similar to Earth's in many respects, such as the typical densities, altitudes, and so on. "Earth's ionosphere is a bit more complex in terms of its structure, and has a larger number of distinct layers," says Andrews. "This is partly due to Earth's atmosphere being a mix of nitrogen and oxygen, unlike the CO2-dominated martian atmosphere." Mars' crustal fields affect the motion and dynamics of its ionospheric plasma, influencing how it circulates, accumulates, and escapes to space. For example, plasma soars to far higher altitudes than expected in regions with vertically-oriented crustal fields, and areas with stronger crustal fields are topped by denser and more extensive layers of ionosphere than weaker or absent fields. Mars' ionosphere sits at the boundary between Mars' lower atmosphere and the solar wind, which floods out into space from the Sun. The solar wind also drags the solar magnetic field out into the Solar System as it travels, creating the interplanetary magnetic field (IMF). When dragged into Mars' vicinity, IMF field lines can connect with the field lines emanating from some regions of Mars' crust (a process known as 'magnetic reconnection'). This process allows plasma to race upwards along the newly-created lines and escape to space, creating narrow cavities within Mars' ionosphere that are comparatively lacking in electrons. "The big question, however, is whether or not these crustal fields affect the rate at which Mars loses its atmosphere to space and if so, how," says Andrews. "It's likely that while plasma is reconfigured in regions where the field is strong, the long-term averages of atmospheric escape are not massively different – but we're unsure." The behaviour and properties of the ionosphere differ between the region nearest the Sun (the 'dayside', between Mars and the Sun) and that stretching away from it (the 'night side', tailing away from Mars towards the outer Solar System). Mars Express data have shown the dayside ionosphere to be surprisingly complex and variable, with electron densities and structured layers of plasma that change abruptly and inconsistently. The satellite has also flagged up how much there is to understand about the night side, and why some of its properties differ considerably from the dayside. The process of plasma escape via magnetic reconnection, for example, is especially efficient at the day-night boundary (the regions surrounding this boundary, or terminator, are sometimes named 'morning' and 'evening' or 'dawn' and 'dusk'). Similarly, the ionosphere on the dayside is both denser and stretches to higher altitudes over crustal anomalies than on the night side. Plasma also appears to flow towards Mars on the dayside, and away at the day-night boundary. In general, the number and density of electrons in the ionosphere increases with field strength during the day and at the boundary between day and night – but on the night side, the opposite is true. Mars' night side ionosphere is patchy; it is replenished by some of the plasma from the dayside ionosphere, and by precipitating electrons from the solar wind and magnetosphere (the region of space over which Mars' small intrinsic magnetic field dominates). "This all reinforces the idea that Mars' plasma environment is strongly influenced by both the levels of incoming solar radiation, and the strength and distribution of the planet's crustal fields," says Eduard Dubinin of the Max Planck Institute for Solar System Research in Göttingen, Germany. "We need to understand much more about these interactions and about Mars' ionosphere in general to paint a detailed picture of Mars' longer-term evolution in terms of climate, habitability, loss of water and atmosphere, and more." As well as forming a better scientific understanding of Mars as a planet, knowing more about the martian ionosphere and crustal fields is vital for missions currently at Mars, and for those planned in the future (including crewed missions). For example, the ionosphere dictates how, when, and where Mars Express' radar equipment (MARSIS) can operate. The dayside ionosphere of Mars is denser and more reflective of radio waves. MARSIS can thus probe Mars' ionosphere on the dayside, as the plasma there reflects incoming radar pulses at the appropriate frequencies (~MHz). On the night side, however, MARSIS performs subsurface sounding. The instrument's radio waves reach through the comparatively sparse ionosphere and can make it far further before being reflected, reaching Mars' surface and up to about 10 km below. "MARSIS can exploit the varying properties of the ionosphere, making it a great instrument to probe both the ionosphere and subsurface of Mars," says Dmitri Titov, project scientist for ESA's Mars Express. The variability of the martian ionosphere could be an issue, however, for any communications on the surface of Mars. Landers and rovers on Mars communicate with Earth via an orbiter, which in turn uses high enough radio frequencies (GHz) that the ionosphere is not a huge obstacle. However, this may become a larger issue if and when humans set foot on the planet. "Shortwave radio communications (MHz) on the surface may be affected by variability of Mars' ionosphere, especially around stronger crustal fields, and our understanding here is still incomplete," adds Titov. "Understanding more about Mars' magnetic and plasma environment is key. Findings such as these from Mars Express are crucial to our continued exploration of the Solar System, whether with robots or human crews." Explore further: Are mystery Mars plumes caused by space weather? More information: E. Dubinin et al. Martian ionosphere observed by Mars Express. 1. Influence of the crustal magnetic fields, Planetary and Space Science (2016). DOI: 10.1016/j.pss.2016.02.004 F. Němec et al. Empirical model of the Martian dayside ionosphere: Effects of crustal magnetic fields and solar ionizing flux at higher altitudes, Journal of Geophysical Research: Space Physics (2016). DOI: 10.1002/2015JA022060 Paul Withers et al. The morphology of the topside ionosphere of Mars under different solar wind conditions: Results of a multi-instrument observing campaign by Mars Express in 2010, Planetary and Space Science (2016). DOI: 10.1016/j.pss.2015.10.013 D. J. Andrews et al. Control of the topside Martian ionosphere by crustal magnetic fields, Journal of Geophysical Research: Space Physics (2015). DOI: 10.1002/2014JA020703


Lundin R.,Swedish Institute of Space Physics
Space Science Reviews | Year: 2011

Solar wind forcing of Mars and Venus results in outflow and escape of ionospheric ions. Observations show that the replenishment of ionospheric ions starts in the dayside at low altitudes (≈300-800 km), ions moving at a low velocity (5-10 km/s) in the direction of the external/ magnetosheath flow. At high altitudes, in the inner magnetosheath and in the central tail, ions may be accelerated up to keV energies. However, the dominating energization and outflow process, applicable for the inner magnetosphere of Mars and Venus, leads to outflow at energies ≈5-20 eV. The aim of this overview is to analyze ion acceleration processes associated with the outflow and escape of ionospheric ions from Mars and Venus. Qualitatively, ion acceleration may be divided in two categories: (a) Modest ion acceleration, leading to bulk outflow and/or return flow (circulation). (b) Acceleration to well over escape velocity, up into the keV range. In the first category we find a processes denoted "planetary wind", the result of e.g. ambipolar diffusion, wave enhanced planetary wind, and mass-loaded ion pickup. In the second category we find ion pickup, current sheet acceleration, wave acceleration, and parallel electric fields, the latter above Martian crustal magnetic field regions. Both categories involve mass loading. Highly mass-loaded ion energization may lead to a low-velocity bulk flow-A consequence of energy and momentum conservation. It is therefore not self-evident what group, or what processes are connected with the low-energy outflow of ionospheric ions from Mars. Experimental and theoretical findings on ionospheric ion acceleration and outflow from Mars and Venus are discussed in this report. © 2011 Springer Science+Business Media B.V.


Andre M.,Swedish Institute of Space Physics | Cully C.M.,Swedish Institute of Space Physics
Geophysical Research Letters | Year: 2012

Ions with energies less than tens of eV originate from the Terrestrial ionosphere and from several planets and moons in the solar system. The low energy indicates the origin of the plasma but also severely complicates detection of the positive ions onboard sunlit spacecraft at higher altitudes, which often become positively charged to several tens of Volts. We discuss some methods to observe low-energy ions, including a recently developed technique based on the detection of the wake behind a charged spacecraft in a supersonic flow. Recent results from this technique show that low-energy ions typically dominate the density in large regions of the Terrestrial magnetosphere on the nightside and in the polar regions. These ions also often dominate in the dayside magnetosphere, and can change the dynamics of processes like magnetic reconnection. The loss of this low-energy plasma to the solar wind is one of the primary pathways for atmospheric escape from planets in our solar system. We combine several observations to estimate how common low-energy ions are in the Terrestrial magnetosphere and briefly compare with Mars, Venus and Titan. Copyright 2012 by the American Geophysical Union.


Wintoft P.,Swedish Institute of Space Physics
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2011

The sunspot number (SSN), 10.7cm radio flux (F 10.7), MgII index, and SOHO/SEM EUV flux have been studied, using wavelet analysis, in order to describe how the first three parameters are related to EUV on scales of days to years. The wavelet transform decomposes the time series into series which captures variability on different temporal scales. The three proxies show weak correlation on time scales of days, thus they are of limited use in space weather when the day-to-day variability is considered. However, the underlying modulation due to the solar rotation and the solar activity cycle is so strong that there is a big influence on the daily values. Both F 10.7 and MgII show a more persistent increase in correlation with scale than SSN and should be the preferred proxies. When a linear regression model is used for SEM/EUV the RMS error is about 26%lower, for the analysed period (1996-2010), for MgII compared to F 10.7. However, when only the long term is considered (scale -1.4yr) the RMS error is 20%larger when MgII is used compared to F 10.7. This is caused by an offset between MgII and SEM that appears around the cycle 23 maximum. This offset is not seen between F 10.7 and SEM. For space weather purposes, although none of the studied proxies works on a daily basis, the MgII index performs the best, but for the longer time scales F 10.7 is the most suitable. © 2011 Elsevier Ltd.

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