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  • 1. Cael, B. B.
    et al.
    Heathcote, A. J.
    Seekell, David
    Umeå University, Arctic Research Centre at Umeå University. Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Climate Impacts Research Centre, Umeå University, Abisko, Sweden.
    The volume and mean depth of Earth's lakes2017In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, no 1, p. 209-218Article in journal (Refereed)
    Abstract [en]

    Global lake volume estimates are scarce, highly variable, and poorly documented. We developed a rigorous method for estimating global lake depth and volume based on the Hurst coefficient of Earth's surface, which provides a mechanistic connection between lake area and volume. Volume-area scaling based on the Hurst coefficient is accurate and consistent when applied to lake data sets spanning diverse regions. We applied these relationships to a global lake area census to estimate global lake volume and depth. The volume of Earth's lakes is 199,000km(3) (95% confidence interval 196,000-202,000km(3)). This volume is in the range of historical estimates (166,000-280,000km(3)), but the overall mean depth of 41.8m (95% CI 41.2-42.4m) is significantly lower than previous estimates (62-151m). These results highlight and constrain the relative scarcity of lake waters in the hydrosphere and have implications for the role of lakes in global biogeochemical cycles.

  • 2.
    Ekenbäck, Andreas
    et al.
    Institutet för rymdfysik (IRF).
    Holmström, Mats
    Barabash, Stas
    Gunell, Herbert
    Energetic neutral atom imaging of comets2008In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 35, no L05103Article in journal (Refereed)
    Abstract [en]

    We present the first computer simulated images of energetic neutral atoms (ENAs) produced by charge exchange between solar wind protons and cometary neutrals. Proton bulk flow and temperature from MHD simulations are used along with a model of cometary neutral densities. The emission of hydrogen ENAs from the comet is then calculated using cross sections for the charge exchange collisions. The ENA production rate is integrated along lines of sight to produce an image of the fluxes. We find detection of hydrogen ENAs feasible and most favorable at solar zenith angles between 80 and 130 degrees.

  • 3.
    Eliasson, Bengt
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Full-scale simulation study of the generation of topside ionospheric turbulence using a generalized Zakharov model2008In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 35, no L11104, p. 5-Article in journal (Refereed)
    Abstract [en]

    We present a full-scale simulation study of the generation of topside electrostatic turbulence observed near the F region peak during ground based ionospheric heating experiments. The study is performed using a generalized Zakharov model. The nonlinear tunneling of electromagnetic waves through the ionospheric layer is attributed to multiple-stage parametric decay and mode-conversion processes. At the bottomside of the F region, electrostatic turbulence excited by the parametric instability results in the conversion of the ordinary (O mode) wave into a large amplitude extraordinary (Z mode) wave propagating through the F peak. At the topside interaction region, the Z mode undergoes parametric decay process that results in the generation of the topside electrostatic turbulence and then conversion process yielding O waves that escape the plasma. This study may explain the observed topside ionospheric turbulence during ground based low duty cycle ionospheric heating experiments where the heater beam is directed far away from the Spitze region.

  • 4.
    Eliasson, Bengt
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Thidé, Bo
    Reply to comment by N. A. Gondarenko et al. on "Simulation study of the interaction between large-amplitude HF radio waves and the ionosphere"2007In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 34, p. L23105-Article in journal (Refereed)
  • 5.
    Englund, Göran
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Eriksson, Håkan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Nilsson, Mats B.
    The birth and death of lakes on young landscapes2013In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 40, no 7Article in journal (Refereed)
    Abstract [en]

    Ongoing land uplift caused by postglacial isostatic rebound creates strong landscape-age gradients alongside the Gulf of Bothnia, northern Scandinavia. Lakes are continuously generated on this dynamic landscape as the uplift isolates bays from sea inundation. However, concomitant with this process older lakes are lost as the basins are filled with sediments, creating a continuum of lake ages on the landscape. We studied the lake size and depth distributions and lake densities, along an age gradient covering 0-4500 years. Map data on the density, area, and elevation of lakes were combined with field-based measurements of maximum basin depth. We find that young lake populations are densely distributed and dominated by small and shallow lakes. Over time, small and shallow lakes are lost by complete sediment filling, resulting in lower lake density and a shift in size and depth distributions towards larger, deeper lakes. Since lake filling is a universal process, we propose that these findings can be generalized to other gradients in landscape age.

  • 6. Fatemi, S
    et al.
    Holmström, M
    Futaana, Y
    Barabash, S
    Lue, Charles
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    The lunar wake current systems2013In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 40, no 1, p. 17-21Article in journal (Refereed)
    Abstract [en]

    We present the lunar wake current systems when the Moon is assumed to be a non-conductive body, absorbing the solar wind plasma. We show that in the transition regions between the plasma void, the expanding rarefaction region, and the interplanetary plasma, there are three main currents flowing around these regions in the lunar wake. The generated currents induce magnetic fields within these regions and perturb the field lines there. We use a three-dimensional, self-consistent hybrid model of plasma (particle ions and fluid electrons) to show the flow of these three currents. First, we identify the different plasma regions, separated by the currents, and then we show how the currents depend on the interplanetary magnetic field direction. Finally, we discuss the current closures in the lunar wake.

  • 7. Fu, HS
    et al.
    Cao, JB
    Khotyaintsev, Yu V
    Sitnov, MI
    Runov, A
    Fu, SY
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Andre, M
    Retino, A
    Ma, YD
    Lu, HY
    Wei, XH
    Huang, SY
    Dipolarization fronts as a consequence of transient reconnection: in situ evidence2013In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 40, no 23, p. 6023-6027Article in journal (Refereed)
    Abstract [en]

    Dipolarization fronts (DFs) are frequently detected in the Earth's magnetotail from X-GSM=-30 R-E to X-GSM=-7 R-E. How these DFs are formed is still poorly understood. Three possible mechanisms have been suggested in previous simulations: (1) jet braking, (2) transient reconnection, and (3) spontaneous formation. Among these three mechanisms, the first has been verified by using spacecraft observation, while the second and third have not. In this study, we show Cluster observation of DFs inside reconnection diffusion region. This observation provides in situ evidence of the second mechanism: Transient reconnection can produce DFs. We suggest that the DFs detected in the near-Earth region (X-GSM>-10 R-E) are primarily attributed to jet braking, while the DFs detected in the mid- or far-tail region (X-GSM<-15 R-E) are primarily attributed to transient reconnection or spontaneous formation. In the jet-braking mechanism, the high-speed flow pushes the preexisting plasmas to produce the DF so that there is causality between high-speed flow and DF. In the transient-reconnection mechanism, there is no causality between high-speed flow and DF, because the frozen-in condition is violated.

  • 8. Futaana, Y
    et al.
    Barabash, S
    Wieser, M
    Lue, C
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wurz, P
    Vorburger, A
    Bhardwaj, A
    Asamura, K
    Remote energetic neutral atom imaging of electric potential over a lunar magnetic anomaly2013In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 40, no 2, p. 262-266Article in journal (Refereed)
    Abstract [en]

    The formation of electric potential over lunar magnetized regions is essential for understanding fundamental lunar science, for understanding the lunar environment, and for planning human exploration on the Moon. A large positive electric potential was predicted and detected from single point measurements. Here, we demonstrate a remote imaging technique of electric potential mapping at the lunar surface, making use of a new concept involving hydrogen neutral atoms derived from solar wind. We apply the technique to a lunar magnetized region using an existing dataset of the neutral atom energy spectrometer SARA/CENA on Chandrayaan-1. Electrostatic potential larger than +135 V inside the Gerasimovic anomaly is confirmed. This structure is found spreading all over the magnetized region. The widely spread electric potential can influence the local plasma and dust environment near the magnetic anomaly. Citation: Futaana, Y., S. Barabash, M. Wieser, C. Lue, P. Wurz, A. Vorburger, A. Bhardwaj, and K. Asamura (2013), Remote energetic neutral atom imaging of electric potential over a lunar magnetic anomaly, Geophys. Res. Lett., 40, 262-266, doi:10.1002/grl.50135.

  • 9.
    Haei, Mahsa
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Öquist, Mats G.
    Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
    Buffam, Ishi
    Ecosystem and Landscape Ecology Lab, Department of Zoology, University of Wisconsin, Madison, Wisconsin, USA.
    Ågren, Anneli
    Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
    Blomkvist, Peder
    Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
    Bishop, Kevin
    Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden.
    Ottosson Löfvenius, Mikaell
    Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
    Laudon, Hjalmar
    Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
    Cold winter soils enhance dissolved organic carbon concentrations in soil and stream water2010In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 37, p. L08501-Article in journal (Refereed)
    Abstract [en]

    Concentrations of dissolved organic carbon ([DOC]) have increased in lakes, streams and rivers across a large part of the northern hemisphere and raised an animated scientific debate about the underlying mechanisms. The lack of consensus about the role of climate in controlling the DOC trends highlights the need for understanding the regulation of surface water DOC. We found that longer and colder winters result in higher [DOC] in a boreal headwater stream during the subsequent snowmelt. In addition, prolonged soil frost increases the spring and summer [DOC] in the riparian soil water, which is a major contributor of stream water DOC in the studied area. We conclude that winter climatic conditions can play a substantial role in controlling stream [DOC] in ways not previously understood. These findings are especially important for northern latitude regions expected to be most affected by climate change.

  • 10.
    Hamrin, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Andre, M
    Norqvist, Patrik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    The importance of a dark ionosphere for ion heating and auroral arc formation2000In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 27, no 11, p. 1635-1638Article in journal (Refereed)
    Abstract [en]

    We present observations from the Freja Satellite to show that density reductions and ion heating at Freja heights are anticorrelated with solar illumination of the ionosphere. When the ionospheric foot-point of a flux-tube is in shadow, the ambient density is lower, transverse ion energization is more common and more intense, and the associated density cavities are deeper. In combination with the suggestion that the electrons must be accelerated to keV energies to carry an imposed current in a low density auroral cavity, these observations may explain the recent observation that auroras are more common when the ionosphere below is in darkness.

  • 11.
    Karlsson, Jan
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Giesler, Reiner
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Persson, Jenny
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Lundin, Erik
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    High emission of carbon dioxide and methane during ice thaw in high latitude lakes2013In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 40, no 6, p. 1123-1127Article in journal (Refereed)
    Abstract [en]

    The winter period is seldom included in the estimates of aquatic-atmospheric carbon exchange. In this study we quantified the flux of carbon dioxide (CO2) and methane (CH4) over 3 years from 12 small subarctic lakes. The lakes accumulated consistent and high amounts of CO2 and CH4 (56–97% as CO2) over the winter, resulting in a high flux during ice thaw. The CO2 flux during ice thaw increased with increasing mean depth of the lakes, while the CH4 flux was high in lakes surrounded by mires. The ice thaw period was quantitatively important to the annual gas balances of the lakes. For nine of the lakes, 11 to 55% of the annual flux occurred during thaw. For three of the lakes with an apparent net annual CO2 uptake, including the thaw period reversed the balance from sink to source. Our results suggest that the ice thaw period is critically important for the emissions of CO2 and CH4 in small lakes.

  • 12. Karlsson, Tomas
    et al.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nilsson, Hans
    Kullen, Anita
    Pitkänen, Timo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Magnetic forces associated with bursty bulk flows in Earth's magnetotail2015In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 42, no 9, p. 3122-3128Article in journal (Refereed)
    Abstract [en]

    We present the first direct measurements of magnetic forces acting on bursty bulk flow plasma in the magnetotail. The magnetic forces are determined using Cluster multispacecraft measurements. We analyze 67 bursty bulk flow (BBF) events and show that the curvature part of the magnetic force is consistently positive, acting to accelerate the plasma toward Earth between approximately 10 and 20 R-E geocentrical distances, while the magnetic field pressure gradient increasingly brakes the plasma as it moves toward Earth. The net result is that the magnetic force accelerates the plasma at distances greater than approximately 14 R-E, while it acts to decelerate it within that distance. The magnetic force, together with the thermal pressure gradient force, will determine the dynamics of the BBFs as they propagate toward the near-Earth tail region. The determination of the former provides an important clue to the ultimate fate of BBFs in the inner magnetosphere.

  • 13.
    Kero, Johan
    et al.
    Umeå University, Faculty of Science and Technology, Physics. Institutet för rymdfysik.
    Szasz, Csilla
    Umeå University, Faculty of Science and Technology, Physics. Institutet för rymdfysik.
    Pellinen-Wannberg, Asta
    Umeå University, Faculty of Science and Technology, Physics. Institutet för rymdfysik.
    Wannberg, Gudmund
    Institutet för rymdfysik.
    Westman, Assar
    EISCAT Scientific Association.
    Meisel, David
    SUNY-Geneseo, NY, USA.
    Three-dimensional radar observation of a submillimeter meteoroid fragmentation2008In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 35, no L04101, p. 5-Article in journal (Refereed)
    Abstract [en]

    We present and discuss two examples of pulsating meteor events observed with the tristatic 930 MHz EISCAT UHF radar system. One of them provides the first strong observational evidence of a submillimeter-sized meteoroid breaking apart into two distinct fragments. The received power fluctuates regularly in the time profiles of all three receivers, but the fluctuation frequencies are different. The pulsations are interpreted as being due to interference from two distinct scattering centers and the three detected pulsation rates are utilized to calculate the differential velocity of the fragments. The result is consistent with interference from two fragments of unequal cross-sectional area over mass ratio, separating from each other due to different deceleration along the trajectory of their parent meteoroid. The other meteor event is an example of a meteoroid undergoing quasi-continuous disintegration. This manifests itself as simultaneous pulsations at all three receivers. Both observations indicate a head echo target upper size limit of the order of half the 32 cm radar wavelength.

  • 14.
    Kero, Johan
    et al.
    Umeå University, Faculty of Science and Technology, Physics. Institutet för rymdfysik.
    Szasz, Csilla
    Umeå University, Faculty of Science and Technology, Physics. Institutet för rymdfysik.
    Wannberg, Gudmund
    Institutet för rymdfysik.
    Pellinen-Wannberg, Asta
    Umeå University, Faculty of Science and Technology, Physics. Institutet för rymdfysik.
    Westman, Assar
    EISCAT Scientific Association.
    On the meteoric head echo radar cross section angular dependence2008In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 35, no L07101, p. 5-Article in journal (Refereed)
    Abstract [en]

    We present radar cross section (RCS) measurements of meteor head echoes observed with the tristatic 930 MHz EISCAT UHF radar system. The three receivers offer a unique possibility to accurately compare the monostatic RCS of a meteor target with two simultaneously probed bistatic RCSs at different aspect angles. Meteoroids from all possible directions entering the common volume monitored by the three receivers are detected, out to an aspect angle of 130° from the meteoroid trajectories. The RCS of individual meteors as observed by the three receivers are equal within the accuracy of the measurements. This is consistent with an essentially isotropic scattering process as has previously been inferred from polarization measurements by S. Close et al. (2002). There is a very weak trend present in our data suggesting that the RCS may decrease at a rate of 0.2 dB per 10° with increasing aspect angle.

  • 15.
    Lindkvist, Jesper
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna.
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna.
    Fatemi, Shahab
    Space Sciences Laboratory, UC Berkeley.
    Wieser, Martin
    Swedish Institute of Space Physics, Kiruna.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna.
    Ceres interaction with the solar wind2017In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, no 5, p. 2070-2077Article in journal (Refereed)
    Abstract [en]

    The solar wind interaction with Ceres is studied for a high water vapor release from its surface using a hybrid model including photoionization. We use a water vapor production rate of 6 kg/s, thought to be due to subsurface sublimation, corresponding to a detection on 6 March 2013 by the Herschel Space Observatory. We present the general morphology of the plasma interactions, both close to Ceres and on a larger scale. Mass loading of water ions causes a magnetic pileup region in front of Ceres, where the solar wind deflects up to 15 ∘ and slows down by 15%. The global plasma interaction with Ceres is not greatly affected by the source location of water vapor nor on gravity, only on the production rate of water vapor. On a global scale, Ceres has a comet-like interaction with the solar wind with observable perturbations farther than 250 Ceres radii downstream of the body.

  • 16. Lindstedt, T
    et al.
    Khotyaintsev, YV
    Vaivads, A
    André, M
    Nilsson, H
    Waara, Martin
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Oxygen energization by localized perpendicular electric fields at the cusp boundary2010In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 37, no 9, article id L09103Article in journal (Refereed)
    Abstract [en]

    We report Cluster observations of oxygen energization by several keV at the boundary between the high latitude cusp and lobe. A localized electric field at the cusp/lobe boundary is responsible for a significant part of the observed energization. Such electric fields can be related to the separatrix region of reconnection at the magnetopause. Ions are accelerated as they move non-adiabatically in the spatially inhomogeneous electric field. Additional heating may be provided by low frequency waves at the oxygen gyrofrequency. Citation: Lindstedt, T., Y. V. Khotyaintsev, A. Vaivads, M. Andre, H. Nilsson, and M. Waara (2010), Oxygen energization by localized perpendicular electric fields at the cusp boundary.

  • 17.
    Lue, Charles
    et al.
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Futaan, Yoshifumi
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Wieser, Martin
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Bhardwaj, Anil
    Space Physics Laboratory, Vikram Sarabhai Space Center, Trivandrum, India.
    Dhanya, M. B.
    Space Physics Laboratory, Vikram Sarabhai Space Center, Trivandrum, India.
    Wurz, Peter
    Physikalisches Institut, University of Bern, Bern, Switzerland.
    Strong influence of lunar crustal fields on the solar wind flow2011In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 38, article id L03202Article in journal (Refereed)
    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.

  • 18.
    Lundin, Erik J
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Department of Environmental Science and Analytical Chemistry (ACES), Stockholm University, Stockholm, Sweden.
    Klaminder, Jonatan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Giesler, Reiner
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Persson, Andreas
    Olefeldt, David
    Heliasz, Michal
    Christensen, Torben R.
    Karlsson, Jan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Is the subarctic landscape still a carbon sink?: Evidence from a detailed catchment balance2016In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 43, no 5, p. 1988-1995Article in journal (Refereed)
    Abstract [en]

    Climate warming raises the question whether high-latitude landscape still function as net carbon (C) sinks. By compiling an integrated C balance for an intensely studied subarctic catchment, we show that this catchment's C balance is not likely to be a strong current sink of C, a commonly held assumption. In fact, it is more plausible (71% probability) that the studied catchment functions as a C source (-1120gCm(-2)yr(-1)). Analyses of individual fluxes indicate that soil and aquatic C losses offset C sequestering in other landscape components (e.g., peatlands and aboveground forest biomass). Our results stress the importance of fully integrated catchment C balance estimates and highlight the importance of upland soils and their interaction with the aquatic network for the catchment C balance.

  • 19. MacIntyre, Sally
    et al.
    Jonsson, Anders
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Jansson, Mats
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Åberg, Jan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Turney, Damon E.
    Miller, Scott D.
    Buoyancy flux, turbulence, and the gas transfer coefficient in a stratified lake2010In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 37, no L24604Article in journal (Refereed)
    Abstract [en]

    Gas fluxes from lakes and other stratified water bodies, computed using conservative values of the gas transfer coefficient k600, have been shown to be a significant component of the carbon cycle. We present a mechanistic analysis of the dominant physical processes modifying k600 in a stratified lake and resulting new models of k600 whose use will enable improved computation of carbon fluxes. Using eddy covariance results, we demonstrate that i) higher values of k600 occur during low to moderate winds with surface cooling than with surface heating; ii) under overnight low wind conditions k600 depends on buoyancy flux β rather than wind speed; iii) the meteorological conditions at the time of measurement and the inertia within the lake determine k600; and iv) eddy covariance estimates of k600 compare well with predictions of k600 using a surface renewal model based on wind speed and β.

  • 20. Madsen, B.
    et al.
    Wedlund, C. Simon
    Eriksson, A.
    Goetz, C.
    Karlsson, T.
    Gunell, Herbert
    Umeå University, Faculty of Science and Technology, Department of Physics. Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium.
    Spicher, A.
    Henri, P.
    Vallieres, X.
    Miloch, W. J.
    Extremely Low-Frequency Waves Inside the Diamagnetic Cavity of Comet 67P/Churyumov-Gerasimenko2018In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 45, no 9, p. 3854-3864Article in journal (Refereed)
    Abstract [en]

    The European Space Agency/Rosetta mission to comet 67P/Churyumov-Gerasimenko has provided several hundred observations of the cometary diamagnetic cavity induced by the interaction between outgassed cometary particles, cometary ions, and the solar wind magnetic field. Here we present the first electric field measurements of four preperihelion and postperihelion cavity crossings on 28 May 2015 and 17 February 2016, using the dual-probe electric field mode of the Langmuir probe (LAP) instrument of the Rosetta Plasma Consortium. We find that on large scales, variations in the electric field fluctuations capture the cavity and boundary regions observed in the already well-studied magnetic field, suggesting the electric field mode of the LAP instrument as a reliable tool to image cavity crossings. In addition, the LAP electric field mode unravels for the first time extremely low-frequency waves within two cavities. These low-frequency electrostatic waves are likely triggered by lower-hybrid waves observed in the surrounding magnetized plasma. Plain Language Summary As sunlight heats a comet nucleus, frozen volatile gases sublimate are ionized and interact with the solar wind and its embedded magnetic field, inducing a dynamical plasma environment around the comet. With the cornerstone European mission Rosetta and its 2years of near-continuous orbiting of comet 67P/Churyumov-Gerasimenko, the origin, structure, and evolution of this environment are only starting to be unveiled. Exciting are the numerous crossings of the diamagnetic cavity, the innermost plasma region from which the solar wind magnetic field is excluded. Whilst the magnetic field structure of the cavity crossings is well studied, the related electric field activity remains until now unexplored. Studying the electric field with the Langmuir probes onboard Rosetta, we find that whereas the large-scale electric field structure agrees well with the observed magnetic field behavior during cavity crossings, unexpected short-lived low-frequency electric field signals manifest themselves within the cavity. We interpret these as electrostatic waves triggered by a modulating of the cavity boundary caused by observed electrostatic waves at the same frequency in the surrounding magnetized plasma. This unravels a new aspect of the electromagnetic activity in the inner cometary environment, which is crucial for our understanding of the comet-solar wind-induced plasma environment.

  • 21. Nakamura, R.
    et al.
    Karlsson, T.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nilsson, H.
    Swedish Inst Space Phys, S-98128 Kiruna, Sweden.
    Marghitu, O.
    Amm, O.
    Bunescu, C.
    Constantinescu, V.
    Frey, H. U.
    Keiling, A.
    Semeter, J.
    Sorbalo, E.
    Vogt, J.
    Forsyth, C.
    Kubyshkina, M. V.
    Low- altitude electron acceleration due to multiple flow bursts in the magnetotail2014In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 41, no 3, p. 777-784Article in journal (Refereed)
    Abstract [en]

    At 10:00 UT on 25 February 2008, Cluster 1 spacecraft crossed the near-midnight auroral zone, at about 2R(E) altitude, while two of the Time History of Events and Macroscale Interactions During Substorms (THEMIS) spacecraft, THD and THE, observed multiple flow bursts on the near-conjugate plasma sheet field lines. The flow shear pattern at THEMIS was consistent with the vortical motion at duskside of a localized flow channel. Coinciding in time with the flow bursts, Cluster 1 observed bursts of counterstreaming electrons with mostly low energies (441eV), accompanied by short time scale (<5s) magnetic field disturbances embedded in flow-associated field-aligned current systems. This conjugate event not only confirms the idea that the plasma sheet flows are the driver of the kinetic Alfven waves accelerating the low-energy electrons but is a unique observation of disturbances in the high-altitude auroral region relevant to the multiple plasma sheet flows. Key Points <list list-type="bulleted"> <list-item id="grl51326-li-0001">First observation of multiple flow signatures on near-conjugate flux tubes <list-item id="grl51326-li-0002">Low-energy electron profile suggests Alfvenic acceleration due to fast flow <list-item id="grl51326-li-0003">Multiple flow bursts are obtained to extend over large radial distance in tail

  • 22.
    Pitkänen, Timo
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Kullen, A.
    Maggiolo, R.
    Karlsson, T.
    Nilsson, H.
    Norqvist, Patrik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Response of magnetotail twisting to variations in IMF B-y: a THEMIS case study 1-2 January 20092016In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 43, no 15, p. 7822-7830Article in journal (Refereed)
    Abstract [en]

    Theoretical considerations, observations, and simulations have shown that the B-y component of the interplanetary magnetic field (IMF) may cause twisting of the magnetotail. However, the fundamental issues, the temporal and spatial responses of the magnetotail in the twisting process, are still unresolved. We report unique multipoint observations of the response of the magnetotail to the variations in IMF B-y on 1-2 January 2009. For the first time, estimates of the tail twisting response time at different (Time History of Events and Macroscale Interactions during Substorms, THEMIS) distances in the same event are inferred. Using cross-correlation and timing analyses, we find that the tail twisting propagates from farther out toward the Earth and the response time increases significantly to the inner magnetosphere.

  • 23.
    Pitkänen, Timo
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Norqvist, Patrik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Karlsson, Tomas
    Space and Plasma Physics, School of Electrical Engineering, Royal Institute of Technology, Stockholm, Sweden.
    Nilsson, Hans
    Swedish Institute of Space Physics, Kiruna, Sweden.
    IMF dependence of the azimuthal direction of earthward magnetotail fast flows2013In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 40, no 21, p. 5598-5604Article in journal (Refereed)
    Abstract [en]

    Cluster magnetotail data together with ACE solar wind data from 2001 to 2009 are used to investigate the dependence of the azimuthal flow direction of earthward magnetotail fast flows on the interplanetary magnetic field (IMF). We find an indication that fast flows have favored azimuthal directions that have dependence on the IMF. Our results suggest that for positive IMF By, the favored azimuthal direction of the fast flows is dawnward in the northern plasma sheet and duskward in the southern plasma sheet. For negative IMF By, an opposite situation takes place, the favored azimuthal flow directions are then duskward and dawnward in the northern and southern plasma sheet, respectively. As a possible explanation for the results, it is suggested that the untwisting reconnected magnetic field lines may direct the fast flows in the magnetotail, the field line twist itself being dependent on the IMF.

  • 24.
    Ramstad, Robin
    et al.
    Swedish Institute of Space Physics, Kiruna.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna.
    Futaana, Yoshifumi
    Swedish Institute of Space Physics, Kiruna.
    Yamauchi, Masatoshi
    Swedish Institute of Space Physics, Kiruna.
    Nilsson, Hans
    Swedish Institute of Space Physics, Kiruna.
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna.
    Mars Under Primordial Solar Wind Conditions: Mars Express Observations of the Strongest CME Detected at Mars Under Solar Cycle #24 and its Impact on Atmospheric Ion Escape2017In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007Article in journal (Refereed)
    Abstract [en]

    An extremely strong Coronal Mass Ejection (CME) impacted Mars on 12 July 2011, while theMars Express spacecraft was present inside the nightside ionosphere. Estimated solar wind density andspeed during the event are 39 particles cm−3 and 730 km/s, corresponding to nominal solar wind fluxat Mars when the solar system was ∼1.1 Ga old. Comparing with expected average atmospheric heavy ionfluxes under similar XUV conditions, the CME impact is found to have no significant effect on the escaperate 3.3 × 1024 s−1, with an upper limit at 1025 s−1 if the observed tail contraction is not taken into account.On the subsequent orbit, 7 h later after magnetosphere response, fluxes were only 2.4% of average. As such,even under primordial solar wind conditions we are unable to find support for a strong solar wind-driven ion escape, rather the main effect appears to be acceleration of the escaping ions by ×10–×20 typicalcharacteristic energy.

  • 25.
    Ramstad, Robin
    et al.
    Swedish Institute of Space Physics, Kiruna.
    Futaana, Yoshifumi
    Swedish Institute of Space Physics, Kiruna.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna.
    Nilsson, Hans
    Swedish Institute of Space Physics, Kiruna.
    del Campo B., Sergio Martin
    Department of Computer Science, Electrical and Space Engineering,Luleå University of Technology, Kiruna, Sweden..
    Schwingenschuh, Konrad
    Space Research Institute, Austrian Academy of Sciences, Graz, Austria..
    Phobos 2/ASPERA data revisited: Planetary ion escape rate from Mars near the 1989 solar maximum2017In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 40, no 3, p. 477-481Article in journal (Refereed)
    Abstract [en]

    Insights about the near-Mars space environment from Mars Express observations have motivated a revisit of the Phobos 2/ASPERA ion data from 1989. We have expanded the analysis to now include all usable heavy ion(O+, O2+ , CO2+) measurements from the circular orbits of Phobos 2. Phobos 2/ASPERA ion fluxes in the Martian tailare compared with previous results obtained by the instruments on Phobos 2. Further validation of the measurement results is obtained by comparing IMP-8 and Phobos 2/ASPERA solar wind ion fluxes, taking into account the time lag between Earth and Mars. Heavy ion flux measurements from 18 circular equatorial orbits around Mars are bin-averaged to a grid, using the MSE (electric field) frame of reference. The binned data are subsequently integrated to determine the total escape rate of planetary ions. From this we derive a total planetary heavy ion escape rate of (2–3)1025 s-1 from Mars for the 1989 solar maximum. 

  • 26.
    Rocher-Ros, Gerard
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Giesler, Reiner
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Lundin, Erik
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Salimi, Shokoufeh
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Division of Water Resources Engineering, Faculty of Engineering, Lund University, Lund, Sweden.
    Jonsson, Anders
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Karlsson, Jan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Large lakes dominate CO2 evasion from lakes in an arctic catchment2017In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, no 24, p. 12254-12261Article in journal (Refereed)
    Abstract [en]

    CO2 evasion from freshwater lakes is an important component of the carbon cycle. However, the relative contribution from different lake sizes may vary, since several parameters underlying CO2 flux are size dependent. Here we estimated the annual lake CO2 evasion from a catchment in northern Sweden encompassing about 30,000 differently sized lakes. We show that areal CO2 fluxes decreased rapidly with lake size, but this was counteracted by the greater overall coverage of larger lakes. As a result, total efflux increased with lake size and the single largest lake in the catchment dominated the CO2 evasion (53% of all CO2 evaded). By contrast, the contribution from the smallest ponds (about 27,000) was minor (<6%). Our results emphasize the importance of accounting for both CO2 flux rates and areal contribution of various sized lakes in assessments of CO2 evasion at the landscape scale.

  • 27.
    Seekell, David A.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia, USA.
    Carr, Joel A.
    Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia, USA.
    Gudasz, Cristian
    Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey, USA.
    Karlsson, Jan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Upscaling carbon dioxide emissions from lakes2014In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 41, no 21, p. 7555-7559Article in journal (Refereed)
    Abstract [en]

    Quantifying CO2 fluxes from lakes to the atmosphere is important for balancing regional and global-scale carbon budgets. CO2 emissions are estimated through statistical upscaling procedures that aggregate data from a large number of lakes. However, aggregation can bias flux estimates if the physical and chemical factors determining CO2 exchange between water and the atmosphere are not independent. We evaluated the magnitude of aggregation biases with moment expansions and pCO(2) data from 5140 Swedish lakes. The direction of the aggregation bias depends on lake size; mean flux was overestimated by 4% for small lakes (0.01-0.1 km(2)) but underestimated by 13% for large lakes (100-1000 km(2)). Simple covariance-based correction factors were generated to adjust for upscaling biases. These correction factors represent an easily interpretable and implemented approach to improving the accuracy of regional and global estimates of lake CO2 emissions.

  • 28.
    Seekell, David A.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Gudasz, Cristian
    Department of Ecology and Genetics/Limnology, Uppsala University, Uppsala, Sweden.
    Long-term pCO(2) trends in Adirondack Lakes2016In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 43, no 10, p. 5109-5115Article in journal (Refereed)
    Abstract [en]

    Lakes are globally significant sources of CO2 to the atmosphere. However, there are few temporally resolved records of lake CO2 concentrations and long-term patterns are poorly characterized. We evaluated annual trends in the partial pressure of CO2 (pCO2) based on chemical measurements from 31 Adirondack Lakes taken monthly over an 18 year period. All lakes were supersaturated with CO2 and were sources of CO2 to the atmosphere. There were significant pCO2 trends in 29% of lakes. The median magnitude of significant positive trends was 32.1 µatm yr−1. Overall, 52% of lakes had pCO2 trends greater than those reported for the atmosphere and ocean. Significant trends in lake pCO2 were attributable to regional recovery from acid deposition and changing patterns of ice cover. These results illustrate that lake pCO2 can respond rapidly to environmental change, but the lack of significant trend in 71% of lakes indicates substantial lake-to-lake variation in magnitude of response.

  • 29. Slapak, R.
    et al.
    Gunell, H.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Observations of multiharmonic ion cyclotron waves due to inverse ion cyclotron damping in the northern magnetospheric cusp2017In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, no 1, p. 22-29Article in journal (Refereed)
    Abstract [en]

    We present a case study of inverse ion cyclotron damping taking place in the northern terrestrial magnetospheric cusp, exciting waves at the ion cyclotron frequency and its harmonics. The ion cyclotron waves are primarily seen as peaks in the magnetic-field spectral densities. The corresponding peaks in the electric-field spectral densities are not as profound, suggesting a background electric field noise or other processes of wave generation causing the electric spectral densities to smoothen out more compared to the magnetic counterpart. The required condition for inverse ion cyclotron damping is a velocity shear in the magnetic field-aligned ion bulk flow, and this condition is often naturally met for magnetosheath influx in the northern magnetospheric cusp, just as in the presented case. We note that some ion cyclotron wave activity is present in a few similar shear events in the southern cusp, which indicates that other mechanisms generating ion cyclotron waves may also be present during such conditions.

  • 30.
    Vedin, Jörgen
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bunescu, Costel
    Marghitu, Octav
    Estimating properties of concentrated parallel electric fields from electron velocity distributions2007In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 34, no 16, article id L16107Article in journal (Refereed)
    Abstract [en]

    Information about the magnitude of the field-aligned potential drop along auroral field lines is usually derived from the velocity distribution of the particles. When the electrons are accelerated by a strong double layer their velocity distribution will have features different from those produced by a weak, spread-out, electric field. Quantifying these features, we obtain information about the strength and thickness of the double layer.

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