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O Amm - One of the best experts on this subject based on the ideXlab platform.
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Ionospheric response to oscillatory flow braking in the magnetotail
Journal of Geophysical Research, 2013Co-Authors: E V Panov, M V Kubyshkina, Anatoli Petrukovich, Vassilis Angelopoulos, Wolfgang Baumjohann, R Nakamura, J M Weygand, O Amm, K H Glassmeier, V. A. SergeevAbstract:[1] We study the Ionospheric response to oscillatory braking of bursty bulk flow observed by THEMIS on 17 March 2008 between 10:22 and 10:36 UT. By calculating different Current components generated in the plasma sheet and correlating the space and ground observations, we discriminate the Ionospheric Current relevant to the large-scale substorm wedge Currents produced by the general reconfiguration of the magnetotail pressure gradient from the Currents that appeared as a result of the flow oscillation. While the former Currents are large and quasi-stable, the latter (oscillating) Currents are substantially (2–3 times) weaker and flow in opposite directions during earthward and tailward flow bursts. The oscillating Currents include the polarization Current and the Current generated by the oscillating part of the pressure gradient. The two oscillating Currents appear to produce modulation of the Ionospheric Currents (with about 2.5 min period) that was seen as Pi2 pulsations in the ground magnetometer observations. Our estimates of the Ionospheric conductance suggest that the damping of the plasma sheet flow oscillation is due to heating the ionosphere through Pedersen Currents. We also found that the all-sky imager at Fort Yukon observed four auroral forms during the first two periods of the oscillatory flow braking: two auroral forms related to the earthward plasma sheet flows and the other two auroral forms related to the tailward rebounds of the earthward flow. The auroral forms evolve in accordance with the appearance and motion of the upward field-aligned Current spot of the modulated part of the Ionospheric field-aligned Current.
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one dimensional spherical elementary Current systems and their use for determining Ionospheric Currents from satellite measurements
Earth Planets and Space, 2006Co-Authors: Liisa Juusola, O Amm, Ari ViljanenAbstract:The method of 1D spherical elementary Current systems (SECS) is a new way for determining Ionospheric and field-aligned Currents in spherical geometry from magnetic field measurements made by a low-orbit satellite. In contrast to earlier methods, the full Ionospheric Current distribution, including both divergence-free and curl-free horizontal Currents, as well as field-aligned Currents, can be determined. Placing infinitely many 2D SECSs of identical amplitudes at a constant latitude results in two types of 1D SECSs, which are independent of longitude, and by superposition can reproduce any Ionospheric and field-aligned Current system with the same property. One type of the 1D SECSs is divergence-free and toroidal with a poloidal magnetic field, and the other type is curl-free and poloidal. Associated with the divergence of the curl-free type are radial Currents. The magnetic field of the combined curl-free 1D SECS and field-aligned Currents is toroidal and restricted to the region above the ionosphere. Ionospheric Currents are determined by placing several 1D SECSs at different latitudes and choosing their amplitudes in such a way that their combined magnetic field as closely as possible fits the one measured by the satellite. The 1D SECS method has been tested using both modeled and real data from the CHAMP satellite, and found to work excellently in 1D cases.
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Ionospheric equivalent Current distributions determined with the method of spherical elementary Current systems
Journal of Geophysical Research, 2003Co-Authors: Antti Pulkkinen, O Amm, A ViljanenAbstract:[1] The ground magnetic field disturbance caused by Ionospheric Currents can be represented by equivalent Currents placed to the Ionospheric plane. Equivalent Currents provide valuable information about the Ionospheric electrodynamics, and thus they can be used, for example, in studies of space weather, ionosphere-magnetosphere coupling, and the magnetotelluric source effect. We derive equivalent Currents by using the spherical elementary Current system method. The applicability of the method for the Baltic Electromagnetic Array Research (BEAR) magnetometer array is validated by means of synthetic Ionospheric Current models and by investigating the goodness of the fit between the modeled and measured ground magnetic field. The applicability of the method for the sparser International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer network is also proved. In addition, the combination of the elementary Current system method and the complex image method, used for the calculation of the induced electromagnetic fields on ground, is introduced, and the combination of the methods is tested by using geoelectric field data from the BEAR project. Our special interest is in the effects that rapidly varying Ionospheric Currents have on technological conductor systems at the surface of the Earth due to geomagnetically induced Currents. Comparison between equivalent Currents and the time derivative vector of the horizontal magnetic field emphasizes the importance of small-scale structures.
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the elementary Current method for calculating Ionospheric Current systems from multisatellite and ground magnetometer data
Journal of Geophysical Research, 2001Co-Authors: O AmmAbstract:The recently launched Cluster II mission provides for the first time the possibility to instantaneously obtain spatially distributed measurements of field-aligned Currents from a fleet of satellites. We present the “elementary Current method” that combines such measurements mapped to the ionosphere with two-dimensional ground magnetic data, to calculate actual (not equivalent) Ionospheric Currents, without the need of further assumptions. If additional two-dimensional measurements of the Ionospheric electric field from coherent scatter radars are available, the Ionospheric Hall and Pedersen conductances can also be inferred. The applicability of the method is demonstrated for a passage of the Cluster II spacecraft over the Multi-Instrument Array for Ionosphere-Magnetosphere Coupling Studies (MIRACLE) network of ground-based instruments in northern Fennoscandia on February 1, 2001, using preliminary but realistic orbit parameters. The geophysical situation of Ionospheric electrodynamic parameters that we model during this passage refers to a real event of a plasma vortex propagating eastward over the MIRACLE field of view, as studied by Kosch et al. [2000]. The application of the elementary Current method to the simulated ground magnetometer and satellite measurements calculated from this model shows that the modeled Ionospheric Currents, as well as the Hall and Pedersen conductances, can be reconstructed by the method to good accuracy.
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Ionospheric disturbance magnetic field continuation from the ground to the ionosphere using spherical elementary Current systems
Earth Planets and Space, 1999Co-Authors: O Amm, A ViljanenAbstract:A new technique for continuation of the ground magnetic field caused by Ionospheric Currents to the ionosphere in spherical geometry is presented that makes use of elementary Ionospheric Current systems, which were introduced by Amm (1997) in extension of an earlier work by Fukushima (1976). The measured ground magnetic disturbance is expanded in terms of the ground magnetic effect of a spatial distribution of such elementary Current systems. Using a matrix inversion technique, the scaling factors for each elementary Current system, and therefrom the Ionospheric equivalent Currents are calculated. The technique can be applied to both global and local scales. Its advantages compared to the common field continuation techniques with Fourier (local scale), spherical cap (local to medium scale), or spherical (global scale) harmonic expansions are: 1) No fixed limitation of the spectral content has to be given for the whole analysis area, as it has to be done for the other techniques by truncation of a series expansion. 2) The locations of the elementary Current systems can be chosen freely, such that they are most suitable with respect to the available measurement sites or the type of Current system to be analysed. Results of the new technique are discussed in comparison to results of the spherical cap harmonic expansion method for a model of a Cowling channel.
A Viljanen - One of the best experts on this subject based on the ideXlab platform.
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application and validation of the spherical elementary Currents systems technique for deriving Ionospheric equivalent Currents with the north american and greenland ground magnetometer arrays
Journal of Geophysical Research, 2011Co-Authors: J M Weygand, A Viljanen, V Angelopoulos, D Murr, M J Engebretson, Hans Gleisner, I R MannAbstract:[1] With data from the Canadian Magnetic Observatory System, Canadian Array for Real time Investigations of Magnetic Activity, Geophysical Institute Magnetometer Array, Greenland, Time History of Events and Macroscale Interactions during Substorms (THEMIS), and Magnetometer Array for Cusp and Cleft Studies ground magnetometer arrays, we applied the state-of-art technique on the basis of spherical elementary Currents systems (SECS) developed by Amm and Viljanen (1999) in order to calculate maps of Ionospheric equivalent Currents over the whole North American auroral region. This study is the first to apply the SECS technique to a large nonrectangular area with widely separated ground magnetometers (∼350 km). For this study we will first demonstrate the validity of the technique using synthetic data and then examine equivalent Ionospheric Currents associated with a Harang discontinuity for a case study on 10 December 2007. The results show in detail the dynamic evolution of the Currents over the entire North American ground magnetometer network. Equivalent Ionospheric Current (EIC) maps inferred at the minimum resolution of the database, in this case 10 s, can thus be analyzed further in conjunction with near-simultaneous images of the THEMIS all-sky imager mosaics and Super Dual Auroral Radar Network radar data. The EIC maps represent a value-added product from the raw magnetometer database and can be used for contextual interpretation as well as help with our understanding of magnetosphere-ionosphere coupling mechanisms using the ground arrays and the THEMIS spacecraft data.
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Evaluation of the geometry of Ionospheric Current systems related to rapid geomagnetic variations
Annales Geophysicae, 2004Co-Authors: S. V. Apatenkov, V. A. Sergeev, R. Pirjola, A ViljanenAbstract:To learn about the geometry and sources of the Ionospheric Current systems which generate strong geomagnetically induced Currents, we categorize differential equivalent Current systems (DEC) for events with strong dB/dt by decomposing them into the contributions of electrojet-type and vortex-type elementary systems. By solving the inverse problem we obtain amplitudes and locations of these elementary Current systems. One-minute differences of the geomagnetic field values at the IMAGE magnetometer network in 1996?2000 are analysed to study the spatial distributions of large dB/dt events. The relative contributions of the two components are evaluated. In particular, we found that the majority of the strongest dB/dt events (100?1000nT/min) appear to be produced by the vortex-type Current structures and most of them occur in the morning LT hours, probably caused by the Ps6 pulsation events associated with auroral omega structures. For strong dB/dt events the solar wind parameters are shifted toward strong (tens nT) southward IMF, enhanced velocity and dynamic pressure, in order for the main phase of the magnetic storms to occur. Although these events appear mostly during magnetic storms when the auroral oval greatly expands, the area of large dB/dt stays in the middle part of the auroral zone; therefore, it is connected to the processes taking part in the middle of the magnetosphere rather than in its innermost region populated by the ring Current. Key words. Geomagnetism and paleomagnetism (rapid time variations) ? Ionosphere (auroral ionosphere; Ionospheric disturbances)
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Ionospheric equivalent Current distributions determined with the method of spherical elementary Current systems
Journal of Geophysical Research, 2003Co-Authors: Antti Pulkkinen, O Amm, A ViljanenAbstract:[1] The ground magnetic field disturbance caused by Ionospheric Currents can be represented by equivalent Currents placed to the Ionospheric plane. Equivalent Currents provide valuable information about the Ionospheric electrodynamics, and thus they can be used, for example, in studies of space weather, ionosphere-magnetosphere coupling, and the magnetotelluric source effect. We derive equivalent Currents by using the spherical elementary Current system method. The applicability of the method for the Baltic Electromagnetic Array Research (BEAR) magnetometer array is validated by means of synthetic Ionospheric Current models and by investigating the goodness of the fit between the modeled and measured ground magnetic field. The applicability of the method for the sparser International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer network is also proved. In addition, the combination of the elementary Current system method and the complex image method, used for the calculation of the induced electromagnetic fields on ground, is introduced, and the combination of the methods is tested by using geoelectric field data from the BEAR project. Our special interest is in the effects that rapidly varying Ionospheric Currents have on technological conductor systems at the surface of the Earth due to geomagnetically induced Currents. Comparison between equivalent Currents and the time derivative vector of the horizontal magnetic field emphasizes the importance of small-scale structures.
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Ionospheric disturbance magnetic field continuation from the ground to the ionosphere using spherical elementary Current systems
Earth Planets and Space, 1999Co-Authors: O Amm, A ViljanenAbstract:A new technique for continuation of the ground magnetic field caused by Ionospheric Currents to the ionosphere in spherical geometry is presented that makes use of elementary Ionospheric Current systems, which were introduced by Amm (1997) in extension of an earlier work by Fukushima (1976). The measured ground magnetic disturbance is expanded in terms of the ground magnetic effect of a spatial distribution of such elementary Current systems. Using a matrix inversion technique, the scaling factors for each elementary Current system, and therefrom the Ionospheric equivalent Currents are calculated. The technique can be applied to both global and local scales. Its advantages compared to the common field continuation techniques with Fourier (local scale), spherical cap (local to medium scale), or spherical (global scale) harmonic expansions are: 1) No fixed limitation of the spectral content has to be given for the whole analysis area, as it has to be done for the other techniques by truncation of a series expansion. 2) The locations of the elementary Current systems can be chosen freely, such that they are most suitable with respect to the available measurement sites or the type of Current system to be analysed. Results of the new technique are discussed in comparison to results of the spherical cap harmonic expansion method for a model of a Cowling channel.
Gary D Egbert - One of the best experts on this subject based on the ideXlab platform.
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an application of principal component analysis to the interpretation of Ionospheric Current systems
Journal of Geophysical Research, 2017Co-Authors: Patrick Alken, Astrid Maute, A D Richmond, Heikki Vanhamaki, Gary D EgbertAbstract:Ionospheric Currents are driven by several different physical processes and exhibit complex spatial and temporal structure. Magnetic field measurements of Ionospheric sources are often spatially sparse, causing significant challenges in visualizing Current flow at a specific time. Standard methods of fitting equivalent Current models to magnetic observations, such as line Currents, spherical harmonic analysis, spherical cap harmonic analysis, and spherical elementary Current systems (SECS), are often unable to capture the full spatial complexity of the Currents, or require a large number of parameters which cannot be fully determined by the available data coverage. These methods rely on a set of generic basis functions which contain limited information about the geometries of the various Ionospheric sources. In this study, we develop new basis functions for fitting ground and satellite measurements, which are derived from physics-based Ionospheric modeling combined with principal component analysis (PCA). The physics-based modeling provides realistic Current flow patterns for all of the primary Ionospheric sources, including their daily and seasonal variability. The PCA technique extracts the most relevant spatial geometries of the Currents from the model run into a small set of equivalent Current modes. We fit these modes to magnetic measurements of the Swarm satellite mission at low and mid-latitudes and compare the resulting model with independent measurements and with the SECS approach. We find that our PCA method accurately reproduces features of the equatorial electrojet and Sq Current systems with only 10 modes, and can predict Ionospheric fields far from the data region.
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Ionospheric Current source modeling and global geomagnetic induction using ground geomagnetic observatory data
Journal of Geophysical Research, 2015Co-Authors: Jin Sun, Anna Kelbert, Gary D EgbertAbstract:Long-period global-scale electromagnetic induction studies of deep Earth conductivity are based almost exclusively on magnetovariational methods and require accurate models of external source spatial structure. We describe approaches to inverting for both the external sources and three-dimensional (3-D) conductivity variations and apply these methods to long-period (T≥1.2 days) geomagnetic observatory data. Our scheme involves three steps: (1) Observatory data from 60 years (only partly overlapping and with many large gaps) are reduced and merged into dominant spatial modes using a scheme based on frequency domain principal components. (2) Resulting modes are inverted for corresponding external source spatial structure, using a simplified conductivity model with radial variations overlain by a two-dimensional thin sheet. The source inversion is regularized using a physically based source covariance, generated through superposition of correlated tilted zonal (quasi-dipole) Current loops, representing Ionospheric source complexity smoothed by Earth rotation. Free parameters in the source covariance model are tuned by a leave-one-out cross-validation scheme. (3) The estimated data modes are inverted for 3-D Earth conductivity, assuming the source excitation estimated in step 2. Together, these developments constitute key components in a practical scheme for simultaneous inversion of the catalogue of historical and modern observatory data for external source spatial structure and 3-D Earth conductivity.
Y. Yamazaki - One of the best experts on this subject based on the ideXlab platform.
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day to day variability of midlatitude Ionospheric Currents due to magnetospheric and lower atmospheric forcing
Journal of Geophysical Research, 2016Co-Authors: Y. Yamazaki, Kathrin Hausler, J A WildAbstract:As known from previous studies on the solar quiet (Sq) variation of the geomagnetic field, the strength and pattern of Ionospheric dynamo Currents change significantly from day to day. The present study investigates the relative importance of two sources that contribute to the day-to-day variability of the Ionospheric Currents at middle and low latitudes. One is high-latitude electric fields that are caused by magnetospheric convection, and the other is atmospheric waves from the lower atmosphere. Global Ionospheric Current systems, commonly known as Sq Current systems, are simulated using the National Center for Atmospheric Research thermosphere-ionosphere-mesosphere-electrodynamics general circulation model. Simulations are run for 1–30 April 2010 with a constant solar energy input but with various combinations of high-latitude forcing and lower atmospheric forcing. The model well reproduces geomagnetic perturbations on the ground, when both forcings are taken into account. The contribution of high-latitude forcing to the total Sq Current intensity (Jtotal) is generally smaller than the contribution of wave forcing from below 30 km, except during active periods (Kp≥4), when Jtotal is enhanced due to the leakage of high-latitude electric fields to lower latitudes. It is found that the penetration electric field drives Ionospheric Currents at middle and low latitudes not only on the dayside but also on the nightside, which has an appreciable effect on the Dst index. It is also found that quiet time day-to-day variability in Jtotal is dominated by symmetric-mode migrating diurnal and semidiurnal tidal winds at 45–60° latitude at ∼110 km.
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Ionospheric Current system during sudden stratospheric warming events
Journal of Geophysical Research, 2012Co-Authors: Y. Yamazaki, Daniel J Mcnamara, Kenji Yumoto, Toshihiko Hirooka, K Kitamura, Takayuki Uozumi, A IkedaAbstract:[1] This paper describes Ionospheric Current systems associated with the counter-electrojet during sudden stratospheric warming (SSW) events in the northern winter months of 2001–2002 and 2002–2003. Magnetic data from 20 stations in the East Asian region, covering both the Northern Hemisphere and the Southern Hemisphere, are analyzed. Additional Current systems that are superposed on the normal Sq Current system and related to the counter-electrojet during the SSW events show a global semidiurnal Current pattern, which shifts to later local times approximately by 0.8 hour/day. The results indicate that abnormally large lunar tidal winds played a main role to produce the additional Current system and counter-electrojet during the SSW events.
Patrick Alken - One of the best experts on this subject based on the ideXlab platform.
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an application of principal component analysis to the interpretation of Ionospheric Current systems
Journal of Geophysical Research, 2017Co-Authors: Patrick Alken, Astrid Maute, A D Richmond, Heikki Vanhamaki, Gary D EgbertAbstract:Ionospheric Currents are driven by several different physical processes and exhibit complex spatial and temporal structure. Magnetic field measurements of Ionospheric sources are often spatially sparse, causing significant challenges in visualizing Current flow at a specific time. Standard methods of fitting equivalent Current models to magnetic observations, such as line Currents, spherical harmonic analysis, spherical cap harmonic analysis, and spherical elementary Current systems (SECS), are often unable to capture the full spatial complexity of the Currents, or require a large number of parameters which cannot be fully determined by the available data coverage. These methods rely on a set of generic basis functions which contain limited information about the geometries of the various Ionospheric sources. In this study, we develop new basis functions for fitting ground and satellite measurements, which are derived from physics-based Ionospheric modeling combined with principal component analysis (PCA). The physics-based modeling provides realistic Current flow patterns for all of the primary Ionospheric sources, including their daily and seasonal variability. The PCA technique extracts the most relevant spatial geometries of the Currents from the model run into a small set of equivalent Current modes. We fit these modes to magnetic measurements of the Swarm satellite mission at low and mid-latitudes and compare the resulting model with independent measurements and with the SECS approach. We find that our PCA method accurately reproduces features of the equatorial electrojet and Sq Current systems with only 10 modes, and can predict Ionospheric fields far from the data region.
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observations and modeling of the Ionospheric gravity and diamagnetic Current systems from champ and swarm measurements
Journal of Geophysical Research, 2016Co-Authors: Patrick AlkenAbstract:The CHAMP and Swarm satellites, which provide high-quality magnetic field measurements in low-altitude polar orbits, are ideally suited for investigating Ionospheric Current systems. In this study, we focus on the F region low-latitude gravity and diamagnetic Currents which are prominent in the equatorial ionization anomaly (EIA) region in the North and South Hemisphere. During its 10 year mission, CHAMP has sampled nearly the entire altitude range of the EIA, offering the opportunity to study these Currents from above, inside, and below their source region. The Swarm constellation offers the unique opportunity to study near-simultaneous measurements of the Current systems at different longitudinal separations. In this study, we present new observations of these Current systems, investigate their seasonal and local time dependence, investigate the use of in situ electron density measurements as a proxy for the magnetic perturbations, and compute the longitudinal self correlation of these Currents. We find that these Currents are strongest during spring and fall, produce nighttime magnetic fields at satellite altitude of up to 5–7 nT during solar maximum, 2–3 nT during solar minimum, and are highly correlated with in situ electron density measurements. We also find these Currents are self-correlated above 70% up to 15° longitude in both hemispheres during the evening.
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relationship between the Ionospheric eastward electric field and the equatorial electrojet
Geophysical Research Letters, 2010Co-Authors: Patrick Alken, Stefan MausAbstract:[1] The equatorial electrojet (EEJ) is a strong Ionospheric Current along the magnetic equator driven by the day side eastward electric field. Current strength is affected by two-stream and gradient-drift instabilities which pose a serious obstacle to quantitative electrodynamic modeling of the equatorial ionosphere. Using highly accurate radar and magnetic field measurements taken over the past decade, we deduce empirical relations between the observed EEJ and the driving eastward electric field. These indicate that the Current strength is largely unaffected by instabilities for eastward fields in the range of −0.4 mV/m to 0.07 mV/m. This is followed by a band of moderate Current reduction from 0.07 mV/m to 1 mV/m, consistent with predictions for the gradient drift instability. At even stronger eastward fields, a further Current reduction is observed, as expected for the two-stream instability. These non-linear empirical relations can be used to correct Ionospheric electric field strengths inferred from space and ground-based magnetic field measurements.