Large-scale Current Systems Research Articles

Dynamics of the magnetosphere during geomagnetic storms on January 21–22, 2005 and December 14–15, 2006

The dynamics of large-scale magnetospheric current systems during geomagnetic storms on January 21–22, 2005 and December 14–15, 2006 is investigated using the A2000 model of the magnetospheric magnetic field. Storm development is controlled by both the interplanetary magnetic field and solar wind pressure that create conditions for injection of plasma into the inner magnetosphere. It is demonstrated that the main role in the development of the January 21–22, 2005 magnetic storm was played by a strong impulse of solar wind pressure, while the December 14–15, 2006 storm was initiated by a changed orientation of the interplanetary magnetic field. As a consequence, the Dst variation of the geomagnetic field during the January 21–22, 2005 storm is determined basically by ring current development. On December 14–15, 2006 it is determined by comparable contributions of the ring current and of the magnetotail currents. The results of modeling are confirmed by data on dynamic properties of the fluxes of three populations of ions with energies 30–80 keV (at low latitudes L < 2, and at latitudes below and above the isotropic precipitation boundary) measured by the solar-synchronous satellites of NOAA (POES 15, POES 16, and POES 17).

Quasi-synoptic transport, budgets and water mass transformation in the Azores–Gibraltar Strait region during summer 2009

We describe the circulation patterns in the Azores–Gibraltar Strait region (North-Eastern Atlantic) during the 2009 CAIBOX cruise on the basis of hydrographic and direct current velocity measurements. This study offers new data for a region where importation of central waters (subpolar and subtropical modes of Eastern North Atlantic Central Water) and exports of Mediterranean Water are strongly related to large-scale dynamics in the North Atlantic Ocean (Azores Current-Mediterranean Water system). The description is backed up quantitatively by the results of a box inverse model, which was used to obtain absolute water mass transport values consistent with thermal wind equations and with conservation of volume, salt and heat. The contributions of water masses were determined in an extended Optimum Multiparameter Analysis from a quasi-synoptic point of view, providing detail in addition to volume, salt and heat transport.The surface–subsurface large-scale current system in the region consists of the Azores Current (13.1±2.5 Sverdrup [Sv], 1Sv=106m3s−1), the Azores Counter-Current (5.2±2.1Sv), the Portugal Current (4.5±1.4Sv) and the Canary Current (7.1±1.1Sv). Broadly speaking, central waters are imported into the CAIBOX region at a rate of 1.6±0.9Sv, and Mediterranean Water is exported at a rate of 1.5±0.4Sv. The downwelling of central waters west of Gibraltar Strait was quantified at 1.1Sv. Not all this volume participates in MW formation, but 0.8Sv of entrained central waters; of which 0.5Sv are from central waters of subpolar origin and 0.3Sv from subtropical central waters. Of the 4.9Sv of subtropical central waters advected by the Azores Current, about 0.7Sv would reach the Gulf of Cadiz region either to take part in central water entrainment or to flow across the Gibraltar Strait as part of the Atlantic inflow to the Mediterranean Sea.

Comets as solar probes

By rapidly evaporating as they fly through the Sun’s hot outer atmosphere, sungrazing comets reveal an otherwise invisible magnetic field that shapes the very beginnings of the solar wind.

Magnetohydrodynamics (MHD) numerical simulations on the interaction of the solar wind with the magnetosphere: A review

The magnetosphere is the outermost layer of the geospace, and the interaction of the solar wind with the magnetosphere is the key element of the space weather cause-and-effect chain process from the Sun to Earth, which is one of the most challenging scientific problems in the geospace weather study. The nonlinearity, multiple component, and time-dependent nature of the geospace make it very difficult to describe the physical process in geospace using traditional analytic analysis approach. Numerical simulations, a new research tool developed in recent decades, have a deep impact on the theory and application of the geospace. MHD simulations started at the end of the 1970s, and the initial study was limited to two-dimensional (2D) cases. Due to the intrinsic three-dimensional (3D) characteristics of the geospace, 3D MHD simulations emerged in the 1980s, in an attempt to model the large-scale structures and fundamental physical processes in the magnetosphere. They started to combine with the space exploration missions in the 1990s and make comparisons with observations. Physics-based space weather forecast models started to be developed in the 21st century. Currently only a few space-power countries such as USA and Japan have developed 3D magnetospheric MHD models. With the rapid advance of space science in China, we have developed a new global MHD model, namely PPMLR-MHD, which has high order spatial accuracy and low numerical dissipation. In this review, we will briefly introduce the global 3D MHD modeling, especially the PPMLR-MHD code, and summarize our recent work based on the PPMLR-MHD model, with an emphasis on the interaction of interplanetary shocks with the magnetosphere, large-scale current systems, reconnection voltage and transpolar potential drop, and Kelvin-Helmholtz (K-H) instability at the magnetopause.

C/NOFS measurements of magnetic perturbations in the low-latitude ionosphere during magnetic storms

The Vector Electric Field Investigation suite on the C/NOFS satellite includes a fluxgate magnetometer to monitor the Earth s magnetic fields in the low-latitude ionosphere. Measurements yield full magnetic vectors every second over the range of +/-45,000 nT with a one-bit resolution of 1.37 nT (16 bit A/D) in each component. The sensor s primary responsibility is to support calculations of both V x B and E x B with greater accuracy than can be obtained using standard magnetic field models. The data also contain information about large-scale current systems that, when analyzed in conjunction with electric field measurements, promise to significantly expand understanding of equatorial electrodynamics. We first compare in situ measurements with the POMME (Potsdam Magnetic Model of the Earth) model to establish in-flight sensor calibrations and to compute magnetic residuals. At low latitudes the residuals are predominately products of the storm time ring current. Since C/NOFS provides a complete coverage of all local times every 97 min, magnetic field data allow studies of the temporal evolution and local time variations of storm time ring current. The analysis demonstrates the feasibility of using instrumented spacecraft in low-inclination orbits to extract a timely proxy for the provisional Dst index and to specify the ring current s evolution.

How ocean ridges affect large‐scale ocean circulation

Driving the flow of heat and nutrients around the ocean is the meridional overturning circulation, a large‐scale current system that in the Atlantic Ocean carries warm surface waters northward and cold, dense water to the south along the bottom of the ocean. An important leg of this journey is the conversion of light warm water into North Atlantic Deep Water (NADW), which occurs along the coasts of Greenland and Labrador, Canada. The temperature and density profile of the ocean is important for climate and ecological systems, so understanding any physical features that may impinge on the properties of NADW, such as ocean ridges, is particularly important. Using experiments in a laboratory water tank, Stewart et al. identified how a ridge could affect the circulation patterns in a model ocean. The authors imposed temperature and heat flux gradients on the surface of the ocean analogue and tracked how differences in ridge size, shape, and location affected the temperature and density stratification. They found that when the ridge is sufficiently shallow, such that the depth of the water between the top of the ridge and the surface of the ocean is less than twice as deep as the ocean thermocline—a near‐surface band of the ocean in which water temperature decreases rapidly with depth—the ridge can significantly affect downstream properties. The authors found that when the deep water is forced up and over a shallow ridge, it tends to mix with the warmer water of the thermocline, increasing the average temperature and density stratification of water downstream from the ridge. (Journal of Geophysical Research‐Oceans, doi:10.1029/2011JC006980, 2011

The dynamics of the Mississippi River plume: Impact of topography, wind and offshore forcing on the fate of plume waters

[1] High-resolution numerical simulations of the northern Gulf of Mexico region using the Hybrid Coordinate Ocean Model (HYCOM) were employed to investigate the dynamical processes controlling the fate of the Mississippi River plume, in particular the conditions that favor cross-marginal transport. The study focuses on the effects of topography, wind-driven and eddy-driven circulation on the offshore removal of plume waters. A realistically forced simulation (nested in a data-assimilative regional Gulf of Mexico HYCOM model) reveals that the offshore removal is a frequent plume pathway. Eastward wind-driven currents promote large freshwater transport toward the shelf break and the DeSoto Canyon, where eddies with diameters ranging from 50 to 130 km interact with the buoyant plume and effectively entrain the riverine waters. Our estimates show that the offshore removal by eddies can be as large as the wind-driven shelf transport. The proximity of eddies to the shelf break is a sufficient condition for offshore removal, and shelf-to-offshore interaction is facilitated by the steep bottom topography near the delta. Strong eddy-plume interactions were observed when the Loop Current System impinged against the shelf break, causing the formation of coherent, narrow low-salinity bands that extended toward the gulf interior. The offshore pathways depend on the position of the eddies near the shelf edge, their life span and the formation of eddy pairs that generate coherent cross-shelf flows. This study elucidates the dynamics that initiate a unique cross-marginal removal mechanism of riverine low-salinity, nutrient-rich waters, allowing their export along connectivity pathways, induced by a large-scale current system.

On the stability of the Atlantic meridional overturning circulation

One of the most important large-scale ocean current systems for Earth's climate is the Atlantic meridional overturning circulation (AMOC). Here we review its stability properties and present new model simulations to study the AMOC's hysteresis response to freshwater perturbations. We employ seven different versions of an Ocean General Circulation Model by using a highly accurate tracer advection scheme, which minimizes the problem of numerical diffusion. We find that a characteristic freshwater hysteresis also exists in the predominantly wind-driven, low-diffusion limit of the AMOC. However, the shape of the hysteresis changes, indicating that a convective instability rather than the advective Stommel feedback plays a dominant role. We show that model errors in the mean climate can make the hysteresis disappear, and we investigate how model innovations over the past two decades, like new parameterizations and mixing schemes, affect the AMOC stability. Finally, we discuss evidence that current climate models systematically overestimate the stability of the AMOC.

Evaluation of Global Ocean Data Assimilation Experiment products on South Florida nested simulations with the Hybrid Coordinate Ocean Model

The South Florida Hybrid Coordinate Ocean Model (SoFLA-HYCOM) encompasses a variety of coastal regions (the broad Southwest Florida shelf, the narrow Atlantic Keys shelf, the shallow Florida Bay, and Biscayne Bay) and deep regions (the Straits of Florida), including Marine Protected Areas (the Florida Keys Marine Sanctuary and the Dry Tortugas Ecological Reserve). The presence of the strong Loop Current/Florida Current system and associated eddies connects the local and basin-wide dynamics. A multi-nested approach has been developed to ensure resolution of coastal-scale processes and proper interaction with the large scale flows. The simulations are free running and effects of data assimilation are introduced through boundary conditions derived from Global Ocean Data Assimilation Experiment products. The study evaluates the effects of boundary conditions on the successful hindcasting of circulation patterns by a nested model, applied on a dynamically and topographically complex shelf area. Independent (not assimilated) observations are employed for a quantitative validation of the numerical results. The discussion of the prevailing dynamics that are revealed in both modeled and observed patterns suggests the importance of topography resolution and local forcing on the inner shelf to middle shelf areas, while large scale processes are found to dominate the outer shelf flows. The results indicate that the successful hindcasting of circulation patterns in a coastal area that is characterized by complex topography and proximity to a large scale current system requires a dynamical downscaling approach, with simulations that are nested in a hierarchy of data assimilative outer models.

Horizontal amplitude and phase structure of low-latitude Pc 3 pulsations around the dawn terminator

[1] The horizontal spatial structure of Pc 3 pulsations observed at low geomagnetic latitude (22–46°) around dawn is studied statistically using data acquired by the Circum-pan Pacific Magnetometer Network (CPMN). It is found that while the phase of the H component of low-latitude Pc 3 pulsations remains largely unchanged with the passing of dawn, the D component undergoes a phase shift of ca. 180°. This phase variation across dawn is related to the abrupt change in the major axis orientation of polarization ellipses observed in previous studies. Both the H and D components have higher amplitude after dawn than before dawn. This horizontal amplitude and phase structure is well explained by the response of a nonuniform ionosphere around dawn to incident Alfven waves, where the secondary electric field caused by charge accumulation at the dawn terminator plays an important role in deformation of the current system. Enhancement of the D/H ratio is also observed just after dawn at very low latitudes (22°). As Alfven waves are not excited efficiently at very low geomagnetic latitudes, including the magnetic equator, the observed horizontal structure in such regions may be due to a large-scale current system originating at higher latitudes.

Relative importance of ring and tail currents to Dst under extremely disturbed conditions

Relative ring current (RC) and tail current (TC) contributions to Dst were investigated on the basis of the statistical study of 70 magnetic storms of different intensities. Special attention was paid to the extremely disturbed conditions during magnetic storms in October–November 2003. Variations of the magnetic field produced by magnetospheric currents on the Earth's surface were calculated using paraboloid model of the magnetosphere A2000 [Alexeev, I.I., Belenkaya, E.S., Kalegaev, V.V., Feldstein, Y.I., Grafe, A., 1996. Journal of Geophysical Research 101,7737; Alexeev, I.I., Kalegaev, V.V., Belenkaya, E.S., Bobrovnikov, S.Yu., Feldstein, Ya.I., Gromova, L.I., 2001. Journal of Geophysical Research 106, 25683], taking into account the effect of terrestrial induced currents. For each magnetic storm we calculated Dst and contributions produced by large-scale magnetospheric current systems. The relative RC and TC contributions for each event at the storm maximum were examined in relationship to the peak pressure-corrected Dst value. Analysis of Dst sources confirms the conclusions of Kalegaev and Ganushkina [2005. In: Pulkkinen, T., Tsyganenko, N.A., Friedel, R.H.W. (Eds.), Physics and Modeling of the Inner Magnetosphere, AGU Geophysical Monograph 155. AGU, Washington, DC, p. 293] and Kalegaev and Makarenkov [2006. Geomagnetism and Aeronomy 46, 570] about saturation of the TC effect under extremely disturbed conditions. The RC becomes the dominant Dst source during severe magnetic storms, but during moderate storms its contribution to Dst is comparable with TC's contribution. The RC injection amplitude increases with the growth of magnetospheric disturbance level.

Analysis of high-latitude current systems during the BEAR experiment based on the IZMEM model

Based on the model of large-scale high-latitude current systems developed at IZMIRAN (IZMEM model), it has been indicated that auroral electrojets and current systems concentrated in the polar cap were the generators of long-period geomagnetic variations during the BEAR experiment on the electromagnetic field registration at the Scandinavian test site on June 1–July 15, 1998. Precisely circumpolar current systems, prevailing in the high-latitude ionosphere during the periods of a quiet magnetospheric state, which is characterized by the presence of the northern vertical (B z >0) component of the IMF vector in the solar wind, are responsible for the magnetotelluric fields.

Coronal mass ejections: Some possibilities for prediction

The onset stage of coronal mass ejections (CMEs) is difficult to observe and is poorly studied. In spite of their practical importance, methods for CME predictions with sufficient lead times are only in the nascent stages of development. The most probable CME mechanism is a catastrophic loss of equilibrium of a large-scale current system in the corona (a flux rope). A twisted magnetic rope is maintained by the tension of field lines of photospheric sources until parameters of the system reach critical values and the equilibrium is lost. Unfortunately, there is low-density plasma (coronal cavity) in most of the rope volume; thus, it is difficult to observe a rope. However, the lower parts of the helical field lines of a rope are fine traps for the dense cold plasma of prominences. Thus, prominences are the best tracers of flux ropes in the corona. The maximal height up to which the rope is in stable equilibrium can be found by analyzing the distribution of the magnetic field generated by photospheric sources in the corona. Comparing this critical height with the actually observed prominence height, one can estimate the probability of the loss of equilibrium by a magnetic rope with a following eruption of prominences and coronal mass ejections.

Estimating ocean transports with dynamic height moorings: An application in the Atlantic Deep Western Boundary Current at 26°N

Efficient monitoring of large-scale current systems for climate research requires the development of new techniques to estimate ocean transports. Here, a methodology for continuous estimation of dynamic height profiles and geostrophic currents from moored temperature sensors is presented. The technique is applied to moorings deployed in the Atlantic Deep Western Boundary Current at 26.5°N, off Abaco, the Bahamas (WOCE ACM-1 array). Relative geostrophic currents are referenced using bottom pressure sensors and available shipboard direct velocity (lowered-ADCP) sections over the period of the deployment, to obtain a time series of absolute volume transport. Comparison with direct velocity measurements from a complete array of current meters shows good agreement for the mean transport and its variablity on time scales longer than 10 days, but larger variability in the current meter derived transport at time scales shorter than 10 days. A rigorous error analysis assesses the contributions of various error sources in the geostrophic as well as direct transport estimates. Low-frequency drift of the bottom pressure sensors is found to be the largest error source in the geostrophic transport estimates and recommendations for improvement of the technique and related measurement technologies are made.

Pseudo-field line resonances in ground Pc5 pulsation events

Abstract. In this work we study four representative cases of Pc5 ground pulsation events with discrete and remarkably stable frequencies extended at least in a high-latitude range of ~20°; a feature that erroneously gives the impression for an oscillation mode with "one resonant field line". Additionally, the presented events show characteristic changes in polarization sense, for a meridian chain of stations from the IMAGE array, and maximize their amplitude at or close to the supposed resonant magnetic field shell, much like the typical FLR. Nevertheless, they are not authentic FLRs, but pseudo-FLRs, as they are called. These structures are produced by repetitive and tilted twin-vortex structures caused by magnetopause surface waves, which are probably imposed by solar wind pressure waves. The latter is confirmed with in-situ measurements obtained by the Cluster satellites, as well as the Geotail, Wind, ACE, and LANL 1994-084 satellites. This research effort is largely based on two recent works: first, Sarafopoulos (2004a) has observationally established that a solar wind pressure pulse (stepwise pressure variation) produces a twin-vortex (single vortex) current system over the ionosphere; second, Sarafopoulos (2004b) has studied ground events with characteristic dispersive latitude-dependent structures and showed that these are associated with twin-vortex ionosphere current systems. In this work, we show that each pseudo-FLR event is associated with successive and tilted large-scale twin-vortex current systems corresponding to a magnetopause surface wave with wavelength 10-20RE. We infer that between an authentic FLR, which is a spatially localized structure with an extent 0.5RE in the magnetospheric equatorial plane, and the magnetopause surface wavelength, there is a scale factor of 20-40. A chief observational finding, in this work, is that there are Pc5 ground pulsation events showing two gradual and latitude dependent phase-shifts of 180°, at the same time.

Relation between the ring current and the tail current during magnetic storms

Abstract. We study the dynamics of the magnetospheric large-scale current systems during storms by using three different magnetospheric magnetic field models: the paraboloid, event-oriented, and Tsyganenko T01 models. We have modelled two storm events, one moderate storm on 25-26 June 1998, when Dst reached -120nT and one intense storm on 21-23 October 1999, when Dst dropped to -250nT. We compare the observed magnetic field from GOES 8, GOES 9, and GOES 10, Polar and Geotail satellites with the magnetic field given by the three models to estimate their reliability. All models demonstrated quite good agreement with observations. Since it is difficult to measure exactly the relative contributions from different current systems to the Dst index, we compute the contributions from ring, tail and magnetopause currents given by the three magnetic field models. We discuss the dependence of the obtained contributions to the Dst index in relation to the methods used in constructing the models. All models show a significant tail current contribution to the Dst index, comparable to the ring current contribution during moderate storms. The ring current becomes the major Dst source during intense storms.

Energetics of the magnetosphere during the magnetic storm

The most detailed studies of the energetic budget of the magnetosphere during the magnetic storms were done on the basis of the paraboloid model using the November 23–27, 1986 and May 6–8, 1988 magnetic storms. Calculations have shown that the energy injected in the course of the magnetic storms into the inner magnetosphere and ionosphere of both hemispheres amounts to ∼ 0.9 –2.2% of the solar wind kinetic energy on the magnetospheric cross section. The total energy injected into the magnetosphere from a distance of 60 R E in the tail down to the ionosphere is ∼4.0–7.5% of the solar wind kinetic energy during the main phase of the two magnetic storms. The injected energy into the tail E TL is 1.03–1.18 of the total energy input into the inner magnetosphere and ionosphere of both hemispheres at the main phase of the two storms. The decay parameter for the energy stored in the magnetospheric tail is ∼5 h . The total energy dissipated in the ionosphere of both hemispheres, in the inner magnetosphere and in the tail during the two storms, is 1.85×10 17 and 3.24×10 17 J , respectively. The total energy input into the magnetosphere is calculated to be 1.77×10 17 and 3.16×10 17 J . The discrepancies of 0.08×10 17 and 0.10×10 17 J amount to 4.3% and 3.1% of the total energy input and characterize the accuracy of the magnetospheric energy budget calculation. In the magnetotail the balance between the injected and dissipated energy of ∼1.09×10 17 J for one storm and ∼1.7×10 17 J for the other is preserved as well. We conclude that one-half of the energy which is injected into the magnetosphere from the solar wind during the storms enters the magnetotail and dissipates there. The coupling parameter ε PA is widely considered to be a measure of the energy dissipation in the inner magnetosphere. The dissipation energy in the inner magnetosphere U T= U J+ U A+ U DR is defined as the sum of the contributions of the Joule dissipation U J, the energy of auroral particle precipitation U A, and the energy injection into the ring current U DR . In this paper, we find that ε PA in the two storms investigated is substantially different from U T. The energy injected into the ring current region at the main phase of the storm amounts to ∼10 16 J . It is nearly 3 or 4 times smaller than the energy input into the magnetosphere via the field-aligned currents or the energy dissipated in the ionosphere by Joule dissipation. The energy injected into the magnetosphere is transferred mainly into processes different from the ring current generation. During the development of intensive auroral electrojets, the energy dissipation in the magnetotail and the increase in the energy of the tail current system occur simultaneously. The energy dissipation in the inner magnetosphere and ionosphere U S occurs not only at the expense of energy previously stored in the magnetotail, but rather at the expense of energy that is injected into the near-Earth tail. This energy transfer from the solar wind into the magnetotail and the energy dissipation in the ionosphere increased simultaneously. Thus, during disturbances in the magnetosphere, simultaneously loading–unloading and directly driven processes occur. Loading- and unloading processes manifest themselves both in the storage of the solar wind energy in the magnetotail and the ring current, and subsequent dissipation. The directly driven processes become manifest in the direct dissipation of the energy which enters into the ionosphere through large-scale field-aligned current systems.

The Fields of Magnetospheric Current Systems on the Earth's Surface in an Interval of the Magnetic Storm Studied under the International Program on Solar-Terrestrial Physics

Using the magnetic storm in January 1997 as an example, we examined the possibilities to employ the magnetospheric field T96 [1, 2] and the dynamic paraboloid model PM of the magnetosphere [3] for modeling the Dst variation. We have revealed the necessity to refine the results of normalizing the free parameters of the model T96 according to the solar wind parameters. The contributions to the Dst variation of magnetic fields of basic large-scale magnetospheric current systems (the field DCF on the magnetopause, the field DR of the ring current, and the field DT in the magnetotail) are estimated for different phases of the storm from model calculations. Possible causes of a discrepancy between the results of modeling Dst using the T96 and PM models are discussed. Special emphasis is made on the ratios of contributions into the Dst variation of the fields of the magnetotail and the ring current in the main phase of magnetic storms and on the contributions to Dst of the fields of various current systems at the recovery phase.

Warm and cold water routes of an O.G.C.M. thermohaline conveyor belt

A global general circulation model analyzed with a Lagrangian methodology is used to describe and quantify the paths, transports, and characteristics of the “warm” waters forming the upper branch of the conveyor belt in the North Atlantic Ocean. The total transport for this branch turns out to be 17.8 Sv in the North Atlantic at 20°N: 11.8 Sv are composed of waters coming from the two classical origins, the Drake Passage and the Indonesian Throughflow, which contribute with 6.5 and 5.3 Sv respectively. The remaining 6 Sv find their origins partly in the passage between Antarctica and the Australian Continent (with 3.1 Sv) and partly in the Indo‐Atlantic sector itself (i.e., with 2.9 Sv). The geographical structure of the different routes emphasizes the role of the Southern Ocean and large‐scale current systems in water mass transformation and distribution.

A Test to Confirm the Source of Energy for Solar Flares

AbstractA test of the hypothesis that flares derive their energy from large scale current systems inferred from active region vector magnetograms is proposed. The test involves a statistical comparison of the flarerelated change in coronal magnetic energy (based on the magnetohydrodynamic virial theorem) and an independent measure of the energy of the flare. A simulation suggests that — assuming the hypothesis is correct—the test requires around 50 flares with energy greater than 5×1023 J to return a significant result. Existing archives of vector magnetograms should provide sufficient data for such a study.