The Experts below are selected from a list of 321 Experts worldwide ranked by ideXlab platform
B J Lynch - One of the best experts on this subject based on the ideXlab platform.
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Magnetic Clouds: Solar Cycle Dependence, Sources, and GeoMagnetic Impacts
Solar Physics, 2018Co-Authors: Y. Li, J G Luhmann, B J LynchAbstract:Magnetic Clouds (MCs) are transient Magnetic structures giving the strongest southward Magnetic field (Bz south) in the solar wind. The sheath regions of MCs may also carry a southward Magnetic field. The southward Magnetic field is responsible for space-weather disturbances. We report a comprehensive analysis of MCs and Bz components in their sheath regions for 1995 to 2017. 85% of 303 MCs contain a south Bz up to 50 nT. Sheath Bz during the 23 years may reach as high as 40 nT. MCs of the strongest Magnetic magnitude and Bz south occur in the declining phase of the solar cycle. Bipolar MCs depend on the solar cycle in their polarity, but not in the occurrence frequency. Unipolar MCs show solar-cycle dependence in their occurrence frequency, but not in their polarity. MCs with the highest speeds, the largest total- B $B$ magnitudes, and sheath Bz south originate from source regions closer to the solar disk center. About 80% of large Dst storms are caused by MC events. Combinations of a south Bz in the sheath and south-first MCs in close succession have caused the largest storms. The solar-cycle dependence of bipolar MCs is extended to 2017 and now spans 42 years. We find that the bipolar MC Bz polarity solar-cycle dependence is given by MCs that originated from quiescent filaments in decayed active regions and a group of weak MCs of unclear sources, while the polarity of bipolar MCs with active-region flares always has a mixed Bz polarity without solar-cycle dependence and is therefore the least predictable for Bz forecasting.
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Magnetic Clouds solar cycle dependence sources and geoMagnetic impacts
arXiv: Solar and Stellar Astrophysics, 2018Co-Authors: J G Luhmann, B J LynchAbstract:Magnetic Clouds (MCs) are transient Magnetic structures giving the strongest southward Magnetic field (Bz south) in the solar wind. The sheath regions of MCs may also carry southward Magnetic field. Southward Magnetic field is responsible for causing space-weather disturbances. We report a comprehensive analysis of MCs and Bz components in their sheath regions during 1995 to 2017. Eighty-five percent of 303 MCs contain a south Bz up to 50 nT. Sheath Bz during the 23 years may reach as high as 40 nT. The MCs of strongest Magnetic magnitude and Bz south occur in the declining phase of the solar cycle. The bipolar MCs have solar-cycle dependence in their polarity, but not in the occurrence frequency. Unipolar MCs show solar-cycle dependence in their occurrence frequency but not in their polarity. MCs with the highest speeds, largest total B magnitudes and sheath Bz south are from source regions closer to the solar disk center. About 80% of large Dst storms are caused by MC events. The combinations of south Bz in the sheath and the south-first MCs in close succession have given the largest storms. The solar-cycle dependence of bipolar MCs is extended to 2017, spanning 42 years. We find that the bipolar MC Bz polarity solar-cycle dependence is given by MCs originated from quiescent filaments in decayed active regions and a group of weak MCs of unclear sources, while the polarity of bipolar MCs with active-region flares always has mixed Bz polarity without solar-cycle dependence and is therefore the least predictable for Bz forecasting.
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multispacecraft observations of Magnetic Clouds and their solar origins between 19 and 23 may 2007
Solar Physics, 2009Co-Authors: B J Lynch, E. K. J. Kilpua, P C Liewer, C J Farrugia, J G Luhmann, Christian Mostl, Ying Liu, C T RussellAbstract:We analyze a series of complex interplanetary events and their solar origins that occurred between 19 and 23 May 2007 using observations by the STEREO and Wind satellites. The analyses demonstrate the new opportunities offered by the STEREO multispacecraft configuration for diagnosing the structure of in situ events and relating them to their solar sources. The investigated period was characterized by two high-speed solar wind streams and Magnetic Clouds observed in the vicinity of the sector boundary. The observing satellites were separated by a longitudinal distance comparable to the typical radial extent of Magnetic Clouds at 1 AU (fraction of an AU), and, indeed, clear differences were evident in the records from these spacecraft. Two partial-halo coronal mass ejections (CMEs) were launched from the same active region less than a day apart, the first on 19 May and the second on 20 May 2007. The clear signatures of the Magnetic cloud associated with the first CME were observed by STEREO B and Wind while only STEREO A recorded clear signatures of the Magnetic cloud associated with the latter CME. Both Magnetic Clouds appeared to have interacted strongly with the ambient solar wind and the data showed evidence that they were a part of the coronal streamer belt. Wind and STEREO B also recorded a shocklike disturbance propagating inside a Magnetic cloud that compressed the field and plasma at the cloud’s trailing portion. The results illustrate how distant multisatellite observations can reveal the complex structure of the extension of the coronal streamer into interplanetary space even during the solar activity minimum.
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solar cycle dependent helicity transport by Magnetic Clouds
Journal of Geophysical Research, 2005Co-Authors: B J Lynch, J R Gruesbeck, T H Zurbuchen, S K AntiochosAbstract:[1] Magnetic Clouds observed with the Wind and ACE spacecraft are fit with the static, linear force-free cylinder model to obtain estimates of the chirality, fluxes, and Magnetic helicity of each event. The fastest Magnetic Clouds (MCs) are shown to carry the most flux and helicity. We calculate the net cumulative helicity which measures the difference in right- and left-handed helicity contained in MCs over time. The net cumulative helicity does not average to zero; rather, a strong left-handed helicity bias develops over the solar cycle, dominated by the largest events of cycle 23: Bastille Day 2000 and 28 October 2003. The majority of MCs (“slow” events, 〈Vr〉 < 500 km/s) have a net cumulative helicity profile that appears to be modulated by the solar activity cycle. This is far less evident for “fast” MC events (〈Vr〉 ≥ 500 km/s), which were disproportionately left-handed over our data set. A brief discussion about the various solar sources of CME helicity and their implication for dynamo processes is included.
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Internal structure of Magnetic Clouds: Plasma and composition
Journal of Geophysical Research, 2003Co-Authors: B J Lynch, Thomas H. Zurbuchen, Lennard A. Fisk, Spiro K. AntiochosAbstract:[1] A comprehensive analysis of Magnetic Clouds observed by the Advanced Composition Explorer (ACE) spacecraft from February 1998 to July 2001 is presented. The Magnetic field data from the MAG instrument is fit with the cylindrically symmetric, linear force-free model and the fit parameter distributions are examined. This Magnetic field model enables us to map plasma data from the SWEPAM and SWICS instruments to a position within the model cylinder. A superposed epoch analysis of all our Magnetic cloud events is used to construct diameter cuts through an “average” cloud profile in any desired plasma, elemental composition, or charge state quantity. These diameter cuts are found to have nontrivial structure and there appears to be significant composition and structural differences between Clouds of different speeds. The slow Magnetic Clouds (〈Vrad〉 < 500 km/s) have an almost constant proton density profile whereas the fast Magnetic cloud (〈Vrad〉 ≥ 500 km/s) profile is depleted throughout with symmetric dips and a local maximum at the cloud center. The fast Magnetic Clouds have a slightly higher Nα/Np ratio than the slow Clouds. Both the fast and slow events have enhanced oxygen and iron charge states compared to the slow solar wind. The fast events have a slightly increased O7+/O6+ average profile and a much stronger Fe≥16+/Fetotal profile than the slow events. We briefly discuss the implications for physical conditions at the Sun, the role these coronal mass ejections (CMEs) may play in transporting Magnetic flux, and the application of our structure results to the current flux rope CME modeling effort.
J G Luhmann - One of the best experts on this subject based on the ideXlab platform.
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Magnetic Clouds: Solar Cycle Dependence, Sources, and GeoMagnetic Impacts
Solar Physics, 2018Co-Authors: Y. Li, J G Luhmann, B J LynchAbstract:Magnetic Clouds (MCs) are transient Magnetic structures giving the strongest southward Magnetic field (Bz south) in the solar wind. The sheath regions of MCs may also carry a southward Magnetic field. The southward Magnetic field is responsible for space-weather disturbances. We report a comprehensive analysis of MCs and Bz components in their sheath regions for 1995 to 2017. 85% of 303 MCs contain a south Bz up to 50 nT. Sheath Bz during the 23 years may reach as high as 40 nT. MCs of the strongest Magnetic magnitude and Bz south occur in the declining phase of the solar cycle. Bipolar MCs depend on the solar cycle in their polarity, but not in the occurrence frequency. Unipolar MCs show solar-cycle dependence in their occurrence frequency, but not in their polarity. MCs with the highest speeds, the largest total- B $B$ magnitudes, and sheath Bz south originate from source regions closer to the solar disk center. About 80% of large Dst storms are caused by MC events. Combinations of a south Bz in the sheath and south-first MCs in close succession have caused the largest storms. The solar-cycle dependence of bipolar MCs is extended to 2017 and now spans 42 years. We find that the bipolar MC Bz polarity solar-cycle dependence is given by MCs that originated from quiescent filaments in decayed active regions and a group of weak MCs of unclear sources, while the polarity of bipolar MCs with active-region flares always has a mixed Bz polarity without solar-cycle dependence and is therefore the least predictable for Bz forecasting.
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Magnetic Clouds solar cycle dependence sources and geoMagnetic impacts
arXiv: Solar and Stellar Astrophysics, 2018Co-Authors: J G Luhmann, B J LynchAbstract:Magnetic Clouds (MCs) are transient Magnetic structures giving the strongest southward Magnetic field (Bz south) in the solar wind. The sheath regions of MCs may also carry southward Magnetic field. Southward Magnetic field is responsible for causing space-weather disturbances. We report a comprehensive analysis of MCs and Bz components in their sheath regions during 1995 to 2017. Eighty-five percent of 303 MCs contain a south Bz up to 50 nT. Sheath Bz during the 23 years may reach as high as 40 nT. The MCs of strongest Magnetic magnitude and Bz south occur in the declining phase of the solar cycle. The bipolar MCs have solar-cycle dependence in their polarity, but not in the occurrence frequency. Unipolar MCs show solar-cycle dependence in their occurrence frequency but not in their polarity. MCs with the highest speeds, largest total B magnitudes and sheath Bz south are from source regions closer to the solar disk center. About 80% of large Dst storms are caused by MC events. The combinations of south Bz in the sheath and the south-first MCs in close succession have given the largest storms. The solar-cycle dependence of bipolar MCs is extended to 2017, spanning 42 years. We find that the bipolar MC Bz polarity solar-cycle dependence is given by MCs originated from quiescent filaments in decayed active regions and a group of weak MCs of unclear sources, while the polarity of bipolar MCs with active-region flares always has mixed Bz polarity without solar-cycle dependence and is therefore the least predictable for Bz forecasting.
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Solar cycle evolution of the structure of Magnetic Clouds in the inner heliosphere
Geophysical Research Letters, 1998Co-Authors: T. Mulligan, Christopher T. Russell, J G LuhmannAbstract:Nearly ten years of continuous Magnetic field observations by the Pioneer Venus spacecraft allows us to study the correlation between the structure of Magnetic Clouds in the inner heliosphere and the phase of the solar cycle. Fifty-six Magnetic Clouds have been identified in the PVO data at .7AU during 1979–1988. As this period spans nearly two solar maxima and one solar minimum we can study the evolution of the structure of these Magnetic Clouds through varying solar activity and under various orientations of the coronal streamer belt. Until shortly after the 1979 solar maximum the majority of Clouds had an initially southward Magnetic field which turned northward as the cloud was traversed, while in the period leading up to the 1988 solar maximum the majority had a northward field that turned southward. In the declining phase of solar activity Magnetic Clouds continued to occur, but only a minority can be classified as having south-to-north and north-to-south rotations. The majority of these Clouds occurred with the field remaining entirely north or south relative to the solar equator. These results confirm observations using Helios and ISEE data indicating that the structure of Magnetic Clouds varies in response to changes in the Magnetic structure of the source region. By interpreting these observations to imply that the leading Magnetic field in Magnetic Clouds is controlled by the polarity of the sun's global field and that the inclination of the coronal streamer belt controls the axis of symmetry of the Clouds, we can predict preferred Magnetic cloud structure and orientation during varying phases of the solar cycle. The helicity of the observations does not seem to be ordered by the solar cycle.
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GeoMagnetic response to Magnetic Clouds of different polarity
Geophysical Research Letters, 1998Co-Authors: F. R. Fenrich, J G LuhmannAbstract:The polarity of a Magnetic cloud refers to its changing Magnetic field direction. It is classified as S-N polarity when the Magnetic field rotates from southward to northward and N-S polarity when the field is initially northward and rotates southward. A study of 29 Magnetic cloud events has found that 40–45% of Magnetic Clouds, independent of polarity, are followed by a fast solar wind stream which compresses the tail end of the cloud. The compression results in an increase in the solar wind plasma density and in 64% of the cases an increase in the Magnetic field strength towards the latter part of the cloud. Such tail end compression can have a significant effect upon geoMagnetic storm intensity if the Magnetic cloud is of N-S polarity. This is because only in the N-S polarity case does the compression coincide with the southward IMF portion of the cloud. To test the “geoeffectiveness” of N-S versus S-N Magnetic Clouds three selected Magnetic cloud events, two of S-N polarity and one of N-S polarity, are investigated in terms of their geoMagnetic response through measured and estimated Dst values. It is found that there is an increased geoeffectiveness of N-S polarity Clouds due to both an increased solar wind dynamic pressure and a compressed southward field associated with a following fast solar wind stream.
Vladimir A. Osherovich - One of the best experts on this subject based on the ideXlab platform.
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Solar Wind Quasi-invariant for Slow and Fast Magnetic Clouds
Solar Physics, 2011Co-Authors: Alla Webb, Joseph Fainberg, Vladimir A. OsherovichAbstract:The solar wind quasi-invariant (QI) has been defined by Osherovich, Fainberg, and Stone (Geophys. Res. Lett.26, 2597, 1999) as the ratio of Magnetic energy density and the energy density of the solar wind flow. In the regular solar wind QI is a rather small number, since the energy of the flow is almost two orders of magnitude greater than the Magnetic energy. However, in Magnetic Clouds, QI is the order of unity (less than 1) and thus Magnetic Clouds can be viewed as a great anomaly in comparison with its value in the background solar wind. We study the duration, extent, and amplitude of this anomaly for two groups of isolated Magnetic Clouds: slow Clouds (360
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mhd of gas with polytropic index below unity and classification of Magnetic Clouds
The solar wind nine conference, 2008Co-Authors: Vladimir A. Osherovich, Joseph Fainberg, R. G. Stone, Adolfo F. Viñas, R. J. Fitzenreiter, Charles J. FarrugiaAbstract:The self-similar Magnetic cloud model of Osherovich, Farrugia and Burlaga (1,2) is based on the exact class of MHD solutions for a Magnetic flux rope with polytropic index γ below unity. The problem in this model is reduced to a second order dynamic equation for the nonlinear oscillator. The corresponding effective potential for the case γ>1 has only an oscillating mode and the case for γ<1 may have two modes: oscillatory and expansion. This model suggests three classes of Magnetic Clouds with different evolutionary patterns. For the first class (those which cannot overcome the threshold), the profile of the Magnetic cloud is rather flat at 1 AU, and the velocity of expansion is small or even shows signs of contraction. The second class (those which have energy sufficient to overcome the threshold) is well described by a free expanding flux tube. The third class has a potential without a well suggesting expansion for all energies. The non-Maxwellian electron distribution function in a cloud explains the o...
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Multi-tube model for interplanetary Magnetic Clouds
Geophysical Research Letters, 1999Co-Authors: Vladimir A. Osherovich, Joseph Fainberg, R. G. StoneAbstract:Measurements of the polytropic index γ inside a Magnetic cloud showed that there are two non-equal tubes inside the cloud [Fainberg et al., 1996; Osherovich et al., 1997]. For both tubes, γ < 1, but each tube has its own polytrope. We test equilibrium solutions which are a superposition of solutions with cylindrical and helical symmetry [Krat and Osherovich, 1978] as a new paradigm for a multi-tube model. Comparison of Magnetic and gas pressure profiles for these bounded MHD states with observations suggests that complex Magnetic Clouds can be viewed as multiple helices embedded in a cylindrically symmetric flux rope.
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Self-similar evolution of interplanetary Magnetic Clouds and Ulysses measurements of the polytropic index inside the cloud
1997Co-Authors: Vladimir A. Osherovich, Joseph Fainberg, R. G. Stone, Robert J. Macdowall, Daniel B. BerdichevskyAbstract:A self similar model for the expanding flux rope is developed for a magnetohydrodynamic model of interplanetary Magnetic Clouds. It is suggested that the dependence of the maximum Magnetic field on the distance from the sun and the polytropic index gamma has the form B = r exp (-1/gamma), and that the ratio of the electron temperature to the proton temperature increases with distance from the sun. It is deduced that ion acoustic waves should be observed in the cloud. Both predictions were confirmed by Ulysses observations of a 1993 Magnetic cloud. Measurements of gamma inside the cloud demonstrate sensitivity to the internal topology of the Magnetic field in the cloud.
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Magnetic flux rope versus the spheromak as models for interplanetary Magnetic Clouds
Journal of Geophysical Research, 1995Co-Authors: Charles J. Farrugia, Vladimir A. Osherovich, L. F. BurlagaAbstract:Magnetic Clouds form a subset of interplanetary ejecta with well-defined Magnetic and thermodynamic properties. Observationally, it is well established that Magnetic Clouds expand as they propagate antisunward. The aim of this paper is to compare and contrast two models which have been proposed for the global Magnetic field line topology of Magnetic Clouds: a Magnetic flux tube geometry, on the one hand, and a spheromak geometry (including possible higher multiples), on the other. Traditionally, the Magnetic structure of Magnetic Clouds has been modeled by force-free configurations. In a first step, we therefore analyze the ability of static force-free models to account for the asymmetries observed in the Magnetic field profiles of Magnetic Clouds. For a cylindrical flux tube the Magnetic field remains symmetric about closest approach to the Magnetic axis on all spacecraft orbits intersecting it, whereas in a spheromak geometry one can have asymmetries in the Magnetic field signatures along some spacecraft trajectories. The duration of typical Magnetic cloud encounters at 1 AU (1 to 2 days) is comparable to their travel time from the Sun to 1 AU and thus Magnetic Clouds should be treated as strongly nonstationary objects. In a second step, therefore, we abandon the static approach and model Magnetic Clouds as self-similarly evolving MHD configurations. In our theory, the interaction of the expanding Magnetic cloud with the ambient plasma is taken into account by a drag force proportional to the density and the velocity of expansion. Solving rigorously the full set of MHD equations, we demonstrate that the asymmetry in the Magnetic signature may arise solely as a result of expansion. Using asymptotic solutions of the MHD equations, we least squares fit both theoretical models to interplanetary data. We find that while the central part of the Magnetic cloud is adequately described by both models, the “edges” of the cloud data are modeled better by the Magnetic flux tube. Further comparisons of the two models necessarily involve thermodynamic properties, since real Magnetic configurations are never exactly force-free and gas pressure plays an essential role. We consider a poly tropic gas. Our theoretical analysis shows that the self-similar expansion of a Magnetic flux tube requires the poly tropic index γ to be less than unity. For the spheromak, however, self-similar, radially expanding solutions are known only for γ equal to 4/3. This difference, therefore, yields a good way of distinguishing between the two geometries. It has been shown recently (Osherovich et al., 1993a) that the polytropic relationship is applicable to Magnetic Clouds and that the corresponding polytropic index is ∼0.5. This observational result is consistent with the self-similar model of the Magnetic flux rope but is in conflict with the self-similar spheromak model.
Ronald P. Lepping - One of the best experts on this subject based on the ideXlab platform.
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Evidence in Magnetic Clouds for systematic open flux transport on the Sun
Journal of Geophysical Research: Space Physics, 2008Co-Authors: N. U. Crooker, Stephen W. Kahler, J. T. Gosling, Ronald P. LeppingAbstract:[1] Most Magnetic Clouds encountered by spacecraft at 1 AU display a mix of unidirectional suprathermal electrons signaling open field lines and counterstreaming electrons signaling loops connected to the Sun at both ends. Assuming the open fields were originally loops that underwent interchange reconnection with open fields at the Sun, we determine the sense of connectedness of the open fields found in 72 of 97 Magnetic Clouds identified by the Wind spacecraft in order to obtain information on the location and sense of the reconnection and resulting flux transport at the Sun. The true polarity of the open fields in each Magnetic cloud was determined from the direction of the suprathermal electron flow relative to the Magnetic field direction. Results indicate that the polarity of all open fields within a given Magnetic cloud is the same 89% of the time, implying that interchange reconnection at the Sun most often occurs in only one leg of a flux rope loop, thus transporting open flux in a single direction, from a coronal hole near that leg to the foot point of the opposite leg. This pattern is consistent with the view that interchange reconnection in coronal mass ejections systematically transports an amount of open flux sufficient to reverse the polarity of the heliospheric field through the course of the solar cycle. Using the same electron data, we also find that the fields encountered in Magnetic Clouds are only a third as likely to be locally inverted as not. While one might expect inversions to be equally as common as not in flux rope coils, consideration of the geometry of spacecraft trajectories relative to the modeled Magnetic cloud axes leads us to conclude that the result is reasonable.
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Slow Magnetic Clouds
2001Co-Authors: Bruce T. Tsurutani, Walter D. Gonzalez, X. Y. Zhou, Ronald P. Lepping, Volker BothmerAbstract:Slow Magnetic Clouds have been analyzed to determine their characteristics and geoeffectiveness.
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Counterstreaming electrons in Magnetic Clouds
Journal of Geophysical Research: Space Physics, 2000Co-Authors: S. Shodhan, Stephen W. Kahler, R. J. Fitzenreiter, Ronald P. Lepping, N. U. Crooker, Davin Larson, George Siscoe, J. T. GoslingAbstract:Two widely used signatures of interplanetary coronal mass ejections are counterstreaming suprathermal electrons, implying Magnetic structures connected to the Sun at both ends, and Magnetic Clouds, characterized by large-scale field rotations, low temperature, and high field strength. In order to determine to what extent these signatures coincide, electron heat flux data were examined for 14 Magnetic Clouds detected by ISEE 3 and IMP 8 near solar maximum and 34 Clouds detected by Wind near solar minimum. The percentage of time during each cloud passage that counterstreaming electrons were detected varied widely, from 6 Clouds with essentially no counterstreaming to 8 Clouds with nearly 100% counterstreaming. All of the former but less than half of the latter occurred near solar minimum, suggesting a possible solar cycle dependence on the degree of Magnetic openness. The counterstreaming intervals were distributed randomly throughout the Clouds, with a median length of 2.5 hours. A plot of counterstreaming percentages against cloud diameter for 33 Clouds modeled as cylindrical flux ropes indicates a linear dependence of the percentage of closed flux on cloud size, with the largest Clouds being the most closed. Overall the results are consistent with the view that although Magnetic field lines within a Magnetic cloud can form a large-scale, coherent structure, reconnection in remote regions of the structure, presumably near the Sun, sporadically alters its topology from closed to open until the cloud assimilates into the ambient solar wind.
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On the Relationship Between Coronal Mass Ejections and Magnetic Clouds
Geophysical Research Letters, 1998Co-Authors: Nat Gopalswamy, Ronald P. Lepping, B J Thompson, Y. Hanaoka, T. Kosugi, J. T. Steinberg, Simon Plunkett, Russell A. Howard, Joseph B. GurmanAbstract:We compare the substructures of the 1997 February 07 coronal mass ejection (CME) observed near the Sun with a corresponding event in the interplanetary medium to determine the origin of Magnetic Clouds (MCs). We find that the eruptive prominence core of the CME observed near the Sun may not directly become a Magnetic cloud as suggested by some authors and that it might instead become the ”pressure pulse” following the Magnetic cloud. We substantiate our conclusions using time of arrival, size and composition estimates of the CME-MC substructures obtained from ground based, SOHO and WIND observations.
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Magnetic Storms - Magnetic Clouds and the quiet-storm effect at Earth
Magnetic Storms, 1997Co-Authors: Charles J. Farrugia, Leonard F. Burlaga, Ronald P. LeppingAbstract:In this review, we discuss first Magnetic Clouds in the context of other interplanetary causes of geoMagnetic storms. We then describe work on the global field line topology of Magnetic Clouds, focussing on information gained by the use of energetic particles. We then give a summary of theoretical and simulation work on the dynamics of Magnetic Clouds. In one approach, based on self-similar evolution of radially expanding Magnetic flux ropes, the role of electrons is central. A section on the boundaries of Magnetic Clouds is followed by one on Magnetic field line draping around these ejecta, including the formation of a Magnetic barrier. In the aspect of the study dealing with the geoMagnetic response to Magnetic Clouds, we discuss effects on the dayside magnetosheath; ionosphere; and nightside magnetosphere at geostationary orbit and beyond, utilizing primarily observations made during Earth's encounter with a Magnetic cloud on January, 13 - 14, 1988. A case study is mentioned where solar energetic particles, injected into a Magnetic cloud and then guided along its helical field lines, entered the magnetosphere through interconnection of the cloud's field lines with those of Earth. Simulation work on the geoMagnetic response to Magnetic Clouds is briefly reviewed. We finally consider studies specifically correlating Magnetic Clouds, in isolation or as part of compound streams, with geoMagnetic storm activity. Throughout, we indicate areas where further work is needed.
T H Zurbuchen - One of the best experts on this subject based on the ideXlab platform.
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solar cycle dependent helicity transport by Magnetic Clouds
Journal of Geophysical Research, 2005Co-Authors: B J Lynch, J R Gruesbeck, T H Zurbuchen, S K AntiochosAbstract:[1] Magnetic Clouds observed with the Wind and ACE spacecraft are fit with the static, linear force-free cylinder model to obtain estimates of the chirality, fluxes, and Magnetic helicity of each event. The fastest Magnetic Clouds (MCs) are shown to carry the most flux and helicity. We calculate the net cumulative helicity which measures the difference in right- and left-handed helicity contained in MCs over time. The net cumulative helicity does not average to zero; rather, a strong left-handed helicity bias develops over the solar cycle, dominated by the largest events of cycle 23: Bastille Day 2000 and 28 October 2003. The majority of MCs (“slow” events, 〈Vr〉 < 500 km/s) have a net cumulative helicity profile that appears to be modulated by the solar activity cycle. This is far less evident for “fast” MC events (〈Vr〉 ≥ 500 km/s), which were disproportionately left-handed over our data set. A brief discussion about the various solar sources of CME helicity and their implication for dynamo processes is included.
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a statistical study of the geoeffectiveness of Magnetic Clouds during high solar activity years
Journal of Geophysical Research, 2004Co-Authors: Jichun Zhang, M W Liemohn, J U Kozyra, Benjamin J Lynch, T H ZurbuchenAbstract:[1] Using the Dst value corrected for the effects of magnetopause currents (Dst*) and solar wind Magnetic field and plasma data from 1 January 1998 to 30 April 2002, during elevated solar conditions, we have statistically examined the relationship of 271 storms (Dst* ≤ −30 nT) to 104 Magnetic Clouds. It is found that most of the Magnetic Clouds result in geoMagnetic storms, but only about 30% of storms are due to Magnetic Clouds. A storm can be driven by a cloud's various regions or their combinations with dissimilar occurrence percentages. These percentages change as a function of geoMagnetic activity levels as well. It is found that the leading field is the most geoeffective region and the sheath region is equally effective at causing Magnetic storms during solar maximum (42%) compared to solar minimum (43%) as a percentage of Magnetic cloud-induced storms. The occurrence percentage of intense storms caused by Clouds is 72%, which is much higher than the ∼20% occurrence percentage of smaller storms caused by Clouds. It is also found that “unipolar Bz” and “bipolar Bz” Clouds have different geoeffectiveness percentages, depending on the Bz orientation. The long-known control of Magnetic activity mainly by southward Bz is supported by the results of this study. It is also shown that multistep development storms can result not only from both the combinations of sheath and cloud fields but also from different fields within a cloud. A new name, quasi-cloud, is proposed for those cloud-like solar wind structures which show evidence of relatively organized field rotations.
