Causes and consequences of magnetic cloud expansion

Démoulin, P.; Dasso, S.

doi:10.1051/0004-6361/200810971

Cited by 148 publications

(214 citation statements)

References 69 publications

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“…In particular, during the MC travel through the heliosphere, different parts of the MC boundary could be in contact with different parcels of SW having different pressure, therefore changing the original shape of the MC. These changes of the MC boundary drive a re-configuration of the internal magnetic field, in a similar way to the global expansion of MCs proposed by Démoulin & Dasso (2009). Thus, the shape of the MC boundary given by Eq.…”

Section: Discussionsupporting

confidence: 61%

“…Moreover, the plasma β is low in MCs (typically β ≈ 0.1, with values ranging from less than ≈10 −2 to a few times 0.1, e.g., Lepping et al 2003;Feng et al 2007;Wu & Lepping 2007, and references therein). Other forces such as gravity are also negligible with respect to the magnetic pressure gradient, therefore the magnetic field evolution can be described, to first a approximation, by a sequence of force-free equilibria ( j × B ≈ 0), e.g., as proposed by Démoulin & Dasso (2009).…”

Section: Force-free Field Evolutionmentioning

confidence: 99%

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Magnetic cloud models with bent and oblate cross-section boundaries

Démoulin¹,

Dasso

2009

A&A

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Context. Magnetic clouds (MCs) are formed by magnetic flux ropes that are ejected from the Sun as coronal mass ejections. These structures generally have low plasma beta and travel through the interplanetary medium interacting with the surrounding solar wind. Thus, the dynamical evolution of the internal magnetic structure of a MC is a consequence of both the conditions of its environment and of its own dynamical laws, which are mainly dominated by magnetic forces. Aims. With in-situ observations the magnetic field is only measured along the trajectory of the spacecraft across the MC. Therefore, a magnetic model is needed to reconstruct the magnetic configuration of the encountered MC. The main aim of the present work is to extend the widely used cylindrical model to arbitrary cross-section shapes. Methods. The flux rope boundary is parametrized to account for a broad range of shapes. Then, the internal structure of the flux rope is computed by expressing the magnetic field as a series of modes of a linear force-free field. Results. We analyze the magnetic field profile along straight cuts through the flux rope, in order to simulate the spacecraft crossing through a MC. We find that the magnetic field orientation is only weakly affected by the shape of the MC boundary. Therefore, the MC axis can approximately be found by the typical methods previously used (e.g., minimum variance). The boundary shape affects the magnetic field strength most. The measurement of how much the field strength peaks along the crossing provides an estimation of the aspect ratio of the flux-rope cross-section. The asymmetry of the field strength between the front and the back of the MC, after correcting for the time evolution (i.e., its aging during the observation of the MC), provides an estimation of the cross-section global bending. A flat or/and bent cross-section requires a large anisotropy of the total pressure imposed at the MC boundary by the surrounding medium. Conclusions. The new theoretical model developed here relaxes the cylindrical symmetry hypothesis. It is designed to estimate the cross-section shape of the flux rope using the in-situ data of one spacecraft. This allows a more accurate determination of the global quantities, such as magnetic fluxes and helicity. These quantities are especially important for both linking an observed MC to its solar source and for understanding the corresponding evolution.

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Section: Discussionsupporting

confidence: 61%

Section: Force-free Field Evolutionmentioning

confidence: 99%

Magnetic cloud models with bent and oblate cross-section boundaries

Démoulin¹,

Dasso

2009

A&A

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“…The speed of an ICME often exceeds the magnetosonic speed in the solar wind frame, and a shock wave and a sheath form upstream of the ICME. A large fraction of ICMEs expand as they propagate away from the Sun, primarily due to the decrease in the total solar wind pressure (e.g., Démoulin and Dasso, 2009). Observations show that the expansion is still significant at the orbit of the Earth (e.g., Klein and Burlaga, 1982;Kilpua et al, 2017) and ceases at about 10-15 AU (e.g., von Steiger and Richardson, 2006, and references therein).…”

Section: M Ala-lahti Et Al: Mirror Mode Wave Occurrence In Icme-mentioning

confidence: 99%

Statistical analysis of mirror mode waves in sheath regions driven by interplanetary coronal mass ejection

et al. 2018

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Abstract. We present a comprehensive statistical analysis of mirror mode waves and the properties of their plasma surroundings in sheath regions driven by interplanetary coronal mass ejection (ICME). We have constructed a semiautomated method to identify mirror modes from the magnetic field data. We analyze 91 ICME sheath regions from January 1997 to April 2015 using data from the Wind spacecraft. The results imply that similarly to planetary magnetosheaths, mirror modes are also common structures in ICME sheaths. However, they occur almost exclusively as dip-like structures and in mirror stable plasma. We observe mirror modes throughout the sheath, from the bow shock to the ICME leading edge, but their amplitudes are largest closest to the shock. We also find that the shock strength (measured by Alfvén Mach number) is the most important parameter in controlling the occurrence of mirror modes. Our findings suggest that in ICME sheaths the dominant source of free energy for mirror mode generation is the shock compression. We also suggest that mirror modes that are found deeper in the sheath are remnants from earlier times of the sheath evolution, generated also in the vicinity of the shock.

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“…Then, the observations on Ulysses show a flux rope with characteristics significantly different from a "standard" flux rope such as observed on ACE. A magnetic field strength flatter than on ACE is expected if the flux rope transverse size increases much less rapidly than its length, since it implies that the axial component becomes dominant with increasing distance to the Sun so that the magnetic tension becomes relatively weaker in the force balance (Démoulin & Dasso 2009a). The strong discontinuity followed by a strong field is more peculiar.…”

Section: Magnetic Cloud At the Position Of Ulyssesmentioning

confidence: 99%

Dynamical evolution of a magnetic cloud from the Sun to 5.4 AU

Nakwacki

Dasso

Démoulin

et al. 2011

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Context. Significant quantities of magnetized plasma are transported from the Sun to the interstellar medium via interplanetary coronal mass ejections (ICMEs). Magnetic clouds (MCs) are a particular subset of ICMEs, forming large-scale magnetic flux ropes. Their evolution in the solar wind is complex and mainly determined by their own magnetic forces and the interaction with the surrounding solar wind. Aims. Magnetic clouds are strongly affected by the surrounding environment as they evolve in the solar wind. We study expansion of MCs, its consequent decrease in magnetic field intensity and mass density, and the possible evolution of the so-called global ideal-MHD invariants. Methods.In this work we analyze the evolution of a particular MC (observed in March 1998) using in situ observations made by two spacecraft approximately aligned with the Sun, the first one at 1 AU from the Sun and the second one at 5.4 AU. We describe the magnetic configuration of the MC using different models and compute relevant global quantities (magnetic fluxes, helicity, and energy) at both heliodistances. We also tracked this structure back to the Sun, to find out its solar source. Results. We find that the flux rope is significantly distorted at 5.4 AU. From the observed decay of magnetic field and mass density, we quantify how anisotropic is the expansion and the consequent deformation of the flux rope in favor of a cross section with an aspect ratio at 5.4 AU of ≈1.6 (larger in the direction perpendicular to the radial direction from the Sun). We quantify the ideal-MHD invariants and magnetic energy at both locations, and find that invariants are almost conserved, while the magnetic energy decays as expected with the expansion rate found. Conclusions. The use of MHD invariants to link structures at the Sun and the interplanetary medium is supported by the results of this multi-spacecraft study. We also conclude that the local dimensionless expansion rate, which is computed from the velocity profile observed by a single-spacecraft, is very accurate for predicting the evolution of flux ropes in the solar wind.

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Causes and consequences of magnetic cloud expansion

Cited by 148 publications

References 69 publications

Magnetic cloud models with bent and oblate cross-section boundaries

Magnetic cloud models with bent and oblate cross-section boundaries

Statistical analysis of mirror mode waves in sheath regions driven by interplanetary coronal mass ejection

Dynamical evolution of a magnetic cloud from the Sun to 5.4 AU

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