Alfvén Profile in the Lower Corona: Implications for Shock Formation

Evans, R. M.; Opher, M.; Manchester, W.; Gombosi, T. I.

doi:10.1086/592016

Cited by 46 publications

(38 citation statements)

References 51 publications

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“…The trajectory shown in Figure 5 should then reflect a rather special distribution of magnetic field strength or coronal density around the active region, so that the local Alfvén speed drops strongly to very low values (around 20 km s −1 ). Such values of the Alfvén speed are not reported for the low coronal heights where EIT waves usually propagate (see, e.g., Evans et al, 2008). Indeed, taking order of magnitude values for the coronal magnetic field in the quiet Sun, B = 1 G, and a rather high (for the quiet Sun regions) density n = 10 9 cm −3 , we can estimate an Alfvén speed of v A = B/ 4πm p n ∼ 70 km s −1 (where m p is the proton mass).…”

Section: Discussionmentioning

confidence: 98%

STEREO/SECCHI Observations on 8 December 2007: Evidence Against the Wave Hypothesis of the EIT Wave Origin

2009

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The physical nature of EIT waves, large-scale bright fronts propagating in the solar corona, remains a subject of a continuing debate. Two main ways of interpreting this phenomenon have been suggested. One of them describes an EIT wave as a fast mode magnetosonic wave freely propagating in the corona. The other interpretation does not consider an EIT wave a true magnetohydrodynamic wave but instead invokes several possibilities linked to the magnetic field restructuring during the coronal mass ejection (CME) evolution in the low corona. We investigate an EIT wave observed by the SECCHI/EUVI telescopes onboard the STEREO spacecraft on 8 December 2007. The wave front had a nearly symmetric shape and exhibited a peculiar velocity profile measured by two independent methods. After an initial short propagation at a speed of around 100 km s −1 , the wave was moving at a very low velocity (around 20 -40 km s −1 ) for about 30 minutes, and then was reaccelerated up to speeds of around 200 km s −1 . It is difficult to envisage such a velocity change for a freely propagating coronal wave. However, such a behavior is possible, for example, for erupting prominences. We conclude that this event provides observational evidence that even EIT waves with a symmetric front can be produced by a magnetic field restructuring during the CME eruption.

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Section: Discussionmentioning

confidence: 98%

STEREO/SECCHI Observations on 8 December 2007: Evidence Against the Wave Hypothesis of the EIT Wave Origin

2009

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“…Finally, it should be noted that the modeled Alfvén speed values extrapolated close to the Sun (between 1000 and 10000 km/s) in Figure are on the order of those given by dedicated models of the Alfvén speed profile in the near‐Sun corona, in the 1000–5000 km/s range [cf. Evans et al ., ]. While the Leitner et al .…”

Section: Discussion Of Some Limitations Of the Approachmentioning

confidence: 99%

Geo‐effectiveness and radial dependence of magnetic cloud erosion by magnetic reconnection

et al. 2014

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[1] Magnetic flux erosion by magnetic reconnection occurs at the front of at least some magnetic clouds (MCs). We first investigate how erosion influences the geo-effectiveness of MCs in a general sense and using a south-north magnetic polarity MC observed on 18-20 October 1995. Although the magnetic shear at its front may not be known during propagation, measurements at 1 AU show signatures of local reconnection. Using a standard MC model, an empirical model of the geomagnetic response (Dst), and an observational estimate of the magnetic flux erosion, we find that the strength of the observed ensuing storm was~30% lower than if no erosion had occurred. We then discuss the interplay between adiabatic compression and magnetic erosion at the front of MCs. We conclude that the most geo-effective configuration for a south-north polarity MC is to be preceded by a solar wind with southward IMF. This stems not only from the formation of a geo-effective sheath ahead of it but also from the adiabatic compression and reduced (or lack thereof) magnetic erosion which constructively conspire for the structure to be more geo-effective. Finally, assuming simple semiempirical and theoretical Alfvén speed profiles expected from expansion to 1 AU, we provide first-order estimates of the erosion process radial evolution. We find that the expected reconnection rates during propagation allow for significant erosion, on the order of those reported. Calculations also suggest that most of the erosion should occur in the inner heliosphere, and up to~50% may yet occur beyond Mercury's orbit.

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“…In this picture, the plasma is accelerated due to gradients in the wave pressure, while heating is achieved by dissipation of wave energy. The importance of wave-driven models was further demonstrated in Evans et al (2008), who compared the Alfvén speed profiles predicted by ten different solar models against type II radio bursts observations. The survey included 1D wave-driven models, global MHD models with ad-hoc heating functions, as well as empirically derived profiles.…”

Section: Wave-driven Solar Modelsmentioning

confidence: 99%

A Global Wave-Driven Magnetohydrodynamic Solar Model With a Unified Treatment of Open and Closed Magnetic Field Topologies

Oran

Holst

Landi

et al. 2013

ApJ

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105

112

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We describe, analyze and validate the recently developed Alfvén Wave Solar Model (AWSoM), a 3D global model starting from the top of the chromosphere and extending into interplanetary space (up to 1-2 AU). This model solves the extended twotemperature magnetohydrodynamics equations coupled to a wave kinetic equation for low frequency Alfvén waves. In this picture, heating and acceleration of the plasma are due to wave dissipation and wave pressure gradients, respectively. The dissipation process is described by a fully developed turbulent cascade of counter-propagating waves. We adopt a unified approach for calculating the wave dissipation in both open and closed magnetic field lines, allowing for a self-consistent treatment of any magnetic topology. Wave dissipation is the only heating mechanism assumed in the model, and no geometric heating functions are invoked. Electron heat conduction and radiative cooling are also included. We demonstrate that the large-scale, steady-state (in the co-rotating frame) properties of the solar environment are reproduced, using three adjustable parameters: the Poynting flux of chromospheric Alfvén waves, the perpendicular correlation length of the turbulence, and a pseudo-reflection coefficient. We compare model results for Carrington Rotation 2063 (November-December 2007) to remote observations in the EUV and X-ray ranges from STEREO, SOHO and Hinode spacecraft, as well as to in-situ measurements performed by Ulysses. The model results are in good agreement with observations. This is the first global model capable of simultaneously reproducing the multi-wavelength observations of the lower corona and the wind structure beyond Earth's orbit.

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Alfvén Profile in the Lower Corona: Implications for Shock Formation

Cited by 46 publications

References 51 publications

STEREO/SECCHI Observations on 8 December 2007: Evidence Against the Wave Hypothesis of the EIT Wave Origin

STEREO/SECCHI Observations on 8 December 2007: Evidence Against the Wave Hypothesis of the EIT Wave Origin

Geo‐effectiveness and radial dependence of magnetic cloud erosion by magnetic reconnection

A Global Wave-Driven Magnetohydrodynamic Solar Model With a Unified Treatment of Open and Closed Magnetic Field Topologies

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