On the topology of filaments and chromospheric fibrils near sunspots

Nakagawa, Y.; Raadu, Michael A.; Billings, Donald E.; McNamara, Daniel E.

doi:10.1007/bf00148825

Cited by 106 publications

(33 citation statements)

References 10 publications

(2 reference statements)

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“…These structures are seen in almost all chromospheric spectral lines, including Hα 6563 Å, Ca ii 8542 Å and Lyα 1216 Å and, though more rarely, also in Ca ii H and K (see Pietarila et al 2009, and references therein). A common assumption is that these chromospheric "fibrils" outline the direction of closed magnetic field structures in the upper photosphere and chromosphere, linking the spot with the surrounding flux of opposite polarity (Nakagawa et al 1971;Woodard and Chae 1999), allowing a mass flow away from the spot (Evershed 1909) or into the spot ("inverse Evershed effect"; St. John 1913, and for a review see Solanki 2003). In a similar fashion, fibrils (seen, e.g., in Hα) are thought to connect opposite polarity magnetic flux elements in the QS Beck et al 2014), although some fibrils may follow the chromospheric part of magnetic field lines that continue into the corona (see also Sect.…”

Section: Characteristic Chromospheric Magnetic Structuresmentioning

confidence: 99%

The magnetic field in the solar atmosphere

Wiegelmann

Thalmann

Solanki

2014

Astron Astrophys Rev

183

125

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This publication provides an overview of magnetic fields in the solar atmosphere with the focus lying on the corona. The solar magnetic field couples the solar interior with the visible surface of the Sun and with its atmosphere. It is also responsible for all solar activity in its numerous manifestations. Thus, dynamic phenomena such as coronal mass ejections and flares are magnetically driven. In addition, the field also plays a crucial role in heating the solar chromosphere and corona as well as in accelerating the solar wind. Our main emphasis is the magnetic field in the upper solar atmosphere so that photospheric and chromospheric magnetic structures are mainly discussed where relevant for higher solar layers. Also, the discussion of the solar atmosphere and activity is limited to those topics of direct relevance to the magnetic field. After giving a brief overview about the solar magnetic field in general and its global structure, we discuss in more detail the magnetic field in active regions, the quiet Sun and coronal holes.

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Section: Characteristic Chromospheric Magnetic Structuresmentioning

confidence: 99%

The magnetic field in the solar atmosphere

Wiegelmann

Thalmann

Solanki

2014

Astron Astrophys Rev

183

125

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“…Order of magnitude analysis 4 shows that prominences are low ␤ and quasi-steady so that they are in a force-free equilibrium, i.e., satisfy J؋Bϭ0. Thus the current and magnetic field must be parallel and from Ampere's law the magnetic field must therefore satisfy ٌϫBϭ B, ͑1͒…”

Section: Brief Overview Of Prominencesmentioning

confidence: 99%

“…Twisted prominences have currents, contain magnetic helicity, and have free energy compared to untwisted ͑vacuum field͒ prominences satisfying the same boundary conditions. Nakagawa et al 4 assumed that is uniform over the extent of a prominence in which case analytic solutions of Eq. ͑1͒ can be found.…”

Section: Brief Overview Of Prominencesmentioning

confidence: 99%

Laboratory simulations of solar prominence eruptions

Bellan

Hansen

1998

Physics of Plasmas

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Spheromak technology is exploited to create laboratory simulations of solar prominence eruptions. It is found that the initial simulated prominences are arched, but then bifurcate into twisted secondary structures which appear to follow fringing field lines. A simple model explains many of these topological features in terms of the trajectories of field lines associated with relaxed states, i.e., states satisfying ٌϫBϭ B. This model indicates that the field line concept is more fundamental than the flux tube concept because a field line can always be defined by specifying a starting point whereas attempting to define a flux tube by specifying a starting cross section typically works only if is small. The model also shows that, at least for plasma evolving through a sequence of force-free states, the oft-used line-tying concept is in error. Contrary to the predictions of line-tying, direct integration of field line trajectories shows explicitly that when is varied, both ends of field lines intersecting a flux-conserving plane do not remain anchored to fixed points in that plane. Finally, a simple explanation is provided for the S-shaped magnetic structures often seen on the sun; the S shape is shown to be an automatic consequence of field line arching and the parallelism between magnetic field and current density for force-free states.

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“…where α is a scalar of proportionality (e.g., Nakagawa et al 1971). Extrapolations of coronal magnetic fields usually imply solving this equation using photospheric data as a boundary condition.…”

Section: The Methodsmentioning

confidence: 99%

Direct Measurements of Magnetic Twist in the Solar Corona

Malanushenko

Yusuf

Longcope

2011

ApJ

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In the present work, we study the evolution of magnetic helicity in the solar corona. We compare the rate of change of a quantity related to the magnetic helicity in the corona to the flux of magnetic helicity through the photosphere and find that the two rates are similar. This gives observational evidence that helicity flux across the photosphere is indeed what drives helicity changes in the solar corona during emergence. For the purposes of estimating coronal helicity, we neither assume a strictly linear force-free field nor attempt to construct a nonlinear force-free field. For each coronal loop evident in extreme ultraviolet, we find a best-matching line of a linear force-free field and allow the twist parameter α to be different for each line. This method was introduced and its applicability discussed in Malanushenko et al. The object of this study is emerging and rapidly rotating AR 9004 over about 80 hr. As a proxy for coronal helicity, we use the quantity α i L i /2 averaged over many reconstructed lines of magnetic field. We argue that it is approximately proportional to the "flux-normalized" helicity H/Φ 2 , where H is the helicity and Φ is the total enclosed magnetic flux of the active region. The time rate of change of such a quantity in the corona is found to be about 0.021 rad hr −1 , which is comparable with the estimates for the same region obtained using other methods, which estimated the flux of normalized helicity to be about 0.016 rad hr −1 .

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On the topology of filaments and chromospheric fibrils near sunspots

Cited by 106 publications

References 10 publications

The magnetic field in the solar atmosphere

The magnetic field in the solar atmosphere

Laboratory simulations of solar prominence eruptions

Direct Measurements of Magnetic Twist in the Solar Corona

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