Magnetoacoustic Portals and the Basal Heating of the Solar Chromosphere

Jefferies, S. M.; McIntosh, Scott W.; Armstrong, J. D.; Bogdan, Thomas J.; Cacciani, A.; Fleck, B.

doi:10.1086/508165

Cited by 193 publications

(238 citation statements)

References 30 publications

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“…The role of the fibrils in the dynamics is two-fold: firstly, they are associated with the channeling of low (below acoustic cutoff) frequency waves propagating into the chromosphere (e.g., Jefferies et al 2006). An inclined magnetic field lowers the effective gravity and also the acoustic cutoff frequency of a wave propagating along the field line (Bel & Leroy 1977).…”

Section: Discussionmentioning

confidence: 99%

Bright fibrils in Ca II K

Pietarila

Hirzberger

Zakharov

et al. 2009

A&A

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Context. Except for the Ca ii resonance lines, fibrils are ubiquitously present in most high-resolution observations of chromospheric lines.Aims. We show that fibrils are also a prevailing feature in Ca ii K, provided the spatial-resolution is sufficiently high. Methods. We present high spatial resolution observations of an active region in the Ca ii K line from the Swedish Solar Telescope.Through a comparison between photospheric intensity and magnetic field data, we study the connection between bright chromospheric fibrils and photospheric structures. Additionally, using Fourier analysis we study how the fibrils are linked to the observed dynamics. Results. We find that very narrow, bright fibrils are a prevailing feature over large portions of the observed field. We also find a clear connection between the fibril footpoints and photospheric magnetic features. We show that the fibrils play two distinct roles in the observed dynamics: depending on their location they can act as a canopy suppressing oscillations or they can channel low-frequency oscillations into the chromosphere.Conclusions. The Ca ii K fibrils share many characteristics with fibrils observed in other chromospheric lines, but some features, such as the very small widths, are unique to these observations.

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Section: Discussionmentioning

confidence: 99%

Bright fibrils in Ca II K

Pietarila

Hirzberger

Zakharov

et al. 2009

A&A

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“…In the low-β approximation (e.g., in sunspots or active regions), the cut-off frequency is modified by the magnetic inclination ν mac = ν 0 cos φ (Bel & Leroy 1977) 1 . This explains the existence of low-frequency compressive waves in the corona (e.g., De Moortel & Nakariakov 2012), which are believed to carry a larger portion of energy flux than their highfrequency counterpart (e.g., Fontenla et al 1993;Jefferies et al 2006). The cut-off frequency of compressive waves could also be used as a seismological tool to probe the local magnetic topology.…”

Section: Introductionmentioning

confidence: 99%

Multi-height observations of magnetoacoustic cut-off frequency in a sunspot atmosphere

Yuan

Сыч

Reznikova

et al. 2013

A&A

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Context. The cut-off frequency of magnetoacoustic gravity (MAG) waves could be decreased by the inclined magnetic field, and therefore, low-frequency waves could penetrate into the upper atmosphere. Aims. We observe the distribution of the cut-off frequency of compressive waves at various heights and reconstruct the magnetic field inclination, according to the MAG wave theory in a stratified atmosphere permeated by a uniform magnetic field. Methods. We analysed the emission intensity oscillations of sunspot AR11131 (08 Dec. 2010) observed at the 1700 Å, 1600 Å, and 304 Å bandpasses of the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO), and computed the narrow-band power maps with the pixelised wavelet filtering method. The distribution of the cut-off frequency was defined as the median contour in the azimuthally-averaged oscillation power. The magnetic field inclination was estimated with the local cut-off frequency according to the MAG wave theory in the low-β limit and was compared to the potential field extrapolation. Results. Shorter period oscillations dominate in the sunspot umbra, while longer period oscillations form an annular shape approximately concentric with the sunspot. Oscillations with longer periods are distributed further away from the sunspot centre. The 5 min oscillations appear to originate at or lower than the photosphere. The magnetic field inclinations determined with the cut-off frequency theory are about 30−40% larger than the values obtained by the potential field extrapolation. Conclusions. The oscillation power distribution in a sunspot atmosphere reflects its magnetic and thermal structure. The cut-off frequency could be used to probe the magnetic field inclination, however, other factors have to be included to fully understand this phenomenon. The existence of return magnetic flux at the outer penumbra was evidenced by the cut-off frequency distribution.

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“…It has been shown that in the presence of inclined magnetic fields, near the magnetic concentrations of the network where plasma β is of order unity or lower, p-modes may propagate (leak) to higher atmospheric layers, especially, if the inclination of the magnetic field is significant (Bel & Leroy 1977;Suematsu 1990;De Pontieu et al 2004). Thus the presence of inclined magnetic field lines near the network boundaries provides a way of channeling otherwise evanescent waves into the chromosphere through what have been called "magneto-acoustic portals" (Jefferies et al 2006). This mechanism allows the propagation of 5 min and even higher period waves at the chromosphere.…”

Section: Power Maps and β Transition Heightsmentioning

confidence: 99%

“…This condition is altered by the presence of the magnetic field which affects the wave propagation properties. This is the case of "p-mode leakage" and "magneto-acoustic portals" (DePontieu et al 2004;Jefferies et al 2006), i.e. the channeling of low frequency waves at higher layers through areas with highly inclined magnetic fields.…”

Section: Introductionmentioning

confidence: 99%

Oscillations in a network region observed in the Hαline and their relation to the magnetic field

Kontogiannis

Tsiropoula

Tziotziou

et al. 2010

A&A

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Aims. Our aim is to gain a better understanding of the interaction between acoustic oscillations and the small-scale magnetic fields of the Sun. To this end, we examine the oscillatory properties of a network region and their relation to the magnetic configuration of the chromosphere. We link the oscillatory properties of a network region and their spatial variation with the variation of the parameters of the magnetic field. We investigate the effect of the magnetic canopy and the diverging flux tubes of the chromospheric network on the distribution of oscillatory power over the network and internetwork. Methods. We use a time series of high resolution filtergrams at five wavelengths along the Hα profile observed with the Dutch Open Telescope, as well as high resolution magnetograms taken by the SOT/SP onboard HINODE. Using wavelet analysis, we construct power maps of the 3, 5 and 7 min oscillations of the Doppler signals calculated at ±0.35 Å and ±0.7 Å from the Hα line center. These represent velocities at chromospheric and photospheric levels respectively. Through a current-free (potential) field extrapolation we calculate the chromospheric magnetic field and compare its morphology with the Hα filtergrams. We calculate the plasma β and the magnetic field inclination angle and compare their distribution with the oscillatory power at the 3, 5 and 7 min period bands. Results. Chromospheric mottles seem to outline the magnetic field lines. The Hα ± 0.35 Å Doppler signals are formed above the canopy, while the Hα ± 0.7 Å corresponding ones below it. The 3 min power is suppressed at the chromosphere around the network, where the canopy height is lower than 1600 km, while at the photosphere it is enhanced due to reflection. 3, 5 and 7 min oscillatory power is increased around the network at the photosphere due to reflection of waves on the overlying canopy, while increased 5 and 7 min power at the chromosphere is attributed mainly to wave refraction on the canopy. At these high periods, power is also increased due to p-mode leakage because of the high inclinations of the magnetic field. Conclusions. Our high resolution Hα observations and photospheric magnetograms provide the opportunity to highlight the details of the interaction between acoustic oscillations and the magnetic field of a network region. We conclude that several mechanisms that have been proposed such as p-mode leakage, mode conversion, reflection and refraction of waves on the magnetic canopy may act together and result to the observed properties of network oscillations.

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Magnetoacoustic Portals and the Basal Heating of the Solar Chromosphere

Cited by 193 publications

References 30 publications

Bright fibrils in Ca II K

Bright fibrils in Ca II K

Multi-height observations of magnetoacoustic cut-off frequency in a sunspot atmosphere

Oscillations in a network region observed in the Hαline and their relation to the magnetic field

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