Orphan penumbrae: Submerging horizontal fields

Abstract

We investigate the properties of orphan penumbrae, which are photospheric filamentary structures observed in active regions near polarity inversion lines that resemble the penumbra of regular sunspots but are not connected to any umbra. We use Hinode data from the Solar Optical Telescope to determine the properties of orphan penumbrae. Spectropolarimetric data are employed to obtain the vector magnetic field and line-of-sight velocities in the photosphere. Magnetograms are used to study the overall evolution of these structures, and G-band and Ca II H filtergrams are to investigate their brightness and apparent horizontal motions. Orphan penumbrae form between regions of opposite polarity in places with horizontal magnetic fields. Their magnetic configuration is that of $\Omega$-shaped flux ropes. In the two cases studied here, the opposite-polarity regions approach each other with time and the whole structure submerges as the penumbral filaments disappear. Orphan penumbrae are very similar to regular penumbrae, including the existence of strong gas flows. Therefore, they could have a similar origin. The main difference between them is the absence of a "background" magnetic field in orphan penumbrae. This could explain most of the observed differences. The fast flows we detect in orphan penumbrae may be caused by the siphon flow mechanism. Based on the similarities between orphan and regular penumbrae, we propose that the Evershed flow is also a manifestation of siphon flows.

15 pages, 15 figures

References in corpus (24)

• The Solar Optical Telescope for the Hinode Mission: An Overview
• Penumbral fine structure and driving mechanisms of large-scale flows in simulated sunspots
• Peripheral downflows in sunspot penumbrae
• An active region filament studied simultaneously in the chromosphere and photosphere: I - Magnetic structure
• Can High Frequency Acoustic Waves Heat the Quiet Sun Chromosphere?
• Structure of sunspot penumbral filaments: a remarkable uniformity of properties
• The origin of the reversed granulation in the solar photosphere
• On the fine structure of the sunspot penumbrae. II. The nature of the Evershed flow
• The quiet solar atmosphere observed and simulated in NaD1
• Propagating Wave Phenomena Detected in Observations and Simulations of the Lower Solar Atmosphere
• 3D view of transient horizontal magnetic fields in the photosphere
• Evidence of magnetic field wrapping around penumbral filaments
• Moat flow in the vicinity of sunspots for various penumbral configurations
• Reversal-free CaIIH profiles: a challenge for solar chromosphere modeling in quiet inter-network
• The Analysis of Penumbral Fine Structure Using an Advanced Inversion Technique
• Sunspots: from small-scale inhomogeneities towards a global theory
• On the Moat-Penumbra Relation
• Study of single-lobed circular polarization profiles in the quiet Sun
• Correlations between sunspots and their moat flows
• Are there field-free gaps near tau=1 in sunspot penumbrae?
• The Fine-Structure of the Net-Circular Polarization in a Sunspot Penumbra
• High-resolution proper motions in a sunspot penumbra
• On the polarimetric signature of emerging magnetic loops in the quiet-Sun
• Large-scale horizontal flows in the solar photosphere V: Possible evidence for the disconnection of bi-polar sunspot groups from their magnetic roots

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• The European Solar Telescope
• Flux cancellation and the evolution of the eruptive filament of 2011 June 7
• The magnetic nature of umbra-penumbra boundary in sunspots
• A distinct magnetic property of the inner penumbral boundary
• A distinct magnetic property of the inner penumbral boundary. II. Formation of a penumbra at the expense of a pore
• Penumbral decay observed in active region NOAA 12585
• Formation of a solar Ha filament from orphan penumbrae
• Onset of penumbra formation
• The magnetic origin of the outer boundaries of sunspots
• Magnetic properties of orphan penumbrae
• Sunspot simulations with MURaM -- I. Parameter study using potential field initial conditions