The flow field in the sunspot canopy

Rezaei, Reza; Schlichenmaier, R.; Beck, C.; Rubio, L. R. Bellot

doi:10.1051/0004-6361:20065180

Cited by 33 publications

(42 citation statements)

References 30 publications

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“…The only influence of the moat flow could then be differential moat velocities across the FOV. Assuming a horizontal moat flow of around 0.5 km s −1 (Balthasar et al 1996;Rezaei et al 2006), we find that the maximum error due to differential velocities cannot be larger than 0.25 km s −1 at the heliocentric angle of the observations. The accuracy of the velocity scale thus only depends on how appropriate the convective blueshift values predicted by the QS model are.…”

Section: Wavelength Scalementioning

confidence: 52%

Magnetic properties of G-band bright points in a sunspot moat

Beck

Rubio

Schlichenmaier

et al. 2007

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We present simultaneous spectropolarimetric observations of four visible (630 nm) and three infrared (1565 nm) spectral lines from the German Vacuum Tower Telescope, together with speckle-reconstructed filtergrams in the G-band and the Ca ii H line core from the Dutch Open Telescope. After alignment of the data sets, we used the G-band intensity to locate bright points (BPs) in the moat of a regular sunspot. With the cospatial and cotemporal information provided by the polarimetric data, we characterize the magnetic, kinematic, and thermal properties of the BPs. We find that (a) 94% of the BPs are associated with magnetic fields; (b) their field strengths range between 500 and 1400 G, with a rather flat distribution; (c) the contrast of BPs in the G-band depends on the angle between the vector magnetic field and the line of sight; (d) the BPs harbor downflows of magnetized plasma and exhibit Stokes V profiles with large area and amplitude asymmetries; (e) the magnetic interior of BPs is hotter than the immediate field-free surroundings by about 1000 K at equal optical depth; and (f) the mean effective diameter of BPs in our data set is 150 km, with very few BPs larger than 300 km. Most of these properties can be explained by the classical magnetic flux tube model. However, the wide range of BP parameters found in this study indicates that not all G-band BPs are identical to stable long-lived flux tubes or sheets of kG strength.

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Section: Wavelength Scalementioning

confidence: 52%

Magnetic properties of G-band bright points in a sunspot moat

Beck

Rubio

Schlichenmaier

et al. 2007

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“…(1), Paper I) assuming the strong-field approximation (Stix 2002;Landi degl'Innocenti & Landolfi 2004). This assumption is fulfilled in the near-infrared wavelength range in which the lobe separation is significantly larger than the Doppler width of the line (e.g., Rezaei et al 2006). …”

Section: Field Strength Measurementmentioning

confidence: 99%

Variation in sunspot properties between 1999 and 2014

Rezaei

Beck

Lagg

et al. 2015

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Aims. We study the variation in the magnetic field strength, area, and continuum intensity of umbrae in solar cycles 23 and 24. Methods. We analyzed a sample of 374 sunspots observed from 1999 until 2014 with the Tenerife Infrared Polarimeter at the German Vacuum Tower Telescope and the Facility InfRared Spectropolarimeter at the Dunn Solar Telescope. The sample of field strength, area, and intensities was used to trace any long-term or cyclic trend of umbral properties in the last 15 years. Results. Sunspots are systematically weaker, that is, have a weaker field strength and stronger continuum intensity, toward the end of cycle 23 than they had at the maximum of cycle 23. The linear trend reverses with the onset of cycle 24. We find that the field strength decreases in the declining phase of cycle 23 by about 112 (±16) G yr −1 , while it increases in the rising phase of cycle 24 by about 138 (±72) G yr −1 . The umbral intensity shows the opposite trend: the intensity increases with a rate of 0.7 (±0.3)% of I c yr −1 toward the end of cycle 23 and decreases with a rate of 3.8 (±1.5)% of I c yr −1 toward the maximum of cycle 24. The distribution of the umbral maximum field strength in cycle 24 is similar to that of cycle 23, but is slightly shifted toward lower values by about 80 G, corresponding to a possible long-term gradient in umbral field strength of about 7 ± 4 G yr −1 . If instead of the maximum umbral field we consider the average value over the entire umbra, the distribution shifts by about 44 Gauss. Conclusions. The umbral brightness decreases in the rising stage of a solar cycle, but increases from maximum toward the end of the cycle. Our results do not indicate a drastic change of the solar cycle toward a grand minimum in the near future.

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“…The Evershed flow seems to stop abruptly at the white light boundary of the spot (Wiehr & Degenhardt 1992;Schlichenmaier & Schmidt 1999), which is in line with downward flows in the outer penumbra. Only a small fraction of the flow continues to flow into the magnetic canopy that surrounds the spot (Rezaei et al 2006). …”

Section: Introductionmentioning

confidence: 99%

Correlations between sunspots and their moat flows

Löhner-Böttcher

Schlichenmaier

2013

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Context. The presence of the moat flow around sunspots is intimately linked to the mere existence of sunspots. Aims. We characterize the moat flow (MF) and Evershed flow (EF) in sunspots to enhance our knowledge of sunspot structures and photospheric flow properties. Methods. We calibrated HMI synoptic Doppler maps and used them to analyze 3 h time averages of 31 circular, stable, and fully developed sunspots at heliocentric angles of some 50 • . Assuming axially symmetrical flow fields, we infer the azimuthally averaged horizontal velocity component of the MF and EF from 51 velocity maps. We studied the MF properties (velocity and extension) and elaborate on how these components depend on sunspot parameters (sunspot size and EF velocity). To explore the weekly and monthly evolution of MFs, we compare spots rotating from the eastern to western limbs and spots that reappear on the eastern limb. Results. Our calibration procedure of HMI Doppler maps yields reliable and consistent results. In 3 h averages, we find the MF decreases on average from some 1000 ± 200 m/s just outside the spot boundary to 500 m/s after an additional 4 Mm. The average MF extension lies at 9.2 ± 5 Mm, where the velocity drops below some 180 m/s. Neither the MF velocity nor its extension depend significantly on the sunspot size or EF velocity. But, the EF velocity does show a tendency to be enhanced with sunspot size. On a time scale of a week and a month, we find decreasing MF extensions and a tendency for the MF velocity to increase for strongly decaying sunspots, whereas the changing EF velocity has no impact on the MF. Conclusions. On 3 h averages, the EF velocity scales with the size of sunspots, while the MF properties show no significant correlation with the EF or with the sunspot size. This we interpret as a hint that the physical origins of EF and MF are distinct.

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The flow field in the sunspot canopy

Cited by 33 publications

References 30 publications

Magnetic properties of G-band bright points in a sunspot moat

Magnetic properties of G-band bright points in a sunspot moat

Variation in sunspot properties between 1999 and 2014

Correlations between sunspots and their moat flows

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