Shibu K. Mathew

Properties of sunspots in cycle 23. I. Dependence of brightness on sunspot size and cycle phase

Aims. In this paper we investigate the dependence of umbral core brightness, as well as the mean umbral and penumbral brightness on the phase of the solar cycle and on the size of the sunspot. Methods. Albregtsen & Maltby (1978) reported an increase in umbral core brightness from the early to the late phase of solar cycle from the analysis of 13 sunspots which cover solar cycles 20 and 21. Here we revisit this topic by analysing continuum images of more than 160 sunspots observed by the MDI instrument on board the SOHO spacecraft for the period between 1998 March to 2004 March, i.e. a sizable part of solar cycle 23. The advantage of this data set is its homogeneity, with no seeing fluctuations. A careful stray light correction, which is validated using the Mercury transit of 7th May, 2003, is carried out before the umbral and penumbral intensities are determined. The influence of the Zeeman splitting of the nearby Ni I spectral line on the measured “continuum” intensity is also taken into account. Results. We did not observe any significant variation in umbral core, mean umbral and mean penumbral intensities with solar cycle, which is in contrast to earlier findings for the umbral core intensity. We do find a strong and clear dependence of the umbral brightness on sunspot size, however. The penumbral brightness also displays a weak dependence. The brightness-radius relationship has numerous implications, some of which, such as those for the energy transport in umbrae, are pointed out.

On the fine structure of sunspot penumbrae. I. A quantitative comparison of two semiempirical models with implications for the Evershed effect

Sunspot penumbrae exhibit prominent fine structure. Different interpretations of spectropolarimetric observations suggest different, sometimes contradictory, properties of this fine structure. In this paper we show that the results of inversions of penumbral infrared profiles based on one-component models with gradients of the atmospheric parameters and two-component models without gradients are compatible with each other. Our analysis reconciles the results of previous investigations and provides further support for the picture that sunspot penumbrae are composed of penumbral flux tubes embedded in a magnetic background. The magnetic field in the tubes is more horizontal and weaker than that of the background atmosphere. While the tubes carry most of the Evershed flow, the background is essentially at rest. We notice also that the magnetic field strength in the flux tubes drops much more slowly with radial distance than the background field. This finding is discussed as a possible driver for the Evershed flow.

Thermal-magnetic relation in a sunspot and a map of its Wilson depression

We present relations between thermal and magnetic quantities in a simple, isolated sunspot, as deduced from the inversion of 1.56 µm spectropolarimetric data. We used a combination of two infrared Fe I lines at 15 648.5 A and 15 652.8 A in the inversions. Due to the high Zeeman sensitivity of these lines, we can study this relationship in the entire sunspot. The relevant parameters were derived both as a function of location within the sunspot and of height in the atmosphere using an inversion technique based on response functions. In this paper we relate the magnetic vector with temperature. We find a non- linear relationship between the various components of the magnetic vector and temperature, which confirm the results from earlier investigations. We also computed the Wilson depression and the plasma β for the observed sunspot and compare our results with earlier findings.

Stray light correction and contrast analysis of Hinode broad-band images

The contrasts of features in the quiet Sun are studied using filtergrams recorded by the broad-band filter imager mounted on the Hinode/Solar Optical Telescope. In a first step, the scattered light originating in the instrument is modeled using Mercury transit data. Combinations of four two-dimensional Gaussians with different widths and weights were employed to retrieve the point-spread functions (PSF) of the instrument at different wavelengths, which also describe instrumental scattered light. The parameters of the PSFs at different wavelengths are tabulated. The observed images were then deconvolved using the PSFs. The corrected images were used to determine the contrasts of features such as bright points and granulation in different wavelength bands. After correction, rms contrasts of the granulation of between 0.11 (at 668 nm) and 0.22 (at 388 nm) were obtained. Similarly, bright point contrasts ranging from 0.07 (at 668 nm) to 0.78 (at 388 nm) are found, which are a factor of 1.8 to 2.8 higher than those obtained before PSF deconvolution. The mean contrast of the bright points is found to be somewhat higher in the CN-band than in the G-band, which confirms theoretical predictions.

Source region of the 18 November 2003 coronal mass ejection that led to the strongest magnetic storm of cycle 23

[1] The superstorm of 20 November 2003 was associated with a high-speed coronal mass ejection (CME) which originated in the NOAA AR 10501 on 18 November. This coronal mass ejection had severe terrestrial consequences leading to a geomagnetic storm with Dst index of -472 nT, the strongest of the current solar cycle. In this paper, we attempt to understand the factors that led to the coronal mass ejection on 18 November. We have also studied the evolution of the photospheric magnetic field of NOAA AR 10501, the source region of this coronal mass ejection. For this purpose, the Michelson Doppler Imager line-of-sight magnetograms and vector magnetograms from Solar Flare Telescope, Mitaka, obtained during 17-19 November 2003 were analyzed. In particular, quantitative estimates of the temporal variation in magnetic flux, energy, and magnetic field gradient were estimated for the source active region. The evolution of these quantities was studied for the 3-day period with an objective to understand the preflare configuration leading up to the moderate flare which was associated with the geoeffective coronal mass ejection. We also examined the chromospheric images recorded in H α from Udaipur Solar Observatory to compare the flare location with regions of different magnetic field and energy. Our observations provide evidence that the flare associated with the CME occurred at a location marked by high magnetic field gradient which led to release of free energy stored in the active region.

Supersonic Downflows in a Sunspot Light Bridge

We report the discovery of supersonic downflows in a sunspot light bridge using measurements taken with the spectropolarimeter onboard the Hinode satellite. The downflows occur in small patches close to regions where the vector magnetic field changes orientation rapidly, and are associated with anomalous circular polarization profiles. An inversion of the observed Stokes spectra reveals velocities of up to 10 km s–1, making them the strongest photospheric flows ever measured in light bridges. Some (but not all) of the downflowing patches are cospatial and cotemporal with brightness enhancements in chromospheric Ca II H filtergrams. We suggest that these flows are due to magnetic reconnection in the upper photosphere/lower chromosphere, although other mechanisms cannot be ruled out.

Analysis of a Fragmenting Sunspot using Hinode Observations

We employ high-resolution filtergrams and polarimetric measurements from Hinode to follow the evolution of a sunspot for eight days starting on 2007 June 28. The imaging data were corrected for intensity gradients, projection effects, and instrumental stray light prior to the analysis. The observations show the formation of a light bridge at one corner of the sunspot by a slow intrusion of neighboring penumbral filaments. This divided the umbra into two individual umbral cores. During the light bridge formation, there was a steep increase in its intensity from 0.28 to 0.7 I QS in nearly 4 hr, followed by a gradual increase to quiet-Sun (QS) values in 13 hr. This increase in intensity was accompanied by a large reduction in the field strength from 1800 G to 300 G. The smaller umbral core gradually broke away from the parent sunspot nearly two days after the formation of the light bridge, rendering the parent spot without a penumbra at the location of fragmentation. The penumbra in the fragment disappeared first within 34 hr, followed by the fragment whose area decayed exponentially with a time constant of 22 hr. In comparison, the parent sunspot area followed a linear decay rate of 0.94 Mm2 hr–1. The depleted penumbra in the parent sunspot regenerated when the inclination of the magnetic field at the penumbra-QS boundary became within 40° from being completely horizontal and this occurred near the end of the fragments lifetime. After the disappearance of the fragment, another light bridge formed in the parent which had similar properties as the fragmenting one, but did not divide the sunspot. The significant weakening in field strength in the light bridge along with the presence of granulation is suggestive of strong convection in the sunspot, which might have triggered the expulsion and fragmentation of the smaller spot. Although the presence of QS photospheric conditions in sunspot umbrae could be a necessary condition for fragmentation, it is not a sufficient one.

PROPERTIES OF UMBRAL DOTS FROM STRAY LIGHT CORRECTED HINODE FILTERGRAMS

High resolution blue continuum filtergrams from Hinode are employed to study the umbral fine structure of a regular unipolar sunspot. The removal of scattered light from the images increases the rms contrast by a factor of 1.45 on average. Improvement in image contrast renders identification of short filamentary structures resembling penumbrae that are well separated from the umbra-penumbra boundary and comprise bright filaments/grains flanking dark filaments. Such fine structures were recently detected from ground based telescopes and have now been observed with Hinode. A multi-level tracking algorithm was used to identify umbral dots in both the uncorrected and corrected images and to track them in time. The distribution of the values describing the photometric and geometric properties of umbral dots are more easily affected by the presence of stray light while it is less severe in the case of kinematic properties. Statistically, umbral dots exhibit a peak intensity, effective diameter, lifetime, horizontal speed and a trajectory length of 0.29 I_QS, 272 km, 8.4 min, 0.45 km/s and 221 km respectively. The 2 hr 20 min time sequence depicts several locations where umbral dots tend to appear and disappear repeatedly with various time intervals. The correction for scattered light in the Hinode filtergrams facilitates photometry of umbral fine structure which can be related to results obtained from larger telescopes and numerical simulations.

Automated Detection of Filaments and Their Disappearance Using Full-Disc Hα Images

A new algorithm is presented that automatically detects filaments on the Sun in full-disc Hα images. Pre-processing of Hα images includes corrections for limb darkening and foreshortening. Further, by applying suitable intensity and size thresholds, filaments are extracted, while other solar features, e.g. sunspots and plages, are removed. Filament attributes such as their position on the solar disc, total area, length, and number of fragments are determined. In addition, every filament is also labelled with a unique number for identification. The algorithm is capable of following a particular filament through successive images, which allows us to detect their changes and disappearance. We have analysed ten cases of filament eruption from different observatories, and the results obtained are presented. The algorithm will eventually be integrated with an upcoming telescope at the Udaipur Solar Observatory for real-time monitoring of activated/eruptive filaments. This aspect should prove to be of particular importance in studies pertaining to space weather.

SUPERSONIC DOWNFLOWS AT THE UMBRA-PENUMBRA BOUNDARY OF SUNSPOTS

High-resolution spectropolarimetric observations of three sunspots taken with Hinode demonstrate the existence of supersonic downflows at or close to the umbra-penumbra boundary which have not been reported before. These downflows are confined to large patches, usually encompassing bright penumbral filaments, and have lifetimes of more than 14 hr. The presence of strong downflows in the center-side penumbra near the umbra rules out an association with the Evershed flow. Chromospheric filtergrams acquired close to the time of the spectropolarimetric measurements show large, strong, and long-lived brightenings in the neighborhood of the downflows. The photospheric intensity also exhibits persistent brightenings comparable to the quiet Sun. Interestingly, the orientation of the penumbral filaments at the site of the downflows is similar to that resulting from the reconnection process described by Ryutova et al. The existence of such downflows in the inner penumbra represents a challenge for numerical models of sunspots because they have to explain them in terms of physical processes likely affecting the chromosphere.