Luis R. Bellot Rubio

Cold, Supersonic Evershed Downflows in a Sunspot

We report here on the discovery of supersonic Evershed downflows in the penumbra of a sunspot. These flows are shown to occur along spatially unresolved, very cold magnetic flux tubes whose downflowing footpoints are found from the middle penumbra outward. Evershed flows along magnetic field lines returning to the solar surface were discovered by Westendorp Plaza and coworkers, but only in the outer parts of the penumbra and beyond its visible boundary; on the other hand, no supersonic flows of any type have ever been reported in the photosphere of sunspots, except for the very different case of the delta spot analyzed by Martinez Pillet and coworkers. We present unequivocal evidence of such supersonic motions, already predicted theoretically by the siphon-flow model, from the interpretation of infrared spectropolarimetric observations of a sunspot with unprecedented spatial resolution.

Observation of Convective Collapse and Upward-moving Shocks in the Quiet Sun

We present spectropolarimetric evidence of convective collapse and destruction of magnetic flux by upward-moving fronts in the quiet Sun. The observational material consists of time series of the full Stokes vector of two infrared spectral lines emerging from regions associated with Ca II K network points. The amplitude of the circular polarization profiles of a particular spatial point is seen to increase while the profiles are redshifted. It then decreases during a much shorter phase characterized by large blueshifts. Inspection of the data indicates that the blueshift occurs because of the sudden appearance of a new, strongly displaced Stokes V profile of the same polarity. The amplification of the magnetic signal takes place in a time interval of about 13 minutes, while blueshifts and the concomitant decreasing Stokes V amplitudes last for only 2 minutes. An inversion code based on the thin flux-tube scenario has been applied to the data in order to derive the thermal, magnetic, and dynamic structures of the atmosphere. According to our results, the field strength undergoes a moderate increase from 400 to 600 G at z = 0 km during the phase in which redshifts are present. The observed redshifts are produced by internal downflows of up to 6 km s-1 at z = 0 km. After ~13 minutes, the material falling down inside the tube appears to bounce off in the deeper layers, originating an upward-propagating front whose manifestation on the Stokes V profiles is a large blueshift. The front moves with a speed of 2.3 km s-1 and has a downflow-to-upflow velocity difference of about 7 km s-1 initially and some 4 km s-1 after 2 minutes. It strongly weakens the magnetic field strength and may be responsible for the complete destruction of the magnetic feature. The observed behavior is in general agreement with theoretical predictions of flux expulsion, convective collapse, and development of shocks within magnetic flux tubes.

The Rotation Period of C/1995 O1 (Hale-Bopp)

C/1995 O1 (Hale-Bopp) was observed in daylight on 16 days between 1997 April 1 and 1997 April 28, five of which had long time sequences (up to 10 hr of data), using the near infrared CAIN camera on the 1.5 m Carlos Sanchez Telescope at Teide Observatory (Tenerife, Canary Islands, Spain). Three spiral dust jet structures were observed for several almost complete rotations. A nucleus rotation period of 11.34±0.02 hr was determined from two different methods. No variations of the rotation period with time due to precessional effects were found in our data. However, the time sampling of the observations, similar to the suggested spin precession period, prevents us from ascertaining whether such variations exist. We note, though, that the good agreement of our results with the rotation periods at different epochs reported by other groups suggests that if they exist, these variations must be small, hence the rotation cannot be very complex.

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.

TEMPORAL EVOLUTION OF VELOCITY AND MAGNETIC FIELD IN AND AROUND UMBRAL DOTS

We study the temporal evolution of umbral dots (UDs) using measurements from the CRISP imaging spectropolarimeter at the Swedish 1 m Solar Telescope. Scans of the magnetically sensitive 630 nm iron ...

MAGNETIC FLUX CANCELLATION IN THE MOAT OF SUNSPOTS: RESULTS FROM SIMULTANEOUS VECTOR SPECTROPOLARIMETRY IN THE VISIBLE AND THE INFRARED

We analyze multiwavelength observations of the cancellation of a moving magnetic feature and a plage element at the outer edge of the moat of an isolated, round sunspot. The event lasted for some 35 minutes until the smaller flux concentration disappeared completely from the photosphere. The data set consists of high-resolution, full vector spectropolarimetric measurements of four visible lines and two near-infrared lines, along with speckle-reconstructed G-band and Ca II H filtergrams. The observations reveal strong chromospheric emission at the neutral line separating the two magnetic poles; it becomes visible 18 minutes after the cancellation has started and persists for 25 minutes. We have carried out an inversion of the observed Stokes profiles to determine the variation of the vector magnetic field, temperature, and line-of-sight velocity during the cancellation. No significant changes in field strength, field inclination, or temperature are observed in either of the two opposite-polarity patches. The decrease in magnetic flux is primarily due to a decrease in magnetic filling factor, which is accompanied by strong upflows (of at least 1.1 km s-1) in the smaller flux concentration. These results suggest that the cancellation is due to magnetic reconnection in the photosphere.

Observation and Interpretation of Meteoroid Impact Flashes on the Moon

The first unambiguous detection of meteoroids impacting the night side of the Moon was obtained during the 1999 Leonid storm. Up to eight optical flashes were recorded with CCD video cameras attached to small telescopes on November 18, 1999. Six impacts were videotaped by at least two independent observers at the same times and lunar locations, which is perhaps the strongest evidence for their collisional nature. The flashes were clearly above the noise and lasted for less than 0.02 s. Although previous observational efforts did not succeed in detecting impact flashes, additional candidates have been reported in the literature. The evidence accumulated so far implies that small telescopes equipped with high speed cameras can be used as a new tool for studying meteoroid streams, sporadic meteoroids, and hypervelocity collisions. In this review we discuss the various intervening parameters for detectability of flashes on the night side of the Moon (geometrical effects, contamination by scattered light from the day side, and properties of the meteoroids such as speed and flux of particles). Particular emphasis is placed on the analysis of the observations in order to derive relevant physical parameters such as luminous efficiencies, impactor masses, and crater sizes. Some of these parameters are of interest for constraining theoretical impact models. From a simple analysis, it is possible to derive the mass distribution of the impactors in the kg range. A more elaborate analysis of the data permits an estimate of the fraction of kinetic energy converted to radiation (luminous efficiency) if the meteoroid flux on the Moon is known. Applied to the 1999 lunar Leonids, these methods yield a mass index of 1.6 ± 0.1 and luminous efficiencies of 2 × 10−3 with an uncertainty of about one order of magnitude. Predictions of visibility of the major annual meteor showers are given for the next few years. These include the forthcoming 2001 Leonid return, for which we estimate detection rates in the visible.

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.

The Analysis of Penumbral Fine Structure Using an Advanced Inversion Technique

We present a method to study the penumbral fine structure using data obtained by the spectropolarimeter on board Hinode. For the first time, the penumbral filaments can be considered as being resolved in spectropolarimetric measurements. This enables us to use inversion codes with only one-component model atmospheres, and thus to assign the obtained stratifications of the plasma parameters directly to the penumbral fine structure. This approach was applied to the limb-side part of the penumbra in the active region NOAA 10923. Preliminary results show a clear dependence of the plasma parameters on the continuum intensity in the inner penumbra, i.e., a weaker and horizontal magnetic field along with an increased line-of-sight velocity are found in the low layers of the bright filaments. The results in the mid penumbra are ambiguous, and future analyses are necessary to unveil the magnetic field structure and other plasma parameters there.