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[en] Two large northern polar crown prominences that erupted on 2010 April 13 and 2010 August 1 were analyzed using images obtained from the Extreme UltraViolet Imager on the twin Solar Terrestrial Relations Observatory spacecraft. Several features along the prominence legs were reconstructed using a stereoscopic reconstruction technique developed by us. The three-dimensional changes exhibited by the prominences can be explained as an interplay between two different motions, namely helical twist in the prominence spine, and overall non-radial equatorward motion of the entire prominence structure. The sense of twist in both the prominences is determined from the changes in latitudes and longitudes of the reconstructed features. The prominences are observed starting from a few hours before the eruption. Increase in height before and during the eruption allowed us to study the kinematics of the prominences in the two phases of eruption, the slow-rise and the fast-eruptive phase. A constant value of acceleration was found for each reconstructed feature in each phase, but it showed a significant change from one leg to the other in both the prominences. The magnitude of acceleration during the eruptive phase is found to be commensurate with the net effect of the two motions stated above.
[en] High-degree solar mode frequencies as measured by ring diagrams are known to change in the presence of the strong magnetic fields found in active regions. We examine these changes in frequency for a large sample of active regions analyzed with data from the Michelson Doppler Imager on board the Solar and Heliospheric Observatory spacecraft, spanning most of solar cycle 23. We confirm that the frequencies increase with increasing magnetic field strength, and that this dependence is generally linear. We find that the dependence is slightly but significantly different for active regions with different sunspot types.
[en] Even after more than 160 years of observations and modelling of solar prominences their true nature contains many open questions. In this work we argue that current 2D prominence fine structure models can help us to understand the puzzling connection between quasi-vertical fine structures often seen in quiescent prominences observed on the solar limb and horizontally aligned dark fibrils representing the fine structures of prominences observed in absorption against the solar disk (filaments).
[en] I present a study of high-resolution time series of Ca II H images and Fe I 630.15 nm spectra taken with the Solar Optical Telescope on the Hinode spacecraft. There is excellent correspondence between the Ca II H and the Fe I line core intensity, except tenuous emission around the network field concentrations in the former that is absent in the latter. Analysis of on-disk observations and a comparison with limb observations suggests that this 'network haze' corresponds to spicules, and likely to type-II spicules in particular. They are known to appear in emission in on-disk broadband Ca II H diagnostics and the network haze is strongest in those areas where features similar to type-II spicules are produced in simulations.
[en] Recent studies revealed a controversy in long-term variations in sunspot field strengths. On one hand, the sunspot field strengths computed by averaging both large and small sunspots and pores show a gradual decrease over the declining phase of solar Cycle 23 and the rising phase of Cycle 24. On the other hand, the strongest sunspot field strengths demonstrate only solar cycle variations with no long-term decline. Here, we investigate the field strength and area properties of sunspots in an attempt to reconcile the presence of both tendencies in recent sunspot field strength measurements. First, we analyze the data set from Penn and Livingston, and we show that in addition to the previously reported long-term decline, the data show the solar cycle variation when only sunspots with the strongest magnetic fields are included. Next, we investigate the variations in the number of sunspots of different sizes, and we find a negative correlation between the numbers of small and large sunspots. Finally, we show that during the period of 1998-2011, the number of large sunspots gradually decreased, while the number of small sunspots steadily increased. We suggest that this change in the fraction of small and large sunspots (perhaps, due to changes in the solar dynamo) can explain the gradual decline in average sunspot field strength as observed by Penn and Livingston.
[en] We present an analysis of facular/network and sunspot areas (and their ratio) covering most of cycle 22 and all of cycle 23. The data are corrected areas (in microhemispheres) from full-disk solar images using two photometric telescopes at the San Fernando Observatory, CFDT1 and CFDT2. Images from CFDT2 have approximately twice the spatial resolution of CFDT1. Sunspot areas are obtained from red images where spots are determined as those pixels darker than -8.5%. Facular/network areas are from Ca II K-line images where facular/network pixels are brighter than 4.8%. Regressions of facular area versus spot area for CFDT1 give a slope term of 25. For CFDT2, the slope term is 33. The average ratio of facular to spot area for cycle 22 is 45 and for cycle 23 the ratio is 42. These values are substantially higher than those from earlier studies. The increase is due to a combination of higher spatial resolution and the removal of a correction factor in μ. For the 0.3 nm K-line images, the spot to facular/network ratio is 138 for six years of cycle 23. A relation is given for the dependence of facular/network area on contrast. The relationship of facular/network area to sunspot area is linear for data from both telescopes.
[en] The Maunder minimum forms an archetype for the Grand minima, and detailed knowledge of its temporal development has important consequences for the solar dynamo theory dealing with long-term solar activity evolution. Here, we reconsider the current paradigm of the Grand minimum general scenario by using newly recovered sunspot observations by G. Marcgraf and revising some earlier uncertain data for the period 1636-1642, i.e., one solar cycle before the beginning of the Maunder minimum. The new and revised data dramatically change the magnitude of the sunspot cycle just before the Maunder minimum, from 60-70 down to about 20, implying a possibly gradual onset of the minimum with reduced activity started two cycles before it. This revised scenario of the Maunder minimum changes, through the paradigm for Grand solar/stellar activity minima, the observational constraint on the solar/stellar dynamo theories focused on long-term studies and occurrence of Grand minima.