Sympathetic eruptions on the Sun have been observed for several decades, but the mechanisms by which one eruption can trigger another one remain poorly understood. We present a 3D MHD simulation that suggests two possible magnetic trigger mechanisms for sympathetic eruptions. We consider a configuration that contains two coronal flux ropes located within a pseudo-streamer and one rope located next to it. A sequence of eruptions is initiated by triggering the eruption of the flux rope next to the streamer. The expansion of the rope leads to two consecutive reconnection events, each of which triggers the eruption of a flux rope by removing a sufficient amount of overlying flux. The simulation qualitatively reproduces important aspects of the global sympathetic event on 2010 August 1 and provides a scenario for so-called twin filament eruptions. The suggested mechanisms are applicable also for sympathetic eruptions occurring in other magnetic configurations. Subject headings: Sun: corona -Sun: coronal mass ejections (CMEs) -Sun: flares -Sun: filaments, prominences -Sun: magnetic topology -Methods: numerical 1 We do not distinguish here between sympathetic flares and sympathetic CMEs, since both are part of the same eruption process.

[1] We construct a solar cycle strength prediction tool by modifying a calibrated flux-transport dynamo model, and make predictions of the amplitude of upcoming solar cycle 24. We predict that cycle 24 will have a 30-50% higher peak than cycle 23, in contrast to recent predictions by Svalgaard et al. and Schatten, who used a precursor method to forecast that cycle 24 will be considerably smaller than 23. The skill of our approach is supported by the flux transport dynamo model's ability to correctly 'forecast' the relative peaks of cycles 16-23 using sunspot area data from previous cycles. Citation: Dikpati, M., G. de Toma, and P. A. Gilman (2006), Predicting the strength of solar cycle 24 using a flux-transport dynamo-based tool, Geophys. Res. Lett., 33, L05102,

Motivated by observed anomalous features in cycle 23, as inferred from records of photospheric magnetic flux, we develop a flux transport dynamo-based scheme in order to investigate the physical cause of such anomalies. In this first study we focus on understanding anomalies occurring in the polar field evolutionary pattern in cycle 23, namely, why the polar reversal in cycle 23 was slow, why after reversal the buildup of the polar field was slow, and why the south pole reversed approximately a year after the north pole did. We construct a calibrated flux transport dynamo model that operates with dynamo ingredients such as differential rotation, meridional circulation, and large-scale poloidal field source derived from observations. A few other dynamo ingredients, such as diffusivity and quenching pattern, for which direct observations are not possible, are fixed by using theoretical guidance. By showing that this calibrated model can reproduce major longitude-averaged solar cycle features, we initialize the model at the beginning of cycle 22 and operate by incorporating the observed variations in meridional circulation and large-scale surface magnetic field sources to simulate the polar field evolution in cycle 23. We show that a 10%-20% weakening in photospheric magnetic flux in cycle 23 with respect to that in cycle 22 is the primary reason for a $1 yr slowdown in polar reversal in cycle 23. Weakening in this flux is also the reason for slow buildup of polar field after reversal, whereas the observed north-south asymmetry in meridional circulation in the form of a larger decrease in flow speed in the northern hemisphere than that in the southern hemisphere during 1996-2002 and the appearance of a reverse, high-latitude flow cell in the northern hemisphere during 1998-2001 caused the north polar field to reverse before the south polar field.