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The Solar Rotation in the 1930s from the Sunspot and Flocculi Catalogs of the Ebro Observatory

Abstract

The tables of sunspot and flocculi heliographic positions included in the catalogs published by the Ebro Observatory in the 1930s have recently been recovered and converted into digital format by using optical character recognition (OCR) technology. We here analyzed these data by computing the angular velocity of several sunspot and flocculi groups. A difference was found in the rotational velocity for sunspots and flocculi groups at high latitudes, and we also detected an asymmetry between the northern and southern hemispheres, which is especially marked for the flocculi groups. The results were then fitted with a differential-rotation law [\(\omega=a+b \sin^{2} B\)] to compare the data obtained with the results published by other authors. A dependence on the latitude that is consistent with former studies was found. Finally, we studied the possible relationship between the sunspot/flocculi group areas and their corresponding angular velocity. There are strong indications that the rotational velocity of a sunspot/flocculi group is reduced (in relation to the differential rotation law) when its maximum area is larger.

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Acknowledgements

First of all, this work has been possible thanks to the recovery and digitalization of some sunspot/flocculi catalogs published at the beginning of the twentieth century by the Ebro Observatory. The authors would like to thank Jordi Vega and María José Blanca for their effort in digitalizing Ebro solar bulletins, and Alberto Gómez and Núria de Paula for their support. Ricard Casas acknowledges the financial support received from the Spanish Ministerio de Ciencia e Innovación (MICINN), project ESP2013-48274.

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Correspondence to J. J. Curto.

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de Paula, V., Curto, J.J. & Casas, R. The Solar Rotation in the 1930s from the Sunspot and Flocculi Catalogs of the Ebro Observatory. Sol Phys 291, 2269–2279 (2016). https://doi.org/10.1007/s11207-016-0974-2

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Keywords

  • Rotation
  • Active regions, velocity field
  • Sunspots, velocity
  • Solar cycle, observations