Search for torsional oscillations in isolated sunspots

Abstract

In this work we seek evidence for global torsional oscillations in alpha sunspots. We have used long time series of continuum intensity and magnetic field vector maps from the Helioseismic and Magnetic Imager (HMI) instrument on board the Solar Dynamics Observatory (SDO) spacecraft.The time series analysed here span the total disk passage of 25 isolated sunspots. We found no evidence of global long-term periodic oscillations in the azimuthal angle of the sunspot magnetic field within ∼ 1 degree. This study could help us to understand the sunspot dynamics and its internal structure.

Discovery of long-period magnetic field oscillations and motions in isolated sunspots

“…The results presented in the following refer to the sunspot of NOAA AR12218, which has been taken as a representative of the sample described in Griñón-Marín et al (2017). The results of the other sunspots of the sample are shown in Appendix A.…”

“…To perform this study we applied the same steps as in the paper Griñón-Marín et al (2017) to the data. Briefly, the magnetic field vector was deprojected by transforming from the line-ofsight coordinate reference system to the local solar reference frame applying two reference system rotations.…”

Torsional Alfvénic Oscillations Discovered in the Magnetic Free Energy during Solar Flares

“…Torsional oscillations of sunspots, which harbour the footpoints of many active region loops have been observed to rotate on much longer time scales. For instance, rotational periods in the range of P ≈ 2 − 8 days were reported (Gopasyuk and Kosovichev 2011;Grinon-Marin et al 2017), and thus cannot be considered as the driver of the azimuthal oscillations observed here (with P ≈ 10 − 22 min), although they are located at the right place.…”

“…Today, various types of torsional oscillations have been studied in solar physics, in the context of: (i) the solar dynamo that oscillates between the global poloidal and toroidal magnetic field in an 11-year solar cycle (Howard and LaBonte 1980;Kitchatinov et al 1999;Durney 2000;Chakraborty et al 2009;Gurerrero et al 2016;Lekshmi et al 2018;Kosovichev and Pipin 2019) or stellar cycle (Lanza 2007); (ii) the differential solar rotation, where zonal latitude bands on the solar surface exhibit slower-and fasterthan average rotation speeds (Spruit 2003;Rempel 2006Rempel , 2007Rempel , 2012Howe et al 2006Howe et al , 2009Howe et al , 2013; (iii) meridional circulation (Gonzalez-Hernandez et al 2010); (iv) torsional oscillations in the solar (or stellar) convection zone (Covas et al 2000(Covas et al , 2004Noble et al 2003;Zhao and Kosovichev 2004); (v) helioseismic measurements of torsional oscillations inside the Sun, using time-distance helioseimology measurements and inversions of Michelson Doppler Imager (MDI) data onboard the Solar and Heliospheric Observatory (SoHO) spacecraft, MDI/SOHO data (Zhao and Kosovichev 2004;Vorontsov et al 2002), or ring-diagram analysis of Global Oscillation Network Group (GONG) data (Howe et al 2006;Lekshmi et al 2018); (vi) torsional and compressional waves in photospheric flux tubes (Sakai et al 2001;Luo et al 2002;Routh et al 2007Routh et al , 2010 and intergranular magnetic flux concentrations (Shelyag 2015); (vii) torsional oscillations of sunspots (Gopasyuk and Kosovichev 2011;Grinon-Marin et al 2017), (viii) torsional oscillations in the generation of active regions (Petrovay and Forgacs-Dajka 2002); (viii) torsional oscillations of coronal loops or threads (Zaqarashvili et al 2003…”