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Reconciling various solar Rossby wave observations using global (M)HD models

Presentation #113.19 in the session “Solar Physics Division (SPD): Photosphere & Chromosphere, Solar Interior, and Solar Cycle”.

Published onJun 18, 2021
Reconciling various solar Rossby wave observations using global (M)HD models

Various observational analyses indicate that the Sun has Rossby waves, which include hydrodynamic Rossby waves like that on the Earth’s atmosphere as well as magnetohydrodynamic ones, which do not have their counterparts on the Earth. Many of us are showing, from observations and model-calculations, that solar hydrodynamic Rossby waves have retrograde speed and they follow Rossby-Haurwitz type dispersion relation. However, it has also recently been shown MHD Rossby waves can have both retrograde as well as prograde speed; if MHD Rossby waves are retrograde they are more retrograde than their HD counterparts, whereas if they are prograde, they are relatively slow. Helioseismically determined Rossby waves detected so far are HD waves that are energetically neutral. Coronal bright points as well as long-lived coronal holes’ longitudinal drift-patterns with time show evidence of HD as well as MHD Rossby waves. We know from nonlinear simulations of global MHD waves and instabilities that Rossby waves can be energetically active and hence, can nonlinearly interact with the solar differential rotation and spot-producing toroidal magnetic fields, very much like nonlinear Orr mechanism in fluid dynamics. Is it possible to reconcile the various solar Rossby waves observations? Certainly different observational techniques applied to different elevations on the Sun may be measuring different Rossby waves. Furthermore, the different measurements may be detecting waves that originate at different depths inside the Sun. Solar atmosphere magnetic features may have roots deep in the convection zone and reflect MHD Rossby waves at those depths. We will present model-simulations to show what physical conditions are responsible for producing sectoral modes and what are responsible for generating energetically active Rossby waves, which have important implications in causing short-term variability in solar activity, and in turn, in space weather. This work is supported by the NCAR, sponsored by the NSF under cooperative agreement 1852977. MD acknowledges support from several NASA grants, namely the LWS award 80NSSC20K0355 to NCAR, subaward to NCAR from NASA’s DRIVE Center award 80NSSC20K0602 (originally awarded to Stanford) and NASA-HSR subaward 80NSSC18K1206 (originally awarded to NSO).

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