Simulating Solar Near-surface Rossby Waves by Inverse Cascade from Supergranule Energy

Dikpati, Mausumi; Gilman, Peter A.; Gómez, Gutiérrez; Kosovichev, A. G.; McIntosh, Scott W.; Sreenivasan, Katepalli R.; Warnecke, Jörn; Zaqarashvili, T. V.

doi:10.3847/1538-4357/ac674b

Cited by 10 publications

(8 citation statements)

References 49 publications

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“…What causes the Rossby waves to be stronger in the magnetic active phase? Excitation of the equatorial Rossby waves is not conclusively understood, although it is suspected that inverse cascading from supergranular energy is the main driving mechanism (Dikpati et al 2022). Supergranular energy being modified during the solar cycle could therefore serve as a mechanism of equatorial Rossby wave enhancement.…”

Section: Discussionmentioning

confidence: 99%

“…While, by now, global properties of equatorial Rossby waves are well identified, their interactions with mechanisms that act on the Sun of similar scales, such as large-scale flows or magnetic fields, are not. Rossby waves are thought to be excited within the convection zone, either within the tachocline or at supergranular depths (Dikpati et al 2022), but their coupling with the solar dynamo, magnetic fields or flows are still unknown, although some coupling must happen directly or indirectly on some scales. In fact, Liang et al (2019) already hinted at a possible connection between Rossby wave amplitudes and the progression of the solar cycle.…”

Section: Introductionmentioning

confidence: 99%

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Observed Power and Frequency Variations of Solar Rossby Waves with Solar Cycles

Waidele,

Zhao

2023

ApJL

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Several recent studies utilizing different helioseismic methods have confirmed the presence of large-scale vorticity waves known as solar Rossby waves within the Sun. Rossby waves are distinct from acoustic waves, typically with longer periods and lifetimes, and their general properties, even if only measured at the surface, may be used to infer properties of the deeper convection zone, such as the turbulent viscosity and entropy gradients that are otherwise difficult to observe. In this study, we utilize 12 yr of inverted subsurface velocity fields derived from the Solar Dynamics Observatory/Helioseismic and Magnetic Imager’s time–distance and ring-diagram pipelines to investigate the properties of the solar equatorial Rossby waves. By covering the maximum and the decline phases of Solar Cycle 24, these data sets enable a systematic analysis of any potential cycle dependence of these waves. Our analysis provides evidence of a correlation between the average power of equatorial Rossby waves and the solar cycle, with stronger Rossby waves during the solar maximum and weaker waves during the minimum. Our result also shows that the frequency of the Rossby waves is lower during the magnetic active years, implying a larger retrograde drift relative to the solar rotation. Although the underlying mechanism that enhances the Rossby wave power and lowers its frequency during the cycle maximum is not immediately known, this observation has the potential to provide new insights into the interaction of large-scale flows with the solar cycle.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Observed Power and Frequency Variations of Solar Rossby Waves with Solar Cycles

Waidele,

Zhao

2023

ApJL

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“…In the absence of a destabilising mechanism, the turbulent motions associated with solar convection provide an excitation mechanism, as in the case of solar p-modes (Lighthill 1967;Goldreich & Keeley 1977) or gravito-inertial waves (Mathis et al 2014;Augustson et al 2020). Non-linear 3D simulations of the convection zone can help us assess the importance of this mechanism (Bekki et al 2022a;Dikpati et al 2022). In this paper, we follow a different approach and present a theoretical model for the turbulent stochastic excitation of purely toroidal inertial modes in 2D in order to test the hypothesis that this mechanism is indeed responsible for the amplitude level at which inertial modes are observed on the Sun.…”

Section: Introductionmentioning

confidence: 99%

Interaction of solar inertial modes with turbulent convection

J.¹,

L.²

2023

A&A

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Context. Inertial modes have been observed on the Sun at low longitudinal wavenumbers. These modes probe the dynamics and structure of the solar convective zone down to the tachocline. While linear analysis allows the complex eigenfrequencies and eigenfunctions of these modes to be computed, it gives no information about their excitation nor about their amplitudes. Aims. We tested the hypothesis that solar inertial modes are stochastically excited by the turbulent motions entailed by convection. Unlike the acoustic modes, which are excited by vertical turbulent motions, the inertial modes are excited by the radial vorticity of the turbulent field. Methods. We have developed a theoretical formalism where the turbulent velocity fluctuations provide the mechanical work necessary to excite the modes. The modes are described by means of a 2D linear wave equation with a source term, under the β plane approximation. This wave equation restrained to a spherical surface is relevant for the quasi-toroidal inertial modes that are observed on the Sun. Latitudinal differential rotation is included in the form of a parabolic profile that approximates the solar differential rotation at low and mid latitudes. The turbulent vorticity field underlying the source term is treated as an input to the model and is constrained by observations of the solar surface. The solution to the linear inhomogeneous wave equation is written in terms of a Green function, which is computed numerically. Results. We obtain synthetic power spectra for the wave's latitudinal velocity, longitudinal velocity, and radial vorticity, with azimuthal orders between 1 and 20. The synthetic power spectra contain the classical equatorial Rossby modes, as well as a rich spectrum of additional modes. The mode amplitudes are found to be of the same order of magnitude as observed on the Sun (∼ 1 m/s). There is a qualitative transition between low and high azimuthal orders: the power spectra for m 5 show modes that are clearly resolved in frequency space, while the power spectra for m 5 display regions of excess power that consist of many overlapping modes.Conclusions. The general agreement between the predicted and observed inertial mode amplitudes supports the assumption of stochastic excitation by turbulent convection. Our work shows that the power spectra are not easily separable into individual modes, thus complicating the interpretation of the observations.

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“…They include other inertial oscillations too (Hathaway et al 2022). These Rossby waves can be understood from simulations also near the surface via the inverse cascade of kinetic energy, such as horizontal supergranular motions (Dikpati et al 2022). Most recently, theory has shown the possible existence of thermal Rossby waves, whose properties are tied to the outward density decline in the convection zone coupled with rotation and differential rotation (Hindman & Jain 2022).…”

Section: Introductionmentioning

confidence: 99%

Deciphering the Pre–solar-storm Features of the 2017 September Storm From Global and Local Dynamics

Raphaldini,

Dikpati,

Norton

et al. 2023

ApJ

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We investigate whether global toroid patterns and the local magnetic field topology of solar active region (AR) 12673 together can hindcast the occurrence of the biggest X-flares of solar cycle (SC)-24. Magnetic toroid patterns (narrow latitude belts warped in longitude, in which ARs are tightly bound) derived from the surface distributions of ARs, prior and during AR 12673 emergence, reveal that the portions of the south toroid containing AR 12673 was not tipped away from its north-toroid counterpart at that longitude, unlike the 2003 Halloween storms scenario. During the minimum phase there were too few emergences to determine multimode longitudinal toroid patterns. A new emergence within AR 12673 produced a complex nonpotential structure, which led to the rapid buildup of helicity and winding that triggered the biggest X-flare of SC-24, suggesting that this minimum-phase storm can be anticipated several hours before its occurrence. However, global patterns and local dynamics for a peak-phase storm, such as that from AR 11263, behaved like the 2003 Halloween storms, producing the third biggest X-flare of SC-24. AR 11263 was present at the longitude where the north and south toroids tipped away from each other. While global toroid patterns indicate that prestorm features can be forecast with a lead time of a few months, their application to observational data can be complicated by complex interactions with turbulent flows. Complex nonpotential field structure development hours before the storm are necessary for short-term prediction. We infer that minimum-phase storms cannot be forecast accurately more than a few hours ahead, while flare-prone ARs in the peak phase may be anticipated much earlier, possibly months ahead from global toroid patterns.

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Simulating Solar Near-surface Rossby Waves by Inverse Cascade from Supergranule Energy

Cited by 10 publications

References 49 publications

Observed Power and Frequency Variations of Solar Rossby Waves with Solar Cycles

Observed Power and Frequency Variations of Solar Rossby Waves with Solar Cycles

Interaction of solar inertial modes with turbulent convection

Deciphering the Pre–solar-storm Features of the 2017 September Storm From Global and Local Dynamics

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