Space Weather Challenge and Forecasting Implications of Rossby Waves

Abstract

Rossby waves arise in thin layers within fluid regions of stars and planets. These global wave‐like patterns occur due to the variation in Coriolis forces with latitude. In the past several years observational evidence has indicated that there are also Rossby waves in the Sun. Although Rossby waves have been detected in the Sun's photosphere and corona, they most likely originate in the solar tachocline, the sharp shear layer at the base of the solar convection zone, where the differential rotation driven by convection transitions to the solidly rotating radiative interior. These waves differ from their Earth's counterparts by being strongly modified by toroidal magnetic fields in the solar tachocline. Recent simulations of magnetohydrodynamics of tachocline Rossby waves and magnetic fields are demonstrated to produce strong “tachocline nonlinear oscillations,” which have periods similar to those observed in the solar atmosphere—enhanced periods of solar activity, or “seasons”—occurring at intervals between six months and two years. These seasonal/subseasonal bursts produce the strongest eruptive space weather events. Thus, a key to forecasting the timing, amplitude, and location of future activity bursts, and hence space weather events, could lie in our ability to forecast the phase and amplitude of Rossby waves and associated tachocline nonlinear oscillations. Accurately forecasting the properties of solar Rossby waves and their impact on space weather will require linking surface activity observations to the magnetohydrodynamics of tachocline Rossby waves, using modern data assimilation techniques. Both short‐term (hours to days) and long‐term (decadal to millennial) forecasts of space weather and climate are now being made. We highlight in this article the potential of solar Rossby waves for forecasting space weather on intermediate time scales, of several weeks to months up to a few years ahead.

Simulating Properties of “Seasonal” Variability in Solar Activity and Space Weather Impacts

“…With these considerations, the evolution of latitude-longitude locations of flux-emergence have been compared with emerged active regions from observations for several Carrington rotations in figure 12 of Dikpati and McIntosh (2020). Therefore, we can understand our simulation results for a broad magnetic band using similar physical arguments, which we present below in a compact way.…”

“…While we are familiar with Earth's Rossby waves, which play crucial roles in creating "jet stream" patterns that are used to forecast our daily weather, solar Rossby waves are no longer mere hypothesis; they have recently been observed using coronal brightpoints movements (McIntosh et al, 2017), coronal holes migration (Krista et al, 2018) and helioseismic measurements (Loptien et al, 2018;Hanasoge and Mandal, 2019). But a major difference between solar and planetary Rossby waves is that the Sun has strong magnetic fields, and hence solar Rossby waves are magnetically modified (see, e.g., Dikpati and McIntosh 2020;Zaqarashvili et al, 2021).…”

Variations in Wave Energy and Amplitudes along the Ray Paths of Barotropic Rossby Waves in Horizontally Non-Uniform Basic Flows

“…Actually, we know that MHD Rossby waves also have small wavenumbers, representing a large scale motion [37]. Finally, the total energy (potential plus kinetic plus magnetic) is conserved for MHD Rossby waves although potential vorticity is not conserved [33]. This implies that we also can build conserved variables as we do for Rossby waves in the Earth's atmosphere.…”

“…On the other hand, all rotating celestial fluid bodies should contain Rossby waves because they are inherent to rotating stratified spheres of fluid or plasma [31,32]. For example, solar Rossby waves propagate and are strongly influenced by the magnetic fields that are almost ubiquitous in the Sun's atmosphere [33]. The resulting magnetohydrodynamic (MHD) Rossby waves can interact with differential rotation and toroidal fields to sustain quasi-periodic tachocline non-linear oscillations (TNOs) that occur due to back-and-forth energy exchanges among potential, kinetic and magnetic energies [34,35].…”

The High Inclination Solar Mission

“…small dipolar regions (EUV brightpoints) in the composite observations, it was discovered that the Sun's magnetic activity bands displayed the characteristic signature of (magnetized) Rossby waves [McIntosh et al, 2017] that drove the longitudinal behavior of magnetic flux emergence and hence the key ingredients of space weather. Subsequent effort invested in understanding the nature of these waves [Dikpati and McIntosh, 2020] has demonstrated the potential of breakthrough by developing greater predictability of space weather phenomena and the magnetic activity in which it is rooted. A high latitude observing platform like HISM will have the opportunity to observe the development of these waves, their longitudinal structure and evolution and finally the organization of phenomena that they drive.…”