Using Kinematic Dynamo Modeling to Probe the Solar Magnetic Cycle

Chandra, Sanghita (2022) Using Kinematic Dynamo Modeling to Probe the Solar Magnetic Cycle. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS dissertation of Sanghita Chandra (17MS135)) 17MS135_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (18MB)

Abstract

Solar activity encompasses a lot of interesting dynamical aspects. Over and beyond its gravitational influence on the solar system, the Sun is the primary source of energy, and therefore life on Earth. But our host star is more than that — it has magnetic properties as well. When the Sun is magnetically active, numerous dark spots known as sunspots appear on the photosphere or the solar surface. This is occasionally accompanied by the release of flares and coronal mass ejections (CMEs). Even in the absence of such high magnetic activity, the Sun continuously releases particles and plasma into space via the solar wind. The thin and wispy outermost layer of the solar atmosphere is the corona. The temperatures here are in the order of millions of degrees. This imparts enough kinetic energy to the plasma and particles in the solar atmosphere to escape the gravitational influence of the Sun. This fuels the solar wind, which impacts the near-Earth space conditions and other planets in various ways. Our magnetic host star is thus quite active in shaping the space environment in the heliosphere. To comprehend what drives such solar activity, it is crucial to understand the magnetic nature of the Sun. The first step towards this is to explore the mechanism by which magnetic fields are generated. The current perception is that the Sun harbors a magnetohydrodynamic dynamo mechanism in its interior. This allows for the generation and recycling of magnetic fields. The physical manifestation of such dynamics is observed through sunspots, solar eruptions, and radiation released by the Sun. The global solar magnetic field has two components – the toroidal and poloidal fields, which are mutually coupled. The interplay of these two magnetic field components drives the solar magnetic cycle. The 11-year solar cycle is central to analyzing solar magnetic behavior. Although we have made significant progress in understanding the dynamics of the Sun, a lot remains unknown. Predicting future solar cycle activity is itself a daunting task. This is true for short-term (tens of years) and long-term (hundreds of years) solar activity. Short-term solar cycle activity shapes space weather, whereas long-term activity falls in the domain of space climate studies. Solar magnetic activity peaks following the ascending phase of the solar cycle. With a larger number of sunspots, there is a higher probability of solar eruptions such as flares and CMEs, accompanied by sudden release of energy through the reorganization of magnetic fields. These transient events constitute a major aspect of space weather. Space weather, in turn, influences planetary systems such as the geomagnetic field. The geomagnetic field shapes the Earth’s magnetosphere, which acts as a protective sheath against energetic plasma and particles ejected from the Sun. High magnetic activity could have hazardous impacts. These include damaging satellites, hindering space missions, perturbing the Earth’s upper atmosphere, and damaging electric grids on Earth — potentially leading to radio or telecommunication blackouts. Furthermore, the longterm magnetic activity governs the interaction of the solar ejecta with planets and their atmospheres. Properties of the host star and star-planet interactions in stellar systems help probe habitability conditions in (exo)planets. Therefore, monitoring and predicting the solar activity cycle is crucial for assessing the impacts of solar activity over a range of timescales. A priori information on high magnetic activity also aids in space mission planning and mission lifetime estimates. In the context, forecasting future solar cycle amplitudes is important because solar activity governs our space environmental conditions. The absence of observational data from the solar interior makes it difficult to constrain the exact dynamics playing out deep within the Sun. From observations, it is well known that a causal connection exists between the poloidal field strength at the end of a solar cycle and the subsequent cycle amplitude. This enables prediction techniques like the precursor method. But is there any feedback mediated via interaction of toroidal field belts in the solar interior on the ampplitude of the upcoming solar cycle? The first part of this thesis aims at deciphering the answer to this question. Using solar dynamo simulations we investigate whether the toroidal flux lying at the base of the convection zone impacts the toroidal field (and hence the amplitude) of the next sunspot cycle. This study could help refine solar cycle predictions and account for any toroidal field interactions in the deep interior of the Sun. We find that indeed there are signatures of interaction of toroidal field belts of consecutive cycles leading to flux cancellation which impact the next solar cycle amplitude. It further underscores the importance of utilizing physics-based models of the solar cycle for making solar cycle predictions, as opposed to empirical or statistical techniques. Our theoretical findings and analysis from observations are in tandem, which further authenticates the work. We are also interested in probing the long-term behavior of the solar cycle. Sunspot observations over several centuries reveal that the Sun occasionally slips into quiescent phases, known as solar grand minima. We use a solar dynamo model to study these phases of solar dormancy. To simulate grand minima episodes, we employ stochastic fluctuation in the source term of the polar fields. Simulations detect a gradual decay of the polar field at the onset of a solar grand minimum followed by a halt in the polar field reversals. But, the large-scale meridional circulation continuously dredges up magnetic fields to the solar surface and advects them further to the polar caps. This eventually builds up polar magnetic fields, strong enough to sustain the regular surface activity again, aiding in the recovery from the grand minimum. We have performed a spectral analysis of the hemispheric polar flux time series during simulated grand minima revealing significant signature of multiple frequencies apart from the 11-year sunspot cycle. In this work, we focus on a ∼ 5-year component and establish its causal connection with the meridional circulation characteristic timescale. The simulation results we obtain are in good agreement with a comparative analysis utilizing using long-term reconstructed solar activity data. Through this project, we conclude that there is a persistence of weak magnetic cycles during solar grand minima episodes. With these studies, we are able to understand the physical drivers of solar activity better, both on short and long timescales. Using numerical simulations, we have identified patterns in solar cycle behavior, which could help in predicting solar cycle activity. It is our hope that this theoretical exercise will catalyze further research in the field of solar and stellar magnetism.

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