Physical Drivers of Long Term Solar Magnetic Variability

Saha, Chitradeep (2025) Physical Drivers of Long Term Solar Magnetic Variability. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Chitradeep Saha (20RS018)) 20RS018.pdf - Submitted Version Restricted to Repository staff only Download (51MB)

Abstract

The Sun is a dynamic star whose dynamism is reflected in its evolving magnetic fields and radiative and particulate fluxes. These measurable outputs fluctuate across a wide range of timescales – spanning minutes to millennia. The most prominent signature of solar magnetic variability is the 11-year sunspot cycle – a rhythmic surge and ebb in the solar magnetic activity. However, successive cycles vary in amplitude and duration, and this variation creates a longer-term pattern – a ‘supradecadal’ modulation – in solar magnetic activity. Understanding the physical origin of supradecadal solar modulation remains a subject of intense debate. This modulation also encapsulates distinct epochs of extremely low (grand minimum) and heightened activity (grand maximum). The Sun’s behaviour, especially during grand minima – such as the Maunder Minimum (1645-1715 AD) – is poorly understood due to sparse observational records that predate modern instrumentation. When there is an apparent magnetic lull on the surface of the Sun, little is known about what occurs inside. Neither is it fully understood how the star has consistently risen to its full magnetic glory every time it slipped into such long, deep slumber. Magnetic fields in the solar polar regions play an integral role in driving solar magnetic variability, yet solar poles remain a less explored territory of the Sun due to observational constraints. In the first part of the thesis, we explore the dynamics of solar interior and polar magnetic fields during solar grand minima phases. Whether the solar dynamo ceases entirely to work during such magnetic quiescence on the visible solar surface, or whether there are activities that still go on deep in the interior – is what we probe in Chapter 3, through multi-millennial scale solar dynamo simulations. In the second part of the thesis, we investigate in greater detail the plausible mechanism of recovery of the Sun from an ongoing grand minimum and return to its full glory. We also quantitatively estimate the polar magnetic fields necessary to facilitate such recovery. The findings are presented in Chapter 4. In the third part of the thesis, we take a step back and pin down the physical origin of such long-term magnetic variability in the Sun, a specific manifestation of which is the solar grand minimum. Competing theories involve stochastic fluctuations and nonlinear dynamo processes occurring at the depth of the solar convective envelope, resulting in a long-term solar activity modulation. Using different kinds of dynamo frameworks, our research presented in Chapter 5 concludes that nonlinearity alone – in the absence of stochasticity – cannot explain solar magnetic variability beyond decadal scales. And finally, in the last part of the thesis, we use satellite observations of the solar polar magnetic landscape over the last decade to understand the solar cycle evolution of kilogauss magnetic patches in the solar polar regions. The motivation is to understand their origin and significance for the solar cycle. The results are presented in Chapter 6. In Chapter 7, we summarise all the research work included in this thesis and discuss further scopes of studies.

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