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Quasi-Biennial Oscillations and Rieger-type Periodicities in a Babcock-Leighton Solar Dynamo

Pawan Kumar, Belur Ravindra, Partha Chowdhury, Bidya Binay Karak

TL;DR

The study demonstrates that a three-dimensional Babcock–Leighton dynamo (STABLE) can generate Rieger-type periodicities and quasi-biennial oscillations when stochastic fluctuations are applied to BMR properties (tilt, flux, time delay, latitude). Using Morlet wavelet analysis and global wavelet power spectra, the authors identify mid-term periods in model outputs and show that combined parameter fluctuations maximize the occurrence of these signals, with tilt primarily driving QBOs and flux fluctuations driving Rieger-type periods. They further find that increasing dynamo supercriticality suppresses Rieger-type periodicities, leaving QBO-like signals, and shortens the main solar-cycle period, supporting the view that the solar dynamo is not highly supercritical. Overall, the work provides a plausible, physically grounded mechanism for observed mid-term solar activity and highlights the importance of multi-parameter stochasticity in the BL process for reproducing such variability.

Abstract

The Sun's magnetic field shows the 11-year solar cycle and shorter periodicities, popularly known as the quasi-biennial oscillations (QBOs) and Rieger-type periods, or ``season of the Sun." Although several theories have been proposed to explain the origin of QBOs and Rieger-type periods, no single theory has widespread acceptance. We explore whether the \bl\ dynamo can produce Rieger-type periodicity and QBOs and investigate their underlying physical mechanisms. We use the observationally guided three-dimensional kinematic \bl\ dynamo model, which has emerged as a successful model for reproducing many characteristic features of the solar cycle. We use Morlet wavelet and global wavelet power spectrum techniques to analyze the data obtained from the model. In our model, we report QBOs and Rieger-type periods for the first time. Further, we investigate the individual \bl\ parameters (fluctuations in flux, latitude, time delay and tilt scatter) role in the occurrence of QBOs and Rieger-type periods. We find that while fluctuations in the individual parameters of the \bl\ process can produce QBOs and Rieger-type periodicity, their occurrence probability is enhanced when we consider combined fluctuations of all parameters in the \bl\ process. Finally, we find that with the increase of dynamo supercriticality, the model tends to suppress the generation of Rieger-type periodicity. Thus, this result supports earlier studies that suggest the solar dynamo is not highly supercritical.

Quasi-Biennial Oscillations and Rieger-type Periodicities in a Babcock-Leighton Solar Dynamo

TL;DR

The study demonstrates that a three-dimensional Babcock–Leighton dynamo (STABLE) can generate Rieger-type periodicities and quasi-biennial oscillations when stochastic fluctuations are applied to BMR properties (tilt, flux, time delay, latitude). Using Morlet wavelet analysis and global wavelet power spectra, the authors identify mid-term periods in model outputs and show that combined parameter fluctuations maximize the occurrence of these signals, with tilt primarily driving QBOs and flux fluctuations driving Rieger-type periods. They further find that increasing dynamo supercriticality suppresses Rieger-type periodicities, leaving QBO-like signals, and shortens the main solar-cycle period, supporting the view that the solar dynamo is not highly supercritical. Overall, the work provides a plausible, physically grounded mechanism for observed mid-term solar activity and highlights the importance of multi-parameter stochasticity in the BL process for reproducing such variability.

Abstract

The Sun's magnetic field shows the 11-year solar cycle and shorter periodicities, popularly known as the quasi-biennial oscillations (QBOs) and Rieger-type periods, or ``season of the Sun." Although several theories have been proposed to explain the origin of QBOs and Rieger-type periods, no single theory has widespread acceptance. We explore whether the \bl\ dynamo can produce Rieger-type periodicity and QBOs and investigate their underlying physical mechanisms. We use the observationally guided three-dimensional kinematic \bl\ dynamo model, which has emerged as a successful model for reproducing many characteristic features of the solar cycle. We use Morlet wavelet and global wavelet power spectrum techniques to analyze the data obtained from the model. In our model, we report QBOs and Rieger-type periods for the first time. Further, we investigate the individual \bl\ parameters (fluctuations in flux, latitude, time delay and tilt scatter) role in the occurrence of QBOs and Rieger-type periods. We find that while fluctuations in the individual parameters of the \bl\ process can produce QBOs and Rieger-type periodicity, their occurrence probability is enhanced when we consider combined fluctuations of all parameters in the \bl\ process. Finally, we find that with the increase of dynamo supercriticality, the model tends to suppress the generation of Rieger-type periodicity. Thus, this result supports earlier studies that suggest the solar dynamo is not highly supercritical.
Paper Structure (11 sections, 9 equations, 8 figures, 1 table)

This paper contains 11 sections, 9 equations, 8 figures, 1 table.

Table of Contents

  1. Introduction
  2. Model
  3. Methods
  4. Results and Discussion
  5. Effects of fluctuations in Babcock--Leighton process
  6. Time delay
  7. Flux variation
  8. Tilt scatter around Joy's law
  9. Combined fluctuations in Babcock--Leighton process
  10. Effect of dynamo supercriticality
  11. Conclusions

Figures (8)

  • Figure 1: Solar cycles obtained from the STABLE dynamo model. (a) The azimuthally average surface radial field as functions of latitude and time. (b) Temporal variation of the total (over the full-disk) monthly sunspot number.
  • Figure 2: Morlet and global wavelet power spectra of monthly sunspot numbers obtained from the dynamo model with fluctuations in the BMR time delay to study the short-term/Rieger-type periodicities (a-b) and QBOs and other long-term periods including the main cycle period corresponding to the 11-year solar cycle (c-d). The red dotted lines in all global wavelet power spectra represent the 95% confidence level and black dotted lines in Morlet wavelet spectra represent COI.
  • Figure 3: Same as Figure \ref{['fig:fig1']}, but from the case in which we consider fluctuation in flux only.
  • Figure 4: Same as Figure \ref{['fig:fig1']}, but fluctuation only in the tilt around Joy's law with $\sigma_\delta = 15^{\circ}$ with fixed time delay and flux.
  • Figure 5: The figure shows the dynamical behavior of various periodicities found for the northern hemisphere using wavelet power spectra for the monthly sunspot number time series data obtained from the model with fluctuations in all the parameters of Babcock--Leighton process with tilt scatter $\sigma_\delta = 15^{\circ}$. (a) Morlet wavelet spectra to identify short-term periods (Rieger-type). (b) Global power spectra for short-term periods (Rieger-type). The red dotted line represents the 95% confidence level. (c) Similar to panel (a) but for the QBOs and long-range periods including the cycle corresponding to the 11-year solar cycle. (d) Similar to panel (b) but for the study of QBOs and long periods.
  • ...and 3 more figures