arXiv subjects

Anu Sreedevi

Publications and source records attributed to Anu Sreedevi.

10 recordsLinked to original sources

arXiv2026

Large ephemeral regions and their tilt angles

The ephemeral regions (ERs), which are short-lived bipolar magnetic regions that emerge across the solar cycle but do not appear as sunspots, play a crucial role in the Sun's magnetic flux budget. However, their properties, particularly the tilt distribution, are poorly constrained by observations. In this study, we isolate ERs from the Automatic Tracking Algorithm for Bipolar Magnetic Regions (AutoTAB) catalog during Solar Cycles 24 and 25 by applying flux and footpoint-separation thresholds. Although AutoTAB was designed to track high-flux regions, it also records ephemeral regions with fluxes of 10^19 to 10^20 Mx, placing them at the upper end of the ER spectrum. The isolated ERs have an average lifetime of 1.2 days. Footpoint separation begins at supergranular scales (about 20 Mm), grows during the first half of the lifetime, and then saturates. ERs occur most frequently near solar minima, consistent with earlier studies and likely reflecting AutoTAB's greater sensitivity to weaker regions when strong BMRs are scarce. Tilt properties reveal a more complex picture. For lifetimes shorter than two days, ERs show a broad, noisy distribution with no systematic latitude dependence. Including longer-lived ERs produces a weak, though statistically insignificant, increasing trend with latitude, suggesting that short-lived ERs are shaped by turbulent convection, while stronger, longer-lived ERs may retain Coriolis-imparted tilts. Overall, these results support the view that ERs occupy the low-flux end of the BMR spectrum and contribute meaningfully to the solar dynamo.

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arXiv2026

Variation of the sunspot area during the rising and declining phases of the solar cycle supports the toroidal flux loss due to flux emergence

Sunspots are obvious observable manifestations of the toroidal magnetic field generated through the dynamo in the convection zone. They appear in different sizes, having a wide distribution in their area. We analyse the sunspot group area of the past 13 cycles and the Bipolar Magnetic Region (BMR) flux for Cycles 23 and 24 to explore their area and flux distributions and connect with the theory. We find that, in general, the group area and BMR flux are statistically larger in the rising phase than in the declining phase of the solar cycle. This implies that the rising phase of the solar cycle is prone to drive more intense space weather. We further show that the mean and median of the area distribution during the rising phase are dependent on cycle strength. However, the distribution mean and median are cycle strength-independent or weakly dependent during the decline phases of the solar cycles, particularly during the last three years when the latitudinal bands of all cycles migrate towards the equator along the same trajectory. These results support the theoretical model of nonlinear flux loss due to flux emergence, which explains why solar cycles rise differently but decay similarly.

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arXiv2025

Backtracking Bipolar Magnetic Regions to their emergence: Two groups and their implication in the tilt measurements

Bipolar Magnetic Regions (BMRs) that appear on the solar photosphere are surface manifestations of the Suns internal magnetic field. With modern observations and continuous data streams, the study of BMRs has moved from manual sunspot catalogs to automated detection and tracking methods. In this work, we present an additional module to the existing BMR tracking algorithm, AutoTAB, that focuses on identifying emerging signatures of BMRs. Specifically, for regions newly detected on the solar disk, this module backtracks the BMRs to their point of emergence. From a total of about 12,000 BMRs identified by AutoTAB, we successfully backtracked 3,080 cases. Within this backtracked sample, we find two distinct populations. One group shows the expected behavior of emerging regions, in which the magnetic flux increases significantly during the emerging phase. The other group consists of BMRs whose flux, however, does not exhibit substantial growth during their evolution, the instances where our algorithm fails to capture the initial emergence of the BMRs. We classify these as discarded BMRs and examine their statistical properties separately. Our analysis shows that these discarded BMRs do not display any preferred tilt angle distribution and do not show systematic latitudinal tilt dependence, in contrast to the trends typically associated with emerging BMRs. This indicates that including such regions in statistical studies of BMR properties can distort or mask the underlying physical characteristics. We therefore emphasise the importance of excluding the discarded population from the whole dataset when analysing the statistical behavior of BMRs.

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arXiv2025

Role of sunspot latitude versus tilt in determining the polar field and amplitude of the next cycle: Cause of the weak Solar Cycle 20

One prominent feature of solar cycle is its irregular variation in its cycle strength, making it challenging to predict the amplitude of the next cycle. Studies show that fluctuations and nonlinearity in generating poloidal field throughout the decay and dispersal of tilted sunspots produce variation in the solar cycle. The flux, latitudinal position, and tilt angle of sunspots are the primary parameters that determine the polar field and, thus, the next solar cycle strength. By analysing the observed sunspots and polar field proxy, we show that the nonlinearity in the poloidal field generation becomes important for strong cycles. Except for strong cycles, we can reasonably predict the polar field at the end of the cycle (and thus the next cycle strength) using the total sunspot area alone. Combining the mean tilt angle and latitude positions with the sunspot area, we can predict the polar field of Cycles 15 -- 24 (or the amplitude of sunspot Cycles 16-25) with reasonable accuracy except for Cycle 23 for which the average tilt angle cannot predict the polar field. For Cycles 15--22, we show that the average tilt angle variation dominates over the latitude variation in determining the polar field of a cycle. In particular, the reduction of tilt in Cycle 19 was the primary cause of the following weak cycle (Cycle 20). Thus, we conclude that tilt quenching is essential in regulating the solar cycle strength in the solar dynamo.

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arXiv2025

Observed Joys law of Bipolar Magnetic Region tilts at the emergence supports the thin flux tube model

Bipolar sunspots, or more generally, Bipolar Magnetic Regions, BMRs, are the dynamic magnetic regions that appear on the solar surface and are central to solar activity. One striking feature of these regions is that they are often tilted with respect to the equator, and this tilt increases with the latitude of appearance, popularly known as Joys law. Although this law has been examined for over a century through various observations, its physical origin is still not established. An attractive theory that has been put forward behind Joys law is the Coriolis force acting on the rising flux tube in the convection zone, which has been studied using the thin flux tube model. However, observational support for this theory is limited. If the Coriolis force is the cause of the tilt, then we expect BMRs to hold Joys law at their initial emergence on the surface. By automatically identifying the BMRs over the last two solar cycles from high resolution magnetic observations, we robustly capture their initial emergence signatures on the surface. We find that from their appearance, BMRs exhibit tilts consistent with Joys law. This early tilt signature of BMRs suggests that the tilt is developed underneath the photosphere, driven by the Coriolis force and helical convection, as predicted by the thin flux tube model. Considerable scatter around Joys law observed during the emergence phase, which reduces in the post emergence phase, reflects the interaction of the vigorous turbulent convection with the rising flux tubes in the near surface layer.

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arXiv2025

Variations of the Ca II K Line Profile Parameters with Solar Latitude and Time Observed from Kodaikanal Solar Observatory

The Calcium-K line profiles as functions of solar latitude and time were obtained through our observations from the Kodaikanal Solar Observatory using the solar tunnel telescope and spectrograph with a CCD detector. Observations were conducted on all days with favourable sky conditions. We analysed the data collected over a period of about ten years to study the variations in the Ca II K line profiles recorded between 2015 and 2024, of which 709 days of data were found useful. The temporal and time-averaged latitudinal variations of the K$_{1}$ width, K$_{2}$ width, K$_{3}$ intensity and the intensity ratios of K$_{2v}$/K$_{2r}$ and K$_{2v}$/K$_{3}$ were computed using a semi-automated program. The parameters showed asymmetric increases towards the higher latitudes, with the rates of increase being higher in the southern hemisphere. The temporal plots for K$_{1}$ width and K$_{3}$ intensity showed positive correlations with the plage and spot filling factors, whereas the temporal plots for K$_{2}$ width, K$_{2v}$/K$_{2r}$ and K$_{2v}$/K$_{3}$ intensity ratios showed negative correlations. The time-averaged latitudinal plot for K$_{1}$ width has small peaks near 25°N and 20°S. The K$_{2}$ width has a small peak at 0°. The K$_{3}$ intensity has peaks at 20°N and 15°S. The K$_{2v}$/K$_{2r}$ intensity ratio shows peaks at 50$\degree$N, 0° and 40°S. The K$_{2v}$/K$_{3}$ intensity ratio shows peaks at 60°N, 0° and 60°S. Slope profiles show spectral response to magnetic activity peaks near K$_{3}$ with north-south asymmetries. Such variations in the line profiles are important in the studies of solar irradiance, surface flux transport and solar dynamo.

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arXiv2024

Variabilities in the polar field and solar cycle due to irregular properties of Bipolar Magnetic Regions

Decay and dispersal of the tilted Bipolar Magnetic Regions (BMRs) on the solar surface are observed to produce large-scale poloidal field, which acts as the seed for the toroidal field and, thus, the next sunspot cycle. However, various properties of BMR, namely, the tilt, time delay between successive emergences, location, and flux, all have irregular variations. Previous studies show that these variations can lead to changes in the polar field. In this study, we first demonstrate that our 3D kinematic dynamo model, STABLE, reproduces the robust feature of the surface flux transport (SFT) model, namely the variation of the generated dipole moment with the latitude of the BMR position. Using STABLE in both SFT and dynamo modes, we perform simulations by varying the individual properties of BMR and keeping their distributions the same in all the cycles as inspired by the observations. We find that randomness due to the distribution in either the time delay or the BMR latitude produces negligible variation in the polar field and the solar cycle. However, randomness due to BMR flux distribution produces substantial effects, while the scatter in the tilt around Joy law produces the largest variation. Our comparative analyses suggest that the scatter of BMR tilt around Joy law is the major cause of variation in the solar cycle. Furthermore, our simulations also show that the magnetic field-dependent time delay of BMR emergence produces more realistic features of the magnetic cycle, consistent with observation.

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arXiv2024

Analysis of BMR tilt from AutoTAB catalog: Hinting towards the thin flux tube model?

One of the intriguing mechanisms of the Sun is the formation of the bipolar magnetic regions (BMRs) in the solar convection zone which are observed as regions of concentrated magnetic fields of opposite polarity on photosphere. These BMRs are tilted with respect to the equatorial line, which statistically increases with latitude. The thin flux tube model, employing the rise of magnetically buoyant flux loops and their twist by Coriolis force, is a popular paradigm for explaining the formation of tilted BMRs. In this study, we assess the validity of the thin flux tube model by analyzing the tracked BMR data obtained through the Automatic Tracking Algorithm for BMRs (AutoTAB). Our observations reveal that the tracked BMRs exhibit the expected collective behaviors. We find that the polarity separation of BMRs increases over their normalized lifetime, supporting the assumption of a rising flux tube from the CZ. Moreover, we observe an increasing trend of the tilt with the flux of the BMR, suggesting that rising flux tubes associated with lower flux regions are primarily influenced by drag force and Coriolis force, while in higher flux regions, magnetic buoyancy dominates. Furthermore, we observe Joy's law dependence for emerging BMRs from their first detection, indicating that at least a portion of the tilt observed in BMRs can be attributed to the Coriolis force. Notably, lower flux regions exhibit a higher amount of fluctuations associated with their tilt measurement compared to stronger flux regions, suggesting that lower flux regions are more susceptible to turbulent convection.

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arXiv2024

AutoTAB: An Algorithm to Track Bipolar Magnetic Regions and Initial results from the Tracked BMRs

AutoTAB is a state-of-the-art, fully automatic algorithm that tracks the Bipolar Magnetic Regions (BMRs) in magnetogram observations. AutoTAB employs identified BMR regions from Line-of-Sight magnetograms from MDI and HMI (1996--2022) to track the BMRs through their evolution on the nearside of the Sun. AutoTAB enables us to create a comprehensive and unique catalog of tracked information of 9232 BMRs in the mentioned time period. This dataset is used to study the collective statistical properties of BMRs and particularly to identify the correct theory for the BMR formation. Here, we discuss the algorithm's functionality and the initial findings obtained from the AutoTAB BMRs catalog.

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arXiv2023

AutoTAB: Automatic Tracking Algorithm for Bipolar Magnetic Regions

Bipolar Magnetic Regions (BMRs) provide crucial information about solar magnetism. They exhibit varying morphology and magnetic properties throughout their lifetime, and studying these properties can provide valuable insights into the workings of the solar dynamo. The majority of previous studies have counted every detected BMR as a new one and have not been able to study the full life history of each BMRs. To address this issue, we have developed an Automatic Tracking Algorithm (AutoTAB) for BMRs, that tracks the BMRs for their entire lifetime or throughout their disk passage. AutoTAB uses the binary maps of detected BMRs to automatically track the regions. This is done by differentially rotating the binary maps of the detected regions and checking for overlaps between them. In this first article of this project, we provide a detailed description of the working of the algorithm and evaluate its strengths and weaknesses by comparing it with existing algorithms. AutoTAB excels in tracking even for the small BMRs (with flux $\sim 10^{20}$ Mx) and it has successfully tracked 9152 BMRs over the last two solar cycles (1996--2020), providing a comprehensive dataset that depicts the evolution of various properties for each BMR. The tracked BMRs exhibit the well-known latitudinal-time distribution and 11 year cyclic variation except for small BMRs which appear at all phases of the solar cycle and show weak latitudinal dependency. Finally, we discuss the possibility of adapting our algorithm to other datasets and expanding the technique to track other solar features in the future.

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