arXiv subjects

Sanghita Chandra

Publications and source records attributed to Sanghita Chandra.

6 recordsLinked to original sources

arXiv2026

Sunrise III/SUSI reveals extended and spectrally structured Ca II K emission above the solar limb

The Sunrise III/SUSI instrument performed a sit-and-stare observation of the off-limb emission in the Ca II K line core, with a combination of excellent temporal and spectral resolution. The emission reaches up to 6 Mm above the solar limb, indicating the existence of off-limb structures akin to spicules. In this letter, we describe the spatio-spectral properties of the observed emission, focusing on the spectral line shape resolved in detail by Sunrise III/SUSI, and compare the observation with simple modeling that relies on a numerical simulation of the solar chromosphere. We find that the simulation contains enough cool and dense material at Mm heights to produce off-limb emission, but that the spectral width of these features is slightly lower than observed, necessitating more detailed modeling.

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arXiv2026

Diversity of morphology of type II spicules in MURaM-ChE simulations

Spicules are ubiquitous, small-scale features in the solar atmosphere, exhibiting a jet-like appearance most clearly identified by their apparent motion in off-limb observations. While they are often interpreted as narrow, thread-like structures, their true three-dimensional (3D) structure remains unknown. We aim to uncover the 3D morphology and dynamics of fast-evolving spicules (type II) using a MURaM-ChE simulation. We use a H$α$ proxy that has been developed using non-equilibrium (NE) hydrogen populations in MURaM-ChE. The proxy, modelled as an escape probability, is synthesised to isolate on-disc as well as off-limb H$α$ wing features. The 3D structure of these features is investigated using the 3D information on opacity in H$α$. We identify type II spicules, with unique 3D morphologies: the dominant being thread-like and slab-like. The appearance of spicules as slabs or threads is a function of time and Doppler velocity. The spicules extending above the spicule-forest (2Mm -3Mm above the surface) tend to be located at quasi-separatrix layers (QSLs). We find that the spatially resolved contributions to the opacity of spicules are often similar for spicules synthesised in the horizontal direction, and their on-disk rapid blueshifted excursion (RBE) synthesised in the vertical direction at the same Doppler velocity of 37km/s. This confirms that RBEs are indeed the on-disc counterparts of spicules. Furthermore, our analysis indicates that cross-field motions can significantly contribute to spicule dynamics. Spicules exhibit a range of morphologies, including both slab-like and thread-like structures. Their observed appearance depends strongly on line-of-sight projection and Doppler sampling. Spicules are preferentially located at QSLs, highlighting the role of magnetic topology in driving spicular dynamics.

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arXiv2025

Statistical properties of spicules in MURaM-ChE

Numerical simulations of the solar chromosphere have progressed towards reproducing spicules, which are transient features observed at the solar limb using spectral lines such as H$α$, Ca II H&K, or Mg II h&k. Two types of spicules, referred to as types I and II, have been identified in observations and studied in previous numerical works. The statistics of type II spicules in 3D numerical simulations have not yet been studied. We aim to compare the statistics of properties such as lengths, lifetimes, widths, heights, inclinations, and maximum velocities of self-consistently formed spicules in a MURaM-ChE simulation with observations. We employ a H$α$ proxy to identify fine-scale structures at the solar limb resembling spicules in a simulation of an enhanced network region. We track the evolution of 58 such features found in a 21-minute time sequence, and compare their dynamical and morphological properties with those derived from quiet-Sun observations using the Solar Optical Telescope (SOT) onboard the Hinode mission in the Ca II H spectral line. Previous studies have shown that spicules show very similar properties in Ca II H and H$α$. The spicule-like structures found in the simulation have statistical properties which are broadly consistent with those observed with Hinode/SOT. In particular, we find evidence for the self-consistent formation of both type I and type II spicules within the simulation, even in the absence of ambipolar diffusion. We also investigate the properties of rapid blueshifted and redshifted excursions (RBEs and RREs) in the simulation in relation to the spicules.

astro-ph.SR↗

arXiv2025

Probing chromospheric fine structures with a Hα proxy using MURaM-ChE

H$α$ observations of the solar chromosphere reveal dynamic small-scale structures known as spicules at the limb and rapid blue and red shifted excursions (RBEs/RREs) on-disk. We want to understand what drives these dynamic features, their magnetohydrodynamic (MHD) properties and their role in energy and heat transport to the upper solar atmosphere. To do this, we aim to develop a proxy for synthetic H$α$ observations in radiative-MHD simulations to help identify these features. We use the chromospheric extension to the MURaM code (MURaM-ChE) to simulate an enhanced network region. We develop a proxy for H$α$ based on a photon escape probability. This is a Doppler-shifted proxy that we use to identify fine structures in the line wings. We study on-disk features in 3D, obtaining their 3D structure from the absorption coefficient. We validate the H$α$ proxy by comparing it against features detected in the wings of H$α$ synthesized using MULTI3D. We detect numerous small-scale structures rooted at the network patches, similar to observations in H$α$. The dynamics of an example feature (RBE) at a Doppler shift of 37 km/s show that flux emergence and consequent reconnection drive the formation of this feature. Pressure gradient forces build up to drive a flow along the field line carrying the feature, making it a jet. There is strong viscous and resistive heating at the first appearance of the feature associated with the flux emergence. At the same time and location, a heating front appears and propagates along the field lines at speeds comparable to the Alfven velocity. We show that a synthetic observable based on an escape probability is able to reliably identify features observed with the H$α$ spectral line. We demonstrate its applicability by studying the formation, dynamics and properties of an RBE.

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arXiv2025

Recovery of the Solar Cycle from Maunder-like Grand Minima Episodes: A Quantification of the Necessary Polar Flux Threshold through Solar Dynamo Simulations

The 11-yr cycle of sunspots undergo amplitude modulation over longer timescales. As a part of this long-term modulation in solar activity, the decennial rhythm occasionally breaks, with quiescent phases with very few sunspots observed over multiple decades. These episodes are termed as solar grand minima. Observation of solar magnetic activity proxies complemented by solar dynamo simulations suggests that the large-scale solar polar fields become very weak during these minima phases with a temporary halt in the polar field reversal. Eventually, with the accumulation of sufficient polar fluxes, the polarity reversal and regular cyclic activity is thought to resume, Using multi-millennial dynamo simulations with stochastic forcing, we quantify the polar flux threshold necessary to recover global solar polarity reversal and surmount grand minima phases. We find that the duration of a grand minimum is independent of the onset rate and does not affect the recovery rate. Our results suggest a method to forecast the Sun's recovery from a grand minima phase. However, based on our approach, we could not identify specific precursors that signal entry in to a grand minima phase -- implying that predicting the onset of grand minima remains an outstanding challenge.

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arXiv2022

Evidence of persistence of weak magnetic cycles driven by meridional plasma flows during solar grand minima phases

Long-term sunspot observations and solar activity reconstructions reveal that the Sun occasionally slips into quiescent phases known as solar grand minima, the dynamics during which is not well understood. We use a flux transport dynamo model with stochastic fluctuations in the mean-field and Babcock-Leighton poloidal field source terms to simulate solar cycle variability. Our long-term simulations detect a gradual decay of the polar field during solar grand minima episodes. Although regular active region emergence stops, compromising the Babcock-Leighton mechanism, weak magnetic activity continues during minima phases sustained by a mean-field $α$-effect; surprisingly, periodic polar field amplitude modulation persist during these phases. A spectral analysis of the simulated polar flux time series shows that the 11-year cycle becomes less prominent while high frequency periods and periods around 22 years manifest during grand minima episodes. Analysis of long-term solar open flux observations appears to be consistent with this finding. Through numerical experimentation we demonstrate that the persistence of periodic amplitude modulation in the polar field and the dominant frequencies during grand minima episodes are governed by the speed of the meridional plasma flow -- which appears to act as a clock.

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