arXiv subjects

Abhijit Bendre

Publications and source records attributed to Abhijit Bendre.

4 recordsLinked to original sources

arXiv2024

Shedding light on the MRI driven dynamo in a stratified shearing box

We study the magneto-rotational instability (MRI) driven dynamo in a geometrically thin disc ($H/R\ll 1$) using stratified zero net flux (ZNF) shearing box simulations. We find that mean fields and EMFs oscillate with a primary frequency $f_{\rm dyn} = 0.017$ ($\approx 9$ orbital period), but also have higher harmonics at $3f_{\rm dyn}$. Correspondingly, the current helicity, has two frequencies $2f_{\rm dyn}$ and $4f_{\rm dyn}$ respectively, which appear to be the beat frequencies of mean fields and EMFs as expected from the magnetic helicity density evolution equation. Further, we adopt a novel inversion algorithm called the `Iterative Removal Of Sources' (IROS), to extract the turbulent dynamo coefficients in the mean-field closure using the mean magnetic fields and EMFs obtained from the shearing box simulation. We show that an $α-$effect ($α_{yy}$) is predominantly responsible for the creation of the poloidal field from the toroidal field, while shear generates back a toroidal field from the poloidal field; indicating that an $α-Ω$-type dynamo is operative in MRI-driven accretion discs. We also find that both strong outflow ($\bar{v}_z$) and turbulent pumping ($γ_z$ ) transport mean fields away from the mid-plane. Instead of turbulent diffusivity, they are the principal sink terms in the mean magnetic energy evolution equation. We find encouraging evidence that a generative helicity flux is responsible for the effective $α$-effect. Finally, we point out potential limitations of horizontal ($x-y$) averaging in defining the `mean' on the extraction of dynamo coefficients and their physical interpretations.

astro-ph.HE↗

arXiv2020

Characterising the Dynamo in a Radiatively Inefficient Accretion Flow

We explore the MRI driven dynamo in a radiatively inefficient accretion flow (RIAF) using the mean field dynamo paradigm. Using singular value decomposition (SVD) we obtain the least squares fitting dynamo coefficients $α$ and $γ$ by comparing the time series of the turbulent electromotive force and the mean magnetic field. Our study is the first one to show the poloidal distribution of these dynamo coefficients in global accretion flow simulations. Surprisingly, we obtain a high value of the turbulent pumping coefficient $γ$ which transports the mean magnetic flux radially outward. This would have implications for the launching of magnetised jets which are produced efficiently in presence a large-scale poloidal magnetic field close to the compact object. We present a scenario of a truncated disc beyond the RIAF where a large scale dynamo-generated poloidal magnetic field can aid jet-launching close to the black hole. Magnitude of all the calculated coefficients decreases with radius. Meridional variations of $α_{ϕϕ}$, responsible for toroidal to poloidal field conversion, is very similar to that found in shearing box simulations using the `test field' (TF) method. By estimating the relative importance of $α$-effect and shear, we conclude that the MRI driven large-scale dynamo, which operates at high latitudes beyond a disc scale height, is essentially of the $α-Ω$ type.

astro-ph.HE↗

arXiv2019

GMRT observations of a first sample of Extremely Inverted Spectrum Extragalactic Radio Sources (EISERS) candidates in the Northern sky

We present an extension of our search for Extremely Inverted Spectrum Extragalactic Radio Sources (EISERS) to the northern celestial hemisphere. With an inverted radio spectrum of slope $α$ > +2.5, these rare sources would either require a non-standard particle acceleration mechanism (in the framework of synchrotron self-absorption hypothesis), or a severe free-free absorption which attenuates practically all of their synchrotron radiation at metre wavelengths. By applying a sequence of selection filters, a list of 15 EISERS candidates is extracted out by comparing two large-sky radio surveys, WENSS (325 MHz) and TGSS-ADR1 (150 MHz), which overlap across 1.03$π$ steradian of the sky. Here we report quasi-simultaneous GMRT observations of these 15 EISERS candidates at 150 MHz and 325 MHz, in an attempt to accurately define their spectra below the turnover frequency. Out of the 15 candidates observed, two are confirmed as EISERS, since the slope of the inverted spectrum between these two frequencies is found to be significantly larger than the critical value $α_c$ = +2.5: the theoretical limit for the standard case of synchrotron self-absorption (SSA). For another 3 sources, the spectral slope is close to, or just above the critical value $α_c$. Nine of the sources have GPS type radio spectra. The parsec-scale radio structural information available for the sample is also summarised.

astro-ph.GA↗

arXiv2012

On the magnetic quenching of mean-field effects in supersonic interstellar turbulence

The emergence of large-scale magnetic fields observed in the diffuse interstellar medium is explained by a turbulent dynamo. The underlying transport coefficients have previously been extracted from numerical simulations. So far, this was restricted to the kinematic regime, but we aim to extend our analysis into the realm of dynamically important fields. This marks an important step on which derived mean-field models rely to explain observed equipartition strength fields. As in previous work, we diagnose turbulent transport coefficients by means of the test-field method. We derive quenching functions for the dynamo α effect, diamagnetic pumping, and turbulent diffusivity, which are compared with theoretical expectations. At late times, we observe the suppression of the vertical wind. Because this potentially affects the removal of small-scale magnetic helicity, new concerns arise about circumventing constraints imposed by the conservation of magnetic helicity at high magnetic Reynolds numbers. While present results cannot safely rule out this possibility, the issue only becomes important at late stages and is absent when the dynamo is quenched by the wind itself.

astro-ph.GA↗