arXiv subjects

Rakesh K. Yadav

Publications and source records attributed to Rakesh K. Yadav.

At least 19 recordsLinked to original sources

arXiv2023

Strong resemblance between surface and deep zonal winds inside Jupiter revealed by high-degree gravity moments

Jupiter's atmosphere-interior is a coupled fluid dynamical system strongly influenced by the rapid background rotation. While the visible atmosphere features east-west zonal winds on the order of 100 m/s (Tollefson et al. 2017), zonal flows in the dynamo region are significantly slower, on the order of 1 cm/s or less, according to the latest magnetic secular variation analysis (Bloxham et al. 2022). The vertical profile of the zonal flows and the underlying mechanism remain elusive. The latest Juno radio tracking measurements afforded the derivation of Jupiter's gravity field to spherical harmonic degree 40. Here, we use the latest gravity solution to reconstruct Jupiter's deep zonal winds without a priori assumptions about their latitudinal profile. The pattern of our reconstructed deep zonal winds strongly resembles that of the surface wind within $\pm$ 35 degrees latitude from the equator, in particular the northern off-equatorial jet (NOEJ) and the southern off-equatorial jet (SOEJ) (Kulowski et al. 2021). The reconstruction features larger uncertainties in the southern hemisphere due to the north south asymmetric nature of Juno's trajectory. Amplitude of the reconstructed deep NOEJ matches that of the surface wind when the wind is truncated at a depth around 2500 km, and becomes twice that of the surface wind if the truncation depth is reduced to about 1500 km. Our analysis supports the physical picture in which prominent part of the surface zonal winds extends into Jupiter's interior significantly deeper than the water cloud layer.

astro-ph.EP↗

arXiv2022

Coronal and chromospheric emission in A-type stars

Cool stars on the main sequence generate X-rays from coronal activity, powered by a convective dynamo. With increasing temperature, the convective envelope becomes smaller and X-ray emission fainter. We present Chandra/HRC-I observations of four single stars with early A spectral types. Only the coolest star of this sample, $τ^3$ Eri ($T_\mathrm{eff}\approx8\,,000$ K), is detected with $\log(L_X/L_\mathrm{bol})=-7.6$ while the three hotter stars ($T_\mathrm{eff}\geq8\,,000$ K), namely $δ$ Leo, $β$ Leo, and $ι$ Cen, remain undetected with upper limits $\log(L_X/L_\mathrm{bol})<-8.4$. The drop in X-ray emission thus occurs in a narrow range of effective temperatures around $\sim 8100$ K and matches the drop of activity in the C III and O VI transition region lines.

astro-ph.SR↗

arXiv2022

A dynamo simulation generating Saturn-like small magnetic dipole tilts

Among planetary dynamos, the magnetic field of Saturn stands out in its exceptional level of axisymmetry. One of its peculiar features is that the magnetic dipole mode is tilted with respect to the planetary rotation axis by only $\approx 0.007^{\circ}$ or less. Numerical dynamo simulations performed in this context have had great difficulty in producing such small dipole tilt angles without introducing ad hoc ingredients such as a latitudinally varying heat flux pattern in the outer layers or stably stratified layers (SSL). Here we present a numerical dynamo simulation that generates a highly axisymmetric dynamo with a dipole tilt of about $\approx 0.0008^{\circ}$ on average. The model consists of a deep dynamo layer and an overlying low-conductivity layer but without any SSL. We highlight a novel mechanism where strong differential rotation generated in the atmospheric layer penetrates into the dynamo region, helping to maintain a very small magnetic dipole tilt.

astro-ph.EP↗

arXiv2021

Investigating Barotropic Zonal Flow in Jupiter's Deep Atmosphere using Juno Gravitational Data

The high-precision Juno gravitational measurements allow us to infer the structure of Jupiter's deep atmospheric zonal flow. Since this inference is nonunique, it is important to explore the space of possible solutions. In this paper, we consider a model in which Jupiter's deep atmospheric zonal flow is barotropic, or invariant along the direction of the rotation axis, until it is truncated at depth by some dynamical process (e.g., Reynolds stress, Lorentz or viscous force). We calculate the density perturbation produced by the $z$-invariant part of the flow using the thermal wind equation and compare the associated odd zonal gravitational harmonics ($J_{3}$, $J_{5}$, $J_{7}$, $J_{9}$) to the Juno-derived values. Most of the antisymmetric gravitational signal measured by Juno can be explained by extending observed winds between $20.9^{\circ}\rm{S}-26.4^{\circ}\rm{N}$ to depths of $\sim 1000$ km. Because the small-scale features of the mid/high latitude zonal flow may not persist to depth, we allow the zonal flow in this region to differ from the observed surface winds. We find that the Juno odd zonal gravitational harmonics can be fully explained by $\sim 1000$ km deep barotropic zonal flows involving the observed winds between $20.9^{\circ}\rm{S}-26.4^{\circ}$N and a few broad mid/high latitude jets.

astro-ph.EP↗

arXiv2020

An Earth-like stellar wind environment for Proxima Centauri c

A new planet has been recently discovered around Proxima Centauri. With an orbital separation of $\sim$$1.44$ au and a minimum mass of about $7$ $M_{\oplus}$, Proxima c is a prime direct imaging target for atmospheric characterization. The latter can only be performed with a good understanding of the space environment of the planet, as multiple processes can have profound effects on the atmospheric structure and evolution. Here, we take one step in this direction by generating physically-realistic numerical simulations of Proxima's stellar wind, coupled to a magnetosphere and ionosphere model around Proxima c. We evaluate their expected variation due to the magnetic cycle of the host star, as well as for plausible inclination angles for the exoplanet orbit. Our results indicate stellar wind dynamic pressures comparable to present-day Earth, with a slight increase (by a factor of 2) during high activity periods of the star. A relatively weak interplanetary magnetic field at the distance of Proxima c leads to negligible stellar wind Joule heating of the upper atmosphere (about $10\%$ of the solar wind contribution on Earth) for an Earth-like planetary magnetic field ($0.3$ G). Finally, we provide an assessment of the likely extreme conditions experienced by the exoplanet candidate Proxima d, tentatively located at $0.029$ au with a minimum mass of $0.29$ $M_{\oplus}$.

astro-ph.SR↗

arXiv2020

Tuning the Exo-Space Weather Radio for Stellar Coronal Mass Ejections

Coronal mass ejections (CMEs) on stars other than the Sun have proven very difficult to detect. One promising pathway lies in the detection of type II radio bursts. Their appearance and distinctive properties are associated with the development of an outward propagating CME-driven shock. However, dedicated radio searches have not been able to identify these transient features in other stars. Large Alfvén speeds and the magnetic suppression of CMEs in active stars have been proposed to render stellar eruptions "radio-quiet". Employing 3D magnetohydrodynamic simulations, we study here the distribution of the coronal Alfvén speed, focusing on two cases representative of a young Sun-like star and a mid-activity M-dwarf (Proxima Centauri). These results are compared with a standard solar simulation and used to characterize the shock-prone regions in the stellar corona and wind. Furthermore, using a flux-rope eruption model, we drive realistic CME events within our M-dwarf simulation. We consider eruptions with different energies to probe the regimes of weak and partial CME magnetic confinement. While these CMEs are able to generate shocks in the corona, those are pushed much farther out compared to their solar counterparts. This drastically reduces the resulting type II radio burst frequencies down to the ionospheric cutoff, which impedes their detection with ground-based instrumentation.

astro-ph.SR↗

arXiv2019

(Simulating) Coronal Mass Ejections in Active Stars

The stellar magnetic field completely dominates the environment around late-type stars. It is responsible for driving the coronal high-energy radiation (e.g. EUV/X-rays), the development of stellar winds, and the generation transient events such as flares and coronal mass ejections (CMEs). While progress has been made for the first two processes, our understanding of the eruptive behavior in late-type stars is still very limited. One example of this is the fact that despite the frequent and highly energetic flaring observed in active stars, direct evidence for stellar CMEs is almost non-existent. Here we discuss realistic 3D simulations of stellar CMEs, analyzing their resulting properties in contrast with solar eruptions, and use them to provide a common framework to interpret the available stellar observations. Additionally, we present results from the first 3D CME simulations in M-dwarf stars, with emphasis on possible observable signatures imprinted in the stellar corona.

astro-ph.SR↗

arXiv2019

Coronal response to magnetically-suppressed CME events in M-dwarf stars

We report the results of the first state-of-the-art numerical simulations of Coronal Mass Ejections (CMEs) taking place in realistic magnetic field configurations of moderately active M-dwarf stars. Our analysis indicates that a clear, novel, and observable, coronal response is generated due to the collapse of the eruption and its eventual release into the stellar wind. Escaping CME events, weakly suppressed by the large-scale field, induce a flare-like signature in the emission from coronal material at different temperatures due to compression and associated heating. Such flare-like profiles display a distinctive temporal evolution in their Doppler shift signal (from red to blue), as the eruption first collapses towards the star and then perturbs the ambient magnetized plasma on its way outwards. For stellar fields providing partial confinement, CME fragmentation takes place, leading to rise and fall flow patterns which resemble the solar coronal rain cycle. In strongly suppressed events, the response is better described as a gradual brightening, in which the failed CME is deposited in the form of a coronal rain cloud leading to a much slower rise in the ambient high-energy flux by relatively small factors ($\sim2-3$). In all the considered cases (escaping/confined) a fractional decrease in the emission from mid-range coronal temperature plasma occurs, similar to the coronal dimming events observed on the Sun. Detection of the observational signatures of these CME-induced features requires a sensitive next generation X-ray space telescope.

astro-ph.SR↗

arXiv2018

The stellar rotation--activity relationship in fully convective M dwarfs

The coronal activity-rotation relationship is considered to be a proxy for the underlying stellar dynamo responsible for magnetic activity in solar and late-type stars. While this has been studied in considerable detail for partly-convective stars that are believed to operate an interface dynamo, it is poorly unconstrained in fully-convective stars that lack the necessary shear layer between radiative core and the convective envelope. We present new X-ray observations of 19 slowly-rotating fully-convective stars with rotation periods from the MEarth Project. We use these to calculate X-ray luminosities (or upper limits for undetected sources) and combine these with existing measurements from \citet{wrig16b}. We confirm the existence of fully-convective stars in the X-ray unsaturated regime and find that these objects follow the same rotation-activity relationship seen for partly-convective stars. We measure the power-law slope of the relationship between Rossby number (the ratio of the rotation period to the convective turnover time) and the fractional X-ray luminosity for X-ray unsaturated fully-convective stars for the first time, and find it to be consistent with that found for partly-convective stars. We discuss this implications of this result for our understanding of stellar magnetic dynamos in fully- and partly-convective stars. Finally, we also use this data to improve empirical estimates of the convective turnover time for fully-convective stars.

astro-ph.SR↗

arXiv2017

Estimating the magnetic field strength in hot Jupiters

A large fraction of known Jupiter like exoplanets are inflated as compared to Jupiter. These "hot" Jupiters orbit close to their parent star and are bombarded with intense starlight. Many theories have been proposed to explain their radius inflation and several suggest that a small fraction of the incident starlight is injected in to the planetary interior which helps to puff up the planet. How will such energy injection affect the planetary dynamo? In this Letter, we estimate the surface magnetic field strength of hot Jupiters using scaling arguments that relate energy available in planetary interiors to the dynamo generated magnetic fields. We find that if we take into account the energy injected in the planetary interior that is sufficient to inflate hot Jupiters to observed radii, then the resulting dynamo should be able generate magnetic fields that are more than an order of magnitude stronger than the Jovian values. Our analysis highlights the potential fundamental role of the stellar light in setting the field strength in hot Jupiters.

astro-ph.EP↗

arXiv2016

Magnetic cycles in a dynamo simulation of fully convective M-star Proxima Centauri

The recent discovery of an Earth-like exoplanet around Proxima Centauri has shined a spot light on slowly rotating fully convective M-stars. When such stars rotate rapidly (period $\lesssim 20$ days), they are known to generate very high levels of activity that is powered by a magnetic field much stronger than the solar magnetic field. Recent theoretical efforts are beginning to understand the dynamo process that generates such strong magnetic fields. However, the observational and theoretical landscape remains relatively uncharted for fully convective M-stars that rotate slowly. Here we present an anelastic dynamo simulation designed to mimic some of the physical characteristics of Proxima Centauri, a representative case for slowly rotating fully convective M-stars. The rotating convection spontaneously generates differential rotation in the convection zone which drives coherent magnetic cycles where the axisymmetric magnetic field repeatedly changes polarity at all latitudes as time progress. The typical length of the `activity' cycle in the simulation is about nine years, in good agreement with the recently proposed activity cycle length of about seven years for Proxima Centauri. Comparing our results with earlier work, we hypothesis that the dynamo mechanism undergoes a fundamental change in nature as fully convective stars spin down with age.

astro-ph.SR↗

arXiv2016

Approaching a realistic force balance in geodynamo simulations

Earth sustains its magnetic field by a dynamo process driven by convection in the liquid outer core. Geodynamo simulations have been successful in reproducing many observed properties of the geomagnetic field. However, while theoretical considerations suggest that flow in the core is governed by a balance between Lorentz force, rotational force and buoyancy (called MAC balance for Magnetic, Archimedean, Coriolis) with only minute roles for viscous and inertial forces, dynamo simulations must employ viscosity values that are many orders of magnitude larger than in the core due to computational constraints. In typical geodynamo models viscous and inertial forces are not much smaller than the Coriolis force and the Lorentz force plays a sub-dominant role. This has led to conclusions that these simulations are viscously controlled and do not represent the physics of the geodynamo. Here we show by a direct analysis of the relevant forces that a MAC balance can be achieved when the viscosity is reduced to values close to the current practical limit. Lorentz force, buoyancy and the uncompensated (by pressure) part of the Coriolis force are of very similar strength, whereas viscous and inertia are smaller by a factor of at least 20 in the bulk of the fluid volume. Compared to non-magnetic convection at otherwise identical parameters, the dynamo flow is of larger scale, less invariant parallel to the rotation axis (less geostrophic) and convection transports twice as much heat, all of which is expected when the Lorentz force strongly influences the convection properties.

physics.geo-ph↗

arXiv2015

Effect of shear and magnetic field on the heat-transfer efficiency of convection in rotating spherical shells

We study rotating thermal convection in spherical shells. We base our analysis on a set of about 450 direct numerical simulations of the (magneto)hydrodynamic equations under the Boussinesq approximation. The Ekman number ranges from $10^{-3}$ to $10^{-5}$. The supercriticality of the convection reaches about 1000 in some models. Four sets of simulations are considered: non-magnetic simulations and dynamo simulations with either free-slip or no-slip flow boundary conditions. The non-magnetic setup with free-slip boundaries generates the strongest zonal flows. Both non-magnetic simulations with no-slip flow boundary conditions and self-consistent dynamos with free-slip boundaries have drastically reduced zonal-flows. Suppression of shear leads to a substantial gain in heat-transfer efficiency, increasing by a factor of 3 in some cases. Such efficiency enhancement occurs as long as the convection is significantly influenced by rotation. At higher convective driving the heat-transfer efficiency tends towards that of the classical non-rotating Rayleigh-Bénard system. Analysis of the latitudinal distribution of heat flow at the outer boundary reveals that the shear is most effective at suppressing heat-transfer in the equatorial regions. Furthermore, we explore the influence of the magnetic field on the {\em non-zonal} flow components of the convection. For this we compare the heat-transfer efficiency of no-slip non-magnetic cases with that of the no-slip dynamo simulations. We find that at $E=10^{-5}$ magnetic field significantly affects the convection and a maximum gain of about 30\% (as compared to the non-magnetic case) in heat-transfer efficiency is obtained for an Elsasser number of about 3. Our analysis motivates us to speculate that convection in the polar regions in dynamos at $E=10^{-5}$ is probably in a `magnetostrophic' regime.

astro-ph.EP↗

arXiv2015

Explaining the coexistence of large-scale and small-scale magnetic fields in fully convective stars

Despite the lack of a shear-rich tachocline region low-mass fully convective stars are capable of generating strong magnetic fields, indicating that a dynamo mechanism fundamentally different from the solar dynamo is at work in these objects. We present a self-consistent three dimensional model of magnetic field generation in low-mass fully convective stars. The model utilizes the anelastic magnetohydrodynamic equations to simulate compressible convection in a rotating sphere. A distributed dynamo working in the model spontaneously produces a dipole-dominated surface magnetic field of the observed strength. The interaction of this field with the turbulent convection in outer layers shreds it, producing small-scale fields that carry most of the magnetic flux. The Zeeman-Doppler-Imaging technique applied to synthetic spectropolarimetric data based on our model recovers most of the large-scale field. Our model simultaneously reproduces the morphology and magnitude of the large-scale field as well as the magnitude of the small-scale field observed on low-mass fully convective stars.

astro-ph.SR↗

arXiv2014

Formation of starspots in self-consistent global dynamo models: Polar spots on cool stars

Observations of cool stars reveal dark spot-like features on their surfaces. Compared to sunspots, starspots can be bigger or cover a larger fraction of the stellar surface. While sunspots appear only at low latitudes, starspots are also found in polar regions, in particular on rapidly rotating stars. Sunspots are believed to result from the eruption of magnetic flux-tubes rising from the deep interior of the Sun. The strong magnetic field locally reduces convective heat transport to the solar surface. Such flux-tube models have also been invoked to explain starspot properties. However, these models use several simplifications and so far the generation of either sunspots or starspots has not been demonstrated in a self-consistent simulation of stellar magnetic convection. Here we show that direct numerical simulations of a distributed dynamo operating in a density-stratified rotating spherical shell can spontaneously generate cool spots. Convection in the interior of the model produces a large scale magnetic field which interacts with near surface granular convection leading to strong concentrations of magnetic flux and formation of starspots. Prerequisites for the formation of sizeable high-latitude spots in the model are sufficiently strong density stratification and rapid rotation. Our model presents an alternate mechanism for starspot formation by distributed dynamo action.

astro-ph.SR↗

arXiv2013

Bridging planets and stars using scaling laws in anelastic spherical shell dynamos

Dynamos operating in the interiors of rapidly rotating planets and low-mass stars might belong to a similar category where rotation plays a vital role. We quantify this similarity using scaling laws. We analyse direct numerical simulations of Boussinesq and anelastic spherical shell dynamos. These dynamos represent simplified models which span from Earth-like planets to rapidly rotating low-mass stars. We find that magnetic field and velocity in these dynamos are related to the available buoyancy power via a simple power law which holds over wide variety of control parameters.

astro-ph.SR↗

arXiv2013

Supercriticality to subcriticality in dynamo transitions

Evidence from numerical simulations suggest that the nature of dynamo transition changes from supercritical to subcritical as the magnetic Prandtl number is decreased. To explore this interesting crossover we first use direct numerical simulations to investigate the hysteresis zone of a subcritical Taylor-Green dynamo. We establish that a well defined boundary exists in this hysteresis region which separates dynamo states from the purely hydrodynamic solution. We then propose simple dynamo models which show similar crossover from supercritical to subcritical dynamo transition as a function of the magnetic Prandtl number. Our models show that the change in the nature of dynamo transition is connected to the stabilizing or de-stabilizing influence of governing non-linearities.

physics.flu-dyn↗

arXiv2013

Consistent scaling laws in anelastic spherical shell dynamos

Numerical dynamo models always employ parameter values that differ by orders of magnitude from the values expected in natural objects. However, such models have been successful in qualitatively reproducing properties of planetary and stellar dynamos. This qualitative agreement fuels the idea that both numerical models and astrophysical objects may operate in the same asymptotic regime of dynamics. This can be tested by exploring the scaling behavior of the models. For convection-driven incompressible spherical shell dynamos with constant material properties, scaling laws had been established previously that relate flow velocity and magnetic field strength to the available power. Here we analyze 273 direct numerical simulations using the anelastic approximation, involving also cases with radius-dependent magnetic, thermal and viscous diffusivities. These better represent conditions in gas giant planets and low-mass stars compared to Boussinesq models. Our study provides strong support for the hypothesis that both mean velocity and mean magnetic field strength scale as a function of power generated by buoyancy forces in the same way for a wide range of conditions.

astro-ph.SR↗