Rotational Modulation Research Articles

Intrinsic rotation modulation by diffusive neutral particles in tokamaks

The modulated transport model, a model kinetic ion transport equation for the pedestal and scrape-off layer (SOL), is generalized to self-consistently include effects of a single neutral particle species. The neutrals contribute additional transport terms, modifying the v∥ -dependent orbit-averaged ion diffusivities of the original work and the resulting predicted intrinsic rotation of the ions. After making simplifying assumptions of the neutral transport, in particular taking the continuous transport limit via a short charge-exchange step expansion, we derive relatively simple analytic expressions that capture the diffusive neutral physics. Within the scope of the model’s validity, the neutral-driven intrinsic rotation can compete with the turbulence-driven intrinsic rotation. However, for physically motivated parameters, the neutral-driven intrinsic rotation appears negligible, either on a term-by-term basis or due to a strong cancellation between the neutral-driven momentum diffusion and pinch terms. It appears that a treatment containing finite charge-exchange steps is necessary to capture neutral transport of strong flow momentum into the confined region from the SOL.

Variability in SSTc2d J163134.1-240100, a brown dwarf with quasi-spherical mass loss

We report on a search for variability in the young brown dwarf SST1624 ( ∼M7 spectral type, M∼0.05M⊙), previously found to feature an expanding gaseous shell and to undergo quasi-spherical mass loss. We find no variability on timescales of 1-6,hours. Specifically, on these timescales, we rule out the presence of a period with amplitude >1%. A photometric period in that range would have been evidence for either pulsation powered by Deuterium burning or rotation near breakup. However, we see a 3% decrease in the K-band magnitude between two consecutive observing nights (a 10, σ result). There is also clear evidence for variations in the WISE lightcurves at 3.6 and 4.5 μm on timescales of days, with a tentative period of about 6,d (with a plausible range between 3 and 7,d). The best explanation for the variations over days is rotational modulation due to spots. These results disfavour centrifugal winds driven by fast rotation as mechanism for the mass loss, which, in turn, makes the alternative scenario – a thermal pulse due to Deuterium burning – more plausible.

Time-resolved Hubble Space Telescope Wide Field Camera 3 Spectrophotometry Reveals Inefficient Day-to-night Heat Redistribution in the Highly Irradiated Brown Dwarf SDSS 1557B

Brown dwarfs (BDs) in ultra-short-period orbits around white dwarfs (WDs) offer a unique opportunity to study the properties of tidally locked, fast-rotating (1–3 hr), and highly irradiated atmospheres. Here we present phase-resolved spectrophotometry of the WD–BD binary SDSS 1557, which is the fifth WD–BD binary in our six-object sample. Using the Hubble Space Telescope Wide Field Camera 3 Near-infrared G141 instrument, the 1.1–1.7 μm phase curves show rotational modulations with semiamplitudes of 10.5% ± 0.1%. We observe a wavelength-dependent amplitude, with longer wavelengths producing larger amplitudes, while no wavelength-dependent phase shifts were identified. The phase-resolved extracted BD spectra exhibit steep slopes and are nearly featureless. A simple radiative energy redistribution atmospheric model re-creates the hemisphere-integrated brightness temperatures at three distinct phases and finds evidence for weak redistribution efficiency. Our model also predicts a higher inclination than previously published. We find that SDSS 1557B, the second most irradiated BD in our sample, is likely dominated by clouds on the nightside, whereas the featureless dayside spectrum is likely dominated by H− opacity and a temperature inversion, much like the other highly irradiated BD EPIC 2122B.

Some Findings from the Longitudinal Migration of Starspots

Abstract We present results regarding the longitudinal migrations of cool stellar spots that exhibit remarkable oscillations and explore their possible causes. We conducted analyses using high-quality data from nine target systems of various spectral types, spanning from F to M, which were observed by the Kepler Satellite. The systems in which the behaviour of the spots was examined are as follows: KIC 4357272, KIC 6025466, KIC 6058875, KIC 6962018, KIC 7798259, KIC 9210828, KIC 11706658, KIC 12599700, and KIC 8669092. Basic stellar parameters were calculated from light curve analysis using the PHOEBE V.0.32 software, and light curves were modelled to obtain sinusoidal variations occurring out-of-eclipses phases, induced by rotational modulation. Subsequently, we calculated the minimum times of the obtained sinusoidal variations using the Fourier transform. The distributions of θ min corresponding to these minimum times over time were computed using linear fits to determine the longitudinal migrations of the spotted areas. We then compared the longitudinal migration periods with the stellar parameters found in the literature. In addition, we also found a secondary variation in the spot migrations apart from the linear models. Our results revealed that the longitudinal migration periods vary in relation to the B – V colour index of the stars.

The SRG/eROSITA all-sky survey

Context. The first eROSITA all-sky survey (eRASS1) performed on board the Spectrum-Roentgen-Gamma mission (SRG) provides more than 900 000 X-ray sources in the 0.2–2.3 keV band located in the western hemisphere. Aims. We present identifications of the eRASS1 sources obtained using our HamStar method, which was designed for the identification of coronal X-ray sources. Methods. HamStar is a Bayesian framework that estimates coronal probabilities for each eRASS1 source based on a cross-match with optical counterparts from Gaia DR3. It considers geometric properties, such as angular separation and positional uncertainty, as well the additional properties of fractional X-ray flux, color, and distance. Results. We identify 138 800 coronal eRASS1 sources and estimate a completeness and reliability of about 91.5% for this sample, which we confirmed with Chandra detections. This is the largest available sample of coronal X-ray emitters and we find nearly five times as many coronal sources as in the ROSAT all-sky survey. The coronal eRASS1 sources are made up of all spectral types and the onset of convection and the saturation limit are clearly visible. As opposed to previous samples, rare source types are also well populated. About 10% of the coronal eRASS1 sources have a correlated secondary counterpart, which is a wide binary companion or belongs to the same stellar cluster. We also identify 6700 known unresolved binaries, and an excess of fast binary periods below 10 d. Furthermore, the binary sequence is clearly visible in a color–magnitude diagram. When combining the coronal eRASS1 sources with rotation modulations from Gaia DR3, we find 3700 X-ray sources with known rotation periods, which is the largest sample of this kind. We fitted the rotation–activity relation and convection turnover times for our flux-limited sample. We do not detect the low-amplitude fast rotators discovered in the Gaia DR3 sample in X-rays.

Latitude-dependent Atmospheric Waves and Long-period Modulations in Luhman 16 B from the Longest Light Curve of an Extrasolar World

In this work, we present the longest photometric monitoring of up to 1200 hr of the strongly variable brown dwarf binaries Luhman 16 AB and provide evidence of ±5% variability on a timescale of several to hundreds of hours for this object. We show that short-period rotational modulation around 5 hr (k = 1 wavenumber) and 2.5 hr (k = 2 wavenumber) dominate the variability under 10 hr, where the planetary-scale wave model composed of k = 1 and k = 2 waves provides good fits to both the periodograms and light curve. In particular, models consisting of three to four sine waves could explain the variability of the light-curve durations up to 100 hr. We show that the relative range of the k = 2 periods is narrower compared to the k = 1 periods. Using simple models of zonal banding in solar system giants, we suggest that the difference in period range arises from the difference in wind-speed distribution at low and mid-to-high latitudes in the atmosphere. Last, we show that Luhman 16 AB also exhibits long-period ±5% variability, with periods ranging from 15 hr up to 100 hr over the longest monitoring of this object. Our results for the k = 1 and k = 2 waves and long-period evolution are consistent with previous 3D atmosphere simulations, demonstrating that both latitude-dependent waves and slow-varying atmospheric features are potentially present in Luhman 16 AB atmospheres and are a significant contribution to the light-curve modulation over hundreds of rotations.

Influence of Two-Frequency Rotational Modulation on the Dynamics of the Rayleigh–Bénard Convection in Water-Based Nanoliquids with Either AA7072 or AA7075 Nanoparticles

The effect of time-periodic two-frequency rotation modulation on Rayleigh–Bénard convection in water with either AA7072 or AA7075 nanoparticles is investigated. The single-phase description of the Khanafer–Vafai–Lightstone model is used for modeling the nanoliquids. An asymptotic expansion procedure is adopted in the case of the linear stability to obtain the correction (due to modulation) to the Rayleigh number at marginal stability of unmodulated convection. A nonlinear regime of convection is considered with a nonautonomous generalized Lorenz model as the governing equation. The method of multiscales is then employed to obtain the coupled nonautonomous Ginzburg–Landau equations with cubic nonlinearity from the Lorenz model. These equations are presented in the phase-amplitude form and the amplitude is used to quantify the heat transport. The modulation amplitude is considered to be small (of order less than unity) and moderate frequencies of modulation are considered. We found that there is a threshold frequency beyond which the system behavior reverses. At frequencies below the threshold, the mean Nusselt number increases with an increase in the amplitude of modulation while an opposite influence is seen for values above the threshold. Such a behavior is a consequence of what is analogously seen in the case of the critical Rayleigh number. The influence of two-frequency modulation is more pronounced on the results of the linear and nonlinear regimes compared to that of the single-frequency one. The heat transport is enhanced due to the presence of dilute concentration of suspended nanoparticles (either AA7072 or AA7075 nanoalloys) in water. The influence of nanoparticles is to modify the threshold values generating chaos but it does not qualitatively alter the dynamical behavior of the system. The plots of Lyapunov exponents reveal that there is no possibility of hyper-chaos in the generalized Lorenz model when there is a rotational modulation.

Characterizing the dynamical magnetosphere of the extremely slowly rotating magnetic O9.7 V star HD 54879 using rotational modulation of the Hα profile

AbstractThe magnetic field in the O9.7 V star HD 54879 has been monitored for almost a decade. Spectropolarimetric observations reveal a rather strong mean longitudinal magnetic field that varies with a period of about 7.41 years. Observations in the H line show a variation with the same period, while the H line shows only little variation. Assuming the periodic variation to be caused by a slow rotation and a dipolar magnetic field, we find a magnetic field strength of 2 kG at the magnetic poles. With the relatively low mass loss rate of year−1, this star is a case of extremely strong magnetic confinement. Both theoretical arguments and numerical simulations indicate the presence of an extended disk of increased gas density in the equatorial plane of the magnetic field, where gas from the line‐driven stellar wind is trapped. This disk is likely to be the origin of the observed H emission, which peaks together with the strongest line‐of‐sight magnetic field. The profile of the H line is resolved in several components and shows a remarkable variability with the rotation period.

Design of a Multi-Position Alignment Scheme.

The current new type of inertial navigation system, including rotating inertial navigation systems and three-autonomy inertial navigation systems, has been increasingly widely applied. Benefited by the rotating mechanisms of these inertial navigation systems, alignment accuracy can be significantly enhanced by implementing IMU (Inertial Measurement Unit) rotation during the alignment process. The principle of suppressing initial alignment errors using rotational modulation technology was investigated, and the impact of various component error terms on alignment accuracy of IMU during rotation was analyzed. A corresponding error suppression scheme was designed to overcome the shortcoming of the significant scale factor error of fiber optic gyroscopes, and the research content of this paper is validated through corresponding simulations and experiments. The results indicate that the designed alignment scheme can effectively suppress the gyro scale factor error introduced by angular motion and improve alignment accuracy.

The nature of medium-period variables on the extreme horizontal branch

A fraction of the extreme horizontal branch stars of globular clusters exhibit a periodic light variability that has been attributed to rotational modulation caused by surface spots. These spots are believed to be connected to inhomogeneous surface distribution of elements. However, the presence of such spots has not been tested against spectroscopic data. We analyzed the phase-resolved ESO X-shooter spectroscopy of three extreme horizontal branch stars that are members of the globular cluster ω Cen and also display periodic light variations. The aim of our study is to understand the nature of the light variability of these stars and to test whether the spots can reproduce the observed variability. Our spectroscopic analysis of these stars did not detect any phase-locked abundance variations that are able to reproduce the light variability. Instead, we revealed the phase variability of effective temperature and surface gravity. In particular, the stars show the highest temperature around the light maximum. This points to pulsations as a possible cause of the observed spectroscopic and photometric variations. However, such an interpretation is in a strong conflict with Ritter’s law, which relates the pulsational period to the mean stellar density. The location of the ω Cen variable extreme horizontal branch stars in HR diagram corresponds to an extension of PG 1716 stars toward lower temperatures or blue, low-gravity, large-amplitude pulsators toward lower luminosities, albeit with much longer periods. Other models of light variability, namely, related to temperature spots, should also be tested further. The estimated masses of these stars in the range of 0.2–0.3 M⊙ are too low for helium-burning objects.

Meteoroid rotation and quasi-periodic brightness variation of meteor light curves

Meteor light curves are sometimes known to display flickering: rapid, quasi-periodic variations in brightness. This effect is generally attributed to the rotational modulation of the ablation rate, which is caused by the time-varying cross section area presented by a nonspherical rotating meteoroid to the oncoming airflow. In this work we investigate the effects that the rotation of a meteoroid of given shape (spherical, cubic, or cylindrical) has on the meteor’s light curve, given state-of-the-art experimental laboratory estimates of the drag and lift coefficients of hypersonic flow (Mach number > 5) around various shaped objects. The meteoroid’s shape is important in determining these two forces, due to the different response of the drag and lift coefficients according to the angle of attack. As a case study, the model was applied to a fireball observed on 2018 April 17 by the PRISMA network, a system of all-sky cameras that achieves a systematic monitoring of meteors and fireballs in the skies over the Italian territory. The results show that this methodology is potentially able to yield a powerful diagnostic of the rotation rate of meteoroids prior to their encounter with the atmosphere, while also providing essential information on their pre-fall actual shapes.

A hidden population of massive white dwarfs: two spotted K+WD binaries

Abstract The identification and characterization of massive (≳ 0.8 M⊙) white dwarfs is challenging in part due to their low luminosity. Here we present two candidate single-lined spectroscopic binaries, Gaia DR3 4014708864481651840 and 5811237403155163520, with K-dwarf primaries and optically dark companions. Both have orbital periods of P ∼ 0.45 days and show rotational variability, ellipsoidal modulations, and high-amplitude radial velocity variations. Using light curves from the Transiting Exoplanet Survey Satellite (TESS), radial velocities from ground-based spectrographs, and spectral energy distributions, we characterize these binaries to describe the nature of the unseen companion. We find that both systems are consistent with a massive white dwarf companion. Unlike simple ellipsoidal variables, star spots cause the light curve morphology to change between TESS sectors. We attempt to constrain the orbital inclination using PHOEBE binary light curve models, but degeneracies in the light curves of spotted stars prevent a precise determination. Finally, we search for similar objects using Gaia DR3 and TESS, and comment on these systems in the context of recently claimed compact object binaries.

MEMS strapdown inertial attitude measurement system using rotational modulation technology.

Attitude determination involves the integration of methodologies and systems for estimating the time varying attitude of moving objects. Strapdown Inertial Attitude Measurement System (SIAMS) is among the most widely used navigation systems. The development of cost effective Micro Electro Mechanic System (MEMS) based inertial sensors has made attitude measurement system more affordable. However, MEMS sensors suffer from various errors that have to be calibrated and compensated to get acceptable attitude results. Given the auto-compensation of inertial sensor bias in rotation error modulation, the objective of this paper is to develop a MEMS-based rotary SIAMS, in which the significant sensor bias is automatically compensated by rotating the IMU, to offer comparable performance with respect to a tactical-grade Inertial Measurement Unit (IMU). With the analysis of the relationship between the MEMS error and misalignment, a MEMS calibration model is derived, and a combined calibration method of multi position rotation is applied to estimate the deterministic sensor errors such as bias, scale factor, and misalignment. Simulation and experiment results indicate that the proposed method can further modulate and compensate the MEMS errors, thereby improving the MEMS attitude accuracy.

Quantifying the Transit Light Source Effect: Measurements of Spot Temperature and Coverage on the Photosphere of AU Microscopii with High-resolution Spectroscopy and Multicolor Photometry

AU Microscopii (AU Mic) is an active 24 ± 3 Myr pre-main-sequence M dwarf in the stellar neighborhood (d = 9.7 pc) with a rotation period of 4.86 days. The two transiting planets orbiting AU Mic, AU Mic b and c, are warm sub-Neptunes on 8.5 and 18.9 day periods and are targets of interest for atmospheric observations of young planets. Here we study AU Mic’s unocculted starspots using ground-based photometry and spectra in order to complement current and future transmission spectroscopy of its planets. We gathered multicolor Las Cumbres Observatory (LCO) 0.4 m SBIG photometry to study the star's rotational modulations and LCO Network of Robotic Echelle Spectrographs high-resolution spectra to measure the different spectral components within the integrated spectrum of the star, parameterized by three spectral components and their coverage fractions. We find AU Mic’s surface has at least two spectral components: a T amb = 4003−14+15 K ambient photosphere and cool spots that have a temperature of T spot = 3003−71+63 K, covering a globally averaged area of 39% ± 4% which increases and decreases by 5.1% ± 0.3% from the average throughout a rotation. We also detect a third flux component with a filling factor less than 0.5% and a largely uncertain temperature between 8500 and 10,000 K that we attribute to flare flux not entirely omitted when time averaging the spectra. We include measurements of spot characteristics using a two-temperature model, which we find agree strongly with the three-temperature results. Our expanded use of various techniques to study starspots will help us better understand this system and may have applications for interpreting the transmission spectra for exoplanets transiting stars of a wide range of activity levels.

Unveiling the spectacular over 24-hour flare of star CD-36 3202

We studied the light curve of the star CD-36 3202, which was observed by TESS for the presence of stellar spots and to analyze the rotationally modulated flare that took place on TESS Barycentric Julian Date 1486.93. Our main aims are to model the light curve of this flare and to estimate its location regarding stellar spots. The flare lasted approximately 27 h. Using our new tool, findinc_mc, we managed to estimate the inclination angle of the star to 70° ± 8°. With BASSMAN, we modeled the light curve of the CD-36 3202 and estimated that three spots are present on its surface. The mean temperature of the spots was about 4000 ± 765 K, and their total area amounted to 11.61% ± 0.13% on average. We created a new tool, named MFUEA, to model rotationally modulated flares, and used it to estimate the latitude of the long-duration flare event, finding 69−1+2 deg. Our estimation of the flare location is the first recreation of the exact position of a flare in relation to starspots. The flare is placed 12° from the center of the coolest spot. This means that the flare is related to the magnetic processes above the active region represented by the spot. Removing the effects of rotational modulation from the flare light curve allowed us to correct the estimation of bolometric energy released during the event from (1.15 ± 0.35) × 1035 erg to (3.99 ± 1.22) × 1035 erg.

Distribution of Cool Starspots on the Surface of the Red Dwarf V647 Her

An analysis of photometric observations of the star V647 Her (M3.5V) obtained at the 1.25-m telescope of the Crimean Astrophysical Observatory in 2022 is presented. The presence of a low-amplitude variability in brightness of the star with a period of 20.69 d, found from observations in 2019 was confirmed. It is shown that as the brightness decreases, the star becomes redder. The observed kind of photometric variability is due to the presence of cool spots on the surface of the star and rotational modulation of brightness with a full amplitude of no more than 0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{{\ ext{m}}}_{.}$$\\end{document}05. We perform a comparison of the photometric results obtained in 2019, 2022, and 2004. The locations of starspots at different epochs were determined from the analysis of phase curves. The distribution of starspots has been maintained for 40–100 days. Starspot parameters were estimated in the framework of the zonal model. The temperature of starspots is 2700–2800 K. The area they occupied in 2004 is 15% of the total surface area of the star. According to the 2019 and 2022 data, it increases up to 30%. The seasonal redistribution of starspots leads to the difference between the spottedness of hemispheres, which is less than 2%.

Using finite element simulation to evaluate charge measurement precision for space inertial sensors

Non-contact measurement and control are essential for accumulated charges on the test mass (TM) of space inertial sensors, as these charges can worsen the sensitivity of space-based gravitational wave detection. However, it is a challenge to evaluate measurement precision due to the limitations of experimental methods. In this study, the charge measurement process is described using an electrostatic force model, and five influence factors in terms of measurement precision are quantitatively evaluated through finite element simulation. The results indicate that the main contributors to mean relative errors (MREs) are the geometric structure of inertial sensors and the charge distribution on the TM. By correcting the capacitance gradient coefficient in the measurement model, the MRE caused by geometric structure can be reduced from 48% to 2%. Furthermore, the rotational modulation scheme demonstrates lower MREs and relative standard deviations, making it a preferred scheme for charge measurement. This study provides a feasible approach to designing and evaluating the charge measurement scheme for space-based gravitational wave detection.

Modelling stellar variability in archival HARPS data: I - Rotation and activity properties with multidimensional Gaussian processes

ABSTRACT Although instruments for measuring the radial velocities (RVs) of stars now routinely reach sub-metre per second accuracy, the detection of low-mass planets is still very challenging. The rotational modulation and evolution of spots and/or faculae can induce variations in the RVs at the level of a few m s–1 in Sun-like stars. To overcome this, a multidimensional Gaussian Process framework has been developed to model the stellar activity signal using spectroscopic activity indicators together with the RVs. A recently published computationally efficient implementation of this framework, S + LEAF 2, enables the rapid analysis of large samples of targets with sizeable data sets. In this work, we apply this framework to HARPS observations of 268 well-observed targets with precisely determined stellar parameters. Our long-term goal is to quantify the effectiveness of this framework to model and mitigate activity signals for stars of different spectral types and activity levels. In this first paper in the series, we initially focus on the activity indicators (S-index and Bisector Inverse Slope), and use them to (a) measure rotation periods for 49 slow rotators in our sample, (b) explore the impact of these results on the spin-down of middle-aged late F, G, and K stars, and (c) explore indirectly how the spot to facular ratio varies across our sample. Our results should provide valuable clues for planning future RV planet surveys such as the Terra Hunting Experiment or the PLATO ground-based follow-up observations programme, and help fine-tune current stellar structure and evolution models.

A long-term study of the magnetic field and activity in the M giant RZ Ari

Aims. We present a detailed long-term study of the single M6 III giant RZ Ari to obtain direct and simultaneous measurements of the magnetic field, activity indicators, and radial velocity in order to infer the origin of its activity. We study its magnetic activity in the context of stellar evolution, and for this purpose, we also refined its evolutionary status and Li abundance. In general, for the M giants, little is known about the properties of the magnetic activity and its causes. RZ Ari possess the strongest surface magnetic field of the known Zeeman-detected M giants and is bright enough to allow a deep study of its surface magnetic structure. The results are expected to shed light on the activity mechanism in these stars. Methods. We used the spectropolarimeter Narval at the Télescope Bernard Lyot (Observatoire du Pic du Midi, France) to obtain a series of Stokes I and Stokes V profiles for RZ Ari. Using the least-squares deconvolution technique, we were able to detect the Zeeman signature of the magnetic field. We measured its longitudinal component by means of the averaged Stokes V and Stokes I profiles. In addition, we also applied Zeeman-Doppler imaging (ZDI) to search for the rotation period of the star, and we constructed a tentative magnetic map. It is the first magnetic map for a star that evolved at the tip of red giant branch (RGB) or even on the asymptotic giant branch (AGB). The spectra also allowed us to monitor chromospheric emission lines, which are well-known indicators of stellar magnetic activity. From the observations obtained between September 2010 and August 2019, we studied the variability of the magnetic field of RZ Ari. We also redetermined the initial mass and evolutionary status of this star based on current stellar evolutionary tracks and on the angular diameter measured from CHARA interferometry. Results. Our results point to an initial mass of 1.5 M⊙ so that this giant is more likely an early-AGB star, but a lotaction at the tip of the RGB is not completely excluded. With a v sin i of 6.0 ±0.5 km s−1, the upper limit for the rotation period is found to be 909 days. On the basis of our dataset and AAVSO photometric data, we determined periods longer than 1100 days for the magnetic field and photometric variability, and 704 days for the spectral line activity indicators. The rotation period determined on the basis of the Stokes V profiles variability is 530 days. A similar period of 544 days is also found for the photometric data. When we take this rotation period and the convective turnover time into account, an effective action of an α-ω-type dynamo seems to be unlikely, but other types of dynamo could be operating there. The star appears to lie outside the two magnetic strips on the giant branches, where the α-ω-type dynamo is expected to operate effectively, and it also has a much higher lithium content than the evolutionary model predicts. These facts suggest that a planet engulfment could speed up its rotation and trigger dynamo-driven magnetic activity. On the other hand, the period of more than 1100 days cannot be explained by rotational modulation and could be explained by the lifetime of large convective structures. The absence of linear polarization at the time the magnetic field was detected, however, suggests that a local dynamo probably does not contribute significantly to the magnetic field, at least for that time interval.

EFFECTS OF ROTATIONAL MODULATION ON CONVECTION IN ETHYLENE GLYCOL-BASED HYBRID NANOFLUIDS WITH INTERNAL HEATING

The rotational modulation effects on Rayleigh-Bénard convection in ethylene glycol-based hybrid nanofluids with internal heating are investigated. Due to their improved thermophysical properties as compared to base fluid, nanofluids are frequently used in numerous heat transfer applications. Hybrid nanofluids with suitable nanoparticle combinations can have better thermophysical characteristics than mono nanofluids. As a result, this study investigates the impact of hybridizing the base fluid on system stability and heat transfer. A single-phase model is employed to perform a linear and weakly nonlinear stability analysis of the nanofluid. The nonautonomous Ginzburg-Landau equation is derived and solved, and the solution is used to obtain the Nusselt number expression. Based on the linear analysis, the critical Rayleigh number attained in the case of hybrid nanofluids is less than the value found in the case of mono nanofluids. Therefore, the convection onset is faster in a hybrid nanofluid than in a mono nanofluid. The study further shows that hybrid nanofluid ethylene glycol-alumina-copper increases the heat transportation rate as compared to the mono nanofluid ethylene glycol-alumina, presenting evidence that hybrid nanofluid facilitates heat transfer better than the mono nanofluid. Increasing the volume of hybrid nanoparticles qualitatively improved heat transfer by up to 5.96%. Further, the effects of important fluid parameters on heat transfer are presented. Among other results, we found that increasing the modulation's amplitude improves heat transmission in the hybrid nanofluid.