arXiv subjects

K. M. Hiremath

Publications and source records attributed to K. M. Hiremath.

15 recordsLinked to original sources

arXiv2022

A possible solution for the faint young Sun paradox: Clues from the exoplanetary data

Faint young Sun paradox (FYSP) is one of the unsolved problem in solar physics. The present study aims to get a possible solution for the FYSP through sun-like G stars and their exoplanetary systems. Using physical properties of exoplanetary data, an empirical relationship between the rate of mass loss ($\frac{dM}{dt}$) with stellar mass (M$_{\star}$) and age ({\em t}) is obtained. We found mass loss rate varies with stellar mass as $\propto$ $(M_{\star}/M_\odot)^{-3.788}$ and proportional to the age as $\propto$ t$^{-1.25}$, which indicates rate of mass loss is higher during early evolutionary stages. Then we applied mass loss corrections to stellar masses of G-type stars with planets and obtained their initial masses at the early evolutionary stages. Subsequently, we applied these relationships to calculate the mass loss rate and mass of Sun at the early evolutionary stage, which is found to be $\sim$ 10$^{-11}$ solar mass per year and $\sim$ (1.061$\pm$0.006) solar mass respectively. The higher solar mass can probably alleviate the problem of the faint young Sun paradox. Then the estimated initial stellar masses of the host stars are used to obtain a best power law relationship with the planetary masses that supports the hypothesis that the {\em massive stars harbour massive planets.} Finally, by using the same empirical power law, planetary mass in the vicinity of Sun is estimated to be $\sim$ (0.84$\pm$0.19) Jupiter mass, which is much higher compared to the present solar terrestrial planetary mass. Hence, this study also suggests that there is a missing planetary mass in the vicinity of the Sun, which can solve the FYSP problem.

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arXiv2022

Rotation rate of high latitude and near polar coronal holes

For the period of 1997-2006, coronal holes detected in the SOHO/EIT 195 $Å$ full disk calibrated images are used to compute the rotation rates of high latitude and near polar coronal holes and, their latitudinal variation is investigated. We find that, for different latitude zones between $80^{o}$ north and $75^{o}$ south, for all their area, the number of days observed on the solar disk, and their latitudes, coronal holes rotate rigidly. Estimated magnitudes of sidereal rotation rate of the coronal holes are: $13.051 \pm 0.206$ deg/day for the equator, $12.993 \pm 0.064$ deg/day in the region of higher latitudes and, $12.999 \pm 0.329$ deg/day near the polar regions. For all the latitudes and the area, we have also investigated the annual variation of rotation rates of these coronal holes. We find that, for all the years, coronal holes rotate rigidly and their magnitude of equatorial, high latitude and polar region rotation rates are independent of magnitude of solar activity.

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arXiv2022

Thermal and magnetic field structure of near equatorial coronal holes

We use full-disk, SOHO/EIT 195 $Å$ calibrated images to measure latitudinal and day to day variations of area and average photon fluxes of the near equatorial coronal holes. In addition, energy emitted by the coronal holes with their temperature and strength of magnetic field structures are estimated. By analyzing data of 2001-2008, we find that variations of average area (A), photon flux (F), radiative energy (E) and temperature (T) of coronal holes are independent of latitude. Whereas inferred strength of magnetic field structure of the coronal holes is dependent on the latitudes and varies from low near the equator to high near both the poles. Typical average values of estimated physical parameters are: $A \sim 3.8(\pm0.5)\times10^{20}~cm^{2}, F \sim 2.3(\pm0.2)\times10^{13}~photons\;cm^{-2}\;sec^{-1}, E \sim 2.32(\pm0.5)\times 10^{3}~ergscm^{-2}sec^{-1} \ and \ T \sim 0.94(\pm0.1)\times10^{6} ~$ K. Average strength of magnetic field structure of coronal hole at the corona is estimated to be $\sim$ $0.08 \pm 0.02$ Gauss. If coronal holes are anchored in the convection zone, one would expect they should rotate differentially. Hence, thermal wind balance and isorotation of coronal holes with the solar plasma implies the temperature difference between the equator and both the poles. Contrary to this fact, variation of thermal structure of near equatorial coronal holes is independent of latitude leading to a conclusion that coronal holes must rotate rigidly that are likely to be anchored initially below the tachocline confirming our previous study (ApJ, 763, 137, 2013).

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arXiv2020

Nearly Century-scale Variation of the Sun's Radius

The Kodaikanal Archive Program (India) is now available to the scientific community in digital form as daily digitized solar white light pictures, from 1923 to 2011. We present here the solar radius data, obtained after a painstaking effort to remove all effects that contribute to the error in their measurements (limb darkening, distortion of the objective lens, refraction, other instrumental effects, etc.). These data were analyzed to reveal any significant periodic variations, after applying a multi-taper method with red noise approximation and the Morlet wavelet transform analysis. After removing obvious periodic variations (such as solar rotation and Earth annual rotation), we found a possible cycle variation at 11.4 yr, quasi biennial oscillations at 1.5 and 3.8 yr, and Rieger-type periodicity at ~159, 91, and 63 days. Another ~7.5 yr periodicity (as a mean) resulting from two other main periodicities detected at 6.3-7.8 yr can be identified as an atmospheric component. The detrending data show, over a mean radius of 959".7 +/- 0".7, a residual of less than ~(-)1 mas over the time period of analysis: if not spurious, this estimate indicates a faint decline, but probably confirms more the constancy of the solar diameter during the considered ranging time, within instrumental and methodological limits. The Kodaikanal long quality observations contribute to international efforts to bring past solar data measurements to the community to further explore issues, for instance, those of the luminosity/radius properties that could be used to pinpoint the "seat of the solar cycle."

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arXiv2019

Kodaikanal calcium images: Detection of plages, Fixing the heliographic coordinates and Estimation of Area

Kodaikanal Observatory is a veritable treasure trove of data, with the data repository covering almost 100 years of observations. For the years 1909-2007, we use calibrated Ca II K spectroheliograms from the Kodaikanal Observatory to detect the plages, fix their heliographic coordinates and also estimate the plage areas. We adopt the following procedure. After ensuring that, for all the years, Kodai calcium images have very negligible ellipcity, a circle is fitted and two central coordinates and radius of calcium images are determined uniquely. For each pixel of the calcium image, we then fix heliographic coordinates and extract plages along with their weighted average coordinates. The heliographic coordinates of these extracted plages are then compared with the heliographic coordinates of photospheric sunspots from the Greenwich sunspot database and chromospheric magnetic plages detected from the SOHO/MDI magnetograms. We find that the heliographic coordinates of calcium plages match very well with the heliographic coordinates of sunspots and magnetic plages authenticating our method of detection of plages and computation of positional coordinates. A code is developed in Python and all the nearly century scale plages data, with accurately estimated heliographic coordinates and areas, is available to the public.

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arXiv2012

Rotation rates of the coronal holes and their probable anchoring depths

For the years 2001-2008, we use full-disk, SOHO/EIT 195 $Å$ calibrated images to determine latitudinal and day to day variations of the rotation rates of coronal holes. We estimate the weighted average of heliographic coordinates such as latitude and longitude from the central meridian on the observed solar disk. For different latitude zones between $40^{o}$ north - $40^{o}$ south, we compute rotation rates, and find that, irrespective of their area, number of days observed on the solar disk and latitudes, coronal holes rotate rigidly. Combined for all the latitude zones, we also find that coronal holes rotate rigidly during their evolution history. In addition, for all latitude zones, coronal holes follow a rigid body rotation law during their first appearance. Interestingly, average first rotation rate ($\sim 438 nHz$) of the coronal holes, computed from their first appearance on the solar disk, match with rotation rate of the solar interior only below the tachocline.

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arXiv2012

Seismology of the Sun : Inference of Thermal, Dynamic and Magnetic Field Structures of the Interior

Recent overwhelming evidences show that the sun strongly influences the Earth's climate and environment. Moreover existence of life on this Earth mainly depends upon the sun's energy. Hence, understanding of physics of the sun, especially the thermal, dynamic and magnetic field structures of its interior, is very important. Recently, from the ground and space based observations, it is discovered that sun oscillates near 5 min periodicity in millions of modes. This discovery heralded a new era in solar physics and a separate branch called helioseismology or seismology of the sun has started. Before the advent of helioseismology, sun's thermal structure of the interior was understood from the evolutionary solution of stellar structure equations that mimicked the present age, mass and radius of the sun. Whereas solution of MHD equations yielded internal dynamics and magnetic field structure of the sun's interior. In this presentation, I review the thermal, dynamic and magnetic field structures of the sun's interior as inferred by the helioseismology.

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arXiv2010

A New Approach in Understanding Growth and Decay of the Sunspots

From the previous study (Hiremath 2009b; Hiremath 2010), on the genesis of solar cycle and activity phenomena, it is understood that sunspots are formed at different depths by superposition of Alfven wave perturbations of a strong toroidal field structure in the convective envelope and after attaining a critical strength, due to buoyancy, raise toward the surface along the rotational isocontours that have positive (0.7-0.935 $R_{\odot}$) and negative (0.935-1.0 $R_{\odot}$) rotational gradients. Owing to physical conditions in these two rotational gradients, from the equation of magnetic induction, sunspot's area growth and decay problem is solved separately. It is found that rate of growth of sunspot's area during its evolution at different depths is function of steady and fluctuating parts of Lorentzian force of the ambient medium, fluctuations in meridional flow velocity, radial variation of rotational gradient and $cot(\vartheta)$ (where $\vartheta$ is co-latitude). While rate of decay of sunspot's area at different depths during its evolution mainly depends upon magnetic diffusivity, rotational gradient and $sin^{2}(\vartheta)$. Gist of this study is that growth and decay of area of the sunspot mainly depends upon whether sunspot is originated in the region of either positive or negative rotational gradient. For different latitudes and life spans of the sunspots on the surface during their evolutionary history, both the analytically derived theoretical area growth and decay curves match reasonably well with the observed area growth and decay curves.

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arXiv2009

Periodicities in the occurrence of solar coronal mass ejections

The Fourier analysis of the CME occurrence data observed by the SOHO satellite shows significant power around 1.9 yr., 1.2 yr., 265 day, 39 day and 26 day periodicities which are almost similar to the periodicities detected in the Fourier analysis of underlying activities of the photosphere. The wavelet analysis of CME occurrences also shows significant power around such periods which occur near the peak of solar activity. For the sake of comparison, the occurrences of H-alpha flares are subjected to Fourier and wavelet analyses. The well-known periods (1.3 yr., 152 day, 27 day) in the flare occurrences are detected. The wavelet analyses of both the occurrences yield the following results : (i) in the both CME and Flare activities, long period (~ 1.3 yr) activity occurs around solar maximum only and, (ii) flare activity of long period, especially for the period 1.3 year, lags behind the long period CME activity nearly by six months. Possible physical explanation for the 1.2 yr CME quasi-periodicity is briefly discussed.

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arXiv2009

Physics of the Solar Cycle : New Views

Two well known turbulent dynamo and MHD oscillation mechanisms are critically examined and fundamental difficulties are presented. Following new ideas on the genesis of the solar cycle and activity phenomena are presented. Inevitability of most likely existence of a combined steady poloidal and toroidal magnetic field structure in the solar interior. Owing to suitable steady poloidal field structure, Alfven wave perturbations of long periods (~ 22 yrs) that excite in the solar core travel first to the poles in both the hemispheres and later reach the equator. While traveling towards the surface, Alfven wave perturbations along the weak poloidal field structure in turn perturb the embedded strong toroidal field structure producing sunspots, especially in the convective envelope, that travel to the surface due to buoyancy along isorotational contours. With realistic density structure of the solar interior, computation of Alfven wave travel times along different field lines of the poloidal field structure (Hiremath and Gokhale 1995) yields almost similar periods (~ 22 yr) explaining the constancy of 22 yr period of the odd degree modes obtained from the Spherical Harmonic Fourier analysis of the surface magnetic field. The observed quasi-periodicities of solar activity indices in the range of 1-5 years are explained due to perturbation of the strong toroidal field structure and, variation of very long period solar cycle and activity phenomena such as the Maunder and grand minima is explained to be due to coupling of long period poloidal and toroidal MHD oscillations.

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arXiv2009

Primordial flares, flux tubes, MHD waves in the early universe and genesis of cosmic gamma ray bursts

It is conjectured that energy sources of the gamma ray bursts are similar to energy sources which trigger solar and stellar transient activity phenomena like flares, plasma accelerated flows in the flux tubes and, dissipation of energy and acceleration of particles by the MHD waves. Phenomenologically we examine in detail the following energy sources which may trigger gamma ray bursts : (i) cosmic primordial flares which could be solar flare like phenomena in the region of inter galactic or inter galactic cluster regions, (ii) primordial magnetic flux tubes that might have been formed from the convective collapse of the primordial magnetic flux (iii) nonlinear interaction and dissipation of MHD waves that are produced from the perturbations of large-scale inter galactic or inter cluster magnetic field of primordial origin. We examine in detail each of the afore mentioned phenomena keeping in mind that whether such processes are responsible for energy sources of the gamma ray bursts. By considering the similarity of observations and prevailing physical conditions in the cosmic environment, the following study suggests that most likely and a promising energy source for creation of the gamma ray bursts may be due to primordial flares.

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arXiv2009

Cosmic GRB energy-redshift relation and Primordial flares as possible energy source for the central engine

By considering similar observed properties of gamma ray bursts (GRB) and solar flares with the prevailing physical conditions in the cosmic environment, the following study suggests that most likely and promising energy source for the central engine which triggers GRB may be due to primordial flares, solar flare like phenomena, at the sites of inter galactic or inter galactic clusters in the early universe. The derived energy-redshift relation, E = E_{0}{(1+z)}^3 (where E is the amount of energy released, z is the redshift of GRB and E_{0} is a constant which is estimated to be ~ 10^{52} ergs), from the simple flare mechanism, is confirmed from the least square fit with the observed energy-redshift relation. Some of the physical parameters like length scale, strength of magnetic field, etc., of the flaring region of the GRB are estimated.

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arXiv2009

Solar Forcing on the Changing Climate

The influence of solar cycle and activity phenomena on the two climatic variabilities such as the rainfall and the surface temperature of the Indian subcontinent are critically examined. It is concluded from this study that the sun indeed strongly influences both of these climatic variabilities and the sun's influence simply cannot be ignored.

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arXiv2008

Deep meridional circulation below the solar convective envelope

With reasonable assumptions and approximations, we compute the velocity of the meridional flow $U$ in the convective envelope by modified Chandrasekhar's (1956) MHD equations. The analytical solution of such a modified equation is found to be $U(x,μ) = \sum_{n=0}^\infty \bigl[u1_n x^n + u2_n x^{-(n+3)}\bigr] C_n^{3/2}(μ)$, where $x$ is non-dimensional radius, $μ= cos{\vartheta}$, ${\vartheta}$ is the co-latitude, $C_n^{3/2} {(μ)}$ are the Gegenbaur polynomials of order 3/2, $u1_n$ and $u2_n$ are the unknown constants. The results show that meridional velocity flow from the surface appears to penetrates deep below base of the convective envelope and at outer part of the radiative zone. With such a deep flow velocity below the convective envelope and a very high density stratification in the outer part of the radiative zone with likely existence of a strong ($\sim$ $10^{4}$ G) toroidal magnetic field structure, the velocity of transport of meridional flow is considerably reduced. Hence, it is very unlikely that the return flow will reach the surface (with a period of solar cycle) as required by some of the flux transport dynamo models. On the other hand, deep meridional flow is required for burning of Lithium at outer part of the radiative zone supporting the observed Lithium deficiency at the surface.

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arXiv2007

Prediction of future fifteen solar cycles

In the previous study (Hiremath 2006a), the solar cycle is modeled as a forced and damped harmonic oscillator and from all the 22 cycles (1755-1996), long-term amplitudes, frequencies, phases and decay factor are obtained. Using these physical parameters of the previous 22 solar cycles and by an {\em autoregressive model}, we predict the amplitude and period of the future fifteen solar cycles. Predicted amplitude of the present solar cycle (23) matches very well with the observations. The period of the present cycle is found to be 11.73 years. With these encouraging results, we also predict the profiles of future 15 solar cycles. Important predictions are : (i) the period and amplitude of the cycle 24 are 9.34 years and 110 ($\pm 11$), (ii) the period and amplitude of the cycle 25 are 12.49 years and 110 ($\pm$ 11), (iii) during the cycles 26 (2030-2042 AD), 27 (2042-2054 AD), 34 (2118-2127 AD), 37 (2152-2163 AD) and 38 (2163-2176 AD), the sun might experience a very high sunspot activity, (iv) the sun might also experience a very low (around 60) sunspot activity during cycle 31 (2089-2100 AD) and, (v) length of the solar cycles vary from 8.65 yrs for the cycle 33 to maximum of 13.07 yrs for the cycle 35.

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