• K. M. Hiremath

Articles written in Journal of Astrophysics and Astronomy

• Emergence of twisted magnetic flux related sigmoidal brightening

• The flares associated with the dynamics of the sunspots

In the present study, we consider six years data of spot groups that have well developed leading and following spots obtained from the Kodaikanal Observatory white light pictures and occurrence of Hα flares. From the daily observations, we compute the variations in rotation rates, meridional velocity, the areas and longitudinal separations. We find that among all these variations, the occurrence of abnormal rotation rates (the rotation rates that have greater than 1σ and longitudinal minimum separation during the course of their evolution eventually lead to triggering of flares. We also find that the events of abnormal rotation rates, longitudinal minimum separation and the flares occur mainly during the 50–80% of the sunspots’ life span indicating magnetic reconnection probably below (0.935R⊙) the solar surface. Relevance of these results with the conventional theory of magnetic reconnection is briefly discussed.

• Observational searches for chromospheric G-mode oscillations from CaII H-line observations

We have used a high spatial and temporal resolution of long time sequence of spectra in CaII H-line obtained at the Vacuum Tower Telescope (VTT) of the Sacramento Peak Observatory on a quiet region at the center of the solar disk over a large number of bright points and network elements to search for atmospheric (chromospheric) g-mode oscillations. An important parameter of the H-line profile, intensity at H2v(Ih2V), has been derived from a large number of line profiles. We derived the light curves of all the bright points and network elements. The light curves represent the main pulse with large intensity amplitude and followed by several follower pulses with lower intensity amplitudes. The light curves of these bright points would give an impression that one can as well draw curves towards and away from the highest peak (main pulse) showing an exponential growth and decay of the amplitudes. An exponential decaying function has been fitted for all the light curves of the bright points to determine the damping time of the modes that are more or less the same, and one value of the coefficient of exponent can represent reasonably well the decay for all the cases. The FFT analysis of temporal variation of both the bright points and the network elements indicates around 10-min periodicity. We speculate that this longer period of oscillation may be related to chromospheric g-mode oscillations.

• The extreme solar activity during october–november 2003

The positional measurements of sunspots from the Kodaikanal Observatory and Solar Geophysical data are used to study the association between occurrence of the abnormal activities of big sunspot groups that were observed during the period of October–November 2003 and occurrence of the flares. During the evolution of the sunspot groups, we have investigated the temporal variations in (i) areas; (ii) rotation rates; (iii) longitudinal extents; and (iv) number of small spots produced in a sunspot group. Among all these activity variations, we find that the spot groups that experience abnormal rotation rates during their evolutionary phases eventually trigger the flares.

• The influence of solar activity on the rainfall over India: Cycle-to-cycle variations

We use 130 years data for studying correlative effects due to solar cycle and activity phenomena on the occurrence of rainfall over India. For the period of different solar cycles, we compute the correlation coefficients and significance of correlation coefficients for the seasonal months of Jan–Feb (JF), Mar–May (MAM), June–Sept (JJAS) and Oct–Dec (OND) and,annual mean data. We find that: (i) with a moderate-to-high significance, Indian rainfall is correlated with the sunspot activity and, (ii) there is an overall trend that during the period of low sunspot activity, occurrence of rainfall is high compared to the period of high sunspot activity.

We speculate in this study a possible physical connection between the occurrence of the rainfall and the sunspot activities and, the flux of galactic cosmic rays. Some of the negative correlations between the occurrences of the sunspot and rainfall activities obtained for different solar cycle periods are interpreted as effects of aerosols on the rain forming clouds due to either intermittent volcanic eruptions or due to intrusion of interstellar dust particles in the Earth’s atmosphere.

• Solar Wind Associated with Near Equatorial Coronal Hole

Present study probes temporal changes in the area and radiative flux of near equatorial coronal hole associated with solar wind parameters such as wind speed, density, magnetic field and temperature. Using high temporal resolution data from SDO/AIA for the two wave-lengths 193 Å and 211 Å, area and radiative flux of coronal holes are extracted and are examined for the association with high speed solar wind parameters. We find a strong association between different parameters of coronal hole and solar wind. For both the wavelength bands, we also compute coronal hole radiative energy near the earth and it is found to be of similar order as that of solar wind energy. However, for the wavelength 193 Å, owing to almost similar magnitudes of energy emitted by coronal hole and energy due to solar wind, it is conjectured that solar wind might have originated around the same height where 193 Å line is formed in the corona.

• Development of a Code to Analyze the Solar White-Light Images from the Kodaikanal Observatory: Detection of Sunspots, Computation of Heliographic Coordinates and Area

Sunspots are the most conspicuous aspects of the Sun. They have a lower temperature, as compared to the surrounding photosphere; hence, sunspots appear as dark regions on a brighter background. Sunspots cyclically appear and disappear with a 11-year periodicity and are associated with a strong magnetic field $(\sim 10^3$ G) structure. Sunspots consist of a dark umbra, surrounded by a lighter penumbra. Study of umbra–penumbra area ratio can be used to give a rough idea as to how the convective energy of the Sun is transported from the interior, as the sunspot’s thermal structure is related to this convective medium. An algorithm to extract sunspots from the white-light solar images obtained from the Kodaikanal Observatory is proposed. This algorithm computes the radius and center of the solar disk uniquely and removes the limb darkening from the image. It also separates the umbra and computes the position as well as the area of the sunspots. The estimated results are compared with the Debrecen photoheliographic results. It is shown that both area and position measurements are in quite good agreement.

• Lowering of Asymmetry

Asymmetry, a well established fact, can be extracted from various solar atmospheric activity indices. Although asymmetry is being localized within short time scale, it also persists at different time scales. In the present study we examine the character and nature of asymmetry at various time scales by optimizing the data set, in units of Carrington Rotations (CRs), for Sunspot Area (SA) and soft X-ray flare index (FISXR). We find from three solar cycles (21-23) that at a small time scale (viz., daily, CRs and monthly) activity appears to be asymmetric with less significance. At larger time scales (≥01 CRs) strength of asymmetry enhances. Number of significant asymmetry points probably depends upon the solar heights. For different combination of data, asymmetry strength appears to be lowered at certain periods ∼06, ∼12, ∼18 CRs (164, 327 and 492 days i.e., harmonics of ∼1.3 years. Owing to similar behavior of emergence of magnetic flux, it is conjectured that emergence of flux on the surface probably contributes to the asymmetry of the solar activity.

• Metallicity of Sun-like G-stars that have Exoplanets

By considering the physical and orbital characteristics of G type stars and their exoplanets, we examine the association between stellar mass and its metallicity that follows a power law. Similar relationship is also obtained in case of single and multiplanetary stellar systems suggesting that, Sun′s present mass is about 1% higher than the estimated value for its metallicity. Further, for all the stellar systems with exoplanets, association between the planetary mass and the stellar metallicity is investigated, that suggests planetary mass is independent of stellar metallicity. Interestingly, in case of multiplanetary systems, planetary mass is linearly dependent on the stellar absolute metallicity, that suggests, metal rich stars produce massive (≥1 Jupiter mass) planets compared to metal poor stars. This study also suggests that there is a solar system planetary missing mass of ∼0.8 Jupiter mass. It is argued that probably 80% of missing mass is accreted onto the Sun and about 20% of missing mass might have been blown off to the outer solar system (beyond the present Kuiper belt) during early history of solar system formation. We find that, in case of single planetary systems, planetary mass is independent of stellar metallicity with an implication of their non-origin in the host star’s protoplanetary disk and probably are captured from the space. Final investigation of dependency of the orbital distances of planets on the host stars metallicity reveals that inward migration of planets is dominant in case of single planetary systems supporting the result that most of the planets in single planetary systems are captured from the space.

• # Journal of Astrophysics and Astronomy

Volume 43, 2022
All articles
Continuous Article Publishing mode

• # Continuous Article Publication

Posted on January 27, 2016

Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019