• Santosh Kumar

      Articles written in Pramana – Journal of Physics

    • Study of solar features causing GMSs with 250γ<H<400γ

      Santosh Kumar Mahendra Pratap Yadav

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      The effect of solar features on geospheric conditions leading to geomagnetic storms (GMSs) with planetary index,AP ≥ 20 and the range of horizontal component of the Earth’s magnetic fieldH such that 250γ <H < 400γ has been investigated using interplanetary magnetic field (IMF), solar wind plasma (SWP) and solar geophysical data (SGD) during the period 1978–99. Statistically, it is observed that maximum number of GMSs have occurred during the maximum solar activity years of 21st and 22nd solar cycles. A peculiar result has been observed during the years 1982, 1994 when sunspot numbers (SSNs) decrease very rapidly while numbers of GMSs increase. No distinct association between yearly occurrence of disturbed days and SSNs is observed. Maximum number of disturbed days have occurred during spring and rainy seasons showing a seasonal variation of disturbed days. No significant correlation between magnitude (intensity) of GMSs and importance ofHα, X-ray solar flares has been observed. Maximum number of GMSs is associated with solar flares of lower importance, i.e., SF during the period 1978-93.Hα, X-ray solar flares occurred within lower helio-latitudes, i.e., (0–30)°N to (0–30)°S are associated with GMSs. NoHα, X-ray solar flares have occurred beyond 40°N or 40°S in association with GMSs. In helio-latitude range (10–40)°N to (10–40)°S, the 89.5% concentration of active prominences and disappearing filaments (APDFs) are associated with GMSs. Maximum number of GMSs are associated with solar flares. Coronal mass ejections (CMEs) are related with eruptive prominences, solar flares, type IV radio burst and they occur at low helio-latitude. It is observed that CMEs related GMS events are not always associated with high speed solar wind streams (HSSWSs). In many individual events, the travel time between the explosion on the Sun and maximum activity lies between 58 and 118 h causing GMSs at the Earth.

    • Associated single photons and doubly-charged scalars at linear $e^{-} e^{-}$ colliders

      Biswarup Mukhopadhyaya Santosh Kumar Rai

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      Doubly-charged scalars, predicted in many models having exotic Higgs representations, can in general have lepton-number violating (LFV) couplings. We show that by using an associated monoenergetic final state photon seen at a future linear $e^{-} e^{-}$ collider, we can have a clear and distinct signature for a doubly-charged resonance. The strength of the $\Delta L = 2$ coupling can also be probed quite effectively as a function of the recoil mass of the doubly-charged scalar.

    • Identifying new physics contributions in the Higgs sector at linear $e^{+} e^{-}$ colliders

      Santosh Kumar Rai

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      Loop-driven decay modes of the Higgs are sensitive to new physics contributions because of new particles in the loops. To highlight this we look at the dilepton-dijet signal in the dominant Higgs production channel at a linear $e^{+} e^{-}$ collider. We show that by taking a simple ratio between cross-sections of two different final states such contributions can be very easily identified.

    • Effect of solar features and interplanetary parameters on geomagnetosphere during solar cycle-23

      Santosh Kumar Amita Raizada

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      The dependence of geomagnetic activity on solar features and interplanetary (IP) parameters is investigated. Sixty-seven intense (−200 nT $\leq$ Dst < −100 nT) and seventeen superintense (Dst < −200 nT) geomagnetic storms (GMSs) have been studied from January 1996 to April 2006. The number of intense and superintense GMSs show three distinct peaks during the 11-year period of 23rd solar cycle. The largest number of high strength GMSs are observed during maximum phase of solar cycle. Halo and partial halo CMEs are likely to be the major cause for these GMSs of high intensity. No relationship is observed between storm duration and the number of CMEs involved in its occurrence. The intensity of the GMS is also independent of the number of CMEs causing the occurrence of storm. These geoeffective CMEs show western and northern bias. Majority of the geoeffective CMEs are associated with X-ray solar flares (SFs). Solar and IP parameters, e.g., $V_{\text{CME}}$, $V_{\text{SW}}$, 𝐵, $B_{z}$ (GSE and GSM coordinates) and their products, e.g., $V_{\text{SW}}·B$ and $V_{\text{SW}}·B_{z}$ are observed and correlated to predict the occurrence of intense GMSs. V CME does not seem to be the appropriate parameter with the correlation coefficient, $r = −0.2$ with Dst index, whereas the correlation coefficient, $r = −0.57$, −0.65, 0.75, −0.68 and 0.77 of the parameters $V_{\text{SW}}$, 𝐵, $B_{z}$, $V_{\text{SW}}·B$ and $V_{\text{SW}}·B_{\text{z}}$ respectively, with Dst indicating that $V_{\text{SW}}·B_{\text{z}}$ and $B_{\text{z}}$ may be treated as the significant contributors in determining the strength of GMSs.

    • Solidification and microstructural aspects of laser-deposited Ni–Mo–Cr–Si alloy on stainless steel

      Reena Awasthi Santosh Kumar D Srivastava G K Dey

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      Laser cladding of stainless steel substrate was carried out using Ni–32Mo–15Cr–3Si (wt%) alloy powder. Laser cladding parameters were optimized to obtain defect-free and metallurgically bonded clad. Variation in solidification rate, cooling rate and compositional variation resulted in heterogeneous microstructure. Microstructure was found to be distinctly different in regions of clad cross-section. Majority of the region was found to consist of eutectic of Mo-rich hcp intermetallic Laves phase and NiFe fcc gamma solid solution phases. Extensive microstructural examinations of different clad regions have been carried out using microscopy and microanalysis techniques.

    • High-speed optimisation of an all-optical half adder using a T-shaped photonic crystal waveguide with an improved contrast ratio


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      In digital the detection techniques, the half adder is a component of electronic systems that performs calculations faster than the other logic gates. This article focusses on an all-optical half adder logic gate using a two dimensional T-shaped photonic crystal waveguide and silicon in an air medium. The half adder design is predicated on the concept of constructive (in-phase) and destructive (out-of-phase) interferences. The high-intensity output is achieved by carefully selecting the lattice constant, rod radius and refractive index of the half adder structure. The effectiveness of the half adder is investigated using finite-difference time-domain and plane-wave expansion techniques at 1.55 μm wavelength. The suggested structure features a minimal size of 8.4 μm × 8.4 μm. For this structure, the CARRY has a greater contrast ratio of 18.96 dB and SUM of 8.7 dB, as expressed by the results. SUM and CARRY have bit rates of 28.5 Tbps and 23.8 Tbps, respectively. The proposed circuit’s primary objective is to be compact and to have a high contrast ratio.

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