arXiv subjects

B. N. Dwivedi

Publications and source records attributed to B. N. Dwivedi.

At least 37 records · Page 2Linked to original sources

arXiv2017

Understanding disk galaxy rotation velocities without dark matter contribution--a physical process for MOND?

An impact model of gravity designed to emulate Newton's law of gravitation is applied to the radial acceleration of disk galaxies. Based on this model (Wilhelm et al. 2013), the rotation velocity curves can be understood without the need to postulate any dark matter contribution. The increased acceleration in the plane of the disk is a consequence of multiple interactions of gravitons (called "quadrupoles" in the original paper) and the subsequent propagation in this plane and not in three-dimensional space. The concept provides a physical process that relates the fit parameter of the acceleration scale defined by McGaugh et al. (2016) to the mean free path length of gravitons in the disks of galaxies. It may also explain the modification of the gravitational interaction at low acceleration levels in MOND (Milgrom 1983, 1994, 2015, 2016). Three examples are discussed in some detail: The spiral galaxies NGC 7814, NGC 6503 and M 33.

astro-ph.GA↗

arXiv2017

On the potential energy in a gravitationally bound two-body system with arbitrary mass distribution

The potential energy problem in a gravitationally bound two-body system has recently been studied in the framework of a proposed impact model of gravitation \citep{WilDwi}. The result was applied to the free fall of the so-called Mintrop--Ball in Göttingen with the implicit assumption that the mass distribution of the system is extremely unbalanced. An attempt to generalize the study to arbitrary mass distributions indicated a conflict with the energy conservation law in a closed system. This necessitated us to reconsider an earlier assumption made in selecting a specific process out of two options \citep{Wiletal}. With the result obtained here we can now make an educated selection and reverse our choice. The consequences are presented and discussed in detail for several processes. Energy and momentum conservation could now be demonstrated in all cases.

physics.gen-ph↗

arXiv2016

The Aharonov-Bohm effect: A quantum or a relativistic phenomenon?

The Aharonov-Bohm effect is considered by most authors as a quantum effect, but a generally accepted explanation does not seem to be available. The phenomenon is studied here under the assumption that hypothetical electric dipole distributions configured by moving charges in the solenoid act on the electrons as test particles. The relative motions of the interacting charged particles introduce relativistic time dilations. The massless dipoles are postulated as part of an impact model that has recently been proposed to account for the far-reaching electrostatic forces between charged particles described by Coulomb's law. The model provides a quantitative explanation of the Aharonov-Bohm effect.

quant-ph↗

arXiv2016

Inference of Magnetic Field in the Coronal Streamer Invoking Kink Wave Motions generated by Multiple EUV Waves

Using MHD seismology by observed kink waves, the magnetic field profile of a coronal streamer has been investigated. STEREO-B/EUVI temporal image data on 7 March 2012 shows an evolution of two consecutive EUV waves that interact with the footpoint of a coronal streamer evident in the co-spatial and co-temporal STEREO-B/COR-I observations. The evolution of EUV waves is clearly evident in STEREO-B/EUVI, and its energy exchange with coronal streamer generates kink oscillations. We estimate the phase velocities of the kink wave perturbations by tracking it at different heights of the coronal streamer. We also estimate the electron densities inside and outside the streamer using SSI of polarized brightness images in STEREO-B/COR-1 observations. Taking into account the MHD theory of kink waves in a cylindrical waveguide, their observed properties at various heights, and density contrast of the streamer, we estimate the radial profile of magnetic field within this magnetic structure. Both the kink waves diagnose the exponentially decaying radial profiles of the magnetic field in coronal steamer upto 3 solar radii. Within the limit of uncertainties in the measurements, it is indicated that coronal magnetic field of the streamer varies slowly in time at various heights, although its nature always remains exponentially decaying. It is seen that during the evolution of second kink motion in the streamer, it increases in brightness, and also in areal extent slightly, which may be associated with the decreased photospheric magnetic flux at its footpoint. As a result, the magnetic field profile produced by the second kink wave is reduced within the streamer compared to the one diagnosed by the first one. The precisely estimated magnetic field profiles with the uncertainty less than 10% match well with the empirical profile and various observational estimations of the outer coronal magnetic field.

astro-ph.SR↗

arXiv2015

Study of multi-periodic coronal pulsations during an X-class solar flare

We investigate quasi-periodic coronal pulsations during the decay phase of an X 3.2 class flare on 14 May 2013, using soft X-ray data from the RHESSI satellite. Periodogram analyses of soft X-ray light curves show that 53 s and 72 s periods co-exist in the 3-6, 6-12 and 12-25 KeV energy bands. Considering the typical length of the flaring loop system and observed periodicities, we find that they are associated with multiple (first two harmonics) of fast magnetoacoustic sausage waves. The phase relationship of soft X-ray emissions in different energy bands using cross-correlation technique show that these modes are standing in nature as we do not find the phase lag. Considering the period ratio, we diagnose the local plasma conditions of the flaring region by invoking MHD seismology. The period ratio P1/2P2 is found to be 0.65, which indicates that such oscillations are most likely excited in longitudinal density stratified loops.

astro-ph.SR↗

arXiv2015

Spectroscopic Observations and Modelling of Impulsive Alfvén Waves Along a Polar Coronal Jet

Using the Hinode/EIS 2$"$ spectroscopic observations, we study the intensity, velocity, and FWHM variations of the strongest Fe XII 195.12 Å line along the jet to find the signature of Alfvén waves. We simulate numerically the impulsively generated Alfvén waves within the vertical Harris current-sheet, forming the jet plasma flows, and mimicking their observational signatures. Using the FLASH code and the atmospheric model with embedded weakly expanding magnetic field configuration within a vertical Harris current-sheet, we solve the two and half-dimensional (2.5-D) ideal magnetohydrodynamic (MHD) equations to study the evolution of Alfvén waves and vertical flows forming the plasma jet. At a height of $\sim 5~\mathrm{Mm}$ from the base of the jet, the red-shifted velocity component of Fe XII 195.12 Å line attains its maximum ($5~\mathrm{km\,s}^{-1}$) which converts into a blue-shifted one between the altitude of $5-10~\mathrm{Mm}$. The spectral intensity continously increases up to $10~\mathrm{Mm}$, while FWHM still exhibits the low values with almost constant trend. This indicates that the reconnection point within the jet's magnetic field topology lies in the corona $5-10~\mathrm{Mm}$ from its footpoint anchored in the Sun's surface. Beyond this height, FWHM shows a growing trend. This may be the signature of Alfvén waves that impulsively evolve due to reconnection and propagate along the jet. From our numerical data, we evaluate space- and time- averaged Alfvén waves velocity amplitudes at different heights in the jet's current-sheet, which contribute to the non-thermal motions and spectral line broadening. The synthetic width of Fe XII $195.12~\mathrmÅ$ line exhibits similar trend of increment as in the observational data, possibly proving the existence of impulsively generated (by reconnection) Alfvén waves which propagate along the jet.

astro-ph.SR↗

arXiv2015

Numerical simulations of multi-shell plasma twisters in the solar atmosphere

We perform numerical simulations of impulsively generated Alfvén waves in an isolated photospheric flux tube, and explore the propagation of these waves along such magnetic structure that extends from the photosphere, where these waves are triggered, to the solar corona, and analyze resulting magnetic shells. Our model of the solar atmosphere is constructed by adopting the temperature distribution based on the semi-empirical model and specifying the curved magnetic field lines that constitute the magnetic flux tube which is rooted in the solar photosphere. The evolution of the solar atmosphere is described by 3D, ideal magnetohydrodynamic equations that are numerically solved by the FLASH code. Our numerical simulations reveal, based on the physical properties of the multi-shell magnetic twisters and the amount of energy and momentum associated with them, that these multi-shell magnetic twisters may be responsible for the observed heating of the lower solar corona and for the formation of solar wind. Moreover, it is likely that the existence of these twisters can be verified by high-resolution observations.

astro-ph.SR↗

arXiv2015

Formation of a rotating jet during the filament eruption on 10-11 April 2013

We analyze multi-wavelength and multi-viewpoint observations of a helically twisted plasma jet formed during a confined filament eruption on 10-11 April 2013. Given a rather large scale event with its high spatial and temporal resolution observations, it allows us to clearly understand some new physical details about the formation and triggering mechanism of twisting jet. We identify a pre-existing flux rope associated with a sinistral filament, which was observed several days before the event. The confined eruption of the filament within a null point topology, also known as an Eiffel tower (or inverted-Y) magnetic field configuration results in the formation of a twisted jet after the magnetic reconnection near a null point. The sign of helicity in the jet is found to be the same as that of the sign of helicity in the filament. Untwisting motion of the reconnected magnetic field lines gives rise to the accelerating plasma along the jet axis. The event clearly shows the twist injection from the pre-eruptive magnetic field to the jet.

astro-ph.SR↗

arXiv2015

Solar coronal plumes and the fast solar wind

The spectral profiles of the coronal Ne viii line at 77 nm have different shapes in quiet-Sun regions and coronal holes (CHs). A single Gaussian fit of the line profile provides an adequate approximation in quiet-Sun areas, whereas a strong shoulder on the long-wavelength side is a systematic feature in CHs. Although this has been noticed since 1999, no physical reason for the peculiar shape could be given. In an attempt to identify the cause of this peculiarity, we address three problems that could not be conclusively resolved in a review article by a study team of the International Space Science Institute (ISSI; Wilhelm et al. 2011) : (1) The physical processes operating at the base and inside of plumes as well as their interaction with the solar wind (SW). (2) The possible contribution of plume plasma to the fast SW streams. (3) The signature of the first-ionization potential (FIP) effect between plumes and inter-plume regions (IPRs). Before the spectroscopic peculiarities in IPRs and plumes in polar coronal holes (PCHs) can be further investigated with the instrument Solar Ultraviolet Measurements of Emitted Radiation (SUMER) aboard the Solar and Heliospheric Observatory (SOHO), it is mandatory to summarize the results of the review to place the spectroscopic observations into context. Finally, a plume model is proposed that satisfactorily explains the plasma flows up and down the plume field lines and leads to the shape of the neon line in PCHs.

astro-ph.SR↗

arXiv2014

MHD Seismology of a Coronal Loop System by the First Two Modes of Standing Kink Waves

We report the observation of the first two harmonics of the horizontally polarized kink waves excited in a coronal loop system lying at south-east of AR 11719 on 2013 April 11. The detected periods of the fundamental mode ($P_1$), its first overtone ($P_2$) in the northern half, and that in the southern one are $530.2 \pm 13.3$, $300.4 \pm 27.7$, and $334.7 \pm 22.1$ s, respectively. The periods of the first overtone in the two halves are the same considering uncertainties in the measurement. We estimate the average electron density, temperature, and length of the loop system as $(5.1 \pm 0.8) \times 10^8$ cm$^{-3}$, $0.65 \pm 0.06$ MK, and $203.8 \pm 13.8$ Mm, respectively. As a zeroth order estimation, the magnetic field strength, $B = 8.2 \pm 1.0$ G, derived by the coronal seismology using the fundamental kink mode matches with that derived by a potential field model. The extrapolation model also shows the asymmetric and nonuniform distribution of the magnetic field along the coronal loop. Using the amplitude profile distributions of both the fundamental mode and its first overtone, we observe that the antinode positions of both the fundamental mode and its first overtone shift towards the weak field region along the coronal loop. The results indicate that the density stratification and the temperature difference effects are larger than the magnetic field variation effect on the period ratio. On the other hand, the magnetic field variation has a greater effect on the eigenfunction of the first overtone than the density stratification does for this case.

astro-ph.SR↗

arXiv2014

On the potential energy in an electrostatically bound two-body system

The potential energy problem in an electrostatically bound two-body system is studied in the framework of a recently proposed impact model of the electrostatic force and in analogy to the potential energy in a gravitationally bound system. The physical processes are described that result in the variation of the potential energy as a function of the distance between the charged bodies. The energy is extracted from distributions of hypothetical interaction entities modified by the charged bodies.

physics.gen-ph↗

arXiv2014

On Thermal-Pulse-Driven Plasma Flows in Coronal Funnels as Observed by Hinode/EUV Imaging Spectrometer (EIS)

Using one-arcsecond-slit scan observations from the Hinode/EUV Imaging Spectrometer (EIS) on 05 February 2007, we find the plasma outflows in the open and expanding coronal funnels at the eastern boundary of AR 10940. The Doppler velocity map of Fe XII 195.120 A shows that the diffuse close-loop system to be mostly red-shifted. The open arches (funnels) at the eastern boundary of AR exhibit blue-shifts with a maximum speed of about 10-15 km/s. This implies outflowing plasma through these magnetic structures. In support of these observations, we perform a 2D numerical simulation of the expanding coronal funnels by solving the set of ideal MHD equations in appropriate VAL-III C initial temperature conditions using the FLASH code. We implement a rarefied and hotter region at the footpoint of the model funnel, which results in the evolution of slow plasma perturbations propagating outward in the form of plasma flows. We conclude that the heating, which may result from magnetic reconnection, can trigger the observed plasma outflows in such coronal funnels. This can transport mass into the higher corona, giving rise to the formation of the nascent solar wind.

astro-ph.SR↗

arXiv2011

Transient induced MHD oscillations : A tool to probe the solar active regions

Solar transients and eruptive phenomena which are ubiquitous in the solar atmosphere, can shed new light to the understanding of the outstanding problems like coronal heating and the solar wind acceleration. Observations in the entire electromagnetic spectrum of such dynamical processes of large and small-scale transient/eruptive events, with highly dynamic magnetic field configuration, and energetic particles, provide crucial information about the plasma processes at mega-Kelvin temperature embedded in a complex magnetic field, and also energy build-up/energy-release processes, taking place in such events. One of the most important phenomenological aspects of solar eruptive phenomena is the induced magnetohydrodynamic (MHD) waves generated during these energetic processes, which carry a potential signature to probing the solar active regions. In this paper, we briefly review the recent trends of the transient (e.g., flares) induced quasi-periodic oscillations in the solar atmosphere and discuss their implications in diagnosing the solar active regions, providing the clue to understanding local plasma dynamics and heating.

astro-ph.SR↗

arXiv2009

Network Loop Oscillations with EIS/Hinode

We analyze a time sequence of He II 256.32 angstrom images obtained with EIS/Hinode, sampling a small magnetic loop in magnetic network. Wavelet analysis indicates 11-min periodicity close to the loop apex. We interprete this oscillation as forcing through upward leakage by the fundamental acoustic eigenmode of the underlying field-free cavity. The observed loop length corresponds to the value predicted from this mechanism.

astro-ph.SR↗

arXiv2009

Spectroscopic Diagnostics of Polar Coronal Plumes

Polar coronal plumes seen during solar eclipses can now be studied with space-borne telescopes and spectrometers. We briefly discuss such observations from space with a view to understanding their plasma characteristics. Using these observations, especially from SUMER/SOHO, but also from EUVI/STEREO, we deduce densities, temperatures, and abundance anomalies in plumes and inter-plume regions, and discuss their implications for better understanding of these structures in the Sun's atmosphere.

astro-ph.SR↗

arXiv2009

The redshifted network contrast of transition region emission

Aims: We study the VUV emission of the quiet Sun and the net redshift of transition region lines in the SUMER spectral range. We aim at establishing a link with atmospheric processes and interpreting the observed downflow as the most evident part of the prevailing global coronal mass transport. Methods: We rank and arrange all pixels of a monochromatic raster scan by radiance and define equally-sized bins of bright, faint, and medium-bright pixels. Comparing the bright pixels with the faint pixels, we determine the spectrally-resolved network contrast for 19 emission lines. We then compare the contrast centroids of these lines with the position of the line itself. We establish a relationship between the observed redshift of the network contrast with the line formation temperature. Results: We find that the network contrast is offset in wavelength compared to the emission line itself. This offset, if interpreted as redshift, peaks at middle transition region temperatures and is 10 times higher than the previously reported net redshift of transition region emission lines. We demonstrate that the brighter pixels are more redshifted, causing both a significant shift of the network contrast profile and the well-known net redshift. We show that this effect can be reconstructed from the radiance distribution. This result is compatible with loop models, which assume downflows near both footpoints.

astro-ph.SR↗

arXiv2008

On active region loops: Hinode/EIS observations

Coronal loops are fundamental building blocks of the solar active regions and the corona. Therefore, a clear understanding of the physics of coronal loops will help us understand the physics of active region heating in particular and coronal heating in general. This requires a precise measurement of physical quantities such as electron densities and filling factors, temperatures, and flows in coronal loops. In this paper we have carried out an investigation of a spatially well resolved coronal loop using the EIS onboard Hinode to measure the above mentioned physical quantities. Based on this study we find that a nano-flare model could explain most of the observed characteristics of this loop.

astro-ph.SR↗

arXiv2008

Observation of multiple sausage oscillations in cool postflare loop

Using simultaneous high spatial (1.3 arc sec) and temporal (5 and 10 s) resolution H-alpha observations from the 15 cm Solar Tower Telescope at ARIES, we study the oscillations in the relative intensity to explore the possibility of sausage oscillations in the chromospheric cool postflare loop. We use standard wavelet tool, and find the oscillation period of ~ 587 s near the loop apex, and ~ 349 s near the footpoint. We suggest that the oscillations represent the fundamental and the first harmonics of fast sausage waves in the cool postflare loop. Based on the period ratio P1/P2 ~ 1.68, we estimate the density scale height in the loop as ~ 17 Mm. This value is much higher than the equilibrium scale height corresponding to H-alpha temperature, which probably indicates that the cool postflare loop is not in hydrostatic equilibrium. Seismologically estimated Alfvén speed outside the loop is ~ 300-330 km/s. The observation of multiple oscillations may play a crucial role in understanding the dynamics of lower solar atmosphere, complementing such oscillations already reported in the upper solar atmosphere (e.g., hot flaring loops).

astro-ph↗