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Numerical simulations of impulsively generated Alfvén waves in solar magnetic arcades  

Dr. A.K. Srivastava   Submitted: 2014-08-05 21:08

We perform numerical simulations of impulsively generated Alfvén waves in an isolated solar arcade, which is gravitationally stratified and magnetically confined. We study numerically the propagation of Alfvén waves along such magnetic structure that extends from the lower chromosphere, where the waves are generated, to the solar corona, and analyze influence of the arcade size and width of the initial pulses on the wave propagation and reflection. Our model of the solar atmosphere is constructed by adopting the temperature distribution based on the semi-empirical VAL-C model and specifying the curved magnetic field lines that constitute the asymmetric magnetic arcade. The propagation and reflection of Alfvén waves in this arcade is described by 2.5D magnetohydrodynamic equations that are numerically solved by the FLASH code. Our numerical simulations reveal that the Alfvén wave amplitude decreases as a result of a partial reflection of Alfvén waves in the solar transition region, and that the waves which are not reflected leak through the transition region and reach the solar corona. We also find the decrement of the attenuation time of Alfvén waves for wider initial pulses. Moreover, our results show that the propagation of Alfvén waves in the arcade is affected by spatial dependence of the Alfvén speed, which leads to phase-mixing that is stronger for more curved and larger magnetic arcades. We discuss processes that affect the Alfvén wave propagation in an asymmetric solar arcade and conclude that besides phase-mixing in the magnetic field configuration, plasma properties of the arcade and size of the initial pulse as well as structure of the solar transition region all play a vital role in the Alfvén wave propagation.

Authors: P. Chmielewski, K. Murawski, Z.E. Musielak, A.K. Srivastava
Projects: None

Publication Status: ApJ (in press)
Last Modified: 2014-08-06 09:04
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Fast Magnetic Twister and Plasma Perturbations in a 3-D Coronal Arcade  

Dr. A.K. Srivastava   Submitted: 2014-04-16 21:01

We present results of 3-D numerical simulations of a fast magnetic twister excited above a foot-point of the potential solar coronal arcade that is embedded in the solar atmosphere with the initial VAL-IIIC temperature profile, which is smoothly extended into the solar corona. With the use of the FLASH code, we solve 3-D ideal magnetohydrodynamic equations by specifying a twist in the azimuthal component of magnetic field in the solar chromosphere. The imposed perturbation generates torsional Alfvén waves as well as plasma swirls that reach the other foot-point of the arcade and partially reflect back from the transition region. The two vortex channels are evident in the generated twisted flux-tube with a fragmentation near its apex that results from the initial twist as well as from the morphology of the tube. The numerical results are compared to observational data of plasma motions in a solar prominence. The comparison shows that the numerical results and the data qualitatively agree even though the observed plasma motions occur over comparatively large spatio-temporal scales in the prominence.

Authors: K. Murawski, A.K. Srivastava, Z. E. Musielak
Projects: None

Publication Status: ApJ (in press)
Last Modified: 2014-04-20 19:16
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Evidence of Multiple Slow Acoustic Oscillations in the Stellar Flaring Loops of Proxima Centauri  

Dr. A.K. Srivastava   Submitted: 2013-10-28 05:27

We present the first observational evidence of multiple slow acoustic oscillations in the post flaring loops of the corona of Proxima Centauri using XMM-Newton observations. We find the signature of periodic oscillations localized in the decay phase of the flare in its soft (0.3-10.0 keV) X-ray emissions. Using the standard wavelet tool, we find the multiple periodicities of 1261 s and 687 s. These bursty oscillations persist for durations of 90 minutes and 50 minutes, respectively, for more than 4 cycles. The intensity oscillations with the period of 1261 s may be the signature of the fundamental mode of slow magnetoacoustic waves with the phase-speed of 119 km s-1 in the loop of the length 7.5 imes 109 cm heated initially to obtain the flare peak temperature of 33 MK and later cooled down in the decay phase maintained at the average temperature of 7.2 MK. The other period of 687 s may be associated with the first overtone of slow magnetoacoustic oscillations in the flaring loop. The fundamental mode oscillations show a dissipation with damping time of 47 min. The period ratio P1/P2 is found to be 1.83 indicating that such oscillations are most likely excited in longitudinal density stratified stellar loops. We estimate the density scale height of stellar loop system as 22.6 Mm, which is smaller than the hydrostatic scale height of the hot loop system, and implies the existence of non-equilibrium conditions.

Authors: Srivastava, A. K.; Lalitha, S.; Pandey, J. C.
Projects: None

Publication Status: ApJL; in press
Last Modified: 2013-10-28 23:02
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MHD Seismology as a Tool to Diagnose the Coronae of X-ray Active Sun-like Flaring Stars  

Dr. A.K. Srivastava   Submitted: 2013-09-03 10:22

It is now well accepted that the detection of impulsively generated multiple MHD modes are potentially used in diagnosing the local plasma conditions of the solar corona. Analogously, such analyses can also be significantly used in diagnosing the coronae of X-ray active Sun-like stars. In the present paper, we briefly review the detection of MHD modes in coronae of some X-ray active Sun-like stars, e.g. Proxima Centauri, XI-Boo etc using XMM-Newton observations, and discuss the implications in deriving physical information about their localized magnetic atmosphere. We conclude that the refinement in the MHD seismology of solar corona is also providing the best analogy to develop the stellar seismology of magnetically active and flaring Sun-like stars to deduce the local physical conditions of their coronae.

Authors: A.K. Srivastava, S. Lalitha
Projects: None

Publication Status: Accepted for the Publication in The Proceedings of ISSTP-2012
Last Modified: 2013-09-03 12:21
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X6.9-class Flare Induced Vertical Kink Oscillations in a Large-Scale Plasma Curtain as Observed by SDO/AIA  

Dr. A.K. Srivastava   Submitted: 2013-09-03 10:18

We present rare observational evidence of vertical kink oscillations in a laminar and diffused large-scale plasma curtain as observed by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). The X6.9 class flare in the Active Region 11263 on 09 August 2011, induces a global large-scale disturbance that propagates in a narrow lane above the plasma curtain and creates a low density region that appears as a dimming in the observational image data. This large-scale propagating disturbance acts as a non-periodic driver that interacts asymmetrically and obliquely with the top of the plasma curtain, and triggers the observed oscillations. In the deeper layers of the curtain, we find evidence of vertical kink oscillations with two periods (795 s and 530 s). On the magnetic surface of the curtain where the density is inhomogeneous due to the coronal dimming, non-decaying vertical oscillations are also observed (period approx 763-896 s). We infer that the global large-scale disturbance triggers vertical kink oscillations in the deeper layers as well as on the surface of the large-scale plasma curtain. The properties of the excited waves strongly depend on the local plasma and magnetic field conditions.

Authors: A.K. Srivastava, M. Goossens
Projects: SDO-AIA

Publication Status: Accepted for the Publication in ApJ
Last Modified: 2013-09-03 12:21
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Three-dimensional numerical simulation of magnetohydrodynamic-gravity waves and vortices in the solar atmosphere  

Dr. A.K. Srivastava   Submitted: 2013-09-03 10:15

With the adaptation of the FLASH code we simulate magnetohydrodynamic-gravity waves and vortices as well as their response in the magnetized three-dimensional (3D) solar atmosphere at different heights to understand the localized energy transport processes. In the solar atmosphere strongly structured by gravitational and magnetic forces, we launch a localized velocity pulse (in horizontal and vertical components) within a bottom layer of 3D solar atmosphere modelled by initial VAL-IIIC conditions, which triggers waves and vortices. The rotation direction of vortices depends on the orientation of an initial perturbation. The vertical driver generates magnetoacoustic-gravity waves which result in oscillations of the transition region, and it leads to the eddies with their symmetry axis oriented vertically. The horizontal pulse excites all magnetohydrodynamic-gravity waves and horizontally oriented eddies. These waves propagate upwards, penetrate the transition region, and enter the solar corona. In the high-beta plasma regions the magnetic field lines move with the plasma and the temporal evolution show that they swirl with eddies. We estimate the energy fluxes carried out by the waves in the magnetized solar atmosphere and conclude that such wave dynamics and vortices may be significant in transporting the energy to sufficiently balance the energy losses in the localized corona. Moreover, the structure of the transition region highly affects such energy transports, and causes the channelling of the propagating waves into the inner corona.

Authors: K. Murawski, I. Ballai, A.K. Srivastava, D. Lee
Projects: None

Publication Status: Accepted for the Publication in MNRAS
Last Modified: 2013-09-03 12:22
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Origin of Macrospicule and Jet in Polar Corona by A Small-scale Kinked Flux-Tube  

Dr. A.K. Srivastava   Submitted: 2013-05-06 10:21

We report an observation of a small scale flux-tube that undergoes kinking and triggers the macrospicule and a jet on November 11, 2010 in the north polar corona. The small-scale flux-tube emerged well before the triggering of macrospicule and as the time progresses the two opposite halves of this omega shaped flux-tube bent transversely and approached towards each other. After sim 2 minutes, the two approaching halves of the kinked flux-tube touch each-other and internal reconnection as well as energy release takes place at the adjoining location and a macrospicule was launched which goes upto a height of 12 Mm. Plasma starts moving horizontally as well as vertically upward along with the onset of macrospicule and thereafter converts into a large-scale jet which goes up to sim 40 Mm in the solar atmosphere with a projected speed of sim 95 km s-1. We perform 2-D numerical simulation by considering the VAL-C initial atmospheric conditions to understand the physical scenario of the observed macrospicule and associated jet. The simulation results show that reconnection generated velocity pulse in the lower solar atmosphere steepens into slow shock and the cool plasma is driven behind it in form of macrospicule. The horizontal surface waves also appeared with the shock fronts at different heights, which most likely drove and spread the large-scale jet associated with the macrospicule.

Authors: Kayshap, P.; Srivastava, A. K.; Murawski, K.; Tripathi, D.
Projects: SDO-AIA

Publication Status: 22 pages, 5 figures, ApJL (in press)
Last Modified: 2013-05-06 11:08
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Study of Failed CME Core Associated with Asymmetric Filament Eruption  

Dr. A.K. Srivastava   Submitted: 2013-04-28 05:50

We present the multi-wavelength observations of asymmetric filament eruption, associated CME and coronal downflows on 2012 June 17-18 during 20:00-05:00 UT. We use SDO/AIA, STEREO-B/SECCHI observations to understand the filament eruption scenario and its kinematics. While LASCO C2 observations have been analyzed to study the kinematics of the CME and associated downflows. SDO/AIA limb observations show that the filament exhibits whipping like asymmetric eruption. STEREO/EUVI disk observations reveal a two ribbon flare underneath the south-eastern part of the filament that is most probably occurred due to reconnection process in the coronal magnetic field in the wake of the filament eruption. The whipping like filament eruption later gives a slow CME in which the leading edge and the core propagate respectively with the average speed of approx 540 km s-1 and approx 126 km s-1 as observed in the LASCO C2 coronagraph. The CME core formed by the eruptive flux-rope shows the outer coronal downflows with the average speed of approx 56 km s-1 after reaching up to approx4.33 Rsun. Initially, the core decelerates with approx 48 m s-2. The plasma first decelerates gradually up to the height of approx4.33 Rsun and then starts accelerating downward. We suggest a self-consistent model of a magnetic flux rope representing the magnetic structure of the CME core formed by eruptive filament that lost its previous stable equilibrium when reach at a critical height. With some reasonable parameters, and inherent physical conditions the model describes the non-radial ascending motion of the flux rope in the corona, its stopping at some height, and thereafter the downward motion, which are in good agreement with the observations.

Authors: Joshi, N. C.; Srivastava, A. K.; Filippov, B.; Uddin, W.; Kayshap, P.; Chandra, R.
Projects: None

Publication Status: The Astrophysical Journal (in press)
Last Modified: 2013-04-30 12:03
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Simulation of the Observed Coronal Kink Instability and Its Implications for the SDO/AIA  

Dr. A.K. Srivastava   Submitted: 2013-04-28 05:47

Srivastava et al. (2010) have observed a highly twisted coronal loop, which was anchored in AR10960 during the period 04:43 UT-04:52 UT on 4 June 2007. The loop length and radius are approximately 80 Mm and 4 Mm, with a twist of 11.5 pi. These observations are used as initial conditions in a three dimensional nonlinear magnetohydrodynamic simulation with parallel thermal conduction included. The initial unstable equilibrium evolves into the kink instability, from which synthetic observables are generated for various high-temperature filters of SDO/AIA. These observables include temporal and spatial averaging to account for the resolution and exposure times of SDO/AIA images. Using the simulation results, we describe the implications of coronal kink instability as observables in SDO/AIA filters.

Authors: Srivastava, A. K.; Botha, G. J. J.; Arber, T. D.; Kayshap, P.
Projects: None

Publication Status: AdSpR (In Press)
Last Modified: 2013-04-30 12:03
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Observations of intensity oscillations in a prominence-like cool loop system as observed by SDO/AIA: evidence of multiple harmonics of fast magnetoacoustic waves  

Dr. A.K. Srivastava   Submitted: 2013-04-28 05:42

Using SDO/AIA 304 Å channel, we study the evolution of weak intensity oscillations in a prominence like cool loop system observed at North-West limb on 7 March 2011. We use the standard wavelet tool to produce statistically significant power spectra of AIA 304 Å normalized fluxes derived respectively near the apex and footpoint of the fluxtube. We find periodicities of ≈667 s and ≈305 s respectively near apex and above footpoint with significance level >98 %. Observed statistically significant periodicities in the tube of projected length ≈170 Mm and width ≈10 Mm, are interpreted as most likely signature of evolution of various harmonics of tubular fast magnetoacoustic waves. Sausage modes are unlikely though they are compressive as they need bulky and highly denser loop system for their evolution for sustaining such large periods. We interpret the observed periodicities as multiple harmonics (fundamental and first) of fast magnetoacoustic kink waves that can generate some weak density perturbations (thus intensity oscillations) in the tube and can be observed pertaining to periodic variation in plasma column depth as tube is oblique in projection with respect to line-of-sight. The period ratio P 1/ P 2=2.18 is observed in the fluxtube, which is the signature of the magnetic field divergence of the cool loop system. We estimate tube expansion factor as 1.27 which is typical of EUV bipolar loops in the solar atmosphere. We estimate the lower bound average magnetic fields ranging from ≈9 to 90 Gauss depending upon typical densities as 109-1011 cm-3 in the observed prominence-like cool loop system. We also observe the first signature of lowering fundamental mode period by a factor 0.85 due to cooling of this loop system.

Authors: Srivastava, A. K.; Dwivedi, B. N.; Kumar, Mukul
Projects: SDO-AIA

Publication Status: Astrophysics and Space Science, Volume 345, Issue 1, pp.25-32
Last Modified: 2013-04-30 12:03
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Observational Evidence of Sausage-pinch Instability in Solar Corona by SDO/AIA  

Dr. A.K. Srivastava   Submitted: 2013-04-28 05:39

We present the first observational evidence of the evolution of sausage-pinch instability in active region 11295 during a prominence eruption using data recorded on 2011 September 12 by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). We have identified a magnetic flux tube visible in AIA 304 Å that shows curvatures on its surface with variable cross-sections as well as enhanced brightness. These curvatures evolved and thereafter smoothed out within a timescale of a minute. The curved locations on the flux tube exhibit a radial outward enhancement of the surface of about 1-2 Mm (a factor of two larger than the original thickness of the flux tube) from the equilibrium position. AIA 193 Å snapshots also show the formation of bright knots and narrow regions in-between at the four locations as that of 304 Å along the flux tube where plasma emission is larger compared to the background. The formation of bright knots over an entire flux tube as well as the narrow regions in <60 s may be the morphological signature of the sausage instability. We also find the flows of confined plasma (propagation of brightness) in these bright knots along the field lines, which indicates the dynamicity of the flux tube that probably causes the dominance of the longitudinal field component over short temporal scales. The observed longitudinal motion of the plasma frozen in the magnetic field lines further vanishes the formed curvatures and plasma confinements as well as growth of instability to stabilize the flux tube.

Authors: Srivastava, A. K.; Erdélyi, R.; Tripathi, Durgesh; Fedun, V.; Joshi, N. C.; Kayshap, P.
Projects: SDO-AIA

Publication Status: The Astrophysical Journal Letters, Volume 765, Issue 2, article id. L42, 7 pp. (2013).
Last Modified: 2013-04-30 12:03
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The Kinematics and Plasma Properties of a Solar Surge Triggered by Chromospheric Activity in AR11271  

Dr. A.K. Srivastava   Submitted: 2013-04-28 05:36

We observe a solar surge in NOAA AR11271 using the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly 304 Å image data on 2011 August 25. The surge rises vertically from its origin up to a height of ≈65 Mm with a terminal velocity of ≈100 km s-1, and thereafter falls and fades gradually. The total lifetime of the surge was ≈20 minutes. We also measure the temperature and density distribution of the observed surge during its maximum rise and find an average temperature and a density of 2.0 MK and 4.1 ? 109 cm-3, respectively. The temperature map shows the expansion and mixing of cool plasma lagging behind the hot coronal plasma along the surge. Because SDO/HMI temporal image data do not show any detectable evidence of significant photospheric magnetic field cancellation for the formation of the observed surge, we infer that it is probably driven by magnetic-reconnection-generated thermal energy in the lower chromosphere. The radiance (and thus the mass density) oscillations near the base of the surge are also evident, which may be the most likely signature of its formation by a reconnection-generated pulse. In support of the present observational baseline of the triggering of the surge due to chromospheric heating, we devise a numerical model with conceivable implementation of the VAL-C atmosphere and a thermal pulse as an initial trigger. We find that the pulse steepens into a slow shock at higher altitudes which triggers plasma perturbations exhibiting the observed features of the surge, e.g., terminal velocity, height, width, lifetime, and heated fine structures near its base.

Authors: Kayshap, P.; Srivastava, Abhishek K.; Murawski, K.
Projects: SDO-AIA

Publication Status: The Astrophysical Journal, Volume 763, Issue 1, article id. 24, 12 pp. (2013).
Last Modified: 2013-04-30 12:04
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Observations of Multiple Surges Associated with Magnetic Activities in AR10484 on 25 October 2003  

Dr. A.K. Srivastava   Submitted: 2012-04-10 23:31

We present a multiwavelength study of recurrent surges observed in Hα , UV (SOHO/EIT) and Radio (Learmonth, Australia) from the super-active region NOAA 10484 on 25 October, 2003. Several bright structures visible in Hα and UV corresponding to subflares are also observed at the base of each surge. Type III bursts are triggered and RHESSI X-ray sources are evident with surge activity. The major surge consists of the bunches of ejective paths forming a fan-shape region with an angular size of (approx 65degree) during its maximum phase. The ejection speed reaches upto sim200 km s-1. The SOHO/MDI magnetograms reveal that a large dipole emerges east side of the active region on 18-20 October 2003, a few days before the surges. On October 25, 2003, the major sunspots were surrounded by ''moat regions'' with moving magnetic features (MMFs). Parasitic fragmented positive polarities were pushed by the ambient dispersion motion of the MMFs and annihilated with negative polarities at the borders of the moat region of the following spot to produce flares and surges. A topology analysis of the global Sun using PFSS shows that the fan structures visible in the EIT 171 Å images follow magnetic field lines connecting the present AR to a preceding AR in the South East. Radio observations of type III bursts indicate that they are coincident with the surges, suggesting that magnetic reconnection is the driver mechanism. The magnetic energy released by reconnection is transformed into plasma heating and provides the kinetic energy for the ejections. A lack of a radio signature in the high corona suggests that the surges are confined to follow the closed field lines in the fans. We conclude that these cool surges may have some local heating effects in the closed loops, but probably play a minor role in global coronal heating and the surge material does not escape to the solar wind.

Authors: Wahab Uddin, B. Schmieder, R. Chandra, Abhishek K. Srivastava, Pankaj Kumar, S. Bisht
Projects: None

Publication Status: Accepted for the Publication in ApJ; 25 pages, 10 Figures, and 1 Table
Last Modified: 2012-04-11 09:37
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Multiwavelength Observations of Supersonic Plasma Blob Triggered by Reconnection Generated Velocity Pulse in AR10808  

Dr. A.K. Srivastava   Submitted: 2012-03-29 01:50

Using multi-wavelength observations of Solar and Heliospheric Observatory (SoHO)/Michelson Doppler Imager (MDI), Transition Region and Coronal Explorer (TRACE) 171 AA, and Hα from Culgoora Solar Observatory at Narrabri, Australia, we present a unique observational signature of a propagating supersonic plasma blob before an M6.2 class solar flare in AR10808 on 9th September 2005. The blob was observed between 05:27 UT to 05:32 UT with almost a constant shape for the first 2-3 minutes, and thereafter it quickly vanished in the corona. The observed lower bound speed of the blob is estimated as sim215 km s-1 in its dynamical phase. The evidence of the blob with almost similar shape and velocity concurrent in Hα and TRACE 171 Å supports its formation by multi-temperature plasma. The energy release by a recurrent 3-D reconnection process via the separator dome below the magnetic null point, between the emerging flux and pre-existing field lines in the lower solar atmosphere, is found to be the driver of a radial velocity pulse outwards that accelerates this plasma blob in the solar atmosphere. In support of identification of the possible driver of the observed eruption, we solve the two-dimensional ideal magnetohydrodynamic equations numerically to simulate the observed supersonic plasma blob. The numerical modelling closely match the observed velocity, evolution of multi-temperature plasma, and quick vanishing of the blob found in the observations. Under typical coronal conditions, such blobs may also carry an energy flux of 7.0 imes106 ergs cm-2 s-1 to re-balance the coronal losses above active regions.

Authors: A.K. Srivastava, R Erdelyi, K. Murawski, P. Kumar
Projects: None

Publication Status: Sol. Phys.
Last Modified: 2012-03-29 09:45
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A multiwavelength study of an M-class flare and the origin of an associated eruption from NOAA AR 11045  

Dr. A.K. Srivastava   Submitted: 2012-03-14 04:24

In this paper, we study multiwavelength observations of an M6.4 flare in Active Region NOAA 11045 on 7 February 2010. The space- and ground-based observations from STEREO, SoHO/MDI, EIT, and Nobeyama Radioheliograph were used for the study. This active region rapidly appeared at the north-eastern limb with an unusual emergence of a magnetic field. We find a unique observational signature of the magnetic field configuration at the flare site. Our observations show a change from dipolar to quadrapolar topology. This change in the magnetic field configuration results in its complexity and a build-up of the flare energy. We did not find any signature of magnetic flux cancellation during this process. We interpret the change in the magnetic field configuration as a consequence of the flux emergence and photospheric flows that have opposite vortices around the pair of opposite polarity spots. The negative-polarity spot rotating counterclockwise breaks the positive-polarity spot into two parts. The STEREO-A 195 Å and STEREO-B 171 Å coronal images during the flare reveal that a twisted flux tube expands and erupts resulting in a coronal mass ejection (CME). The formation of co-spatial bipolar radio contours at the same location also reveals the ongoing reconnection process above the flare site and thus the acceleration of non-thermal particles. The reconnection may also be responsible for the detachment of a ring-shaped twisted flux tube that further causes a CME eruption with a maximum speed of 446 km s-1 in the outer corona.

Authors: Dwivedi, B. N.; Srivastava, Abhishek K.; Kumar, Mukul; Kumar, Pankaj
Projects: None

Publication Status: New Astr.
Last Modified: 2012-03-14 14:08
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Observations of Post-flare Plasma Dynamics during an M1.0 Flare in AR11093 by SDO/AIA  

Dr. A.K. Srivastava   Submitted: 2011-10-02 21:45

We observe the motion of cool and hot plasma in a multi-stranded post flareloop system that evolved in the decay phase of a two ribbon M1.0 class flare inAR 11093 on 7 August 2010 using SDO/AIA 304 Å and 171 Å filters. Themoving intensity feature and its reflected counterpart are observed in the loopsystem at multi-temperature. The observed hot counterpart of the plasma thatprobably envelopes the cool confined plasma, moves comparatively faster(sim34 km s-1) to the later (29 km s-1) in form of the spreadedintensity feature. The propagating plasma and intensity reflect from the regionof another footpoint of the loop. The subsonic speed of the moving plasma andassociated intensity feature may be most likely evolved in the post flare loopsystem through impulsive flare heating processes. Complementing ourobservations of moving multi-temperature intensity features in the post flareloop system and its reflection, we also attempt to solve two-dimensional idealmagnetohydrodynamic equations numerically using the VAL-IIIC atmosphere as aninitial condition to simulate the observed plasma dynamics. We consider alocalized thermal pulse impulsively generated near one footpoint of the loopsystem during the flare processes, which is launched along the magnetic fieldlines at the solar chromosphere. The pulse steepens into a slow shock at higheraltitudes while moving along this loop system, which triggers plasmaperturbations that closely exhibit the observed plasma dynamics.

Authors: Abhishek K. Srivastava, K. Murawski
Projects: SDO-AIA

Publication Status: The Astrophysical Journal (ApJ)
Last Modified: 2011-10-03 11:11
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Numerical simulations of solar macrospicules  

Dr. A.K. Srivastava   Submitted: 2011-08-29 23:59

Context. We consider a localized pulse in the component of velocity, parallelto the ambient magnetic field lines, that is initially launched in the solarchromosphere. Aims. We aim to generalize the recent numerical model of spiculeformation (Murawski & Zaqarashvili 2010) by implementing a VAL-C model of solartemperature. Methods. With the use of the code FLASH we solve two-dimensionalideal magnetohydrodynamic equations numerically to simulate the solarmacrospicules. Results. Our numerical results reveal that the pulse locatedbelow the transition region triggers plasma perturbations, which exhibit manyfeatures of macrospicules. We also present an observational (SDO/AIA 304 Å)case study of the macrospicule that approximately mimics the numericalsimulations. Conclusions. In the frame of the model we devised, the solarmacrospicules can be triggered by velocity pulses launched from thechromosphere.

Authors: Murawski, K.; Srivastava, Abhishek K.; Zaqarashvili, T. V.
Projects: SDO-AIA

Publication Status: A&A
Last Modified: 2011-08-30 10:49
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Observations of a pulse driven cool polar jet by SDO/AIA  

Dr. A.K. Srivastava   Submitted: 2011-08-23 03:41

Context. We observe a solar jet at north polar coronal hole (NPCH) using SDOAIA 304 {deg}A image data on 3 August 2010. The jet rises obliquely above thesolar limb and then retraces its propagation path to fall back. Aims. Wenumerically model this observed solar jet by implementing a realistic (VAL-C)model of solar temperature. Methods. We solve two-dimensional idealmagnetohydrodynamic equations numerically to simulate the observed solar jet.We consider a localized velocity pulse that is essentially parallel to thebackground magnetic field lines and initially launched at the top of the solarphotosphere. The pulse steepens into a shock at higher altitudes, whichtriggers plasma perturbations that exhibit the observed features of the jet.The typical direction of the pulse also clearly exhibits the leading front ofthe moving jet. Results. Our numerical simulations reveal that a largeamplitude initial velocity pulse launched at the top of the solar photosphereproduces in general the observed properties of the jet, e.g., upward andbackward average velocities, height, width, life-time, and ballistic nature.Conclusions. The close matching between the jet observations and numericalsimulations provides first strong evidence for the formation of this jet by asingle velocity pulse. The strong velocity pulse is most likely generated bythe low- atmospheric reconnection in the polar region which results intriggering of the jet. The downflowing material of the jet most likely vanishesin the next upcoming velocity pulses from lower solar atmosphere, and thereforedistinctly launched a single jet upward in the solar atmosphere is observed.

Authors: Srivastava, Abhishek K.; Murawski, Kris
Projects: SDO-AIA

Publication Status: A&A
Last Modified: 2011-08-23 08:55
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Multi-Wavelength Observations of a Flux Rope Failed in the Eruption and Associated M-Class Flare from NOAA AR 11045  

Dr. A.K. Srivastava   Submitted: 2011-07-07 23:40

We present the multi-wavelength observations of a flux rope that was tryingto erupt from NOAA AR 11045 and the associated M-class solar flare on 12February 2010 using space and ground based observations from TRACE, STEREO,SOHO/MDI, Hinode/XRT and BBSO. While the flux rope was rising from the activeregion, an M1.1/2F class flare was triggered nearby one of its footpoints. Wesuggest that the flare triggering was due to the reconnection of a rising fluxrope with the surrounding low-lying magnetic loops. The flux rope reached aprojected height of ~0.15 Rs with a speed of ~90 km s-1 while the soft X-ray fluxenhanced gradually during its rise. The flux rope was suppressed by anoverlying field and the filled plasma moved towards the negative polarity fieldto the west of its activation site. We find the first observational evidence ofthe initial suppression of a flux rope due to a remnant filament visible bothat chromospheric and coronal temperatures that evolved couple of days before atthe same location in the active region. SOHO/MDI magnetograms show theemergence of a bipole ~12 h prior to the flare initiation. The emerged negativepolarity moved towards the flux rope activation site, and flare triggering nearthe photospheric polarity inversion line (PIL) took place. The motion of thenegative polarity region towards PIL helped in the build-up of magnetic energyat the flare and flux rope activation site. This study provides a uniqueobservational evidence of a rising flux rope that failed to erupt due to aremnant filament and overlying magnetic field, as well as associated triggeringof an M-class flare.

Authors: Kumar, Pankaj; Srivastava, A. K.; Filippov, B.; Erdélyi, R.; Uddin, Wahab
Projects: None

Publication Status: 20 pages, 11 figures for Solar Physics
Last Modified: 2011-07-08 11:23
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Evidence of the Leakage of MHD Oscillations above On-Disk Coronal Hole  

Dr. A.K. Srivastava   Submitted: 2011-06-22 02:57

Not Available

Authors: Kumar, Mukul; Dwivedi, B. N.; Srivastava, A. K.
Projects: None

Publication Status: Advances in Geosciences, Vol. 27, p. 113-127
Last Modified: 2011-06-22 12:24
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Abstracts by Author
Numerical simulations of impulsively generated Alfv?n waves in solar magnetic arcades
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Observation of Kink Instability as Driver of Recurrent Flares in AR 10960
Evidence of Solar Flare Triggering due to Loop-Loop Interaction Caused by Footpoint Shear-Motion
On the Observations of Multiple MHD Oscillations in the Solar Loops
Observations from Hinode/EIS of intensity oscillations above a bright point: signature of the leakage of acoustic oscillations in the inner corona
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