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Srivastava A.K.,Aryabhatta Research Institute of Observational science ARIES | Dwivedi B.N.,Banaras Hindu University
Monthly Notices of the Royal Astronomical Society | Year: 2010

We study the intensity oscillations in the upper chromosphere/transition region (TR) and corona, above a bright point (BP) in the solar atmosphere. We analyse the time series of He ii 256 Å, Fe xii 195 Å and Fe xv 284 Å, observed in a 40-arcsec slot close to the centre of the Sun above the BP by the extreme ultraviolet (EUV) imaging spectrometer (EIS) on board Hinode. Using standard wavelet and periodogram tools, we produce power spectra of intensity oscillations. In the He ii 256.32 Å and Fe xii 195.12 Å EUV light curves, we detect intensity oscillations of the periods ∼ 263 ± 80 s and ∼ 241 ± 60 s, respectively, with a probability >95 per cent in wavelets, which are also consistent with their periodograms. This provides the most likely signature of the propagation of acoustic oscillations around the ∼5.0-min period from the photosphere to the inner corona. The radiative cooling and thus the finite radiative relaxation time are found to be the most likely mechanisms for the reduced cut-off frequency environment above the observed BP. This may allow the transfer of ∼5.0-min acoustic oscillations from the upper chromosphere/TR into the corona. We find that intensity oscillations in He ii 256.32 Å show temporal damping during the total span of the observation. This may be the first most likely observational signature of acoustic wave damping in the upper chromosphere caused by the radiative cooling effect. The intensity oscillations in Fe xii 195.12 Å show an amplification, which may be a most likely signature of the mode-coupling (two-wave interaction) and then resonant energy conversion, probably from transverse magnetohydrodynamic (MHD) waves of the double period (e.g. Alfvén waves) to the observed acoustic waves in the lower solar atmosphere where the plasma beta tends to unity. However, we find no evidence of real oscillations around the ∼5.0-min period with its amplification in the higher corona where the Fe xv 284.16 Å line is formed, which rules out this type of wave activity there. Almost 1.6 per cent of the solar surface is covered with small BPs, probably associated with the small-scale closed-loop system, which may be a subset of expanding flux tubes. Hence, the leakage of ∼5.0-min oscillations above such BPs, which is associated with the highest powers of strong convective motions, and probably resonantly amplified by transverse MHD waves (e.g. Alfvén waves), may be significant for heating the solar atmosphere locally. © 2010 The Authors. Journal compilation © 2010 RAS.

Srivastava A.K.,Aryabhatta Research Institute of Observational science ARIES | Goossens M.,Catholic University of Leuven
Astrophysical Journal | Year: 2013

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 on board the Solar Dynamics Observatory. The X6.9-class flare in active region 11263 on 2011 August 9 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 coronal dimming, non-decaying vertical oscillations are also observed (period 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. © 2013. The American Astronomical Society. All rights reserved.

Gupta A.C.,Aryabhatta Research Institute of Observational science ARIES
Journal of Astrophysics and Astronomy | Year: 2011

It is well established that the blazars show flux variations in the complete electromagnetic (EM) spectrum on all possible time scales ranging from a few tens of minutes to several years. Here we report the review of various UV and X-ray flux variability properties of blazars. Our analysis show that UV variability amplitude is smaller than X-rays, mostly soft X-rays hardness ratio show correlations with blazar luminosity and different modes of variability might be operating for different time scales and epochs. Quasi periodic oscillations are seen on a few occasions in blazars, higher fraction of high energy peaked blazars show intra day and short term variabilities in X-rays but variability duty cycle is much less in optical bands on intra day time scale compared to low energy peaked blazars. But these results are yet to be established. © 2011 Indian Academy of Sciences.

Srivastava A.K.,Aryabhatta Research Institute of Observational science ARIES | Murawski K.,Group of Astrophysics
Astrophysical Journal | Year: 2012

We observe the motion of cool and hot plasma in a multi-stranded post-flare loop (PFL) system that evolved in the decay phase of a two-ribbon M1.0 class flare in AR 11093 on 2010 August 7 using the Solar Dynamics Observatory/ Atmospheric Imaging Assembly 304 Å and 171 Å filters. The moving intensity feature and its reflected counterpart are observed in the loop system at multiple temperatures. The observed hot counterpart of the plasma probably envelopes the cool confined plasma and moves comparatively faster (∼34kms-1) than the latter (29kms-1) in the form of a spreading intensity feature. The propagating plasma and intensity reflect from the region of another footpoint of the loop. The subsonic speed of the moving plasma and associated intensity feature may be most likely evolved in the PFL system through impulsive flare heating processes. Complementing our observations of moving multi-temperature intensity features in the PFL system and its reflection, we also attempt to solve two-dimensional ideal magnetohydrodynamic equations numerically using the VAL-IIIC atmosphere as an initial condition to simulate the observed plasma dynamics. We consider a localized thermal pulse impulsively generated near one footpoint of the loop system during the flare processes, which is launched along the magnetic field lines at the solar chromosphere. The pulse steepens into a slow shock at higher altitudes while moving along this loop system, which triggers plasma perturbations that closely exhibit the observed plasma dynamics. © 2012 The American Astronomical Society. All rights reserved.

Kayshap P.,Aryabhatta Research Institute of Observational science ARIES | Srivastava A.K.,Aryabhatta Research Institute of Observational science ARIES | Murawski K.,Group of Astrophysics
Astrophysical Journal | Year: 2013

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. © 2013. The American Astronomical Society. All rights reserved.

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