Was the cosmic ray burst detected by the GRAPES-3 on 22 June 2015 caused by transient weakening of geomagnetic field or by an interplanetary anisotropy? |
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Prasad Subramanian Submitted: 2018-03-29 00:18
The GRAPES-3 muon telescope in Ooty, India had claimed detection of a 2 hour (h) high-energy (∼20 GeV) burst of galactic cosmic-rays (GCRs) through a >50σ surge in GeV muons, was caused by reconnection of the interplanetary magnetic field (IMF) in the magnetosphere that led to transient weakening of Earth's magnetic shield. This burst had occurred during a G4-class geomagnetic storm (storm) with a delay of 12h relative to the coronal mass ejection (CME) of 22 June 2015 (Mohanty et al., 2016). However, recently a group interpreted the occurrence of the same burst in a subset of 31 neutron monitors (NMs) to have been the result of an anisotropy in interplanetary space (Evenson et al., 2017) in contrast to the claim in (Mohanty et al., 2016). A new analysis of the GRAPES-3 data with a fine 10.6∘ angular segmentation shows the speculation of interplanetary anisotropy to be incorrect, and offers a possible explanation of the NM observations. The observed 28 minutes (min) delay of the burst relative to the CME can be explained by the movement of the reconnection front from the bow shock to the surface of Earth at an average speed of 35 km s-1, much lower than the CME speed of 700 km s-1. This measurement may provide a more accurate estimate of the start of the storm.
Authors: P.K. Mohanty, K.P. Arunbabu, T. Aziz, S.R. Dugad, S.K. Gupta, B. Hariharan, P. Jagadeesan, A. Jain, S.D. Morris, P.K. Nayak, P.S. Rakshe, K. Ramesh, B.S. Rao, M. Zuberi, Y. Hayashi, S. Kawakami, P. Subramanian, S. Raha, S. Ahmad, A. Oshima, S. Shibata, H. Kojima
Projects: None
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Publication Status: To appear in Phys. Rev D
Last Modified: 2018-03-29 09:41
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The structure of solar radio noise storms |
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Prasad Subramanian Submitted: 2014-12-30 06:51
The Nançay Radioheliograph (NRH) routinely produces snapshot images of the full sun at frequencies between 150 and 450 MHz, with typical resolution 3 arcmin and time cadence 0.2 s. Combining visibilities from the NRH and from the Giant Meterwave Radio Telescope (GMRT) allows us to produce images of the sun at 236 or 327 MHz, with a large FOV, high resolution and time cadence. We seek to investigate the structure of noise storms (the most common non-thermal solar radio emission). We focus on the relation of position and altitude of noise storms with the observing frequency and on the lower limit of their sizes. We present results for noise storms on four days. The results consist of an extended halo and of one or several compact cores with relative intensity changing over a few seconds. We found that core sizes can be almost stable over one hour, with a minimum in the range 31-35 arcsec (less than previously reported) and can be stable over one hour. The heliocentric distances of noise storms are ∼1.20 and 1.35 R⊙ at 432 and 150 MHz, respectively. Regions where storms originate are thus much denser than the ambient corona and their vertical extent is found to be less than expected from hydrostatic equilibrium. The smallest observed sizes impose upper limits on broadening effects due to scattering on density inhomogeneities in the low and medium corona and constrain the level of density turbulence in the solar corona. It is possible that scatter broadening has been overestimated in the past, and that the observed sizes cannot only be attributed to scattering. The vertical structure of the noise storms is difficult to reconcile with the classical columnar model.
Authors: Claude Mercier, Prasad Subramanian, Gilbert Chambe, P. Janardhan
Projects: None
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Publication Status: Accepted, Astronomy and Astrophysics
Last Modified: 2014-12-31 13:43
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Coronal turbulence and the angular broadening of radio sources - the role of the structure function |
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Prasad Subramanian Submitted: 2014-12-22 21:43
The amplitude of density turbulence in the extended solar corona, especially near the dissipation scale, impinges on several problems of current interest.
Radio sources observed through the turbulent solar wind are broadened due to refraction by and scattering off density inhomogeneities, and observations of scatter broadening are often employed to constrain the turbulence amplitude. The extent of such scatter broadening is usually computed using the structure function, which gives a measure of the spatial correlation measured by an interferometer. Most such treatments have employed analytical approximations to the structure function that are valid in the asymptotic limits s \gg li or s ≪ li, where s is the interferometer spacing and li is the inner scale of the density turbulence spectrum. We instead use a general structure function (GSF) that straddles these regimes, and quantify the errors introduced by the use of these approximations. We have included the effects of anisotropic scattering for distant cosmic sources viewed through the solar wind at small elongations. We show that the regimes where the GSF predictions are more accurate than those of the asymptotic expressions are not only of practical relevance, but are where inner scale effects influence estimates of scatter broadening.
Taken together, we argue that the GSF should henceforth be used for scatter broadening calculations and estimates of turbulence amplitudes in the solar corona and solar wind.
Authors: M. Ingale, Prasad Subramaian, Iver Cairns
Projects: None
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Publication Status: Accepted, Monthly Notices of the Royal Astronomical Society
Last Modified: 2014-12-23 14:49
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High rigidity Forbush decreases: due to CMEs or shocks? |
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Prasad Subramanian Submitted: 2013-04-25 10:20
We seek to identify the primary agents causing Forbush decreases (FDs) observed at the Earth in high rigidity cosmic rays. In particular, we ask if such FDs are caused mainly by coronal mass ejections (CMEs) from the Sun that are directed towards the Earth, or by their associated shocks. We use the muon data at cutoff rigidities ranging from 14 to 24 GV from the GRAPES-3 tracking muon telescope to identify FD events. We select those FD events that have a reasonably clean profile, and can be reasonably well associated with an Earth-directed CME and its associated shock. We employ two models: one that considers the CME as the sole cause of the FD (the CME-only model) and one that considers the shock as the only agent causing the FD (the shock-only model). We use an extensive set of observationally determined parameters for both these models. The only free parameter in these models is the level of MHD turbulence in the sheath region, which mediates cosmic ray diffusion (into the CME, for the CME-only model and across the shock sheath, for the shock-only model).
We find that good fits to the GRAPES-3 multi-rigidity data using the CME-only model require turbulence levels in the CME sheath region that are only slightly higher than those estimated for the quiet solar wind. On the other hand, reasonable model fits with the shock-only model require turbulence levels in the sheath region that are an order of magnitude higher than those in the quiet solar wind.
This observation naturally leads to the conclusion that the Earth-directed CMEs are the primary contributors to FDs observed in high rigidity cosmic rays.
Authors: Arun Babu, H. M. Antia, S. R. Dugad, S. K. Gupta, Y. Hayashi, S. Kawakami, P. K. Mohanty, T. Nonaka, A. Oshima, P. Subramanian
Projects: None
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Publication Status: Accepted for publication in Astronomy and Astrophysics
Last Modified: 2013-04-26 10:15
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Constraints on coronal turbulence models from source sizes of noise storms at 327 MHz |
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Prasad Subramanian Submitted: 2010-12-17 05:19
We seek to reconcile observations of small source sizes in the solar corona at 327 MHz with predictions of scattering models that incorporate refractive index effects, inner scale effects and a spherically diverging wavefront. We use an empirical prescription for the turbulence amplitude CN2(R) based on VLBI observations by Spangler and coworkers of compact radio sources against the solar wind for heliocentric distances R approx 10-50 Rodot. We use the Coles & Harmon model for the inner scale li(R), that is presumed to arise from cyclotron damping. In view of the prevalent uncertainty in the power law index that characterizes solar wind turbulence at various heliocentric distances, we retain this index as a free parameter. We find that the inclusion of spherical divergence effects suppresses the predicted source size substantially. We also find that inner scale effects significantly reduce the predicted source size. An important general finding for solar sources is that the calculations substantially underpredict the observed source size. Three possible, non-exclusive, interpretations of this general result are proposed. First and simplest, future observations with better angular resolution will detect much smaller sources. Consistent with this, previous observations of small sources in the corona at metric wavelengths are limited by the instrument resolution. Second, the spatially-varying level of turbulence CN2(R) is much larger in the inner corona than predicted by straightforward extrapolation Sunwards of the empirical prescription, which was based on observations between 10-50 Rodot. Either the functional form or the constant of proportionality could be different. Third, perhaps the inner scale is smaller than the model, leading to increased scattering.
Authors: Prasad Subramanian (IISER Pune, India), Iver Cairns (U. Sydney, Australia)
Projects: None
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Publication Status: Accepted for publication in the Journal of Geophysical Research (Space Physics)
Last Modified: 2010-12-17 09:41
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Forbush decreases and turbulence levels at CME fronts |
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Prasad Subramanian Submitted: 2008-10-16 22:17
We seek to estimate the average level of MHD turbulence near coronal mass ejection (CME) fronts as they propagate from the Sun to the Earth. We examine the cosmic ray data from the GRAPES-3 tracking muon telescope at Ooty, together with the data from other sources for three well observed Forbush decrease events. Each of these events are associated with frontside halo Coronal Mass Ejections (CMEs) and near-Earth magnetic clouds. In each case, we estimate the magnitude of the Forbush decrease using a simple model for the diffusion of high energy protons through the largely closed field lines enclosing the CME as it expands and propagates from the Sun to the Earth. We use estimates of the cross-field diffusion coefficient Dperp derived from published results of extensive Monte Carlo simulations of cosmic rays propagating through turbulent magnetic fields. Our method helps constrain the ratio of energy density in the turbulent magnetic fields to that in the mean magnetic fields near the CME fronts. This ratio is found to be sim 2% for the 11 April 2001 Forbush decrease event, sim 6% for the 20 November 2003 Forbush decrease event and sim 249% for the much more energetic event of 29 October 2003.
Authors: Prasad Subramanian, H. M. Antia, S. R. Dugad, U. D. Goswami, S. K. Gupta, Y. Hayashi, N. Ito, S. Kawakami, H. Kojima, P. K. Mohanty, P. K. Nayak, T. Nonaka, A. Oshima, K. Sivaprasad, H. Tanaka, S. C. Tonwar
Projects: None
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Publication Status: Accepted for publication in Astronomy & Astrophysics
Last Modified: 2008-10-17 06:51
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Energetics of Solar Coronal Mass Ejections |
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Prasad Subramanian Submitted: 2007-01-08 21:39
Aims: To investigate if solar coronal mass ejections are driven mainly by coupling to the ambient solar wind, or through the release of internal magnetic energy.
Methods: We examine the energetics of 39 flux-rope like coronal mass ejections (CMEs) from the Sun using data in the distance range sim 2-20 R{odot} from the Large Angle Spectroscopic Coronograph (LASCO) aboard the Solar and Heliospheric Observatory (SOHO). This comprises a complete sample of the best examples of flux-rope CMEs observed by LASCO in 1996-2001.
Results: We find that 69% of the CMEs in our sample experience a clearly identifiable driving power in the LASCO field of view. For these CMEs which are driven, we examine if they might be deriving most of their driving power by coupling to the solar wind. We do not find conclusive evidence in favor of this hypothesis. On the other hand, we find that their internal magnetic energy is a viable source of the required driving power. We have estimated upper and lower limits on the power that can possibly be provided by the internal magnetic field of a CME. We find that, on the average, the lower limit on the available magnetic power is around 74% of what is required to drive the CMEs, while the upper limit can be as much as an order of magnitude larger.
Authors: Prasad Subramanian (1), Angelos Vourlidas (2) ((1) Indian Institute of Astrophysics, Bangalore, India; (2) Naval Research Laboratory, Washington, DC, USA)
Projects: SoHO-LASCO
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Publication Status: Accepted for publication in Astronomy and Astrophysics
Last Modified: 2007-01-08 22:20
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