Petrology and geochemistry of the Grubergebirge anorthosite and marginal rocks,central Dronning Maud Land: Further characterization of the Late Neoproterozoic magmatic event in East Antarctica     

Vadlamani Ravikant  A. Dharwadkar  P. R. Golani  R. Ravindra 《Journal of the Geological Society of India》2011,78(1):7-18

The Grubergebirge anorthosite, a Late Neoproterozoic massif-type anorthosite, was emplaced into Late Mesoproterozoic-aged metamorphosed orthogneisses and supracrustal rocks in the Wohlthat Mountains. Mineralogically and chemically, the marginal rocks to the anorthosite massif classify as ferromonzodiorite and ferromonzonite. Variations in trace and rare earth element abundances and normalized patterns between the anorthosite and associated marginal ferromonzodiorite (and minor ferromonzonite) and distinct differences between the ferromonzodiorite and ferromonzonite have been observed. Whereas the magmas from which the anorthosite crystallized have been slightly contaminated by incorporating crustal material (now occurring as enclaves), the marginal ferromonzodiorite represent rocks that originated due to mixing (hybridization) of injected primitive ferrodiorite magma(s) with preexisting crustal material, resulting in the hybrid ferromonzodiorite.  相似文献   

Modeling and Measuring the Flux Reconnected and Ejected by the Two-Ribbon Flare/CME Event on 7 November 2004     

Dana Longcope  Colin Beveridge  Jiong Qiu  B. Ravindra  Graham Barnes  Sergio Dasso 《Solar physics》2007,244(1-2):45-73

Observations of the large two-ribbon flare on 7 November 2004 made using SOHO and TRACE data are interpreted in terms of a three-dimensional magnetic field model. Photospheric flux evolution indicates that ?1.4×10⁴³ Mx² of magnetic helicity was injected into the active region during the 40-hour buildup prior to the flare. The magnetic model places a lower bound of 8×10³¹ ergs on the energy stored by this motion. It predicts that 5×10²¹ Mx of flux would need to be reconnected during the flare to release the stored energy. This total reconnection compares favorably with the flux swept up by the flare ribbons, which we measure using high-time-cadence TRACE images in 1?600 Å. Reconnection in the model must occur in a specific sequence that would produce a twisted flux rope containing significantly less flux and helicity (10²¹ Mx and ?3×10⁴² Mx², respectively) than the active region as a whole. The predicted flux compares favorably with values inferred from the magnetic cloud observed by Wind. This combined analysis yields the first quantitative picture of the flux processed through a two-ribbon flare and coronal mass ejection.  相似文献   

Analysis of enhanced velocity signals observed during solar flares     

Brajesh Kumar  B. Ravindra 《Journal of Astrophysics and Astronomy》2006,27(4):425-438

Solar flares are known to release a large amount of energy. It is believed that the flares can excite velocity oscillations in active regions. We report here the changes in velocity signals in three active regions which have produced large X-class flares. The enhanced velocity signals appeared during the rise time of the GOES soft X-ray flux. These signals are located close to the vicinity of the hard X-ray source regions as observed with RHESSI. The power maps of the active region show enhancement in the frequency regime 5–6.5 mHz, while there is feeble or no enhancement of these signals in 2–4 mHz frequency band. High energy particles with sufficient momentum seem to be the cause for these observed enhanced velocity signals.  相似文献   

Moving Magnetic Features in and out of Penumbral Filaments     

B. Ravindra 《Solar physics》2006,237(2):297-319

A time sequence of high-resolution SOHO/MDI magnetograms, Dopplergrams, and continuum images is used to study the moving magnetic features (MMFs) in and out of penumbral filaments. Precursors of MMFs have been observed inside the penumbral filaments. One hundred and fifteen out of 127 well-observed individual MMFs in the moat of two sunspots have been identified to have precursors at an average distance of 4″ inside the penumbral filaments. The velocity of these precursors is small inside the penumbral filaments and becomes large once the MMFs cross the outer penumbra. The paths followed by the MMFs exhibit large fluctuations in their magnetic field strength values, with an additional hike in the fluctuations near the outer penumbra. It is also observed that the path followed by the MMFs appear as a cluster of fibrils which could be traced back inside the penumbra. The appearance of MMFs and their azimuthal velocity is position and time dependent. Electronic Supplementary Material Electronic Supplementary Material is available for this article at  相似文献   

Role of debris cover to control specific ablation of adjoining Batal and Sutri Dhaka glaciers in Chandra Basin (Himachal Pradesh) during peak ablation season     

Parmanand Sharma  Lavkush K Patel  Rasik Ravindra  Ajit Singh  Mahalinganathan K  Meloth Thamban 《Journal of Earth System Science》2016,125(3):459-473

As part of the on-going annual mass balance measurements on Batal and Sutri Dhaka glaciers, observations were made during peak ablation (August–September) season in 2013 to understand the response of debris covered and clean-ice (debris free) glacier surface to melting processes. Though, both the Batal and Sutri Dhaka glaciers have almost similar geographical disposition, Batal shows extensive debris cover (90% of the ablation area), while the latter is free from debris (only 5% of the ablation area). The thickness of debris in Batal glacier is inversely proportional to altitude, whereas Sutri Dhaka mostly experienced debris-free zone except snout area. Observation revealed that the vertical gradient of ablation rate in ablation area is contrastingly opposite in these two glaciers, reflecting significant control of debris thickness and their distribution over glacier surface on the ablation rates. While different thickness (2–100 cm) of debris have attenuated melting rates up to 70% of total melting, debris cover of <2 cm thickness has accelerated melting up to 10% of the total melting. Estimated melt ratio reveals that about 90% of the ablation area has experienced inhibited melting in Batal glacier, whereas only less than 5% ablation area of Sutri Dhaka has undergone inhibited melting. Comparison of topographical maps of 1962 with successive satellite images of the area demonstrates a terminus retreat of 373 ± 33.5 m and 579 ± 33.5 m for Batal and Sutri Dhaka glaciers for the period 1962–2013, respectively.  相似文献   

Intra-event characteristics of extreme erosive rainfall on Mauritius     

Werner Nel  Alexia Hauptfleisch  Paul D. Sumner  Ravindra Boojhawon  Soonil D. D. V. Rughooputh  Kumar R. Dhurmea 《自然地理学》2016,37(3-4):264-275

Mauritius is a volcanic island with a raised interior where extreme rainfall events dominate rainfall erosivity. Intra-event characteristics of the 120 highest erosive events at six selected locations between 2004 and 2008 were analyzed to provide the first detailed intra-storm data for a tropical island environment. On Mauritius, spatial variation is evident in the characteristics of extreme erosive rainfall recorded at the stations, with a noticeable increase in rainfall depth, duration, kinetic energy, and erosivity of extreme events with altitude. Extreme events in the raised interior (central plateau) show high variability of peak intensity over time as well as a higher percentage of events in which the greatest intensity occurs in the latter part of the event. Intra-event distribution of rainfall in the interior of the island shows that rainfall there has a higher potential to exceed infiltration rates as well as the ability to generate high peak runoff rates and cause substantial soil loss. The study suggests that even though within-event rainfall characteristics are complex, they have implications for soil erosion risk, and that, in tropical island environments, the within-storm distribution of rainfall should be incorporated in soil-loss modeling.  相似文献   

Simplified SMA-inspired 1-parameter SCS-CN model for runoff estimation     

Sangeeta Verma  Pushpendra Kumar Singh  Surendra Kumar Mishra  Sanjay Kumar Jain  Ronny Berndtsson  Anju Singh  Ravindra Kumar Verma 《Arabian Journal of Geosciences》2018,11(15):420

This study proposes a simplified 1-parameter SCS-CN model (M5) based on Mishra-Singh (2002) model and soil moisture accounting (SMA) procedure for surface runoff estimation and compares its performance with the existing SCS-CN method (SCS, 1956) (M1), Michel 1-P model (Water Resour Res 41:1-6, 2005) (M2), Sahu 1-P model (Hydrol Process 21:2872-2881, 2007) (M3), and Ajmal et al. model (J Hydrol 530:623-633, 2015) (M4) using large rainfall–runoff dataset of 48,763 events from123 USDA-ARS watersheds. The performance of models was evaluated using three statistical error indices such as Nash-Sutcliffe efficiency (NSE), root mean square error (RMSE), percentage bias (PBIAS), and rank and grading system (RGS). Based on the results obtained, the models can be ranked as follows: M5?>?M4?>?M3?>?M1?>?M2, i.e., model M5 outperformed all the remaining four models M1–M4 and hence is recommended for field applications.  相似文献   

Dynamics of Coronal Bright Points as Seen by Sun Watcher Using Active Pixel System Detector and Image Processing (SWAP), Atmospheric Imaging Assembly (AIA), and Helioseismic and Magnetic Imager (HMI)     

K. Chandrashekhar  S. Krishna Prasad  D. Banerjee  B. Ravindra  Daniel B. Seaton 《Solar physics》2013,286(1):125-142

The Sun Watcher using Active Pixel system detector and Image Processing (SWAP) onboard the PRoject for OnBoard Autonomy-2 (PROBA2) spacecraft provides images of the solar corona in EUV channel centered at 174 Å. These data, together with the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamics Observatory (SDO), are used to study the dynamics of coronal bright points. The evolution of the magnetic polarities and associated changes in morphology are studied using magnetograms and multi-wavelength imaging. The morphology of the bright points seen in low-resolution SWAP images and high-resolution AIA images show different structures, whereas the intensity variations with time show similar trends in both SWAP 174 Å and AIA 171 Å channels. We observe that bright points are seen in EUV channels corresponding to a magnetic flux of the order of 10¹⁸ Mx. We find that there exists a good correlation between total emission from the bright point in several UV–EUV channels and total unsigned photospheric magnetic flux above certain thresholds. The bright points also show periodic brightenings, and we have attempted to find the oscillation periods in bright points and their connection to magnetic-flux changes. The observed periods are generally long (10?–?25 minutes) and there is an indication that the intensity oscillations may be generated by repeated magnetic reconnection.  相似文献   

Double-filter model with modified Kalman filter for baseband signal pre-processing with application to ultra-tight GPS/INS integration     

Yong Luo  Ravindra Babu  Wen-qi Wu  Xiao-feng He 《GPS Solutions》2012,16(4):463-476

In federated design of ultra-tight GPS/INS integrated system, the baseband signal pre-processing is completed in a single pre-filter assigned for each channel. As the state space model of this single pre-filter includes the code tracking errors coupled with carrier tracking errors, ionospheric errors and normalized signal amplitude, the carrier tracking process may be destroyed. Also, the measurement noises are not independent any longer after passing through the code and carrier discriminators. Therefore, we propose a double-filter-based pre-filter model that distributes the carrier and code tracking into two independent filters: a conventional pre-filter, where the normalized signal amplitude is excluded from the state space and tracks only the code signal, and a 3-dimension state filter, tracking the carrier signal. The measurement information from both filters is a scalar quantity, which removes most of the noise correlation. To further improve the performance of the double-filter-based pre-filter model, we propose a modified Kalman filter algorithm. Simulation and field tests have been conducted, and the performance analysis has been done for the following configurations in a vector-tracking mode: double-filter model with modified Kalman filter, double-filter model with conventional Kalman filter and traditional single-filter model. The preliminary analysis indicates that the double-filter model with modified Kalman filter shows the best performance in tracking and navigation domains, while the traditional single-filter model shows a sub-optimal performance.  相似文献   

High-speed solar wind streams of two different origins and cosmic-ray variations during 1980–1986     

Pankaj K. Shrivastava  Ravindra P. Shukla 《Solar physics》1994,154(1):177-185

There are two types of high-speed solar wind streams classified in two categories:coronal-hole and solar-flare-generated streams. These two types are classified in two categories considering the bulk speed, proton density, temperature and magnetic field in the interplanetary medium. Their effects on cosmic ray intensity have been studied on a short-term basis during 1980–1986. Daily means of one middle and one low-latitude set of neutron monitor data have been taken for analysis using the Chree method of superposed epochs. The investigation indicates that the solar-flare-generated high-speed solar wind streams are more effective in producing cosmic ray decreases than are the coronal-hole generated streams.  相似文献