Seminar

Title : Meteorological observations of convective vortices and simulation of dust lifting within a dust devil

Date : 12-03-2020
Time : 16:00:00
Speaker : Shefali Uttam
Area : Planetary Sciences Division
Venue : Seminar Room # 113/114 (Thaltej Campus)

Abstract

The lower atmosphere of Mars is highly turbulent. Vortices form in the lower part of boundary layer due to convective heating of the surface by solar flux. A pressure drop is observed within a vortex which is accompanied by a rise in air temperature, and change in wind velocity. If the winds are strong enough and dust is available, then the vortex lifts dust into the atmosphere and are known as dust devils. On Mars, dust devils play an important role to inject dust grains into the atmosphere to maintain the haze and can significantly affect the global dust loading. I will discuss about the detection of these vortices using REMS data on board Curiosity. My initial analysis of data from mission sol 1545 to 1660 shows a detection of 80 convective vortices. The observations shows a distinct rise in vortex activity around noon hours between 12:00 and 15:00 Local Mean Solar Time (LMST). These vortices are also accompanied by UV intensity drop giving a possibility of occurrence of dust devil. I will also discuss about the modelling of the dust distribution within a dust devil. Our model results show a concentration of ~〖10〗^3 particles/cc near the surface during a dust devil condition on Mars. We also estimate a dust flux of ~3×10-3 kgm-2s-1, which lies in the estimated range of dust fluxes from dust devils at the Mars Pathfinder site varying between ~6×10-4 kgm-2s-1 to ~5×10-3 kgm-2s-1.

Title : Triggering of a solar filament eruption and associated flare

Date : 12-03-2020
Time : 16:00:00
Speaker : Mr. Suraj Sahu (USO, PRL Ahmedabad)
Area : Astronomy & Astrophysics Division
Venue : Seminar Room # 113/114 (Thaltej Campus)

Abstract

Solar filaments are large magnetic structures confining cool (T ~ 10^4 K) and dense (ne ~ 10^17 m^{-3} ) plasma in the hot solar corona. Typically, the filament plasma is 100 times cooler and denser than its coronal surroundings. Depending upon the type of magnetic environment, in which filaments form, they are classified as: active region filament (ARF), intermediate filament (IF), and quiescent filament (QF). Study of active region filaments is key to understand the evolution of magnetic fields of active regions and its role in powering solar eruptive phenomena, viz., flares and coronal mass ejections (CMEs). With these motivations, we study the eruption of an active region (AR) filament from AR NOAA 12371 on 22 June 2015, which led to a major M6.6 solar flare and a halo fast CME. The study utilizes data from Atmospheric Imaging Assembly (AIA) and Helioseismic Magnetic Imager (HMI) on board Solar Dynamics Observatory (SDO), Big Bear Solar observatory (BBSO), and Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The event exhibited an active pre-flare phase during which a hot EUV coronal channel (co-spatial with filament channel) was in build-up stage and displayed hard X-ray emission up to 25 keV. As such, this is the first evidence of HXR coronal channel. The Non-linear-force-free-field (NLFFF) modeling of coronal magnetic field exhibited a magnetic flux rope (MFR) oriented along the polarity inversion line (PIL) and co-spatial with the coronal channel. We observe significant changes in the AR’s photospheric magnetic field at the activity site during an extended period of about 42 hours in the form of rotation of sunspots, moving magnetic features, and flux cancellation along the PIL. Prior to the flare onset, the hot channel underwent a slow rise phase (~14 km/s) for about 13 min which is followed by a fast rise (~ 90 km/s). The slow to fast transition of the hot channel precisely divides the pre-flare and impulsive phase of the flare which points toward the feedback process between the early dynamics of the eruption and the strength of the flare magnetic reconnection.

Title : Multi-isotopic and (S)TEM Investigations of Presolar Silicates

Date : 06-03-2020
Time : 16:00:00
Speaker : Dr. Manish N. Sanghani
Area : Planetary Sciences Division
Venue : Seminar Room # 113/114 (Thaltej Campus)

Abstract

Primitive meteorites, interplanetary dust particles and comets contain tiny dust grains that have unique isotopic characteristics very distinct from the normal solar system values. Highly abnormal isotopic compositions of these grains can not be explained by any physical or chemical processes within the solar system. These grains formed in the winds of massive stars and in the ejecta of stellar explosions even before the sun was born, known as presolar grains. Presolar ‘stardust’ grains are the direct snapshots of the stellar nucleosynthesis, and laboratory analyses of these grains provide excellent opportunity to better understand stellar and interstellar processes of grain formation, alteration and destruction. Studying presolar silicates has some advantages over studying other refractory phases. Unlike the other refractory phases, presolar silicates can only be discovered in situ within the fine grained matrices of primitive extraterrestrial samples by high resolution isotope imaging technique using the NanoSIMS ion probe, and hence they are not altered by any chemical treatments. In addition to that, presolar silicate abundances are higher as compared to other presolar phases and they also register a variety of stellar sources. The first part of this talk will describe the in-situ discovery of presolar silicate and oxide grains from the fine grained matrices of two different carbonaceous chondrites Isheyevo and NWA 801. Stellar sources of oxygen anomalous grains found in this study will be discussed. Results of multi-isotopic and (S)TEM investigations of some of the presolar silicates will be presented and grain condensation and alteration will be discussed. Lastly, results of high resolution, multi-isotopic measurements of some of the presolar silicates will be discussed and some new insights on the galactic chemical evolution will be presented. The talk will be concluded by summarizing the main results and brief discussions on the future prospects of this work.

Title : Source extraction algorithms for deep radio surveys

Date : 05-03-2020
Time : 16:00:00
Speaker : Mr. Sushant Dutta (PRL Ahmedabad)
Area : Astronomy & Astrophysics Division
Venue : Seminar Room # 113/114 (Thaltej Campus)

Abstract

The source extraction in deep radio surveys pose challenges as, unlike optical/IR surveys, the PSF and the background RMS vary across the image. Since the source density is large in deep radio surveys, there is always a chance of source confusion, which in turn affects the background RMS estimation. Rudimentary source extraction algorithms available in the literature are inefficient to produce a complete and reliable source catalogue, with accurate astrometry and flux estimation, in case of diffuse and extended radio sources. These factors can be taken care of in Python Blob Detection and Source Finder (PyBDSF) algorithm developed mainly to serve the need of low-frequency radio surveys. I shall discuss the working principle of PyBDSF and the results of source extraction algorithm applied to deep 325 MHz GMRT image.