P. Janardhan - Physical Research Laboratory (original) (raw)

Papers by P. Janardhan

The Astrophysical Journal, 2021

The Solar X-ray Spectrometer (XSM) payload onboard Chandrayaan-2 provides disk-integrated solar s... more The Solar X-ray Spectrometer (XSM) payload onboard Chandrayaan-2 provides disk-integrated solar spectra in the 1-15 keV energy range with an energy resolution of 180 eV (at 5.9 keV) and a cadence of 1 second. During the period from September 2019 to May 2020, covering the minimum of Solar Cycle 24, it observed nine B-class flares ranging from B1.3 to B4.5. Using time-resolved spectroscopic analysis during these flares, we examined the evolution of temperature, emission measure, and absolute elemental abundances of four elements-Mg, Al, Si, and S. These are the first measurements of absolute abundances during such small flares and this study offers a unique insight into the evolution of absolute abundances as the flares evolve. Our results demonstrate that the abundances of these four elements decrease towards their photospheric values during the peak phase of the flares. During the decay phase, the abundances are observed to quickly return to their pre-flare coronal values. The depletion of elemental abundances during the flares is consistent with the standard flare model, suggesting the injection of fresh material into coronal loops as a result of chromospheric evaporation. To explain the quick recovery of the so-called coronal "First Ionization Potential (FIP) bias" we propose two scenarios based on the Ponderomotive force model.

The European Physical Journal D

Icy mantles of benzonitrile, an aromatic with a cyanide side chain that has recently been detecte... more Icy mantles of benzonitrile, an aromatic with a cyanide side chain that has recently been detected in the interstellar medium, were subjected to vacuum ultraviolet photon irradiation and found to form a residue. The residue was removed from the substrate and placed on a Quantifoil grid for electron microscopy analysis. Transmission electron microscopy showed Quantum Dot (QD) and Nitrogendoped Graphene (N-Graphene) sheets. Diffraction and Energy Dispersive X-ray Spectroscopy revealed the crystalline nature and carbon-nitrogen composition, of the observed graphene sheet. This is the first result showing QD and N-Graphene synthesis in ice irradiation at interstellar temperatures.

Shock processing of amorphous carbon nanodust

Advances in Space Research

Shock-induced transformation of non-magnetic to magnetic ISM dust analogue

Monthly Notices of the Royal Astronomical Society

The fate of organometallic dust subjected to extreme conditions, especially the shock fronts, in ... more The fate of organometallic dust subjected to extreme conditions, especially the shock fronts, in the interstellar medium (ISM) has not been explored to date. Iron and cyclopentadiene, which have been found to be present in the ISM, are known to react and produce the organometallic compound ferrocene under terrestrial conditions. In our experiment we subjected ferrocene, a possible proxy of such dust, to ∼5.6 Mach (M) shock commensurate with conditions encountered in the ISM, leading to a temperature rise up to 7300 K within 2 ms. Analysis of the post-shock residue showed the presence of a α-Fe and Fe3C composite that responded to an external magnetic field. These results show that a non-magnetic dust composed of molecules containing transition metals undergoing shock processing in the ISM can dissociate and synthesize dust that is then magnetic. Such drastic transformations from non-magnetic to magnetic dust induced by shocks might be of importance in interstellar polarization.

Special Edition “2015 UN/Japan Workshop on Space Weather I” 147 Solar and Interplanetary Signatur... more Special Edition “2015 UN/Japan Workshop on Space Weather I” 147 Solar and Interplanetary Signatures of a Maunder-like Grand Solar Minimum around the Corner Implications to Near-Earth Space P. Janardhan, Susanta Kumar Bisoi , S. Ananthakrishnan, R. Sridharan, L. Jose 1 Physical Research Laboratory, India. 2 National Astronomical Observatories, Chinese Academy of Sci., Beijing China 3 Department of Electronic Science, University of Pune, India 4 Space & Atmospheric Sciences Division, Navrangpura, Ahmedabad, India.

20 year decline in solar photospheric magnetic fields:

2016 URSI Asia-Pacific Radio Science Conference (URSI AP-RASC), 2016

To overcome blind spots of an ordinary weather radar which scans horizontally at a high altitude,... more To overcome blind spots of an ordinary weather radar which scans horizontally at a high altitude, a weather radar which operates vertically, so called an atmospheric profiler, is needed. In this paper, a K-band radar for observing rainfall vertically is introduced, and measurement results of rainfall are shown and discussed. For better performance of the atmospheric profiler, the radar which has high resolution even with low transmitted power is designed. With this radar, a melting layer is detected and some results that show characteristics of the meting layer are measured well.

2019 URSI Asia-Pacific Radio Science Conference (AP-RASC), 2019

Air travel has become one of the most common means of transportation. The most common question wh... more Air travel has become one of the most common means of transportation. The most common question which is generally asked is: How does an airplane gain lift? And the most common answer is via the Bernoulli principle. It turns out that it is wrongly applied in common explanations, and there are certain misconceptions. In an alternative explanation the push of air from below the wing is argued to be the lift generating force via Newton's law. There are problems with this explanation too. In this paper we try to clear these misconceptions, and the correct explanation, using the Lancaster-Prandtl circulation theory, is discussed. We argue that even the Lancaster-Prandtl theory at the zero angle of attack needs further insights. To this end, we put forward a theory which is applicable at zero angle of attack. A new length scale perpendicular to the lower surface of the wing is introduced and it turns out that the ratio of this length scale to the cord length of a wing is roughly $0.493...

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2019

Astrochemical ices are known to undergo morphological changes, from amorphous to crystalline, upo... more Astrochemical ices are known to undergo morphological changes, from amorphous to crystalline, upon warming the ice from lower (10 K) to higher temperatures. Phase changes are mostly identified by the observation of significant changes in the InfraRed (IR) spectrum, where the IR bands that are broad in the amorphous phase are narrower and split when the ice turns crystalline. To-date all the molecules that are studied under astrochemical conditions are observed to follow such a behaviour without significant attenuation in the IR wavelength. However, in this paper we report a new observation when propargyl ether (C 3 H 3 OC 3 H 3) is warmed from the amorphous phase, at 10 K, through the phase transition temperature of 170 K, the crystalline ice being found to strongly attenuate IR photons at the mid-IR wavelengths.

Astronomy & Astrophysics, 2018

Context. It is well known that the polarity of the Sun’s magnetic field reverses or flips around ... more Context. It is well known that the polarity of the Sun’s magnetic field reverses or flips around the maximum of each 11 year solar cycle. This is commonly known as polar field reversal and plays a key role in deciding the polar field strength at the end of a cycle, which is crucial for the prediction of the upcoming cycle. Aims. We aim to investigate solar polar fields during cycle 24, using measurements of solar magnetic fields in the latitude range 55°–90° and 78°–90°, to report a prolonged and unusual hemispheric asymmetry in the polar field reversal pattern in solar cycle 24. Methods. This study was carried out using medium resolution line-of-sight synoptic magnetograms from the magnetic database of the National Solar Observatory at Kitt Peak (NSO/KP), USA for the period between February 1975 and October 2017, covering solar cycles 21–24 and high-resolution line-of-sight synoptic magnetograms from the Michaelson Doppler Imager instrument onboard the Solar Heliospheric Observator...

The Astrophysical Journal, 2016

We present a detailed multiwavelength study of observations from X-ray, near-infrared, and centim... more We present a detailed multiwavelength study of observations from X-ray, near-infrared, and centimeter wavelengths to probe the star formation processes in the S237 region. Multiwavelength images trace an almost sphere-like shell morphology of the region, which is filled with the 0.5-2 keV X-ray emission. The region contains two distinct environments-a bell-shaped cavity-like structure containing the peak of 1.4 GHz emission at center, and elongated filamentary features without any radio detection at edges of the sphere-like shell-where Herschel clumps are detected. Using the 1.4 GHz continuum and 12 CO line data, the S237 region is found to be excited by a radio spectral type of B0.5V star and is associated with an expanding H II region. The photoionized gas appears to be responsible for the origin of the bell-shaped structure. The majority of molecular gas is distributed toward a massive Herschel clump (M clump ∼260  M), which contains the filamentary features and has a noticeable velocity gradient. The photometric analysis traces the clusters of young stellar objects (YSOs) mainly toward the bell-shaped structure and the filamentary features. Considering the lower dynamical age of the H II region (i.e., 0.2-0.8 Myr), these clusters are unlikely to be formed by the expansion of the H II region. Our results also show the existence of a cluster of YSOs and a massive clump at the intersection of filamentary features, indicating that the collisions of these features may have triggered cluster formation, similar to those found in the Serpens South region.

Advances in Space Research, 2017

We connect the time sequence of changes in the IMF-Bz to the development of spread-F at an equato... more We connect the time sequence of changes in the IMF-Bz to the development of spread-F at an equatorial station Kwajalein on three different nights in November 2004, one during a geomagnetic quiet period and other two during geomagnetic disturbed periods. The chosen days show clear and smooth variations of IMF-Bz without any large fluctuations thereby enabling one to correlate changes in equatorial spread-F with corresponding changes in IMF-Bz. It is shown that a slow and continuous increase in the IMF-Bz over a duration of few hours has a similar effect on the equatorial ionosphere as of a sudden northward turning of the IMF-Bz in causing an electric field through the polar region and then to the equator. We conclude that the Spread-F at equatorial and low latitudes are due to echoes from ionization irregularities that arise due to the plasma instabilities generated by an eastward electric field on the large plasma density gradient in or below the base of the F-layer during any period of the night time along with the gravity driven Rayleigh-Taylor instability.

Solar Physics, 2006

Observations of a solar flare at 617 MHz with the Giant Meter-wave Radio Telescope are used to st... more Observations of a solar flare at 617 MHz with the Giant Meter-wave Radio Telescope are used to study the morphology of flare radio emssion at decimetric wavelengths. There has been very little imaging in the 500-1000 MHz frequency range, but it is of great interest since it corresponds to densities at which energy is believed to be released in solar flares. This event has a very distinctive morphology at 617 MHz: the radio emission is clearly resolved by the 30 beam into arc-shaped sources seeming to lie at the tops of long loops, anchored at one end in the active region in which the flare occurs, with the other end lying some 200000 km away in a region of quiet solar atmosphere. Microwave images show fairly conventional behaviour for the flare in the active region: it consists of two compact sources overlying regions of opposite magnetic polarity in the photosphere. The decimetric emission is confined to the period leading up to the impulsive phase of the flare, and does not extend over a wide frequency range. This fact suggests a flare mechanism in which the magnetic field at considerable height in the corona is destabilized a few minutes prior to the main energy release lower in the corona. The radio morphology also suggests that the radiating electrons are trapped near the tops of magnetic loops, and therefore may have pitch angles near 90 • .

The Solar Wind and Interplanetary Disturbances

The Astrophysical Journal, 2004

We discuss a filament eruption/coronal mass ejection (CME) event associated with a flare of GOES ... more We discuss a filament eruption/coronal mass ejection (CME) event associated with a flare of GOES class M2.8 that occurred on 2001 November 17. This event was observed by the Nobeyama Radio Heliograph (NoRH) at 17 and 34 GHz. NoRH observed the filament during its eruption both as a dark feature against the solar disk and a bright feature above the solar limb. The high cadence of the radio data allows us to follow the motion of the filament at high time resolution to a height of more than half a solar radius. The filament eruption shows a very gradual onset and then a rapid acceleration phase coincident with the launch of a fast halo CME. Soft X-ray and extreme-ultraviolet (EUV) images show heating in a long loop underneath the filament prior to the flare. The NoRH height-time plot of the filament shows a roughly constant gradual acceleration for 1 hr, followed by a very abrupt acceleration coincident with the impulsive phase of the associated flare, and then a phase of constant velocity or much slower acceleration. This pattern is identical to that recently found to occur in the motion of flare-associated CMEs, which also show a sharp acceleration phase closely tied to the impulsive phase of the flare. When the rapid acceleration occurs in this event, the flare site and the filament are separated by~0.5 R , making it unlikely that a disturbance propagates from one location to the other. Models in which a disruption of the large-scale coronal magnetic field simultaneously permits the acceleration of the filament and the flare energy release seem to be a better explanation for this event.

Astronomy & Astrophysics, 2006

We report first results from an ongoing program of combining visibilities from the Giant Meterwav... more We report first results from an ongoing program of combining visibilities from the Giant Meterwave Radio Telescope (GMRT) and the Nancay Radio Heliograph (NRH) to produce composite snapshot images of the sun at meter wavelengths. We describe the data processing, including a specific multi-scale CLEAN algorithm. We present results of a) simulations for two models of the sun at 327 MHz, with differing complexity b) observations of a complex noise storm on the sun at 327 MHz on Aug. 27, 2002. Our results illustrate the capacity of this method to produce high dynamic range snapshot images when the solar corona has structures with scales ranging from the image resolution of 49 to the size of the whole sun. We emphasize that snapshot images of a complex object such as the sun, obtained by combining data from both instruments, are far better than images from either instrument alone, because their uv-coverages are very complementary.

The Astrophysical Journal Letters, 2021

Solar flares, with energies ranging over several orders of magnitude, result from impulsive relea... more Solar flares, with energies ranging over several orders of magnitude, result from impulsive release of energy due to magnetic reconnection in the corona. Barring a handful, almost all microflares observed in X-rays are associated with the solar active regions. Here we present, for the first time, a comprehensive analysis of a large sample of quiet-Sun microflares observed in soft X-rays by the Solar X-ray Monitor (XSM) on board the Chandrayaan-2 mission during the 2019–2020 solar minimum. A total of 98 microflares having peak flux below GOES A-level were observed by the XSM during observations spanning 76 days. By using the derived plasma temperature and emission measure of these events obtained by fitting the XSM spectra along with volume estimates from concurrent imaging observations in EUV with the Solar Dynamics Observatory/Atmospheric Imaging Assembly, we estimated their thermal energies to be ranging from 3 × 1026 to 6 × 1027 erg. We present the frequency distribution of the q...

Solar Physics, 2020

The Solar X-ray Monitor (abbreviated as XSM) on board India's Chandrayaan-2 mission is designed t... more The Solar X-ray Monitor (abbreviated as XSM) on board India's Chandrayaan-2 mission is designed to carry out broadband spectroscopy of the Sun from lunar orbit. It observes the Sun as a star and measures the spectrum every second in the soft X-ray band of 1-15 keV with an energy resolution better than 180 eV at 5.9 keV. The primary objective of the XSM is to provide the incident solar spectrum for the X-ray fluorescence spectroscopy experiment on the Chandrayaan-2 orbiter, which aims to generate elemental abundance maps of the lunar surface. However, observations with the XSM can independently be used to study the Sun as well. The Chandrayaan-2 mission was launched on 22 July 2019, and the XSM began nominal operations, in lunar orbit, from September 2019. The in-flight observations, so far, have shown that its spectral performance has been identical to that on the ground. Measurements of the effective area from ground calibration were found to require some refinement, which has been carried out using solar observations at different incident angles. It also has been shown that the XSM is sensitive enough to detect solar activity well below A-class. This makes the investigations of microflares and the quiet solar corona feasible in addition to the study of the evolution of physical parameters during intense flares. This article presents the in-flight performance and

Multi-directional measurements of high energy particles from the Sun-Earth L1 point with STEPS

Space Telescopes and Instrumentation 2016: Ultraviolet to Gamma Ray, 2016

Aditya Solar wind Particle EXperiment (ASPEX) is one of the scientific experiments onboard the Ad... more Aditya Solar wind Particle EXperiment (ASPEX) is one of the scientific experiments onboard the Aditya-L1 mission, the first Indian solar mission planned to be launched in the year of 2019. The primary objective of the ASPEX experiment is to carry out in-situ, multi-directional measurements of solar wind ions in the energy range of 100 eV/n to 5 MeV/n. ASPEX instrument has been configured into two subsystems: Solar Wind Ion Spectrometer (SWIS) and Supra Thermal & Energetic Particle Spectrometer (STEPS). SWIS will measure the angular and energy distribution of solar wind ions in the energy range of 100 eV to 20 keV and STEPS will measure the energy spectrum of high energetic particles from six directions covering the energy range of 20 keV/n to 5 MeV/n. This paper presents the overall configuration of the STEPS subsystem with preliminary results obtained from the bread board model.