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  Janardhan Padmanabhan

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Name Professor J Padmanabhan
(Professor Janardhan Padmanabhan)
FNA ID P20-1846
Address Senior Professor (H) and Dean (Retd.), Physical Research Laboratory, Navrangpura,
City Ahmedabad
Pin Code 380009
Country India
Gender Male
Specialization Solarwind, Space Weather, Solar Radio Studies, Solar Magnetic Fields, Comets
Service in the Council
Qualification PhD
Membership Individual Member URSI (MURSI).; Member National Committee of COSPAR-URSI-SCOSTEP.; Member Union Radio-Scientifique Internationale (URSI).; Member of the International Astronomical Union (IAU).; Member of the American Geophysical Union (AGU).
  Year of Election 2020  
E-mail janardhan.padmanabhan@gmail.com, jerry@prl.res.in
Personal Website https://www.prl.res.in/~jerry/

Brief Summary:

I am an experienced Senior Professor (H) with a demonstrated history of working in Fundamental Research in Solar and Space Science research. Skilled in Physics, Space Technology, Lecturing, and Science,  with a PhD focused in Solar Radio Astronomy, Solar Magnetic Fields, Solar Wind, and Space Weather studies from Physical Research Laboratory, Ahmedabad, India. 

I am the Principle Investigator (PI) of the Aditya Solar Wind Particle Experiment (ASPEX) on India\'s first dedicated Solar Mission ADITYA-L1, due for launch in 2021.

I have worked at the Radioastroomisches Institut, Universitat Bonn (1996-1997); and as a Research Associate at the Department of Astronomy, University of Maryland, USA (1999-2000).  I have been a visiting professor at ISEE, Japan (2003 and 2018); Visiting Professor at INPE, Brazil (2007), Visiting Professor at DAMTP, Cambridge, UK (2008).

Other research interests include Comets, Exo-Planets, Star-Formation, and Astrophysical Ices. 

Apart from my research interests, I also have a keen interest in ornithology and nature photography - Those interested may see the link:  http://www.flickr.com/photos/jerryprl

Highlights of Some Important Scientific Contributions:

The Aditya Solarwind Particle Experiment (ASPEX) to be flown onboard the ADITYA-L1 Mission of ISRO in 2021: - Principal Investigator (PI): Janardhan, P.

One of the most important features of the ASPEX experiment is that it will be able to identify the arrival time of ICMEs at L1 accurately by measuring the He++/H+number density ratios or helium abundance enhancements (HAE).  A He++/H+ ratio greater than 0.08 is known to be the most reliable markers of CME arrival at 1 AU, ASPEX will thus be unique in its ability to predict Space Weather events caused by CME’s, a vital input for potentially harmful space weather events.

The uniqueness of our experiment lies in the fact that Time-resolved energy spectral measurements of both protons and alpha particles from the four directions will provide one

The ability to address the anisotropy in the energy distribution of particles in the direction of the Parker spiral vis-a-vis other directions. This, in turn, will help to trace the origin of supra-thermal particles which could not be explained by only solar wind propagation. 


The ASPEX payload onboard the Aditya mission consists of a set of two separate particle analyzers to take advantage of the unique location of the spacecraft at the L1 Lagrangian point of the Sun-Earth system to carry out systematic and continuous observations of particle fluxes over an energy range spanning 100 eV to 5 MeV. The payload consisting of two separate components will cover the entire energy range – the Solar Wind Ion Spectrometer (SWIS) covering the low energy range (100 eV to 20 keV) using an electrostatic analyzer and the Suprathermal Energetic Particle Spectrometer (STEPS) covering the high energy range (20 keV to 5 MeV) using solid-state detectors.

The primary focus of the ASPEX payload is to understand the solar and interplanetary processes (like shock effects, wave-particle interactions, etc.) in the acceleration and energization of the solar wind particles. In order to achieve that it is necessary that ASPEX intends to measure low as well as high energy particles that are associated with slow and fast components of the solar wind, suprathermal population, shocks associated with CME, and CIR, and solar energetic particles (SEPs). Among these, it is expected that the slow and fast components of the solar wind and some part of the suprathermal population can be measured in a predominantly radial direction. In addition, a part of the suprathermal population, CME and CIR-accelerated particles, and SEPs are expected to arrive at the detectors along the Parker spiral. The He++/H+ ratio will be used as a compositional “flag” to differentiate (and identify) the arrivals of CME, CIR, SEP-related particles from those of the quiet solar wind origin.

Solar Magnetic Fields:

The sunspot minimum at the end of Solar Cycle 23 has been one of the deepest experienced in the past 100 years. A detailed study of polar magnetic fields during the last three solar cycles, viz., Cycles 21, 22 and 23, and the ongoing cycle 24 using both ground-based and space-based data and has established that solar polar fields have been dropping steadily for the past 25 years starting from ~1995.  In addition, my observations of the current cycle-24 has shown a pronounced hemispheric asymmetry in field reversal times with the solar northern hemisphere reversing polarity 2.5 years after the south.  I have also been able to develop a new tool for predicting the amplitude of the next solar cycle, using a correlation between magnetic flux canceled at the equator and the flux transported to the solar poles.  

Extensive interplanetary scintillation (IPS) observations at 327 MHz spanning the period between 1983 and 2014 have also been analyzed to show a steady and significant drop in the turbulence levels in the entire inner heliosphere also starting from around ~1995.  My work has shown that this largescale IPS signature, in the inner heliosphere, coupled with the declining solar polar fields since ~1995, shows that the build-up to the deepest minimum in 100 years actually began in the mid-1990s.  These studies led to a prediction that a Maunder like solar minimum epoch (when the sun was completely devoid of sunspots in the period 1645 to 1715) is on the anvil, wherein, sun-spots are entirely absent for many decades thereby impacting global climate rainfall events, radio communication and other non-anthropogenic climate effects associated with global cooling and unusual weather patterns.

Theoretical support for this scenario has come from other researchers who have used theoretical computations of sunspot numbers and a kinematic dynamo simulation model to show that very deep minima are generally associated with weak solar polar fields and changes in the solar meridional flow speeds. Both of these theoretical predictions have been confirmed by my work using innovative, systematic, large-scale, observations of solar photospheric magnetic fields and IPS observations of solar wind turbulence levels in the inner heliosphere

Solar Wind Disappearance Events:

Extremely rare and enigmatic solar wind disappearance events have taken place on three separate occasions in Solar Cycle 23.  In fact, these events are so rare that in the past three solar cycles, spanning 33 years, only seven such events have been recorded, of which three have occurred in SC23.  These enigmatic and large-scale density anomalies in the interplanetary medium are periods of time when the average solar wind densities at 1 AU drop by over two orders of magnitude for periods exceeding 24 hours.  I have, in a series of papers provided the first comprehensive understanding of these unique events.  He has shown that disappearance events are solar surface phenomena, not linked to global solar events like solar polar field reversals, as speculated by many other researchers.  Using extensive IPS observations and spacecraft data it has been shown conclusively that such events originate at short-lived active-region-coronal-hole (AR-CH) boundaries located at central meridian on the Sun. I have further shown that these events are all associated with highly non-linear solar wind flows and extended Alfvén radii.  My model invokes interchange reconnection processes driven by large magnetic flux expansion factors (between 100 - 1000) at the solar source region and is the first and only one to satisfactorily explain all the observational peculiarities of disappearance events at 1 AU.  One very significant finding in this extensive study is that apart from co-rotating interaction regions, disappearance events are the only other non-explosive solar events that can cause space weather effects at 1 AU.

Tracking Interplanetary Disturbances from the Sun to 1 AU:

I have carried out extensive IPS observations, at 327 MHz to evolve a unique technique to track interplanetary (IP) disturbances through space from day-to-day.  The technique, known as the “picket-fence” method, first uses a theoretical model to predict the location in space of a flare generated shock and then uses the IPS sources as a rapidly movable picket fence in the sky to pinpoint and track the propagating shock front between 0.2 and 0.8 AU.  This method was the first successful day-to-day tracking of IP disturbances with the ORT and has shown that X-class flares are associated with IP shocks on a one-to-one basis.  The uniqueness of the method lies in the fact that the locations in space of temporally distinct IP transients were predicted in advance by a simple theoretical model of propagation of IP shocks, based on real-time observations of temporally separated events on the Sun.  Signatures of these shocks were then detected unambiguously using IPS observations on a large grid of spatially distributed compact radio sources that were used as a rapidly movable picket-fence in the sky. 

Radio Observations of the Sun:

Our work was one of the pioneering studies in carrying out solar imaging observations with the Giant Meterwave Radio Telescope (GMRT). Radio maps of a flare-CME event that took place on 17 November 2001 have provided the first and only observational evidence in support of the breakout model of flare-CME initiation.  The breakout model is different from other models in that the reconnection process that initiates a mass ejection begins from the top of the coronal loop as opposed to models that initiate the process by reconnections occurring lower in the corona. The results are significant in that it is the earliest observational evidence that could favor such a scenario.  Systematic observations at 327 and 236 MHz, coordinated with the Nançay Radioheliograph (NRH), France, have been carried out on several occasions at GMRT to observe the quiet sun with an aim to produce composite synthesis maps from NRH and GMRT, with both wide field of view and high spatial resolution. A method to combine the two sets of visibilities and obtain a spectacular increase in the quality and dynamic range of solar images at several frequencies has been developed. Dynamic ranges between 250 and 400, the highest ever, dynamic range achieved in radio images so far have been obtained.  A study of noise storms using the GMRT and the NRH has shown that storms are produced in regions much denser than the ambient corona; the structure of source at one frequency is now resolved and can be down to ~15 arcsec at 327 MHz. Our observations also show that the size of noise storm sources may significantly change from one case to another one. Such changes cannot be explained by propagation effects and indicate that the resolution of our observations allows us to reach down to the intrinsic features of the structure of storm sources; our observations have provided values for the scale-height of the electron density distribution in noise storm sources that are significantly less than in the ambient corona implying that the sources regions emitting at different frequencies are not magnetically connected. This contradicts the classical columnar model and questions the currently used theories for emission mechanisms, which imply magnetic trapping of supra-thermal electrons. Recent GMRT observations have also shown how the ducting effect can produce radio emission far away from the flaring site.

In the light of the discovery of fast Moreton waves and EIT waves in the solar corona, extensive P-band (333 MHz) Very Large Array (VLA) solar data from the VLA archives were analyzed to search for very fast propagating coronal disturbances. Solar surface maps produced with high time-resolution were used to detect motion associated with a solar flare at a speed of 26000 km s-1. The observations have a significant impact because the inferred velocity is larger than any previously inferred velocity of a disturbance in the solar atmosphere, with the exception of beams of freely streaming accelerated electrons.

Sounding the Solar Corona:

Using dual-frequency (S-band & X-band) Doppler sounding data from the Ulysses satellites Solar Corona Experiment (SCE), I have developed a method that uses, for the first time, the deep-space-network (DSN) of telescopes to cross-correlate spacecraft sounding data across intercontinental baselines.  I was thus able to derive solar wind velocities and study the fine structure of solar wind turbulence in this inaccessible region between 4Rs and 40 Rs, where Rs is the solar radius.  Since sounding data close to the Sun is heavily affected by solar noise, I developed a digital filtering technique to extract cross-correlation amplitudes and time-lags that would otherwise be buried in the noise.  Apart from yielding solar wind velocities and columnar electron densities at southern solar latitudes between the pole and the equator, and in an otherwise inaccessible region of the IP medium, the method has been employed to identify various types of solar wind flows in the acceleration regime that depends on the nature and morphology of the underlying photospheric magnetic fields. The results, thus, have a significant impact on our understanding of the origin and nature of different types of solar wind flows in the low corona.

Cometary Studies:

During the perihelion passage of comet Hale-Bopp (C/1995 OI) in March-April 1997, we made K-band radio observations with the 100-meter telescope of the Max-Planck-Institüt für Radioastronomie. Emission was firmly detected, for the first time, from all of the five lowest metastable (J=K) inversion transitions of ammonia. Assuming a thermal distribution for the metastable states of ammonia, a rotational temperature of 104 ±30 K was derived and an ammonia production rate at the perihelion of 6.6 ±1.3 × 1028s-1was determined. The results are significant in that ammonia has been discovered in only two other comets so far and these observations, the first to detect all five metastable inversion transitions of ammonia, has yielded the only reliable estimate of the rotational temperature in a cometary coma, thereby giving a probable kinetic temperature of the inner coma of comets.

I have also used cometary plasma tail occultations of extragalactic radio sources to study electron density fluctuations well downstream (~ 2×106 km) of the nucleus.  It has been shown that only under very specific conditions of occulting geometry, cometary tail plasma can cause enhanced scintillation at Earth.  In the case of comet Halley, his studies have shown that there exists a fine-scale size region near the tail axis with scale sizes in the range 9 km - 27 km and an rms electron density fluctuation Delta Nrms between 2 cm-3 and 5 cm-3 and a second zone near the tail edges with the scale sizes and Delta Nrms in the range 100 – 265 km s-1 and 0.4 cm-3 to 0.8 cm-3 respectively.  The importance of accounting for the tail-lag, which can cause the tail to deviate from the true radial direction by as much as 3-5 degrees, was first demonstrated by me and is crucial in determining the exact occultation time and geometry.

Awards and Recognitions:

1.     ISRO Merit Award - 2015             

2.     The - Vikram Sarabhai Research Award in Space Sciences for the year 2003.

3.     The Alexander Von Humboldt Research Fellowship in Astrophysics for the year 1996 by the Alexander Von Humboldt Foundation, Bonn, Germany.

4.     The \"Young Astronomer\" award in1988 from the National Science Foundation (NSF, U.S.A.) to attend the Twentieth General Assembly of the International Astronomical Union, Baltimore, USA and to visit the Mullard Radio Astronomy Observatory, Cambridge, UK.

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