Janardhan Padmanabhan, FNA                                                                       
INSA Senior Scentist
Physical Research Laboratory
Ahmedabad - 380 009, India

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<Ooty IPS Survey>

An all sky map of the scintillating sources detected the Ooty Radio Telescope (ORT).
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<Ulysses-orbit>

The trajectory of the Ulysses spacecraft, the first to leave the ecliptic plane and make a polar pass of the Sun.
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<Thaltej-Dipole-Array>

A view of the 20,000 m2 dipole array at Thaltej, Ahmedabad, that was operated by PRL and used for IPS observations of the solar wind at 103 MHz.

Click on the picture for a full size image of the array.




<The VLA>

The Very Large Array in the "A" configuration, Soccoro, New Mexico, USA.
















<GMRT - 45m Dish>

One of the 45 meter diameter GMRT dishes in the central square at Kodhad, Pune.

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<Hale-Bopp and Effelsberg Telescope>

Comet Hale-Bopp. The Effelsberg 100 m dish is in the bottom left corner of the picture. Photo Credit: Bill-Sherwood, MPIFR, Bonn., Germany.




















































































































































































































































































































































































<Aspex team>

The ASPEX team members at Thaltej in 2019, PRL.

click on the image above to enlarge.

 
 

<elec-button>  Current Research Interests:


My research covers a wide range of interests from radio studies of the sun and the solarwind, with emphasis on "Space Weather" studies using observations of Interplanetary Scintillation (IPS) and meter wavelength solar imaging observations. Aditionally, I have an interest in studies of solar photospheric magnetic fields, radio observations of comets, experimental studies of astrochemical ices, shock processing of molecules, and exo-planets. Over the past decade I have been involved in scentific payload development for solar studies and led the development, as Principal Investigator (PI), of the ASPEX payload onboard India's first observatory-class Mission ADITYA-L1 to study the sun from the L1 Lagrangian Point of the sun-earth system. ADITYA-L1 was launched on 02 September 2023.

<sun> Payload Development for Solarwind Studies Aditya Solar Wind Particle Experiment (ASPEX).

Apart from the above I have also done some work on optical spectroscopy of Be stars - Studies of H-alpha emission profiles. A brief list highlighting various reserch interests are given below


<sun> IPS studies of the Solar wind and Interplanetary Medium.

<sun> IPS studies of the Solar Wind at High Latitudes.

<sun> VLA Imaging of Solar Flares and Compact Radio Sources.

<sun> Solar Imaging Observations with the Giant Meterwave Radio Telescope (GMRT).

<sun> IPS studies of Interstellar Scattering.

<sun> IPS and Emission Line studies of Cometary Ion Tails and Coma.

<sun> Very Long Baseline Interferometric (VLBI) Studies at 327 MHz.

<sun> Studies of H-alpha Emission Line profiles of Be Stars.

<sun> Geoeffectiveness of Corotating Interacting Regions CIR.

<sun> Studies of Solar Photospheric Magnetic Fields.




IPS Studies of the Solar Wind and Interplanetary Medium:


Understanding the nature and exact solar origins of sudden and intense geomagnetic activity on the earth has been a topic of current research interest globally with a number of spacecraft like SOHO and YOHKOH devoting a large fraction of their time to this particular aspect. Apart from space based platforms ground based efforts involving interplanetary scintillation (IPS) measurements have also yielded important insights and IPS is the most cost effective method to study the large scale properties of the interplanetary medium from the ground.

Since 1992, I have been interested in addressing the problem regarding the solar sources of intense geomagnetic storms. In the past decade, the cause of such storms has been attributed, by different groups of research workers, to a variety of solar surface features ranging from solar flares and coronal holes to disappearing filaments and coronal mass ejection's (CME's). To study and understand this problem, I have made extensive interplanetary scintillation (IPS) observations with the the Ooty Radio Telescope (ORT), which has a large effective collecting area of 8000 m2, and have monitored density enhancements and solar wind velocities in the directions of a large grid of compact extragalactic radio sources. Using the scintillating sources as a movable picket fence in the sky and using a theoretical model to predict the location in space of flare generated shocks I have been able to track, on a daily basis, traveling interplanetary disturbances from 0.2 to 0.8 AU. Such tracking of interplanetary disturbances coupled with spacecraft data has been shown to be an efficient way of monitoring interplanetary transients and traveling interplanetary disturbances. However, to be able to conduct such an experiment on a regular basis one requires to have a known grid of compact scintillators distributed all over the sky. Towards achieving this end I first undertook an extensive IPS survey between 1992-94 (using the ORT) to identify a grid of spatially well distributed compact extragalactic radio sources that showed strong scintillation at 327 MHz.

The major objectives of this IPS survey of about 5000 radio sources with flux densities > 1.5 Jy were:

  • to obtain a spatially well distributed list of scintillating sources around the ecliptic plane for interplanetary weather mapping
  • to obtain a finding list of compact milli arc second sources for the space VLBI mission "Radio Astron"
  • to study the high latitude solar wind as a comparison to the in-situ observations by the Ulysses satellite.
  • to have a complete sample of sub arc second extragalactic compact sources for studies of interstellar scattering in the inner galaxy.

Apart from the grid of known scintillators distributed in the sky, the IPS survey analysis yielded a large number of measurements of compact component sizes for the radio sources at 327 MHz. This data was then used as a basis the selection of sources for 92 cm VLBI observations.

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Solar Wind Studies Very Close to the Sun and at High Latitudes:


The phenomenon of IPS at meter wavelengths can be exploited to study the solar wind at distances beyond approximately 40 solar radii. To study the plasma properties and structure of the solar wind at high latitudes and at distances < 40 solar radii I have used dual-frequency Doppler sounding data from the Ulysses satellite's Solar Corona Experiment (SCE). The sounding data yielded solar wind velocities and measurements of columnar electron densities at southern solar latitudes between the pole and the equator in the distance range 4 to 40 solar radii.

The Ulysses SCE was performed at the spacecraft's two solar conjunctions in summer 1991 and winter 1995. During the two conjunctions dual-frequency ranging and Doppler observations were conducted on a nearly continuous basis at the NASA Deep Space Network and other ground stations. The differential Doppler data, which are sensitive to the changes in electron column density along the radio ray path from spacecraft to ground station, were used to determine coronal plasma velocities. The measurement technique, based on a cross correlation analysis, can be applied whenever data were obtained simultaneously from two well-separated ground stations. Because the observations are made so close to the Sun (ray path offsets: 4-40 solar radii, the data are often quite noisy. A ` ``filtering'' technique was thus developed in order to enhance the two-station correlations. A method of combining such two-station cross correlations with uplink-downlink correlations was also used to determine the exact location of disturbances along the radio ray path from the spacecraft to the earth. For further details refer the following papers by Janardhan et al., (1999) Sol. Phys., 184, 157. and Lotova et al., (2002) Sol. Phys., 205, 149.

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Studies of Interstellar ScatteringUsing IPS


IPS observations of compact radio sources yield estimates of the compact component sizes in the sources. A comparison of the measured compact component sizes of a set of sources at two different observing frequencies can be used to estimate the interstellar scatter broadening of compact radio sources. I have made extensive IPS observations of compact component sizes at 103 MHz, using a large dipole array having a physical area of 10,000 m2. A comparison of these observations with 151 MHz IPS measurements from Cambridge was used to estimate the contribution of interstellar scattering at 103 MHz and to show the enhanced scattering in the plane of the Galaxy . (Please refer Janardhan & Alurkar (1993), A&A 269, 119--127. for more details of the work.

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VLA Imaging of Radio Sources and Solar Flares:.


(i) Dual Frequency VLA Imaging of Compact Sources:.

A set of 59 ultra compact (< 100 milli arcseconds (mas)) have been identified at meterwavelengths using an extensive ~ 4000 radio sources interplanetary scintillation (IPS) survey made by the Ooty Radio Telescope (ORT) in India during 1992-'95. In view of the rarity of compact structures at low frequencies, it is important to study the detailed structure and nature of these components, besides their direct relevance to the study of solar wind and interplanetary medium. The spectral indices, of the radio sources in this small sub-set varies in the range 0.17 to -1.49 between 80 and 5000 MHz, assuming S(ν) is proportional to ν−&alpha. Since the IPS observations do not identify the nature of the compact components, they may arise from compact cores in the centre of active galaxies or hot spots in the extended lobes of radio galaxies or may arise from compact steep spectrum (CSS) sources. In order to study the nature of these sources, imaging observations of these objects at 3.5 and 20 cm using the Very Large Array (VLA) `A' configuration were carried out in July and September 1999 in three separate observing runs of 3 hours each. The data is now being reduced and analized.


(ii) P-Band Very Large Array (VLA) Imaging of Solar Flares:.

P-Band (0.333 GHz) VLA observations of a solar flare showed motion associated with a solar flare at a speed of 2600 km s-1. The observed speed the highest speed yet directly measured for a disturbance other than freely streaming energetic electrons. Details of the observations can be found in White, Janardhan and Kundu (2000), ApJL, 533, L167--L170.



GMRT Imaging Observations of Solar Flares:.


An M2.8 flare observed at 1060 MHz with the Giant Meter Wave Radio Telescope (GMRT) on Nov 17 2001 was associated with a prominence eruption observed at 17 GHz by the Nobeyama radioheliograph and the initiation of a fast partial halo CME observed with the LASCO C2 coronograph. Together with observations from the Hiraiso spectrograph, these data offer a detailed insight into the initiation process of this flare-CME event. It was found that the nonthermal radiation signatures of the reconnection processes leading to the initiation of this event started at the 200 MHz level before proceeding lower into the corona and initiating the prominence eruption and the associated flare. This is consistent with one of the key predictions of the magnetic breakout model of Antiochos et al. (1999).

<GMRT 16s Snapshot> <GMRT 16s Snapshot> <GMRT 16s Snapshot>


The three panels show 16s snapshot images at times corresponding to each of the three peaks in the lightcurvr of the flare. Note that during the third peak, the emitting region is different from that in the first two snapshots. The restoring beam was 34"x24" in size and the images have a dynamic range of about 10.

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VLBI Studies at 327 MHz:


The IPS survey (mentioned briefly earlier) yielded a grid of about 600 compact scintillators of which approximately 80 sources yielded consistent measurements of compact component sizes of < 100 milli arc seconds. These 80 sources were further studied using the the European VLBI Network (EVN) at 92 cm with the dual aim of identifying the compact components giving rise to scintillation and to make a finding list for the space VLBI mission "Radio Astron". The analysis if the EVN data is still underway. A small subset of the identified strong scintillators were found to be extremely steep spectrum sources and these need to be mapped to study and identify their compact components.

From October 28, 18:00 UT to October 31, 13:00 UT, 1998, VLBI observations on a number of sources at 327 MHz were carried out using the following stations: Ooty Radio Telescope, Green Bank RT-43 (USA), Noto RT-32 (Italy), Urumqi RT-25 (China), Evpatoria RT-70 (Ukraine), Bear Lakes RT-64 (Russia, near Moscow), Puschino RT-22 (Russia, near Moscow), St.Pustyn RT-14 (Russia, near N.Novgorod), Zimenki RT-15 (Russia, near N.Novgorod). A number of the sources observed in this run were taken from the Ooty IPS survey.


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IPS and Emission Line Studies of Cometary Ion Tails and Coma:


(i). At Meter Wavelengths − Using IPS:

The phenomenon of IPS has been exploited to study cometary ion tail plasma via occultation of one or more radio sources and scintillation of their electromagnetic emission as it passed through the tail of the comet. It has been shown that under specific conditions of occulting geometry the rms electron density fluctuations in cometary ion tails is sufficient to cause scintillation at the earth. IPS at meter wavelengths can be thus used to determine the variation in the rms electron density fluctuation in cometary tail plasma at distances well downstream of the nucleus. Further details of the this work can be found in the following references Janardhan et. al., (1991), Aust. Jou. Phys., 44, 565 and Janardhan et. al., (1992)  Aust. Jou. Phys., 45, 115.





The figure above shows the path of the radio source 3C459 (2314+038) across the tail of comet Halley on seven days between 16 and 22 December 1985 when the radio source was occulted by the comet tail. The path is computed for a tail lag of -3o. The tail-lag results from the interaction of the radially directed solar wind and the orbital motion of the comet. Neglecting the tail-lag will cause large errors in the computation of the occultation as shown in the figure below where the location of the tail axis of the comet has been shown for a tail lag of -3o, 0o and +3o at three different times as indicated.





The table below gives the measured scintillation index on each day. Since the scintillation index is directly proportional to the rms electron density fluctuation along the line-of-sight to the source, these measurements can estimate the rms electron density in the cometary tail plasma provided the contribution from the solar wind is negligable.





For this it is necessary to have the occultation take place at large distances from the Sun as was the case here, with the solar elongation of the occulted source being well beyond 80o when the scintillation from the solar wind is very weak or negligable. This work was carried out using the large Thaltej Radio Telescope, operating at 103 MHz. which was part of a three-station IPS observatory of the Physical Research Laboratory, India. The network of telescopes consisted of a large dipole array having a physical area of 10,000 m2 and two smaller but similar 5,000 m2 arrays separated by an average baseline of about 200 km.


(ii). At Centimeter Wavelengths:

During its recent perihelion passage in March/April 1997, K-band radio observations of comet Hale-Bopp (C/1995 O1) were conducted at the 100 meter Effelsberg Radio Telescope of the Max-Planck-Institut für Radioastronomie. Emission was firmly detected from the five lowest metastable (J=K) inversion transitions of ammonia. Assuming a thermal distribution for the metastable states of NH3, a rotational temperature of 104䕂 K was derived and an ammonia production rate at perihelion of 6.6 ± 1.3× 1028 s-1. The ammonia-to-water abundance ratio was found to be of the order of 1.0%. A marginal detection of the 616-523 transition line of water at a wavelength of 1.35 cm was also made. Go here for further details or refer the following papers by Bird et al., (1997; 1999) AAL, 325, L5--L9. and Earth, Moon and Planets, 78, 21

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Studies of H−α Emission Line Profiles of Be Stars:


Be stars are rapid rotating, early type stars characterised by Balmer line emission and IR excess. They exhibit irregular variability both, spectroscopically and photometrically. They are surrounded by a gaseous envelope, which is photoionized by the radiation from the central star. The subsequent recombination process gives rise to the hydrogen line emission. The emission line profiles of Be stars show a variety of shapes. One of the earliest models to explain these profiles was the rotational model by Struve, 1931 (Please refer Banerjee, Rawat and Janardhan, (2000), A&A Suppl. Ser. 147, 229-242.,for details of the references quoted here). Since then many emission line spectroscopic studies have been made to understand the Be phenomenon - particularly the process of envelope formation - and the physical parameters of the disk like its shape, size and kinematics.

Though great progress has been made in explaining the line profiles, by invoking general physical mechanisms, there are still certain aspects which are not well understood. Hummel & Dachs, 1992 and Hummel, 1994 have proposed an elegant mechanism which largely explains as to how symmetric profiles of the winebottle through the shell type, and their many variations in between, can be generated by the same optically thick Keplerian envelope when viewed from different angles.

One of the objectives of the present study was to see how observational data agree with the scenario put forward by Hummel, 1994. Towards this end, we have acquired high resolution H−α profiles for a sample of 44 Be stars. Given the latitude of our observatory (24.653oN), both, the northern and the mid-southern declination sources are accessible in the sky, and this is reflected in the choice of our sources. The other objective behind this study was the idea of building up, or filling in the gaps in, the database of the emission line profiles of Be stars, particularly that for the temporal variability of the line profiles. The atlas by Hanuschik et al., 1996 shows that significant variations can occur in the profiles over protracted periods. Our data is intended to make an additional input for any comprehensive model, that may be constructed, for explaining the observed variability. See also the paper by Banerjee, Janardhan and Ashok, (2001) AAL 380, L13--L16.

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The Geo-effectiveness of Solar Wind Flows Caused By Co-rotating Interaction Regions


A systematic study of Co-rotating Interaction Regions has shown that their geo-effectiveness is governed by their azimuthal or non-radial flow angle. Only those CIR associated solar wind flows that deviate with respect to the radial direction by less than 6o in the azimuthal plane are seen to be geo-effective and show a causal relationship between Bz and the equatorial electrojet (EEJ).

This provides a quick way to predict Geo-effective solar wind outflows from L1 and is the first observational method of predicting geo-effectiveness with a lead time of 30 minutes to an hour.

This is for the first time ever that we have an observational parameter to specify if a given solar wind outflow observed at L1 and associated with co-rotating interaction regions will have an impact on the terrestrial magnetosphere or in other words be geo-effective. This work was also reported in Nature India under the title “A new angle on the effects of solar wind” - Nature India

CIR’s and Geo-effectiveness:

The magnetic field in the heliosphere continuously evolves in response to the solar photospheric field at its base. Together with the rotation of the Sun, this evolution drives space weather through the continually changing conditions of the solar wind and the magnetic field embedded within it. The aim of space weather studies has therefore been to try and predict the geo-effectiveness of solar wind streams interacting with the terrestrial magnetosphere and ionospheric system. In other words, space weather studies try to establish a causal relationship between solar wind events or disturbances taking place outside the terrestrial magnetosphere with events occurring within the terrestrial magnetosphere or the earth’s ionosphere.

Solar rotation coupled with the fact that solar wind streams can have different flow velocities will yield interaction regions, in the inner-heliosphere, where the different flow streams will interact. Such interaction regions are commonly referred to as co-rotating interaction regions (CIR) and are identified by rapid fluctuations in the z-component of the interplanetary magnetic field (Bz). We have identified a large number of such CIR’s at the L1 Lagrangian point of the sun-earth system and shown that their geo-effectiveness is governed by their azimuthal or non-radial flow angle. Interestingly, only those CIR associated solar wind flows that deviate with respect to the radial direction by less than 6o in the azimuthal plane are seen to be geo-effective and show a causal relationship between Bz and the equatorial electrojet (EEJ).

These results thus, provide one an easy and quick method of predicting the geo-effectiveness of solar wind outflows by merely examining their degree of deviation from the radial direction. See the paper by Rout, Janardhan, Sekar, et al., (2017) GRL, 45, 4532 -- 4539.


The schematic above (not to the scale; viewed from above the ecliptic plane) depicts the Co-rotating Interaction Region (CIR) caused by the different velocity outflows from the Sun which reveals the efficacy of geo-effectiveness of the solar wind flows caused by CIR. For azimuthal flow angle below 6o, measured at the first Lagrangian point (L1),with respect to the Sun-Earth line, the fluctuations in interplanetary magnetic field (Bz) and equatorial electrojet (EEJ) are causally connected.



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A long-term study of declining solar photospheric magnetic fields: inner-heliospheric signatures and possible implications


Important findings:

  • This study indicates that a grand minimum akin to a Maunder like minimum may be in progress if the decline in solar fields continues beyond 2020.
  • Solar photospheric fields and solar wind micro-turbulence levels have been steadily declining from ~1995 and the decline is continuing. The declining trend is likely to continue at least until the minimum of cycle 24 in 2020.
  • The heliospheric Magnetic Field, based on the correlation between the high-latitude magnetic field and the HMF at the solar minima, is expected to decline to a value of ~4.0 (±0.6) nT by 2020.
  • The peak 13 month smoothed sunspot number of Cycle 25 is likely to be ~69 ± 12, thereby making Cycle 25 a slightly weaker cycle than Cycle 24, and only a little stronger than the cycle preceding the Maunder Minimum and comparable to cycles in the 19th century.
  • Solar cycle 24 showed pronounced asymmetry in the polar field reversals between the two solar hemispheres with the northern hemisphere reversing polarity 2.5 years after the southern hemisphere.

Impact on the field:

The study predicts the onset of a Maunder like grand minimum, with all the associated climate effects, if the decline in solar fields continues beyond 2020. This is a very significant conclusion as the last Grand minimum occurred over 400 years ago and we will have the opportunity to study it in detail.

Declining solar photospheric fields

Sunspots or dark regions of strong magnetic fields on the sun are generated via magneto-hydrodynamic processes involving the cyclic generation of toroidal, sunspot fields from pre-existing poloidal fields and their eventual regeneration through a process, referred to as the solar dynamo. This leads to the well known and periodic 11-year solar cycle of waxing and waning sunspot numbers. However, studies of past sunspot activity reveal periods like the Maunder minimum (1645 -- 1715) when the sunspot activity was extremely low or virtually non-existent. Using 14C records from tree rings going back 11, 000 years in time, 27 such prolonged or grand solar minima have been identified, implying that conditions existed in these 17% -- 18% of solar cycles to force the sun into grand minima. The current solar cycle 24 was preceded by one of the deepest solar minima in the past 100 years, with sunspot numbers continuously remaining well below 25, and thereby causing cycle 24 to start ~1.3 years later than expected. Also, solar cycle 24, with a peak smoothed sunspot number ~75 in November 2013, has been the weakest since cycle 14 in the early 1900's.

The work on solar photospheric magnetic fields, using synoptic magnetograms from the National Solar Observatory (NSO), Kitt Peak (NSO/KP), between 1975—2010, spanning the last three solar cycles, have shown a steady decline in solar photospheric magnetic fields at helio-latitudes (≥45ο) until 2010, with the observed decline having begun in the mid-1990's. Also, recent studies of the sunspot umbral field strengths have shown that it has been decreasing by ~50 G per year. It is known that for field strengths below about 1500 G, there would be no contrast between the photosphere and sunspot regions, thereby making the later invisible. Some authors have claimed that the umbral field strengths in cycle 25 would be around 1500 G, and thus there would be very little/no sunspots visible on the solar photosphere. Studies of the heliospheric magnetic fields (HMF), using in-situ measurements at 1 AU, have also shown a significant decline in their strength. In addition, using 327 MHz observations from the four station IPS observatory of the Institute of Space Earth Environmental Sciences (ISEE), Nagoya University, Japan, we have examined solar wind micro-turbulence levels in the inner-heliosphere and have found a similar steady decline, continuing for the past 18 years, and in sync with the declining photospheric fields. A study, covering solar cycle 23, of the solar wind density modulation index, ЄN ≡ ∆N/N, where, ∆N is the rms electron density fluctuations in the solar wind and N is the density, has reported a decline of around 8% from which the authors attributed to the declining photospheric fields.

In light of the very unusual nature of the minimum of solar cycle 23 and the current weak solar cycle 24, solar photospheric magnetic fields between 1975—2013, the HMF between 1975—2014, and the solar wind micro-turbulence levels between 1983—2013 were reexamined. An estimate of the peak sunspot number of solar cycle 25 was made and the question of whether we are heading towards a grand minimum much like the Maunder minimum was addressed. The cyclic magnetic activity of the Sun, manifested via sunspot activity, modulates the heliospheric environment, and the near-Earth space. It was, therefore, felt that it was imperative that one examine how the recent changes in solar activity have influenced the near-Earth space environment. We therefore examined the response of the Earth's ionosphere, for the period 1994—2014, to assess the possible impact of such a Maunder minimum on the Earth's ionospheric current system. It may be noted that a recent study, reported that the solar activity in Cycle 23 and that in the current Cycle 24 is close to the activity on the eve of Dalton and Gleissberg-Gnevyshev minima, and claimed that a Grand Minimum may be in progress. Also, a recent analysis of yearly mean sunspot-number data covering the period 1700 to 2012 showed that it is a low-dimensional deterministic chaotic system. Their model for sunspot numbers was able to successfully reconstruct the Maunder Minimum period and they were hence able to use it to make future predictions of sunspot numbers. Their study predicts that the level of future solar activity will be significantly decreased leading us to another prolonged sunspot minimum lasting several decades. The study on the other hand, using an entirely different approach, also suggests a long period of reduced solar activity.

Modelling studies of the solar dynamo invoking meridional flow variations over a solar cycle have successfully reproduced the characteristics of the unusual minimum of sunspot cycle 23 and have also shown that very deep minima are generally associated with weak polar fields. Attempts to model grand minima, seen in ~11000 years of past sunspot records using 14C data from tree rings have found that gradual changes in meridional flow velocity lead to a gradual onset of grand minima while abrupt changes lead to an abrupt onset. In addition, these authors have reported that one or two solar cycles before the onset of grand minima, the cycle period tends to become longer. It is noteworthy that surface meridional flows over cycle 23 have shown gradual variations from 8.5 ms-1 to 11.5 ms-1 and 13.0 ms-1 (Hathaway and Rightmire, 2010) and cycle 24 started ~1.3 years later than expected. There is also evidence of longer cycles before the start of the Maunder and Sporer minimum. It may also be noted that the current cycle 24 is already weak and the analysis suggests a similar weak cycle 25. All these indicate that a grand minimum akin to a Maunder like minimum may be in progress. See the papers by Janardhan, et al., (2015a) JGR, 120, 5306 -- 5317. 
Janardhan, et al., (2015b) Sun and Geosphere, 10, No. 2, 147 -- 156. 
Janardhan, et al., (2018) A & A 618, A148. 
and references therein.

The Figure above (upper panel) shows photospheric magnetic fields in the latitude range 45o-78o for the period Feb1975 - Dec2016 while the lower panel shows the decline in micro-turbulence levels from 327 MHz IPS observations in the period 1983 - 2016 and in the distance range 0.2 - 0.8 AU. The filled grey dots in both panels are actual measurements of magnetic fields (top) and solar wind micro-turbulence (bottom), while the filled blue circles are annual means with 1 σ error bars. The solid red line in both panels is a best fit to the declining trends for the annual means while the dotted red lines are extrapolations of the best fit until 2034 for the photospheric fields (top) and the IPS observations (bottom). The horizontal dashed line is drawn at 1.8 G and is explained in the text. The decline begs the question, indicated by a '?' in both panels, as to whether we are headed towards a Maunder type minimum beyond cycle 25. The vertical red dashed line in both panels indicates the expected minimum of the current solar cycle 25 in 2020.

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The Aditya Solarwind Particle Experiment (ASPEX)
onboard ISRO's ADITYA-L1 Mission
Launched: 02 Sept. 2023


Principal Investigator [PI: 2008 - 2020]:

Janardhan, P.



Summary:
  • The ADITYA-L1 mission is the first observatory class, 3-axis stabilized Indian spacecraft launched, on 02 September 2023, with the primary goal of studyingthe Sun, the solar wind and inner heliospheric magnetic fields from the L1 Lagrangian point of the Sun-Earth System. The ADITYA-L1 spacecraft carries onboard seven payloads, of which one, viz. the Aditya Solar Wind Particle Experiment (ASPEX), was designed, developed and built by the Physical Research Laboratory, Ahmedabad, Gujarat, India. The ASPEX payload aims to study solar and interplanetary processes that accelerate and energize Protons and Alpha particles in the inner heliosphere and importantly, their energies and directions of arrival both in and out of the ecliptic plane. In addition, it will observe these energetic particles over a very wide range of energies from 100 eV to 5 MeV. Since it is not possible to cover such a large energy range using a single instrument, ASPEX has been configured as two separate instruments viz. the Solar Wind Ion Spectrometer (SWIS), that measures the energy distribution and direction of arrival of Protons and Alpha particles in the energy range 100 eV to 20 keV, using two independent Top Hat Analyzers, THA1 and THA2. While THA1 has a 360o field-of-view (FOV) in the ecliptic plane, THA2 has a 360o FOV perpendicular to the ecliptic plane. The second instrument, called the Suprathermal and Energetic Particle Spectrometer (STEPS) measures Proton and Alpha particle energy spectra, at a cadence of 1 sec, in the energy range from 20 keV to 5 MeV, using custom-built solid state Si detectors. Measurements using a 3-axis stabilized spacecraft at the L1 Lagrangian point, covering such a large energy range and from multiple directions have not been attempted so far.  

    One of the most import 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 marker 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. 

ASPEX:

The ASPEX payload onboard the Aditya mission consists of two 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 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 solar wind, suprathermal population, shocks associated with CME and CIR, and solar energetic particles (SEPs). Among these, it is expected that 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.



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