Author name code: sridharan
ADS astronomy entries on 2022-09-14
author:"Sridharan, R."
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Title: Measurements of atmospheric turbulence parameters at Vainu
Bappu Observatory using short-exposure CCD images
Authors: Sreekanth, Reddy V.; Banyal, Ravinder Kumar; Sridharan, R.;
Selvaraj, Aishwarya
Bibcode: 2019RAA....19...74S
Altcode: 2019RAA....19...74V
We report atmospheric turbulence parameters, namely atmospheric seeing,
tilt-anisoplanatic angle (θ 0) and coherence time (τ
0), measured under various sky conditions, at Vainu Bappu
Observatory in Kavalur. Bursts of short exposure images of selected
stars were recorded with a high-speed, frame-transfer CCD mounted
on the Cassegrain focus of a newly commissioned 1.3 m telescope. The
estimated median seeing is ≈ 1.85″ at wavelength of ∼ 600 nm,
the image motion correlation between different pairs of stars is ∼
44% for θ 0 ≈ 36″ and mean τ 0 is ≈ 2.4
ms. This work was motivated by the design considerations and expected
performance of an adaptive optics system that is currently being
planned for the telescope.
Title: Wavelength calibration of a tunable spatial heterodyne
spectrometer
Authors: Nirmal, K.; Sridharan, R.; Sriram, S.; Ambily, Suresh;
Mathew, Joice; Sarpotdar, Mayuresh; Murthy, Jayant; Gopalakrishnan,
Binukumar; Safonova, Margarita
Bibcode: 2018SPIE10702E..4VN
Altcode:
Spatial Heterodyne Spectroscopy (SHS) is a relatively novel
interferometric technique similar to Fourier transform spectroscopy
and shares design similarities with a Michelson Interferometer. An
Imaging detector is used at the output of a SHS to record the spatially
heterodyned interference pattern. The spectrum of the source is obtained
by Fourier transforming the recorded interferogram. The merits of the
SHS -its design, including the lack of moving parts, compactness, high
throughput, high SNR and instantaneous spectral measurements - makes it
suitable for space as well as ground observatories. The small bandwidth
limitation of the SHS can be overcome by building it in tunable
configuration (Tunable Spatial Heterodyne Spectrometer(TSHS)). In
this paper, we describe the wavelength calibration of the tunable SHS
using a Halogen lamp and Andor monochromator setup. We found a relation
between the fringe frequency and the wavelength.
Title: Upper limit of helium-4 in the sunlit lunar exosphere during
magnetotail passage under low solar wind condition: Result from
CHACE aboard MIP in Chandrayaan-1
Authors: Das, Tirtha Pratim; Thampi, Smitha V.; Dhanya, M. B.;
Bhardwaj, Anil; Ahmed, S. M.; Sridharan, R.
Bibcode: 2017Icar..297..189D
Altcode:
The outcome of efforts to detect He-4 in the sunlit lunar exosphere as
evinced by the CHACE mass spectrometer aboard the Moon Impact Probe
in Chandrayaan-1 is reported. The in situ observations by CHACE were
carried out in the lunar dayside, covering a broad range of lunar
latitudes, when the Moon was on the verge of exiting the Earth's
magnetotail. A combination of daytime He depletion and decrease during
magnetotail passage of the Moon, along with low flux of alpha particles
in the solar wind at the time of CHACE observations present a case when
the He abundance in the Moon had hit one of its lowest values. CHACE,
thus had the opportunity to explore the lunar exosphere in an extreme
combination of the factors that control the lunar He abundance. Based
on the observations and instrument sensitivity, an upper limit of ∼
8.0 × 102 cm-3 for the surface density of lunar
He-4 in the sunlit hemisphere is proposed. This result is expected to
provide realistic constraints to the lunar He exosphere models under
similar extreme conditions.
Title: Further refinements to the spatiotemporal forecast model for
L-band scintillation based on comparison with C/NOFS observations
Authors: Yadav, Sneha; Sridharan, R.; Sunda, Surendra; Pant, Tarun K.
Bibcode: 2017JGRA..122.5643Y
Altcode:
The model-generated spatiotemporal maps to forecast the occurrence
pattern of plasma density irregularities in the nightside equatorial
F region that are responsible for the L-band scintillations have
been put to test, in both space and time, by comparing it with actual
observations by the Communication/Navigation Outage Forecasting System
satellite. The forecast model is based on (i) the temporal variations of
the density perturbations during daytime in the Nmax region
and (ii) the a priori knowledge of zonal velocity of the perturbations
in the postsunset hours. The present study not only substantiates the
hypothesis used for the generation of the scintillation forecast but
also suggests that the equatorial plasma bubbles remain tied-up with
the initial perturbations which trigger the primary Rayleigh-Taylor
instability. The outcome highlights the need to take into account
the altitudinal profile of the topside F region electron density as
it could modify the zonal extent of the plasma bubbles that support
the generation of the density irregularities and the consequent L-band
scintillations. The present study takes us one more step closer toward
the realization of an operational forecast system for satellite-based
navigation.
Title: Corrigendum to "Observation of Neon at mid and high
latitudes in the sunlit Lunar Exosphere: Results from CHACE
aboard MIP/Chandrayaan-1", Vol. 272 (2016), pp 206-211, doi
10.1016/j.icarus.2016.02.030
Authors: Pratim Das, Tirtha; Thampi, Smitha V.; Bhardwaj, Anil; Ahmed,
S. M.; Sridharan, R.
Bibcode: 2017Icar..284..489P
Altcode:
Our paper titled "Observation of Neon at mid and high latitudes in the
sunlit Lunar Exosphere: Results from CHACE aboard MIP/Chandrayaan-1"
(Icarus 272 (2016) 206-211) presents the results of the observations
on the distribution of neutral Neon in the mid and high lunar
latitudes by the CHACE instrument aboard Moon Impact Probe (MIP)
in Chandrayaan-1. The authors recently noticed two errors in the
representation of the results in two figures, although there is no
change in the reported number densities and the other interpretations
of the results.
Title: The impact of the 17 March 2015- St. Patrick's Day storm on
the evolutionary pattern of Equatorial Ionization Anomaly over the
Indian longitudes using high resolution spatio-temporal TEC maps -
New insights
Authors: Yadav, S.; Sunda, S.; Sridharan, R.
Bibcode: 2016AGUFMSA31A2389Y
Altcode:
The impact of the St. Patrick's Day storm (17 March 2015) on the major
equatorial electro-dynamical process viz., the Equatorial Ionization
Anomaly (EIA) has been assessed using 2D (lat. x long.) total electron
content (TEC) maps generated from the ground based SBAS (Satellite
Based Augmentation System) enabled receiver data. The various aspects
of EIA viz., i) evolution/devolution, ii) longitudinal structure,
and iii) its variability during different phases of a geomagnetic
storm have been brought out. These 2D TEC maps, which have a large
latitudinal (5̊ S-45° N) and longitudinal (55-110° E) coverage,
show the complete reversal in the longitudinal structure of EIA during
the recovery phase of the storm as compared to the quiet day. These
results have been explained in the light of the combined effects of the
storm associated processes viz., i) the penetration electric fields of
magnetosphere origin, ii) storm induced thermospheric winds, and iii)
activation of the consequent disturbance dynamo, effectively distorting
the longitudinal wave number 4 (WN4) structure of the EIA. It has been
shown unambiguously that even a separation of few degrees in longitude (
30̊) could experience significantly different forcings. The relevance
and the far reaching consequences of the study in the light of the
current trends and requirements for reliable satellite based navigation
are highlighted.
Title: Observation of Neon at mid and high latitudes in the sunlit
lunar exosphere: Results from CHACE aboard MIP/Chandrayaan-1
Authors: Das, Tirtha Pratim; Thampi, Smitha V.; Bhardwaj, Anil; Ahmed,
S. M.; Sridharan, R.
Bibcode: 2016Icar..272..206D
Altcode:
The distribution of neutral Neon at the mid and high latitudes in the
sunlit lunar exosphere observed by CHandra's Altitudinal Composition
Explorer (CHACE) aboard the Moon Impact Probe (MIP) of the Chandrayaan-1
is reported. The CHACE observation was made when Moon was in the Earth's
magnetotail. The upper limits of the surface number density are found
to vary from (7-22) × 103 cm-3 at the pole,
to (3-5) × 103 cm-3 in mid (50°S) latitudes
and to (0.5-1.1) × 103 cm-3 in lower (20°S)
latitudes. The surface number densities estimated at lower latitudes
from CHACE observations are consistent with the LADEE Neutral Mass
Spectrometer (NMS) observations.
Title: Impact of the perturbation zonal velocity variation on
the spatio/temporal occurrence pattern of L-band scintillation -
a case study
Authors: Bagiya, Mala; Pant, Tarun; Choudhary, Raj Kumar; Sunda,
Surendra; Sridharan, R.
Bibcode: 2016cosp...41E.123B
Altcode:
The earlier evolved method for the forecast of the spatio-temporal
variation of L-band scintillation based on the expected variation
of the perturbations [Bagiya et al., 2014; Sridharan et al., 2014],
under favorable ionospheric/thermospheric conditions, has been refined
by duly accounting for the local time variation of the zonal velocity
of the perturbations. The unique combination of the two geostationary
satellites (GSAT-8 and GSAT-10) over the Indian zone has been used
to estimate the typical LT dependence of the perturbation velocities
by closely following identifiable features in the scintillation
pattern. The measured velocities, that registered a steady decrease
with the progression of night, had been shown to significantly alter
the forecast pattern of the scintillations with respect to longitude
and local time. The significant improvement in the forecast pattern
has been demonstrated through a case study putting the forecast
method on a firmer footing. References: Bagiya, M. S., R. Sridharan,
S. Sunda, L. Jose, T. K. Pant and R. Chaudhary, Critical assessment of
the forecasting capability of L-band scintillations over the magnetic
equatorial region - Campaign results, J. Atmos. and Sol. Terr. Phys.,
110-111, 15-20 2014 Sridharan, R., Mala S. Bagiya, Surendra Sunda,
Rajkumar Choudhary, Tarun K. Pant, Lijo Jose, First results on
forecasting the spatial occurrence pattern of L-band scintillation and
its temporal evolution, J. Atmos. and Sol. Terr. Phys., 119, 53-62 2014
Title: Solar and Interplanetary Signatures of a Maunder-like Grand
Solar Minimum around the Corner - Implications to Near-Earth Space
Authors: Janardhan, P.; Bisoi, S. K.; Ananthakrishnan, S.; Sridharan,
R.; Jose, L.
Bibcode: 2015SunGe..10..147J
Altcode:
Our study of a steady decline of solar high-latitude (?45?) photospheric
magnetic fields for the past 20 years combined with the fact that cycle
24 is already past its peak, implies that high-latitude fields are
likely to decline until ∼2020, the expected minimum of cycle 24.Also,
interplanetary scintillation (IPS) observations, at 327 MHz, of solar
wind micro-turbulence levels during 1983-2013, have shown a steady
decline, in sync with the declining solar high-latitude fields. An
estimateof both the heliospheric magnetic field (HMF) strength in 2020
and the floor value of the HMF, using the correlation between the polar
field and the HMF at solar minimum, was found to be 4.0 (±0.6) nT and
3.2 (±0.4) nT, respectively. Using the estimated value of the HMF in
2020, the peak sunspot number for solar Cycle 25 was estimated to be 69
(±12). These results and the fact that solar magnetic fields continue
to decline at present, begs the question as to whether we are headed
towards a long period of very low sunspot activity similar to the well
known Maunder minimum between 1645-1715. An assessment of possible
impact of such a likely grand minimum on terrestrial ionospheric
current systems, based on the one-to-one correlation of sunspot number
and night time F-region maximum electron density, reveals that the
period post 2020 will be useful for undertaking systematic ground
based low-frequency radio astronomy observations, as the night time
ionospheric cutoff-frequency could be well below 10 MHz.
Title: Refinement of the background ionospheric conditions
and plausible explanation based on neutral dynamics for the
occurrence/non-occurrence of L-band scintillation patches against
forecast.
Authors: Sridharan, R.; Jose, Lijo; Bagiya, Mala S.; Sunda, Surendra;
Chaudhary, R. K.; Pant, Tarun K.
Bibcode: 2015JASTP.133...18S
Altcode:
The recently evolved L-band scintillation forecast mechanism based on
the characteristic features of the daytime F-region electron density
fluctuations and also on the basic ionospheric conditions had been
successful to a reasonable extent in forecasting the spatio-temporal
map of scintillation patches. There had been a few non-compliances
in the expected pattern within/outside the forecast windows. The
present paper attempts to address such non-compliances and offers
a plausible explanation based on neutral dynamics, especially the
local time variation of vertical winds over the magnetic equator,
while at the same time refining the earlier stipulated background
ionospheric conditions. With the above refinements, it is anticipated
that the forecast mechanism would become very robust. The present
results highlight the importance of the neutral dynamical parameters
and the urgent need to concentrate on the efforts to make systematic
measurements of the same in order to characterize their variability.
Title: Impact of the perturbation zonal velocity variation on the
spatio/temporal occurrence pattern of L band scintillation—A
case study
Authors: Bagiya, Mala S.; Sridharan, R.; Sunda, Surendra; Jose, Lijo;
Pant, Tarun K.; Choudhary, Rajkumar
Bibcode: 2015JGRA..120.5882B
Altcode:
The earlier evolved method for the forecast of the spatiotemporal
variation of L band scintillation based on the expected variation of the
perturbations, under favorable ionospheric/thermospheric conditions,
has been refined by duly accounting for the local time variation of
the zonal velocity of the perturbations. The unique combination of
the two geostationary satellites (GSAT-8 and GSAT-10) over the Indian
zone has been used to estimate the typical local time dependence of the
perturbation velocities by closely following identifiable features in
the scintillation pattern. The measured velocities, that registered
a steady decrease with the progression of night, had been shown to
significantly alter the forecast pattern of the scintillations with
respect to longitude and local time. The significant improvement in
the forecast pattern has been demonstrated through a case study putting
the forecast method on a firmer footing.
Title: A 20 year decline in solar photospheric magnetic fields:
Inner-heliospheric signatures and possible implications
Authors: Janardhan, P.; Bisoi, Susanta Kumar; Ananthakrishnan, S.;
Tokumaru, M.; Fujiki, K.; Jose, L.; Sridharan, R.
Bibcode: 2015JGRA..120.5306J
Altcode: 2015arXiv150603589J
We report observations of a steady 20 year decline of solar photospheric
fields at latitudes ≥45° starting from ∼1995. This prolonged and
continuing decline, combined with the fact that cycle 24 is already
past its peak, implies that magnetic fields are likely to continue
to decline until ∼2020, the expected minimum of the ongoing solar
cycle 24. In addition, interplanetary scintillation observations of
the inner heliosphere for the period 1983-2013 and in the distance
range 0.2-0.8 AU have also shown a similar and steady decline in solar
wind microturbulence levels, in sync with the declining photospheric
fields. Using the correlation between the polar field and heliospheric
magnetic field (HMF) at solar minimum, we have estimated the value of
the HMF in 2020 to be 3.9 (±0.6) nT and a floor value of the HMF of
∼3.2 (±0.4) nT. Given this floor value for the HMF, our analysis
suggests that the estimated peak sunspot number for solar cycle 25 is
likely to be 62 (±12).
Title: Corrigendum to "The Sunlit lunar atmosphere: A Comprehensive
study by CHACE on the Moon Impact Probe of Chandrayaan-1"
[Planet. Space Sci. 58 (2010) 1567-1577]
Authors: Sridharan, R.; Ahmed, S. M.; Das, Tirtha Pratim; Sreelatha,
P.; Pradeep Kumar, P.; Naik, Neha; Supriya, Gokulapati
Bibcode: 2015P&SS..111..167S
Altcode:
The authors regret, an inadvertent error that could creep in while
one attempts to estimate the number densities based on the reported
lunar atmospheric composition data from the CHACE measurements in
CHANDRAYAAN-1, the first Indian mission to the moon has been identified.
Title: The spatial distribution of molecular Hydrogen in the lunar
atmosphere-New results
Authors: Thampi, Smitha V.; Sridharan, R.; Das, Tirtha Pratim; Ahmed,
S. M.; Kamalakar, J. A.; Bhardwaj, Anil
Bibcode: 2015P&SS..106..142T
Altcode:
The measurements carried out by Chandra's Altitudinal Composition
Explorer (CHACE) onboard the Moon Impact Probe (MIP) of Chandrayaan
I mission is used to obtain information on the 2-D distribution of
the lunar atmospheric H2 by a novel approach that makes
use of the basic fact that the Moon has a Surface Boundary Exosphere
(SBE).These are the 'first' daytime in situ measurements of lunar
H2 covering the 20°S to 88°S latitude region centered
~14°E longitude. A critical examination of the observed spatial
features of the surface number density of H2 vis-à-vis
the surface topography delineated from the Lunar Laser Ranging
Instrument (LLRI) in the main orbiter Chandrayaan-I, indicates that
that lunar surface process may be important in introducing small scale
variations in the H2 number density. Another constituent
which exhibited spatial variation in the observed partial pressure is
40Ar and it was hypothesized that it is indicative of the
spatial heterogeneity in the radiogenic activity of the Lunar interior
(Sridharan et al., 2013a). The absolute number density at the surface
and also the latitude/altitude variation of the densities that are
reported for the first time, highlight the complexities of the sunlit
lunar atmosphere.
Title: First results on forecasting the spatial occurrence pattern
of L-band scintillation and its temporal evolution
Authors: Sridharan, R.; Bagiya, Mala S.; Sunda, Surendra; Choudhary,
Rajkumar; Pant, Tarun K.; Jose, Lijo
Bibcode: 2014JASTP.119...53S
Altcode:
After a fairly successful attempt to forecast the temporal
evolution of L-band scintillation over a given location, Trivandrum
(8.5° N, 76.91° E, dip latitude 0.5° N) (Sridharan et al.,
2012, J. Atmos. Sol. Terr. Phys. 80 230-238; Bagiya et al., 2014,
J. Atmos. Sol. Terr. Phys. 110-111, 15-22), an attempt has been made
here to generate the spatial-temporal maps of the occurrence pattern
of L-band scintillation over the Indian region. To start with, the
day time fluctuations in [f0F2]2 are
used to forecast the temporal evolution of perturbations during the
course of the night over Trivandrum. Similar to the earlier studies,
here too it is taken that the electron density perturbations retain
their characteristics throughout night and traverse with a uniform
velocity. This implies that when the integrity of wave train of electron
density perturbations is retained, any particular feature that passes
over Trivandrum would have crossed over another location west of
Trivandrum at an earlier time only dictated by the zonal velocity. With
this assumption it becomes feasible to generate the probable spatial and
temporal pattern of L-band scintillation. The consequences/limitations
of the above assumptions are discussed in detail. The observed
relation between the total duration of spread-F and the base height
of the F-region (h'F) at 1930 LT has been explained in terms of the
favourable background neutral atmospheric conditions. Following Bagiya
et al. (2013, J. Geophys. Res. 118, 1-8), the relation between h'F at
1930 LT and the probable maximum latitudinal extent of the spread-F
enables specification of the upper limit for the latitudes likely to be
affected by the scintillation. It is believed that the presented results
hold enough potential to generate the reliable L-band scintillation
forecast maps and provide the necessary alerts to the satellite based
air navigation users.
Title: Effects of prolonged southward interplanetary magnetic field
on low-latitude ionospheric electron density
Authors: Bagiya, Mala S.; Hazarika, Rumajyoti; Laskar, Fazlul I.;
Sunda, Surendra; Gurubaran, S.; Chakrabarty, D.; Bhuyan, P. K.;
Sridharan, R.; Veenadhari, B.; Pallamraju, D.
Bibcode: 2014JGRA..119.5764B
Altcode:
The present work describes the low-latitude ionospheric variability
during an unusually prolonged (~33 h) geomagnetically disturbed
condition that prevailed during 15-16 July 2012. The low-latitude
electron density in summer hemisphere, investigated using ground- and
satellite-based observations, responded to this by generating strong
negative ionospheric storm on 16 July. The maximum electron density
on 16 July over Indian low latitudes was reduced by more than 50%
compared to that on a geomagnetically quiet day (14 July 2012). In
contrast to the extreme reduction in total electron content (TEC)
in the Northern Hemisphere, TEC from a winter hemispheric station
revealed substantial (~23 total electron content unit, 1 TECU =
1016 el m-2) enhancements on the same day. This
contrasting hemispherical response in TEC is suggested to be due to the
combined effects of strong interhemispheric and solar-driven day-night
winds. Further, very weak equatorial electrojet (EEJ) strength on
16 July indicated that the westward electric field perturbations
in the low-latitude ionosphere were possibly due to the disturbance
dynamo effect associated with meridional circulation from polar to
equatorial latitudes. Interestingly, despite reduction in the integrated
EEJ strength on 15 July, the low-latitude electron density showed
substantial enhancement, highlighting the significant effect of the
positive ionospheric storm on the low-latitude ionosphere. The roles
of electrodynamical/neutral-dynamical and compositional disturbances
are discussed in view of these observations to understand low-latitude
ionospheric response when geomagnetic disturbance persists for longer
duration.
Title: Gravity wave signatures in the dip equatorial
ionosphere-thermosphere system during the annular solar eclipse of
15 January 2010
Authors: Manju, G.; Madhav Haridas, M. K.; Ramkumar, G.; Pant, Tarun
K.; Sridharan, R.; Sreelatha, P.
Bibcode: 2014JGRA..119.4929M
Altcode:
The present work pertains to the eclipse-induced gravity wave
modulations in the ionosphere-thermosphere region over Trivandrum
(8.5°N, 77°E, dip 2°N) during the annular solar eclipse of 15
January 2010. Electron density and neutral wind rocket payload
measured horizontal winds and electron densities at E region
altitudes, and ionosonde-derived foF1 and
foF2 parameters are used to analyze the
characteristics of the eclipse-induced gravity waves. The analysis
reveals an intensification of gravity waves with periods around 30-100
min during the peak phase of the eclipse. The vertical wavelength of
the prevalent wave is found to be around 2 km. The role of gravity
wave-induced winds in generating blanketing Es over the
equator is also examined.
Title: Critical assessment of the forecasting capability of L-band
scintillation over the magnetic equatorial region - Campaign results
Authors: Bagiya, Mala S.; Sridharan, R.; Sunda, Surendra; Jose, Lijo;
Pant, Tarun K.; Choudhary, Rajkumar
Bibcode: 2014JASTP.110...15B
Altcode:
A critical evaluation of the novel method suggested by Sridharan et
al. (2012) to forecast L-band scintillation is made using the results
from a special campaign conducted from Trivandrum (8.5°N, 76.91°E,
dip latitude 0.5°N), India, during April 2012. The significance of the
campaign lies in the fact that, (1) efforts are made to minimise the
uncertainties due to the movement of the satellite platform (TEC and
S4 observations from GNSS satellites) by choosing a recently launched
GSAT-8 geostationary satellite for ionospheric scintillation in L1
band, (2) unlike the previous study (Sridharan et al., 2012) wherein
the GPS derived TEC fluctuations were treated as representative of
ionospheric perturbations, in the present exercise, the fluctuating
component of the foF2 data from the ground based digital Ionosonde
have been taken as a measure of the perturbations and (3) though
both the GSAT and Ionosonde are stationary, still the ionospheric
regions they represent are physically separated and in order to
correlate the scintillation over the GSAT location to the forecast
perturbations over the ionosonde location, the required zonal velocity
of the perturbations/irregularities is estimated using GSAT and GPS
scintillation data during one of the close-by GPS passes and this is
taken to represent the particular solar epoch and season. Following
the earlier method of Sridharan et al. (2012) the relative amplitudes
and phase integrity of the perturbations have been maintained and
extended throughout night. By adopting the above changes, it has
been noted that the forecasting capability of L band scintillation
has remarkably improved vindicating the role of perturbations in the
evolution of the scintillation, thus making it more useful for practical
applications. The nonoccurrence of scintillation on occasions in the
prescribed time windows has also been understood based on the changes
in the background conditions. A threshold upward velocity for the
evening F-region as early as 1730-1830 LT, has been worked out to be 5
ms-1 for the ESF to get triggered and for its sustenance,
a plasma scale length of <20 km along with a critical base height
(h'F) of 225 km have been evaluated as necessary background conditions.
Title: Ionospheric impact on the geopotential height profile of the
temperature by balloon-borne GPS radiosondes?
Authors: Choudhary, R. K.; Rajeev, K.; Krishna Moorthy, K.;
Sridharan, R.
Bibcode: 2013GeoRL..40..239C
Altcode:
<p label="1">We have studied the potential inaccuracies in
the geopotential height, its manifestation in the altitude profile
of temperature measured by balloon-borne GPS radiosonde (GPS sonde),
and its association with the variations in ionospheric plasma density,
particularly in the Indian low latitude region. Using results from a GPS
sonde inter-comparison campaign, conducted at Gadanki (13.5°N, 79.2°E,
6.4°N geomagnetic latitude) at different local times of a day, we show
that differences in temperature at different geopotential heights vary
with respect to the local time of the day in October 2010. However,
differences in temperature become negligible and within the limits of
experimental uncertainty if the comparison is made with respect to
the time of measurements during the ascent rather than with respect
to the GPS-derived geopotential height. We show that the origin of
the mismatch rests with the inaccurate estimation of geopotential
height by the two GPS sondes and not due to any inaccuracy in their
measurements itself. Differences in the geopotential height estimates
are larger during late afternoon, resulting in larger temperature
differences between the sondes, and least during late evening/early
morning hours. The uncertainty in the geopotential height estimation
remains well synchronized with the magnitude and variations in the
total electron content (TEC) of the ionosphere. This study exhorts us
to account for the local ionospheric variations while extracting the
geopotential height profiles of temperature from GPS sonde measurements,
especially in the low and equatorial regions where there are large
spatial and temporal variations in the ionospheric densities.
Title: Spatial heterogeneity in the radiogenic activity of the lunar
interior: Inferences from CHACE and LLRI on Chandrayaan-1
Authors: Sridharan, R.; Das, Tirtha Pratim; Ahmed, S. M.; Supriya,
Gogulapati; Bhardwaj, Anil; Kamalakar, J. A.
Bibcode: 2013AdSpR..51..168S
Altcode: 2013arXiv1302.5772S
In the past, clues on the potential radiogenic activity of the
lunar interior have been obtained from the isotopic composition of
noble gases like Argon. Excess Argon (40) relative to Argon (36),
as compared to the solar wind composition, is generally ascribed to
the radiogenic activity of the lunar interior. Almost all the previous
estimates were based on, 'on-the-spot' measurements from the landing
sites. Relative concentration of the isotopes of 40Ar
and 36Ar along a meridian by the Chandra's Altitudinal
Composition Explorer (CHACE) experiment, on the Moon Impact Probe (MIP)
of India's first mission to Moon, has independently yielded clues on the
possible spatial heterogeneity in the radiogenic activity of the lunar
interior in addition to providing indicative 'antiquity' of the lunar
surface along the ground track over the near side of the moon. These
results are shown to broadly corroborate the independent topography
measurements by the Lunar Laser Ranging Instrument (LLRI) in the main
orbiter Chandrayaan-1. The unique combination of these experiments
provided high spatial resolution data while indicating the possible
close linkages between the lunar interior and the lunar ambience.
Title: Pre-assessment of the "strength" and "latitudinal extent"
of L-band scintillation: A case study
Authors: Bagiya, Mala S.; Sridharan, R.; Sunda, S.
Bibcode: 2013JGRA..118..488B
Altcode:
Forecasting the strength of nighttime L-band scintillation well in
advance of their actual occurrence by making use of the background
conditions has been attempted over the Indian zone with an eye
on operational forecasting capability. It has been shown that the
base height of the equatorial F-region as early as 1930 h LT shows
a linear relation to the maximum altitudinal extent of medium-scale
irregularities that are responsible for the L-band scintillation during
the course of the night and these in turn get mapped to latitudes away
from the equator defining the latitude range that gets affected by
their presence. On the other hand, the average value of the vertical
total electron content between 1830 h LT and 1930 h LT on a given
day is shown to control the maximum strength of the scintillation
represented by S4 index on the day. The above analysis,
although it has been carried out for the equatorial station Trivandrum,
is applicable to other latitudes as well. Having demonstrated the
control of the background ionospheric-thermospheric conditions, it has
been shown that the averaged vertical total electron content around the
prereversal enhancement (1830 h LT to 1930 h LT) along with the actual
base height information at 1930 h LT over the magnetic equator would
enable one to forecast the maximum possible S4 in a given
latitudinal region as early as 1930 h and issue necessary forewarning
alerts wherever needed.
Title: Moving towards operational forecasting of occurrence of
L-band scintillations based on background ionospheric variability:
a case study using GSAT platform
Authors: Bagiya, M. S.; Sridharan, R.; Sunda, S.
Bibcode: 2012AGUFMSA13A2159B
Altcode:
An operational forecasting of L band scintillation is very vital
for real time satellite based communication and navigation. A novel
method based on GPS-TEC (GTEC) data several hours before the actual
event has been worked out (Sridharan et al., 2012) and two of the many
fundamental questions viz., 'when' and for 'how long' the scintillation
patches are likely to be present have been answered conclusively. The
close linkage between the perturbation features and the evolutionary
pattern of the GPS L-band scintillation enables us to forecast 'when'
and for 'how long' the L band scintillations could occur, in addition
to their occurrence pattern. But as GPS is the moving system, a time
lag is observed in forecasted evolutionary pattern of scintillation
and actual observed scintillation which was expected also. To simplify
this problem, the method has been updated with GSAT L1 - scintillation
and ionosonde fof2 observations. Both these instruments are referred
to fixed locations, but the only care has to be taken is that of the
physical separation of the ionospheric regions referred by them. The
perturbation features in terms of dfof2 now are used to forecast the
evolutionary pattern of the scintillation couple of hours before the
actual event. An excellent agreement between forecasted evolutionary
pattern of scintillation and actual observed one take us one more step
closer towards operational forecasting of L band scintillations. There
are some occasions when scintillation was forecasted but it did not
appear which attributed to the background conditions during that period.
Title: Imaging the Circumstellar Environment of ω CMa with the VLTI
Authors: Sridharan, R.; Štefl, S.; Rivinius, Th.
Bibcode: 2012ASPC..464..231S
Altcode:
Interferometric imaging in the near-infrared allows us to probe the
circumstellar environments of nearby bright stars with milli-arc-seconds
resolution. In this paper, we present the first synthesized spectral
line (Brγ) image of the Be star ω(28) CMa, obtained with the
AMBER/VLTI high resolution data. Images were synthesized in 100
spectral channels covering the Brγ line using the squared visibilities
and the closure phases. These images were further refined using the
self-calibration procedure proposed by Millour et al. (2011). We present
both the continuum image and the image obtained within the Brγ line.
Title: Interaction of Solar Wind with Moon: AN Overview on the
Results from the SARA Experiment Aboard Chandrayaan-1
Authors: Bhardwaj, Anil; Dhanya, M. B.; Sridharan, R.; Barabash, Stas;
Yoshifumi, Futaana; Wieser, Martin; Holmström, Mats; Lue, Charles;
Wurz, Peter; Schaufelberger, Audrey; Asamura, Kazushi
Bibcode: 2012aogs...30...35B
Altcode: 2012agos...30...35B
The results from the Sub-keV Atom Reflecting Analyzer (SARA) experiment
onboard Chandrayaan-1 have revealed several hitherto unknown and
interesting aspects about the interaction of solar wind with the
Moon. The SARA experiment had two sensors — CENA and SWIM. The
Chandrayaan-1 energetic neutrals analyzer (CENA), detected energetic
neutral atoms (ENAs), and the Solar Wind Monitor (SWIM) measured ions
of solar wind origin. In this review, we summarize the observations
made by the SARA experiment, which are: (1) substantial ( 20%) and
sustained backscattering of solar wind protons from lunar surface as
energetic neutral hydrogen,1 (2) minimagnetosphere around
magnetic anomalies on Moon using the backscattered ENAs,2
(3) reflection of solar wind protons from the Moon surface,3
(4) huge ( 50%) deflection of solar wind protons over strong magnetic
anomalies,4 and (5) presence of protons in the near-lunar
plasma wake.5 These results have implications on the lunar
plasma environment, implantation of solar wind hydrogen on lunar
surface, and behavior of small scale magnetic anomalies on planetary
bodies. The SARA observations suggest that similar processes may happen
on other airless bodies covered with regolith in the solar system as
well as in extra-solar system. This paper presents a review of the
results obtained from the SARA observation.
Title: Rocket borne in-situ Electron density and Neutral Wind
measurements in the equatorial ionosphere—Results from the January
2010 annular solar eclipse campaign from India
Authors: Manju, G.; Sridharan, R.; Ravindran, Sudha; Madhav Haridas,
M. K.; Pant, Tarun K.; Sreelatha, P.; Mohan Kumar, S. V.
Bibcode: 2012JASTP..86...56M
Altcode:
One of the recent developments in the form of a novel probe for
simultaneous in-situ measurements of Electron density and Neutral Wind
(ENWi) parameters was extensively made use of during the annular solar
eclipse that passed right over the magnetic dip equatorial station,
Trivandrum in India. The response of the equatorial ionosphere to the
annular solar eclipse of January 2010 is investigated using the data
from ENWi and other ground based instruments like digital Ionosonde,
HF radar, magnetometer etc. Significant effects on the neutral wind
components and electron densities during the peak phase of the eclipse
are brought out by the ENWi data. A clear reversal in the zonal neutral
wind is observed during the rocket flights at the peak phase of the
eclipse as well as in the flight conducted 3 h later. The ground
magnetic field measurements revealed the reversal of electrojet
during the eclipse alongwith blanketing Es layers vindicating the
earlier hypothesis that blanketing Es layers manifest over magnetic
equatorial regions only during periods of either weak or reversed
electrojet (counter electrojet). The HF radar results independently
substantiate the in situ measurements with regard to the location of the
irregularities and their Doppler velocities. The regional scale changes
that have been brought about by the Solar eclipse both in the neutral
and ionized medium and the consequent electrodynamics are highlighted.
Title: Study of Lunar Atmosphere by CHACE aboard Chandrayaan-1 and
a Follow-up by the CHACE-2 onboard Chandrayaan-2
Authors: Pratim Das, Tirtha; Bhardwaj, Anil; Mohankumar, Sv; Chace-2
Team; Ahmed, Sm; Sridharan, R.
Bibcode: 2012cosp...39..407P
Altcode: 2012cosp.meet..407P
A quadrupole mass spectrometer-based payload CHACE (CHandra's
Altitudinal Composition Explorer) flown in the Moon Impact Probe
(MIP) of Chandrayaan-1 in 2008 provided the first measurement on the
neutral composition in the mass range of 1 to 100 amu from the sunlit
lunar atmosphere. The in-situ measurements were made continuously
as the MIP descended from ~100 km to the lunar surface, providing
very valuable 44 minutes of data around 14 degree E lunar meridian,
with a broad latitudinal (from 45 degree N to 90 degree S) coverage
and good spatial (~0.1 degree latitudinal) and ~ 250 m altitudinal
resolution. The important results of the CHACE include : i) measurement
of the total pressure of the sunlit lunar atmosphere of around 5x10
^{-7} Torr, which is around two to three orders of magnitude more than
that predicted based on the earlier nightside measurements by the
Apollo missions; ii) direct detection of water vapour in the sunlit
lunar atmosphere; iii) detection of carbon dioxide as one of the major
constituent in the sunlit lunar atmosphere; iv) presence of heavier
(mass >60 amu) constituents in trace amount; and v) variability
in the radiogenic activity of the lunar interior observed through
the latitudinal variation of the 40Ar/36Ar ratio. Since the MIP was
a short duration mission, the CHACE observations were limited to only
44 minutes. The break-through results obtained by the CHACE have also
raised few questions that call for a detailed orbiter-based observation
using a similar instrument. Hence, the CHACE-2 (CHandra's Atmospheric
Composition Explorer-2) is selected to fly on Chandrayaan-2, which is an
orbiter in ~200 km circular polar orbit. CHACE-2 will explore the lunar
neutral atmosphere in the range of 1 to 300 amu and will have multiple
operating modes to optimise the performance in different conditions
in the dayside and nightside of the lunar atmosphere. The detailed
observations by CHACE-2 are expected to unravel many aspects of the
lunar neutral density and composition. *CHACE-2 Team: P. Sreelatha,
P. Pradeepkumar, B. Sunder, Amarnath Nandi, Neha Naik, G. Supriya,
R. Satheesh Thampi, Vipin K. Yadav, M. B. Dhanya, G. P. Padmanabhan,
N. Raghu, and A.V. Aliyas
Title: On the multifaceted role played by the Neutral Wind in the
Equatorial Electrodynamics during Magnetically Disturbed times
Authors: Choudhary, Raj Kumar; St-Maurice, Jean-Pierre; Ambili, M. K.;
Sridharan, R.
Bibcode: 2012cosp...39..337C
Altcode: 2012cosp.meet..337C
A disturbance dynamo was observed in the magnetic equatorial region
of the Asian subcontinent on May 31, 2005, one day after an intense
magnetic storm of relatively short duration. At first sight, there
was nothing unusual about the event on that particular day as the
ground-based magnetograms revealed very strong reduction in the normal
mid-day electrojet with the essential disappearance of the Equatorial
Ionization Anomaly, as revealed by the Total Electron Content (TEC)
data. A closer look at the ionosonde data however revealed that,
contrary to conventional thinking, the zonal electric field had remained
eastward for most of the day in spite of the evidence for westward
currents. There was, in addition, a clear mid afternoon amplification
of the eastward zonal field followed by a westward turning field after
1600 IST. This electric field oscillation triggered a depletion in the
equatorial TEC, but without an accompanying modulation of the TEC in
the anomaly crest region. Interestingly, the equatorial TEC returned to
its pre-depletion value when the plasma came through the downward phase
of its oscillation. We argue that these observations can be explained
by two important disturbed dynamo neutral wind properties that have
not been emphasized before, namely,(1) the neutral wind circulation
cell due to Joule heating did not reach the magnetic equator but came
close enough to it to essentially kill the equatorial fountain effect
while maintaining an eastward zonal field; (2) a weakening of the high
latitude-driven Hadley cell circulation in the mid-afternoon hours could
have easily produced the observed equatorial F region zonal electric
field oscillation creating a situation that was highly reminiscent of
the equatorial `Pre-Reversal-Enhancement' signature normally observed
near the terminator instead of the mid-afternoon
Title: Investigation of Lunar plasma wake with the SARA experiment
on Chandrayaan-1
Authors: Dhanya, M. B.; Barabash, Stas; Wieser, Martin; Futaana,
Yoshifumi; Holmström, Mats; Bhardwaj, Anil; Wurz, Peter; Satheesh
Thampi, R.; Alok, Abhinaw; Sridharan, R.
Bibcode: 2012cosp...39..451D
Altcode: 2012cosp.meet..451D
The analysis of the observations of the protons in the near-lunar plasma
wake by the SWIM sensor of the SARA experiment on Chandrayaan-1 mission
had revealed the following: 1) Dawn-dusk asymmetry in the differential
flux of protons in the wake, with the dusk flux 30% lower than the dawn
flux. The asymmetry possibly linked to the plasma expansion process
(parallel to IMF) with the IMF oriented dominantly along the Parker
spiral. The asymmetry is more pronounced near the wake edges. 2)
Comparison of the density of the protons computed as a function of
distance along the direction of IMF with the 1-D plasma expansion
model supports the observation of protons up to distances of ~500 km
from wake edge where as in the deeper wake, processes other than plasma
expansion dominates. 3) The velocity distribution of the protons in the
wake also supports the above inference. Since the protons originated
by different entry mechanisms differ in their energies, the existence
of protons of different population in the near lunar wake alters the
wake plasma environment.
Title: Forecasting L band scintillations 'when' and for 'how long':
a reality?
Authors: Sridharan, R.; Sunda, Surendra; Bagiya, Mala
Bibcode: 2012cosp...39.1873S
Altcode: 2012cosp.meet.1873S
No abstract at ADS
Title: Towards forecasting the "strength" of L band scintillation
based on background TEC variability: a case study
Authors: Bagiya, Mala; Sunda, Surendra; Sridharan, R.
Bibcode: 2012cosp...39...85B
Altcode: 2012cosp.meet...85B
The L band scintillation is one of the manifestations of ESF
irregularities which typically span over several orders of
scale sizes. The importance of operational forecasting of L band
scintillation occurrence lies in its affects on the satellite to
ground communication and vice versa. In addition to occurrence, the
other fundamental question is the severity of scintillation i.e. its
strength, especially in the light of increasing solar activity. It
is well known that the ionosphere scintillations very often become
so strong that the ground receivers lose their lock resulting in
disruption in the communication link. After providing a satisfactory
answer to the occurrence pattern of the scintillations, the present
study attempts to elucidate on the strength of the scintillations based
on the background ionospheric/thermospheric conditions represented by
GPS-TEC and the F10.7 through case studies.
Title: A novel method based on GPS TEC to forecast L band
scintillations over the equatorial region through a case study
Authors: Sridharan, R.; Bagiya, Mala S.; Sunda, Surendra
Bibcode: 2012JASTP..80..230S
Altcode:
Forecasting the occurrence of L-band scintillations has been a
challenging task and, this challenge has been tackled by evolving a
simple method using GPS-TEC data. For given background conditions, it
has been shown that the fluctuations in the GPS-TEC truly represent the
characteristic features of the perturbations that are responsible for
the initiation of the plasma instability that finally culminates in to
the observed irregularities. The close linkage between the perturbation
features and the evolutionary pattern of the scintillations enable us to
forecast ‘when’, and ‘for how long’ the L-band scintillations
would occur, in addition to their ‘occurrence pattern’. The first
of their kind of results take us a step closer towards operational
forecasting of L-band scintillations for real time navigational
purposes.
Title: Energetic Neutral Atom Imaging of the Lunar Surface
Authors: Vorburger, A.; Wurz, P.; Barabash, S.; Wieser, M.; Futaana,
Y.; Holmström, M.; Bhardwaj, A.; Dhanya, M. B.; Sridharan, R.;
Asamura, K.
Bibcode: 2012EGUGA..1412220V
Altcode:
Since the Moon is not shielded by a global magnetic field or by an
atmosphere, solar wind plasma will impinge onto the lunar surface
almost unhindered. Until recently it was assumed that almost all of
the impinging solar wind ions are absorbed by the surface. However,
McComas et al., GRL 2009, Wieser et al., PSS 2009, and Rodríguez
et al., PSS 2012, presented observations made by the Interstellar
Boundary Explorer (IBEX) and by Chandrayaan-1 that showed that up to
20% of the impinging solar wind ions are backscattered as energetic
neutral atoms (ENAs). Since the energy of these backscattered ENAs
is sufficiently high, the ENAs leave the lunar surface on ballistic
trajectories and their trajectory can be reconstructed from the point of
measurement to its origin on the lunar surface. This allows recording
maps of ENAs backscattered from the lunar surface. For a quantitative
analysis of the backscattered ENAs the angular scattering function
has to be known. Schaufelberger et al., GRL 2011, recently published
a mathematical description of the angular scattering function of the
ENAs leaving the lunar surface based on CENA (Chandrayaan-1 Energetic
Neutral Analyzer) measurements. This scattering function allows us to
map the particles, which are measured at an altitude of 100 km and 200
km, respectively, back onto the lunar surface. The 7 angular sectors
of CENA and Chandrayaan-1's polar orbit give us a good coverage of the
lunar surface. We analysed all available CENA measurements and produced
a global map of the neutral hydrogen atoms with energies in the range
(19 - 740 eV) that are coming off the lunar surface.
Title: Effects of a magnetic cloud simultaneously observed on the
equatorial ionosphere in midday and midnight sectors
Authors: Rastogi, R. G.; Chandra, H.; Das, A. C.; Sridharan, R.;
Reinisch, B. W.; Ahmed, Khurshid
Bibcode: 2012EP&S...64..353R
Altcode:
An impact of a magnetic cloud on the Earth's magnetosphere occurred
at 1636 UT on 25 June, 1998, associated with a sudden increase of
the solar wind density and velocity, as well as a sudden increase
of the zenithal component of the interplanetary magnetic field (IMF-
B z). Following the impact of the magnetic cloud, IMF- B
z was northward (10 nT) and remained steadily strong (about
15 nT) for the next six hours. IMF- B z turned southward at
2330 UT on 25 June, 1998, and remained strongly southward (-15 nT) for
the next four hours. During the positive phase of IMF- B z,
both the Auroral index and ring current index SYM/H remained steadily
low indicating complete isolation of the Earth's magnetosphere from the
solar wind and no significant changes were observed in the equatorial
ionosphere. After the southward turning, the steady southward IMF- B
z permitted solar wind energy to penetrate the magnetosphere
and caused the generation of a magnetic storm associated with strong
auroral electrojet activity ( A E index). Strong southward
IMF- B z corresponds to the dawn-dusk interplanetary electric
field (eastward on the dayside and westward on the night side). The
ionograms at Jicamarca (night side) showed strong spread- F and at
Thumba (dayside) showed an absence of equatorial type of sporadic-
E, indicating a dusk-to-dawn electric field. Thus, the observations
point to an electric field opposite in direction to that expected
by the prompt penetration of the interplanetary electric field. An
abnormally-large Auroral index ( A E) associated with the
start of the storm suggests that the cause of the equatorial electric
field changes is due to the disturbance dynamo effect.
Title: Direct Observations of Magnetic Anomalies on the Lunar Surface
under Varying Solar Wind Conditions
Authors: Vorburger, A.; Wurz, P.; Barabash, S.; Wieser, M.; Futaana,
Y.; Holmström, M.; Bhardwaj, A.; Dhanya, M. B.; Sridharan, R.;
Asamura, K.
Bibcode: 2012EGUGA..1412169V
Altcode:
In contrast to Earth, the Moon does not have a global dipolar
magnetic field. Since the first lunar landing with Apollo 11,
we know, though, that localised magnetic fields exist on the lunar
surface. Measurements conducted by the Lunar Prospector magnetometer
and electron reflectometer suggested that these localised magnetic
fields are able to deflect the impinging solar wind in favourable cases
(Lin et al., Science 1998). Magnetohydrodynamic simulations support the
implication that mini-magnetospheres are formed above the locations of
strong localised magnetic fields and can hold off the impinging solar
wind (Harnett and Winglee, JGR 2002). Analysis of magnetic field data
from Lunar Prospector of the Reiner Gamma anomaly region showed that the
distortion of the magnetic field of this anomaly strongly depends on the
impinging solar wind parameters, which was interpreted that the size and
shape of the mini-magnetosphere changed with the solar wind parametes
(Kurata et al., GRL 2005). Wieser et al., GRL 2010 showed that SARA,
the Sub-KeV Atom Analyzer on board Chandrayaan-1, is able to detect an
ENA image of the mini-magnetosphere in the measured energetic neutral
atom flux. Here we analysed all orbits where CENA, the Chandrayaan-1
Energetic Neutral Analyzer, recorded data when a magnetic anomaly was in
CENA's field-of-view. Our goal was to determine if 1) a signature of the
magnetic anomaly is always visible in the ENA signal and if 2) there is
a correlation between the solar wind dynamic pressure, the solar wind
magnetic field, the local magnetic field strength and the reduction in
the reflected ENA flux. Our results show that for the simplest case,
i.e., the Gerasimovich anomaly, there is indeed a clear correlation
between the shielding efficiency, the magnetic field strength and the
solar wind dynamic pressure. For the other observed magnetic anomalies,
for which the magnetic fields are not only weaker but also spatially
more variable than that of the Gerasimovich anomaly, only in about
half of the cases such a correlation was found. We therefore conclude
that the magnetic anomaly interaction is in general quite complex and
that data with higher spatial resolution and more detailed modelling
is required to understand this process better.
Title: Angular Scattering of Energetic Neutral Hydrogen Atoms off
the Lunar Surface
Authors: Vorburger, A.; Wurz, P.; Barabash, S.; Wieser, M.; Futaana,
Y.; Holmström, M.; Bhardwaj, A.; Dhanya, M. B.; Sridharan, R.;
Asamura, K.
Bibcode: 2012EGUGA..1412204V
Altcode:
Planetary surfaces, such as the lunar surface, which are not shielded
by an atmosphere or a global magnetic field, are constantly bombarded by
the surrounding plasma, e.g. by solar wind ions. When these ions hit the
lunar surface, a large percentage is backscattered as energetic neutral
atoms (ENAs). Measurements conducted by IBEX and Chandrayaan-1 showed
that the backscatter fraction lies in the range 10-20% of the impinging
solar wind ions (McComas et al., GRL 2009, Wieser et al., PSS 2009,
and Rodríguez et al., PSS 2012). The energy of the backscattered ENAs
is sufficiently high so that the individual trajectories can be mapped
back onto the lunar surface in a straight forward manner and imaging
of the surface using the ENAs is possible. To guarantee a quantitative
imaging of the lunar surface, the angular distribution of the released
ENAs has to be taken into account, though. It was formerly assumed
that most of the impinging solar wind ions are absorbed by the lunar
surface. Before CENA, no analysis of in-flight measurements concerning
the angular scattering profile was available. We therefore analysed all
available measurements conducted by CENA, the Chandrayaan-1 Energetic
Neutral Analyzer, to derive the scattering profile of low energetic
hydrogen atoms coming off the lunar surface. Our analysis shows that the
angular scattering profile exhibits 4 distinct features for increasing
solar zenith angle: 1) amplitude decrease, 2) increased azimuthal
structure, 3) bigger ratio of sunward versus anti-sunward flux and 4)
shallower polar scattering. We derived four mathematical functions,
each of which describes one feature, and the product of which describes
the ENA angular distribution function. The directional ENA flux can
then be described as the product of the impinging solar wind flux, the
reflection ratio at the sub solar point and the ENA angular distribution
function. This function has been derived from lunar backscatter data,
but it should be applicable to mapping of any planetary surface that
is not protected by an atmosphere or a global magnetic field.
Title: An overview of CAWSES-India program with emphasis to equatorial
atmospheric coupling processes
Authors: Rao, P. B.; Beig, G.; Dabas, R. S.; Ramkumar, Geetha;
Gurubaran, S.; Rao, Kusuma G.; Manoharan, P. K.; Patra, A. K.;
Ravindran, Sudha; Pant, Tarun K.; Venkat Ratnam, M.; Chakravarty,
S. C.; Sridharan, R.
Bibcode: 2012JASTP..75...98R
Altcode:
This paper presents an overview of CAWSES—India program, highlighting
some of the Indian coordinated efforts to understand the Sun-Earth
system as a whole, with special emphasis to the equatorial and
low latitude phenomena. Two multi-institutional, multi-instrument
campaigns, one under 'Space Weather: Science and Applications' and
the other emphasizing tides under 'Atmospheric Coupling Processes'
were conducted during February-April 2006. The highlights of the
campaigns include improved prediction of equatorial spread-F (ESF)
based on a 'new factor' combining the strength and asymmetry of
the equatorial ionization anomaly (EIA); simultaneous detection of
quasi-2-day wave at both E and F region heights; tomographic image
showing a Traveling Ionospheric Disturbance (TID) in association with
a counter electrojet (CEJ) event—all highlighting the neutral and
electrodynamical coupling of the equatorial ionosphere-thermosphere
regions; an unusual lowering of mesospheric temperature during a
moderate geomagnetic storm—highlighting a new space weather effect;
diurnal tide showing a peak amplitude of 35 m/s at 45 km in zonal
wind and a significantly lower amplitude (∼15 m/s) in the meridional
wind and also evidence of convective activity in the lower atmosphere
influencing the tidal variability in the Mesosphere-Lower Thermosphere
(MLT) region highlighting the vertical coupling of the atmospheric
regions. The development of models for equatorial and low latitude
ionosphere has been among the prime activities taken up under 'Space
Climate'. Two models have been developed towards this end: one based
on Second Degree (SD) relationships of F region parameters to the mean
sun spot number and the other a Multiple Regression Analysis (MRA)
based model involving expressions relating F region parameters to that
representing solar and geomagnetic activities. The two models, intended
for long and short term predictions, have been found to represent
the equatorial and low latitude ionosphere over Indian longitudes
better than the commonly used International Reference Ionosphere
(IRI). Under the theme of 'Solar Influence on Atmospheric Climate',
a study has been made on the effects of solar variability on middle
atmosphere using satellite data and model simulations, highlighting
the significant differences between them.
Title: A Novel probe for in-situ Electron density and Neutral Wind
(ENWi) measurements in the near Earth space
Authors: Manju, G.; Sridharan, R.; Sreelatha, P.; Ravindran, Sudha;
Madhav Haridas, M. K.; Pant, Tarun K.; Pradeep Kumar, P.; Satheesh
Thampi, R.; Naik, Neha; Mridula, N.; Jose, Lijo; Sumod, S. G.
Bibcode: 2012JASTP..74...81M
Altcode:
A novel method for the in-situ measurement of thermospheric and
ionospheric parameters has successfully been tried out by means of
a new probe ENWi. The probe has been designed for neutral wind and
electron density measurements up to ∼115 km. ENWi was successfully
flight tested on sounding rockets from Thumba, India during the recent
annular solar eclipse (January 2010) that passed right over the rocket
range. The present paper deals with the basic concept that has been
successfully put to test for neutral wind measurements in a critical
height region, thus highlighting its potential to fill up a void in
the Aeronomic studies.
Title: Scattering function for energetic neutral hydrogen atoms off
the lunar surface
Authors: Schaufelberger, A.; Wurz, P.; Barabash, S.; Wieser, M.;
Futaana, Y.; Holmström, M.; Bhardwaj, A.; Dhanya, M. B.; Sridharan,
R.; Asamura, K.
Bibcode: 2011GeoRL..3822202S
Altcode:
The Sub-keV Atom Reflecting Analyzer instrument on board the lunar
orbiter Chandrayaan-1 provided a large number of measurements of
lunar energetic neutral atoms (ENAs). These ENAs were formerly solar
wind ions, which were neutralized and backscattered from the lunar
surface. The angles under which the ENAs are scattered strongly
depend on the solar wind ions' incidence angle, which corresponds
to the solar zenith angle (SZA). Our large dataset provides us
with a complete coverage of the SZA and almost complete coverage of
the scattering angles. When combining all available measurements,
four distinct features are discernible with SZA increase: amplitude
decrease, less azimuthal uniformity, bigger ratio of sunward versus
anti-sunward flux and shallower scattering. We analyzed more than
290‧000 measurements and derived a mathematical description of the
features and their dependencies on the SZA.
Title: Shielding efficiency of lunar magnetic anomalies: Observations
from SARA on board Chandrayaan-1
Authors: Schaufelberger, A.; Wurz, P.; Barabash, S.; Wieser, M.;
Futaana, Y.; Holmström, M.; Bhardwaj, A.; Dhanya, M. B.; Sridharan,
R.; Asamura, K.
Bibcode: 2011epsc.conf..959S
Altcode: 2011DPS....43..959S
SARA produced the first image of a mini-magnetosphere above a lunar
magnetic anomaly using energetic neutral atoms. It was shown that this
magnetosphere is able to reduce the impinging solar wind flux onto the
surface by more than 50%. Having analyzed all available observations
by SARA, we report on the shielding efficiency of these anomalies by
presenting correlations between solar wind flux, local magnetic field
strength and reduction in reflected flux.
Title: Scattering function for lunar ENAs: Observations from SARA
on board Chandrayaan-1
Authors: Schaufelberger, A.; Wurz, P.; Barabash, S.; Wieser, M.;
Futaana, Y.; Holmström, M.; Bhardwaj, A.; Dhanya, M. B.; Sridharan,
R.; Asamura, K.
Bibcode: 2011epsc.conf..954S
Altcode: 2011DPS....43..954S
When solar wind particles hit the lunar surface, a large amount is
backscattered as neutral hydrogen. The backscattered ENAs exhibit a
distinct angular distribution, depending on the solar zenith angle. We
analysed ENAs, measured by SARA on board Chandrayaan-1, to determine
a mathematical description of this scatter distribution as a function
of the solar zenith angle and the observation angles.
Title: Evolutionary phases of equatorial spread F including L band
scintillations and plumes in the context of GPS total electron
content variability: A case study
Authors: Bagiya, Mala S.; Sridharan, R.
Bibcode: 2011JGRA..11610304B
Altcode:
The evolution of large-scale (few kilometers), medium-scale (few
hundreds of meters), and small-scale (meters) size plasma density
irregularities in the postsunset equatorial F region, in the context
of characteristic GPS total electron content (GTEC) variations, are
reported from Indian longitudes. The ionograms and GTEC from a GPS
receiver installed as a part of the GPS Aided Geo Augmentation Network
(GAGAN) project for satellite-based navigation are obtained from
an equatorial station at Trivandrum (8.5°N, 76.91°E, dip latitude
0.5°N). The variations in the GTEC with respect to TEC are considered
to represent the seed perturbations for the plasma instability that
results in the equatorial spread F (ESF) irregularities and are treated
as a perturbation factor (P). The VHF radar at Gadanki (13.5°N,
79.17°E, dip latitude 6.4°N) provided the small-scale structures of
ESF. The background thermospheric conditions that affect the growth
of the plasma instability through ion-neutral collision frequency
(νin) are estimated using the F region base height (h'F)and
the representative scale height of the neutral atmosphere and are
represented by a growth factor (G). The present case study reveals
a close coupling between the background ionospheric conditions and
the baseline perturbations in deciding the evolutionary phases of
ESF. It has been shown that although large-scale (kilometer scale)
irregularities are formed without any constraints when the background
ionospheric-thermospheric conditions are favorable in the presence of
fluctuations in GTEC, consistently, the medium-scale and small-scale
irregularities show remarkable similarity with the variations in the
product of the perturbation and growth factors.
Title: Erratum to “Extremely high reflection of solar wind protons
as neutral hydrogen atoms from regolith in space” [Planet. Space
Sci. 57 (2009) 2132-2134]
Authors: Wieser, Martin; Barabash, Stas; Futaana, Yoshifumi;
Holmström, Mats; Bhardwaj, Anil; Sridharan, R.; Dhanya, M. B.; Wurz,
Peter; Schaufelberger, Audrey; Asamura, Kazushi
Bibcode: 2011P&SS...59..798W
Altcode:
No abstract at ADS
Title: Daytime upper mesospheric energetics over a tropical station,
Trivandrum (8.5°N, 77°E): An investigation using the multiwavelength
dayglow photometry
Authors: Vineeth, C.; Pant, T. K.; Sridharan, R.
Bibcode: 2011JGRA..116.1304V
Altcode:
This paper presents the first systematic measurements of “daytime”
upper mesosphere temperatures over Trivandrum (8.5°N, 77°E), a near
equatorial station in India. The measurements were made using a unique
meridional scanning multiwavelength dayglow photometer. The first ever
observation of the short-scale (within a day), seasonal (during a year),
and spatiotemporal (spanning over a region of ∼300 km centered at
Trivandrum) variation of the daytime upper mesospheric temperature
have been discussed. In general, the daytime mesospheric temperatures
over this latitude exhibit large variability in their short scale
seasonal, and spatiotemporal characteristics. The seasonal variation
of temperature during the year 2005 exhibited a clear-cut semiannual
oscillation pattern with minima around day numbers 75 and 250, which
corroborate well with the earlier measurements of temperature over
Thumba using the sounding rockets.
Title: Low-latitude ionospheric-thermospheric response to storm time
electrodynamical coupling between high and low latitudes
Authors: Bagiya, Mala S.; Iyer, K. N.; Joshi, H. P.; Thampi, Smitha
V.; Tsugawa, Takuya; Ravindran, Sudha; Sridharan, R.; Pathan, B. M.
Bibcode: 2011JGRA..116.1303B
Altcode:
Using multi-instrumental and multistation data, we present low-latitude
ionospheric-thermospheric behavior during the geomagnetic storm of 15
May 2005. The diurnal pattern of total electron content (TEC) at a chain
of equatorial to low-latitude stations shows strong positive ionospheric
storm on 15 May. Latitudinal variation of TEC shows development of
strong equatorial ionization anomaly (EIA) on the same day. Evidence,
in terms of equatorial electrojet (EEJ) and magnetogram signatures, is
presented for the prompt penetration of interplanetary electric field
(IEF) as the cause of the positive ionospheric storm. Consequent to
the storm time circulation resulting from the extra energy deposition
via Joule heating over high latitudes, compositional changes occur in
the global thermosphere. TEC enhancements on 16 May are attributed
to enhancement of atomic oxygen at equatorial and low latitudes and
the negative ionospheric storm on 17 May observed beyond certain low
latitudes is explained in terms of enhancement of molecular species
because of the storm time neutral composition changes. Strong ESF plume
structures on range time intensity (RTI) map and L-band scintillation
and TEC depletions in GPS measurements are observed in the longitude
sectors where the local time of sudden storm commencement (SSC) falls
after the post sunset hours. The ionospheric zonal electric fields
are altered by the combined effects of eastward disturbance dynamo
electric fields and direct prompt penetration of eastward electric
fields associated with the northward turning of interplanetary magnetic
field (IMF) Bz leading to subsequent development of ESF
after midnight.
Title: Planetary wave-tidal interactions over the equatorial
mesosphere-lower thermosphere region and their possible implications
for the equatorial electrojet
Authors: Vineeth, C.; Pant, T. K.; Sumod, S. G.; Kumar, K. K.;
Gurubaran, S.; Sridharan, R.
Bibcode: 2011JGRA..116.1314V
Altcode:
Optically measured daylight mean mesopause temperatures over a dip
equatorial station, Trivandrum (8.5°N 77°E dip lat. 0.5°N),
have been analyzed in conjunction with simultaneously measured
equatorial electrojet (EEJ)-produced magnetic field at the surface. The
signature of planetary wave-tidal interactions in the mesosphere-lower
thermosphere (MLT) region has been observed for the first time in
the day-to-day variability in the EEJ, i.e., the time of its peaking
and the duration, as inferred from the EEJ-produced magnetic field
on the ground. The present study shows that the planetary wave of
quasi 16 day periodicity plays an important role in causing these
variabilities, especially during the winter months. The quasi 16
day wave is found to be modulating the mesopause temperature (MT),
duration, and time of the maximum EEJ intensity (DEEJ and
TEEJ). During positive excursions of the planetary wave,
TEEJ showed a shift toward evening, while the MT showed an
increase and DEEJ showed a broadening. Similarly, all these
parameters exhibited an opposite trend during negative excursions. The
planetary wave-tidal interactions and subsequent modification of the
tidal components have been shown to be responsible for the observed
variations. This study presents a new perspective addressing the
day-to-day variability of the EEJ.
Title: The sunlit lunar atmosphere: A comprehensive study by CHACE
on the Moon Impact Probe of Chandrayaan-1
Authors: Sridharan, R.; Ahmed, S. M.; Pratim Das, Tirtha; Sreelatha,
P.; Pradeepkumar, P.; Naik, Neha; Supriya, Gogulapati
Bibcode: 2010P&SS...58.1567S
Altcode:
The altitudinal/latitudinal profile of the lunar atmospheric composition
on the sunlit side was unraveled for the first time by the Chandra's
Altitudinal Composition Explorer (CHACE) on the Moon Impact Probe, a
standalone micro-satellite that impacted at the lunar south pole, as a
part of the first Indian mission to Moon, Chandrayaan-1. Systematic
measurements were carried out during the descent phase of the
impactor with an altitude resolution of ∼250 m and a latitudinal
resolution of ∼0.1°. The overall pressure on the dayside and the
neutral composition in the mass range 1-100 amu have been measured by
identifying 44 and 18 amu as the dominant constituents. Significant
amounts of heavier (>50 amu) species also have been detected,
the details of which are presented and discussed.
Title: Protons in the near-lunar wake observed by the Sub-keV Atom
Reflection Analyzer on board Chandrayaan-1
Authors: Futaana, Y.; Barabash, S.; Wieser, M.; Holmström, M.;
Bhardwaj, A.; Dhanya, M. B.; Sridharan, R.; Wurz, P.; Schaufelberger,
A.; Asamura, K.
Bibcode: 2010JGRA..11510248F
Altcode: 2010arXiv1011.4448F
Significant proton fluxes were detected in the near-wake region of the
Moon by an ion mass spectrometer on board Chandrayaan-1. The energy
of these nightside protons is slightly higher than the energy of
the solar wind protons. The protons are detected close to the lunar
equatorial plane at a 140° solar zenith angle, that is, ∼50°
behind the terminator at a height of 100 km. The protons come from
just above the local horizon and move along the magnetic field in the
solar wind reference frame. We compare the observed proton flux with
the predictions from analytical models of an electrostatic plasma
expansion into a vacuum. The observed velocity is higher by a factor
of 2 to 3 than the velocity predicted by analytical models. The simple
analytical models cannot explain the observed ion dynamics along the
magnetic field in the vicinity of the Moon.
Title: Mini-magnetospheres above magnetic anomalies on the Moon
Authors: Wieser, M.; Schaufelberger, A.; Barabash, S.; Holmström,
M.; Futaana, Y.; Bhardwaj, A.; Sridharan, R.; Dhanya, M. B.; Wurz,
P.; Asamura, K.
Bibcode: 2010epsc.conf..619W
Altcode:
No abstract at ADS
Title: Seeing-Induced Errors in Solar Doppler Velocity Measurements
Authors: Padinhatteeri, Sreejith; Sridharan, R.; Sankarasubramanian, K.
Bibcode: 2010SoPh..266..195P
Altcode: 2010SoPh..tmp..120P; 2010arXiv1009.2966P; 2010SoPh..tmp..132P
Imaging systems based on a narrow-band tunable filter are used to
obtain Doppler velocity maps of solar features. These velocity
maps are created by taking the difference between the blue- and
red-wing intensity images of a chosen spectral line. This method
has the inherent assumption that these two images are obtained under
identical conditions. With the dynamical nature of the solar features
as well as the Earth's atmosphere, systematic errors can be introduced
in such measurements. In this paper, a quantitative estimate of the
errors introduced due to variable seeing conditions for ground-based
observations is simulated and compared with real observational data
for identifying their reliability. It is shown, under such conditions,
that there is a strong cross-talk from the total intensity to the
velocity estimates. These spurious velocities are larger in magnitude
for the umbral regions compared to the penumbra or quiet-Sun regions
surrounding the sunspots. The variable seeing can induce spurious
velocities up to about 1 km s−1. It is also shown that
adaptive optics, in general, helps in minimising this effect.
Title: A comparison of optically measured daytime OH temperatures
over the tropics during solar maximum and minimum periods
Authors: Vineeth, C.; Pant, T. K.; Gurubaran, S.; Hossain, M. M.;
Sridharan, R.
Bibcode: 2010EP&S...62..647V
Altcode:
This paper deals with the variability of optically measured daytime
OH temperatures during two different solar activity epochs, over
Trivandrum (8.5°N, 76.5°E), a tropical station in India. The data
used for the present study span the period of February-March, during
the solar maximum year of 2001 and solar minimum year of 2005. The
mean daytime temperature (180±5 K) during the 2001 study period was
found to be lower than the mean temperature (195±9 K) of the 2005
study period by ∼15 K. However, apart from this noticeable shift in
mean temperature level, the variabilities in the temperature of both
years were strikingly similar. Moreover, the wave periodicities present
during these periods were also quite similar. Possible reasons for these
observations were investigated in the context of the various forcings
that control the energetics of the equatorial mesopause region. The
observations presented in this study are unique and reveal a number
of newer aspects of the energetics of the tropical mesopause.
Title: Global Mapping of Backscattered Energetic Neutral Hydrogen
at the Moon: Observations from SARA/Chandrayaan-1
Authors: Schaufelberger, Audrey; Wurz, Peter; Barabash, Stas; Wieser,
Martin; Futaana, Yoshifumi; Holmström, Mats; Bhardwaj, Anil; Dhanya,
M. B.; Sridharan, R.; Asamura, Kazushi
Bibcode: 2010EGUGA..1214269S
Altcode:
The Indian spacecraft Chandrayaan-1 carried the Sub-keV Atom Reflecting
Analyzer (SARA) to investigate the solar wind - moon interaction
by comparing impinging solar wind ions to energetic neutral atoms
(ENAs) originating from the lunar surface. Solar wind backscattering
contributes substantially to the observed signal, with up to 20% of
the solar wind protons reflected as hydrogen ENAs. Angular resolution
of SARA allows to create images or maps of the energetic neutral
flux from the surface. Besides of pure geometrical effects like solar
zenith angle, the intensity of the backscattered signal shows variations
correlated to the location of the reflection point on the surface. Such
variations are e.g. observed at local magnetic anomalies, which shield
the surface from solar wind by forming a mini-magnetosphere under
certain solar wind conditions (Futaana et al., Planet. Space Sci. 2006
and Wieser et al., GRL 2010). We report on global imaging of the
features of lunar magnetic anomalies from a large dataset recorded by
SARA. We compare the geometries of different lunar magnetic anomalies
and discuss differences as well as similarities. We also discuss
correlations between albedo features and ENA fluxes which are valuable
for evaluating the space weathering effect by the solar wind protons.
Title: Observation of a lunar mini-magnetosphere above a magnetic
anomaly using energetic neutral atoms
Authors: Wieser, Martin; Barabash, Stas; Futaana, Yoshifumi;
Holmström, Mats; Bhardwaj, Anil; Sridharan, R.; Dhanya, M. B.;
Schaufelberger, Audrey; Wurz, Peter; Asamura, Kazushi
Bibcode: 2010EGUGA..12.8590W
Altcode:
The Sub-keV Atom Reflecting Analyzer (SARA) instrument on the Indian
Chandrayaan-1 spacecraft has resulted in a comprehensive data set
about interaction of solar wind with the lunar surface. When solar
wind hits the lunar surface, it is partly backscattered as energetic
neutral atoms. The intensity of the backscattered energetic neutral
atoms is a measure of the intensity of the solar wind reaching the
surface. We report on the imaging of a lunar magnetic anomaly in
backscattered neutral hydrogen atoms: The image shows the formation of a
partial void of the solar wind, a mini-magnetosphere, above the strong
magnetic anomaly near the Crisium antipode on the lunar farside. The
mini-magnetosphere is 360 km across at the surface and surrounded by
a 300-km-thick region of enhanced plasma flux that results from the
solar wind flowing around the mini-magnetosphere. These observations
demonstrate a new observational technique to study airless bodies,
imaging in backscattered neutral atoms, and its application to a new
class of objects, mini-magnetospheres.
Title: ‘Direct’ evidence for water (H2O) in the sunlit
lunar ambience from CHACE on MIP of Chandrayaan I
Authors: Sridharan, R.; Ahmed, S. M.; Pratim Das, Tirtha; Sreelatha,
P.; Pradeepkumar, P.; Naik, Neha; Supriya, Gogulapati
Bibcode: 2010P&SS...58..947S
Altcode:
Direct detection of water in its vapour phase in the tenuous lunar
environment through in situ measurements carried out by the Chandra's
Altitudinal Composition Explorer (CHACE) payload, onboard the Moon
Impact Probe (MIP) of Chandrayaan I mission vindicates the presence
of water on the surface of the moon in form of ice at higher lunar
latitudes inferred from IR absorption spectroscopy, (especially that
of OH), by the Moon Mineralogy Mapper (M 3) of Chandrayaan
I. The quadrupole mass spectrometer based payload, CHACE, sampled the
lunar neutral atmosphere every 4 s with a broad latitudinal (∼40°N
to 90°S, with a resolution of ∼0.1°) and altitudinal (from 98 km
up to impact on the lunar surface with a resolution of ∼0.25 km)
coverage in the sunlit side of the moon for the first time. These two
(CHACE and M 3) complementary experiments are shown to
collectively provide unambiguous signatures for the distribution of
water in solid and gaseous phases in Earth's moon.
Title: The Sub-Kev Atom Reflecting Analyzer (sara) Experiment Aboard
CHANDRAYAAN-1 Mission: Instrument and Observations
Authors: Bhardwaj, Anil; Dhanya, M. B.; Sridharan, R.; Wieser, Martin;
Barabash, Stas; Yoshifumi, Futaana; Holmström, Mats; Wurz, Peter;
Schaufelberger, Audrey; Kazushi, Asamura
Bibcode: 2010aogs...19..151B
Altcode: 2010arXiv1012.1527B
SARA experiment aboard the first Indian lunar mission Chandrayaan-1
had the objective to explore the solar wind-lunar interaction using
energetic neutral atoms (ENA) from the lunar surface as diagnostic
tool. SARA consisted of an ENA imaging mass analyzer CENA (Chandrayaan-1
Energetic Neutral Analyzer) and an ion mass analyser SWIM (Solar Wind
Monitor), along with a digital processing unit (DPU) which commands and
controls the sensors and provides the interface to the spacecraft. Both
sensors have provided excellent observational data. CENA has observed
ENAs from the lunar surface and found that 20% of the incident solar
wind ions get backscattered as ENAs from the lunar surface. This is
contrary to the previous assumptions of almost complete absorption
of solar wind by the lunar surface. The observation is relevant for
other airless bodies in the solar system.
Title: Protons observed in the near lunar wake by the SARA/SWIM
sensor onboard Chandrayaan-1
Authors: Futaana, Yoshifumi; Barabash, Stas; Wieser, Martin;
Holmström, Mats; Bhardwaj, Anil; Dhanya, M. B.; Sridharan, R.; Wurz,
Peter; Schaufelberger, Audrey; Asamura, Kazushi
Bibcode: 2010EGUGA..12.5202F
Altcode:
We report significant proton fluxes detected in the near wake region
of the Moon by the ion mass spectrometer SWIM on board Chandrayaan-1
spacecraft. These nightside protons are even detected close to the
lunar equatorial plane at a solar zenith angle of 140˚, i.e. about
50˚ behind the terminator at a height of 100 km. The observed energy
of these nightside protons is slightly higher than that of the upstream
solar wind protons. The density of them is 1000 times less than that
of the upstream solar wind. The arrival direction of the nightside
proton fluxes is just above the local horizon, but in the solar wind
stationary frame the fluxes move along the magnetic field. Therefore,
we concluded that the nightside proton fluxes are of the solar wind
origin, but are accelerated along the magnetic field during their
expansion into the lunar wake. We have then compared the observed
proton fluxes with the predictions from analytical models of an
electrostatic plasma expansion into a vacuum. Two analytical models
are examined. Both uses a set of equations based on 1-D gas-dynamics,
but one assumes a Maxwell-type electron velocity distribution function
and the other assumes a Kappa-distribution. It is found that both models
qualitatively agree with the observations, however, the observed proton
density is lower by a factor of 5-10, and the observed velocity in the
solar wind reference frame is higher by a factor of 2-3 compared to
the model predictions. This discrepancy between the observations and
the models may be explained by the absorption of the plasma particles
by the lunar surface at the terminator region where the solar wind
proton expansion is initiated.
Title: Studying the Lunar-Solar Wind Interaction with the SARA
Experiment aboard the Indian Lunar Mission Chandrayaan-1
Authors: Bhardwaj, Anil; Barabash, Stas; Dhanya, M. B.; Wieser,
Martin; Yoshifumi, Futaana; Holmström, Mats; Sridharan, R.; Wurz,
Peter; Schaufelberger, Audrey; Kazushi, Asamura
Bibcode: 2010AIPC.1216..518B
Altcode: 2010arXiv1012.1687B
The first Indian lunar mission Chandrayaan-1 was launched on 22 October
2008. The Sub-keV Atom Reflecting Analyzer (SARA) instrument onboard
Chandrayaan-1 consists of an energetic neutral atom (ENA) imaging mass
analyzer called CENA (Chandrayaan-1 Energetic Neutrals Analyzer), and
an ion-mass analyzer called SWIM (Solar wind Monitor). CENA performed
the first ever experiment to study the solar wind-planetary surface
interaction via detection of sputtered neutral atoms and neutralized
backscattered solar wind protons in the energy range ~0.01-3.0 keV. SWIM
measures solar wind ions, magnetosheath and magnetotail ions, as well as
ions scattered from lunar surface in the ~0.01-15 keV energy range. The
neutral atom sensor uses conversion of the incoming neutrals to positive
ions, which are then analyzed via surface interaction technique. The
ion mass analyzer is based on similar principle. This paper presents
the SARA instrument and the first results obtained by the SWIM and CENA
sensors. SARA observations suggest that about 20% of the incident solar
wind protons are backscattered as neutral hydrogen and ~1% as protons
from the lunar surface. These findings have important implications
for other airless bodies in the solar system.
Title: First observation of a mini-magnetosphere above a lunar
magnetic anomaly using energetic neutral atoms
Authors: Wieser, Martin; Barabash, Stas; Futaana, Yoshifumi;
Holmström, Mats; Bhardwaj, Anil; Sridharan, R.; Dhanya, M. B.;
Schaufelberger, Audrey; Wurz, Peter; Asamura, Kazushi
Bibcode: 2010GeoRL..37.5103W
Altcode: 2010GeoRL..3705103W; 2010arXiv1011.4442W
The Sub-keV Atom Reflecting Analyzer (SARA) instrument on the Indian
Chandrayaan-1 spacecraft has produced for the first time an image of
a lunar magnetic anomaly in backscattered hydrogen atoms. The image
shows that a partial void of the solar wind, a mini-magnetosphere, is
formed above the strong magnetic anomaly near the Crisium antipode. The
mini-magnetosphere is 360 km across at the surface and is surrounded
by a 300-km-thick region of enhanced plasma flux that results
from the solar wind flowing around the mini-magnetosphere. The
mini-magnetosphere is visible only in hydrogen atoms with energy
exceeding 150 eV. Fluxes with energies below 100 eV do not show
corresponding spatial variations. While the high-energy atoms
result from the backscattering process, the origin of the low-energy
component is puzzling. These observations reveal a new class of objects,
mini-magnetospheres, and demonstrate a new observational technique to
study airless bodies, imaging in backscattered neutral atoms.
Title: A new Ionospheric Probe for insitu measurements of neutral
wind and electron density
Authors: Manju, G.; Sreelatha, P.; Ravindran, Sudha; John, Rosmy;
Naik, Neha; Thampi, Satheesh; P, Pradeep Kumar; Mohan Kumar, S. V.;
Sridharan, R.
Bibcode: 2010cosp...38..950M
Altcode: 2010cosp.meet..950M
A novel Electron Density and Neutral Wind Probe (ENWi) has been
designed and developed for undertaking rocket borne upper atmospheric
studies. This probe consists of a first of its kind neutral
wind/electron density probe and a Langmuir probe. The neutral wind
probe is designed for measurement of ionospheric E region ( 90-120 km)
neutral winds and F-region ( 120-500 km) ion drift. The ionic motion
is controlled by the neutrals due to frequent collisions between
the two species, in the E region. Hence the neutral wind velocity is
equal to the ionic velocity in this altitude region. This principle is
used to derive the neutral winds in the E region. The field of view
of the instrument is 10o . The instrument is capable of making wind
measurements in the range of 10-400 m/s. The Langmuir probe will be
operated in fixed bias mode for measurement of ionospheric electron
density and irregularity strength. The electron density measurements
can be made in the range 0f 103-(5 x 105)/cm3. This probe was used
for insitu measurements during the annular solar eclipse campaign of
January 14-17, 2010. The major scientific results to be presented
are a) Investigations on the effects of the eclipse on the neutral
winds and electron density in the E region and their implications for
occurrence/non occurrence of the ionospheric irregularities and Counter
Electrojet. b) Investigations on the modulations in the electron
density and neutral wind/ion drift in the post eclipse period from
the rocket flight after the eclipse. These results are the first ever,
simultaneous in-situ neutral wind and electron density measurements,
during an eclipse at the magnetic equator.
Title: Solar Wind Monitoring with SWIM-SARA Onboard Chandrayaan-1
Authors: Bhardwaj, A.; Barabash, S.; Sridharan, R.; Wieser, M.; Dhanya,
M. B.; Futaana, Y.; Asamura, K.; Kazama, Y.; McCann, D.; Varier, S.;
Vijayakumar, E.; Mohankumar, S. V.; Raghavendra, K. V.; Kurian, T.;
Thampi, R. S.; Andersson, H.; Svensson, J.; Karlsson, S.; Fischer,
J.; Holmstrom, M.; Wurz, P.; Lundin, R.
Bibcode: 2010ASSP...19..531B
Altcode: 2010mcia.conf..531B
The SARA experiment aboard the Indian lunar mission Chandrayaan-1
consists of two instruments: Chandrayaan-1 Energetic Neutral Analyzer
(CENA) and the SolarWind Monitor (SWIM). CENA will provide measurements
of low energy neutral atoms sputtered from lunar surface in the 0.01-3.3
keV energy range by the impact of solar wind ions. SWIM will monitor
the solar wind flux precipitating onto the lunar surface and in the
vicinity of moon. SWIM is basically an ion-mass analyzer providing
energy-per-charge and number density of solar wind ions in the energy
range 0.01-15 keV. It has sufficient mass resolution to resolve
H+ , He++, He+, O++,
O+, and >20 amu, with energy resolution 7% and angular
resolution 4:5° × 22:5. The viewing angle of the instrument is 9°
× 180°.Mechanically, SWIM consists of a sensor and an electronic
board that includes high voltage supply and sensor electronics. The
sensor part consists of an electrostatic deflector to analyze the
arrival angle of the ions, cylindrical electrostatic analyzer for
energy analysis, and the time-of-flight system for particle velocity
determination. The total size of SWIM is slightly larger than a credit
card and has a mass of 500 g.
Title: Atmosphere-Ionosphere coupling -manifestations in the low
latitude ionosphere
Authors: Pant, Tarun; Sumod, S. G.; Vineeth, C.; Thampi, Smitha;
Jose, Lijo; Kishore Kumar, K.; Sreeja, V.; Manju, G.; Ravindran,
Sudha; Sridharan, R.; Niranjan, K.; Dabas, R. S.; Das, Rupesh; Alex,
S.; Tiwari, Diwakar
Bibcode: 2010cosp...38.1257P
Altcode: 2010cosp.meet.1257P
As is known, the extent of the impact of the geomagnetic storms on the
vertical coupling of the MLTI regions, especially over low latitudes,
is not very well understood. To investigate this aspect of the MLTI
coupling, a multi-instrument campaign was conducted during March-April
2006, a period when a few moderate geomagnetic storms occurred,
in India. During the campaign period, a Multiwavelength dayglow
Photometer (MWDPM) was operated along with ionosonde, magnetometer
and a meteor radar from Trivandrum (8.5oN, 76.5oE, 0.5oN diplat.),
the dip equatorial station in India. The MWDPM provided the optically
estimated daytime mesopause temperature while the meteor radar measured
the wind and temperature in the lower thermosphere-upper mesosphere
region. The MWDPM also provided the near simultaneous measurements on
the thermospheric dayglow (O1D 630 nm). GPS satellite based measurements
of total electron content (TEC) over a number of locations in India
provided the latitudinal distribution of ionization. The new and
important observations made during this campaign are the following:
(a) The optically measured daytime mesopause temperature shows a
prominent decrease in the afternoon hours during the initial and
main phases of the geomagnetic storms. This observation, perhaps,
is new and unique. (b) Quasi 2 and 5 day oscillations appear to be
modulating the mesopause temperature indicating the presence of
planetary waves therein. The wind and temperature variability in
the lower thermosphere, obtained using the collocated radar further
corroborate with these optical measurements and establish the origin
of these waves to be in the lower atmosphere. (c) The Equatorial
Electrojet (EEJ) induced magnetic field on the ground also exhibit
similar variability indicating the presence of these planetary
scale oscillations, also in the ionospheric dynamo region. (d) The
simultaneously measured thermospheric dayglow (O1D 630 nm) also shows
an intensification of a quasi 2-and 5-day wave activity which in turn
indicates towards the possible wave modulations in the daytime upward
drift over the equator. (e) Interestingly, these modulations in the MTI
region are not restricted to the region around equator. The ionization
density over a station, almost 25o away in latitude, also exhibits
their presence. In this context, these measurements as presented here,
are new and will be discussed in context of the vertical coupling of
the equatorial MLTI region.
Title: Energetic neutral atom imaging of the moon: Observation of
a mini-magnetosphere above a lunar magnetic anomaly
Authors: Wieser, Martin; Barabash, Stas; Futaana, Yoshifumi;
Holmström, Mats; Bhardwaj, Anil; Sridharan, R.; Dhanya, B.;
Schaufelberger, Audrey; Wurz, Peter; Asamura, Kazushi
Bibcode: 2010cosp...38..424W
Altcode: 2010cosp.meet..424W
The Sub-keV Atom Reecting Analyzer (SARA) instrument on the Indian
Chandrayaan-1 space-craft has resulted in a comprehensive data set about
interaction of solar wind with the lunar surface. When solar wind hits
the lunar surface, it is partly backscattered as energetic neutral
atoms. The intensity of the backscattered energetic neutral atoms is
a measure of the intensity of the solar wind reaching the surface. We
report on the imaging of a lunar magnetic anomalies in backscattered
neutral hydrogen atoms. At the example of the strong magnetic anomaly
near the Crisium antipode on the lunar farside we show that a partial
void of the solar wind, a mini-magnetosphere, is formed above the
magnetic anomaly. The mini-magnetosphere is 360 km across at the surface
and surrounded by a 300-km-thick region of enhanced plasma ux that
results from the solar wind owing around the mini-magnetosphere. These
observations demonstrate a new observational technique to study airless
bodies, imaging in ackscattered neutral atoms, and its application to
a new class of objects, mini-magnetospheres.
Title: Plasma expansion into the lunar wake: Observation of protons
by the SARA/SWIM sensor onboard Chandrayaan-1
Authors: Futaana, Yoshifumi; Barabash, Stas; Wieser, Martin;
Holmström, Mats; Bhardwaj, Anil; Dhanya, B.; Sridharan, R.; Wurz,
Peter; Schaufelberger, Audrey; Asamura, Kazushi
Bibcode: 2010cosp...38..427F
Altcode: 2010cosp.meet..427F
Since the Moon absorbs most of the solar wind protons and electrons, a
void region is formed behind its body. Here we report significant proton
fluxes in the lunar wake detected by the ion mass spectrometer SWIM on
board Chandrayaan-1 spacecraft, and discuss the structure of the lunar
wake. These nightside protons have been commonly detected in the lunar
wake region, however, we investigated in-depth one remarkable event
when the interplanetary magnetic field is perpendicular to the solar
wind. In this event, the protons are found close to the lunar equatorial
plane at about 50° behind the terminator at a height of 100 km. The
observed energy of these nightside protons is slightly higher than that
of the upstream solar wind protons. The density of them is 1000 times
less than that of the upstream solar wind. The arrival direction of
the proton fluxes is slightly above the local horizon, however, as we
convert the reference frame into the solar wind stationary frame the
fluxes are moving along the magnetic field. Therefore, we concluded
that the nightside proton fluxes are of the solar wind origin that
have been accelerated along the magnetic field during their expansion
into the lunar wake. We have then compared the observed proton fluxes
with the predictions from two rather simple analytical models of an
electrostatic plasma expansion into a vacuum. Both model uses a set
of quasi-equilibrium 1-D electrostatic equations to model the plasma
expansion into vacuum along the magnetic field. The first model assumes
a Maxwell-type electron velocity distribution function and the second
assumes a Kappa-distribution. It is found that both models qualitatively
agree with the observations, however, the observed proton density is
lower by a factor of 5-10, and the observed velocity in the solar
wind reference frame is higher by a factor of 2-3 compared to the
model predictions. The discrepancy between the observation and the
models may be explained by the absorption of the plasma particles by
the lunar surface at the terminator region where the solar wind proton
expansion is initiated.
Title: Extremely high reflection of solar wind protons as neutral
hydrogen atoms from regolith in space
Authors: Wieser, Martin; Barabash, Stas; Futaana, Yoshifumi;
Holmström, Mats; Bhardwaj, Anil; Sridharan, R.; Dhanya, M. B.; Wurz,
Peter; Schaufelberger, Audrey; Asamura, Kazushi
Bibcode: 2009P&SS...57.2132W
Altcode: 2010arXiv1012.2972W
We report on measurements of extremely high reflection rates of solar
wind particles from regolith-covered lunar surfaces. Measurements by
the Sub-keV Atom Reflecting Analyzer (SARA) instrument on the Indian
Chandrayaan-1 spacecraft in orbit around the Moon show that up to
20% of the impinging solar wind protons are reflected from the lunar
surface back to space as neutral hydrogen atoms. This finding, generally
applicable to regolith-covered atmosphereless bodies, invalidates the
widely accepted assumption that regolith almost completely absorbs
the impinging solar wind.
Title: Electrodynamical response of the Indian low-mid latitude
ionosphere to the very large solar flare of 28 October 2003 -
a case study
Authors: Manju, G.; Pant, T. K.; Devasia, C. V.; Ravindran, S.;
Sridharan, R.
Bibcode: 2009AnGeo..27.3853M
Altcode:
The electrodynamic effects on the low-mid latitude ionospheric region
have been investigated using GPS (global positioning system) data,
ionosonde data and ΔH values, during the very large solar flare
(X17.2/4B) of 28 October 2003. The results bring out the flare induced
unusual behaviour of the equatorial ionosphere on this day just prior
to sunset. The important observations are i) Large and prolonged
Ne enhancements observed from ionosonde data just after the
flare-related peak enhancement in EUV flux. The observed enhancement in
Ne is due to the increase in ionization production due to the
enhanced EUV flux and the persistence of the enhancement is probably due
to the prompt penetration related upliftment of the F layer (just prior
to the flare peak phase) to higher altitudes, where recombination rates
are lower. ii) A significant enhancement in total electron content (TEC)
(~10 TEC units) at regions around the Equatorial Ionization Anomaly
(EIA) crest region (Ahmedabad) during the flare in association with
the flare related EUV flux enhancement. iii) Similar enhancements seen
at stations of Jodhpur and Delhi in the mid latitude sector. iv)The
flare related flux enhancements in different longitude sectors in the
equatorial electrojet region have been shown to produce positive and
negative variations in electrojet strength indicating the presence of
current systems having positive and negative polarities in different
longitude sectors. Thus the flare effect reveals the longitudinal
variation of the counter electrojet events in the Equatorial Electrojet
(EEJ) region.
Title: Signatures of low latitude-high latitude coupling in the
tropical MLT region during sudden stratospheric warming
Authors: Vineeth, C.; Pant, T. K.; Kumar, K. K.; Ramkumar, G.;
Sridharan, R.
Bibcode: 2009GeoRL..3620104V
Altcode:
The present paper analyses the variations of daytime mesopause
temperature over an equatorial station during polar Stratospheric
Sudden Warming (SSW) events, indicating a possible dynamical coupling
between the two regions, through the intensification of planetary
wave activity. The amplified wave signatures of quasi-16 day period
have been seen in the Equatorial Mesopause Temperature (EMT) and
Polar Stratospheric Temperature (at 10 hPa) during the course of the
SSW. The EMT measurements were made from Trivandrum (8.5°N, 76.5°E),
an equatorial station in India and the stratospheric temperatures were
obtained from NCEP/NCAR reanalysis. This investigation reveals (1)
an overall enhancement and amplified wave activity in the EMT prior
to the SSW and (2) propagation of a wave disturbance from equatorial
stratosphere to North Pole in about a month. These results are new
and reveal some newer aspects of the SSW and its equatorial effects.
Title: Observations by the Sub-keV Atom Reflecting Analyzer (SARA)
on board of Chandrayaan-1
Authors: Wieser, M.; Barabash, S.; Bhardwaj, A.; Sridharan, R.;
Futaana, Y.; Holmström, M.; Lundin, R.; Dhanya, M. B.; Asamura, K.;
Wurz, P.
Bibcode: 2009epsc.conf..736W
Altcode:
No abstract at ADS
Title: Additional stratifications in the equatorial F region at dawn
and dusk during geomagnetic storms: Role of electrodynamics
Authors: Sreeja, V.; Balan, N.; Ravindran, Sudha; Pant, Tarun Kumar;
Sridharan, R.; Bailey, G. J.
Bibcode: 2009JGRA..114.8309S
Altcode: 2009JGRA..11408309S
The role of electrodynamics in producing additional stratifications in
the equatorial F region (F 3 layer) at dawn and dusk during
geomagnetic storms is discussed. Two cases of F 3 layer at
dawn (0600-0730 LT on 5 October 2000 and 8 December 2000) and one case
of F 3 layer at dusk (1600-1730 LT on 5 October 2000) are
observed, for the first time, by the digital ionosonde at the equatorial
station Trivandrum (8.5°N 77°E dip ∼ 0.5°N) in India. The unusual
F 3 layers occurred during the geomagnetic storms and are
associated with southward turning of interplanetary magnetic field B
z , suggesting that eastward prompt penetration electric
field could be the main cause of the F 3 layers. The dawn
F 3 layer on 5 October is modeled using the Sheffield
University Plasmasphere-Ionosphere Model by using the E × B drift
estimated from the real height variation of the ionospheric peak
during the morning period. The model qualitatively reproduces the
dawn F 3 layer. While the existing F 2 layer
rapidly drifts upward and forms the F 3 layer and topside
ledge, a new layer forming at lower heights develops into the normal
F 2 layer.
Title: Response of the equatorial and low-latitude ionosphere in
the Indian sector to the geomagnetic storms of January 2005
Authors: Sreeja, V.; Devasia, C. V.; Ravindran, Sudha; Pant, Tarun
Kumar; Sridharan, R.
Bibcode: 2009JGRA..114.6314S
Altcode: 2009JGRA..11406314S
The equatorial and low-latitude ionospheric response to three
moderate geomagnetic storms (17, 18, and 22 January) during the
period from 16 to 23 January 2005 is investigated in the context of
development/inhibition of the Equatorial Ionization Anomaly (EIA)
and the subsequent occurrence/nonoccurrence of Equatorial Spread F
(ESF) irregularities on these days. The study is carried out using the
Total Electron Content (TEC) measured with the GPS receivers along the
∼80°E longitude sector and the F-layer bottom height obtained from
the Ionosonde located over the dip equatorial location of Trivandrum
(8.5°N, 77°E, dip latitude ∼0.5°N) in India. It is observed that,
for the storms on days 17 and 22, the development of the anomaly was
inhibited, probably due to the westward disturbance dynamo electric
fields. Subsequently, the post sunset enhancement in the vertical
drift of the equatorial F region was also inhibited significantly
compared to the quiet day pattern and, as anticipated, no ESF was
observed on these days. A large vertical drift of the equatorial F
region followed by nearly simultaneous onset of weak ESF was observed
on day 18. The late development of the EIA on this day could be due
to the eastward prompt penetration electric field associated with
the southward turning of the interplanetary magnetic field. Also,
strong and distinct F3 layer appeared for a short time in the morning,
reappeared later in the noon time, and then quickly ascended to the
topside ionosphere during the main phase of the storm on day 18.
Title: Role of gravity wavelike seed perturbations on the triggering
of ESF - a case study from unique dayglow observations
Authors: Sreeja, V.; Vineeth, C.; Pant, Tarun Kumar; Ravindran, Sudha;
Sridharan, R.
Bibcode: 2009AnGeo..27..313S
Altcode:
First observational evidence, from the Indian longitudes, for the
presence of gravity wavelike perturbations with periods of 20-30 min,
acting as probable seeds for Equatorial Spread F (ESF) irregularities
is described. The study is based on the daytime optical measurements
of the mesopause temperature and the intensity of the thermospheric
O(1D) 630.0 nm dayglow emissions using the unique
MultiWavelength Dayglow PhotoMeter from Trivandrum (8.5° N; 77° E;
dip lat ~0.5° N), a dip equatorial station. Measurements during the
equinoctial months of a solar maximum (2001) and a solar minimum year
(2006) have been used in this study. It is shown that under identical
background ionospheric conditions within a solar epoch, the power of the
gravity waves have a deterministic role in the generation of ESF. The
mesopause temperature simultaneously observed, indicate that possible
source regions for these perturbations lie in the lower atmosphere.
Title: Determination of day-time OH emission heights using
simultaneous meteor radar, day-glow photometer and TIMED/SABER
observations over Thumba (8.5°N, 77°E)
Authors: Kumar, Karanam Kishore; Vineeth, C.; Antonita, T. Maria;
Pant, Tarun Kumar; Sridharan, R.
Bibcode: 2008GeoRL..3518809K
Altcode:
Simultaneous day-glow photometer and meteor radar observations are
used for determining the day-time OH emission height for the first
time over Thumba (8.5°N, 77°E). The meteor radar observations
are extensively used to obtain the ambipolar diffusion coefficient
(Da) profile in the 82-98 km height region and are compared
with OH rotational temperature (TOH) measurements. The height profile
of correlation coefficient between Da and TOH showed a
maximum at 90 km. These ground based estimates are then compared
with space based OH volume emission rate (VER) observations by
TIMED/SABER satellite over this latitude, which showed an excellent
agreement. Further, the temporal variation of OH emission heights
showed tidal modulation of OH emission heights. The significance of
the present results lies in determining the day-time OH emission height
for the first time using collocated radar and photometer observations,
which will have implications in interpreting TOH in the MLT region.
Title: Crowded-Field Astrometry with SIM PlanetQuest. II. An Improved
Instrument Model
Authors: Sridharan, R.; Allen, Ronald J.
Bibcode: 2008PASP..120..791S
Altcode: 2008arXiv0805.4622S
In a previous paper we described a method of estimating the
single-measurement bias to be expected in astrometric observations
of targets in crowded fields with the future Space Interferometry
Mission PlanetQuest. That study was based on a simplified model of the
instrument and the measurement process involving a single-pixel focal
plane detector, an idealized spectrometer, and continuous sampling
of the fringes during the delay scanning. In this paper we elaborate
on this “instrument model” to include the following additional
complications: spectral dispersion of the light with a thin prism,
which turns the instrument camera into an objective prism spectrograph;
a multiple-pixel detector in the camera focal plane; and binning of
the fringe signal during scanning of the delay. The results obtained
with this improved model differ in small but systematic ways from those
obtained with the earlier simplified model. We conclude that it is the
pixelation of the dispersed fringes on the focal plane detector that is
responsible for the differences. The improved instrument model described
here suggests additional ways of reducing certain kinds of confusion,
and provides a better basis for the evaluation of instrumental effects
in the future.
Title: Local time dependent response of postsunset ESF during
geomagnetic storms
Authors: Tulasi Ram, S.; Rama Rao, P. V. S.; Prasad, D. S. V. V. D.;
Niranjan, K.; Gopi Krishna, S.; Sridharan, R.; Ravindran, Sudha
Bibcode: 2008JGRA..113.7310T
Altcode:
Development or inhibition of ESF during magnetically active periods has
been an important space weather topic of interest during the recent
past in view of its applications in the satellite based navigational
systems. Particularly, the postsunset period exhibits significant
variability for storm time development of ESF versus longitude. In
this paper, we report the results of a multi-instrumental (ground based
and space-borne) and multistation study on the development/inhibition
of postsunset ESF during five moderate to intense geomagnetic storms
occurred during the low and descending phase of the solar activity
period, 2004-2006. It has been observed that, the prompt penetration of
eastward electric fields into low latitudes and subsequent development
of ESF occurred in all longitudinal sectors where the local time
corresponds to postsunset hours during the entire main phase of
the storm. In this paper, we show the development of plasma bubble
irregularities over a wide longitudinal extent of 92° owing to the dusk
time penetration of eastward electric fields into low latitudes. Either
the sudden increase in AE-index and/or a marked decrease in Sym-H
index may be used as proxies to determine the occurrence as well
as the time of penetration of electric fields into equatorial and
low latitudes. However, in such cases where the AE-index does not
represent any sudden increase, the dSymH/dt seems to be the better
index to determine the time of penetration. In this paper, is also
presented an interesting case where the prompt penetration eastward
electric fields dominated the existing strong westward electric fields
and subsequently caused the onset of spread-F and scintillations at
both VHF (244 MHz) as well as L-band (1.5 GHz) frequencies.
Title: Plasmaspheric electron content (PEC) over low latitude regions
around the magnetic equator in the Indian sector during different
geophysical conditions
Authors: Manju, G.; Ravindran, Sudha; Devasia, C. V.; Thampi, Smitha
V.; Sridharan, R.
Bibcode: 2008JASTP..70.1066M
Altcode:
The variation of plasmaspheric electron content (PEC) is an important
parameter for studying the effects of space weather events in the low
latitude ionosphere. In the present study, the vertical TEC (VTEC)
measurements obtained from co-located dual-frequency Global Positioning
System (GPS) and Coherent Radio Beacon Experiment (CRABEX) systems
have been used. The daytime PEC variations under different geophysical
conditions have been estimated (around the magnetic equator) over
the Indian sector, for the first time. The first observations of the
nighttime PEC variations over the Indian sector are also estimated from
the simultaneous measurements of Faraday rotation, differential Doppler
and modulation phase delay made using the CRABEX system on-board the
Indian geostationary satellite GSAT2. The study shows that the PEC
varies over a range of 10-22% (of the total electron content (TEC))
during daytime of magnetically quiet period. There is an increase in PEC
with latitude during magnetically quiet period. During a magnetically
disturbed period of 9 November 2004, the PEC increased to ~30% of
the TEC over the magnetic equatorial location of Trivandrum (8.5°N,
76.9°E, dip 0.5°N), while at Bangalore (13°N, 78°E, dip 10°N)
it showed a large depletion. The implications of the new observations
are discussed.
Title: Investigation of the response of equatorial MLTI region during
a partial solar eclipse through ground-based daytime optical technique
Authors: Vineeth, C.; Pant, Tarun K.; Thampi, Smitha V.; Sridharan,
R.; Ravindran, Sudha; Devasia, C. V.; Kishore Kumar, K.; Alex, S.
Bibcode: 2008JGRA..113.3302V
Altcode:
First experimental evidence for solar eclipse induced changes in the
equatorial mesopause and the thermosphere ionosphere regions using
daytime optical photometer is presented. The photometer was operated
in scanning mode to probe the thermosphere and mesopause regions
simultaneously in North-South direction over Trivandrum (8.5°N; 77°E;
dip lat. 0.5°N) during a partial solar eclipse event on 03 October
2005. This provided a unique data set of thermosphere and mesopause
regions over a horizontal distance of 600 and 240 km respectively from
zenith during this event. The striking feature observed during the
eclipse is the enhancement in the thermospheric O(1D) 630
nm dayglow intensity and its equatorward movement. Another noteworthy
observation is an overall enhancement in the mesopause temperature,
more pronounced (∼25-30 K) over zenith. These observations are
discussed in context of the vertical coupling of the mesopause with
thermosphere-ionosphere region.
Title: Mid-Infrared Photometry and Spectra of Three High-Mass
Protostellar Candidates at IRAS 18151-1208 and IRAS 20343+4129
Authors: Campbell, M. F.; K., T.; Sridharan; Beuther, H.; Lacy, J. H.;
Hora, J. L.; Zhu, Q.; Kassis, M.; Saito, M.; De Buizer, J. M.; Fung,
S. H.; Johnson, L. C.
Bibcode: 2008ApJ...673..954C
Altcode: 2007arXiv0710.3754C
We present arcsecond-scale mid-IR photometry (in the 10.5 μm N band
and at 24.8 μm), and low-resolution spectra in the N band (R simeq 100)
of a candidate high-mass protostellar object (HMPO) in IRAS 18151-1208
and of two HMPO candidates in IRAS 20343+4129, IRS 1 and IRS 3. In
addition, we present high-resolution mid-IR spectra (R simeq 80,000)
of the two HMPO candidates in IRAS 20343+4129. These data are fitted
with simple models to estimate the masses of gas and dust associated
with the mid-IR-emitting clumps, the column densities of overlying
absorbing dust and gas, the luminosities of the HMPO candidates,
and the likely spectral type of the HMPO candidate for which [Ne II]
12.8 μm emission was detected (IRAS 20343+4129 IRS 3). We suggest that
IRAS 18151-1208 is a pre-ultracompact H II region HMPO, IRAS 20343+4129
IRS 1 is an embedded young stellar object with the luminosity of a B3
star, and IRAS 20343+4129 IRS 3 is a B2 ZAMS star that has formed an
ultracompact H II region and disrupted its natal envelope.
Title: Small Satellites for Atmospheric and near earth Space sciences
- the Indian perspectives of a low cost access to Space
Authors: Sridharan, R.
Bibcode: 2008cosp...37.3008S
Altcode: 2008cosp.meet.3008S
Small satellites of 100-400 kg class are expected to play bigger roles
in the years to come. With the advancement of technology in terms
of miniaturization and also reliability, it has become possible to
configure small satellites which otherwise would have demanded larger
platforms, both in terms of weight and power. The atmospheric and near
Earth space processes are truly multi-dimensional and are extremely
complex with large temporal and spatial variability and also respond
closely to the processes in the Sun. As a consequence, no single
satellite mission would be able to provide the complete information
thus warranting multiple missions. With the successful demonstration
of multiple satellites launching capability, the spare capacity of
the launch vehicles could be effectively and judiciously used for
launching dedicated small scientific satellites as co passengers
with negligible cost factor. This is viewed as an opening up of an
otherwise difficult opportunity involving dedicated launches. With
the prospect of multiple satellites for science missions becoming a
reality the overall mission with an active life of 2-3 years could
be realized with judicious choice of components. This is expected to
bring in a larger user community in the country. The first step in this
direction is the configuration of a modular micro and small satellite
bus. The upcoming missions of TWSAT (Third world satellite), Youth Sat
(active participation of the student community), SARAL (Satellite for
ARGOs and Altimetry), SENSE/E and SENSE/P (Satellite for Earth's Near
Space environment), ISTAG (Indian Satellite for Aerosols and Gases),
are utilizing the above concepts. ISRO has also come out with AO's
for Astronomy and Astrophysics payloads, as most of the stringent
requirements of various experiments could be met with the small
satellite platforms themselves. A brief outline of the upcoming and
proposed activities would be presented and discussed in the talk.
Title: Geomagnetic storm effect on low-latitude total electron content
Authors: Bagiya, Mala; Thampi, Smitha; Aggarwal, Malini; Ravindran,
Sudha; Joshi, H. P.; Iyer, K. N.; G, Manju; Sridharan, R.
Bibcode: 2008cosp...37..158B
Altcode: 2008cosp.meet..158B
During geomagnetically disturbed conditions electrodynamic coupling
between high and lowlatitude ionosphere dominates the ionospheric
behavior at low latitudes. The interplanetary electric field coupled to
low-latitude ionosphere plays a significant role in low-latitude Total
Electron Content (TEC) variation during geomagnetic storms. In the
present paper, TEC observations using GPS satellites from a chain of
GPS receivers extending from the magnetic equator to the crest of the
Equatorial Ionization Anomaly (EIA) and beyond, in the Indian region
during three storm periods in May 2005, a low solar activity period,
are used to bring out for the first time, the TEC behavior during
storms over such a wide geographical region. The EIA development is
found to be strongly modulated by the storm-time electric field. The
double main phase moderate geomagnetic storm on 8 May 2005 causes
an enhancement and poleward movement of EIA crest on 9 and 10 May,
which recovers to normal values on 11 May, in accordance with the
storm evolution. Similar analysis for the strong storm of 15 May and
weak storm of 20 May are also presented. The reasons for the observed
differences in the TEC behavior between these storms are discussed in
detail in terms of the physical processes.
Title: The polar Sudden Stratospheric Warming (SSW) and it's possible
manifestations in the equatorial Mesosphere-Thermosphere-Ionosphere
Authors: Pant, Tarun; Vineeth, C.; Sridharan, R.
Bibcode: 2008cosp...37.2343P
Altcode: 2008cosp.meet.2343P
In this study, the variations in daytime mesopause temperature and the
Equatorial Electrojet over equator during Sudden Stratospheric Warming
(SSW) events over high latitudes have been investigated. To reflect
upon the stratospheric conditions NCEP-NCAR reanalysis data have also
been used. This study indicates a possible dynamical coupling between
the two regions through the planetary wave activity. The amplified
wave signatures of quasi-16 day period are seen in the equatorial
mesopause temperature and zonal mean polar stratospheric temperature
(at 10 hPa) during the course of SSW. The possibility that the planetary
waves over the polar stratosphere, which play an important role in the
generation of SSW, could also have contribution from the tropics has
been indicated through numerical simulations in the past [Dunkerton,
1981], but due to the paucity of global measurements it could not
be established unequivocally. These simulations also indicated the
presence of a zero-wind line whose real counterparts were not observed
in the atmosphere. The NCEP-NCAR reanalysis of stratospheric wind
and temperatures clearly shows that (i) a dynamical feature similar
to the zero-wind line appears over the tropics 60 days prior to the
major warming and progresses poleward and, (ii) enhanced PW activity
is seen almost simultaneously. This study shows that the recent SSW
events had tropical associations. Further, favored occurrences of
Equatorial Counter Electrojets (CEJs) with a quasi 16-day periodicity
over Trivandrum (8.5oN, 76.5oE, 0.5oN diplat.) in association with the
polar Stratospheric Sudden Warming (SSW) events are presented. It is
seen that, the stratospheric temperature at 30 km over Trivandrum showed
a sudden cooling prior to the SSW and the first bunch of CEJs occurred
around this time. Stratospheric zonal mean zonal wind at 30 km exhibited
a distinctly different pattern during the SSW period. These circulation
changes are proposed to be conducive for the upward propagation of
the westward waves (both gravity and planetary) over the equatorial
latitudes. The interaction of such waves with the tidal components at
the upper mesosphere and its subsequent modification are suggested
to be responsible for the occurrence of CEJs having planetary wave
periods. This result is significant as it presents a new perspective
for understanding the mechanism that causes the SSW and its subsequent
global manifestations.
Title: On the response of the Indian low latitude ionospheric regions
to the magnetic disturbances of October 2003
Authors: G, Manju; Pant, Tarun; Ravindran, Sudha; Sridharan, R.
Bibcode: 2008cosp...37..956G
Altcode: 2008cosp.meet..956G
: The response of the equatorial and low latitude ionosphere to
the magnetic disturbances of 29 October 2003 are presented.This
study has been carried out using TEC values derived from GPS data
at the locations of Jodhpur (26.3 ° N, 73° E) and Delhi (28 ° N,
77° E). The equatorial F-region response has been studied using the
ionosonde data at Trivandrum (8.5 ° N, 77° E) / SHAR. The magnetic
field data from Trivandrum has also been used. The GPS TEC data show
large enhancements following the storm sudden commencement. The IMF
Bz fluctuations are also observed periodically on this day after the
sudden commencement. The ionosonde data derived real height profiles
at Trivandrum, the magnetic equatorial location show large increases
periodically. The TEC enhancements at the regions around and beyond
the anomaly crest are attributed to the storm induced super fountain
effect at Indian longitudes. The details will be presented.
Title: Seeing induced cross-talks in filter-based velocity
measurements
Authors: Sreejith, P.; Sridharan, R.; Sankarasubramanian, K.
Bibcode: 2008BASIP..25...40S
Altcode: 2008BASI...25S..40S
Narrow band imaging using Universal Birefringent Filter (UBF) can
be used to measure velocity features on the Sun. Velocity maps,
in filter-based observations, are created by taking the intensity
differences between the blue and red wing images of a chosen spectral
line. In the case of variable seeing, there is a large probability
that one of the (red- or blue-) wing images can be affected more
compared to the other. Such differential seeing leads to spurious
velocity values which are contributed due to cross-talk from the
intensity, and found to be more pronounced in umbra and penumbra of
sunspots. Simulations are carried out to estimate and understand the
sources of the cross-talk. Variable seeing conditions are created
using the Adaptive Optics Performance Evaluator (AOPE) software. The
point-spread-function (PSF) produced by the AOPE is used to generate the
red- and blue-wing images. In this paper, we also discuss the advantages
of using AO corrected images in such variable seeing scenario.
Title: The Sub-keV Atom Reflecting Analyzer on Chandrayaan-1
Authors: Wieser, Martin; Barabash, Stas; Bhardwaj, Anil; Sridharan,
R.; Futaana, Yoshifumi; Holmström, Mats; Asamura, Kazushi; Wurz, Peter
Bibcode: 2008cosp...37.3464W
Altcode: 2008cosp.meet.3464W
The Sub-keV Atom Reflecting Analyzer (SARA) on the Indian Chandrayaan-1
mission to the Moon will image the solar wind-surface interaction to
study surface composition and surface magnetic anomalies and associated
mini-magnetospheres. SARA consists of a Digital Processing Unit (DPU)
and two sensors, the Solar WInd Monitor (SWIM) for solar wind monitoring
and the Chandrayaan-1 Energetic Neutrals Analyzer (CENA), an imaging
neutral atom spectrometer to observe atoms sputtered from the lunar
surface by the solar wind as well as backscattered hydrogen. We review
the status of the instrument and give an overview of science targets.
Title: Crowded-Field Astrometry with SIM PlanetQuest. I. Estimating
the Single-Measurement Astrometric Bias Arising from Confusion
Authors: Sridharan, R.; Allen, Ronald J.
Bibcode: 2007PASP..119.1420S
Altcode: 2007arXiv0710.4262S
The accuracy of position measurements of stellar targets with the
future Space Interferometry Mission PlanetQuest (SIM) will be limited
not only by photon noise and by the properties of the instrument
(design, stability, etc.) and the overall measurement program (observing
strategy, reduction methods, etc.), but also by the presence of other
"confusing" stars in the field of view (FOV). We use a simple phasor
model as an aid to understanding the main effects of this confusion
bias in single observations with SIM. This analytic model has been
implemented numerically in computer code and applied to a selection
of typical SIM target fields drawn from some of the key projects
already accepted for the mission. We expect that less than 1% of all
SIM targets will be vulnerable to confusion bias; we show that for
the present SIM design, confusion may be a concern if the surface
density of field stars exceeds 0.4 star <mml:msup> arcsec -
2 </mml:msup> . We have developed a software tool as an aid
to ascertaining the possible presence of confusion bias in single
observations of any arbitrary field. Some a priori knowledge of the
locations and spectral energy distributions of the few brightest stars
in the FOV is helpful in establishing the possible presence of confusion
bias, but the information is in general not likely to be available with
sufficient accuracy to permit its removal. We discuss several ways of
reducing the likelihood of confusion bias in crowded fields. Finally,
several limitations of the present semianalytic approach are reviewed,
and their effects on the present results are estimated. The simple model
presented here provides a good physical understanding of how confusion
arises in a single SIM observation, and has sufficient precision to
establish the likelihood of a bias in most cases. We close this paper
with a list of suggestions for future work on this subject.
Title: Estimation of confusion in SIM targets with a detailed focal
plane model
Authors: Sridharan, R.; Allen, R. J.
Bibcode: 2007SPIE.6693E..1HS
Altcode: 2007SPIE.6693E..45S
The Space Interferometry Mission is an unique interferometer capable of
performing narrow and wide angle astrometry on a few thousands of stars,
distributed all over the Galaxy. It will be designed to achieve a single
epoch precision of 10 micro arc seconds and an end of mission accuracy
of 4 micro arc seconds in position and a similar accuracy in parallax
and proper motions. The presence of confusing background and foreground
stars might impose a limitation on the astrometric accuracy. We estimate
the expected single measurement position uncertainty of the targets,
owing to the presence of the confusing stars, from the knowledge of
the dispositions and the spectral energy distributions (SEDs) of the
stars within and just outside the field-of-view (FOV) of SIM. Our
model also includes details of the instrumental parameters and the
measurement process. The estimated uncertainties can in turn be used
to correct the bias in the single measurement astrometric delay and,
thus the final astrometric accuracy can be improved. We estimate
the offsets from the zero delay position of the instrument and the
projected separation of the components of binary stars in an elemental
observation, following an one-dimensional synthesis imaging approach
and a model fit to the absolute visibility data. These simulations
help us to explore the strategies that can be followed to extract the
details of the field through suitable model parameters in future.
Title: An image stabilization system for solar observations
Authors: Sridharan, R.; Raja Bayanna, A.; Louis, Rohan Eugene; Kumar,
Brajesh; Mathew, Shibu K.; Venkatakrishnan, P.
Bibcode: 2007SPIE.6689E..0ZS
Altcode: 2007SPIE.6689E..29S
An image stabilization system has been developed and demonstrated
for solar observations in the visible wave-length at Udaipur Solar
Observatory (USO) with a 15 cm Coudé-refractor. The softwa4re and
hardware components of the system are similar to that of the low cost
solar adaptive optics system developed for the 1.5 m McMath-Pierce
solar telescope at Kitt Peak observatory for solar observations in the
infrared. The first results presented. The system has a closed loop
correction bandwidth in the range of 70 to 100 Hz. The root mean by a
factor of 10 to 20. The software developes and key issues concerning
optimum system performance have been addressed.
Title: Investigation on the mesopause energetics and its possible
implications on the equatorial MLTI processes through coordinated
daytime airglow and radar measurements
Authors: Pant, Tarun Kumar; Tiwari, Diwakar; Vineeth, C.; Thampi,
Smitha V.; Sridharan, S.; Devasia, C. V.; Sridharan, R.; Gurubaran,
S.; Sekar, R.
Bibcode: 2007GeoRL..3415102P
Altcode:
This paper investigates the simultaneous presence of a prominent
planetary scale modulations of quasi 16-day period observed in the
daytime mean mesopause temperature and in the phase velocity of
Type I electrojet irregularities during January-March 2001 over
Trivandrum (8.5°N, 76.5°E, 0.5°N diplat.). The temperature
was estimated from the Multiwavelength Dayglow Photometer (MWDPM)
measurements of OH (8-3) Meinel band dayglow emissions. The radar
measured phase velocity of the irregularities is found not only
exhibiting ~8 and quasi 16-day modulations, but also exceeding the
isothermal ion-acoustic threshold i.e. ~360 m/s for Trivandrum. The
modulation in the mesopause temperature is attributed to the presence
of planetary wave of quasi 16-day periodicity. Further, it is proposed
that the temperature modulations which cause composition changes in
the mesopause and equatorial electrojet region manifest as changes
in the phase velocity through fluctuations in ion-neutral collisions
corroborating with St.-Maurice et al. (2003).
Title: Highly localized cooling in daytime mesopause temperature
over the dip equator during counter electrojet events: First results
Authors: Vineeth, C.; Pant, Tarun Kumar; Devasia, C. V.; Sridharan, R.
Bibcode: 2007GeoRL..3414101V
Altcode:
The first observations of lowering of mesopause temperature during
Counter Electrojet (CEJ) events over a narrow region of ~ +/-150 km
centered at around magnetic equator are presented. The daytime mesopause
temperature is measured over Trivandrum (8.5°N 77°E dip lat. 0.5°N),
India using the ground based Multiwavelength Dayglow Photometer. The
unique meridional scanning capability of the instrument is extensively
used in this study. A lowering of temperature by as much as ~25 K
has been observed during certain CEJ events, which includes a few
partial CEJs. The gravity wave tidal interaction through vertically
upward wind is proposed to be manifesting as lowering in the mesopause
temperature and also as CEJ. These observations are `new' and address
to the issues concerning the vertical coupling processes prevailing in
the equatorial Mesosphere Lower Thermosphere Ionosphere (MLTI) region.
Title: Atmosphere-Ionosphere coupling observed over the dip equatorial
MLTI region through the quasi 16-day wave
Authors: Vineeth, C.; Pant, Tarun Kumar; Devasia, C. V.; Sridharan, R.
Bibcode: 2007GeoRL..3412102V
Altcode:
The present paper analyses the variations of daytime mesopause
temperature and the Equatorial electrojet strength over the dip equator
during December 2005-March 2006 period, indicating a possible strong
dynamical coupling between the two region through the intensification
of planetary wave activity. The wave signatures (quasi 16-day period)
are seen in the mesopause temperature and the electrojet-induced
surface magnetic field, measured from Trivandrum (8.5°N, 76.5°E,
0.5°N diplat.), a geomagnetic dip equatorial station in India. This
investigation reveals (1) amplification of the quasi 16-day wave in
the equatorial mesopause temperature and the EEJ induced magnetic field
(2) regular occurrence of Counter Electrojet (CEJ), with a periodicity
of ~16 days (3) diminishing CEJ strength concurrent with the damping
of the amplitude of wave oscillation with time. These results are new
and reveal some newer aspects of the Mesosphere Lower Thermosphere
Ionosphere (MLTI) dynamics.
Title: An additional layer in the low-latitude ionosphere in Indian
longitudes: Total electron content observations and modeling
Authors: Thampi, Smitha V.; Balan, N.; Ravindran, Sudha; Pant, Tarun
Kumar; Devasia, C. V.; Sreelatha, P.; Sridharan, R.; Bailey, G. J.
Bibcode: 2007JGRA..112.6301T
Altcode: 2007JGRA..11206301T
The paper presents the observations and modeling of an additional
layer in the low-latitude ionosphere in Indian longitudes. The
signatures of the additional layer are observed as ledges or humps
between the equatorial ionization anomaly trough and crest (EIA) in
the latitudinal profiles of total electron content (TEC), measured
using a single ground-based beacon receiver located at Trivandrum
(8.5°N, 77°E, dip 0.5°N) in India. The ground-based ionograms also
show the presence of the so-called F3 layer for a short
duration corresponding to these signatures, and the layer is found
to drift upward to the topside ionosphere. The study provides first
observational evidence that the so-called "humps" in the latitudinal
variation of TEC are nothing but the upward propagating F3
layer. This conclusion is supported by theoretical modeling using the
Sheffield University Plasmasphere Ionosphere Model. It is shown that
upward ExB drift and strong equatorward neutral wind (perturbed by
atmospheric waves) can produce the humps in the latitudinal variation
of TEC through the reduction in the downward diffusion of ionization
along geomagnetic field lines. The model results also show that the
F3 layer drifts to the topside and forms topside ledges.
Title: Local time dependant response of Indian equatorial ionosphere
to the moderate geomagnetic storms
Authors: Tulasi Ram, S.; Rama Rao, P. V. S.; Prasad, D. S. V. V. D.;
Niranjan, K.; Sridharan, R.; Devasia, C. V.; Ravindran, Sudha
Bibcode: 2007AdSpR..39.1304T
Altcode:
The electrodynamics and neutral dynamics of the equatorial ionosphere
undergo significant changes during geo-magnetically disturbed
periods as a result of the high and low latitude electro-dynamical
coupling. These changes may sometimes trigger the plasma instabilities
that lead to the onset of spread-F and scintillations even at the L-band
frequencies. The response of the equatorial ionosphere-thermosphere
system over the Indian sector to moderate geomagnetic storms of 11th
Feb 2004 and 9th Mar 2004 has been studied using the ground based
measurements from the Indian equatorial and low latitude region. A
large vertical drift of the equatorial F-layer followed by nearly
simultaneous onset of Spread-F on ionograms and scintillations at
VHF (244 MHz) and L-band (1.5 GHz) frequencies, have been observed
during the early evening periods that correspond to the time of rapid
decrease (-25 to -30 nT/h) in Sym-H index due to prompt penetration
of eastward electric fields into equatorial and low latitudes. The
Spread-F and scintillations continued to exist for longer durations
up to post-midnight to pre-dawn hours, possibly due to the combined
action of prompt penetration and long-lived Disturbance Dynamo electric
fields. Due to prolonged geomagnetic activity during the subsequent days
(morning-to-noon hours), strong reversal in the Equatorial ElectroJet
current is observed and the Equatorial Ionization Anomaly (EIA) is
significantly suppressed under the influence of westward disturbance
dynamo electric fields. The post-sunset enhancements in the vertical
drift of the equatorial F-layer are also reduced significantly from
their quiet day patterns and the subsequent occurrence of Spread-F
and scintillations is inhibited.
Title: Coherent Radio Beacon Experiment (CRABEX) for tomographic
imaging of the equatorial ionosphere in the Indian longitudes
Preliminary results
Authors: Thampi, Smitha V.; Ravindran, Sudha; Devasia, C. V.;
Sreelatha, P.; Pant, Tarun K.; Sridharan, R.; Ratnam, D. Venkata;
Sarma, A. D.; Reddi, C. Raghava; Jose, Jessy; Sastri, J. H.
Bibcode: 2007AdSpR..40..436T
Altcode:
To investigate the equatorial ionospheric processes like the equatorial
ionization anomaly (EIA), equatorial spread F (ESF) and their inter
relationships, a network of ground based stations receiving the
150 and 400 MHz transmissions from the low earth orbiting satellites
(LEOS) is set up along the 77-78°E longitudes covering the equatorial
and low-latitude region. The receivers at these stations measure the
relative phase of the 150 MHz beacon with respect to that at 400 MHz,
which is proportional to the slant total electron content (TEC) along
the line of sight. These simultaneously measured TECs are used to obtain
the latitude-altitude distribution of electron densities (tomograms) in
the meridional plane. The tomograms are generated using the Algebraic
reconstruction technique (ART), a standard inversion algorithm. In
this paper, the first tomographic images of the equatorial ionosphere
along the 77-78°E meridian are presented, signalling the addition of a
new dimension to the investigation of equatorial ionosphere. The images
indicate the temporal evolution of the equatorial ionosphere and EIA, as
well as the variability in the strength of EIA at various local times.
Title: Interplay Between the Equatorial Geophysical Processes
Authors: Sridharan, R.
Bibcode: 2006ihy..workE..52S
Altcode:
r_sridharanspl@yahoo.com With the sun as the main driving force,
the Equatorial Ionosphere- thermosphere system supports a variety of
Geophysical phenomena, essentially controlled by the neutral dynamical
and electro dynamical processes that are peculiar to this region. All
the neutral atmospheric parameters and the ionospheric parameters show a
large variability like the diurnal, seasonal semi annual, annual, solar
activity and those that are geomagnetic activity dependent. In addition,
there is interplay between the ionized and the neutral atmospheric
constituents. They manifest themselves as the Equatorial Electrojet
(EEJ), Equatorial Ionization Anomaly (EIA), Equatorial Spread F (ESF),
Equatorial Temperature and Wind Anomaly (ETWA). Recent studies have
revealed that these phenomena, though apparently might show up as
independent ones, are in reality interlinked. The interplay between
these equatorial processes forms the theme for the present talk.
Title: Deterministic prediction of post-sunset ESF based on the
strength and asymmetry of EIA from ground based TEC measurements:
Preliminary results
Authors: Thampi, Smitha V.; Ravindran, Sudha; Pant, Tarun Kumar;
Devasia, C. V.; Sreelatha, P.; Sridharan, R.
Bibcode: 2006GeoRL..3313103T
Altcode:
This paper provides the first observations of EIA asymmetry by receiving
beacon transmissions onboard low earth orbiting satellites from a
single station ground-based receiver. The EIA strength and asymmetry
are derived from the latitudinal profiles of TEC obtained from a radio
beacon receiver at Trivandrum (8.5°N, 77°E, diplat ~0.5°N). These two
parameters, obtained well ahead of the onset time of ESF, are shown to
have a definite role on the subsequent ESF activity. In the present
paper, both these factors are combined to define a new `threshold
parameter' for the generation of ESF. It has been shown that this
parameter can define the state of the `background ionosphere' conducive
for the generation of ESF irregularities much prior to its onset.
Title: Energetic neutral atom imaging mass spectroscopy of the Moon
and Mercury environments
Authors: Kazama, Yoichi; Barabash, Stas; Bhardwaj, Anil; Asamura,
Kazushi; Futaana, Yoshifumi; Holmström, Mats; Lundin, Rickard;
Sridharan, R.; Wurz, Peter
Bibcode: 2006AdSpR..37...38K
Altcode:
Low-energy neutral atom (LENA) imaging is an important technique for
doing planetary sciences at magnetized and unmagnetized planets. In
the case of the Moon, the precipitating solar-wind causes sputtering,
which releases surface atoms as LENAs into space. Moreover, the
solar-wind ions may be back-scattered from the surface into space
as neutral atoms. At Mercury, in addition to the above processes,
LENAs are also generated by the charge-exchange of energetic ions with
the exospheric gasses. Global LENA mass spectroscopic imagery at the
Moon and at Mercury provides us information on their surfaces and the
interaction processes between energetic particles and the surfaces
via remote-sensing using LENAs. We are developing a state-of-the-art
LENA instrument for the Indian lunar exploration mission Chandrayaan-1
and the Mercury exploration mission BepiColombo. The instrument is
light-weight and capable of mass discrimination, including heavy
components such as iron, and has high sensitivity to fulfill various
scientific objectives in the area of planetary sciences.
Title: Panel discussion: ILWS Panel Discussion on Future
Collaborations
Authors: Spann, J. F.; Sridharan, R.
Bibcode: 2006ilws.conf...13S
Altcode:
No abstract at ADS
Title: Prediction of post-sunset ESF based on the strength and
asymmetry of EIA from ground based TEC measurements
Authors: Thampi, S. V.; Ravindran, S.; Devasia, C. V.; Pant, T. K.;
Sreelatha, P.; Sridharan, R.
Bibcode: 2006ilws.conf..414T
Altcode:
The Coherent Radio Beacon Experiment (CRABEX) is aimed
at investigating the equatorial ionospheric processes like the
Equatorial Ionization Anomaly (EIA) and Equatorial Spread F (ESF)
and their inter relationships. As a part of CRABEX program, a
network of six stations covering the region from Trivandrum (8.5°N)
to Nainital (29.3°N) is set up along the 77-78° E meridian. These
ground receivers basically measure the slant Total Electron Content
(TEC) along the line of sight from the Low Earth Orbiting satellites
(NIMS). These simultaneous TEC measurements are inverted to obtain
the tomographic image of the latitudinal distribution of electron
densities in the meridional plane. In this paper, the tomographic
images of the equatorial ionosphere along the 77-78°E meridian
are presented. The crest intensities in the southern and northern
hemispheres also show significant differences with seasons, showing
the variability in the EIA asymmetry. The evening images give an
indication of the prevailing electrodynamical conditions on different
days, preceding the occurrence/non-occurrence of ESF. Apart from this,
the single station TEC measurements from the Trivandrum station itself
is used to estimate the EIA strength and asymmetry. Since this station
is situated at the trough of the EIA, right over the dip equator,
the latitudinal gradients on both northern (N) and southern (S)
sides can be used to compute the EIA strength and asymmetry. These
two parameters, obtained well ahead of the onset time of ESF, are
shown to have a definite role on the subsequent ESF activity. Hence,
both these factors are combined to define a new `forecast parameter'
for the generation of ESF. It has been shown that this parameter can
uniquely define the state of the `background ionosphere' conducive for
the generation of ESF irregularities as early as 1600 IST. A critical
value for the `forecast parameter' has been identified such that when
the estimated value for `forecast parameter' exceeds it, the ESF is
seen to occur. It is also observed that this critical value varies
with season. All these aspects are studied in detail and the results
are presented.
Title: Investigation of the mesopause energetics and its possible
implications on the mesosphere-lower thermosphere -ionosphere (MLTI)
processes
Authors: Pant, T.; Vineeth, C.; Sridharan, R.; Devasia, C.
Bibcode: 2006cosp...36.1405P
Altcode: 2006cosp.meet.1405P
A significant contribution to the energy and momentum budget of the
MLTI region comes from processes like the tides and the gravity waves
The measured temperature and wind fields around mesopause reflect the
changes in the energetics and the dynamics of this region brought about
by various competing insitu and non-local processes at any given time
and location The measurements on the night airglow emission intensities
OH band emissions in particular had been traditionally used to get
an indirect estimate of the temperature in the source region close
to mesopause Similar measurements during daytime have been proved
to be technically challenging The unique Multiwavelength dayglow
photometer MWDPM operating from Thiruvananthapuram 8 5oN 76 5oE 0 5odip
an equatorial station in India has been providing these much needed
continuous daytime measurements in recent times Measurements involving
MWDPM VHF radar and ionosonde from this station has brought out some
interesting results on the mesopause 87km temperature equatorial
electrojet and thermospheric dayglow O1D 630 0nm intensity vis-a-vis
the ionospheric parameters measured by radar at altitudes slightly
higher above Few of these results are mentioned below Significant
day-to-day variations in the overall mesopause temperature are observed
The monthly mean temperatures exhibit distinct trends The mean mid-day
temperatures during winter months exhibit the presence of waves with
an approximate period of 16 days While short-term periodic trends are
quite frequent in the estimated
Title: Plasmaspheric electron content variation in the magnetic
equatorial region during space weather events: Results from the
CRABEX (Coherent Radio Beacon Experiment) using the beacon onboard
the Indian geostationary satellite (GSAT - 2)
Authors: Ravindran, Sudha; Manju, G.; Devasia, C. V.; Sridharan,
R.; Thampi, S. V.; Sreelatha, P.; Sreeja, V.; Pant, T. K.; Raghava
Reddi, C.
Bibcode: 2006cosp...36.1784R
Altcode: 2006cosp.meet.1784R
CRABEX is a national scientific program for the investigation of
the unique features associated with the equatorial and low latitude
ionosphere in the Indian zone using the technique of ionospheric
tomography It consists of a beacon transmitter onboard the Indian
geostationary satellite GSAT-2 which transmits four coherently generated
frequencies - 150 012MHz 400 032MHz along with 1 MHz modulation of
these frequencies i e 149 01192 MHz and 399 03192 MHz and a unique
ground receiver system designed and set up at Trivandrum dip 0 3 r
N to receive these beacon transmissions The data obtained from the
measurement of the differential phase between 400 MHz and 150 MHz
gives the relative Total Electron Content TEC along the line of sight
between the satellite and the ground receiver and the measurement of
modulation phase delay of 1 MHz on the above frequencies provides
a coarse estimate of TEC These two measurements together give an
accurate estimate of TEC along the line of sight from the satellite
to the ground receiver and the Faraday rotation measurements give
a reliable estimate of electron content upto sim 2000 km i e the
ionospheric content IEC The simultaneous measurements of IEC and TEC
upto the geo-stationary altitude of 36000 km is used to determine the
plasmaspheric electron content PEC The CRABEX program with its another
segment consisting of a network of 6 receiver stations over the Indian
subcontinent established along 77-78 r E meridian also makes use of
the data obtained by receiving the 150 and 400 MHz
Title: Seasonal variation of the 'forecast parameter' for the
occurrence of post-sunset ESF using ground based TEC measurements
Authors: Thampi, S. V.; Ravindran, Sudha; Pant, Tarun; Sreelatha,
P.; Devasia, C. V.; Sridharan, R.
Bibcode: 2006cosp...36.1781T
Altcode: 2006cosp.meet.1781T
Even though the importance of ambient ionospheric and thermospheric
conditions in the initiation and non-linear development of ESF and
its dynamics has been well recognized the day-to-day variability of
ESF has been remaining an enigma All the studies demonstrate that the
daytime electrodynamics play a decisive role in the initiation of ESF
on a given day and hence parameters like EIA strength have been used for
forecasting the ESF However all such predictions were only probabilistic
This paper attempts to give deterministic prediction of ESF using
the observations of EIA strength and asymmetry These observations are
made using the Coherent Radio Beacon receiver CRABEX receiver located
at Trivandrum textsc 8 5 r N 77 r E D iplat textsc sim 0 5 r N which
basically receives the 150 and 400 MHz transmissions from the NIMS
satellites and measures the differential Doppler between them This can
be converted to the latitudinal profile of the Vertical Total Electron
Content TEC The TEC data in the time interval 1600-1845 IST has been
chosen for the analysis The EIA strength and asymmetry two parameters
obtained well ahead of the onset time of ESF are combined to define
a new forecast parameter for the generation of ESF It has been shown
that this parameter can uniquely define the state of the background
ionosphere conducive for the generation of ESF irregularities as early
as 1600 IST A critical value for this parameter has been identified such
that if the estimated value exceeds it on a given day the ESF is seen to
Title: On the Characteristic Scales of Solar Convection
Authors: Sridharan, R.
Bibcode: 2005ASPC..346...47S
Altcode:
Granulation, mesogranulation, supergranulation and giant cells are
the four different characteristic scales of convection believed to
exist on the Sun. Three of these, granulation, mesogranulation and
supergranulation, have been clearly identified through ground and space
based observations. There is no clear observational evidence for the
existence of giant cells so far. In this paper, we propose a method
of estimating various spatial scales available in a two dimensional
data using generalised structure functions. We demonstrate the method
through simulations and apply it to explore the spatial scales present
in solar data obtained from Solar and Heliospheric Observatory (SOHO).
Title: Low energy neutral atom imaging on the Moon with the SARA
instrument aboard Chandrayaan-1 mission
Authors: Bhardwaj, Anil; Barabash, Stas; Futaana, Yoshifumi; Kazama,
Yoichi; Asamura, Kazushi; McCann, David; Sridharan, R.; Holmstrom,
Mats; Wurz, Peter; Lundin, Rickard
Bibcode: 2005JESS..114..749B
Altcode:
No abstract at ADS
Title: East-west asymmetries of the equatorial electrojet 8.3 m
type-2 echoes observed over Trivandrum and a possible explanation
Authors: Patra, A. K.; Tiwari, D.; Devasia, C. V.; Pant, T. K.;
Sridharan, R.
Bibcode: 2005JGRA..11011305P
Altcode:
The east-west asymmetries in the spectral parameters of the type-2
echoes from the equatorial electrojet plasma irregularities observed
using a 18 MHz radar from Trivandrum are presented. Observations
show that the difference in signal strength, velocity, and spectral
width of the type-2 echoes observed in the west and east beam are
as high as 15 dB, 60 m s-1, and 70 m s-1,
respectively. Further, the asymmetry in velocity increases with height,
while the asymmetries in signal strength and spectral width decrease
with height. While the velocity asymmetry is consistent with the past
results, the asymmetries in signal strength and spectral width of the
type-2 echoes are significant new results, not reported earlier. Finite
vertical drift velocities of the irregularities, associated with the
primary wave structures, are found to be responsible for the observed
east-west velocity asymmetry. The asymmetries in signal strength and
spectral width are attributed to the orientation of kilometer-scale
plasma waves present in the equatorial electrojet. Finally, a unified
picture is presented to show that the properties of the kilometer-scale
waves are the ones that are responsible for all three asymmetries
observed in the spectral parameters of the type-2 echoes.
Title: A comparative study of daytime mesopause temperatures obtained
using unique ground based optical and meteor wind radar techniques
over the magnetic equator
Authors: Vineeth, C.; Pant, Tarun Kumar; Antonita, Maria; Ramkumar,
Geetha; Devasia, C. V.; Sridharan, R.
Bibcode: 2005GeoRL..3219101V
Altcode:
This paper presents the first ever comparison of daytime Mesopause
OH rotational temperatures as determined using Meinel (8-3) dayglow
emissions with those obtained using the recently established and
collocated, meteor radar (SKiYMET) over the magnetic equator in
India. The measurements were made over Thiruvananthapuram (8.5°N,
76.5°E, 0.5°N diplat.) for January-May 2005 period. Overall
agreement had been good (+/-20 K) with all the gross features coming
out fairly well. Further, the temperatures estimated using these two
techniques, exhibit significant wavelike modulations with periods
spanning ~3-65 days with varying phase differences, presumably due to
their interaction with the longer period waves like the semiannual
oscillation (SAO). The consistency of the temperature variabilities
at two different altitudes viz. 87 and 94 km enables investigation of
the mesosphere lower thermosphere dynamic coupling.
Title: Phase Diversity Technique for High Resolution Solar Imaging
Authors: Bayanna, A. Raja; Sridharan, R.; Venkatakrishnan, P.
Bibcode: 2005BASI...33..413B
Altcode:
No abstract at ADS
Title: Imaging with insolated mirrors
Authors: Venkatakrishnan, P.; Sridharan, R.; Gupta, S. K.
Bibcode: 2005BASI...33..265V
Altcode:
Modern solar telescope designs are different from the conven- tional
concept of vacuum telescopes. These new designs are "open" telescopes
which try to minimize the temperature difference between various parts
of the telescope and the ambient air. In this paper, we address a
few issues related to the thermal response and image quality of such
insolated mirrors. We estimate the distortion produced by thermal
and material inhomogeneities and present limiting values of allowable
temperature differences and percentage change of expansion coefficients
for different aperture diameters, for typical materials under best
possible seeing conditions. We predict the evolution of surface
temperature of an insolated mirror using a simplified theoretical
approach and show that it is compatible with the experimental values
to a large extent. The results indicate the possibility of avoiding
active cooling of the mirror surfaces, at least for primary mirrors
with aperture diameter less than or equal to 50 cm.
Title: Imaging with Insolated Mirrors
Authors: Venkatakrishnan, P.; Sridharan, R.; Gupta, S. K.
Bibcode: 2005BASI...33..415V
Altcode:
No abstract at ADS
Title: An Image Stabilization System for Solar Observations
Authors: Sridharan, R.; Bayanna, A. Raja; Kumar, Brajesh;
Venkatakrishnan, P.
Bibcode: 2005BASI...33..414S
Altcode:
No abstract at ADS
Title: Preliminary Results on the Calibration and Control of an
Adaptive Optics System
Authors: Kumar, Brajesh; Sridharan, R.; Bayanna, A. Raja;
Venkatakrishnan, P.
Bibcode: 2005BASI...33..413K
Altcode:
No abstract at ADS
Title: First observation of topside ionization ledges using radio
beacon measurements from low Earth orbiting satellites
Authors: Thampi, Smitha V.; Ravindran, Sudha; Devasia, C. V.; Pant,
Tarun K.; Sreelatha, P.; Sridharan, R.
Bibcode: 2005GeoRL..3211104T
Altcode:
Ionization ledges in the topside ionosphere have been observed from
Trivandrum (8.5°N, 77°E, 0.5°N dip) using single station Total
Electron Content (TEC) measurements. These measurements are made
using a ground based radio beacon receiver. The receiver measures the
relative phase between 150 MHz and 400 MHz beacon transmissions from a
Low Earth Orbiting Satellite (LEOS). The ionization ledges, which could
be observed, using only topside sounders so far, are seen to have their
clear signatures in relative TECs derived using differential Doppler
measurements. Simulations were also carried out to understand how the
ionospheric topside ledges of various intensities would manifest in the
latitudinal profiles of the relative TEC. The magnetometer observations
indicate the presence of the Equatorial Counter Electrojet (CEJ) on the
days when the ledge is observed. The study demonstrates the potential
of radio beacon measurements for observing topside ionization ledges,
and phenomena similar to it.
Title: Ultimate Limits to Precision Astrometry with the SIM
PlanetQuest Mission
Authors: Sridharan, R.; Allen, R. J.
Bibcode: 2005AAS...206.1402S
Altcode: 2005BAAS...37..453S
We are initiating a program to study factors which limit the ultimate
astrometric accuracy of the Space Interferometry Mission. Our study
is aimed in particular at modelling and quantifying the disturbances
caused by the presence of additional (faint) confusing sources within
the SIM field of view. An important question to address is whether this
confusion will limit the ultimate accuracy of planet search projects
with SIM before we reach the limits imposed by the SIM instrument
itself. This work will be funded by the NASA SIM Project Office
at JPL.
Title: Simultaneous radar observations of meter-scale F region
irregularities at and off the magnetic equator over India
Authors: Patra, A. K.; Tiwari, D.; Sripathi, S.; Rao, P. B.; Sridharan,
R.; Devasia, C. V.; Viswanathan, K. S.; Subbarao, K. S. V.; Sekar,
R.; Kherani, E. A.
Bibcode: 2005JGRA..110.2307P
Altcode: 2005JGRA..11002307P
Simultaneous observations of equatorial spread F irregularities made
with an 18 MHz radar from Trivandrum, located at the geomagnetic
equator, and a 53 MHz radar from Gadanki, located at a magnetic
latitude of 6.5°N, corresponding to nearly the same longitude zone, are
presented. The observations correspond to 8.3 and 2.8 m irregularities,
respectively. The spread F irregularities at both the locations are
found to occur nearly at the same time but are observed for longer
duration at Gadanki than at Trivandrum. The spread F structures
as observed in the intensity maps corresponding to Gadanki are
characterized by multiple periodic plumes in contrast to a limited
number of plumes observed over Trivandrum. The Doppler velocities
associated with these irregularities corresponding to Trivandrum are in
the range of -100-150 m s-1, whereas they are in the range
of -100-250 m s-1 at Gadanki. Further, the fluctuating
velocity fields are much stronger in the Gadanki observations than
in the Trivandrum observations. Remarkably, the spectral widths are
<100 m s-1 in Trivandrum observations in contrast to those
observed at Gadanki with values as high as 300 m s-1. The
observations are compared with those made elsewhere and are discussed
in the light of current understanding of the meter-scale irregularities
responsible for the radar backscatter.
Title: Simulations of Solar AO Systems
Authors: Sridharan, R.; Bayanna, A. Raja; Venkatakrishnan, P.
Bibcode: 2005sao..conf..132S
Altcode:
In this paper, first we compare the two kinds of algorithms that are
being used in solar AO systems to sense a distorted wave-front through
simulations. Then, we comment on the various issues related to solar
AO systems and describe solar features that can be studied using AO
as a tool. Then we briefly describe the laboratory model of AO that
is being built at the Udaipur Solar Observatory (USO), India.
Title: Simulation studies on the tomographic reconstruction of the
equatorial and low-latitude ionosphere in the context of the Indian
tomography experiment: CRABEX
Authors: Thampi, S.; Pant, T.; Ravindran, S.; Devasia, C.; Sridharan,
R.
Bibcode: 2004AnGeo..22.3445T
Altcode:
Equatorial ionosphere poses a challenge to any algorithm that is
used for tomographic reconstruction because of the phenomena like the
Equatorial Ionization Anomaly (EIA) and Equatorial Spread F (ESF). Any
tomographic reconstruction of ionospheric density distributions in
the equatorial region is not acceptable if it does not image these
phenomena, which exhibit large spatial and temporal variability,
to a reasonable accuracy. The accuracy of the reconstructed image
generally depends on many factors, such as the satellite-receiver
configuration, the ray path modelling, grid intersections and
finally, the reconstruction algorithm. The present simulation study is
performed to examine these in the context of the operational Coherent
Radio Beacon Experiment (CRABEX) network just commenced in India. The
feasibility of using this network for the studies of the equatorial and
low-latitude ionosphere over Indian longitudes has been investigated
through simulations. The electron density distributions that are
characteristic of EIA and ESF are fed into various simulations and the
reconstructed tomograms are investigated in terms of their reproducing
capabilities. It is seen that, with the present receiver chain existing
from 8.5° N to 34° N, it would be possible to obtain accurate images
of EIA and the plasma bubbles. The Singular Value Decomposition (SVD)
algorithm has been used for the inversion procedure in this study. As
is known, by the very nature of ionospheric tomography experiments,
the received data contain various kinds of errors, like the measurement
and discretization errors. The sensitivity of the inversion algorithm,
SVD in the present case, to these errors has also been investigated
and quantified.
Title: Evidence for direct solar control of the mesopause dynamics
through dayglow and radar measurements
Authors: Pant, T.; Tiwari, D.; Sridharan, S.; Sridharan, R.; Gurubaran,
S.; Subbarao, K.; Sekar, R.
Bibcode: 2004AnGeo..22.3299P
Altcode:
Available from http://www.copernicus.org/site/EGU/annales/22/9/3299.htm?FrameEngine=false;
Title: Estimation of Fried's Parameter from Long-Exposure Solar Images
Authors: Sridharan, R.; Dashora, Nirvikar; Venkatakrishnan, P.
Bibcode: 2004SoPh..222...35S
Altcode:
We propose a criterion for extending the parameter search method
(Krishnakumar and Venkatakrishnan, 1997) of estimating the point spread
function to solar data. In the parameter search method, the number of
pixels with negative intensity values in the restored object is used
as an estimator for determining the unknown parameters of the point
spread function. As a solar image has a high background, the restored
object does not contain negative values, thereby making the method
unsuitable for solar data. We propose to use the intrinsic contrast of
solar features as a criterion for identifying the unknown parameter. We
validate our method through simulations. This method can not be used
for image restoration but can be used for monitoring daytime seeing.
Title: Threshold height (h'F)c for the meridional wind
to play a deterministic role in the bottom side equatorial spread
F and its dependence on solar activity
Authors: Jyoti, N.; Devasia, C. V.; Sridharan, R.; Tiwari, Diwakar
Bibcode: 2004GeoRL..3112809J
Altcode:
Detailed investigations have been carried out on the occurrence
of bottom side Equatorial Spread F (ESF) and the thermospheric
meridional wind characteristics just before the former's initiation
using ground based ionospheric data corresponding to the equinoctial
periods of 1993-1998, from Trivandrum (8.5°N, 76.5°E, dip =
0.5°N) and Sriharikota (13.7°N, 80.2°E, dip ~ 10°N) in the Indian
longitudes. Critical analysis of the base height of the F-region h'F
at the time of triggering of ESF and the polarity of the meridional
winds revealed that, if the h'F is above a certain level ESF occurred
under both equatorward and poleward wind conditions. Below that level,
ESF occurred only when equatorward winds were present implying that the
equatorward winds must somehow be able to offset the reduced growth
rate of the plasma instability responsible for ESF. A plausible
explanation linking Equatorial Ionization Anomaly (EIA) and the
consequent Equatorial Temperature and Wind Anomaly (ETWA) and the
consequent neutral dynamics effectively enabling the instability even at
lower height has been offered. The threshold height (h'F)c
gleaned out on the basis of the polarity of the meridional winds has
been shown to bear a linear relation to the solar activity and sheds
light on the enigmatic short and long term variability of ESF.
Title: Radar Observations of 8.3-m scale equatorial spread F
irregularities over Trivandrum
Authors: Tiwari, D.; Patra, A.; Devasia, C.; Sridharan, R.; Jyoti,
N.; Viswanathan, K.; Subbarao, K.
Bibcode: 2004AnGeo..22..911T
Altcode:
Available from http://www.copernicus.org/site/EGU/annales/22/3/911.htm?FrameEngine=false;
Title: Low-order adaptive optics for the meter aperture solar
telescope of Udaipur Solar Observatory
Authors: Sridharan, Rengaswamy; Bayanna, A. R.
Bibcode: 2004SPIE.5171..219S
Altcode:
Meter Aperture Solar Telescope (MAST) is a proposed modern solar
telescope equipped with Adaptive Optics (AO) facility for observing
the Sun in Optical and infra-red wavelengths. It is planned to develop
a low-order AO system at the re-imaged pupil plane of the MAST. Before
developing such an AO system, one would like to answer a few questions
like what is the size of the sub-apertures required to achieve optimum
performance under typical seeing conditions? What is the required
bandwidth? Is it possible to operate the system with a narrow bandwidth
of 0.1 nm? Is it possible to achieve diffraction limited imaging by
using speckle imaging on the low-order AO corrected images? In this
paper, we try to answer these questions through extensive computer
simulations and arrive at a final optimal specification ot the low-order
AO system of the MAST. We simulate distorted wave-fronts for various
seeing conditions (for both Kolmogorov and von Karman spectrum) using
large phase screens generated using Fourier transfrom method. We find
the local slopes of the distorted wave-front over the sub-apertures
of different lenslet array geometries using a least square modal
recontruction method. Then we estimate the structure functions, optical
transfer functions, Strehl resolution of the corrected wave-front
and evaluate the performance. We have developed a speckle-masking
code and parallelised it using a 16-processor IBM-SP machine. We use
a series of AO corrected images to obtain a speckle reconstruction of
the object. We evaluate the performance of this hybrid imaging system
in achieving diffraction limited imaging of small-scale solar features.
Title: Radar observations of 8.3 m scale equatorial spread f
irregularities over Trivandrum
Authors: Devasia, C. V.; Tiwari, D.; Patraa, A. K.; Sridharan, R.;
Jyoti, N.; Viswanathan, K. S.; Subbarao, K. S. V.
Bibcode: 2004cosp...35.2566D
Altcode: 2004cosp.meet.2566D
In this paper, we present observations of equatorial spread F (ESF)
irregularities made using a newly installed 18 MHz radar located at
Trivandrum. We characterize the morphology and the spectral parameters
of the 8.3 m ESF irregularities which are found to be remarkably
different from that observed so extensively at 3 m scale size. We
also present statistical results of the irregularities in the form
of percentage occurrence of the echoes and spectral parameters (SNR,
Doppler velocity, Spectral width). The Doppler spectra are narrower,
less structured and less variable in time as compared to those observed
for 3 m scale size. We have never observed the ESF irregularity
velocities to be supersonic here unlike those at Jicamarca and the
velocities are found to be within ± 200 m s-1. The spectral widths
are found to be less than 150 m s-1. Hence the velocities and spectral
width both are smaller than those reported for 3 m scale size. The
velocities and spectral widths are further found to be much smaller
than those of the American sector. These observations are compared
with those reported elsewhere and discussed in the light of present
understanding on the ESF irregularities at different wavelengths.
Title: An observational study on the Strength and Movement of EIA
in the Indian zone - Results from the Indian Tomography Experiment
(CRABEX)
Authors: Thampi, S. V.; Devasia, C. V.; Ravindran, S.; Pant, T. K.;
Sridharan, R.
Bibcode: 2004cosp...35.2169T
Altcode: 2004cosp.meet.2169T
To investigate the equatorial ionospheric processes like the Equatorial
Ionization Anomaly (EIA) and Equatorial Spread F and their inter
relationships, a network of five stations receiving the 150 and 400 MHz
transmissions from the Low Earth Orbiting Satellites (LEOs) covering
the region from Trivandrum (8.5°N, Dip ∼0.3N°) to New Delhi (28°N,
Dip ∼20°N) is set up along the 77-78°E longitude. The receivers
measure the relative phase of 150 MHz with respect to 400 MHz, which is
proportional to the slant relative Total Electron Content (TEC) along
the line of sight. These simultaneous TEC measurements are inverted
to obtain the tomographic image of the latitudinal distribution of
electron densities in the meridional plane. The inversion is done
using the Algebraic Reconstruction Technique (ART). In this paper,
the tomographic images of the equatorial ionosphere along the 77-78°
E meridians are presented. The images indicate the movement of the
anomaly crest, as well as the strength of EIA at various local times,
which in turn control the over all electrodynamics of the evening
time ionosphere, favoring the occurrence of Equatorial Spread F (ESF)
irregularities. These features are discussed in detail under varying
geophysical conditions. The results of the sensitivity analysis of
the inversion algorithm using model ionospheres are also presented.
Title: Causative mechanism for the variabilities exhibited by o1d
630.0nm dayglow through coordinated optical and radar measurements
Authors: Kumar Pant, T.; Tiwari, D.; Subbarao, K. S.; Devasia, C. V.;
Sridharan, R.; Sekar, R.
Bibcode: 2004cosp...35.2467K
Altcode: 2004cosp.meet.2467K
As is well known, the equatorial electrojet parameters have their
imprint on the thermospheric O1D 630nm dayglow intensity through
electro-dynamical coupling. The Equatorial ionization anomaly (EIA)
is the process that links the changes of measured dayglow intensity
to the equatorial electrodynamics. In addition to the modulation
of the quiet time electrodynamical processes, O1D 630.0nm airglow
is controlled by photochemistry and also the large and small scale
neutral dynamical processes. The apportionment due to these processes
under different geophysical conditions is an unexplored topic and is
extremely important in the investigation of the thermosphere-ionosphere
system. In this context, this paper presents the variabilities in the
daytime thermospheric airglow vis-à-vis the evolution and growth of
Equatorial ionization anomaly (EIA) and Equatorial electrojet (EEJ)
during varying geophysical conditions, through systematic coordinated
measurements sing multiwavelength dayglow photometer and VHF/HF radars,
ground magnetometers and ionosonde from Trivandrum, a dip equatorial
station in India.
Title: Investigation of the mesopause energetics and its possible
implications on the mesosphere-lower thermosphere-ionosphere (MLTI)
processes through coordinated daytime airglow and radar measurements
Authors: Kumar Pant, T.; Tiwari, D.; Vineeth, C.; Subbarao, K. S.;
Devasia, C. V.; Sridharan, R.; Sekar, R.
Bibcode: 2004cosp...35.2463K
Altcode: 2004cosp.meet.2463K
In recent years, investigations based on the daytime thermospheric
and mesospheric airglow i.e. O1D 630 nm and OH (8-3) Meinel Band
intensity measurements from India have led to some new insight to the
coupling processes of the mesosphere, thermosphere and ionosphere (MTI)
over the equatorial latitudes. It has been shown through coordinated
airglow and adar studies that the planetary waves have a significant
impact on the mesopause dynamics. On occasions significant short-term
variabilities (10min-2hours) are also seen in the dayglow intensities
that owe their existence to both locally and remotely generated gravity
waves. While the interaction of these long and short period waves with
mean atmospheric flow at mesopause altitude alters the dynamics, the
odulations in the energetics get reflected in terms of the variations
in neutral temperature having far reaching consequences higher
above. Simultaneous measurements pertaining to the MLTI region are
needed to evaluate the significance of the observed variabilities. In
this context, coordinated measurements of various neutral and plasma
parameters are routinely done using multiwavelength dayglow photometer
and VHF/HF radars, ground magnetometers and ionosonde at Trivandrum,
a dip equatorial station in India become a unique set. This paper
presents some recent results on the day-to-day, long and short term
variation of mesospheric dayglow (OH (8-3) Meinel Band emission)
and the estimated mesopause neutral temperatures in addition to the
estimation of the possible implication that the mesopause temperature
has in the processes coupling the equatorial MTI region.
Title: East-west Asymmetry in the 8.3 m scale size EEJ irregularities
over dip equator Trivandrum, India
Authors: Patra, A. K.; Pant, T. K.; Devasia, C. V.; Sridharan, R.;
Subbarao, K. S. V.
Bibcode: 2004cosp...35.2563P
Altcode: 2004cosp.meet.2563P
In this paper we present the east-west asymmetry in the 8.3 m
scale size EEJ irregularities observed using 30° West and 30°
East from the zenith beam of the newly established 18 MHz radar at
Trivandrum. East-west asymmetry in the EEJ refers to the dissimilarity
in the observed spectral parameters observed using west and east
pointing radar beams. East-West asymmetry has been observed in
all the measured spectral parameters i.e. signal-to-noise ratio
(SNR), irregularity phase velocity and spectral width as a function
of height. The westward observed SNR is more than eastward and it
decreases with increase in altitudes. At lower heights, the observed
SNR difference is found to be as high as 10 dB and decreases to ∼2
dB at an altitude of 105 km. Up to 99 km, the westward and eastward
observed irregularity drift velocities agree with each other, while
the westward measured drifts are more than those of eastward higher
above. Up to 105 km, the westward measured spectral widths are more
than those of eastward. However, above 105 km the westward measured
spectral widths are lesser than that of measured eastward. Day-to-day
variability has also been observed in the east-west asymmetry. To the
best of our knowledge there is no observations related to East-West
asymmetry at 8.3 m EQUATORIAL ELECTROJET irregularities. Further, there
are first observations on this aspect from the Indian longitude sector
Title: Inhibition/Development of equatorial Spread F on magnetically
disturbed days - A case study
Authors: Devasia, C. V.; Jyothi, N.; Pant, K. T.; Diwakar, T.;
Sridharan, R.
Bibcode: 2004cosp...35.2164D
Altcode: 2004cosp.meet.2164D
A case study of occurrence/ non-occurrence of Equatorial Spread F
(ESF) events on several magnetically disturbed days over the magnetic
equatorial location of Trivandrum (8.5°N; 77°E; dip 0.5°N) in India
was conducted during March-April 1998. This study carried out under
the ISTEP (Indian-STEP) program brought out some interesting aspects
of the occurrence/non occurrence of ESF in relation to the nature of
equatorial ionospheric response to the geomagnetic disturbance. The
study indicated that the polarity and strength of the electric field
disturbances which become active around noon hours on these days
have an important role in modulating the development of Equatorial
Ionization Anomaly (EIA). These electric field disturbances of larger
timescales that are associated with ionospheric disturbance dynamo
effects are shown to have a controlling effect on the F-region height
rise, which in turn characterise the occurrence/ non- occurrence of ESF
on different disturbed days. These aspects are discussed and presented.
Title: A system for predicting close approaches and potential
collisions in geosynchronous orbits
Authors: Beusch, J.; Abbot, R.; Sridharan, R.
Bibcode: 2004cosp...35...88B
Altcode: 2004cosp.meet...88B
The geosynchronous orbit is getting crowded with over 300 active,
revenue producing large satellites and over 500 inactive dead resident
space objects that pose a physical collision threat to the active
satellites. The in situ demise of a particular satellite, Telstar 401,
followed by a similar demise of SOLIDARIDAD 1, initiated a research
and development effort at MIT Lincoln Laboratory to address this
threat. This work with commercial satellite operators is accomplished
using the mechanism of Cooperative Research and Development
Agreements. Initial work to detect and warn of close approaches
with these two failed satellites led to more extensive research
on the collision threat over the entire geosynchronous belt. It is
apparent that: a) There is a significant probability of collision; b)
The probability has increased considerably in the last decade or so;
c) The continuing failure of geosynchronous satellites and injection
of rocket bodies into or near geosynchronous orbit will increase the
threat; d) Debris in or near geosynchronous orbit poses another problem
that has to be addressed. This paper surveys what has been achieved so
far in predicting the threat and protecting satellites. An assessment
of the probability of collision is presented as well as a description
of the Geosynchronous Monitoring and Warning System. The operations
of the GMWS are described as well as some of the results achieved so
far. Areas of current research are mentioned.
Title: On the equatorial ionospheric response to the severe Space
Weather event of October - November 2003
Authors: Devasia, C. V.; Manju, G.; Sudha Ravindran, S. V.; Thampi,
T. K.; Pant, D. T.; Sridharan, R.
Bibcode: 2004cosp...35.2162D
Altcode: 2004cosp.meet.2162D
We have investigated in detail the equatorial ionospheric response
to the space weather events corresponding to certain key intervals
during the progress of the severe magnetic disturbance period of
Oct.-Nov. 2003 using the high time resolution magnetometer data
and the ionospheric data for the equatorial location of Trivandrum
(8.5°N; 77°E; dip 0.5°) and SHAR (13.7°N; 80.2°E; dip ∼10°N),
an off equatorial location in India. We have observed significant
anomaly in the general behavioural pattern of the equatorial and low
latitude ionosphere caused by the great magnetic storm of Oct. 28-30,
2003 in terms of the F-layer height changes and the F-layer critical
frequency changes observed over Trivandrum and SHAR. Significantly,
the daytime equatorial E and F region responses were marked by the
periodic appearance and disappearance of E and F layers, sometimes
with time scales of about an hour, probably in tune with the storm
time features in response to the rapid changes of the solar wind and
associated magnetic fields. The features of perturbed thermospheric
meridional winds in relation to the occurrence/non occurrence of
Equatorial Spread F (ESF) during this disturbance period is also
investigated. Main results of these investigations are presented
and discussed.
Title: Simultaneous radar observations of the electrojet plasma
irregularities at 18 and 54.95 MHz over Trivandrum, India
Authors: Tiwari, Diwakar; Patra, A. K.; Viswanathan, K. S.; Jyoti,
N.; Devasia, C. V.; Subbarao, K. S. V.; Sridharan, R.
Bibcode: 2003JGRA..108.1368T
Altcode:
Observations of equatorial electrojet (EEJ) plasma irregularities
made using a newly established 18 MHz coherent backscatter radar
at Trivandrum are presented. Also presented are the simultaneous
observations of the electrojet plasma irregularities made using a
collocated 54.95 MHz radar. These correspond to 8.3 m and 2.7 m scale
irregularities, respectively. The statistics of spectral parameters
observed at the two scale sizes are also presented and compared in
detail. An interesting and new aspect of these observations is that
the 8.3 m irregularities have significant anisotropy in the plane
perpendicular to magnetic field in contrast to that reported for 3 m
irregularities. While type-1 and type-2 spectra could be distinctly
observed at 2.7 m, it is rather difficult to distinguish at 8.3 m. The
type-2 velocities for the two scale sizes are pretty close to each other
below 102 km, but the velocities for 8.3 m are less than that of 2.7 m
above. Type-1 velocities are quite close to each other only at altitudes
of the peak electrojet when clear type-1 echoes are observed. Spectral
widths are remarkably different at the two scale sizes. The ratio
of spectral width corresponding to 2.7 m to that at 8.3 m is close
to 1.4, quite similar to that reported earlier from Jicamarca. The
observed difference in the velocities and spectral width at the two
scale sizes are attributed to the effects of anomalous collision of
electrons caused by the large-scale turbulence present in the EEJ. The
above observations are found to be consistent with the EEJ instability
theories and related simulation works.
Title: Performance Evaluation of Adaptive Optics Systems
Authors: Sridharan, Rengaswamy; Raja Bayanna, A.; Srivastava, Nandita;
Kumar, Brajesh; Ravindra, B.; Gupta, S. K.; Jain, Naresh; Ambastha,
A.; Venkatakrishnan, P.
Bibcode: 2003BASI...31..455S
Altcode:
Adaptive Optics (AO) systems improve the resolution of ground based
telescopes and allow for long exposure images. Their performance
depends on the seeing conditions at the time of observations. In this
paper, we evaluate the performance of an AO system under various seeing
conditions through simulations. Then we present the wave-front sensing
and correction schemes that would be used in the first phase of the
AO system to be developed at the Udaipur Solar Observatory.
Title: Significant upper thermospheric contribution to the O
(1S)557.7nm dayglow emission: first ground based evidence
Authors: Taori, Alok; Sridharan, R.; Chakrabarty, D.; Modi, N. K.;
Narayanan, R.
Bibcode: 2003JASTP..65..121T
Altcode: 2003JATP...65..121T
Out of the two source regions of O (1S) green line
557.7nm emission namely the one in the lower thermosphere and the
other in the upper thermosphere, the upper thermospheric source is
known to contribute only 15-20% of the total 557.7nm volume emission
during nighttime. On the other hand, successful, simultaneous ground
based measurements of daytime 557.7nm O (1S) and 630.0nm O
(1D) thermospheric emissions with the multiwavelength daytime
photometer (MWDPM) reveal striking similarities in their variabilities,
the inference being the dominance of thermospheric contribution in
557.7nm O (1S) emission throughout the day, in conformity
with the space borne measurements made else where. The first set of
ground-based measurements are presented and discussed.
Title: Estimation of Fried's Parameter From Specklegrams of Solar
Features
Authors: Sridharan, R.; Venkatakrishnan, P.; Verma, V. K.
Bibcode: 2002SoPh..211..395S
Altcode:
A few methods of estimating Fried's parameter (r0) from
specklegrams of solar features are described. Some of these methods
were used to estimate r0 for the speckle data obtained from
Kodaikanal Observatory (KO), Uttar Pradesh State Observatory (UPSO)
and Udaipur Solar Observatory (USO). The average value of r0
was found to be ∼ 3 cm at USO and UPSO during our observations. At
KO, values of r0 ranging from 6 to 10 cm were estimated.
Title: Effects of solar eclipse on the electrodynamical processes
of the equatorial ionosphere: a case study during 11 August 1999
dusk time total solar eclipse over India
Authors: Sridharan, R.; Devasia, C. V.; Jyoti, N.; Tiwari, Diwakar;
Viswanathan, K. S.; Subbarao, K. S. V.
Bibcode: 2002AnGeo..20.1977S
Altcode:
The effects on the electrodynamics of the equatorial E- and F-regions of
the ionosphere, due to the occurrence of the solar eclipse during sunset
hours on 11 August 1999, were investigated in a unique observational
campaign involving ground based ionosondes, VHF and HF radars from
the equatorial location of Trivandrum (8.5° N; 77° E; dip lat. 0.5°
N), India. The study revealed the nature of changes brought about by
the eclipse in the evening time E- and F-regions in terms of (i) the
sudden intensification of a weak blanketing ES-layer and
the associated large enhancement of the VHF backscattered returns,
(ii) significant increase in h' F immediately following the eclipse
and (iii) distinctly different spatial and temporal structures in the
spread-F irregularity drift velocities as observed by the HF radar. The
significantly large enhancement of the backscattered returns from the
E-region coincident with the onset of the eclipse is attributed to
the generation of steep electron density gradients associated with
the blanketing ES , possibly triggered by the eclipse
phenomena. The increase in F-region base height immediately after
the eclipse is explained as due to the reduction in the conductivity
of the conjugate E-region in the path of totality connected to the
F-region over the equator along the magnetic field lines, and this,
with the peculiar local and regional conditions, seems to have reduced
the E-region loading of the F-region dynamo, resulting in a larger post
sunset F-region height (h' F) rise. These aspects of E-and F-region
behaviour on the eclipse day are discussed in relation to those observed
on the control day.
Title: Observations on Stratospheric-Mesospheric-Thermospheric
temperatures using Indian MST radar and co-located LIDAR during
Leonid Meteor Shower (LMS)
Authors: Selvamurugan, R.; Devasia, C. V.; Jain, A. R.; Raghava Reddi,
C.; Rao, P. B.; Sridharan, R.
Bibcode: 2002AnGeo..20.1869S
Altcode:
The temporal and height statistics of the occurrence of meteor trails
during the Leonid meteor shower revealed the capability of the Indian
MST radar to record large numbers of meteor trails. The distribution
of radio meteor trails due to a Leonid meteor shower in space and
time provided a unique opportunity to construct the height profiles
of lower thermospheric temperatures and winds, with good time and
height resolution. There was a four-fold increase in the meteor
trails observed during the LMS compared to a typical non-shower
day. The temperatures were found to be in excellent continuity with
the temperature profiles below the radio meteor region derived from
the co-located Nd-Yag LIDAR and the maximum height of the temperature
profile was extended from the LIDAR to ~110 km. There are, how-ever,
some significant differences between the observed profiles and the
CIRA-86 model profiles. The first results on the meteor statistics
and neutral temperature are presented and discussed below.
Title: On the plausible linkage of thermospheric meridional winds
with the equatorial spread F
Authors: Devasia, C. V.; Jyoti, N.; Subbarao, K. S. V.; Viswanathan,
K. S.; Tiwari, Diwakar; Sridharan, R.
Bibcode: 2002JASTP..64....1D
Altcode: 2002JATP...64....1D
Some of the characteristic features of thermospheric meridional winds
during equinoctial period, associated with equatorial spread F (ESF)
and their possible role in the triggering of ESF are presented through
case studies of observational events under different geophysical
conditions that essentially control the post-sunset F-layer height
(/h'F) rise. The present study reveals that the polarity and magnitude
of the meridional winds become significant with the equatorward
wind being present when the /h'F is below a critical height for the
instability to get triggered. The distinctly different characteristic
features of the meridional winds during ESF and non-ESF events are
presented and discussed.
Title: An observation-driven model of the equatorial ionosphere -
DEOS rocket campaign study
Authors: Sojka, J. J.; Eccles, J. V.; Thiemann, H.; Sridharan, R.;
Lakhina, G. S.; Rao, P. B.; Schunk, R. W.
Bibcode: 2002AdSpR..29..899S
Altcode:
An ionosphere model for a single, low-latitude, magnetic meridion is
driven with observation-determined drivers to produce electron density
specifications containing day to day ionospheric weather. The drivers
are vertical plasma velocity and cross-equator neutral wind. These
are determined from ground-based ionosondes and magnetometers. Data
from the DEOS German-Indian Rocket Campaign of 1998 in India is used
to test the accuracy of the model ionosphere using data-determined
drivers. The rocket campaign was supported by ground-based magnetometers
and ionosondes. The ground-based observations are used to estimate the
vertical plasma drift and cross equator neutral wind. The ionosphere
model calculates the time history of the ionosphere from 35°N to
35°S using the observation-determined drivers. The resulting electron
densities in the meridion are compared against the in situ observations
of electron density from the DEOS rockets.
Title: Daytime optical and radar measurements of the equatorial MLTI
regions - new results
Authors: Pant, T.; Tiwari, D.; Vijayalakshmi, V.; Subbarao, K.;
Devasia, C.; Sridharan, R.; Sekar, R.
Bibcode: 2002cosp...34E1839P
Altcode: 2002cosp.meetE1839P
In the recent past, investigations based on the daytime thermospheric
airglow (O1 D 630nm) intensity measurements, from India, have led to
some new insight into the dynamics of the thermosphere ionosphere system
(TIS) over low and equatorial latitudes. It has been shown through
coordinated studies that the equatorial electrojet parameters have
their imprint on the thermospheric O1 D 630nm dayglow intensity through
electro-dynamical coupling. However, it is important to mention that on
occasions significant short term variabilities (20min-2hours) have also
been seen in the dayglow intensities not completely corroborating with
the electrojet strength. The observation that the thermospheric dayglow
emission maximizes at different times on different days indicates
towards a day-to-day change in the equatorial electrodynamics and
also in the consequent development of phenomenon like the Equatorial
Ionization Anomaly (EIA). The reasons for this change in the relative
influence of processes like photochemistry and electrodynamics remain
largely unexplained. In the recent times an awareness has evolved that,
the state of the atmosphere around mesopause and slightly higher above
affect the ionosphere through the modulations of its energetics and
dynamics caused by the tides and the gravity waves that penetrate up
into this region. Further, while the interaction of these waves with
mean atmospheric flow at this altitude would alter the dynamics,
the modulations in the energetics would get reflected in terms of
the variations in neutral temperature. The present paper aims at
studying the possible implications that the mesopause temperature
has in the evolution and growth of the various large scale processes
in the equatorial MLTI region, through coordinated measurements
using multiwavelength dayglow photometer and VHF/HF radars, ground
magnetometers and ionosonde, from a dip equatorial station, Thumba,
Trivandrum, India.
Title: Solar activity linked variabilities in the Thermospheric
meridional winds and their control on the occurrence of Equatorial
Spread-F
Authors: Sridharan, R.; Jyoti, N.; Devasia, C.
Bibcode: 2002cosp...34E1381S
Altcode: 2002cosp.meetE1381S
The thermospheric wind system, through interactions with the ionospheric
layers control many of the electrodynamic processes of the equatorial
ionosphere thermosphere system; the occurrence of equatorial Spread
F (ESF) being a manifestation of one of these processes. One of the
important aspects of a plausible linkage of Thermospheric meridional
winds with the ESF has recently been identified as through the
polarity of the winds just before the onset of ESF. The importance of
winds of equatorward/poleward polarity would matter only in relation
with the Fregion base height as to whether it is below or above a
characteristic- critical height. It is found that there is a strong
solar cycle modulation of the critical base height of the F-region
by as much as +/-50km for the occurrence of ESF with the associated
meridional winds also responding to the solar variabilities. The
solar activity dependence of the meridional wind characteristics and
the F-region critical height variations are discussed in detail in
relation to their possible roles in the variations of ESF occurrence.
Title: On the role of vertical electron density gradients in the
generation of type II irregularities associated with blanketing Es
during counter electrojet events - a case study
Authors: Devasia, C.; Jyoti, N.; Sridharan, R.; Raghava Reddi, C.;
Diwakar, T.; Subba Rao, K.
Bibcode: 2002cosp...34E1380D
Altcode: 2002cosp.meetE1380D
The characteristics of different types of Sporadic E (ES) layers and the
associated plasma density irregularities over the magnetic equator have
been studied in a campaign mode, using VHF backscatter radar, digital
ionosonde and ground magnetometer data from Trivandrum (dip lat. 0.5°N,
geog. lat. 8.5°N, geog. long. 77°E), India. Blanketing type Es (ESb)
with varying intensity and duration were observed in association with
afternoon counter electrojet (CEJ). ESb was associated with intense
backscatter returns and with either very low zonal electric fields
and/or with distortion present in the altitude profile of the phase
velocity of the type II irregularities. The results of the coordinator
study indicate the possible role of electron density gradients and the
role of local winds in their generation, eventually resulting in the
ESb layers. Evidences for the local winds to be responsible for the
generation of steep vertical gradients based on the VHF backscatter
radar data are provided and discussed.
Title: Parameterisation of the local and non-local processes in the
thermospheric energy budget based on DE-2 satellite data
Authors: Pant, T. K.; Sridharan, R.
Bibcode: 2001JASTP..63.1715P
Altcode: 2001JATP...63.1715P
Though the mechanism redistributing the energy and momentum between the
neutral and ionised species of the upper atmosphere i.e. thermosphere
and ionosphere system (TIS) over low and equatorial latitudes are
understood reasonably well, only few individual processes could be
modeled so far. As a result, the most widely used thermospheric model
(MSIS) is found to fall short of the true prediction of low latitude
thermospheric temperatures and its variabilities. Large scale local
processes like equatorial ionisation anomaly (EIA) and equatorial
temperature and wind anomaly (ETWA) do affect the thermal structure
of this region significantly. Apart from that, the effect of the
processes which are essentially non-local in origin like the one that
could be represented by the variabilities in earth's ring currents,
also contribute to the thermal structure and its variabilities. The
present paper deals with parameterisation of these local and non-local
processes and suggests ways of incorporating the same in the existing
MSIS model. It has been demonstrated that the whole exercise leads to a
significant improvement in the MSIS which is now able to make very good
predictions, with regard to the variabilities and in terms of absolute
magnitudes on more than 50-60% of the events studied. The present
results take us closer towards a realistic model for low/equatorial
latitudes.
Title: Probability of collision in the geostationary orbit
Authors: Leclair, Raymond A.; Sridharan, Ramaswamy
Bibcode: 2001ESASP.473..463L
Altcode: 2001spde.conf..463L
The advent of geostationary satellite communication 37 years ago,
and the resulting continued launch activity, has created a population
of active and inactive geosynchronous satellites which will interact,
with genuine possibility of collision, for the foreseeable future. As
a result of the failure of Telstar 401 three years ago, MIT Lincoln
Laboratory, in cooperation with commercial partners, began an
investigation into this situation. Under the agreement, Lincoln worked
to ensure a collision did not occur between Telstar 401 and partner
satellites and to understand the scope and nature of the problem. The
results of this cooperative activity and recent results to carefully
characterize the actual probability of collision in the geostationary
orbit are described.
Title: Seasonal dependence of the response of the low latitude
thermosphere for external forcings
Authors: Pant, T. K.; Sridharan, R.
Bibcode: 2001JASTP..63..987P
Altcode: 2001JATP...63..987P
Using the in situ measured neutral temperatures by the DE-2 satellite,
the Dst geomagnetic index which is a measure of the intensity
of earth's ring current along with the MSIS model predictions, the
low latitude thermosphere has been shown to have a delayed response
to any energy input over polar regions with the time delay revealing
a seasonal dependence. The response time has been observed to vary
between 8 and 20h from summer to winter. Interestingly, the response
sensitivity of the thermosphere for any change in Dst index
remained nearly the same throughout the year implying other forcings
to be responsible for the observed seasonal dependence. These aspects
are presented and discussed.
Title: Coordinated thermospheric day-night airglow and ionospheric
measurements from low latitudes—First results
Authors: Taori, Alok; Sridharan, R.; Chakrabarty, D.; Narayanan, R.;
Ramarao, P. V. S.
Bibcode: 2001GeoRL..28.1387T
Altcode:
The Dayglow photometer which monitors the O(¹D) 630.0 nm thermospheric
airglow emission, has been converted into a day-night photometer
and was operated from Waltair (17.7°N, 83.3°E dip. Lat. 10.09°N)
during March 1998 as a part of the Indian Solar Terrestrial Energy
Programme (ISTEP) campaign. The first results on the day-night airglow
measurements together with ionosonde data from the same location are
scrutinized in the present exercise. The base height of the F-layer
(h'F) showed a strong negative correlation with dayglow intensity
similar to nighttime conditions and a new empirical relation (Barbier
type) linking the F-region electron densities and the h'F with the
dayglow intensity has been worked out highlighting the importance of
chemical processes in OI 630.0 nm dayglow and its variabilities.
Title: Sporadic-E associated with the Leonid meteor shower event of
November 1998 over low and equatorial latitudes
Authors: Chandra, H.; Sharma, S.; Devasia, C. V.; Subbarao, K. S. V.;
Sridharan, R.; Sastri, J. H.; Rao, J. V. S. V.
Bibcode: 2001AnGeo..19...59C
Altcode:
Rapid radio soundings were made over Ahmedabad, a low latitude station
during the period 16 20 November 1998 to study the sporadic-E layer
associated with the Leonid shower activity using the KEL Aerospace
digital ionosonde. Hourly ionograms for the period 11 November to 24
November were also examined during the years from 1994 to 1998. A
distinct increase in sporadic-E layer occurrence is noticed on 17,
18 and 19 November from 1996 to 1998. The diurnal variations
of f0Es and fbEs also
show significantly enhanced values for the morning hours of 18 and 19
November 1998. The ionograms clearly show strong sporadic-E reflections
at times of peak shower activity with multiple traces in the altitude
range of 100 140 km in few ionograms. Sporadic-E layers with multiple
structures in altitude are also seen in some of the ionograms (quarter
hourly) at Thumba, situated near the magnetic equator. Few of ionograms
recorded at Kodaikanal, another equatorial station, also show sporadic-
E reflections in spite of the transmitter power being significantly
lower. These new results highlighting the effect of intense meteor
showers in the equatorial and low latitude E-region are presented.
Title: Indo-German low-latitude project deos: plasma bubbles in the
post sunset and nighttime sector
Authors: Thiemann, H.; Sojka, J. J.; Eccles, J. J.; Rao, P. B.;
Rama Rao, P. V. S.; Sridharan, R.; Lakhina, G. S.
Bibcode: 2001AdSpR..27.1065T
Altcode:
Two spread-F flights were performed under equinox conditions in April
and September 1998 from the Indian low-latitude station SHAR. Both
flights detected plasma bubbles confined to a narrow longitudinal
extent. Plasma parameters are measured under two different conditions
in the post-sunset ionosphere; when the F-layer is moving upward (19:21
IST launch) and when the F-layer is stationary (20:41 IST launch). Low
altitude bubbles during upwelling of the F-region in the prereversal
current enhancement phase are characterized by turbulent, non Maxwellian
regions. Transitional scale waves with k=-3.6 are observed in the
bubbles. This flight also detects strong sharp E-layers during downleg
and an intermediate layer at 170km. Nighttime bubbles during almost
stationary F-layer conditions show turbulent, Maxwellian features with
moderate cooling compared to the undisturbed environment. Transitional
scale waves in the bubble region show a spectral index k=-3.1. The
spectral power is reduced by around 20dB compared to the sunset flight.
Title: Plausible explanation for the equatorial temperature and wind
anomaly (ETWA) based on chemical and dynamical processes
Authors: Pant, T. K.; Sridharan, R.
Bibcode: 2001JASTP..63..885P
Altcode: 2001JATP...63..885P
It has been proposed earlier through independent studies that
either `chemical heating' or `ion drag' might be responsible for
the generation of a large-scale low-latitude process, namely the
Equatorial Temperature and Wind Anomaly (ETWA). The present study
based on simultaneous measurements of ionospheric and thermospheric
parameters by DE-2 satellite alongwith some of the standard models
shows that both the processes are important and necessary for the
generation of ETWA. A proper quantification has been attempted with
reasonable success by taking into account both the chemical heating
and ion drag. The chemical heating which operates irrespective of
the time of the day, has been shown to account for 25-30 K (/~30%)
of the neutral temperature enhancements in ETWA. On the other hand,
the ion-drag associated heating, which is operative only during daytime
conditions could raise the temperature by 60-80 K. Thus the relative
importance of ion drag and chemical heating have been evaluated in
arriving as a plausible explanation of ETWA.
Title: Imprint of equatorial electrodynamical processes in the OI
630.0 nm dayglow
Authors: Sridharan, R.; Pallam Raju, D.; Somayajulu, V. V.; Taori,
A.; Chakrabarty, D.; Raghavarao, R.
Bibcode: 1999JASTP..61.1143S
Altcode: 1999JATP...61.1143S
Results from coordinated measurements of OI 630.0 nm dayglow intensities
(centered on ~220 km altitude), along with VHF (50 MHz) coherent
backscatter returns from Thiruvananthapuram, a dip equatorial station in
India, revealed that the temporal variability at short periods (<4
h) of the Doppler frequency of the coherently backscattered 50 MHz
radar signal in the electrojet region (~101 km altitude) preceded the
dayglow variations. The time delay was found to be inversely related
to the electric field magnitude inferred from the Doppler frequency
and also with the independently estimated electrojet strength inferred
from the ground magnetic data. These results are presented as direct
evidence for the prevailing electrodynamic coupling between the E-
and F-region of the ionosphere over the dip equator.
Title: First results on daytime mesopause OH rotational temperatures
using ground-based photometry from equatorial latitudes
Authors: Sridharan, R.; Taori, A.; Gurubaran, S.; Rajaram, R.;
Shepherd, M. G.
Bibcode: 1999JASTP..61.1131S
Altcode: 1999JATP...61.1131S
Daytime mesospheric OH (8,3) rotational temperatures were estimated
from the measurements of the relative intensities of the rotational
lines in the same vibrational band, using the unique multiwavelength
daytime photometer, from Tirunelveli (8.7°N 77.8°E 0.5°N dip
latitude), an equatorial station in the Indian zone. On a couple of
days when simultaneous measurements were available in the vicinity of
the experimental site, the estimated temperatures compared well with
those measured by the Wind Imaging Interferometer (WINDII) onboard
the Upper Atmosphere Research Satellite (UARS). These results, the
first of their kind, revealed: (1) large day-to-day variabilities;
(2) oscillatory features with periodicities ranging from 0.5 to 6
h and (3) large deviations of +/-30-50 K, on occasions, from the
MSIS-90 model temperatures. Comparisons have also been made with
simultaneous measurements of neutral winds around mesopause altitudes
using the Partial Reflection radar from the same site to look for any
possible association of neutral dynamical parameters with the mesopause
temperature variabilities. These results are presented and discussed.
Title: DEOS: Rocket Investigations of the Indian Low-Latitude
Ionosphere
Authors: Thiemann, H.; Mayer, G.; Piel, A.; Steigies, C.; Rama Rao,
P. V. S.; Rao, P. B.; Sridharan, R.; Somayajulu, V. V.
Bibcode: 1999ESASP.437..429T
Altcode: 1999erbp.conf..429T
No abstract at ADS
Title: Effects of 6 January 1997 space weather related processes in
the low latitude thermosphere-ionosphere system
Authors: Sridharan, R.; Taori, A.; Chakrabarty, D.; Chandra, H.;
Sharma, S.; Narayanan, R.; Modi, N. K.
Bibcode: 1999JASTP..61.1001S
Altcode: 1999JATP...61.1001S
OI 630.0 nm dayglow data from Mt. Abu, India, along with ground based
ionosonde data, were scrutinized for the 6-12 January 1997 space
weather related event. The OI 630.0 nm dayglow intensities registered
an enhancement by a factor of 2-2.5 on 11 January, in the morning
hours, as compared to 9 and 12 January, i.e. one day later than when
auroral activities were noted at unusual latitudes and on the day when
the magnetosphere was compressed to <6RE. The ionosonde
data did not show any such dramatic increase in the F-region electron
densities for the same period. Various plausible mechanisms that
could cause the observed enhancement in the dayglow without altering
the F-region of the ionosphere are discussed in the context of the
actual measurements.
Title: A case-study of the low-latitude thermosphere during
geomagnetic storms and its new representation by improved MSIS model
Authors: Pant, T. K.; Sridharan, R.
Bibcode: 1998AnGeo..16.1513P
Altcode:
The thermospheric temperatures from low and equatorial latitudes
during geomagnetically disturbed periods are known to exhibit
significant deviations from atmospheric model predictions. Also,
the oscillatory features seen in the observations are not accounted
for by the models. A simple relation has been established between
the difference in the observed and model-predicted temperatures and
the rate of change of Dst, the magnetic index representing the ring
current variabilities. Using this relation, a correction term has been
added to the latest MSIS-90 model algorithm and almost all the observed
variations in neutral temperatures spectroscopically determined from
Mt.Abu, a low-latitude station in India, are successfully reproduced
for two moderate geomagnetic storms.
Title: Joule heating due to vertical ion currents in the lower
thermosphere over the dip equator
Authors: Raghavarao, R.; Sridharan, R.; Suhasini, R.
Bibcode: 1998EP&S...50..833R
Altcode:
The theory of equatorial electrojet predicts the presence of vertical
ion currents (Pedersen currents) as a part of the electrojet current
system. The vertical ion current density profile over the dip equator,
that forms a part of the meridional current system is derived from an
electrojet model. The joule heating due to these currents flowing upward
during daytime for a local time for 1100 hrs has been estimated. The
primary east-west current density of the model is kept at the same
value as that measured by means of rocket-borne magnetometer on
one occasion. The electrical power dissipated as heat in the narrow
belt in the height region of 100-180 km is estimated and found to be
significant. The height of maximum power dissipation coincides with
the altitude of maximum ion velocity i.e. 122 km. By solving the heat
conduction equation we obtain a maximum temperature increase of 8°K
around 135 km. The importance of this localized heating in the lower
thermosphere around ±2° of the dip equator is discussed.
Title: High resolution 2-D maps of OI 630.0 nm thermospheric dayglow
from equatorial latitudes
Authors: Pallam Raju, D.; Sridharan:, R.
Bibcode: 1998AnGeo..16..997P
Altcode:
The first-ever high resolution 2-D maps of OI 630.0 nm dayglow
obtained from equatorial latitudes clearly reveal the movement as a
large-scale feature of the equatorial ionization anomaly (EIA). These
also show the presence of wave-like features classified as gravity
waves presumably originating at the crest of the EIA, similar to the
equatorial electrojet acting as a source of these waves. These results
are presented and discussed.
Title: Monitoring of Objects in Space with the US Space Surveillance
Network
Authors: Pensa, A. F.; Sridharan, R.
Bibcode: 1998mdis.conf..305P
Altcode:
No abstract at ADS
Title: Ionization hole campaign-a coordinated rocket and ground-based
study at the onset of equatorial spread-F: first results
Authors: Sridharan, R.; Chandra, H.; Das, S. R.; Sekar, R.; Sinha,
H. S. S.; Raju, D. Pallam; Narayanan, R.; Raizada, Shika; Misra,
R. N.; Raghavarao, R.; Vyas, G. D.; Rao, P. B.; Ramarao, P. V. S.;
Somayajulu, V. V.; Babu, V. V.; Danilov, A. D.
Bibcode: 1997JASTP..59.2051S
Altcode: 1997JATP...59.2051S
A comprehensive multi-technique campaign involving the launch of two
high altitude RH-560 rockets was carried out from Sriharikota (SHAR),
India, a near-equatorial rocket launching station at the onset of
equatorial spread-F, along with a host of ground-based complementary
experiments at other locations spread over the country. The main
objectives were to obtain the background ionospheric and thermospheric
conditions at the onset of equatorial spread-F, and to be able to
evaluate the relative importance of the various agencies, the neutral
dynamics in particular. Multiple barium cloud releases, in situ plasma
diagnostic measurements, along with complementary optical and radio
probing experiments were carried out as a part of this campaign. The
presence of large scale gradients in the ambient electric fields at ~
185 km altitude and also of vertical winds of significant magnitudes at
higher altitudes were found. The presence of large scale irregularities
in the ion densities at heights above 250 km in a region of negative
background density gradient is one of the significant new results. The
details of the Ionization hole campaign along with the first results
are presented and discussed in the context of the present knowledge
of the phenomenon of equatorial spread-F.
Title: DEOS: Dynamics of the Equatorial Ionosphere over SHAR:
Indo-German Low-Latitude Rocket Project
Authors: Thiemann, H.; Mayer, G. V.; Piel, A.; Steigies, C.; Olsen,
N.; Primdahl, F.; Sridharan, R.; Gupta, S. P.; Rangarajan, G. K.;
Rao, D. R. K.; Rao, P. B.
Bibcode: 1997ESASP.397..349T
Altcode: 1997erbp.conf..349T
No abstract at ADS
Title: Equatorial plasma bubble evolution and its role in the
generation of irregularities in the lower F region
Authors: Sekar, R.; Sridharan, R.; Raghavarao, R.
Bibcode: 1997JGR...10220063S
Altcode:
Rocket measurements from equatorial regions during equatorial spread F
(ESF) consistently reveal the presence of plasma density irregularities
in the lower F region (200-300 km) where the initial conditions
including the vertical electron density gradient are not favorable for
the development of linear instability. An investigation carried out
using a nonlinear numerical simulation model and the results obtained
from the Ionization Hole campaign revealed that the altitude variation
of the recombination coefficient and the vertical polarization velocity
due to fringe fields associated with the plasma bubble at the base of
the F layer are responsible for changing the polarity of the vertical
gradient in the plasma density profile in the lower F region, while
the penetration of the fringe fields is mainly responsible for the
development of zonal electron density gradient which plays a crucial
role in the secondary plasma instabilities resulting in the generation
of kilometer scale size irregularities in the vertical direction.
Title: Remote Sensing and Characterization of Anomalous Debris
Authors: Sridharan, R.; Beavers, W.; Lambour, R.; Gaposchkin, E. M.;
Kansky, J.
Bibcode: 1997ESASP.393..261S
Altcode: 1997spde.conf..261S
No abstract at ADS
Title: Searching for Satellite Ejecta with Ground-Based Radars
Authors: Andrews, S.; Hall, D.; Sridharan, R.
Bibcode: 1997ESASP.393...65A
Altcode: 1997spde.conf...65A
No abstract at ADS
Title: Photometry of solar corona to study the coronal structures.
Authors: Chakraborty, P.; Sankarasubramanian, K.; Majumder, S.;
Nayak, K. R.; Sridharan, R.; Jana, A. D.; Rajalakshmi, G.; Suresh,
D.; Asgekar, A.; Geetha, K. P.; Kunnumpuram, R. C.
Bibcode: 1997KodOB..13...69C
Altcode:
White light images of the solar corona were taken on KODAK 2415 films
using 3-inch (f/11.5) telescopes fitted with SLR cameras. Observations
were made from 4 different sites spread over the totality belt by four
different teams.
Title: First results from ground-based daytime optical investigation
of the development of the equatorial ionization anomaly
Authors: Pallam Raju, D.; Sridharan, R.; Gurubaran, S.; Raghavarao, R.
Bibcode: 1996AnGeo..14..238P
Altcode:
A meridional scanning OI 630.0-nm dayglow photometer was operated from
Ahmedabad (17.2°N dip lat.) scanning a region towards the south in
the upper atmosphere extending over ~5° in latitude from 10.2°N to
15.2°N dip latitude. From the spatial and temporal variabilities
of the dayglow intensity in the scanning region we show for the
first time, evidence for the passage of the crest of the equatorial
ionization anomaly (EIA) in the daytime by means of a ground-based
optical technique. The relationship between the daytime eastward
electric field over the dip equator in the same longitude zone as
inferred from the equatorial electrojet strength and the evolutionary
pattern of EIA is clearly demonstrated. The latter as inferred from
the dayglow measurements is shown to be consistent with our present
understanding of the electrodynamical processes in the equatorial
region. The present results reveal the potential of this ground-based
optical technique for the investigation of ionospheric/thermospheric
phenomena with unprecedented spatial and temporal resolution.
Title: Ground-based optical observations of daytime auroral emissions
from Antarctica
Authors: Raju, D. P.; Sridharan, R.; Narayanan, R.; Modi, N. K.;
Raghavarao, R.; Subbaraya, B. H.
Bibcode: 1995JATP...57.1591R
Altcode:
First results obtained from ground-based observations on daytime amoral
emission intensities from the Indian station, Maitri, in Antarctica,
are presented and discussed. These results show striking increases
in intensities in all the emissions (viz. low- and high-energy
electron-induced and proton-induced emissions), during noon hours in a
latitudinal region near the plasmapause and far from the conventional
cusp zone. Wave-like disturbances are seen on moderately disturbed days
in proton-induced emissions. Plausible causes for such enhancements
and disturbances are discussed.
Title: Ground-based optical observations of daytime auroral emissions
from Antarctica.
Authors: Pallam Raju, D.; Sridharan, R.; Narayanan, R.; Modi, N. K.;
Raghavarao, R.; Subbaraya, B. H.
Bibcode: 1995JATP...57.1591P
Altcode:
First results obtained from ground-based observations on daytime
auroral emission intensities from the Indian station, Maitri, in
Antarctica, are presented and discussed. These results show striking
increases in intensities in all the emissions (viz. low- and high-energy
electron-induced and proton-induced emissions), during noon hours in a
latitudinal region near the plasmapause and far from the conventional
cusp zone. Wave-like disturbances are seen on moderately disturbed days
in proton-induced emissions. Plausible causes for such enhancements
and disturbances are discussed.
Title: Effects of neutral temperature on meridional winds estimated
from ionospheric data
Authors: Gurubaran, S.; Sridharan, R.; Raghavarao, R.
Bibcode: 1995JATP...57.1095G
Altcode:
Indirect determination of meridional winds using ground-based
ionosonde data from low latitude regions, under the assumption that the
thermosphere and the F-region of the ionosphere behave as a closely
coupled system, has been critically examined. The significance of
neutral temperature and its variations in the above estimates has been
demonstrated through individual case studies after duly validating the
procedure adopted. Since the measured neutral temperatures have shown
large deviations from the existing atmospheric models on many occasions
and more so during high solar activity periods, it has been shown
that the neutral temperature effects on the F-region heights should
be properly accounted for before one attempts to estimate meridional
winds. However, it has also been shown that during low solar activity
periods, use of atmospheric models may still provide a fairly reasonable
average picture. Examples of these effects are presented and discussed.
Title: Variabilities in the thermospheric temperatures in the region
of the crest of the equatorial ionization anomaly--a case study
Authors: Gurubaran, S.; Sridharan, R.; Suhasini, R.; Jani, K. G.
Bibcode: 1995JATP...57..695G
Altcode:
Results on spectroscopic measurements of thermospheric temperatures made
from a low latitude station, Mt Abu (24.6 deg N, 72.7 deg E, geographic;
18 deg N dip latitude), India, situated in the crest region of the
equatorial ionization anomaly (EIA), are presented. On many occasions,
these measurements reveal large deviations from the predictions of
the neutral atmospheric model, MSIS-86, bringing out its limitations
as applied to the equatorial and low latitude thermosphere. The role
played by large-scale geophysical processes like the EIA, equatorial
spread F(ESF) and the midnight temperature maximum (MTM), all of which
influence the thermal structure of the upper atmosphere, is examined
in the context of explaining the differences between the measured
temperatures and model predictions. It has been conclusively shown
that Joule heating associated with ESF irregular electric fields is
not solely responsible for the observed deviations, and the possibility
of a significant role by the EIA related processes is indicated.
Title: Precursor to equatorial spread-F in OI 630.0 nm dayglow
Authors: Sridharan, R.; Raju, D. Pallam; Raghavarao, R.; Ramarao,
P. V. S.
Bibcode: 1994GeoRL..21.2797S
Altcode:
A unique OI 630.0 nm dayglow photometer operated from Waltair (10.0°
N dip lat.), a-low-latitude station in India, in a bidirectional mode,
i.e., over zenith and at 20° elevation pointing north, has revealed
features associated with the evolution of the Equatorial Ionization
Anomaly (EIA) The estimated strength of the EIA on a particular day
based on these features reveal that significant differences exist
in the EIA contribution to OI 630.0 nm on equatorial spread-F (ESF)
and non-ESF days. There exists a precursor in the OI 630.0 nm dayglow
which enables the prediction of ESF at least 3 hours prior to its
actual occurrence and hence points to the significant control of
daytime EIA-related processes in the triggering of the post-sunset ESF.
Title: Two-dimensional high-resolution imaging of the equatorial
plasma fountain.
Authors: Sridharan, R.; Sekar, R.; Gurubaran, S.
Bibcode: 1993JATP...55.1661S
Altcode:
The first visual representation of the reverse equatorial plasma
fountain during night-time has been made using images obtained by an
All Sky Imaging Fabry-Perot Spectrometer observing the O I 630.0 nm
airglow emission line from the thermosphere; the intensity of this
line emission is directly related to the F-region plasma densities
during night-time. From the identifiable features when the enhanced
airglow emitting region moves overhead and when it completely leaves the
field-of-view, the equatorwards velocity of the EIA has been estimated
to be ≍150 km/h at the latitude of the measurements (Mt Abu, 20°20'
dip latitude, India). Lower limits for the latitudinal and longitudinal
extents of the EIA crest have each been inferred to be 6°.
Title: Effect of meridional winds and neutral temperatures on the
F layer heights over low latitudes
Authors: Gurubaran, S.; Sridharan, R.
Bibcode: 1993JGR....9811629G
Altcode:
Results of coordinated measurements of thermosphere and F region
parameters with an aim to study the extent of the prevailing coupling
between these two regions at low latitudes (Mount Abu/Ahmedabad,
India) are presented through individual case studies. Spectroscopically
measured neutral temperature and meridional winds are used in the servo
equations to estimate the balance height of the F region ionization
maximum and compared with independently measured Hmax by
means of ground-based ionosonde. The comparison reveals fairly good
agreement, reproducing most of the observed features and providing
direct experimental evidence for these two regions to behave as a
closely coupled system. The electric fields, though they appear to be
less significant at these latitudes, could still be inferred once the
thermospheric data become available. The important conclusion is that
the effect of the neutral temperature and its variabilities should be
properly accounted for in the determination of meridional winds from
the existing ground-based ionosonde data.
Title: Validity of the estimates of night-time meridional winds made
from bottomside ionograms
Authors: Sekar, R.; Sridharan, R.
Bibcode: 1992JATP...54.1197S
Altcode:
The thermospheric and ionospheric data from a rocket experiment over
SHAR are used to make a direct comparison between meridional winds
estimated from bottomside ionograms and results of coordinated in situ
measurements. The meridional wind 'U' is estimated to be poleward
over SHAR with a magnitude of 43 +/- 22 m/s. This value is in good
agreement with the directly measured meridional wind velocity (54 +/-
10 m/s) using the neutral strontium cloud.
Title: OI 630.0-nm dayglow in the region of equatorial ionization
anomaly: Temporal variability and its causative mechanism
Authors: Sridharan, R.; Haider, S. A.; Gurubaran, S.; Sekar, R.;
Narayan, R.
Bibcode: 1992JGR....9713715S
Altcode:
Results of the continuous measurements of OI 630.0-nm dayglow
from Mount Abu (20°20'N dip latitude), a station in the region of
equatorial ionization anomaly (EIA) by means of a recently developed
ground-based dayglow photometer are presented. The measurements
on magnetically quiet and moderately active days reveal that there
are strong noontime bite-outs in 630.0-nm intensities. the temporal
variabilities in the intensities show a good correlation with those
in electron densities in the lower F region over Ahmedabad (18°23'N
dip latitude) with a time delay of 1 hour. Theoretical estimation
of the intensities indicates that the dissociative recombination of
O2+ with ambient electrons is mainly responsible
for the observed temporal variability in the dayglow emission. The time
delay between the theoretically evaluated and the measured noontime
bite-outs is discussed in the context of the development of the EIA.
Title: Co-ordinated thermospheric and F-region measurements from
low latitudes
Authors: Sridharan, R.; Gurubaran, S.; Raghavarao, R.; Suhasini, R.
Bibcode: 1991JATP...53..515S
Altcode:
Coordinated measurements of thermospheric temperatures by a ground-based
Fabry-Perot spectrometer from Mt. Abu and F-region base height by
ionosonde from Ahmedabad reveal very close coupling between them. There
is a one-to-one correspondence between the neutral temperatures and
the F-region base height, showing a relation of 11 +/-4 km/100 K. The
behavior agrees with that expected from the servo model of Rishbeth
(1967). Results of two days of measurements during geomagnetically
quiet periods, for which simultaneous data were available, are presented
and the various related physical processes are discussed.
Title: First results of OI 630.0 nm dayglow measurements from
equatorial latitudes
Authors: Sridharan, R.; Raghavarao, R.; Gurubaran, S.; Narayanan, R.
Bibcode: 1991JATP...53..521S
Altcode:
First results of the campaign to measure the OI 630.0 nm dayglow
emission continuously from an equatorial station, Sriharikota
(5.5 deg dip latitude), India, during January-February 1989 are
presented. Results from zenith observations during five days reveal
close coupling with the F-region electron densities controlled by
the electrodynamical processes, showing features like the noon-time
'bite-outs' and several other fine features. These new results give
us clues for identifying the significant processes that result in OI
630.0 nm dayglow emission.
Title: Winds, wind-shears and plasma densities during the initial
phase of a magnetic storm from equatorial latitudes
Authors: Sridharan, R.; Raghavarao, R.; Suhasini, R.; Narayanan, R.;
Sekar, R.
Bibcode: 1989JATP...51..169S
Altcode:
A sodium vapor release experiment was performed at an equatorial rocket
launching station during the initial phase of a magnetic storm. The
formation of sharp layers of ionization can be explained by the
electrodynamical processes associated with wind shears at a location
close to the edge of the equatorial electrojet belt. The significance
of the change in the neutral composition due to the enhanced neutral
temperature and the low turbopause level is explored.
Title: Latitudinal variation in the ionospheric parameters - A
Soviet-Indian experiment by simultaneous launchings
Authors: Danilov, A. D.; Pokhunkov, A. A.; Varfolomeev, V. A.;
Sridharan, R.; Gupta, S. P.
Bibcode: 1988JATP...50..175D
Altcode:
Coordinated ion-neutral composition and electron density measurements
have been carried out over Thumba (India) and Volgograd (USSR), near
sunrise. One of the launchings from Thumba revealed the turbopause to
be around 110 km. Large fluctuations in ion and electron densities
were also registered in the altitude region 105-125 km, along with
oscillatory structures in the neutral composition, indicating that
unusual conditions prevailed during this measurement. Contrary to
expectations, 'time synchronous' launchings from Thumba and Volgograd
revealed nearly identical distributions of neutral species. The
turbopause altitudes during these flights were also the same within
the limits of experimental uncertainty. These measurements confirm that
the role of the turbopause and temperature are mutually independent in
governing the distribution of neutral composition in the thermosphere.
Title: Excess heating over the equatorial latitudes during storm
sudden commencement
Authors: Gupta, Ranjan; Desai, J. N.; Raghavarao, R.; Sekar, R.;
Sridharan, R.; Narayanan, R.
Bibcode: 1986GeoRL..13.1055G
Altcode:
Temperatures of the equatorial thermosphere were obtained from the
rocket released vapour clouds during (i) geomagnetically quiet and
(ii) just after (<2 hrs) a sudden commencement. These measurements
of thermospheric temperatures at different altitudes during sudden
commencement are the first of their kind. The results reveal larger
temperatures (>400 - 500°K) than that expected by the Jacchia 77
model during disturbed periods, while the quite day measurements agree
well with the model predictions. The possible sources of energy that
account for the excess temperatures are discussed in the context of
the present measurements.
Title: Relative ion composition variation over the dip equator -
A comparison of measurements with IRI
Authors: Sridharan, R.; Raghavarao, R.; Pokhunkov, A. A.; Varfolomeev,
V. A.
Bibcode: 1985JATP...47.1081S
Altcode:
A graphical comparison of four ion composition measurements, obtained
in the early morning at the Thumba equatorial station during 1978-1982,
with an IRI model (1981) is presented. Underestimation, with the IRI
model, of the NO(+)/O2(+) ratios by a factor of at least 1.6 except
in the midlatitudes is observed; ion dynamics are studied to explain
this occurrence. The electron density is decreased and the molecular
ion concentration increased proportionately at night. NO(+) ions drift
downward due to Pedersen currents prevalent as a part of the electrojet
current system, thereby, increasing the ion concentration downward.
Title: The importance of vertical ion currents on the nighttime
ionization in the equatorial electrojet
Authors: Raghavarao, R.; Sridharan, R.; Suhasini, R.
Bibcode: 1984JGR....8911033R
Altcode:
Plasma density profiles in the equatorial electrojet region reveal
that the ionization density at 100 km increases by a factor of 2 to 10
from the time of sunset to midnight. In addition, a valley in plasma
density centered around 120 km altitude deepens by a factor of 2 to 5
during the same period. These two aspects are explained on the basis
of the dynamical features peculiar to the electrojet region. It is
demonstrated that the vertically downward motion of positive ions
due to Pedersen currents is very important in the redistribution of
ionization. The valley is shown to be formed at 120 km, where the
vertical ion velocity maximizes.
Title: Evidence for turbopause control on neutral density variations
in the lower thermosphere
Authors: Sridharan, R.; Raghavarao, R.
Bibcode: 1984JATP...46..851S
Altcode:
Data obtained from in situ neutral composition measurements over the
dip equator are used in explaining the altitude variation of relative
composition. The turbopause height h(t) and the model exospheric
temperatures T(inf) are used in explaining the altitude variation
of O2/N2. Although the measured O2/N2 ratio at any altitude in the
lower thermosphere does not show any direct relation to either the
exospheric temperature or the turbopause height, it shows a fairly
linear relation to the product of h(t) x T(inf). This is clear evidence
for the altitude distribution of the neutral species being controlled
by the mutually independent variables, viz. the turbopause height and
the exospheric temperature.
Title: An intermediate-averaged theory for high altitude orbits
Authors: Sridharan, R.; Seniw, W. P.
Bibcode: 1979STIN...8012104S
Altcode:
An analytic theory for the evolution of high altitude satellite orbits
is developed in this note. The distinctive feature of the theory lies
in the double averaging of the differential equations - once over the
period of the orbit, and secondly over the period of the moon. This
technique is called intermediate averaging to distinguish it from the
conventional doubly averaged theories, and to denote the time scales
inherent in the averaging technique.
Title: Real time astrometry
Authors: Taff, L. G.; Poirier, I. M.; Freed, A.; Sridharan, R.
Bibcode: 1978STIN...7925952T
Altcode:
This report provides estimates of the positional accuracy for celestial
objects which have been obtained at the Experimental Test Site of
the Ground-based Electro-Optical Deep Space Surveillance (GEODSS)
program. Optical observations made at the ETS have been compared with
radar measurements performed by the Millstone Hill Radar. A brief
description of the equipment and theoretical basis of the optical
data reduction is included too. It appears that optical observations
of artificial satellites, with real time reduction, are accurate to
approximately 5 seconds.
Title: Non-Numeric Computation for High Eccentricity Orbits
Authors: Sridharan, R.; Renard, M. L.
Bibcode: 1975CeMec..11..179S
Altcode:
Geocentric orbits of large eccentricity (e=0.9 to 0.95) are
significantly perturbed in cislunar space by the Sun and Moon. The
time-history of the height of perigee, subsequent to launch, is
particularly critical. The determination of ‘launch windows’
is mostly concerned with preventing the height of perigee from
falling below its low initial value before the mission lifetime has
elapsed. Between the extremes of high accuracy digital integration
of the equations of motion and of using an approximate, but very
fast, stability criteria method, this paper is concerned with the
development of a method of intermediate complexity using non-numeric
computation. The computer is used as the theory generator to generalize
Lidov's theory using six osculating elements. Symbolic integration
is completely automatized and the output is a set of condensed
formulae well suited for repeated applications in launch window
analysis. Examples of applications are given.