[Show abstract][Hide abstract] ABSTRACT: The geomagnetic field response to a moderate-amplitude negative sudden impulse (SI-) that occurred on 14 May 2009 at 10: 30 UT was examined at 97 geomagnetic observatories situated all over the globe. The response signature contains a contribution from magnetospheric as well as ionospheric currents. The main impulse (MI) is defined as the maximum depression in the observed geomagnetic field. It is observed that for low-to-high latitudes, the amplitude of the MI is larger in the afternoon to post-dusk sector than in the dawn-noon sector, indicating asymmetry in the MI amplitude. We estimated the contribution at various observatories due to the Chapman-Ferraro magnetopause currents using the Tsyganenko model (T01) and subtracted this from the observed MI amplitude to obtain the contribution due to ionospheric currents. It is found that the ionospheric currents contribute significantly to the MI amplitude of moderate SI-even at low-to-mid latitudes and that the contribution is in the same direction as that from the magnetopause currents near dusk and in the opposite direction near dawn. The equivalent current vectors reveal a clockwise (anticlockwise) ionospheric current loop in the afternoon (morning) sector during the MI of the negative pressure impulse. This evidences an ionospheric twin-cell-vortex current system (DP2) due to field-aligned currents (FACs) associated with the dusk-to-dawn convection electric field during the MI of an SI-. We also estimated the magnetic field variation due to prompt penetration electric fields, which is found to be very small at low latitudes in the present case. The studied SI-is not associated with shock, and hence no preliminary reverse impulse was evident. In addition, the summer hemisphere reveals larger MI amplitudes than the winter hemisphere, indicating once again the role of ionospheric currents.
Full-text · Article · Aug 2014 · Earth Planets and Space
[Show abstract][Hide abstract] ABSTRACT: The present investigation brings out, in contrast to the earlier works, the changes in the equatorial electrojet (EEJ) current in response to a few moderate (M-class) and low (C and B class) intensity solar flares during 2005–2010. Special care is taken to pick these flare events in the absence of prompt electric field perturbations associated with geomagnetic storms and substorms that also affect the electrojet current. Interestingly, only the normalized (with respect to the pre-flare level) deviations of daytime EEJ (and not the deviations alone) change linearly with the increases in the EUV and X-ray fluxes. These linear relationships break down during local morning hours when the E-region electric field approaches zero before reversal of polarity. This elicits that the response of EEJ strength corresponding to less-intense flares can be appropriately gauged only when the local time variation of the quiet time E-region zonal electric field is taken into account. The flare events enhanced the EEJ strength irrespective of normal or counter electrojet (CEJ) conditions that shows that solar flares change the E-region ionization density and not the electric field. In addition, the enhancements in the X-ray and EUV fluxes, for these flares occurring during this solar minimum period, are found to be significantly correlated as opposed to the solar maximum period, indicating the differences in the solar processes in different solar epochs.
Full-text · Article · Dec 2013 · Journal of Atmospheric and Solar-Terrestrial Physics
[Show abstract][Hide abstract] ABSTRACT: Systematic ground-based magnetometer measurements from Thumba (8.47°N, 76.6°E) and Tirunelveli (8.73°N, 77.8°E), India, revealed a movement of the magnetic dip equator towards the south. The magnetic dip angle measured over Thumba increased by about 2° during 1985–2010. In view of the movement of the dip equator over Thumba, the dependence of the generation of streaming plasma waves on the dip angle is examined. An order of magnitude calculation using the results obtained from the thin-shell model of the electrojet is performed. The calculation suggests that the streaming waves over Thumba during magnetically quiet periods at noontime exist when the dip angle is <1.5° and that these waves are generally absent whenever the dip angle is ≥1.5°. Evidence based on in situ rocket and ground-based radar measurements is provided by comparing earlier and recent observations.
Full-text · Article · Oct 2013 · Journal of Atmospheric and Solar-Terrestrial Physics
[Show abstract][Hide abstract] ABSTRACT:  Systematic investigations of optical dayglow emissions at OI 557.7, OI 630.0, and OI 777.4 nm have been carried out simultaneously over a large field of view (~140°). These emission intensities are obtained during January–March in the years 2011 and 2012 from Hyderabad (17.5°N, 78.5°E), India, using a high spectral resolution multiwavelength imaging echelle spectrograph. Spectral analyses of planetary wave type periodicities in all the dayglow emission intensities are performed, and their association with lower atmospheric and direct solar forcings is presented. This analysis revealed that periods near the atmospheric free normal modes of 5, 10, 16, and 25 days (which are produced mainly in the troposphere) are found to register their presence in the upper atmospheric emission intensities. In an earlier study during high solar activity period (2001), sunspot numbers (SSNs) and the daily averaged OI 630.0 nm dayglow intensities were seen to be covarying. In contrast, the variability in the dayglow emission intensities during relatively low solar activity epoch (2011) shows no or weaker correlation with that of the SSN but a greater similarity with that of the equatorial electrojet strength. Periodicities of both lower atmospheric normal modes and those related to sunspots are found during moderate solar activity (2012). Based on this analysis, it appears that the upper atmospheric dayglow emissions respond mainly to lower atmospheric forcing during low solar activity, solar forcing in high solar activity, and both during moderate solar activity level.
Full-text · Article · Jul 2013 · Journal of Geophysical Research: Space Physics
[Show abstract][Hide abstract] ABSTRACT: Onset of a geomagnetic substorm often intensifies the westward auroral electrojet, as well as produces asymmetric magnetic field at low/mid latitudes. Auroral electrojet and low latitude asymmetric indices are known to correlate well during substorms. These indices have been widely used to monitor the duration and strength of substorm activities. However, several processes, other than substorms, introduce local time asymmetry in magnetic field at low latitudes, which can substantially influence the ASY indices. Large number of substorms are observed in association with changes in the interplanetary magnetic field (IMF). It is known that sharp IMF Bz orientation changes result in penetration of interplanetary electric field (IEF) to lower latitudes, which affects the geomagnetic fields to different degree in different local times. In the present study, we demonstrate that sharp IMF Bz fluctuations during the expansion phases of substorms introduce additional asymmetry at low latitudes. The effect is clearly seen in ASYH, whereas ASYD remains almost unaltered.
Full-text · Article · Mar 2013 · Journal of Atmospheric and Solar-Terrestrial Physics
[Show abstract][Hide abstract] ABSTRACT: Cosmic noise absorption (CNA) measurred by imaging Riometer, is an excellent tool to passively study the high latitude D-region ionospheric conditions and dynamics. An imaging Riometer has been installed at Indian Antarctic station Maitri (Geographic 70.75 degree S, 11.75 degree E; corrected geomagnetic 63.11 degree S, 53.59 degree E) in February 2010. This is the first paper using the imaging Riometer data from Maitri. Present paper introduces the detail of this facility, including its instrumentation, related CNA theory and its applications. Sidereal shift of around 2 hours in the diurnal pattern validates the data obtained from the newly installed instrument. Moreover, the strength of cosmic noise signal on quiet days also varies with months. This is apparently due to solar ionization of D-region ionosphere causing enhanced electron density where collision frequency is already high. The main objective of installing the imaging Riometer at Maitri is to study magneotspheric-ionospheric coupling during substorm processes. In the current study, we present two typical examples of disturbed time CNA associated with storm-time and non-storm time substorm. Results reveal that CNA is more pronounced during storm-time substorm as compared to non-storm time substorm. The level of CNA strongly depends upon the strengthening of convectional electric field and the duration of south-ward turning of interplanetary magnetic field before the substorm onset.
Full-text · Article · Jan 2013 · Journal of Earth System Science
[Show abstract][Hide abstract] ABSTRACT: Magnetic data from a newly commissioned Indian Antarctic station Bharati (corrected geomagnetic (CGM) coordinates 74.7°S, 97.2°E) and closely-spaced IMAGE chain observatories (∼100° magnetic meridian in Northern hemisphere) has been analyzed to study the climatology of substorms which were localized poleward of the standard auroral oval. We considered four austral summers (year 2007–2010) when data from Bharati was available. Several very high latitude substorms were observed in this duration when the solar activity remained unexpectedly low for a long time. Various features of very high latitude substorms, e.g., local time dependence, interplanetary state, hemispherical asymmetry and their nightside low latitude signatures are examined. Events studied here, suggested the following properties of substorms occurring at very high latitudes: (1) maximum occurrence was observed near magnetic midnight (21:00–02:00 MLT). (2) In contradiction to earlier reports, many substorms were observed even during negative IMF Bz condition. In addition, majority of substorms occurred during low or moderate solar wind streams. (3) Magnetic signatures were often pronounced in the winter hemisphere. (4) Even if widely used standard AE indices fail to monitor very high latitude substorms, their low latitude signatures are often evident.
Full-text · Article · Dec 2012 · Advances in Space Research
[Show abstract][Hide abstract] ABSTRACT: The quiet-time (K p ≤ 3) daily variations of the geomagnetic field at the Indian Antarctic station, Maitri (Geographic Coord.: 70.75 • S, 11.73 • E; Geomagnetic Coord.: 66.84 • S, 56.29 • E) during two consecutive years of a solar minimum are considered in order to investigate the characteristics of the solar quiet (S q) current system. The present work reports the signatures of the south limb of the S q current loop of the southern hemisphere over a sub-auroral station. It is observed that the seasonal variation of the S q current strength over Maitri is strongest during the summer months and weakest during the winter months. In spite of the total darkness during the winter months, an S q pattern is identified at Maitri. The range of the horizontal field variation in the daily S q pattern during summer is one order higher than that during winter. An interesting feature regarding the phase of the local time variation in the seasonal pattern is found here. A sharp shift in the time of the peak S q current to later local times (>1 hour per month) is observed during January–February and July–August, which may correspond to the transition from the complete presence, or absence, of sunlight to partial sunlight. The differences in the incoming solar UV radiation during such transitions can cause a sudden change in the local ionospheric conductivity pattern, and can also trigger some unusual thermo-tidal activity, that might be responsible for modifying the global S q pattern.
Full-text · Article · Nov 2012 · Earth Planets and Space
[Show abstract][Hide abstract] ABSTRACT: Schumann resonances (SR) are lightening generated global electromagnetic
modes in extremely low frequency (ELF) range operating in the
Earth-ionosphere cavity. The 1st harmonic is at nearly 7.8 Hz and
subsequent harmonics are separated by approximately 6 Hz. In this paper
an attempt has been made to study the characteristics of SR using ground
magnetic data from searchcoil magnetometers operating at a low latitude
station Shillong (geographic co-ordinate lat 25.92 N, long 91.88 E) and
a sub-auroral Indian Antarctic station Maitri (geographic co-ordinate
lat 70.75 S, long 11.75 E). The universal time (UT) diurnal patterns of
amplitudes of different harmonics show the dominance of thunderstorm
activities in three major sectors viz. south-east Asia, Africa and South
America, whereas the frequency variation pattern suggests the tidal
influence. The seasonal variation of SR amplitude shows increase during
local summer at Shillong as well as Maitri. Amplitudes as well as
frequency exhibit distinct seasonal characteristics at both the
stations, there by exhibiting the local influence on global SR modes.
[Show abstract][Hide abstract] ABSTRACT: In the present study, we investigate the geomagnetic field response to
positive and negative solar dynamic pressure pulses. The positive
(negative) pressure pulse compresses (enlarges) the Earth's
magnetosphere, resulting in the enhancement (reduction) in the
horizontal component of the Earth's magnetic field. It has been observed
that the signature of the geomagnetic field response to the solar wind
pressure pulse varies with latitude, as well as with magnetic local
time. Our analysis shows that between 40° and 70° geomagnetic
latitude, preliminary reverse impulse (PRI) occurs in the dawn- noon
sector. Thus, asymmetry in PRI occurrence is evident between dawn-noon
and dusk-midnight sector. The amplitude of main impulse is found to be
larger in dusk-midnight sector than dawn-noon sector.
[Show abstract][Hide abstract] ABSTRACT: Substorms at the auroral latitudes trigger a peculiar dynamics where a
sharp depression in the AL index is observed along with an abrupt
absorption in the cosmic noise due to ionospheric disturbances caused by
high electron precipitation. However, such abrupt absorption may not
always be seen during a substorm. Maitri (70°45' S, 11°45'E,
geographic; 66°S, 53°21E, geomagnetic) is considered to be
located at sub-auroral latitude and behaves as an auroral station during
disturbed period. India has recently installed an Imaging Riometer at
this sub auroral location. Using the data of cosmic noise intensity from
Imaging riometer and the simultaneous magnetic field variation from
digital flux gate magnetometer (DFM), a statistical study was performed
in corroboration with the prevailing interplanetary conditions. Pi2
geomagnetic pulsations often accompanied by pulsations in particle
precipitation flux have been thoroughly analyzed. Diurnal and seasonal
patterns of such events were also statistically examined.
[Show abstract][Hide abstract] ABSTRACT: Low latitude asymmetric (ASY) indices, in addition to auroral indices,
have been widely used to monitor the phenomena of magnetic substorm.
However, low latitude ASY indices get strongly affected by various
current systems in the magnetosphere and the ionosphere and may not have
any direct relation with substorm activity. Intense and long-lasting
solar flares are known to produce significant magnetic field
disturbances on the dayside of the Earth, which could be an additional
source of low latitude asymmetry. We examine the effect of solar flares
on non-substorm and substorm times ASY indices using solar flare and
indices data during the solar cycles 22 and 23 (year 1986 - 2008). It is
observed that long-lasting intense solar flares during non-substorm time
generate asymmetry similar to substorm events in ASY indices. In
addition, substorm time ASY indices are also substantially affected by
intense solar flares. We suggest that the occurrence of solar flares
must be taken into account, especially in case studies where some
conclusions are made based on the enhancements in ASY indices.
Full-text · Article · Mar 2012 · Journal of Atmospheric and Solar-Terrestrial Physics
[Show abstract][Hide abstract] ABSTRACT: A case of the drastic effects of an eastward prompt penetration and a
westward overshielding electric field successively affecting the daytime
equatorial ionosphere during the space weather event that occurred on 24
November 2001 is presented. Under the influence of the strong eastward
prompt penetration electric field starting from 11:25 Indian standard
time (IST), the equatorial electrojet (EEJ) strength reached the maximum
value of 225 nT at 12:42 IST, almost 7 times greater than the monthly
quiet time mean at the same time. This peak EEJ value exceeds the
maximum observed values during the month of November for the entire
solar cycle by more than 100 nT, irrespective of quiet or disturbed
conditions. Further, owing to an ensuing overshielding event that
occurred during the main phase of the storm rather than the end of the
main phase, this unusually large EEJ showed an equally strong polarity
reversal along with a weakening of the sporadic E layer over the
equator. The EEJ strength was reduced from +225 to -120 nT at
˜13:45 IST, resulting in a strong counter electrojet condition. The
latitudinal variation of the F region electron density data from the
CHAMP satellite reveal an ill-developed equatorial ionization anomaly at
17:00 IST (11:24 UT) over the Indian sector due to this significant
weakening of the zonal electric field. These observations showcase the
significant degree to which the low-latitude ionosphere can be affected
by the interplanetary electric field.
No preview · Article · Mar 2012 · Journal of Geophysical Research Atmospheres
[Show abstract][Hide abstract] ABSTRACT: The westward flowing toroidal ring current at about 2–7 R E in the Earth's equatorial plane consists of sym-metric and asymmetric parts. Zonal mean of H disturbances from longitudinally distributed low latitude stations repre-sents the symmetric contribution, whereas departure from the zonal mean gives local time dependent asymmetric com-ponent at each of the stations. Through a standard analysis of closely spaced low latitude geomagnetic data we demon-strate 24 h periodicity in the asymmetric component of the storm-time ring current, which is related to the changing lo-cal time due to rotation of the Earth. Detailed examination of shorter period oscillations, when observed globally, often show westward propagating modes. Eastward propagating mode was also observed in one case. Based on satellite and radar observations covering a narrow longitude region, west-ward and eastward propagating modes had been reported in earlier studies. In this study, we report that similar propagat-ing modes which are available on global scale, can be iden-tified using ground-based magnetometer data. These glob-ally propagating modes, observed from ground-based stud-ies, find obvious practical application in diagnostics of the magnetosphere, especially the ring current region. Simulta-neous use of satellite and ground-based data should establish the morphology of such modes.
Full-text · Article · Jan 2012 · Annales Geophysicae
[Show abstract][Hide abstract] ABSTRACT: Geomagnetic data collected during magnetic storm over magnetically conjugate pair (according to IGRF 2000) of high latitude stations viz., Maitri (70° 45′ S, 11° 42′ E) and Tromso (69° 40′ N, 18° 56′ E) reveal that amplitudes of Pc6 pulsation characteristically differ. The amplitude obtained from horizontal magnetic field for the Pc6 pulsation frequency range between 0.6 and 1.6 mHz significantly differs in time corresponding to peak amplitude. The relative differences in its time of occurrence found to gradually increase around initial phase of storm and remain exactly out of phase at peak amplitude of storm. Thence, it is found to be in agreement in phase gradually until storm unwinds. This indicates that simultaneous amplitude of Pc6 pulsation at conjugate pair of stations and its time of occurrence could be a key factor to infer storm arrivals somewhat prior to its peak effects. The emphasize remains on prediction of storm arrivals only by utilizing ground based magnetometer observations. However, it is necessary to understand differences on the basis of weak, moderate, strong, and super strong cases and more exactly how they behave along the line of magnetic Meridian. Nevertheless, the analysis implies that geo-effective magnetic ejecta/clouds/CIRs/sheaths/CMEs/ICMEs giving rise to geomagnetic storm can be predicted ahead of its peak effects by having magnetometer data over conjugate locations.
No preview · Article · Nov 2011 · Advances in Space Research
[Show abstract][Hide abstract] ABSTRACT: Geomagnetic substorms are triggered on the nightside of the earth's magnetosphere and the most dramatic effect is observed at the auroral latitudes (60°-70° magnetic). Magnetic field disturbances observed at a set of longitudinally distributed auroral stations are used to derive auroral electrojet (AE) indices being widely used to monitor substorm activities. We present observations of magnetic substorms having more prominent effect poleward of the standard auroral oval. Magnetic data from the third Indian Antarctic station, Bharati (BHA; corrected geomagnetic (CGM) coordinates: 74.7°S, 96.6°E) in conjunction with IMAGE chain data (near conjugate station Hornsund (HOR; CGM coordinates: 74.3N, 108.5°E) have been subjected to detailed examination to study such substorms. The substorms presented in this study were mainly localized to high latitudes and hence the standard AE indices failed to monitor such substorm activities. Nevertheless, typical low-latitude features of substorm, for example, positive bay and Pi2 burst on the nightside were distinctly evident.
[Show abstract][Hide abstract] ABSTRACT: The January 15, 2010, solar annular eclipse crossed the magnetic equator in the middle of the day over India, in a region instrumented with several magnetometers, Total Electron Content stations using GPS data, and an ionosonde located very near the center of the eclipse. With the help of a one-dimensional model appropriate for the region of interest we show that the ionosonde data was consistent with a lower F region plasma that was moving upwards with only modest velocities in the morning hours and moving resolutely downwards in the afternoon hours. This motion agreed well with the local magnetometer data which revealed a weakened electrojet taking place in the morning hours while a full-blown counter-electrojet was present in the afternoon hours. We show that the unusual solar eclipse-induced electrodynamics resulted in a reduction in the Total Electron Content depletion not just at the magnetic equator but also, more markedly, in the Equatorial Ionization Anomaly (EIA) zone, a further 10 degrees to the north. This latter point clearly shows that the eclipse led to a cut-off in the supply of plasma provided through the equatorial fountain, by altering a fundamental aspect of the equatorial electrodynamics.
Full-text · Article · Sep 2011 · Journal of Geophysical Research Atmospheres
[Show abstract][Hide abstract] ABSTRACT: The global component of fairweather electricity is subject to special attention to watch the solar-terrestrial effects and
secular changes in climate. It is generally considered that the diurnal variation of atmospheric electricity parameters, if
they are not following the Carnegie pattern, are not representative of the global thunderstorm activity. Some of the results
obtained from Maitri (70°45′54″S, 11°44′03″), are discussed here in context with global thunderstorm activity and space weather
influences. The diurnal pattern of the Potential Gradient and current density strongly deviate from the Carnegie curve. We
have showed that this deviation is not due to the local electrical influence but due to the global thunderstorm activity.
During fairweather condition the parameters are representing the global thunderstorm activity and to some extent they respond
to the upper atmospheric electro dynamic phenomenon. The mean value of the potential gradient (77.7 V/m) and current density
(2.13 pA/m2) well below the expected global mean but close to the value reported from the same location and season in the past years.
The mean conductivity, 3.34 × 10−14 mhom−1, is slightly at higher side and they exhibit a different diurnal trend comparing to the past measurements at this location.
KeywordsConductivity–Conduction current–Potential Gradient–Global Lightning Flash numbers–Geomagnetic substorm
Full-text · Article · Sep 2011 · Journal of the Geological Society of India