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°S, 11.75°E; corrected geomagnetic 63.11°S, 53.59°E) in February 2010. This is the first paper using the imaging riometer data from Maitri. The present paper introduces the details 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 southward turning of interplanetary magnetic field before the substorm onset.

Geomagnetic Pc5–6 (1–4 mHz) pulsations during three magnetic storms have been analyzed using ground observations at an array of 5 Indian stations ranging from 0.8° to 25° dip latitudes and two pairs of low-latitudes (~20°)—near equator (~4–5°) stations, located at ~4 hours to the West and East from this meridian. The pulsations are found to occur as individual wave packets that are characterized by similar spectra at all stations with two dominate bands of frequencies: ~1.5–2.0 mHz and ~2.5–4.0 mHz. Very strong daytime equatorial enhancement of the wave intensity has been noted. The higher enhancement is observed near noon while it is reduced towards the morning and evening sectors. The polarization of both frequency bands at the equator is found to be almost linear, mainly along the H-direction. It has been found that the amplitude of the D-component decreased with decrease in the latitude towards the equator. One of the spectral peaks is observed exclusively at the dip equator station and it has been attributed to the generation/amplification of MHD waves by the equatorial electrojet instabilities. The mechanism of day time equatorial enhancement of long period geomagnetic pulsations and the propagation characteristics are discussed.

The paper describes the study of solar flare effects ( sfe ) on horizontal ( H ), eastward ( Y ) and vertical ( Z ) components of the geomagnetic field at the Indo-USSR chain of magnetic observatories extending from the magnetic latitudes 0° to 45°N, a network not available any where else in the world. Events are selected when strong, normal and reversed (counter) equatorial electrojet are in existence as well as when only a partial counter electrojet is present. During a strong and normal electrojet event time, sfe consists of a positive impulse in H at all stations, the amplitude of H following the latitudinal variation similar to that of the quiet day monthly mean, Sq ( H ). The sfe in Y is negative at all the stations. Sfe in Z shows positive impulse at the four-electrojet stations and negative at other stations. During a counter electrojet period the effect of solar flare on H field is negative impulse at electrojet stations, positive at low latitude and again negative at stations north of Sq focus. Sfe in Y is small at all the equatorial nd negative at higher latitude stations. However, sfe ( Z ) is negative at the equatorial latitudes. During a partial counter electrojet period the observed effect is the combination of an increase of the planetary current component and the decrease of the electrojet component, giving rise to a negative impulse in H at equatorial stations and apositive impulse in H , increasing with increasing distance from the equator. These results are presented and discussed.

The unique network of geomagnetic observatories along 145°E geomagnetic longitude extending from the magnetic equator to the north pole has enabled to study the latitudinal profiles of Storm Sudden Commencement (SSC) amplitudes in the three components H , Y and Z of the geomagnetic field separately for the daytime and nighttime events. An abnormally large positive impulse of Z is observed at the equatorial stations with maximum at Trivandrum during the daytime as well as the night time hours suggesting large induced current within the earth’s crust south of Indian continent. The daytime enhancement of SSC ( H ) at the extended equatorial latitudes is undoubtedly due to the disturbed electric field generated by the magnetopause current communicated to the equator through polar latitudes. A prominent decrease of SSC ( H ) during night hours and the ‘induction vector’ at SSC frequencies at equatorial latitudes are indicative of the concentration of induced current from source fields extended in altitudes.

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.

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.

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.

The standard auroral electrojet (AE) indices are based on magnetic disturbance data from 10 to 12 northern auroral observatories. Recently, Newell and Gjerloev (2011a) computed equivalent SuperMAG electrojet (SME) indices using data from around 100 mid latitude to high latitude observatories in the Northern Hemisphere. The SME indices certainly have advantage over the AE indices in terms of number as well as temporal resolution of substorm onsets due to better latitudinal and longitudinal coverage. The UT and seasonal variations of geomagnetic activity have been extensively examined in the past. However, particularly for the AE indices, these variations have remained elusive due to sparse distribution of the AE observatories. In this study, we examine what effect the inclusion of large number of stations would make on the UT and seasonal variations of the auroral electrojets activities. For this purpose, data for years 1997–2009 have been considered when consistently many stations (> 70) were available for the computation of the SME indices. We demonstrate that the SME indices exhibit grossly similar UT and seasonal variations as observed in the AE indices. However, there are subtle differences which arise due to difference in number of stations. Our study suggests that most of the UT and seasonal variations of the AE indices, reported earlier, were mainly not due the sparse distribution of stations, but rather to the actual physical processes that control them.

Magnetic Pulsations recorded on the ground are the signatures of the integrated signals from the earth’s magnetosphere. Pc4 geomagnetic pulsations are quasi-sinusoidal variations in the earth’s magnetic field in the period range 45-150 seconds. The magnitude of these pulsations ranges from fraction of a nano Tesla (nT) to several nT. Although these pulsations can be observed in a number of ways, yet the application of ground-based magnetometer arrays has proven to be the most successful methods of studying the spatial structure of hydromagnetic waves in the earth’s magnetosphere. The solar wind provides the energy for the earth’s magnetospheric processes. The source of Pc4 magnetic pulsations can either be internal to the magnetosphere (endogenic) or external to it, transmitted through the magnetopause (exogenic). Most of the Pc4 studies undertaken in the past have been confined to middle and high latitudes. The spatial and temporal variations observed in the Pc4 occurrence are of vital importance because these provide evidence that can be directly related to both endogenic and exogenic wave generation mechanisms. At low latitudes ( L< 2), the wave energy predominates in the Pc4 band. However the spatial characteristics of these pulsations have received little attention in the past. The present study is undertaken for describing the dependence of low latitude Pc4 occurrence on the Solar Wind Velocity (VSW) and the Interplanetary Magnetic Field (IMF) over the period range 01 January to 31 December, 2005 employing an array of three low latitude recording stations at Hanley, Nagpur and Pondicherry. Analysis of the data for the whole year 2005 provided similar patterns of Pc4 occurrence for VSW at all the three stations. Although Pc4 occurrence was reported for VSW ranging from 250 to 1000 Km/s, yet the major Pc4 events occurred for a VSW range of 300-700 Km/sec. The IMF dependence of Pc4 occurrence for the year 2005 has shown that even though at all the three stations, it spread for IMF magnitude of up to 22 nT, yet the majority of Pc4 events occurred for a narrower range of 2-10 nT. However it is important to note that at all the three stations, the peak in the occurrence of Pc4 events was observed for IMF range of 3 to 5 nT. The results suggest that the solar wind controls Pc4 occurrence through a mechanism in which Pc4 wave energy is convected through the magnetosheath and coupled to the standing oscillations of the magnetospheric field lines.

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.

Low latitude asymmetric (ASY) indices in addition to auroral electrojet (AE) indices have been widely used to monitor magnetic substorm phenomenon. However, low latitude ASY indices are strongly affected by various current systems in the magnetosphere and the ionosphere which may not have any direct relation with substorm activity. In this paper we investigate 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). We observe that during quiet magnetospheric conditions, long-lasting intense solar flares generate asymmetry similar to substorm events in ASY indices. In addition, substorm time ASY indices are also substantially affected by intense solar flares.

The westward flowing toroidal ring current at about 2–7 RE in the Earth's equatorial plane consists of symmetric and asymmetric parts. Zonal mean of H disturbances from longitudinally distributed low latitude stations represents the symmetric contribution, whereas departure from the zonal mean gives local time dependent asymmetric component at each of the stations. Through a standard analysis of closely spaced low latitude geomagnetic data we demonstrate 24 h periodicity in the asymmetric component of the storm-time ring current, which is related to the changing local 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, westward and eastward propagating modes had been reported in earlier studies. In this study, we report that similar propagating modes which are available on global scale, can be identified using ground-based magnetometer data. These globally propagating modes, observed from ground-based studies, find obvious practical application in diagnostics of the magnetosphere, especially the ring current region. Simultaneous use of satellite and ground-based data should establish the morphology of such modes.

The quiet-time (ΣKp ≤ 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 (Sq) current system. The present work reports the signatures of the south limb of the Sq current loop of the southern hemisphere over a sub-auroral station. It is observed that the seasonal variation of the Sq 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 Sq pattern is identified at Maitri. The range of the horizontal field variation in the daily Sq 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 Sq 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 Sq pattern.