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INTRODUCTION

Significant contribution to research in geomagnetism started from India as back as in 19th century with the pioneering work of Brown and Chambers and Moos. Institute started work in the areas of solar and lunar geomagnetic variations, equatorial electrojet studies, annual and semi-annual variations of the geomagnetic field, complexities of the recurrent geomagnetic field signatures etc. Solar-terrestrial-physics associated studies were mainly utilising long series of geomagnetic field observations at the Indian Observatories and also worldwide network of geomagnetic data. The Geomagnetic Observatory data were also used for studies on Solar wind plasma and Interplanetary Magnetic Field (IMF) associations, Solar flare effects etc., thus understanding the Sun-Earth interaction phenomena as inferred from the ground magnetic variations over the equatorial and low latitude locations.

 

Digital magnetic records at Tirunelveli for three consecutive days during different solar activity conditions in April 2001. Solar flare effect is seen as a spurt in the amplitude of the Horizontal component (‘H’) on April 10th, Normal day ( April 11th ) and an intense storm development on April 11th . The impact of the shock associated with the coronal mass ejection following the solar flare on April 10th, is seen as storm Sudden commencement and subsequent development of an intense main phase in the ground magnetic field.

The continuous recorded data gives an opportunity to decipher the long-term secular changes as well as the daily variations of the magnetic components that is basically the reflection of the ionospheric and magnetospheric change occurring above a particular observatory. Thus the variations in the geomagnetic field can be used as a diagnostic tool for understanding the internal structure of the Earth as well as the dynamics of the upper atmosphere and magnetosphere.

Secular variation of the three components of the earth’s magnetic field from the Colaba-Alibag combined observations for 160 years The geographical location of India plays a pivotal role with the latitudinal coverage existing from equator to the focus of the low latitude Sq current system. India has the long lasting history of measuring the earth's magnetic field using classical instruments. With the advancement of instrumentation in the field of magnetic measurements, the observatories operated by IIG are being modernized using fluxgate magnetometers for variation recordings and Declination Inclination Magnetometer (DIM) for Absolute observations.

 

SOLAR QUIET (Sq) AND EQUATORIAL ELECTROJET VARIATIONS

Locations of the Indian permanent observatories are such as to cover a wide range of latitudes extending from the dip equator up to the focus of the Sq current system, which flows in the E-region of the ionosphere. Thus, the magnetic data recorded continuously at these observatories provides a unique opportunity to study the associated phenomena of ionospheric Sq current system as well as the equatorial electrojet (EEJ). The EEJ flows eastward within a narrow region of about ? 50 latitude centered over the dip equator. Scientists at the Institute have been working towards understanding the detailed nature of these current systems, including its large day-to-day variability, solar activity and seasonal dependence, as well as the reverse equatorial electrojet, better known as equatorial counter electrojet (CEJ) phenomenon. An empirical model of the equatorial electrojet has been successfully set up based on the surface magnetic data recorded at various stations located in six different longitude sectors, and able to reproduce the characteristic signatures of the EEJ-associated horizontal and vertical magnetic field components at ground level. Besides this, researchers at the institute are also engaged in studying ionospheric electrodynamics using various satellite magnetic field observations such as Magsat, Oersted, Champ, SAC-C etc. The ground magnetic field data from Indian stations have been used to support the satellite findings.

EFFECTS OF MAGNETIC DISTURBANCES

The importance of geomagnetic data in exploring the space weather phenomena of varied timescales is becoming crucial.

 

The ionosphere and magnetosphere are a closely coupled system that channels energy and momentum from the solar wind to the upper atmosphere. A number of coupled current systems flow in these regions of highly conducting plasmas. These currents are responsible for most of the temporal changes in the geomagnetic field that occur on time scales of seconds to days, including magnetic pulsations. Studies of the ionosphere and magnetosphere seek to obtain a quantitative understanding of the flow of energy and momentum through the solar wind, magnetosphere and ionosphere systems. This study in turn supports the physics of magnetic reconnection at the magnetopause, the response of the magnetosphere to changes in solar wind pressure, the processes responsible for viscous like interactions. Investigations towards this direction elucidate the physical mechanisms responsible for generating pulsations and controlling their cross-field transport in the magnetosphere.

GEOMAGNETIC FIELD FLUCTUATIONS OF VARIOUS PERIODICITIES

Study of micropulsations, planetary waves, and long period oscillations such as quasi-biennial (QBO), annual (AO), and semi-annual (SAO) oscillations, is being carried out in the institute using high resolution magnetometers.

IIG scientists have attempted to establish the spatial and frequency characteristics of the equatorial enhancement for several period bands from minutes range to sq harmonics using geomagnetic data from upgraded/new networks. Using data adaptive singular spectrum analysis technique, the regular Sq harmonics and Pc pulsations can be isolated. The characteristics of geomagnetic pulsations undergo appreciable changes as they pass through the ionosphere. These changed properties at the low and equatorial stations are distinctly different from those at the high latitudes. It is found that polarization directions of PC3-4 (period 10 to 100 seconds) pulsation changed during the counter electrojet time. The amplitude of these pulsations is enhanced by equatorial electrojet.

Geomagnetic pulsation in PC5-6 range is being studied using the magnetic records. It is seen that these pulsations are enhanced in the equatorial region by a factor of about four during daytime. Recently one search coil magnetometer is being installed at Geomagnetic Research Laboratory Allahabad. This system will help in studying pulsations in various frequency bands in the low latitudes.

TOPICS OF INVESTIGATIONS ARE MAINLY:

  • The process of understanding the manifestation and development of the geomagnetic storm process under varied interplanetary conditions following rapid eruptions from the active Sun as solar flares and gigantic Coronal mass ejections (CMEs) etc.
  • The effect of the impingement of the solar energetic particles (SEP), following active solar flare occurrences in the Sun and fast coronal mass ejections leading to dynamic changes in the earth’s magnetosphere.
  • Estimation of storm time energy budget by computing solar wind energies, magnetospheric coupling energies, auroral and Joule heating energies and also ring current energies.
  • The extreme magnetic storm of September 1-2, 1859 was one of the most intense events in recorded history, based on the data deduced from the reported ground magnetic observations from the Colaba magnetic observatory. Using empirical results on the interplanetary magnetic field strengths of magnetic clouds versus velocity, it was shown that the well known September 1, 1859 Carrington solar flare most likely had an associated intense magnetic cloud ejection which led to the magnetic storm on Earth, which was observed as a large decrease in the Horizontal Component (Dst˜-1600 nT) during the main phase, as recorded at Colaba (Bombay).
  • The intense storm events on October 29 and October 31, 2003 resulted from earth directed strong eruption of Coronal Mass Ejections (CMEs) at a speed around 2000km/s. Due to the occurrence of large proton events following the flare events, solar wind detectors onboard NASA/ESA’s ACE/SOHO satellites at L1 – Lagrangian point- were affected, thus reporting saturated values in the data stretch for sometime.

Figure displays the respective growth of the main phase events during the two storm sudden commencements occurred on October 29 and October 30, 2003, following the highly eruptive solar events. Solar wind and interplanetary magnetic field parameters (ACE) during the storm events are also given.

    • During solar maximum, solar flares, geo-effective CMEs and the intense southward IMF are responsible for the development of non-recurrent geomagnetic storms. Dominance of high speed streams from coronal holes is a known feature during solar minimum. If the high speed streams overtake slower speed streams, the magnetic field and plasma compressions results at their interfaces known as Co-rotating Interaction Regions (CIRs) which are responsible for recurrent geomagnetic storms during the descending phase of the solar cycle. Multiple peak signatures of the occurrence of geomagnetic activity seem to dominate during recent solar cycles. Main sources of origin for these structured activity maxima are investigated by studying the variable characteristics observed in the low latitude storm development pattern.
    • Low latitude magnetograms often display sporadic positive increase in the Horizontal component of the geomagnetic field, named as positive bays and at times as negative bays. These bay structures are believed to be associated with the onset of the substorms and a quantitative estimate of the asymmetric development of the magnitude and phase of the bay events are brought out depending on the level of magnetic activity.

  • Long series of magnetic data recorded at Indian magnetic observatories are used to quantify the occurrence characteristics of geomagnetic storms.
 

 B. Veenadhari, Geeta Vichare, Rajesh Singh, B. D. Kadam, S. Mukherjee

 

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