Abstract
The present study mainly focuses on the anomalous characteristics observed in some abnormal quiet day (AQD) variations of the north–south (H) and east–west (D) components of geomagnetic field at 11 Indian stations for the years 2004, 2005, and 2009 during low solar activity period for summer and winter months. In this study, during some quiet days, horizontal component (H) at stations situated near equator to the Sq focus latitudes has shown double peak structure, exhibiting maximum in the forenoon and afternoon hours. Correspondingly, declination component (D) over the low latitudes has shown features outside the normal trend, i.e., westward-directed field in the morning hours and eastward-directed field in the afternoon hours and shows negative bay-type of variations. The technique of principal component analysis (PCA) has been applied to the data sets for presenting a quantitative estimate of the influence of day-to-day variability in the Sq current system on normal (NQD) and abnormal quiet (AQD) days. AQDs observed at the Indian stations are reflected in the second principal component PC-2. Anomalous changes in day-to-day variations (H and D) are interpreted as an influence of high latitude magnetospheric current systems as well as due to single current vortex (SCV) located in the ionosphere whose focus lie between 10° and 15° N geomagnetic latitude for the northern hemisphere winter AQDs.
Highlights
Most smooth and regular variations recorded on magnetograms during magnetically quiet days are known as solar quiet daily variations or Sq (Campbell 1989)
The equatorial electrojet (EEJ) strength, which is calculated by taking the difference of H-component at Tirunelveli and Alibag on these days are shown at the bottom panel of these figures which is more on abnormal quiet day (AQD) in comparison with normal quiet day (NQD)
From principal component analysis (PCA), we suggest that the possible source of the AQD on 11 December 2009 event may be located in the ionosphere with an equivalent current system in the form of a single current vortex (SCV) that flows in a clockwise direction as evident from Fig. 9b in which the focus is shifted to lower latitudes between SHL and NGP
Summary
Most smooth and regular variations recorded on magnetograms during magnetically quiet days are known as solar quiet daily variations or Sq (Campbell 1989) These types of variations were first observed by Graham (1724). The major driving force for quiet day variations are due to X-rays (1–170 A°) and extreme ultraviolet rays (170–1750 A°) from the Sun that ionizes the upper atmosphere (known as ionosphere). These ionized gases known as plasma are forced to move across the Earth’s main magnetic field producing electromagnetic forces (EMFs). The observed quiet day variations at the ground is a result of several parameters like ionospheric conductivity, ionospheric winds, geomagnetic field configuration, etc. and the ionospheric dynamo is not the only source of these Sq variations but some contributions from magnetospheric origin is present in this Sq field
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