عنوان مقاله [English]
The daily record of geomagnetic variations at any world location typically shows a multitude of irregular changes in the field that represent the superposition of many spectral components whose amplitudes generally increase with increasing period. It has been a long established fact that variations in ground magnetic records are caused by the dynamo action in the upper atmosphere. These daily variations in the geomagnetic fields at the earth's surface during geomagnetically quiet conditions are known to be associated with the dynamo currents which are driven by winds and thermal tidal motions in the E-region of the ionosphere.
Unique current sources in the upper atmosphere and magnetosphere have been identified as origins of many of these spectral field variations. On occasion there are days when the magnetic records smoothly change with primarily 24-, 12-, 8-, and 6-hour period spectral components dominating the field composition and few of the irregularly appearing, shorter or longer period changes are present. On these days, the oscillations of three orthogonal field components produce records that are predictably similar to others recorded many days earlier or later and follow a pattern of gradual change through the seasons of the year. Such records describe the quiet daily geomagnetic variations. When the small but persistent effects ascribed to the lunar- tidal current system have been removed, the changes are called Sq for solar quiet fields, referring to their local-time changes when solar-terrestrial disturbances are absent.
Solar activity, identified with the sunspot number, controls the percentage of magnetically quiet days in a year as an inverse relationship. The quietest geomagnetic levels usually occur on, or a year after, the minimum in sunspot number. Because of the 10.6-year cycle in solar activity, a similar cycle of geomagnetically quiet years occurs.
The quasilogarithmic Kp is a convenient three-hour index for selecting the quiet conditions; its linear counterpart is the Ap index. Some authors prefer to select quiet days by a limiting value of the day's Ap (e.g., Ap = 10). Others take a fixed number (e.g., five) of the quietest days (judged by the day's Ap) for a given month, whatever the values may be.
The 19 North American observatories are selected in this study. There are 60 days in 1997 in which the global geomagnetic disturbance index, Kp, have all 8 daily values less than 2+. These days are taken as preliminary “quiet day” recordings. All observatories have 60-min sample records. The original recordings of field are in Universal Time (UT) as orthogonal north, east, and into-the-earth components of field as X, Y, and Z. The data for each component are Fourier analyzed for each quiet day.
A systematic change in pattern of the daily variation with change in latitude is clearly evident. The large amplitude of X occurs at all observatories near the geomagnetic dip equator because of existence of the equatorial electrojet. The relation between the Chapman factor (cosχ)0.5 and Sq (Z) is investigated and concluded that the Sq (Z), corresponds in onset and subsidence with Chapman-factor change.
It has been concluded that the maximum amplitudes of magnetic potentialoccur near the midlatitudes which are the locations of the external current foci. The internal Z is in the opposite direction to that of the external Z. The external and internal currents must be oppositely directed to obtain the similar pattern of X and Y but opposite pattern of Z. It seems that all three internal variation amplitudes are less than the corresponding external amplitudes.