Physics
Scientific paper
Aug 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994angeo..12..612t&link_type=abstract
Annales Geophysicae (ISSN 0992-7689), vol. 12, no. 7, p. 612-624
Physics
30
Interplanetary Magnetic Fields, Magnetic Storms, Solar Activity, Solar Terrestrial Interactions, Solar Wind, Solar Wind Velocity, Time Measurement, Classifications, Data Bases, Data Correlation, Magnitude, Time Series Analysis
Scientific paper
A superposed epoch analysis of geomagnetic storms has been undertaken. The storms are categorized via their intensity (as defined by the Dst index). Storms have also been classified here as either storm sudden commencements (SSCs) or storm gradual commencements (SGCs, that is all storms which did not begin with a sudden commencement). The prevailing solar wind conditions defined by the parameters solar wind speed nu(sub sw), density (rho(sub sw)) and pressure (P(sub sw)) and the total field and the components of the interplanetary magnetic field (IMF) during the storms in each category have been investigated by a superposed epoch analysis. The southward component of the IMF, appears to be the controlling parameter for the generation of small SGCs (-100 nT less than minimum Dst less than or = -50 nT for greater than or = 4 h), but for SSCs of the same intensity solar wind pressure is dominant. However, for large SSCs (minimum Dst less than or = -100 nT for greater than or = 4 h) the solar wind speed is the controlling parameter. It is also demonstrated that for larger storms magnetic activity is not solely driven by the accumulation of substorm activity, but substantial energy is directly input via the dayside. Furthermore, there is evidence that SSCs are caused by the passage of a coronal mass ejection, whereas SGCs result from the passage of a high speed/low speed coronal stream interface. Storms are also grouped by the sign of B(sub z) during the first hour epoch after the onset. The sign of B(sub z) at t = +1 h is the dominant sign of the B(sub z) for approximately 24 h before the onset. The total energy released during storms for which B(sub z) was initially positive is, however, of the same order as for storms where B(sub z) was initially negative.
Lester Mark
Taylor Jaime R.
Yeoman Timothy K.
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