Solar causes of the long-term increase in geomagnetic activity Export

@article{citeulike:227376, abstract = {We analyze the causes of the century-long increase in geomagnetic activity, quantified by annual means of the <i>aa</i> index, using observations of interplanetary space, galactic cosmic rays, the ionosphere, and the auroral electrojet, made during the last three solar cycles. The effects of changes in ionospheric conductivity, the Earth\&\#8217;s dipole tilt, and magnetic moment are shown to be small; only changes in near-Earth interplanetary space make a significant contribution to the long-term increase in activity. We study the effects of the interplanetary medium by applying dimensional analysis to generate the optimum solar wind-magnetosphere energy coupling function, having an unprecedentedly high correlation coefficient of 0.97. Analysis of the terms of the coupling function shows that the largest contributions to the drift in activity over solar cycles 20\&\#8211;22 originate from rises in the average interplanetary magnetic field (IMF) strength, solar wind concentration, and speed; average IMF orientation has grown somewhat less propitious for causing geomagnetic activity. The combination of these factors explains almost all of the 39\% rise in <i>aa</i> observed over the last three solar cycles. Whereas the IMF strength varies approximately in phase with sunspot numbers, neither its orientation nor the solar wind density shows any coherent solar cycle variation. The solar wind speed peaks strongly in the declining phase of even-numbered cycles and can be identified as the chief cause of the phase shift between the sunspot numbers and the <i>aa</i> index. The rise in the IMF magnitude, the largest single contributor to the drift in geomagnetic activity, is shown to be caused by a rise in the solar coronal magnetic field, consistent with a rise in the coronal source field, modeled from photospheric observations, and an observed decay in cosmic ray fluxes. \&copy; 1999 American Geophysical Union}, author = {Stamper, R. and Lockwood, M. and Wild, M. N. and Clark, T. D. G.}, citeulike-article-id = {227376}, citeulike-linkout-0 = {http://www.agu.org/pubs/crossref/1999/1999JA900311.shtml}, journal = {Journal of Geophysical Research}, keywords = {aa, coupling\_function, geophysical\_indices, long-term\_change, magnetosphere, solar\_variability, solar\_wind}, number = {A12}, pages = {28325--28342}, posted-at = {2005-06-14 12:07:21}, priority = {0}, title = {Solar causes of the long-term increase in geomagnetic activity}, url = {http://www.agu.org/pubs/crossref/1999/1999JA900311.shtml}, volume = {104}, year = {1999} }

TY - JOUR ID - citeulike:227376 L3 - citeulike-article-id:227376 N2 - We analyze the causes of the century-long increase in geomagnetic activity, quantified by annual means of the <i>aa</i> index, using observations of interplanetary space, galactic cosmic rays, the ionosphere, and the auroral electrojet, made during the last three solar cycles. The effects of changes in ionospheric conductivity, the Earth&#8217;s dipole tilt, and magnetic moment are shown to be small; only changes in near-Earth interplanetary space make a significant contribution to the long-term increase in activity. We study the effects of the interplanetary medium by applying dimensional analysis to generate the optimum solar wind-magnetosphere energy coupling function, having an unprecedentedly high correlation coefficient of 0.97. Analysis of the terms of the coupling function shows that the largest contributions to the drift in activity over solar cycles 20&#8211;22 originate from rises in the average interplanetary magnetic field (IMF) strength, solar wind concentration, and speed; average IMF orientation has grown somewhat less propitious for causing geomagnetic activity. The combination of these factors explains almost all of the 39% rise in <i>aa</i> observed over the last three solar cycles. Whereas the IMF strength varies approximately in phase with sunspot numbers, neither its orientation nor the solar wind density shows any coherent solar cycle variation. The solar wind speed peaks strongly in the declining phase of even-numbered cycles and can be identified as the chief cause of the phase shift between the sunspot numbers and the <i>aa</i> index. The rise in the IMF magnitude, the largest single contributor to the drift in geomagnetic activity, is shown to be caused by a rise in the solar coronal magnetic field, consistent with a rise in the coronal source field, modeled from photospheric observations, and an observed decay in cosmic ray fluxes. &copy; 1999 American Geophysical Union IS - A12 JF - Journal of Geophysical Research EP - 28342 TI - Solar causes of the long-term increase in geomagnetic activity VL - 104 SP - 28325 KW - aa KW - coupling_function KW - geophysical_indices KW - long-term_change KW - magnetosphere KW - solar_variability KW - solar_wind AU - Stamper, R AU - Lockwood, M AU - Wild, MN AU - Clark, TDG PY - 1999/// UR - http://www.agu.org/pubs/crossref/1999/1999JA900311.shtml ER -