Interplanetary shocks and sudden impulses during solar maximum (2000) and solar minimum (1995–1996) E. Echer a, * , W.D. Gonzalez a , A. Dal Lago a , L.E.A. Vieira a , F.L. Guarnieri a , A.L.C. Gonzalez a , N.J. Schuch b a Instituto Nacional de Pesquisas Espaciais – INPE, P.O. Box 515, 12245-970 Sa ˜ o Jose ´ dos Campos, SP, Brazil b Centro Regional Sul de Pesquisas Espaciais, CRSPE/INPE – Santa Maria, RS, Brazil Received 15 November 2002; received in revised form 8 April 2003; accepted 9 April 2005 Abstract In this work a study is performed on the correlation between fast forward interplanetary shock parameters at 1 Astronomical Unit and sudden impulse (SI) amplitudes in the H-component of the geomagnetic field, for periods of solar activity maximum (year 2000) and minimum (year 1995–1996). Solar wind temperature, density and speed, and total magnetic field, were taken to calculate the static pressures (thermal and magnetic) both in the upstream and downstream sides of the shocks. The variations of the solar wind parameters and pressures were then correlated with SI amplitudes. The solar wind speed variations presented good correlations with sudden impulses, with correlation coefficients larger than 0.70 both in solar maximum and solar minimum, whereas the solar wind density presented very low correlation. The parameter better correlated with SI was the square root dynamic pressure varia- tion, showing a larger correlation during solar maximum (r = 0.82) than during solar minimum (r = 0.77). The correlations of SI with square root thermal and magnetic pressure were smaller than with the dynamic pressure, but they also present a good corre- lation, with r > 0.70 during both solar maximum and minimum. Multiple linear correlation analysis of SI in terms of the three pres- sure terms have shown that 78% and 85% of the variance in SI during solar maximum and minimum, respectively, are explained by the three pressure variations. Average sudden impulse amplitude was 25 nT during solar maximum and 21 nT during solar mini- mum, while average square root dynamic pressure variation is 1.20 and 0.86 nPa 1/2 during solar maximum and minimum, respec- tively. Thus on average, fast forward interplanetary shocks are 33% stronger during solar maximum than during solar minimum, and the magnetospheric SI response has amplitude 20% higher during solar maximum than during solar minimum. A comparison with theoretical predictions (TsyganenkoÕs model corrected by EarthÕs induced currents) of the coefficient of sudden impulse change with solar wind dynamic pressure variation showed excellent agreement, with values around 17 nT/nPa 1/2 . Ó 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Interplanetary shocks; Sudden impulses; Solar maximum; Solar minimum; Space weather 1. Introduction Interplanetary shock waves are generated when the relative speed between a fast stream (coronal mass ejec- tion) and the slow solar wind is greater than the charac- teristic speed of the medium-magnetosonic speed (Kivelson and Russell, 1995). When a shock wave reaches the terrestrial magnetosphere, compressing it, the Chapman–Ferraro current that flows in the magne- topause is intensified (Nishida, 1978; Kivelson and Rus- sell, 1995). This intensified magnetopause current generates a magnetic field that increases the geomagnetic field inside the magnetosphere. Because a shock is a dis- continuous variation in solar wind parameters, a discon- tinuous variation in the low latitude geomagnetic field H-component is also observed and it is called a sudden impulse – SI (Chapman and Ferraro, 1931). If a SI is 0273-1177/$30 Ó 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2005.04.030 * Corresponding author. Fax: +55 12 3945 6810. E-mail address: eecher@dge.inpe.br (E. Echer). www.elsevier.com/locate/asr Advances in Space Research 36 (2005) 2313–2317