Abstract. The magnetic structure and geomagnetic response of 73 magnetic clouds (MC) observed by the WIND and ACE satellites in solar cycle 23 are examined. The results have been compared with the surveys from the previous solar cycles. The preselected candidate MC events were investigated using the minimum variance analysis to determine if they have a flux-rope structure and to obtain the estimation for the axial orientation (θ C , φ C ). Depending on the calculated inclination relative to the ecliptic we divided MCs into "bipolar" (θ C <45 • ) and "unipolar" (θ C >45 • ). The number of observed MCs was largest in the early rising phase, although the halo CME rate was still low. It is likely that near solar maximum we did not identify all MCs at 1 AU, as they were crossed far from the axis or they had interacted strongly with the ambient solar wind or with other CMEs. The occurrence rate of MCs at 1 AU is also modified by the migration of the filament sites on the Sun towards the poles near solar maximum and by the deflection of CMEs towards the equator due to the fast solar wind flow from large polar coronal holes near solar minimum. In the rising phase nearly all bipolar MCs were associated with the rotation of the magnetic field from the south at the leading edge to the north at the trailing edge. The results for solar cycles 21-22 showed that the direction of the magnetic field in the leading portion of the MC starts to reverse at solar maximum. At solar maximum and in the declining phase (2000)(2001)(2002)(2003) we observed several MCs with the rotation from the north to the south. We observed unipolar (i.e. highly inclined) MCs frequently during the whole investigated period. For solar cycles 21-22 the majority of MCs identified in the rising phase were bipolar while in the declining phase most MCs were unipolar. The geomagnetic response of a given MC depends greatly on its magnetic structure and the orientation of the sheath fields. For each event we distinguished the effect of the sheath fields and the MC fields. All unipolar MCs with magnetic field southward at the axis were geoeffective (D st <−50 nT) while those with Correspondence to: K. E. J. Huttunen (emilia.huttunen@helsinki.fi) the field pointing northward did not cause magnetic storms at all. About half of the all identified MCs were not geoffective or the sheath fields preceding the MC caused the storm. MCs caused more intense magnetic storms (D st <−100 nT) than moderate magnetic storms (−50 nT ≥D st ≥−100 nT).

This paper introduces and describes the radio and plasma wave investigation on the STEREO Mission: STEREO/WAVES or S/WAVES. The S/WAVES instrument includes a suite of state-of-the-art experiments that provide comprehensive measurements of the three components of the fluctuating electric field from a fraction of a hertz up to 16 MHz, plus a single frequency channel near 30 MHz. The instrument has a direction finding or goniopolarimetry capability to perform 3D localization and tracking of radio emissions associated with streams of energetic electrons and shock waves associated with Coronal Mass Ejections (CMEs). The scientific objectives include: (i) remote observation and measurement of radio waves excited by energetic particles throughout the 3D heliosphere that are associated with the CMEs and with solar flare phenomena, and (ii) in-situ measurement of the properties of CMEs and interplanetary shocks, such as their electron density and temperature and the associated plasma waves near 1 Astronomical Unit (AU). Two companion papers provide details on specific aspects of the S/WAVES instrument, namely the electric antenna system (Bale et al., Space Sci. Rev., 2007) and the direction finding technique (Cecconi et al., Space Sci. Rev., 2007).

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[1] We have investigated the solar wind drivers of magnetic storms during the rising phase of solar cycle 23 from January 1996 to December 1999. We used observations of coronal mass ejections (CMEs) by the Large Angle and Spectrometric Coronagraph instrument on SOHO and in situ solar wind observations by Wind, IMP 8, and ACE spacecraft. The storms were determined from both the Dst and Kp indices, and the study was limited to storms with Dst À50 nT or Kp ! 5. We show examples of different behavior of Dst and Kp indices during magnetic storms caused by different types of solar wind drivers. Furthermore, we have investigated cross-correlation between peak Dst and Kp values of storms organized according to the associated solar wind driver. It makes a difference whether a sheath region or the following ejecta causes the storm. We found that almost all intense and stronger magnetic storms (Dst À100 nT, or Kp ! 7À) were associated with shocks and CMEs, but for moderate storms, driver statistics were different in different phases of the solar cycle. We found different behavior of the Kp and Dst indices during different types of solar wind drivers. Intense and short-time disturbances, like postshock streams and sheath regions, generated more Kp storms, and ejecta generated more Dst storms. Thus one should be careful when comparing studies based on any single activity index.