Browsing by Subject "MINIMUM"

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  • Andreeova, K.; Kilpua, E. K. J.; Hietala, H.; Koskinen, H. E. J.; Isavnin, A.; Vainio, R. (2013)
    In this paper we have analyzed a substructure found within a leading part of a north–south-oriented magnetic cloud (MC) observed on 3–4 September 2008 in the near-Earth solar wind by multiple spacecraft (ACE, Wind, THEMIS B and C). The MC was preceded by a stream interface (SI) and followed by a high-speed stream (HSS). The identified substructure featured a strong depletion of suprathermal halo electrons and showed distinct magnetic field and plasma signatures. It occurred where suprathermal electron flow within a cloud changed from bidirectional to unidirectional, indicating change in the field line connectivity to the Sun. We found that the substructure maintained roughly its integrity from the first Lagrangian point to the vicinity of the Earth's bow shock in the front edge of the MC, but revealed small changes in the structure which could be explained either by temporal evolution or spatial configuration of the spacecraft.
  • Grandin, Maxime; Aikio, Anita T.; Kozlovsky, Alexander (2019)
    We study the properties and geoeffectiveness of solar wind high-speed streams (HSSs) emanating from coronal holes and associated with stream interaction regions (SIRs). This paper presents a statistical study of 588 SIR/HSS events with solar wind speed at 1 AU exceeding 500 km/s during 1995-2017, encompassing the decline of solar cycle 22 to the decline of cycle 24. Events are detected using measurements of the solar wind speed and the interplanetary magnetic field. Events misidentified as or interacting with interplanetary coronal mass ejections are removed by comparison with an existing interplanetary coronal mass ejection list. Using this SIR/HSS event catalog (list given in the supporting information), a superposed epoch analysis of key solar wind parameters is carried out. It is found that the number of SIR/HSSs peaks during the late declining phase of solar cycle (SC) 23, as does their velocity, but that their geoeffectiveness in terms of the AE and SYM-H indices is low. This can be explained by the anomalously low values of magnetic field during the extended solar minimum. Within SC23 and SC24, the highest geoeffectiveness of SIR/HSSs takes place during the early declining phases. Geoeffectiveness of SIR/HSSs continues to be up to 40% lower during SC24 than SC23, which can be explained by the solar wind properties.
  • Willamo, T.; Usoskin, I. G.; Kovaltsov, G. A. (2017)
    Aims. Sunspot number series are composed from observations of hundreds of different observers that require careful normalization to standard conditions. Here we present a new normalized series of the number of sunspot groups for the period 1749-1996. Methods. The reconstruction is based on the active day fraction (ADF) method. which is slightly updated with respect to previous works, and a revised database of sunspot group observations. Results. Stability of some key solar observers has been evaluated against the composite series. The Royal Greenwich Observatory dataset appears relatively stable since the 1890s but is approximately 10% too low before that. A declining trend of 10-15% in the quality of Wolfer's observations is found between the 1880s and 1920s, suggesting that using him as the reference observer may lead to additional uncertainties. Wolf (small telescope) appears relatively stable between the 1860s and 1890s, without any obvious trend. The new reconstruction reflects the centennial variability of solar activity as evaluated using the singular spectrum analysis method. It depicts a highly significant feature of the modern grand maximum of solar activity in the second half of the 20th century, being a factor 1.33-1.77 higher than during the 18 and 19th centuries. Conclusions. The new series of the sunspot group numbers with monthly and annual resolution is provided forming a basis for new studies of the solar variability and solar dynamo for the last 250 yr.