caa a22 4500 44534699X CHVBK 20180317142832.0 cr unu---uuuuu 170323e20111201xx s 000 0 eng 10.1007/s11207-010-9646-9 doi (NATIONALLICENCE)springer-10.1007/s11207-010-9646-9 Magnetic Clouds at/near the 2007 - 2009 Solar Minimum: Frequency of Occurrence and Some Unusual Properties [Elektronische Daten] [R. Lepping, C.-C. Wu, D. Berdichevsky, A. Szabo] Magnetic clouds (MCs) have been identified for the period 2007 - 2009 (at/near the recent solar minimum) from Wind data, then confirmed through MC parameter fitting using a force-free model. Adramatic increase in the frequency of occurrence of these events took place from the two early years of 2007 (with five MCs) and 2008 (one MC) compared to 2009 (12MCs). This pattern approximately mirrors the occurrence-frequency profile that was observed over a three-year interval 12 years earlier, with eight events in 1995, four in 1996, and 17 in 1997, but decreased overall by a factor of 0.62 in number. However, the average estimated axial field strength [〈|B O|〉] taken over all of the 18 events of 2007 - 2009 (called the "recent period” here) was only 11.0nT, whereas 〈|B O|〉 for the 29 events of 1995 - 1997 (called the "earlier period”) was 16.5nT. This 33% average drop in 〈|B O|〉 is more or less consistent with the decreased three-year average interplanetary magnetic field intensity between these two periods, which shows a 23% drop. In the earlier period, the MCs were clearly of mixed types but predominantly of the South-to-North type, whereas those in the recent period are almost exclusively the North-to-South type; this change is consistent with global solar field changes predicted by Bothmer and Rust (Geophys. Monogr. Ser. 99, 139, 1997). As we have argued in earlier work (Lepping and Wu, J. Geophys. Res. 112, A10103, 2007), this change should make it possible to carry out (accurate short-term) magnetic storm forecasting by predicting the latter part of an MC from the earlier part, using a good MC parameter-fitting model with real-time data from a spacecraft at L1, for example. The recent set's average duration is 15.2 hours, which is a 27% decrease compared to that of the earlier set, which had an average duration of 20.9 hours. In fact, all physical aspects of the recent MC set are shown to drop with respect to the earlier set; e.g., as well as the average internal magnetic field drop, the recent set had a somewhat low average speed of 379km s−1 (5% drop), and the average diameter had a 24% drop. Hence, compared to the earlier set, the recent set consists of events that are smaller, slightly slower, and weaker in every respect (and fewer in number), but in a relative sense the two three-year sets have similar frequency-of-occurrence profiles. It is also interesting that the two sets have almost the same average axial inclinations, i.e., axial latitude ≈31° (in GSE). These MC characteristics are compared to relevant solar features and their changes. Apreliminary assessment of the statistics on possible shocks and pressure pulses upstream of these recent MCs yields the following: About 28% of the MCs, at most, had shocks, and 33% had shocks and/or pressure pulses. These are low values, since typically the percentage of cases with shocks is about 50%, and the percentage with shocks and/or pressure pulses is usually about75%. Springer Science+Business Media B.V., 2010 Lepping R. Heliosphysics Science Division, NASA/Goddard Space Flight Center, 20771, Greenbelt, MD, USA aut Wu C.-C Naval Research Laboratory, 20735, Washington, DC, USA aut Berdichevsky D. Department of Electrical and Computer Engineering, University of the District of Columbia, 20008, Washington, DC, USA aut Szabo A. Heliosphysics Science Division, NASA/Goddard Space Flight Center, 20771, Greenbelt, MD, USA aut Solar Physics Springer Netherlands 274/1-2(2011-12-01), 345-360 0038-0938 274:1-2<345 2011 274 11207 https://doi.org/10.1007/s11207-010-9646-9 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/s11207-010-9646-9 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Lepping R. Heliosphysics Science Division, NASA/Goddard Space Flight Center, 20771, Greenbelt, MD, USA aut NATIONALLICENCE 700 1- Wu C.-C Naval Research Laboratory, 20735, Washington, DC, USA aut NATIONALLICENCE 700 1- Berdichevsky D. Department of Electrical and Computer Engineering, University of the District of Columbia, 20008, Washington, DC, USA aut NATIONALLICENCE 700 1- Szabo A. Heliosphysics Science Division, NASA/Goddard Space Flight Center, 20771, Greenbelt, MD, USA aut NATIONALLICENCE 773 0- Solar Physics Springer Netherlands 274/1-2(2011-12-01), 345-360 0038-0938 274:1-2<345 2011 274 11207 Metadata rights reserved Springer special CC-BY-NC licence nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer