Magnetic Clouds at/near the 2007 – 2009 Solar Minimum: Frequency of Occurrence and Some Unusual Properties

Abstract

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%.