  The space weather is extremely mild during solar cycle 24: the number of
major geomagnetic storms and high-energy solar energetic particle events are at
the lowest since the dawn of the space age. Solar wind measurements at 1 AU
using Wind and ACE instruments have shown that there is a significant drop in
the density, magnetic field, total pressure, and Alfven speed in the inner
heliosphere as a result of the low solar activity. The drop in large space
weather events is disproportionately high because the number of energetic
coronal mass ejections that cause these events has not decreased significantly.
For example, the rate of halo CMEs, which is a good indicator of energetic
CMEs, is similar to that in cycle 23, even though the sunspot number has
declined by about 40%. The mild space weather seems to be a consequence of the
anomalous expansion of CMEs due to the low ambient pressure in the heliosphere.
The anomalous expansion results in the dilution of the magnetic contents of
CMEs, so the geomagnetic storms are generally weak. CME driven shocks
propagating through the weak heliospheric field are less efficient in
accelerating energetic particles, so the particles do not attain high energies.
Finally, we would like to point out that extreme events such as the 2012 July
23 CMEs that occurred on the backside of the Sun and did not affect Earth
except for a small proton event.
