The quasi-5-day wave at mid- and high-latitudes in the mesosphere and lower-thermosphere was compared between the hemispheres using meteor radar horizontal wind measurements, spanning June 2010 to December 2012. Variances of the quasi-5-day wave showed a wave activity from July to August in both hemispheres and in April 2012 in the Northern Hemisphere and November 2012 in the Southern Hemisphere with unique characteristics at each site.

A number of current systems exist in the Earth's magnetosphere. It is very difficult to identify local measurements as belonging to a specific current system. Therefore, there are different definitions of supposedly the same current, leading to unnecessary controversy. This study presents a robust collection of these definitions of current systems in geospace, particularly in the near-Earth nightside magnetosphere, as viewed from a variety of observational and computational analysis techniques.

A lot of efforts have been put into understanding the turbulence structure in space and astrophysical plasmas, in particular how the filamentary structure develops as the length scale of the turbulent fluctuations changes from large to smaller ones. Motivated by the recent spacecraft observations in the solar wind, an analytic model is proposed to explain the nature of filament-formation processes in space plasma turbulence with a successful test against the spacecraft observations.

Our study demonstrates a remarkable association between the earthward penetration of ULF waves and radiation belt electron enhancements during four magnetic storms that occurred in 2001. In the past, ULF waves had been observed at unusual depths during rare superstorms. But ULF wave activity, reaching magnetic shells as low as 2, was also observed during relatively intense storms when it played a key role in diffusing electrons radially inward and thereby accelerating them to higher energies.

Polar Cap (PC) index values are derived from polar magnetic variations and are important for the scaling of solar-terrestrial relations. The International Association of Geomagnetism and Aeronomy (IAGA) has recently endorsed a new PC index version to supersede the many previous index versions. However, the new PC index procedure has some adverse features which should be known and taken into account by users of the index. These features are discussed and alternative procedures are suggested.

We study the effect of geomagnetic storms on the sub-ionospheric VLF/LF propagations based on 2-year data from the network observations in Japan. Three statistical parameters such as average signal amplitude, variability of the signal amplitude, and nighttime fluctuation were calculated. These statistical parameters indicate significant anomalies during the geomagnetic storm in the high-latitude paths. While mid-latitude and low-mid latitude paths have smaller influence from the storm activity.