Swedish Institute of Space Physics (IRF), Kiruna Semi-annual Geomagnetic (Aurora) activities derived from Ionospheric sP M.Yamauchi Swedish Institute of Space Physics (IRF), Kiruna Kp & nightside AL Þ semi-annual peak AU & dayside AL Þ annual peak

Decay of magnetospheric energy by sP of entire hemisphere High dissipation during winter through the summer hemisphere. Equinox converts convection energy to the aurora effectively. Calculate the accumulated input energy (e·t) for different seasons using a simple circuit model. e : instantaneous input energy e µ sP. t : decay time of total energy t µ 1/∫N+SsP Þ Simply calculate e·t µ sP / ∫N+SsP

Solstice sP * Chapman a layer sP µ √cos(q) Lat 90° 60° 90° 50° Solstice sP 12 LT 18 LT 24 LT * Chapman a layer sP µ √cos(q) * Chapman b layer sP µ cos(q) where q is the solar zeneath angle 12 LT 18 LT 24 LT

Map of sP Faint precipitation case Strong precipitation case inc=5° Lat 90° 50° 90° inc=23° inc=14° inc=5°

Result ! 24 LT 22 LT 20 LT 18 LT 16 LT

Faint precipitation case Strong precipitation case 24 LT 22 LT 20 LT 18 LT 16 LT

Summery of the simple circuit model (1) The semi-annual variation of the geomagnetic activity is reproduced from a very simple circuit model using conductivity effect (SP/∫N+SSP) only. (2) Semi-annual variation of SP/∫N+SSP is found in wide range of LT, and is most prominent at 60-70° latitude. (3) Since large activity corresponds to wide area of "decay" region (integral becomes wide), we expect that intensity of substorms for same input has more clearly semi-annual variation. Þ Need to examine the Kp dependence of the semi-annual variation to check Kp variation is consistent with the SP/∫N+SSP model.

High Kp is more often near equinox than near solstice It is not a linear lift up the activity for equinox, but the large activities are selectively enhanced during equinox.

axis = number of 3-hour periods during a 4-month period Each x represents 1-year data