1. 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

2. 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

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

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

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

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

7. 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.

8. 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.

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