Latitudinal variation of foF2 hysteresis for solar cycles 20, 21 and 22

NIEVES ORTIZ DE ADLER; ANA G. ELIAS

Sodankyla, Finlandia

Workshop; 4th Workshop on Long-Term Changes and Trends in the Atmosphere; 2006

IAGA - ICMA - CAWSES, Sodankyla Geophysical Observatory

Noon foF2 monthly median values for equinoctial months of solar cycles 20, 21 and 22, from several worldwide stations were analyzed. For each solar cycle the difference between foF2 for a given Rz in the falling branch of the cycle and the corresponding value of the rising branch is evaluated. The average difference for the whole solar cycle is considered as the hysteresis magnitude, which is found to vary systematically with geomagnetic latitude. It is positive between 40ºS and 40ºN, with minimum values at equatorial latitudes and maximum at mid-latitudes on either side of the equator. For latitudes greater than 50º, negative values are observed. This pattern is the same for every cycle but with different magnitudes. Cycle 20, with a maximum Rz around 120, presents the lowest hysteresis magnitudes, while solar cycles 21 and 22, with maximum Rz around 190, present similar hysteresis magnitudes and greater, on average, than those of cycle 20. For mid-latitudes the hysteresis values are around 0.5 MHz. For low and high latitudes it can be around 2 MHz, which represents 20% of foF2 at high latitudes. Although the phenomenon of ionospheric hysteresis has been known for a long time, a linear relationship between foF2 and Rz is used in forecasting and long-term trend estimations. While the inclusion of the hysteresis into long-term ionospheric predictions remains questionable, in long-term trend assessments it may induce spurious trends in filtering processes.