1. Are small-scale irregularities in a predominantly non-linear state? Evidence from Dynasonde measurements N A Zabotin and J W Wright Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado, 80309-0216 N A Zabotin and J W Wright Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado, 80309-0216 Results presented here have been obtained through support from the National Science Foundation, grant ATM0125297

2. Basic idea of the phase structure function method Because the ionospheric plasma drifts, the radio sounding signal encounters different realizations of the irregularity field even at only slightly different times. The consequent phase fluctuations can be measured by the dynasonde. The temporal Structure Function is a very appropriate statistical characteristic of the phase fluctuations (see Zabotin & Wright, Radio Sci., 36, 757, 2001). Because the ionospheric plasma drifts, the radio sounding signal encounters different realizations of the irregularity field even at only slightly different times. The consequent phase fluctuations can be measured by the dynasonde. The temporal Structure Function is a very appropriate statistical characteristic of the phase fluctuations (see Zabotin & Wright, Radio Sci., 36, 757, 2001).

3. Theory (built for a power irregularity spectrum) and experiment both suggest that the small-lag part of the full-scale SF is well approximated by a log-log linear law: log (D (φ) / rad 2 ) = SIA+SIB· log(τ/sec). Theory (built for a power irregularity spectrum) and experiment both suggest that the small-lag part of the full-scale SF is well approximated by a log-log linear law: log (D (φ) / rad 2 ) = SIA+SIB· log(τ/sec).

5. Irregularity amplitude ΔN/N, gradients and GDI F region E region

6. Summary 1. Small-scale irregularity spectrum is practically always approximated by power law. 2. There is not asymmetry between irregularity growth and decay. 3. Irregularity amplitude never drops down to the thermal fluctuations level. 4. Average irregularity spectrum index is close to Sudan and Keskinen’s 8/3. 5. Horizontal gradients are not the main controlling factor for the irregularity amplitude. 1. Small-scale irregularity spectrum is practically always approximated by power law. 2. There is not asymmetry between irregularity growth and decay. 3. Irregularity amplitude never drops down to the thermal fluctuations level. 4. Average irregularity spectrum index is close to Sudan and Keskinen’s 8/3. 5. Horizontal gradients are not the main controlling factor for the irregularity amplitude.

8. Practical Consequences When modeling evolution of the small-scale irregularities one should not assume “clean” initial state with thermal fluctuations only. It is useful to permit a possibility that initial state was already non- linear.