C. Manoj*, S. Maus and Patrick Alken NGDC/CIRES, Boulder, Colorado, USA

1. Penetration Characteristics of the Interplanetary Electric Field to the Day-time Equatorial Ionosphere C. Manoj*, S. Maus and Patrick Alken NGDC/CIRES, Boulder, Colorado, USA (* On leave from, NGRI-Hyderabad, India) H. Lühr GeoForschungsZentrum-Potsdam, Germany ISEA, Crete, Greece. S10 Ionospheric storms and space weather effects

2. The ionospheric equatorial electric field (EEF) exhibits large day-to-day variability. • Wind forced diurnal variations (~50% of the variance) • Influence of interplanetary variations on EEF • Wind forced (disturbance dynamo) • Prompt penetration • Other ISEA, Crete, Greece. S10 Ionospheric storms and space weather effects

3. Prompt penetration, some questions • Frequency dependence of the prompt penetrating electric field? • Coherence, phase relation • Does the prompt penetration depend on local time, solar flux, season, polarity of IMF Bz, etc ? • What is the period range of prompt penetration effect on EEF? ISEA, Crete, Greece. S10 Ionospheric storms and space weather effects

4. Data during 2001 to 2008 Interplanetary electric field (IEF) data • Advance Composition Explorer (ACE) satellite at L1 point • Time-shifted to the magnetosphere’s bow-shock nose by OMNI Equatorial ionospheric electric field (EEF) data • Jicamarca Unattended Long-term Investigations of the Ionosphere and Atmosphere (JULIA) radar, Peru. • 1002 days ISEA, Crete, Greece. S10 Ionospheric storms and space weather effects

5. Example of data processing • Diurnal variation of JULIA data is removed using the model by *Alken (2008) • Eastward electric field at JULIA is calculated as, • 3. The ionospheric field variations are correlated with the interplanetary E-field (IEF). * manuscript in preparation ISEA, Crete, Greece. S10 Ionospheric storms and space weather effects

6. Average power spectra of IEF and JULIA electric fields Spectra are estimated from pairs of daily EEF and IEF data, each 6 hours long. 265 pairs of data. The power spectra and cross spectra are computed by Welch's averaged periodogram method (Welch, 1967). Both power spectra show monotonous increase in power with period. Dependence on activity level (Ap). Power is higher by factor of 3. ISEA, Crete, Greece. S10 Ionospheric storms and space weather effects

7. Coherence between IEF and EEF Significance level (Thompson, 1979) Coherence is significant for periods above 20 minutes. It peaks around 2 hours (0.5 cycles / hour). Coherence is slightly higher during active days |<- Period in minutes -> | <- Period in hours ->| ISEA, Crete, Greece. S10 Ionospheric storms and space weather effects

8. 250 Delay in Minutes 200 0 150 100 phase difference (degrees) 50 0 -50 -100 6 10 20 30 1 2 4 6 10 |<- Period in minutes -> | <- Period in hours ->| Cross Phase spectra Cross-phase spectra is the IEF phase minus the EEF phase as a function of frequency. Unshifted IEF data show monotonous decrease. 10 17 2πf.Δt Δt = 17 min 25 When delayed by 17 minutes, the phase spectra have negligible values for all the periods we consider. A process that causes coherent EEF signals over the whole range is prompt penetration. In the subsequent analysis, we always delay IEF data by 17 min. ISEA, Crete, Greece. S10 Ionospheric storms and space weather effects

9. Dependence on local time Using 3-hour long windows of EEF and IEF data. Coherence is maximum for a window centered on local noon. Coherence at 40 minutes period seems to be independent of LT ISEA, Crete, Greece. S10 Ionospheric storms and space weather effects

10. Dependence on IMF Bz The whole data set is divided into two groups. Prompt penetration shows no significant dependence on IMF Bz polarity. ISEA, Crete, Greece. S10 Ionospheric storms and space weather effects

11. Dependence on season The coherence functions for June and Dec. solstice are almost identical. The coherence functions during the two equinox periods are slightly different. (small sample number) No significant dependence on season is observed ISEA, Crete, Greece. S10 Ionospheric storms and space weather effects

12. Dependence on solar flux level The coherence between IEF and JULIA electric fields is lower for high solar flux (EUVAC > 120). EUVAC (Extreme Ultraviolet (EUV) flux model for aeronomic calculations (Richards et al., 1994). EUVAC = 0.5*(F10.7+F10.7A), where F10.7A is the 81-day moving average of F10.7 ISEA, Crete, Greece. S10 Ionospheric storms and space weather effects

13. 0.178 0.100 0.056 0.032 Ratio of EEF/IEF 0.018 0.010 0.006 0.003 0.002 100 10 1 0.1 Frequency (Cycles per hour) 50 phase difference (degrees) 0 This study Nicolls et al. (2007) -50 6 10 20 30 1 2 4 6 10 |<- Period in minutes -> | <- Period in hours ->| Signal Transfer Function To predict EEF variations from interplanetary electric field (IEF) data Transfer function magnitude is ratio of EEF to IEF as a function of frequency. TF phase is the EEF phase minus the IEF phase. Maximum admittance around 2 hours. The transfer function does not introduce a phase modulations. The magnitude of our transfer function is higher than that by Nicolls et al. (2007). The difference increases towards shorter periods. ISEA, Crete, Greece. S10 Ionospheric storms and space weather effects

14. Conclusions • The coherence between IEF and EEF peaks around 2 hours period at a magnitude squared coherence of 0.6. • The lack of a frequency-dependent phase shift between IEF and EEF indicates that the coupling process between IEF and EEF signals is prompt penetration. • Coherence peaks at local noon, Coherence is lower on days with high solar flux. • We find that the penetration of interplanetary electric fields to the equatorial ionosphere shows no significant dependence on the polarity of IMF Bz. • The transfer function can be used to predicted the non-diurnal variations of equatorial electric fields up to 38%. ISEA, Crete, Greece. S10 Ionospheric storms and space weather effects