Radar Observations in the Vicinity of pre-noon Auroral Arcs

1. Radar Observations in the Vicinity of pre-noon Auroral Arcs Hans Nilsson and Tima Sergienko Swedish Institute of Space Physics, Kiruna, Sweden Vladimir Safargaleev Polar Geophysical Institute, Apatity, Russia Alexander Kozlovsky and Anita Aikio Oulu University, Oulu, Finland Andrei Kotikov St Petersburg State University, St Petersburg, Russia

2. Campaign periods • Data from 1998-12-21 (published in Annales Geophysicae, 2005) • ESR - optical campaign January 2004 (Safargaleev 2005, Kozlovsky 2005) • ESR - optical campaign December 2004

3. EISCAT Field of View • Convection (CP4) • Temperature enhancements • Density enhancements (ESR field-aligned)

4. View from the Polar satellite

5. EISCAT data from 981221

6. Close look at density data • Structured / intensified precipitation associated with low density regions • Energy of precipitating particles lower in structured precipitation cases

7. Model of luminosity

8. Luminosity / EISCAT comparison • Luminosity and electron density varies together - thus low density regions are mainly caused by low precipitation flux • Any cavity formation processes in downward current regions cannot contribute significantly to low conductance regions on the radar resolution scale length

9. Cavities • E region cavities represent conductance structures and may thus influence ionospheric feedback • Often thought that imposed downward current may cause cavities; cavities are caused by arc related current system • Through ionospheric feedback, decreased ionospheric background conductance may lead to favorable conditions for the ionospheric feedback instability; low conductance structure is cause of arc formation

10. Precipitation energy spectra derived from EISCAT • Structured precipitation shows lower energy of source population than background (diffuse) precipitation • Likely the arc source population is accelerated ionospheric electrons 1 keV 10 keV

11. Event 1

12. Event 2

13. Electric fields, currents and arcs • Electric field intensifies followed by arc brightening • Current intensifies as electric field intensifies (driven by magnetosphere) • Electric field enhanced only poleward of arc which could be studied in detail

14. Conclusions • Transverse electric field enhanced only on poleward side of arc - typical morning side situation • “Cavities” on radar resolution scale length are absence of precipitation • Association arc - cavity in radar data because diffuse precipitation is “stabilizing”, structured precipitation occur in its absence • Ionospheric feedback important for structuring and small scale features

15. 20040124 07:07:40Double arc system Longyearbyen Barentsburg

16. Quantitative estimates • Pedersen conductance is 3 mho in background diffuse precipitation, down to 0.5 mho in low density regions and about 2 mho in structured precipitation • Background electric field is typically 50 mV/m, sometimes much more • Structured precipitation is comparatively cold (500 eV average energy, vs 2 keV for diffuse precipitation)

17. Ionospheric feedback “Lumped circuit” Magnetosphere model From PhD thesis: Thayer School of Engineering Dartmouth College Effects of the Active Auroral Ionosphere on Magnetosphere - Ionosphere Coupling Dimitri Pokhotelov

18. Electrodynamics of an arc(The very simple version) Very low conductance (Cavity formation in Downward current region?) Background electric field Strongly enhanced electric field Downward current Upward current Weak field inside arc, often difficult to resolve Using EISCAT measurements

19. Mechanism of ionospheric feedback Figure from Pokhotelov thesis

20. Slow and fast feedback Alfvén wave speed Fast feedback (Ionospheric Alfvén Resonator) Slow feedback (Field-line Eigen modes)

21. Ionospheric feedback • Atkinson, J. Geophys. Res, 1970 • Sato, J. Geophys. Res, 1978 • Miura and Sato, J. Geophys. Res, 1980 • Lysak and Song, J. Geophys. Res, 2002 • Pokhotelov et al., J. Geophys. Res. 2002 • Streltsov and Lotko, J. Geophys. Res., 2003