Dynamic subauroral ionospheric electric fields observed by the

Dynamic sub-auroral ionospheric electric fields observed by the Falkland Islands radar during the course of a geomagnetic storm Adrian Grocott*, Steve Milan, Mark Lester, Tim Yeoman University of Leicester, U. K. *currently visiting NIPR, Japan Mervyn Freeman British Antarctic Survey, U. K. Jo Baker Virginia Tech. , U. S. 日本学術振興会 Japan Society for the Promotion of Science

Polar Cap and Auroral Convection • Observations of ionospheric electric fields in the auroral zone and polar cap provide direct evidence for magnetospheric driving by the solar wind 12 MLT Auroral Oval Ionospheric Plasma Streamlines 18 MLT 06 MLT Convection Electric Field E 00 MLT

Polar Cap and Auroral Convection • During magnetospheric substorms, processes in the inner magnetosphere drive sub-auroral electric fields which couple to the auroral zones adding to the complexity of the electrodynamics [e. g. Grocott et al. , 2006, 2010]. 12 MLT Auroral Oval Ionospheric Plasma Streamlines 18 MLT 06 MLT Sub-auroral Electric Field Substorm ‘Harang’ Electric Fields E 00 MLT

Sub-Auroral Ionospheric Convection Polarisation jets (PJ) [Galperin et al. , 1973] Sub-auroral ion drifts (SAID) [Spiro et al. , 1979] Substorm associated radar auroral surges (SARAS) [Freeman et al. , 1992] Sub-auroral electric fields (SAEF) [Karlsson et al. , 1998] Sub-auroral polarisation streams (SAPS) [Foster and Burke, 2002] Auroral westward flow channels (AWFC) [Parkinson et al. , 2003] Generally observed to be westward, pre-midnight phenomena The term SAPS used to encompass the full range of electric fields observed in the sub-auroral region including the broad (∼ 5◦), weak (∼ 100 m s− 1) background flows which persist beyond midnight into the predawn sector. SAID are fast (1 − 4 km s− 1), latitudinally narrow (∼ 1◦ − 2◦) regions of rapid westward ion drift explicitly associated with substorm electrodynamics.

Sub-Auroral Ionospheric Convection • This study focusses on storm-time radar observations of an interval of dynamic sub-auroral electric fields and their relationship to the prevailing interplanetary conditions, auroral particle precipitation, and geomagnetic activity 12 MLT Auroral Oval Ionospheric Plasma Streamlines 18 MLT 06 MLT Sub-auroral Electric Field E Falkland Islands 00 MLT Radar (FIR) field-of-view Substorm ‘Harang’ Electric Fields

Instrumentation • Super. DARN FIR and BKS radar 0300 UT observations of sub-auroral electric fields (BKS is northern hemisphere near-conjugate radar to FIR) • Upstream Driving: ACE solar wind and IMF data • Magnetospheric Morphology: DMSP auroral precipitation boundaries • Ring Current Dynamics: Ground geomagnetic storm-time indices BKS FIR

Observations: Storm Overview • Time-series of data spanning 5 days between 3 and 7 August 2010 • Solar wind and IMF data indicate the arrival of a fast solar wind front at ~1800 UT on 3 August • AE indices reveal intervals of enhanced auroral electrojet activity • Sym-H index shows the characteristic signature of a geomagnetic storm • At the peak of storm activity on Aug 4 th an interval of ionospheric Pwr (d. B)

Observations: Storm Main Time-series of data from 1600 Phase UT on 3 August to 0900 UT on 4 th i. Solar wind shock and sudden storm commencement (SSC) ii. Start of a ~3 h interval of strongly southward IMF iii. Peak in Asym-H index iv. Start of a sequence of 4 substorms and appearance of ionospheric radar scatter v. Start of storm recovery phase Between (iv) and (v) the radar scatter, IMF, and ground magnetic indices all exhibit oscillatory behaviour Vel (ms-1)

sub-auroral ion drifts (SAID) SAID SA PS Observations: Flow Channel Characteristics sub-auroral polarisation streams (SAPS) • Comparing the FIR data with BKS data reveals a similar fast flow region at similar latitudes in both hemispheres. • BKS observed a wider band of scatter, illustrating that the high speed, narrow flow channel is embedded within a wider, slower band of flow. • This is consistent with previous studies of sub-auroral ion drifts (SAID) [Spiro et al. , 1979] and sub-auroral polarisation streams (SAPS) [Foster and Burke, 2002].

Observations: Flow Channel Location Fast westward flows Electron precipitation boundary Circle fits to electron and ion equatorward boundaries a. Polar projections of FIR data with DMSP overpasses b. Electron and c. ion energy spectrograms

Observations: Flow Channel Location Comparison of FIR backscatter location with DMSP spectrogram data e- i+ reveals the location of the flow channel to be close to, or equatorward of, the electron precipitation boundary and generally poleward of the ion boundary

Observations: Flow Channel Velocity • The velocity varies across the field-of-view such that with increasing MLT it changes from strongly negative to positive • i. e. the direction of flow is 2 6 10 14 generally westward • Time series reveal smaller -scale variations within the flow channel • The data from all beams Vel (ms-1) reveal the evolution of the flow with time along the length of the channel Vel (ms-1)

Observations: Flow Channel Velocity • The lowest velocities were observed in beam 10 • We therefore assume that the direction of the flow is perpendicular to this direction • A simple beam swinging analysis then enables estimation of the full vector velocity along each beam • Non-uniformity of the velocity estimated on each beam indicates variability within the flow Vel (ms-1)

Observations: Flow Channel Velocity • The lowest velocities were observed in beam 10 • We therefore assume that the direction of the flow is perpendicular to this direction • A simple beam swinging analysis then enables estimation of the full vector velocity along each beam • Non-uniformity of the velocity estimated on each beam indicates variability within the flow

Reconnection rates and changes in open flux Reconnection rates [after Milan et al. 2006, 2007] : VD = vsw B� L sin 4(θ/2) : commonly used empirical formula for dayside reconnection rate VN = −AL × c : proxy for nightside reconnection rate, derived from studies of flux closure observed during substorms c, a constant, was chosen to give quasi-steady long-term level of open flux over the whole interval

Summary and Conclusions 1. Continuous observations of a highly dynamic, narrow channel of enhanced flow, spanning a number of substorm cycles during the main phase of a geomagnetic storm 2. Conjugate, hemisphere observations place the flows at the poleward edge of the SAPS, consistent with satellite observations of SAID [e. g. Foster and Burke, 2002] 3. Simultaneous particle precipitation data place the SAID close to the equatorward boundary of auroral electron precipitation, but generally poleward of the ion boundary 4. The separation of these boundaries increases following the onset of substorm activity; the electron boundary moves poleward with the SAID, whereas the ion boundary remains at lower latitudes 5. This latitudinal motion of the SAID is well correlated with changes in polar cap open flux content, substorm injections, small-scale velocity variations, and peaks in Asym-H - indicates a

Future Work • Relationship between polar cap and auroral convection, and the largescale modification of this relationship during magnetospheric substorms is well understood • Next is to study the coupling of the polar cap, auroral and sub-auroral convection • With the FIR radar we now have continuous coverage from 40◦ magnetic latitude to the pole