Statistical analysis of SAR arcs detached from auroral oval based on all-sky imaging observations Yuki Takagi1*, Kazuo Shiokawa1, Yuichi Otsuka1, and Martin Connors2   1. Institute for Space-Earth Environmental Research, Nagoya University, Japan 2. Athabasca University, Canada My name is Yuki Takagi, Today, I’d like to talk about SAR arc detachment. The tiltle of this presentation is “Statistical analysis of SAR arcs detached from auroral oval based on all-sky imaging observations”.

Introduction Ionosphere Plasmasphere Ring current [Kozyra et al., 1987] Stable Auroral Red (SAR) arcs are the optical phenomenon caused by low-energy electron precipitation into the ionospheric F layer from the interaction region between the ring current and the plasmasphere. Coulomb collisions between ring current and thermal electrons were first proposed as the energy source for SAR arcs.[Cole, 1965] Observations and theory of the formation of SAR arcs were constructed [Rees & Roble, 1975] At first, I introduce about the SAR arc. Stable Auroral Red arcs (called SAR arc) are the optical phenomenon caused by low-energy electron precipitation into the ionospheric F layer from the interaction region between the ring current and the plasmasphere. Coulomb collisions between ring current and thermal electrons were first proposed as the energy source for SAR arcs by Cole in 1965. Observations and theory of the formation of SAR arcs were constructed by Rees & Roble in 1975.

Introduction SAR arcs have been observed at mid-latitudes during geomagnetic storms (e.g., Rees and Roble, 1975; Mendillo et al., 2016). Shiokawa et al. (2009) reported an event of SAR arc detachment at Athabasca. However, statistical analysis of SAR arc detachment has not been done yet. SAR arcs have been observed at mid-latitudes during geomagnetic storms (e.g., Rees and Roble, 1975; Mendillo et al., 2016). Shiokawa et al. (2009) reported an event of SAR arc detachment at Athabasca. This figure show the keogram observed at Athabasca in 2005. In this figure, this arc is SAR arc. However, statistical analysis of SAR arc detachment has not been done yet. So in our study, we make a statistical analysis of SAR arc detachment observed at Athabasca in 2006-2016. [Shiokawa et al., 2009] Purpose: We make a statistical analysis of SAR arc detachment observed at Athabasca in 2006-2016.

Observation Place: Athabasca Observatory, Canada (61.1MLAT, L=4.2) Instrument: an all-sky imager with a narrowband interference filter (630.0nm) Periods: 2006–2016 Total: 163 events In this study, we used an all-sky imager with a narrowband interference filter with the wavelength of 630.0nm from 2006 to 2016. we found 163 events.

Observation Event1 (Sept. 14, 2009) Event2 (Oct. 2, 2008) Next, we show examples of SAR arc detachments. First event was observed on 14th September in 2009. left figure shows the keogram. Vertical axis show th geographic latitude and horizontal axis shows universal time from 2 hours before to 4 hours after the start time. Right oanel show the movie of all-sky camera on this day. In keogram, we define the SAR arc is this. And in the movie, this arc is SAR arc detachment. Second event was observed on 2nd October in 2008.

Criteria of SAR Arc Event Selection Definition of SAR arc in our study (1)intensification and lower-latitude expansion of high-latitude aurora and subsequent separation of SAR arcs in the 630-nm keogram (2)the corresponding emission is not clearly observed in the 557.7 nm keogram Extracted parameters (1) start time: start time of intensification of high-latitude aurora (2) end time: the time at which the SAR arc emission disappears or it is masked by other auroral activities or clouds. (3) Lowest latitude of SAR arcs 630.0 nm In this study, we used keograms with a color scale of 0--500 R, and determined the occurrence of SAR arc detachment based on visual inspection under the following criteria: (1)intensification and lower-latitude expansion of high-latitude aurora and subsequent separation of SAR arcs in the 630-nm keogram, and (2)the corresponding emission is not clearly observed in the 557.7 nm keogram, indicating that the 630 nm light is the main emission of the arc. And we extracted 3 parameters from these keogram, Start time, end time and lowest latitude. 557.7 nm

MLT and yearly variation The occurrence rate of SAR arc detachment was highest in 20-22 MLT. →This could be due to the duskward drift of the ring current ions, which are the energy source of a SAR arc. The occurrence rate of SAR arc detachment was low in the solar maximum and minimum, and high during the period of increase or decrease of solar activity. The occurrence rate also correlated well with geomagnetic activities represented by the Ap index. →Alexeyev and Ievenko [2007] showed that the occurrence rate of SAR arcs in the solar cycle 23 has a maximum in the declining phase of solar activity and has a good correlation with the Ap index, similar to the present result. I will talk about the results from now. At first, I talke about the results of MLT and yearly variation. Top figure shows the occurrence rate of SAR arc detachment as a function of MLT. The numbers above each time represent the total time (hours) for which SAR arc detachment was observed. The occurrence rate is highest at 20-22 MLT, before midnight. This could be due to the duskward drift of the ring current ions, which are the energy source of a SAR arc. Bottom figure shows the yearly variation of the SAR arc occurrence rate, geomagnetic Ap index, and the solar F10.7 flux. The occurrence rate of SAR arc detachment was low in the solar maximum and minimum, and high during the period of increase or decrease of solar activity. The occurrence rate also correlated well with geomagnetic activities represented by the Ap index. Alexeyev and Ievenko [2007] showed that the occurrence rate of SAR arcs in the solar cycle 23 has a maximum in the declining phase of solar activity and has a good correlation with the Ap index, similar to the present result.

Superposed epoch analysis for AU, AL, SYM-H indices The AL index begins to decrease at ~1 hour before the time of SAR arc detachment, with its minimum value occurring around the detachment time. →The SAR arc detachment occurs when the substorm recovery phase starts. No clear change in the SYM-H index is observed before or after the SAR arc detachment. The average value of the SYM-H index remains near -20 nT. →The SAR arc detachment is likely to occur during geomagnetically disturbed periods. Next, I talke about the results of Superposed epoch analysis for AU, AL, SYM-H indices. These figure shows the results of superposed epoch analysis of AU and AL indices, and the SYM-H index from 2 hours before to 4 hours after the SAR arc detachment. The average values defined every 1 minutes are indicated by the solid lines, and the standard deviations for every 30 minutes are indicated by the error bars. The AL index begins to decrease at ~1 hour before the time of SAR arc detachment, with its minimum value occurring around the detachment time. →The SAR arc detachment occurs when the substorm recovery phase starts. No clear change in the SYM-H index is observed before or after the SAR arc detachment. The average value of the SYM-H index remains near -20 nT. →The SAR arc detachment is likely to occur during geomagnetically disturbed periods.

Equatorward velocity of SAR arc and corresponding electric field in the magnetosphere The equatorward drift velocity of SAR arcs and electric field in the magnetosphere tend to be higher in the dusk and dawn local times compared with those around midnight. →When the SAR arc detachment appears in the dusk sector, it is expected that the magnetospheric electric field is strong and the ring current ion is accelerated to a higher energy and go to the dusk side. In that case, the SAR arc velocity toward the equator might be faster because of the stronger magnetospheric electric field. This velocity has no significant relationship with the geomagnetic activity. Since the position of this SAR arc is an ionospheric mapping of the position of energetic ions in the magnetosphere, there is a possibility that the equatorward velocity of the detached SAR arcs indicates the inward velocity of high energy ions in the magnetosphere. In this study, the equatorward velocity of the SAR arc was estimated from the change of the latitude of the SAR arc peak intensities at 0.5 and 1 hour after the SAR arc detachment. Then these estimated velocities in the ionosphere were projected to the velocity in the magnetosphere by using the Tsyganenko-06 model (Tsyganenko, 2006), and the magnetospheric electric field was estimated by assuming that the movement of the SAR arc is caused by the ExB drift (below equation).

Summary and Conclusions We have made a statistical analysis of SAR arc detachments using all-sky camera data at Athabasca, Canada from 2006 to 2016. The yearly variation of SAR arc detachment has a better correlation with the geomagnetic Ap index than the solar F10.7 index. SAR arc detachment tends to occur at pre-midnight local times, indicating ring-current ion drift to the dusk sector. The SAR arc detachment tends to occur at the beginning of a substorm recovery phase. This probably indicates that the SAR arcs detach from the main oval as the main auroral oval returns to higher latitudes at the beginning of the recovery phase. The equatorward velocities of SAR arcs are higher in the dusk and dawn local times compared with those around midnight. We suggest that when the magnetosphere electric field is stronger, the ring current ions are accelerated to higher energies and drift more to the dusk side. We have made a statistical analysis of SAR arc detachments using all-sky camera data at Athabasca, Canada from 2006 to 2016. The yearly variation of SAR arc detachment has a better correlation with the geomagnetic Ap index than the solar F10.7 index. SAR arc detachment tends to occur at pre-midnight local times, indicating ring-current ion drift to the dusk sector. The SAR arc detachment tends to occur at the beginning of a substorm recovery phase. This probably indicates that the SAR arcs detach from the main oval as the main auroral oval returns to higher latitudes at the beginning of the recovery phase. The equatorward velocities of SAR arcs are higher in the dusk and dawn local times compared with those around midnight. We suggest that when the magnetosphere electric field is stronger, the ring current ions are accelerated to higher energies and drift more to the dusk side.