DARTS of the Month

► DARTS of the Month

Figure 1. (Top) Auroral images of Arc 1 and Arc 2 obtained at ~10:58:17.91 UT and at ~10:58:28.34 UT, respectively, on Jan. 28, 2008. (Bottom) Energy-time spectrogram of precipitating electrons. The start time is 10:58:05 UT on Jan. 28, 2008.

"Zoom-in" on a substorm onset with Reimei

The Reimei (INDEX) satellite has been operated for more than four years since the successful launch in August 2005, and obtained huge amount of auroral data. These data are open in the DARTS database in ISAS.

A substorm is one of the most intriguing phenomena in the research field of terrestrial and planetary magnetosphere and ionosphere. To understand its trigger process, namely, the rapid and global evolutions at the onset timing, it is important to interpret comprehensively using combined data of satellite with ground-based experiments.

A substorm onset is identified from the data obtained by Reimei and THEMIS ground-based stations (GBOs) over Canada and Alaska at ~10:58 UT on January 28, 2008. Figure 1 shows image data taken by Reimei/MAC, and the energy-time spectrogram of precipitating electrons by Reimei/ESA. MAC measured the intense auroral arc extending east-westward with a typical width of 20 km from ~10:58:13 UT to ~10:58:26 UT (Arc 1), followed by the multiple folded aurora from ~10:58:23 UT to ~10:58:37 UT (Arc 2). It is found that the intense poleward arc (Arc 1) corresponds to high-energy monoenergetic inverted-V electrons with the peak energy exceeding the energy range of ESA (12 keV/e) measured from 10:58:13 UT to 10:58:17 UT. The folded aurora (Arc 2) corresponds to the multiple inverted-Vs seen from 10:58:21 UT to 10:58:29 UT. It is worth noting that there are significant supra-thermal electron bursts in the most poleward region from ~10:58:09 UT to ~10:58:14 UT, suggesting the existence of Alfvenic acceleration.

Figure 2. Mosaic images obtained from THEMIS GBO all-sky images at Fort Yukon, Inuvik, and Forth Smith with 1 min interval from 10:57 UT to 11:01 UT on January 28, 2008.Reimei image data are overlapped on the mosaic plot at 10:58 UT, and the region of arcs 1 and 2 is expanded in the bottom-right.

Concerning on the auroral evolutions in a wide range during this period, we plot successive mosaic images from all-sky data at THEMIS GBO stations, and compare them with the Reimei data. Figure2 shows the auroral mosaic images at Fort Yukon, Inuvik and Fort Smith from 10:57 UT to 11:01 UT with every 1 min. The Reimei data are overlapped on the mosaic image at 10:58 UT since Reimei is mostly immediate to Inuvik at this time. We see Arc 1 newly appeared at the time between 10:57 UT to 10:58 UT in the limited east-west area extending from Inuvik to Fort Yukon. Regarding on Arc 2 at 10:57 UT and 10:58 UT, the stable Arc 2 exists equatorward of Arc 1. Further, the precise dynamics of auroral expansion in the poleward and east-west directions can be revealed from the mosaic images after 10:59 UT.

Here are the auroral movies taken by Reimei/MAC and THEMIS GBOs .

It is unique that Reimei passed over the vicinity of initial brightening region just after stating the substorm expansion onset. Reimei revealed the small-scale characteristics of poleward expanding intense arc (Arc 1) as well as the equatorward stable growth phase aurora. All-sky image data at THEMIS GBOs showed the complicated auroral expansion seen at 15-20 sec after the Reimei passage. Further, by combining the Reimei and ground-based data with simultaneous in-situ data by THEMIS probes in the magnetotail, we will investigate the relationship between a large-scale change in the magnetosphere and small-scale aurora and currents in the ionosphere.

Ken Sakanoi (Planetary Plasma and Atmospheric Research Center, Graduate School of Science, Tohoku Univ)
Hirahara Masafumi (Department of Earth and Planetary Science, Graduate School of Science, Tokyo Univ)
February, 2010