12. Low Latitude A.urorae on October 21, I

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1 No. 3] Proc. Japan Acad., 66, Ser. B (199) Low Latitude A.urorae on October 21, I By Hiroshi MIYAOKA, *) Takeo HIRASAWA, *) Kiyohumi and Yoshihito TANAKA**> (Communicated by Takesi NAGATA, M. J. A., March 12, YUMOTO,**) 199) Bright red aurorae were observed during a severe magnetic storm in the northern area of Hokkaido, Japan on October 21, The storm started with a sudden commencement (SC) at 917UT, October 2, resulted from a remarkable solar flare occurred at 1229UT on October 19 with the largest class of 4B/X13 near the central meridian of the sun. Outstanding visible red aurorae were detected twice in the intervals of UT and UT on October 21, 1989, as shown in Fig, 1. A 4-channel scanning photometer and a $uxgate magnetometer at Moshiri Observatory (44 22'N, 'E) of Nagoya University located in Hokkaido recorded the auroral events successfully. Aurorae are usually high latitude phenomena observed within the region beyond 65 in the geomagnetic latitude. According to a statistical global distribution of aurorae by Vestine,l1 the probability of aurora appearance was estimated to be less than only one night in 2 years at such a low latitude region as Japan. Low latitude aurorae have been recorded for many centuries in the past over the world, and reviews of such observations were given by Loomis,=~ Chamberlain3) and others. A comprehensive summary of low latitude aurorae has been presented also by Tinsley et al.,4> with an elaborate list of references. Since low latitude aurorae appear during magnetic storms of the severest class, they usually take place during the maximum period of solar activity in every 11 years. in Japan, the following 3 events were reported as low latitude aurorae since 1958, i.e., on February 11, 1958, April 3 and November 13, 196. Because of a very rare occurrence of low latitude aurorae, its generation mechanism remains still unknown even today. It is expected that our observational results will become a clue to clarifying the mechanism, utilizing also the available simultaneous data of precipitating particles detected by polar orbiting satellites during the aurora break-up, which are shown below. The meridian scanning photometer at Moshiri is designed to record aurora emission intensities, with 2A bandwidth interference filters, in the following ( different wavelengths simultaneously: 4278A (N1N), 5577A ( 1D-1S), 63A 3P-1D) and 6683A (N~ 1P). The collimator with 3 fields of view scans 18 degrees in the geomagnetic meridian plane in every 64 seconds. The fluxgate magnetometer for 3 components of the geomagnetic field recorded the magnetic disturbances accompanying aurora break-up with an accuracy of.1 nt. The time variations of the maximum auroral luminosity in the geomagnetic meridian plane are shown in Fig. 2(a) for 63A and 5577A wavelengths. The intensity of 63A emission had already been enhanced over 1 kr from the normal airglow level at 11 UT on October 21. A sudden brightening occurred *) National Institute of Polar Research, Kaga, Itabashi, Tokyo 173, Japan. **) Research Institute of Atmospherics, Nagoya University, Toyokawa, Aichi 442, Japan.

2 48 H. MIYAOKA rt al. [Vol. 66(B), Fig. 1. A low latitude aurora photographed from W akkanai, HoKKalflo on Vet. Z1, The red colour is almost pure 63 A emission. We can also recognize several faint ray structures along geomagnetic field lines, embedded in the red aurora. These ray columns are mainly emitting in 5577A. (Photo by T. Maruyama, lookiiig north at about UT, Oct. 21) Fig. 2. (a) Auroral intensity of 63 A and 5577 A photometer during the aurora break-up on Oct. 21, the photometer was turned off due to adjustments. netogramme observed on Oct. 21 at Moshiri. A time is marked with a thick bar. The aurora break-up positive bay over 2 nt. lines measured by a scanning In the hatched interval, (b) An H-component of maginterval of aurora appearance clearly corresponds to a large

3 No. 31 Low Latitude Aurorae. I 49 Fig. 3. Electron precipitations measured by the DMSP-F9 satellite during the low latitude aurora brightening. Top panel shows energy-time spectrum of electrons in 3 ev-3 kev energy range. A middle panel shows an integrated electron energy flux, and a bottom panel presents count rates in four energy channels. Satellite positions are given in the following coordinates; geographic latitude and longitude (GLAT, GLON), geomagnetic latitude and longitude (MLAT, MLON), and magnetic local time (MLT) with universal time (UT). The time interval with a large energy flux exceeding.1 erg/cm2c sr. sec is marked by a broken line as a reference of an aurora oval. The existence of electron precipitations with low energy is confirmed on the low latitude side of an ordinary auroral oval. at 1137 UT, i.e., simultaneously with a remarkable magnetic positive bay of about 2 nt, as shown in Fig. 2(b). In a few minutes after the sudden brighten- ing, the 63A intensity was enhanced abruptly and reached a saturation level of 8.8 kr. On the other hand, the 5577A intensity, stayed at the airglow level during most of time, except for an enhancement about 8 minutes after the 63A break-up. At the time of 5577A enhancement, we could recognize several ray columns along the line of geomagnetic field with whity colour and a life time about several tens of seconds. A positive bay in an H-component magnetogramme of mid/low latitude region usually means an enhancement of westward electrojet currents flowing in the high latitude ionosphere, in association with a break-up of magnetospheric substorm. The 63A and 5577A enhancements in this event are, therefore, considered to be directly connected with a substorm activity in higher latitudes. The intensity of 63A emission gradually decreased after the maximum around at 1146 UT, and returned to the pre-breakup level at 125 UT. During this event, intensities of 4278A(N2+ 1N) and 6683A (N21P) indicated no brightening with an almost constant level less than 2 R. Concerning the generation mechanisms of low latitude aurorae, Tinsley et al.j)

4 5 H. MIYAOKA et al. [Vol. 66(B), Fig. 4. (a) A schematic picture of a location of the low latitude aurora with an auroral oval derived from the EXOS-D/ATV during the low latitude aurora break-up. The trajectory of the DMSP-F9 satellite is also shown in the figure. (b) A meridional crass section of Fig. 4(a) with view angles from Moshiri Observatory. The height of aurorae are emissions assumed as 1-3 km for 4278 A or 5577 A emissions, and 15-5 km for a 63 A emission. predominant proposed two types of excitation mechanisms from their spectroscopic observations. The first is a precipitation of heavy particles (ions or neutral atoms of H, He, ) producing the high vibrational/rotational excitation of N2+ 1N band emissions. The second type is an excitation by low energy electrons causing a enhancement of 63A [OI ] emissions. To identify the generation mechanism contributing to the low latitude aurora on October 21, 1989, we examined precipitating particle data measured by DMSP-F9 satellite with a sun-synchronous circular polar orbit of 85 km in altitude. Fig. 3 shows the precipitating electron data across the auroral oval almost along the meridian about 2 east of Japan just during the brightening of the low latitude aurora. The onboard particle detector measures every second the downward fluxes of electrons and ions in 2 steps within the energy range from 3 ev to 3 kev. The top panel in Fig. 3 shows energy-time spectrum of precipitating electrons, the middle panel shows electron energy flux integrated over the range of 3 ev- 3 kev, and the bottom panel indicates electron count rates of four energy channels for 3 ev, 3 ev, 3 kev, and 3 kev, respectively. The latitude ranges with energy influx exceeding.1 erg/cm2 sr sec are indicated by vertical broken lines. We notice here the existence of a region for low energy electrons (almost registered in 3 and 3 ev count rates) just on the low latitude side of the ordinary

5 No. 3] Low Latitude Aurorae. I 51 o a auroral oval defined by an enhancement of precipitating electrons in several kev (shaded interval in Fig. 3). As the cross-section for (1D) has a peak value at 5-4 ev, ~ a large amount of low energy electron precipitation is highly effective for exciting a 63A emission line. Hence, a bright and visible 63A aurora is reasonably expected to appear in this region. Since there were no significant ion precipitations in the energy range above 1 kev, it is concluded that a possibility of ion/neutral atom precipitation must be small for a generation mechanism of this low latitude aurora event. Fig. 4(a) is a schematical picture of the low latitude aurora in reference to an auroral oval derived from the auroral imager (ATV) on the EXOS-D (Akebono) satellite, which was launched on Feb. 22, 1989 for studying auroral phenomena. The EXOS-D satellite took UV auroral images in every 8 seconds sequentially from 1213 UT on Oct. 21.7),8) Since an UV aurora is produced by precipitating electrons with almost the same energy range as a visible aurora, the auroral oval indicated by the EROS-D/ATV data must almost coincide with that determined from precipitating energetic electrons with energy above 1 kev. The shaded region marked as a low latitude aurora in Fig. 4(a) corresponds to the region of low energy electron precipitation on the equatorward side of the auroral oval in Fig. 3. The meridional cross section of Fig. 4(a) is given in Fig. 4(b) in order to examine whether this geometrical configuration still holds for the results from the scanning photometer. The height of auroral emissions are assumed here to be 1-3 km for 4278A or 5577A, and 15-5 km for 63A respectively. It is reasonable from this figure that we could not observe intense 4278A or 5577A emissions excited in the auroral oval because the emission region was under 5 in elevation angle from Moshiri. The elevation angle ranges between and 18 as for the 63A emission; this result agrees well with the scanning data at Moshiri. The intensified 5577A emission, which was observed with whity ray structures at the maximum brightening of the aurora, still has some questions to be explained. A possible interpretation for this event is that there were some occasional precipitations of high energy (several kev) electrons at the maximum stage of 63A aurora brightening. The authors' thanks are due to J. H. Allen, WDC-A for Solar-Terrestrial Physics, Boulder for supplying us with the DMSP-F9 particle data promptly and to T. Maruyama, Communications Research Laboratory, Wakkanai for use of his valuable photograph of the aurora in Hokkaido. References 1) 2) 3) 4) 5) 6) 7) 8) E. H. Vestine (1944) : Terr. Magn. Atmos. Electr., 49, E. Loomis (1859) : Am. J. Sci., 2nd ser., 28, 385. J. W. Chamberlain (1961) : Physics of the Aurora and Airglow. Academic Press, New York, p. 14. B. A. Tinsley et al. (1986) : J. Geophys. Res., 91, B. A. Tinsley et al. (1984) : Geophys. Res. Lett., 11, S. C. Solomon et al. (1988) : J. Geophys. Res., 93, E. Kaneda et al. (1989) : EOS, 7, 129. T. Yamamoto et al. (1989) : ibid., 7', 129.