Low-latitude auroras observed in Japan:

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110,, doi: /2004ja010706, 2005 Low-latitude auroras observed in Japan: K. Shiokawa, T. Ogawa, and Y. Kamide Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa, Japan Received 28 July 2004; revised 17 January 2005; accepted 9 February 2005; published 4 May [1] From routine observations by means of highly sensitive all-sky cameras and tiltingfilter photometers, a total of 20 low-latitude aurora events in Japan were identified during the high solar activity period of This indicates that the invisible-level auroras appear rather frequently in the northern sky of Japan during magnetic storms. These auroras are characterized by an enhancement in red (630.0 nm) emissions in the northern sky during magnetic storms, probably at geomagnetic latitudes higher than We show examples of stable auroral red (SAR) arcs, which are typically observed during the recovery phase of storms. We also found several red aurora events that occurred at the initial phase of magnetic storms. Intensifications as well as equatorward motion of the auroras were observed in association with storm time substorms. Green line emissions (557.7 nm) were often enhanced (1 kr) not only in the northern sky but also in the whole sky. Some auroras show weak enhancements (10 R) of N 2 + 1N (427.8 nm) in the northern sky in the morning hours. We discuss possible causes of these emissions in light of magnetospheric disturbances and thermospheric dynamics. Citation: Shiokawa, K., T. Ogawa, and Y. Kamide (2005), Low-latitude auroras observed in Japan: , J. Geophys. Res., 110,, doi: /2004ja Introduction [2] Auroras at middle and low latitudes have been observed visually for many centuries in southern Europe, the United States (including Hawaii), Japan, India, and Cuba [e.g., Loomis, 1861; Chapman, 1957; Tinsley et al., 1986; Rassoul et al., 1992; Miyaoka et al., 1990; Shiokawa et al., 1994]. Tinsley et al. [1984] pointed out that the low-latitude auroras are typically characterized by (1) N 2 + 1N emission at high vibrational levels and (2) a high (>10) ratio of red (630.0 nm) to green (557.7 nm) atomic oxygen lines. The latter aurora sometimes shows intense visible emissions, producing a spectacular red aurora in the sky. [3] Recent studies report that these red auroras have several characteristics in common: they appear during the main phases of geomagnetic storms, they last for a few hours, and their appearance coincides with the occurrence of magnetospheric substorms [Tinsley et al., 1986; Miyaoka et al., 1990; Rassoul et al., 1992; Shiokawa et al., 1994, 1995]. Shiokawa et al. [1997] indicated that broadband electrons, which are the electron precipitation observed by the DMSP satellites at a broad energy range of 30 ev to 30 kev at subauroral latitudes, can be a cause of the low-latitude red auroras during storm time substorms. [4] On the other hand, stable auroral red (SAR) arcs are often observed at subauroral latitudes, predominantly during the recovery phase of magnetic storms [e.g., Rees and Akasofu, 1963; Rees and Roble, 1975]. The SAR arcs Copyright 2005 by the American Geophysical Union /05/2004JA feature monochromatic emissions at a wavelength of nm and can last for more than 10 hours. The SAR arcs probably correspond to low-energy (<10 ev) electron precipitation (heat conduction) produced by interactions between high-energy ring current particles and the plasmasphere that is refilled during the recovery phase of storms [e.g., Kozyra et al., 1987; Fok et al., 1993]. [5] In this paper, we report 20 low-latitude auroral events in Japan, which were observed by using highly sensitive optical instruments (all-sky cooled CCD imagers and tilting photometers) between 35.5 and 25.4 N in geomagnetic latitudes during the high solar activity period of Emissions at wavelengths of 630.0, 557.7, (N 2 + 1N), and nm (Hb) were measured on a routine basis. In addition to the above mentioned characteristics, we found low-latitude auroral events occurring at the initial phase of magnetic storms and events with intense nm emissions over the whole sky at midlatitudes. 2. Instrumentation and Data Examples [6] Routine measurements obtained by using highly sensitive all-sky cooled CCD imagers and filter-tilting meridian-scanning photometers have been conducted at Rikubetsu (43.5 N, E, dipole geomagnetic latitude (MLAT) = 34.7 N) and Shigaraki (34.8 N, E, MLAT = 25.4 N) since October A condition required for automatic measurement is that the elevation angles of the sun and moon be less than 12 and 0, respectively. A three-channel northward looking photometer and a panchromatic image-intensified camera [Shiokawa et al., 1996a] were installed and were in operation from October 1994 at Moshiri (44.4 N, E, MLAT = 35.5 N). The three- 1of15

2 Table 1. List of Optical Instruments Used for Measurements of Low-Latitude Auroras Instruments Wavelength Time Resolution Sky Pointing Installation Rikubetsu Imager nm, nm, OH-band 5.5 min all-sky Oct Meridian-scanning photometer nm, nm, nm 1 min at zenith angles of ±75, ±37.5, and 0 Oct Meridian-scanning photometer nm, nm 10 min at zenith angles of ±75 and 0 Oct Northward looking photometer nm, nm, nm 1 s at a zenith angle of 70 Oct Moshiri Northward looking photometer nm, nm, nm 1 s at a zenith angle of 70 Oct Northward looking camera panchromatic 4 s northward with a field-of-view of 94 Oct Shigaraki Imager nm, nm, OH-band 5.5 min all-sky Oct channel photometer and the panchromatic camera are automatically turned on when the elevation angle of the sun is less than 12 (even when the moon is in the sky). Moshiri and Rikubetsu are in Hokkaido (northern Japan), and Shigaraki is central Japan, near Kyoto. Table 1 lists the instruments that were used for measurements of low-latitude auroras at these stations. [7] The cooled CCD imagers at Rikubetsu (imager 3) and Shigaraki (imager 1) are part of the Optical Mesosphere Thermosphere Imagers (OMTIs) [Shiokawa et al., 1999, 2000a]. The imagers use a thinned and back-illuminated cooled CCD with pixels. They measure airglow and auroral emissions of atomic oxygen (wavelengths: nm and nm) sequentially, with exposure times of 165 s (630.0 nm) and 105 s (557.7 nm) and a time resolution of 5.5 min. Background emission was monitored as well at a wavelength of nm every 30 min with an exposure time of 105 s. Band-pass filters with a width (full width at half maximum) of 2.0 nm are used to detect these emissions. [8] Figure 1 shows examples of all-sky images at a wavelength of nm obtained by imager 3 at Rikubetsu during the low-latitude auroral events on 29 November 2000 (Figure 1a) and 30 October 2003 (Figure 1b). Top is to the north, and left is to the east. In Figure 1a the lowlatitude aurora is seen in the northern edge of the images during a moderate storm (jdstj 100 nt). The emission at the northwestern edge of the images is contaminated by city lights from the town of Rikubetsu. The aurora was suddenly intensified at 1357 UT (2257 LT). This intensification occurred just after the onset of a storm time substorm, as described below. Figure 1b shows a lowlatitude auroral event during an intense storm (jdstj nt). A stable aurora was observed continuously on this night in the northern sky of Rikubetsu. The aurora rapidly shifted southward in association with the beginning of a new storm from 1712 UT (0212 LT) to 1834 UT (0334 LT) and showed a clear east-west arc-like feature in the images. [9] The filter-tilting meridian-scanning photometers at Rikubetsu (Nos. 2 and 3) are also part of OMTIs. Each photometer measures two airglow/aurora emissions simultaneously. The background continuum emission from the sky is monitored by tilting the band-pass filter. Photometer 2 measures emissions of atomic oxygen (630.0 nm) and OH molecule (843.0 nm and nm). It has a field of view of 3 and operates at 5 points (zenith angle: ±75, ±37.5 and 0 ) in the north-south meridian with a time resolution of 1 min. Photometer 3 measures emissions of molecular nitrogen ions (N 2 + 1N at nm) and hydrogen Balmer b (486.1 nm) with a time resolution of 10 min. This photometer has a field of view of 8 and measures the intensity at 3 points (zenith angle: ±75 and 0 ) in the north-south meridian. Detailed descriptions of the photometers were given by Katoh et al. [1999]. The tilting photometer gives more reliable data in the absolute intensities than the imager, because the photometer measures the sky background intensity simultaneously at nearby wavelengths by tilting the interference filter, while the imagers take the background intensity every 30 min at a fixed wavelength of nm. [10] Figure 2 shows an example of auroral luminosity data obtained by photometer 2 (630.0 nm) and 3 (427.8 nm) at Rikubetsu during the low-latitude auroral event of 29 November 2000 (same as that in Figure 1a). Airglow/ auroral intensities obtained by all-sky imager 3 are also shown (north-south meridian plots, i.e., keograms, in the top two panels and nm intensities in three directions in the middle panel). The H component magnetic field variations at Rikubetsu are plotted in the bottom panel. The Van Rhijin effect, which is the integration effect through the emission layer at large zenith angles, is not corrected in these intensity data. [11] The nm intensity is relatively high for the whole sky at the beginning of the measurement before 1300 UT, because of the high electron density in the ionospheric F layer after sunset. It suddenly increases in the northern sky up to 0.5 kr at around 1400 UT (2300 LT), as shown in the top panel (keogram) and in the line plot (photometer data). This increase is also identifiable in Figure 1a, and it coincides with the positive enhancement of the H component magnetic field (midlatitude positive bay), indicating the expansion onset of a magnetospheric substorm. [12] The nm intensity increases in the northern sky at this time, but the increase is over the whole sky at around 1600 UT, with a peak zenith intensity of 0.8 kr. It is highly unusual to observe such an intense airglow intensity at midlatitudes. This enhancement at 1600 UT seems to propagate from north to the south in the keogram. It is similar to the large-scale traveling ionospheric disturbance (LSTID) [e.g., Shiokawa et al., 2003], although the LSTID is seen in the nm airglow emission in the ionosphere at altitudes of km. The nm airglow height is 2of15

3 Figure 1. All-sky airglow images (raw counts) at nm at Rikubetsu, Japan (a) at UT ( LT) on 29 November 2000 and (b) at UT ( LT) on 30 October Top is to the north, and left is to the east. White dots are stars. The low-latitude auroras are seen in the northern sky. The emission at the northwestern edge of the images on 29 November 2000 is city lights km in the mesopause region. We discuss this nm enhancement in section 5.6. [13] In Figure 2 the nm intensity measured by photometer 3 is fairly weak (less than the noise level of the photometer) throughout the night, except for the end of the observation (after 1930 UT). This enhancement at the end of the observation is always observed when the sky is clear, even without storms, and is probably due to the twilight N + 2 emission from resonance scattering of sunlight by N + 2 ions in the thermosphere [Chamberlain, 1961; Torr and Torr, 1982], since the photometer measurement continues until the solar zenith angle becomes less than 102. Note that the noise level of photometer 3 (427.8 nm) is very small (a few R). [14] The three-channel northward looking photometers at Moshiri and Rikubetsu measure the intensities of oxygen (630.0 nm and nm) and molecular nitrogen ions (N + 2 1N, nm) at an elevation angle of 20 from the northern horizon, using three interference filters and three photomultipliers [Shiokawa et al., 1996a]. We use these photometers only to detect auroral emissions and not for airglow, because the sensitivity of the photometers is lower than that of the tilting photometers and imagers, and because it does not measure contamination from sky background emission. The advantage of these photometers is that they can be operated even when the moon is in the sky, because the moon does not get into the field of view of the northward looking photometers. [15] Figure 3 shows an example of the low-latitude aurora detected by the three-channel photometer at Moshiri during the recovery phase of the intense magnetic storm of 31 March 2001 (jdstj 200 nt). The H component magnetic field variations obtained at Moshiri are shown as well. To reduce photometer noise, we show 1-min averages calculated from the 1-s sampled photometer data. The nm intensity clearly increased up to 5 kr after 1700 UT, indicating that a visible red aurora appeared in the northern sky of Moshiri. The nm 3of15

4 listed. Table 2 clearly indicates that low-latitude auroras frequently appeared near Japan during the solar maximum period. The auroras were observed mostly in the northern sky of Hokkaido (Moshiri and Rikubetsu). For three events (events 7, 11, and 16), however, the auroras were also observed by an all-sky imager at Shigaraki (central Japan [Shiokawa et al., 2003]). [17] The intensities at nm usually lie in the 1 5 kr range in the northern sky. Note that the Van Rhijin effect is not corrected in this list, because we do not have accurate knowledge of the latitudes and altitude ranges of the auroras. The intensities at nm are mostly less than 1 kr. However, they sometimes exceed nm (e.g., events 14 and 19). Although the intensities at nm are very weak (<0.1 kr), the high sensitivity of photometer 3 makes it possible to record the enhancements for several cases. Photometer 3 also measures the nm emission with a similar high sensitivity. However, any detectable nm emissions were not seen during these low-latitude auroral events. We will revisit these spectral characteristics in the next section with detailed plots of representative events. [18] Figure 4 shows 3-day Dst variations during the lowlatitude auroral events listed in Table 2 for The periods during which the auroras were observed in Japan are indicated by horizontal bars, where a dashed bar indicates that the observation was terminated because of sunrise or moonrise. It is seen that the low-latitude auroras were observed not only during great geomagnetic storms, with Dst < 300 nt (31 March 2001, October 2003, and November 2003), but also during small storms of Dst > 100 nt (13 May 1999, 28 April 2001, and Figure 2. From top to bottom, north-south cross sections (keograms) of all-sky images for emissions at and nm, intensities at 427.8, 557.7, and nm, and H component magnetic field observed at Rikubetsu on 29 November The line plot intensities at and nm are measured by two tilting photometers, where north and south intensities are those at a zenith angle of 75. The line plot intensities at nm are obtained from allsky image data, where north and south intensities are those at a zenith angle around 66. The north and south intensities tend to be larger than that at the zenith because the Van Rhijin effect (the integration effect through the emission layer at large zenith angles) is not corrected at these intensities. and nm intensities increased slightly during this auroral event. 3. Event List and Relation to Magnetic Disturbances [16] From routine measurements of the highly sensitive imagers and photometers described above, we observed a total of 20 low-latitude auroral events during Table 2 lists these events. The auroral events detected by less sensitive photometers and cameras before 1998 are also Figure 3. Intensities at 427.8, 557.7, and nm measured by a northward looking photometer and variations of H component magnetic field at Moshiri on 31 March The photometer data are at a zenith angle of 70. 4of15

5 Table 2. Low-Latitude Aurora Measured by STELAB in a Date and Time Event mindst Station nm nm nm Source b 21 Oct UT 268 nt Moshiri >8.8 kr c 4.0 kr ND (<20 R) M90 26 Feb UT 174 nt Moshiri >2 kr c 0.1 kr ND (<50 R) S94 27 Feb UT 174 nt Moshiri >2 kr c 0.1 kr ND (<50 R) S94 29 Feb UT 118 nt Moshiri >1 kr c ND (<0.1 kr) ND (<50 R) S94 10 May UT 288 nt Rikubetsu no obs. no obs. no obs. S94 13 Sept UT 161 nt Rikubetsu >25 R d no obs. no obs. S95 18 Feb UT nt Rikubetsu >0.8 kr e 0.7 kr (NPE) ND (<2 R) S00 13 May UT 2 49 nt Rikubetsu 0.8 kr e 0.5 kr 14 R e S01 6 April UT nt Rikubetsu >1.7 kr e 0.5 kr 14 R e 7 April UT nt Rikubetsu >4.2 kr e 0.2 kr (NPE) ND (<2 R) 6 Nov UT nt Rikubetsu >2.2 kr e 0.4 kr (NPE) 11 R e 29 Nov UT nt Rikubetsu 0.45 kr 1.1 kr (NPE) ND (<2 R) 31 March UT nt Moshiri >5.0 kr d,e 0.2 kr d,e 0.2 kr d,e 31 March UT nt Rikubetsu >1.0 kr d 0.3 kr d (NPE) 12 R e 31 March UT nt Shigaraki >2.1 kr c 0.4 kr no obs. S03 28 April UT 8 47 nt Rikubetsu >0.6 kr e 0.1 kr (NPE) ND (<2 R) 21 Oct UT nt Rikubetsu >1.0 kr e >0.7 kr e ND (<2 R) 6 Nov UT nt Moshiri 4.0 kr ND (<0.5 kr) ND (<0.2 kr) 24 Nov UT nt Moshiri 4.0 kr 0.2 kr ND (<0.2 kr) 24 Nov UT nt Rikubetsu 2.0 kr 0.7 kr no obs. 24 Nov UT nt Shigaraki >0.3 kr e 0.3 kr no obs. 17 April UT nt Rikubetsu 0.7 kr 0.3 kr (NPE) 10 R 29 May UT nt Rikubetsu 0.7 kr e 0.8 kr e 13 R e 24 Oct UT nt Rikubetsu 0.17 kr 1.1 kr (NPE) ND (<2 R) 29 Oct UT nt Rikubetsu 2.2 kr d,e >1.5 kr d,e 88 R d,e 29 Oct UT nt Moshiri 0.9 kr 1.0 kr ND (<0.2 kr) 30 Oct UT nt Moshiri >3.5 kr e 0.4 kr e ND (<0.2 kr) 30 Oct UT nt Rikubetsu >2.4 kr e >1.6 kr e 5R e 30 Oct UT nt Shigaraki 0.1 kr 0.4 kr no obs. 31 Oct UT nt Moshiri 4.0 kr e 0.5 kr e ND (<0.2 kr) 31 Oct UT nt Rikubetsu 0.7 kr e >1.2 kr e ND (<2 R) 20 Nov UT nt Moshiri 1.4 kr d ND (<0.2 kr) ND (<0.2 kr) 21 Nov UT nt Rikubetsu 0.3 kr d 1.0 kr ND (<2 R) 8 Nov UT nt Rikubetsu 2.2 kr e no obs. ND (<4 R) a LT = UT + 9 hours. A plus in the time indicates event may last longer (measurements were terminated due to sunrise or moonrise). ND is not detected (less than noise level of measurement). NPE is no particular enhancement at north (northward and southward intensities are comparable. b Sources are as follows: M90, Miyaoka et al. [1990]; S94, Shiokawa et al. [1994]; S95, Shiokawa et al. [1995]; S00, Shiokawa et al. [2000b]; S01, Shiokawa et al. [2001]; S03, Shiokawa et al. [2003]. c Maximum intensity should be larger (instrumental saturation). d Maximum intensity should be larger (measurements through cloudy sky). e Maximum intensity should be larger (measurements were terminated due to sunrise or moonrise). 24 October 2003). They occurred during the main and recovery phases of the storms. In some cases, they occurred at the initial phase of the storms (6 April 2000, 6 November 2000, and 21 October 2001). Note that the time intervals without horizontal bars do not indicate that the aurora did not appear, because we cannot measure auroras during daytime and cloudy nights. [19] Figure 5 shows 2-day H component magnetic field variations observed at Rikubetsu and Moshiri during the low-latitude auroral events of The horizontal bars again indicate the time periods of the auroras. It is evident that the auroral events at the initial phase of storms occurred just after (within a few hours of) the storm sudden commencements (6 April 2000, 21 October 2001, and 24 October 2003). It is noteworthy that some low-latitude auroral events coincide with the onset of storm time substorms, which are identified by midlatitude positive-h bays, during the main phase of storms (13 May 1999, 6 November 2000, and 20 November 2003; see also the example in Figure 2). 4. Event Studies [20] In section 3 we demonstrated that low-latitude auroras can occur at practically any time during magnetic storms: at the beginning of a storm as well as during the main and recovery phases. In this section we show some representative events of these low-latitude auroras in detail The 6 7 April 2000 Event [21] The magnetic storm of 6 7 April 2000 was very intense, with a minimum Dst index of 288 nt. At the beginning and during the recovery phase of this storm, lowlatitude auroras were observed by the imager and the photometers at Rikubetsu. [22] Figure 6 shows keograms and intensities at 630.0, 557.7, and nm at Rikubetsu on 6 April 2000, with the H component magnetic field variations, in the same format as that of Figure 2. The sky was fairly clear on this night. The northward moving structures continuously seen in the nm keogram are atmospheric gravity waves in the nm airglow emission from the mesopause region at an altitude of km [e.g., Taylor et al., 1995]. [23] The magnetic storm started at 1638 UT with a clear sudden commencement. High-latitude magnetograms indicate that an intense substorm with a maximum AE index (provisional) of 2000 nt (at around 1730 UT) took place just after this sudden commencement. At 1700 UT, both the nm and nm intensities began to increase in the 5of15

6 the north increased much earlier than the others, indicating that this auroral event contained a weak nm emission. [24] Figure 7 shows the keograms and intensities at Rikubetsu on the next night (7 April 2000). The sky was Figure 4. Three days Dst indices during the low-latitude auroral events for the period of Thick horizontal bars indicate the time intervals when the auroras were observed in Japan. Thick dashed bars indicate that the auroral observation was terminated because of sunrise or moonrise. northern sky. At 1800 UT, the nm intensity drastically increased up to 1.7 kr, when the measurement was terminated in the morning. Such a drastic enhancement was not seen in the nm data. Although the nm intensity increased in the morning in the whole sky, the intensity in Figure 5. Two days variations of the H component magnetic field during the low-latitude auroral events for the period of The data were obtained at Rikubetsu except for the top three curves, which were obtained at Moshiri. Thick horizontal bars indicate the time intervals when the auroras were observed in Japan. Thick dashed bars indicate that the auroral observation was terminated because of sunrise or moonrise. 6of15

7 satellite at UT on 7 April The satellite traversed relatively close to Japan ( E) from south to north: the geographic longitude of Rikubetsu is E. [26] In Figure 8, low-energy (<1 kev) electron precipitation is observed above 69.0 (64.5 MLAT) geographic latitudes (GLATs). High-energy ions are also observed above these latitudes. On the other hand, the nm emission intensity of 4.2 kr (at 1200 UT) in Figure 7 was observed by the tilting photometer at an elevation angle of 15 from the northern horizon of Rikubetsu. If we assume the altitude of the aurora to be 600 km, the latitude of the observed aurora must be located at about 54 GLAT (shown in Figure 14). Significant electron precipitation, however, was not observed at this latitude in the DMSP data. This is probably because the typical electron precipitation that causes SAR arcs is less than 30 ev, which is the lowestenergy limit of the DMSP particle detectors. [27] The DMSP F15 satellite also has the Special Sensor- Ions, Electrons, and Scintillation (SSIES) package, which consists of a retarding potential analyzer (RPA), ion drift meter (IDM), Langmuir probe, and scintillation meter. Figure 6. From top to bottom, north-south cross sections (keograms) of all-sky images for emissions at and nm, intensities at 427.8, 557.7, and nm, and H component magnetic field observed at Rikubetsu on 6 April 2000 in the same format as that of Figure 2. mostly clear except for UT. The H component magnetic field at Rikubetsu gradually increased at the recovery phase of the intense storm. Intense nm emission was observed in the northern sky. The maximum intensity of 4.2 kr was observed at the beginning of the measurement, and the intensity decreased gradually toward midnight. No discernible nm enhancement was seen in the data. These emission features are consistent with those of the SAR arcs in the recovery phase of storms. The enhancement in nm intensity at the end of the observation is probably due to the thermospheric twilight emission, because the intensity increased in all directions simultaneously at the very end of the observation. [25] During the SAR-arc event of 7 April 2000, the DMSP F15 satellite traversed close to the Japanese longitudinal sector and measured precipitating particles and ion/ electron features in the topside ionosphere at an altitude of 850 km. Figure 8 shows data for precipitating electrons and ions obtained by the SSJ4 instruments, which monitor precipitating ions and electrons, on board the DMSP F15 Figure 7. From top to bottom, north-south cross sections (keograms) of all-sky images for emissions at and nm, intensities at 427.8, 557.7, and nm, and H component magnetic field observed at Rikubetsu on 7 April 2000 in the same format as that of Figure 2. 7of15

8 Figure 8. Number flux, average energy, and energy-time spectra of precipitating electrons and ions at energies of 30 ev to 30 kev observed by the DMSP F15 satellite during the SAR arc event on 7 April 2000 in the Japanese longitudinal sector. Ion data were not obtained below 1 kev because of degradation of a particle detector. Satellite locations are shown at the bottom, where MLAT, GLAT, GLON, and MLT are magnetic latitude, geographic latitude, geographic longitude, and magnetic local time, respectively. Figure 9 shows ion and electron temperatures, ion drift velocities, and ion density measured by the SSIES instruments on board the DMSP F15 satellite on the same orbit as that in Figure 8. The electron temperatures are clearly enhanced to more than 6000 K at GLAT (39 60 MLAT), which was located at the lower latitudes of the particle precipitation region. This enhancement of electron temperature is consistent with previous observations of electron temperatures in SAR arcs by the DE satellite, suggesting the idea of SAR arc generation by the heat flow in thermal electrons from the plasmasphere [Rees and Roble, 1975; Kozyra et al., 1987]. The ion temperatures, too, slightly increased in the same latitude range. The northward ion drift velocity decreased at the region of temperature enhancement, while the signature of storm time Subauroral Ion Drift (SAID, westward ion drift at subauroral latitudes) was not seen in the V y data. The ion density is seen to slightly increase at the lower-latitude side of the temperature enhancement (45 52 GLAT) The 21 October 2001 Event [28] As shown in the previous section, the low-latitude aurora of 7 April 2000 (Figure 7), was a typical SAR arc event during the recovery phase of magnetic storms. On the other hand, the aurora of 6 April 2000 (Figure 6), took place just after the onset of a magnetic storm. This is not likely to be a typical SAR arc. Low-latitude auroras just after storm onset were in fact observed several times, as shown in the Dst and magnetogram plots in Figures 4 and 5. Here we show another example of such an event, observed on 21 October [29] Figure 10 shows keograms and intensities at 630.0, 557.7, and nm at Rikubetsu on 21 October 2001, with the H component magnetic field variations, in the same format as that in Figures 2, 6, and 7. The sky was partially cloudy before 1700 UT (particularly at UT), as shown in the keograms, but bright stars could be seen in allsky images throughout the night. [30] The magnetic storm started at 1648 UT with a clear sudden commencement. High-latitude magnetograms indicate that a large substorm with a maximum AE index (provisional) of 1400 nt (around 1730 UT) took place just after this sudden commencement. At around 1640 UT, both nm and nm intensities increased in the whole sky, although this enhancement was probably due to the sky becoming clear at this time. At 1810 UT, the 8of15

9 Figure 9. Ion and electron temperatures, ion drift velocities, and ion density observed by the DMSP F15 satellite during the SAR arc event on 7 April 2000 in the Japanese longitudinal sector. For the ion drift velocity, Vx and Vy are in the horizontal direction of the satellite track (positive: northward) and perpendicular to the track (positive: westward), respectively. Positive Vz is in the direction away from the center of the Earth. Satellite locations are shown at the bottom, where ALT, GLAT, GLON, MLAT, and MLT are satellite altitude, geographic latitude, geographic longitude, magnetic latitude, and magnetic local time, respectively nm intensity drastically increased up to 1.0 kr. Such a drastic enhancement was not seen in the nm data. The nm intensity increased in all directions after 1900 UT, probably because of the thermospheric twilight emission. This event is quite similar to the event of 6 April 2000, shown in Figure 6, indicating that there is a common feature in low-latitude red auroras just after storm initiation The October 2003 Event [31] The magnetic storms of October 2003, were extremely intense, with a minimum Dst index (provisional) of 401 nt. As shown in the Dst variations in Figure 4, there was a sequence of three storms. Since, fortunately, the sky at Rikubetsu was mostly clear on these three nights, we could observe the development and decay of low-latitude auroras throughout this historically record-making activity. [32] Figures 11, 12, and 13 show keograms and intensities at 630.0, 557.7, and nm at Rikubetsu on October 2003, along with the H component magnetic field variations. The sky was intermittently cloudy on October 29 but was clear on the other two nights. [33] The magnetic storms started at 0611 UT on October 29, 2003, while optical observation began at 1000 UT (Figure 11), when the storm had already developed. Although the sky was partially cloudy on 29 October, the nm emission was evidently intensified in the northern sky from 1400 UT. After 1830 UT, the sky became clear, and the nm emission was intensified up to 2.2 kr in the northern sky. The nm emission was intensified simultaneously, with a peak (88 R) at 1900 UT. This is the highest record of nm intensity at Rikubetsu since the beginning of tilting-photometer observation in October The nm emission showed an enhancement of up to 1.5 kr in the northern sky. It should be noted that the nm emission at the zenith was intense (0.6 kr) throughout the night as well. [34] The sky on 30 October 2003 was quite clear at Rikubetsu (see Figure 12). A weak nm emission (0.5 kr) was continuously observed in the northern sky of Rikubetsu during the recovery phase of the second storm. High-latitude magnetograms indicate that from 1640 UT, an extremely intense substorm took place with the AE index (provisional) value reaching 3000 nt at around 1800 UT. The third storm started around this time. The H component magnetic field in the bottom panel decreased rapidly from 1730 UT. The all-sky images at nm at this time are shown in Figure 1b. An equatorward shift of an east-west arc structure of the aurora is observed from 1712 UT in the images. Among the high-sensitive imaging measurements at Rikubetsu since October 1998 this was the only event for which the poleward edge of the low-latitude aurora was observed by the imager. The nm emission was further enhanced up to 2.4 kr in the northern sky from 1900 UT, as shown in Figure 12. The nm emission intensity seemed to increase (5 R) in the northern sky at UT. The nm intensity did not increase at this time. The nm emission, however, was unusually intense not only in the northern sky (maximum 1.6 kr), but also in the whole sky ( kr at zenith). 9of15

10 at invisible intensity levels. In this section, we discuss various aspects of the observed auroras Location of Auroras [37] From the measurements made since October 1998 we have observed 20 low-latitude auroras in Japan. All of them were observed in the northern sky of Hokkaido (Rikubetsu and Moshiri). Figure 14 shows the relationship of auroral latitudes and elevation angles from the northern horizon of Rikubetsu (GLAT = 43.5 N) for different emission heights. The meridian-scanning filter-tilting photometer looks at an elevation angle of 15. In case of SAR arcs, Okano and Kim [1987] determined the altitudes of SAR arcs to be km, using a triangulation technique from two ground-based stations. Kozyra et al. [1990] calculated the typical altitude of SAR arcs during the solar maximum summer to be 600 km. In case of auroras caused by electron precipitation, the nm emission altitude ranges from 200 to 300 km [e.g., Solomon et al., 1988]. If we assume the altitude of auroras to be 600 (200) km, the elevation angle of 15 corresponds to 54 Figure 10. From top to bottom, north-south cross sections (keograms) of all-sky images for emissions at and nm, intensities at 427.8, 557.7, and nm, and H component magnetic field observed at Rikubetsu on 21 October 2001 in the same format as that of Figure 2. [35] The sky condition on the third night (October 31, 2003) was also fine at Rikubetsu (see Figure 13). This night was in the recovery phase of the largest storm (minimum Dst = 401 nt (provisional)), as shown in Figure 4. The nm emission was 0.7 kr at the beginning of the observation and gradually decreased toward midnight. This feature is quite similar to the SAR arc event of 7 April 2000 (Figure 7). However, the intensity of the nm emission in the northern sky showed variations somewhat similar to these of the nm emission, particularly before midnight. The nm emission was intense not only in the north ( kr) but also for the whole sky ( kr at the zenith), as in the previous two nights. 5. Discussion [36] The automated routine measurements using highly sensitive optical instruments in Japan have shown that storm time auroras frequently appear in the northern sky of Japan Figure 11. From top to bottom, north-south cross sections (keograms) of all-sky images for emissions at and nm, intensities at 427.8, 557.7, and nm, and H component magnetic field observed at Rikubetsu on 29 October 2003 in the same format as that of Figure of 15

11 [39] The origin of the SAR arcs was studied in detail by previous researchers. Rees and Roble [1975] modeled SAR arcs as an interaction between ring current particles and plasmaspheric electrons through the Landau damping of ion cyclotron waves. Kozyra et al. [1987, 1993] pointed out the importance of Coulomb collisions between ring current O + and H + ions and plasmaspheric low-energy electrons in generating the SAR arcs. These studies indicate that the overlap region of storm time ring current and plasmasphere is crucial for generating low-energy electron precipitation that causes nm emissions at subauroral latitudes. Sazykin et al. [2002] pointed out that the SAID (polarization jet) can be an additional source of weak SAR arcs through the ion-neutral collisional heating and ion composition changes Substorm-Associated Red Auroras [40] Some low-latitude auroras appeared and were intensified just after substorm onset, as shown in the example of 29 November 2000 (event 6, Figures 1a and 2). The sub- Figure 12. From top to bottom, north-south cross sections (keograms) of all-sky images for emissions at and nm, intensities at 427.8, 557.7, and nm, and H component magnetic field observed at Rikubetsu on 30 October 2003 in the same format as that of Figure 2. (48 ) GLAT, which is 45 (39 ) MLAT at the Earth s surface and 47 (41 ) MLAT at the 600-km (200-km) altitude. From these considerations, we conclude that the low-latitude auroras observed at Rikubetsu were located at latitudes higher than MLAT SAR Arcs [38] Quite a few of the events we observed are probably SAR arcs. They are characterized by long-lasting red auroras during the recovery phase of large storms, as shown in Figure 7 (7 April 2000), and Figure 12 (30 October 2003). During the event of 30 October 2003, a red emission was observed throughout the night at the recovery phase of the large storm (minimum Dst (provisional) = 363 nt). It suddenly moved equatorward at 1712 UT, as shown in Figure 1b, associated with the start of a new storm and the intense substorm with the maximum AE (provisional) value reaching 3000 nt. Such an intensification and the subsequent equatorward shift of SAR arcs have in fact been reported by Prasad et al. [1975] and Okano and Kim [1987]. Figure 13. From top to bottom, north-south cross sections (keograms) of all-sky images for emissions at and nm, intensities at 427.8, 557.7, and nm, and H component magnetic field observed at Rikubetsu on 31 October 2003 in the same format as that of Figure of 15

12 In case of auroras, the excited state O( 1 D) is generated by electrons and ions precipitating from the magnetosphere. On the other hand, at midlatitudes this emission exists continuously as an airglow emission with a typical intensity of <100 R at an altitude of km. The nm airglow is caused by the dissociative recombination of O 2 + as [e.g., Link and Cogger, 1988], O þ þ O 2! O þ 2 þ O ð2þ O þ 2 þ e! O 1 D þ O ð3þ Figure 14. Relations of auroral latitudes and elevation angles from the northern horizon of Rikubetsu (GLAT = 43.5 N) for emission altitudes of 200, 400, 600, 800, and 1000 km. The geomagnetic (dipole) latitudes in the bottom axis indicate those at the Earth s surface, calculated on the basis of the IGRF2000 model (epoch: 2000). storms are identified by the positive-h bay at Rikubetsu. Similarly, events 2 (13 May 1999), 5 (6 November 2000), and 18 (20 November 2003) occurred coincident with the onset of substorms (positive H bays), as shown in Figure 5. Such features of substorm-associated low-latitude auroras have been reported previously [e.g., Tinsley et al., 1986; Rassoul et al., 1992; Shiokawa et al., 1994, 1995]. [41] These substorm-associated low-latitude auroras may correspond to the equatorward shift of SAR arcs, as reported by Prasad et al. [1975] and Okano and Kim [1987]. The other possibility is the precipitation of broadband electrons at the equatorward edge of the auroral oval. Broadband electrons were reported by Shiokawa et al. [1996b] as an intense electron precipitation over a broadband energy range of 30 ev to 30 kev during storm time substorms. Shiokawa et al. [1997] have shown that broadband electrons can cause low-latitude red auroras, because the green (557.7-nm) and blue (427.8-nm) emissions from these electrons are confined to lower altitudes, which are below the northern horizon from Rikubetsu. To identify the source particles of the auroras, we need to examine particle data obtained by conjugate satellites, such as DMSP satellites (Figure 8). Note, however, that such conjugate groundsatellite measurements are very rare, because the DMSP satellites traverse only at limited local times of 18, 21, and 06 LT in the nightside sector Enhanced Red Airglow Through Thermospheric Dynamics [42] The red line at a wavelength of nm is emitted from the atomic oxygen as O 1 D! O 3 P þ hn630:0nm ð1þ Because reaction (2) dominates the whole process, the production of the nm airglow is proportional to the molecular oxygen density [O 2 ] and the oxygen ion density [O + ]. The oxygen ion density [O + ] is nearly equal to the electron density in the F layer. Thus the nm airglow emission is a sensitive indicator of the variations of electron density and O 2 density in the bottomside of the F layer at km. [43] During magnetic storms, the upper atmosphere is heated through the energy input from the magnetosphere, causing enhancements of the O 2 and N 2 density in the nm airglow layer. The enhanced O 2 density might produce more O( 1 D) states through the reactions (2) and (3). However, the enhanced N 2 density would quench the excited state O( 1 D) through, O 1 D þ N2! O 3 P þ N2 : ð4þ Through the faster process (4), the heating of the upper atmosphere would result in the decrease of the nm airglow. Prölss [1995] pointed out that the heating of the upper atmosphere causes negative ionospheric storms, during which the enhanced neutral density in the ionosphere reduces the electron density according to the quenching O + ions by molecular nitrogen and oxygen. [44] Prölss [1995] also discussed the positive effect of storms, which is an electron density increase by the F layer height increase due to equatorward neutral wind (traveling atmospheric disturbance (TAD)) or eastward electric field. However, such layer rise would cause a decrease of the nm airglow intensity, because the nm airglow is proportional to the O 2 density, which decreases significantly at higher altitudes. [45] The nm airglow does intensify according to the height decrease of the F layer by poleward neutral winds associated with TADs. However, the TAD-related nm enhancement can clearly be distinguished from the lowlatitude auroras, because TADs exhibit an equatorward propagating feature in the all-sky airglow images, as reported by Shiokawa et al. [2002, 2003]. If a westward electric field is imposed to the ionosphere, it would also cause the F layer decrease and the subsequent nm airglow enhancement. However, such an airglow enhancement by the penetration of magnetospheric electric field would be on a global scale, and be more strengthened at lower latitudes [see Shiokawa et al., 2000b]. Note that all the low-latitude aurora events reported in this paper are confined in the northern sky of the observatories. From 12 of 15

13 these considerations, the thermospheric airglow is not likely to be the cause of the observed low-latitude auroras Auroras at the Initial Phase of Storms [46] As shown in the examples of 6 April 2000 (event 4, Figure 6) and 21 October 2001 (event 9, Figure 10), several low-latitude auroral events occurred during the initial phase of magnetic storms. The other example of this type of event took place on 24 October 2003 (event 14, see Figure 5). As shown in Figure 4, these events occurred when the Dst index was not yet well developed. It should also be noted that for these three events, intense substorms with maximum AE values of 2000, 1400, and 2000 nt, respectively, took place coincident with the storm sudden commencements. [47] These initial phase auroras are characterized by red (630.0 nm) emissions, as is the case during the main and recovery phases. It is quite possible that the plasmasphere and ring current particles interact each other at the initial phase of the storms, causing the observed emissions similar to the SAR arcs during the recovery phase. If this scenario is correct, the high-energy ring current particles must have penetrated deep into the inner magnetosphere (L 2) within two hours from the sudden commencement. However, a common view about convection which has been employed to model the motion of the ring current particles [e.g., Volland, 1973; Stern, 1975; Maynard and Chen, 1975; Rowland and Wygant, 1998] does not appear to allow such a sudden change in the magnetosphere. The intense substorms just after sudden commencement may also play a role in creating the high-energy particles near L 2. Lowenergy plasmaspheric electrons may drift out during the initial phase of storms, i.e., erosion of the plasmasphere. However, this type of process would be much slower than the ring current particle motion, since the particle drift velocities due to the gradient and curvature of the magnetic field becomes smaller with decreasing particle energy. [48] The initial phase red auroras reported in this paper may also have some relation to the auroras associated with the solar wind pressure pulse [e.g., Spann et al., 1998]. Chua et al. [2001] investigated ultraviolet images obtained by the Polar spacecraft and particle data obtained by the Fast Auroral Snapshot (FAST) spacecraft for the pressure pulse event of 26 August They concluded that the pressure pulse aurora is characterized by greater flux of lowenergy electrons (average energy <7 kev) compared to the substorm auroras. These particles would cause the nm and nm emissions as well as the nm emission. However, only the nm emission might be detected from the lower latitudes, since the other emissions are at lower altitudes and are below the horizon The nm Emission at Low Latitudes [49] In the present study, we identified low-latitude auroras from an enhancement of nm emission. As shown in Table 2, in many cases the nm emission does not increase at the time of the nm enhancements. Nevertheless, the nm emission intensity is often larger than the ordinary airglow intensity ( R at zenith). Sometimes it exceeds 1 kr in the northern sky. Examples of such events are shown in Figures 12 and 13. The nm emission is intense in the northern sky, but at the zenith, it exceeds the ordinary airglow level as well. A similarly intense nm emission of 1 kr was reported by Shiokawa et al. [2003] in southern Japan during the magnetic storm of 31 March [50] Tinsley et al. [1984] suggested two types of lowlatitude auroras: one is generated by low-energy electrons and characterized by a red color, while the other is generated by precipitation of energetic neutral atoms from the ring current. The latter may generate nm emissions. Tinsley et al. showed that such precipitation of heavy particles must cause nm emissions as well. However, we did not observe any corresponding nm and nm emissions during these intensified nm events (see Figures 12 and 13). [51] During several events with enhanced nm emission in the northern sky, kev-order electrons might have precipitated at latitudes higher than Rikubetsu, producing the observed nm emission at altitudes of km. Such precipitation would also cause nm emissions at altitudes of 100 km, which may be, however, under the northern horizon of Rikubetsu. [52] For the nm enhancements at zenith or south of the observatories, they are not likely to be generated by the kev-order particle precipitation, since they are not accompanied by 630.0/427.8/486.1-nm emissions. Because the intensity variation of the enhanced nm emission is rather slow, it may be the enhanced airglow originating from the lower thermosphere and mesopause region, which is caused by thermospheric dynamics and composition changes during large magnetic storms [e.g., Jaccha et al., 1976; Prölss, 1980]. A detailed comparison with globalscale modeling of the thermosphere, including nm emission processes, will be needed for further study The nm Emission [53] Using the highly sensitive filter-tilting photometer 3 at Rikubetsu, we have identified eight low-latitude auroral events that were accompanied by weak nm emissions in the northern sky, as listed in Table 2. The intensities of the nm emissions were mostly 10 R. It should be noted that all these events were observed in the morning hours as a nm enhancement in the north just before the twilight enhancement, as shown in Figures 6, 11, and 12. Photometer 3 also measures Hb emission at a wavelength of nm with a noise level of a few R. However, Hb emissions were not found in the low-latitude auroral events reported in this paper. [54] Similar highly sensitive measurements of nm and nm emissions have been reported by Tinsley et al. [1984] for the magnetic storms of 13 April 1981, 14 July 1982, 22 September 1982, and 13 June 1983 at the McDonald Observatory, Texas (30.7 N, E, 39.6 MLAT). For three events they reported nm intensities to be R. Tinsley et al. also reported weak nm emission of a few R for all four events. These differences in nm and nm emissions between Japan and Texas may be due to the latitude difference (Rikubetsu: 34.7 MLAT, McDonald Observatory: 39.6 MLAT). [55] It is important to point out that the nm enhancements were observed only in the morning sector before the twilight enhancement. If the emission was caused 13 of 15

14 by precipitation of energetic neutral atoms from ring current particles, as suggested by Tinsley et al. [1984], it should be enhanced in the midnight and premidnight sectors, because ring current ions are injected from midnight and drift toward dusk. The excitation of twilight nm emission (N 2 + (B 2 S u + )) is the resonance scattering of sunlight by N 2 + ions in the lower thermosphere, typically at altitudes of km [Chamberlain, 1961]. However, the N 2 concentration in the thermosphere drastically increases at subauroral latitudes during magnetic storms, because of the strong upwelling of the N 2 -rich atmosphere in the auroral oval and subsequent global transport to the subauroral latitudes [e.g., Prölss, 1995]. Thus the twilight N 2 + 1N emissions at higher altitudes at subauroral latitudes can be the cause of the observed nm emissions in the northern sky of Rikubetsu. 6. Conclusions [56] We have conducted automated routine measurements of low-latitude auroras using highly sensitive optical instruments (all-sky cooled CCD imagers and filter-tilting meridian scanning photometers) since October 1998 at Rikubetsu (dipole geomagnetic latitude: 34.7 ) and other stations in Japan. Twenty low-latitude auroral events with enhanced nm emissions were identified during This indicates that the invisible-level auroras appear rather frequently in the northern sky of Japan during magnetic storms, although it has been thought that auroras are rare in Japan. The characteristics of the observed auroras are summarized as follows. [57] 1. SAR arcs were observed during the main and recovery phases of the storms. They were long-lasting (more than a few hours) nm emissions in the northern sky. For the SAR arc event of 7 April 2000, conjugate DMSP F15 data showed no particle precipitation in a 30 ev to 30 kev energy range, but the data show a clear enhancement of electron temperatures (>6000 K) in the SAR arc latitudes. [58] 2. Some low-latitude auroras appeared or were intensified coincident with the onset of storm time substorms. [59] 3. Several red auroras appeared at the initial phase of magnetic storms (within 2 hours of the storm sudden commencements). These events were accompanied by intense substorms with maximum AE values of nt. [60] 4. During a few cases, green line emissions (557.7 nm) were also enhanced over the whole sky (>0.5 kr at the zenith and >1.0 kr in the northern sky). [61] 5. The nm emission sometimes increases (10 R) in the northern sky in the morning hours just before the twilight enhancement. We suggest that this increase is caused not by particle precipitation but by resonant scattering of sunlight by high-altitude N + 2 ions, which are supplied to subauroral latitudes by the dynamic behavior of the storm time neutral atmosphere. [62] Acknowledgments. We thank Y. Otsuka, Y. Katoh, M. Satoh, and T. Katoh of the Solar-Terrestrial Environment Laboratory, Nagoya University, for their kind assistance in the development and operation of the all-sky imagers. We are also grateful to K. Hanano of the Rikubetsu Observatory and M. Sera and Y. Ikegami of the Moshiri Observatory for their continuous support of the measurements. The observations at Shigaraki were carried out in collaboration with the Research Institute for Sustainable Humanosphere, Kyoto University. The all-sky imagers and the photometers were calibrated using optical facilities at the National Institute of Polar Research, Japan. The DMSP particle detectors were designed by Dave Hardy of AFRL, and data were obtained from JHU/ APL. We thank Dave Hardy, Fred Rich, and Patrick Newell for providing the DMSP particle data. We gratefully thank the Center for Space Sciences at the University of Texas at Dallas and the US Air Force for providing the DMSP thermal plasma data. This work was supported by Grant-in-Aid for Scientific Research ( and ) and Dynamics of the Sun- Earth-Life Interactive System (G-4, the 21st Century COE Program) of the Ministry of Education, Culture, Sports, Science and Technology of Japan. [63] Arthur Richmond thanks Nicola J. Fox and Yogeshwar Sahai for their assistance in evaluating this paper. References Chamberlain, J. W. (1961), Physics of Aurora and Airglow, Elsevier, New York. Chapman, S. (1957), The aurora in middle and low latitudes, Nature, 179, Chua, D., G. Parks, M. Brittnacher, W. Peria, G. Germany, J. Spann, and C. 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