Azimuthal structures of ray auroras at the beginning of auroral substorms

GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L23106, doi:10.1029/2009gl041252, 2009 Azimuthal structures of ray auroras at the beginning of auroral substorms K. Sakaguchi, 1 K. Shiokawa, 1 and E. Donovan 2 Received 5 October 2009; revised 11 November 2009; accepted 18 November 2009; published 12 December 2009. [1] The time evolution of optical structures in brightening auroras was examined for two substorm events that occurred on January 15, 2008 and February 27, 2009. The onsets of both auroral substorms were captured in the field of view of an all-sky TV camera at Gillam, Canada. We found that in both cases, the brightening auroras formed vertical structures, or so-called ray auroras, which lined up longitudinally for several tens of seconds before luminosity increased sharply. The ray structure implies precipitating electrons accelerated over a broad energy range at substorm onset. Two dimensional Fourier transform analysis of the auroral images showed the ray auroras have characteristic wavenumbers and harmonics that develop within a few seconds of the initial brightening. These results suggest that dispersive Alfvén waves containing turbulent parallel electric fields form in brightening regions at the beginning of auroral substorms. Citation: Sakaguchi, K., K. Shiokawa, and E. Donovan (2009), Azimuthal structures of ray auroras at the beginning of auroral substorms, Geophys. Res. Lett., 36, L23106, doi:10.1029/2009gl041252. 1. Introduction [2] Auroras at substorm onset become active and their luminosity rapidly increases in a fraction of a second. Recent high time resolution imaging has shown that brightening region expands rapidly westward/eastward along the onset arc at a speed that starts at about 20 km/s, and the speed exponentially decreases over time [Sakaguchi et al., 2009]. During such rapid longitudinal expansion, brightening auroras feature wave-like (in longitude) formations with wavelengths of 50 200 km [Donovan et al., 2006; Liang et al., 2008; Rae et al., 2009]. It is thought that the intensification process of upward field-aligned currents in the plasma sheet and/or generation of parallel electric fields at high altitudes that accelerate auroral electrons are mapped to the brightening auroral forms at substorm onset. [3] The purpose of this study is to focus on the initial (tens of seconds) spatio-temporal evolutions of the optical structures observed in the brightening auroras, by using video-rate imaging data obtained from the ground. The likelihood of observing a substorm aurora from its initial brightening is low, and obtaining imaging data of it with a high sampling rate is even more difficult. However, in order to examine the brightening dynamics on the time scale of 1 Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan. 2 Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada. Copyright 2009 by the American Geophysical Union. 0094-8276/09/2009GL041252 seconds, time resolutions of less than a second are necessary. The characteristic spatio-temporal scales of a brightening aurora are expected to be observable in this initial interval before other onset-related processes affect the aurora. These characteristic scales may constrain theories describing the acceleration mechanisms of auroral electrons [Borovsky, 1993] and/or the possible plasma instabilities that trigger the substorm onset [Lui, 2004]. 2. Observations [4] In this study we used auroral data obtained by an allsky TV camera during the THEMIS-ground campaign at Gillam (geographic latitude 56.4 N, geographic longitude 265.4 E, dipole geomagnetic latitude 65.6 ), Canada. This all-sky TV camera observes white-light auroral images at a sampling rate of 29.97 frames/s. A detailed description of the campaign observations and instrumentation has been reported by Shiokawa et al. [1996, 2009]. [5] The events presented herein were observed at the beginning of auroral intensifications that developed into substorms within the field of view of the Gillam all-sky TV camera on January 15, 2008 and February 27, 2009. An eastwest keogram of the first event was shown by Sakaguchi et al. [2009]. Figure 1 shows the x-component magnetic field recorded at the auroral onset site of Gillam (GILL) with a magnetometer in the CARISMA array [Mann et al., 2008] and the x-component magnetic field fluctuations recorded by UCLA magnetometers [Russell et al., 2008] at mid-latitude sites at Pine Ridge (PINE) and Hot Springs (HOTS). The magnetic fluctuations are filtered for Pi2 frequency range (period: 40 150 s). Onset times defined as the moment of brightening identified at Gillam are indicated by dashed lines. Figure 1 shows that two typical magnetic-field signatures during substorms become remarkable after the optical onsets. The first is the Pi2 geomagnetic pulsations at mid-latitudes. The second is the decrease of the x-component magnetic field in the auroral zone. The maximum amplitudes of magneticfield depressions were approximately 150 nt for the January 15, 2008 event and 50 nt for the February 27, 2009 event. 3. Fourier Analysis [6] In order to investigate the optical structures and their evolution in brightening auroras at substorm onset, auroral images were analyzed using a two-dimensional Fourier transform method. Figures 2 (middle) and 3(middle) show time sequences of the auroral intensifications at intervals of 3 and 5 s, respectively. The vertical (geographic northsouth) and horizontal (geographic east-west) dimensions of the images are given in kilometers, assuming an emission L23106 1of5

Figure 1. (a, c) X-component magnetic fields observed at onset site and (b, d) fluctuations of x-component magnetic field in the Pi2 range observed at mid-latitude site on January 15, 2008 and February 27, 2009. Dashed lines indicate onset times of auroral intensification. altitude of 100 km, and magnetic zenith is indicated by the asterisk. For both events brightening auroras have wavelike structures and appear to radiate from the magnetic zenith. Such auroral structures are called ray auroras. Figures 2 (bottom) and 3 (bottom) show power spectral densities in the wavenumber domain obtained by the Fourier method using the auroral images shown in Figures 2 (middle) and 3 (middle). Hanning windows were applied to each auroral image before the Fourier transform. The vertical and horizontal dimensions indicate the number of sinusoidal waves in 256-km segments in directions of geographic latitude and longitude, respectively. [7] Strong spectral peaks are visible at the center of all wavenumber domain images in Figures 2 and 3. In addition, vertically aligned second-, third-, and higher-order spectral peaks are clearly visible in the wavenumber images except for plot a for both events (Figures 2 and 3). These vertically aligned peaks indicate small-scale longitudinal structures in the brightening auroras. The third- and higher-order spectral peaks have wavenumbers in multiples of the second-order spectral peak in each image, indicating harmonic structures. These harmonic structures correspond to new auroral rays that appear between the pre-existing auroral rays shown in Figures 2 (middle) and 3 (middle). [8] Figures 2 (top) and 3 (top) show high-resolution dynamic spectra of wavenumber (corresponding wavelengths are shown on the right axis) in the geographic longitudinal direction. One spectral line was obtained from the first and second quadrants of the wavenumber domain image in Figures 2 (bottom) and 3 (bottom), in which all latitudinal components were integrated. Figures 2 (top) and 3 (top) show the time variations of the spectral peak of wavenumber during a 60-s period. The times of Figures 2 (middle), 2 (bottom), 3 (middle), and 3 (bottom) are indicated by dashed lines in the dynamic spectra. [9] In the case of the January 15, 2008 event (Figure 2), the wavenumber spectrum has several discrete peaks at the beginning of auroral intensification at 04:55:40 04:55:48 UT. Figure 2. Brightening auroral images and wavenumber spectra at the beginning of a substorm on January 15, 2008. (top) Dynamic spectra of wavenumeber/wavelength (left/right axis) in the east-west direction at 04:55:30 04:56:30 UT, (middle) white-light auroral images every 3 s, and (bottom) power spectral densities in wavenumber domain of Figure 2 (middle) obtained by Fourier translation. A yellow asterisk in Figure 2 (middle) indicates the location of the magnetic zenith. 2of5

Figure 3. Brightening auroral images and wavenumber spectra at the beginning of a substorm on February 27, 2009. (top) Dynamic spectra of wavenumeber/wavelength (left/right axis) in the east-west direction at 05:26:30 05:27:30 UT, (middle) white-light auroral images every 3 s, and (bottom) power spectral densities in wavenumber domain of Figure 2 (middle) obtained by Fourier translation. A yellow asterisk in Figure 2 (middle) indicates the location of the magnetic zenith. The wavenumbers (wavelengths) of the spectral peaks are 3.5 (73 km) and 7 (36 km). The harmonic 36-km structure appears a few seconds after the appearance of the 73-km structure, however, these structures last only a few seconds. The wavenumbers of these spectral peaks decrease gradually. Decreasing wavenumbers indicated by the downward-sloping trend, is a general feature on this dynamic spectrum. [10] In the case of the February 27, 2009 event (Figure 3), the wavenumber spectrum also has clear discrete peaks during auroral intensification after 05:26:42 UT. The spectral peaks are located at wavenumbers (wavelengths) of 6(43 km) and 11 (23 km). Here, we also find a trend of decreasing wavenumbers similar to that seen for the first event (Figure 2). The wavelength (43 km) of the initial structures stayed more or less constant for about 10 seconds, and then subsequently increased (decreasing wavenumber). 4. Discussion and Summary [11] We identified commonly observed optical structures that are characteristic of brightening auroras for two isolated substorm onsets. Herein, the onset denotes the time of initial enhancement of auroral luminosity with fast longitudinal development before major poleward expansion. Time variations of auroral luminosities integrated over an area of 256 256 km 2 are shown in Figure 4. The line plots of both events have two kinks, The kinks indicating that the substorm auroras intensified through two phases, gradually at first and rapidly later. The characteristic spectra seen in Figures 2 and 3 were observed during the initial gradual phase of auroral intensification. [12] The time sequences of auroral images for both events show that from pre-existing weak auroral arcs, onset aurora appeared as rays emitted from the magnetic zenith. It is noteworthy that filtered auroral intensities observed by the meridian scanning photometer (http://aurora.phys.ucalgary. ca/norstar/). The data show that proton (H b ) auroras are on the equatorward of the pre-existing weak auroral arcs, indicating that the ray auroras mapped to the central plasma sheet. Since the stopping heights of precipitating electrons in the ionosphere are determined by electron energies, the vertical depth of the excitation segment of auroral particles becomes wider as the energy spectrum of electrons becomes broader [e.g., Rees, 1963], so the ray auroras are almost certainly the result of the precipitation of electrons with Figure 4. Auroral luminosities observed on (a) January 15, 2008 and (b) February 27, 2009, integrated over an area of 256 256 km 2 corresponding to image size of Figures 2 (middle) and 3 (middle). 3of5

broadband energy spectra. Recent particle observations of auroral electrons at initial brightening have demonstrated that auroras surging poleward were dominated by a burst of superthermal electrons with a field-aligned pitch angle distribution [Mende et al., 2003]. In addition, Mende et al. [2003] confirmed signatures of dispersive Alfvén waves with superthermal electrons from the pitch-angle distribution of ions and magnetometer measurements. The dispersive Alfvén waves carry the turbulent electric field parallel to the geomagnetic field and are able to accelerate electrons to superthermal energies [Chaston et al., 2000]. For the event investigated by Mende et al. [2003], a detailed structure of the aurora that was probably produced by superthermal electrons was not observed since they used global auroral images obtained by the IMAGE satellite. However, we speculate that ray auroras can be caused by precipitation of superthermal electrons since under the Alvénic accelerations electrons have multiple energy fluxes with small wavelengths transverse to magnetic fields. [13] Ground-based multi-spectral observations of auroras by Deehr and Lummerzheim [2001] have consistently shown a different aspect of the above scenario of Alfvénic electron precipitation into onset auroras. They reported enhancement of the 630-nm emission in the onset arc relative to other emissions just prior to onset, indicating an increase of electron number flux with average energies less than 100 ev. These observations of the broadband energy spectrum of auroral electrons at substorm onset agree with the expected electron energy spectrum from our observations of ray auroras at the time of initial brightening. Thus, the auroral ray structures, which can be formed under Alfvénic accelerations at the substorm onset, indicate incoming bursty Alfvén waves prior to the expansion phase of the auroral substorm. [14] The ray auroras observed at substorm onset were aligned longitudinally. The number of visible rays in the longitudinal segment became denser after a few seconds due to appearance of smaller rays in between. The results of two-dimensional Fourier transforms applied to the auroral images showed that wavenumber spectra contained several peaks after initial brightening. These wavenumbers were found to be harmonics of the fundamental mode. Dynamic spectra showing time variations of longitudinal wavelengths demonstrated that harmonic wavenumbers appeared after a few seconds. In other words, the brightening aurora immediately cascaded into smaller structures after a few seconds. Similar dynamics of dispersive Alfvén wave propagation in the ionospheric Alfvén resonator (IAR) have been observed in the simulations by Lysak and Song [2008]. In their simulation, the scaling transition of Alfvén waves occurred after a few seconds when the reflected wave from the ionosphere first interfered with the incident waves. Then, the perpendicular structure of Poynting fluxes decayed to half and half in the density cavities. The time delay of wave interference in the IAR is consistent with the time scale (a few seconds) of the appearance of harmonic wavenumbers in ray auroras for the present events. Generation of harmonic structures in the brightening auroras was repeatedly observed in dynamic spectra, especially in Figure 3. The lifetime of these fine structures ranged from a few seconds to ten seconds. This is consistent with the expected lifetime of parallel electric fields carried by dispersive Alfvén waves with no time variation [Stasiewicz et al., 2000]. [15] The findings obtained from the observations of substorm onsets presented in this paper are summarized as follows. [16] 1. Auroral brightening arises from ray auroras that have characteristic wavenumbers in the longitudinal direction; in the present cases, the wavelengths were 73 km and 43 km, respectively. [17] 2. The distance between rays becomes shorter due to the growth of new small rays; several harmonics were observed in the wavenumber domain spectra. [18] 3. Small-scale structures last less than 10 s and gradually become larger structures. [19] These structures and the time variations observed for brightening auroras suggest that dispersive Alfvén waves are present along the field line above the brightening aurora. In future studies, we plan to investigate the origin of these Alfvén waves, specifically, where and how they are launched with characteristic azimuthal structures. [20] Acknowledgments. The auroral campaign observations at Gillam were carried out by the Solar-Terrestrial Environment Laboratory, Nagoya University in collaboration with the Canadian Space Agency (CSA), and with the support from the University of Calgary NORSTAR team. We acknowledge S. Mende, C. T. Russell, and I. R. Mann for the use of GMAG data, CARISMA is operated and deployed by the University of Alberta, and funded by the CSA. This work was supported by a Grant-in-Aid for Scientific Research (19403010) and Special Funds for Education and Research (Energy Transport Processes in Geospace) from MEXT, Japan and JSPS Research Fellowships for Young Scientists. References Borovsky, J. (1993), Auroral arc thicknesses as predicted by various theories, J. Geophys. Res., 98, 6101 6138. Chaston, C. C., C. W. Carlson, R. E. Ergun, and J. P. McFadden (2000), Alfvén waves, density cavities and electron acceleration observed from the FAST spacecraft, Phys. Scr. T, 84, 64 68. Deehr, C., and D. 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Sakaguchi, K., K. Shiokawa, A. Ieda, R. Nomura, A. Nakajima, M. Greffen, E. Donovan, I. R. Mann, H. Kim, and M. Lessard (2009), Fine structures and dynamics in auroral initial brightening at substorm onsets, Ann. Geophys., 27, 623 630. Shiokawa, K., et al. (1996), Auroral observations using automatic optical instruments: Relations with multiple Pi 2 magnetic pulsations, J. Geomagn. Geoelectr., 48, 1407 1419. Shiokawa, K., et al. (2009), Longitudinal development of a substorm brightening arc, Ann. Geophys., 27, 1935 1940. Stasiewicz, K., et al. (2000), Small scale Alfvénic structure in the aurora, Space Sci. Rev., 92, 423 533. E. Donovan, Department of Physics and Astronomy, University of Calgary, Calgary, AB T2N 1N4, Canada. K. Sakaguchi and K. Shiokawa, Solar-Terrestrial Environment Laboratory, Nagoya University, Furo-cho, Chikusa, Nagoya, Aichi 464-8601, Japan. (kaori@stelab.nagoya-u.ac.jp) 5of5