Annales Geophysicae www.ann-geophys.net 0992-7689 1432-0576 27 5 2009 10.5194/angeo-27-2191-2009 http://www.ann-geophys.net/27/2191/2009/ http://www.ann-geophys.net/27/2191/2009/angeo-27-2191-2009.html http://www.ann-geophys.net/27/2191/2009/angeo-27-2191-2009.pdf 2191 2204 2009-05-13 In-situ observation of ULF wave activities associated with substorm expansion phase onset and current disruption J. Liang jliang@phys.ucalgary.ca W. W. Liu E. F. Donovan E. Spanswick Space Science Branch, Canadian Space Agency, St-Hubert, Quebec, Canada Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada In this paper we present two substorm events with coordinated ground-based and in-situ THEMIS observations, and focus our interest on the wave activities in Pi1 and Pi2 bands from minutes before the substorm expansion phase (EP) onset to minutes after the local current disruption (CD). We find that Pi2 band (40–100 s) wave appears 1–2 min before the substorm onset and last over the entire EP interval, while higher-frequency wave within Pi1 band (10–30 s) emerges within few tens of seconds after the EP onset, intensifies during the local CD, and fades afterwards. The pre-onset Pi2 waves are attributed to a ballooning mode which acts as the seed perturbation to the substorm EP onset process. The azimuthal wavenumber estimated from the Doppler shift nature of the ballooning mode is consistent with the longitudinal "wavelength" inferred from the onset auroral structures. The Pi1 waves appearing within few tens of seconds after the EP onset are interpreted as supportive of a two-fluid instability mode of thin current sheet investigated in an accompanying paper (Liu and Liang, 2009). During the local CD, broadband wave activities from Pi2 band to well above the ion gyrofrequency are observed, suggesting the coexistence of various plasma instabilities featuring different frequency ranges. Arnoldy, R., Posch, J., Engebretson, M., Fukunishi, H., and Singer, H.: Pi1 magnetic pulsations in space and at high latitudes on the ground, J. Geophys. Res., 103(A10), 23581–23591, 1998. Asano, Y., Mukai, T., Hoshino, M., Saito, Y., Hayakawa, H., and Nagai, T.: Evolution of the thin current sheet in a substorm observed by Geotail, J. Geophys. Res., 108(A5), 1189, doi:10.1029/2002JA009785, 2003. Auster, H. U., Glassmeier, K. H., Magnes, W., et al.: The THEMIS fluxgate magnetometer, Space Sci. Rev., 141, 235–264, doi:10.1007/s11214-008-9365-9, 2008. Cheng, C. Z.: Physics of substorm growth phase, onset, and dipolarization, Space Sci. Rev., 113, 207–270, 2004. Donovan, E., Liu, W., Liang, J., Spanswick, E., et al.: Simultaneous THEMIS in situ and auroral observations of a small substorm, Geophys. Res. Lett., 35, L17S18, doi:10.1029/2008GL033794, 2008. Erickson, G. M., Maynard, N. C., Burke, W. J., Wilson, G. R., and Heinemann, M. A.: Electromagnetics of substorm onsets in the near-geosynchronous plasma sheet, J. Geophys. Res., 105, 25265–25290, 2000. Holter, Ø., Altman, C., Roux, A., et al.: Characterization of low frequency oscillations at substorm breakup, J. Geophys. Res., 100(A10), 19109–19119, 1995. Lessard, M. R., Lund, E. J., Jones, S. L., Arnoldy, R. L., Posch, J. L., Engebretson, M. J., and Hayashi, K.: Nature of Pi1B pulsations as inferred from ground and satellite observations, Geophys. Res. Lett., 33, L14108, doi:10.1029/2006GL026411, 2006. Liang, J., Donovan, E. F., Liu, W. W., Jackel, B., Syrjäsuo, M., Mende, S. B., Frey, H. U., Angelopoulos, V., and Connors, M.: Intensification of preexisting auroral arc at substorm expansion phase onset: Wave-like disruption during the first tens of seconds, Geophys. Res. Lett., 35, L17S19, doi:10.1029/2008GL033666, 2008. Liang, J., Liu, W. W., and Donovan, E. F.: Ion temperature drop and quasi-electrostatic electric field at the current sheet boundary minutes prior to the local current disruption, J. Geophys. Res., in review, 2009. Liou, K., Meng, C.-I., Lui, A. T. Y., Newell, P. T., and Wing, S.: Magnetic dipolarization with substorm expansion onset, J. Geophys. Res., 107(A7), 1131, doi:10.1029/2001JA000179, 2002. Liu, W. W.: Physics of the explosive growth phase: Ballooning instability revisited, J. Geophys. Res., 102, 4927–4931, 1997. Liu, W. W., Liang, J., and Donovan, E. F.: Interaction between kinetic ballooning mode and thin current sheet: Quasielectrostatic field, local onset, and global characteristics, Geophys. Res. Lett., 35, 35, L20107, doi:10.1029/2008GL035757, 2008. Liu, W. W. and Liang, J.: Disruption of magnetospheric current sheet by quasi-electrostatic field, Ann. Geophys., 27, 1941–1950, 2009. Lui, A. T. Y., Chang, C.-L., Mankofsky, A., Wong, H.-K., and Winske, D.: A Cross-Field Current Instability for Substorm Expansions, J. Geophys. Res., 96(A7), 11389–11401, 1991. Lui, A. T. Y.: A synthesis of magnetospheric substorm models, J. Geophys. Res. 96, 1849–1856, 1991. Lui, A. T. Y. and Najmi, A.-H.: Time-frequency decomposition of signals in a current disruption event, Geophys. Res. Lett., 24, 3157–3160, 1997. Lui, A. T. Y.: Multiscale phenomena in the near-Earth magnetosphere, J. Atmos. Solar-Terr. Phys., 64, 125–143, 2002. Lui, A. T. Y., Yoon, P. H., Mok, C., and Ryu, C.-M.: Inverse cascade feature in current disruption, J. Geophys. Res. 113, A00C06, doi:10.1029/2008JA013521, 2008. McFadden, J. P., Carlson, C. W., Larson, D., et al.: The THEMIS ESA plasma instrument and in-flight calibration, Space Sci. Rev., 141, 277–302, doi:10.1107/s11214-008-9440-2, 2008. Mende, S. B., Harris, S. E., Frey, H. U., et al.: The THEMIS array of ground-based observatories for the study of auroral substorms, Space Sci. Rev., 141, 357–387, doi:10.1007/s11214-008-9380, 2008. Murphree, J., Johnson, M., Cogger, L., and Hearn, D.: Freja UV Imager observations of spatially periodic suroral distortions, Geophys. Res. Lett., 21(17), 1887–1890, 1994. Ohtani, S.: On the tailward expansion of current disruption during substorms, Adv. Space. Res., 13, 265–268, 1993. Pu, Z. Y., Korth, A., Chen, Z. X., et al.: MHD drift ballooning instability near the inner edge of the near-Earth plasma sheet and its application to substorm onset, J. Geophys. Res., 102(A7), 14397–14406, 1997. Roux, A., Le Contel, O., Coillot, C., et al.: The search coil magnetometer for THEMIS, Space Sci. Rev., 141, 265–275, doi:10.1007/s11214-008-9455-8, 2008. Saito, M. H., Miyashita, Y., Fujimoto, M., Shinohara, I., Saito, Y., and Mukai, T.: Modes and characteristics of low-frequency MHD waves in the near-Earth magnetotail prior to dipolarization: Fitting method, J. Geophys. Res., 113, A06201, doi:10.1029/2007JA01277, 2008a. Saito, M. H., Miyashita, Y., Fujimoto, M., Shinohara, I., Saito, Y., Liou, K., and Mukai, T.: Ballooning mode waves prior to substorm-associated dipolarizations: Geotail observations, Geophys. Res. Lett., 35, L07103, doi:10.1029/2008GL033269, 2008b. Savitzky, A. and Golay, M. J. E.: Smoothing and Differentiation of Data by Simplified Least Squares Procedures, Analyt. Chem., 36, 1627–1639, doi:10.1021/ac60214a047, 1964. Shiokawa, K., Shinohara, I., Mukai, T., Hayakawa, H., and Cheng, C. Z.: Magnetic field fluctuations during substorm-associated dipolarizations in the nightside plasma sheet around $X=-$10 $R_E$, J. Geophys. Res., 110, A05212, doi:10.1029/2004JA010378, 2005. Sigsbee, K., Cattell, C. A., Fairfield, D., Tsuruda, K., and Kokubun, S.: Geotail observations of low-frequency waves and high-speed earthward flows during substorm onsets in the near magnetotail from 10 to 13 $R_E$, J. Geophys. Res., 107(A7), 1141, doi:10.1029/2001JA000166, 2002. Trondsen, T. and Cogger, L.: A survey of small-scale spatially periodic distortions of auroral forms, J. Geophys. Res., 103(A5), 9405–9415, 1998. Volwerk, M.: Multi-satellite observations of ULF waves, Magnetospheric ULF waves: synthesis and new directions, p. 109, edited by: Takahashi, K., Chi, P. J., Denton, R. E., and Lysak, R. L., AGU Monograph, 169, 2006. Volwerk, M., Nakamura, R., Baumjohann, W., Uozumi, T., Yumoto, K., Klecker, B., Balogh, A., and Reme, H.: Pi2 Observations by Cluster, ICS-9, Graz, 2008. Voronkov, I., Donovan, E. F., Jackel, B. J., and Samson, J. C.: Large-scale vortex dynamics in the evening and midnight auroral zone: Observations and simulations, J. Geophys. Res., 105(A8), 18505–18518, 2000. Voronkov, I., Rankin, R., Frycz, P., Tikhonchuk, V., and Samson, J.: Coupling of shear flow and pressure gradient instabilities, J. Geophys. Res., 102(A5), 9639–9650, 1997.