JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, A00C27, doi:10.1029/2008JA013553, 2009 Click Here for Full Article Surface waves and field line resonances: A THEMIS case study Oleksiy Agapitov,1 Karl-Heinz Glassmeier,2,3 Ferdinand Plaschke,2 Hans-Ulrich Auster,2 Dragos¸ Constantinescu,2 Vassilis Angelopoulos,4 Werner Magnes,5 Rumi Nakamura,5 Charles W. Carlson,6 Sabine Frey,6 and James P. McFadden6 Received 30 June 2008; revised 29 July 2009; accepted 6 August 2009; published 18 December 2009. [1] Using magnetic field and plasma observations from four of the five Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft, a surface wave at the dawn flank of the magnetopause was identified on 25 April 2007. The wave had an amplitude of about 1 RE and propagated tailward with a velocity of about 190–240 km/s. Its azimuthal wavelength was in the range of 9–11 RE. Magnetosheath velocity values support the hypothesis that this surface wave was generated by the Kelvin-Helmholtz instability. Simultaneously, an ultralow-frequency (ULF) pulsation event was detected by the fifth THEMIS spacecraft deeper in the magnetosphere, at a distance of about 5–7 RE from the magnetopause. This ULF event showed all the signatures predicted for waves generated by the classical field line resonance process. Frequency and phases of the detected ULF oscillations were found to be in close agreement with the magnetopause surface periodic disturbances. We conclude that the observed ULF wave event was most likely directly generated by the magnetopause surface wave and thus represents one of the few known manifestations of the classical field line resonance process in space directly observed, a conclusion made possible due to the special configuration of the THEMIS mission and its five spacecraft. Citation: Agapitov, O., et al. (2009), Surface waves and field line resonances: A THEMIS case study, J. Geophys. Res., 114, A00C27, doi:10.1029/2008JA013553. 1. Introduction decaying toward the inner magnetosphere, and its coupling to a localized Alfve´nic perturbation at a point where the local [2] Ultralow-frequency (ULF) pulsations in the period field line shell eigenfrequency equals the frequency of the range from 1 s to more then 600 s are one of the modes via driving surface wave. The typical characteristics of a global which the magnetosphere reacts to solar wind dynamics and FLR are predominantly toroidal magnetic field oscillations, associated instabilities of the magnetospheric system. One of localized in radial direction within the magnetosphere. the paradigms of ULF pulsation theory is the resonant Across the position of maximum field amplitude, the phase coupling of a compressional surface wave with toroidal of the toroidal component changes by 180° and the direction oscillations somewhere deeper in the magnetosphere. This of polarization is reversed [e.g., Nishida, 1978]. The spatially field line resonance (FLR) mechanism was first suggested by decaying surface mode is thought to be generated by the Tamao [1965] and later used by Southwood [1974] and Chen Kelvin-Helmholtz instability (KHI) of the dawn and dusk and Hasegawa [1974] to interpret observational results by magnetopause [e.g., Southwood, 1968; Pu and Kivelson, Samson et al. [1971]. A basic ingredient of the process is the 1983; Fujita et al., 1996], conditions for which are best when resonant interaction of a compressional mode, spatially passing through a high-speed solar wind stream [e.g., Seon et al., 1995; Engebretson et al., 1998]. A ULF pulsation 1Astronomy and Space Physics Department, National Taras generated by the described resonant coupling between a Shevchenko University of Kiev, Kiev, Ukraine. compressional surface wave and a toroidally polarized and 2Institut fu¨r Geophysik und extraterrestrische Physik, Technische spatially confined Alfvenic perturbation we define here as a Universita¨t Braunschweig, Braunschweig, Germany. classical field line resonance. 3Max-Planck-Institut fu¨r Sonnensystemforschung, Katlenburg-Lindau, Germany. [3] At the ground and in the ionosphere field line reso- 4Department of Earth and Space Sciences, University of California, nance associated oscillations are usually identified as lati- Los Angeles, California, USA. tudinally localized oscillations of the H component of the 5Space Research Institute, Austrian Academy of Sciences, Graz, magnetic field [e.g., Samson et al., 1971; Green, 1982; Austria. Glassmeier, 1988; Mathie et al., 1999a; Rae et al., 2005] or 6Space Sciences Laboratory, University of California, Berkeley, California, USA. in the NS component of the ionospheric electric field [e.g., Walker et al., 1979; Fenrich et al., 1995], associated with a Copyright 2009 by the American Geophysical Union. 180° phase shift across this resonantly oscillating field line 0148-0227/09/2008JA013553$09.00 A00C27 1of10 A00C27 AGAPITOV ET AL.: FIELD LINE RESONANCE A00C27 shell. In space, however, direct observations of field line bations, are discussed as an alternative to the KHI source resonances are very sparse. As direct observations we define mechanism [e.g., Kivelson and Southwood, 1985; Sinha and spatially and temporally resolved observations of wave- Rajaram, 2003]. Such perturbations can generate natural fields exhibiting typical spatial and temporal characteristics modes of the magnetospheric resonator with the polariza- of field line resonances. tion depending on the propagation direction and magnetic [4] Further possibilities to drive discrete frequency per- field disturbance vector [Agapitov and Cheremnykh, 2008]. turbations with frequencies in the ULF range are cavity and In their paper, these authors analyze periodic pulsations waveguide modes [e.g., Mann et al., 1999; Mills et al., associated with sudden impulse in spacecraft magnetic field 1999], sudden impulses of the solar wind [e.g., Saito and measurements and in ground based measurements at the Matsushita, 1967; Kivelson and Southwood, 1985], and magnetic conjugated point. Also, ULF pulsations with quasiperiodic changes of the solar wind dynamic pressure different frequencies were observed simultaneously on [e.g., Kepko and Spence, 2003]. different magnetic latitudes after a sudden impulse [e.g., [5] The waveguide theory predicts that the magneto- Glassmeier et al., 1984]. The existence of such spectral sphere can act as a cavity which traps discrete frequency maxima affirms the magnetospheric property of selecting compressional mode energy between the magnetosphere particular spectral peaks with global modes coupling to boundary and the reflection region inside the magneto- corresponding local field line shell oscillations. Qualitative sphere [e.g., Mann et al., 1999]. Most of the events confirmation of the experimental results were obtained analyzed by [e.g., Mathie et al., 1999b] using ground based extensive numerical modeling in a dipole magnetosphere observations can be explained in terms of such waveguide [e.g., Lysak and Lee, 1992]. modes and agree closely with the theory proposed by [9] Nevertheless, the many observations of FLRs at the Samson et al. [1992] and Mann et al. [1999], having ground and in the ionosphere on the one hand and the discrete frequencies of oscillation. CLUSTER measure- sparsity of space observations of classical field line reso- ments [Mann et al., 2002] furthermore support the hypoth- nances on the other hand is worthwhile to be noted. Here, esis that, during intervals of fast solar wind speed, the KHI we present observations from the five Time History of can excite magnetospheric waveguide modes which bathe Events and Macroscale Interactions during Substorms the flank magnetosphere with discrete frequency ULF wave (THEMIS) spacecraft [Angelopoulos, 2008], traversing the power and drive large amplitude resonant ULF pulsations. dusk magnetosphere on a highly elliptic orbit with the [6] Probably the best documented example of a KHI- spacecraft very often aligned in almost radial direction driven resonant ULF pulsation in space is the long-lasting during the coast phase of the mission. Such radial conjuga- event of 25 November 2001, reported about and analyzed in tions are ideal for studying surface waves and associated detail by Rae et al. [2005]. Multipoint observations from the field line resonances. We consider global ULF pulsations in CLUSTER and POLAR spacecraft allow one to demon- a study similar to Mann et al. [2002] and Rae et al. [2005] strate that KHI-driven surface waves drive resonant field with detail analysis of the magnetopause surface perturba- line oscillations. The data presented in this study provide tions. The simultaneous analysis of magnetopause pertur- evidence for an observational link between wave activity at bation and ULF waves inside the magnetosphere gave an the magnetopause, in the magnetosphere, and in the iono- opportunity to explain the properties of observed ULF sphere. The 25 November 2001 event certainly qualifies to waves and distinguish the different possible generation directly demonstrate the functionality of the classical field mechanisms. line resonance process. [7] Earlier studies by, e.g., Singer [1982] also reported 2. Observations radially localized, but poloidally polarized ULF waves using ISEE1 and ISEE2 observations and that allowed one to [10] THEMIS consists of five identically instrumented estimate the resonance region width to be about 0.7 RE. spacecraft, launched on 17 February 2007. The main goal Cramm et al. [2000] also reported such localized poloidal of this mission