Distinct Scaling Regimes of Energy Release Dynamics in the Nighttime Magnetosphere

arXiv:0807.0631v1 [physics.space-ph] 3 Jul 2008 GEOPHYSICAL RESEARCH LETTERS, VOL. ???, XXXX, DOI:10.1029/, Distinct Scaling Regimes of Energy Release Dynamics in the 1 Nighttime Magnetosphere 2 V. M. Uritsky1, E. Donovan1, A. J. Klimas2, and E. Spanswick1 Based on a spatiotemporal analysis of POLAR UVI 3 images, we show that the auroral emission events that 4 initiate equatorward of the isotropic boundary (IB) ob- 5 tained from a time-dependent empirical model, have sys- 6 tematically steeper power-law slopes of energy, power, 7 area and lifetime probability distributions compared to 8 the events that initiate poleward of the IB. The low- 9 latitude group of events contains a distinct subpopulation 10 of substorm-scale disturbances violating the power-law 11 behavior, while the high latitude group is described by 12 nearly perfect power-law statistics over the entire range 13 of scales studied. The results obtained indicate that the 14 inner and outer portions of the plasma sheet are charac- 15 terized by substantially different scaling regimes of bursty 16 energy dissipation suggestive of different physics in these 17 regions. 18 1. Introduction The activity of the nighttime auroral oval represents a 19 wide range of dynamical processes in the magnetotail, 20 including substorm expansion onsets, pseudobreakups, 21 steady magnetospheric convection events with or with- 22 out substorms, bursty bulk flows, and sawtooth events 23 (see e.g., Zesta et al. [2000]; Lui [2001]; Frey et al. [2004]; 24 Henderson et al. [2006]). Despite the diversity of physical 25 conditions associated with each particular type of auroral 26 activity, their net energy output can be described by a 27 set of apparently universal power-laws (Lui et al. [2000]; 28 Lui [2002]; Uritsky et al. [2003, 2002, 2006]) signaling 29 the existence of a organizing dynamical principle arrang- 30 ing intermittent magnetospheric dissipation across vast 31 ranges of spatial and temporal scales. 32 Power-law intermittency of energy dissipation has at- 33 tracted significant attention in modern statistical me- 34 chanics (see Dhar [2006] and refs therein) and is often 35 considered a hallmark of turbulent and/or critical phe- 36 nomena with no characteristic scales other than those 37 dictated by the finite size of the system (Sreenivasan 38 et al. [2004]; Lubeck [2004]). Examples of such behav- 39 ior in geo- and space sciences include fully developed 40 turbulence in hydrodynamic or magnetized flows (Lazar- 41 ian [2006]), Guttenberg-Richter statistics of earthquake 42 magnitudes (Turcotte [1989]), scale-invariance in the so- 43 lar corona (Charbonneau et al. [2001]). In this context, 44 the auroral activity provides one of the most impressive 45 1Physics and Astronomy Department, University of Calgary, Calgary, AB, Canada 2UMBC at NASA / Goddard Space Flight Center, Greenbelt, Maryland, USA Copyright 2018 by the American Geophysical Union. 0094-8276/18/$5.00 1 X - 2 URITSKY ET AL.: SCALING REGIMES IN EARTH’S MAGNETOSPHERE examples of scale-free behavior in nature. The energy 46 distribution of electron emission regions exhibits a power- 47 law shape over a range of 6 orders of magnitude (Uritsky 48 et al. [2002]) which can be extended to up to 11 orders 49 by combining the satellite data with ground-based TV 50 observations (Kozelov et al. [2004]). 51 The auroral emission statistics reported so far repre- 52 sent global long-term properties of nighttime magneto- 53 spheric disturbances. The fact that these properties are 54 dominated by power-law scaling does not eliminate the 55 possibility of a more complex behavior on the level of 56 specific plasma sheet structures described by drastically 57 different physical conditions and geometry. In this study, 58 we are taking a step toward a better understanding of 59 the relationship between the scale-free auroral precipita- 60 tion statistics and the underlying central plasma sheet 61 (CPS) morphology. We suggest that the inner and the 62 outer CPS regions are responsible for three distinct scal- 63 ing modes of the auroral precipitation dynamics, and pro- 64 vide a possible physical interpretation for the observed 65 differences. 66 2. Data and Algorithm We have studied time series of digital images of night- 67 time northern aurora (55-80 MLat, 2000 - 0400 MLT) 68 taken by the Ultraviolet Imager (UVI) onboard the PO- 69 LAR spacecraft in the 165.5 to 174.5 nm portion of the 70 Lyman-Birge-Hopfield spectral band (integration time 71 36.5 s, time resolution 184 s). The data analyzed in- 72 clude 16,000 images covering two observation periods: 73 01/01/1997 - 02/28/1997 and 01/01/1998 - 02/28/1998. 74 Our analysis was based on spatiotemporal tracking of au- 75 roral emission events (Uritsky et al. [2002, 2003]). The 76 UV luminosity w(t, r) was studied as a function of time t 77 and position r on the image plane. First, active auroral 78 regions were identified by applying an activity threshold 79 wa representing

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