2016 THEMIS SCIENCE NUGGETS

Magnetotail energy dissipation during an auroral substorm

by Evgeny V. Panov
Space Research Institute, Graz, Austria

Introduction

Violent releases of space plasma energy from the Earth’s magnetotail during substorms produce strong electric currents and bright aurora. But what modulates these currents and aurora and controls dissipation of the energy released in the ionosphere? Multispacecraft observations have revealed that a substorm onset sequence is initiated by magnetotail reconnection. Newly reconnected field lines relax their magnetic tension, generating series of earthward flow bursts (bursty bulk flows or BBFs). Rapid modification of pressure and entropy distributions in the inner magnetosphere by a slowing flow burst has been suggested to generate an enduring substorm current wedge. Using simultaneous observations in the near-Earth plasma sheet by five Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes, conjugate ground all-sky camera observations from Canada, and magnetometer arrays over North America on 23 March 2009 between 6:00 and 6:40 UT, we investigate the source of dissipation of magnetotail energy released when flow bursts stop.


Figure 1. Overview of substorm parameters during substorm expansion and recovery phases: (a) AL auroral electrojet index. (b) (∇Vx∇P)Z, where P is plasma sheet pressure and V is flux tube volume. (c) Time-integrated average meridional auroral velocity at SNAP, RANK, and SNKQ (red curve), and geographic latitudes covered by the five THEMIS footprints, as predicted by the AM-03 model (pink area). (d) Total auroral luminosity from SNAP, RANK, and SNKQ. (e) Total upward and downward SECS scaling factors and horizontal EICs (see legend) on 23 March 2009 between 6:00 and 6:40 UT.

Results

Since auroral and ionospheric current dynamics originate in the magnetosphere, observations in the plasma sheet may provide a clue about the substorm generation process. The auroral electrojet (AL) index (Figure 2a) reveals substorm expansion (between about 6:03 UT and 6:15 UT) and recovery (after 6:15 UT) phases. Magnetospheric perpendicular currents may divert to parallel because of zero divergence of total current density. The divergent (to parallel) diamagnetic perpendicular current is proportional to ∇Vx∇P, where where P is plasma sheet pressure and V is flux tube volume. The triangular configuration of THEMIS probes in the xy plane (see ionospheric footprints allowed us to calculate the Z GSM component of ∇Vx∇P in the region between the probes. Indeed, Figure 2b shows that (∇Vx∇P)Z grew rapidly during the substorm expansion phase and decreased slowly during the substorm recovery phase. This behavior agrees with development of substorm currents in the ionosphere, Figure 2e.

Figure 2. Overview of substorm parameters during substorm expansion and recovery phases: (a) AL auroral electrojet index. (b) (∇Vx∇P)Z, where P is plasma sheet pressure and V is flux tube volume. (c) Time-integrated average meridional auroral velocity at SNAP, RANK, and SNKQ (red curve), and geographic latitudes covered by the five THEMIS footprints, as predicted by the AM-03 model (pink area). (d) Total auroral luminosity from SNAP, RANK, and SNKQ. (e) Total upward and downward SECS scaling factors and horizontal EICs (see legend) on 23 March 2009 between 6:00 and 6:40 UT.

The time-integrated average meridional auroral velocity at SNAP, RANK, and SNKQ ∫VNS dt (red curve in Figure 2c) indicates that during the substorm expansion phase, auroral activity moved poleward by about 9 degrees (from about 55 to 64 deg latitude). During the substorm recovery phase, ∫VNS dt slowly returned. The ratio between the velocity of tailward expansion of plasma sheet dipolarization (~35 km/s) and the poleward propagation velocity of the auroral activity (~1.8 km/s) is close to the ratio between the velocity of inner plasma sheet restretching (~13km/s) and the equatorward propagation velocity of the auroral activity (~0.7 km/s). The geographic latitude of the five THEMIS footprints (pink area in Figure 2c) appeared to partly follow the auroral activity location. In effect, Figures 2 c,d show that a poleward fast-moving bright aurora slows down and dims after the expansion phase, recedes to lower latitudes, then fades away during late substorm recovery phase.

These observations indicate that non-collinear pressure and flux tube volume gradients in the magnetotail indeed feed the direct part of the ionospheric currents (DC). However, as seen from the ASI observations, auroral activity within the auroral bulge near the maximal ionospheric currents (near RANK) is quite complex (Figure 3d), and alternating ionospheric cur rents exist there (Figure 3e). The amplitudes of the alternating currents are significantly smaller than those of the DC currents; thus, they do not change the direction of the total current.

Figure 3. THEMIS space and ground observations on 23 March 2009 between 6:16 UT and 6:36 UT: (a) Time-integrated oscillations of radial ion velocity VR at P1 (red) and P2 (green), (b) time derivative of oscillations in VR at P1 (red) and P2 (green), (c) (∇Vx∇P)Z, where P is plasma sheet pressure and V is flux tube volume, (d) total upward Jup SECS scaling factors around Rankin Inlet, (d) total auroral luminosity from SNAP, RANK, and SNKQ, (e) meridional (red) and longitudinal (blue) auroral velocity components. Auroral speed and velocity components were averaged over field of view of RANK.

The footprints of THEMIS probes P1 and P2 at the auroral bulge location most of the time. In Figure 3a we show ∫δVR dt – time-integrated oscillations of the radial ion velocity VR, where δ indicates band pass filtering at periods between 10 and 500 s, and positive VR means earthward. The location of the oscillating magnetic flux tube with respect to its equilibrium position is indicated by ∫VR dt. When the oscillating flux tube was earthward of this position (i.e., when red and green curves in Figure a were above zero), the force acting on it (Figure 3b) was directed tailward (toward the equilibrium position). During such intervals (∇Vx∇P)Z in Figure 3c exhibited peaks. Hence, the plasma sheet field-aligned current is modulated by the oscillating magnetic flux tube, getting stronger or weaker depending on the location of the oscillating magnetic flux tube with respect to its point of equilibrium. In contrast to the steady (DC) component of (∇Vx∇P)Z from Figure 3b, the alternate (AC) component of (∇Vx∇P)Z in Figure 3c may be partly balanced by inertial currents, agreeing with thin filament simulations. Nonetheless, ground Jup (Figure 2d) still reveals significant (up to 15% of an average magnitude) oscillations. We correlated space ∫δVR dt and ground Jup observations. We found that the ionospheric current dynamics lags behind THEMIS observations by about 45s (e.g., a plot of ∫δVR dt against Jup (not shown) reveals a linear dependence, with the correlation coefficients exceeding 0.9). This time delay is about 15%; the observed oscillation period of ∫δVR dt is about 5 minutes. This represents a phase lag of about 1 radian, which is consistent with a level of an average Pedersen conductance in the ionosphere of 3 S.

The intervals of positive ∫δVR dt correspond to about 10% increases in the ionospheric field-aligned currents (Figure d3). Every peak in the field-aligned currents corresponds to enhanced auroral luminosity (Figure 3e) and velocity (Figure 3f) of the auroral arcs like the one shown in Figure at RANK (two more arc examples are given in auxiliary material for ground ASI and current observations at 6:17:30 UT and at 6:21:00 UT). The arcs were longitudinally oriented and moved equatorward at a velocity up to 200km/min (with an average value of the order of 50km/min, Figure 3f). The velocity of the auroral activity (Figure 3f) peaked when the magnetic flux tube moved earthward from its equilibrium position. Hence, magnetic flux tube oscillations during fast flow braking in the near-Earth plasma sheet modulated the ionospheric current and auroral dynamics during the substorm under study.

Figure 4. Top right: animated computer model of the magnetic field line oscillating once every six minutes, streaming electrons Earthward with every contraction. Bottom right: actual spacecraft data plot showing auroral brightness, electric current over time, and magnetic field line distance from Earth. Bottom left: ionospheric electric current map showing inflowing (red) and outflowing (blue) electrons, derived from ground magnetometer observations. Top left: all-sky camera composite video showing auroral brightening, coinciding with inward magnetic field motion and electron influx into the ionosphere every six minutes. The five colored crosses represent the THEMIS probe positions with respect to the magnetic field line’s origin in the magnetosphere. Credit: E. Panov, IWF Graz.

Conclusion

The auroral bulge appears to map in the flow braking region where the plasma sheet pressure and flux tube volume gradients are non-collinear. A fast tailward expansion of magnetotail dipolarization and then slower inner plasma sheet restretching during substorm expansion and recovery phases cause faster poleward, then slower equatorward movement of the substorm auroral bulge. Plasma sheet parcels, oscillating anharmonically around their equilibrium position and building up stronger pressure and flux tube volume gradients earthward of this position, are responsible for discrete longitudinal auroral arcs within the bulge that move equatorward at a velocity of the order of 1 km/s. The observed auroral activity appears to consume sufficient energy to dissipate the released magnetotail energy.

Reference

Panov, E.V., W. Baumjohann, R. A. Wolf, R. Nakamura, V. Angelopoulos, J. M. Weygand, M. V. Kubyshkina (2016), Magnetotail energy dissipation during an auroral substorm, Nat. Phys., doi:10.1038/nphys3879

Biographical Note

E.V. Panov is a research fellow at the Space Research Institute, Austrian Academy of Sciences, Graz, Austria. His current research activities are focused on BBFs’ oscillatory braking and ballooning waves in the near-Earth plasma sheet.

Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu