David G. Sibeck

Tail Reconnection Triggering Substorm Onset

Magnetospheric substorms explosively release solar wind energy previously stored in Earths magnetotail, encompassing the entire magnetosphere and producing spectacular auroral displays. It has been unclear whether a substorm is triggered by a disruption of the electrical current flowing across the near-Earth magnetotail, at ∼10 RE (RE: Earth radius, or 6374 kilometers), or by the process of magnetic reconnection typically seen farther out in the magnetotail, at ∼20 to 30 RE. We report on simultaneous measurements in the magnetotail at multiple distances, at the time of substorm onset. Reconnection was observed at 20 RE, at least 1.5 minutes before auroral intensification, at least 2 minutes before substorm expansion, and about 3 minutes before near-Earth current disruption. These results demonstrate that substorms are likely initiated by tail reconnection.

Geodynamo, Solar Wind, and Magnetopause 3.4 to 3.45 Billion Years Ago

Early Origin of Earths Magnetic Field Earths magnetic field protects us from stellar winds and radiation from the Sun. Understanding when, during the Earths formation, the large-scale magnetic field was established is important because it impacts understanding of the young Earths atmosphere and exosphere. By analyzing ancient silicate crystals, Tarduno et al. (p. 1238; see the Perspective by Jardine) demonstrate that the Earths magnetic field existed 3.4 to 3.45 billion years ago, pushing back the oldest record of geomagnetic field strength by 200 million years. This result combined with estimates of the conditions within the solar wind at that time implies that the size of the paleomagnetosphere was about half of that typical today, but with an auroral oval of about three times the area. The smaller magnetosphere and larger auroral oval would have promoted loss of volatiles and water from the early atmosphere. Analysis of ancient silicate crystals indicates that Earth’s magnetic field existed 3.40 to 3.45 billion years ago. Stellar wind standoff by a planetary magnetic field prevents atmospheric erosion and water loss. Although the early Earth retained its water and atmosphere, and thus evolved as a habitable planet, little is known about Earth’s magnetic field strength during that time. We report paleointensity results from single silicate crystals bearing magnetic inclusions that record a geodynamo 3.4 to 3.45 billion years ago. The measured field strength is ~50 to 70% that of the present-day field. When combined with a greater Paleoarchean solar wind pressure, the paleofield strength data suggest steady-state magnetopause standoff distances of ≤5 Earth radii, similar to values observed during recent coronal mass ejection events. The data also suggest lower-latitude aurora and increases in polar cap area, as well as heating, expansion, and volatile loss from the exosphere that would have affected long-term atmospheric composition.

The link between shocks, turbulence, and magnetic reconnection in collisionless plasmas

Global hybrid (electron fluid, kinetic ions) and fully kinetic simulations of the magnetosphere have been used to show surprising interconnection between shocks, turbulence, and magnetic reconnection. In particular, collisionless shocks with their reflected ions that can get upstream before retransmission can generate previously unforeseen phenomena in the post shocked flows: (i) formation of reconnecting current sheets and magnetic islands with sizes up to tens of ion inertial length. (ii) Generation of large scale low frequency electromagnetic waves that are compressed and amplified as they cross the shock. These “wavefronts” maintain their integrity for tens of ion cyclotron times but eventually disrupt and dissipate their energy. (iii) Rippling of the shock front, which can in turn lead to formation of fast collimated jets extending to hundreds of ion inertial lengths downstream of the shock. The jets, which have high dynamical pressure, “stir” the downstream region, creating large scale disturbances ...

On the electron diffusion region in planar, asymmetric, systems

Particle-in-cell simulations and analytical theory are employed to study the electron diffusion region in asymmetric reconnection, which is taking place in planar configurations without a guide field. The analysis presented here focuses on the nature of the local reconnection electric field and on differences from symmetric configurations. Further emphasis is on the complex structure of the electron distribution in the diffusion region, which is generated by the mixing of particles from different sources. We find that the electric field component that is directly responsible for flux transport is provided not by electron pressure-based, “quasi-viscous,” terms but by inertial terms. The quasi-viscous component is shown to be critical in that it is necessary to sustain the required overall electric field pattern in the immediate neighborhood of the reconnection X line.

Simultaneous Ground- and Space-Based Observations of the Plasmaspheric Plume and Reconnection

Persistent Plume Energy from the solar wind is carried into Earths magnetosphere and ionosphere through the reconnection of magnetic lines. When this coupling is strong, changes in the solar wind cause geomagnetic storms that perturb the magnetosphere and allow a plume of cool plasma to be released outward. As the plume rises from the ionosphere, it extends the boundary of its plasmasphere origin toward the magnetopause. Using simultaneous ground-based and satellite observations, Walsh et al. (p. 1122; see the Perspective by Borovsky) show that this plume can persist for hours, extending all the way from the ionosphere to the magnetopause. Ground-based and satellite observations show that a plume extends all the way to the magnetopause and can persist for hours. [Also see Perspective by Borovsky] Magnetic reconnection is the primary process through which energy couples from the solar wind into Earth’s magnetosphere and ionosphere. Conditions both in the incident solar wind and in the magnetosphere are important in determining the efficiency of this energy transfer. In particular, the cold, dense plasmaspheric plume can substantially impact the coupling in the dayside reconnection region. Using ground-based total electron content (TEC) maps and measurements from the THEMIS spacecraft, we investigated simultaneous ionosphere and magnetosphere observations of the plasmaspheric plume and its involvement in an unsteady magnetic reconnection process. The observations show the full circulation pattern of the plasmaspheric plume and validate the connection between signatures of variability in the dense plume and reconnection at the magnetopause as measured in situ and through TEC measurements in the ionosphere.

Magnetopause expansions for quasi‐radial interplanetary magnetic field: THEMIS and Geotail observations

We report THEMIS and Geotail observations of prolonged magnetopause (MP) expansions during long-lasting intervals of quasi-radial interplanetary magnetic field (IMF) and nearly constant solar wind dynamic pressure. The expansions were global: the magnetopause was located more than 3 RE and ~7 RE outside its nominal dayside and magnetotail locations, respectively. The expanded states persisted several hours, just as long as the quasi-radial IMF conditions, indicating steady-state situations. For an observed solar wind pressure of ~1.1-1.3 nPa, the new equilibrium subsolar MP position lay at ~14.5 RE, far beyond its expected location. The equilibrium position was affected by geomagnetic activity. The magnetopause expansions result from significant decreases in the total pressure of the high-beta magnetosheath, which we term the low-pressure magnetosheath (LPM) mode. A prominent LPM mode was observed for upstream conditions characterized by IMF cone angles less than 20 ~ 25 grad, high Mach numbers and proton plasma beta<1.3. The minimum value for the total pressure observed by THEMIS in the magnetosheath adjacent to the magnetopause was 0.16 nPa and the fraction of the solar wind pressure applied to the magnetopause was therefore 0.2, extremely small. The equilibrium location of the magnetopause was modulated by a nearly continuous wavy motion over a wide range of time and space scales.The pressure balance at the magnetopause is formed by magnetic field and plasma in the magnetosheath, on one side, and inside the magnetosphere, on the other side. In the approach of dipole earths magnetic field configuration and gas-dynamics solar wind flowing around the magnetosphere, the pressure balance predicts that the magnetopause distance R depends on solar wind dynamic pressure Pd as a power low R ~ Pd^alpha, where the exponent alpha=-1/6. In the real magnetosphere the magnetic filed is contributed by additional sources: Chapman-Ferraro current system, field-aligned currents, tail current, and storm-time ring current. Net contribution of those sources depends on particular magnetospheric region and varies with solar wind conditions and geomagnetic activity. As a result, the parameters of pressure balance, including power index alpha, depend on both the local position at the magnetopause and geomagnetic activity. In addition, the pressure balance can be affected by a non-linear transfer of the solar wind energy to the magnetosheath, especially for quasi-radial regime of the subsolar bow shock formation proper for the interplanetary magnetic field vector aligned with the solar wind plasma flow.

THEMIS observations of extreme magnetopause motion caused by a hot flow anomaly

[1] On 30 October 2007, the five THEMIS spacecraft observed the cause and consequence of extreme motion of the dawn flank magnetopause, displacing the magnetopause outward by at least 4.8 RE in 59 s, with flow speeds in the direction normal to the model magnetopause reaching 800 km/s. While the THEMIS A, C, D, and E observations allowed the determination of the velocity, size, and shape of a large bulge moving tailward along the magnetopause at a speed of 355 km/s, THEMIS B observed the signatures of a hot flow anomaly (HFA) upstream of the bow shock at the same time, indicating that the pressure perturbation generated by the HFA may be the source of the fast compression and expansion of the magnetosphere. The transient deformation of the magnetopause generated field-aligned currents and created traveling convection vortices which were detected by ground magnetometers. This event demonstrates that kinetic (non-MHD) effects at the bow shock can have global consequences on the magnetosphere.

Determination of the substorm initiation region from a major conjunction interval of THEMIS satellites

[1] We investigate in detail the time history of substorm disturbances in the magnetotail observed during a major tail conjunction of Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites on 29 January 2008, 0700―0900 UT. During this interval, all THEMIS satellites were closely aligned along the tail axis near midnight and were bracketed in local time by GOES 11 and 12. The radial distance covered ranges from the geosynchronous altitude to ∼30 R E in the tail. This interval consists of three activations detected by the THEMIS satellites with good ground all-sky-camera observations of auroral activity. The first activation is a small substorm with spatially limited disturbance in the tail. The onset arc was equatorward of an undisturbed arc. The second activation is a moderate size substorm with the onset arc also being equatorward of an undisturbed arc. The third activation is an intensification of the substorm with its onset indicated by the second activation. The active auroral arc for this intensification was near the poleward boundary of the auroral oval. Analysis of these observations indicates that the first activation is a small substorm initiated in the near-Earth plasma sheet and does not involve magnetic reconnection of open magnetic field lines. Magnetic reconnection on closed field lines can be ruled out for this substorm because it cannot generate the observed high-speed plasma flow. The second and third activations are part of a moderate size substorm initiated also in the near-Earth plasma sheet, with a subsequent substorm intensification involving activity initiated tailward of ∼30 R E . Overall, the time history of substorm activity for these two substorms is consistent with the near-Earth initiation model.

Time History of Events and Macroscale Interactions during Substorms observations of a series of hot flow anomaly events

[1] A series of seven hot flow anomaly (HFA) events has been observed by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) C spacecraft just upstream from the subsolar bow shock from 0100 to 1300 UT on 19 August 2008. Both young (no shocks at edges, two distinct ion populations) and mature (strong shocks at edges, a single hot ion population) HFAs have been observed. Further upstream, THEMIS B observed four proto‐HFAs (density and magnetic field strength depletions, plasma heating but no flow deflections) which later developed into HFAs observed by THEMIS C. We present evidence indicating that electromagnetic right‐hand resonant ion beam instabilities heat ions inside HFAs. Observations of small‐amplitude perturbations (DB/B < 50%) consistent with the resonant ion beam instability in a proto‐HFA, 30 s electromagnetic waves (DB/B ∼ 1) in a young HFA, and magnetic pulsations in a mature HFA (DB/B ∼ 4) indicate that they are at early, middle, and late (nonlinear) stages of the electromagnetic right‐hand resonant ion beam instabilities. Both young and mature HFAs are associated with strong electromagnetic waves near the lower hybrid frequency (0.1–1 Hz). The lower hybrid waves are the likely source of the electron heating inside HFAs. THEMIS B observations of four proto‐HFAs which later developed into HFAs observed by THEMIS C indicate that these four HFAs might extend beyond 14 RE upstream from the bow shock, while the other three HFAs may extend between 5 and 14 RE upstream from the bow shock. We present an example of an HFA that lies displaced toward the side of the tangential discontinuity with a quasi‐parallel bow shock configuration rather than lying centered on the driving interplanetary magnetic field discontinuity.

Cluster four spacecraft measurements of small traveling compression regions in the near-tail

Cluster four spacecraft measurements of small travelling compression regions in the near-tail