Space Physics Seminar - winter-2016

Jan. 15, 2016
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• Xiangning Chu - UCLA

Is the pressure gradient a driver of the substorm current wedge?

Substorm current wedge (SCW) formation is believed to be related to the flow braking and diversion process. In this work, good temporal and spatial correlations are found between earthward flows during five THEMIS tail seasons and substorm onsets identified using the midlatitude positive bay index. Flow occurrence is found to peak at substorm onset. More than half the flows observed within one hour of substorm onsets occur within ten minutes of onsets. In addition, Most of these flows (85%) are found inside an SCW between its upward and downward field-aligned currents (FACs). It has been suggested that these FACs are generated either by flow vortices, pressure gradient, or both. It is shown that the flow speed (related to the flow vortices) decays quickly within several minutes. On the other hand, the equatorial thermal pressure (related to the pressure gradient) increases and persists for about an hour, and has a trend similar to that for the westward electrojet and FACs of the SCW. Therefore, the SCW is likely sustained by the pressure gradient rather than short-lived flow vortices. The pressure gradient, calculated when three THEMIS probes were distributed in a triangular configuration in the equatorial plane, was found to be well organized relative to the central meridian (CM) of the SCW. The component ?P_x increases for all substorms; while ?P_y increases in magnitude and points toward the center of the current wedge. The non-alignment of ?P and ?V should generate an SCW with a quadrupole FAC pattern, similar to that seen in global MHD and RCM-E simulations. In these simulations the inner current loop is weaker than the outer loop so that the magnetic effect at geosynchronous orbit and on the ground is that of the outer loop diminished in strength by the inner loop, which resembles a classic SCW.

Jan. 22, 2016
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• San Lu - UCLA

Three-dimensional global hybrid simulation of the Earth’s magnetosphere

Three-dimension (3-D) global hybrid simulation model is developed and performed to investigate the whole Earth’s magnetosphere for the first time, with physics from the ion kinetic to the global Alfvenic convection scales. Under the pure southward interplanetary magnetic field (IMF), it is found that the dayside reconnection leads to the penetration of the dawn-dusk electric field through the magnetopause and thus a thinning of the plasma sheet, followed by the multiple X-line reconnection in the magnetotail. The following results are going to be presented: (1) The magnetotail reconnection layer is turbulent with a nonuniform structure and unsteady evolution, and exhibits properties of typical collisionless fast reconnection with the Hall effect. (2) Hall electric fields in the thin current layer cause a systematic dawnward ion drift motion and thus a dawn-dusk asymmetry of the plasma sheet with a higher (lower) density on the dawnside (duskside). Correspondingly, more reconnection as well as the relevant phenomena occurs on the duskside. (3) A number of small-scale flux ropes (FRs) are generated through the multiple X-line reconnection. The nonuniform and unsteady multiple X-line reconnection with particle kinetic effects leads to various kinds of FR evolution: earthward/tailward propagation, coalescence, merging, and tilt. (4) The earthward propagating FRs become highly asymmetric due to the imbalance of the reconnection rates between the multiple X-lines. The earthward propagating asymmetric FRs can fully reproduce the observational features of the dipolarization fronts (DFs). Therefore, the earthward propagating FRs can be used to explain the observed DFs in the magnetotail. (5) A shear flow type instability is found on the duskside flank of the ring current plasma, whereas a kinetic ballooning instability appears on the dawnside. (6) Ion velocity distributions and energy spectra reveal multiple beams/populations and energization at various regions in the magnetotail.

Jan. 26, 2016
4 p.m. - 5 p.m.
Geology 6704

Presented By:

• Heli Hietala - UCLA

Ion Heating in Mid-Magnetotail Reconnection Jets

Magnetic reconnection redistributes energy by releasing magnetic energy into particle energies-high speed bulk flows, heating, and particle acceleration. With near-Earth in situ observations, we have access to different parameter regimes: The magnetotail has typically a very large magnetic shear and symmetric boundary conditions. Reconnection at the magnetopause, in contrast, usually takes place under asymmetric boundary conditions and a variety of shear angles. Finally, reconnecting current sheets in the solar wind are typically large scale and not affected by nearby obstacles, and observations are typically made far downstream from the X-line. As such, magnetotail reconnection, especially at lunar distances where the effect of the Earth's dipole is small, should be closest to simple models.

Ion heating has recently been studied systematically in solar wind and magnetopause reconnection, but not in the magnetotail. The energetics of magnetotail reconnection jets are particularly interesting as the available magnetic energy per particle (B_in^2/mu0*n_in = m_iVA,in^2) is typically orders of magnitude higher and the inflow plasma beta much lower than in the solar wind and at the magnetopause.

We use ARTEMIS observations of fast jets at the lunar distance to study ion heating in reconnection. In particular, we address (i) the ion temperature increase (ii) ion temperature anisotropy and firehose instability, and (iii) the underlying ion dynamics. We examine the spatial structure of the ion temperature across the exhaust, and compare with particle-in-cell simulations. We find that the temperature parallel to the magnetic field dominates near the edges of the jet, while the very center of the exhaust has Tperp > Tpara, indicating Speiser-like ion motion.

Jan. 29, 2016
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• Anton Artemyev - UCLA

Configuration of the Magnetotail Current Sheet

My presentation is devoted to the magnetic field configuration and distributions of plasma thermal pressure in the magentotail current sheet. We use THEMIS spacecraft observations of the magnetotail current sheet within the downtail region 35RE>r>9R. We collect statistics of current sheets where the current density amplitude jy (in the GSM system) can be estimated owing to measurements of the flapping velocity of the current sheet vertical motion. The current sheet thickness LCS is restored using estimates of the current density amplitude and magnetic field measurements. Observed current sheets are very stretched with the typical scale of the inhomogeneity along x-axis much larger than LCS. The direct comparison of the tension force ~jyBz/c (with Bz is the magnetic field component) and the radial gradient of the plasma pressure demonstrates that this gradient cannot balance the magnetotail current sheet. We discuss other possible mechanisms responsible for the balancing of the magnetotail current sheet and compare obtained results with existing current sheet models.

Feb. 5, 2016
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• Heather Elliott - SwRi

Solar Wind and Pluto Observations from the New Horizons Mission

In this presentation we describe the Solar Wind Around Pluto (SWAP) instrument on the New Horizons spacecraft. The SWAP instrument response model is based on extensive lab and inflight calibration, and this response is used to forward model the observed count rate observations in order to obtain the density, speed, and temperature. In preparation for the Pluto flyby, we used the solar wind observations to test the calibration and validate our analysis techniques to prepare for the Pluto flyby. For validation purposes, the SWAP observations were compared to both current 1 AU propagated solar wind observations, and prior Voyager 2 observations. We examined radial trends in the SWAP observations since many radial trends are well known in the inner heliosphere. In addition to the radial trends in the individual solar wind parameters, we also examined the radial trends in the solar wind speed-temperature relationship, and the periodicities present in the wind parameters. This extensive solar wind analysis not only allows us to confirm our techniques, but also provides context for the solar wind conditions during the Pluto flyby, which occurred during a solar wind compression. By comparing the Pluto flyby observations to observations from the flyby rehearsal that occurred in 2013 in the solar wind, it is clear that for more than 100 Pluto radii Pluto excludes the solar wind. We observed a change in the ratio of the primary and secondary detectors rates, and are currently performing additional laboratory calibration to determine if this is an indication of a heavy ions originating from Pluto. If so, then we observed a long Pluto tail more than 100 Pluto radii long.

Feb. 12, 2016
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• Yuki Harada - UCB

The Foremoon: A Complex Upstream Region of the Moon

As a consequence of the absence of shielding by a global magnetic field and a dense atmosphere, the Moon interacts directly with its ambient plasma. The classical picture of the Moon–-solar-wind interaction implies that the dayside lunar surface simply absorbs the incident charged particles, leaving a plasma void behind the Moon referred to as the "lunar wake." Although the upstream plasma and fields are not perturbed at all in this zeroth-order picture of the complete "passive absorber"–-solar-wind interaction, a variety of waves and modified particle-velocity distributions have been observed by a number of spacecraft in the upstream region magnetically connected to the Moon and to the lunar wake. In many respects, these lunar upstream waves and particles resemble those seen in the terrestrial foreshock. In this seminar, I will present an overview of this "foremoon," where multiple categories of Moon-related waves and particles coexist. I also show different characteristics of the Moon-plasma interaction in the solar wind, terrestrial tail lobes, and plasma sheet.

Feb. 19, 2016
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• Chuck Swenson - USU

Tackling the Spatial Temporal Ambiguity, or Launching the Auroral Spatial Structures Probe (and some results)

The most significant advances in Earth, solar, and space physics over the next decades will originate from new observational techniques. The most promising observation technique yet to be fully developed are multi-point or large distributed constellation-based observations of the Earth system. This approach is required to understand the “big picture” and “systems-level” coupling between disparate regions such as the solar-wind, magnetosphere, ionosphere, thermosphere, mesosphere, atmosphere, land, and ocean on a planetary scale. One challenge to deploying satellite constellations been cost but this is now being mitigated by the recent development of miniature instrumentation and miniature spacecraft such as CubeSats.

The NASA Auroral Spatial Structures Probe, rocket 49.002, was launched January 28, 2015 from the Poker Flat Research Range into active aurora over the northern coast of Alaska. It is one of the first NASA sounding rocket missions to attempt a set of multipoint measurements within the aurora. It consisted of a formation of 7 miniature spacecraft (a main payload with 6 miniature sub-payloads). Five of the payload created a string of pearls along the rocket trajectory and the other two were deployed to either side. Each payload included magnetometers, electric field double probes, Langmuir probes. An impedance probe was included on the main payload. The objective has been to unravel the difference between temporal and spatial variations of the auroral fields through a set of observations at different times of the same volume of space. Ultimately one wants to determine the contribution of small scale fluctuations of the electromagnetic fields within the aurora to the larger-scale energy deposition processes. We discuss how such an experimental rocket campaign is conducted when one must predict the occurrence of aurora in advance of the rocket launch. We present preliminary results that hint at the underlying spatial structures of the fields. currents, and energy deposition within the active aurora.

Feb. 26, 2016
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• David Malaspina - LASP

Plasma Boundaries: A Bridge Between Macro-Scale and Micro-Scale Physics

Space plasma physics research is frequently focused on either macro-scale or micro-scale processes, often treating them as independent. Yet recent advances in spacecraft instrumentation, simulation, and laboratory studies are breaking down that paradigm, demonstrating that interactions between large and small scales are critical for understanding and predicting the behavior of systems as diverse as the solar wind, the terrestrial magnetosphere, and laboratory plasmas. In each of these systems, plasma boundaries act as a bridge between physical scales. Macro-scale plasma motions drive boundary formation, micro-scale instabilities develop or are spatially sorted as a consequence of these boundaries, and finally, the aggregate effects of many micro-scale interactions modify the macro-scale system. Examples of interaction between macro- and micro-scale physical processes mediated by plasma boundaries in terrestrial inner magnetosphere, the solar wind, and laboratory plasmas will be discussed, with a focus on observational data.

March 4, 2016
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• Steve Schwartz - Imperial

Shocking stuff: MMS targets for electron processes at the Earth’s Bow Shock

Despite four decades of in situ study of the Earth’s bow shock, and collisionless shocks elsewhere within the interplanetary medium, there is still no quantitative explanation for the way the incident bulk flow energy is divided amongst electrons, thermal protons and other ions, energetic particles, and electromagnetic Poynting flux. In physical terms, although the total shock energetics can be calculated, the equation of state necessary to describe the internal processes does not exist. Statistically, the relative electron heating decreases as the inverse of the shock number – a result that seems to hold not only for the rather modest values at Earth, but also higher Mach numbers in the outer solar system and supernovae. Recently we emphasised the non-local nature of electron dynamics at the bow shock, suggesting that the local shock parameters were insufficient to determine the local shock properties. In fact, the controlling influence takes place near the point of tangency between the interplanetary magnetic field and the curved bow shock. This puts MMS in a prime position to explore the detailed physics occurring at such nearly-exactly-perpendicular shocks where the role of adiabatic vs non-adiabatic particle motion, contributions due to electric field spikes, and pre-conditioning electron motion through the extended shock foot all probably play a role.

March 11, 2016
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• Yuri Shprits - MIT

The Versatile Electron Radiation Belt (VERB) code: Long-term simulations during the Van Allen Probes mission

The Versatile Electron Radiation Belt (VERB) code solves the Fokker-Planck equation, taking into account radial diffusion and local pitch-angle, energy and mixed scattering. Using the VERB code, we performed several long-term simulations during the first year of the Van Allen Probes mission. We considered the energetic (>300 KeV), relativistic (~1 MeV) and ultra-relativistic (>3 MeV) electrons. The measurements of the energetic and relativistic electrons were well reproduced by the simulation during a period of various geomagnetic activity. However, for ultra-relativistic energies, the VERB code simulation significantly overestimates the observations. Since the additional losses were required only at very high energies, we concluded that EMIC waves are the most likely additional source of scattering to explain the observed decay rates. We included various parameterizations of the long-term EMIC waves based on the solar wind parameters and geomagnetic indexes, and found that simulation with EMIC waves provided a better agreement with the observations.