Space Physics Seminar - winter-2017

Jan. 13, 2017
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• Ivan Vasko - UCB

Diffusive scattering of electrons by electron holes around injection fronts

Van Allen Probes have detected nonlinear electrostatic spikes around injection fronts in the outer radiation belt. These spikes include electron holes (EH), double layers and more complicated solitary waves. We show that EHs can efficiently scatter electrons due to their substantial transverse electric fields. Although the electron scattering driven by EHs is diffusive, it cannot be evaluated via the standard quasi-linear theory. We derive analytical formulas describing local electron scattering by a single EH and verify them via test-particle simulations. We show that the most efficiently scattered are gyroresonant electrons (crossing EH on a time scale comparable to the local electron gyroperiod). We compute bounce-averaged diffusion coefficients and demonstrate their dependence on the EH spatial distribution (latitudinal extent and spatial filling factor) and individual EH parameters (amplitude of electrostatic potential, phase velocity, spatial scales). We show that EHs can drive pitch-angle scattering of . 5 keV electrons at rates 10?2 ? 10?4 s?1 and, hence, can contribute to electron losses and conjugated diffuse aurora brightenings. The momentum and pitch-angle scattering rates can be comparable, so that EHs can also provide efficient electron heating. The scattering rates driven by EHs at L?shells L ? 5 ? 8 are comparable to those due to chorus waves and may exceed those due to electron cyclotron harmonics.

Jan. 17, 2017
3:30 p.m. - 5 p.m.
Slichter 3853

Presented By:

• Alex Lazarian - University of Wisconsin

Turbulence and physics of energetic particles

I shall discuss a few processes of particle acceleration (a) acceleration by turbulent reconnection, (b) acceleration by shocks with enhancing magnetic field by turbulent dynamo, (c) stochastic acceleration in Alfvenic turbulence. I shall also discuss the suppression of the streaming instability by turbulence and show the effects of this on the observed cosmic ray anisotropies.

Jan. 20, 2017
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• Oleksiy Agapitov - UCB

Oblique Whistler-Mode Waves in the Earth's Inner Magnetosphere

The recent spacecraft observations of oblique whistler-mode waves in the Earth inner magnetosphere, as well as the various consequences of the presence of such waves for electron scattering and acceleration, are presented. The statistics of occurrences and intensity of oblique chorus waves in the region of the outer radiation belt, comprised between the plasmapause and geostationary orbit is performed. On this basis, we demonstrate that varying amounts of oblique waves can significantly change the rates of particle scattering, acceleration, and precipitation into the atmosphere during quiet times as well as in the course of a storm. The significant parallel electric field component provides the favorable conditions for nonlinear wave-particles interactions processes and recent spacecraft measurements allowed to study the effects for the radiation belts particles. Huge numbers of different nonlinear structures (double layers, electron holes, non-linear whistlers, etc. referred to as Time Domain Structures - TDS) have been observed by the electric field experiment on board the Van Allen Probes and THEMIS. A large part of the observed non-linear structures are associated with whistler waves and some of them can be directly driven by whistlers. Observations of electron velocity distributions and chorus waves by the Van Allen Probes provided long-lasting signatures of electron Landau resonant interactions with oblique chorus waves in the outer radiation belt. In the inhomogeneous geomagnetic field, such resonant interactions then lead to the formation of a plateau in the parallel (with respect to the geomagnetic field) velocity distribution due to trapping of electrons into the wave effective potential. The feedback from trapped particles provides steepening of the parallel electric field and development of TDS seeded from initial the whistler structure (well explained in terms of Particle-In-Cell model). We demonstrate that oblique whistler-mode chorus waves can be considered as an important ingredient of the radiation belt system and can continuously play a key role in many aspects of wave-particle resonant interactions.

Jan. 27, 2017
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• Christina Cohen - Caltech

Current Multi-Spacecraft Studies of Solar Energetic Particle Acceleration and Transport

Through new missions and unusual solar conditions, solar cycle 24 has afforded the opportunity for expanding our understanding of solar energetic particle (SEP) acceleration and transport. With complementary SEP observations from multiple spacecraft separated significantly in longitude, it has been possible to examine the longitudinal distribution of energetic particles in individual events, rather than relying on statistical event studies. Unprecedented 360° views of the Sun, in multiple wavelengths and coronagraphs, has made it possible to identify solar source regions regardless of where they are located and more accurately determine the properties of related coronal mass ejections. The unusually quiet conditions during the onset of cycle 24 allowed smaller SEP events to be examined and their source regions to be unambiguously identified. This talk will review some of the unexpected SEP observations made over this solar cycle and discuss what we have subsequently learned about particle acceleration near the Sun and transport through the inner heliosphere.

Feb. 3, 2017
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• Xiaojia Zhang - AOS/EPSS

Kinetics of sub-ion scale magnetic holes in the near-Earth plasma sheet

In collisionless space plasmas, the energy cascade from larger to smaller scales requires an effective interaction between ions and electrons. These interactions are organized by sub-ion scale kinetic plasma structures, where strong electric fields connect the decoupling motion of demagnetized ions and magnetized electrons. In this study, we consider one such example of sub-ion scale kinetic structures – magnetic holes, observed by THEMIS spacecraft in the dipolarized hot plasma sheet. Magnetic holes represent localized depressions of the magnetic field with strong currents at the boundaries. Taking the advantage of a very slow plasma convection (~10-20 km/s), we reconstruct the electron velocity distribution within magnetic holes and demonstrate that the current at boundaries is predominantly carried by magnetized thermal electrons. The electron motion is dominated by E×B drifts in a Hall electric field. The corresponding scalar potential drop across the hole is about a fraction of the electron temperature. We also show that magnetic holes can effectively modulate the intensity of electron cyclotron harmonic (ECH) waves, and thus modulate the spatial distribution of hot electron precipitations. The configuration of magnetic holes contains field-aligned currents with amplitudes of ~ 5 nA/m2 (one order of magnitude smaller than the amplitude of Hall current density). Therefore, these sub-ion scale magnetic holes can be important for ionosphere-magnetosphere coupling.

Feb. 10, 2017
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• Qianli Ma - BU

Radial diffusion and decay of energetic electrons in Earth’s outer radiation belt and slot region

During non-storm periods, the radiation belt electron evolutions are mostly controlled by the source from radial diffusion transport and the pitch angle scattering loss by magnetospheric waves. The Van Allen Probes measurements have provided detailed description about the radiation belt evolution and structure, including the inward intrusion of several MeV electrons in the outer radiation belt and several hundred keV electrons in the slot region, the decay of the energetic electrons during their inward transport, and the ‘S-shaped’ radiation belt structure formed as a consequence of the electron decay. Our 3D radiation belt simulation including radial diffusion and pitch angle and energy diffusion by plasmaspheric hiss and Electromagnetic Ion Cyclotron (EMIC) waves reproduces the essential features of the observed electron flux evolution. The recent improvements in radial diffusion models provide reasonable estimates on the radial intrusion timescale of energetic electrons. The wave-induced electron decay timescales and pitch angle distributions in our simulation are consistent with the Van Allen Probes observations over multiple energy channels. In addition, the energy-dependent electron scattering due to plasmaspheric hiss leads to the formation of ‘S-shaped’ electron flux contours across the radiation belts. Our study quantitatively evaluates the roles of ULF waves, plasmaspheric hiss, and EMIC waves in the transport and loss of radiation belt electrons.

Feb. 17, 2017
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• Norbert Krupp - Max Planck Lindau

Energetic particles measurements on Cassini as tools for probing global parameters of Saturn’s magnetosphere and the interaction with icy moons in the Saturnian system

In the first part of the presentation the major findings based on energetic particles in the Saturnian magnetosphere will be presented. The second part summarizes the responses in energetic particles near the moons Enceladus, Dione, Rhea.

Feb. 24, 2017
3:30 p.m. - 5 p.m.
Geology 6704

Presented By:

• Yingdong Jia - UCLA EPSS

The Enceladus Plume

Orbiting in the quiet inner magnetosphere of Saturn, the small moon Enceladus has a plume at its south pole. Water vapor is spread around Enceladus orbit, ionized, then spun up by the magnetosphere of Saturn. This plume fills Saturn's inner magnetoshpere with water group ions, and then drives the circulation of plasma and field between inner and outer magnetosphere. My talk reviews the contribution of the Enceladus plume to Saturn's magnetosphere, the generation of the plume, and local interactions around Enceladus.

March 3, 2017
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• Chih-Ping Wang - AOS

Structure and Dynamics of the Earth’s Mid-Tail

The Earth’s magnetotail plays an essential role in regulating energy and particle flows from the solar wind to the magnetosphere. New results from ARTEMIS observations and global simulations indicated that the structures and dynamics of the mid-tail (X ~40-100 RE) are quite different from the tail inside 30 RE. In this talk I will review recent progress in understanding the mid-tail. I will present investigations of mesoscale variations observed in two mid-tail regions: (1) hot electron enhancements in the magnetosheath and (2) plasma pressure enhancement in the mantle/lobe.

March 10, 2017
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• Quanming Lu - USTC

The mechanisms of electron acceleration during magnetic reconnection and its applications in the Earth’s magnetotail

Magnetic reconnection converts magnetic energy rapidly into plasma kinetic energy, and the production of energetic electrons is also one of the most important signatures during magnetic reconnection. In this talk, I will report the recent progress from our research group on electron acceleration during magnetic reconnection. First, I will introduce how electrons are accelerated in the vicinity of the X line, the jet front, the separatrix region, and the magnetic island, which is based on particle-in-cell simulations. Then, based on satellite observations in the earth’s magnetotail, I will give evidence of the reported mechanisms of electron acceleration during magnetic reconnection.

March 17, 2017
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• Haoming Liang - P&A

Oxygen Ions in Magnetotail Reconnection

Spacecraft have observed a significant fraction of oxygen ions (O+) in Earth’s magnetotail X-line during the periods of enhanced geomagnetic activity. It is important to understand how such O+ influences the reconnection process and how the O+ ions are accelerated due to reconnection. To this end we have used a 2.5D implicit Particle-in-Cell simulation (iPIC3D) in a 2D Harris current sheet in the presence of proton and O+ ions. By comparing the simulation runs for oxygen concentrations of 50%, 5% and 0% (only protons), we found that (1) the dipolarization front (DF) propagation is encumbered by the current sheet O+ inertia, which reduces the DF speed and delays the fast reconnection phase; (2) the reconnection rate in the 50% O+ Run is much less than the 0% O+ Run, which can be attributed to the O+ drag on the convective magnetic flux via an ambipolar electric field in the O+ diffusion region; (3) without entering the exhaust, the lobe O+ can be accelerated near the separatrices away from the X-point by the Hall electric field and form the hot population downstream of the DFs; (4) the pre-existing current sheet O+ ions are reflected by the DFs and form a hook-shaped distribution in phase space, from which the DF speed history can be deduced; (5) the DF thickness is proportional to the O+ concentration in the pre-existing current sheet. These results illustrate the differences between storm-time and non-storm substorms due to a significant concentration of oxygen ions. The oxygen heating results are expected to be observable by the Magnetospheric Multiscale (MMS) mission in the magnetotail.