Space Physics Seminar - fall-2017

Sept. 29, 2017
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• Victor Reville - UCLA-EPSS

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Seminar Description coming soon.

Oct. 6, 2017
3:30 p.m. - 5 p.m.
Geology 6704

Presented By:

• Xiangning Chu - UCLA AOS

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The plasmasphere is a region of cold dense plasma in the inner magnetosphere, extending from Earth’s upper atmosphere to the plasmapause. It is constantly in a dynamic state, with erosion and refilling occurring during geomagnetic storms. The plasmaspheric dynamics are important in understanding radiation belt physics because plasmaspheric density strongly influences energetic particle scattering, as well as plasma wave excitation and propagation. Previous empirical density models can provide statistically averaged density profiles that, however, do not resolve the dynamic evolution of the plasma density. In this presentation, I will introduce the data-driven dynamic empirical models of the plasma density recently developed using a neural network approach. By taking time series of solar and geomagnetic indices as input, instead of using instantaneous values, these models are not only time-dependent but also history-dependent. They have good predictive abilities on both training database and out-of-sample data with a correlation coefficient of 0.95. Given their good predictive performance, they could provide unprecedented opportunities to gain insight into the plasmaspheric dynamics at any time and location. As an example, we will show that these models succeeded in reproducing various dynamic plasmaspheric features such as the erosion and recovery of the plasmasphere, as well as the plume formation. We will also show the dependence of the plasmasphere on geomagnetic activity such as magnetic storms, substorms, and enhanced convection and demonstrate the ability to reproduce plasmaspheric refilling with the model. Finally, we will show how these models could be used to discover new unexpected phenomena that are difficult to find otherwise.

Oct. 13, 2017
3:30 p.m. - 5 p.m.
Geology 6704

Presented By:

• Robert L. McPherron - UCLA - EPSS

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Magnetic activity on the Earth's surface is driven by the solar wind through the process of magnetic reconnection. A southward interplanetary magnetic field in the solar wind merges with the Earth's magnetic field and is transported over the poles and stretched into a long magnetic tail. The field eventually reconnects in the tail and returns in a convection system to the dayside. When the IMF is fluctuating this process takes about 3 hours and is called a magnetospheric substorm. If instead the IMF turns steadily southward and the solar wind speed is slow then the magnetosphere transitions from the substorm state to steady magnetospheric convection (SMC). In an SMC the reconnection rates at the front and rear of the magnetosphere are balanced so there are no changes in the configuration of the magnetosphere and no substorms occur after an initial substorm. If instead the IMF is strongly southward and the wind speed is high the magnetosphere enters a state of very large quasi-periodic substorms called sawtooth events. These three states are the fundamental modes of response of the magnetosphere to the solar wind. The magnetospheric substorm has three phases of growth, expansion, and recovery. The expansion phase is characterized by extremely active aurora near midnight that expands from the equatorward edge of the auroral oval to its poleward edge. Often a substorm will begin with a pseudo breakup which looks like it will become an expansion phase but then appears to be quenched. At any time but most often at the end of the expansion and in the recovery phase there will be intensifications of a band of aurora at the poleward edge of the oval that often develop into equatorward directed auroral streamers. These are called poleward boundary intensifications (PBI). The cause of the substorm expansion and the role of PBI in structuring activity are outstanding questions in magnetospheric physics and the subject of much controversy. This seminar will present examples of the three response modes and the two phenomenon which confuse the simple classification.

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

Presented By:

• Franco Rappazzo - UCLA - EPSS

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The magnetic field is fundamental to solar activity and shapes the interplanetary environment, as clearly shown by the full three dimensional monitoring of the heliosphere provided by the measurements of the Helios, Ulysses, SOHO, ACE, Wind, STEREO, Hinode, IRIS, SDO, and Voyager spacecraft. Magnetic fields are also the source for coronal heating and the very existence of the solar wind; produced by the sun’s dynamo and emerging into the corona, magnetic fields become a conduit for waves, act to store energy, and then propel plasma into the heliosphere in the form of Coronal Mass Ejections (CMEs). In 2018 the NASA Parker Solar Probe (PSP) mission will launch to carry out the first in situ exploration of the outer solar corona and inner heliosphere, soon to be followed by ESA’s Solar Orbiter (SO). Direct measurements of the plasma in the closest atmosphere of our star should lead to a new understanding of the questions of coronal heating and solar wind acceleration. I will describe the PSP and SO scientific objectives, instrument suites, and models of solar magnetic activity, coronal heating, and solar wind acceleration that SPP may confirm or falsify. Connections to relevant laboratory plasma experiments will be discussed: these include Alfvén wave turbulence, magnetic reconnection, and electron and ion heating and acceleration in complex magnetic fields.

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

Presented By:

• Xin An - UCLA AOS

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Motivated by the puzzles of chorus wave excitation in space and by their recognized importance in radiation belt dynamics, whistler-mode chorus waves are studied in the Large Plasma Device at UCLA, by the injection of a helical electron beam into a cold plasma. Incoherent broadband whistler waves similar to magnetospheric hiss are observed in the laboratory plasma whose mode structure is identified by the phase-correlation technique. It is experimentally demonstrated that the waves are excited through a combination of Landau resonance, cyclotron resonance and anomalous cyclotron resonance. To account for the finite size effect of the electron beam, linear unstable eigenmodes of whistler waves are calculated by matching the eigenmode solution at the boundary. It is shown that the perpendicular wave number inside the beam is quantized due to the constraint imposed by the boundary condition. Darwin particle-in-cell simulations are carried out to study the simultaneous excitation and interaction of Langmuir and whistler waves in a beam-plasma system. The electron beam is first slowed down and relaxed by the rapidly growing Langmuir wave parallel to the background magnetic field. Subsequently, electrons that compose the high-energy tail of the core plasma are trapped by the large amplitude Langmuir wave and are accelerated in the parallel direction. The excitation of whistler waves through Landau resonance is limited by the saturation of Langmuir waves, due to a faster depletion rate of the beam free energy from the inverted population by the latter compared to the former. Last but not least, a recent computational study relevant to the excitation of chorus waves in space will be discussed briefly.

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

Presented By:

• James Shirley - JPL/Caltech

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Solar active regions, torsional oscillations, and meridional flows:
Understanding and modeling the variability with time of the solar differential rotation

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Observations reveal coherent large-scale flows in the solar convection zone that are intimately related to the solar magnetic activity cycle. Solar active regions, the birthplaces of sunspots and sunspot groups, are generally flanked by bands of faster-than-average and slower-than-average zonal motions that extend to great depths within the convective zone. These ‘torsional oscillations’ may be viewed as perturbations of the mean differential rotation of the Sun. The torsional oscillations coexist with two types of meridional motions. The first of these flows generally poleward from the equator in each hemisphere, with velocities of 10-20 m s-1. Smaller scale flows (with velocities of 5-10 m s-1) are in addition seen to converge toward the active regions; the speeds of these flows vary with the phase of the sunspot cycle. The meridional flows and torsional oscillations exhibit spatiotemporal correlations with each other and with the progressive evolution of the magnetic activity cycle. Such correlations suggest the existence of physical relationships linking the phenomena of the active regions, the torsional oscillations, and the meridional flows. These linked phenomena are likely to be closely connected with the mechanism(s) responsible for the excitation of the solar dynamo.

The chicken-and-egg problem of determining causality has proven to be a difficult one. Does the progressive motion with time of the active regions (as seen in the butterfly diagram) arise due to the large scale differential motions of solar materials, or is it the other way around? Or might these intertwined and mutually-correlated phenomena arise due to some other cause? A recently developed dynamical hypothesis may potentially shed light on this question. We have traditionally considered the rotational motions and the orbital motions of the Sun (and other extended bodies) to be entirely independent and uncoupled, aside from certain weak effects attributed to tidal friction. This has allowed us to treat the angular momenta of the solar rotation and the solar orbital motion separately as closed systems. This long-standing assumption has recently been called into question. An equation describing a weak non-tidal coupling of the orbital and rotational motions was derived in Shirley (Plan. Space Sci. 141, 1-16, 2017). In the current presentation we will describe the nature and likely consequences of the proposed orbit-spin coupling mechanism for the solar differential rotation. In addition we make note of some earlier solar-physical observations and studies that may be cited in support of the physical hypothesis. If time permits we may additionally review pertinent results obtained from general circulation modeling of the Mars atmosphere with orbit-spin coupling (Mischna, M.A., and J. H. Shirley, Plan. Space Sci 141, 45-72, 2017).

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

Presented By:

• Zoran Mikic - Predictive Sciences, Inc.

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I will describe how we used a magnetohydrodynamic (MHD) model based on measurements of photospheric magnetic fields on the Sun to predict the structure of the corona for the total solar eclipse that swept across the United States on August 21, 2017. To make a more realistic prediction, we used two innovations: the energization of the magnetic field within filament channels, and the use of a wave-turbulence-driven (WTD) model to accelerate the solar wind and heat the corona. I will show how the prediction compared with observations.

Dec. 1, 2017
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• Lauren Blum - NASA/GSFC

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Seminar Description coming soon.

Dec. 8, 2017
3:30 p.m. - 4:30 p.m.
Geology 6704

Presented By:

• Munehito Shoda - Tokyo University

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Full title: New 1D modeling of the coronal heating and solar wind acceleration including both shock and turbulence heating