Jacob Bortnik, Ph.D. - US grants

Diffuse auroral precipitation provides a major source of energy input to the Earth's upper atmosphere. The precipitated particles lead to changes in the rate of ionization, which can modulate large natural current systems and induce electrical currents along transmission lines and pipelines. The additional ionization can also result in chemical changes to the neutral atmosphere, which has been linked with ozone-depletion. The precipitated particles also lead to changes in the electrical conductivity of the ionosphere, which maps along magnetic-field lines and further affect the transport of plasma in the magnetosphere.

The diffuse aurora is primarily caused by rapid pitch-angle scattering of plasma sheet electrons. Because the magnetosphere is essentially collisionless, the only viable mechanism that can cause pitch-angle scattering is wave-particle interactions. Previous studies have shown that scattering by both electrostatic electron cyclotron harmonic (ECH) waves and whistler-mode chorus could be important, but there is still no general consensus on the dominant process. This project will examine the roles of ECH and chorus waves by looking at the rate of wave-induced particle scattering by both classes of wave. Pitch-angle diffusion rates will be calculated with existing codes, based on a comprehensive analysis of the spectral properties of chorus and ECH waves observed by the CRRES satellite in the outer magnetosphere, under different levels of magnetic activity. Computed rates of electron scattering will be used to determine the equilibrium pitch-angle distribution and the rate of precipitation loss to the atmosphere compared to the limit imposed by strong diffusion. The scattering results will be used, together with statistical data on the global distribution of trapped electrons in the near-Earth plasma sheet, to model the global distribution of diffuse auroral emissivity. This study will provide a quantitative understanding of how microphysical processes regulate the transfer of energy in geospace. The proposed research is central to the primary science objectives of the new GEM Focus Group on Diffuse Auroral Precipitation.

Much of the research will be undertaken by a graduate student and a young research scientist. The scientific results obtained will be made available to other members of the magnetospheric community interested in modeling the global distribution of the ring current electron population and diffuse auroral precipitation under different geomagnetic conditions. The results of the study can be used to model the global distribution of ionospheric conductivity, which regulates the rate of magnetospheric convection, and is critical for the development of a Geospace General Circulation Model. Quantification of electron scattering rates in the near-Earth plasmasheet is also important for understanding the evolution of electron flux and pitch angle distributions during injection events, which provide the source of free energy for the excitation of waves affecting energetic radiation belt dynamics. The important physical processes studied at Earth, can be applied to other magnetized planets such as Jupiter and Saturn, where similar scattering can occur.

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). The project is being funded jointly by the Division of Atmospheric Sciences of the Geosciences Directorate and by the Division of Physics in the Mathematical and Physical Sciences Directorate.

This project will study nonlinear wave-particle interactions between relativistic electrons and whistler mode waves with a combination of theory/numerical simulation and experiment via the Large Plasma Device (LAPD) at UCLA. The motivation is to understand the dynamics of Earth's radiation belts, which is a critical unresolved problem in magnetospheric physics. There is a dichotomy between the two methods used to model wave-particle interactions: namely quasilinear diffusion theory and the test-particle simulations. The quasilinear theory breaks down in the outer radiation belt where large amplitude, narrow-band coherent waves have recently been observed. The porject will extend a recent test particle approach using a distribution of electrons and wave amplitudes and phases in order to derive a more self-consistent treatment of the interaction of energetic electrons with large amplitude coherent whistler waves. Experiments will be carried out on the LAPD involving the interaction of an energetic electron beam propagating from one end of the device with waves launched from the other end. The change in the pitch-angle and energy of the beam will be measured. The experimental results will compared to the theoretical/numerical simulations to improve and validate the theory.

The project is headed by a young research scientist and will also involve a postdoctoral researcher and a graduate student. The project has societal impacts related to space weather.

Plasmaspheric hiss is an incoherent, band-limited emission found predominantly in the dense-plasma region surrounding the Earth, known as the plasmasphere. Hiss propagates as an electromagnetic plasma wave in the whistler mode, and is able to resonate with the high-energy electrons in the Van Allen radiation belts. The importance of hiss in controlling the structure and dynamics of the radiation belts has long been established, but the origin of hiss itself has been an open problem for over four decades. This project will develop a newly discovered mechanism involving chorus waves that appears to be able to naturally account for the observed frequency band, the incoherent nature, the day/night asymmetry, and the dependence on geomagnetic activity. An existing numerical ray tracing code will be used to calculate wave propagation, together with Landau damping using fluxes measured by the Combined Release and Radiation Effects Satellite (CRRES). By properly weighting the power distribution of chorus waves and tracking the evolution of chorus into hiss using ray tracing, the distribution of hiss intensity, frequency band, and wave normal angle will be quantified as a function of distance from the Earth, magnetic local time, and latitude. The effects of geomagnetic activity on the distribution of hiss characteristics will then be analyzed by recalculating the ray database under a number of different magnetospheric conditions. In the final year of the study, the project will determine contribution to hiss from (a) global lightning activity, (b) the effects of azimuthal propagation, and (c) propagation in plasmaspheric plumes. The computed hiss distributions will be compared with observations from satellite measurements.

The research will be done primarily by a young research scientist and a postdoctoral scholar. Understanding the physics of the radiation belts is important to understanding and being able to forecast space weather.

The Earth's radiation belts consist of a population of energetic electrons that is trapped by the Earth's nominally dipole magnetic field. A major challenge in radiation belt physics is identifying and describing the various processes that contribute to the energization of the electrons, and their loss, particularly uner highly dynamic geomagnetic conditions. The aim of this 3-year project is to quantitatively understand a newly discovered wave-scattering mechanism, termed transit-time diffusion, which has the potential to play an important role in radiation belt energization. This involves scattering by fast magnetosonic waves, which is a class of waves that has not previously been considered important for this problem. A combination of modeling and analytical approaches will be used to answer a number of fundamental questions about this new mechanism and its relative importance for radiation belt dynamics compared to other wave processes.

The project will be led and carried out mainly by a team of three early-career scientist and a graduate student will also be trained as part of the project. Energetic electron fluxes in the radiation belts constitute an important space weather concern, as they are known to adversely affect space-based assets upon which modern society is increasingly dependent. Consequently, better understanding and prediction of radiation belt dynamics would be of benefit to society at large.

Plasma density is one of the fundamental quantities of the magnetosphere-ionosphere (M-I) coupling that affects the growth and propagation of various plasma wave modes, magnetic reconnection rate, and ionospheric conductance; all of which strongly influence energy and mass transport in the M-I system. By taking advantage of simultaneous satellite-ground conjunctions in recent years, this award will help determining the source region of dayside density modulations, specifically addressing three outstanding scientific questions: Where does the enhanced density originate? How do enhanced density regions evolve in time? And what is the typical size of the enhanced density regions?

The plasma density in the dayside magnetosphere is highly structured, and this structure can have a large impact on the excitation of whistler-mode waves that in turn scatter plasma sheet electrons drifting from the nightside and accelerate electrons in the Earth's radiation belts. It has been recently found that whistler-mode waves drive structured patches of the diffuse aurora; this can be used to highlight enhanced density regions in the dayside magnetosphere. The dayside 'aurora-wave-density' correlations lead to questions about the origin of enhanced plasma density patches and their propagation in the dayside magnetosphere. Satellite observations alone have difficulties separating spatial and temporal effects in tracing the motion of enhanced density regions, but ground-based 2D auroral imaging could offer an excellent technique for monitoring the shape and motion of diffuse aurora that is driven by precipitating energetic electrons interacting with whistler-mode waves.

The proposed investigation will use a creative approach for understanding dayside magnetospheric density evolution by using Antarctic-based auroral observations. In particular, South Pole is an ideal dayside auroral observatory due to its longest polar night in the world. The wave-particle interaction producing whistler-mode waves will be used as a tool for imaging dayside plasma density structures using correlated South Pole all-sky auroral imager and THEMIS spacecraft observations. This research may influence not only its own field of diffuse auroral studies, but also related fields such as dayside magnetospheric dynamics, wave particle interactions, and excitation of plasma waves.

This interesting and important scientific research provides an ideal opportunity to train a graduate student, further scientific collaboration and cooperation in Antarctica, and create a list of THEMIS-South Pole auroral imager 'dayside conjunction' events and respective geomagnetic field mapping results for the use by a broader geospace science community.

Waves exist in space plasmas just as in the oceans and the atmosphere. In these plasmas, collisions between charged particles are rare. As a result, plasma waves are a major means of transferring energy from one charged particle population to another. Charged particles "surf" the waves. To first order, those that are moving slightly faster than the waves are energized, while those moving slower lose energy to the waves causing them to grow. There are a wide variety of plasma waves with different properties and different source mechanisms. Three of these (plasmaspheric hiss, chorus, and electromagnetic ion cyclotron (EMIC) waves) are widely believed to play significant roles in the depletion of the electron radiation belts but how this happens and how each contributes with local time and radial distance are still-open and strongly debated questions of fundamental importance. During their interactions with the waves, electrons are scattered out of their trapped orbits and sent on trajectories into the dense atmosphere where they are lost through collisions. The work will independently examine experimental observations and, most importantly, use theoretical tools to understand the interactions leading to the precipitation. The science questions to be addressed in this proposal are particularly important, since electron precipitation leads to chemical changes in the upper atmosphere, and is critical in regulating ring current and radiation belt electron dynamics. The grant will support the further training and development of a promising female early-career scientist. The results will be useful to the broader space physics and upper atmosphere communities, to researchers studying the chemistry of the middle atmosphere, and for space environment applications, such as active mitigation techniques for both natural and artificial radiation in space.

Testing theoretical ideas about particular wave-particle interactions and the variations in the space environment that effect them has been difficult because the waves are measured at large radial distances in the magnetosphere while the electron precipitation that they produce must be viewed from low-earth orbit. To complicate matters, the mix of plasma waves depends on the radial distance and magnetic local time but in addition is an as yet to be determined function of the severity of space weather storming, and the phase of the storm. The principal investigator (PI) has developed an innovative technique to analyze the physical relationship between wave intensity and wave-driven electron pitch angle scattering loss, which can be directly implemented using conjugate observations from near-equatorial and low-altitude satellites. This project, which uses both theory and observation, will provide a definitive understanding of the quantitative contribution of each type of plasma wave to electron precipitation within various energy ranges and in different L-MLT regions. The results will provide a highly important contribution to our wider understanding of the mechanisms that regulate the hazardous radiation environment surrounding the Earth.

This project will examine the mechanisms by which charged particles in Earth's magnetic field travel both out of and through the atmosphere. Identifying these mechanisms is important for understanding how the various layers of the near-Earth environment are coupled. Understanding the neutral and charged atmosphere has become of increasing interest for understanding the background in which space weather events play. Data from NSF supported facilities such as the Advanced Modular Incoherent Scatter Radar (AMISR) will be analyzed in conjunction with space craft data. This award will also fund two postdoctoral researchers.

Current particle precipitation theories assume that energetic particles in the magnetosphere precipitate into the atmosphere after undergoing pitch-angle scattering into the loss cone due to interactions with waves at or near the geomagnetic equator. However, the experimental verification of the effect of the multiple candidate waves on the particles' pitch-angle distributions has not been definitively established. This project brings together height-resolved measurements of ionization in the ionosphere, in situ measurements of pitch-angle scattering in the inner magnetosphere, and multidisciplinary expertise to achieve closure for all wave modes that may be involved in electron precipitation mechanisms. Instances of temporally coincident Van Allen Probes' measurements of particles and waves in the radiation belt (or ring current) and Poker Flat Incoherent Scatter Radar (PFISR) measurements of particles precipitating into the ionosphere near the foot-point of the Van Allen Probes' flux tubes will be identified. The loss-cone distribution functions created by candidate waves for pitch-angle scattering using the quasi-linear diffusion equations will be calculated. Changes in the phase-space distribution of electrons that are produced by the electric and magnetic wave fields in the frequency range from a few Hz to several kHz will be quantified by applying UCLAs Full Diffusion Code to Van Allen Probes' particle and field measurements. Finally, a comparison of the ionization profiles produced in the atmosphere by loss cone distributions calculated from Van Allen Probes' data with the ionization profiles directly measured with PFISR.

Radio waves are naturally produced in the near-Earth space environment. Chorus waves are a type of such radio waves that interact with relativistic particles in the radiation belts. Understanding the interactions between these waves and particles is important for predicting the changing space environment and its societal impacts. In particular, this allows us to predict space weather and prepare for impacts to national commercial and defense satellites and communications - a priority set out in the National Space Weather Action Plan.

This project aims to explore the origin of banded chorus waves in Earth's radiation belt by analyzing data from the Van Allen Probes and performing particle-in-cell simulations to aid in interpretation. Naturally occurring chorus waves are believed to play a key role in accelerating relativistic electrons in the Earth's outer radiation belt, and in precipitating ~keV (and higher) electrons to the upper atmosphere, causing diffuse aurorae and pulsating aurorae. Chorus typically occurs in two distinct frequency bands separated by a gap at half of the electron gyro-frequency. The origin of this two-band structure has been a half-century-long open scientific question. This project, by systematically surveying the chorus waves and their properties right in the source region, will explore whether the spectral gap at half the electron gyro-frequency is generated at the magnetic equator, or is formed by damping as the waves propagate to higher latitudes. The electron distributions that accompany banded chorus waves will be statistically investigated and linear growth calculations made to investigate the direct energy source of banded chorus waves. Based on typical observations of waves and particle distribution, 1-Dimensional and 2-Dimensional particle-in-cell simulations will be performed to explore the multi-stage interaction processes that could account for the banded chorus waves.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

The dynamical evolution of the Earth?s radiation belts is governed by the relative balance between particles being accelerated and transported into and lost from the magnetosphere. Relativistic electron microbursts are one source of particle losses in the magnetosphere. This work is a modeling effort to better understand the physical process of losses from microbursts. The work will have a broader impact on the technological infrastructure that supports our society and national security, due to the significant space weather hazard that radiation belt electrons pose to Earth-orbiting spacecraft. Two undergraduates and one graduate student researcher will be supported.

The effort will develop a computational framework to identify the physical mechanisms that produce relativistic electron microbursts in the Earth?s outer radiation belt and to quantify, for the first time, accurate loss estimates due to this scattering process. The science investigation will be carried out as follows. Numerical ray tracing will be used to calculate the chorus wave power spectrum (frequency and wave normal angle) as a function of time and location along a given magnetic field line, including Landau damping. Then, the wave power distribution will be used to calculate the resultant pitch-angle change near the loss cone at each point along the field line due to resonant interactions with the obtained chorus wave field, including nonlinear effects. This is then repeated for a set of test-particles (i.e., energetic electrons) over a range of initial energies. For an assumed radiation belt electron distribution taken from an empirical model, the work calculates the spatio-temporal dependence of the electron flux precipitated into the ionosphere due to the derived resonant pitch-angle scattering. Repeating this entire procedure across a range of locations provides a map of the electron flux precipitated into the ionosphere as a function of time, position, and energy, due to resonant interactions with chorus waves. With these model calculations in hand, the work explores and evaluates the various potential mechanisms involved in the scattering through parameter variation, for example by increasing/suppressing Landau damping, or by restricting/including higher order resonances. These mechanisms will then be additionally scrutinized through data-model comparisons, both statistical and for an individual event.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

Energetic precipitation that rains down on the atmosphere from space is known to cause a whole host of space weather effects, including damage to low-Earth orbiting spacecraft, location signal disruptions, and danger to trans-polar aircraft and passengers. Additionally, the atmospheric effects of nitric oxides from particle precipitation play a role in our understanding of weather and climate - a topic that is particularly relevant right now. The linkage between space and climate is not well-known. This study aims to investigate this link directly by analyzing data from spacecraft, ground-based instrumentation, and using atmospheric modeling to uncover the cause and effect of particle precipitation on the atmosphere and climate. This proposal involves graduate student training and participation by several early- and mid-career researchers who will benefit from the collaboration in a new direction. Furthermore, the team plans to host a short, virtual seminar series to be advertised to the full community that will illuminate this science topic, highlight the interdisciplinary nature of the work, and discuss new findings.<br/><br/>Energetic particle precipitation (EPP) is both a cause and a consequence of a myriad different processes throughout geospace. It is the result of loss mechanisms that occur throughout the inner magnetosphere, contributing to global dynamics of the trapped radiation belt and ring current particle populations. It is also the cause of several effects on the Earth's upper and middle atmosphere, as the energy deposited to these regions create changes in chemical constituents. As such, EPP is a process that crosses the traditionally-defined boundaries between space science and earth science, and has wide-ranging implications on both systems. The research project aims to advance the frontiers of our knowledge on the drivers and effects of EPP on the coupled magnetosphere-atmosphere system. In summary, this project is to quantify the loss that occurs in the magnetospheric particle population through ground- and space-based observations and use that information to assess the effects of that precipitation on the Earth's atmosphere using whole atmosphere modeling. This approach will allow us to advance understanding of this process within the context of the space science and atmospheric science fields.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.