Photodetachment chamber

A view down the photodetachment chamber


Current research

Astrochemistry (atom-molecule collisions)

The extremely cold and tenuous clouds of gas and dust that occupy interstellar space -- the space between the stars -- host a rich chemistry. This research project will study chemical reactions of the simplest polyatomic molecular ions that drive the chemistry of the interstellar gas. These ions can exist at the very low temperatures at which most other molecules freeze out. The research team will use a sophisticated laboratory experiment to accurately determine reaction rates of the ions at very low temperatures. Results from the study will allow astronomers to better understand the abundance of these ions in space and to use them as probes of the coldest and densest regions within interstellar clouds, which are birthplaces of stars and planets. The project will contribute to the training and professional development of a postdoctoral research scientist. The project will also support a program that is known to have a positive impact on middle school and high school science education.

The research team will perform laboratory studies of the gas-phase reactions of atomic deuterium with H3+ -- the simplest polyatomic ion found in the interstellar gas -- and its deuterated forms H2D+ and D2H+. The three reactions are important in the chemistry that drastically increases the deuterium content of interstellar molecules; and the H2D+ and D2H+ ions probe the densest regions of cold dark clouds where most other molecules freeze out. Understanding the chemistry of these ions is therefore essential to understanding these regions, which are potential sites of star and planet formation. The proposed studies are challenging because atoms and molecular ions are difficult to produce in the laboratory. The team will measure rate coefficients of the reactions of D atoms with H3+, H2D+, and D2H+ using a dual-source, merged fast-beams apparatus, which was built with prior NSF support (AST-0905832), that enables them to produce and react beams of atoms and molecular ions. Collisional cross sections extracted from the measured rate coefficients will be used to generate thermal rate coefficients at temperatures relevant to cold interstellar clouds (10-100 K). The rate coefficients are expected to be accurate to within 15% -- about an order of magnitude better than those calculated for the three reactions. The project will support a postdoctoral research scientist at Columbia University.

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Negative ion sputter source.

Beam merger simulation for the D3 apparatus

Astrochemistry (electron-molecule collisions)

Molecules play an important role in the modern universe where they are a key component for a wide range of sources including diffuse, translucent, and dense molecular clouds; hot cores; photon dominated regions (PDRs); protostellar disks; protoplanetary disks; planetary and satellite ionospheres; cometary comae; and circumstellar envelopes around dying stars. As we strive to improve our understanding of these objects, it is necessary to be able to model and interpret their chemical composition, charge balance, emission and/or absorption spectra, and thermal structure. This, in turn, requires reliable knowledge of the underlying molecular collisions which control these properties. Of particular astrophysical importance is dissociative recombination (DR) which is the primary neutralizing reaction for molecules in cosmic plasmas. For chemical networks involving ion-molecule reactions, this process is often the terminating step for particular synthesis pathways. Knowing branching ratios for final products is critical as they can determine the viability of the pathway in question as well as whether or not a compound can be produced in the gas phase or if unknown surface chemistry must be invoked. The end products of DR may be energetic, in which case they can collisionally heat the plasma. Or they may be in excited states, in which case they can cool the gas through radiative relaxation.

We are carrying out a series of DR studies for selected ions of importance to the various NASA Astrophysics missions. Our work is designed to improve the DR data used in astrophysical and astrochemical models for the molecular objects listed above and thereby improve our understanding of these sources. We will deepen our understanding of halogen chemistry in the cold interstellar medium (ISM). This will enable the development of new proxies for H2 abundance determinations in the cold ISM. The data for our halogen chemistry studies were collected using the recently decommissioned heavy ion test storage ring (TSR) at the Max Planck Institute for Nuclear Physics in Heidelberg (MPIK), Germany.

Our proposed research addresses NASA ’s Strategic Goal 2.4: "Discover how the universe works, explore how it began and evolved, and search for Earth-like planets". More specifically, the expected advances in our knowledge of star formation physics resulting from our proposed work meets NASA’s Objective 2.4.2 "Improve understanding of the many phenomena and processes associated with galaxy, stellar, and planetary system formation and evolution from the earliest epochs to today".

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Negative ion sputter source.

The Test Storage Ring (TSR) at the MPIK Heidelberg.

The Solar wind

This project aims to investigate the wave-induced heating of the solar corona by extending the research analysis to include quiet Sun regions and coronal hole plumes. Currently, there is a dearth of observational results to resolve the debate over whether waves can heat the entire solar corona, or other heating mechanisms need to be invoked. The project team will use spectroscopic line-profile measurements to infer the wave energy within various structures and determine if it is sufficient for heating of the solar corona. The project will enable several educational broader impacts. Much of the analysis during the project will be conducted by a postdoctoral research scientist who will develop experience in diverse areas of solar physics and related techniques. Undergraduate research participation is also expected, for which the students will receive course credit through the Columbia Astronomy Department. The research and EPO agenda of this project supports the Strategic Goals of the NSF Atmospheric & Geospace Sciences Division in discovery, learning, diversity, and interdisciplinary research.

We will aim to infer the wave amplitudes in the solar corona and determine how the damping rate varies with plasma properties, inhomogeneity, and magnetic geometry, thereby constraining the damping mechanism for the waves. Some of the dissipated wave energy appears to heat the minor ions in the solar corona. Numerous numerical models have been developed to describe processes that could transfer energy to the ions via turbulence or wave-particle interactions. The predicted ion heating rate depends on both the specific energy transfer mechanism as well as the frequency distribution of wave power. Thus, the ion temperature data constrain theories of the ion heating and also provide insight into the wave spectrum. The project team will measure the ion temperatures in the quiet Sun corona and in plumes, building on the novel techniques developed during previous our previous NSF grant (AGS-1060194). The results of this project are expected to provide critical tests of the existing ion-heating theories. Accurate ion temperatures are also needed for the analysis of wave amplitudes. By determining the temperature and wave amplitudes self-consistently, the project team will obtain accurate measurements of both quantities. These are crucial for proving, or disproving, the existing wave-heating mechanisms of the solar corona.

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D3 Apparatus

EIS observations are illustrated here overlayed on an EIT/SOHO image in the 195 Å band

Atomic physics studies for solar physics

We propose to use laboratory measurements to calibrate spectroscopic electron density diagnostics relevant for solar physics to accuracies of better than 20%. Our results will be directly applicable to solar spectroscopy and can also be used to test theoretical calculations. Improving the accuracy of density diagnostics will increase the scientific return of current and planned solar missions such as Hinode, SDO, Solar Orbiter, Solar-C, and sounding rocket observations such as EUNIS. This work will address the Laboratory Nuclear, Atomic, and Plasma Physics element of the Heliophysics Technology and Instrument Development for Science program.

Density is a key parameter for solar physics. It is used to determine the energy and force balance in various solar regions and to understand the nature of solar structures. Among the numerous areas in which accurate density measurements are needed are coronal heating, coronal seismology, coronal mass ejections, solar flares, and understanding the nature of inhomogeneous structures in the solar atmosphere.

The primary density diagnostics for solar plasmas use ratios of emission line intensities, at least one of which is density sensitive. This sensitivity arises due to the atomic physics of the system. It depends on the collisional excitation and deexcitation rates and radiative transition rates for the several atomic levels directly involved in the transition, as well as cascade contributions from many higher energy levels.

Essentially all of these data comes from theoretical calculations, which have not been adequately tested. The theoretical results are usually compared to other calculations or to observed solar spectra, neither of which independently tests the theory. Moreover, the calculations rarely provide any uncertainty estimates and large systematic errors are possible depending on the complexity of the atomic model used.

A recent comparison of several diagnostic line ratios showed discrepancies in the inferred density of factors of 2 to 10. This implies that observations are unable to accurately interpret spectra in order to describe solar structures, which severely limits our ability to model the underlying physics. Our measurements will reduce the uncertainty of density diagnostics by an order of magnitude.

We will calibrate density sensitive line intensity ratios using the Electron Beam Ion Trap (EBIT) at the Lawrence Livermore National Laboratory (LLNL). EBIT is a cylindrical trap, in which an axial magnetic field guides an electron beam running along the axis. The electron beam forms an electric potential well that confines the ions in the radial direction, while biased electrodes at each end provide axial confinement. Collisions between the ions and the electron beam ionize and excite the trapped ions.

By adjusting the electron beam parameters we can vary the density in the trap and measure how the various line intensity ratios change. The ion emission line spectra will be measured using high resolution ultraviolet spectrometers. The electron beam density will be derived from the electron current and X-ray or extreme ultraviolet images of the beam. The effective density experienced by the ions depends on the overlap of the ion cloud with the electron beam. To determine the overlap, the geometry of the ion cloud will be measured using an optical CCD. The resulting effective electron density will range from 1E8 to 1013 cm-3.

We will also compare our results to new theoretical calculations and to published atomic data in order to identify the underlying causes of any discrepancies we find. We will concentrate on ions most relevant for solar physics. For example, we will measure diagnostics from Fe IX - XIII found in the 170-210 Angstrom wavelength band, as these lines and wavelengths are observed by various solar spectrometers. We will also measure diagnostic line ratios from other ions and wavelengths that are important for specific instruments. Jump to top

EBIT Apparatus

Cut-away view of LLNL EBIT.

Plasma physics simulations of the solar wind

Motivated by recent observations suggesting that the solar corona is heated by Alfvén wave damping at unexpectedly low heights, we propose experiments to study the basic plasma physics of Alfvén wave dissipation. Proposed coronal damping mechanisms rely on inhomogeneities in the plasma, but these theories have not been systematically studied in the laboratory. We will perform experiments using the Large Plasma Device (LAPD) to test these theories.

A major unsolved problem in astrophysics is to determine what heats the solar corona. One proposed solution is that magnetohydrodynamic Alfvén waves carry the energy from below the solar surface and deposit it in the corona. Such low frequency small amplitude Alfvén waves are predicted to damp only over length scales too long to to heat the corona. Recent observations, though, show that these waves are actually damped in the low corona. The plasma physics of the damping process has not been determined. There are several proposed mechanisms; all rely on inhomogeneities to drive flows and currents at small length scales where the energy can be more easily dissipated. We will study Alfvén wave damping in a laboratory plasma scaled to be similar to the solar corona. Experiments will be performed using LAPD at the Basic Plasma Science Facility at UCLA. LAPD produces a 20 m long column of plasma in which the magnetic field and density can be controlled over a wide range. Shear Alfvén waves will be launched and the wave magnetic fields, plasma properties, and flows will be measured with high temporal and spatial resolution.

Our experiments will focus on wave reflection from a gradient in the Alfvén speed. Reflection is a requirement for producing plasma turbulence, which is believed to be a fundamental process in many plasma environments. However, predictions of Alfvén wave reflection from an Alfvén-speed gradient have not been systematically tested. Previously, Alfvén wave reflection has been studied experimentally using conducting grids and other artificial configurations to reflect the waves. Such setups may be relevant to the Earths magnetosphere, but not the solar corona. Our preliminary LAPD experiments have demonstrated wave reflection from a gradient. We will measure the wave reflection and transmission for different frequencies and gradients. The second task is to study plasmas with gradients transverse to the magnetic field. Cross-field variations in the wave phase speed cause waves on neighboring field lines to propagate out of phase, distorting the wave fronts and leading to dissipation by phase mixing. By itself, this is likely too slow to explain the damping in the corona. But some theories predict the damping rate to be enhanced by the generation of cross-field propagating waves, resonant absorption, and nonlinear processes. A systematic study will be initiated using controlled radial density gradients. Measurements will quantify phase mixing in the gradient layer, any new wave modes or flows that are generated, and heating. The only previous comparable experiment was also done in LAPD and looked at waves at the edge of the plasma column. But this was neither a systematic study nor relevant for the corona.

This proposal will increase collaboration between the astrophysics and plasma physics communities by addressing issues relevant to the solar corona. It will support several educational broader impacts. Much of the research will be conducted by a postdoctoral research scientist who will gain experience in diverse areas of plasma and solar physics.

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LAPD

Top-down view of LAPD.

Determining the source regions of the solar wind

A major unsolved problem in solar physics is determining the source regions of the solar wind. We propose to develop new composition diagnostics for in situ solar wind measurements that will enable Solar Orbiter, Solar Probe Plus, and other missions to identify the sources of the fast wind. This work addresses the science element "Physics of the Inner Heliosphere" in the Living With a Star program.

The fast solar wind originates in coronal holes. Within coronal holes two major structures are observed: plumes and interplume regions. Whether plumes or interplumes are the source of thefast solar wind is the subject of much debate. Some have argued that plumes are a major contributor to the solar wind, while others question whether plumes are present in the solar wind at all.

In order to resolve this issue, it is necessary to determine the coronal sources of the observed in situ solar wind. Such measurements, mainly by Ulysses, have found structures in fast solar wind streams, notably the microstreams and pressure balanced structures. These might be the remnants of plumes, but their connection remains ambiguous.

One challenge for relating these observations to coronal hole structure is that existing observations were made far from the Sun, where mixing of plume and interplume properties might have occured. Solar Orbiter and Solar Probe Plus are designed to address this problem by going much closer to the Sun.

The other major challenge has been that observations focussed on plasma properties, such as pressure and magnetic field, which are not uniquely related to solar structures. That is, the structures observed in situ could have formed in interplanetary space rather than originating at the Sun. To unambiguously connect observed solar wind material to plumes and interplumes, it is necessary to identify signatures that persist from the corona into the heliosphere.

The objective of our proposed work is to provide definitive composition diagnostics that will identify the coronal sources of the fast solar wind. The solar wind composition is fixed at low heights in the corona and thereby provides a clear link between the solar wind and the source region.

Ion abundances are one such diagnostic. The charge state distribution becomes frozen-in as the plasma flows away from the Sun. Thus, the charge balance reflects the temperature and density structure of the corona. Because plumes and interplumes have substantial differences in temperature, density, and outflow velocity, it is expected that their frozen-in charge states will also differ in solar wind.

We will calculate the time-dependent ionization balance and predict frozen-in ion abundances based on various plume and interplume models and observations. In preparation for analyzing Solar Orbiter and Solar Probe Plus data, we will apply our results to published in situ data from Ulysses and other missions.

Elemental abundances are also fixed at the Sun. In interplume material, the abundances are similar to the photosphere. But in plumes there are indications of a first ionization potential (FIP) effect, in which elements with a low FIP have elemental abundances that are enhanced relative to their photospheric values.

We will infer elemental abundances in plumes and interplumes using largely archival spectroscopic observations. It is likely that the elemental abundances of plumes vary as a function of the age of the plume. We will determine whether there is a time-dependent FIP effect in plumes. Our inferred elemental abundances will be compared with to existing published in situ measurements.

The outcome of this work will be a set of derived ion abundances and measured elemental abundances, which will be available to analyze in situ measurements from Solar Orbiter and Solar Probe Plus, as well as other in situ data. These data will enable determining whether structures seen in the fast wind are due to plumes or not.

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Solar Orbiter

Artist's impression of European Space Agency’s Solar Orbiter.