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NASA NTRS2012

Monte Carlo Model Insights into the Lunar Sodium Exosphere

Sodium in the lunar exosphere is released from the lunar regolith by several mechanisms. These mechanisms include photon stimulated desorption (PSD), impact vaporization, electron stimulated desorption, and ion sputtering. Usually, PSD dominates; however, transient events can temporarily enhance other release mechanisms so that they are dominant. Examples of transient events include meteor showers and coronal mass ejections. The interaction between sodium and the regolith is important in determining the density and spatial distribution of sodium in the lunar exosphere. The temperature at which sodium sticks to the surface is one factor. In addition, the amount of thermal accommodation during the encounter between the sodium atom and the surface affects the exospheric distribution. Finally, the fraction of particles that are stuck when the surface is cold that are rereleased when the surface warms up also affects the exospheric density. In [1], we showed the "ambient" sodium exosphere from Monte Carlo modeling with a fixed source rate and fixed surface interaction parameters. We compared the enhancement when a CME passes the Moon to the ambient conditions. Here, we compare model results to data in order to determine the source rates and surface interaction parameters that provide the best fit of the model to the data.

Hurley, Dana M.↗

NASA NTRS2019

Satellite Orbital Drag During Magnetic Storms

We investigate satellite orbital drag effects at lowEarth orbit associated with thermosphere heating during magnetic storms caused by coronal mass ejections. CHAllenge Minisatellite Payload (CHAMP) and Gravity Recovery And Climate Experiment (GRACE) neutral density data are used to compute orbital drag. Stormtoquiet density comparisons are performed with background densities obtained by the JacchiaBowman 2008 (JB2008) empirical model. Our storms are grouped in different categories regarding their intensities as indicated by minimum values of the SYMH index. We then perform superposed epoch analyses with storm main phase onset as zero epoch time. In general, we find that orbital drag effects are larger for CHAMP (lower altitudes) in comparison to GRACE (higher altitudes). Results show that storm time drag effects manifest first at high latitudes, but for extreme storms, particularly observed by GRACE, stronger orbital drag effects occur during early main phase at low/equatorial latitudes, probably due to heating propagation from high latitudes. We find that storm time orbital decay along the satellites' path generally increases with storm intensity, being stronger and faster for the most extreme events. For these events, orbital drag effects decrease faster probably due to elevated cooling effects caused by nitric oxide, which introduce modeled density uncertainties during storm recovery phase. Errors associated with total orbit decay introduced by JB2008 are generally the largest for the strongest storms and increase during storm times, particular during recovery phases. We discuss the implication of these uncertainties for the prediction of collision between space objects at lowEarth orbit during magnetic storms.

Oliveira, D. M.↗

NASA NTRS2016

Correlation of Coronal Plasma Properties and Solar Magnetic Field in a Decaying Active Region

We present the analysis of a decaying active region observed by the EUV Imaging Spectrometer on Hinode during 2009 December 7-11. We investigated the temporal evolution of its structure exhibited by plasma at temperatures from 300,000 to 2.8 million degrees, and derived the electron density, differential emission measure, effective electron temperature, and elemental abundance ratios of Si/S and Fe/S (as a measure of the First Ionization Potential (FIP) Effect). We compared these coronal properties to the temporal evolution of the photospheric magnetic field strength obtained from the Solar and Heliospheric Observatory Michelson Doppler Imager magnetograms. We find that, while these coronal properties all decreased with time during this decay phase, the largest change was at plasma above 1.5 million degrees. The photospheric magnetic field strength also decreased with time but mainly for field strengths lower than about 70 Gauss. The effective electron temperature and the FIP bias seem to reach a basal state (at 1.5 x 10(exp 6) K and 1.5, respectively) into the quiet Sun when the mean photospheric magnetic field (excluding all areas <10 G) weakened to below 35 G, while the electron density continued to decrease with the weakening field. These physical properties are all positively correlated with each other and the correlation is the strongest in the high-temperature plasma. Such correlation properties should be considered in the quest for our understanding of how the corona is heated. The variations in the elemental abundance should especially be considered together with the electron temperature and density.

Sun: abundances↗

NASA NTRS1990

Coronal proton transport in the presence of Alfven waves and implications for impact line linear polarization

The transport of fast (about 1 MeV) protons in the corona is considered when their density and ansiotropy are sufficiently large to excite Alfven waves. The Alfven waves scatter the protons and make them almost isotropic. The Alfven wave instability remains in a marginally stable state in which the proton anisotropy is just large enough to excite Alfven waves to a level which would eliminate any greater anisotropy. The protons propagate diffusively down a coronal loop and have a path length of order 100 times their length in the absence of Alfven waves. This increases their Coulomb losses, and much higher energy protons (2-20 MeV) are required initially to produce H-alpha impact line linear polarization in the chromosphere as compared to the case in the absence of Alfven waves (about 200 keV) for loop lengths about 10 to the 9th cm. The interpretation of the observed polarization becomes less direct because it depends on more poorly known parameters than in the case of the absence of Alfven waves.

Smith, Dean F.↗

NASA NTRS1979

On the ionization equilibrium balance

Ionization equilibrium for the most astrophysically abundant elements are calculated using multilevel atomic models for several stages of ionization. It is found that the relative abundance of the lithium-, beryllium-, and boron-like ions at electron densities as low as 1 million per cu cm is density-dependent. Because of this dependence, the intensity of lines arising from ions of the Li, Be, and B isoelectronic sequences are density-sensitive. The general implications of these results as applied to solar and stellar atmospheric models, coronal abundances, and spectroscopic diagnostic techniques are discussed.

Vernazza, J. E.↗

NASA NTRS1979

Hydrodynamic simulations of flare/surge events

A one-dimensional, hydrodynamic, time-dependent model which simulates the major observed dynamics of flare associated surges was developed. The thermodynamics, the surge mass, the time scales, the physical dimensions, and the velocities of typical surge events are reproduced. The surge is created by a sudden pressure increase at the top of the chromosphere, and this pressure pulse produces a disturbance which is followed, with a time-dependent numerical solution, as it propagates upward through the transition region and into the corona. The leading edge of the disturbance is a weak shock which has only a slight effect on the original transition region and coronal thermodynamics. The major effect occurs behind the shock where the temperature is decreased and the density is increased, and this cool, dense region moves upward then falls downward. After the material begins returning to the chromosphere, a second shock is formed which propagates upward, brings the infalling material to rest, and returns the atmosphere to hydrostatic equilibrium.

Steinolfson, R. S.↗

NASA NTRS1985

Simultaneous Solar Maximum Mission and Very Large Array (VLA) observations of solar active regions

Simultaneous observations of solar active regions with the Solar Maximum Mission (SMM) Satellite and the Very Large Array (VLA) have been obtained and analyzed. Combined results enhance the scientific return for beyond that expeted from using either SMM or VLA alone. A total of two weeks of simultaneous SMM/VLA data were obtained. The multiple wavelength VLA observations were used to determine the temperature and magnetic structure at different heights within coronal loops. These data are compared with simultaneous SMM observations. Several papers on the subject are in progress. They include VLA observations of compact, transient sources in the transition region; simultaneous SMM/VLA observations of the coronal loops in one active region and the evolution of another one; and sampling of the coronal plasma using thermal cyclotron lines (magnetic field - VLA) and soft X ray spectral lines (electron density and electron temperaure-SMM).

Lang, K. R.↗

NASA NTRS1990

Soft X-ray spectroscopy of solar flares - An overview

An overview of the current status of high spectral resolution soft X-ray observations of solar flares is given. The review concentrates primarily on recent results and interpretations of results obtained from orbiting Bragg crystal spectrometers flow during the last solar maximum on the US DoD P78-1 spacecraft, the NASA SMM, and the ISAS Hinotori spacecraft. Results and several key issues regarding interpretation of the spectra are presented. Specifically, the dynamics of coronal flare plasmas as revealed by X-ray line profiles and wavelength shifts are discussed. Recent results concerning the theory of chromospheric evaporation are given. The temperature of coronal flare plasma is discussed within the context of a differential mission measure. Results concerning electron density measurements, nonequilibrium processes, and relative element abundances are also reviewed.

Doschek, G. A.↗

NASA NTRS1992

Three-dimensional kinematic reconnection of plasmoids with nulls

The global nonlinear dynamics of magnetic field lines in plasmoids with a pair of nulls, where B = 0, is studied. The aim of this analysis is to describe the separatrix surfaces on which singularities can occur in ideal magnetohydrodynamics because of topological changes in the field. These separatrix surfaces should locate the boundary layers associated with 3D reconnection in the presence of resistivity or inertia. It is found that the field lines exhibit chaotic scattering with several properties in common with plasmoid models without nulls (in which one component of the magnetic field never changes sign). In particular, the singular surfaces can be fractal, implying complex current density structures down to the dissipation scale. These generic features are expected to exist in typical coronal magnetic geometries exhibiting three-dimensional reconnection and the formation of current sheets.

Lau, Yun-Tung↗

NASA NTRS1994

Correlative analysis of hard and soft x ray observations of solar flares

We have developed a promising new technique for jointly analyzing BATSE hard X-ray observations of solar flares with simultaneous soft X-ray observations. The technique is based upon a model in which electric currents and associated electric fields are responsible for the respective heating and particle acceleration that occur in solar flares. A useful by-product of this technique is the strength and evolution of the coronal electric field. The latter permits one to derive important flare parameters such as the current density, the number of current filaments composing the loop, and ultimately the hard X-ray spectrum produced by the runaway electrons. We are continuing to explore the technique by applying it to additional flares for which we have joint BATSE/Yohkoh observations. A central assumption of our analysis is the constant of proportionality alpha relating the hard X-ray flux above 50 keV and the rate of electron acceleration. For a thick-target model of hard X-ray production, it can be shown that cv is in fact related to the spectral index and low-energy cutoff of precipitating electrons. The next step in our analysis is to place observational constraints on the latter parameters using the joint BATSE/Yohkoh data.

Zarro, Dominic M.↗

NASA NTRS1996

Plasma Properties and Magnetic Field Structure of the Solar Corona, Based on Coordinated Max '91 Observations from SERTS, the VLA, and Magnetographs

The plasma properties and magnetic field structure of the solar corona were determined using coordinated observations obtained with NASA/GSFC's Solar EUV Rocket Telescope and Spectrograph (SERTS), the Very Large Array (VLA), and Kitt Peak photospheric longitudinal magnetograms. A problem was identified with the SERTS calibration as determined from laboratory measurements. A revised calibration curve was derived by requiring that the numerous available measured line intensity ratios agreed with their respective theoretical values. Densities were derived from line intensity ratios, and active region densities were found to typically exceed quiet Sun densities by factors of only about 2. The active region density was found to remain constant across the SERTS slit, despite the fact that the emission line intensities vary significantly. This indicates that the product of the path length and the volume filling factor must vary significantly from the active region outskirts to the central core. Filling factors were derived and found to range from much less than one to nearly unity. Wavelength shifts were examined along the SERTS slit in the spatially resolved spectra, but no evidence was found for significant Doppler shifts in active region 7563 or in the quiet Sun. The numerical procedure developed by Monsignori-Fossi and Landini was used to derive the active region and quiet sun differential emission measure (DEM) from the spatially averaged spectra. A DEM was estimated for each spatial pixel in the two dimensional active region images by scaling the averaged active region DEM based upon corresponding pixel intensities of SERTS Mg IX, Fe XV, and Fe XVI images. These results, along with density measurements, were used in an IDL computer code which calculated the temperature dependence of the coronal magnetic field in each spatial pixel by minimizing the difference between the observed and calculated 20 and 6 cm microwave brightness temperatures.

Brosius, Jeffrey W.↗

NASA NTRS1997

UVCS/SOHO empirical models of solar coronal holes

A self-consistent empirical model for the major plasma parameters in a solar-minimum coronal hole was developed using the ultraviolet coronagraph spectrometer (UVCS) operating onboard the Solar and Heliospheric Observatory (SOHO). The radial and latitudinal distribution of density, velocity, and kinetic temperature for electrons, neutral hydrogen, and ionized oxygen was obtained. The data were acquired during November 1996 and April 1997. The model provides experimental values which can be used to constrain theoretical models of the fast solar wind. The implications on various models of coronal heating and acceleration are discussed.

Cranmer, S. R.↗

NASA NTRS2012

Inferring Lower Boundary Driving Conditions Using Vector Magnetic Field Observations

Low-beta coronal MHD simulations of realistic CME events require the detailed specification of the magnetic fields, velocities, densities, temperatures, etc., in the low corona. Presently, the most accurate estimates of solar vector magnetic fields are made in the high-beta photosphere. Several techniques have been developed that provide accurate estimates of the associated photospheric plasma velocities such as the Differential Affine Velocity Estimator for Vector Magnetograms and the Poloidal/Toroidal Decomposition. Nominally, these velocities are consistent with the evolution of the radial magnetic field. To evolve the tangential magnetic field radial gradients must be specified. In addition to estimating the photospheric vector magnetic and velocity fields, a further challenge involves incorporating these fields into an MHD simulation. The simulation boundary must be driven, consistent with the numerical boundary equations, with the goal of accurately reproducing the observed magnetic fields and estimated velocities at some height within the simulation. Even if this goal is achieved, many unanswered questions remain. How can the photospheric magnetic fields and velocities be propagated to the low corona through the transition region? At what cadence must we observe the photosphere to realistically simulate the corona? How do we model the magnetic fields and plasma velocities in the quiet Sun? How sensitive are the solutions to other unknowns that must be specified, such as the global solar magnetic field, and the photospheric temperature and density?

Schuck, Peter W.↗

NASA NTRS1973

A coronal hole and its identification as the source of a high velocity solar wind stream

X-ray images of the solar corona showed a magnetically open structure in the low corona which extended from N20W20 to the south pole. Analysis of the measured X-ray intensities shows the density scale heights within the structure to be typically a factor of two less than that in the surrounding large scale magnetically closed regions. The structure is identified as a coronal hole. Wind measurements for the appropriate period were traced back to the sun by the method of instantaneous ideal spirals. A striking agreement was found between the Carrington longitude of the solar source of a recurrent high velocity solar wind stream and the position of the hole.

Krieger, A. S.↗

NASA NTRS1981

Generation of coronal electric currents due to convective motions on the photosphere

Generation of electric currents in a magnetized plasma overlying a dense convective layer is studied, assuming that the magnetic field perturbation is small and satisfies the force-free equation. Currents are produced by rotational motions on the boundary in the case of a uniform equilibrium field. In a simple two-dimensional bipolar configuration, however, both irrotational and incompressible motions give rise to currents, and the current density has a peak at the magnetic neutral line. Scaling laws for the current density as well as for the stored magnetic energy are derived, and the possibility of heating the solar corona through the dissipation of coronal currents generated in this way is discussed.

Sakurai, T.↗

NASA NTRS1985

On the formation of coronal cavities

The formation of a coronal cavity and its relation to a quiescent prominence is studied theoretically. The stability of condensation modes of a plasma in the coronal streamer model (Steinolfson et al., 1982) is considered using a two-dimensional time-dependent ideal MHD numerical simulation. It is found that a plasma with beta = 0.5 is unstable but one with beta = 4 is stable because the density enhancement of the plasma trapped by the closed fields increases with the strength of the magnetic field. The means by which condensation modes can produce a coronal cavity and/or initiate the formation of a prominence (depending on the field configuration) are discussed. It is argued that prominence and cavity material is all supplied from the chromospheric level in the form of spicules.

An, C.-H.↗

NASA NTRS1985

Thermal phases of interstellar and quasar gas

Interstellar gas may be in a variety of thermal phases, depending on how it is heated and ionized; here a unified picture of the equation of state of interstellar and quasar gas is presented for a variety of such mechanisms over a broad range of temperatures, densities, and column densities of absorbing matter. It is found that for select ranges of gas pressure, photoionizing flux, and heating, three thermally stable phases are allowed: coronal gas (T above 100,000 K); warm gas (T about 10,000 K); and cold gas (T less than 100 K). With attenuation of ultraviolet and X-ray radiation, the cold phase may undergo a transition to molecules. In quasar broad-line clouds, this transition occurs at column density N(H) = about 10 to the 23rd/sq cm and could result in warm molecular cores and observable emission from H2 and OH. The underlying atomic physics behind each of these phase transitions and their relevance to interstellar matter and quasars are discussed.

Lepp, S.↗

NASA NTRS2017

Reconnection-Driven Coronal-Hole Jets with Gravity and Solar Wind

Coronal-hole jets occur ubiquitously in the Sun's coronal holes, at EUV and X-ray bright points associated with intrusions of minority magnetic polarity. The embedded-bipole model for these jets posits that they are driven by explosive, fast reconnection between the stressed closed field of the embedded bipole and the open field of the surrounding coronal hole. Previous numerical studies in Cartesian geometry, assuming uniform ambient magnetic field and plasma while neglecting gravity and solar wind, demonstrated that the model is robust and can produce jet-like events in simple configurations. We have extended these investigations by including spherical geometry,gravity, and solar wind in a nonuniform, coronal hole-like ambient atmosphere. Our simulations confirm that the jet is initiated by the onset of a kink-like instability of the internal closed field, which induces a burst of reconnection between the closed and external open field, launching a helical jet. Our new results demonstrate that the jet propagation is sustained through the outer corona, in the form of a traveling nonlinear Alfven wave front trailed by slower-moving plasma density enhancements that are compressed and accelerated by the wave. This finding agrees well with observations of white-light coronal-hole jets, and can explain microstreams and torsional Alfven waves detected in situ in the solar wind. We also use our numerical results to deduce scaling relationships between properties of the coronal source region and the characteristics of the resulting jet, which can be tested against observations.

Karpen, J. T.↗