Engineering topics

Ofman, L.

Publications and source records attributed to Ofman, L..

At least 19 records

NASA NTRS2019

Understanding the Role of 𝛼 Particles in Oblique Heliospheric Shock Oscillations

Recent observations by DSCOVR provide high temporal resolution (50 samples per second) magnetic vector field data that allows investigating the details of oblique heliospheric shock oscillations. It was found that some of these shocks exhibit magnetic oscillations, both downstream and upstream of the shock front. The DSCOVR/MAG magnetic field data are supplemented by an extensive database of low Mach number (M < 3) low‐β (<1) shock data observed by Wind albeit with lower temporal resolution. Motivated by the observations, we use the 2.5D hybrid model of the oblique shocks with α particles in addition to kinetic protons and electron fluid. We model the properties of the oblique shocks for a number of typical parameters found in observations and study the effects of the shock parameters and the relative α particle abundances on the properties of the shock magnetic field, density, and velocity oscillations. We find the α particles “surf” on the shock front and produce a wake of density oscillations. We examine the details of the phase space of the ions as well as the ion velocity distribution functions in various parts of the shock and study their nonthermal properties. We determine the effects of the α particle kinetic properties and abundances on the structure and dynamics of the shock downstream oscillations for a range of parameters relevant to low Mach number low‐β heliospheric shocks.

Ofman, L.↗

NASA NTRS2018

Motions in Prominence Barbs Observed on the Solar Limb

We analyze and discuss an example of prominence barbs observed on the limb on 2016 January 7 by the Hinode/Solar Optical Telescope in Ca II and H alpha, the Interface Region Imaging Spectrograph, with slit jaw images and Mg II spectral data, and the Solar Dynamics Observatory's Atmospheric Imaging Assembly. In the recent literature there has been a debate concerning whether these features, sometimes referred to as "tornadoes," are rotating. Our data analysis provides no evidence for systematic rotation in the barbs. We do find line-of-sight motions in the barbs that vary with location and time. We also discuss observations of features moving along the barbs. These moving features are elongated parallel to the solar limb and tend to come in clusters of features moving along the same or similar paths in the plane of the sky during a period of 10 minutes to an hour, moving toward or away from the limb. The motion may have a component along the line of sight as well. The spectral data indicate that the features are Doppler shifted. We discuss possible explanations for these features.

Kucera, T. A.↗

NASA NTRS2017

Energy Release in the Solar Atmosphere from a Stream of Infalling Prominence Debris

Recent high-resolution and high-cadence extreme-ultraviolet (EUV) imaging has revealed a new phenomenon, impacting prominence debris, where prominence material from failed or partial eruptions can impact the lower atmosphere, releasing energy. We report a clear example of energy release and EUV brightening due to infalling prominence debris that occurred on 2011 September 7-8. The initial eruption of material was associated with an X1.8-class flare from AR 11283, occurring at 22:30 UT on 2011 September 7. Subsequently, a semicontinuous stream of this material returned to the solar surface with a velocity v greater than 150 km/s, impacting a region remote from the original active region between 00:20 and 00:40 UT on 2011 September 8. Using the Solar Dynamics Observatory/Atmospheric Imaging Assembly, the differential emission measure of the plasma was estimated throughout this brightening event. We found that the radiated energy of the impacted plasma was L(sub rad) approx. 10(exp 27) erg, while the thermal energy peaked at approx.10(exp 28) erg. From this we were able to determine the mass content of the debris to be in the range 2 x 10(exp 14) less than m less than 2 x 10(exp 15) g. Given typical prominence masses, the likely debris mass is toward the lower end of this range. This clear example of a prominence debris event shows that significant energy release takes place during these events and that such impacts may be used as a novel diagnostic tool for investigating prominence material properties.

Inglis, A. R.↗

NASA NTRS2017

The Effects of Inhomogeneous Proton-Alpha Drifts on the Heating of the Solar Wind

Previous modeling studies have demonstrated that waves and super-Alfvenic drift can lead to perpendicular preferential heating of the alpha particles with respect to protons. Using a 2.5-D hybrid model of the solar wind alpha-proton plasma, we study the effects of inhomogeneous (across the magnetic field) background streaming focusing on the fast solar wind. We explore the effects of an initial relative, inhomogeneous ion drift on the perpendicular ion heating and cooling and consider the effects of solar wind expansion. We study the spectrum of the magnetic fluctuations in the inhomogeneous background solar wind and demonstrate the generation of oblique waves and their effects on enhanced resonant anisotropic ion heating. The model reproduces the typical ion temperature anisotropy values seen in observations. Using our model, we find that inhomogeneous super-Alfvenic ion drift in the plasma generates significant power of oblique waves in the solar wind plasma, in addition to enhanced heating compared to the non-drifting populations. We demonstrate the effects of various inhomogeneity profiles and regions of the drift on the ion anisotropic heating in super-Alfvenic and near-Alfvenic drifts. We find that the cooling effect due to the solar wind expansion is not significant when super-Alfvenic drifts are considered.

proton↗

NASA NTRS2016

Slow Solar Wind: Observations and Modeling

While it is certain that the fast solar wind originates from coronal holes, where and how the slow solar wind (SSW) is formed remains an outstanding question in solar physics even in the post-SOHO era. The quest for the SSW origin forms a major objective for the planned future missions such as the Solar Orbiter and Solar Probe Plus. Nonetheless, results from spacecraft data, combined with theoretical modeling, have helped to investigate many aspects of the SSW. Fundamental physical properties of the coronal plasma have been derived from spectroscopic and imaging remote-sensing data and in situ data, and these results have provided crucial insights for a deeper understanding of the origin and acceleration of the SSW. Advanced models of the SSW in coronal streamers and other structures have been developed using 3D MHD and multi-fluid equations.

Abbo, L.↗

NASA NTRS2013

Energy Release from Impacting Prominence Material Following the 2011 June 7 Eruption

Solar filaments exhibit a range of eruptive-like dynamic activity, ranging from the full or partial eruption of the filament mass and surrounding magnetic structure as a coronal mass ejection to a fully confined or failed eruption. On 2011 June 7, a dramatic partial eruption of a filament was observed by multiple instruments on board the Solar Dynamics Observatory (SDO) and Solar-Terrestrial Relations Observatory. One of the interesting aspects of this event is the response of the solar atmosphere as non-escaping material falls inward under the influence of gravity. The impact sites show clear evidence of brightening in the observed extreme ultraviolet wavelengths due to energy release. Two plausible physical mechanisms for explaining the brightening are considered: heating of the plasma due to the kinetic energy of impacting material compressing the plasma, or reconnection between the magnetic field of low-lying loops and the field carried by the impacting material. By analyzing the emission of the brightenings in several SDO/Atmospheric Imaging Assembly wavelengths, and comparing the kinetic energy of the impacting material (7.6 × 10(exp 26) - 5.8 × 10(exp 27) erg) to the radiative energy (approx. 1.9 × 10(exp 25) - 2.5 × 10(exp 26) erg), we find the dominant mechanism of energy release involved in the observed brightening is plasma compression.

Sun: corona↗

NASA NTRS2012

Slow Magnetosonic Waves and Fast Flows in Active Region Loops

Recent extreme ultraviolet spectroscopic observations indicate that slow magnetosonic waves are present in active region (AR) loops. Some of the spectral data were also interpreted as evidence of fast (approx 100-300 km/s) quasiperiodic flows. We have performed three-dimensional magnetohydrodynamic (3D MHD) modeling of a bipolar AR that contains impulsively generated waves and flows in coronal loops. The model AR is initiated with a dipole magnetic field and gravitationally stratified density, with an upflow-driven steadily or periodically in localized regions at the footpoints of magnetic loops. The resulting flows along the magnetic field lines of the AR produce higher density loops compared to the surrounding plasma by injection of material into the flux tubes and the establishment of siphon flow.We find that the impulsive onset of flows with subsonic speeds result in the excitation of damped slow magnetosonic waves that propagate along the loops and coupled nonlinearly driven fast-mode waves. The phase speed of the slow magnetosonic waves is close to the coronal sound speed. When the amplitude of the driving pulses is increased we find that slow shock-like wave trains are produced. When the upflows are driven periodically, undamped oscillations are produced with periods determined by the periodicity of the upflows. Based on the results of the 3D MHD model we suggest that the observed slow magnetosonic waves and persistent upflows may be produced by the same impulsive events at the bases of ARs.

Magnetohydrodynamics-Sun Activity↗

NASA NTRS2011

Pulsed Flows Along a Cusp Structure Observed with SOO/AIA

We present observations of a cusp-shaped structure that formed after a flare and coronal mass ejection on 14 February 2011. Throughout the evolution of the cusp structure, blob features up to a few Mm in size were observed flowing along the legs and stalk of the cusp at projected speeds ranging from 50 to 150 km/sec. Around two dozen blob features, on order of 1 - 3 minutes apart, were tracked in multiple AlA EUV wavelengths. The blobs flowed outward (away from the Sun) along the cusp stalk, and most of the observed speeds were either constant or decelerating. We attempt to reconstruct the 3-D magnetic field of the evolving structure, discuss the possible drivers of the flows (including pulsed reconnect ion and tearing mode instability), and compare the observations to studies of pulsed reconnect ion and blob flows in the solar wind and the Earth's magnetosphere.

Thompson, Barbara↗

NASA NTRS2011

SDO/AIA Observation of Kelvin-Helmholtz Instability in the Solar Corona

We present observations of the formation, propagation and decay of vortex-shaped features in coronal images from the Solar Dynamics Observatory (SDO) associated with an eruption starting at about 2:30UT on Apr 8, 2010. The series of vortices formed along the interface between an erupting (dimming) region and the surrounding corona. They ranged in size from several to ten arcseconds, and traveled along the interface at 6-14 km s-1. The features were clearly visible in six out of the seven different EUV wavebands of the Atmospheric Imaging Assembly (AIA). Based on the structure, formation, propagation and decay of these features, we identified these features as the first observations of the Kelvin- Helmholtz (KH) instability in the corona in EUV. The interpretation is supported by linear analysis and by MHD model of KH instability. We conclude that the instability is driven by the velocity shear between the erupting and closed magnetic field of the Coronal Mass Ejection (CME).

Ofman, L.↗

NASA NTRS2010

Hybrid Model of Inhomogeneous Solar Wind Plasma Heating by Alfven Wave Spectrum: Parametric Studies

Observations of the solar wind plasma at 0.3 AU and beyond show that a turbulent spectrum of magnetic fluctuations is present. Remote sensing observations of the corona indicate that heavy ions are hotter than protons and their temperature is anisotropic (T(sub perpindicular / T(sub parallel) >> 1). We study the heating and the acceleration of multi-ion plasma in the solar wind by a turbulent spectrum of Alfvenic fluctuations using a 2-D hybrid numerical model. In the hybrid model the protons and heavy ions are treated kinetically as particles, while the electrons are included as neutralizing background fluid. This is the first two-dimensional hybrid parametric study of the solar wind plasma that includes an input turbulent wave spectrum guided by observation with inhomogeneous background density. We also investigate the effects of He++ ion beams in the inhomogeneous background plasma density on the heating of the solar wind plasma. The 2-D hybrid model treats parallel and oblique waves, together with cross-field inhomogeneity, self-consistently. We investigate the parametric dependence of the perpendicular heating, and the temperature anisotropy in the H+-He++ solar wind plasma. It was found that the scaling of the magnetic fluctuations power spectrum steepens in the higher-density regions, and the heating is channeled to these regions from the surrounding lower-density plasma due to wave refraction. The model parameters are applicable to the expected solar wind conditions at about 10 solar radii.

Ofman, L.↗

NASA NTRS2010

Winds from Luminous Late-Type Stars: II. Broadband Frequency Distribution of Alfven Waves

We present the numerical simulations of winds from evolved giant stars using a fully non-linear, time dependent 2.5-dimensional magnetohydrodynamic (MHD) code. This study extends our previous fully non-linear MHD wind simulations to include a broadband frequency spectrum of Alfven waves that drive winds from red giant stars. We calculated four Alfven wind models that cover the whole range of Alfven wave frequency spectrum to characterize the role of freely propagated and reflected Alfven waves in the gravitationally stratified atmosphere of a late-type giant star. Our simulations demonstrate that, unlike linear Alfven wave-driven wind models, a stellar wind model based on plasma acceleration due to broadband non-linear Alfven waves, can consistently reproduce the wide range of observed radial velocity profiles of the winds, their terminal velocities and the observed mass loss rates. Comparison of the calculated mass loss rates with the empirically determined mass loss rate for alpha Tau suggests an anisotropic and time-dependent nature of stellar winds from evolved giants.

Airapetian, V.↗

NASA NTRS2008

Development of Solar Wind Model Driven by Empirical Heat Flux and Pressure Terms

We are developing a time stationary self-consistent 2D MHD model of the solar corona and solar wind as suggested by Sittler et al. (2003). Sittler & Guhathakurta (1999) developed a semiempirical steady state model (SG model) of the solar wind in a multipole 3-streamer structure, with the model constrained by Skylab observations. Guhathakurta et al. (2006) presented a more recent version of their initial work. Sittler et al. (2003) modified the SG model by investigating time dependent MHD, ad hoc heating term with heat conduction and empirical heating solutions. Next step of development of 2D MHD models was performed by Sittler & Ofman (2006). They derived effective temperature and effective heat flux from the data-driven SG model and fit smooth analytical functions to be used in MHD calculations. Improvements of the Sittler & Ofman (2006) results now show a convergence of the 3-streamer topology into a single equatorial streamer at altitudes > 2 R(sub S). This is a new result and shows we are now able to reproduce observations of an equatorially confined streamer belt. In order to allow our solutions to be applied to more general applications, we extend that model by using magnetogram data and PFSS model as a boundary condition. Initial results were presented by Selwa et al. (2008). We choose solar minimum magnetogram data since during solar maximum the boundary conditions are more complex and the coronal magnetic field may not be described correctly by PFSS model. As the first step we studied the simplest 2D MHD case with variable heat conduction, and with empirical heat input combined with empirical momentum addition for the fast solar wind. We use realistic magnetic field data based on NSO/GONG data, and plan to extend the study to 3D. This study represents the first attempt of fully self-consistent realistic model based on real data and including semi-empirical heat flux and semi-empirical effective pressure terms.

Sittler, Edward C., Jr.↗

NASA NTRS2002

The Three-Dimensional EIT Wave

An EIT wave is an impulsive disturbance which has been observed in the EUV, Soft X-ray and white light corona, with corresponding observations in the chromosphere. The effects of these disturbances can be observed across the entire solar disk of the Sun, and throughout the inner heliosphere as well. However, the picture is not complete; observations alone do not establish a complete understanding of the nature of this three-dimensional phenomenon. A number of associated phenomena have been documented, though in most cases causality has not determined. Additionally, it is unclear which factors govern the impulse's ability to affect regions of the corona and heliosphere. We discuss the various observations and the models which provided links between the associated phenomena.

Thompson, B. J.↗

NASA NTRS2002

EUV Dimmings: Simple or Enigmatic?

EUV dimmings are large-scale depletions in coronal EUV emission associated with coronal mass ejections. Their occurrence is nearly 100% correlated with fast CMEs. Many of the EUV dimming observations appear to correspond well in appearance and behavior to SXR dimmings. It has been assumed that the dimmings are merely the location of the CME mass removal in the corona, and that their similar appearance to coronal holes is because they are transient coronal holes with fieldlines opened by the coronal mass ejection. However, not all CMEs have dimmings, and large-scale E W dimmings have also been caused by heating of local plasma, and due to absorption by cool material "clouds" evolving in the wake of an eruption. Additionally, there are several ways in which these regions "heal," presumably due to the closing down of the open field lines. The presentation will include a range of EUV dimming observations, a discussion of their origin, and what they might imply about the nature of the associated CMEs.

Thompson, B. J.↗

NASA NTRS2000

Winds From Luminous Late-Type Stars: The Effects of Nonlinear Alfven Waves - 1

We present the results of magnetohydrodynamic (MHD) modeling of winds from luminous late-type stars using a 2.5-dimensional, nonlinear MHD computer code. We assume that the wind is generated within an initially hydrostatic atmosphere and is driven by torsional Alfven waves generated at the stellar surface. Two cases of atmospheric topology are considered: case I has longitudinally uniform density distribution and isotropic radial magnetic field over the stellar surface, and case II has an isotropic, radial magnetic field with a transverse density gradient, which we refer to as an "atmospheric hole." We use the same set of boundary conditions for both models. The calculations are designed to model a cool luminous star, for which we assume an initial hydrostatic pressure scale height of 0.072 Stellar Radius, an Alfven wave speed of 92 km/s at the surface, and a wave period of 76 days, which roughly corresponds with the convective turnover time. For case I the calculations produce a wind with terminal velocity of about 22 km/s and a mass loss rate comparable to the expected value of 10(exp -6) Solar Mass/yr. For case II we predict a two-component wind: a fast (25 km/s) and relatively dense wind outside of the atmospheric hole and a slow (1.5 km/s), rarefied wind inside of the hole.

Airapetian, V. S.↗

NASA NTRS1999

TRACE Observation of Damped Coronal Loop Oscillations: Implicating for Coronal Heating

The imaging telescope on board the Transition Region and Coronal Explorer (TRACE) spacecraft observed the decaying transversal oscillations of a long [(130 +/- 6) x 10(exp 6) meters], thin [diameter (2.0 +/- 0.36) x 10(exp 6) meters], bright coronal loop in the 171 angstrom Fe-IX emission line. The oscillations were excited by a solar flare in the adjacent active region. The decay time of the oscillations is 12.1 +/- 6.7 minutes for an oscillation with a frequency 3.90 +/- 0.13 millihertz The coronal dissipation coefficient is estimated to be eight to nine orders of magnitude larger than the theoretically predicted classical value. The larger dissipation coefficient may solve existing difficulties with wave heating and reconnection theories.

Nakariakov, V. M.↗

NASA NTRS1997

IPS observations of the solar wind velocity and the acceleration mechanism

Coronal holes are well know sources of high speed solar wind, however, the exact acceleration mechanism of the wind is still unknown. Interplanetary scintillation (IPS) observations indicate that the fast solar wind reaches an average velocity of 800 km s(exp -1) within several solar radii with large velocity fluctuations. However, the origin of the IPS velocity spread below 10 solar radii is unclear. A previously developed coronal home model with a more realistic initial state is applied, and time-dependent, nonlinear, resistive 2.5-DMHD equations are numerically solved. It is found that nonlinear solitary-like waves with a supersonic phase speed are generated in coronal holes by torisonal Alfven waves in the radial flow velocity. The outward propagating nonlinear waves are similar in properties to sound solitons. When these waves are present, the solar wind speed and density fluctuate considerably on a time scale of an hour and on spatial scales of several solar radii in addition to the Alfvenic fluctuations. This is in qualitative agreement with the IPS velocity observations beyond 10 solar radii.

Ofman, L.↗

NASA NTRS1997

Possible signatures of nonlinear MHD waves in the solar wind: UVCS observations and models

Recent ultraviolet coronagraph spectrometer (UVCS) white light channel observations are discussed. These data indicated quasi-periodic variations in the polarized brightness in the polar coronal holes. The Fourier power spectrum analysis showed significant peaks at about six minutes and possible fluctuations on longer time scales. The observations are consistent with the predictions of the nonlinear solitary-like wave model. The purpose of a planned study on plume and inter-plume regions of coronal holes, motivated by the result of a 2.5 magnetohydrodynamic model (MHD), is explained.

Ofman, L.↗