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NASA NTRS1978

Magnetic loops, downflows, and convection in the solar corona

Optical and extreme-ultraviolet observations of solar loop structures show that flows of cool plasma from condensations near the loop apex are a common property of loops associated with radiations whose maximum temperature is greater than approximately 7000 K and less than approximately 3,000,000 K. It is suggested that the mass balance of these structures indicates reconnection by means of plasma motion across field lines under rather general circumstances (not only after flares). It is shown that the cool material has lower gas pressure than the surrounding coronal medium. The density structure of the bright extreme ultraviolet loops suggests that downflows of cool gas result from isobaric condensation of plasma that is either out of thermal equilibrium with the local energy deposition rate into the corona, or is thermally unstable. The evidence is thought to indicate that magnetic fields act to induce a pattern of forced convection.

Foukal, P.↗

NASA NTRS2013

Global-Scale Consequences of Magnetic-Helicity Injection and Condensation on the Sun

In the recent paper of Antiochos, a new concept for the injection of magnetic helicity into the solar corona by small-scale convective motions and its condensation onto polarity inversion lines (PILs) has been developed. We investigate this concept through global simulations of the Sun's photospheric and coronal magnetic fields and compare the results with the hemispheric pattern of solar filaments. Assuming that the vorticity of the cells is predominately counter-clockwise/clockwise in the northern/southern hemisphere, the convective motions inject negative/positive helicity into each hemisphere. The simulations show that: (i) On a north-south orientated PIL, both differential rotation and convective motions inject the same sign of helicity which matches that required to reproduce the hemispheric pattern of filaments. (ii) On a high latitude east-west orientated polar crown or sub-polar crown PIL, the vorticity of the cells has to be approximately 2-3 times greater than the local differential rotation gradient in order to overcome the incorrect sign of helicity injection from differential rotation. (iii) In the declining phase of the cycle, as a bipole interacts with the polar field, in some cases helicity condensation can reverse the effect of differential rotation along the East-West lead arm, but not in all cases. The results show that this newly developed concept of magnetic helicity injection and condensation is a viable method to explain the hemispheric pattern of filaments in conjunction with the mechanisms used in Yeates et al. (2008). Future observational studies should focus on determining the vorticity component within convective motions to determine, both its magnitude and latitudinal variation relative to the differential rotation gradient on the Sun.

magnetic fields↗

NASA NTRS2017

The Role of Magnetic Helicity in Structuring the Solar Corona

Two of the most widely observed and striking features of the Suns magnetic field are coronal loops, which are smooth and laminar, and prominences or filaments, which are strongly sheared. Loops are puzzling because they show little evidence of tangling or braiding, at least on the quiet Sun, despite the chaotic nature of the solar surface convection. Prominences are mysterious because the origin of their underlying magnetic structure filament channels is poorly understood at best. These two types of features would seem to be quite unrelated and wholly distinct. We argue that, on the contrary, they are inextricably linked and result from a single process: the injection of magnetic helicity into the corona by photospheric motions and the subsequent evolution of this helicity by coronal reconnection. In this paper, we present numerical simulations of the response of a Parker (1972) corona to photospheric driving motions that have varying degrees of helicity preference. We obtain four main conclusions: (1) in agreement with the helicity condensation model of Antiochos (2013), the inverse cascade of helicity by magnetic reconnection in the corona results in the formation of filament channels localized about polarity inversion lines; (2) this same process removes most complex fine structure from the rest of the corona, resulting in smooth and laminar coronal loops; (3) the amount of remnant tangling in coronal loops is inversely dependent on the net helicity injected by the driving motions; and (4) the structure of the solar corona depends only on the helicity preference of the driving motions and not on their detailed time dependence. We discuss the implications of our results for high-resolution observations of the corona.

Knizhnik, K. J.↗

NASA NTRS1987

Coloration and darkening of methane clathrate and other ices by charged particle irradiation - Applications to the outer solar system

The results of laboratory experiments simulating the irradiation of hydrocarbon-H2O or hydrocarbon-H2O/NH3 clathrates by charged particles in the outer solar system are reported. Ices produced by condensing and boiling liquid CH4 on an H2O frost surface at 100 K or by cocondensing frosts from gaseous mixtures were exposed to coronal-discharge electron irradiation at 77 K, and the spectral properties of the irradiated surfaces were determined. Significant darkening of the initially white ices was observed at doses of 1 Gerg/sq cm, corresponding to 8-500 yrs of irradiation by Uranian magnetospheric electrons on the surfaces of the principal Uranian satellites, or to total destruction of CH4 in the upper 1 mm of the satellite surfaces after 0.05-3.0 Myr. It is estimated that 10 m or more of icy satellite or comet surfaces would be radiation-hardened to a CH4-free ice-tholin mixture over 4 Gyr.

Thompson, W. Reid↗

NASA NTRS1993

ICE/ISEE plasma wave data analysis

This report is one of the final processing of ICE plasma wave (pw) data and analysis of late ISEE 3, ICE cometary, and ICE cruise trajectory data, where coronal mass ejections (CME's) were the first locus of attention. Interest in CME's inspired an effort to represent our pw data in a condensed spectrogram format that facilitated rapid digestion of interplanetary phenomena on long (greater than 1 day) time scales. The format serendipitously allowed us to also examine earth-orbiting data from a new perspective, invigorating older areas of investigation in Earth's immediate environment. We, therefore, continued to examine with great interest the last year of ISEE 3's precomet phase, when it spent considerable time far downwind from Earth, recording for days on end conditions upstream, downstream, and across the very weak, distant flank bow shock. Among other motivations has been the apparent similarity of some shock and post shock structures to the signatures of the bow wave surrounding comet Giacobini-Zinner, whose ICE-phase data we revisited.

Greenstadt, E. W.↗

NASA NTRS2013

Prominence Mass Supply and the Cavity

A prevalent but untested paradigm is often used to describe the prominence-cavity system; the cavity is under-dense because it it evacuated by supplying mass to the condensed prominence. The thermal non-equilibrium (TNE) model of prominence formation offers a theoretical framework to predict the thermodynamic evolutin of the prominence and the surrounding corona. We examine the evidence for a prominence-cavity connection by comparing the TNE model and diagnostics of dynamic extreme ultraviolet (EUV) emission surrounding the prominence, specifically prominence horns. Horns are correlated extensions of prminence plasma and coronal plasma which appear to connect the prominence and cavity. The TNE model predicts that large-scale brightenings will occur in the Solar Dynamics Observatory Atmospheric Imaging Assembly 171 A badpass near he prominence that are associated with the cooling phase of condensation formation. In our simulations, variations in the magnitude of footpoint heating lead to variations in the duration, spatial scale, and temporal offset between emission enhancements in the other EUV bandpasses. While these predictions match well a subset of the horn observations, the range of variations in the observed structures is not captured by the model. We discuss the implications of one-dimensional loop simulations for the three-dimensional time-averaged equilibrium in the prominence and the cavity. Evidence suggests that horns are likely caused by condensing prominence plasma, but the larger question of whether this process produces a density-depleted cavity requires a more tightly constrained model of heating and better knowledge of the associated magnetic structure.

PROMINENCES↗

NASA NTRS1984

Condensation modes in magnetized cylindrical plasmas

In order to understand the formation and stability of solar prominences, attention is given to the condensation modes of the thermal instability in a cylindriical plasma whose magnetic field has both potential (longitudinal) and nonpotential (poloidal) components forming helically twisted field lines. It is noted that the field line twist has a significant effect on the stability of condensation modes which are unstable if field lines are straight, but become stable as the twist increases. The problem is treated in a fully self-consistent way, to derive a second-order ordinary differential equation for radiative MHD stability. The approximation made by neglecting inertial terms is valid for coronal loop conditions if there is no singular surface in the plasma, and a simplified differential equation is derived. Attention is given to stability for the m = zero, 1, and 2 modes numerically, for various loop parameters.

An, C.-H.↗

NASA NTRS1989

H-alpha redshifts as a diagnostic of solar flare heating

The dynamics of chomospheric condensations formed during chromospheric evaporation are studied using coordinated X-ray and H-alpha observations of five solar flares. It is shown that the peak downflow velocity of condensations predicted by simple hydrodynamic compression of the chromosphere is in good agreement with empirical downflow velocities implied by impulsive phase H-alpha red wing Doppler shifts. It is found that the H-alpha wing redshift provides a diagnostic of the pressure excess in the evaporating region and the coronal energy flux driving chromospheric evaporation. Because preflare coronal pressures derived for each of the flares are high, it is suggested that the chromosphere succeeds in radiating excess flare heat energy without undergoing explosive chromospheric evaporation.

Zarro, Dominic M.↗

NASA NTRS2019

A Model for Energy Buildup and Eruption Onset in Coronal Mass Ejections

Coronal mass ejections (CMEs) and eruptive flares (EFs) are the most energetic explosions in the solar system. Their underlying origin is the free energy that builds up slowly in the sheared magnetic field of a filament channel. We report the first end-to-end numerical simulation of a CME/EF, from zero-free-energy initial state through filament channel formation to violent eruption, driven solely by the magnetic-helicity condensation process. Helicity is the topological measure of linkages between magnetic flux systems, and is conserved in the corona, building up inexorably until it is ejected into interplanetary space. Numerous investigations have demonstrated that helicity injected by small-scale vortical motions, such as those observed in the photosphere, undergoes an inverse cascade from small scales to large, “condensing” at magnetic-polarity boundaries. Our new results verify that this process forms a filament channel within a compact bipolar region embedded in a background dipole field, and show for the first time that a fast CME eventually occurs via the magnetic-breakout mechanism. We further show that the trigger for explosive eruption is reconnection onset in the flare current sheet that develops above the polarity inversion line: this reconnection forms flare loops below the sheet and a CME flux rope above, and initiates high-speed outward flow of the CME. Our findings have important implications for magnetic self organization and explosive behavior in solar and other astrophysical plasmas, as well as for understanding and predicting explosive solar activity.

filaments↗

NASA NTRS1988

Thermal instability of a radiative and resistive coronal plasma

Thermal instability is believed to determine the evolution and formation of cool structures in the solar atmosphere such as the transition region and prominences (or filaments). The linear modes that arise in a sheared, force-free, magnetic field due to thermal instability are studied numerically. Previous studies have considered separately modes that arise due to the effects of radiation, compression, anisotropic thermal conduction, and ohmic heating. Here the results of such studies are integrated, first by presenting simple arguments that illustrate the essential physics of ideal, sheared-field, condensation modes, and second by showing numerically how finite resistivity affects the condensational instability in parameter regimes applicable to the solar corona.

Sparks, L.↗

NASA NTRS1990

Radiative magnetized thermal conduction fronts

The evolution of plane-parallel magnetized thermal conduction fronts in the interstellar medium (ISM) was studied. Separating the coronal ISM phase and interstellar clouds, these fronts have been thought to be the site of the intermediate-temperature regions whose presence was inferred from O VI absorption-line studies. The front evolution was followed numerically, starting from the initial discontinuous temperature distribution between the hot and cold medium, and ending in the final cooling stage of the hot medium. It was found that, for the typical ISM pressure of 4000 K/cu cm and the hot medium temperature of 10 to the 6th K, the transition from evaporation to condensation in a nonmagnetized front occurs when the front thickness is 15 pc. This thickness is a factor of 5 smaller than previously estimated. The O VI column densities in both evaporative and condensation stages agree with observations if the initial hot medium temperature Th exceeds 750,000 K. Condensing conduction fronts give better agreement with observed O VI line profiles because of lower gas temperatures.

Borkowski, Kazimierz J.↗

NASA NTRS1984

The effect of line-tying on the radiative MHD stability of coronal plasmas with radial pressure profile

The role of photospheric line-tying, i.e., solar coronal loop structures, was investigated in terms of the effect on radiative modes and the influence that different radial pressure profiles exert on the effects of line-tying on radiative MHD stability. Energy is assumed dissipated by heat conduction and radiation and zero- and first-order solutions are obtained for the radiative time scales. Line-tying is a magnetic tension in the zero-order MHD mode and produces stability. Heat conduction occurs along bent field lines in first-order MHD modes when plasmas cross the field lines. Irradiated cool-core loops can experience MHD instabilities in the cylinder center, while line-tying can stabilize the plasma in the surrounding hot medium. Line-tying also adds stability to magnetosonic and condensation modes.

An, C.-H.↗

NASA NTRS1992

ICE/ISEE plasma wave data analysis

The interval reported on, from Jan. 1990 to Dec. 1991, has been one of continued processing and archiving of ICE plasma wave (pw) data and transition from analysis of ISEE 3 and ICE cometary data to ICE data taken along its cruise trajectory, where coronal mass ejections are the focus of attention. We have continued to examine with great interest the last year of ISEE 3's precomet phase, when it spent considerable time far downwind from Earth, recording conditions upstream, downstream, and across the very weak, distant flank bow shock. Among other motivations was the apparent similarity of some shock and post shock structures to the signatures of the bow wave surrounding comet Giacobini-Zinner, whose ICE-phase data was revisited. While pursuing detailed, second-order scientific inquiries still pending from the late ISEE 3 recordings, we have also sought to position ourselves for study of CME's by instituting a data processing format new to the ISEE 3/ICE pw detector. Processed detector output has always been summarized and archived in 24-hour segments, with all pw channels individually plotted and stacked one above the next down in frequency, with each channel calibrated separately to keep all data patterns equally visible in the plots, regardless of gross differences in energy content at the various frequencies. Since CME's, with their preceding and following solar wind plasmas, can take more than one day to pass by the spacecraft, a more condensed synoptic view of the pw data is required to identify, let alone assess, CME characteristics than has been afforded by the traditional routines. This requirement is addressed in a major new processing initiative in the past two years. Besides our own ongoing and fresh investigations, we have cooperated, within our resources, with studies conducted extramurally by distant colleagues irrespective of the phase of the ISEE 3/ICE mission under scrutiny. The remainder of this report summarizes our processing activities, our investigations, both internal and cooperative, our scientific results, and our publication activity.

Greenstadt, E. W.↗