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Linker, J. A.

Publications and source records attributed to Linker, J. A..

17 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2011

A Model for Magnetically Coupled Sympathetic Eruptions

Sympathetic eruptions on the Sun have been observed for several decades, but the mechanisms by which one eruption can trigger another remain poorly understood. We present a three-dimenSional MHD simulation that suggests two possible magnetic trigger mechanisms for sympathetic eruptions. We consider a configuration that contains two coronal flux ropes located within a pseudo-streamer and one rope located next to it. A sequence of eruptions is initiated by triggering the eruption of the flux rope next to the streamer. The expansion of the rope leads to two consecutive reconnection events, each of which triggers the eruption of a flux rope by removing a sufficient amount of overlying flux. The simulation qualitatively reproduces important aspects of the global sympathetic event on 2010 August 1 and provides a scenario for the so-called twin filament eruptions. The suggested mechanisms are also applicable for sympathetic eruptions occurring in other magnetic configurations.

Torok, T.↗
NASA NTRS2011

A Model fot the Sources of the Slow Solar Wind

Models for the origin of the slow solar wind must account for two seemingly contradictory observations: the slow wind has the composition of the closed-field corona, implying that it originates from the continuous opening and closing of flux at the boundary between open and closed field. On the other hand, the slow wind also has large angular width, up to approx.60deg, suggesting that its source extends far from the open-closed boundary. We propose a model that can explain both observations. The key idea is that the source of the slow wind at the Sun is a network of narrow (possibly singular) open-field corridors that map to a web of separatrices and quasi-separatrix layers in the heliosphere. We compute analytically the topology of an open-field corridor and show that it produces a quasi-separatrix layer in the heliosphere that extends to angles far from the heliospheric current sheet. We then use an MHD code and MDI/SOHO observations of the photospheric magnetic field to calculate numerically, with high spatial resolution, the quasi-steady solar wind, and magnetic field for a time period preceding the 2008 August 1 total solar eclipse. Our numerical results imply that, at least for this time period, a web of separatrices (which we term an S-web) forms with sufficient density and extent in the heliosphere to account for the observed properties of the slow wind. We discuss the implications of our S-web model for the structure and dynamics of the corona and heliosphere and propose further tests of the model. Key words: solar wind - Sun: corona - Sun: magnetic topology

Antiochos, S. K.↗
NASA NTRS2010

A Model for the Sources of the Slow Solar Wind

Models for the origin of the slow solar wind must account for two seemingly contradictory observations: The slow wind has the composition of the closed-field corona, implying that it originates from the continuous opening and closing of flux at the boundary between open and closed field. On the other hand, the slow wind has large angular width, up to approximately 60 degrees, suggesting that its source extends far from the open-closed boundary. We propose a model that can explain both observations. The key idea is that the source of the slow wind at the Sun is a network of narrow (possibly singular) open-field corridors that map to a web of separatrices and quasi-separatrix layers in the heliosphere. We compute analytically the topology of an open-field corridor and show that it produces a quasi-separatrix layer in the heliosphere that extends to angles far front the heliospheric current sheet. We then use an MHD code and MIDI/SOHO observations of the photospheric magnetic field to calculate numerically, with high spatial resolution, the quasi-steady solar wind and magnetic field for a time period preceding the August 1, 2008 total solar eclipse. Our numerical results imply that, at least for this time period, a web of separatrices (which we term an S-web) forms with sufficient density and extent in the heliosphere to account for the observed properties of the slow wind. We discuss the implications of our S-web model for the structure and dynamics of the corona and heliosphere, and propose further tests of the model.

Antiochos, Spiro K.↗
NASA NTRS2010

Magnetic Topology of Coronal Hole Linkages

In recent work, Antiochos and coworkers argued that the boundary between the open and closed field regions on the Sun can be extremely complex with narrow corridors of open ux connecting seemingly disconnected coronal holes from the main polar holes, and that these corridors may be the sources of the slow solar wind. We examine, in detail, the topology of such magnetic configurations using an analytical source surface model that allows for analysis of the eld with arbitrary resolution. Our analysis reveals three important new results: First, a coronal hole boundary can join stably to the separatrix boundary of a parasitic polarity region. Second, a single parasitic polarity region can produce multiple null points in the corona and, more important, separator lines connecting these points. Such topologies are extremely favorable for magnetic reconnection, because it can now occur over the entire length of the separators rather than being con ned to a small region around the nulls. Finally, the coronal holes are not connected by an open- eld corridor of finite width, but instead are linked by a singular line that coincides with the separatrix footprint of the parasitic polarity. We investigate how the topological features described above evolve in response to motion of the parasitic polarity region. The implications of our results for the sources of the slow solar wind and for coronal and heliospheric observations are discussed.

Titov, V. S.↗
NASA NTRS2008

1997 May 12 Coronal Mass Ejection Event. I. A Simplified Model of the Preeruptive Magnetic Structure

A simple model of the coronal magnetic field prior to the coronal mass ejection (CME) eruption on 1997 May 12 is developed. First, the magnetic field is constructed by superimposing a large-scale background field and a localized bipolar field to model the active region (AR) in the current-free approximation. Second, this potential configuration is quasi-statically sheared by photospheric vortex motions applied to two flux concentrations of the AR. Third, the resulting force-free field is then evolved by canceling the photospheric magnetic flux with the help of an appropriate tangential electric field applied to the central part of the AR. To understand the structure of the modeled configuration, we use the field line mapping technique by generalizing it to spherical geometry. We demonstrate that the initial potential configuration contains a hyperbolic flux tube (HFT) which is a union of two intersecting quasi-separatrix layers. This HFT provides a partition of the closed magnetic flux between the AR and the global solar magnetic field. Such a partition is approximate since the entire flux distribution is perfectly continuous. The vortex motions applied to the AR inter-lock the field lines in the coronal volume to form additionally two new HFTs pinched into thin current layers. Reconnection in these current layers helps to redistribute the magnetic flux and current within the AR in the flux-cancellation phase. In this phase, a magnetic flux rope is formed together with a bald patch separatrix surface wrapping around the rope. Other important implications of the identified structural features of the modeled configuration are also discussed.

Titov, V. S.↗
NASA NTRS2006

Maximizing the Scientific Return of the Sentinels Mission Using Global MHD Models

The Sentinels mission promises to provide a unique view of the acceleration and transport processes of energetic particles as well as the initiation and evolution of coronal mass ejections (CMEs). A crucial component in understanding the physics associated with these processes lies in the large-scale structure of the corona and heliosphere, particularly during the eruption and propagation of fast CMEs. In this talk we review the current status of our MHD modeling efforts, focusing on Sentinels-specific science, and project forward to envisage what capabilities we may have developed by the time that the Sentinels launch (2012). In conjunction with Solar Orbiter, the farside and near-Earth Sentinels spacecraft will provide simultaneous photospheric magnetograph measurements at multiple longitudes, which will lead to major improvements in our ability to prescribe accurate, time-dependent global boundary conditions. Data returned from the inner heliospheric Sentinels will be used to validate these results. The model results can be used in a variety of ways to interpret the observations. For example, products from the model, such as the properties of CME-driven shocks and CME-associated reconnection sites can be used to interpret complex energetic particle profiles. Also, by tracing along magnetic field lines, the inferred sites of the energetic particles can be connected directly with the in situ measurements at each spacecraft. Perhaps more so than any previous mission, sophisticated models will be required to unravel the broad and disparate measurements returned by the suite of Sentinels spacecraft.

Riley, P.↗
NASA NTRS2004

Fitting Flux Ropes to a Global MHD Solution: A Comparison of Techniques

Flux rope fitting (FRF) techniques are an invaluable tool for extracting information about the properties of a subclass of CMEs in the solar wind. However, it has proven difficult to assess their accuracy since the underlying global structure of the CME cannot be independently determined from the data. In contrast, large-scale MHD simulations of CME evolution can provide both a global view as well as localized time series at specific points in space. In this study we apply 5 different fitting techniques to 2 hypothetical time series derived from MHD simulation results. Independent teams performed the analysis of the events in "blind tests", for which no information, other than the time series, was provided. F rom the results, we infer the following: (1) Accuracy decreases markedly with increasingly glancing encounters; (2) Correct identification of the boundaries of the flux rope can be a significant limiter; and (3) Results from techniques that infer global morphology must be viewed with caution. In spite of these limitations, FRF techniques remain a useful tool for describing in situ observations of flux rope CMEs.

Riley, Pete↗
NASA NTRS2004

Fitting Flux Ropes to a Global MHD Solution: A Comparison of Techniques

Flux rope fitting (FRF) techniques are an invaluable tool for extracting information about the properties of a sub-class of coronal mass ejections (CMEs) in the solar wind. However, it has proven difficult to assess their accuracy since the underlying global structure of the CME cannot be independently determined from the data. In contrast, large-scale MHD simulations of CME evolution can provide both a global view as well as localized time series at specific points in space. In this study we apply five different fitting techniques to two hypothetical time series derived from MHD simulation results. Independent teams performed the analysis of the events in "blind tests", for which no information, other than time series, was provided. From the results, we infer the following: 1) Accuracy decreases markedly with increasingly glancing encounters; 2) Correct identification of the boundaries of the flux rope can be a significant limiter; 3) Results from techniques that infer global morphology must be viewed with caution. In spite of these limitations, FRF techniques remain a useful tool for describing in situ observations of flux rope CMEs.

Riley, Pete↗
NASA NTRS2003

Using an MHD simulation to interpret the global context of a coronal mass ejection observed by two spacecraft

In late February 1999 the ACE spacecraft observed a coronal mass ejection (CME) at 1 AU, in the ecliptic plane. Thirteen days later, Ulysses observed a CME at 5 AU and 22"s. We present a detailed analysis of the plasma, magnetic field, and composition signatures of these two events. On the basis of this comparison alone, it is not clear that the two spacecraft observed the same solar event. However, using a generic MHD simulation of a fast CME initiated at the Sun by magnetic flux cancellation and propagated out into the solar wind, together with additional evidence, we argue that indeed the same CME was observed by both spacecraft. Although force-free models appear to fit the observed events well, our simulation results suggest that the ejecta underwent significant distortion during its passage through the solar wind, indicating that care should be taken when interpreting the results of force-he models. Comparison of composition measurements at the two spacecraft suggests that significant spatial inhomogeneities can exist within a single CME.

Riley, Pete↗
NASA NTRS2003

Dynamical Evolution of the Inner Heliosphere Approaching Solar Activity Maximum: Interpreting Ulysses Observations Using a Global MHD Model

In this study we describe a series of MHD simulations covering the time period from 12 January 1999 to 19 September 2001 (Carrington Rotation 1945 to 1980). This interval coincided with: (1) the Sun s approach toward solar maximum; and (2) Ulysses second descent to the southern polar regions, rapid latitude scan, and arrival into the northern polar regions. We focus on the evolution of several key parameters during this time, including the photospheric magnetic field, the computed coronal hole boundaries, the computed velocity profile near the Sun, and the plasma and magnetic field parameters at the location of Ulysses. The model results provide a global context for interpreting the often complex in situ measurements. We also present a heuristic explanation of stream dynamics to describe the morphology of interaction regions at solar maximum and contrast it with the picture that resulted from Ulysses first orbit, which occurred during more quiescent solar conditions. The simulation results described here are available at: http://sun.saic.com.

Riley, Pete↗
NASA NTRS2002

EVIDENCE OF POSTERUPTION RECONNECTION ASSOCIATED WITH CORONAL MASS EJECTIONS IN THE SOLAR WIND

Using a coupled 2.5-dimensional, time-dependent MHD model of the solar corona and inner heliosphere, we have simulated the eruption and evolution of a coronal mass ejection containing a flux rope all the way from the Sun to 1 AU. Although idealized, we find that the simulation reproduces many generic features of magnetic clouds. In this paper we report on a new, intriguing aspect of these comparisons. Specifically, the results suggest that jetted outflow, driven by posteruptive reconnection underneath the flux rope, occurs and may remain intact out to 1 AU and beyond. We present an example of a magnetic cloud with precisely these signatures and show that the velocity perturbations are consistent with reconnection outflow. We suggest that other velocity and/or density enhancements observed trailing magnetic clouds may be signatures of such reconnection and, in some cases, may not be associated with prominence material, as has previously been suggested.

Riley, Pete↗
NASA NTRS1995

Large-scale structure of the solar corona and inner heliosphere

The large-scale structure of the solar corona influences solar activity particularly coronal mass ejections (CMEs). The helmet streamers that are observed to dominate the structure of the inner corona are formed by the interaction of the solar wind with coronal magnetic fields. We have simulated this interaction in three dimensions using the magnetohydrodynamic (MHD) equations. In order to create a realistic model, we use the magnetic field that is observed at the Sun's surface (deduced from daily Wilcox Solar Observatory magnetograms) as input, in combination with specified density and temperature profiles at the surface. A self-consistent 3D solar-wind solution is developed by integrating the MHD equations in time to steady state. Such solutions can reproduce the observed structures that are seen in coronagraph images and eclipse photographs of the corona. This model allows us to accurately determine the position of the heliospheric current sheet. We will compare the results obtained from our model with Ulysses observations during the period May-June 1993, and with an eclipse photograph of the corona on November 3, 1994.

Mikic, Z.↗
NASA NTRS1995

Coronal mass ejections and the evolution of the large-scale corona

Coronal mass ejections (CMEs) are dynamic, large-scale events in the solar corona that expel plasma and magnetic fields into the solar wind. The structure of the large-scale corona influences CME occurrence; for example, CMEs are most frequently observed to be disruptions of coronal or helmet streamer configurations on the Sun. We have investigated the evolution of the large-scale corona in the presence of differential rotation, using time-dependent magnetohydrodynamic (MHD) simulations in two and three dimensions. An equilibrium coronal configuration is first computed by superimposing a Parker solar wind flow on an initial potential field, the equations are integrated in time until a steady-state is reached. When differential rotation is introduced, we find that the configuration does not reach a steady state; the closed field regions (helmet streamers) disrupt recurrently, ejecting plasmoids into the solar wind. Our results suggest that differential rotation may be one of the mechanisms by which mass ejections are initiated. We will describe the evolution of the polarization brightness (pB) for the resulting configurations, as well as the propagation of the ejected plasmoids in the solar wind, and we will compare the effects of differential rotation with other possible initiation mechanisms, such as the emergence of new magnetic flux.

Linker, J. A.↗
NASA NTRS1990

MHD simulations of coronal mass ejections - Importance of the driving mechanism

The importance of the form of the driving mechanism in MHD simulations of coronal mass ejections is investigated. A model simulation problem is devised, and it is found that the use of a simple form for the initial corona, with an upward moving parcel of cold, dense plasma as the driving mechanism, can produce results that are consistent with many of the features observed by coronagraphs. The results imply that the nature of the driving mechanism may play an important role in determining the dynamical evolution of mass ejections.

Linker, J. A.↗
NASA NTRS1990

Effects of the driving mechanism in MHD simulations of coronal mass ejections

Results of time-dependent MHD simulations of mass ejections in the solar coronal are presented. Previous authors have shown that results from simulations using a thermal driving mechanism are consistent with the observations only if an elaborate model of the initial corona is used. The first simulation effort, using a simple model of a plasmoid as the driving mechanism and a simple model of the initial corona, produces results that are also consistent with many observational features, suggesting that the nature of the driving mechanism plays an important role in determining the subsequent evolution of mass ejections. First simulations are based on the assumption that mass ejections are driven by magnetic forces.

Linker, J. A.↗
NASA NTRS1985

Explanation of the inward displacement of Io's hot plasma torus and consequences for sputtering sources

Radial profiles of the ion density and flux-tube content in the Io torus have peak values inside Io's orbit, even though Io is the effective source of these ions. Formation of an inward peak constrains either the velocity distributions or source regions of sputtered neutrals. A further constraint is that the ionization of neutrals on trapped trajectories that return to Io's surface must be limited. A dominant sulphur source is most easily reconciled with these constraints.

Linker, J. A.↗