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At least 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
NASA NTRS1997

The quiescent corona and slow solar wind

The observations of the ultraviolet coronagraph spectrometer (UVCS), operating onboard the Solar and Heliospheric Observatory (SOHO) spacecraft, are discussed. The purpose of the UVCS is the study of the quiescent coronal streamer and the slow solar wind. The observations started in January 1996. Polarized radiance data in the visible continuum were obtained. Some characteristics of the coronal streamer from the UVCS recorded data are discussed. A model for the source of the slow solar wind in the inner corona is proposed.

Noci, G.↗
NASA NTRS2017

Is There a CME Rate Floor? CME and Magnetic Flux Values for the Last Four Solar Cycle Minima

The recent prolonged activity minimum has led to the question of whether there is a base level of the solar magnetic field evolution that yields a ''floor'' in activity levels and also in the solar wind magnetic field strength. Recently, a flux transport model coupled with magneto-frictional simulations has been used to simulate the continuous magnetic field evolution in the global solar corona for over 15 years, from 1996 to 2012. Flux rope eruptions in the simulations are estimated (Yeates), and the results are in remarkable agreement with the shape of the SOlar Heliospheric Observatory/Large Angle and Spectrometric Coronagraph Experiment coronal mass ejection (CME) rate distribution. The eruption rates at the two recent minima approximate the observed-corrected CME rates, supporting the idea of a base level of solar magnetic activity. In this paper, we address this issue by comparing annual averages of the CME occurrence rates during the last four solar cycle minima with several tracers of the global solar magnetic field. We conclude that CME activity never ceases during a cycle, but maintains a base level of 1 CME every 1.5 to approx. 3 days during minima. We discuss the sources of these CMEs.

Webb, D. F.↗
NASA NTRS2023

Deriving Large Coronal Magnetic Loop Parameters Using LOFAR J Burst Observations

Large coronal loops around one solar radius in altitude are an important connection between the solar wind and the low solar corona. However, their plasma properties are ill-defined, as standard X-ray and UV techniques are not suited to these low-density environments. Diagnostics from type J solar radio bursts at frequencies above 10 MHz are ideally suited to understand these coronal loops. Despite this, J-bursts are less frequently studied than their type III cousins, in part because the curvature of the coronal loop makes them unsuited for using standard coronal density models. We used LOw-Frequency-ARray (LOFAR) and Parker Solar Probe (PSP) solar radio dynamic spectrum to identify 27 type III bursts and 27 J-bursts during a solar radio noise storm observed on 10 April 2019. We found that their exciter velocities were similar, implying a common acceleration region that injects electrons along open and closed magnetic structures. We describe a novel technique to estimate the density model in coronal loops from J-burst dynamic spectra, finding typical loop apex altitudes around 1.3 solar radius. At this altitude, the average scale heights were 0.36 solar radius, the average temperature was around 1 MK, the average pressure was 0.7mdyn cm -2 , and the average minimum magnetic field strength was 0.13 G. We discuss how these parameters compare with much smaller coronal loops.

Jinge Zhang↗
NASA NTRS1977

A survey on initial results of the Helios plasma experiment

The reported results are mainly based on data obtained during the first part of the Helios-1 mission. The sun was very quiet during the considered period. The data are, therefore, representative for comparatively stationary conditions in the solar corona. Data obtained concerning the protons were evaluated by means of a special computer routine. The significance of the data is discussed, taking into account the status of the corona and the interplanetary plasma during the considered part of the Helios-1 mission, fast stream structures in the region between 0.3 and 1 AU, radial gradients of fast and slow solar wind, and the separation of proton double streams and alpha-particles. Attention is also given to the 'strahl' in the electron distribution, differences between fast streams and slow plasma on the basis of the observed electron distributions, and radial gradients in the case of solar wind electron parameters.

Rosenbauer, H.↗
NASA NTRS2023

Feasibility Study of Measuring Degree of Linear Polarization of the Solar F-Corona Using Filter Observations on the COronal Diagnostic EXperiment

One of the most unrevealed pieces of information about solar F-corona is its polarization. We propose the possibility of measuring the degree of linear polarization (D F ) of the F-corona along the radial distance from the Sun using the signal of two filters installed on the COronal Diagnostic EXperiment (CODEX), which will be mounted on board the International Space Station in December 2023. By analyzing the signal and noise of CODEX with Monte-Carlo simulations, we can derive D F with a 1.4 nm-width narrow bandpass filter centered at 393.55 nm and a 10 nm-width broad bandpass filter centered at 393.5 nm by stacking six images and integrating over 1R ⊙ × 1R ⊙ . The D F measured by CODEX will help reduce the uncertainty of the K-coronal polarization (pB K ), a main target of the mission, as well as to provide a better understanding of the F-corona.

Solar F corona↗
NASA NTRS1983

Solar maximum mission: Ground support programs at the Harvard Radio Astronomy Station

Observations of the spectral characteristics of solar radio bursts were made with new dynamic spectrum analyzers of high sensitivity and high reliability, over the frequency range 25-580 MHz. The observations also covered the maximum period of the current solar cycle and the period of international cooperative programs designated as the Solar Maximum Year. Radio data on shock waves generated by solar flares were combined with optical data on coronal transients, taken with equipment on the SMM and other satellites, and then incorporated into computer models for the outward passage of fast-mode MHD shocks through the solar corona. The MHD models are non-linear, time-dependent and for the most recent models, quasi-three-dimensional. They examine the global response of the corona for different types of input pulses (thermal, magnetic, etc.) and for different magnetic topologies (for example, open and closed fields). Data on coronal shocks and high-velocity material ejected from solar flares have been interpreted in terms of a model consisting of three main velocity regimes.

Maxwell, A.↗
NASA NTRS1994

Coronal streamers as detected with the SPARTAN 201-01 white light coronagraph

The preliminary results concerning the fine scale structure analysis of the streamers on the east limb and the region between these streamers, as well as their morphological and physical characteristics are presented. The results were obtained using white light polarized brightness data. The solar corona was observed with an externally occulted white light coronagraph carried on the SPARTAN 201-01 spacecraft for a 47 h period beginning on DOY 101, 1993. At this phase of the solar magnetic activity cycle there were well developed coronal helmet streamers located over both the east and west limbs of the sun. The photometric properties of one streamer found near the south east limb of the sun are similar to those measured of helmet streamers at the time of the 1973 total eclipse by both the ground based white light coronal camera and the Skylab externally occulted coronagraph.

Fisher, Richard↗
NASA NTRS1995

Inference of 3-dimensional structure underlying large-scale coronal events observed by Yohkoh and Ulysses

The Yohkoh/SXT images provide full-disk coverage of the solar corona, usually extending before and after one of the large-scale eruptive events that occur in the polar crown These produce large arcades of X-ray loops, often with a cusp-shaped coronal extension, and are known to be associated with coronal mass ejections. The Yohkoh prototype of such events occurred 12 Nov. 1991. This allows us to determine heights from the apparent rotation rates of these structures. In comparison v with magnetic-field extrapolations from Wilcox Solar Observatory. use use this tool to infer the three dimensional structure of the corona in particular cases: 24 Jan. 1992, 24 Feb. 1993, 14 Apr. 1994, and 13 Nov. 1994. The last event is a long-duration flare event.

Slater, G. L.↗
NASA NTRS1988

Estimated wave speeds in coronal holes and streamers

In this paper, characteristic wave speeds in the solar corona are made based on published models of a coronal hole and streamer. It is found that the Alfven speed varies from over 4000 km/s at the center of the coronal hole at 2.0 solar radii down to about 100 km/s in the center of the streamer at 5.0 solar radii. Taking into account the bias in the streamer model, a more realistic estimate of the minimum Alfven speed is 800 km/s at the same location. The sound speed lies between 100 and 200 km/s everywhere between 2.0 and 5.0 solar radii while the flow speed is generally less than the sound speed at 2.0 solar radii and greater than the sound speed at 5.0 solar radii.

Suess, S. T.↗
NASA NTRS1993

Chromospheric and coronal observations with multilayer optics

The first high resolution X-ray images of an astronomical object (the solar corona) formed with normal incidence multilayer optics, were obtained in late 1987. We review the developments which have occurred in multilayer optics technology since 1987, and discuss the advantages that these developments present for solar observations. The most significant advantages of multilayer optics are: (1) telescopes with modest apertures (about 0.1-0.5 meters) can achieve images with very high (about 0.1-0.3 arcsec) resolution; and (2) the spectral selectivity of multilayers permits the investigation of thermal structures with resolution T/(Delta)T is about 5-10. We describe the analysis of polar plumes observed in 1987 and of small X-ray emitting regions called 'bright points' observed in 1991 to illustrate the power of multilayer optics for astronomical studies.

Walker, Arthur B. C., Jr.↗
NASA NTRS1995

Large-scale structure of solar wind as observed on the Prognoz 7 satellite

Properties of different solar wind streams depend on the large scale structure of coronal magnetic field and dynamical phenomena in the solar atmosphere. We present average values and distributions of MHD parameters (density, velocity, temperature, fluxes of mass, momentum and energy, ratio of thermal and magnetic pressures, as well as helium abundance) as observed on board the Prognoz 7 satellite in the different types of the solar wind streams connected with solar corona structure and phenomena: (1) heliospheric current sheet, (2) streams from coronal holes, (3) streams from coronal streamers, (4) plasma disturbed by interplanetary shocks, and (5) coronal mass ejections. As for quasistationary streams of solar wind, maximum mass flux is recorded in the streams emanating from the coronal streamers while maximum thermal and kinetic energy fluxes are observed in the streams from the coronal holes. The momentum fluxes are equal in both types of streams. Maximum ratio of thermal and magnetic pressures is observed in heliospheric current sheet. The maximum helium abundance is observed in coronal mass ejection, it is higher in streams from coronal holes than in streams from streamers, and its dependences on density and mass flux are different in different types of the streams. Dynamics of alpha-particle velocity and temperature relative to protons in different streams is discussed.

Yermolaev, Yu. I.↗
NASA NTRS2023

Simulation of the Solar Energetic Particle Event on 2020 May 29 Observed by Parker Solar Probe

This paper presents a stochastic three-dimensional focused transport simulation of solar energetic particles (SEPs) produced by a data-driven coronal mass ejection (CME) shock propagating through a data-driven model of coronal and heliospheric magnetic fields. The injection of SEPs at the CME shock is treated using diffusive shock acceleration of post-shock suprathermal solar wind ions. A time-backward stochastic simulation is employed to solve the transport equation to obtain the SEP time–intensity profile at any location, energy, and pitch angle. The model is applied to a SEP event on 2020 May 29, observed by STEREO-A close to ∼1 au and by Parker Solar Probe (PSP) when it was about 0.33 au away from the Sun. The SEP event was associated with a very slow CME with a plane-of-sky speed of 337 km s −1 at a height below 6 RS as reported in the SOHO/LASCO CME catalog. We compute the time profiles of particle flux at PSP and STEREO-A locations, and estimate both the spectral index of the proton energy spectrum for energies between ∼2 and 16 MeV and the equivalent path length of the magnetic field lines experienced by the first arriving SEPs. We find that the simulation results are well correlated with observations. The SEP event could be explained by the acceleration of particles by a weak CME shock in the low solar corona that is not magnetically connected to the observers.

Solar energetic particles↗
NASA NTRS1999

A New View of the Origin of the Solar Wind

This paper uses white-light measurements made by the SOHO LASCO coronagraph and HAO Mauna Loa Mk III K-coronameter to illustrate the new view of solar wind structure deduced originally from radio occultation measurements. It is shown that the density profile closest to the Sun at 1.15 Ro, representing the imprint of the Sun, is carried essentially radially into interplanetary space by small-scale raylike structures that permeate the solar corona and which have only been observed by radio occultation measurements. The only exception is the small volume of interplanetary space occupied by the heliospheric plasma sheet that evolves from coronal streamers within a few solar radii of the Sun. The radial preservation of the density profile also implies that a significant fraction of field lines which extend into interplanetary space originate from the quiet Sun, and are indistinguishable in character from those emanating from polar coronal holes. The white-light measurements dispel the long-held belief that the boundaries of polar coronal holes diverge significantly, and further support the view originally proposed that the fast solar wind originates from the quiet Sun as well as polar coronal holes.

Woo, Richard↗
NASA NTRSDate not supplied

Initial Results from NuSTAR Co-Observation with MaGIXS-2 at Solar Maximum

NuSTAR is a solar-capable hard x-ray (HXR) astrophysical observatory sensitive to emission from hot (> MK) plasma and accelerated particles. These are both crucial for understanding energy release in the solar corona, including the processes that heat active regions during both flaring and quiescent times. As NuSTAR was optimized for much fainter astrophysical sources, it best observes the Sun when it is relatively quiet: most NuSTAR solar studies have occurred when the baseline GOES 1-8 Å flux ranges from sub-A to mid-B level. MaGIXS is a soft x-ray (SXR) rocket-borne imaging spectrometer, which is also sensitive to very hot plasma. The mutual high-temperature sensitivity of NuSTAR and MaGIXS (unique to instruments that directly focus x-ray light) makes co-observation highly desirable for understanding active region heating. Because of this, the NuSTAR Heliophysics team elected to conduct a campaign for the MaGIXS-2 flight despite a high level of solar activity (C-level baseline GOES flux) – conditions in which we would normally not observe. We discuss the potential of the resulting dataset, which is significantly impacted by NuSTAR’s limitations in the high-flux regime. We will present initial results from this co-observation campaign, discussing both the scientific potential of the NuSTAR observations as well as new insights into NuSTAR’s performance in high-rate conditions.

Jessie Duncan↗
NASA NTRS2015

An MHD Code for the Study of Magnetic Structures in the Solar Wind

We have developed a 2.5D MHD code designed to study how the solar wind influences the evolution of transient events in the solar corona and inner heliosphere. The code includes thermal conduction, coronal heating and radiative cooling. Thermal conduction is assumed to be magnetic field-aligned in the inner corona and transitions to a collisionless formulation in the outer corona. We have developed a stable method to handle field-aligned conduction around magnetic null points. The inner boundary is placed in the upper transition region, and the mass flux across the boundary is determined from 1D field-aligned characteristics and a 'radiative energy balance' condition. The 2.5D nature of this code makes it ideal for parameter studies not yet possible with 3D codes. We have made this code publicly available as a tool for the community. To this end we have developed a graphical interface to aid in the selection of appropriate options and a graphical interface that can process and visualize the data produced by the simulation. As an example, we show a simulation of a dipole field stretched into a helmet streamer by the solar wind. Plasmoids periodically erupt from the streamer, and we perform a parameter study of how the frequency and location of these eruptions changed in response to different levels of coronal heating. As a further example, we show the solar wind stretching a compact multi-polar flux system. This flux system will be used to study breakout coronal mass ejections in the presence of the solar wind.

corona↗
NASA NTRS1983

Quiescent coronae of active chromosphere stars

Quiescient X-ray emission from M dwarfs and the changes in quiescent levels on time scales from hours to about one year are discussed based on Einstein Observatory data. The coronal temperatures for many of these stars are determined; they are generally hotter than the solar corona and some of the more active dM stars have coronal temperatures of about 10 million K. Arguments in support of the hypothesis that M-dwarf coronae are magnetically dominated are presented. The usefulness of loop model atmosphere calculations in elucidating the coronal heating mechanism and the ways in which observations may be used to test competing theories are examined. The X-ray measuremnts can be used to predict magnetic field strengths on these stars, with testable implications.

Golub, L.↗
NASA NTRS2011

Solar Probe Plus

The NASA Solar Probe Plus mission is planned to be launched in 2018 to study the upper solar corona with both.in-situ and remote sensing instrumentation. The mission will utilize 6 Venus gravity assist maneuver to gradually lower its perihelion to 9.5 Rs below the expected Alfven pOint to study the sub-alfvenic solar wind that is still at least partially co-rotates with the Sun. The detailed science objectives of this mission will be discussed. SPP will have a strong synergy with The ESA/NASA Solar orbiter mission to be launched a year ahead. Both missions will focus on the inner heliosphere and will have complimentary instrumentations. Strategies to exploit this synergy will be also presented.

Szabo, Adam↗
NASA NTRS1995

Solar Probe Technology Challenges

A mission close to the sun is only possible if new spacecraft technologies can be developed and incorporated into a state-of-the-art spacecraft concept. The perihelion goal of 4 solar radii requires a shielded spacecraft that can tolerate the 3000 suns solar flux while maintaining the electronics components at room temperature. In addition, the shield surface should sublimate at a rate of less than 3 mg/s at perihelion. Many shield configuration designs have been studied and the most promising is a parabolic shape that functions as both a shield and a large high gain antenna. The shield material chosen for this design is a carbon-carbon material with highly emissive surface properties. A mission requirement for a high telecommunications power stems from the expected interference when attempting to transmit data through the solar corona. It is expected that the large carbon-carbon shield/antenna will have a large power gain even at high temperatures and will return adequate telemetry at the X-band radio frequency chosen for the Solar Probe mission. Other key technology needs include a non-nuclear power subsystem that can function in the extreme environments of the mission from Earth to Jupiter and onward to a 4 solar radii perihelion.

Solar↗