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All sourcesNASA NTRSDOE OSTI
NASA NTRS2023

The 18–19 March 2022 Series of 3He-Rich Events Observed By Solar Orbiter at 0.36 Au Compared With EUV, X-Ray, and Radio Observations

Context. During the first close perihelion pass of Solar Orbiter, a series of impulsive 3He-rich solar particle events was observed on 18–19 March 2022 from a distance of 0.36 au. In addition to the energetic particle, radio, and X-ray data from Solar Orbiter, the events were observed in radio and/or extreme ultraviolet by STEREO-A, SDO, Wind, and Parker Solar Probe. Aims. Observations of the event series along with remote sensing of flaring and radio emission with only small timing delays due to the close distance allow the association with energetic particles to be determined with much higher accuracy than previously possible from 1 au. Methods. By comparing the onsets of type-III bursts with the arrival of electrons of tens of keV at Solar Orbiter only a few minutes later, it can be seen that, overall, each of the more intense type-III bursts was associated with an electron and ion injection. Extreme ultraviolet data show that the times of the type-III bursts coincide with emission from a small (approximately Earth-sized) loop to the west of a nearby active region. Results. The energetic particle spectra and abundances show typical properties of impulsive 3He-rich flares and, when combined with the remote sensing observations, establish that the particle-accelerating mechanism in this series of events operates near the solar surface in association with magnetic loops, and in the absence of other phenomena such as jets and small coronal mass ejections.

acceleration of particles↗
NASA NTRS1986

Particle propagation effects on wave growth in a solar flux tube

The evolution of a distribution of electrons is followed after they are injected impulsively at the top of a coronal magnetic loop, with the objective of studying the plasma instabilities which result. At early times the downgoing electrons have beamlike distributions and amplify electrostatic waves via the Cerenkov resonance; the anomalous Doppler resonance is found to be less important. Slightly later, while the electrons are still predominantly downgoing, they are unstable to cyclotron maser generation of z-mode waves with omega(p) much less than Omega, or to second harmonic x-mode waves. The energetics of these instabilities, including saturation effects and heating of the ambient plasma, are discussed. It is suggested that coalescence of two z-mode waves generated by cyclotron maser emission of the downgoing electrons may produce the observed microwave spike bursts.

White, S. M.↗
NASA NTRS1984

Alfvenic resonant cavities in the solar atmosphere - Simple aspects

It is noted in the present investigation of Alfven wave propagtion in a simple medium consisting of three uniform layers, each of which is characterized by a different Alfven speed value, that the central layer can, under general conditions, act as a resonant cavity. When this cavity is externally driven by an incident wave on one of the outer layers, resonant transmission peaks are generated which allow large energy fluxes to enter the cavity from outside. Two types of resonance are distinguished: one which occurs when the cavity has the largest or the smallest of the three Alfven speeds, as in coronal loops, and another which is generated when the cavity Alfven speed is intermediate between the two outer Alfven speed values, as may occur on solar spicules. It is also shown that if the energy lost to heat exceeds that lost through leakage out of the cavity, cavity heating can be independent of the damping rate.

Hollweg, J. V.↗
NASA NTRS1988

Magnetohydrodynamic (MHD) modelling of solar active phenomena via numerical methods

Numerical ideal MHD models for the study of solar active phenomena are summarized. Particular attention is given to the following physical phenomena: (1) local heating of a coronal loop in an isothermal and stratified atmosphere, and (2) the coronal dynamic responses due to magnetic field movement. The results suggest that local heating of a magnetic loop will lead to the enhancement of the density of the neighboring loops through MHD wave compression. It is noted that field lines can be pinched off and may form a self-contained magnetized plasma blob that may move outward into interplanetary space.

Wu, S. T.↗
NASA NTRS1991

Non-Maxwellian electron distributions in models of the solar atmosphere

In this paper, the solar models of Fontenla et al. (1990) are extended to coronal temperatures using optically thin radiative losses and a semicircular loop geometry. The effects of a non-Maxwellian high-energy tail of the electron distribution function is tested by performing a Fokker-Planck calculation of the tail. The results show that the downward conductive heat flux is very clsoe to that given by Braginskii (1965) for small departures from a Maxwellian distribution. The effects of the high-energy tail of the electron distribution on element ionization are computed and found to be negligible for all ionization states of O, Ne, and Si. It is concluded that, for quiet sun loop models in stationary energy balance, the ionization state of all elements is not significantly affected by the non-Maxwellian tail of the electron velocity distribution, and that the heat flux can be computed by considering only small first-order departures from the Maxwellian distribution.

Macneice, P.↗
NASA NTRS1987

Solar active region physical parameters inferred from a thermal cyclotron line and soft X-ray spectral lines

Simultaneous high-resolution observations of coronal loops were made at the 20-cm wavelength with the VLA and at soft X-ray wavelengths with the SMM FCS. The images obtained at both wavelengths have nearly identical sizes and ellipsoidal shapes, with the emission stretching between and across regions of opposite magnetic polarity in the underlying photosphere. The results indicate that the radiation at 20 cm and soft X-ray wavelengths originates from the same region, and that 20 cm maps can image X-ray coronal loops. The X-ray spectral lines were used to obtain values of electron temperature, T(e), of about 2.6 x 10 to the 6th K and electron density, N(e), of about 3.1 x 10 to the 9th/cu cm. These parameters were used to show that the layers emitting 20-cm radiation can be optically thick to either thermal bremsstrahlung or thermal gyroresonance radiation, depending upon unknown but plausible values of loop thickness, magnetic scale height, and magnetic field strength.

Lang, Kenneth R.↗
NASA NTRS2007

"Bursty" Reconnection Following Solar Eruptions: MHD Simulations and Comparison with Observations

Posteruptive arcades are frequently seen in the aftermath of coronal mass ejections (CMEs). The formation of these loops at successively higher altitudes, coupled with the classic "two-ribbon" flare seen in H-alpha, are interpreted as reconnection of the coronal magnetic field that has been dragged outward by the CME. White-light observations of "rays," which have been interpreted as being coincident with the current sheet at the reconnection site underneath the erupting CME, also provide evidence for its occurrence. "Blobs" occasionally seen within these rays suggest an even richer level of structure. In this report, we present numerical simulations that reproduce both the observed rays and the formation and evolution of the blobs. We compare their properties with SOHO/LASCO observations of similar structures, and relate their formation to standard theories of reconnection,

Riley, Pete↗
NASA NTRS2014

Verification of Loop Diagnostics

Many different techniques have been used to characterize the plasma in the solar corona: density-sensitive spectral line ratios are used to infer the density, the evolution of coronal structures in different passbands is used to infer the temperature evolution, and the simultaneous intensities measured in multiple passbands are used to determine the emission measure. All these analysis techniques assume that the intensity of the structures can be isolated through background subtraction. In this paper, we use simulated observations from a 3D hydrodynamic simulation of a coronal active region to verify these diagnostics. The density and temperature from the simulation are used to generate images in several passbands and spectral lines. We identify loop structures in the simulated images and calculate the loop background. We then determine the density, temperature and emission measure distribution as a function of time from the observations and compare with the true temperature and density of the loop. We find that the overall characteristics of the temperature, density, and emission measure are recovered by the analysis methods, but the details of the true temperature and density are not. For instance, the emission measure curves calculated from the simulated observations are much broader than the true emission measure distribution, though the average temperature evolution is similar. These differences are due, in part, to inadequate background subtraction, but also indicate a limitation of the analysis methods.

Winebarger, A.↗
NASA NTRS1977

Comments regarding energy release and transfer in solar flares

Skylab observations of the high-temperature coronal plasma are used in characterizing energy released in the corona during flare-like events. The loop structure of the soft X-ray flare features is reviewed; short-term exposures of several soft X-ray images are employed in suggesting that energy, and possibly material as well, is added to the apex of loop systems during flare events. An accompanying restriction in the flow of mass and energy from the loop apex is also hypothesized to account for the recorded observations. Further investigations of the confining ability of the magnetic fields that contain the X-ray emitting plasma are proposed.

Vorpahl, J. A.↗
NASA NTRS1985

Ion energy storage for post-flare loops

Low-energy non-thermal protons may have long lifetimes in coronal loops with low density and high temperature. If energy were stored in such protons in the initial phases of a solar flare, it could be released slowly during the later phases. Within the present observational limits for post-flare loops, this mechanism should be considered in addition to a field-line reconnection theory of the Kopp and Pneuman type. The thin-target gamma ray emission from the trapped protons is below present limits, but more sensitive observations can test the hypothesis.

Hudson, H. S.↗
NASA NTRS1994

Coronal observations from the soft x-ray telescope on Yohkoh

The Yohkoh payload includes a Soft X-ray Telescope (SXT) which is capable of taking high-resolution (2.5 arcsec) images of the Sun through several X-ray and optical filters. A Charge Coupled Devices (CCD) camera provides images with a low background, and large dynamic range and at a rapid cadence (greater than 2 s), which enables the SXT to observe the effects of flares while continuing to observe the fainter quiet-Sun features. We present the observations from the SXT that illustrate how the faint coronal loops that make up the complex structure of the quiet Sun evolve on a variety of timescales from seconds to months. We describe a variety of coronal structures, such as coronal holes, bright points, the large-scale quiet coronal loops, and active regions, to show the wide range of opportunities that Yohkoh presents to further our understanding of the solar corona.

Strong, K. T.↗
NASA NTRS1996

Structure and dynamics of the coronal magnetic field

The last few years have seen a marked increase in the sophistication of models of the solar corona. This has been brought about by a confluence of three key elements. First, the collection of high-resolution observations of the Sun, both in space and time, has grown tremendously. The SOHO (Solar Heliospheric Observatory) mission is providing additional correlated high-resolution magnetic, white-light and spectroscopic observations. Second, the power and availability of supercomputers has made two- and three-dimensional modeling routine. Third, the sophistication of the models themselves, both in their geometrical realism and in the detailed physics that has been included, has improved significantly. The support from our current Space Physics Theory grant has allowed us to exploit this confluence of capabilities. We have carried out direct comparisons between observations and models of the solar corona. The agreement between simulated coronal structure and observations has verified that the models are mature enough for detailed analysis, as we will describe. The development of this capability is especially timely, since observations obtained from three space missions that are underway (Ulysses, WIND and SOHO) offer an opportunity for significant advances in our understanding of the corona and heliosphere. Through this interplay of observations and theory we can improve our understanding of the Sun. Our achievements thus far include progress modeling the large-scale structure of the solar corona, three-dimensional models of active region fields, development of emerging flux and current, formation and evolution of coronal loops, and coronal heating by current filaments.

VanHoven, Gerard↗
NASA NTRS1993

Observational evidence for hotter transition region loops within the supergranular network

We correlate EUV spectroheliograms of the quiet sun taken by the Harvard EUV Spectroheliometer on Skylab with a magnetogram taken within 15-20 minutes by the 40 channel magnetograph at Kitt Peak. Focus is on the O VI and Ne VII ions which are formed in the hotter transition region and the Mg x ion which is formed in the corona. Since no imaged observations in lines from T = 2 x 10 exp 5 - 10 exp 6 K temperature region have been made since Skylab, these data are unique in providing opportunities to investigate the hotter transition region and its relationship to the corona. The signature of an EUV intranetwork loop is a small EUV brightening located on the neutral line of a small magnetic bipole within the supergranular network. The data clearly show such features. Some of these features are visible at coronal temperatures, indicating that network loops can reach 10 exp 6 K. Others are bright in the lines formed below 10 exp 6 K but occur in regions of depleted coronal emission, which cannot be explained by models based on back-heating from a large-scale corona. We conclude that some fraction of the quiet solar EUV output is generated within network loops that are effectively insulated from the corona.

Dowdy, James F., Jr.↗
NASA NTRS1981

On the relationship between soft X-rays and H-alpha-emitting structures during a solar flare

Based on data obtained during a solar flare on March 31, 1979, soft X-ray (SXR) and hard X-ray (HXR) bursts are analyzed and compared with other available data in order to identify structures in H-alpha that may correspond to the SXR-emitting site. Measurements taken with the X-ray telescope and the XUV spectroheliograph flown on Skylab, have shown that the SXR emission from many flares comes from rather small structures of about 10-20 arcsec across. These structures appear to be loops that cross the magnetic neutral line. Understanding of the morphology of SXR was based on data of the solar flare of June 15, 1973, observed from Skylab, and the work of Moore et al., (1980). Dense, highly emissive coronal structures, not suggested to be the X-ray source, were forming, lost energy rapidly by emission and conduction, and finally formed the loops. It is concluded that bright H-alpha loops form rapidly as the SXR emission rises, and the overall decay (cooling rate) of SXR emission is much slower than the formation time of individual loops.

Zirin, H.↗
NASA NTRS1987

The coronal mass ejection of July 6, 1980 - A candidate for interpretation as a coronal shock wave

The July 6, 1980 looplike coronal mass ejection observed with the Solar Maximum Mission coronagraph is described and analyzed. Two observations are given special consideration: (1) the sides of the loop moved laterally while they were visible and did not form stationary bright columns, or 'legs', in the late stage, and (2) raylike features initially outside the mass ejection loop were not pushed aside or bent until the laterally moving loop sides passed over them. These observations, as well as the results of quantitative analysis of the motion of the bright loop, are consistent with the identification of the bright loop as a shock front. The loop top moved outward faster (at about 1000 km/s) than most mass ejections studied previously, and possibly faster than the Alfven speed in the corona, while the loop sides moved laterally at a speed of about 800 km/s and showed no evidence of deceleration. The excess mass in the bright loop is estimated to be less than about 2 x 10 to the 14th g, well below the value found to be typical of coronal mass ejections.

Sime, D. G.↗
NASA NTRS1977

Solar sources of the interplanetary magnetic field and solar wind

Open magnetic field lines, those which extend from the solar photosphere to interplanetary space, are traced in the current-free (potential field) approximation using measured photospheric fields as a boundary condition. It is found that (1) only a relatively small fraction of the photospheric area connects via open field lines to the interplanetary magnetic field; (2) those photospheric areas which do contribute open field lines lie beneath coronal holes and within the boundaries of the holes as projected onto the photosphere or else between loop systems of an active region; (3) the interplanetary magnetic field in the plane of the sun's equator, essentially the field in the ecliptic plane, may connect to photospheric regions of high latitude; and (4) the fastest solar wind streams are correlated with those magnetic flux tubes which expand least in cross-sectional area over the distance between the photosphere and the coronal height where the solar wind begins.

Levine, R. H.↗
NASA NTRS1995

Structure and dynamics of coronal plasmas

The Normal Incidence X-ray Telescope (NIXT) obtained a unique set of high resolution full disk solar images which were exposed simultaneously by X-rays in a passband at 63.5 A and by visible light. The perfect alignment of a photospheric visible light image with a coronal X-ray image enables us to present observations of X-ray intensity vs an accurately determined height above the visible limb. The height at which the observed X-ray intensity peak varies from 4000 km in active regions to 9000 km in quiet regions of the sun. The interpretation of the observations stems from the previously established fact that, for the coronal loops, emission in the NIXT bandpass peaks sharply just above the footpoints. Because there is not a sharp peak in the observed X-ray intensity vs off limb height, we conclude that the loop footpoints, when viewed at the limb, are obscured by absorption in chromospheric material along the line of sight. We calculate the X-ray intensity vs height predicted by a number of different idealizations of the solar atmosphere, and we compare these calculations with the observed X-ray intensity vs height. The calculations use existing coronal and chromospheric models. In order for the calculations to reproduce the observed off limb X-ray intensities, we are forced to assume an atmosphere in which the footpoints of coronal loops are interspersed along the line of sight with cooler chromospheric material extending to heights well above the loop footpoints. We argue that the absorption coefficient for NIXT X-rays by chromospheric material is roughly proportional to the neutral hydrogen density, and we estimate an average neutral hydrogen density and scale height implied by the data.

Golub, Leon↗