Solar coronal streamers

The solar disk locations of 13 coronal streamers were determined from a combination of eclipse, K-coronameter (1 1/8r1 1/2 R ), and balloon-borne coronagraph (2<r<6 R ) observations taken during 1964 and 1965. Of this sample, three were observed twice on photographs taken over intervals of four and 28 days. Most of these streamers could be structurally associated with K-coronameter enhancements to establish their disk locations.Those features having known disk locations all lay above some stage of chromospheric disk activity in the form of active regions and prominences. The average lifetime of three K-coronameter streamer-enhancements, for which all or nearly all of their lifetimes were known, was about 4 solar rotations. Rotation rates for the lower latitude streamer-enhancements (30°) were essentially identical to the underlying surface. One high latitude feature ( 50°) which overlay a quiescent prominence had a rate equivalent to the surface rate at 30° latitude. In general those K-coronameter enhancements associated with streamers came into existence over time periods of 14 days and disappeared by gradually blending into the background coronal pattern. All the observed structures are explained by a model consisting of localized, high density features (streamers) which overlie disk activity and are imbedded in a uniform but weaker azimuthally-symmetric quiet corona.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Solar coronal streamers

A unique combination of photographic and K-coronameter data were used to study the structure and evolution of two known coronal streamers. In addition, two other K-coronameter enhancements were studied as representing ideal second examples of the known streamers. As a general rule the observations indicate that these features were direct coronal manifestations of photospheric bipolar magnetic regions (BMR) and were of two basic types:active region, by which is meant a coronal streamer which develops radially over a low-latitude active region; andhelmet which denotes a streamer whose structure and development appear to be a consequence of a long-lived complex of activity, composed of both trailing magnetic fields and a parent center of disk activity.The similarity of growth rates during the first solar rotation of life led to derivation of a total streamer density of 4–5 × 10⁸ cm^–3 atr = 1.125R . This density may represent a characteristic maximum density at the base of streamers. The intensity gradient of the inner (r1.5R ) corona was used to establish a qualitative evolutionary model of streamers which synthesizes the observations. Briefly, streamers initially develop over active regions; the streamer growth rate may be as rapid as the disk activity, or at worst lags flare activity by solar rotation. The streamer can be the cause of interplanetary and geomagnetic effects at 1 AU within a solar rotation after birth. Thereafter the streamer follows an evolution dictated by the underlying solar magnetic fields. In any case the lowest level of the coronal enhancement has a lifetime not exceeding that of the solar disk activity.     

Post-flare coronal loop interaction

High-resolution images of post-flare loop systems in Fexiv (5303 ) and Fex (6374 ) display occasional transient enhancements at the projected intersection of some loops. The brightness of a green-line enhancement gradually increases to a marked maximum and then fades with a lifetime of the order of thirty minutes. The red-line image at the same location, although fainter, shows the same overall characteristics, its maximum following that of the green-line on average by 8.6 min. H then becomes more evident and reaches a maximum in extent on average 9.3 min after the red-line maximum. The phenomenon is interpreted as a process of localized loop coalescence involving partial magnetic reconnection. Estimates of the electron density are derived from the cooling time following the initial heating of the plasma in the immediate vicinity of the X-point of interaction. Similar estimates for the energy dissipated, equivalent to a very small flare, are derived by two independent methods.Operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.     

Monochromatic observations of a coronal loop

An isophotal map of a small coronal loop, obtained from a coronagraph observation through a solid Fabry-Perot interferometer, is used to estimate the variation of emission per unit volume and the pressure gradient at the top and sides of the loop. The magnitude of the magnetic field necessary to maintain the estimated pressure gradients is found to be ¦H ²¦ = 30 G².     

Solar mass ejections and coronal holes

In this paper we present observations of two types of solar mass ejections, which seem to be associated with the location of coronal, holes. In the first type, a filament eruption was observed near a coronal hole, which gave rise to a strong interplanetary scintillations. as detected by IPS observations. In the second type, several large scale soft X-ray blow-outs were observed in the YOHKOH SXT X-ray movies, in all the cases they erupted from or near the boundary of coronal holes and over the magnetic neutral line. It is proposed that the open magnetic field configuration of the coronal hole provides, the necessary field structure for reconnection to take place, which in turn is responsible for filament eruption, from relatively lower heights. While, in the case of X-ray blow-outs, the reconnection takes place at a greater height, resulting in high temperature soft X-ray emission visible as X-ray blow-outs.     

Solar coronal heating by magnetosonic waves

Solar coronal heating by magnetohydrodynamic (MHD) waves is investigated. ultraviolet (UV) and X-ray emission lines of the corona show non-thermal broadenings. The wave rms velocities inferred from these observations are of the order of 25–60 km s⁻¹ . Assuming that these values are not negligible, we solved MHD equations in a quasi-linear approximation, by retaining the lowest order non-linear term in rms velocity. Plasma density distribution in the solar corona is assumed to be inhomogeneous. This plasma is also assumed to be permeated by dipole-like magnetic loops. Wave propagation is considered along the magnetic field lines. As dissipative processes, only the viscosity and parallel (to the local magnetic field lines) heat conduction are assumed to be important. Two wave modes emerged from the solution of the dispersion relation. The fast mode magneto-acoustic wave, if originated from the coronal base can propagate upwards into the corona and dissipate its mechanical energy as heat. The damping length-scale of the fast mode is of the order of 500 km. The wave energy flux associated with these waves turned out to be of the order of 2.5×10⁵ ergs cm⁻² s⁻¹ which is high enough to replace the energy lost by thermal conduction to the transition region and by optically thin coronal emission. The fast magneto-acoustic waves prove to be a likely candidate to heat the solar corona. The slow mode is absent, in other words cannot propagate in the solar corona.     

Two-dimensional pressure structure of a coronal loop

The steady-state pressure structure of a solar coronal loop is discussed using the theory of magneto-hydrodynamical turbulence in cylindrical geometry. The steady state is represented by the superposition of two Chandrasekhar-Kendall functions. This representation, in principle can delinetate the three dimensional temperature structure of the coronal loop. In this paper, we have restricted ourselves to a two dimensional modeling since only this structure submits itself to the scrutiny of the available observations. The radial as well as the axial variations of the pressure in a constant density loop are calculated. These variations are found to conform to the observed features of cool core and hot sheath of the loops as well as to the location of the temperature maximum at the apex of the loop. We find that these features are not present uniformly all along either the length of the loop or across the radius as will be shown in the text. We have also discussed the possible oscillatory nature of these pressure variations and the associated time periods have been estimated.     

Mass flow in loop type coronal transients

The white light coronagraph on Skylab observed many loop type coronal transients. These loops travel through the coronagraph's field of view (2–6R ) over a period of a few hours, after which the legs of the loops usually remain visible for a few days. In this paper we investigate the temporal changes in density and mass per unit length measured along the legs of such loops during the several days after the initial eruption. Examination of 8 transients shows that the mass and density in the legs decrease during the few hours after the top of the loop has travelled beyond the coronagraph's field of view. The mass and density then increase slowly, during the next one half to one day, then decrease again over approximately the same period. These changes are generally shown to be too rapid to be explained by solar rotation, indicating that the transient legs have a lifetime of only a few days.The results of a detailed study of the transient of 10 August 1973 are compared with the results from theoretical calculations. For the top of the loop a one-dimensional flow problem is solved, assuming a balance between gravity, inertia, and pressure gradients. The legs are modeled by a flow in a tube of constant cross section. Models for the flow in the legs were calculated under the assumption that the mass distribution is close to hydrostatic equilibrium. Using these models we can estimate that approximately 5 × 10¹⁴ g of material flow outward through the legs of this transient. We also find that the best fit to the observed average density gradient is obtained with a temperature of 1.7 × 10⁶ K.On leave from Max-Planck Institut für Physik und Astrophysik, Munich, Germany.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Energy equilibrium in a coronal magnetic loop

Temperature distribution in the cylindrically symmetric coronal magnetic loop, (i) with constant pressure and (ii) with the pressure varying along the radial distance, of the (a) hotter apex and (b) cooler apex than base is investigated analytically by considering the equilibrium between the heat conduction and radiation loss. If the temperature of the loop does not lie within one of the specified temperature ranges, then the distribution is calculated numerically.The effect of the inclusion of heating due to an external source is studied and found that it increases the length of the loop. On the basis of the observed phenomenon, that the magnetic field varies along the loop, the temperature distribution in the loop is investigated for the loop-geometries proposed by Antiochos and Sturrock (1976). It is concluded that for the larger compression in the area of cross section, the height of the loop decreases.Present investigation shows that no loop with equal apex and base temperatures can exist, but a small variation between the two temperatures supports the existence of the loop, which can be observed in nature.     

A magnetohydrodynamic theory of coronal loop transients

A magnetohydrodynamic theory is presented for coronal loop transients. It is shown that the heliocentrifugal motion of a transient loop, as exhibited by the translational displacement of the axis of the loop, is driven by the magnetohydrodynamic buoyancy force exerted by the ambient medium. Self-induced hydromagnetic force, which includes the magnetic force produced by the internally driven current and the thermal force produced by the pressure imbalance between the internal and external gas pressures, causes the peripheral expansion of the loop, as exhibited by the lateral broadening and longitudinal stretching. This contention is substantiated by an analysis based on a model structure for a coronal loop.Besides accounting for the acceleration and expansion of a transient loop, this magnetohydrodynamic theory also provides an explanation for the initial ejection of a coronal loop from stationary equilibrium. Magnetic unwinding in consequence of abrupt magnetic activities at the solar surface will cause the periphery of a stationary coronal loop to expand. The increase in volume will enhance the magnetohydrodynamic buyoyancy force to exceed the gravitational force. Once a coronal loop is ejected from the solar surface, it will be continually accelerated and undergo expansion. Eventually a transient loop will blend with the ambient solar wind. This is also indicated by the theory presented in this paper.     

Solar cycle dependence of polar coronal holes

A study has been made of the polar coronal holes in relation to solar cycle activity. Important results obtained are: (i) the peak of the frequency distribution of coronal hole size shifts towards lower values as the solar cycle advances towards maximum, this being true for both the north and south polar holes, (ii) coronal hole size decreases with the increase of sunspot number.     

Solar cycle variation of large-scale coronal structures

A green line intensity variation is associated with the interplanetary and photospheric magnetic sector structure. This effect depends on the solar cycle and occurs with the same amplitude in the latitude range 60° N–60° S. Extended longitudinal coronal structures are suggested, which indicate the existence of closed magnetic field lines over the neutral line, separating adjacent regions of opposite polarities on the photospheric surface.Supported by an ESRO/NASA fellowhip.On leave from Torino University, Italy; now at Istituto di Fisica, Universita di Torino, Italy.     

Can coronal loop transients be driven magnetically?

In this note we investigate the possibility of magnetic driving of loop transients. The action of local magnetic forces to balance gravity in a coasting loop and to confine the loop has been proposed by Mouschovias and Poland (1977). In this paper we use similar configurations but deal with the global field structure and present models which show both the initial phase of large acceleration and the later phase of almost constant velocity. We use very simple one-dimensional models consisting of a ring current which is subjected to gravitational attraction. The velocity curves calculated for these models are in good agreement with the observations. Therefore we conclude that if such ring currents can be produced fast enough in the solar corona, they are capable of driving the loop transients observed in the ATM white light coronagraph.On leave from Max-Planck Institut für Physik und Astrophysik, Föhringer Ring 6, 8 München 40, F.R.G.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Two-dimensional steady-state pressure structure in a coronal loop

By expressing the magnetic field and fluid velocity in terms of two Chandrasekhar-Kendall functions (n = 0, m = 0; n = 1, m = 0) we investigated the steady-state pressure profile inside a solar coronal loop. For constant density loops, we found a two-dimensional (radial and axial) structure of pressure. This work is the modified version of the work of Krishan (1985). At the base of the loop, the pressure is found to increase steeply outwards along the radius, whereas at the apex it decreases slowly. The radial variation of pressure is found to be minimum around L/5, where L is the length of the loop measured from one foot to another one. But Krishan (1985) found that the rate of increase of pressure at the base was nearly equal to the rate of decrease of pressure at the apex, and the pressure was found nearly constant at L/4. For axial variation, we found that along the loop axis the pressure increases from the base up to z = 3L/8 and then decreases up to the apex, whereas at the surface, the pressure decreases from the base up to the apex. Krishan (1985), however, found the axial variation to be linear.     

Solar wind heavy ions from flare-heated coronal plasma

Information concerning the coronal expansion is carried by solar wind heavy ions. Distinctly different energy-per-charge ion spectra are found in two classes of solar wind having the low kinetic temperatures necessary for E/q resolution of the ion species. Heavy ion spectra which can be resolved are most frequently observed in the low-speed interstream (IS) plasma found between high speed streams; the streams are thought to be coming from coronal holes. Although the sources of the IS plasma are uncertain, the heavy ion spectra found there contain identifiable peaks of O, Si, and Fe ions. Such spectra indicate that the IS ionization state of O is established in coronal gas at T 2.1 × 10⁶ K while that of Fe is frozen in farther out at 1.5 × 10⁶ K. On occasion anomalous spectra are found outside IS flows in solar wind with abnormally depressed local kinetic temperatures. The anomalous spectra contain Fe¹⁶⁺ ions, not usually found in IS flows, and the derived coronal freezing in temperatures are significantly higher; for two of the best cases values of 3.4 × 10⁶ K were found for the O ions and 2.9 × 10⁶ K for Fe ions. The coronal sources of some of these ionizationally hot flows are identified as solar flares. The appearance of abnormally depressed kinetic temperatures in solar wind coming from flare-heated coronal gas lends support to earlier speculation that flares can expel plasma enclosed in magnetic bottles or bubbles. In transit to 1 AU the gas is sufficiently isolated from the hot corona that it cools anomalously.The Los Alamos Scientific Laboratory requests that the publisher identify this article as work performed under the auspices of the Department of Energy.By acceptance of this article, the publisher recognizes that the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or to allow others to do so, for U.S. Government purposes.     

Formation of neutral current sheet and loop coronal transients

An eruption of opposite magnetic flux into a bipolar background field is likely to lead to the formation of a natural current sheet between the new emerging field and the background. A numerical study is made on this process, based on the ideal MMD equations, taking into account the interaction between the magnetic field and the coronal plasma. The result shows that a subsonic eruption will give rise to a four region structure; 1) a cool and dense prominence made of the erupting material in the innermost region; 2) a cool and tenuous region further out; 3) a hot and dense loop formed by the concentration of both the erupting material and the coronal material in the neutral current sheet; and 4) a forerunner region outside the loop with density slightly above the background, due to fast magneto-acoustic waves. This structure agrees with the observed features of typical loop coronal transients. Therefore the eruption of opposite magnetic flux into a bipolar background is probably an important mechanism for triggering off such transients.     

A numerical hydrodynamic model of a heated coronal loop

The fluid equations describing a fully ionized single temperature (i.e., electron and proton temperatures assumed identical) hydrogen plasma in a coronal loop subject to a transient heating pulse (2 × 10⁹ ergs cm^–2 s^–1) centred about the loop apex have been solved numerically. An adaptive regriding scheme was used to ensure adequate spatial resolution throughout the transition region, and due regard paid to the numerical time constants. Because of the fine gridding made possible by this scheme these results represent the first reliable simulation of the impact of a downward propagating conduction front on the transition region, and the early stages of the development of the downward moving compression and upward ablation. Intensities in the O v (1371 Å) transition region line were calculated from the model results. Finally estimates have been made of the importance of the downward-streaming collisionless high-energy tail of the distribution in the transition region resulting from the very steep temperature gradients. It is shown that the mass and energy densities are not substantially altered by the non-Maxwellian tail except in so far as they are coupled to higher moments of the distribution function such as the heat flux through the fluid equations.     

Characteristics of transverse oscillations in a coronal loop arcade

TRACE observations from 15 April 2001 of transverse oscillations in coronal loops of a post-flare loop arcade are investigated. They are considered to be standing fast kink oscillations. Oscillation signatures such as displacement amplitude, period, phase and damping time are deduced from 9 loops as a function of distance along the loop length. Multiple oscillation modes are found with different amplitude profile along the loop length, suggesting the presence of a second harmonic. The damping times are consistent with the hypothesis of phase mixing and resonant absorption, although there is a clear bias towards longer damping times compared with previous studies. The coronal magnetic field strength and coronal shear viscosity in the loop arcade are derived.     

High-frequency oscillations in a solar active region coronal loop

The Solar Eclipse Corona Imaging System (SECIS) was used to record high-cadence observations of the solar corona during the total solar eclipse of 1999 August 11. During the 2 min 23.5 s of totality, 6364 images were recorded simultaneously in each of the two channels: a white light channel, and the Fe  xiv (5303 Å) 'green line' channel ( T ∼2 MK) . Here we report initial results from the SECIS experiment, including the discovery of a 6-s intensity oscillation in an active region coronal loop.     

Prominence condensation and magnetic levitation in a coronal loop

We describe the results of a model dynamic simulation of the formation and support of a narrow prominence at the apex of a coronal magnetic loop or arcade. The condensation process proceeds via an initial radiative cooling and pressure drop, and a secondary siphon flow from the dense chromospheric ends. The anti-buoyancy effect as the prominence forms causes a bending of the confining magnetic field, which propagates toward the semi-rigid ends of the magnetic loop. Thus, a wide magnetic hammock or well (of the normal-polarity Kippenhahn-Schlüter-type) is formed, which supports the prominence at or near the field apex. The simplicity of this 1.5-dimensional model, with its accompanying diagnostics, allows one to comprehend the various contributions to the nonlinear dynamics of prominence condensation and levitation.