摘要:

Recently I have published supersonic solutions of the inviscid solar-wind equations, obtained with an iterative approach that relates boundary conditions at the supersonic level to those at infinity. In the present paper, I examine further the nature of the solutions at infinity, and introduce a revised definition for one of the asymptotic coefficients. For Scarf-Noble solutions, the ratio of thermal energy to gravitational potential at the supersonic level,&tgr;s/&lgr;s≡(kTr/GMm)s,must lie in the narrow range between 0.4375 and about 0.4135. These limits are fixed by, respectively, infinite and zero conductive heat flow at infinity. The kinetic energy in the solar wind at infinity is only 10 to 14 times its value at the supersonic level,&lgr;s,regardless of the temperature at&lgr;s.At the higher temperatures in this range the vast majority of the available energy is transported to infinity by conduction. Finally, I have formulated the related problem of catastrophic blowoff for planetary atmospheres composed of neutral diatomic molecules. As with the solar problem, the presumption is that no radiant heat is absorbed above the supersonic level. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51358

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The evolution of the distribution function for a test population of electrons in an isothermal electron-proton corona has been studied using a Fokker-Planck description. The aim is to investigate whether a suprathermal tail forms due to the energy dependence of the Coulomb cross section. We find that a Maxwellian test population, injected into this background close to the coronal base with a temperature equal to that of the background electrons, maintains its shape throughout the transition from collision-dominated to collisionless flow. No significant suprathermal tail in the electron distribution function is seen in the outer corona. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51359

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We used type III radio bursts recorded by the ARTEMIS spectrograph on the ground (100–500 MHz), and by the URAP radio receiver on the Ulysses spacecraft (1–940 kHz), to follow energetic electrons ejected from solar flares into the interplanetary medium (IPM). Out of about 200 large events observed over 1991 1994, we found≈50&percent;frequency associations. Typically, a group of many bursts on ARTEMIS gives rise to one isolated burst on Ulysses. This demonstrates that some of the flare electrons themselves make it all the way to the IPM. We confirm that speeds deduced from frequency drifts are≈c/3in the corona andc/10in the IPM. We find that this deceleration occurs before about 0.1 AU, and that there is no significant correlation between the initial and final speeds. We derive the average radiation pattern of type III bursts at low frequencies and find that the maximum intensity direction is shifted≈40°east of the radial direction, which we interpret as resulting from an ambient density gradient directed in the opposite direction. We suggest that right above “open” coronal magnetic field lines imbedded inside active regions, the solar wind presents a local maximum speed; then interaction with the relatively slower wind around results in a westward density gradient. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51360

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Spectral observations of cool stars enable the study of the presence and character of winds and the mass loss process in objects with effective temperatures, gravities, and atmospheric compositions which differ from that of the Sun. A wealth of recent spectroscopic measurements from theHubble Space Telescope, and theExtreme Ultraviolet Explorersatellite complement high resolution ground-based spectra in the optical and infrared spectral regions. Such observations when combined with realistic semi-empirical atmospheric modeling allow us to estimate the physical conditions in the atmospheres and winds of many classes of cool stars. In low gravity stars, evidence is found for relatively fast (≈200 km s-1), warm winds with rapid acceleration occurring in the chromosphere. In some cases, outflows commensurate with stellar escape velocities are present. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51361

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We review the observed properties of the solar wind with the aim of finding a simple and straightforward understanding of its origin and acceleration. A theory is developed for the high speed solar wind based on a simple dissipation length assumption for hydromagnetic wave heating of the coronal plasma close to the Sun. Solutions with the correct particle and energy fluxes and with a realistic magnetic field, match the requirements on the density at the base of the corona provided the dissipation length is relatively small (∼0.25−0.5solar radii). The significant features are that the acceleration is rapid, with the sonic point within∼2solar radii, and the proton temperatures are high, namely8−10×106 K.Such efficient dissipation requires any Alfven waves responsible to have frequencies in the range 0.01 Hz–10 kHz. This has implications for the nature of the plasma and energy source in the supergranular network. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51363

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Coronal magnetic field models developed over the years reproduce the static characteristics of coronal and heliospheric structures fairly well; however, limitations of spatial and temporal resolution and nonuniform quality of the input data are important factors limiting investigions of the response of the corona to rapidly changing photospheric conditions. The Solar Oscillations Investigation (SOI) Michelson Doppler Imager (MDI) experiment on SOHO, scheduled for launch in November 1995, will produce a series of full-disk photospheric magnetic field observations with4″resolution about every 96 minutes for the next several years. Higher resolution observations of the center of the disk will be available several times per day. These data should provide a basis for predicting the coronal and heliospheric field and their changes with unprecedented accuracy during the rising phase of Solar Cycle 23. Improved understanding of the structure and dynamics of the magnetized corona will require more realistic coronal magnetic field models. Our horizontal current-current sheet coronal field model includes large-scale, low-altitude, horizontal currents and the effect of thin current sheets in the streamer belt on the field above cusp type neutral points. To be self-consistent, the model now includes the effect of the streamer current sheet on the field below the cusp point. An overlying source surface can be added to simulate the effect of the accelerating solar wind. To suggest the potential of the MDI photospheric magnetic field data, we show calculations of the steady coronal magnetic field using low spatial resolution data and compare results with various observations. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51362

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The polar crown event of 1994 April 14 is one of the largest scale eruptive events observed by theYohkoh/SXT. Associated with the formation of an arcade of soft X-ray loops at the Sun was the detection of an interplanetary forward/reverse shock event by theUlyssesspacecraft some 7 days later. The relationship between the coronal and interplanetary signatures of these events is important if we are to address fully the initialisation and consequent development of interplanetary phenomena, such as CMEs and counter-streaming electrons, originating at the Sun. We investigate the development of the energetics of the 1994 April 14 event and their relationship to the dynamics of the eruption are investigated. The arcade formation, together with the eruption of material into interplanetary space, suggests a large-scale temporary reconfiguration of the coronal magnetic field. We examine the effects of the formation of such a coronal arcade structure on the HCS and discuss the dynamics involved with the passage of a large scale disturbance through the interplanetary magnetic field. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51455

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Two extremely similar interplanetary forward/reverse shock events, with bidirectional electron streaming, were detected byUlyssesin 1994 [Gosling&etal;, 1994]. Both events resulted in geomagnetic storms and presumably were associated with coronal mass ejections. In this paper we use theYohkohsoft X-ray observations to characterize the conditions in the lower corona at the times appropriate for the launching of these two events. We find two strikingly different solar events to be the likeliest candidates: an LDE flare on 20 Feb. 1994, and a extremely large-scale arcade event on 14 April 1994. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51456

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

With soft X-ray imaging we can study the entire coronal volume, except for cold inclusions such as prominences, as a function of time. This should allow us to observe the origins of coronal mass ejections. We report here an initial survey of theYohkoh/SXT observations at the times of reported or apparent mass ejections: three LDE flare events and two large-scale arcade formations. For each of the events we can easily detect sudden coronal dimming, which we interpret as the launch interval of a CME. In one of the flare events we have found a well-defined plasma cloud, apparently formed from a set of loop structures, which rises and disappears during the growth phase of the flare emission. Its mass amounted to some4×1014 gwith a density of3×108 cm−3and a temperature of 2.8 MK before its disappearance. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51457

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

TheUlyssespolar passages are producing a unique set of observations of solar-wind disturbances at high heliographic latitudes. In this paper we use theYohkohsoft X-ray telescope (SXT) to locate some of these events, as defined by theUlysses/SWICS data, in the solar corona. Of 8 events, we identify two with flares, three with front-side large arcade events, two with far-side events, and one was not seen in theUlyssesdata. The arcade events generally resemble long-duration flares seen in active regions, but are larger, slower, and cooler. We presentYohkohimages of each of these events. In the large arcade events (seeAlexander&etal;, 1996, for a detailed look at one of them) the magnetic morphology at the location of theYohkoharcade is generally consistent with the development of a large system of loops. Some of the identifications are ambiguous, and we summarize the reasons for this. From the SWICS data we have obtained ionization temperatures for several events, and find that they have no obvious pattern in relation to the X-ray temperatures; this may be expected on the basis that the interplanetary plasma cloud is physically distinct from the plasma trapped in the corona. Soft X-ray observations of the solar corona show occasional occurrences of large-scale brightenings in the form of arcades of loops. Such structures have been known since Skylab (e.g.,Sturrock, 1980), and have a clear relationship with coronal mass ejections (e.g.,Kahler, 1977). We now may study this phenomenon statistically with the much more comprehensiveYohkohobservations; withYohkohmovies we can also begin to extend our knowledge to the three-dimensional development of the structures. At the same timeUlysseshas sampled the latitude dependence of the interplanetary effects. With this paper we introduce this subject and provide a preliminary listing of events from the passage ofUlyssesthrough high heliographic latitudes. The starting point of the present survey is a list of interplanetary plasma clouds (IPC’s) derived fromUlysses/SWICS data. These are essentially the same as the events termed CMEs byGosling&etal; (1994a, 1994b). For this identification the presence of bidirectional streaming in the suprathermal electron distribution is one of the main criteria. We note that there are no direct coronagraph observations, however. TheYohkohobservations were examined at the apparent time of origin of eachUlyssesevent, resulting in some clear and some less-certain identifications. We also studied the ionization temperatures of the IPC material as a beginning step to give the identifications a physical basis. There has been little study thus far of theYohkohsoft X-ray observations in relationship to CMEs, which we believe to be closely related to the interplanetary disturbances.Hiei&etal; (1993) reported the onlyYohkohevent yet studied in conjunction with white-light coronagraph observations. HoweverKlimchuk&etal; (1994) showed that X-ray eruptive phenomena with parameters similar to those of CMEs occur frequently at the limb, and there have been several studies of individual eruptive events (e.g.,Watanabe&etal;, 1992). Presently there is no systematic knowledge of the X-ray coronal counterparts of CMEs, and the survey represented here is part of the effort to rectify this situation. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51364

出版商:AIP    

年代:1996

数据来源: AIP