S. T. Suess

Ulysses solar wind plasma observations from pole to pole

We present Ulysses solar wind plasma data from the peak southerly latitude of −80.2° on 12 September 1994 through the corresponding northerly latitude on 31 July 1995. Ulysses encountered fast wind throughout this time except for a 43° band centered on the solar equator. Median mass flux was nearly constant with latitude, while speed and density had positive and negative poleward gradients, respectively. Solar wind momentum flux was highest at high latitudes, suggesting a latitudinal asymmetry in the heliopause cross section. Solar wind energy flux density was also highest at high latitudes.

Ulysses Solar Wind Plasma Observations at High Southerly Latitudes

Solar wind plasma observations made by the Ulysses spacecraft through –80.2� solar latitude and continuing equatorward to –40.1� are summarized. Recurrent high-speed streams and corotating interaction regions dominated at middle latitudes. The speed of the solar wind was typically 700 to 800 kilometers per second poleward of –35�. Corotating reverse shocks persisted farther south than did forward shocks because of the tilt of the heliomagnetic streamer belt. Sporadic coronal mass ejections were seen as far south as –60.5�. Proton temperature was higher and the electron strahl was broader at higher latitudes. The high-latitude wind contained compressional, pressure-balanced, and Alfv�nic structures.

The Sun and the Heliosphere as an Integrated System

Preface.- 1 Hydrogen Walls: Mass Loss of Dwarf Stars and the Young Sun Jeffrey L. Linsky, Brian E. Wood.- 2 The Heliospheric Interface: Models and Observations Vladislav V. Izmodeno.- 3 Radiation from the Outer Heliosphere and Beyond Iver H. Cairns.- 4 Ulysses at Solar Maximum Richard G. Marsden.- 5 Propagation of Energetic Particles to High Latitudes T.R. Sanderson.- 6 Solar Wind Properties from IPS Observations Masayoshi Kojima et al.- 7 The Dynamically Coupled Heliosphere Nathan Schwadron.- 8 A Global Picture of CMEs in the Inner Heliosphere N. Gopalswamy.- 9 MHD Turbulence in the Heliosphere: Evolution an Intermittency Bruno Bavassano et al.- 10 Waves and Turbulence in the Solar Corona Eckart Marsch.- 11 The Influence of the Chromosphere-Corona Coupling on Solar Wind and Heliospheric Parameters Oystein Lie-Svendsen.- 12 Elemental Abundances in the Solar Corona John C. Raymond.- 13 The Magnetic Field from the Solar Interior to the Heliosphere Sami K. Solanki.- 14 Magnetic Reconnection E.R. Priest and D.I. Pontin

Ulysses observations of microstreams in the solar wind from coronal holes

During its south polar passage in 1994, the Ulysses spacecraft continuously sampled the properties of the solar wind emanating from the south polar coronal hole. At latitudes poleward of ∼−60°, the solar wind speed had an average value of 764 km/s and a range of 700–833 km/s. The principal variations in the vector velocity were associated with either outward propagating Alfven waves with periods up to about half a day or with longer-period high- or low-speed “microstreams.” The microstreams had an amplitude of ∼40 km/s and a mean half width of 0.4 days, and they recurred on timescales of 2–3 days (power spectral peaks at 1.9 and 3.3 days). The density and temperature profiles showed the expected evidence of pileup and compression on the leading edges of high-speed microstreams, although no forward or reverse shocks were observed. The particle fluxes were nearly the same for both the fast and slow microstreams. The higher-speed microstreams had higher proton temperatures and higher alpha-particle abundances than did the slower microstreams. The absence of latitude variations in the thickness or the recurrence rate suggests that the microstreams are caused by temporal rather than long lived (> a few days) spatial variations in the source region at the Sun. Some speculations are made about the possible cause of the microstreams.

Volumetric heating in coronal streamers

The addition of a volumetric heat source to a coronal streamer model produces distinct, important changes in the model. Originally, such heating was added to meet the observational requirement for a thin current sheet above streamers. Here we report additional consequences of a volumetric heat source, together with the effects of redistribution of heat through thermal conduction. Specifically, we address the question of whether a heat source will allow a truly steady state to be achieved in the presence of thermal conduction, something known to be impossible for an adiabatic gas. The heat source causes a slow, continuing expansion and stripping of magnetic flux from the top of a streamer until the streamer essentially evaporates and the field is fully open to the interplanetary medium after an interval which depends on the magnitude of the source but typically varies from weeks to months. We find that thermal conduction does not quench the evaporation. We also find that the heat source, in the absence of other processes, must depend on the magnetic field geometry to simulate both the thinness of current sheets above streamers and the low density/high flow speed in coronal holes. Finally, we find that the expansion is not necessarily continuous.

Current Sheet Evolution in the Aftermath of a CME Event

We report on SOHO UVCS observations of the coronal restructuring following a coronal mass ejection (CME) on 2002 November 26, at the time of a SOHO-Ulysses quadrature campaign. Starting about 1.5 hr after a CME in the northwest quadrant, UVCS began taking spectra at 1.7 R� , covering emission from both cool and hot plasma. Observations continued, with occasional gaps, for more than 2 days. Emission in the 974.8 8 line of [Fe xviii], indicating temperatures above 6 ; 10 6 K, was observed throughout the campaign in a spatially limited location. Comparison with EITimages shows the [Fe xviii] emission to overlie a growing post-flare loop system formed in the aftermath of the CME. The emission most likely originates in a current sheet overlying the arcade. Analysis of the [Fexviii] emission allows us to infer the evolution of physical parameters in the current sheet over the entire span of ourobservations:inparticular,wegivethetemperatureversustimeinthecurrentsheetandestimateitsdensity.Atthe timeofthequadrature,UlysseswasdirectlyabovethelocationoftheCMEandinterceptedtheejecta.Highionization state Fe was detected by the Ulysses SWICS throughout the magnetic cloud associated with the CME, although its rapid temporal variation suggests bursty, rather than smooth, reconnection in the coronal current sheet. The SOHOUlyssesdatasetprovideduswiththeuniqueopportunityofanalyzingacurrentsheetstructurefromitslowest coronal levels out to its in situ properties. Both the remote and in situ observations are compared with predictions of theoretical CME models.

Global model of the corona with heat and momentum addition

We have been developing a series of global coronal models directed at a better simulation of empirical coronal hole and streamer properties. In a previous study, a volumetric heat source was used to produce a thin current sheet above streamers and high solar wind speed in the coronal hole. This improved the preexisting coronal structure for coronal mass ejection simulations even when not using a polytropic energy equation. Here we report on the addition of a momentum source to the model with volumetric heating and thermal conduction. Most theoretical acceleration models in coronal holes are driven either by thermal pressure or waves (magnetosonic, Alfven, and sonic waves). In the thermal pressure driven models an artificially high effective temperature is assumed. In the wave driven models the force is generally not big enough to accelerate the solar wind as quickly as observed. In the present model, in comparison to earlier calculations [Suess et al., 1996], we reduce the heat source and add momentum. These changes appear to further improve the numerical simulation results in comparison to empirical properties. We have high solar wind speed in the hole without using unrealistic high plasma temperature. We also demonstrates that the deposition height of the momentum addition affects the mass flux. The model still predicts a slow-speed solar wind source in the streamer and high plasma β at the top of the streamer.

On Heating the Sun’s Corona by Magnetic Explosions: Feasibility in Active Regions and Prospects for Quiet Regions and Coronal Holes

We build a case for the persistent strong coronal heating in active regions and the pervasive quasi-steady heating of the corona in quiet regions and coronal holes being driven in basically the same way as the intense transient heating in solar flares: by explosions of sheared magnetic fields in the cores of initially closed bipoles. We begin by summarizing the observational case for exploding sheared core fields being the drivers of a wide variety of flare events, with and without coronal mass ejections. We conclude that the arrangement of an events flare heating, whether there is a coronal mass ejection, and the time and place of the ejection relative to the flare heating are all largely determined by four elements of the form and action of the magnetic field: (1) the arrangement of the impacted, interacting bipoles participating in the event, (2) which of these bipoles are active (have sheared core fields that explode) and which are passive (are heated by injection from impacted active bipoles), (3) which core field explodes first, and (4) which core-field explosions are confined within the closed field of their bipoles and which ejectively open their bipoles. We then apply this magnetic-configuration framework for flare heating to the strong coronal heating observed by the Yohkoh Soft X-ray Telescope in an active region with strongly sheared core fields observed by the Marshall Space Flight Center vector magnetograph. All of the strong coronal heating is in continually microflaring sheared core fields or in extended loops rooted against these active core fields. Thus, the strong heating occurs in field configurations consistent with the heating being driven by frequent core-field explosions that are smaller than but similar to those in confined flares and flaring arches. From analysis of the thermal and magnetic energetics of two selected core-field microflares and a bright extended loop, we find that (1) it is energetically feasible for the sheared core fields to drive all of the coronal heating in the active region via a staccato of magnetic microexplosions, (2) the microflares at the feet of the extended loop behave as the flares at the feet of flaring arches in that more coronal heating is driven within the active bipole than in the extended loop, (3) the filling factor of the X-ray plasma in the core field microflares and in the extended loop is ~ 0.1, and (4) to release enough magnetic energy for a typical microflare (1027-1028 ergs), a microflaring strand of sheared core field need expand and/or untwist by only a few percent at most. Finally, we point out that (1) the field configurations for strong coronal heating in our example active region (i.e., neutral-line core fields, many embedded in the feet of extended loops) are present in abundance in the magnetic network in quiet regions and coronal holes, and (2) it is known that many network bipoles do microflare and that many produce detectable coronal heating. We therefore propose that exploding sheared core fields are the drivers of most of the heating and dynamics of the solar atmosphere, ranging from the largest and most powerful coronal mass ejections and flares, to the vigorous microflaring and coronal heating in active regions, to the multitude of fine-scale explosive events in the magnetic network, which drive microflares, spicules, global coronal heating, and, consequently, the solar wind.

Alfvén wave trapping, network microflaring, and heating in solar coronal holes

Fresh evidence that much of the heating in coronal holes is provided by Alfven waves is presented. This evidence comes from examining the reflection of Alfven waves in an isothermal hydrostatic model coronal hole with an open magnetic field. Reflection occurs if the wavelength is as long as the order of the scale height of the Alfven velocity. For Alfven waves with periods of about 5 min, and for realistic density, magnetic field strength, and magnetic field spreading in the model, the waves are reflected back down within the model hole if the coronal temperature is only slightly less than 1.0 x 10 to the 6th K, but are not reflected and escape out the top of the model if the coronal temperature is only slightly greater than 1.0 x 10 to the 6th K. Because the spectrum of Alfven waves in real coronal holes is expected to peak around 5 min and the temperature is observed to be close to 1.0 x 10 to the 6th K, the sensitive temperature dependence of the trapping suggests that the temperature in coronal holes is regulated by heating by the trapped Alfven waves.

Latitudinal dependence of the radial IMF component: Coronal imprint

Abstract. Measurements by Ulysses have confirmedthat there is no significant gradient with respect to he-liomagnetic latitude in the radial component, Br, of theinterplanetary magnetic field. In the corona, the plasma/3 is << l, except directly above streamers, so longitudi-nal and latitudinal gradients in field strength will relaxdue to the transverse magnetic pressure gradient forceas the solar wind carries magnetic flux away from theSun. This happens quickly enough so that the field isessentially uniform by 5- 10 Re, apparently remainingso as it is carried to beyond 1 AU. Here, we illustrate thecoronal relaxation with a qualitative physical argumentand by reference to a detailed MHD simulation. 1. Introduction Ulysses in 1993-1995 [Smith and Balogh, 1995; Baloghet al., 1995] and ICE and IMP-8 in 1984-1988 [Burtonet al., 1995] observed no significant gradient in helio-magnetic latitude in the radial component, Br, of theinterplanetary magnetic field (IMF) at heliocentric dis-tances of 1-4 AU. These observations were made nearsolar minimum when the Suns magnetic field is mostnearly an axially aligned dipole. Figure 1 shows theresults from IMP-8 and ICE scaled to 1 AU and plot-ted against magnetic latitude. The data are five degreebin averages. Curves (a), (b), and (c) are from mod-els which will be discussed below. The apparent smallgradient in Br near the magnetic equator is probablydue to small errors in sector identification so that thetrue gradient is completely negligible. Essentially nogradient was observed by Ulysses in 1993-1995 up to aheliographic latitude of 80 °. Here we discuss the rea-son why the magnetic field at the top of the corona,i.e. at 10 Ro, does not have any significant gradient inlatitude or longitude outside of the heliospheric plasmasheet (HPS) [Gosling et al., 1981] surrounding the he-liospheric current sheet (HCS). In §2, we explain why