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after more than a half - century of study , the basic physical processes that are responsible for heating the million - degree corona and accelerating the supersonic solar wind are now beginning to be pinned down .
different mechanisms are probably dominant for different regions ( see reviews by * ? ? ?
* ; * ? ? ?
* ; * ? ? ?
* ; * ? ? ?
for example , it seems increasingly clear that bright euv and x - ray loops are heated by small - scale , intermittent magnetic reconnection that is driven by the continual stressing of their magnetic footpoints @xcite .    for the open - field regions that link the corona and the solar wind ,
there is still disagreement about the relative contributions of different processes ( see   4 below ) .
however , we are rapidly approaching a time when these processes can be included in self - consistent models that can make testable predictions .
this paper attempts to summarize some recent work that is helping us to bring observations and theoretical models to the point of straightforward comparison and testing . because the coronal magnetic field varies so substantially both as a function of position ( on the sun at any one time ) and as a function of time ( over the solar cycle ) , there is always a broad range of solar wind source regions available for comparison with model predictions . in some ways ,
these variations give us something approaching the turnable `` parameter knobs '' of a laboratory experiment .
this paper emphasizes the example of differences between the previous solar minimum ( 19961997 ) and the present minimum ( 20072009 ) .
the most definitive link between a particular type of coronal structure ( measured via remote sensing ) and a specific type of quasi - steady solar wind flow ( measured _ in situ _ ) is the connection between large coronal holes and high - speed streams @xcite .
coronal holes are generally interpreted as bundles of open flux tubes that flare out superradially with increasing distance .
observations from the uvcs instrument on _ soho _ suggest that the range of heights over which the wind s acceleration occurs in coronal holes can vary greatly , even when the wind at 1 au is identically fast @xcite .
the denser slow - speed solar wind appears to come from many different coronal sources .
two regions that are often cited as sources of slow wind are : ( 1 ) boundaries between coronal holes and streamers , and ( 2 ) narrow plasma `` stalks '' that extend out from the tops of streamer cusps ( e.g. , * ? ? ?
* ; * ? ? ?
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however , during active phases of the solar cycle there is evidence that slow wind also originates in small coronal holes and active regions @xcite .
the remainder of this paper will discuss the fast solar wind that emerges from polar coronal holes at solar minimum @xcite .
however , as the soho23 meeting has demonstrated , not all solar minima are created equal .
the morphology of the coronal magnetic field exhibited some interesting differences from the previous minimum to the present minimum .
polar coronal holes on the disk in 20072009 are smaller in area ( by about 15% ) in comparison to those from 19961997 , and their mean photospheric magnetic fields are lower by about 40% as well ( e.g. , * ? ? ? * ; * ? ? ?
* ) . the magnetic field measured at 1
au is also lower , but by only about 20% @xcite .
the streamer belt observed in the extended corona has a broader latitudinal extent than it did in 19961997 @xcite .
it is likely that this is the result of two contributing factors .
( 1 ) the weaker polar field does not exert as much transverse pressure , which acts to confine the streamer belt to low latitudes ( see , e.g. , * ? ? ?
( 2 ) there are more small and transient coronal holes at low latitudes during the present minimum ; these can also deform the streamer belt .
some likely consequences of the above differences in the coronal magnetic field are discussed below .
low - density coronal holes exhibit a complex array of plasma parameters due to their nearly collisionless nature . as a result
, every particle species evolves towards having its own unique temperature , its own type of departure from a maxwellian velocity distribution , and its own outflow speed .
remote - sensing measurements of the low corona ( i.e. , @xmath01.3 @xmath1 ) and the extended corona ( @xmath2 to 10 @xmath1 ) , as well as _ in situ _ particle and field detection in the heliosphere ( @xmath3 ) , can be combined to follow this evolution .
the extended corona is particularly important to study in this regard , since it is not only where most of the wind s acceleration occurs , but it is also where many plasma species undergo their transition from collisional to collisionless dynamics ( see * ? ? ?
as an example of how different types of measurements can help paint a more complete picture , figure 1 shows temperature measurements in polar coronal holes from the last solar minimum in 19961997 .
the one - fluid temperatures shown at the lowest heights come from semi - empirical ( * ? ? ?
* dashed curve ) and theoretical ( * ? ? ?
* solid curve ) models of the photosphere , chromosphere , and low corona .
electron temperatures in the low corona were measured by sumer/_soho _ and reported by ( * ? ? ?
* black bars ) and ( * ? ? ?
* gray bars ) .
uvcs - derived proton temperatures in the extended corona are also shown ( * ? ? ?
* triangles ) with an attempt to remove the model - dependent alfvn wave broadening .
perpendicular temperatures of a representative minor ion ( o@xmath4 ) are shown from sumer measurements at low heights ( * ? ? ?
* open circles ) and uvcs measurements in the extended corona ( * ? ? ?
* filled circles ) . _ in situ _ electron and proton temperatures in the fast wind ( dotted and dot - dashed curves , respectively ) are taken from @xcite .
the detailed radial dependences of minor ion temperatures have not yet been precisely measured _ in situ , _ but the gray region above shows a likely range of values for the abundant o@xmath5 ion ( see , e.g. , * ? ? ?
coronal holes tend to exhibit preferential ion heating ( i.e. , @xmath6 ) that primarily occurs in the direction perpendicular to the background magnetic field ( @xmath7 ) .
because of these departures from thermal equilibrium , the fast solar wind is an optimal `` proving ground '' for studies of collisionless kinetic processes that many believe are the ultimate dissipation mechanisms of coronal heating .
it was noticed several decades ago that the damping of ion cyclotron resonant alfvn waves could naturally give rise to the observed plasma properties ( see reviews by * ? ? ?
* ; * ? ? ?
* ; * ? ? ?
however , many other dissipation processes have been proposed as well , and they often involve multiple steps of energy conversion between waves , reconnection structures , and other nonlinear plasma features .
taking all of the above complexities into account and producing a self - consistent model of coronal heating and solar wind acceleration ( for all particle species ) has still not been accomplished . however , if an assumption is made to consider only the _ total _ energy content of the plasma  and not its `` partitioning '' into protons , electrons , and other ions  then the problem becomes more tractable . at this level of detail ,
there are two general types of physics - based model that attempt to explain the overall flows of energy :    1 .
there are _ wave / turbulence - driven _ ( wtd ) models in which open magnetic flux tubes rooted to the photosphere are jostled by convection , leading to waves that propagate up into the corona .
these waves ( usually alfvn waves ) are often proposed to partially reflect back down toward the sun , develop into strong mhd turbulence , and dissipate over a range of heights .
these models also tend to explain the differences between fast and slow solar wind _ not _ by any major differences in the lower boundary conditions , but instead as an outcome of different rates of lateral flux - tube expansion over several solar radii @xcite .
2 .   there are _ reconnection / loop - opening _ ( rlo ) models in which the flux tubes feeding the solar wind are influenced by impulsive bursts of mass , momentum , and energy addition .
there is often assumed to be strong coupling between closed , loop - like magnetic flux systems ( that are in the process of emerging , fragmenting , and diffusing across the surface ) and the open flux tubes that connect to the solar wind .
these models tend to explain the differences between fast and slow solar wind as a result of qualitatively different rates of flux emergence , reconnection , and coronal heating at the basal footpoints of different regions on the sun @xcite .
@xcite presented a set of wtd models in which the one - fluid equations of mass , momentum , and energy conservation were solved simultaneously with transport equations for alfvnic and acoustic wave energy .
the coronal heating rate was computed self - consistently from a phenomenological description of turbulent dissipation of partially reflected alfvn waves ( see also * ? ? ?
* ; * ? ? ?
* ; * ? ? ?
it is important to note that these models were run with lower boundary conditions in the optically thick solar photosphere , and _ not _ in the transition region ( tr ) or low corona .
thus , the properties of the chromosphere , the height of the tr , and the mass flux of plasma entering the solar wind were all computed robustly and consistently in these models .
the @xcite models of polar coronal holes produced conditions at 1 au that appeared consistent with fast solar wind measurements at the 19961997 solar minimum .
more recently , we computed a new set of wtd models that were designed to correspond to the 20072009 solar minimum
. the _ only change _ made to the models was in the radial dependence of the imposed magnetic field strength .
the @xcite magnetic field profile was multiplied by a smooth function constructed to produce the following effects : ( 1 ) no change to the @xmath81 kg field strength in the intergranular flux tubes in the photosphere ; ( 2 ) a 40% reduction in the field strength at the base of the corona ( @xmath9 ) to account for the lower polar field strengths measured by low - resolution magnetograms ; and ( 3 ) an 18% reduction in the field strength in interplanetary space from one minimum to the next ( see , e.g. , * ? ? ?
figure 2 shows the relative changes in the resulting wtd models of polar coronal holes .
all quantities are ratios of the form @xmath10 , where the subscript `` old '' refers to the 19961997 minimum and `` new '' refers to the 20072009 minimum .
the change in the input magnetic field strength , described above , is shown as a solid line .
the other quantities are all self - consistent _ outputs _ of the model .
the relative changes at 1 au can be compared with measured changes in the plasma parameters from @xcite and @xcite .
table 1 shows this comparison and confirms that the modeled solar wind responds in a very similar way to the changing magnetic field as does the actual solar wind . in both the models and the _ ulysses _ polar pass data ,
the solar wind speed @xmath11 is relatively unchanged , but the density @xmath12 and temperature @xmath13 decrease by factors that hover around 20% and 10% , respectively .
the decreases in gas pressure ( proportional to @xmath14 ) and dynamic pressure ( proportional to @xmath15 ) are between 20% and 30% for both the observations and models .
lcc & _ ulysses _ polar data & wtd model output + speed & 03% & + 01% + density & 17% & 22% + temperature & 14% & 08% + gas pressure & 28% & 21% + dynamic pressure & 22% & 27% +    figure 2 also shows that the models predict the present solar minimum should have a smaller temperature in the low corona and a slightly higher density .
nobeyama measurements of the radio `` brightness temperature '' ( yashiro et al .
, this meeting ) do indicate a slightly lower temperature in the upper chromosphere during 20072009 , in comparison to 19961997 , but there are not yet any firm comparisons from measurements in the low corona . at the heights observable by uvcs ,
the predicted temperature decline in figure 2 is about 20% and the predicted density change has shifted from an increase to a slight decrease .
( at these heights we expect the proton , electron , and heavy ion temperatures to be behaving differently from one another . )
preliminary reports of minimum - to - minimum variations from uvcs indicate that the h  i ly@xmath16 intensities are higher in 20072009 than in 19961997 , and the o  vi intensities are lower ( gardner et al .
, this meeting ; miralles et al . , this meeting )
. both of these changes may be consistent with lower electron temperatures and electron densities , but only detailed empirical modeling of the spectral line properties will reveal whether that is the case .
the _ soho _ era has seen significant progress toward identifying and characterizing the physical processes responsible for coronal heating and solar wind acceleration . as
remote - sensing measurements have become available in the collisionless extended corona , the traditional gap between solar physics and _ in situ _ space physics has become narrower . however , there are still many unanswered questions : how and where in the solar atmosphere are the relevant waves and turbulent motions generated ? which kinds of fluctuation modes ( i.e. , linear or nonlinear ; alfvn , fast , or slow ; high @xmath17 or high @xmath18 ) are most important ? what frequencies dominate the radially evolving power spectrum ? what fraction of the interplanetary solar wind comes from filamentary structures such as coronal reconnection events and/or plumes and jets ?    answering the above questions involves moving forward in both the theoretical and observational directions . a key step to making further progress
is the ability to include both the wtd and rlo processes in existing 3d numerical simulations of the sun - heliosphere system .
some recent progress in producing computationally efficient approximations to the rates of wtd wave reflection has been reported by @xcite and @xcite .
studies of the connection between the evolving `` magnetic carpet '' and the open flux tubes that feed the solar wind are also ongoing .
the plasma parameters of both the major species ( protons , electrons , and he@xmath19 ) and minor ions are not yet known in coronal holes to the accuracy required to determine the relative contributions of the proposed physical processes .
as figure 1 shows , even quantities as basic as the ratio @xmath20 are not yet known with sufficient accuracy because measurements of the proton and electron temperatures have not yet been made over the same ranges of heights .
minor ion measurements need to be extended to a larger number of ions ( i.e. , a wider range of ion cyclotron frequencies ) so that the ultimate kinetic damping mechanisms of waves and turbulence can be determined ( see , e.g. , * ? ? ?
also , we do not yet have a good enough observational `` lower boundary condition '' on the energetics of waves and turbulence in the photosphere .
existing measurements of the sun s convective granulation with sub - arcsecond spatial resolution need to be matched by sub - second _ time resolution , _ so that the power spectrum of the motions of small - scale magnetic flux tubes ( e.g. , g - band bright points ) can be extended to higher frequencies .
this paper reviews our growing understanding of the physics behind coronal heating ( in open - field regions ) and the acceleration of the solar wind . 
 many new insights have come from the last solar cycle s worth of observations and theoretical work . 
 measurements of the plasma properties in the extended corona , where the primary solar wind acceleration occurs , have been key to discriminating between competing theories . 
 we describe how uvcs / soho measurements of coronal holes and streamers over the last 14 years have provided clues about the detailed kinetic processes that energize both fast and slow wind regions . 
 we also present a brief survey of current ideas involving the coronal source regions of fast and slow wind streams , and how these change over the solar cycle . 
 these source regions are discussed in the context of recent theoretical models ( based on alfvn waves and mhd turbulence ) that have begun to successfully predict both the heating and acceleration in fast and slow wind regions with essentially no free parameters . 
 some new results regarding these models  including a quantitative prediction of the lower density and temperature at 1  au seen during the present solar minimum in comparison to the prior minimum  are also shown .