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space - based coronagraphs have observed thousands of coronal mass ejections ( cmes ) since their discovery in the early 1970s .
the observations show that hundreds of cmes can attain speeds of 1000 km / sec or more within a few r@xmath0 which exceed the local alfvn speed at those heights .
hence , fast cmes can drive shocks which are detected sometimes in the radio wavelengths as drifting type - ii bursts @xcite .
such indirect observations , however , provide only limited information about the shock ; namely , the driver speed and , the onset and duration of the radio emission .    fortunately , direct imaging and spectroscopic measurements of the density compression at the shock front have been made possible in the last ten years thanks to the lasco coronagraphs and uvcs spectrometer , both instruments aboard the solar heliospheric observatory ( _ soho _ ) . as we will explain , the combination of white light and uv observations can provide the full range of physical parameters ( density , temperature , magnetic field ) across the shock in addition to spatial information about the shock shape and its relation to the driving cme .
now , the physics of coronal shocks can be studied in detail .
the off - limb spectroscopy and coronagraphy are , and will be , our only means for studying shocks where they can accelerate particles with the highest energies ( @xmath1 r@xmath0 ) .
even the forthcoming _ solar probe plus _ mission will only reach to 9.5 r@xmath0 .
( @xmath2 ) on 03/07 and @xmath2 ( @xmath3 ) on 06/07 , respectively .
several streamers are clearly deflected by the shock .
some examples are denoted with dfl. for the june 6 cme , the driver can not be readily distinguished from the deflected streamers . ]
because we have reviewed many aspects of coronagraphic imaging of cme shocks in a recent volume of this conference series @xcite , we present here only a brief overview of the progress made over the last two years . the early work on shock detection @xcite has motivated an ever expanding number of detections of cme - driven shocks @xcite .
these works are based mostly on observations by the secchi instruments which have higher sensitivites than the lasco coronagraphs .
the white light shocks can now be routinely detected in the inner corona ,
@xmath4 r@xmath0 @xcite , and can be followed out to at least 0.5 au @xcite .
the secchi and radio observations show that shocks tend to form at 1.5 r@xmath0 and can accelerate electrons out to 4 @xmath5 @xcite .
the standoff distance between the shock and the driver appears to be 1 - 2 r@xmath0 within the coronagraph field of view @xcite .
clear white light shock signatures were rare during 2007 - 2010 due to the prolonged activity minimum but there have been several fast events since february 2011 .
we present two recent examples in figs .  [
fig : secchi]-[fig : shock ] .
the three viewpoints afforded by the secchi and lasco telescopes easily reveal the global reach of cme - driven shocks in the middle corona .
measurements of the shock longitudinal extent allow us to localize with better precision the injection of solar energetic particles ( seps ) along magnetic field lines @xcite and connect it to the cme source region @xcite .
shocks can be identified in uv spectra by means of heating , density compression and high speeds associated with them . the first spectral detection of a cme driven shock was made by uvcs in 2000 @xcite .
the authors measured enhancements in the o@xmath6 and si@xmath7 that were consistent with an mhd shock .
more than a dozen of shocks have been identified since @xcite .
a review of plasma physical parameters derived by uvcs across shocks is given in table  [ tab : a ] .
the plasma heating and compression across the shock lead typically to strong brightening of the si@xmath7 ( si  xii @xmath8521 ) line , brightening or dimming of the o@xmath6 ( o  vi @xmath91032 - 1037 ) lines and dimming of the h  i ly@xmath10 @xmath81216 line .
these differences are related to the different physical processes involved in the atomic excitations , because coronal ions are mainly excited by both collisions with electrons ( collisional excitation ) and/or by resonant scattering of photons emitted from the underlying layers of the solar atmosphere ( radiative excitation ) ; the latter process is also related to the doppler dimming effect ( see * ? ? ?
1032 - 1037 intensities observed during the transit of the cme - driven shock .
top right : line profile broadenings observed at the shock transit ( average over the spatial region shown in the middle panel by the two solid white lines ; * ? ? ?
bottom : same as top panels , but for the may 7 , 2004 event @xcite . ]
the most evident signature of a shock in euv spectra is a broadening of the o  vi @xmath91032 - 1037 line profiles in close correspondence to the shock transit , as derived from shock speeds either measured in the lasco coronagraph images or derived from the frequency slope of accompanying type - ii radio bursts ( fig .
[ fig : uvcsshock ] ) .
spectral signatures of the shocks have been detected from about 1.7 r@xmath0 up to 4.3 r@xmath0 ( table  [ tab : a ] ) .
the observed broadening is usually reproduced with superposition of a narrower component due to emissison from coronal plasma located along the line of sight ( los ) , and a broader component due to the shock emission ( e.g. , fig .
[ fig : uvcsshock ] bottom right panel ) .
the latter is often red- or blue - doppler shifted by up to 0.4  with respect to the coronal emission because of the los component of shock velocity , resulting in asymmetric observed profiles ( see e.g. , * ? ? ?
* ; * ? ? ?
* and fig .
[ fig : uvcsshock ] , top right panel ) .
fwhms of broad components typically correspond to post - shock o@xmath6 kinetic temperatures @xmath11k .
.cme - driven shock parameters derived from uvcs data [ cols= " < , > , > , > , > , > , > " , ]     there is a high degree of complementarity between the coronagraphic ( spatial distribution , speed of driver and shock ) and spectroscopic measurements ( pre- and post - shock plasma parameters ) . indeed , combined analyses of lasco and uvcs observations can obtain the full range of plasma parameters upstream and downstream of the shock , including the magnetic field in both regimes .
the mhd rankine - hugoniot jump relations for an oblique shock provide not only the post - shock plasma parameters , but also the pre- and post - shock magnetic field strength when the pre - shock coronal plasma parameters , with the exception of the magnetic field , can be determined by spectroscopic measurements ( i.e. , electron density and temperature , pre - shock and shock velocity ) and the density compression ratio across the shock is determined by coronagraphic measurements .
such results derived by @xcite are summarized in table [ tab : b ] .
interestingly , the magnetic and kinetic energy density increases are nearly equal ( as expected for equipartition of energy ) , and are both more than two times larger than the thermal energy increase .
this trailblazing study demonstrates the power behing off - limb spectroscopic observations when combined with coronagraphic cme images .
in the short space of this article , we tried to demonstrate the ability of coronagraphic and spectroscopic instruments to remotely detect coronal shocks and measure their properties . despite a slow start during the 2000s ,
shocks are now detected and analyzed with increasing frequency .
coronagraphs , especially the simultaneous observations from secchi and lasco , are capable of deriving the 3d structure of the shock , its standoff distance from the driver , its speed and even the density compression ratio across the shock boundary .
but they do not measure magnetic field or temperature conditions .
hence , coronagraphs can not directly discriminate between a shock or a fast mode wave since the observed density compression could be the same .
however , shocks can now be followed to at least 0.5 au and possibly to earth thanks to recent imag processing developments @xcite .
more details can be found in @xcite .
the spectroscopic observations have both advantages and disadvantages compared to the obsevations in the radio or in the visible . on one hand ,
the narrow slits provide only one - dimensional spatial information and the results are quite sensitive to the orientation of the slit relative to the shock front
. measurements of very fast shocks may also be compromised due to the relatively long integrations for the uvcs spectra ( @xmath12 s ) .
spectroscopic detection of coronal shocks occurs only by chance , because the field of view of the 40 
long uvcs slit covers only a limited latitudinal region ( e.g. 75@xmath13 when the slit is positioned at 1.7 r@xmath0 ) . on the other hand ,
spectral measurements are the only means to study the heating of individual ion species and electrons in the collisionless coronal shocks .
measurement of physical properties in cme - driven shocks and their correlations with sep events will be very useful in establishing the necessary conditions for shocks to accelerate ambient solar wind or suprathermal ions to seps .
fortunately , off - limb spectroscopic capability will be offered by the forthcoming _ solar orbiter _
mission with the metis ( multi - element telescope for imaging and spectroscopy ; see * ? ? ?
* ) instrument , that will acquire simultaneously off - limb spectra at three heights , thus helping us to disentangle spatial and temporal evolution of plasma parameters across shocks .
part of this work was funded by various nasa grants .
soho is an international collaboration between nasa and esa .
lasco was constructed by a consortium of institutions : nrl ( washington , dc , usa ) , mps ( katlenburg- lindau , germany ) , lam ( marseille , france ) and univ.of birmingham ( birmingham , uk ) .
the secchi data are produced by an international consortium of the nrl , lmsal and nasa gsfc ( usa ) , ral and univ .
bham ( uk ) , mps ( germany ) , csl ( belgium ) , iota and ias ( france ) .
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shocks driven by coronal mass ejections ( cmes ) are primary agents of space weather . 
 they can accelerate particles to high energies and can compress the magnetosphere thus setting in motion geomagnetic storms . for many years , these shocks were studied only in - situ when they crossed over spacecraft or remotely through their radio emission spectra . 
 neither of these two methods provides information on the spatial structure of the shock nor on its relationship to its driver , the cme . in the last decade , we have been able to not only image shocks with coronagraphs but also measure their properties remotely through the use of spectroscopic and image analysis methods . 
 thanks to instrumentation on _ stereo _ and _ soho _ we can now image shocks ( and waves ) from the low corona , through the inner heliosphere , to earth . here , we review the progress made in imaging and analyzing cme - driven shocks and show that joint coronagraphic and spectrscopic observations are our best means to understand shock physics close to the sun .     
 address = space sciences division , naval research laboratory , washington dc , usa ,     address = inaf - osservatorio astronomico di torino , pino torinese , italy