You are an expert at summarizing long articles. Proceed to summarize the following text: 
the crab nebula has been the best laboratory for investigating the mechanism linking the pulsar wind nebula ( pwn ) with the pulsar . the energetics confirms that the pwn is powered by the spin - down energy of the pulsar , and it is generally believed that this energy is transported by a relativistic wind ( e.g. kennel & coroniti 1984 ) . hester ( 1998 ) and tanvir , thomson , & tsikarishvili ( 1997 ) showed that the `` wisps '' , which are elliptical ripples around the pulsar ( scargle 1969 ) , are moving outwards with a speed of about 0.5_c_. such high energy phenomena must be associated with x - ray emission . here , we present the results of a series of monitoring x - ray observations of the crab nebula with _ chandra _ , whose spatial resolution of @xmath0 is comparable to that of ground - based optical telescopes . we adopt a distance of 2 kpc to the crab nebula throughout this paper . the crab nebula was observed with acis - s3 ( the back - illuminated ccd chip ) eight times , once every three weeks from 2000 november 3 to 2001 april 6 . these observations were coordinated with the _ hubble space telescope _ ( hst ) ( hester et al . 2002 ) . each observation has approximately 2.6 ksec of effective exposure time . we employed a 0.2 sec frame time to reduce pileup and a restricted window to reduce dead - time . the window size is only slightly larger than the x - ray extent of the crab nebula ( see fig  [ fig:2ndimage ] ) . all of the images shown were made using the 0.210 kev band . figure  [ fig:2ndimage ] shows one of eight images of the crab nebula . it clearly shows an axisymmetric structure about the polar jet with the torus and the inner ring resolved in an early _ chandra _ observation ( weisskopf et al . fine fibrous structures are also resolved at periphery of such large - scale structures . they show clear correlation with a optical polarization measurement ( hickson & van den bergh 1990 ) , indicating that they trace local magnetic field structures . figure  [ fig:2468image ] shows a series of the observations . two wisps moving outward are detected through all of eight observations . they can also be seen in simultaneous hst observations ( hester et al . 2002 ) . here , we denote them as `` wisp a '' and `` wisp b '' . they appear to break off from the inner ring . with respect to the inner ring s elliptical shape , the shapes of the wisps look warped due to the time delay of light travel ( hester et al . 2002 ) . we measured the speed of the wisps , assuming that they are moving in the equatorial plane which includes the inner ring , and taking the inclination of the plane ( the inclination angle was derived assuming the inner ring is circular ) and the time delays into account . in spite of the difference in the directions and the birth times , the speeds of the two wisps are almost same , @xmath10.43_c_. their similarity indicates the existence of the continuous isotropic pulsar wind in the equatorial plane . the inner ring is also variable , but unlike the wisps , it preserves its overall ring - like shape and relative position with respect to the pulsar . the ring never forms a continuous loop , but appears intermittent and mostly consists of knot - like features . among them , three knots , which lie along the southeast portion of the ring and are symmetric about the axis of the jet , gradually brighten by factor @xmath11.5 within our observational period of 6 months . however , we note that these knots were bright enough to be detected 1 year before our 1st observation ( weisskopf et al . additionally , some blob - like features appear to move outward along the jet , with a speed of @xmath10.34_c_.    temporal variations can also be seen in the torus . figure  [ fig : diff ] shows the difference images of the 2nd@xmath23rd and the 2nd@xmath28th observations . although the variation is not strongly pronounced in the 3-week difference image , except for the wisps , differences within the torus and along its boundary are quite substantial over a duration of 19 weeks . the torus seems to be expanding at 0.10.2_c_. due to the small displacement and the ambiguous boundary , relatively large uncertainties remain . the fact that the angular extent along the major axis measured with _ chandra _ agrees well with those measured with _ ( brinkmann et al . 1985 ) , _ rosat _ ( hester et al . 1995 ; greiveldinger & aschenbach 1999 ) , and even from lunar occultation 25 years ago ( aschenbach & brinkmann 1975 ) suggests that the torus is stable on tens of arcsecond scales over decades , but varies on arcsecond scales on several months . the northwestern region and the end of the jet do not exhibit any strong variations . these x - ray observational pictures generally match the canonical scenario that the pwn is confined by the supernova ejecta . the relativistic winds from the pulsar are continuously transported in the equatorial plane and accumulate at the torus , outside of which the optical filaments  ejecta  can be seen . aschenbach , b. , & brinkmann , w. 1975 , , 41 , 147 brinkmann , w. , aschenbach , b. , & langmeier , a. 1985 , nature , 313 , 662 greiveldinger , c. , & aschenbach , b. 1999 , , 510 , 305 hickson , p. & van den bergh , s. 1990 , , 365 , 224 hester , j. j. et al . 1995 , , 448 , 240 hester , j. j. 1998 , in neutron stars and pulsars : thirty years after the discovery , ed . n. shibazaki et al . , 431 hester , j. j. , et al . 2002 , in neutron stars in supernova remnants , ed . slane & b.m . gaensler , in press . kennel , c. f. , & coroniti , f. v. 1984 , , 283 , 694 pavlov , g. g. , kargaltsev , o. y. , sanwal , d. , & garmire , g. p. 2001 , 554 , 189 scargle , j. d. 1696 , , 156 , 401 tanvir , n. r. , thomson , r. c. , & tsikarishvili , e. g. 1997 , new astronomy , 1 , 311 weisskopf , m. , et al . 2000 , , 536 , 81
we present a series of monitoring observations of the crab nebula with the _ chandra x - ray observatory _ , focusing on the temporal evolution of the structure . 
 this series of 8 observations , spanning a period of approximately six months , shows the dynamic nature of the inner x - ray structures . 
 we detected outward moving `` wisps '' from the recently discovered inner ring seen in optical observations . 
 we also find that the inner ring itself shows temporal variations in structure . 
 the torus also appears to be expanding . 
 such temporal variations generally match the canonical scenario that an expanding synchrotron nebula injected from the pulsar is confined by the supernova ejecta . 
 # 1_#1 _ # 1_#1 _ =    # 1 1.25 in .125 in .25 in