BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Talks.cam//talks.cam.ac.uk//
X-WR-CALNAME:Talks.cam
BEGIN:VEVENT
SUMMARY:Near fields\, rays\, and multipoles - John Chapman (Keele Universi
 ty)
DTSTART:20230302T133000Z
DTEND:20230302T150000Z
UID:TALK197875@talks.cam.ac.uk
DESCRIPTION:Although we normally think of acoustic waves as travelling `at
  the speed of sound'\,&nbsp\; nevertheless there are perfectly good `subso
 nic waves' which can travel at any speed less than this.&nbsp\; Such waves
  are often called `inhomogeneous'\, because their amplitude necessarily va
 ries in the transverse direction\, and they are essential for describing n
 ear fields (and also edge waves).&nbsp\;\nFrom this familiar starting poin
 t\, I shall present the complete ray structure of a number of three-dimens
 ional fields which I believe are not well-known\, despite being of fundame
 ntal importance to multiple wave scattering problems.&nbsp\;&nbsp\; These 
 suggest a number of new canonical scattering problems in which a highly ge
 ometric ray theory approach is possible for determining fine details of sc
 attering in the near- and mid-field.&nbsp\;\nA basic object for our purpos
 es is the three-dimensional near-field ray structure of a high-order rotat
 ing multipole.&nbsp\; The near field is a `spinning orange'\, somewhat fla
 ttened to ellipsoidal shape\, in which the segments are&nbsp\; regions of 
 alternating high and low pressure\, and the `peel' marks the transition to
  propagating spiral waves in the far field.&nbsp\; Although this structure
  is easily deducible from the Debye approximation to a Hankel function of 
 arbitrary order\, I believe it will not be known to everyone in the audien
 ce.\nI'll report on the first steps of progress with the new canonical pro
 blems\, begun in collaboration with Stuart Hawkins in this Multiple Wave S
 cattering programme.&nbsp\; It seems certain that the new geometrical stru
 ctures disclosed by the numerical codes will all yield to a high-precision
  analytical description in the fullness of time.
LOCATION:Seminar Room 2\, Newton Institute
END:VEVENT
END:VCALENDAR