P41C-­‐1628	  How	  magne*c	  ﬁeld	  direc*on	  inﬂuences	  lunar	  swirl	  morphology	  Doug	  Hemingway	  (djheming@ucsc.edu)	  and	  Ian	  Garrick-­‐Bethell,	  Earth	  &	  Planetary	  Sciences,	  University	  of	  California,	  Santa	  Cruz	  At	  Airy,	  the	  rock	  magnetization	  points	  mainly	  downward	  [10]	  such	  that	   magnetic	   ﬁeld	   lines	   at	   the	   spacecraft	   altitude	   tend	   to	   point	  inward	   and	   then	   downward	   toward	   the	   centre	   of	   the	   anomaly	  (Figure	   3a).	   As	   with	   Reiner	   Gamma,	   a	   map	   of	   the	   horizontal	  magnetic	  ﬁeld	  (Figure	  3c)	  shows	  potentially	  relevant	  structure	  not	  seen	   in	   a	   map	   of	   the	   total	   ﬁeld	   (Figure	   3b).	   The	   relationship	  between	   the	   form	   of	   the	   albedo	   anomaly	   and	   the	   horizontal	  magnetic	  ﬁeld	  is	  most	  strikingly	  revealed	  in	  a	  map	  of	  the	  east-­‐west	  component	   of	   the	   magnetic	   ﬁeld	   (Figure	   4).	   Here,	   the	   dark	   lane	  through	   the	   centre	   of	   the	   anomaly	   shows	   exceptional	   alignment	  with	   the	   line	   representing	   zero	   east-­‐west	   magnetic	   ﬁeld	   strength	  (dashed	   white	   line).	   Near	   solar	   noon	   (when	   darkening	   eﬀects	  should	   be	   most	   intense),	   solar	   wind	   ions	   falling	   along	   this	   line	  would	   experience	   little	   deﬂection,	   leaving	   them	   to	   concentrate	  along	  the	  line	  itself.	  	  Latitude (degrees N)−17.5−18−18.5−1910km

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Longitude (degrees E)c	  b	  c	  4.	  Discussion	  	  Figure	  1:	  MagneKc	  ﬁeld	  maps	  derived	  from	  Lunar	  Prospector	  data	  over	  ClemenKne	  750	  nm	  reﬂectance	  maps	  at	  the	  Reiner	  Gamma	  anomaly.	  Panel	  (a)	  shows	  the	  direcKon	  of	  ﬁeld	  lines	  at	  the	   spacecraT	   alKtude	   (36	   km)	   with	   arrow	   lengths	   showing	   relaKve	   horizontal	   ﬁeld	   strength.	  
Panel	   (b)	   shows	   contours	   of	   the	   total	   magneKc	   ﬁeld	   strength	   (min	   contour:	   5	   nT;	   contour	  interval:	   1	   nT).	   Panel	   (c)	   shows	   contours	   of	   the	   horizontal	   component	   alone	   (min	   contour:	   5	   nT;	  contour	  interval:	  1.5	  nT).	  The	  transect	  labeled	  AA	  is	  for	  use	  with	  Figure	  5.	  	  Figure	  3:	  MagneKc	  ﬁeld	  maps	  derived	  from	  Lunar	  Prospector	  data	  over	  ClemenKne	  750	  nm	  reﬂectance	   maps	   at	   the	   Airy	   anomaly.	   Panel	   (a)	   shows	   the	   direcKon	   of	   ﬁeld	   lines	   at	   the	  spacecraT	  alKtude	  (28	  km)	  with	  arrow	  lengths	  showing	  relaKve	  horizontal	  ﬁeld	  strength.	  Panel	  (b)	  shows	  contours	  of	  the	  total	  magneKc	  ﬁeld	  strength	  (contour	  interval:	  1	  nT).	  Panel	  (c)	  shows	  contours	  of	  the	  horizontal	  component	  alone	  (contour	  interval:	  0.5	  nT).	  The	  transect	  labeled	  BB	  is	  for	  use	  with	  Figure	  6.	  	  Figure	  4:	  East-­‐west	  Figure	  2:	  CorrelaKon	  120.3

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Total Field Strength (nT)between	  magneKc	  ﬁeld	  direcKon	  and	  reﬂectance	  at	  Reiner	  Gamma.	  ‘Angle	  from	  verKcal’	  is	  90º	  where	  ﬁeld	  lines	  are	  horizontal.	  
Data	  points	  are	  colour-­‐coded	  by	  total	  ﬁeld	  strength	  and	  lines	  are	  ﬁced	  for	  the	  weakest	  third	  (blue),	  middle	  third	  (green),	  and	  strongest	  third	  (red)	  of	  the	  data	  points,	  with	  doced	  lines	  showing	  95%	  slope	  conﬁdence	  intervals.	  

component	  of	  magneKc	  ﬁeld	  (measured	  by	  Lunar	  Prospector)	  over	  ClemenKne	  750	  nm	  reﬂectance	  map	  at	  Airy	  (contour	  interval:	  1	  nT).	  
Contour	  line	  colours	  indicate	  magnitude	  of	  eastward	  ﬁeld	  strength,	  with	  warm	  colours	  indicaKng	  east-­‐poinKng	  ﬁelds	  and	  cool	  colours	  indicaKng	  west-­‐poinKng	  ﬁelds.	  The	  dashed	  white	  line	  indicates	  zero	  ﬁeld	  strength	  in	  the	  east-­‐west	  direcKon.	  

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801bright	  	  lobe	  0.50.2bright	  	  lobe	  bright	  	  lobe	  0.50.2Clementine V2 750nm Reflectancebright	  	  lobe	  dark	  lane	  bright	  	  lobe	  Normalized field intensity

1e	  ctanc
ReﬂeAt	  Reiner	  Gamma,	  the	  rock	  magnetization	  is	  dominantly	  horizontal	  and	   north-­‐pointing	   [9,10]	   such	   that	   ﬁeld	   lines	   are	   south-­‐pointing	   at	  the	  spacecraft	  altitude	  over	  the	  anomaly	  (Figure	  1a).	  While	  a	  map	  of	  total	  magnetic	  ﬁeld	  strength	  (Figure	  1b)	  fails	  to	  match	  the	  form	  of	  the	  albedo	  anomaly,	  a	  map	  of	  the	  horizontal	  component	  alone	  (Figure	   1c)	   shows	   that	   high	   horizontal	   ﬁeld	   strength	   corresponds	  well	   with	   the	   brightest	   portions	   of	   the	   surface.	   We	   have	   noted	  similar	  correspondences	  at	  other	  swirls,	  as	  have	  others	  [11].	  More	  quantitatively,	   Figure	   2	   illustrates	   that	   as	   ﬁeld	   strength	   increases	  (indicated	   by	   the	   transition	   from	   cooler	   to	   warmer	   colours),	  magnetic	   ﬁeld	   direction	   becomes	   increasingly	   important	   in	  determining	  surface	  reﬂectance.	  	  

6.5Clementine V2 750nm Reflectance	  7

dark	  lane	  2.	  Results	  at	  Reiner	  Gamma	  7.5dark	  lane	  Our	   results	   conﬁrm	   the	   predicted	   relationship	   between	  reﬂectance	   and	   magnetic	   ﬁeld	   direction,	   lending	   strong	  support	   to	   the	   solar	   wind	   deﬂection	   model	   and	   oﬀering	   a	  potential	   explanation	   not	   only	   for	   the	   general	   form	   of	   the	  swirls	   but	   also	   for	   the	   frequently	   observed,	   intra-­‐swirl,	   dark	  lanes.	  

8Clementine V2 750nm Reflectance	  a	  −17@0km	  Swirls	  are	  sinuous	  patterns	  of	  anomalously	  high	  albedo	  and	  distinct	  spectral	   characteristics	   that	   accompany	   many	   of	   the	   Moon’s	  crustal	   magnetic	   anomalies	   [1–4].	   Swirls	   are	   found	   in	   diverse	  locations	   across	   the	   lunar	   surface	   and	   vary	   in	   scale	   with	   some	  spanning	   more	   than	   100	   kilometers.	   Understanding	   these	   unique	  natural	   laboratories,	   where	   space	   weathering	   and	   magnetism	  intersect,	   may	   help	   shed	   light	   on	   important	   questions	   in	   lunar	  science	   including	   the	   Moon’s	   dynamo	   history	   [5,6]	   and	   the	  distribution	  of	  water	  over	  its	  surface	  [4].	  	  One	   model	   for	   swirl	   formation	   suggests	   that	   crustal	   magnetic	  ﬁelds	   act	   as	   mini	   magnetospheres,	   protecting	   localized	   portions	   of	  the	  surface	  from	  the	  optical	  maturation	  (darkening)	  eﬀects	  of	  solar	  wind	  ion	  bombardment	  [7].	  In	  this	  model,	  incoming	  solar	  wind	  ions	  are	   magnetically	   deﬂected	   by	   the	   Lorentz	   force,	   F	   =	   q(v	   ×	   B),	  leaving	   portions	   of	   the	   surface	   relatively	   free	   from	   the	   darkening	  eﬀects	   of	   the	   solar	   wind.	   The	   cross	   product	   in	   the	   Lorentz	   law	  means	   the	   deﬂection	   force	   should	   be	   maximized	   where	   the	  magnetic	   ﬁeld	   is	   perpendicular	   to	   the	   incoming	   solar	   wind	   and	  minimized	   where	   ﬁeld	   lines	   are	   parallel	   to	   the	   solar	   wind.	   Since	  darkening	  will	  be	  most	  pronounced	  near	  solar	  noon,	  this	  translates	  roughly	  to	  horizontal	  and	  vertical	  ﬁelds,	  respectively.	  	  	  Here,	  we	  test	  this	  prediction	  using	  maps	  we	  derive	  [8]	  from	  Lunar	  Prospector	   3-­‐axis	   magnetometer	   measurements	   and	   Clementine	  750	   nm	   reﬂectance	   maps.	   We	   focus	   on	   two	   anomalies,	   Reiner	  Gamma	   and	   Airy,	   which	   exhibit	   distinct	   magnetization	   directions	  [9,10].	   We	   also	   use	   simple	   dipole	   moment	   models	   to	   estimate	   how	  the	  ﬁelds	  might	  appear	  at	  lower	  altitudes.	  

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3.	  Results	  at	  Airy	  These	  two	  examples	  appear	  to	  be	  illustrating	  distinct	  aspects	  of	  the	  solar	   wind	   deﬂection	   phenomenon:	   whereas	   the	   Reiner	   Gamma	  example	   demonstrates	   that	   swirls	   are	   brightest	   where	   magnetic	  ﬁeld	  lines	  are	  horizontal,	  the	  Airy	  case	  shows	  that	  dark	  lanes	  can	  be	  associated	   with	   vertical	   magnetic	   ﬁeld	   lines.	   However,	   a	   look	   at	  magnetic	   ﬁeld	   direction	   closer	   to	   the	   surface	   reveals	   that	   both	  eﬀects	  can	  be	  observed	  at	  each	  of	  the	  two	  anomalies.	  Despite	  the	  single,	  broad	  lobe	  in	  the	  horizontal	  magnetic	  ﬁeld	  proﬁle	  at	  Reiner	  Gamma	   (red	   line	   in	   Figure	   5),	   and	   the	   wide	   spacing	   between	   the	  two	  lobes	  in	  the	  horizontal	  ﬁeld	  proﬁle	  at	  Airy	  (red	  line	  in	  Figure	  6),	  there	   may	   be	   ﬁner	   structure	   and	   closer	   spacing,	   respectively,	   that	  would	   be	   apparent	   in	   lower	   altitude	   measurements.	   Based	   on	  models,	   with	   magnetic	   sources	   approximated	   by	   buried	   dipoles,	  Figures	   5	   and	   6	   show	   that,	   despite	   the	   appearance	   at	   spacecraft	  altitudes	   (orange	   dashed	   lines),	   if	   magnetic	   ﬁeld	   measurements	  could	  be	  taken	  at	  suﬃciently	  low	  altitudes	  (blue	  dashed	  lines),	  the	  positions	   of	   the	   bright	   lobes	   and	   dark	   lanes	   could	   align	   closely	   with	  the	  peaks	  and	  valleys,	  respectively,	  in	  the	  horizontal	  magnetic	  ﬁeld	  proﬁles.	   The	   low	   altitude	   measurements	   required	   to	   test	   this	  hypothesis	   could	   soon	   be	   possible	   as	   NASA	   Ames	   Research	   Center,	  UC	   Berkeley,	   and	   UC	   Santa	   Cruz	   are	   currently	   cooperating	   to	  develop	   a	   low-­‐cost	   spacecraft	   mission	   with	   exactly	   this	   capability	  [12].	  	  While	   our	   models	   for	   magnetization	   are	   not	   unique,	   they	  agree	   with	   observations	   and	   can	   account	   for	   the	   alternating	  bright	  and	  dark	  bands	  while	  making	  it	  clear	  why	  no	  such	  ﬁne	  structure	   would	   be	   observable	   at	   the	   spacecraft	   altitude.	  
We	   conclude	   that	   observations	   of	   magnetic	   ﬁeld	   direction	  strongly	   support	   the	   solar	   wind	   deﬂection	   model	   for	   swirl	  formation.	  We	  further	  predict	  that	  very	  close	  to	  the	  surface,	  ﬁeld	  lines	  will	  be	  horizontal	  over	  the	  brightest	  parts	  of	  swirls	  and	  vertical	  over	  the	  intra-­‐swirl	  dark	  lanes.	  

Model	  	  a	  Airy	  8.5Latitude (degrees N)

1.	  Introduction	  9

Normalized field intensity

The	  bright	  lobes	  and	  dark	  lanes	  of	  lunar	  swirls	  are	  associated	  with	  horizontal	  and	  vertical	  magnetic	  ﬁeld	  lines,	  respectively	  Reiner	  Gamma	  00

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Transect Distance (km)Figure	   5:	   Proﬁles	   of	   normalized	   horizontal	   magneKc	   ﬁeld	   intensity	   (leT	   axis)	   and	  reﬂectance	  (right	  axis)	  along	  the	  transect	  labeled	  AA	  in	  Figure	  1c.	  Red	  line:	  the	  horizontal	  ﬁeld	  as	   measured	   by	   the	   Lunar	   Prospector	   magnetometer	   at	   the	   spacecraT	   alKtude.	   Grey	   line:	  ClemenKne	  750	  nm	  reﬂectance.	  The	  dashed	  lines	  represent	  the	  horizontal	  ﬁeld	  predicted	  at	  the	  spacecraT	  alKtude	  (orange)	  and	  at	  the	  surface	  (blue)	  based	  on	  a	  model	  of	  two	  north-­‐poinKng	  dipoles	  buried	  beneath	  the	  13	  km	  and	  35	  km	  points	  along	  the	  transect.	  

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Transect Distance (km)Figure	   6:	   Proﬁles	   of	   normalized	   horizontal	   magneKc	   ﬁeld	   intensity	   (leT	   axis)	   and	  reﬂectance	  (right	  axis)	   	  along	  the	  transect	  labeled	  BB	  in	  Figure	  3c.	  Red	  line:	  the	  horizontal	  ﬁeld	  as	   measured	   by	   the	   Lunar	   Prospector	   magnetometer	   at	   the	   spacecraT	   alKtude.	   Grey	   line:	  ClemenKne	  750	  nm	  reﬂectance.	  The	  dashed	  lines	  represent	  the	  horizontal	  ﬁeld	  predicted	  at	  the	  spacecraT	  alKtude	  (orange)	  and	  at	  the	  surface	  (blue)	  based	  on	  a	  model	  of	  a	  single	  downward-­‐poinKng	  dipole	  beneath	  the	  43	  km	  point	  along	  the	  transect.	  

References/Acknowledgements	  	  [1]	   Hood	   and	   Williams,	   (1989)	   19th	   LPSC.	   [2]	   Richmond	   et	   al.	   (2005)	   JGR	   110,	   E05011.	   [3]	   Garrick-­‐
Bethell	  et	  al.	  (2011)	  Icarus	  212.	  [4]	  Kramer	  et	  al.	  (2011)	  JGR	  116,	  E00G18.	  [5]	  Dwyer	  et	  al.	  (2011)	  Nature	  479.	   [6]	   Le	   Bars	   et	   al.	   (2011)	   Nature	   479.	   [7]	   Hood	   and	   Schubert	   (1980)	   Science	   208.	   [8]	   Hood	   et	   al.	  
(2001)	   JGR	   106,	   E11.	   [9]	   Kurata	   et	   al.	   (2005)	   GRL	   32.	   [10]	   Hemingway	   and	   Garrick-­‐Bethell	   (2012)	   JGR	  117.	   [11]	   Shibuya	   et	   al.	   (2010)	   EPSC	   abstract	   #166.	   [12]	   Garrick-­‐Bethell	   et	   al.	   (2011)	   LEAG	   abstract	  #2038.	  We	  would	  like	  to	  acknowledge	  Lon	  Hood	  for	  his	  assistance	  early	  in	  our	  work.	  

AGU	  2011-­‐12-­‐08