Spectropolarimetry of Na I emission in Mercury exosphere

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Spectropolarimetry of Na I emission in Mercury
exosphere

• John Allen (GSFC), Detrick Branston (NSO),
  Pat Michaels (GSFC), Jose Ceja (CSUN)

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Outline

• Mercury exosphere
• Why polarimetry?
• Observations
• Instrumental polarization
• Results

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Mercury Exosphere

• Tenuous not gravitationally bound
• H, He, O, Na, K emission lines
• Na
• number density 108 cm-3
• atmosphere is optically thick
• changes with time, solar wind
• distribution not uniform across disk
• anti-Sunward tail observed

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Mercury Exosphere

• Typical day in a Na atom
• Ejected from surface E k ( T 2-3 103 K)
• Several hops with duration 1000s decreasing to
  250s
• Ionization and re-implantation or neutralization
  at the surface
• After 10 ionizations follows B field line and
  escapes planet

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Mercury Exosphere

• Na spectrum
• D2 589.0nm delta J1
• D1 589.6nm delta J0

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Mercury Exosphere

• Na spectrum level lifetimes
• D1, D2 Upper levels
• Lower level
• Order 104 photon absorptions per collision

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Why polarimetry?

• The Mercury B field has only been measured a few
  times during Mariner 10 flybys, yet B plays a key
  role.
• B 400 nTesla 4 milliGauss
• Sodium emission is bright can it be used to make
  measurements of B field?
• The Hanle effect on scattered radiation provides
  a possible measurement technique

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Why polarimetry?

• The Hanle effect is
• important for measuring weak fields where the
  Zeeman splitting is smaller than the natural line
  width
• a decrease in the linear polarization of the
  scattered radiation compared to no B case
• a rotation of the polarization azimuth is seen
  for some geometry

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Why polarimetry?

• The Hanle effect can be described with the
  classical multiple oscillator picture.
• An important idea is that linear oscillators can
  be decomposed into circular oscillators

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Why polarimetry?

• The frequency of the two circular oscillators
  must be equal in order to reproduce the linear
  oscillator.
• A properly oriented magnetic field will alter the
  frequency of a circular oscillator
• Strong magnetic field large shift (Zeeman)
• Weak magnetic field small shift (Hanle)

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Why polarimetry?

• This shift is the Larmor frequency

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Why polarimetry?

• No shift linear oscillator reproduced

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Why polarimetry?

• With shift, direction precesses with time

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Why polarimetry?

• When all directions total depolarization

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Why polarimetry?

• If the lifetime of the level is much greater than
  the precession rate then the emitted photon can
  have any polarization azimuth the signal is
  completely depolarized.
• However, if the level lifetime is comparable to
  the rate, then a net polarization change is seen
  (in total polarization or azimuth).

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Why polarimetry?

• Net polarization change

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Why polarimetry?

• Net polarization change

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Why polarimetry?

• The Mercury B field has only been

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Why polarimetry?
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Why polarimetry?

• Each transition with a specific lifetime then has
  a characteristic magnetic field for which the
  Hanle effect is most sensitive
• D1, D2 Bchar 8 Gauss
• Lower level Bchar 0.01 milliGauss
• (Remember BM 4 milliGauss)
• There are angular effects if B is not oriented
  along the line of sight.

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Why polarimetry?

• In standard polarization theory, we expect no
  Hanle effect in D1 line since delta J0
• However the standard theory neglects ground state
  polarization (atomic polarization) solar
  observations show strong atomic polarization in
  the solar atmosphere.
• With atomic polarization, dependence of Hanle
  effect on B angle changes too.

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Why polarimetry?

• On Mercury we expect
• No upper level Hanle effect
• Ground state effect results in depolarization
  and/or rotation in the D2 line.
• Perhaps changes in D1 if ground state
  polarization exists.

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Observations

• NSO Sac Peak Dunn Solar Telescope 29 May 4
  June 2000
• NSO Kitt Peak McMath/Pierce Telescope 13 19
  Jan 2002

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Observations

• DST observations used Fabry-Perot filter system
  and rotating waveplate/linear polarizer.
• The FP was scanned in wavelength to cover both
  lines.
• The polarization analysis package was changed
  between scans in a symmetric pattern.

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Observations
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Observations

• DST observations were hand-guided and had poor
  seeing images were spatially averaged across the
  whole disk.
• Weather was poor and variable telluric absorption
  lines are suspected to interfere with the data.

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Observations
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Observations

• DST Results (Allen, Penn, Branston, Ceja,
  Patrick, in preparation)
• Strange line ratios D2/D1 0.4 to 1.3
• Continuum polarized at 4.5 (not 6)
• D2 polarized at continuum level.
• D1 depolarized relative to the continuum.
• No polarization azimuth changes between lines and
  continuum (but high measurement noise).

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Observations

• DST Results (Allen, Penn, Branston, Ceja,
  Patrick, in preparation)

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Observations

• The DST results motivated observations at McM/P
  at Kitt Peak.
• Observations very different slit spectrograph
  modified with dual-beam polarization analysis.
• Slow-chopping of the polarization was done in a
  symmetric pattern.

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Observations
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Observations

• McM/P Results (Allen, Penn, Branston, in
  preparation) (disk integrated again)
• Normal line ratios D2/D1 1.5
• Continuum polarized at 6.7
• D2 polarized at continuum level.
• D1 depolarized relative to the continuum.
• Continuum azimuth normal to scattering plane as
  is D2 azimuth
• D1 azimuth rotated 60o wrt scattering normal

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Observations
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Observations
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Observations
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Instrumental Polarization

• The agreement of the continuum total polarization
  and azimuth angle with previous measurements
  suggests minimal instrumental polarization
  contamination.
• Flat fielding technique has been shown to largely
  remove Stokes I ? Stokes Q, U crosstalk terms.
• Sky subtraction must help to remove instrumental
  polarization.

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Instrumental Polarization

• However, azimuth angle change in D1 line suggests
  instrumental contamination may be present.
• Three sequences at different spectrograph table
  angles show the same effect implies telescope
  effect rather than instrument.
• The Mueller matrix for the McM/P has been
  thoroughly investigated, in particular by
  Bernasconi (1997).

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Instrumental Polarization

• Telescope Mueller matrix modifies the incoming
  Stokes vector
• The Mueller matrix is 4x4, depends on the angles
  between mirrors and the indicies of refraction of
  the mirrors (n, k).

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Instrumental Polarization
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Instrumental Polarization
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Instrumental Polarization

• One term in this matrix looks like

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Instrumental Polarization

• Because alpha depends on the hour angle and
  declination, the Mueller matrix is time
  dependent.
• However during the 20min required for one scan,
  the matrix changes are roughly linear.
• Symmetric pattern can remove linear changes
  Q,U,-Q, -U, -U, -Q, U, Q pattern averages to one
  time.

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Instrumental Polarization

• While removing trends, the measurement technique
  is still subject to any polarization that is
  introduced.
• For a zero polarization input, the telescope will
  output a non-zero polarization.
• Unfortunately the Mueller matrix of the telescope
  changes over time as n, k vary and no
  calibration (at Mercury HA, dec) performed during
  observing (need Sun?).

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Instrumental Polarization

• Conclusion simulations of the azimuth angle
  produced by instrumental polarization do not
  reproduce the azimuth measured in the D1 emission
  line.

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Results

• In both telescopes with both instruments at the
  different times we see the same thing
• D2 is polarized indistinguishably from the
  continuum
• D1 is depolarized relative to the continuum
• The polarization azimuths at McM/P are consistent
  with the noise in DST data.

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Results

• Two scenarios are in agreement with these
  observations
• J-level quantum interference (non-magnetic)
• Ground-state Hanle depolarization of D2
  (magnetic)

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Results

• J-level quantum interference states that the
  upper levels of the D-lines cannot be treated
  independently.

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Results

• Stenflo (1980) and recently many others have used
  J-level interference to fit the linear
  polarization in Ca II H,K lines and Na D1, D2
  lines in solar atmosphere.
• This effect is independent of Doppler width of
  the line.
• Effects are not detectable in intensity, but
  large in linear polarization spectrum.

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Results
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Results

• Stenflo (1980) fit uses 3 parameters
• continuum polarization (measured)
• continuum to line ratio between lines (2000)
• constant k equals the product of function
  related to illumination and fractional importance
  of depolarizing collisions (0.1)

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Results
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Results

• Standard polarization can also explain these
  observations
• D2 is depolarized by factor of 100 by
  ground-state Hanle effect
• B field is mostly tangential to line of sight
  when averaged over disk so no rotation expected
  D2 azimuth is the same as the continuum.

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Results
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Results

• Problems
• Depolarization should (?) be larger for B fields
  at 4 milliGauss strength.
• Neither idea yet explains the D1 azimuth
  measurement (atomic polarization?).

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Results

• Need QM model using
• ground-level Hanle effect
• atomic polarization
• J-level interference
• dipole Mercury field, atmosphere RT
• to predict depolarization and azimuth.

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Web acknowledgements

• Arturo Lopez Ariste, THEMIS Hanle figs