Inner Source Diffuse Helium Observations Elena Moise 1, Jeff Kuhn 1, and John Raymond 2 1 Institute for Astronomy, University of Hawai’i 2 Harvard-Smithsonian Center for Astrophysics 22 septembrie 2010, Bucuresti

Abstract  Sensitive SOLARC imagining spectropolarimetric observations from Haleakala reveal a diffuse coronal surface brightness in the He I 1083nm line. A series of observations suggests that this signal originates from an “inner source” of neutral helium atoms in the solar corona. Here we explore the possibility that this cold coronal component originates from helium ions that are neutralized by the near-sun dust and subsequently excited to the metastable 1s2s 3 S state, which then scatters photons from the solar disk. We find a strong correlation between the polarized He brightness and coronal white light brightness that supports the argument that electronic collisional excitation of the metastable helium triplet level is responsible for our polarization signal.

Eclipse Observations, 1994 Angle from limb (arcsec) Brightness (disk units) He I Fe XIII He I FeXIII FeXIII

Detection of Neutral Helium (He I)  He I 58.4nm 1s2p 1 P - ground level SOHO/UVCS, ~ 9 R Θ  He I 1083nm 1s2p 3 P - metastable 1s2s 3 S SOLARC + IR spectropolarimetry

Why 1083nm and not 58.4nm?  Sun radiates about 10 7 more photons in IR than in UV  Spurious scattered light from the atmosphere and telescope optics declines with increasing wavelength  Possible drawback: Relative density of triplet He I (responsible for 1083nm scattering) to singlet He I (responsible for the 58.4nm resonance line) is likely to be small  By using SOLARC and IR Imaging Spectropolarimetry, the He I line at 1083nm is detectable

SOLARC + IR Imaging Spectropolarimeter LCVR Polarimeter Input array of fiber optics bundle Re-imaging lens Prime focus inverse occulter/field stop Secondary mirror Primary mirror Fiber Bundle Collimator Echelle Grating Camera Lens NICMOS3 IR camera

Raw Intensity λ nm 16 fibers (‘rows’) 8 ‘columns’ He 1083nm 14 Feb 2005 E limb

Stokes Q Scattered photospheric Mg & Si He 1083 nm Q < 0  perpendicular polarization to limb 14 Feb 2005 E limb

Average Polarized He I Flux Diffuse component:  A - Stokes U  B - Stokes Q Localized component (chromospheric light - prominence)  C - Stokes Q  D - Stokes U  Diffuse He I signal is not scattered chromospheric light or a result of near-by (e.g. geocoronal) helium forward scattering He Si

New Observations QQ 23 Jan 07 QQ 6 Feb 07 Q Q U U 15 Feb 07 Q U Q U 21 Feb 07 Q U Q&UQ&U Q&UQ&U 20 Mar 07 W S; +Q N; +Q E; -Q +U -U

Interpretation  Each time He I is observed, it has the right polarization state  So far, each time observations were taken in the ecliptic diffusive He I signal was detected  No significant Doppler shift  This He I is cool, much colder than any He I in thermal and ionization equilibrium with the corona (for 1MK He I FWHM=0.39nm while our observations are consistent with instrumental resolution of 0.05nm)  Comparison with active regions show no correlation

Interpretation  Weak fall off of diffusive He I with R Θ agreement with eclipse observations  Comparison with streamers shows an 80% correlation a probability of less then 0.1% for being a simple coincidence 21 Feb 07 E limb W limb

Possible sources for He I  Scattered chromospheric He I  Geocoronal  Equilibrium coronal component  Local interstellar neutral He wind  Non-equilibrium coronal component = Inner source Ion absorption, subsequent deionization, and desorption of neutrals into solar wind environment

Interstellar Neutral He in the Heliosphere  Neutrals are ionized by charge exchange, photoionization, and electron impact  Interstellar neutrals are ionized  pickup ions (PUI)  PUI are accelerated at TS  anomalous cosmic rays (ACR)  He is focused by the Sun’s gravitational field on the downwind side 26km/s

Summary  There’s a neutral He corona at I >10 -8 solar disk units near the Sun  This He I is cool - much colder than any He I in thermal and ionization equilibrium with the corona  The most likely origin of the near-Sun He I is desorption of cool atomic helium from the circumsolar dust (inner source)

Mahalo!

Average Polarized He I Flux: SE vs. NW He Intensity at R~1.3R Θ SE limb: 4x10 -7 NW limb: 2x10 -7 If He were 100% polarized Q=1dn -> I~10 -8 Disk brightness units 10% polarization -> I~10 -7 Si He SE limb NW limb