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the mechanism governing the formation of spicules is still uncertain .
none of the theoretical models proposed until now ( see e.g. the review by * ? ? ?
* ) is able to explain in a satisfactory way all observational facts including ubiquity , evolution , energetics and periodicity @xcite and also the magnetic fields , whose presence in spicules is required by all theoretical models .
investigations of such magnetic fields through spectropolarimetric observation were reported recently @xcite . together with a suitable inversion technique , which takes into account the joint action of the hanle and zeeman effects , full - stokes polarimetry is in fact a powerful diagnostic tool to obtain information about the three - dimensional geometry of the magnetic fields that channel the spicular motion . in particular
the helium multiplets at 5876 and 10830    are very useful spectral lines for that purpose .    in the past year
an ambitious observational project of spicules in the helium d@xmath0 multiplet has been stared at the istituto ricerche solari locarno ( irsol ) taking advantage of the good performances of the zurich imaging polarimeter ( zimpol ) @xcite .
a preliminary physical interpretation of our observations based on suitable inversion techniques has been applied in order to obtain information on the magnetic field vectors involved .    with the same technique
we have additionally obtained several prominence observations , which are described also in these proceedings @xcite .
the observations of spicules were performed with the gregory - coud telescope ( gct ) at irsol , whose aperture is 45 cm .
the zimpol polarimeter @xcite allowed precise measurements free from seeing induced spurious effects ( modulation at 42 khz ) .
the solar image was rotated with a dove prism set after the polarization analyzer in order to keep the limb parallel to the spectrograph slit .
a limb tracker was used to maintain the distance from the limb constant .
53 spicules measurements were obtained at different latitudes and limb distances during 15 days in the period between november 2004 and june 2005 .
the total exposure time for each measurement ranged from 10 minutes to about 1 hour .
every 2 minutes simultaneous measurements of stokes @xmath1 , @xmath2 and one linear polarization component ( alternatively @xmath3 and @xmath4 ) was stored allowing to observe the time evolution . despite of the fact that the dynamic jets that we call spicules have a lifetime of 5 - 10 minutes , the general structures of the obtained stokes images remained almost similar even during the long measurements .
calibrations observations were performed regularly .
these included polarimetric efficiency measurements , dark current , flat field and measurements of the instrumental polarization .
measurement of linear to circular crosstalks as a function of declination for he - d@xmath0 .
_ left : _ @xmath5 crosstalk . to obtain this measurement a linear polarizer
is placed before the entrance window of the telescope with the polarization axis parallel to the geographic equator , which is our choice here for the positive direction of stokes @xmath6 .
_ right : _ @xmath7
crosstalk . to obtain this measurement
the linear polarizer is rotated by 45@xmath8 with respect to the previous measurement .
, title="fig : " ]   measurement of linear to circular crosstalks as a function of declination for he - d@xmath0 .
_ left : _ @xmath5
crosstalk . to obtain this measurement a linear polarizer
is placed before the entrance window of the telescope with the polarization axis parallel to the geographic equator , which is our choice here for the positive direction of stokes @xmath6 .
_ right : _ @xmath7 crosstalk . to obtain this measurement
the linear polarizer is rotated by 45@xmath8 with respect to the previous measurement .
, title="fig : " ]    the instrumental polarization could be carefully corrected for , taking advantage of the fact that in a gct the instrumental polarization for a given wavelength is a function of declination and stays therefore almost constant over one day .
the crosstalks @xmath9 , @xmath10 and @xmath11 were determined through the measurements performed in quiet regions near the solar disk center .
the crosstalks @xmath5 and @xmath7 were determined applying a linear polarization filter at different positions before the entrance window of the telescope .
the results as a function of the declination are shown in fig .
[ fitctallwl - qv ] .
the @xmath12 and @xmath13 crosstalks were extracted from the @xmath5 and @xmath7 crosstalk measurements taking into account the symmetries of the theoretical mller matrix of a gct ( see * ? ? ? * ) .
a database containing the theoretical stokes profiles for different limb distances , magnetic field orientations and strengths has been created , with the theoretical stokes profiles calculated via the application of the quantum theory of the hanle and zeeman effects ( see , e.g. , * ? ? ? * ) spicules are assumed to be optically thin .
the theoretical profiles that better fit the measured profiles are carefully searched in the database , in order to infer the magnetic field vector .
future planed improvements in our analysis will include the introduction of non - thermal motions to better fit the intensity profiles and a detailed study of the so - called van vleck ambiguity which may occur in addition to the well known 180@xmath8 ambiguity .
we will eventually evaluate the opportunity to account for radiative transfer effects ( see * ? ? ? * ) , although they are usually small for the he - d@xmath0 line .
a preliminary analysis of our observations in quiet regions shows that generally the measured he - d@xmath0 stokes profiles are compatible with a magnetic field strength of approximately 10 gauss ( see examples in fig .
[ prof - quiet02 ] , [ 5876 - 050618sp5876m2 ] and [ prof - nw ] ) , which is in good agreement with the results obtained from the he i 10830    multiplet by @xcite .
the stokes @xmath14 profiles are usually dominated by the symmetric signature due to the alignment - to - orientation transfer mechanism discussed by @xcite and @xcite .
, title="fig : " ] , title="fig : " ]    -profile is dominated by the alignment - to - orientation conversion mechanism explained in @xcite and @xcite . two regions marked with `` 1 '' and
`` 2 '' are selected for inversions , which are shown in the next figure.[5876 - 050618sp5876m2 ] ]    stokes profiles corresponding to the two regions selected in figure [ 5876 - 050618sp5876m2 ] with the ensuing theoretical fits ( continuous lines ) , title="fig : " ] stokes profiles corresponding to the two regions selected in figure [ 5876 - 050618sp5876m2 ] with the ensuing theoretical fits ( continuous lines ) , title="fig : " ]    only in one measurement the stokes profiles indicate magnetic field strengths as high as 50 - 60 gauss ( figs .
[ 5876 - 050618sp5876m4 ] and [ prof - act-71 - 79 ] ) . in this particular case
the stokes @xmath14 profile shows a typical zeeman - like antisymmetric shape .
note that stokes q is affected by a strong depolarization .
it is very important to point out that this measurement was obtained near the equator in the proximity of an active region ( ar 10776 , 18 june 2005 ) .
profiles , which indicates stronger magnetic fields .
two regions marked with `` 1 '' and `` 2 '' are selected for inversions , which are shown in the next figure.[5876 - 050618sp5876m4 ] ]    stokes profiles obtained after integrating the two regions selected in fig .
[ 5876 - 050618sp5876m4 ] together with fitted theoretical profiles ( solid lines ) , title="fig : " ] stokes profiles obtained after integrating the two regions selected in fig .
[ 5876 - 050618sp5876m4 ] together with fitted theoretical profiles ( solid lines ) , title="fig : " ]
using zimpol at the gct telescope in locarno it was possible to obtain a large set of high quality full stokes spectropolarimetric measurements of spicules in he - d@xmath0 .
the observations of spicules in quiet regions indicate that the magnetic fields involved are around 10 gauss . in one measurement taken in the proximity of an active region ,
magnetic fields up to 50 - 60 gauss were found .
we are grateful for the financial support that has been provided by the canton of ticino , the city of locarno , eth zurich and the fondazione carlo e albina cavargna .
this work has been also partially supported by the spanish ministerio de educacin y ciencia through project aya2004 - 05792 and by the european solar magnetism network .
ramelli , r. , bianda , m. , trujillo bueno , j. , merenda , l. , & stenflo , j. o. 2005 , in esa sp-596 , chromospheric and coronal magnetic fields , ed .
b. fleck & d. danesy ( noordwijk : esa publications division )
a large set of high precision full - stokes spectropolarimetric observations of the he - d@xmath0 line in spicules has been recorded with the zimpol polarimeter at the gregory - coud telescope in locarno . 
 the observational technique allow us to obtain measurements free from seeing induced spurious effects . 
 the instrumental polarization is well under control and taken into account in the data analysis . 
 the observed stokes profiles are interpreted according to the quantum theory of the hanle and zeeman effects with the aim of obtaining information on the magnetic field vector . to this end , we make use of a suitable stokes inversion technique . the results are presented giving emphasis on a few particularly interesting measurements which show clearly the operation of the hanle and zeeman effects in solar chromospheric spicules .