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owing to new observations and a continuous improvement of simulations , in the last years the solar community has published several works concerning the formation and evolution of filaments .
active region ( ar ) filament studies are still scarce in literature , and it is not clear how similar their formation process is to that of the quiescent ( qs ) ones . generally speaking , there are two proposed scenarios that try to explain the formation of filaments : ( 1 ) the sheared arcade ( sa ) model and ( 2 ) the flux rope emergence ( fre ) model .
the difference between both models mainly exists in the atmospheric height of filament formation . on the one hand , in the sa model
the filament is formed in the corona by shearing motions in addition to converging flows at the polarity inversion line ( pil ) and reconnection processes . as a result ,
the magnetic structure can be a flux rope ( e.g. , * ? ? ? * ) or a dipped arcade ( e.g. , * ? ? ?
* ) . on the other hand
, the fre model assumes that a flux rope emerges from below the photosphere ascending into the corona ( e.g. , * ? ? ?
@xcite provide an extensive discussion and references related to these two models . in this work
, we present a multi - height study of the magnetic structure of an ar filament observed in july 2005 and investigate whether this filament fits into the sa or fre model .
the present analysis is mainly based on spectropolarimetric observations acquired with the tenerife infrared polarimeter attached to the vacuum tower telescope ( vtt ) in tenerife ( tip - ii ; * ? ? ?
the data sets of an ar filament belonging to noaa 10781 were taken close to disk center ( @xmath00.91 and 0.95 ) on 2005 july 3 and 5 .
however , h@xmath1 images from big bear solar observatory ( bbso ; * ? ? ?
* ) show that the filament was already present a few days before , on july 1 .
line - of - sight ( los ) magnetograms from the michelson doppler imager ( mdi , * ? ? ?
* ) show that below the filament an extensive facular region is seen with two polarities clearly separated by the pil . between july 17
the opposite polarities became more compact , almost touching each other at the pil , and then broadened again . during this event , with characteristics of the `` sliding door '' effect ( an effect firstly described by * ? ? ? * ) , new pores and orphan penumbrae emerged at the pil ( see * ? ? ?
* ; * ? ? ?
* for a detailed description and images ) .
@xcite related this effect to the emergence of a flux rope below an ar filament .
the spectral region observed with tip - ii comprised the photospheric si i 10827    line , the chromospheric he i 10830    triplet , and two telluric lines with a spectral sampling of @xmath211.04  m  px@xmath3 .
therefore , with this instrument it is possible to simultaneously analyze the vector magnetic field at two different heights in the solar atmosphere . in this work
, we will concentrate on the second set of spectropolarimetric data taken on july 5 between 7:36 and 14:51 ut . the observing strategy was to scan the filament with the slit parallel to the pil with a scanning step size of 0.3@xmath4 .
the pixel size along the slit was 0.17@xmath4 .
the vector magnetic field was inferred by carrying out inversions of the four stokes parameters with two different inversion codes : ( 1 ) for the si  i line we used the sir code ( stokes inversion based on response functions ; * ? ? ?
* ) and ( 2 ) for the he  i triplet we used a milne - eddington - based inversion code ( melanie ; * ? ? ?
the 180@xmath5-ambiguity was solved using the azam code @xcite .
the los velocities were determined from the inversions and converted to an absolute scale , i.e. , corrections for orbital motions and the gravity shift were made ( see * ? ? ? * for a full description of the velocity calibration ) .
the inferred magnetic field strength in this ar filament is 600800  g @xcite , which was the strongest value detected so far inside ar filaments . recently ,
other authors have also reported similar strong magnetic fields inside filaments , ( e.g. , * ? ? ?
* ; * ? ? ?
therefore , it seems to be rather common to find these strong fields inside ar filaments .
the set of inferred chromospheric and photospheric vector magnetograms indicate that the magnetic structure which supports the filament is a flux rope . to substantiate this result ,
non - linear force - free ( nlff ) field extrapolations , starting from the photosphere and chromosphere , were carried out .
the extrapolations confirmed the presence of a flux rope structure , which lay surprisingly low in the atmosphere and had its axis located at @xmath21.4  mm above the solar surface ( see * ? ? ? * for a complete description of the extrapolations and results ) .
the light green field lines on both sides of the filament represent the fields in the positive ( upward arrow ) and negative ( downward arrow ) faculae .
this figure is from @xcite . ]
based on the vector magnetograms , we constructed the sketch presented in fig .
the cartoon shows a gray - scale continuum image from the dutch open telescope ( dot ) on 2005 july 5 . in the middle of the image ,
the aforementioned pores and orphan penumbrae that started to appear on july 4 are seen .
the reconstruction of the filament is represented as a black structure which was extracted from the corresponding dot h@xmath1 image .
the filament follows the pil .
the positive ( negative ) polarity is at the right ( left ) side of the pil when viewing the figure from the lower right corner .
there are three different magnetic field lines represented in the cartoon .
( 1 ) the dark red field lines are based on the photospheric vector magnetograms .
these field lines are aligned along the pil where pores and orphan penumbrae are seen .
however , outside the orphan - penumbrae region , the field lines have an inverse polarity configuration ( pointing from negative to positive polarity ) .
( 2 ) the light blue field lines were derived from the chromospheric vector magnetograms . in this case , the field lines are parallel to the filament axis outside the orphan - penumbrae region , whereas inside , the field lines show a normal configuration ( the field lines point from positive to negative polarity ) .
( 3 ) the green field lines , close to the corners of the image , represent the positive ( upward pointing arrow ) and negative ( downward pointing arrow ) polarity of the faculae .    to shed light on the formation of the flux rope it was crucial to infer the los velocities .
calibrations using two telluric lines and corrections related to orbital motions and the gravity shift were carefully carried out ( see * ? ? ?
the motions of the transverse magnetic fields in the photosphere , in the orphan - penumbrae area , show , on a seven - hour average , a slow upward trend .
above , in the chromosphere , the filament moves on average downward . nevertheless , there are clearly localized upflow areas ( see fig . 3 in * ? ? ? * ) .
this indicated , when looking at the orphan - penumbrae area in fig .
[ fig1 ] , that the axis of the flux rope is slowly rising .
half of the flux rope is below the surface , which seems to be responsible for the formation of the orphan penumbrae .
the upper part of the flux rope reaches the chromosphere where groups of field lines produce the upflow patches detected in the chromosphere . the portion of the filament that does not have orphan penumbrae below behaves differently .
out of seven maps , the first four show on average upward motions of the transverse fields .
the other three show on average velocities close to zero .
the chromospheric counterpart shows downward motions .
the initial formation phase of this ar filament can not be described with the presented data sets because the filament was already present prior to our observations .
however , we have shown that ar filaments can have extremely low - lying flux rope structures ( even as low as the photosphere ) that support the filament .
this structure can eventually emerge from below the photosphere , as seen in our data sets , generating pores and orphan penumbrae along the pil .
therefore , ar filaments can have a photospheric counterpart . in the chromosphere ,
the filament s plasma is being pushed upward by the magnetic field that expands from the emerging flux rope structure . at the same time
, the emerging flux rope supports the filament material against gravity .
altogether , the present results favor a flux rope emergence scenario .
ck greatly acknowledges the travel support received from the iau .
the vtt is operated by the kiepenheuer - institute for solar physics in freiburg , germany , at the spanish observatorio del teide , tenerife , canary islands .
the national center for atmospheric research ( ncar ) is sponsored by the national science foundation ( nsf ) .
the authors would like to thank c. denker for carefully reading the manuscript .
several scenarios explaining how filaments are formed can be found in literature . in this paper 
 , we analyzed the observations of an active region filament and critically evaluated the observed properties in the context of current filament formation models . 
 this study is based on multi - height spectropolarimetric observations . 
 the inferred vector magnetic field has been extrapolated starting either from the photosphere or from the chromosphere . 
 the line - of - sight motions of the filament , which was located near disk center , have been analyzed inferring the doppler velocities . 
 we conclude that a part of the magnetic structure emerged from below the photosphere .