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\begin{opening}         
\title{The X17 flare of October 28: the gradual phase} 
\author{G. \surname{Del Zanna}}  
\runningauthor{Del Zanna et al.}
\runningtitle{October 28 flare}
\institute{MSSL, University College London Holmbury St. Mary Dorking  RH5 6NT UK}
\author{A. \surname{Berlicki}}
\institute{Observatoire de Paris, Section de Meudon, LESIA,
              F-92195 Meudon Principal Cedex, France}
\author{B. \surname{Schmieder}}
\institute{Observatoire de Paris, Section de Meudon, LESIA,
              F-92195 Meudon Principal Cedex, France}
\author{H. \surname{Mason}}
\institute{DAMTP, University of Cambridge UK}

%Berlicki A., Schmieder B., Mason H.,

\date{\today}

\begin{abstract}

Abstract to be written.

\end{abstract}
\keywords{solar flares}


\end{opening}           

\section{Introduction}

On October 28 2003, AR??
unleashed one of the biggest solar flares 
(class X17).
The event was observed with many instruments, 
and presented various phases. 
A description of the event can be found 
in Schmieder et al (2005). 

After a few  filament eruptions and the main 
X 17 X-ray flare,  a multiple-ribbon 
structure formed. The two main ribbons  quickly
separated. An arcade of EUV loops also formed. 
Here, we present results on the gradual phase.


At first, the overall morphology of this event fits
the standard picture of a two-ribbon flare.
Arcades of loops emitting at different temperatures 
and with a potential configuration
are frequently observed. (REFERENCE??)
The footpoints of these loops normally are 
well observed in H$\alpha$, and form the ribbons.
During the formation of the loops,
the ribbons separate at high speeds.

 H$\alpha$ and X-ray observations have often shown 
 the hotter loops  located above the cooler loops 
(Schmieder et al 1996). The density of the loops 
was estimated to be  
relatively high (10$^{11}$ cm-3 to 10$^{10}$ cm-3) according to the time
but usually is lower than the cool loop densities
(REFERENCE??).
%
 The plasma in the cool loops is often observed to flow down in the legs at
 speeds which can reach the free fall (50 to 150 km/s). It has been
 inferred that 10$^{16}$-10$^{17}$ g of materal flows through a flare loop
system during its life time (Kleczek 1964).

These types of observations have suggested 
'standard models' of reconnection of
open magnetic field lines in the corona 
(Kopp and Pneuman 1976, Poletto and Kopp
1986, Forbes and Malherbe 1986, Forbes and Acton 1996).
The reconnection of the open field lines occurs at a magnetic X- line
which moves upward as the region of closed loops grows.
The loops
and ribbons are associated with the field lines which map from the
X-line to the chromosphere.

The reconnection process naturally explains the existence of high
temperature in the hot loops.
Thermal condensation (cf. Antiochos and Sturrock 1978)
could form the cool loops from the hot loops.
Another important process is the evaporation of 
material that fills directly the hot loops.



However, the observational basis of the 'standard models'
have been criticised, and 
various alternative scenarios have been proposed
over the years (see, e.g. Feldman 1996, REFERENCES).
One of the main limitations has been a lack of spectroscopic measurements.

For our understanding of flares, it is 
 important to study the role of the
 mechanisms that replenish/empty the loops 
(free fall, evaporation and condensation),
as well as the processess that create the ribbons 
(thermal conduction, bremshtrahlung of 
energetic particles). 


The first EUV spectroscopic measurements of a 
two-ribbon flare were reported in a {\em letter} by 
Czaykowska et al (1999).
Czaykowska et al (1999) reported on a SOHO/CDS
observation of an arcade of hot loops
 after two hours from the peak of an M6.8 flare. CDS observed
downflows at the ends of the hot loops and upflows 
at the outer edges of the flare ribbons, which coincided with 
the footpoints of the  hot loops.
The upflows were seen in Fe XVI 
($3 \times 10^6$~K) and Fe~XIX ($8 \times 10^6$~K) lines.
As the ribbons continued to separate, the upflows 
continued to remain concentrated at the outer edges of a ribbon.
The observed  pattern of upflows and downflows 
was in agreement with  models that explain 
the creation of flare loops by chromospheric evaporation
 driven by reconnection. 


Here we have the  opportunity to study the flows of the plasma in a system
of postflare loops observed in EUV
(CDS, TRACE) in H$\alpha$ (Bialkov, ??).
The ribbons are clearly seen in 
H$\alpha$ and in TRACE  1600\AA, while the 
 arcade of loops is seen in the 
 TRACE  195\AA\ band.


%\appendix

\section{Observations}

SOHO/CDS performed various scans of the active region 
before and during the flare.
We present here results from one fast (20 minutes)
NIS raster, that we designed to cover a 
wide  range of  temperatures 
(He I 584 \AA, log T =4.5; O IV 625.9 \AA, log T = 5.3; 
O V 630 \AA, log T = 5.4; Mg X 625 \AA, log T = 6.0; 
Si XII 520.8 \AA,  log T = 6.3; Fe XIX 592.2 \AA,  log T = 6.9),
and to provide elctron densities.
This NIS raster observed the active region during the gradual phase
(12:28-12:52 UT, see Fig.~\ref{fig:goes_plot}).


During this time, 
TRACE 195 and 1600~\AA\ images taken at high-cadence 
show that the main
features are approximately stationary (see Fig.~\ref{fig:trace_seq}).
The H$\alpha$ images (see Fig.??) show the locations of the 
ribbons. The  TRACE 1600~\AA\ images (dominated by 
UV continuum and neutral ion emissions) also clearly indicate
the presence of a multiple ribbon structure.

The main two ribbons R1 and R2 (WHERE HARD X-RAY EMISSION SEEN BY RHESSI 
IS PRESENT) are located above the main sunspots of opposite polarity.
A third ribbon structure, that formed in the location
where the filament erupted, is also present. 
The ribbon R3 is seen in the TRACE 1600~\AA\
as an almost straight line of emission, overlying a region
of mixed polarity.
During the CDS scan (20m), there is no measurable motion of 
the ribbons (NEED TO CHECK- PERHAPS JUST A COUPLE OF TRACE PIXELS-GDZ).

In the TRACE 195~\AA, an arcade of hot loops that 
connect the two ribbons R1 and R2 is visible.
The sequence of TRACE 195~\AA\ images show apparent flows 
toward the ribbons, with loops continuously appearing and disappearing.


\begin{figure}[!htbp]
\centerline{\epsfig{file=goes_bw2.ps, width=8.0cm , angle=90 }}
\caption{Evolution of the GOES   X-ray fluxes during the 
October 28, 2003 X17 flare, with the timing of the CDS 
raster.}
  \label{fig:goes_plot}
% Fig.~\ref{fig:goes_plot}
\end{figure}

\begin{figure*}[!htbp]
\centerline{
\epsfig{file=trace_seq_195.ps, width=8.0cm , angle=0 }
\epsfig{file=trace_seq_1600.ps, width=8.0cm , angle=0 }
} 
\caption{Sequence of TRACE 195, 1600 \AA\ images during the CDS
raster, shown with the 
same intensity scale. The FOV of the CDS raster, and the 
position of the slit. Notice that overall the loop structures
seen in TRACE 195~\AA\ and the ribbon structures
seen in TRACE 1600~\AA\ remain largely unchanged.
}
 \label{fig:trace_seq}
% Fig.~\ref{fig:trace_seq}
\end{figure*}

\begin{figure*}[!htbp]
\centerline{
\epsfig{file=trace_195_1240_bw_mdi.ps, width=8.0cm , angle=90 }
\epsfig{file=trace_1600_1240_bw_mdi.ps, width=7.0cm , angle=90 }
} 
\centerline{
\epsfig{file=trace_195_1240_zbw_mdi.ps, width=8.0cm , angle=90 }
\epsfig{file=trace_1600_1240_zbw_mdi.ps, width=8.0cm , angle=90 }
} 
\caption{TRACE 195, 1600 \AA\ images, with contours from the
SOHO/MDI magnetogram (200, 500, 1000 G).
Notice the arcade of loops seen in TRACE 195~\AA\ 
connecting the ribbon structures R1, R2 which are located above the 
sunspot regions of opposite polarity.
Also note the system of loops in TRACE 195~\AA\ 
which connects the ribbon R3
to R1 and R2. The ribbon R3 is seen in TRACE 1600~\AA\
as an almost straight line of emission.
 Flows in TRACE 1600~\AA\ from R3 to R1 are seen.
The central part of R3 is also the location of the 
strongest emission in  TRACE 195~\AA.
}
 \label{fig:trace}
% Fig.~\ref{fig:trace}
\end{figure*}

\begin{figure}[!htbp]
\centerline{\epsfig{file=wro124059_mdimag0935_map.ps, width=12.0cm , angle=0 }}
\caption{H$\alpha$ observation from Bialkov, with contours
of SOHO/MDI magnetograms.
Note the similarity with the TRACE 1600~\AA\ image and 
the presence of the three ribbon structures.}
 \label{fig:halpha}
% Fig.~\ref{fig:halpha}
\end{figure}

\begin{figure}[!htbp]
\centerline{
\epsfig{file=halpha_1235_lfov.ps, width=6.0cm , angle=0 }
\epsfig{file=halpha_1235.ps, width=6.0cm , angle=0 }
}
\caption{H$\alpha$ image obtained during the CDS raster,
showing the entire active region and the area of the 
TRACE field of view. Note the similarities
 with the TRACE 1600~\AA\ image}
 \label{fig:halpha2}
% Fig.~\ref{fig:halpha2}
\end{figure}

CDS  observed the arcade of hot loops,
and also shows that strong emission at 
all temperatures (from O V to Fe XIX) is present.

Strong downflows ($v \simeq 50-100$ km/s ), in 
particular at transition region temperatures
are observed in O~V near the footpoints
of the loops , which are located 
at the ribbons R1 and R2.
Thes flows are close to the free fall velocity.

The free fall velocity depends on the gas pressure height in the loops:
at 10$^6$ K H = 50 000 km then v=0
at 10$^5$ K H=5000 km then v$\simeq$10 km/s
at 10$^4$ K H=500 km then v$\simeq$100 km/s

Strong downflows are also present in two 
regions that have loops structures connected to them,
on the eastern side of R1.


Strong upflows in all TR (e.g. OV)
 and coronal lines (e.g. Mg X) are observed 
in the regions just outside the ribbons, in particular
in the region outside R1, coincident with the 
dark region  in the TRACE 195~\AA\ band.
This region forms immediately after the formation
of the two ribbons and lasts as long as the ribbons.
It is interesting to notice that there is no 
similar region on the outer edge of the other 
ribbon R2.
Also that, contary to what reported by 
Czaykowska et al (1999), no measurable flows in Fe~XIX 
are observed.


I WILL ADD A SEQUENCE OF EIT 195 IMAGES DURING 
THE FLARE AND WITH A LARGE FOV.


Another system of hot loops connecting R3 to 
both R1 and R2 are clearly visible in TRACE 195~\AA.
This suggests the presence of opposite polarities
along R3. It is interesting to notice that R3 
is visible soon after the two main ribbons form,
 in the same location of the filament that previously 
erupted.

 Flows in TRACE 1600~\AA\ from R3 to R1 are seen.
However, flows are not observed in TRACE 195~\AA\ or in
the CDS lines. 
The central part of R3 is also the location of the 
strongest emission in  TRACE 195~\AA\ and in 
all spectral lines above 1 MK. The peak emission
in the flare line Fe~XIX is also at this location.
This suggests that the dominant contribution to the
TRACE 195~\AA\ is due to Fe~XXIV.

The fact that the hottest emission occurs in R3
suggests that the dominant heating takes place there.


WHAT IS R3 ???? 

THE TRACE MOVIES AND THE LOOPS THAT CONNECT 
R2 WITH R3  SEEM TO RULE OUT THAT R3 IS HIGH IN THE CORONA.

I HAVE LOOKED AT H-ALPHA IMAGES 2-3 HOURS LATER BUT IT DOES NOT
APPEAR THAT THE FILAMENT REFORMS WHERE R3 IS. 
R3 SEEMS TO FADE AWAY AS R1 AND R2 DO. 

WHAT ARE THE H-ALPHA DATA TELLING US ?


PLAN OF THINGS TO ADD:

-MEASUREMENTS OF CDS DENSITIES AND FLOWS IN A FEW KEY POINTS:

-ALONG R1, R2, R3

-IN THE REGION OF THE ARCADE

- THE DARK REGION WITH UPFLOWS 

We should be able to give some estimates of cooling times.



\begin{figure*}[!htbp]
\centerline{\epsfig{file=ov_int_1600.ps, width=7.0cm , angle=90 }
\epsfig{file=ov_vel.ps, width=7.0cm , angle=90 } } 
\centerline{\epsfig{file=mgx_int_1600.ps, width=7.0cm , angle=90 }
\epsfig{file=mgx_vel.ps, width=7.0cm , angle=90 } } 
\centerline{\epsfig{file=fexix_int_1600.ps, width=7.0cm , angle=90 }
\epsfig{file=fexix_int_195.ps, width=7.0cm , angle=90 }}
\caption{From top to bottom:
 CDS intensity and velocity maps in O V, Mg X with contours of 
the TRACE 1600 \AA\ intensity; intensity in Fe~XIX with 
contours of TRACE  1600,195 \AA\ intensities.
Strong downflows ($v \simeq 50-100$ km/s ) in the ribbons 
are observed.}
\label{fig:cds_int}
% Fig.~\ref{fig:cds_int}
\end{figure*}


\section{Discussion and conclusions }


%The X17 flare presented a great complexity. 




\acknowledgements

GDZ and HEM acknowledge support from  PPARC. 

SOHO is a project of international collaboration between ESA and NASA.

TRACE 

We acknowledge the use of the  TRACE, SOHO  
on-line databases.


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