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\begin{document}                                                                
                  
\begin{article}
\begin{opening}         
\title{Photospheric Signatures Of Solar Flares} 
\author{Priya~\surname{Desai}$^1$\sep
Richard~\surname{Bogart}$^1$\sep
Sebastien~\surname{Couvidat}$^1$\sep
Hugh~\surname{Hudson}$^2$\sep
Jesper~\surname{Schou}$^1$}
\institute{$^1$ Stanford University, Stanford, CA, USA;
         \ email:priya@solar.stanford.edu\\
           $^2$ Space Sciences Laboratory, University of California, Berkeley,
CA, USA\\}
           
\runningauthor{Priya Desai {\it et al.}}
\runningtitle{Photospheric Signatures Of Solar Flares}

\begin{abstract}
We investigate flare-related changes in a photospheric absorption line (Fe~{\sc
i}  617.3~nm) profile in several solar flares of varying X-ray  and
H-$\alpha$ classes, observed with the Helioseismic and Magnetic Imager (HMI)
instrument aboard the Solar Dynamic Observatory (SDO).

   We present observations from four X-ray flares: SOL2011-02-15T01:56 (X2.2) 
flare which shows distinct continuum and line-core brightening; SOL2011-09-07T22.38 (X1.8) 
which exhibits significant continuum and more pronounced line core brightening; SOL2011-09-24T09:40 (X1.9) where the line-core exceeds the continuum and the absorption line briefly reverses and goes into emission (a phenomenon that may have been observed before, but not in such high resolution); and, SOL2010-11-04T23:58:00 (M1.6)  which shows less pronounced line-core brightening and continuum enhancement. 

   We note that although the HMI produces high resolution line-of-sight(LOS) 
observables including continuum intensity, line width, and line depth at 
a 45 second cadence, the algorithm used to interpret the level 1 
HMI filtergrams is based on assumptions that may not be valid during impulsive phenomenon like flares. In such 
cases it is imperative that we use the level 1 filtergrams for analysis; the LOS observables are at best useful only 
for cursory diagnostics.
 
   
\end{abstract}
 \keywords{Solar Dynamics Observatory, continuum, absorption line, photosphere,
white-light flares, HMI}

\end{opening}           

\section{Introduction and Motivation}  


 A solar flare is an explosive release of energy stored in twisted magnetic
fields, usually above or near sunspots. The term white-light flare (WLF)  has
traditionally  been used to designate solar flares observable as transient 
brightenings in the visible continuum \cite{Carrington59}. 
 WLFs are important in flare research because the continuum represents a large
fraction of the radiated energy of a flare (e.g., Neidig, 1989), and their
properties challenge our current understanding of flare energy transport \cite{Krucker11}.

It is currently believed that the visible continuum is enhanced  in all flares,
but that the weaker ones are not detected as the signal is lost in the normal
spatial and temporal fluctuations  of the photosphere. 
Furthermore, even in strong flares, of small spatial extent ($<$5~Mm), and of
short duration ($<$250~s), a systematic observation of the white-light
component of flares has been historically challenging.
It is difficult to capture a flare serendipitously at the right time and
location for a spectroscopic observation, and most measurements (including
space-based ones) do not have the spatial and temporal resolution to  observe
the WL continuum  near sunspots, which have large image contrast. 
In fact, prior to 1993 \cite{Neidig93}, only $\sim$86 WLFs had been
reported, and white-light emissions were only detected in flares above GOES
magnitude X2. 
Surveys of white-light flares observed by Yohkoh \cite{Matthews03} and by
Hinode \cite{Wang09} found the detectability threshold to be around class M1 flare. 
\inlinecite{Hudson06} used white-light observations from the Transition
Region and Coronal Explorer (TRACE) with a spatial resolution of 1$''$ and
detected white-light emission for events as weak as GOES C2.0. 
\inlinecite{Jess08} detected intense white-light emission in the blue continuum
for a C1.6 flare using the diffraction-limited observations of the Swedish
1-meter Solar Telescope.

Motivated by this renewed interest in white-light flares, which we define as
flares exhibiting enhanced emission in parts of the spectrum originating at or
near the height of the the visible continuum photosphere, we set out to
determine if  we could detect photospheric flare signatures using the
Helioseismic and Magnetic Imager (HMI) instrument \cite{Schou12}  on the Solar Dynamics
Observatory (SDO), launched on February 11, 2010. 
The HMI instrument provides images in a photospheric line, Fe~{\sc  i} 617.3~nm,
at sufficient spatial and temporal resolution to resolve flare signatures and
provide continuous images during the flare.  
Measurements of the line strength and nearby continuum brightness along with
Doppler and polarization information, and covering the whole solar disc, are
available at a spatial scale of 0.5''  once every 45~sec. 
Full-disc filtergrams are available every 3.75~sec, in one of two polarization states,
and one of six wavelengths across the line, offering the potential of high time resolution of
the photospheric signatures of solar flares, provided that they can be detected
and adequately measured. 
Thus, we have  an opportunity to study the profiles of a large sample of flares
of varying magnitudes using the same instrument, unlike previous ground based
observations, which typically were with slit spectrographs providing limited
coverage (e.g. \opencite{Neidig89}) and variable observing conditions.

 The HMI capability provides a first opportunity to systematically study the behavior of the line profile.
Specifically we would like to understand emission in the line itself, along with
the nearby continuum, as a more complete guide to the photospheric participation
in the flare.


\section{Observations and Data Reduction}\label{sec:data}


Since a strong correlation is known to exist between X-ray emission and observed
continuum brightening \cite{Neidig89}, we chose to analyze a large sample of 
candidate flares belonging to GOES X-ray class X6.9 to class C1.5. We primarily 
examined the HMI observables during the flares  in order to
establish the quantities most likely to be affected over the course of a flare,
and their typical behavior. For this paper however, we only present observations from four representative
X class flares that seem to best demonstrate the different observed photospheric
effects: SOL2011-09-24T09:40, an X1.9 flare where the absorption line briefly
goes into emission, with the line profile exhibiting a line width that changes with time; SOL2011-02-15T01:56, a X2.2 flare which shows noticeable
reduction in line-depth, changes in the line width and less significant continuum brightening but for which the line does
not go into emission; SOL2011-09-07T23:23, a X1.8 flare which shows distinct continuum brightening and even more pronounced 
line-core brightening, and, SOL2010-11-04T23:58:00, a M1.6 flare which shows
line-core brightening but less significant continuum enhancement (See Table~1). We note that SOL2011-08-09T08:05:00, the strongest X class flare in the current solar cycle, also showed the line briefly going into emission.   

                                                                      % TABLE 1
\begin{center}
\begin{table*}
\caption[]{Characteristics of the flares chosen for this study. 
}
\begin{tabular*}{\textwidth}{l l l l l l }
\hline
Date & Peak (UT) &AR  &Location&  Class &Comments\\
\hline
 2010.11.04 & 23:58 & 11121 & S20 E76 & M1.6 & reduced continuum and core brightening.\\
 2011.02.15 & 01:56 & 11158 & S21 W14 & X2.2 &   \\
 2011.09.07 & 22:38 & 11283 & N14 W30 & X1.8 & \\
 2011.09.24 & 09:40 & 11302 & N13 E61 & X1.9 & continuum brightening, core in emission.\\
 
\hline
\end{tabular*}
\end{table*}
\end{center}


                      


                                                                      % FIGURE 1
\begin{figure}						
\centerline{\includegraphics[width=0.65\textwidth,clip= ] {filter.ps}}
\caption{ Spectral transmission profile of HMI Fe I line at 6173.34\AA\ .  Filtergrams are available at six wavelength positions (0.68m\AA\ apart) and two polariztaion states across the absorption line.  
}
\label{Transmission Profiles}
\end{figure}

The HMI instrument produces five line-of-sight (LOS) observables: pseudo-continuum intensity (I$_c$), Fe~{\sc I} line depth (L$_d$),
line width (L$_w$), Doppler velocity at solar-surface (Dopplergrams, V), and LOS magnetic-field strength (magnetograms, M) every 45 seconds. 
In order to generate these quantities, HMI produces a nearly continuous stream
of full-disc images of the Sun in a set of six narrow wavelength bands (FWHM 76
m\AA) clustered around the central wavelength of the Fe~{\sc I} photospheric
absorption line at 6173\AA\ \cite{Schou12}. Twelve filtergrams- images taken at six wavelengths 
and two polarizations (left-circular and right-circular, hereafter LCP and RCP) at a cadence    
of 3.75 seconds using the front camera are used to calculate the HMI observables.
However, it is important to note that the accuracy of these LOS observables is dependent 
on the algorithm used to translate a sequence of HMI filtergrams into corresponding 
observables. The algorithm used to calculate the continuum, line width and line depth assumes that the Fe I line 
is a ``pure'' Gaussian, and these quantities are obtained from analytical expressions using estimates of the first and second Fourier 
coefficients of the Fe I line profile \cite{Seb12A}. As such, these proxy quantities 
are more or less close to the actual observables, depending on the shape of the 
Fe I line at a specific HMI pixel and at a given time. However, in the presence 
of a strong magnetic field such as the one in a sunspot, or an active region, or during a flare,    
the LCP and RCP line profiles may be significantly different from a Gaussian profile. 
In such cases, the line width, line depth, and continuum intensities returned by the 
observables code might not bear much resemblance to the actual quantities and hence it is best
to use only the filtergrams for quantitative measurements and analysis. 

     Since flares are impulsive phenomena often accompanied by changes in the local magnetic field; and it is known that algorithms used for calculating the observables  under such conditions often yield spurious results, we use 45s cadence filtergrams taken at six equally-spaced (68.8 m\AA\ apart) tuning positions across the Fe I 6173.34 \AA\ absorption line profile that have been tracked to the Carrington rate (see Figures~1 and 2). We do not use the raw level 1 filtergrams (which are separated by 3.75 seconds) but compute intermediate HMI data that have been interpolated in time to the same target time. Thus, the data used are based on the level 1 filtergrams separated by 3.75 seconds, but linearly interpolated to a specific time so that the twelve measurements all refer to the same target time. A gap-filling routine was also run, to spatially interpolate over bad pixels or cosmic ray hits. The tuning positions are at $\pm$ 34.4 m\AA\ ( positions I3 and I2), $\pm$103.2 m\AA\ (positions I4 and I1), and $\pm$172.0 m\AA\ (positions I5 and I0) about the central wavelength of 6173.3433 \AA\ (see Figure 1). As can be seen from Figures~1 and 2, the two wavelengths at $\pm$  172.0 m\AA\ are close to the continuum when the Fe I line is at rest (i.e. Doppler velocity is zero.). However, when the velocity is different from zero, the line is shifted and in this case, only one of either I5 or I0 is a better estimate of the true continuum. Based on the Doppler velocity of the flaring region before the flare, we use the filtergram values of I5 (or I0) as the proxy continuum values.


\begin{figure}    
\begin{center}
\begin{tabular}{lr}					            		% FIGURE 2
\epsfig{file=2011.02.15_filtergrams.ps,width=.50 \linewidth} &
 \epsfig{file=imgs/fl_215.ps,width=0.40\linewidth}\\
\end{tabular}
\caption{ {\bf Top Left:} The Left Circularly Polarized (LCP) and Right Circularly Polarized (RCP) spectral intensity values during SOL2011.02.15T1:56 at 2011.02.15\_01:57:45 UTC for the pixel at  $12^\circ$.00 W $22^\circ$.44 S. The presence of the magnetic field causes the shifts in the LCP and RCP line profiles (Zeeman Splitting) and the direction of the shift suggests that an average of I1 (i.e the average of LCP1 and RCP1) values would provide the best estimate of the line core. Doppler shift is positive, so I5 is our ``proxy'' continuum value. The error bars are $\pm 6\sigma$ and are calculated over the half hour interval before and after the flare.            
{\bf Top Right: } The section of the active region AR 11158 selected for analysis.
}    
\label{Filtergrams showing the Zeeman splitting in mag field and image of AR }
\end{center}
\end{figure}






     Plots of the line profile during a flare (made using the LCP and RCP filtergrams values) exhibit the Zeeman splitting (see Figure 2) and can be used to determine the direction of the magnetic field. We use the plot to  determine whether the intensity at I2, I3, an average of values at I1 and I2 or some combination thereof is a reasonable estimate for the ``line core'' value. We use the difference between this newly measured line core proxy and the continuum proxy as an estimate of the ``line depth''. 

   Image stabilization provides spatial stability of 0.03$''$ throughout the complete sequences of filtergrams required for the observables. Typical per-pixel statistical noise for one filtergram is approximately 0.3\%, as determined from time-series fluctuations away from the flare time.
Exposure-time knowledge is about 0.002\% and does not contribute to the error appreciably. For the continuum intensity measurement, combining all filtergrams over a 45~s frame sequence, the per-pixel noise is about 0.1\%.

\begin{figure}							% FIGURE 3
\centerline{\includegraphics[width=\linewidth] {2011.02.15_01:30:00_TAI_Linecore_Cont.ps}}
\caption{ Mosaic of the time-variation plots of the normalized continuum (red) and line-core(green) intensity for a 5x5 pixel section of the active region of flare SOL2011-02-15T01:56:00. Each plot corresponds to a single pixel and the central pixel is located at $12^\circ$.00W $22^\circ$.44S. The horizontal axis spans a 60~min interval including the duration of the flare. The continuum and line-core values have been normalized by a linear fit of the continuum intensity 15 minutes before and after the flare. Note, the linear fit was performed to de-trend the spacecraft motion from the continuum and line-core data. Figure~2 shows the selected section of the active region.  
}
\label{confusogram_Ic_Lc 02-15}
\end{figure}
 
                       



     
\begin{figure}							% FIGURE 4
\centerline{\includegraphics[width=\linewidth] {2011.09.07_22:10:30_TAI_Linecore_Cont.ps}}
\caption{ Mosaic of the time variation plots of the  normalized continuum (red) and line-core(green) 
 for a 5 x 5 pixel section of the active region that produced the flare SOL2011-09-07T22:38:00. 
Each plot corresponds to a single pixel. The central pixel,is located at $13^\circ$.76N and
$31^\circ$.84W. The horizontal (time) axis spans 60~min. The continuum and line-core values have been normalized to a linear fit of the continuum intensity 15 minutes before and after the flare. Note, the linear fit was performed to de-trend the spacecraft motion from the continuum and line-core data. Figure~6 shows the selected section of the active region.
}    
\label{confusogram_Ic_Lc 09-07}
\end{figure}



\begin{figure}                                          	% FIGURE 5
\begin{center}
\begin{tabular}{ccc}
\epsfig{file=imgs/09_07/im_35_fp.ps,width=0.29\linewidth} &
\epsfig{file=imgs/09_07/im_36_fp.ps,width=0.29\linewidth} &
\epsfig{file=imgs/09_07/im_37_fp.ps,width=0.29\linewidth} \\
\epsfig{file=imgs/09_07/im_38_fp.ps,width=0.29\linewidth} &
\epsfig{file=imgs/09_07/im_39_fp.ps,width=0.29\linewidth} &
\epsfig{file=imgs/09_07/im_40_fp.ps,width=0.29\linewidth}\\
\epsfig{file=imgs/09_07/im_41_fp.ps,width=0.29\linewidth} &
\epsfig{file=imgs/09_07/im_42_fp.ps,width=0.29\linewidth} &
\epsfig{file=imgs/09_07/Ic_1.ps,width=0.29\linewidth}\\
\end{tabular}
\caption{ 
Brightening observed in the line-core images for SOL2011-09-07T22:38:00. These images have been produced by averaging the left-circularly polarized (LCP) and right-circularly polarized(RCP) filtergrams for wavelength positions corresponding to I2 and I1,i.e. corresponding to 6173.37\AA\ and 6173.443\AA\ which we assume to be a reasonable estimate of the ``true'' line-core intensity. The region shown is a $8.25^\circ$ x $8.25^\circ$ square, suitably tracked and mapped to a grid centered at 14$^\circ$.0N and 30$^\circ$.0W, the nominal location of the flare. The images shown are taken 45s apart starting at 2011.09.07\_22:36:15 UT.
The last image is a intensity image of the same region, provided for spatial reference.
}
\label{Images Lc 09-07}
\end{center}
\end{figure}	





\begin{figure}
\begin{center}
\begin{tabular}{lr}					            		% FIGURE 6
\epsfig{file=2011.09.07_22:10:30_TAIfiltergrams_forpaper.ps,width=.50 \linewidth} &
 \epsfig{file=imgs/fl_0907.ps,width=.45\linewidth}\\
\end{tabular}
\caption{ {\bf Top Left:} Spectral intensity across the HMI line before (2011.09.07\_22:10:30:00 UTC, green color) and 
during the flare (2011.09.07\_22:37:45 UTC, blue color) for the pixel located at $13^\circ$.84N and
31$^\circ$.72W. This pixel corresponds to the fourth pixel in the first column in the mosaic in Figures~4. Intensities in both left-circularly-polarized (LCP) and right-circularly-polarized (RCP) light are shown. The line-core values for this flare seem to be best represented by an average of values at I2 and I1.The Doppler velocity is positive, so I5 values best represent the continuum. The error bars represent $\pm$ six times the pre-flare standard deviation in the intensity values.        
{\bf Top Right:}The section of the active region AR 11283 selected for analysis.
}    
\label{Filtergrams showing the Zeeman splitting in mag field and image of AR }
\end{center}
\end{figure}


					%


\section{Discussion and Results} 

The plots in Figures~2-3 correspond to SOL2011-02-15T01:56:00 (X2.2). Figure~2 (left) shows the spectral intensity across the HMI line (Fe I 6173.34\AA\ ) during the flare in each of the polarization states (LCP and RCP). The Doppler velocity during the flare was positive and hence I5 (average of RCP5 and LCP5) values were taken as the proxy continuum values. The values at I1 (so average of RCP1 and LCP1) is deemed to best represent the line core. The error bars plotted are 6 $\pm\sigma$ and have been calculated over a thirty minute interval before and after the flare. The image on the right shows the section of the active region AR 11158 chosen for analysis.

Figure~3 shows a mosaic of temporal variations in the normalized continuum (red line) and line-core (green line) values for a grid of twenty-five pixels centered at $12^\circ$.0W, $22^\circ$.44S corresponding to the section shown in Figure~2 . Each plot corresponds to one pixel and the horizontal axis spans a period of sixty minutes (-including the flare) starting at 2011.09.07\_22:10:30:00 UTC. The continuum and line core both have been normalized by a linear fit made using continuum data before and after the flare. Both continuum and line core were normalized by the same fit. This was done to both normalize the plots, and de-trend the spacecraft motion from the filtergram data. The central pixels show a distinct continuum and line core enhancement during the flare. 

The images and plots in Figures~4-6 were created using filtergram data corresponding to SOL2011-09-07T22:38:00 (X1.8). Similar to Figure~3, Figure~4 illustrates the normalized temporal variation in the continuum and line core over a period of one hour (-including during the flare) in  a 5x5 grid  of twenty-five pixels centered at $13^\circ$.76N and $31^\circ$.84W. A large number of the pixels show greater than a 3-$\sigma$ increase in the line core, but a more modest corresponding enhancement in the continuum value. Examination of all the pixels where the line core brightening was observed
indicates that the line strength never exceeds the continuum and the line always stays in absorption for this flare. The Doppler velocity is positive and hence average I5 values are taken to represent the continuum values and the average of I2 and I1 (i.e. average of the values at RCP1, LCP1, RCP2, and LCP2) best represent the line-core values.
Figure~5 is a typical time sequence of tracked and mapped images of the flaring ``line core'' as determined by averaging the filtergram values measured at I2 during the flare, and subtracting the preflare I2 image from that. These images are taken 45~s apart starting at 2011.09.07\_22:36:15 UTC and are tracked to the Carrington rate. The last image is a continuum intensity image for the same region, provided for spatial reference. Figure~6(left) shows the spectral intensity across the HMI line before and during the flare in both polarizations. I5 is the continuum approximation and I2 values best represents the line core. We note that the pre-flare ``line width'' is different from the line width during the flare.  

Figures~7-9 correspond to flare SOL2011-09-24T22:38:00 with Figure~7 showing the normalized continuum and line core plots
centered at 12$^\circ$.04 N and 61$^\circ$.32 E. The Doppler velocity for this flare is negative; hence average of the values
at position I0 are taken to best represent the continuum and average of the values at I2 and I3  best represents the line-core estimate.
The mosaic of plots show that the line-core equals of exceeds the continuum at many pixels and we see this phenomenon last for more than 
ninety seconds. The horizontal axis in these plots is 2 hours.   
Figure~9 shows the six spectral intensities (filtergrams) before and during the flare at two consecutive pixels located at 12$^\circ$.08N
 and 61$^\circ$.36E (left) and 12$^\circ$.04 N 61$^\circ$.32 E (right), demonstrating that the line width of the absorption line changes as a function of time over the duration of the flare. Temporal plots of the filtergrams show that, at some pixels, the absorption line
core brightens as the flare progresses, and it temporarily exceeds the continuum: thus the absorption line reverses and actually goes into emission. Figure~8 is an image of the section of AR 11302 chosen for analysis. 
Past observations of flares \cite{Babin07,Lozitsky09} suggest that while central emission features have been observed in the line-core before, such high resolution observations of  the entire line going into emission may be rare. 
We have observed similar line emission profiles in the flare
SOL2011-08-09T08:05:00 (X6.9) suggesting they may be more common than believed in the past. 
The white light flare SOL2010-06-12, as reported by \opencite{Martinez11} did not show this behavior . 


\begin{figure}							% FIGURE 7
\centerline{
  \includegraphics[width=\linewidth]{2011.09.24_08:50:15_TAI_Linecore_Cont.ps}}
\caption{Mosaic of the time variation plots the normalized continuum (red)and line-core (green) intensity for a 5 x 5 section of the active region that produced the flare SOL2011-09-24T09:40:00. Each plot corresponds to a single pixel and the central pixel,is located at $61^\circ$.32 E and $12^\circ$.04 N. The horizontal (time) axis spans 120~min. The continuum and line-core values have been normalized to a linear fit of the continuum intensity 15 minutes before and after the flare. Note, the linear fit was performed to de-trend the spacecraft motion from the continuum and line-core data. Figure~8 shows the selected section of the active region .
}
\label{confusogram_Lc 09-24}
\end{figure}


\begin{figure}							% FIGURE 8
\centerline{\includegraphics[width=0.6\textheight,clip=]{imgs/fl_0924.ps}}
\caption{ Section of active region AR 11302 selected for analysis.
} 
\label{AR region 9.24.2011}
\end{figure}


\begin{figure}							% FIGURE 9
\begin{center}
\begin{tabular}{ll}
  \epsfig{file=2011.09.24_09:02:15_TAIfiltergrams_for_paper.ps,width=0.46\linewidth} &
  \epsfig{file=2011.09.24_09:02:15_TAIfiltergrams_for_paper_ii.ps,width=0.46\linewidth} \\
\end{tabular}
\caption{The six point spectral intensities (in LCP and RCP) of the pixels
located at  latitude 12$^o$.08 N, 61$^o$.32 E{\bf(Top Left)}  and 12$^o$.04 N,
61$^o$.32 E {\bf( Top Right)}  before and during the flare SOL2011-09-24T22:38:00. These
``profiles'' show that the line-core changes to a greater extent than the
continuum during the flare, and the line goes briefly into emission. The Doppler velocity is negative and I0 is considered to be the best estimate of the true continuum with the average of I2 and I3 values as the best estimate of the linecore.}
\label{Filtergrams}
\end{center}
\end{figure}





Figures~8 and 9 show the corresponding plots and images for SOL2010-11-04T23:58:00(M1.6). This is the the only M class flare ahown here but as the plots indicate, here too the line-core enhancement is more pronounced than the continuum enhancement. 



\begin{figure}							% FIGURE 10
\centerline{
  \includegraphics[width=\linewidth]{2010.11.04_23:15:00_TAI_Linecore_Cont.ps}}
\caption{Mosaic of the time variation plots of normalized continuum  and line core intensity for a 5 x 5 section of the active region that produced the flare SOL2010-11-04T23:58:00. Each plot corresponds to a single pixel,and the central pixel is located at 21$^o$.08 S  and 77$^o$.36 E. The continuum and line-core values have been normalized to a linear fit of the continuum intensity 15 minutes before and after the flare. Note, the linear fit was performed to de-trend the spacecraft motion from the continuum and line-core data. The horizontal(time) axis spans 60 minutes. The selected region is shown in Figure~11.
}
\label{confusogram_Lc 11-04}
\end{figure}




\begin{figure}							% FIGURE 11
\begin{center}
\begin{tabular}{ll}
  \epsfig{file=2010.11.04_23:15:00_TAIfiltergrams_for_paper.ps,width=0.46\linewidth} &
  \epsfig{file=imgs/fl_1104.ps,width=0.46\linewidth} \\
\end{tabular}
\caption{{\bf Top Left: }The six point spectral intensities (in LCP and RCP) of the pixels
located at latitude 21$^o$.08 S  and 77$^o$.36 E before and during the flare SOL2011-09-24T22:38:00. This location corresponds to the central pixel in the mosaic in Figure~10. Doppler velocity is negative so I5 values are taken to represent the continuum. Average of values at I2 and I3 are assumed to best represent the line core.{\bf Top Right:} The section of the active region AR 11121 corresponding to the pixels in Figure~10.  }
\label{Filtergrams}
\end{center}
\end{figure}



Analysis of all X class flares observed by HMI ((over nine so far), reveal that an 
increase in the continuum is observed in all the flares, suggesting that all
the X-class flares could be categorized at white-light flares. 
However, the combination of an overall increase in brightness, and the reduced absorption in the line suggests that the
line-core intensity responds more sensitively to the flare than the continuum
intensity. 


\section{Summary}  

We have detected a photospheric continuum brightening in a large number of X ray flares. 
Many of these would traditionally have been classified as WLFs. 
However, the relative enhancement in the line-core profile is significantly
stronger than that observed in the continuum and is sometimes detectable even
when there is no significant continuum enhancement. In a few cases, the line width of the absorption 
line changes so much during a flare that the line ceases to resemble a Gaussian. 
The line-core intensity sometimes temporarily exceeds the continuum emission,
thus reversing the absorption line into an emission line. 
The observed emission may last for over a minute before the line returns
to absorption.

The flares so observed are spatially and temporally well correlated to the GOES
X-ray events, indicating that HMI can be used to systematically study and track
the photospheric effect of flares. 

  To study transient phenomenon like flares using HMI data, it is important to use level 1 
HMI filtergrams since the algorithm used may yield spurious results in the presence of
 strong magnetic fields. 



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