# Flare High-Energy Particles and Photospheric Response

## Summary

Solar flares not only cause various phenomena in the solar upper
atmosphere, such as plasma heating and mass motion, but also produce
some impulsive phenomena in the photosphere. Recently, SOHO/Michelson
Doppler Imager (MDI) Dopplergrams have revealed some new photospheric
phenomena in flares: photospheric upward and downward flows and
Sunquakes. Sunquakes are a kind of pressure waves that propagate
throughout the Sun\'s interior and often manifest themselves with
concentric ring at the solar photosphere. These responses are believed
to be produced by accelerated electrons or protons when they bombard the
lower atmosphere of the Sun.

We propose to study the hydrodynamic effects of solar flares and their
relationship to high-energy particles. The spatial and temporal
relationship between flare X-ray sources and seismic sources will be
investigated. The correlation between the intensity of hard X-ray fluxes
and the strength of photospheric responses will be checked. In
particular, the relationship between the characteristics of energetic
particles obtained from X-ray spectra and seismic phenomena will be
studied. Numerical simulation of flare hydrodynamics will be conducted
assuming electron or proton thick-target heating. In addition, solar
cycle effect on sunquake will be explored.

This study enables us to study the energy release, particle acceleration
and transport, and plasma heating in solar flares. The well-accepted
particle thick-target model, which is critical in modeling the optical
and UV radiation in solar flares, will be tested and extended. This
study will also help us to understand the physical mechanism of the
photospheric phenomena, which is essential for developing new methods of
helioseismology analysis of flare active regions.

## Introduction

Solar flares are impulsive energy release processes on the Sun. The
flare energy is believed to be stored, transferred, and released in and
by the coronal magnetic field. It is the rearrangement of magnetic field
that we believe plays an important role in flares. A well-accepted
scenario for solar eruptive events can be described as follows: energy
is gradually stored in a stressed magnetic system. The balanced system
is suddenly disrupted, and then stored energy is quickly released,
resulting in filament eruptions, flares, and coronal mass ejections
(CMEs). In flares, the particles, i.e. electrons, and ions, are
accelerated very efficiently in short time scales in solar corona (e.g.,
Aschwanden et al. 1996). After that, they stream above and below the
reconnection region, which is believed be located above the flare loops.
The particles moving upwards will propagate away from the Sun, some of
which can be detected near the Earth. While the particles moving
downward will spiral along the magnetic field and reach the footpoints
of the flare loops.

Decades of observations have provided a wealthy of information on solar
flares (see some recent reviews by McKenzie 2002, Priest & Forbes 2002,
Aschwanden 2004). Most of the observed emissions come from solar
atmosphere (i.e., chromosphere, transition region and corona). In upper
atmosphere, flare loops and coronal sources above the loops are often
observed. Cooler loops can also be seen at several EUV bands (e.g., 195
Å). Coronal wave (or so called "EIT wave") was discovered using
SOHO/Extreme ultraviolet Imaging Telescope (EIT) data (Thompson et al.
1999). In lower atmosphere, loop footpoints are seen in hard X-rays
(HXR), Hα and some EUV bands. A chromospheric wave (or called "Moreton
wave"), believed to be originated from corona, was discovered in Hα
observation (Moreton 1960).

Many observations of flaring atmosphere supported the nonthermal
thick-target model (Brown 1971, Lin & Hudson 1976). In the thick-target
model, accelerated particles transport down along the flare loop to the
footpoints of the magnetic tube where they lose all their energies in
Coulomb collisions with dense, ambient plasma. This will causes
intensive plasma heating at the footpoints. The heated plasma at lower
atmosphere evaporates to fill the reconnected flare loops and radiates
soft X-rays (SXRs) as observed. Striking support also comes from the
observation of simultaneous (\< 1 sec) impulsive HXR emissions from two
footpoints of a flare loop (Sakao 1994), as would be expected from the
interaction of electron beams with the chromosphere.

Hydrodynamic and radiative response of loop atmosphere has been modeled
and simulated for more than three decades. Kostyuk & Pikerl'ner (1974)
and Livshits et al. (1981) suggested that the plasma heated by
nonthermal electrons flows upward at a velocity equal to the sound
velocity. In the mean time, the sudden rise in pressure with the heating
of the dense plasma produces a downward motion. When this motion reaches
sufficient velocity it generates a shock wave propagating downwards. The
plasma behind the compression wave is cooler and denser than the
material ahead of it. This cooler region was later called "chromosphere
condensation" by Fisher et al (1985c). Fisher et al (1985a, b, & c)
found that the downward acoustic wave can be only seen for explosive
evaporation cases where electron beam energy flux is greater than
3×10^10^ erg cm^-2^ s^-1^. In that case, the plasma temperature in
chromosphere jumps to coronal values in shorter time scale than the
hydrodynamic expansion time of the evaporated material. Therefore, the
pressure in the explosively evaporated region exceeds both that of the
coronal plasma above it and that of the chromospheric plasma below it.
The high gradients on either side of this pressure maximum drive
material both upwards and downwards. Recently, with the hydrodynamic
code of Carlsson & Stein (1997), Abbett & Hawley (1999) incorporated
both hydrodynamics and radiative transfer and predicted that impulsive
flares consist of two phase: a gentle phase in which hydrodynamic is
near balance between flare heating and radiative cooling, and an
explosive phase in which beam heating dominates over cooling and
characterized by strong hydrodynamic wave. Most recently, Allred et al.
(2005) improved the Abbett & Hawley's work by more accurately modeling
heating from the soft X-ray, extreme ultraviolet and ultraviolet
emission (or called XEUV emission) and electron beam heating. The
electron beam fluxes in their simulation were obtained from flare X-ray
spectra. Allred et al. (2005) found that, in the explosive phase of the
X-class flare they simulated, synthetic profiles for lines formed in the
upper chromosphere and transition region show blueshift corresponding to
a plasma upward velocity of \~120 km s^-1^, and line formed in the lower
chromosphere show redshifts of \~40 km s^-1^ (Figure 1, *left panel*).
This downward velocity was found to be larger than the local sound speed
(Figure 1, *right panel*). Therefore, the chromosphere condensation will
generate shock wave propagating toward photosphere. The predictions on
mass flows in solar atmosphere are supported by some recent observations
with SOHO/Coronal Diagnostics Spectrometer (CDS) (Brosius & Phillips
2004, Milligan et al. 2005) which showed simultaneous plasma upward
motion in corona and downward motion in transition region and
chromosphere.

![](media/image1.png){width="3.0in" height="2.042361111111111in"}

Figure 1 *Left panel*: plasma flow velocity at different loop height
assuming electron beam heating. The negative velocity means moving
downwards. Photosphere is of zero height. *Right panel*: ratio of mass
flow speed and sound speed. Based on the results of Allred et al. (2005)

Besides all these predicted atmospheric response to the flares, it has
been suggested that solar flares will generate some photospheric
responses. As estimated by Wolff (1972), a large solar flare can raise
the temperature of the photosphere by 10 percent. The resulting thermal
expansion exerts upon the solar interior a mechanical impulse which can
stimulate oscillation of the entire Sun. Kosovichev & Zharkova (1995)
have modeled the photospheric oscillations excited inside the sun by
nonthermal electrons. The plasma upward motion resulted from explosive
evaporation is balanced by recoil of plasma downward motion at lower
chromosphere which generates propagating waves in the solar interior.
Because the sound speed increase with depth, the shock wave propagating
downwards will be reflected in the deep layers of the Sun and appear
back on the surface (see Figure 2). Kosovichev & Zharkova predicted that
the shock wave will manifest itself as circular rings propagating away
from the flare region. The speed of the expanding seismic waves increase
with distance because the distant waves propagate into the deep interior
where the sound speed in higher.

![](media/image3.png){width="2.7152777777777777in"
height="2.7090277777777776in"}

Figure 2 Samples of theoretical ray paths of the acoustic waves excited
by flare and propagating through the solar interior after the flare.

There have been some early seismic observations indicating some increase
in power of p mode after flares (Haber 1988a, b, Braun & Duvall 1990).
However, these ground-based observations were not conclusive. The first
unequivocal observation of photospheric disturbance excited by solar
flare was obtained with SOHO/MDI on 1996 July 9 (Kosovichev & Zharkova
1998), which was close to the solar minimum of the last solar cycle. The
flare was an impulsive GOES X2.6 flare. The 25-100 keV light curve
obtained with the Burst and Transient Source Experiment (BATSE) is shown
in Figure 3 (*solid line*). MDI Dopplergrams reveal strong localized
upward and downward mass flows during the flare (Figure 4a). The
velocity variation is also impulsive, with its peak delayed by \~ 1 min
relative to the hard X-ray peak (Figure 1, *dashed curve*). About 20 min
after the HXR peak, the dopplergrams show a circular wave packet
propagating from the flare region which lasted about 35 min (Figure 4b).
The enhanced dopplergrams with Gaussian filter at three times are shown
in Figure 5. Detailed analysis revealed that the velocity increased from
\~30 to 100 km s^-1^. Kosovichev & Zharkova named such photospheric
phenomenon "sunquake". Both the propagating ring-like wave and its
increasing velocity appear to agree with the prediction by Kosovichev &
Zharkova (1995).

![](media/image4.png){width="2.8in" height="2.38125in"}

Figure 3 BATSE 25-100 keV light curve of the flare on 1996 July 9
(*Solid line*) and 1-min averages of the Doppler velocity of the
downflowing plasma (see Figure 4, *left panel*) obtained from SOHO/MDI.
(from Kosovichev & Zharkova 1998).

![](media/image5.png){width="4.749305555555556in"
height="2.4145833333333333in"}

Figure 4 MDI Dopplergrams at the flare region at 09:11 UT (*left panel*)
and 09:37 UT (*right panel*). The Bright and dark areas corresponds to
downflows and upflows, respectively. (from Kosovichev & Zharkova 1998)

One interesting aspect of observations on the sunquake is that they
seemed to only appear near the solar minimum. As we mentioned earlier,
the first sunquake was discovered near the solar minimum. Kosovichev &
Zharkova (1998) then optimistically predicted "...at times of maximum
solar activity, flares several time larger can be observed, which would
shake the Sun more strongly". However, during the solar maximum (i.e.,
from 1999 to 2002), although there were more than 30 X-class flares

![](media/image6.png){width="4.679166666666666in"
height="1.8006944444444444in"}

Figure 5 Remapped and filtered Dopplergrams at 09:36 (*left*), 0946
(*middle*), and 09:56 UT (*right*) on 1996 July 9.

occurred, none sunquake was ever detected. The sunquakes did not
re-appear until the solar activity started to decay at 2003. All the
flares discovered so far showing sunquakes are listed in Table 1.
Instead of looking for seismic wave directly in dopplergrams, Donea et
al. (1999) and Donea & Lindsey (2005) recently discovered strong
acoustic sources in the so-called "egression power images" they
constructed. These acoustic signatures indicate possible occurrence of
weaker sunquakes in more flares, although the dopplergrams did not show
sunquakes explicitly.

Table 1 Solar flares showing sunquakes in SOHO/MDI Dopplergram (from
Kosovichev 2006).

![](media/image7.wmf){width="5.995833333333334in" height="2.0625in"}

Although sunquake has apparent solar cycle effects, the photospheric
mass flows do not. We found that mass flow can be observed in most of
X-class flares, not matter when the flares occur. Figure 6 shows two
Dopplergrams near the impulsive peak of an X2 flare on 2000 November 24.
The upflow and downflow can be clearly identified in both maps, showing
source location changed from one time to the next. However, no sunquake
can be detected in this flare. According to TRACE flare catalogue, there
have been more than 60 X-class flares from 1999 to 2004, not including
those missed by TRACE because of its limited field of view (8.5 × 8.5
arcmin). Moreover, M-class flares have not been surveyed for mass flows
yet. It is likely that some strong M-class flares will show mass flows,
because M9.4 on 2004 August 15 show both mass flows and sun quake.

![](media/image8.png){width="2.1319444444444446in"
height="2.0416666666666665in"}

Figure 6 Two Dopplergrams for the X2.0 flare on 2000 November 24. The
white and black areas represent the downflow and upflow regions,
respectively. No sunquake was detected for this flare.

It is of great interest to understand the seismic effects (i.e., mass
flows and sunquakes) of solar flares, because their discovery "opens
interesting prospects for flare seismology and for determining the
effects of flare process in the Sun's interior" (Kosovichev & Zharkova
1998). One of the most fundamental questions is how such seismic
responses are generated. The smoking gun would be the accelerated,
nonthermal particles, i.e., electrons and protons. It has been predicted
"explosive evaporation" caused by the nonthermal electrons heating can
generate a strong compression (or called chromosphere condensation) in
solar lower atmosphere (Kostiuk & Pikelner 1975, Livshits et al. 1981,
Fisher et al. 1985abc, Kosovichev 1986, Allbett et al. 1999, Allred et
al. 2005).

Information of energetic particles in solar flares can be best obtained
with X-ray and gammy-ray observations. Based on the comparison between
hard X-ray light curve and time profile of mass flow alone (Figure 3),
Kosovichev & Zharkova (1995) claimed that the seismic response is
consistent with the theoretical predictions of the thick-target model.
Unfortunately, BASTE did not have X-ray imaging instrument onboard. With
its unprecedented X-ray and Gamma-ray imaging and spectral capabilities,
the Ramaty High Energy Solar Spectroscopic Imager (RHESS, Lin et al.
2002) provide us an excellent opportunity to study the seismic effect of
solar flares.

Early study of seismic effects with RHESSI has been very productive.
Donea & Lindsey (2005) found the acoustic signature in the "egression
power maps" they discovered co-aligned with RHESSI HXR footpoint
sources, suggesting a direct link between energetic particles and
hydrodynamic response in photosphere. Since flares are seismically quiet
in general, even for strong flares, Donea & Lindsey tentatively suggest
that the link between the seismic emission and energetic protons. They
also argued that more massive proton beam would generate stronger
photospheric compression wave than electron beam. Recently, Kosovichev
(2006) analyzed two more events with both RHESSI X-ray and gamma-ray
images. Figure 7 (*right panel*) shows RHESSI and MDI observations of a
flare on 2005 October 28. Evidently, the hard X-ray and 2.2 MeV
Gammy-ray sources were located very close to the three seismic sources,
whose location were extrapolated from the time-distance diagram shown in
Figure 7 (*left panel*). The existence of 2.2 MeV gamma-ray source
indicate the presence of energetic ions (mostly protons), because this
line emission is radiated when fast neutrons generated by accelerated
ions thermalized and were captured by hydrogen to form deuterium.
Because no 2.2 MeV gamma-ray source was found to be near source 3,
Kosovichev (2006) suggest that the sunquake might not be caused by
protons, which disagrees with Donea & Lindsey (2005).

![](media/image10.wmf){width="2.823611111111111in" height="2.625in"}

![](media/image11.wmf){width="3.123611111111111in"
height="2.4291666666666667in"}

Figure 7 *Left panel*: SOHO/MDI white light image of flaring region and
superimposed images of the Doppler signal at the impulsive phase of the
flare. The blue and yellow spots show upward and downward photospheric
mass motions. Three seismic wave fronts detected about \~ 30 min later
are also overlaid. The locations of RHESSI 50-100 keV sources (yellow
circles) and 2.2 MeV Gamma-ray source (green cicles) are also overlaid.
(Kosovichev 2006)

![](media/image12.png){width="3.1319444444444446in"
height="2.5069444444444446in"}

Figure 8 *Left Panel*: RHESSI light curves at five energy bands for the
flare on 2005 January 15 flare. The two dashed lines indicate the time
interval for the MDI Dopplergram and RHESSI HXR images shown in Figure
8. *Right Panel*: RHESSI spectrum at the peak near 00:40 UT. The
spectrum is fitted with a model consists of a thermal bremsstrahlung
component and a nonthermal thick-target bremsstrahlung component.

![](media/image14.wmf){width="5.995138888888889in"
height="2.895138888888889in"}

Figure 9 Seismic and X-ray sources of the X1.2 flare on January 15,
2005. *Left panel:* Dopplergram difference image near the hard X-ray
peak. The long white feature near the image center corresponds to strong
downflows at the seismic source. RHESSI hard X-ray image is overlaid.
*Right Panel:* MDI magnetogram at flare region (Red: positive region,
blue: negative). RHESSI soft X-ray image (in gray) and hard X-ray image
(white contours) were overlaid.

![](media/image15.wmf){width="2.111111111111111in" height="4.375in"}

Another flare analyzed by Kosovichev (2006) is a moderate X-class flare
on 2005 January 15, but it produced strongest seismic wave observed so
fare by MDI. RHESSI light curve of this flare was shown in Figure 8
(*left panel*). One RHESSI spectrum near flare peak is shown in Figure 8
(*right panel*). It is evident that the high energy part of spectrum has
a power-law distribution, suggesting presence of nonthermal electrons.
The MDI and RHESSI observations of the flare are shown in Figure 9. The
flare morphology in X-rays appears to be simpler than the earlier case.
The *left panel* of Figure 9 shows that the HXR source located above the
elongated photospheric downflow source. The *right panel* shows that the
soft X-ray loop connected the opposite magnetic polarities, with HXR
sources located at the loop footpoint. Figure 10 show the sequence of
events for the flare: hard X-ray flux peaked first, then photospheric
materials was seen to moved downwards and upwards, about 20 min later
the seismic wave was detected to expand with time.

Figure 10 *Top panel:* RHESSI 50-100 keV light curve. *Middle panel*:
Photospheric mass flow velocity with time (negative: downflow, positive:
upflow). *Bottom panel*: time-distance diagram of the observed sunquake.
The dashed curve shows the theoretical relation for helioseismic waves.

**Open Questions**

As we presented above, the preliminary studies on seismic response
reveal some very interesting results. In the mean time, they raise many
more questions. Here are some of the questions we are interested:

*(1) How are the seismic responses being produced? Whether they can be
explained with the thick-target model?*

Previous simulations on flare hydrodynamics assuming electron
thick-target heating (Kostiuk & Pikelner 1975, Livshits et al. 1981,
Fisher et al. 1985abc, Kosovichev 1986, Allbett et al. 1999, Allred et
al. 2005) all indicate a downward propagating shock wave in lower solar
atmosphere. To explain the discovered the seismic phenomena, Kosovichev
& Zharkova (1998), Kosovichev (2006) and Donea & Lindsey (2005) all
suggested the model predicted acoustic wave might propagate through the
transition region and chromosphere and then compress the photospheric
materials to form mass flow and prompt sunquake afterwards. However, so
far, all the simulations assuming electron thick-target heating have not
predicted any photospheric response, either because their models did not
include photosphere or because the model-predicted the shock wave
dissipated to fast to reach photosphere. Using the electron beam flux
obtained from one RHESSI spectrum of an X-class flare, Allred et al.
(2005) found that because the density of lower atmosphere was so high,
the shock wave generated at explosive phase was dissipated before it
reached the photosphere. Moreover, in order to explain the seismic wave
in the 1996 July 9 flare, Kosovichev & Zharkova (1998) estimated the
total shock momentum transported to photosphere should be \~3×10^22^ g
cm s^-1^, which is about one order of magnitude larger than the value
typically estimated from flare spectral observations (Zarro et al.
1988). This raises an important question: are the photospheric response
really caused by the electron thick-target heating? Or is there anything
wrong with the current thick-target model?

*(2) What is the spatial relationship between the seismic sources and
the hard X-ray/gamma-ray sources?*

Donea & Lindsey (2005) found that the acoustic sources were located
close to the HXR sources seen with RHESSI. However, for the events they
analyzed, not every HXR source has a corresponding acoustic source, and
not every acoustic source has a corresponding HXR source either. In the
2003 October 28 flare (Figure 7, *left panel*), the upflow and downflow
regions seem to be separated, so do the HXR and gamma-ray sources.
However, no simple spatial correlation among these sources can be
identified. While for the 2005 January 15 flare (Figure 9), the
situation seem to be simpler, because the mass downflow appeared to be
located right below the HXR source. If the seismic sources and
HXR/gamma-ray sources are physically linked as we assume, there ought to
exist a noticeable spatial relation between them. Right now, with the
limited cases, we do not have a clear answer.

*(3) How do hydrodynamic responses depend on the characteristics of
energetic particles, e.g., spectral index, total beam flux?*

We have known that photospheric mass flows can be observed in most
X-class class flares during both solar maximum and minimum, while the
sunquakes can be detected in some selected strong X-flares only near
solar minimum. Kosovichev (2006) found that it was the very moderate
X1.2 flare on 2005 January 15 that produced the strongest seismic wave
ever observed, not the X17 flare in Table 1. We want to know what
factors determine whether flares show or do not show seismic response.
If the photospheric response is prompted by the energetic particles as
we often assume, the strength of the response should depend on
characteristics of energetic particles. For instance, do those flares
showing seismic response have harder spectra than those seismic quiet
flares? Do those flares showing sunquakes have the larger beam flux?

*(4) What is the relationship between the mass flow and sunquakes? Why
sunquakes appear to have solar cycle effect, but photospheric mass flows
do not?*

All the sunquakes discovered so far (Kosovichev & Zharkova 1998,
Kosovichev 2006) had observable mass flow near their originating point
of seismic wave (see Figure 7, *left panel*). However, most of the
X-flares do show the mass flow, among which only 6 events showed
detectable sunquakes. It is likely that both mass flow and sunquakes are
caused by the shock wave due to the thick-target heating. One possible
explanation for the lack of sunquakes is that only the shock wave is
strong enough to excite the seismic wave. If so, then we do expect the
flares with sunquakes have stronger mass flows. Right now, we do not
know whether or not that is statistically true. It has been noticed that
gamma-ray line flares often occurred during the declining phase of the
solar cycle. Is this coincidence or there are some links between them?
Does that mean the evolution of magnetic field plays some role in both
cases?

**Scientific Goals & Proposed Research**

The main goal of this proposal is to study the hydrodynamic effects of
solar flares and their relationship to high-energy particles so as to
better understand the energy release and transport in solar flares and
flare dynamics. Through this study, the well-accepted thick-target
model, which is an important part of flare standard model (see reviews
by McKenzie 2002 and Aschwanden 2004), will be tested and extended.
Based on the photospheric responses, important constraints for the
energy release, particle acceleration and transport processes will be
drawn. The solar cycle effect on seismic response, flare generation, and
even particle acceleration will also be explored.

Because the seismic phenomena are most likely to be caused by
accelerated particles, which also radiate hard X-rays and gamma-rays
during flares, we should study the seismic observations and hard
X-ray/gamma-ray observations together. Therefore, SOHO/MDI and RHESSI
form a golden match for this study.

This study will consist of two parts of the work: observations and
modeling. In the observational part, we will conduct systematic and
statistical analysis on temporal, spatial relationship between
photospheric phenomena and HXR/gamma-ray. So far, only a few sporadic
analyses on several selected events have been conducted. Although these
analyses did not yield any conclusive result at this stage, they have
demonstrated to us that such coordinated analysis will be very
productive and promising in understanding the flare hydrodynamics.
Moreover, no study has been conducted to relate the characteristics of
energetic particles and the photospheric phenomena. This study will not
only offer a direct link between the seismic signatures and their
perpetrators so as to better understand the photospheric responses, but
also give us a chance to obtain more information on energetic particles
so as to better understand the particle acceleration and transport
mechanisms in solar flares.

Modeling is also a very important part of this proposal. Without model,
it will be very difficult for us to really understand the physics behind
of these observational phenomena.

To our knowledge, no simulation on flare hydrodynamics has predicted any
photospheric responses, which is apparently inconsistent with the
observations. Earlier simulations either did not include photosphere in
the models, or just suggest that the flare disturbance (e.g., waves)
could not pass through the chromosphere and reach the photosphere.
People studying flares and other corona phenomena tend to focus on their
interests in the solar atmosphere and ignore the phenomenon below the
atmosphere, while people studying solar interior focus their interest on
wave propagation inside of the Sun and its significance to
helioseismology. To connect the phenomena at the solar atmosphere and
their ensuing photospheric response will open a new window for us to
better study solar flares and helioseismology in whole.

To make it more specific, here we list some of detailed works we plan
do:

*(1)Study the spatial relationship between the HXR/gamma-ray sources and
the photospheric sources (including both mass flows and sunquake
origination points)*

Such information is crucial to understand the particle transport in
solar atmosphere and flare hydrodynamics. As mentioned earlier, based on
the limited number of events being studied, we still do not know the
exact spatial relationship between these impulsive sources. We know that
most X-class flares (even some strong M-class flares) show photospheric
mass flows during solar minimum and maximum, therefore, more events
should be studied for this purpose.

Besides the RHESSI and MDI, Global Oscillation Network Group (GONG) can
provide photospheric line Ni ~I~ λ6768 intensity and Doppler maps, which
can offer complementary spatial information on photospheric emission
during flares.

*(2) Study the temporal relation between the HXR sources and mass flows*

If both HXR sources and mass flows are caused by energetic particles, we
would expect a temporal correlation between these two sources. We have
seen such correlation in 2005 January 15 event (Figure 10). A delay of
the mass flow peak relative to HXR peak may be expected in the
thick-target model because the shock waves are generated after energetic
particle beam heats the chromosphere (Figure 3). However, mass flows
were also reported to be present even before the impulsive part of the
flare started (Meunier & Kosovichev 2003). It may suggest that the
particle beam heating may not be the only way to produce the mass flow.
Therefore, temper relation between HXR flux and mass flow provide a good
check on the cause of the photospheric phenomena.

*(3) Check the correlation between the intensity of HXR/gamma-ray
sources and strength of phosphoric response*

Multiple HXR/gamma-ray sources in one flare are often found not to be of
the same intensity. Moreover, their intensities vary during the course
of the flare. Therefore, it is interesting to see whether mass flows and
sunquakes are stronger near the stronger HXR/gamma-ray sources. The
strength of the mass flows can be represented by their velocity, while
the strength of sunquakes can be measured by their maximal wave
amplitude.

*(4) Study the relationship between the deduced characteristics of the
energetic particles, e.g. spectral index, beam flux, and strength of
photospheric response*

Since thick-target heating by energetic particles is thought be the
cause of the photospheric phenomena, direct comparison between the
particle characteristics and the photospheric response should provide
very informative connection between them. With the RHESSI spectral
analysis software, by fitting the data with model-predicted thick-target
bremsstrahlung, we are able to obtain the electron flux (electron
s^-1^), power-law index, break energy (for double-power law
distribution), and even low-energy cutoff to the electron distribution.
The total electron beam power (ergs s^-1^) can also be obtained with a
simple integration. Total ions energy can also be estimated with the
method used by Emslie et al. (2004). All these parameters should be
checked with the strength of photospheric response for possible
relationship. A positive correlation between the particle beam power and
the strength of seismic waves or the speed of mass flows would be
expected if these photospheric phenomena are indeed caused by the
energetic particles.

Particle characteristics for the flares showing both sunquakes and mass
flows and for the flares showing mass flows alone will be compared. Such
study can offer us a chance to see whether the apparent solar cycle
effect of sunquakes can be explained in terms of the characteristics of
energetic particles, in stead of real solar cycle effect.

*(6) Conduct flare hydrodynamics simulations to test the thick-target
model*

We plan to use the RADYN code, which was originally developed by
Carlsson & Stein (1997) and extended by Abbett et al. (1999) and Allred
et al. (2005) lately, to simulate the hydrodynamic process during solar
flares. The code assumes electron thick-target heating at the footpoints
of magnetic loops. The equations of hydrodynamics, population
conservation, and radiative transfer are solved simultaneously.
Hydrodynamic effects due to gravity, thermal conduction, and
compressional viscosity are included. The model includes the photosphere
as the lower boundary, therefore photospheric response can be simulated
with the code.

As we mentioned earlier, both chromospheric upflow and downflow have
been predicted as a result of explosive evaporation at solar atmosphere
(e.g., Fisher et al. 1985, Abbett & Hawley 1999, Allred et al. 2005). In
order to explain the photospheric downflow with the thick-target model,
the downflow in chromosphere (or chromospheric condensation) must be
strong enough to generate shock wave and reach the photosphere. The
photospheric upflow could be due to the results of rebounding of
compressed plasma. Similar wave rebounding was once seen in the
chromosphere in the simulation of Allred et al (2005). The photospheric
upflow could also be caused by direct heating of energetic particles
and/or ionization wave preceding the shock.

The simulation will be conducted with two directions: forward and
backward. In the forward simulation, similar to what Allred et al.
(2005) have done, we first obtain the electron distribution by fitting
RHESSI spectra, then input the obtained distribution to the simulation
process to see whether the chromospheric condensation can generate a
shock wave, whether the wave can reach the photosphere, and how
photosphere responds to wave compression. With the electron beam
distribution obtained from RHESSI spectra of one X-class flare, Allred
et al. found that chromospheric condensation was supersonic (Figure 2,
*right panel*). However, when the compression wave reaches photosphere,
it is very weak with a speed of \~0.1 km s^-1^, almost one order of
magnitude less than what was typically observed by MDI. One possible
reason is that they underestimated the electron beam power because the
low-energy cutoff they obtained from Holman et al. (2003) was an upper
limit, therefore, giving a lower limit of energy. Recently, Sui et al.
(2005a, b & c) have found two ways to better estimate the low-energy
cutoff to electron distributions in solar flares. Therefore, the
electron beam power will be better estimated for some events.

In the backward simulation, we can vary some parameters of electron
distribution, such as low-energy cutoff and high-energy cutoff, until
the model can predict the observed seismic downflow with a reasonable
electron beam power. Therefore, we will be able to gain some constraints
on accelerated electrons. If the observed seismic flow and wave can not
be reproduced, we will seek to improve or adjust the existing model. It
is one of the ultimate goals of this study to get feedback from the
coordinated study so as to learn more about the thick-target model and
plasma heating.

The model will also be extended to apply to proton thick-target heating.
As suggested by Donea & Lindsey (2005), photospheric heating by
high-energy protons could play a major factor in seismic emission,
because the protons can penetrate deeper into the chromosphere, even
into the photosphere. The reason why Kosovichev (2005) did not find 2.2
MeV emission source in most of sunquake flares listed in Table 1 could
be due to the fact that the proton spectra are too steep to excite the
2.2 MeV line. Therefore, the proton heating should not be completely
excluded. So far, only electron thick-target heating was ever applied in
the Carlsson & Stein's code. We plan to apply proton thick-target
heating in the code. It is very interesting to how solar atmosphere
respond to proton heating.

**Coordinated Data Sources**

*The Ramaty High Energy Solar Spectroscopic Imager (RHESSI)* provides
X-ray/gamma-ray light curve, images, and spectra. The image resolution
can be as high as \~ 2 arcsec. The X-ray source centroid can be as
precise as sub arcsec. The energy coverage is from 3 keV to 17 MeV. The
temporal resolution can be as high as sub second.

*The Michelson Doppler Imager (MDI)* provides photospheric continuum
image, magnetogram and dopplergram. The continuum image show the
sunspot, active region where flares occur. The Magnetogram shows the
line of sight component of photospheric magnetic field, give us the
magnetic environment where flares occur and its evolution before and
after flares. Its time cadence is 96-minute. Dopplergram shows the
photospheric mass flow and sunquake. The time resolution of dopplergram
is 1 min.

*Global Oscillation Network Group (GONG)* provides both Doppler and
intensity maps of the full solar disk in the photospheric line Ni ~I~
λ6768 with 1 min cadence.

*Hα observations* (e.g. from Big Bear Solar Observatory, Meudon
Observation) provide flare images in *Hα* typically showing flare
ribbons formed in chromosphere and even flare loops at upper atmosphere.
The BBSO has time cadence of \~40 ms.

*Imaging Vector Magnetograph (IVM)* at Mees Solar Observatory provides
spectral map in Na D1 line, which is formed mostly in the low
chromosphere at line center. The effective cadence of the IVM
observations is 2.5 min. The IVM Na D1 line will show us the mass motion
at lower chromosphere.

*Coronal Diagnostic Spectrometer (CDS)* on board of SOHO provides
extreme ultraviolet spectra within its 308-381 and 513-633 wavebands for
each of 143 1\".68 arcsec along its 4\' long slit. It can also provide
raster images of target areas. We can learn blueshift and redshift of
different lines sensitive to different temperature plasma. The time
resolution is \~10 s.

EUV observations from Transition Region and Coronal Explorer (TRACE) and
Extreme ultraviolet Imaging Telescope (EIT) may also be used to provide
information on the evolution of magnetic loop morphology during flares.

# 

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