TITLE: Physical mechanisms driving solar microflares and supergranular
       network dynamics - relevance for coronal heating

SHORT TITLE: Microflares and network dynamics: physical mechanisms

PRINCIPAL INVESTIGATOR: J. Rybak (1), choc at astro.sk

CO-INVESTIGATORS:  P. Gomory (1), A. Veronig (2), M. Temmer (2), S. Stoiser (2) 

AFFILIATION(s): (1) Astronomical Institute, Slovak Academy of Sciences,
     	            SK-05960 Tatranska Lomnica, Slovakia
     	        (2) IGAM/Institute of physics, Karl-Franzens University, 
                    A-8010 Graz, Austria

DATES: August 3-31 2007

TARGETS: 1/ microflares in the active regions
         2/ network of the quiet solar atmosphere; 

	 - both near the disk center, selection depanding on the actual activity
           around the solar disk center

COOPERATING INSTRUMENTS:

- ground: DOT - Dutch Open Telescope (LaPalma), Kanzelhohe (Austria), Hvar (Croatia)
- space: SOHO: CDS, MDI, EIT; TRACE; RHESSI, Hinode: EIS, SOT, XRT

SCIENTIFIC OBJECTIVE:

This proposal merges together attempts to observe two kinds of solar
structures - solar microflares and supergranular network - which are
planned to be investigated to address common open questions of their
drivers and their impact on the coronal heating and dynamics.

MICROFLARES:

Microflares are small-scale dynamic events potentially important for the
heating of the solar corona as well as for the mass supply to the corona
and the solar wind.  At present, mainly two types of models are invoked to
understand the heating of the corona: models in which the energy is
transported by (various) types of waves in a magnetized plasma from the
convection zone to the corona where they dissipate their energy and models
in which magnetic free energy is accumulated in the corona by (slow)
footpoint motions, and explosively released via magnetic reconnection in
numerous small-scale flare events, so-called microflares or nanoflares.

Intrinsically, due to their small sizes and fast dynamics, the analysis of
microflares demands high spatial resolution observations combined with good
temporal cadence. Our objective is to analyse the dynamics and plasma
evolution during microflares by studying the chromospheric response to
electron beam and/or conductive heating (flare footpoints: RHESSI hard
X-rays, DOT H alpha and Ca II H) together with the transition region and
coronal response (flare loops: RHESSI and Hinode XRT soft X-rays,
post-flare loops: TRACE EUV) in imaging data combined with X-ray spectral
analysis (RHESSI soft and hard X-rays). These observations will allow us to
draw inferences on the plasma temperature and emission measure evolution as
well as on the importance and energetics of accelerated electrons in
microflares. The RHESSI instrument has an unprecedented spectral resolution
of 1 keV as well as unprecedented sensitivity in the range 3-20 keV, where
the transition between thermal and non-thermal emissions is supposed to
take place, which makes it a highly valuable instrument for the analysis of
microflares

In addition, we also plan to use CDS spectroscopy in order to study mass
motions related to the chromospheric evaporation process. The comparison of
these observational data with theoretical predictions in the frame of
electron-beam-driven and conductively driven chromospheric evaporation for
individual microflares can help us to better understand: a) whether
non-thermal electrons are present in microflares which hints at magnetic
reconnection as the underlying physical process, b) how much plasma is
brought into the corona by microflares, c) which process (electron beams or
heat conduction from the hot coronal microflare plasma) dominates the mass
transport, d) how much energy is deposited during microflares which is
available for the heating of the corona.
 
Similar studies have been applied by team members to regular, i.e. larger
flares as well as to microflares. It turned out that large uncertainties in
the obtained parameters are introduced by the limited spatial resolution of
X-ray instruments which most probably give upper limits to the flare source
sizes, whereas the high-resolution observations from TRACE hint at very
fine thread-like flare structures. However, due to the temperature coverage
the coronal TRACE observations are generally restricted to the post-flare
phase and do not allow direct insight into the important impulsive flare
phase. In this respect DOT offers the great possibility to study the
chromospheric flare response with very high spatial resolution (as high as
0.2") at two different heights of the chromosphere using the H alpha and Ca
II H lines. Combined with good temporal resolution this will provide us
with better insight into the source sizes related to the impulsive phase as
well as the source evolution and complexity.

SUPERGRANULAR NETWORK:
 
Supergranular network is clearly related to the heating of the corona as
well. On the other hand its close relation to the underlying layers is
obvious today. Photospheric and chromospheric layers are planned to be
investigated in order to identify the most probable physical mechanisms
responsible for the energy transfer and dynamics of the solar corona above
the chromospheric network. Our previous results derived from data of the
SOHO JOP 78 (http://sohowww.nascom.nasa.gov/soc/JOPs/jop078/jop078.html)
indicate the presence of downward propagating waves in/above the
chromospheric network and led to the assumption that reconnection of the
magnetic field lines should be the dominant mechanism to heat the solar
corona above the particular chromospheric network (Gomory at al. 2006, A&A
448, 1169). In contrast, findings of other authors (e.g. Marsh et al. A&A
404, L37 (2003)) show evidence of propagating intensity oscillations
spreading out from the photosphere to the corona and therefore prefer
alternative heating mechanism of the corona, i.e. dissipation of
magneto-hydrodynamic waves which originate from the solar convective
zone. To clarify these findings a new joint observing program (JOP 171) of
the SOHO instruments (CDS, EIT, MDI) and the TRACE satellite was
proposed. First runs of the JOP 171 were already performed but only with
very limited support of the photospheric measurements. As the drivers of
all proposed heating mechanisms are localized in the photosphere,
additional information are necessary to data of the JOP 171 for better
identification of the heating mechanism. We expect that time series of the
speckle-reconstructed DOT filtergrams taken simultaneously with the CDS
spectra will provide an excellent material to study the properties of the
mentioned drivers.

The CDS spectroscopy, although of low spectral resolution, provides a
perfect temperature coverage of the line emission from chromosphere up to
the corona. Therefore, the CDS data are planned to be used for the study of
waves in the upper solar atmosphere and for the determination of direction
of the wave propagation. To do that we will apply cross-correlation
technique (Gomory at al. 2004, ESA SP-575, 400) and wavelet analysis on the
intensities and the Doppler shifts of the selected CDS spectral lines.

Two runs of a similar program were performed on April 12 and 14, 2007
during the first common SOHO-HINODE campaign with SUMER, CDS, MDI
instruments involved from the SOHO side.

Observing procedures and requirements:

DOT:

As the main goal we plan to acquire three-wavelength profile sampling for H
alpha line providing Dopplergrams for the chromospheric layers using
tunable filter available at the DOT for this line. We expect to use also
fixed filters for the blue and red continuum channels as well as G-band and
Ca II H channels. The resulting speckle-restored image sequences for the
FOV of 90" x 70" with cadence of ~15 seconds completely satisfy our demands
on spatial and temporal resolution. Compensation for the solar rotation is
needed during the DOT observing runs. Final co-alignment with other data
will be performed using the white light images.

The DOT time is granted for the whole period Aug 3-31, 2007, using the
external usage of the DOT in a service mode in which the DOT team operates
the telescope with support of one support astronomer from our team.

Kanzelhoehe, Hvar: 

high cadence H alpha imaging - full disk or larger FOV than DOT,
alternative for case of bad weather/seeing conditions at LaPalma

SOHO:

The observing sequences of the CDS, EIT and MDI instruments on-board SoHO
are planned in the same mode as described in the proposal of the JOP 171
and JOP 185.

SOHO/CDS: 1-D measurements in center of previously taken 2-D raster are
planned using the spectral lines forming over an extended temperature
range: He I 584.33A, O III 599.59A, O V 629.74A, Ne VI 562.80A, Mg IX
386.04A, and Si XII 520.67A.  Exposure cadence of only 10s is sufficient to
trace possible variations of emission and dynamics of the outer solar
atmosphere. The CDS programs VARDIN were already used several times (JOP
171, 185).

SOHO/MDI: high-resolution longitudinal magnetograms (0.6") of the 1-min
cadence will be acquired with some intensitygrams. These data will be used
for tracing evolution of the photospheric magnetic flux within the FOV of
the DOT. The WL intensitygrams will be used for post-facto coalignment of
the MDI, TRACE and images from the SST and the DOT.

SOHO/EIT: CME watch in 195 A channel (or 171 channel).

TRACE:

In particular we are interested in the high resolution images (0.5"), high
cadence FeIX 171A images taken with a sequence of other channels (the white
light (WL), UV 1600A continuum, Lyman alpha) each ~10 minutes. The white
light images will be used for the post-facto co-alignment with the DOT
images.

RHESSI:

RHESSI observes the Sun in soft and hard X-rays (as well as gamma rays)
with a full-Sun field of view. The maximum spatial resolution is 2.3" and
the highest time resolution is 2s depending on count statistics. For
microflares the spatial resolution is usually restricted to 7" and the time
resolution in imaging and spectroscopy to 20s. The temporal resolution for
the flux evolution in X-rays may be as good as 2 s also for microflares.
Microflare studies with RHESSI require that there is no attenuator in the
detectors field of view (A0 state) in order to ensure highest sensitivity
at low X-ray energies. The A0 state is the default RHESSI observing mode
during times of low solar activity which is expected to be the case during
the phase of solar cycle minimum in 2007.