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Astronomy & Astrophysics manuscript no. 0503 August 11, 2004
(DOI: will be inserted by hand later)
Transition region small-scale dynamics as seen by SUMER on
SOHO
L. Teriaca 1 , D. Banerjee 2 , A. Falchi 3 , J. G. Doyle 4 and M. S. Madjarska 5
1 Max-Planck-Institut fur Sonnensystemforschung ? , Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
2 Indian Institute of Astrophysics, Koramangala, Bangalore 560034, India
3 INAF-Osservatorio Astrosico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
4 Armagh Observatory, College Hill, Armagh BT61 9DG, UK
5 Mullard Space Science Laboratory, University College London, Holmbury St Mary RH5 6NT, UK
Abstract. High spectral, spatial and temporal resolution UV observations of the quiet Sun transition region show
a highly structured and dynamical environment where transient supersonic ows are commonly observed. Strongly
non-Gaussian line proles are the spectral signatures of these ows and are known in the literature as explosive
events. In this paper we present a high spatial resolution ( 1 00 ) spectroheliogram of a 273 00  291 00 area of the
quiet Sun acquired with SUMER/SOHO in the O vi spectral line at 103.193 nm. The extremely high quality of
these observations allows us to identify tens of explosive events from which we estimate an average size of 1 800 km
and a birthrate of 2 500 s 1 over the entire Sun. Estimates of the kinetic and enthalpy uxes associated with these
events show that explosive events are not important as far as solar coronal heating is concerned. The relationship
with the underlying photospheric magnetic eld is also studied, revealing that explosive events generally occur
in regions with weak (and, very likely, mixed polarity) magnetic ux. By studying the structure of upward and
downward ows exceeding those associated to average quiet Sun proles, we nd a clear correlation between the
\excess" ows and the magnetic network. However, although explosive events are always associated with ow
patterns often covering areas larger than the explosive event itself, the contrary is not true. In particular, almost
all ows associated with the stronger concentrations of photospheric magnetic ux do not show non-Gaussian line
proles. In some cases, non-Gaussian line proles are associated with supersonic ows in small magnetic loops.
The case of a small loop showing a supersonic siphon-like ow of  130 km s 1 is studied in detail. This is, to
our knowledge, the rst detection of a supersonic siphon-like ow in a quiet Sun loop. In other cases, the ow
patterns associated with explosive events may suggest a relation with UV spicules.
Key words. Sun: transition region - Sun: UV radiation - Line: proles
1. Introduction
Extreme-UltraViolet (EUV) spectroscopy of plasmas at
Transition Region (TR) temperatures reveals the exis-
tence of dierent kinds of transient phenomena. Highly
non-Gaussian line proles showing strong Doppler shifts
are frequently observed in quiet Sun areas and within coro-
nal holes in lines formed between 610 4 K and 710 5 K.
First observed by Brueckner & Bartoe (1983) in HRTS
spectra, they are known as UV explosive events (EEs).
Their birthrate on the quiet Sun was estimated to be
around 1  10 16 m 2 s 1 by Dere et al. (1989). EEs are
characterised by spatial scales of  1 600 km, average life-
time of about 60 s and line proles showing Doppler shifts
up to 225 km s 1 (Dere et al. 1989). EEs are generally
Send oprint requests to: L. Teriaca,
e-mail: teriaca@linmpi.mpg.de
? former Max-Planck-Institut fur Aeronomie
observed along the magnetic network at the boundaries
of the super-granulation cells but away from the larger
ux concentrations (Porter & Dere 1991), in regions with
weak and mixed polarity uxes (Chae et al. 1998) and in
association with episodes of photospheric magnetic ux
cancellation (Dere et al. 1991; Chae et al. 1998; Ryutova
& Tarbell 2000). Furthermore, the spectral signature of
EEs show enhancements in both the red and blue wings
of the prole, implying high velocity ows similar to a
bi-directional jet associated with magnetic reconnection
(Innes et al. 1997). Hence, EEs are believed to be the
product of magnetic reconnection, a magnetic cancella-
tion of the photospheric elds with emerging ux (Dere
et al. 1991) or the interaction between separate ux el-
ements driven together by convective motions (Porter &
Dere 1991). The large velocities associated with EEs sug-
gest that kinetic and enthalpy ows could be relevant to
the coronal energy budget. Winebarger et al. (2002) esti-

2 Teriaca et al.: Transition region small-scale dynamics
mated the global energy release associated with explosive
events through an analysis of the velocity dierential emis-
sion measure. They show that EEs can provide an energy
ux  40 W m 2 , implying that the EEs themselves are
not energetically signicant to the solar atmosphere.
More recently another class of transient events, named
blinkers (e.g. Harrison 1997; Harrison et al. 1999; Bewsher
et al. 2002), was discovered and studied with the
CDS spectrometer (Harrison et al. 1995) aboard SOHO.
Blinkers are dened as enhancements of the radiance
of lines formed at mid-TR temperatures (e.g., O v
62.97 nm, formation temperature of 2:510 5 K) primar-
ily occurring at network boundaries (Harrison et al. 1999;
Bewsher et al. 2002), although they have been also ob-
served in the internetwork (Brkovic et al. 2001). Due to the
lower spatial and spectral resolution of CDS with respect
to SUMER, it has been a matter of debate whether blink-
ers are EEs seen at lower resolution or whether they are
dierent phenomena. Although it seems very likely that
some events, classied as short-duration blinkers, could
just be large EEs, recent studies seem to support the idea
that blinkers and EEs are dierent phenomena (Chae et
al. 2000; Peter & Brkovic 2003; Madjarska & Doyle 2003).
The latter study, in particular, shows that blinkers as seen
in SUMER data have at most ows of  25 km s 1 .
Although EEs can be observed up to 2 00 3 00 above
the visible limb (Dere 1992), both EEs and blinkers are
essentially observed on the disk. Going o-limb, UV ob-
servations reveal another type of dynamic transients: UV
spicules. UV spicules are elongated structures extending
above the solar limb up to  15 Mm ( 21 00 , at the SOHO-
Sun distance 1 00  715 km) (Withbroe 1983, Cook et
al. 1984), showing variations indicative of apparent mo-
tions around 30 km s 1 (Wilhelm 2000). Whether UV
spicules are cospatial or the extension of the long known
H spicules (extensively reviewed by Beckers 1968, 1972;
Bray & Loughhead 1974; Athay 1976; Suematsu 1998) is
still uncertain, although there are strong indications that
a close relation may exist (see Wilhelm 2000 and refer-
ences therein). Moreover, it is still not known whether
they have any relation with other UV dynamic events.
Wilhelm (2000) suggested that EEs and spicules may be
closely related, outlining a mechanism where EEs could
be the rst stage of a sequence of events leading to the
formation of a spicule.
In this paper we present high spatial resolution ob-
servations of a large quiet Sun area in the mid-transition
region line O vi 103.193 nm (3  10 5 K). These observa-
tions allow us to identify tens of explosive events and to
investigate their dynamical characteristics and their rela-
tionship with the magnetic network. The birthrate and the
typical size of these events are also found. From our results
we make an estimation of the total energy (kinetic energy
plus enthalpy) ux associated with EEs. A detailed analy-
sis of the mass ows exceeding those associated to average
quiet Sun proles is performed and a possible link between
EEs and spicules is explored. The rst, to our knowledge,
detection of a supersonic ow in a small loop in the quiet
Sun is also studied in detail. The relevance of our results
to the understanding of the structuring of the solar TR is
discussed.
2. Observations
A raster of a 273 00  291 00 quiet region around Sun cen-
tre (see Fig. 1) was obtained on 30 January 1996 by
the SUMER spectrograph (Wilhelm et al. 1995) aboard
SOHO in the O vi 103.193 nm line. The capabilities
of the SUMER experiment are extensively described by
Wilhelm et al. (1997) and Lemaire et al. (1997). The
raster starts at 04:38:01 and ends at 04:57:03 UTC. It
consists of a series of spectra obtained exposing for 3 s
the central part of detector A. After each exposure only
50 of the 1024 spectral pixels of the detector were trans-
mitted to the ground. This, with a spectral resolution
of 4:418  10 3 nm per pixel, corresponds to a spectral
window of  0:22 nm. The observed area was covered
by stepping the 1 00  300 00 slit westward 0.76 00 a total of
360 times. The reduction of SUMER data requires sev-
eral stages. After correcting for dead time and local gain
depression, data were at-elded and destretched using
the provided software. Radiometrically calibrated spectral
and line radiances have been obtained by using the ra-
diometry.pro routine. To give an indication of the noise
level, spectral and line radiances are also shown in total
count per pixel.
Around 25 min later, 15 full disk magnetograms were
acquired with MDI (Scherrer et al. 1995) from 05:22 to
05:38 UTC. No relevant variation is observed during the
16 min separing the rst and the last magnetogram. After
correcting for the solar rotation, an average of the 15
frames was obtained, allowing us to build a good signal-
to-noise magnetogram to superpose to the O vi obser-
vations. The resulting magnetogram was aligned to the
SUMER raster by cross-correlating the absolute magnetic
ux and the SUMER continuum around 103.08 nm, which
forms in the middle chromosphere (around 1.1 Mm above
 5000 = 1, according to model C of Vernazza et al. 1981).
At each spatial location, the continuum level was obtained
by averaging the spectral radiances over the 10 spectral
pixels between 103.062 and 103.106 nm. The latter point
is away from the O vi line centre by  4 times the aver-
age FWHM of the line. The alignement is estimated to be
precise within 3 00 .
A logarithmically-scaled image of the observed area
obtained by integrating over the O vi 103.193 nm line is
shown in Fig. 1. Levels of the longitudinal magnetic ux of
(10, 25 and 40) G are shown with white (positive polarity)
and black (negative polarity) solid lines. The structures
visible in Fig. 1 and their relationship with the underlying
longitudinal magnetic ux have been discussed in detail by
Warren & Winebarger (2000). Here, we want to focus on
a detailed analysis of the line proles in order to study
the small-scale dynamics of the transition region plasma.
In this way we further exploit the potentials of one of the

Teriaca et al.: Transition region small-scale dynamics 3
Fig. 1. Logarithmically-scaled image of
the quiet Sun obtained by integrating
over the O vi 103.193 nm line (3 
10 5 K) after subtracting the continuum.
The minimum and the maximum of the
image correspond to radiances around
14 and 4654 mW m 2 sr 1 , respec-
tively. Levels of the longitudinal mag-
netic ux of 10 G, 25 G and 40 G are
shown with white (positive) and black
(negative polarity) solid lines. The loca-
tions where non-Gaussian line proles
were found are marked with a black +.
best raster sequences taken by the SUMER spectrograph
aboard SOHO.
3. Explosive events identication and general
characteristics
The prole of a line emitted by an optically-thin plasma
is only determined by the dynamics of the emitting ions.
Motions on scales larger than the instrumental spatial
resolution will lead to the shift of the entire line prole,
while thermal and non-thermal motions on scales smaller
than the spatial resolution will determine the shape of
the prole. Moreover, dierent structures with dierent
dynamics may be present along the line of sight (LOS)
over which the line is integrated. However, quiet Sun pro-
les are generally well represented by a single Gaussian
(although a combination of two Gaussians may represent
better the \quiet" TR line prole; see, e.g., Peter 2001),
indicating that the small-scale thermal and non-thermal
motions both follow a Maxwellian distribution in most of
the cases. In this framework, the triggering of high-speed
ows will lead to the non-Gaussian line proles character-
ising the explosive events. It should be noted that ows on
scales larger than the spatial resolution may also lead to
non-Gaussian line proles when their emission is combined
with that from other structures along the LOS.
To identify explosive event proles, a single Gaussian
has been tted to all the  10 5 spectra forming the raster.
All line proles, for which at least one of the tted pa-
rameters (or the  2 ) was diverging by more than 3  from
the average of its distribution, were rst selected. All the
proles forming this sub-sample were, hence, visually in-
spected and all proles for which at least three to four con-
tiguous pixels were consistently diverging by a Gaussian
prole (by more than their Poissonian uncertainties) were
agged as explosive events. The positions of the agged
proles are indicated by black + marks on the radiance
image displayed in Fig. 1.
It is clearly visible that the selected points are not
randomly distributed, but appear to form small patches.
Considering all contiguous points as belonging to the same
event, it is possible to count around NEE = 50 explo-
sive events in the observed area with an average size of
1 800 km. As representative examples we show in Fig. 2
single-pixel line proles from three locations marked as
E1, E2, and E3 on Fig.1. The majority ( 2=3) of the
proles show a stronger blue wing with average bulk ve-
locities jvj  100 km s 1 . The average radiance L at the
EEs locations is 1.08 W m 2 sr 1 , compared to a value
of 0.37 W m 2 sr 1 obtained by averaging over the whole
dataset.

4 Teriaca et al.: Transition region small-scale dynamics
Fig. 2. Spectral radiances, L , at the centre of three of the explosive events identied in Fig. 1. The prole obtained by averaging
the entire dataset is shown by the dotted lines.
Once all the explosive events were identied, it is pos-
sible to estimate their rate of occurrence. The explosive
events birthrate, R, is given by
R = NEE
t exp A obs
; (1)
where t exp is the exposure time, A obs the rastered area and
NEE is the number of observed explosive events. Given an
area of 4:1 10 16 m 2 (273 00 291 00 ), t exp = 3 s and NEE =
50, we obtain an event birthrate R = 4:1 10 16 m 2 s 1
(' 2 500 s 1 over the whole Sun).
Although explosive events leave a weak signature in
chromospheric lines (Teriaca et al. 2002; Madjarska &
Doyle 2002), no change is observable in lines formed at
T  1 MK (Teriaca et al. 2002; Doyle et al. 2004). This
suggests that, at sites of strong velocities, the plasma is
not heated to coronal temperatures, indicating that energy
is mostly used to accelerate it. The presence of upward ve-
locities jvj  100 km s 1 would, within this framework,
suggest a possible role for kinetic and enthalpy energy
uxes associated with explosive events in heating the quiet
Sun corona.
An (order of magnitude) estimate of the kinetic energy
associated with an explosive event, can be written as:
E kin = 0:5mHN e fv 2 jvjA (2)
where mH is the mass of the hydrogen atom, N e the elec-
tron density (N e = NH , fully ionised plasma of pure H)
and v, A and  are the typical speed, area and lifetime of
explosive events, respectively. The volumetric lling fac-
tor, f , accounts for the lamentary structure of the transi-
tion region and is estimated to be around 0.01 (Feldman et
al. 1979). However, for a large explosive event, Dere (1992)
found a value of 10 4 by comparing the electron density
derived from the ratio of density sensitive lines within the
140 nm O iv multiplet with that obtained from the volu-
metric emission measure. Applying the same technique to
another explosive event observed in the same O iv multi-
plet by Teriaca et al. (2001), we nd a value  0:007 for f .
These values also show that EE-like line proles are due
to motions on scales much smaller than the actual spatial
resolution. Let us consider A = 3:210 12 m 2 (1 800 2 km 2 )
and jvj = 100 km s 1 from the present analysis,  = 60 s
(Dere et al. 1989) and the value 0:007 for the lling factor
f . Teriaca et al. (2001) nd values around 2  10 10 cm 3
for the electron density during EEs observed in O iv lines
(formed around 1:7 10 5 K). Assuming constant pressure
through the TR, we obtain an electron density of 1  10 10
cm 3 for the plasma around 310 5 K. Adopting this value
of N e , Eq. 2 yields a kinetic energy E kin = 1  10 16 J for
a typical event.
Similarly, the enthalpy energy ux is:
H = 5N e KBTf jvjA (3)
where KB is the Boltzmann constant and T the line for-
mation temperature. Considering T = 3  10 5 K (O vi),
we obtain H = 3  10 16 J. Finally, the total energy ux
associated with explosive events is obtained by multiply-
ing the typical energy for one event and the birthrate:
ETot = (E kin + H)  R = 20 W m 2 . This value (pro-
vided f is correctly estimated) is at least one order of
magnitude smaller than the quiet Sun radiative losses for
all plasmas hotter than 3  10 5 K ( 400 W m 2 ) calcu-
lated by Dere & Mason (1993). Furthermore, the energy
ux should be further reduced by considering that not all
the explosive events show a dominant blueshifted (upward
directed) component.
4. Relation with the magnetic network and
structure of the velocity eld
From the analysis of Fig. 1 it appears that the observed
events outline the network (as seen in O vi) but generally
do not appear in the brightest regions. Moreover, they
seem to avoid the areas where the longitudinal magnetic
ux is stronger. In fact, for all but two of these events, the
average absolute longitudinal ux in the underlying area

Teriaca et al.: Transition region small-scale dynamics 5
Fig. 3. Radiance image as in Fig. 1. Solid dark-grey and light-grey (red and light-blue in the electronic version) isocontours
show the downward and upward mass uxes, respectively. The contours are traced for levels of < F B;R + 3 (F B;R ) >= 576
(count 1=2 km s 1 ). Levels of the longitudinal magnetic ux of (10, 25 and 40) G are shown with white (positive polarity) and
black (negative polarity) solid lines. The locations where non-Gaussian line proles were found are marked with a black +.
is below 6 G. Although a vector magnetogram with higher
spatial resolution would be necessary to measure the true
magnetic ux, these observations indicate explosive events
to occur in regions away from the strong magnetic eld
concentrations.
However, at the pressure characteristic of the network
mid-transition region (n e T e = 7  10 14 cm 3 K, Teriaca
et al. 2001), the low- assumption holds also for magnetic
elds of a few gauss and the plasma is forced to move along
the magnetic eld lines. In this framework, the study of
plasma ows can provide information about the structur-
ing of the transition region and the conguration of the
magnetic eld also well above the photosphere, where di-
rect measurements are, thus far, not feasible.
In the previous section only the clearly non-Gaussian
line proles were marked as belonging to explosive events.
Proles characterised by large broadenings and/or line
shifts but still retaining a Gaussian shape were not con-

6 Teriaca et al.: Transition region small-scale dynamics
Fig. 4. Detailed view of box A in Fig. 1
together with the line proles at the in-
dicated locations. (a) Logarithmically-
scaled radiance image. Isocontours of
the negative polarity of the longitudinal
magnetic ux at ( 10, 25 and 40) G
are shown with black solid lines (no pos-
itive ux above 10 G is present in the
displayed area). Locations where non-
Gaussian line proles were found are
marked with a black +. The dashed
dark-grey (red) line indicates the pro-
jection (on the plane perpendicular to
the LOS) of a semi-circular loop with
a diameter of 13 00 . The loop is inclined
by 18 ф with respect to the LOS and the
footpoint line rotated by 12.5 ф clock-
wise. The black dots indicate the mea-
sured position of the observed loop. (b)
Line prole at a location on the north-
ern leg of the loop. The bars indicate
the data points. The thin solid lines
show the three components used to t
the data while the resulting tting pro-
le is represented by the thick solid line.
(c) Line prole at the top of the loop.
The dotted line shows the prole ob-
tained averaging over the whole raster
times 4.9. (d) The same as for panel (b)
but for the southern leg of the loop.
sidered. However, these prole could still indicate relevant
mass ows and/or small-scale motions that exceed those
generally observed on the quiet Sun. To identify these \ex-
cess motions" the following quantity was calculated for
each spectrum of coordinates (i; j) forming the raster.
D i;j (v) =

f i;j (v) f i;j
(v) (if f i;j (v) f i;j
(v) > 0)
0 (otherwise) (4)
where f i;j
(v) is the spectrum obtained averaging over the
whole dataset and scaled to match the observed peak am-
plitude at position (i; j).
The mass ux of a plasma of density  moving through
an area A, with a velocity v normal to A, is given by
 vA / N e vA. Assuming the pixel size as the unit area and
considering that for an allowed transition in an optically
thin plasma N e is proportional to the square-root of the
line radiance L, the mass ux can be written as /
p
L
v.
For each spectrum of the raster (of coordinates i, j), the
upward and downward directed mass uxes (F B and F R ,
respectively) were computed as:
F B;R
i;j
=
R v b
va
vD i;j (v)dv
h R v b
va
D i;j (v)dv
i 1=2
where 
F B
i;j : v a = 200; v b = 0
F R
i;j
: v a = 0; v b = 200 (5)
The value of the speed in the blue and red wings, v B and
v R , can be easily calculated as the rst moment of the dif-
ference spectrum D i;j (v). For both distributions the aver-
age F B;R and the standard deviation (F B;R ) were also
calculated. The quantities F B +3(F B ) and F R +3(F R )
are quite similar being equal to 611 and 541 (count 1=2
km s 1 ), respectively. Finally, the speed and the mass
ux are considered as reliable only at those locations were
the radiance of the dierence spectrum is 2.3 times larger
than the uncertainties in the radiance of the wing itself. In
Fig. 3, the isocontours at the < F B;R +3(F B;R ) >= 576
level are shown with dark-grey (F R ) and light-grey (F B )
thick solid lines (red and light-blue in the electronic ver-
sion). From the analysis of Fig. 3 several facts appear evi-
dent. First, a good correlation between the \excess" mass
ows and the magnetic eld is clearly present. However,
the ow patterns associated with the stronger concentra-
tions of the magnetic ux generally does not show any
non-Gaussian (or EE-like) line prole. Second, EEs are
associated with ow patterns often covering areas larger
than those showing strongly non-Gaussian line proles
and occur in areas with weak (and, very likely, mixed po-
larity) magnetic ux.
This seems to indicate a dierent structure of the tran-
sition region at the locations of stronger ux concentration
than in the remaining network. The large ux concentra-
tions could be the roots of coronal funnels either open

Teriaca et al.: Transition region small-scale dynamics 7
or forming the base of large coronal loops, while the re-
maining network is dominated by small TR loops (Dowdy
et al. 1986; Peter 2001). At the regions of strong mag-
netic ux and large ows, a single Gaussian provides a
good t of the line proles showing bulk velocities either
downwards ( 30 km s 1 around X= 140, Y= 60)
or upwards ( 17 km s 1 around X= 60, Y= 10).
Heating/pressure imbalances can easily explain subsonic
ows in large coronal loops (see later in the text). The
EEs would, instead, be typical of small loops that do not
reach coronal temperatures, in agreement with Teriaca et
al. (2002) and Doyle et al. (2004) who report that EEs
are not seen in lines formed at coronal temperatures. In
this context it is interesting to note that the energy ux
associated with EEs (20 W m 2 ), although small with re-
spect to the energy requirements of the whole corona, it
is comparable to the average energy radiated in the O vi
103.193 nm line during EEs: 4L = 13:6 W m 2 .
Now we focus our attention on the ows associated
with EEs. It is important to underline here that the EEs
proles and the \excess" mass ows were studied in a dif-
ferent manner. The EEs were identied by their strongly
non-Gaussian line proles while the \excess" ows were
determined by the moments of the dierence spectra as in
Eq. 5. This means that the \excess" ows with no corre-
sponding EE-like proles are due to a shift and/or broad-
ening of the entire line prole, while the sign of the mass
ow at the location of EEs gives information on the spa-
tial characteristics and extension of the enhanced wings
characterising such events. The majority of the EEs are
dominated by upward ows, conrming that the major-
ity of these events, when observed in O vi, show a preva-
lent blue wing emission, although it should be also noted
the existence of few EEs showing only the red wing. In
some of the cases the topology of the \excess" ows could
indicate siphon-like ows in closed magnetic structures,
while in other cases blue and red patches seem to origi-
nate from the same EE but extend beyond it, and the more
extended blue pattern may be indicative of an association
with spicules.
A particularly interesting example of ows in a closed
magnetic structure is outlined by box A in Fig. 3. Fig. 4a
shows an enlarged view of this region, revealing a small
isolated loop. Three line proles corresponding to the
two legs (panels b and d) and the top (panel c) of
the loop are also shown. Note that the line prole in
Fig. 4b indicates the presence of down ows with velocities
 100 km s 1 in correspondence with one of the legs of the
loop. Whereas the line prole in Fig. 4d, corresponding to
the other leg, shows the presence of up ows with velocities
 100 km s 1 . Furthermore, the line prole correspond-
ing to the top portion of the loop (Fig. 4c) shows a prole
very close in shape to the average quiet Sun prole, in-
dicating no shifts at all. This is clearly a LOS eect and
represents a case of supersonic ows within a magnetic
loop (c s  50 km s 1 ). To test this hypothesis further,
the loop position was found by determining the positions
of the maxima along horizontal cuts in the radiance image
Fig. 5. In the top panel the line radiance, L, along the loop
(following the dotted line in Fig. 4a) is shown. The distance
along the loop is reckoned from the southern extreme of the
loop. In the bottom panel we show the speeds obtained by the
moment analysis (+ symbols) together with the measurements
obtained by the multi-Gaussian tting (triangles). Negative
values indicate blueshifts (up ows). For the sake of clarity, the
two sets of measurements are slightly shifted with respect to
each other in abscissa. A single Gaussian t (square) and the
rst moment (asterisk) of the line prole at the loop top are
also shown. The solid line indicate the LOS component for a
130 km s 1 ow along the semi-circular geometrical loop whose
projection on the plain perpendicular to the LOS is shown by
the dark-grey (red) dashed line in Fig. 4a. The dotted lines are
obtained for velocities of 90 and 170 km s 1 .
(black dots in Fig. 4a). A semi-circular geometrical loop of
13 00 diameter was then considered and it was inclined with
respect to the LOS, and its footpoint line rotated until its
projection on the plane perpendicular to the LOS (dashed
dark-grey/red line in Fig.4a) matched the position of the
observed loop. The match was obtained for an inclination
of 18 ф with respect to the LOS and a rotation of 12.5 ф
clockwise of the footpoint line.
The top panel of Fig. 5 shows the line radiance along
the loop. It is interesting to note that the top of the loop
is brighter than both the two legs. The lower panel of
Fig. 5 shows the value of the wing speed along the loop
(plus sign). Negative values indicate blueshifts (up ows).
v B (up ows) is shown when F B > F R while v R (down-
ows) is shown when F R > F B . F B is much larger than
F R for the southern leg of the loop while the contrary
holds for the northern leg. To verify the values obtained
from the moment analysis of the dierence spectra, a triple
Gaussian t was also performed at the locations on the
legs of the loop and the results for the dominant wing are
shown in the bottom panel of Fig. 5 by triangles. An ex-
ample of the applied tting is shown in Fig. 4b and 4d.
In the lower panel of Fig. 5, the speed measurements at
the top of the loop were obtained from a single Gaussian

8 Teriaca et al.: Transition region small-scale dynamics
Fig. 6.Detailed view of the region
marked as B in Fig. 1. On the radiance
image on the left panel isocontours are
dened as in Fig. 3. The locations where
non-Gaussian line proles were found
are marked with a black +. On the right
panel the line prole corresponding to
one of the marked pixels is shown (solid
line) together with the average prole
over the whole raster (dotted line).
t (square) and from the rst moment (asterisk) of the
line prole at that location. Finally, the speed component
along the LOS was calculated at each point along the geo-
metrical loop for a ow of 130 km s 1 and the results were
compared with the measured values. The agreement is re-
markably good and represents (to our knowledge) the rst
detection of a supersonic ow in a quiet Sun loop. Other
indications of similar ows can be seen in Fig. 3 around
coordinates (30; 95) and ( 160; 55). It is interesting to
note that all these three cases concern loop structures ori-
ented in the north-south direction, i.e. such that the time
to scan through the structure was minimum. This may
be an indication that these supersonic ows are of short
duration. However, observations extended in time are nec-
essary to conrm it.
High-speed ows (50 to 100 km s 1 ) in cold loops
(T  5  10 5 K) have been so far observed only in active
regions with both CDS (Brekke et al. 1997; Kjeldseth-
Moe & Brekke 1998) and SUMER (Wilhelm 1997). Our
observations show such phenomenon to exist also in the
quiet Sun. The short time in which the loop in Fig. 4
was rastered ( 15 s) makes our observations much sim-
ilar to a snapshot and allows direct comparison with nu-
merical models of ows along loops. Flows can be driven
by asymmetries (such as heating or pressure imbalances)
between the two legs of the loop (Boris & Mariska 1982;
Mariska & Boris 1983; McClymont & Craig 1987; Mariska
1988; McClymont 1989; Spadaro et al. 1991; Thomas
& Montesinos 1991; Robb & Cally 1992; Orlando et al.
1995a,b) or by radiatively-cooling condensations (Reale
et al. 1996, 1997; Muller et al. 2003). In the majority of
the cases the derived ows are very small, of the order of
few kilometers per second. However, short duration super-
sonic ows may also be obtained (see, e.g. Orlando et al.
1995a,b; Robb & Cally 1992). It should be noted that a
footpoint pressure imbalance would lead to a ow arising
from the footpoint with the higher pressure. If we assume
the ambient pressure being equal at the two footpoints,
then the stronger magnetic concentration corresponds to
a smaller gas pressure and the ow will be directed up-
wards at the footpoint where the magnetic ux is weaker
and downwards at the footpoint with stronger ux con-
centration, as predicted by Thomas & Montesinos (1991).
This is exactly what we observe in the case illustrated in
Figs. 4 and 5. This nding qualitatively resembles the
siphon ow observed by Ruedi et al. (1992) in the in-
frared. A similar scenario may also characterise the case
at (30; 95).
Although the majority of the EEs visible in Fig. 3 are
located in areas away from strong magnetic ux concen-
trations, a remarkable exception is indicated by the re-
gion within box B in Fig. 3. The left panel of Fig. 6 rep-
resents an enlarged view of this region. The white and
black contours represent the positive and negative lon-
gitudinal magnetic ux, respectively. Thick solid dark-
grey and light-grey (red and light-blue) contours show
the downwards and upwards directed mass uxes, respec-
tively. Finally, the black + mark the location of highly
non-Gaussian line proles, an example of which is pro-
vided in the right panel. A closer inspection of Fig. 6 (left
panel) reveals that the non-Gaussian proles are located
in what appears to be the intersection of two systems of
loops, one connecting the positive polarity P1 to the nega-
tive polarity N1 and the other connecting P2 and N2. The
ow pattern seems to be also organised along the same
directions. The ow pattern and magnetic eld congura-
tion seem shifted a few second of arc with respect to the
other but this may well be a residual error in superposing
the magnetic eld. This is a clear example of a small-scale
magnetic reconnection between two loop systems. The re-
connection site is indicated by the patch of non-Gaussian
line proles at the centre of which the largest radiance of
the entire dataset was recorded (see Fig. 6, right panel).

Teriaca et al.: Transition region small-scale dynamics 9
5. Summary and conclusion
Our careful spectroscopic analysis of a large quiet area
around Sun centre in the mid-transition region line O vi
103.193 nm has revealed substantial mass ows (exceed-
ing those associated with the average quiet Sun proles) to
be strongly correlated to the magnetic network. Line pro-
les from the regions of high magnetic ux concentration
are Gaussian in shape and show bulk ows of the order of
(10 to 30) km s 1 , either upwards or downwards directed.
This seems to indicate mass ows on scales larger than
the instrumental spatial resolution ( 1:5 00 ).
The non-Gaussian line proles typical of the explosive
events (EEs) testify, on the other hand, the presence of
ows on scales smaller than the spatial resolution. Several
explosive events were, hence, found in the observed area
through a search for the non-Gaussian line proles. We
have estimated an event rate of  2 500 s 1 over the whole
Sun. We nd that the kinetic energy and enthalpy uxes
associated with EEs do not play a signicant role in the
energy budget of the outer solar atmosphere. EEs are as-
sociated with mass ow patterns often larger than the area
characterised by the non-Gaussian line proles. However,
although they are located along the network, EEs seem to
avoid regions with strong magnetic ux concentrations.
Regions that, although often characterised by relevant
mass ows, show nearly Gaussian line proles. These re-
sults could support the idea of a multi-component tran-
sition region characterised by large coronal funnel rooted
at the strong ux concentrations and by a series of small
cold loop that outline the magnetic network (as depicted
by Dowdy et al. 1986). The majority of the coronal funnels
would be the bases of large coronal loops, as also sketched
by Peter (2001).
Mass ows seem to be present in all these structures,
with those in the larger loops being characterised by sub-
sonic speeds (at least close to the footpoints) and scales
larger than the spatial resolution. In the smaller loops,
small-scale supersonic ows (witnessed by the EE-like line
proles) seem instead a frequent feature. In, at least, one
case (see Fig. 4) we nd evidence that EE-like line proles
are clearly associated to a supersonic siphon-like ow in a
loop structure. Another two possible cases have also been
found. These ows seem to be triggered by pressure imbal-
ances at the footpoints of small ( 10 00 15 00 ) TR loops.
However, only perhaps three such cases (3 couples of EEs
locations) could be found out of  50, thus siphon-like
ows are likely to explain only a minority of the observed
EEs.
As discussed in Sect. 1, EE-like proles are believed to
be the spectral signature of bi-directional jets generated by
magnetic reconnection between oppositely directed mag-
netic eld lines (Innes et al. 1997). Chae et al. (2000)
considered bi-directional jets to occur as a result of a col-
lision of a network and an internetwork ux thread. At
the intersection point, the two ux threads are almost
anti-parallel, forming an angle that is greater than 90 ф .
An alternative model (Chae 1999) involves a two-step re-
connection process. According to this model, the initial
reconnection occurs low in the atmosphere with the for-
mation of magnetic islands. These islands are observed as
H up- ow events. The magnetic islands are annihilated
by over-lying magnetic eld lines through a second recon-
nection that, being a fast reconnection process, produces
the bi-directional jets. Such a model could explain the ob-
servation (Madjarska & Doyle 2002) that EE's are rst
observed in chromospheric lines then in TR lines. It may
also be consistent with the observed three to ve minute
recurrence rate of explosive events (Chae et al. 1998; Ning
et al. 2004). The latter authors noted that this period is
very close to the period of chromospheric and TR oscil-
lations and suggested that initial reconnection low in the
atmosphere could be triggered at a particular phase of a
wave or oscillation.
EEs have some common aspects with spicules. In fact,
spicules, like EEs, are related to the chromospheric net-
work (Beckers 1968, 1972; Athay 1976; Suematsu 1998).
The number of observed H spicules present on the Sun at
any time is around 6  10 4 , if only spicules higher than 5
Mm are considered (observed heights are between 6.5 and
9.5 Mm). The number increases by one order of magni-
tude if smaller spicules are also considered (Beckers 1972).
An average lifetime of  600 s (Beckers 1972) yields a
birthrate between 2  10 17 and 2  10 16 m 2 s 1 , in-
dicating that between 2 and 20 (0.7 and 6.7 s 1 ) spicules
should be present in our observed FOV (compared to
around 50 EEs). Finally, we note that the upward directed
mass uxes in Fig. 3 cover about the 1.6% of the observed
area. This value is very close to the  1% of the solar sur-
face covered by spicules (Beckers 1968, 1972; Athay 1976).
Wilhelm (2000) hypothesised that the rearrangement
of the magnetic eld lines following a reconnection process
(shown by the EE-like proles) could lead to the lifting
of plasma, thus generating a spicule. In this case, an EE
could be the rst stage in the formation of a spicule. In
most of our cases, blue and red patches seem to originate
from a given EE location but with the blue pattern gener-
ally more extended. This means that the reconnection pro-
cess aects an area much larger than that characterised by
non-Gaussian line proles. Fig. 7 is a close-up of the area
within box C in Fig. 3 and shows several examples of ow
patterns associated to EEs. Particularly interesting is the
velocity structure around (X= 8, Y= 20). In this case
the explosive event is located at the base of a  15 00 long
blueshifted structure that could be related to the upward
apparent motions observed in spicules. The red contour
could be directly related to the plasma accelerated down-
ward from the EE location while the more extended blue
pattern could be due to the upward accelerated plasma
only at the base of the pattern (where the EE-like proles
are observed), while at larger distances it could be due to
plasma lifted by the reconnecting eld lines as suggested
by the spicule model outlined by Wilhelm (2000) in his
Fig. 8a-d. Another example of a ow pattern associated

10 Teriaca et al.: Transition region small-scale dynamics
Fig. 7. Close-up of the area within box C in Fig. 3. Isocontours
are dened as in Fig. 3. The locations where non-Gaussian line
proles were found are marked with a black +.
to a spicule could be that shown on the right panel of
Fig. 6.
From a general comparison of the properties of these
two types of events, we feel that our results indicate a pos-
sible link between the occurrence of the EEs and the for-
mation of spicules. However, further high spectral and spa-
tial resolution UV space observations in TR and, possibly,
chromospheric lines (of a quality comparable to the data
presented here), combined with simultaneous high spatial
resolution images at several wavelengths across the H
line are necessary to give a denitive answer on whether
or not EEs and spicules are eectively related phenomena.
Acknowledgements. The authors thank D.E. Innes for fruit-
ful discussion and K. Wilhelm for careful reading of the
manuscript and for useful comments and suggestions. Many
thanks also to the referee, V. Hansteen, for his comments
that improved the manuscript. DB is grateful to the CPA,
K. U. Leuven University for providing facilities during his
stay at Leuven. DB's work is partially supported by the ESA
PRODEX project (ESA/Contract no. 14815/00/NL/SFe(IC)).
Research at Armagh Observatory is grant-aided by the N.
Ireland Dept. of Culture, Arts and Leisure. The SUMER
project is nancially supported by DLR, CNES, NASA, and
PRODEX. SOHO is a mission of international cooperation be-
tween ESA and NASA.
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