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A&A manuscript no.
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ASTRONOMY
AND
ASTROPHYSICS
June 5, 2000
Research Note:
Physical Parameters of EUV Explosive Events
M.E. P'erez & J.G. Doyle
Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland
email: epp@star.arm.ac.uk, jgd@star.arm.ac.uk
Received date, accepted date
Abstract. Previously published results of EUV explo­
sive events and electron density enhancements in the solar
transition region are revised. An attempt has been made
to correlate both observational phenomena, and to asso­
ciate the observed density enhancements to magnetic re­
connection sites. The corresponding local magnetic field
strength in these sites is estimated. These values are of
the same order as previously measured in photospheric
cancelling flux regions.
Key words: Sun: --Sun: EUV radiation -- transition region
-- line ratio -- electron density -- explosive events
1. Introduction
The so called EUV explosive events represents one of
the smallest scale flare­like phenomena directly observ­
able on the Sun. These events have been observed to
cover areas from one to a few arcsec squared, with life­
times of between 20 and 200 seconds, and kinetic en­
ergies of the order of ¸ 10 23 \Gamma 10 25 erg. These explo­
sive events are mostly seen at transition region temper­
atures (Brueckner & Bartoe 1983, Dere et al. 1989). Re­
cent data suggest that their time variation and spatial
structure are consistent with bi­directional plasma jets
produced by magnetic reconnection (Dere 1991, 1994;
Porter & Dere 1991; Innes et al. 1997; P'erez et al. 1999a;
Benz & Krucker 1999).
Chae et al. (1998) and Dere (1991) observed explosive
events occurring in bursts at intermittent locations along
the boundary separating opposite polarity elements that
were cancelling and reconnecting. Dere (1991) found that
explosive event velocities were comparable to the Alfv'en
velocity, which supported the hypothesis that the accel­
eration of plasma is due to magnetic field annihilation
between the emerging and the pre­existing magnetic field.
In the area where the current sheet is formed, density
and temperature enhancements are expected (Cheng 1980,
Hayes & Shine 1987).
Send offprint requests to: M.E. P'erez
P'erez et al. (1999a), henceforth Paper I, studied EUV
explosive events identified in two observational sequences
made in July 1996. In a second paper (Paper II), P'erez
et al. (1999b) discussed a density diagnostic carried out
in a similar location on the Sun as the explosive events
analysed in Paper I. These latter observational sequences
were obtained very close in time to those presented in
Paper I. Here, a suggestion is made about the possible
connection between the variations observed in the electron
density and the sites of the explosive events. Finally, an
estimate of the magnetic field strength is given for the
area where these events took place.
Table 1. Description of observational data. The slit size and
area rastered are given in arcsec squared.
Date (1996) 10 July 14 July
Paper I : Explosive Events
Start (UT) 07:19:18 06:30:20
End (UT) 07:34:44 06:40:45
Pointing (630,--200) (2,910)
Slit 0:3 \Theta 120 1:0 \Theta 120
Raster 66 \Theta 120 33 \Theta 120
Location AR: NOAA 7978 Northern CH
Exposure time 15 s 20 s
Spectral line O vi 1032 š A C iv 1548 š A
Log (Te/K) 5.5 5.0
Paper II : Density Enhancements
Start (UT) 07:36:15 00:46:42
End (UT) 08:42:56 02:14:04
Pointing (630,­200) (0,910)
Slit 0:3 \Theta 120 1:0 \Theta 120
Location AR: NOAA 7978 Northern CH
Raster ¸ 7 \Theta 82 ¸ 1:5 \Theta 112
Exposure time 20 s 20 s
Line Ratio O iv 1399/1401 O iv 1399/1401
Log (Te/K) 5.2 5.2

2 Physical Parameters of EUV Explosive Events: P'erez & Doyle
Table 2. Observational characteristics of the EUV explosive events with the area given in arcsec squared.
Characteristic Northern CH AR (NOAA 7978)
Number of events 1 – 5
Life­time 160 s 60--90 s
Maximum Doppler 150 (blue) Event1: 150 (blue & red)
velocity (v) 100 (red) Event2: 260 (blue), 215 (red)
in (km s \Gamma1 ) Event3&4: 200 (blue), 180 (red)
Extension Event1: 4 \Theta 7 \Gamma 9
(N­S)\Theta(E­W) 6 \Theta 8 Event2: 4 \Theta 4
Event3&4: 14 \Theta 4
Temperature Mid­Transition Region (10 5 K) High­Transition Region
Not seen at ¸ 2 10 4 K (3.2 10 5 K)
Type of event Bi­directional jet Burst of events
(reconnection jet) Bi­directional jets (reconnection jets)
Inclination ¸ 13 ffi ¸ 10 ffi \Gamma 23 ffi
Length 6 10 3 km 1.5--1.3 10 4 km
Line profiles Asymmetric profiles Asymmetric profiles
Multi­Gaussians fitted (2--4) Multi­Gaussian fitting (2--3)
Other Supersonic mass motions Supersonic mass motions (sound speed ú95 km s \Gamma1 )
(sound speed ú50 km s \Gamma1 ) Associated to photospheric magnetic
flux cancellation & regions with
weak & bipolar magnetic field
2. Observations
The observations discussed here were obtained on 10 and
14 July 1996 with the SUMER (Solar Ultraviolet Mea­
surements of Emitted Radiation) instrument on board the
SOHO satellite (Wilhelm et al. 1997). In Table 1 details
of the observations presented in Paper I & Paper II are
outlined. The dataset centered at (630,\Gamma200) arcsec, in
active region NOAA 7978, was observed on 10 July in
the O vi 1032 š A line and, less than two minutes later, in
O iv 1399 š A & 1401 š A. On 14 July, observations were taken
in the Northern coronal hole with a gap of 4h 16min be­
tween the C iv 1548 š A dataset and the dataset taken in
the O iv lines.
3. Results & Discussion
3.1. Paper I: Explosive Events
Figures 5 & 7 in Paper I, shows the time series for the ex­
plosive events observed in the transition region lines of
O vi 1032 š A and C iv 1548 š A. Table 2 summarizes the main
results presented in Paper I, for the events observed in
the Northern coronal hole and in the active region. Four
events were studied for the active region dataset and in
order to avoid confusion they have been labeled in Ta­
ble 2 as Event n. This table gives the number of events
observed in each region, their corresponding lifetime, the
maximum Doppler velocity observed for each event anal­
ysed, their extension along the slit, and the corresponding
temperatures of the lines in which these events were ob­
served. Other relevant conclusions from this work are also
summarized in Table 2, i.e. the bi­directional nature of the
observed explosive events.
The locations of the explosive event sites found in the
active region, as summarized in Table 3, are in very good
agreement with the locations found in Paper II for the
larger variations in the electron density. However, such a
correlation is not present for the Northern coronal hole
datasets. Therefore, at least in the case of the active re­
gion, given the spatial coincidence between the explosive
event sites and the electron density enhancements, both
phenomena might reasonably be considered to be physi­
cally related.
3.2. Paper II: Density Enhancements
In Paper II we used the density­sensitive line ratio of O iv
1399.8/1401.2 to diagnose the electron density in the tran­
sition region of a coronal hole, an active region and a
`quiet' Sun region at disk center. All the data handling
procedures applied to the original data and the atomic
data used are described in this paper. Since the O iv lines
used are not strong we used binning in time of four min­
utes and a running mean along the slit of five pixels, which
constitutes our resolution element.
Figures 5 & 9 of Paper II show the electron density val­
ues obtained for the coronal hole and active region dataset.

Physical Parameters of EUV Explosive Events: P'erez & Doyle 3
Table 3. A lower limit to the magnetic field strength (B) for the explosive event sites analysed in Paper I and the corresponding
electron density values found in Paper II.
Location Ne VA B (red) VA B (blue)
Event no. (N­S) x (E­W) (cm \Gamma3 ) (red) (G) (blue) (G)
(arcsec 2 ) (km s \Gamma1 ) (km s \Gamma1 )
Event1 [\Gamma172; \Gamma176] max: 3 10 11 150 33 150 33
x [627, 626] mean: 1 10 11 19 19
Event2 [\Gamma172; \Gamma176] max: 3 10 11 215 47 260 57
x [627, 626] mean: 1 10 11 27 33
Event3&4 [\Gamma205; \Gamma210] max: 1 10 11 180 23 200 25
x [625, 623] mean: 7 10 10 19 21
[\Gamma214; \Gamma220] max: 3 10 11 180 39 200 44
x [625, 623] mean: 1 10 11 23 25
These show variations along the slit as well as in the E­W
direction, over time periods of a few minutes. Such varia­
tions can be as large as a factor of two in ¸5 minutes.
One possible explanation could be that explosive
events occurring in the high­transition region (at ¸
3 10 5 K) could be caused by the deposition of energy
by, for example, magnetic reconnection at lower temper­
ature region, (see Sarro et al. 1999). As a consequence,
compression and, therefore, density enhancements would
be observable at mid­transition region temperatures (¸
1.5 10 5 K). From this point of view, the higher frequency
of occurrence of the density enhancements in the active
region with respect to the observed explosive events could
be indicative of a particular distribution/spectra of energy
deposited over a period of time.
Dere et al. (1991) concluded that it is valid to use
the O iv line ratios for the density diagnostic of explo­
sive events, even if these are fast­varying phenomena. The
time­scales needed to keep the hypothesis of ionization
equilibrium, and of collisional and radiative equilibrium,
are fast in comparison with the time­scales observed for
explosive events (20--60 s). Therefore, the electron density
values given in Paper II can be regarded as being valid
even if they correspond to explosive event sites.
3.3. Estimation of the Local Magnetic Field Strength
Imagine an xy­plane which represents a neutral current
sheet with say, thickness of 2l. Thus an inflow of magnetic
field at velocity v from the x­direction is balanced by an
outflow along the y­direction. Integrating over the current
sheet gives
j z = cB y
4úl : (1)
Suppose the plasma is incompressible and in steady­state,
the equilibrium pressure balance can be written as
ffi p = p i \Gamma p o = aev 2
y
2 ; (2)
where p i ; p o are the gas pressure inside and outside the
current sheet, respectively. In an equilibrium state
ffi p ú
B 2
y
8ú ; (3)
i.e., the pressure balance is set equal to the magnetic
pressure outside the reconnection region. Combining these
equations, one arrives at
v y = B y
(4úae) 1=2
= vA = 2:5 10 6 B N e
\Gamma1=2 (km s \Gamma1 ) (4)
where ae is the mass density. In the above we have assumed
N p ¸ 0:8N e where N e is in cgs units of cm \Gamma3 and vA is the
Alfv'en speed. This is the classical Sweet­Parker steady­
state solution which assumes that the outflow pressure is
the same as that at the neutral point. However, if the
outflow pressure was less than the input pressure then
v ? v A , i.e. the reconnection rate is enhanced. On­the­
other­hand, if the outflow pressure is large, then v !
vA (see Priest & Forbes 2000 for further details). These
latter situations are not applicable to explosive events,
due to the time duration of the jets and the lack of an
observed acceleration/de­acceleration in the jet velocity.
Thus in the discussion which follows, we assume v = v A
where the maximum observed Doppler velocity for each
explosive event is given in Table 2. It should also be noted
that these Doppler velocities are only approximate, since
we are unable to correct for line­of­sight effects. Table 3
shows the local magnetic field strengths (B), calculated
using Eq. 4. These values have been estimated for both
the red­shift and blue­shift velocities given in Table 2. The
maximum and averaged electron density values for each
selected region are shown in Table 3.
The electron density values presented in Table 3, for
each observed explosive event in the active region dataset,
are generally very similar except for the event located be­
tween [\Gamma205; \Gamma210] (N­S) and [625, 623] (E­W) which is,

4 Physical Parameters of EUV Explosive Events: P'erez & Doyle
nevertheless, similar in magnitude. Therefore, we may as­
sign a maximum electron density value of 3 10 11 cm \Gamma3 and
an averaged value of 10 11 cm \Gamma3 , for the explosive events
observed in the active region dataset. The corresponding
variation of magnetic field strengths is mainly due to the
changes observed in the Doppler velocities, which could
be affected in part by line­of­sight effects since the ob­
served values are only the projected velocity values in
the radial direction. The estimated magnetic field strength
ranges between 19--57G for the active region. These field
strengths are of the same order as those measured in the
photospheric cancelling flux regions by the Big Bear mag­
netograph (Martin 1984).
Dere et al. (1991), using the same method described
here, inferred field strengths of 15G for the red wing
plasma and 24G for the blue wing plasma of an explo­
sive event seen in the S iv 1406 š A line in a 'quiet' Sun
region. The density values used by Dere et al. were 7 10 10
cm \Gamma3 for the red wing and 6 10 10 cm \Gamma3 for the blue wing.
The averaged density value found here is similar, within
errors, to those calculated by Dere et al. (1991) for an
explosive event occurring in O iv lines. Moreover, their
inferred field strengths are of the same order as those esti­
mated here. Therefore, it seems reasonable to assume that
the observed electron density enhancements coincide with
explosive event sites.
In conclusion, the observed electron density enhance­
ments, in particular in the active region dataset, are
consistent with increases in the electron density due to
transient events (Cheng 1980, Hayes & Shine 1987, Dere
1991). On the other hand, the proximity in time and co­
spatial location of the explosive events described in Pa­
per I, together with their observed burst­like occurrence,
indicates that there is a good probability that these events
are occurring at the same time as the electron density en­
hancements. Finally, the values of the local magnetic field
strength confirm this hypothesis, correlating the observed
density enhancements to regions of magnetic field cancel­
lation and, therefore, with reconnection sites.
Acknowledgements. Research at Armagh Observatory is grant­
aided by the Dept. of Education for N. Ireland while partial
support for software and hardware is provided by the STAR­
LINK Project which is funded by the UK PPARC. MEP is
supported via a studentship from Armagh Observatory. This
work was supported by PPARC grants PPA/G/S/1999/00055
and GR/K43315. We would like to thank the SUMER team at
Goddard Space Flight Center for their help in obtaining the
data and Robert Erd'elyi for helpful suggestions. The SUMER
project is financially supported by DLR, CNES, NASA, and
PRODEX. SUMER is part of SOHO, the Solar and Helio­
spheric Observatory of ESA and NASA.
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