A&A 381, 694-702 (2002)
DOI: 10.1051/0004-6361:20011556
G. Trottet 1 - J.-P. Raulin 2 - P. Kaufmann 2 - M. Siarkowski 3 - K.-L. Klein 1 - D. E. Gary 4
1 - DASOP, CNRS-UMR 8645,
Observatoire de Paris, Section de Meudon,
92195 Meudon, France
2 - CRAAE/CRAAM, Instituto Presbiteriano Mackenzie,
Rua da Consolação 896, 01302-907 São Paulo,
Brazil
3 - Space Research Center, Polish Academy of Sciences,
ul. Kopernika 11, 51-622 Wroclaw, Poland
4 - NJIT, Center for Solar Research, 323 M L King Blvd.,
Newark, NJ 07102, USA
Received 20 September 2001 / Accepted 24 October 2001
Abstract
We present a detailed analysis of radio observations obtained at 212 and
405 GHz during the 2000 March 22 H
2N flare that occured in AR8910
at
1834 UT. These data are compared with microwave, soft X-ray and
hard X-ray measurements of this flare. While the flare emission is not
clearly detected at 405 GHz, the time profile of the 212 GHz emission
exhibits an impulsive burst, associated in time with the 1-18 GHz
impulsive microwave burst and a long-lasting thermal burst which finishes
at about the same time as the soft X-ray emission but reaches
its maximum later. The 212 GHz impulsive
emission and the lack of detection at 405 GHz are consistent
with synchrotron radiation from a population of ultrarelativistic
electrons in an average magnetic field of 400-600 G. This radiating
population of electrons has a hard energy spectrum (power law index
2.7). The expected >1 MeV gamma-ray continuum emission
from the radio emitting electrons is comparable to that detected for
mid-size electron-dominated events and the hard X-ray flux they
would produce at 100 keV is consistently lower than the upper limit
inferred from the observations. It is
shown that the 212 GHz thermal source has to be different from that
radiating the soft X-ray and microwave thermal emission. The present
observations of a solar burst provide the first
evidence of the extension of the gyrosynchrotron spectrum
of an impulsive radio burst in the synchrotron domain above 200 GHz.
Key words: Sun: activity - flares - particle emission - radio radiation
Solar flares are fundamentally high energy phenomena (Ramaty & Mandzhavidze
1993) that produce highly accelerated electrons and ions which
contain a large fraction, if not most, of the released energy.
The understanding of particle acceleration is thus a key issue to
establish the physics of flares. While the knowledge of
the low energy electron and ion distribution functions is a basic ingredient to
study the flare energetics, the characteristics (energy spectrum, upper
energy cut-off, ...) of the most
energetic particles provide among the strongest constraints
to acceleration models. The most direct diagnostics of high energy electrons
are: (i) the high energy (
1 MeV) gamma-ray (GR) continuum from e-p and e-e bremsstrahlung
(e.g. Chupp 1984; Rieger et al. 1998)
and (ii) the radio emission from gyrosynchrotron radiation
in coronal magnetic field structures
(see Pick et al. 1990; Bastian et al. 1998 and
references therein).
High energy GR continua of electronic origin have been
detected for a few tens of flares. In particular the observations
show that: (i) for some flares
the >1 MeV GR continuum from electron bremsstrahlung
shows a high energy rollover in the 50-60 MeV energy range
(Rieger & Marschhäuser 1990; Trottet et al. 1998)
while, for other events, there is no rollover up to 100 MeV (the upper
energy limit of solar dedicated instruments) and even up to
300 MeV (Vilmer et al. 1994, 1999; Leikov
et al. 1993) and (ii) the spectrum of >0.5-1 MeV
GR-producing electrons is harder than that of
lower energy hard X-ray (HXR) producing electrons
(e.g. Vestrand 1988; Rieger et al. 1998;
Trottet et al. 1998; Vilmer et al. 1999).
However, due to the sensitivity of GR detectors,
high energy GR observations have only been obtained
for large flares. The radio emission in the centimeter-millimeter domain
provides of more sensitive diagnostics which, in principle allow us to
investigate if electron acceleration up to relativistic energies also
occurs in weak flares. Short millimeter wave observations obtained at 86
GHz have indeed revealed that energetic electrons with hard spectra, like
those of GR-producing electrons, may be accelerated in weak flares
that would not produce detectable GR emission
(e.g. Kundu et al. 1994). Nevertheless, at 86 GHz, the characteristic
energy of radiating electrons is of the order of a few MeV
for magnetic fiels of a few 100 G
(e.g. Dulk 1985; Ramaty et al. 1994).
Radio observations at higher frequencies are thus needed to probe higher
energy electrons. This is achieved by the Solar Sub-millimeter Telescope
(SST) installed at the El Leoncito
Astronomical Complex in the argentinean Andes
which started solar observations at 212 and 405 GHz mid-1999
(Kaufmann et al. 2000a). First SST observations of a solar
flare that occured on 2000 March 22 at
1834 UT have been
described in Kaufmann et al. (2001). This study emphasizes
the detection of numerous 100-300 ms duration spikes
at both observing frequencies but did not provide clear evidence
that the time evolution of 212 and 405 GHz emission exhibits a
counterpart to the gyrosynchrotron microwave burst observed up
to 18 GHz.
In this paper we show that the time evolution of the
212 GHz radio emission detected by SST during the 2000 March 22 flare
exhibits a short
(3 min) impulsive burst which is the short-millimeter counterpart
of the impulsive microwave burst, followed by a long-lasting (
60 min)
thermal emission. Both components are below the noise level at 405 Ghz.
These findings are discussed in Sect. 3 and a summary of the main results
is given in Sect. 4.
![]() |
Figure 1: From top to bottom, time evolution of: the 1.5-12 keV soft X-ray flux observed by GOES, the 15-30 keV and 120-240 keV hard X-ray emission observed by RF15-I on Interball, the 15.4 GHz radio flux density measured by RSTN and the 212 GHz flux density detected by SST. The data gap in the 212 GHz time profile is due to calibration |
Open with DEXTER |
The 2000 March 22 H
2N (GOES X1.1) flare occurred in active region
NOAA 8910 (N14 W57) at
1834 UT.
The present multi-wavelength study of this flare
combines mm and sub-mm radio
observations with microwave and X-ray measurements.
Figure 1 displays the time evolution of these different emissions
from 1820 UT to 2000 UT.
The soft X-ray emission (SXR) in the 1.5-12 keV and 3-24 keV energy bands was obtained by GOES with a time resolution of 3 s. Assuming an iso-thermal source, these data allow us to estimate the temporal evolution of the temperature T and emission measure EM of the hot flare plasma as a function of time by using the method given in Thomas et al. (1985). The results are discussed in Sect. 3.2.
The hard X-ray (HXR) data were obtained by the RF15-I soft/hard X-ray
photometer aboard the Interball Tail Probe satellite
(Sylvester et al. 2000; Siarkowski et al. 1999).
HXR measurements are performed by a NaI (T1) sintillator in five energy
channels in the 10-240 keV energy range. Figure 1 shows the time
evolution of the 15-30 keV and 120-240 keV count rates (1 s averaged)
obtained after dead time and counter wrapping corrections
(Sylvester et al. 2000). Due to the high flux of low
energy HXR photons all channels are strongly affected by pile-up
during the whole observation period. After 1844 UT, around the maximum
of the 15.4 GHz and 212 GHz burst (see Fig. 1), pile-up, effects are
so strong that they dominate the count rates shown in Fig. 1. Thus: (i)
the time evolution of the measured 120-240 keV HXR rate
(the less affected by pile up) is not the
actual one and (ii) the data cannot be used for spectral analysis.
However we consider that before
1844 UT the HXR rates in the
60-120 keV and 120-240 keV channels provide an upper limit of
the actual rates. Therefore an upper limit
of the 100 keV photon flux
can be estimated by normalizing the 120-240 keV count rate expected
from an incident photon energy spectrum, taken as a power
law of index
,
to the observed count rate.
Moreover, because pile-up effects
are expected to be less important at higher energies, the actual value
of the photon spectral index is expected to be lower than that
obtained from the two highest energy channels. Thus around 1844 UT (maximum
of the microwave and 212 GHz impulsive burst), it is found that
5.4 and that
ranges from 3 to 13 photons cm-2s-1keV-1
for
ranging from 1.5 to 5.4.
Microwave total flux measurements were obtained by the Owens Valley Solar Array (OVSA) in the frequency range 1.2-18 GHz, and with the Radio Solar Telescope Network (RSTN) in the frequency range 1.4-15.4 GHz, with time resolutions of 10 s and 1 s respectively. The time evolution of the 15.4 GHz flux density from RSTN is shown in Fig. 1. In the following this frequency will be used for timing considerations whereas the spectrum of the radio emission will be taken from OVSA observations.
The SST provides observations at 212 and 405 GHz with a time resolution of
1 ms. The multiple-receiver focal array produces 6 beams (1-4 at 212 GHz and
5-6 at 405 GHz), allowing us to estimate the burst location
(Giménez de Castro et al. 1999).
During the 2000 March 22 flare
observations, the projection on the sky of beam 5 is centered on and tracks
AR 8910, while those
of beams 2-4 vary with time but always cover the active region, and those
of beams 1 and 6 are away from AR 8910 (see Fig. 1 in Kaufmann et al.
2001). The half power beamwidths were 7.5
for beams 2, 3, 4, 5 and 10
for beams 1 and 6.
In order to, as much as possible, get rid of
non-solar temporal variations (due to e.g.
atmospheric opacity, a hot calibration source
within the receiver system), the signal S1
from beam 1 (away from
AR 8910) has been subtracted from the signal S3 of beam 3 (always on the solar disk). This difference is referred to as S31
in the following. This procedure also removes the contribution of the
quiet Sun which, at a given frequency,
is expected to be similar in the different beams. Thus S31 will
represent the actual signal from an optically thin source but will
underestimate the signal from an optically thick source.
In addition, the smooth decrease of
S3-S1 due to the progressive
entry of beam 1 onto the solar disk during the flare observation has
been removed from S31(t).
S31 has been calibrated by setting S31 = 0 for
sky observations.
Because beams 2 and 4 projected partly onto the sky, it was not possible to
derive S21 and S41 in a similar way to S31.
The flux density F212 (in units of
10-22 Wm-2Hz-1 = 1 sfu), which is displayed in Fig. 1,
is related to S31 by:
![]() |
(1) |
Figure 1 shows that the 212 GHz radio burst exhibits two components: (i)
an impulsive burst B1 (1843-1846 UT), which is
the counterpart of the impulsive microwave burst, and (ii) a more intense
and time-extended emission B2 (
1846-2000 UT). While B2 and the
1.5-12 keV SXR emission vanish roughly at the same time, B2
reaches its maximum during the decay of the SXR emission.
Such a B1-B2 time sequence has been already observed for a few events
detected at 86 GHz (e.g. Kundu et al.
1994; Raulin et al. 1999).
In the following, B1 and B2 are discussed separately.
![]() |
Figure 2: From top to bottom, time evolution of: the 120-240 keV hard X-ray emission, the time derivative of the 1.5-12 keV soft X-ray intensity, the 15.4 GHz and 212 GH radio flux density and the radio spectral index (see text) during the impulsive burst B1. The vertical dashed lines indicate the onset and the maximum of the 212 GHz burst. |
Open with DEXTER |
Figure 2 shows the time evolution of the 120-240 keV HXR rate, of the time derivative of the 1.5-12 keV SXR emission, of the 15.4 GHz and 212 GHz radio flux densities and of the radio spectral index during B1. The spectral index has been obtained by assuming that the the radio spectrum is represented by a power law between 15.4 and 212 GHz. Because the 120-240 keV rate is strongly affected by pile-up (see Sect. 2), the SXR time derivative is used as a proxy of the HXR rate (e.g. Neupert 1968; Dennis & Zarro 1993).
Figure 2 shows that, although the 212 GHz and 15.4 GHz bursts are globally
time-associated, the onset and the main peak at 212 GHz
are delayed (12 s for the peak) with respect
to those at 15.4 GHz. Previous comparisons between the millimeter,
microwave and HXR emission indicate that the 86 GHz emission
onset appears sometimes to be delayed
with respect to the microwave and HXR onset
(e.g. Kundu et al. 1994; Lim et al. 1992).
Such delays have
been taken as evidence that the acceleration time of relativistic
(a few MeV) electrons, which radiate the millimeter emission,
is longer than that of a few tens to a few hundreds keV electrons
which produce the HXR and microwave emission.
Such an interpretation is not consistent with the observation
of fast correlated time variations of radiative signatures due to
a large range of electron energies, i.e. (i) HXR and GR
from
100 keV to >10 MeV, which are found to co-evolve
within the 1-2 s time resolution of GR detectors
(Forrest & Chupp 1983; Talon et al. 1993) and (ii)
HXR and millimeter waves where common features were reported on sub-second
time scales (Kaufmann et al. 2000b and references therein).
Indeed, such findings likely support the idea that
the acceleration of electrons up to sub-relativistic and relativistic
energies is simultaneous within
1 s.
For the present burst, the time evolution
of the main 15.4 GHz emission
(1842 UT to
1846 UT) is not simple but comprises
several secondary maxima or changes of slope
(during both the rise and the decay of the burst)
which reveal successive episodes of acceleration and injection
of electrons in the radio emitting region.
Multiple energy releases are also supported by the fact
that the evolution of the SXR time derivative exhibits numerous
time structures. Although
it is not possible to estimate how well this time derivative
reproduces the actual HXR time evolution, we note that
most of these time structures find counterparts at
15.4 GHz and at 212 GHz after the maximum.
Due to energy dependent transport effects (energy losses,
pitch angle diffusion, partial trapping) the HXR and radio burst
will last longer than the associated injection of electrons, so that
successive injections may lead to HXR and microwave bursts which overlap
in time (see discussion in Kaufmann et al. 2000b).
This may explain why the maximum of the 15.4 GHz emission falls between
two maxima of the SXR time derivative and why some of the injections show up
as a change of slope of the microwave time profile.
Although this precludes an estimate of the actual start, maximum time and duration
of a given injection, the data suggest that the 212 GHz burst starts
during one of the electron injections which constitute the rise of
the main peak at 15.4 GHz (see Fig. 2).
Figure 2 also shows that the radio spectral index
increases smoothly
from the beginning of the main rise till the maximum
of the 212 GHz emission.
Thus a plausible, though not unique, interpretation of the delays
between the 212 GHz and 15.4 GHz onsets and maxima, is that significant
212 GHz emission is only observed for an injection with a
hard electron spectrum and that such an injection overlaps
with a previous injection of a softer electron spectrum
(in the same magnetic structure or a neighboring one).
This is in line with results of
multi-wavelength studies of HXR/GR bursts
which revealed that these events exhibit sucessive
injections of electrons and ions and that noticeable changes in the
electron spectrum and in the electron to ion ratio
occur from one injection
to the other (Chupp et al. 1993;
Trottet et al. 1994; Trottet el al. 1998).
This interpretation is also consistent
with the observations by White (1999),
Kundu et al. (1994) and
Lim et al. (1992).
Indeed a careful examination of the hard X-ray, microwave and 86 GHz
time profiles shows, for those events where the 86 GHz onsets are
delayed, that these onsets
occur when a new electron injection shows up in the HXR/microwave
time profiles.
![]() |
Figure 3: The radio spectrum measused over a 10 s time interval at the maximum of the B1 burst (triangles). The solid lines are theoretical spectra computed for magnetic fields of 300 G, 500 G and 700 G (see text). |
Open with DEXTER |
Figure 3 displays the radio spectrum measured
over the 10 s interval which covers the maximum of B1.
The shape of this spectrum
is as expected for gyrosynchrotron/synchrotron
emission generated by relativistic/ultra
relativistic electrons in coronal magnetic fields. It
exhibits a turnover frequency
between
9 and 11 GHz, an optically thick
rising part up to
8 GHz and an optically thin decreasing part above
15-18 GHz. Due to lack of radio observations between 18 GHz and 212 GHz,
the optically thin part of the spectrum is poorly defined. As a first
approximation the radio spectrum is assumed to decrease monotonously
above 18 GHz. The solid lines in Fig. 3 are
gyrosynchrotron spectra
calculated for different values of a uniform magnetic field B
ranging from 300 G to 700 G by using
exact gyrosynchrotron codes developed by Ramaty (1969)
and corrected by Ramaty et al. (1994).
These calculations have been performed
for: (i) a radio source size of 10
(typical size for millimeter sources,
e.g. Raulin et al. 1999 and references therein); (ii) a
viewing angle
and (iii)
an instantaneous isotropic
distribution of radiating electrons with an energy spectrum taken as:
(electrons MeV-1).
The ratio
(where
is the
plasma frequency and
gyrofrequency) that
governs the level of the Razin-Tsytovich suppression below
has been set to 1 for all values of B. This choice
does not affect the results below because they are related to
the spectrum above
.
For a given value of B, the free parameters K and
have been estimated by using a minimum
algorithm which
provides the computed spectrum which best fits the observed one above
10-14 GHz. The comparison between the observed and computed radio spectra
(respectively, triangles and solid lines in Fig. 3)
results in the following comments:
Because the effect of partial trapping of electrons within
the radio emitting region is ignored,
the electron flux
entering the thick-target HXR/GR emitting source
may be overestimated. This leads to
(electrons keV-1 s-1),
where L is the radio source scale length (10
), v(E)the electron speed
and
the mean electron pitch angle (taken as 45
for the
assumed isotropic distribution). Thick target calculations taking into
acount both e-p and e-e bremsstrahlung (Vilmer, private communication)
are then performed to comptute
,
F(>1 MeV) and F(>10 MeV).
The obtained HXR/GR spectrum is well represented by a single power law
of index
1.5 in the 100 keV-100 MeV photon energy
range. Such a hard spectrum is characteristic of that measured
for electron-dominated events above 1 MeV (Rieger et al. 1998; Trottet et al. 1998; Vilmer et al. 1999).
In Fig. 4 (top panel) F(>10 MeV) is plotted as a function
of F(>1 MeV) for the events reported by Rieger et al.
The rectangle shows the domain of values of
F(>1 MeV) and F(>10 MeV) estimated for
the present event and for B ranging from 300 G to 600 G. Figure 4 indicates
that the present event should correspond to a mid-size
electron-dominated event.
Figure 4 (bottom panel) displays
for
ranging from 1.5 to 5.4 (solid line)
and the estimated values of
obtained for
= 1.5 and B ranging from 300 G to 600 G. Except for B = 300 G
and
= 1.5,
is always lower than
as it should be. In fact the photon spectrum below 0.5-1 MeV is
generally steeper than above 1 MeV (e.g. Rieger et al.
1998; Trottet et al. 1998 and references therein). Thus it
would be more relevant to compare
the value of
obtained for
= 1.5
to that of
obtained for
1.5, so that
B = 300 G cannot be fully excluded.
In summary, the microwave and short mm-wave impulsive burst emission
observed during the 2000 March 22 X1.1 flare
is consistent with gyrosynchrotron
radiation from an inhomogenous source with an average magnetic field
strength in the 400-600 G range.
The characteristic energy of electrons radiating synchrotron emission at
a given frequency
in a magnetic field B is (e.g. Dulk 1985):
E(MeV)
[(
,
where
is the viewing angle.
For B
400-600 G,
the 212 GHz emission is produced by synchrotron radiation from
ultra-relativistic electrons with E in the 12-15 MeV range.
![]() |
Figure 4: Top panel: the >10 MeV GR continuum fluence as a function of the >1 MeV GR continuum fluence for the electron-dominated events reported in Rieger et al. (1998) (triangles); the solid rectangle shows the domain of values deduced from the present radio observations for a magnetic field ranging from 300 G to 600 G (see text). Bottom panel: the upper limit of the photon flux at 100 keV estimated, as a function of the hard X-ray spectral index, from the RF15-I HXR measurements (solid line); the triangles show the photon flux at 100 keV for a spectral index of 1.5 deduced from the radio observations and for different values of the magnetic field strength. |
Open with DEXTER |
![]() |
Figure 5:
Top panel: time evolution of the 14.8 GHz flux density
observed by OVSA during the decay of the microwave emission and comptuted for
a 30
![]() ![]() |
Open with DEXTER |
The radio spectra measured by OVSA during B2 indicate that both
non thermal gyrosynchrotron and thermal bremsstrahlung radiation
contribute to the observed 1-18 GHz microwave emission. Assuming
that the microwave thermal source is the same as the SXR source the
values of EM and T obtained from the GOES observations (see Sect. 2)
are used to compute the expected microwave free-free emission for a
source diameter ranging from 30
to 45
(typical sizes
for 17 GHz thermal sources; e.g. Raulin et al. 1999). The top panel
of Fig. 5 shows the time evolution of the observed and expected
14.8 GHz flux density during the decay of the microwave emission
after 1850 UT. The contribution of free-free emission to the total
14.8 GHz emission increases with time and becomes dominant
after
1900 UT. As an example, in the bottom panel of Fig. 5, the
thermal spectrum (solid lines)
derived from GOES SXR observations
(
cm-3 and
K)
at 1910:50 UT, when free-free emission is dominant, is
overplotted on the observed one (error bars). The observed
and computed spectra agree reasonably well and exhibit a turnover
frequency around 10 GHz below (above) which free-free emission
is optically thick (thin). As expected, in the optically thin regime
the computed flux density does not depend on the cross section of the
source for a given value of EM (i.e. at a given time;
see e.g. Dulk 1985).
Below
10 GHz (optically thick emission),
a slightly better agreement is obtained for a source diameter of 30
.
The good agreement between observed and computed spectra is
consistent with the assumption that the SXR and microwave thermal sources
are similar.
The smooth time profile of the 212 GHz flux density during B2 suggests that
this radio emission is predominantly of thermal origin. Indeed, after
1846 UT, the OVSA spectra indicate that the contribution
of gyrosynchrotron radiation to the 212 GHz emission is negligeable.
However, the 212 GHz flux density expected from the microwave thermal source
(
20 sfu) is much lower than the observed one (
190 sfu;
see Fig. 2, bottom). This is in agreement with the suggestion by
Kaufmann et al. (2001) that the SXR and 212 GHz emission
arise from different regions. Kaufmann et al. also proposed that the
short mm-wave emission might be produced deep in the chromosphere.
The investigation of such a possibility is beyond the scope of the present study, since it would require a realistic and time dependant flare model of the deep atmosphere, which is poorly constrained by the present observations.
In this paper we have reported and analysed observations of a
radio burst at frequencies larger than 100 GHz.
These observations were obtained at 212 and 405 GHz by the SST
during the 2000 March 22 H
2N (GOES X1.1) flare at
1834 UT.
At 212 GHz the radio event comprises an impulsive part B1 which lasts for
3 min and a time extended burst B2 which lasts for
60 min. Neither
B1 nor B2 produced a significant signal at 405 GHz, most probably because
of the low level of aperture efficiency of the SST at that frequency during
the considered observing period (see Sect. 2).
The time evolution of the
212 GHz radiation has been compared with that of the 1.5-24 keV SXR,
120-240 keV HXR and 1-18 GHz microwave emission and radio frequency spectra
have been analysed. The main results can be summarized as follows:
It is obvious that synchrotron emission from ultra relativistic electrons is expected for GR flares with significant electron bremsstrahlung emission above 10 MeV. The present observations constitute the first detection of such a synchrotron component above 200 GHz and demonstrate the ability of the SST to measure it. Kaufmann et al. (2001) reported that numerous fast subsecond time structures were detected at 212 and 405 GHz during both B1 and B2. Although the physics of such fast time variations is not yet understood, the fact that their flux density increases with frequency provides a clear indication that their origin is different from that of the synchrotron impulsive burst analysed in the present study. The high-energy bremsstrahlung continuum expected from the electrons radiating this synchrotron burst could have been detected by present GR detectors. However, it is obvious that the sensitivity of the SST, which has been substantially improved since the observations discussed in this paper, should allow us to detect much weaker flares than the one studied here and thus to investigate if the acceleration of ultra relativistic electrons occurs only during large (GOES M and X) flares.
Acknowledgements
GOES data were provided by the Solar Data Analysis Center at NASA/GSFC. RSTN data have been retrieved from the National Geophysical Data Center homepage with the assistance of H. Coffey and TRACE images were visualized from the instrument homepage. One of the authors (PK) is also part time with CCS, Unicamp, Campinas, SP, Brazil. The present work was supported by the French-Bresilian program of exchange of scientists (CNRS-CNPq contract 8810 and 910089/99-0) and by FAPESP under grants No 99/06126-7 and 01/03791-1. The OVSA instrument and data analysis were supported through NSF grant AST-9987366 and NASA grant NAG5-9682 to NJIT.