Nancay Radioheliograph Research Articles

Connecting remote and in situ observations of shock-accelerated electrons associated with a coronal mass ejection

Context. One of the most prominent sources for energetic particles in our Solar System are huge eruptions of magnetised plasma from the Sun, known as coronal mass ejections (CMEs), which usually drive shocks that accelerate charged particles up to relativistic energies. In particular, energetic electron beams can generate radio bursts through the plasma emission mechanism, for example, type II and accompanying herringbone bursts. Aims. In this work, we investigate the acceleration location, escape, and propagation directions of various electron beams in the solar corona and compare them to the arrival of electrons at spacecraft. Methods. To track energetic electron beams, we used a synthesis of remote and direct observations combined with coronal modeling. Remote observations include ground-based radio observations from the Nançay Radioheliograph (NRH) combined with space-based extreme-ultraviolet and white-light observations from Solar Dynamics Observatory (SDO), Solar Terrestrial Relations Observatory (STEREO), and Solar Orbiter (SolO). We also used direct observations of energetic electrons from the STEREO and Wind spacecraft. These observations were then combined with a three-dimensional (3D) representation of the electron acceleration locations, including the results of magneto-hydrodynamic models of the solar corona. This representation was subsequently used to investigate the origin of electrons observed remotely at the Sun and their link to in situ electrons. Results. We observed a type II radio burst followed by herringbone bursts that show single-frequency movement through time in NRH images. The movement of the type II burst and herringbone radio sources seems to be influenced by regions in the corona where the CME is more capable of driving a shock. We found two clear distinct regions where electrons are accelerated in the low corona and we found spectral differences between the radio emission generated in these regions. We also found similar inferred injection times of near-relativistic electrons at spacecraft to the emission time of the type II and herringbone bursts. However, only the herringbone bursts propagate in a direction where the shock encounters open magnetic field lines that are likely to be magnetically connected to the same spacecraft. Conclusions. Our results indicate that if the in situ electrons are indeed shock-accelerated, the most likely origin of the in situ electrons arriving first is located near the acceleration site of herringbone electrons. This is the only region during the early evolution of the shock where there is clear evidence of electron acceleration and an intersection of the shock with open field lines, which can be directly connected to the observing spacecraft.

Quantifying the Magnetic Structure of a Coronal Shock Producing a Type II Radio Burst

Type II radio bursts are thought to be produced by shock waves in the solar atmosphere. However, what magnetic conditions are needed for the generation of type II radio bursts is still a puzzling issue. Here, we quantify the magnetic structure of a coronal shock associated with a type II radio burst. Based on multiperspective extreme-ultraviolet observations, we reconstruct the three-dimensional (3D) shock surface. By using a magnetic field extrapolation model, we then derive the orientation of the magnetic field relative to the normal of the shock front (θ Bn) and the Alfvén Mach number (M A ) on the shock front. Combining the radio observations from the Nancay Radio Heliograph, we obtain the source region of the type II radio burst on the shock front. It is found that the radio burst is generated by a shock with M A ≳ 1.5 and a bimodal distribution of θ Bn. We also use the Rankine–Hugoniot relations to quantify the properties of the shock downstream. Our results provide a quantitative 3D magnetic structure condition of a coronal shock that produces a type II radio burst.

Observations of a Flare-Generated Blast Wave in a Pseudo Coronal Mass Ejection Event

We present an analysis of the event near the east limb, SOL2014-03-06T09:23, in which a pseudo coronal mass ejection (CME) was detected by the Large Angle and Spectrometric Coronagraph (LASCO) C2 instrument and indicated as “Poor Event; Only C2” in the Solar and Heliospheric Observatory (SOHO) LASCO CME Catalog. The analysis was performed based on two main methods: 1) investigation of the difference brightness profiles along specific directions in the solar corona using the EUV observations by the Atmospheric Imaging Assembly (AIA) instrument onboard the Solar Dynamics Observatory (SDO); 2) investigation of the spatially-resolved observations of the type II radio bursts made with the Nancay Radioheliograph. Based on the analysis performed we argue that the observed pseudo-CME could be a blast wave caused by impulsive flare energy release in the low corona. We also argue that, in the limited height range of ${\approx}\,0.2 \mathrm{R_{\odot }}\,\mbox{--}\,0.5 \mathrm{R}_{\odot}$, the front of this blast wave could steepen into a shock front.

A New Image Restoration Method for MUSER

Solar radio images in decimeter wave range consist of many complicated components including a disk component, some bright and weak compact sources, and many diffuse features. Complicated structures combining these various components maybe cause restoration failure when using conventional algorithms. Furthermore, the images at different frequencies band are pretty different. Therefore, restoration method for solar radio image is different from other radio sources. Some image restoration methods were applied and obtained good results on Nancay radioheliograph images and Nobeyama radioheliograph images, and some new methods were introduced into processing these complicated solar radio images in recent years. For a new radioheliograph with ultrawide frequency band, new image restoration method which can maximize function of telescope is demanded. Different images could be obtained from the same visibilities data by using different weighting functions in imaging processing. In this paper, a new restoration method for solar radio image was proposed. Two images with different weighting functions from the same data are combined in this method. This restoration method has applied to data processing of Mingantu spectral radioheliograph.

Energetics of small electron acceleration episodes in the solar corona from radio noise storm observations

Observations of radio noise storms can act as sensitive probes of nonthermal electrons produced in small acceleration events in the solar corona. We use data from noise storm episodes observed jointly by the Giant Metrewave Radio Telescope (GMRT) and the Nancay Radioheliograph (NRH) to study characteristics of the nonthermal electrons involved in the emission. We find that the electrons carry $10^{21}$ to $10^{24}$ erg/s, and that the energy contained in the electrons producing a representative noise storm burst ranges from $10^{20}$ to $10^{23}$ ergs. These results are a direct probe of the energetics involved in ubiquitous, small-scale electron acceleration episodes in the corona, and could be relevant to a nanoflare-like scenario for coronal heating.

Coronal mass ejection-related particle acceleration regions during a simple eruptive event

An intriguing feature of many solar energetic particle (SEP) events is the detection of particles over a very extended range of longitudes in the heliosphere. This may be due to peculiarities of the magnetic field in the corona, to a broad accelerator, to cross-field transport of the particles, or to a combination of these processes. The eruptive flare on 26 April 2008 provided an opportunity to study relevant processes under particularly favourable conditions since it occurred in a very quiet solar and interplanetary environment. This enabled us to investigate the physical link between a single well-identified coronal mass ejection (CME), electron acceleration as traced by radio emission, and the production of SEPs. We conduct a detailed analysis, which combines radio observations (Nancay Radio Heliograph and Nancay Decametre Array, Wind/Waves spectrograph) with remote-sensing observations of the corona in extreme ultraviolet (EUV) and white light, as well as in situ measurements of energetic particles near 1AU (SoHO and STEREO spacecraft). By combining images taken from multiple vantage points, we were able to derive the time-dependent evolution of the 3D pressure front that was developing around the erupting CME. Magnetic reconnection in the post-CME current sheet accelerated electrons, which remained confined in closed magnetic fields in the corona, while the acceleration of escaping particles can be attributed to the pressure front ahead of the expanding CME. The CME accelerated electrons remotely from the parent active region, owing to the interaction of its laterally expanding flank, which was traced by an EUV wave, with the ambient corona. SEPs detected at one STEREO spacecraft and SoHO were accelerated later, when the frontal shock of the CME intercepted the spacecraft-connected interplanetary magnetic field line. The injection regions into the heliosphere inferred from the radio and SEP observations are separated in longitude by about 140 •. The observations for this event show that it is misleading to interpret multi-spacecraft SEP measurements in terms of one acceleration region in the corona. The different acceleration regions are linked to different vantage points in the interplanetary space.

Injection of solar energetic particles into both loop legs of a magnetic cloud

Each of the two STEREO spacecraft carries a SEPT Instrument which measures electrons and protons. Anisotropy observations are provided in four viewing directions. The SEP event on 7 Nov 2013 was observed by both STEREO spacecraft, which were longitudinally separated by 68{\deg} at that time. While STEREO A observed the expected characteristics of an SEP event at a well-connected position, STEREO B detected a very anisotropic bi-directional distribution of near-relativistic electrons and was situated inside a magnetic-cloud-like structure during the early phase of the event. We examine the source of the bi-directional SEP distribution at STEREO B. On the one hand this distribution could be caused by a double injection into both loop legs of the MC. On the other hand, a mirroring scenario where the incident beam is reflected in the opposite loop leg could be the reason. Furthermore, the energetic electron observations are used to probe the magnetic structure inside the magnetic cloud. We show that STEREO B was embedded in an MC-like structure ejected three days earlier. We apply a GCS model to the coronagraph observations from three viewpoints as well as the Global Magnetic Cloud model to the in situ measurements at STEREO B to determine the orientation and topology of the MC close to the Sun and at 1 AU. We also estimate the path lengths of the electrons propagating through the MC to estimate the amount of magnetic field line winding inside the structure. The relative intensity and timing of the energetic electron increases in the different SEPT telescopes at STEREO B strongly suggest that the bi-directional electron distribution is formed by SEP injections in both loop legs of the MC separately instead of by mirroring farther away beyond the STEREO orbit. Observations by the Nancay Radioheliograph of two distinct radio sources during the SEP injection further support the above scenario.

Electron density and temperature in the solar corona from multifrequency radio imaging

Context. The 2D images obtained through rotational aperture synthesis with the Nancay Radioheliograph are suitable for quantitative exploitation. First results are presented.Aims. We study the variations of the quiet corona in brightness and size during an 8-year period and derive electron density and temperature in the corona.Methods. Images at 6 frequencies between 150 and 450 MHz for 183 quiet days between 2004 and 2011 were used. Measurements of the brightness temperature T b beyond the limb allowed coronal density models to be derived in both EW and NS radial directions, with a weak dependence on the electron temperature. The total ranges in the heliocentric distance r are 1.15–1.60 R ⊙ (EW) and 1.0–1.4 R ⊙ (NS). The agreement between results from different frequencies, in the ranges of r where there is overlapping shows the robustness of the method. The electron temperature, in turn, can be derived from the comparison of the observed mean spectra on the disk with those predicted through transfer calculations from the density models derived from limb observations. Results. The widths of the brightness profiles that were averaged yearly have minima at cycle minimum (2008–2009). These minima are more pronounced for EW profiles than for NS ones. The derived yearly-averaged density models along equatorial and polar diameters are consistent with isothermal and hydrostatic models. They are characterized by their density value n 0 extrapolated down to the base of the corona and their scale-height temperature T H . Changes in n 0 and T H with solar cycle are given for equatorial and polar regions. The kinetic temperature T e of electrons in the corona (~0.62 MK) is found to be significantly less than T H (~1.5 MK). This implies an ion temperature T i ~ 2.2 MK.Conclusions. The yearly-averaged variations of these models are less than the dispersion between models derived from other techniques, such as white light and EUV observations, partly because these two techniques are not time-averaged, and they refer to particular days. The radio models are generally less dense, which is compatible with isothermal hydrostatic equilibrium in their range of heliocentric distances, and they show different behaviors with the solar cycle in the equatorial or polar radial directions. The electron kinetic temperature T e is substantially less than T H .

Simultaneous Radio and EUV Imaging of a Multi-lane Coronal Type II Radio Burst

A multi-lane solar type II radio burst was observed by several solar spectrographs on 16 February 2011. The event was also recorded by the Nancay Radioheliograph (NRH) at several metric wavelengths, by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO), and by the Extreme Ultraviolet Imager (EUVI) onboard the Solar TErrestrial Relations Observatory (STEREO) in a number of EUV passbands. These multi-wavelength data provide a rare opportunity to reveal the emission source of the multiple type II lanes. Our study shows that all lanes are associated with a single EUV wave, presumably the radio-emitting shock. The EUV wave was driven by a coronal mass ejection (CME) associated with an M1.6 flare and a filament eruption. With the NRH data and the three-dimensional (3D) bow-shock reconstruction that we built using the multi-viewpoint data of the EUV wave, we are able to deduce the 3D coordinates of the radio sources. We conclude that all the three type II lanes originated from the western flank of the shock, with two of them from closely adjacent locations on the southern part, the other one from a distinct location on the northern part. This case study demonstrates how the type II origin can be pinpointed by combining analyses of different data sets.

Spatially resolved observations of a coronal type II radio burst with multiple lanes

Relative dynamics of the radio sources of the metric type II burst with three emission lanes and coronal mass ejection (CME) occurred in the lower corona (r≲1.5R⊙) during the SOL2011-02-16T14:19 event is studied. The observational data of the Nancay Radioheliograph (NRH) and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) are used. These observations are also supplemented by the data sets obtained with the STEREO-A and -B, RHESSI, and GOES spacecraft, as well as with the ground-based solar radio spectrometers. It is found that the sources of the radio burst were located ahead of the expanding CME and had a complex spatial structure. The first and the second lanes were both emitted from the “magnetic funnel” — a bundle of open magnetic field lines separated the south and north systems of magnetic loops of the active region. Due to the projection effect and limited angular resolution of the NRH it is not possible to determine, whether the spatial locations of the radio sources of the two first emission lanes differed or not. It is argued that the observations support the hypothesis that the radio sources of the first and second lanes could be emitted respectively ahead of and behind a front of the same weak (the Alfvén Mach number MA≈1.1–1.2), fast mode, quasi-parallel piston MHD shock wave. However, the third lane of the burst was definitely emitted from a different place. Its radio sources were situated ahead of the north-west part of the CME propagated through the north system of magnetic loops. This indicates clearly that different emission lanes of the same type II burst can be a result of propagation of different parts of a single CME through regions with different physical conditions (geometries and plasma densities) in the lower corona.

Fine structural features of radio-frequency radiation of the solar flare of February 12, 2010

Solar radio emission records received at the IZMIRAN spectrograph (25–270 MHz) during the solar flare event of February 12, 2010 are analyzed. Different fine structures were observed in three large groups of type III bursts against a low continuum. According to data from the Nancay radioheliograph, sources of all three groups of bursts were located in one active region, 11046, and their emissions were accompanied by soft X-ray bursts (GOES satellite): C7.9 at 0721 UT, B9.6 at 0940 UT, and M8.3 at 1125 UT. After the first group of bursts, classical fiber bursts were observed in combination with reverse-drift fiber bursts with unusual arc drift. After the third (the most powerful) group, stable second-length pulsations and slow-drift fiber bursts were observed, the instantaneous frequency bands of which were an order of magnitude larger than the frequency band of classical fiber bursts, and the frequency drift was several times lower. More complex fiber bursts were observed in the weakest group in the time range 0940:39–0942:00 UT. They were narrow-band (∼0.5 MHz) fiber bursts, periodically recurring in a narrow frequency band (5–6 MHz) during several seconds. The presence of many chaotically drifting ensembles of fibers, crossing and superimposing on one another, is a feature of this event. It is assumed that occurrence of these structures can be connected with the existence of many small shock fronts behind the leading edge of a coronal mass ejection.

RADIO IMAGING OF A TYPE IVM RADIO BURST ON THE 14TH OF AUGUST 2010

Propagating coronal mass ejections (CMEs) are often accompanied by burst signatures in radio spectrogram data. We present Nancay Radioheliograph observations of a moving source of broadband radio emission, commonly referred to as a type IV radio burst (type IVM), which occurred in association with a CME on the 14th of August 2010. The event was well observed at extreme ultraviolet (EUV) wavelengths by SDO/AIA and PROBA2/SWAP, and by the STEREO SECCHI and SOHO LASCO white light (WL) coronagraphs. The EUV and WL observations show the type IVM source to be cospatial with the CME core. The observed spectra is well fitted by a power law with a negative slope, which is consistent with optically thin gyrosynchrotron emission. The spectrum shows no turn over at the lowest Nancay frequencies. By comparing simulated gyrosynchrotron spectra with Nancay Radioheliograph observations, and performing a rigorous parameter search we are able to constrain several key parameters of the underlying plasma. Simulated spectra found to fit the data suggest a nonthermal electron distribution with a low energy cutoff of several tens to 100 keV, with a nonthermal electron density in the range 100-102 cm–3, in a magnetic field of a few Gauss. The nonthermal energy content of the source is found to contain 0.001%-0.1% of the sources thermal energy content. Furthermore, the energy loss timescale for this distribution equates to several hours, suggesting that the electrons could be accelerated during the CME initiation or early propagation phase and become trapped in the magnetic structure of the CME core without the need to be replenished.

A DECADE OF SOLAR TYPE III RADIO BURSTS OBSERVED BY THE NANÇAY RADIOHELIOGRAPH 1998-2008

We present a statistical survey of almost 10 000 radio Type III bursts observed by the Nancay Radioheliograph from 1998 to 2008, covering nearly a full solar cycle. In particular, sources sizes, positions, and fluxes were examined. We find an east-west asymmetry in source positions which could be attributed to a 6(+/-)1 degree eastward tilt of the magnetic field, that source FWHM sizes s roughly follow a solar-cycle averaged distribution dN/ds = 14 {\nu}^{-3.3} s^{-4} arcmin^{-1} day^{-1}, and that source fluxes closely follow a solar-cycle averaged dN/dS_{\nu} = 0.34 {\nu}^{-2.9} S_{\nu}^{-1.7} sfu^{-1} day^{-1} distribution (when {\nu} is in GHz, s in arcmin, and S_{\nu} in sfu). Fitting a barometric density profile yields a temperature of 0.6 MK, while a solar wind-like (\propto h^{-2}) density profile yields a density of 1.2x10^6 cm^{-3} at an altitude of 1 RS, assuming harmonic emission. Finally, we found that the solar-cycle averaged radiated Type III energy could be similar in magnitude to that radiated by nanoflares via non-thermal bremsstrahlung processes, and we hint at the possibility that escaping electron beams might carry as much energy away from the corona as is introduced into it by accelerated nanoflare electrons.

RADIO IMAGING OF SHOCK-ACCELERATED ELECTRONS ASSOCIATED WITH AN ERUPTING PLASMOID ON 2010 NOVEMBER 3

We present observations of a metric type II solar radio burst that occurred on the 3rd of November 2010 in association with an erupting plasmoid. The eruption was well observed by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory and the Reuven Ramaty High Energy Solar Spectroscopic Imager, while the burst occurred in the frequency range of the Nancay Radioheliograph (NRH). Such events, where the type II emission occurs in the NRH frequency range, allowing us to image the burst, are infrequent. Combining these data sets, we find that the type II is located ahead of the hot (∼11 MK) core of the plasmoid, which is surrounded by a well-defined envelope of cool (few MK) plasma. Using two methods, we determine the propagation velocity of the shock: (1) fitting the type II emission observed in PHOENIX and HUMAIN radio spectrogram data; (2) direct imaging of the type II source location using NRH observations. We use LASCO C2 polarized brightness images to normalize our coronal density model. However, we find that information from imaging is required in order to fine-tune this normalization. We determine a shock propagation velocity between 1900 km s −1 and 2000 km s −1 . This is faster than the plasmoid observed at extreme-ultraviolet wavelengths by AIA (v = 670-1440 km s −1 , where the cooler plasma propagates faster than the hot core). The positioning of the type II, ahead of the plasmoid, suggests that the electrons are accelerated in a piston-driven shock.

Morphology of the quiet Sun between 150 and 450 MHz as observed with the Nançay radioheliograph

Context. Since its last upgrading in 2004, the Nancay radioheliograph (NRH) is able to produce reliable images of the quiet Sun in the 150–450 MHz range, which corresponds to the low and medium corona. These images are better than those previously obtained with the NRH or other instruments in this range and are suitable for quantitative and systematic exploitation.Aims. We aim to study the radio aspects of the solar atmosphere. We aim to focuss on the description of the morphology and the comparisons with observations in EUV and X-ray ranges and with magnetic structures.Methods. We used the rotational aperture synthesis technique (suitable for non time-varying objects) along with an original self-calibration procedure and a specific deconvolution algorithm (a variant of CLEAN that includes a scale analysis).Results. We present results from radio imaging of the quiet Sun with the NRH. The analysis was carried over about 160 days during the summers (June-August) from 2004 to 2011. We confirms and extend our first results, which were published from a much smaller data sample. We emphasize new aspects of the corona observed in this frequency range, in particular the existence of coronal holes darker than previously reported and dark channels at high frequencies. We give examples that illustrate the complex morphology of coronal structures as revealed by radio imaging, the center-to-limb effects (radio occultation) and the variation with the phase of the solar cycle. We compare our images with large-scale coronal magnetic structures. We show that dark coronal holes and channels seen at high frequencies and bright ribbons seen at low frequencies seem to be associated with particular types of magnetic structures. Conclusions. Detailed radio images in this frequency range are a new tool with high potential for investigating the low and middle corona, since these images can emphasize structures that are not (or much less) visible at other wavelengths. In the near future, there is much to learn from observations with NRH during the ascending part of the cycle (never observed at m/dcm wavelengths ) and from composite images of combined NRH and GMRT data, which will have a much better resolution. In a second step, it will be interesting to obtain circularly polarized images (giving a diagnostic of the coronal magnetic field) and images with new-generation instruments, which will yield detailed images with shorter synthesis or even snapshot images, from which processes evolving on shorter time scales can be studied.

FLARE-GENERATED TYPE II BURST WITHOUT ASSOCIATED CORONAL MASS EJECTION

We present a study of the solar coronal shock wave on 2005 November 14 associated with the GOES M3.9 flare that occurred close to the east limb (S06° E60°). The shock signature, a type II radio burst, had an unusually high starting frequency of about 800 MHz, indicating that the shock was formed at a rather low height. The position of the radio source, the direction of the shock wave propagation, and the coronal electron density were estimated using Nancay Radioheliograph observations and the dynamic spectrum of the Green Bank Solar Radio Burst Spectrometer. The soft X-ray, Hα, and Reuven Ramaty High Energy Solar Spectroscopic Imager observations show that the flare was compact, very impulsive, and of a rather high density and temperature, indicating a strong and impulsive increase of pressure in a small flare loop. The close association of the shock wave initiation with the impulsive energy release suggests that the impulsive increase of the pressure in the flare was the source of the shock wave. This is supported by the fact that, contrary to the majority of events studied previously, no coronal mass ejection was detected in association with the shock wave, although the corresponding flare occurred close to the limb.

The Quiet Sun between 150 and 450 MHz : morphology and modeling

We present results from quiet sun observations in the metric range. Solar images where obtained through rotational aperture synthesis at a number of frequencies, using the Nancay radioheliograph in its latest version, from 2004 to 2011. The main morphology is described, and compared to the magnetic field topology. We also measured on-disk spectra, of the average sun and of coronal holes, and equatorial and polar limb profiles. From these measurements, simple temperature/density atmospheric models are derived and briefly discussed.

Location of Decimetric Pulsations in Solar Flares

This work investigates the spatial relation between coronal X-ray sources and coherent radio emissions, both generally thought to be signatures of particle acceleration. Two limb events were selected during which the radio emission was well correlated in time with hard X-rays. The radio emissions were of the type of decimetric pulsations as determined from the spectrogram observed by Phoenix-2 of ETH Zurich. The radio positions were measured from observations with the Nancay Radioheliograph between 236 and 432 MHz and compared to the position of the coronal X-ray source imaged with RHESSI. The radio pulsations originated at least 30 - 240 Mm above the coronal hard X-ray source. The altitude of the radio emission increases generally with lower frequency. The average positions at different frequencies are on a line pointing approximately to the coronal hard X-ray source. Thus, the pulsations cannot be caused by electrons trapped in the flare loops, but are consistent with emission from a current sheet above the coronal source.

A single picture for solar coronal outflows and radio noise storms

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ORIGIN OF CORONAL SHOCK WAVES ASSOCIATED WITH SLOW CORONAL MASS EJECTIONS

We present a multiwavelength study of five coronal mass ejection/flare events (CME/flare) and associated coronal shock waves manifested as type II radio bursts. The study is focused on the events in which the flare energy release, and not the associated CME, is the most probable source of the shock wave. Therefore, we selected events associated with rather slow CMEs (reported mean velocity below 500 km s–1). To ensure minimal projection effects, only events related to flares situated close to the solar limb were included in the study. We used radio dynamic spectra, positions of radio sources observed by the Nancay Radioheliograph, GOES soft X-ray flux measurements, Large Angle Spectroscopic Coronagraph, and Extreme-ultraviolet Imaging Telescope observations. The kinematics of the shock wave signatures, type II radio bursts, were analyzed and compared with the flare evolution and the CME kinematics. We found that the velocities of the shock waves were significantly higher, up to one order of magnitude, than the contemporaneous CME velocities. On the other hand, shock waves were closely temporally associated with the flare energy release that was very impulsive in all events. This suggests that the impulsive increase of the pressure in the flare was the source of the shock wave. In four events the shock wave was most probably flare-generated, and in one event results were inconclusive due to a very close temporal synchronization of the CME, flare, and shock.