Abstract. Propagating wave-like disturbances associated with solar flares -commonly observed in the chromosphere as Moreton waves -have been known for several decades. Recently, the phenomenon has come back into focus prompted by the observation of coronal waves with the SOHO/EIT instrument ("EIT waves"). It has been suggested that they represent the anticipated coronal counterpart to Moreton waves, but due to some pronounced differences, this interpretation is still being debated. We study 12 flare wave events in order to determine their physical nature, using Hα, EUV, He I 10 830 Å SXR and radioheliographic data. The flare wave signatures in the various spectral bands are found to lie on closely associated kinematical curves, implying that they are signatures of the same physical disturbance. In all events, and at all wavelengths, the flare waves are decelerating, which explains the apparent "velocity discrepancy" between Moreton and EIT waves which has been reported by various authors. In this paper, the focus of the study is on the morphology, the spatial characteristics and the kinematics of the waves. The characteristics of the common perturbation which causes the wave signatures, as well as the associated type II radio bursts, will be studied in companion Paper II, and a consistent physical interpretation of flare waves will be given.

Abstract. Type II radio bursts recorded in the metric wavelength range are excited by MHD shocks traveling through the solar corona. They often expose the fundamental and harmonic emission band, both frequently being split in two parallel lanes that show a similar frequency drift and intensity behaviour. Our previous paper showed that band-splitting of such characteristics is a consequence of the plasma emission from the upstream and downstream shock regions. Consequently, the split can be used to evaluate the density jump at the shock front and to estimate the shock Mach number, which in combination with the shock speed inferred from the frequency drift provides an estimate of the Alfvén velocity and the magnetic field in the ambient plasma. In this paper such a procedure is applied to 18 metric type II bursts with the fundamental band starting frequencies up to 270 MHz. The obtained values show a minimum of the Alfvén velocity at the heliocentric distance R ≈ 2 amounting to v A ≈ 400-500 km s −1 . It then increases achieving a local maximum of v A ≈ 450-700 km s −1 at R ≈ 2.5. The implications regarding the process of formation and decay of MHD shocks in the corona are discussed. The coronal magnetic field in the range 1.3 < R < 3 decreases as R −3 to R −4 , or H −1.5 to H −2 if expressed as a function of the height. The results are compared with other estimates of the coronal magnetic field in the range 1 < R < 10. Combined data show that below H < 0.3 the magnetic field is dominated by active region fields, whereas above H = 1 it becomes radial, behaving roughly as B = 2 × R −2 with a plausible value of B ≈ 5 nT at 1 a.u.

Abstract. The study of solar flare waves -globally propagating wave-like disturbances usually observed in Hα as Moreton waves -has recently come back into focus prompted by the observation of coronal waves in the EUV with the SOHO/EIT instrument ("EIT waves"), and in several additional wavelength channels. We study 12 flare wave events in order to determine their physical nature, using Hα, EUV, helium I, SXR and radioheliographic data. In the companion Paper I, we have presented the observational data and have discussed the morphology, spatial characteristics and the kinematics of the different flare wave signatures. The wavefronts observed in the various spectral bands were found to follow kinematical curves that are closely associated, implying that they are signatures of the same physical disturbance. In the present paper, we continue the study with a close examination of the evolution of the common perturbation that causes the different wave signatures, and with a detailed analysis of the metric type II radio bursts that were associated with all flare wave events. The basic characteristics of the waves are deceleration, perturbation profile broadening, and perturbation amplitude decrease. This behavior can be interpreted in terms of a freely propagating fast-mode MHD shock formed from a large-amplitude simple wave. It is shown that this scenario can account for all observed properties of the flare waves in the various spectral bands, as well as for the associated metric type II radio bursts.

Context. Recent RHESSI observations indicate an apparent altitude decrease of flare X-ray loop-top (LT) sources before changing to the commonly observed upward growth of the flare loop system. Aims. We performed a detailed study of the LT altitude decrease for one well observed flare in order to find further hints on the physics of this phenomenon and how it is related to the magnetic reconnection process in solar flares. Methods. RHESSI X-ray source motions in the 2003 November 3, X3.9 flare are studied together with complementary data from SXI, EIT, and Kanzelhöhe Hα. We particularly concentrate on the apparent altitude decrease of the RHESSI X-ray LT source early in the flare and combine kinematical and X-ray spectral analysis. Furthermore, we present simulations from a magnetic collapsing trap model embedded in a standard 2-D magnetic reconnection model of solar flares. Results. We find that at higher photon energies the LT source is located at higher altitudes and shows higher downward velocities than at lower energies. The mean downward velocities range from 14 km s −1 in the RHESSI 10-15 keV energy band to 45 km s −1 in the 25-30 keV band. For this flare, the LT altitude decrease was also observed by the SXI instrument with a mean speed of 12 km s −1 . RHESSI spectra indicate that during the time of LT altitude decrease the emission of the LT source is thermal bremsstrahlung from a "superhot" plasma with temperatures increasing from 35 MK to 45 MK and densities of the order of 10 10 cm −3 . The temperature does not significantly increase after this early (pre-impulsive superhot LT) phase, whereas the LT densities increase to a peak value of (3-4) × 10 11 cm −3 . Conclusions. Modeling of a collapsing magnetic trap embedded in a standard 2D magnetic reconnection model can reproduce the key observational findings in case that the observed emission is thermal bremsstrahlung from the hot LT plasma. This agrees with the evaluated RHESSI spectra for this flare.Key words. Sun: flares -Sun: X-rays, gamma rays IntroductionThe expansion of flare ribbons and the growth of the flare loop system in dynamical "two-ribbon" flares belong to the most convincing observational signatures of magnetic reconnection in the solar corona. Furthermore, observations of cuspedshaped structures in images from the Soft X-ray Telescope onboard Yohkoh (Tsuneta et al. 1992;Tsuneta 1996), the existence of hard X-ray (HXR) sources above the soft X-ray flare loops (Masuda et al. 1994), and upward ejected plasmoids (Shibata et al. 1995;Ohyama & Shibata 1998) provide observational evidence that the energy release occurs above the flare loops.In the classical "CSHKP" magnetic reconnection model of two-ribbon flares (Carmichael 1964; Sturrock 1966;Hirayama 1974;Kopp & Pneuman 1976; for a review see Priest & Forbes 2002), a sheared or twisted coronal arcade containing a prominence rises slowly during the preflare phase. Then the structure rapidly erupts and the arcade field lines are stretched to form a vertical current sheet above the ma...

Abstract. Patterns analogous to the band-splitting of metric type II bursts are found in a number of type II bursts observed in the dekameter-kilometer wavelength range. A similarity of morphological and frequency-time characteristics of two emission components are indicative of a common source. Relative frequency splits span in the range ∆f/f = 0.05−0.6. At radial distances between 2 and 4 R only small splits around 0.1 can be found. In the interplanetary space the relative split on average increases with the radial distance, whereas the inferred shock velocity decreases. In three events extrapolations of the split components point to the base and the peak of the jump in the local plasma frequency caused by the associated shock passage at 1 AU. This is suggestive of the plasma radiation from the regions upstream and downstream of the shock. Adopting this interpretation, one finds that the drop of ∆f/f at 2-4 R is congruent with the Alfvén velocity maximum expected there. The split increase and the velocity decrease at larger distances can be explained as a consequence of declining Alfvén speed in the interplanetary space.

Abstract. Based on a sample of 1114 flares observed simultaneously in hard X-rays (HXR) by the BATSE instrument and in soft X-rays (SXR) by GOES, we studied several aspects of the Neupert effect and its interpretation in the frame of the electronbeam-driven evaporation model. In particular, we investigated the time differences (∆t) between the maximum of the SXR emission and the end of the HXR emission, which are expected to occur at almost the same time. Furthermore, we performed a detailed analysis of the SXR peak flux -HXR fluence relationship for the complete set of events, as well as separately for subsets of events which are likely compatible/incompatible with the timing expectations of the Neupert effect. The distribution of the time differences reveals a pronounced peak at ∆t = 0. About half of the events show a timing behavior which can be considered to be consistent with the expectations from the Neupert effect. For these events, a high correlation between the SXR peak flux and the HXR fluence is obtained, indicative of electron-beam-driven evaporation. However, there is also a significant fraction of flares (about one fourth), which show strong deviations from ∆t = 0, with a prolonged increase of the SXR emission distinctly beyond the end of the HXR emission. These results suggest that electron-beam-driven evaporation plays an important role in solar flares. Yet, in a significant fraction of events, there is also clear evidence for the presence of an additional energy transport mechanism other than nonthermal electron beams, where the relative contribution is found to vary with the flare importance.

Abstract. We analyse properties of 58 type II radio bursts recorded in the meter-to-kilometer wavelength range, focusing on episodes of band-split emission. The basic two parameters utilized are the frequency drift D f = d f /dt and the relative band-split BDW = ∆ f / f of type II burst emission lanes. On average, in the meter-to-kilometer wavelength range D f increases with the emission frequency as D f ∝ f 1.83 , revealing that source velocities are smaller at larger heliocentric distances. The relative bandsplit shows a weak but statistically significant dependence on the emission frequency, BDW ∝ f −0.06 , indicating an increase of BDW with the heliocentric distance. Combining the shock velocity estimated from the frequency drift, with the Mach number inferred from the band-split, the Alfvén speed and the magnetic field in the ambient plasma can be estimated as a function of the heliocentric distance r. However, the outcome directly depends on the coronal/interplanetary density model used, which is poorly known in the upper corona and the near-Sun interplanetary space. So, we invert the problem: utilizing the results of the previous paper where it was shown that beyond the heliocentric distance of two solar radii (r/r = R > 2) the average magnetic field decreases approximately as B ∝ R −2 , we infer the density n(R) in the upper corona and near-Sun interplanetary space. The obtained empirical dependence n(R) is presented in the analytical form as a four-degree polynomial of 1/R, and is compared with some theoretical n(R) models, considering also a deviation from the B ∝ 1/R 2 scaling used. The model matches the five-fold Saito density model (representing the active region corona) with the n ∝ R −2 regime in the interplanetary space. Furthermore, it is shown that on average the magnetosonic speed attains a local minimum of v ms ≈ 400 km s −1 around R = 3 and a broad local maximum of v ms ≈ 500 km s −1 in the range R = 4−6, beyond which it gradually decreases to several tens km s −1 at 1 a.u. The local minimum becomes even deeper if the super-radial expansion of the magnetic field is taken into account. The implications regarding the formation and evolution of shocks in the corona and upper corona are discussed in the framework of CME-piston and flare-blast scenarios. The inferred general decrease of type II burst source velocities and broadening of band-splits with distance is interpreted in terms of the deceleration of mass ejections driving the shocks in the decreasing v ms environment.

The large flare/CME event that occurred close to the west solar limb on 3 November 2003 launched a large-amplitude large-scale coronal wave that was observed in Hα and Fe xii 195 Å spectral lines, as well as in the soft X-ray and radio wavelength ranges. The wave also excited a complex decimeter-to-hectometer type II radio burst, revealing the formation of coronal shock(s). The back-extrapolation of the motion of coronal wave signatures and the type II burst sources distinctly marks the impulsive phase of the flare (the hard X-ray peak, drifting microwave burst, and the highest type III burst activity), favoring a flare-ignited wave scenario. On the other hand, comparison of the kinematics of the CME expansion with the propagation of the optical wave signatures and type II burst sources shows a severe discrepancy in the CME-driven scenario. However, the CME is quite likely associated with the formation of an upper-coronal shock revealed by the decameter-hectometer type II burst. Finally, some six minutes after the launch of the first coronal wave, another coronal disturbance was launched, exciting an independent (weak) decimeter-meter range type II burst. The back-extrapolation of this radio emission marks the revival of the hard X-ray burst, and since there was no CME counterpart, it was clearly ignited by the new energy release in the flare. Moreton waves and type II bursts are very closely related phenomena (e.g., Harvey et al. 1974;Klassen et al. 2000;Khan & Aurass 2002;Warmuth et al. 2004b), indicating the common nature of the underlying disturbance. The physical background of the relationship was explained by Uchida (1974): the coronal fast-mode MHD shock that propagates along "valleys of low Alfven velocity" excites type II bursts in the corona, whereas the Moreton wave is a "surface track" of the shock front propagation (cf., Uchida 1974, and references therein).Generally, the coronal large-amplitude MHD disturbance could be generated by flares as well as by CMEs, and quite likely, both types of processes happen . The excellent timing association of Moreton waves and type II bursts with the impulsive phase of associated flares (e.g., Harvey 1965;Švestka & Fritzova-Švestkova 1974;Vršnak et al. 1995;Vršnak 2001;Klassen et al. 1999Klassen et al. , 2003) strongly suggests that the waves are ignited by flares. This is also supported by a number of relatively well-defined correlations between various wave characteristics and the flare energy release characteristics (e.g.,Article published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx