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Microwave observations of a large-scale coronal wave with the Nobeyama radioheliograph  

Alexander Warmuth   Submitted: 2016-10-13 01:08

Context. Large-scale globally propagating waves in the solar corona have been studied extensively, mainly using extreme ultraviolet (EUV) observations. In a few events, corresponding wave signatures have been detected in microwave radioheliograms provided by the Nobeyama radioheliograph (NoRH). Several aspects of these observations seem to contradict the conclusions drawn from EUV observations. Aims. We investigate whether the microwave observations of global waves are consistent with previous findings. Methods. We revisited the wave of 1997 Sep 24, which is still the best-defined event in microwaves. We obtained radioheliograms at 17 and 34 GHz from NoRH and studied the morphology, kinematics, perturbation profile evolution, and emission mechanism of the propagating microwave signatures. Results. We find that the NoRH wave signatures are morphologically consistent with both the associated coronal wave as observed by SOHO/EIT and the Moreton wave seen in Hα . The NoRH wave is clearly decelerating, which is typically found for large-amplitude coronal waves associated with Moreton waves, and its kinematical curve is consistent with the EIT wavefronts. The perturbation profile shows a pronounced decrease in amplitude. Based on the derivation of the spectral index of the excess microwave emission, we conclude that the NoRH wave is due to optically thick free-free bremsstrahlung from the chromosphere. Conclusions. The wavefronts seen in microwave radioheliograms are chromospheric signatures of coronal waves, and their characteristics support the interpretation of coronal waves as large-amplitude fast-mode MHD waves or shocks.

Authors: Warmuth, A., Shibasaki, K., Iwai, K., & Mann, G.
Projects: Nobeyama Radioheliograph,SoHO-EIT

Publication Status: Astronomy & Astrophysics (2016), Volume 593, id. A102, 8 pp.
Last Modified: 2016-10-14 09:18
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Constraints on energy release in solar flares from RHESSI and GOES X-ray observations. II. Energetics and energy partition  

Alexander Warmuth   Submitted: 2016-04-20 09:43

Aims: We derive constraints on energy release, transport and conversion processes in solar flares based on a detailed characterization of the physical parameters of both the thermal plasma and the accelerated nonthermal electrons based on X-ray observations. In particular, we address the questions of whether the energy required to heat the thermal plasma can be supplied by nonthermal particles, and how the energetics derived from X-rays compare to the total bolometric radiated energy. Methods: Time series of spectral fits and images for 24 flares ranging from GOES class C3.4 to X17.2 were obtained using RHESSI hard X-ray observations. This has been supplemented by GOES soft X-ray fluxes. In our companion Paper I, we have used this data set to obtain the basic physical parameters for the thermal plasma (using the isothermal approximation) and the injected energetic electrons (assuming the thick-target model). Here, we used this data set to derive the flare energetics, including thermal energy, radiative and conductive energy loss, gravitational and flow energy of the plasma, and kinetic energy of the injected electrons. We studied how the thermal energies compare to the energy in nonthermal electrons, and how the various energetics and energy partition depend on flare importance. Results: All flare energetics show a good to excellent correlation with the peak GOES flux. The gravitational energy of the evaporated plasma and the kinetic energy of plasma flows can be neglected in the discussion of flare energetics. The radiative energy losses are comparable to the maximum thermal energy, while the conductive losses are considerably higher than the maximum thermal energy, especially in weaker flares. The total heating requirement of the hot plasma amounts to ~50% of the total bolometric energy loss, with the conductive losses as a major contribution. The nonthermal energy input by energetic electrons is not sufficient to account for the total heating requirements of the hot plasma or for the bolometric losses, in particular in weak flares. Conclusions: Our results support the standard model of solar flares, with the following modifications. (1) Heating the hot thermal plasma and supplying the bolometric radiated energy requires an additional non-beam heating mechanism. (2) Strong conductive losses are a necessary additional energy transport process that transfers the energy released in the corona to the lower (and denser) atmospheric layers, where the bulk of the released energy is efficiently radiated away at longer wavelengths (EUV, UV, and white light) by cooler material.

Authors: Warmuth, A., Mann, G.
Projects: RHESSI

Publication Status: A&A 588, A116 (2016)
Last Modified: 2016-04-20 13:21
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Constraints on energy release in solar flares from RHESSI and GOES X-ray observations I. Physical parameters and scalings  

Alexander Warmuth   Submitted: 2016-04-20 09:39

Aims. We constrain energy release and particle acceleration processes in solar flares by means of comprehensively characterizing the physical parameters of both the thermal plasma and the accelerated nonthermal particles using X-ray data. Our aim is to bridge the gap between detailed case studies and large statistical studies. Methods. We obtained time series of spectral fits and images for 24 flares ranging from GOES class C3.4 to X17.2 using RHESSI hard X-ray observations. These data were used to derive basic physical parameters for the thermal plasma (using the isothermal approximation) and the injected nonthermal electrons (assuming the thick-target model). For the thermal component, this was supplemented by GOES soft X-ray data. We derived the ranges and distributions of the various parameters, the scaling with flare importance, and the relation between thermal parameters derived from RHESSI and GOES. Finally, we investigated the relation between thermal and nonthermal parameters. Results. Temperature and emission measure of the thermal plasma are strongly correlated with the peak GOES X-ray flux. Higher emission measures result both from a larger source volume and a higher density, with the latter effect being more important. RHESSI consistently gives higher temperatures and lower emission measures than GOES does, which is a signature of a multithermal plasma. The discrepancy between RHESSI and GOES is particularly pronounced in the early flare phase, when the thermal X-ray sources tend to be large and located higher in the corona. The energy input rate by nonthermal electrons is correlated with temperature and with the increase rate of emission measure and thermal energy. Conclusions. The derived relations between RHESSI- and GOES-derived thermal parameters and the relation between thermal Parameters and energy input by nonthermal electrons are consistent with a two-component model of the thermal flare plasma. Both RHESSI and GOES observe a cooler plasma component (10-25 MK) that is generated by chromospheric evaporation caused by a nonthermal electron beam. In addition, a hotter component (>25 MK) is only detected by RHESSI; this component is more consistent with direct in situ heating of coronal plasma. With the exception of the early impulsive phase, RHESSI observes a combination of the evaporated and the directly heated component.

Authors: Warmuth, A., Mann, G.
Projects: RHESSI

Publication Status: A&A 588, A115 (2016)
Last Modified: 2016-04-20 13:22
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Large-scale Globally Propagating Coronal Waves  

Alexander Warmuth   Submitted: 2015-10-01 01:01

Large-scale, globally propagating wave-like disturbances have been observed in the solar chromosphere and by inference in the corona since the 1960s. However, detailed analysis of these phenomena has only been conducted since the late 1990s. This was prompted by the availability of high-cadence coronal imaging data from numerous spaced-based instruments, which routinely show spectacular globally propagating bright fronts. Coronal waves, as these perturbations are usually referred to, have now been observed in a wide range of spectral channels, yielding a wealth of information. Many findings have supported the "classical" interpretation of the disturbances: fast-mode MHD waves or shocks that are propagating in the solar corona. However, observations that seemed inconsistent with this picture have stimulated the development of alternative models in which "pseudo waves" are generated by magnetic reconfiguration in the framework of an expanding coronal mass ejection. This has resulted in a vigorous debate on the physical nature of these disturbances. This review focuses on demonstrating how the numerous observational findings of the last one and a half decades can be used to constrain our models of large-scale coronal waves, and how a coherent physical understanding of these disturbances is finally emerging.

Authors: Alexander Warmuth
Projects: None

Publication Status: Living Reviews in Solar Physics, vol. 12, no. 3
Last Modified: 2015-10-07 12:57
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Kinematical evidence for physically different classes of large-scale coronal EUV waves  

Alexander Warmuth   Submitted: 2011-08-19 01:27

Context. Large-scale wavelike disturbances have been observed in the solar corona in the EUV range since more than a decade. The physical nature of these so-called 'EIT waves' is still being debated controversially. The two main contenders are on the one hand MHD waves and/or shocks, and on the other hand magnetic reconfiguration in the framework of an expanding CME. There is a lot of observational evidence backing either one or the other scenario, and no single model has been able to reproduce all observational constraints, which are partly even contradictory. This suggests that there may actually exist different classes of coronal waves that are caused by distinct physical processes. Then, the problems in interpreting coronal waves would be mainly caused by mixing together different physical processes.
Aims: We search for evidence for physically different classes of large-scale coronal EUV waves.
Methods: Kinematics is the most important characteristic of any moving disturbance, hence we focus on this aspect of coronal waves. Identifying distinct event classes requires a large event sample, which is up to now only available from SOHO/EIT. We analyze the kinematics of a sample of 176 EIT waves. In order to check if the results are severely affected by the low cadence of EIT, we complement this with high-cadence data for 17 events from STEREO/EUVI. In particular, we focus on the wave speeds and their evolution.
Results: Based on their kinematical behavior, we find evidence for three distinct populations of coronal EUV waves: initially fast waves (v >= 320 km s-1) that show pronounced deceleration (class 1 events), waves with moderate (v ≈ 170-320 km s-1) and nearly constant speeds (class 2), and slow waves (v <= 130 km s-1) showing a rather erratic behavior (class 3).
Conclusions: The kinematical behavior of the fast decelerating disturbances is consistent with nonlinear large-amplitude waves or shocks that propagate faster than the ambient fast-mode speed and subsequently slow down due to decreasing amplitude. The waves with moderate speeds are consistent with linear waves moving at the local fast-mode speed. Thus both populations can be explained in terms of the wave/shock model. The slow perturbations with erratic behavior, on the other hand, are not consistent with this scenario. These disturbances could well be due to magnetic reconfiguration.

Authors: Warmuth, A., Mann, G.
Projects: SoHO-EIT,STEREO

Publication Status: A&A 532, A151 (2011)
Last Modified: 2011-08-19 09:20
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Rapid changes of electron acceleration characteristics at the end of the impulsive phase of an X-class solar flare  

Alexander Warmuth   Submitted: 2009-06-11 06:44

We present a detailed spectral analysis of the the X1.3 flare of 2005 Jan 19 using hard X-ray (HXR) spectra obtained with RHESSI. This flare exhibits HXR pulses during the impulsive phase, with a particularly pronounced peak at the end of the impulsive phase. This peak is associated with HXR emission up to high energies (>300 keV) but does not show any Neupert effect (i. e. no simultaneous rise in soft X-rays). Fitting the spatially integrated photon spectra with a Maxwellian plus a nonthermal thick-target component reveals that the data are consistent with a high low-energy cutoff ( 100 keV) of the energetic electrons during the late peak. The high low-energy cutoff straightforwardly explains the lack of a Neupert effect – while highly energetic electrons are produced efficiently, there is a lack of low-energy electrons that usually contain the bulk of the total energy. Hence, the energy input into the chromosphere remains too small to trigger chromospheric evaporation. This observation shows that the characteristics of electron acceleration can change dramatically and rapidly at the end of the impulsive phase of solar flares. This could be evidence for physically distinct acceleration processes acting in the same event, or alternatively for a sudden shift in the characteristic parameters of the accelerator. Using radio observations and comparing HXR images with magnetograms, we conclude that changes in the strength and the topology of the magnetic field in which the accelerator is working are responsible for the profound changes in the injected electron spectrum.

Authors: Alexander Warmuth, Gordon D. Holman, Brian R. Dennis, Gottfried Mann, Henry Aurass, and Ryan O. Milligan
Projects: RHESSI

Publication Status: ApJ, in press
Last Modified: 2009-06-11 08:48
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Modelling shock drift acceleration of electrons at the reconnection outflow termination shock in solar flares. Observational constraints and parametric study  

Alexander Warmuth   Submitted: 2008-10-01 08:39

Context: The acceleration of electrons to nonthermal energies in solar flares is one of the main unsolved questions in solar physics. One possibility for the production of these energetic electrons is acceleration at the reconnection outflow termination shock (TS). Aims: By comparing theoretical results with observations of nonthermal electrons, we determine if shock drift acceleration (SDA) at the TS is a viable electron acceleration mechanism. Methods: We use radio observations to constrain the characteristics of the TS, and hard X-ray observations provided by RHESSI, INTEGRAL and HXRS to obtain the characteristics of the injected electrons. Invoking relativistic shock-drift acceleration at the TS, we calculate electron flux spectra, which are then compared with the corresponding observational results from RHESSI. A parametric study of the model allows us to answer the question under which conditions the TS is a viable electron accelerator. Results: SDA at the TS is able to reproduce the required fluxes and kinetic power of nonthermal electrons in solar flares as long as there is significant heating of the outflow jet and a sufficiently large shock area. A prediction of the model is that the flux and power of injected electrons is larger than the values which are usually given by fitting observed spectra, since the low-energy cutoff is generally below 10 keV. The synthetic spectra are consistent with the observed spectral indices up to ~100 keV ? beyond that, they soften too quickly. Possibly this is because we have not yet considered various additional effects, such as multiple reflections at the shock or some form of preacceleration. The observed relation between electron flux and spectral index is reproduced by the model, as well as the temporal evolution of the energetic electrons. We conclude that SDA at the TS is a viable electron acceleration mechanism which deserves further study.

Authors: A. Warmuth, G. Mann, H. Aurass
Projects: RHESSI,SoHO-EIT,TRACE

Publication Status: A&A, accepted
Last Modified: 2008-10-01 09:47
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Generation of highly energetic electrons at reconnection outflow shocks during solar flares  

Alexander Warmuth   Submitted: 2008-10-01 08:33

Context: During solar flares a large amount of energy is suddenly released and partly transfered into energetic electrons. They are of special interest since a substantial part of the energy released during a flare is deposited into the energetic electrons. RHESSI observations, e.g. of the solar event on October 28, 2003, show that 1036 electrons with energies > 20 keV are typically produced per second during large flares. They are related to a power of about 1022 W. It is a still open question in which way so much electrons are acclerated up to high energies during a fraction of a second. Aims: Within the framework of the magnetic reconnection scenario, jets appear in the outflow region and can establish standing fast-mode shocks if they penetrate with a super-Alfvénic speed into the surrounding plasma. It is the aim to show that this shock can be the source of the energetic electrons produced during flares. Methods: The electrons are regarded to be energized by shock drift acceleration. The process is necessarily treated in a fully relativistic manner. The resulting distribution function of accelerated electrons is a loss-cone one and allows to calculated the differential electron flux, which can be compared with RHESSI observations. Results. The theoretically obtained fluxes of energetic electrons agree with the observed ones as demonstrated for the solar event on October 28, 2003. Conclusions: The shock appearing in the outflow region of the reconnection site is able to produce energetic electrons. Their fluxes are in agreement with RHESSI observations.

Authors: G. Mann, A. Warmuth, H. Aurass
Projects: RHESSI,SoHO-EIT,TRACE

Publication Status: A&A, accepted
Last Modified: 2008-10-01 09:47
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Abstracts by Author
Microwave observations of a large-scale coronal wave with the Nobeyama radioheliograph
Constraints on energy release in solar flares from RHESSI and GOES X-ray observations. II. Energetics and energy partition
Constraints on energy release in solar flares from RHESSI and GOES X-ray observations I. Physical parameters and scalings
Large-scale Globally Propagating Coronal Waves
Kinematical evidence for physically different classes of large-scale coronal EUV waves
Rapid changes of electron acceleration characteristics at the end of the impulsive phase of an X-class solar flare
Modelling shock drift acceleration of electrons at the reconnection outflow termination shock in solar flares. Observational constraints and parametric study
Generation of highly energetic electrons at reconnection outflow shocks during solar flares

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