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Plasma diagnostics of coronal dimming events  

Astrid Veronig   Submitted: 2018-02-27 04:58

Coronal mass ejections (CMEs) are often associated with coronal dimmings, i.e. transient dark regions that are most distinctly observed in Extreme Ultra-violet (EUV) wavelengths. Using Atmospheric Imaging Assembly (AIA) data, we apply Differential Emission Measure (DEM) diagnostics to study the plasma characteristics of six coronal dimming events. In the core dimming region, we find a steep and impulsive decrease of density with values up to 50-70%. Five of the events also reveal an associated drop in temperature of 5-25%. The secondary dimming regions also show a distinct decrease in density, but less strong, decreasing by 10-45%. In both the core and the secondary dimming the density changes are much larger than the temperature changes, confirming that the dimming regions are mainly caused by plasma evacuation. In the core dimming, the plasma density reduces rapidly within the first 20-30 min after the flare start, and does not recover for at least 10 hrs later, whereas the secondary dimming tends to be more gradual and starts to replenish after 1-2 hrs. The pre-event temperatures are higher in the core dimming (1.7-2.6 MK) than in the secondary dimming regions (1.6-2.0 MK). Both core and secondary dimmings are best observed in the AIA 211 Å and 193 Å filters. These findings suggest that the core dimming corresponds to the footpoints of the erupting flux rope rooted in the AR, while the secondary dimming represents plasma from overlying coronal structures that expand during the CME eruption.

Authors: Kamalam Vanninathan, Astrid M. Veronig, Karin Dissauer, Manuela Temmer
Projects: SDO-AIA

Publication Status: Astrophysical Journal, in press
Last Modified: 2018-02-28 13:59
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Reconnection fluxes in eruptive and confined flares and implications for superflares on the Sun  

Astrid Veronig   Submitted: 2017-12-14 08:36

We study the energy release process of a set of 51 flares (32 confined, 19 eruptive) ranging from GOES class B3 to X17. We use Hα filtergrams from Kanzelhöhe Observatory together with SDO HMI and SOHO MDI magnetograms to derive magnetic reconnection fluxes and rates. The flare reconnection flux is strongly correlated with the peak of the GOES 1-8 Å soft X-ray flux (c=0.92, in log-log space), both for confined and eruptive flares. Confined flares of a certain GOES class exhibit smaller ribbon areas but larger magnetic flux densities in the flare ribbons (by a factor of 2). In the largest events, up to 50% of the magnetic flux of the active region (AR) causing the flare is involved in the flare magnetic reconnection. These findings allow us to extrapolate toward the largest solar flares possible. A complex solar AR hosting a magnetic flux of 2 · 1023  Mx, which is in line with the largest AR fluxes directly measured, is capable of producing an X80 flare, which corresponds to a bolometric energy of about 7 · 1032 ergs. Using a magnetic flux estimate of 6 · 1023  Mx for the largest solar AR observed, we find that flares of GOES class ≈X500 could be produced (Ebol ≈ 3 · 1033 ergs). These estimates suggest that the present day's Sun is capable of producing flares and related space weather events that may be more than an order of magnitude stronger than have been observed to date.

Authors: J. Tschernitz, A.M. Veronig, J. Thalmann, J. Hinterreiter, W. Pötzi
Projects: None

Publication Status: Accepted for publication in ApJ
Last Modified: 2017-12-14 13:38
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Ground-based Observations of the Solar Sources of Space Weather (Invited Review)  

Astrid Veronig   Submitted: 2016-02-09 13:33

Monitoring of the Sun and its activity is a task of growing importance in the frame of space weather research and awareness. Major space weather disturbances at Earth have their origin in energetic outbursts from the Sun: solar flares, coronal mass ejections and associated solar energetic particles. In this review we discuss the importance and complementarity of ground-based and space-based observations for space weather studies. The main focus is drawn on ground-based observations in the visible range of the spectrum, in particular in the diagnostically manifold Hα spectral line, which enables us to detect and study solar flares, filaments (prominences), filament (prominence) eruptions, and Moreton waves. Existing Hα networks such as the GONG and the Global High-Resolution Hα Network are discussed. As an example of solar observations from space weather research to operations, we present the system of real-time detection of Hα flares and filaments established at Kanzelhöhe Observatory (KSO; Austria) in the frame of the space weather segment of the ESA Space Situational Awareness programme (swe.ssa.esa.int). An evaluation of the system, which is continuously running since July 2013 is provided, covering an evaluation period of almost 2.5 years. During this period, KSO provided 3020 hours of real-time Hα observations at the ESA SWE portal. In total, 824 Hα flares were detected and classified by the real-time detection system, including 174 events of Hα importance class 1 and larger. For the total sample of events, 95% of the automatically determined flare peak times lie within ± 5 min of the values given in the official optical flares reports (by NOAA and KSO), and 76% of the start times. The heliographic positions determined are better than ± 5 degrees. The probability of detection of flares of importance 1 or larger is 95%, with a false alarm rate of 16%. These numbers confirm the high potential of automatic flare detection and alerting from ground-based observatories.

Authors: Astrid M. Veronig, W. Poetzi
Projects: None

Publication Status: Accepted for "Ground-based Solar Observations in the Space Instrumentation Era", Proceedings of the Coimbra Solar Physics Meeting 2015, ASP Conference Series, Eds. I. Dorotovic, C. Fischer, and M. Temmer
Last Modified: 2016-02-09 15:00
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Magnetic Reconnection Rates and Energy Release in a Confined X-class Flare  

Astrid Veronig   Submitted: 2015-09-23 23:55

We study the energy-release process in the confined X1.6 flare that occurred on 22 October 2014 in AR 12192. Magnetic-reconnection rates and reconnection fluxes are derived from three different data sets: space-based data from the Atmospheric Imaging Assembly (AIA) 1600Å filter onboard the Solar Dynamics Observatory (SDO) and ground-based Hα and Ca II K filtergrams from Kanzelhöhe Observatory. The magnetic-reconnection rates determined from the three data sets all closely resemble the temporal profile of the hard X-rays measured by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI), which are a proxy for the flare energy released into high-energy electrons. The total magnetic-reconnection flux derived lies between 4.1x1021 Mx (AIA 1600Å) and 7.9x1021 Mx (Hα), which corresponds to about 2 to 4% of the total unsigned flux of the strong source AR. Comparison of the magnetic-reconnection flux dependence on the GOES class for 27 eruptive events collected from previous studies (covering B to >X10 class flares) reveals a correlation coefficient of ~0.8 in double-logarithmic space. The confined X1.6 class flare under study lies well within the distribution of the eruptive flares. The event shows a large initial separation of the flare ribbons and no separation motion during the flare. In addition, we note enhanced emission at flare-ribbon structures and hot loops connecting these structures before the event starts. These observations are consistent with the emerging-flux model, where newly emerging small flux tubes reconnect with pre-existing large coronal loops.

Authors: Astrid M. Veronig, Wolfgang Polanec
Projects: RHESSI,SDO-AIA

Publication Status: Solar Physics (in press)
Last Modified: 2015-09-26 14:57
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Relation between the CME acceleration and the non-thermal flare characteristics  

Astrid Veronig   Submitted: 2012-05-14 04:23

We investigate the relationship between the main acceleration phase of coronal mass ejctions (CMEs) and the particle acceleration in the associated flares as evidenced in RHESSI non-thermal X-rays for a set of 37 impulsive flare-CME events. Both CME peak velocity and peak acceleration yield distinct correlations with various parameters characterizing the flare-accelerated electron spectra. The highest correlation coefficient is obtained for the relation of the CME peak velocity and the total energy in accelerated electrons (c = 0.85), supporting the idea that the acceleration of the CME and the particle acceleration in the associated flare draw their energy from a common source, probably magnetic reconnection in the current sheet behind the erupting structure. In general, the CME peak velocity shows somewhat higher correlations with the non-thermal flare parameters than the CME peak acceleration, except for the spectral index of the accelerated electron spectrum which yields a higher correlation with the CME peak acceleration (c = -0.6), indicating that the hardness of the flare-accelerated electron spectrum is tightly coupled to the impulsive acceleration process of the rising CME structure. We also obtained high correlations between the CME initiation height h0 and the non-thermal flare parameters, with the highest correlation of h0 to the spectral index of flare-accelerated electrons (c = 0.8). This means that CMEs erupting at low coronal heights, i.e. in regions of stronger magnetic fields, are accompanied with flares which are more efficient to accelerate electrons to high energies. In the majority of events (~80%), the non-thermal flare emission starts after the CME acceleration, on average delayed by ~6 min, in line with the standard flare model, where the rising flux rope stretches the field lines underneath until magnetic reconnection sets in. We find that the current sheet length at the onset of magnetic reconnection is 21 ± 7 Mm. The flare HXR peaks are well synchronized with the peak of the CME acceleration profile, in 75% of the cases they occur within 5 min. Our findings provide strong evidence for the tight coupling between the CME dynamics and the particle acceleration in the associated flare in impulsive events, with the total energy in accelerated electrons being closely correlated to the peak velocity (and thus the kinetic energy) of the CME, whereas the number of electrons acclerated to high energies is decisively related to the CME peak acceleration and the height of the pre-eruptive structure.

Authors: S. Berkebile-Stoiser, A.M. Veronig, B. Bein, M. Temmer
Projects: RHESSI,STEREO

Publication Status: ApJ, in press
Last Modified: 2012-05-14 12:11
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Plasma diagnostics of an EIT wave observed by Hinode/EIS and SDO/AIA  

Astrid Veronig   Submitted: 2011-11-16 01:46

We present plasma diagnostics of an EIT wave observed with high cadence inHinode/EIS sit-and-stare spectroscopy and SDO/AIA imagery obtained during theHOP-180 observing campaign on 2011 February 16. At the propagating EIT wavefront, we observe downward plasma flows in the EIS Fe XII, Fe XIII, and Fe XVIspectral lines (log T ~ 6.1-6.4) with line-of-sight (LOS) velocities up to 20km s-1. These red-shifts are followed by blue-shifts with upward velocities up to-5 km s-1 indicating relaxation of the plasma behind the wave front. During thewave evolution, the downward velocity pulse steepens from a few km s-1 up to 20km s-1 and subsequently decays, correlated with the relative changes of the lineintensities. The expected increase of the plasma densities at the EIT wavefront estimated from the observed intensity increase lies within the noiselevel of our density diagnostics from EIS XIII 202/203 Å line ratios. Nosignificant LOS plasma motions are observed in the He II line, suggesting thatthe wave pulse was not strong enough to perturb the underlying chromosphere.This is consistent with the finding that no Hα Moreton wave was associatedwith the event. The EIT wave propagating along the EIS slit reveals a strongdeceleration of a ~ -540 m/s2 and a start velocity of v0 ~ 590 km s-1. Thesefindings are consistent with the passage of a coronal fast-mode MHD wave,pushing the plasma downward and compressing it at the coronal base.

Authors: A.M. Veronig, P. G?m?ry, I.W. Kienreich, N. Muhr, B. Vrsnak, M. Temmer, H.P. Warren
Projects: Hinode/EIS,SDO-AIA

Publication Status: ApJ Lett., in press
Last Modified: 2011-11-16 13:19
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Case Study of Four Homologous Large-scale Coronal Waves Observed on 2010 April 28 and 29  

Astrid Veronig   Submitted: 2011-01-26 09:12

On 2010 April 28 and 29, the Solar TErrestrial Relations Observatory B/Extreme Ultraviolet Imager observed four homologous large-scale coronal waves, the so-called EIT-waves, within 8 hr. All waves emerged from the same source active region, were accompanied by weak flares and faint coronal mass ejections, and propagated into the same direction at constant velocities in the range of ~220-340 km s-1. The last of these four coronal wave events was the strongest and fastest, with a velocity of 337 ± 31 km s-1 and a peak perturbation amplitude of ~1.24, corresponding to a magnetosonic Mach number of Mms ~ 1.09. The magnetosonic Mach numbers and velocities of the four waves are distinctly correlated, suggestive of the nonlinear fast-mode magnetosonic wave nature of the events. We also found a correlation between the magnetic energy buildup times and the velocity and magnetosonic Mach number.

Authors: Kienreich, I. W.; Veronig, A. M.; Muhr, N.; Temmer, M.; Vršnak, B.; Nitta, N.
Projects: STEREO

Publication Status: ApJ Letts. 727, L43 (2011)
Last Modified: 2011-01-26 12:04
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Multiwavelength imaging and spectroscopy of chromospheric evaporation in an M-class solar flare  

Astrid Veronig   Submitted: 2010-07-08 01:32

We study spectroscopic observations of chromospheric evaporation mass flows in comparison to the energy input by electron beams derived from hard X-ray data for the white-light M2.5 flare of 2006 July 6. The event was captured in high cadence spectroscopic observing mode by SOHO/CDS combined with high-cadence imaging at various wavelengths in the visible, EUV and X-ray domain during the joint observing campaign JOP171. During the flare peak, we observe downflows in the He I and O v lines formed in the chromosphere and transition region, respectively, and simultaneous upflows in the hot coronal Si XII line. The energy deposition rate by electron beams derived from RHESSI hard X-ray observations is suggestive of explosive chromospheric evaporation, consistent with the observed plasma motions. However, for a later distinct X-ray burst, where the site of the strongest energy deposition is exactly located on the CDS slit, the situation is intriguing. The O v transition region line spectra show the evolution of double components, indicative of the superposition of a stationary plasma volume and upflowing plasma elements with high velocities (up to 280 km s-1) in single CDS pixels on the flare ribbon. However, the energy input by electrons during this period is too small to drive explosive chromospheric evaporation. These unexpected findings indicate that the flaring transition region is much more dynamic, complex, and fine-structured than is captured in single-loop hydrodynamic simulations.

Authors: A.M. Veronig, J. Rybak, P. G?m?ry, S. Berkebile-Stoiser, M. Temmer, W. Otruba, B. Vrsnak, W. Pötzi, D. Baumgartner
Projects: RHESSI,SoHO-CDS

Publication Status: Astrophysical Journal (2010, in press)
Last Modified: 2010-07-09 19:22
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First observations of a dome-shaped large-scale coronal EUV wave  

Astrid Veronig   Submitted: 2010-05-26 06:35

We present first observations of a dome-shaped large-scale extreme-ultraviolet coronal wave, recorded by the Extreme Ultraviolet Imager instrument on board STEREO-B on 2010 January 17. The main arguments that the observed structure is the wave dome (and not the coronal mass ejection, CME) are (1) the spherical form and sharpness of the dome?s outer edge and the erupting CME loops observed inside the dome; (2) the low-coronal wave signatures above the limb perfectly connecting to the on-disk signatures of the wave; (3) the lateral extent of the expanding dome which is much larger than that of the coronal dimming; (4) the associated high-frequency type II burst indicating shock formation low in the corona. The velocity of the upward expansion of the wave dome (v ~ 650 km s-1) is larger than that of the lateral expansion of the wave (v ~ 280 km s-1), indicating that the upward dome expansion is driven all the time, and thus depends on the CME speed, whereas in the lateral direction it is freely propagating after the CME lateral expansion stops. We also examine the evolution of the perturbation characteristics: first the perturbation profile steepens and the amplitude increases. Thereafter, the amplitude decreases with r propto -2.5?0.3, the width broadens, and the integral below the perturbation remains constant. Our findings are consistent with the spherical expansion and decay of a weakly shocked fast-mode MHD wave.

Authors: A. M. Veronig, N. Muhr, I.W. Kienreich, M. Temmer, B. Vrsnak
Projects: STEREO

Publication Status: Astrophys. J. Letts. 716, L57 (2010)
Last Modified: 2010-05-26 07:17
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Large amplitude oscillatory motion along a solar filament  

Astrid Veronig   Submitted: 2007-07-16 02:43

Large amplitude oscillations of solar filaments is a phenomenon known for more than half a century. Recently, a new mode of oscillations, characterized by periodical plasma motions along the filament axis, was discovered. We analyze such an event, recorded on 23 January 2002 in Big Bear Solar Observatory Hα filtergrams, in order to infer the triggering mechanism and the nature of the restoring force. Motion along the filament axis of a distinct buldge-like feature was traced, to quantify the kinematics of the oscillatory motion. The data were fitted by a damped sine function, to estimate the basic parameters of the oscillations. In order to identify the triggering mechanism, morphological changes in the vicinity of the filament were analyzed. The observed oscillations of the plasma along the filament was characterized by an initial displacement of 24 Mm, initial velocity amplitude of 51 km s-1, period of 50 min, and damping time of 115 min. We interpret the trigger in terms of poloidal magnetic flux injection by magnetic reconnection at one of the filament legs. The restoring force is caused by the magnetic pressure gradient along the filament axis. The period of oscillations, derived from the linearized equation of motion (harmonic oscillator) can be expressed as P=pisqrt{2}L/vAphiapprox4.4L/vAphi, where vAphi =Bphi0/sqrt{mu_0 ho} represents the Alfvén speed based on the equilibrium poloidal field Bphi0. Combination of our measurements with some previous observations of the same kind of oscillations shows a good agreement with the proposed interpretation.

Authors: B. Vrsnak, A.M. Veronig, J.K. Thalmann, T. Zic
Projects: None

Publication Status: Astron. Astrophys. (in press)
Last Modified: 2007-07-16 12:12
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Interaction of a Moreton/EIT wave and a coronal hole  

Astrid Veronig   Submitted: 2006-04-28 10:14

We report high-cadence Hα observations of a distinct Moreton wave observed at Kanzelhoehe Solar Observatory associated with the 3B/X3.8 flare and CME event of 2005 January 17. The Moreton wave can be identified in about 40 Hα frames over a period of 7 min. The EIT wave is observed in only one frame but the derived propagation distance is close to that of the simultaneously measured Moreton wave fronts indicating that they are closely associated phenomena. The large angular extent of the Moreton wave allows us to study the wave kinematics in different propagation directions with respect to the location of a polar coronal hole (CH). In particular we find that the wave segment whose propagation direction is perpendicular to the CH boundary (``frontal encounter'') is stopped by the CH which is in accordance with observations reported from EIT waves (Thompson et al. 1998). However, we also find that at a tongue-shaped edge of the coronal hole, where the front orientation is perpendicular to the CH boundary (the wave ``slides along'' the boundary), the wave signatures can be found up to 100 Mm inside the CH. These findings are briefly discussed in the frame of recent modeling results.

Authors: Astrid M. Veronig, Manuela Temmer, Bojan Vrsnak, Julia Thalmann
Projects: None

Publication Status: ApJ (accepted)
Last Modified: 2006-05-02 16:22
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X-ray sources and magnetic reconnection in the X3.9 flare of 2003 November 3  

Astrid Veronig   Submitted: 2005-09-30 06:16

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. 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. 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. 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 1010 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)cdot 1011 cm-3.} 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 is in accordance with the evaluated RHESSI spectra for this flare.

Authors: A.M. Veronig, M. Karlický, B. Vrsnak, M. Temmer, J. Magdalenic, B.R. Dennis, W. Otruba, W. Poetzi
Projects: RHESSI

Publication Status: A&A (accepted)
Last Modified: 2005-09-30 06:16
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Multi-wavelength study of coronal waves associated with the CME-flare event of 03 November 2003  

Astrid Veronig   Submitted: 2005-09-30 03:26

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 which was imaged in Hα and Fe XII 195 A spectral lines, as well as in the soft X-ray and radio wavelength range. The wave excited also 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 the flare-ignited wave scenario. On the other hand, the 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 with 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.

Authors: B. Vrsnak, A. Warmuth, M. Temmer, A. Veronig, J. Magdalenic, A. Hillaris, M. Karlický
Projects: RHESSI,Soho-EIT,Soho-MDI,Soho-LASCO

Publication Status: A&A (in press)
Last Modified: 2005-09-30 03:26
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Broadband Metric-Range Radio Emission Associated with a Moreton/EIT Wave  

Astrid Veronig   Submitted: 2005-05-10 07:16

We present the evolution and kinematics of a broadband radio source that propagated collaterally with an Hα/EIT wave, linking it with the type II burst that was excited higher up in the corona. The NRH wave emission extended from the frequency f~327 to f<151 MHz and was considerably weaker than the flare-related type IV burst. The emission centroid propagated at a height of 0-200 Mm above the solar limb and was intensified when the disturbance passed over enhanced coronal structures. We put forward the ad hoc hypothesis that the NRH wave signature is optically thin gyrosynchrotron emission excited by the passage of the coronal MHD fast-mode shock. The identification of radio emission associated with the coronal wave front is important since it offers us new diagnostic information that could provide us with better insight into the physical conditions in the disturbance itself.

Authors: Vr┼ínak, B., Magdalenić, J., Temmer, M., Veronig, A., Warmuth, A., Mann, G., Aurass, H., Otruba, W.
Projects: None

Publication Status: ApJ 625, L67 (2005)
Last Modified: 2005-05-10 07:16
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A coronal thick target interpretation of two HXR loop events  

Astrid Veronig   Submitted: 2004-01-29 04:24

We report a new class of solar flare hard X-ray (HXR) source where emission is mainly in a coronal loop so dense as to be collisionally thick at electron energies up to gtrsim50 keV. In most events previously reported most of emission is at the dense loop footpoints though sometimes with a faint high altitude component. HXR RHESSI data on loop dimensions and nonthermal electron parameters and GOES soft X-ray (SXR) data on hot loop plasma parameters are used to model coronal thick target physics for two `discovery' events (14 April 2002, 23:56 UT 15 April 2002, 23:05 UT). We show that: loop column densities N are consistent with (a) a nonthermal coronal thick target interpretation of the HXR image and spectrum; (b) chromospheric evaporation by thermal conduction from the hot loop rather than by electron beam heating; and (c) the hot loop temperature being due to balance of thick target collisional heating and (mainly) conductive cooling

Authors: A.M. Veronig and J.C. Brown
Projects: RHESSI

Publication Status: ApJL (accepted)
Last Modified: 2004-02-26 18:18
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A coronal thick target interpretation of two HXR loop events  

Astrid Veronig   Submitted: 2004-01-29 04:24

We report a new class of solar flare hard X-ray (HXR) source where emission is mainly in a coronal loop so dense as to be collisionally thick at electron energies up to gtrsim50 keV. In most events previously reported most of emission is at the dense loop footpoints though sometimes with a faint high altitude component. HXR RHESSI data on loop dimensions and nonthermal electron parameters and GOES soft X-ray (SXR) data on hot loop plasma parameters are used to model coronal thick target physics for two `discovery' events (14 April 2002, 23:56 UT 15 April 2002, 23:05 UT). We show that: loop column densities N are consistent with (a) a nonthermal coronal thick target interpretation of the HXR image and spectrum; (b) chromospheric evaporation by thermal conduction from the hot loop rather than by electron beam heating; and (c) the hot loop temperature being due to balance of thick target collisional heating and (mainly) conductive cooling.

Authors: A.M. Veronig and J.C. Brown
Projects: RHESSI

Publication Status: ApJ 603, L117 (2004).
Last Modified: 2004-11-19 06:47
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Relative timing of solar flares observed at different wavelengths  

Astrid Veronig   Submitted: 2002-09-23 04:49

The timing of 503 solar flares observed simultaneously in hard X-rays, soft X-rays and Hα is analyzed. We investigated the start and the peak time differences in different wavelengths, as well as the differences between the end of the hard X-ray emission and the maximum of the soft X-ray and Hα emission. In more than 90% of the analyzed events, a thermal preheating seen in soft X-rays is present prior to the impulsive flare phase. On average, the soft X-ray emission starts 3 min before the hard X-ray and the Hα emission. No correlation between the duration of the preheating phase and the importance of the subsequent flare is found. Furthermore, the duration of the preheating phase does not differ for impulsive and gradual flares. For at least half of the events, the end of the nonthermal emission coincides well with the maximum of the thermal emission, consistent with the beam-driven evaporation model. On the other hand, for about 25% of the events there is strong evidence for prolonged evaporation beyond the end of the hard X-rays. For these events, the presence of an additional energy transport mechanism, most probably thermal conduction, seems to play an important role.

Authors: A. Veronig, B. Vrsnak, M. Temmer, A. Hanslmeier
Projects:

Publication Status: To appear in Solar Physics 208, 295-313 (September 2002)
Last Modified: 2002-09-23 04:50
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Investigation of the Neupert effect in solar flares. I. Statistical properties and the evaporation model  

Astrid Veronig   Submitted: 2002-09-23 04:42

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 electron-beam-driven evaporation model. In particular, we investigated the time differences (Delta 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 Delta 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 Delta 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.

Authors: A. Veronig, B. Vrsnak, B. R. Dennis, M. Temmer, A. Hanslmeier, and J. Magdalenic
Projects:

Publication Status: A&A 392, 699-712 (2002)
Last Modified: 2002-09-23 04:43
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Abstracts by Author
Plasma diagnostics of coronal dimming events
Reconnection fluxes in eruptive and confined flares and implications for superflares on the Sun
Ground-based Observations of the Solar Sources of Space Weather (Invited Review)
Magnetic Reconnection Rates and Energy Release in a Confined X-class Flare
Relation between the CME acceleration and the non-thermal flare characteristics
Plasma diagnostics of an EIT wave observed by Hinode/EIS and SDO/AIA
Case Study of Four Homologous Large-scale Coronal Waves Observed on 2010 April 28 and 29
Multiwavelength imaging and spectroscopy of chromospheric evaporation in an M-class solar flare
First observations of a dome-shaped large-scale coronal EUV wave
Large amplitude oscillatory motion along a solar filament
Interaction of a Moreton/EIT wave and a coronal hole
X-ray sources and magnetic reconnection in the X3.9 flare of 2003 November 3
Multi-wavelength study of coronal waves associated with the CME-flare event of 03 November 2003
Broadband Metric-Range Radio Emission Associated with a Moreton/EIT Wave
A coronal thick target interpretation of two HXR loop events
A coronal thick target interpretation of two HXR loop events
Relative timing of solar flares observed at different wavelengths
Investigation of the Neupert effect in solar flares. I. Statistical properties and the evaporation model

Related Pages
MSU Solar Physics.
Max Millennium Science Mail Archive.
Max Millennium Message of the Day Mail Archive.
Max Millennium Flare Catalog

Archive Maintainer
Alisdair Davey



© 2003 Solar Physics Group - Montana State University