Neupert Effect Research Articles

Multiwavelength Observations of Flares on AD Leonis

We report results from a multiwavelength observing campaign conducted during 2000 March on the flare star AD Leo. Simultaneous data were obtained from several ground- and space-based observatories, including observations of eight sizable flares. We discuss the correlation of line and continuum emission in the optical and ultraviolet wavelength regimes, as well as the flare energy budget, and we find that the emission properties are remarkably similar even for flares of very different evolutionary morphology. This suggests a common heating mechanism and atmospheric structure that are independent of the detailed evolution of individual flares. We also discuss the Neupert effect, chromospheric line broadening, and velocity fields observed in several transition region emission lines. The latter show significant downflows during and shortly after the flare impulsive phase. Our observations are broadly consistent with the solar model of chromospheric evaporation and condensation following impulsive heating by a flux of nonthermal electrons. These data place strong constraints on the next generation of radiative hydrodynamic models of stellar flares.

Investigations of stellar coronae with XMM-Newton

The XMM-Newton observatory has given a strong new impetus to the investigations of stellar coronae. It carries high-resolution X-ray spectroscopic instruments that give access to information on the physical conditions in coronal plasmas. Furthermore, its imaging detectors produce high-quality X-ray light curves and images with a large field of view. I review results on stellar coronae obtained with XMM-Newton between its launch in late 1999 and the present late 2002. Among other topics, I discuss the elemental composition of stellar coronae and plasma electron densities in quiescence and during flares. I present evidence for various signatures of the Neupert effect in stars, which provides evidence of chromospheric and coronal heating by flares. Coronal structures based on eclipse mapping inferred from light curve inversion are shown.

The Neupert effect and new RHESSI measures of thetotal energy in electrons accelerated in solar flares

Abstract It is believed that a large fraction of the total energy released in a solar flare goes initially into acceleratedelectrons. These electrons generate the observed hard X-ray bremsstrahlung as they lose most of their energy by coulomb collisions in the lower corona and chromosphere. Results from the Solar Maximum Mission showed that there may be even more energy in accelerated electrons with energies above 25 keV than in the soft X-ray emitting thermal plasma. If this is the case, it is difficult to understand why the Neupert Effect — the empirical result that for many flares the time integral of the hard X-ray emission closely matches the temporal variation of the soft X-ray emission — is not more clearly observed in many flares. From recent studies, it appears that the fraction of the released energy going into accelerated electrons is lower, on average, for smaller flares than for larger flares. Also, from relative timing differences, about 25% of all flares are inconsistent with the Neupert Effect. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is uniquely capable of investigating the Neupert Effec since it covers soft X-rays down to 3 keV (when both attenuators are out of the field of view) and hard X-rays with keV energy resolution, arcsecond-class angular resolution, and sub-second time resolution. When combined with the anticipated observations from the Soft X-ray Imager on the next GOES satellite, these observations will provide us with the ability to track the Neupert Effect in space and time and learn more about the relation between plasma heating and particle acceleration. The early results from RHESSI show that the electron spectrum extends down to as low as 10 keV in many flares, thus increasing the total energy estimates of the accelerated electrons by an order of magnitude or more compared with the SMM values. This combined with the possible effects of filling factors smaller than unity for the soft X-ray plasma suggest that there is significantly more energy in nonthermal electrons than in the soft X-ray emitting plasma in many flares.

The neupert effect and new RHESSI measures of the total energy in electrons accelerated in solar flares

The neupert effect and new RHESSI measures of the total energy in electrons accelerated in solar flares

Flares From the Giant ar 9433 on 24 April 2001

We present Hα CCD observations of three small-to-medium-size two-ribbon flares observed in the giant AR 9433 on 24 April 2001. Flare observations at other associated wavelengths (e.g., soft X-rays (SXR), hard X-rays (HXR), microwaves (MW)) obtained from archives are also presented and compared. We have tested the Neupert effect for the most energetic third flare. The flare observations are in agreement with the thick-target model. In the case of this flare the HXR emitting electrons appears to be the heating source of SXR and Hα emissions. The flares are also studied in EUV and UV emissions using TRACE data. We discuss the complexity of the magnetic field using SOHO/MDI magnetograms. The flares are observed to occur in both (f/p) polarity regions in highly sheared magnetic field with emerging flux regions and MMFs.

Detection of the Neupert Effect in the Corona of an RS Canum Venaticorum Binary System byXMM‐Newtonand the Very Large Array

The RS CVn-type binary σ Geminorum was observed during a large, long-duration flare simultaneously with XMM-Newton and the Very Large Array. The light curves show a characteristic time dependence that is compatible with the Neupert effect observed in solar flares: the time derivative of the X-ray light curve resembles the radio light curve. This observation can be interpreted in terms of a standard flare scenario in which accelerated coronal electrons reach the chromosphere, where they heat the cool plasma and induce chromospheric evaporation. Such a scenario can hold only if the amount of energy in the fast electrons is sufficient to explain the X-ray radiative losses. We present a plausibility analysis that supports the chromospheric evaporation model.

Investigation of the Neupert effect in solar flares

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 () 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 . 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 , 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.

Temporal aspects and frequency distributions of solar soft X-ray flares

A statistical analysis of almost 50 000 soft X-ray (SXR) flares observed by GOES during the period 1976–2000 is presented. On the basis of this extensive data set, statistics on temporal properties of soft X-ray flares, such as duration, rise and decay times with regard to the SXR flare classes is presented. Correlations among distinct flare parameters, i.e. SXR peak flux, fluence and characteristic times, and frequency distributions of flare occurrence as function of the peak flux, the fluence and the duration are derived. We discuss the results of the analysis with respect to statistical flare models, the idea of coronal heating by nanoflares, and elaborate on implications of the obtained results on the Neupert effect in solar flares.

RHESSI and Trace Observations of the 21 April 2002 X1.5 Flare

Observations of the X1.5 flare on 21 April 2002 are reviewed using the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and the Transition Region and Coronal Explorer (TRACE). The major findings are as follows: (1) The 3–25 keV X-rays started < 4 min before the EUV (195 A) emission suggesting that the initial energy release heated plasma directly to ≳20 MK, well above the 1.6 MK needed to produce the Fexu (195 A) line. (2) Using coaligned 12–25 keV RHESSI and TRACE images, further evidence is found for the existence of hot (15–20 MK) plasma in the 195 A passband. This hot, diffuse emission is attributed to the presence of the Fe xxiv (192 A) line within the TRACE 195 A passband. (3) The 12–25 keV source centroid moves away from the limb with an apparent velocity of ~ 9.9 km s−1, slowing to ~ 1.7 km s−1 after 3 hours, its final altitude being ~ 140 Mm after ~ 12 hours. This suggests that the energy release site moves to higher altitudes in agreement with classical flare models. (4) The 50–100 keV emission correlates well with EUV flare ribbons, suggesting thick-target interactions at the footpoints of the magnetic arcade. The 50–100 keV time profile matches the time derivative of the GOES light curve (Neupert effect), which suggests that the same electrons that produced the thick-target hard X-ray emission also heat the plasma seen in soft X-rays. (5) X-ray footpoint emission has an E−3 spectrum down to ~ 10 keV suggesting a lower electron cutoff energy than previously thought. (6) The hard X-ray (25–200 keV) peaks have FWHM durations of ~ 1 min suggesting a more gradual energy release process than expected. (7) The TRACE images reveal a bright symmetric front propagating away from the main flare site at speeds of ≥ 120 km s−1. This may be associated with the fast CME observed several minutes later by LASCO. (8) Dark sinuous lanes are observed in the TRACE images that extend almost radially from the post-flare loop system. This ‘fan of spines’ becomes visible well into the decay phase of the flare and shows evidence for both lateral and downward motions.

Hard X-rays from Slow Flares

“Slow LDEs” are those for which the rise phase is slow, as well as the decay phase. Such flares follow the Neupert effect, which implies that the non-thermal energy release has a similar relationship to heating as in a normal impulsive flare. Based on a sample of 53 slow LDEs during the first nine years of Yohkoh observations, we find 19 for which substantial overlap occurs with BATSE hard X-ray observations. These events tend strongly to have extended hard X-ray emission even though their hard X-ray emission does not tend to be “impulsive” in the sense of rapid variation. The hard X-ray fluences for these 19 events correlate with the soft X-ray peak fluxes, implying strong non-thermal particle acceleration even for these relatively slow energy-release rates. These events often correspond to the occurrence of “supra-arcade downflows,” a phenomenon consistent with the classical reconnection model for gradual-phase flare energy release. This correspondence suggests a close relationship, not depending strongly upon time scale, between large-scale reconnection and the acceleration of non-thermal electrons.

Spatial Structure of Simple Spiky Bursts at Microwave/Millimeter Wavelengths

We present the results of a study of spatial structure of sources of microwave and millimeter bursts with simple spiky time profiles at 17 and 34 GHz, similar to those found to be common at 3 mm wavelength. These bursts are of short duration, with fast 2-4 s rise time to peak, followed by a rapid exponential decay. When mapped at high spatial resolution with the Nobeyama Radio Heliograph (NoRH), the radio images show direct evidence that the radio sources are compact bipolar loops: source sizes are less than 5'' and three of the five events studied show closely spaced oppositely polarized components in the circular polarization maps. All five events are located directly over magnetic neutral lines in the photosphere. The soft X-ray behavior is not entirely consistent with the Neupert effect in these events, since all five events show a rise in the soft X-ray flux well before any nonthermal electrons are present in the corona and the ratio of peak soft X-ray flux to peak radio 17 GHz flux may vary by many orders of magnitude from one event to the next. The abrupt time profiles of these events and their physical properties are consistent with a single-loop scenario in which magnetic energy release and acceleration of nonthermal electrons are confined to a compact localized region.

The Solar Flare Soft X‐Ray Differential Emission Measure and the Neupert Effect at Different Temperatures

Data from the Yohkoh Soft X-ray Telescope (SXT) and Bragg Crystal Spectrometer (BCS) are used to obtain the differential emission measure (DEM) of solar flare soft X-ray plasma. The maximum entropy method is used to obtain the DEM for 80 flares observed by SXT and BCS from 1991 October to 1992 November. We find that it is often difficult to describe the DEM using a monotonically decreasing function; functions with a hump at high temperature are occasionally more useful. From the time history of the DEM, we show that higher temperature plasma peaks earlier and cools faster. We also show that high-temperature plasma (>16.5 MK) is more likely than low-temperature plasma to exhibit the so-called Neupert effect, in which the time derivative of the soft X-ray emission measure is similar to the light curve of the impulsive hard X-ray emission for the flare.

High-Energy Aspects of Stellar Coronae

Sun-like stars are the sites of various high-energy processes, many of whichinvolve a release of coronal magnetic energy. Most of our knowledge on high-energymechanisms in stellar atmospheres (other than the Sun's) is based onobservations in the radio and soft x-ray regimes. They indicate a continuouspresence of MeV particles, heating to high (> 10 MK) plasma temperatures, and a close interrelation between particle acceleration and heating, e.g., during chromosphericevaporation as evidenced in observations of the Neupert Effect. Thepresent paper summarizes different aspects of high-energy processes in solar-typestellar coronae from recent, selected observing campaigns.

Theoretical Study of the Spatially Resolved Soft X‐Ray versus Hard X‐Ray Relationship in Solar Flares

We have carried out a series of calculations of the hard (>30 keV) and soft (1-3 keV) X-ray time histories for various parts of a solar flare loop using hydrodynamic simulations of the electron-heated flare atmosphere. We have considered two typical cases: the strongly evaporated flare in which the injected energetic electron flux is large and the initial atmospheric density is small, and the weakly evaporated flare in which the electron flux is small and the initial atmospheric density is large. The spatially unresolved Neupert effect that the time of the soft X-ray time history is proportional to the hard X-ray time history holds for these models; however, in both cases, we find that the footpoint soft X-ray time history is not proportional to the hard X-ray time history at the same location. The model cannot explain recent observations in which a correlation between the soft and hard X-ray time histories (and not a derivative relation) at the flare footpoint was observed. Furthermore, the loop-top soft X-ray source decays much earlier than indicated by observations. In order for these model predictions to be consistent with observations, the footpoint soft X-ray source may have to be nonthermal, excited by low-energy electrons. Also, continuous heating of the plasma at the top of the loop is required in addition to that from energetic electrons.

The Neupert Effect in Active Stellar Coronae: Chromospheric Evaporation and Coronal Heating in the dMe Flare Star Binary UV Ceti

Evidence for coronal heating by chromospheric evaporation in flares of active dMe stars is presented through observations of the Neupert effect in high-frequency microwaves and soft X-rays. The Neupert effect, as originally found in solar flares, manifests itself in a close similarity between the soft X-ray light curve and the time integral of the simultaneous microwave light curve. It is interpreted as the signature of the accumulation of hot plasma due to heating by accelerated electrons in the chromosphere.We used the ROSAT and ASCA soft X-ray observatories and the Very Large Array (VLA) radio telescope (at 6 cm and 3.6 cm wavelengths) to monitor simultaneously the nearby dMe flare star binary Gliese 65 A+ B = UV Ceti during 9 hours on each of two consecutive days. We find several weakly polarized radio events that start contemporaneously (within a few minutes) with X-ray flares and then peak and decay as the X-ray flares develop gradually. A striking similarity to the temporal evolution of solar gradual events is found. We argue that the Neupert effect is best observed in relatively hard bands of the soft X-ray emission, but that its presence can be inferred from the much softer bands commonly used for stellar observations by use of the solar analogy. Together with spectral hardness observations of soft X-rays, the data suggest the operation of chromospheric evaporation on UV Cet. The observations thus indicate a causal relation between the nonthermal and thermal energies of the underlying electron populations. We find that stellar flares are, relative to solar flares, X-ray-weak. The ratio between the total energy radiated into the radio and the soft X-ray bands closely matches the corresponding ratio between the quiescent luminosities of active stars, perhaps implying similar mechanisms and similar efficiencies for the quiescent emission and for larger, single flares. Estimating the total kinetic energy in the electrons from the radio flux, we find that only a part is observed in soft X-rays, a discrepancy well known from solar flares.

Comprehensive Multiwavelength Observations of the 1992 January 7 Solar Flare

Observations of a solar flare that occurred at 2022 UT on 1992 January 7, during the 1991 December/1992 January Max `91 campaign, are presented. This flare was observed simultaneously in Hα, radio (at microwave and millimeter wavelengths), and soft and hard X-rays (by the Yohkoh spacecraft) with high spatial and moderate spectral resolution. A comparison of magneto grams before and after the flare shows evidence of the emergence of new magnetic flux of opposite polarity at the flare site. Although this flare was only of moderate size (GOES classification C8.9 and Hα importance SF), it exhibited several distinct bursts and at least 10 spatially distinct hard/soft X-ray sources. Cospatial Hα brightenings suggest that most of the X-ray sources are located at footpoints of magnetic loops. Two of the hard X-ray sources have no Hα counterparts and are therefore believed to be located at loop tops. The flare consisted of three bursts of particle acceleration followed by a purely thermal phase. High spectral resolution Ca XIX line profiles indicate upflows shortly after the second acceleration phase. Analysis of the microwave/hard X-ray/soft X-ray emission from individual sources provides information on the radio emission mechanisms, the energetic electron population, the magnetic field strength, and the plasma density. These parameters were estimated for the two microwave sources observed during the third acceleration burst; these sources were simultaneously detected in soft X-rays, and one of the sources is also seen in hard X-ray maps. Although the microwave emission is consistent with the gyro synchrotron mechanism, the millimeter emission, which peaks during the thermal phase when all nonthermal activity has ceased, is likely due to thermal bremsstrahlung from the soft X-ray emitting hot plasma. The energy lost to collisions by the energetic (&gt;15 keV) electrons and the energy contained in the thermal plasma are calculated for each source. The energy injected by the nonthermal electrons from all sources is estimated to be 10<SUP>30</SUP> ergs. Only the soft X-ray sources with gradual time profiles seem to show the Neupert effect.

Impulsive X-ray brightenings in solar flares

An analysis of 40 solar flares observed with the Yohkoh satellite was carried out to investigate the temporal relationship between rapid hard and soft X-ray flaring. Using Bragg Crystal Spectrometer (BCS) and Hard X-ray Telescope (HXT) data it was found that in every event the Neupert effect occurred throughout most of the impulsive phase. The effect was weakest late in the impulsive phase. Soft X-ray Telescope (SXT) image sequences showed that this was simply the result of the soft X-ray emission from cooling sources dominating the BCS observed flux, and that all intense non-thermal bursts are temporally associated with impulsive soft X-ray brightenings. Furthermore, at the start of the impulsive phase of these flares the rate of change of the BCS soft X-ray flux was found to be proportional to the hard X-ray intensity. These results demonstrate a causal relationship between thermal and non-thermal emissions, consistent with the thick target model.

Simultaneous Extreme-Ultraviolet Explorer and Optical Observations of AD Leonis: Evidence for Large Coronal Loops and the Neupert Effect in Stellar Flares

We report on the first simultaneous Extreme-Ultraviolet Explorer (EUVE) and optical observations of flares on the dMe flare star AD Leonis. The data show the following features: (1) Two flares (one large and one of moderate size) of several hours duration were observed in the EUV wavelength range; (2) Flare emission observed in the optical precedes the emission seen with EUVE; and (3) Several diminutions (DIMs) in the optical continuum were observed during the period of optical flare activity. To interpret these data, we develop a technique for deriving the coronal loop length from the observed rise and decay behavior of the EUV flare. The technique is generally applicable to existing and future coronal observations of stellar flares. We also determine the pressure, column depth, emission measure, loop cross-sectional area, and peak thermal energy during the two EUV flares, and the temperature, area coverage, and energy of the optical continuum emission. When the optical and coronal data are combined, we find convincing evidence of a stellar 'Neupert effect' which is a strong signature of chromospheric evaporation models. We then argue that the known spatial correlation of white-light emission with hard X-ray emission in solar flares, and the identification of the hard X-ray emission with nonthermal bremsstrahlung produced by accelerated electrons, provides evidence that flare heating on dMe stars is produced by the same electron precipitation mechanism that is inferred to occur on the Sun. We provide a thorough picture of the physical processes that are operative during the largest EUV flare, compare and contrast this picture with the canonical solar flare model, and conclude that the coronal loop length may be the most important factor in determining the flare rise time and energetics.

The Neupert effect: What can it tell us about the impulsive and gradual phases of solar flares?

The Neupert Effect is the name given to the correlation obsrved in many flares between the impulsive time profile of the microwave and hard X-ray emissions and the time derivative of the soft X-ray profile. We have used data collected between 1980 and 1989 from the Hard X-Ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM) and the soft X-ray detector on GOES to determine which events show this correlation and which do not. We have found that of 66 HXRBS events observed in 1980 with a peak rate of >1000 counts/s, 58 (80%) showed good correlation with peaks in the GOES time derivative plot corresponding to peaks in the HXR plots to within ±20 s. The more gradually varying X-ray events that are commonly referred to as Type C flares and that are often accompanied by eruptive flares, tend to show poorer correlation between the SXR time derivative and the HXR time profile. In seve al cases studied, the later, more gradually varying, peaks either did not register at all in the SXR time derivative plots or resulted in very broad peaks that, in one ase on 1981 April 26, was delayed by 13 minutes.