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1.
The solar irradiance in the Extreme Ultraviolet (EUV) spectral bands has been observed with a 15 s cadence by the SOHO Solar EUV Monitor (SEM) since 1995. During remarkably intense solar flares the SEM EUV measurements are saturated in the central (zero) order channel (0.1–50.0 nm) by the flare soft X‐ray and EUV flux. The first order EUV channel (26–34 nm) is not saturated by the flare flux because of its limited bandwidth, but it is sensitive to the arrival of Solar Energetic Particles (SEP). While both channels detect nearly equal SEP fluxes, their contributions to the count rate is sensibly negligible in the zero order channel but must be accounted for and removed from the first channel count rate. SEP contribution to the measured SEM signals usually follows the EUV peak for the gradual solar flare events. Correcting the extreme solar flare SEMEUV measurements may reveal currently unclear relations between the flare magnitude, dynamics observed in different EUV spectral bands, and the measured Earth atmosphere response. A simple and effective correction technique based on analysis of SEM count‐rate profiles, GOES X‐ray, and GOES proton data has been developed and used for correcting EUV measurements for the five extreme solar flare events of July 14, 2000, October 28, November 2, November 4, 2003, and January 20, 2005. Although none of the 2000 and 2003 flare peaks were contaminated by the presence of SEPs, the January 20, 2005 SEPs were unusually prompt and contaminated the peak. The estimated accuracy of the correction is about ±7.5% for large X‐class events. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

2.
The fluxes of extreme ultraviolet (EUV) and soft X-ray emission are key parameters for modelling the ionosphere and upper atmosphere. A new aspect is considered in using these fluxes for diagnostics and short-term prediction of proton radiation danger from the flare. The EUV (λ < 105 nm) and soft X-ray (0.1–0.8 nm) fluxes were compared for two types of solar flares. The first type is followed by a strong enhancement in solar energetic (E >10 MeV) proton flux, the second is not followed by any enhancement in proton flux. It was discovered that the flare UV flux was considerably higher for flares with protons than for those without protons. Soft X-ray fluxes were approximately equal in both cases. An excess of EUV emission in proton flares grows with increasing proton flux. An analytic expression was found for the growth in proton flux as a function of the excess of EUV radiation at a given X-ray flux. These results can be used in predicting flare radiation danger.  相似文献   

3.
With increasing solar activity since 2010, many flares from the backside of the Sun have been observed by the Extreme Ultraviolet Imager (EUVI) on either of the twin STEREO spacecraft. Our objective is to estimate their X-ray peak fluxes from EUVI data by finding a relation of the EUVI with GOES X-ray fluxes. Because of the presence of the Fe xxiv line at 192 Å, the response of the EUVI 195 Å channel has a secondary broad peak around 15 MK, and its fluxes closely trace X-ray fluxes during the rise phase of flares. If the flare plasma is isothermal, the EUVI flux should be directly proportional to the GOES flux. In reality, the multithermal nature of the flare and other factors complicate the estimation of the X-ray fluxes from EUVI observations. We discuss the uncertainties, by comparing GOES fluxes with the high cadence EUV data from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). We conclude that the EUVI 195 Å data can provide estimates of the X-ray peak fluxes of intense flares (e.g., above M4 in the GOES scale) to small uncertainties. Lastly we show examples of intense flares from regions far behind the limb, some of which show eruptive signatures in AIA images.  相似文献   

4.
We describe the design of a balloon-borne Imaging X-ray Polarimeter for Solar flares (IXPS). This novel instrument, a Time Projection Chamber (TPC) for photoelectric polarimetry, will be capable of measuring polarization at the few percent level in the 20?C50 keV energy range during an M- or X-class flare, and will provide imaging information at the ??10 arcsec level. The primary objective of such observations is to determine the directivity of nonthermal high-energy electrons producing solar hard X-rays, and hence to learn about the particle acceleration and energy release processes in solar flares. Secondary objectives include the separation of the thermal and nonthermal components of the flare X-ray emissions and the separation of photospheric albedo fluxes from direct emissions.  相似文献   

5.
Broadband sensors aboard the Naval Research Laboratory's SOLRAD 11 satellites measured solar emission in the 0.5 to 3 Å, 1 to 8 Å, 8 to 20 Å, 100 to 500 Å, 500 to 800 Å, and 700 to 1030 Å bands between March 1976 and October 1979. Measurements of EUV and soft X-ray emission from a large number of solar flares were obtained. Although solar flare measurements in the soft X-ray bands are continuously made and used as a standard of a flare's geophysical significance, direct measurements of flare EUV emission are quite rare. We present measurements of the X-ray and EUV emission from several flares with special emphasis on the relative EUV response associated with flares in different categories determined by 1 to 8 Å soft X-ray flux. An example of a flare exhibiting an impulsive (nonthermal) phase is included.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 Semptember 1980, Scheveningen, The Netherlands.  相似文献   

6.
The solar soft X-ray (XUV) radiation is important for upper atmosphere studies as it is one of the primary energy inputs and is highly variable. The XUV Photometer System (XPS) aboard the Solar Radiation and Climate Experiment (SORCE) has been measuring the solar XUV irradiance since March 2003 with a time cadence of 10 s and with about 70% duty cycle. The XPS measurements are between 0.1 and 34 nm and additionally the bright hydrogen emission at 121.6 nm. The XUV radiation varies by a factor of ∼2 with a period of ∼27 days that is due to the modulation of the active regions on the rotating Sun. The SORCE mission has observed over 20 solar rotations during the declining phase of solar cycle 23. The solar XUV irradiance also varies by more than a factor of 10 during the large X-class flares observed during the May–June 2003, October–November 2003, and July 2004 solar storm periods. There were 7 large X-class flares during the May–June 2003 storm period, 11 X-class flares during the October–November 2003 storm period, and 6 X-class flares during the July 2004 storm period. The X28 flare on 4 November 2003 is the largest flare since GOES began its solar X-ray measurements in 1976. The XUV variations during the X-class flares are as large as the expected solar cycle variations.  相似文献   

7.
Thomas N. Woods 《Solar physics》2014,289(9):3391-3401
The solar extreme-ultraviolet (EUV) observations from the Solar Dynamics Observatory (SDO) have revealed interesting characteristics of warm coronal emissions, such as Fe xvi 335 Å emission, which peak soon after the hot coronal X-ray emissions peak during a flare and then sometimes peak for a second time hours after the X-ray flare peak. This flare type, with two warm coronal emission peaks but only one X-ray peak, has been named the EUV late phase (Woods et al., Astrophys. J. 739, 59, 2011). These flares have the distinct properties of i) having a complex magnetic-field structure with two initial sets of coronal loops, with one upper set overlaying a lower set, ii) having an eruptive flare initiated in the lower set and disturbing both loop sets, iii) having the hot coronal emissions emitted only from the lower set in conjunction with the X-ray peak, and iv) having the first peak of the warm coronal emissions associated with the lower set and its second peak emitted from the upper set many minutes to hours after the first peak and without a second X-ray enhancement. The disturbance of the coronal loops by the eruption is at about the same time, but the relaxation and cooling down of the heated coronal loops during the post-flare reconnections have different time scales with the longer, upper loops being significantly delayed from the lower loops. The difference in these cooling time scales is related to the difference between the two peak times of the warm coronal emission and is also apparent in the decay profile of the X-ray emissions having two distinct decays, with the first decay slope being steeper (faster) and the delayed decay slope being smaller (slower) during the time of the warm-coronal-emission second peak. The frequency and relationship of the EUV late-phase decay times between the Fe xvi 335 Å two flare peaks and X-ray decay slopes are examined using three years of SDO/EUV Variability Experiment (EVE) data, and the X-ray dual-decay character is then exploited to estimate the frequency of EUV late-phase flares during the past four solar cycles. This study indicates that the frequency of EUV late-phase flares peaks before and after each solar-cycle minimum.  相似文献   

8.
Predictions of Energy and Helicity in Four Major Eruptive Solar Flares   总被引：1，自引：0，他引：1  
In order to better understand the solar genesis of interplanetary magnetic clouds (MCs), we model the magnetic and topological properties of four large eruptive solar flares and relate them to observations. We use the three-dimensional Minimum Current Corona model (Longcope, 1996, Solar Phys. 169, 91) and observations of pre-flare photospheric magnetic field and flare ribbons to derive values of reconnected magnetic flux, flare energy, flux rope helicity, and orientation of the flux-rope poloidal field. We compare model predictions of those quantities to flare and MC observations, and within the estimated uncertainties of the methods used find the following: The predicted model reconnection fluxes are equal to or lower than the reconnection fluxes inferred from the observed ribbon motions. Both observed and model reconnection fluxes match the MC poloidal fluxes. The predicted flux-rope helicities match the MC helicities. The predicted free energies lie between the observed energies and the estimated total flare luminosities. The direction of the leading edge of the MC’s poloidal field is aligned with the poloidal field of the flux rope in the AR rather than the global dipole field. These findings compel us to believe that magnetic clouds associated with these four solar flares are formed by low-corona magnetic reconnection during the eruption, rather than eruption of pre-existing structures in the corona or formation in the upper corona with participation of the global magnetic field. We also note that since all four flares occurred in active regions without significant pre-flare flux emergence and cancelation, the energy and helicity that we find are stored by shearing and rotating motions, which are sufficient to account for the observed radiative flare energy and MC helicity.  相似文献   

9.
We analyze the occurrence-frequency distributions of peak fluxes [P], total fluxes [E], and durations [T] of solar flares over the last three solar cycles (during 1980??C?2010) from SMM/HXRBS, CGRO/BATSE, and RHESSI hard X-ray data. From the synthesized data we find powerlaw slopes with mean values of ?? P =1.73±0.07 for the peak flux, ?? E =1.62±0.12 for the total flux, and ?? T =1.99±0.35 for flare durations. We find a tendency of an anti-correlation of the powerlaw slope of peak fluxes with the flare rate or sunspot number as a function of the solar cycle. The occurrence powerlaw slope is always steeper by ??????0.1 during a solar-cycle minimum compared with the previous solar-cycle maximum, but the relative amplitude varies for each cycle or instrument. Since each solar cycle has been observed with a different instrument, part of the variation could be attributed to instrumental characteristics and different event selection criteria used in generating the event catalogs. The relatively flatter powerlaw slopes during solar maxima could indicate more energetic flares with harder electron-energy spectra, probably due to a higher magnetic complexity of the solar corona. This would imply a non-stationarity (or solar-cycle dependence) of the coronal state of self-organized criticality.  相似文献   

10.
The Extreme ultraviolet SpectroPhotometer (ESP) is one of five channels of the Extreme ultraviolet Variability Experiment (EVE) onboard the NASA Solar Dynamics Observatory (SDO). The ESP channel design is based on a highly stable diffraction transmission grating and is an advanced version of the Solar Extreme ultraviolet Monitor (SEM), which has been successfully observing solar irradiance onboard the Solar and Heliospheric Observatory (SOHO) since December 1995. ESP is designed to measure solar Extreme UltraViolet (EUV) irradiance in four first-order bands of the diffraction grating centered around 19 nm, 25 nm, 30 nm, and 36 nm, and in a soft X-ray band from 0.1 to 7.0?nm in?the?zeroth-order of the grating. Each band’s detector system converts the photo-current into a count rate (frequency). The count rates are integrated over 0.25-second increments and transmitted to the EVE Science and Operations Center for data processing. An algorithm for converting the measured count rates into solar irradiance and the ESP calibration parameters are described. The ESP pre-flight calibration was performed at the Synchrotron Ultraviolet Radiation Facility of the National Institute of Standards and Technology. Calibration parameters were used to calculate absolute solar irradiance from the sounding-rocket flight measurements on 14 April 2008. These irradiances for the ESP bands closely match the irradiance determined for two other EUV channels flown simultaneously: EVE’s Multiple EUV Grating Spectrograph (MEGS) and SOHO’s Charge, Element and Isotope Analysis System/Solar EUV Monitor (CELIAS/SEM).  相似文献   

11.
The highly variable solar extreme ultraviolet (EUV) radiation is the major energy input to the Earth’s upper atmosphere, strongly impacting the geospace environment, affecting satellite operations, communications, and navigation. The Extreme ultraviolet Variability Experiment (EVE) onboard the NASA Solar Dynamics Observatory (SDO) will measure the solar EUV irradiance from 0.1 to 105?nm with unprecedented spectral resolution (0.1?nm), temporal cadence (ten seconds), and accuracy (20%). EVE includes several irradiance instruments: The Multiple EUV Grating Spectrographs (MEGS)-A is a grazing-incidence spectrograph that measures the solar EUV irradiance in the 5 to 37?nm range with 0.1-nm resolution, and the MEGS-B is a normal-incidence, dual-pass spectrograph that measures the solar EUV irradiance in the 35 to 105?nm range with 0.1-nm resolution. To provide MEGS in-flight calibration, the EUV SpectroPhotometer (ESP) measures the solar EUV irradiance in broadbands between 0.1 and 39?nm, and a MEGS-Photometer measures the Sun’s bright hydrogen emission at 121.6?nm. The EVE data products include a near real-time space-weather product (Level?0C), which provides the solar EUV irradiance in specific bands and also spectra in 0.1-nm intervals with a cadence of one minute and with a time delay of less than 15?minutes. The EVE higher-level products are Level?2 with the solar EUV irradiance at higher time cadence (0.25?seconds for photometers and ten seconds for spectrographs) and Level?3 with averages of the solar irradiance over a day and over each one-hour period. The EVE team also plans to advance existing models of solar EUV irradiance and to operationally use the EVE measurements in models of Earth’s ionosphere and thermosphere. Improved understanding of the evolution of solar flares and extending the various models to incorporate solar flare events are high priorities for the EVE team.  相似文献   

12.
We investigate the frequency of all (X-ray flare events higher than class B1.0), B, C, M and X-class flares, respectively, derived from the National Geophysical Data Center (NGDC) list of solar flares between May 1983 and September 2014, which corresponds to the two complete solar cycles (SCs) 22 and 23 as well as the rise and maximum phases of SC 24. Analysis shows that the temporal behavior for these various class flares is quite different. The main findings of this study, confirmed by using the Hinode flare catalog where possible, are as follows. (1) The B-class flares are in complete antiphase with all, C, M and X-class flares. (2) While, there is a small decreasing trend in the peak values of the smoothed monthly C-class flare numbers from SC 22 to 24, the occurrence rate of M and X-class flares dropped by almost half and two-thirds, respectively, during SC 23 and remained almost the same during SC 24. This class-dependent temporal behavior provides support for dynamo models that involve the coexistence of a deep global and a superficial local dynamo.  相似文献   

13.
Wang  Haimin 《Solar physics》1997,174(1-2):163-173
This paper reviews studies of the relationship between the evolution of vector magnetic fields and the occurrence of major solar flares. Most of the data were obtained by the video magnetograph systems at Big Bear Solar Observatory (BBSO) and Huairou Solar Observatory (HSO). Due to the favorable weather and seeing conditions at both stations, high-resolution vector magnetograph sequences of many active regions that produced major flares during last solar maximum (1989–1993) have been recorded. We have analyzed several sequences of magnetograms to study the evolution of vector magnetic fields of flare productive active regions. The studies have focused on the following three aspects: (1) processes which build up magnetic shear in active regions; (2) the pre-flare magnetic structure of active regions; and (3) changes of magnetic shear immediately preceding and following major flares. We obtained the following results based on above studies: (1) Emerging flux regions (EFRs) play very important roles in the production of complicated photospheric flow patterns, magnetic shear and flares. (2) Although the majority of flares prefer to occur in magnetically sheared regions, many flares occurred in regions without strong photospheric magnetic shear. (3) We found that photospheric magnetic shear increased after all the 6 X-class flares studied by us. We want to emphasize that this discovery is not contradictory to the energy conservation principle, because a flare is a three-dimensional process, and the photosphere only provides a two-dimensional boundary condition. This argument is supported by the fact that if two initial ribbons of a flare are widely separated (which may correspond to a higher-altitude flare), the correlation of the flare with strong magnetic shear is weak; if the two ribbons of a flare are close (which may correspond to a lower-altitude flare), its correlation with the strong shear is strong. (4) We have analyzed 18 additional M-class flares observed by HSO in 1989 and 1990. No detectable shear change was found for all the cases. It is likely that only the most energetic flares can affect the photospheric magnetic topology.  相似文献   

14.
To better understand long-term flare activity, we present a statistical study on soft X-ray flares from May 1976 to May 2008. It is found that the smoothed monthly peak fluxes of C-class, M-class, and X-class flares have a very noticeable time lag of 13, 8, and 8 months in cycle 21 respectively with respect to the smoothed monthly sunspot numbers. There is no time lag between the sunspot numbers and M-class flares in cycle 22. However, there is a one-month time lag for C-class flares and a one-month time lead for X-class flares with regard to sunspot numbers in cycle 22. For cycle 23, the smoothed monthly peak fluxes of C-class, M-class, and X-class flares have a very noticeable time lag of one month, 5 months, and 21 months respectively with respect to sunspot numbers. If we take the three types of flares together, the smoothed monthly peak fluxes of soft X-ray flares have a time lag of 9 months in cycle 21, no time lag in cycle 22 and a characteristic time lag of 5 months in cycle 23 with respect to the smoothed monthly sunspot numbers. Furthermore, the correlation coefficients of the smoothed monthly peak fluxes of M-class and X-class flares and the smoothed monthly sunspot numbers are higher in cycle 22 than those in cycles 21 and 23. The correlation coefficients between the three kinds of soft X-ray flares in cycle 22 are higher than those in cycles 21 and 23. These findings may be instructive in predicting C-class, M-class, and X-class flares regarding sunspot numbers in the next cycle and the physical processes of energy storage and dissipation in the corona.  相似文献   

15.
A new methodology is given to determine basic parameters of flares from their X-ray light curves. Algorithms are developed from the analysis of small X-ray flares occurring during the deep solar minimum of 2009, between Solar Cycles 23 and 24, observed by the Polish Solar Photometer in X-rays (SphinX) on the Complex Orbital Observations Near-Earth of Activity of the Sun-Photon (CORONAS-Photon) spacecraft. One is a semi-automatic flare detection procedure that gives start, peak, and end times for single (“elementary”) flare events under the assumption that the light curve is a simple convolution of a Gaussian and exponential decay functions. More complex flares with multiple peaks can generally be described by a sum of such elementary flares. Flare time profiles in the two energy ranges of SphinX (1.16?–?1.51 keV, 1.51?–?15 keV) are used to derive temperature and emission measure as a function of time during each flare. The result is a comprehensive catalogue – the SphinX Flare Catalogue – which contains 1600 flares or flare-like events and is made available for general use. The methods described here can be applied to observations made by Geosynchronous Operational Environmental Satellites (GOES), the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and other broad-band spectrometers.  相似文献   

16.
Solar activity during 2007?–?2009 was very low, causing anomalously low thermospheric density. A comparison of solar extreme ultraviolet (EUV) irradiance in the He?ii spectral band (26 to 34 nm) from the Solar Extreme ultraviolet Monitor (SEM), one of instruments on the Charge Element and Isotope Analysis System (CELIAS) on board the Solar and Heliospheric Observatory (SOHO) for the two latest solar minima showed a decrease of the absolute irradiance of about 15±6 % during the solar minimum between Cycles 23 and 24 compared with the Cycle 22/23 minimum when a yearly running-mean filter was used. We found that some local, shorter-term minima including those with the same absolute EUV flux in the SEM spectral band show a higher concentration of spatial power in the global network structure from the 30.4 nm SOHO/Extreme ultraviolet Imaging Telescope (EIT) images for the local minimum of 1996 compared with the minima of 2008?–?2011. We interpret this higher concentration of spatial power in the transition region’s global network structure as a larger number of larger-area features on the solar disk. These changes in the global network structure during solar minima may characterize, in part, the geo-effectiveness of the solar He?ii EUV irradiance in addition to the estimations based on its absolute levels.  相似文献   

17.
It is generally accepted that densities of quiet-Sun and active region plasma are sufficiently low to justify the optically thin approximation, and this is commonly used in the analysis of line emissions from plasma in the solar corona. However, the densities of solar flare loops are substantially higher, compromising the optically thin approximation. This study begins with a radiative transfer model that uses typical solar flare densities and geometries to show that hot coronal emission lines are not generally optically thin. Furthermore, the model demonstrates that the observed line intensity should exhibit center-to-limb variability (CTLV), with flares observed near the limb being dimmer than those occurring near disk center. The model predictions are validated with an analysis of over 200 flares observed by the EUV Variability Experiment (EVE) on the Solar Dynamics Observatory (SDO), which uses six lines, with peak formation temperatures between 8.9 and 15.8 MK, to show that limb flares are systematically dimmer than disk-center flares. The data are then used to show that the electron column density along the line of sight typically increases by \(1.76 \times 10^{19}~\mbox{cm}^{-2}\) for limb flares over the disk-center flare value. It is shown that the CTLV of hot coronal emissions reduces the amount of ionizing radiation propagating into the solar system, and it changes the relative intensities of lines and bands commonly used for spectral analysis.  相似文献   

18.
This paper describes the methods used to obtain the thermal evolution and radiative output during solar flares as observed by the Extreme ultraviolet Variability Experiment (EVE) onboard the Solar Dynamics Observatory (SDO). How EVE measurements, due to the temporal cadence, spectral resolution and spectral range, can be used to determine how the thermal plasma radiates at various temperatures throughout the impulsive and gradual phase of flares is presented and discussed in detail. EVE can very accurately determine the radiative output of flares due to pre- and in-flight calibrations. Events are presented that show that the total radiated output of flares depends more on the flare duration than the typical GOES X-ray peak magnitude classification. With SDO observing every flare throughout its entire duration and over a large temperature range, new insights into flare heating and cooling as well as the radiative energy release in EUV wavelengths support existing research into understanding the evolution of solar flares.  相似文献   

19.
The active region NOAA 6555 had several locations of highly sheared magnetic field structure, yet, only one of them was the site for all the five X-class flares during its disk passage in March 1991. The pre-flare observations of high-resolution H filtergrams, vector magnetograms and H Dopplergrams of the 2B/X5.3 flare on 25 March 1991 show that the flaring site was characterized by a new rising emerging flux region (EFR) near the highly sheared magnetic field configuration. The polarity axis of the emerging flux was nearly perpendicular to the pre-existing magnetic neutral line. The location of the EFR was the site of initial brightening in H. The post-flare magnetograms show higher magnetic shear at the flare location compared to the post-flare magnetograms, which might indicate that the EFR was sheared at the time of its emergence. As the new EFR coincided with the occurrence of the flare, we suggest that it might have triggered the observed flare. Observations from Big Bear Solar Observatory and Marshall Space Flight Center also show that there was emergence of new flux at the same location prior to two other X-class flares. We find that out of five observed X-class flares in NOAA 6555, at least in three cases there are clear signatures of flare-related flux emergence. Therefore, it is concluded that EFRs might play an important role in destabilizing the observed sheared magnetic structures leading to large X-class flares of NOAA 6555.  相似文献   

20.
We investigate the connections between the occurrence of major solar flares and subsurface dynamic properties of active regions. For this analysis, we select five active regions that produced a total of 11 flares with peak X-ray flux intensity higher than M5.0. The subsurface velocity fields are obtained from time–distance helioseismology analysis using SDO/HMI (Solar Dynamics Observatory/Helioseismic and Magnetic Imager) Doppler observations, and the X-ray flux intensity is taken from GOES (Geostationary Operational Environmental Satellites). It is found that among the eight amplitude bumps in the evolutionary curves of subsurface kinetic helicity, five (62.5%) of them had a flare stronger than M5.0 occurring within 8 hours, either before or after the bumps. Another subsurface parameter is the Normalized Helicity Gradient Variance (NHGV), reflecting kinetic helicity spread in different depth layers; it also shows bumps near the occurrence of these solar flares. Although there is no one-to-one correspondence between the flare and the subsurface properties, these observational phenomena are worth further studies to better understand the flares’ subsurface roots, and to investigate whether the subsurface properties can be used for major flare forecasts.  相似文献   

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