X-ray images from the Yohkoh satellite, obtained following occurrences of limb flares sometimes s... more X-ray images from the Yohkoh satellite, obtained following occurrences of limb flares sometimes show coronal fan-like structures extending above a growing post-flare loop system. We show one such event observed in AR 7270 on the east limb of the Sun on 28/29 August 1992. We suggest that these rays are `ministreamers,' formed as a result of the re-structuring of the
Solar-B, the next ISAS mission (with major NASA participation), is designed to address the fundam... more Solar-B, the next ISAS mission (with major NASA participation), is designed to address the fundamental question of how magnetic fields interact with plasma to produce solar variability. The mission has a number of unique capabilities that will enable it to answer the outstanding questions of solar magnetism. First, by escaping atmospheric seeing, it will deliver continuous observations of the solar
Because of the healt hazards of solar activity, human traffic in deep space will require faciliti... more Because of the healt hazards of solar activity, human traffic in deep space will require facilities for monitoring, predicting, and altering the astronauts to potential danger. This suggests a ‘‘solar watch’’ program consisting of a network of platforms at one A.U. from the Sun, capable of monitoring its global behavior. The instrumentation carried by these network platforms can evolve in sophistication with successive launches, in order to lead to a deep understanding of the physical mechanisms of solar activity — the best basis for maximally reliable activity forecasts. The possibility of steroscopic viewing of the solar surface and corona will confer unique advantages for the physical understanding of these physical mechanisms.
We present an analysis of solar flares and its extension to active stars with the goal to investi... more We present an analysis of solar flares and its extension to active stars with the goal to investigate the observability of non-thermal components of stellar flares with future instrumentation, in particular with Simbol-X. We derive a scaling law for the relationship between soft (thermal) and hard (non-thermal) peak X-ray fluxes which we extrapolate from the solar case to energetic stellar flares.
Do solar active regions typically radiate more coronal energy during flares than the quiescent pe... more Do solar active regions typically radiate more coronal energy during flares than the quiescent periods between them? This is a fundamental question for storage and release models of flares and active regions, yet it is presently poorly answered by observations. The EUV Variability Experiment (EVE) on the Solar Dynamics Observatory (SDO) provides spectrally resolved observations of the Sun in the "Sun-as-a-point source" mode. It covers a wide range of temperatures and thus allows a detailed study of thermal emissions. Here we present two approaches for computing the active region luminosity, using EVE observations of fourteen Fe lines (FeIX-FeXXIV). In the first approach, we analyze EVE data in a time-series sense, when only one active region is present on the disk; this allows us to subtract the background due to the quiet sun and get the contribution from the active region alone. In the second approach, we analyze correlations of the radiative signatures with proxy indice...
Preface.- Overview of the Volume.- An Observational Overview of Solar Flares.- Implications of X-... more Preface.- Overview of the Volume.- An Observational Overview of Solar Flares.- Implications of X-ray Observations for Electron Acceleration and Propagation in Solar Flares.- Properties of Energetic Ions in the Solar Atmosphere from Gamma-ray and Neutron Observations.- The Relationship Between Solar Radio and Hard X-ray Emission.- Microflares and the Statistics of X-ray Flares.- Deducing Electron Properties from Hard X-ray Observations.- Recent Advances in Understanding Particle Acceleration Processes in Solar Flares.- Energy Release and Particle Acceleration in Flares: Summary and Future Prospects.- Index.
We present new RHESSI upper limits in the 3-200 keV energy range for solar hard X-ray emission in... more We present new RHESSI upper limits in the 3-200 keV energy range for solar hard X-ray emission in the absence of flares and active regions, i.e., the quiet Sun, using data obtained between 2005 July and 2009 April. These new limits, substantially deeper than any previous ones, constrain several physical processes that could produce hard X-ray emission. These include cosmic-ray effects and the generation of axions within the solar core. The data also limit the properties of "nanoflares," a leading candidate to explain coronal heating. We find it unlikely for nanoflares involving nonthermal effects to heat the corona because such events would require a steep electron spectrum E −δ with index δ > 5 extending to very low energies (<1 keV), into the thermal energy range. We also use the limits to constrain the parameter space of an isothermal model and coronal thin-target emission models (power-law and kappa distributions).
This chapter reviews how our knowledge of CMEs and CME-associated phenomena has been improved, si... more This chapter reviews how our knowledge of CMEs and CME-associated phenomena has been improved, since the launch of the SOHO mission, thanks to multi-wavelength analysis. The combination of data obtained from space-based experiments and ground based instruments allows us to follow the space-time development of an event from the bottom of the corona to large distances in the interplanetary medium. Since CMEs originate in the low solar corona, understanding the physical processes that generate them is strongly dependant on coordinated multi-wavelength observations. CMEs display a large diversity in morphology and kinematic properties, but there is presently no statistical evidence that those properties may serve to group them into different classes. When a CME takes place, the coronal magnetic field undergoes restructuring. Much of the current research
A flare rising from behind the solar limb was recor9eg §imultaneously by the UCSD X-ray detector ... more A flare rising from behind the solar limb was recor9eg §imultaneously by the UCSD X-ray detector on OSO~III (7.7-200 keV) and the-Caltech ph~t~heliograph on Robinson Laboratory roof (Ha). The de-occultation gives excellent spatial resolution of the X-ray source. S:Q_ect~a_ suggest that the material was already heated to 27 000 000 ° and that the increase in flux was due to the de-occultation. The flux-nse io-maxT1rnini. was proportional to the apparent area. The uniformity of this rise shows that there was no special kernel of emission. Comparison of the deduced volume with the bremsstrahlung formula gives a density of about 1010 for the 27 000 000 ° component of the flare; this is confirmed by consideration of the maximum possible coulomb braking. The actual decay is more likely by escape rather than coulomb braking. In the course of studies of soft X-rays observed by the OS0-111 X-ray telescope (Hudson et al., 1969) simultaneous with observations of solar phenomena with the Caltech photoheliograph, a particularly interesting de-occultation event occurred on December 2, 1967. A large bright limb flare rose from behind the limb of the sun, coming from a region that had crossed the limb just a few hours earlier. By comparison of the uncovered area of the flare and the observed X-ray flux, it was possible to draw conclusions on the plasma responsible for the X-ray event. This plasma was found to be uniformly distributed through the arch. The electrons had energy corresponding to 27 000 000° (lower than the typical flare). The density of these energetic electrons was between 6 and 9 x 10 9 cm-3 •
We report on G-band emission observed by the Solar Optical Telescope onboard the Hinode satellite... more We report on G-band emission observed by the Solar Optical Telescope onboard the Hinode satellite in association with the X1.5-class flare on 2006 December 14. The G-band enhancements originate from the footpoints of flaring coronal magnetic loops, coinciding with non-thermal hard X-ray bremsstrahlung sources observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager. At the available 2 minute cadence, the G-band and hard X-ray intensities are furthermore well correlated in time. Assuming that the G-band enhancements are continuum emission from a blackbody, we derived the total radiative losses of the white-light flare (white-light power). If the G-band enhancements additionally have a contribution from lines, the derived values are overestimates. We compare the white-light power with the power in hard X-ray producing electrons using the thick target assumption. Independent of the cutoff energy of the accelerated electron spectrum, the white-light power and the power of accelerated electrons are roughly proportional. Using the observed upper limit of ∼30 keV for the cutoff energy, the hard X-ray producing electrons provide at least a factor of 2 more power than needed to produce the white-light emission. For electrons above 40 keV, the powers roughly match for all four of the time intervals available during the impulsive phase. Hence, the flare-accelerated electrons contain enough energy to produce the white-light flare emissions. The observed correlation in time, space, and power strongly suggests that electron acceleration and
The Atacama Large Millimeter/submillimeter Array (ALMA) is a new powerful tool for observing the ... more The Atacama Large Millimeter/submillimeter Array (ALMA) is a new powerful tool for observing the Sun at high spatial, temporal, and spectral resolution. These capabilities can address a broad range of fundamental scientific questions in solar physics. The radiation observed by ALMA originates mostly from the chromosphere-a complex and dynamic region between the photosphere and corona, which plays a crucial role in the transport of energy and matter and, ultimately, the heating of the outer layers of the solar atmosphere. Based on first solar test observations, strategies for regular solar campaigns are currently being developed. State-of-the-art numerical simulations of the solar atmosphere and modeling of instrumental effects can help constrain and optimize future observing modes for ALMA. Here we present a short technical description of ALMA and an overview of past efforts and future possibilities for solar observations at submillimeter and millimeter wavelengths. In addition, selected numerical simulations and observations at other wavelengths demonstrate ALMA's scientific potential for studying the Sun for a large range of science cases. Keywords Sun • photosphere • chromosphere • corona • magnetohydrodynamics • radiative transfer • flares • prominences
Some CME models involve the opening of previously closed magnetic field lines as the plasma erupt... more Some CME models involve the opening of previously closed magnetic field lines as the plasma erupts through the lower corona. However, the identification of observational signatures of this process has proven illusive. We report on Yohkoh/SXT observations of large-scale arcade events which reveal the dimming, or depletion, of coronal material above the bright long-duration arcade region. Viewed in video form,
X-ray images from the Yohkoh satellite, obtained following occurrences of limb flares sometimes s... more X-ray images from the Yohkoh satellite, obtained following occurrences of limb flares sometimes show coronal fan-like structures extending above a growing post-flare loop system. We show one such event observed in AR 7270 on the east limb of the Sun on 28/29 August 1992. We suggest that these rays are `ministreamers,' formed as a result of the re-structuring of the
Solar-B, the next ISAS mission (with major NASA participation), is designed to address the fundam... more Solar-B, the next ISAS mission (with major NASA participation), is designed to address the fundamental question of how magnetic fields interact with plasma to produce solar variability. The mission has a number of unique capabilities that will enable it to answer the outstanding questions of solar magnetism. First, by escaping atmospheric seeing, it will deliver continuous observations of the solar
Because of the healt hazards of solar activity, human traffic in deep space will require faciliti... more Because of the healt hazards of solar activity, human traffic in deep space will require facilities for monitoring, predicting, and altering the astronauts to potential danger. This suggests a ‘‘solar watch’’ program consisting of a network of platforms at one A.U. from the Sun, capable of monitoring its global behavior. The instrumentation carried by these network platforms can evolve in sophistication with successive launches, in order to lead to a deep understanding of the physical mechanisms of solar activity — the best basis for maximally reliable activity forecasts. The possibility of steroscopic viewing of the solar surface and corona will confer unique advantages for the physical understanding of these physical mechanisms.
We present an analysis of solar flares and its extension to active stars with the goal to investi... more We present an analysis of solar flares and its extension to active stars with the goal to investigate the observability of non-thermal components of stellar flares with future instrumentation, in particular with Simbol-X. We derive a scaling law for the relationship between soft (thermal) and hard (non-thermal) peak X-ray fluxes which we extrapolate from the solar case to energetic stellar flares.
Do solar active regions typically radiate more coronal energy during flares than the quiescent pe... more Do solar active regions typically radiate more coronal energy during flares than the quiescent periods between them? This is a fundamental question for storage and release models of flares and active regions, yet it is presently poorly answered by observations. The EUV Variability Experiment (EVE) on the Solar Dynamics Observatory (SDO) provides spectrally resolved observations of the Sun in the "Sun-as-a-point source" mode. It covers a wide range of temperatures and thus allows a detailed study of thermal emissions. Here we present two approaches for computing the active region luminosity, using EVE observations of fourteen Fe lines (FeIX-FeXXIV). In the first approach, we analyze EVE data in a time-series sense, when only one active region is present on the disk; this allows us to subtract the background due to the quiet sun and get the contribution from the active region alone. In the second approach, we analyze correlations of the radiative signatures with proxy indice...
Preface.- Overview of the Volume.- An Observational Overview of Solar Flares.- Implications of X-... more Preface.- Overview of the Volume.- An Observational Overview of Solar Flares.- Implications of X-ray Observations for Electron Acceleration and Propagation in Solar Flares.- Properties of Energetic Ions in the Solar Atmosphere from Gamma-ray and Neutron Observations.- The Relationship Between Solar Radio and Hard X-ray Emission.- Microflares and the Statistics of X-ray Flares.- Deducing Electron Properties from Hard X-ray Observations.- Recent Advances in Understanding Particle Acceleration Processes in Solar Flares.- Energy Release and Particle Acceleration in Flares: Summary and Future Prospects.- Index.
We present new RHESSI upper limits in the 3-200 keV energy range for solar hard X-ray emission in... more We present new RHESSI upper limits in the 3-200 keV energy range for solar hard X-ray emission in the absence of flares and active regions, i.e., the quiet Sun, using data obtained between 2005 July and 2009 April. These new limits, substantially deeper than any previous ones, constrain several physical processes that could produce hard X-ray emission. These include cosmic-ray effects and the generation of axions within the solar core. The data also limit the properties of "nanoflares," a leading candidate to explain coronal heating. We find it unlikely for nanoflares involving nonthermal effects to heat the corona because such events would require a steep electron spectrum E −δ with index δ > 5 extending to very low energies (<1 keV), into the thermal energy range. We also use the limits to constrain the parameter space of an isothermal model and coronal thin-target emission models (power-law and kappa distributions).
This chapter reviews how our knowledge of CMEs and CME-associated phenomena has been improved, si... more This chapter reviews how our knowledge of CMEs and CME-associated phenomena has been improved, since the launch of the SOHO mission, thanks to multi-wavelength analysis. The combination of data obtained from space-based experiments and ground based instruments allows us to follow the space-time development of an event from the bottom of the corona to large distances in the interplanetary medium. Since CMEs originate in the low solar corona, understanding the physical processes that generate them is strongly dependant on coordinated multi-wavelength observations. CMEs display a large diversity in morphology and kinematic properties, but there is presently no statistical evidence that those properties may serve to group them into different classes. When a CME takes place, the coronal magnetic field undergoes restructuring. Much of the current research
A flare rising from behind the solar limb was recor9eg §imultaneously by the UCSD X-ray detector ... more A flare rising from behind the solar limb was recor9eg §imultaneously by the UCSD X-ray detector on OSO~III (7.7-200 keV) and the-Caltech ph~t~heliograph on Robinson Laboratory roof (Ha). The de-occultation gives excellent spatial resolution of the X-ray source. S:Q_ect~a_ suggest that the material was already heated to 27 000 000 ° and that the increase in flux was due to the de-occultation. The flux-nse io-maxT1rnini. was proportional to the apparent area. The uniformity of this rise shows that there was no special kernel of emission. Comparison of the deduced volume with the bremsstrahlung formula gives a density of about 1010 for the 27 000 000 ° component of the flare; this is confirmed by consideration of the maximum possible coulomb braking. The actual decay is more likely by escape rather than coulomb braking. In the course of studies of soft X-rays observed by the OS0-111 X-ray telescope (Hudson et al., 1969) simultaneous with observations of solar phenomena with the Caltech photoheliograph, a particularly interesting de-occultation event occurred on December 2, 1967. A large bright limb flare rose from behind the limb of the sun, coming from a region that had crossed the limb just a few hours earlier. By comparison of the uncovered area of the flare and the observed X-ray flux, it was possible to draw conclusions on the plasma responsible for the X-ray event. This plasma was found to be uniformly distributed through the arch. The electrons had energy corresponding to 27 000 000° (lower than the typical flare). The density of these energetic electrons was between 6 and 9 x 10 9 cm-3 •
We report on G-band emission observed by the Solar Optical Telescope onboard the Hinode satellite... more We report on G-band emission observed by the Solar Optical Telescope onboard the Hinode satellite in association with the X1.5-class flare on 2006 December 14. The G-band enhancements originate from the footpoints of flaring coronal magnetic loops, coinciding with non-thermal hard X-ray bremsstrahlung sources observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager. At the available 2 minute cadence, the G-band and hard X-ray intensities are furthermore well correlated in time. Assuming that the G-band enhancements are continuum emission from a blackbody, we derived the total radiative losses of the white-light flare (white-light power). If the G-band enhancements additionally have a contribution from lines, the derived values are overestimates. We compare the white-light power with the power in hard X-ray producing electrons using the thick target assumption. Independent of the cutoff energy of the accelerated electron spectrum, the white-light power and the power of accelerated electrons are roughly proportional. Using the observed upper limit of ∼30 keV for the cutoff energy, the hard X-ray producing electrons provide at least a factor of 2 more power than needed to produce the white-light emission. For electrons above 40 keV, the powers roughly match for all four of the time intervals available during the impulsive phase. Hence, the flare-accelerated electrons contain enough energy to produce the white-light flare emissions. The observed correlation in time, space, and power strongly suggests that electron acceleration and
The Atacama Large Millimeter/submillimeter Array (ALMA) is a new powerful tool for observing the ... more The Atacama Large Millimeter/submillimeter Array (ALMA) is a new powerful tool for observing the Sun at high spatial, temporal, and spectral resolution. These capabilities can address a broad range of fundamental scientific questions in solar physics. The radiation observed by ALMA originates mostly from the chromosphere-a complex and dynamic region between the photosphere and corona, which plays a crucial role in the transport of energy and matter and, ultimately, the heating of the outer layers of the solar atmosphere. Based on first solar test observations, strategies for regular solar campaigns are currently being developed. State-of-the-art numerical simulations of the solar atmosphere and modeling of instrumental effects can help constrain and optimize future observing modes for ALMA. Here we present a short technical description of ALMA and an overview of past efforts and future possibilities for solar observations at submillimeter and millimeter wavelengths. In addition, selected numerical simulations and observations at other wavelengths demonstrate ALMA's scientific potential for studying the Sun for a large range of science cases. Keywords Sun • photosphere • chromosphere • corona • magnetohydrodynamics • radiative transfer • flares • prominences
Some CME models involve the opening of previously closed magnetic field lines as the plasma erupt... more Some CME models involve the opening of previously closed magnetic field lines as the plasma erupts through the lower corona. However, the identification of observational signatures of this process has proven illusive. We report on Yohkoh/SXT observations of large-scale arcade events which reveal the dimming, or depletion, of coronal material above the bright long-duration arcade region. Viewed in video form,
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Papers by H. Hudson