The frequency of long-duration solar X-ray events

It is shown that the long-duration X-ray events tend to ignore the sunspot cycle. This is particularly true for events with durations of 6 hr or more.Sachs/Freeman Associates, Inc., Bowie, MD 20715, U.S.A.     

The decay phase of solar flare events

A study of the properties of the cosmic radiation of energy - 10 MeV generated by solar flares is reported. Data from four Pioneer spacecraft in interplanetary orbits, and separated by 180° in heliocentric longitude are employed. Attention is restricted to the properties evident at times in excess of 1 day after the occurrence of the parent flare. The anisotropic character of the radiation; the gradients in heliocentric longitude; the decay time constants; and the energy spectra of the radiation are all studied in detail.It is found that the equilibrium anisotropy assumes a direction - 45° E of the satellite-Sun line at very late times. It is suggested that the anisotropy at such times is parallel to E × B. This observation confirms that convection is the determining process in the escape of the solar cosmic rays from the solar system. It indicates that a positive radial gradient of solar cosmic radiation density has builtup at orbit of Earth some 4 days after a flare. This results in an effective convective velocity of approximately 1/2 the solar wind velocity. Direct measurements indicate the presence of strong gradients in heliocentric longitude even at very late times ( 4 days). These gradients are essentially invariant with respect to time, e-folding angles of n - 30° have been observed at - 10 MeV. The presence of these gradients has a major effect on the temporal variation of the cosmic ray flux during the decay phase of the flare effect. Thus, the observed decay time constant is either increased or decreased relative to the convective value depending on the position of the observer relative to the centroid of the cosmic ray population injected by the flare. The effect of the gradient becomes more pronounced at lower energies, and may even exceed the convective removal rate. The observed decay time constant, the characteristics of the anisotropy, and the gradient in longitude are shown to be inter-related as demanded by theory. It is shown that the exponent of the cosmic ray spectrum is dependent on the location of the observer relative to the centroid of the cosmic ray population injected by the parent flare. At a given point in the frame of reference of the cosmic ray population, the spectral exponent is invariant with time.Now at CSIRO, G.P.O. Box 124, Port Melbourne, Victoria 3207, Australia.On leave from Physical Research Laboratory, Ahmedabad, India.     

The homologous flare events in solar active regions

This paper consists of two parts. We first discuss recent general results on the study of properties of flare homology, and their relevance to the physical interpretation of the flare phenomenon at large. We devote particular attention to the discovery of homologous flares which occur in rapid succession, within a few minutes of each other in many cases. We name these kind of flares rafales. These flares signal the existence of several episodes of energy release within the same magnetic configuration. We also show the existence of particular sites in the solar atmosphere which have peculiar characteristics in terms of solar rotation, and where recurrent flaring may take place over and over again in different solar rotations. This indicates that the disturbance causing the emergence of activity is deep seated, below the solar photosphere. Finally, in the second part, we discuss an extensive set of observations of two homologous flares of a rafale, stressing the dynamic aspects of the observations, particularly the presence of peaks in the vertical component of the velocity field. These results are shown to be in agreement with studies of filament activations and the surging arches which are observed before the flash phase of solar flares.     

Cosmic ray anisotropies observed late in the decay phase of solar flare events

Concurrent interplanetary magnetic field and 0.7–7.6 MeV proton cosmic-ray anisotropy data obtained from instrumentation on Explorers 34 and 41 are examined for five cosmic-ray events in which we observe a persistent eastern-anisotropy phase late in the event (t ? 4 days). The direction of the anisotropy at such times shows remarkable invariance with respect to the direction of the magnetic field (which generally varies throughout the event) and it is also independent of particle species (electrons and protons) and particle speed over the range 0.06 ? β ? 0.56. The anisotropy is from the direction 38.3° ± 2.4° E of the solar radius vector, and is inferred to be orthogonal to the long term, mean interplanetary field direction. Both the amplitude of the anisotropy and the decay time constant show a strong dependence on the magnetic field azimuth. Detailed comparison of the anisotropy and the magnetic field data shows that the simple model of convection plus diffusion parallel to the magnetic field is applicable for this phase of the flare effect. It is demonstrated that contemporary theories do not predict the invariance of the direction as observed, even when the magnetic field is steady; these theories need extension to take into account the magnetic field direction ψ varying from its mean direction ψ _o. It is shown that the late phase anisotropy vector is not expected to be everywhere perpendicular to the mean magnetic field. The suggestion that we are observing kinks in the magnetic field moving radially outwards from the Sun leads to the conclusion that the parallel diffusion coefficient varies as 1/cos² (ψ ? ψ _o). Density gradients in the late decay phase are estimated to be ≈ 700%∣AU for 0.7–7.6 MeV protons. A simple theory reproduces the dependence of the decay time constant on anisotropy; it also leads to a radial density gradient of about 1000%∣AU and diffusion coefficient of 1.3 × 10²⁰ cm² s^?1.     

Laboratory solar flare experiments

Several laboratory experiments on magnetic field line reconnection are briefly reviewed. Emphasis is placed on the double inverse pinch device (DIPD) in which magnetic flux is built up during a quiescent reconnection phase and then abruptly transferred during an impulsive reconnection phase. Scaling estimates show that this impulsive phase corresponds to a solar release of 10³⁰ ergs in 10² seconds with the production of GeV potentials. The trigger for the impulsive flare is a conduction mode instability (ion-acoustic) which abruptly changes the resistance of the neutral point region when the reconnection current density reaches a critical value.Some results are presented from another reconnection device which has exactly antiparallel fields at the boundaries. This flat plate device develops one x-type neutral point rather than tearing into many neutral points. The reconnection rate is more quiescent than in the DIPD. A mild conduction mode instability occurs. The results suggest that regions with flattened boundary fields may not be as conducive to flares as regions with more curved fields.     

A solar flare videometer

A new instrument, called a videometer, has been developed to measure solar flare area, peak intensity and integrated intensity in real time. The videometer uses a closed circuit television system to convert an optical H image into electrical signals for measurement. Observations of two Class I flares with the videometer are discussed.     

Evidence for prolonged acceleration based on a detailed analysis of the long-duration solar gamma-ray flare of June 15, 1991

Gamma-ray emission extending to energies greater than 2 GeV and lasting at least for two hours as well as 0.8–8.1 MeV nuclear line emission lasting 40 min were observed with very sensitive telescopes aboard the GAMMA and CGRO satellites for the well-developed post-flare loop formation phase of the 3B/X12 flare on June 15, 1991. We undertook an analysis of optical, radio, cosmic-ray, and other data in order to identify the origin of the energetic particles producing these unusual gamma-ray emissions. The analysis yields evidence that the gamma-rays and other emissions, observed well after the impulsive phase of the flare, appear to be initiated by prolonged nonstationary particle acceleration directly during the late phase of the flare rather than by a long-term trapping of energetic electrons and protons accelerated at the onset of the flare. We argue that such an acceleration, including the acceleration of protons up to GeV energies, can be caused by a prolonged post-eruptive energy release following a coronal mass ejection (CME), when the magnetic field above the active region, strongly disturbed by the CME eruption, relaxes to its initial state through magnetic reconnection in the coronal vertical current sheet.     

Mars Express and Venus Express multi-point observations of geoeffective solar flare events in December 2006

In December 2006, a single active region produced a series of proton solar flares, with X-ray class up to the X9.0 level, starting on 5 December 2006 at 10:35 UT. A feature of this X9.0 flare is that associated MeV particles were observed at Venus and Mars by Venus Express (VEX) and Mars Express (MEX), which were ∼80° and ∼125° east of the flare site, respectively, in addition to the Earth, which was ∼79° west of the flare site. On December 5, 2006, the plasma instruments ASPERA-3 and ASPERA-4 on board MEX and VEX detected a large enhancement in their respective background count levels. This is a typical signature of solar energetic particle (SEP) events, i.e., intensive MeV particle fluxes. The timings of these enhancements were consistent with the estimated field-aligned travel time of particles associated with the X9.0 flare that followed the Parker spiral to reach Venus and Mars. Coronal mass ejection (CME) signatures that might be related to the proton flare were twice identified at Venus within <43 and <67 h after the flare. Although these CMEs did not necessarily originate from the X9.0 flare on December 5, 2006, they most likely originated from the same active region because these characteristics are very similar to flare-associated CMEs observed on the Earth. These observations indicate that CME and flare activities on the invisible side of the Sun may affect terrestrial space weather as a result of traveling more than 90° in both azimuthal directions in the heliosphere. We would also like to emphasize that during the SEP activity, MEX data indicate an approximately one-order of magnitude enhancement in the heavy ion outflow flux from the Martian atmosphere. This is the first observation of the increase of escaping ion flux from Martian atmosphere during an intensive SEP event. This suggests that the solar EUV flux levels significantly affect the atmospheric loss from unmagnetized planets.     

Correction of SOHO CELIAS/SEM EUV measurements saturated by extreme solar flare events

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)     

The dependence of solar flare energetics on flare volumes

Solar X-ray flare images from Skylab and data from full Sun detectors were used in a statistical analysis to determine the relationship between flare volumes and flare energetics. Data from the rise phases of 45 flares were used in the analysis. For each event the diameter D, length L, and volume V of the flare loops were determined and then compared to the thermal energy, rate of increase of thermal energy, and rise time of the soft X-ray flux. The latter three quantities were all found to be positively correlated with D, L, and V. However, the thermal energy per unit volume and rate of increase of thermal energy per unit volume decrease with increasing volume. No correlation was found between emission measure Y and volume V, indicating that the electron density tends to be smaller for larger flare volumes. We find a larger dynamic range for V than for Y, hence knowledge of V is more critical than that of Y for calculating the thermal energy of the X-ray emitting structure, which is proportional to Y ^0.5 V ^0.5. Using certain assumptions, the results were compared to several flare models. The classical neutral sheet model, the sheared loop model of Spicer and even models using the magnetic field in a passive role for the energy release were all found to be consistent with the results.     

Cooling of solar flare plasmas

A simple model for the cooling of solar flare plasmas is considered. This model predicts that an increase in emission measure with decreasing temperature is a general feature of a cooling flare. The results are compared to solar flare data.     

A comparison of theoretical and experimental estimates of the solar proton diffusion coefficient during three flare events

The propagation time for solar protons observed during the events of January 24, February 25 and March 17, 1969 are compared with those estimated from numerical solutions of the Fokker-Planck transport equation, using values of the diffusion coefficient of the form K _r = K ₀ r ^b where r is radial distance from the Sun, K ₀ is obtained from the plasma-field parameters near the Earth and b varies from - 3 to + 1. K ₀ is derived either by assuming that all the magnetic fluctuation power is in small amplitude transverse waves or alternatively in discontinuous changes in ¦B¦ along the flux tube of propagation. In the first case it is found that the K ₀ values calculated require either b -3, implying a very rapid wave growth with r, or the Fokker-Planck equation reduces to the situation of purely convective transport which is at variance with the experimental observations. More reasonable results are found in the second case although even here K ₀ is probably underestimated. Alternative ways of deducing K ₀ empirically from particle anisotropy measurements are put forward and these seem to favour the discontinuity model.     

Cosmic-ray intensity related to solar and terrestrial activity indices in solar cycle No. 20

The 11-year modulation of cosmic-ray intensity is studied using the data from nine world-wide neutron monitoring station over the period 1965–1975. From this analysis the following relation among the modulated cosmic-ray intensityI, the relative sunspot numberR, the number of proton eventsN _p and the geomagnetic indexA _p has been derived which describes the long-term modulation of cosmic rays $$I = C - 10^{ - 3} (KR + 4N_P + 12A_P ),$$ whereC is a constant which depends on the rigidity of each station, andK is a coefficient related to the diffusion coefficient of cosmic rays and its transition in space. The standard deviation between the observed and calculated values of cosmic-ray intensity is about 5–9%. This relation has been explained by a generalization of the Simpson solar wind model which has been proved by the spherically symmetric diffusion-convection theory.     

Imaging of impulsive solar flare phenomena

We review some recent advances in our understanding of impulsive solar flare phenomena obtained through new hard X-ray and radio imaging instruments (the Solar Maximum Mission and Hinotori satellites, the VLA and VLBI).     

The structure of high-temperature solar flare plasma in non-thermal flare models

We solve the energy equation for the high-temperature (coronal) component of flare plasma for two models of energy input: (i) direct collisional heating by a beam of suprathermal electrons, and (ii) ohmic heating by the beam-neutralizing reverse current. We discuss the regimes where each case is applicable, and solve for the differential emission measure distribution of the coronal plasma in each case. Scaling laws between loop temperatures and injected electron fluxes are derived for both models; these are testable observationally through coordinated soft X-ray and hard X-ray observations, thus providing a method of discriminating between the two cases. We also readdress the question of the energetic importance of a return current which is below the instability threshold for generation of ion-acoustic plasma turbulence. We find that unless the ambient coronal density is very low ( 10⁹ cm ^–3), collisional heating will always dominate there, in agreement with the findings of previous authors. However, in the chromosphere/corona transition region, the relatively low temperature and correspondingly high plasma resistivity imply that reverse current ohmic heating can predominate the flare energetics, by up to an order of magnitude.Presidential Young Investigator.     

Source of the solar flare energy

Recent Skylab and magnetograph observations indicate that strong photospheric electric currents underlie small flare events such as X-ray loops and surges. What is not yet certain, because of the non-local dynamics of a fluid with embedded magnetic field, is whether flare emission derives from the energy of on-site electric currents or from energy which is propagated to the flare site through an intermediary, such as a stream of fast electrons or a group of waves. Nevertheless, occurrences of: (1) strong photospheric electric currents beneath small flares; (2) similar magnetic fine structure inside and outside active regions; (3) eruptive prominences and coronal white light transients in association with big flares; and, (4) active boundaries of large unipolar regions suggest the possibility that all phenomena of solar activity are manifestations of the rapid ejection and/or gradual removal of electric currents of various sizes from the photosphere. The challenge is to trace the precise magnetofluid dynamics of each active phenomenon, particularly the role of electric current build-up and dissipation in the low corona.     

Prospects for solar flare X-ray polarimetry

The history of solar flare X-ray polarimetry is reviewed and it is shown that as yet, there is no experimental evidence for such polarization. The present experimental limits are at the level of a few percent but these results may be biased by a large thermal component at low energies which may decrease the apparent polarization. To avoid this difficulty it will be necessary to make observations at higher energies where thermal emission is less important.The theoretical estimates of the polarization expected in the solar flare are also reviewed. The best present theoretical estimates are in the range of a few percent and are consistent with the present experimental limits.In this paper we discuss a new satellite instrument that has sufficient sensitivity at high energies to detect the polarization that is predicted by the present theories. The instrument sensitivity for a moderate (M class) event approaches polarization levels of 1% in each of 7 energy bins spanning the 10 to 100 keV range for integration times as short as 10 s. Comparable results can be obtained for an X class flare in 1 s.Presidential Young Investigator.     

Potential of interplanetary torques and solar modulation for triggering terrestrial atmospheric and lithospheric events

The Sun is forced into an orbit around the barycenter of the solar system because of the changing mass distributions of the planets. Solar-planetary-lunar dynamic relationships may form a new basis for understanding and predicting cyclic solar forcing functions on the Earth's climate.     

Cosmic-ray intensity versus solar soft X-ray background in cross-correlation analysis

The relationship between the galactic cosmic ray modulation (CR) and the non-flare coronal level, as given by the solar soft X-ray background (XBG), is investigated from 1 July 1968 to 30 June 1980 on a daily basis. The stationarity problem of a multivariate time series, as well as the role of the short- and medium-term corona variability are faced. From them it is found that the CR/XBG relation is variable during the considered heliomagnetic semicycle, while CR and XBG are highly anticorrelated on a long-time scale (12-month averages). The CR/XBG relationship during the declining phase of solar activity shows a moderately strong anticorrelation, on short- and medium-term time scales (coefficient up to –0.77 for 27-day running averages), went towards insignificant values in the minimum phase and is only partially reconstructed during the rise of the following solar cycle. During the solar activity maximum of cycle 20 the cosmic-ray modulation is only related to the short-term coronal fluctuations (no other time scales are significant, supporting the reliability of the so-called `Gnevyshev gap' in solar parameters).     

An overview of solar and stellar flare research

An overview of the many topics discussed at IAU Colloquium No. 104 is presented as an introduction to the Proceedings. Suggested areas for future research emerging from the conference are summarized.