Relativistic electrons from the Sun observed by IMP-4

Data are presented from the IMP-4 satellite of 0.3–12 MeV electrons from the Sun between May 24, 1967 and May 2, 1969. Correlations with contemporary proton intensity increases at energies above 1 MeV are studied. Classical solar flare events such as those frequently observed from 30°W–60°W in solar longitude are not discussed. Categories of unusual events are defined and examples of each type are given. Discussion of these events centers around the emission and propagation of energetic particles from the point of origin on the Sun to the Earth. The results of this study are the following: (1) The differential electron energy spectrum (0.3–12 keV) from solar flares appears to be a constant of the flare process, with the spectral index = (-)3.0 ± 0.2. (2) Particle emission from solar flares contains a prompt component, which is injected into the interplanetary medium beyond the Sun and which is responsible for the diffusion characteristics of solar particle events, and a delayed component which is effectively contained in the lower solar atmosphere where it diffuses typically ± 100° in longitude and gradually escapes into interplanetary space. The delayed component gives rise to the corotating features commonly observed after the impulsive and diffusive onset from the prompt component. This is not the same as the two component model discussed by Lin (1970a) in which 40 keV electrons are often observed as a separate phenomenon and frequently precede higher energy particles observed at 1 AU. (3) Storage of electrons > 300 keV and protons > 1 MeV is essential to explain emission and propagation characteristics of solar particle events. In some rare cases the storage mechanism appears to be very efficient, culminating in a catastrophic decay of the trapping region. (4) The events with low proton/electron ratios all occur at least three weeks after the previous relativistic electron producing flare.     

Evidence for electron excitation of type III radio burst emission

Type III radio bursts observed at kilometric wavelengths ( 0.35 MHz) by the OGO-5 spacecraft are compared with > 45 keV solar electron events observed near 1 AU by the IMP-5 and Explorer 35 spacecraft for the period March 1968–November 1969.Fifty-six distinct type III bursts extending to 0.35 MHz ( 50 R equivalent height above the photosphere) were observed above the threshold of the OGO-5 detector; all but two were associated with solar flares. Twenty-six of the bursts were followed 40 min later by > 45 keV solar electron events observed at 1 AU. All of these 26 bursts were identified with flares located west of W 09 solar longitude. Of the bursts not associated with electron events only three were identified with flares west of W 09, 18 were located east of W 09 and 7 occurred during times when electron events would be obscured by high background particle fluxes.Thus almost all type III bursts from the western half of the solar disk observed by OGO-5 above a detection flux density threshold of the order of 10^–13 Wm^–2 Hz^–1 at 0.35 MHz are followed by > 45 keV electrons at 1 AU with a maximum flux of 10 cm^–2 s^–1 ster^–1. If particle propagation effects are taken into account it is possible to account for lack of electron events with the type III bursts from flares east of the central meridian. We conclude that streams of 10–100 keV electrons are the exciting agent for type III bursts and that these same electrons escape into the interplanetary medium where they are observed at 1 AU. The total number of > 45 keV electrons emitted in association with a strong kilometer wavelength type III burst is estimated to be 5 × 10³².     

Characteristics of solar coronal source regions producing 3He-rich particle events

We use H, X-ray, and kilometric radio data to examine the solar coronal activity associated with energetic (1 MeV/nucl^–1) ³He-rich particle events observed near Earth. The basis of the study is the 12 ³He-rich events observed in association with impulsive 2 to 100 keV electron events reported by Reames et al. (1985). We find that when H and X-ray brightenings can be associated with ³He/electron events, they have onsets coinciding to within 1 min of that of the associated metric type III bursts. In three or four events we found no associated H or X-ray flares, and in two events even the metric type III bursts were weak or absent. The measured low-energy (2 keV) electron spectra for these events show no evidence of a flattening due to Coulomb collisional losses. These results and several other recent findings are consistent with the idea that the ³He/electron events are due to particle acceleration in the corona well above the associated H and X-ray flares.     

The gamma ray spectrometer for the Solar Maximum Mission

The Solar Maximum Mission Gamma Ray Experiment (SMM GRE) utilizes an actively shielded, multicrystal scintillation spectrometer to measure the flux of solar gamma rays. The instrument provides a 476-channel pulse height spectrum (with energy resolution of 7% at 662 keV) every 16.38 s over the energy range 0.3–9 MeV. Higher time resolution (2 s) is available in three windows between 3.5 and 6.5 MeV to study prompt gamma ray line emission at 4.4 and 6.1 MeV. Gamma ray spectral analysis can be extended to 15 MeV on command. Photons in the energy band from 300–350 keV are recorded with a time resolution of 64 ms. A high energy configuration also gives the spectrum of photons in the energy range from 10–100 MeV and the flux of neutrons 20 MeV. Both have a time resolution of 2 s. Auxiliary X-ray detectors will provide spectra with 1-sec time resolution over the energy range of 10–140 keV. The instrument is designed to measure the intensity, energy, and Doppler shift of narrow gamma ray lines as well as the intensity of extremely broadened lines and the photon continuum. The main objective is to use this time and spectral information from both nuclear gamma ray lines and the photon continuum in a direct study of the dynamics of the solar flare/particle acceleration phenomena.     

Solar radio bursts with a spectral flattening at millimeter wavelengths

An analysis of solar radio burst spectra in the range 3–80 GHz is carried out using measurements of the observatories at Bern and Nobeyama supplemented by data from worldwide network stations. Special interest was focused on strong events at frequencies above 30 GHz. It is found that there exists an extended group of events with a flattening of the spectra at millimeter wavelengths. In particular, two types of flattening are observed: (i) a high-frequency flattening either following a monotonic spectral flux increase at cm-waves or forming a flat broad-band spectrum at mm-wavelengths ; (ii) a millimetric flattening as a decrease of the slope (i.e., a hardening) of the descending branch of the spectrum having a peak in the microwave range. Besides this, in complicated bursts a strong temporal evolution of millimeter spectra may occur resulting in either type of the flattening. Some factors capable of producing the millimeter flattening are considered: (1) superposition of multiple source regions of gyrosynchrotron radiation, (2) gyromagnetic radiation from a two-component energy spectrum of the accelerated electrons at high energies, or by a temporal hardening of the electron spectrum during extended flares, and (3) optically thin bremsstrahlung of evaporated plasma.Presented at the CESRA Workshop on Coronal Magnetic Energy Release at Caputh near Potsdam in May 1994.     

Energetic solar electrons in the interplanetary medium

ISEE-3 measurements extending down to 2 keV energy have provided a new perspective on energetic solar electrons in the interplanetary medium. Impulsive solar electron events are observed, on average, several times a day near solar maximum, with 40% detected only below 15 keV. The electron energy spectra have a nearly power-law shape extending smoothly down to 2 keV, indicating that the origin of these events is high in the corona. These coronal flare-like events often produced ³He-rich particle events.In large solar flares which accelerate electrons and ions to relativistic energies, the electron spectrum appears to be modified by a second acceleration which results in a double power-law shape above 10 keV with a break near 100 keV and flattening from 10–100 keV. Large flares result in long-lived (many days) streams of outflowing electrons which dominate the interplanetary fluxes at low energies. Even in the absence of solar activity, significant fluxes of low energy electrons flow out from the Sun.Solar type-III radio bursts are produced by the escaping 2–10² keV electrons through a beam-plasma instability. The detailed ISEE-3 measurements show that electron plasma waves are generated by the bump-on-tail distribution created by the faster electrons running ahead of the slower ones. These plasma waves appear to be converted into radio emission by nonlinear wave-wave interactions.     

Particle acceleration in impulsive solar flares

A model for second-step electron acceleration in impulsive solar flares is presented. We have extended the theory of stochastic particle acceleration to include Coulomb energy losses which become important at low coronal heights. This inclusion successfully explains the observed steepening of interplanetary electron spectra below 3 MeV following impulsive solar flares taking place at low coronal heights. It also explains the observed spectral differences of relativistic electrons in long-duration and impulsive flares.     

A search for solar neutrons during solar flares

The upper limit on the solar neutron flux from 1–20 MeV has been measured, by a neutron detector on the OGO-6 satellite, to be less than 5 × 10^–2 n cm^–2 s^–1 at the 95% confidence level for several flares including two flares of importance 3B and a solar proton event of importance 3B. The measurements are consistent with the models proposed by Lingenfelter (1969) and by Lingenfelter and Ramaty (1967) for solar neutron production during solar flares. The implied upper limit on the flux of 2.2 MeV solar gamma rays is about the same as the 2.2 MeV flux observed by Chupp et al. (1973).     

Enhancement of solar heavy nuclei at high energies in the 4 July 1974 event

Relative abundances of energetic nuclei in the 4 July 1974 solar event are presented. The results show a marked enhancement of abundances that systematically increase with nuclear charge numbers in the range of the observation, 6 Z 26 for energies above 15 MeV nucl.^–1 While such enhancements are commonly seen below 10 MeV nucl^–1, most observations at higher energies are found to be consistent with solar system abundances. The energy spectrum of oxygen is observed to be significantly steeper than most other solar events studied in this energy region. It is proposed that these observations are characteristic of particle populations at energies 1 MeV nucl^–1, and that the anomalous features observed here may be the result of the high energy extension of such a population that is commonly masked by other processes or populations that might occur in larger solar events.     

Solar hard X-ray bursts

The major results from SMM are presented as they relate to our understanding of the energy release and particle transportation processes that lead to the high-energy X-ray aspects of solar flares. Evidence is reviewed for a 152–158 day periodicity in various aspects of solar activity including the rate of occurrence of hard X-ray and gamma-ray flares. The statistical properties of over 7000 hard X-ray flares detected with the Hard X-Ray Burst Spectrometer are presented including the spectrum of peak rates and the distribution of the photon number spectrum. A flare classification scheme introduced by Tanaka is used to divide flares into three different types. Type A flares have purely thermal, compact sources with very steep hard X-ray spectra. Type B flares are impulsive bursts which show double footpoints in hard X-rays, and soft-hard-soft spectral evolution. Type C flares have gradually varying hard X-ray and microwave fluxes from high altitudes and show hardening of the X-ray spectrum through the peak and on the decay. SMM data are presented for examples of type B and type C events. New results are presented showing coincident hard X-rays, O v, and UV continuum observations in type B events with a time resolution of 128 ms. The subsecond variations in the hard X-ray flux during 10% of the stronger events are discussed and the fastest observed variation in a time of 20 ms is presented. The properties of type C flares are presented as determined primarily from the non-imaged hard X-ray and microwave spectral data. A model based on the association of type C flares and coronal mass ejections is presented to explain many of the characteristics of these gradual flares.     

Electron acceleration in solar flares

For the period September 1978 to December 1982 we have identified 55 solar flare particle events for which our instruments on board the ISEE-3 (ICE) spacecraft detected electrons above 10 MeV. Combining our data with those from the ULEWAT spectrometer (MPI Garching and University of Maryland) electron spectra in the range from 0.1 to 100 MeV were obtained. The observed spectral shapes can be divided into two classes. The spectra of the one class can be fit by a single power law in rigidity over the entire observed range. The spectra of the other class deviate from a power law, instead exhibiting a steepening at low rigidities and a flattening at high rigidities. Events with power-law spectra are associated with impulsive (<1 hr duration) soft X-ray emission, whereas events with hardening spectra are associated with long-duration (<1 hr) soft X-ray emission. The characteristics of long-duration events are consistent with diffusive shock acceleration taking place high in the corona. Electron spectra of short-duration flares are well reproduced by the distribution functions derived from a model assuming simultaneous second-order Fermi acceleration and Coulomb losses operating in closed flare loops.     

The solar-flare infrared continuum

We consider potential sources of infrared (1 to 1 mm) continuum in solar flares. Several mechanisms should produce detectable fluxes: in the 350 window for ground-based observations, impulsive emission will arise in synchrotron radiation from 1–10 MeV electrons, and possibly thermal (free-free) continuum from the source of the white-light flare; the hot flare plasma responsible for soft X-ray emission will also emit detectable fluxes of free-free continuum in the largest flares. At shorter wavelengths the dominant infrared emission will come from the H flare itself. Observations in the infrared wavelengths will help to complete our picture of flare structure in both the impulsive and gradual phases.     

Gamma-ray and microwave evidence for two phases of acceleration in solar flares

Relativistic electrons in large solar flares produce gamma-ray continuum by bremsstrahlung and microwave emission by gyrosynchrotron radiation. Using observations of the 1972, August 4 flare, we evaluate in detail the electron spectrum and the physical properties (density, magnetic field, size, and temperature) of the common emitting region of these radiations. We also obtain information on energetic protons in this flare by using gamma-ray lines. From the electron spectrum, the proton-to-electron ratio, and the time dependences of the microwave emission, the 2.2 MeV line and the gamma-ray continuum, we conclude that in large solar flares relativistic electrons and energetic nuclei are accelerated by a mechanism which is different from the mechanism which accelerates 100 keV electrons in flares.Research supported by NASA Grant 21-002-316 at the University of Maryland, College Park.     

Solar X-ray Spectrometer (Soxs) Mission on Board GSAT2 Indian Spacecraft: The Low-Energy Payload

The first space-borne solar astronomy experiment of India, namely Solar X-ray Spectrometer (SOXS), was successfully launched on 08 May 2003 on board geostationary satellite GSAT-2 of India. The SOXS is composed of two independent payloads, viz. SOXS Low-Energy Detector (SLD) Payload and SOXS High-Energy Detector (SHD) Payload. The SOXS aims to study the full-disk integrated X-ray emission in the energy range from 4 keV to 10 MeV. In this paper we present the first report on the SLD instrumentation and its in-orbit performance. The SLD payload was designed and developed at the Physical Research Laboratory in collaboration with various centers of Indian Space Research Organisation (ISRO). The basic scientific aim of the SLD payload is to study solar flares in the energy range from 4 to 60 keV with high spectral and temporal resolution. To meet these requirements, the SLD payload employs state-of-the-art solid state detectors, the first time for a solar astronomy experiment, viz. Si PIN (4 –25 keV), and cadmium–zinc–telluride (4 –60 keV). With their superb high-energy resolution characteristics, SLD can observe iron and iron–nickel complex lines that are visible only during solar flares. In view of its 3.4 FOV, the detector package is mounted on a Sun Aspect System, for the first time, to get uninterrupted observations in a geostationary orbit. The SLD payload configuration, its in-flight operation, and the response of the detectors are presented. We also present the first observations of solar flares made by the SLD payload and briefly describe their temporal and spectral mode results.     

The super-hot thermal component in the decay phase of solar flares

Solar X-ray observations from balloons and from the SMM and HINOTORI spacecraft have revealed evidence for a super-hot thermal component with a temperature of 3 × 10⁷ K in many solar flares, in addition to the usual 10–20 × 10⁶ K soft X-ray flare plasma. We have systematically studied the decay phase of 35 solar flare X-ray events observed by ISEE-3 during 1980. Based on fits to the continuum X-ray spectrum in the 4.8–14 keV range and to the intensity of the 1.9 Å feature of iron lines, we find that 15 (about 43%) of the analyzed events have a super-hot thermal component in the decay phase of the flare. In this paper the important properties of the super-hot thermal component in the decay phase are summarized. It is found that an additional input of energy is required to maintain the super-hot thermal components. Finally, it is suggested that the super-hot thermal component in the decay phase is created through the reconnection of the magnetic field during the decay phase of solar flares.     

A search for neutrons of solar origin using balloon borne detectors 1967–69

A series of telescopes having approximately a 30° half opening angle and responding to neutrons in the energy range 50 MeV to 350 MeV has been flown to the top of the atmosphere on balloons released from an equatorial launching site at Kampala, Uganda, between 1967 and 1969. The aim of the experiment was to attempt to detect solar neutrons during periods of enhanced solar activity. No neutrons of solar origin were detected, but an upper limit of the order of 30 neutrons m^–2 s^–1 at the Earth has been placed on the continuous solar neutron flux in the above energy range, and a limit of four photons m^–2 s^–1 has also been placed on the corresponding -ray flux above 80 MeV. Limits have likewise been placed on the total emission from various flares. For a 1B flare the values were 23 × 10⁴ neutrons m^–2 and 6 × 10⁴ photons m^–2.     

Upper limit on the steady emission of the 2.223 MeV neutron capture γ-ray line from the Sun

Data accumulated by the Solar Maximum Mission Gamma Ray Spectrometer (GRS) have been searched for evidence of the 2.223 MeV neutron capture line from the Sun, outside the times of -ray-emitting solar flares. Background-corrected spectra accumulated over 3-day intervals between 1980 and 1989 show no evidence of the line. Upper limits are reported separately for periods of high and low solar activity.A conservative 3 upper limit of 5.7 × 10^–5 (cm² s)^–1 is placed on the steady flux in the 2.223 MeV line during inactive periods, which is nearly two orders of magnitude lower than previously published results. After correction for limb darkening of the line emission from off-center positions, this upper limit becomes 7.1 × 10^–5 (cm²s)^–1. Our 3 upper limit on the steady flux in the line during periods of high solar activity is 6.9 × 10^–5 (cm² s)^–1, or 8.6 × 10^–5 (cm² s)^–1 after correction for limb darkening. Our results imply that the quiescent solar corona cannot be heated by ions accelerated above 1 MeV in microflares (or a continuous acceleration process), so long as the ion energy spectrum is similar to that measured in large flares. We also use our results to derive the rate of tritium production at the solar surface; our upper limit of 9 nuclei (cm² s)^–1 is about a factor of 9 below the upper limit from searches for ³H in the solar wind. We place upper limits of the order 10³³ on the number of energetic (> 30 MeV) protons which can be stored in active regions prior to being released in solar flares, which imply that the strongest observed flares cannot be produced by such a mechanism.     

Energy Spectra of Accelerated Solar Protons From Different Sources – I. Reconstruction and Properties of the Source Spectrum

It is widely accepted now that a significant fraction of the solar energetic particles (SEPs) observed at 1 AU after major solar flares are actually accelerated at a CME-driven shock. In addition, in the emerging new paradigm for SEP acceleration in different sources at or near the Sun, the existence of two types of flares – impulsive and gradual – is recognized. Within this concept, it is tempting also to separate SEPs into two groups – interacting and escaping – and to derive their 'source spectra' from observational data on various flare emissions (protons, gamma rays, neutrons, etc.). By different techniques, those spectra have been reconstructed for 80 solar proton events (SPE) in 1949–1991. In this paper, all available data on the source spectra of solar protons are summarized and revised. We discuss in detail existing uncertainties in the derived spectral indexes, consider other methodological problems involved in this study, and suggest several possible lines for the future investigations of solar flares and SCRs using the source spectrum data. It is noted that some peculiarities of the spectra, for instance, spectral steepening for high energies, may be characteristic of large events of the 23 February 1956 type.     

Charge dependence of the solar modulation of multiply charged cosmic ray nuclei

The intensity and energy spectra of multiply charged cosmic ray nuclei, in the energy interval 250–1500 MeV/n, were studied at three different levels of solar activity, viz. in 1963, 1964 and 1967. The same detectors, nuclear emulsion stacks flown from Fort Churchill, Canada, were used to determine simultaneouslty the energy spectra of helium, C, N, O as well as H (Z=10–28) nuclei. An analysis of the measured spectra indicates that these can be interpreted in terms of: (a) the source spectrum as a Fermi spectrum with a spectral index of 2.65; (b) the interstellar propagation as in a Gaussian distribution of path lengths with a mean path length of 4 g cm^–2 and (c) the interplanetary propagation as given by the numerical solution of the Fokker-Planck equation incorporating diffusion, convection and adiabatic deceleration. On comparing the measured ratios of He to H-nuclei (mean Z14) with the theoretically calculated values for the three levels of solar activity, it is found that within experimental uncertainties, the solar modulation is essentially the same for nuclei of same mass to charge ratio and is not dependent on the charge of the nuclei.On leave from Tata Institute of Fundamental Research, Bombay.     

The hydrogen emission spectrum in three white-light flares

We present spectral data for three white-light flares (WLFs) showing Balmer continuum at wavelengths 3700 Å. These flares also have a weaker continuum extending toward longer wavelengths, from which, in one flare where this continuum is sufficiently bright, we are able to identify a Paschen jump near 8500 Å. The presence of the latter suggests that the Paschen continuum may be a substantial contributor to the WLF continuum at visible wavelengths. We note the possibility, therefore, that the entire continuum of this particular flare may be dominated by H _fb emission.In all three flares the head of the Balmer continuum, as well as the head of the Paschen continuum in the flare where it was identified, is advanced toward longer wavelengths as a result of the blending of the hydrogen emission lines of the respective series. The principal quantum number of the last resolvable line of the Balmer or Paschen series is approximately 16. The electron density, as measured from the halfwidths of the high Balmer lines in two of the flares, is approximately 5 × 10¹³ cm^–3. Due to possible misplacements of the spectrograph slit, however, the electron density in the brightest kernels of the WLFs may not have been obtained.Operated by the Association of Universities for Research in Astronomy, Inc. under contract AST 78-17292 with the National Science Foundation.