G. Trottet

Energetic Neutrons, Protons, and Gamma Rays during the 1990 May 24 Solar Cosmic-Ray Event

The solar cosmic-ray event on 1990 May 24 can be divided into three phases: a first impulsive production of γ-rays and greater than 200 MeV neutrons; a second slower phase during which there were high-energy protons at the Sun for ~20 minutes producing pions and high-energy neutrons; and a third phase when the protons observed by the IMP 8 and GOES spacecraft and by neutron monitors were injected into interplanetary space. This third phase started after the onset of the event but before the second phase had ceased. We found that high-energy neutron production occurred during the last 60 s of the impulsive phase and at least the first 19 minutes of the second phase. During the second phase the high energy-neutron and γ-ray emissions decayed more slowly than either the 2.2 MeV or 4-7 MeV γ-ray line emissions. A two-component neutron energy spectrum that changes between the first and second phases gives a reasonable fit to the count rate increase recorded by the Climax neutron monitor. From the fit we infer that the integrated neutron emissivity at the Sun was ~3.5 × 1030 sr-1 for E > 100 MeV. The maximum intensity of P > 1.5 GV solar protons near the Earth was 4.5 × 103 (m2 sr s)-1. The differential solar proton energy flux (dJ/dE) as a function of rigidity at the Sun can be described by an evolving power-law spectrum. We estimate that the number of escaping protons with E > 30 MeV in the third phase was 7%-14% of the number of protons required to produce the solar neutron increase at the Earth. Although it is attractive to assume that the interplanetary solar protons simply leaked out from the trapping region at the Sun, the data suggest that the interplanetary solar protons were not from the population of energetic particles that produced the neutron and γ-ray emissions but were freshly accelerated during the third phase of the solar flare.

Hard X-ray and γ-ray observations of solar flares with the Phebus experiment

The Phebus experiment on board the GRANAT satellite provides temporal and spectral observations of solar and cosmic γ-ray bursts in the 0.1 ≃ 100 MeV nominal energy range. The experiment was turned on January 8, 1990 and is still in operation. In this paper we present the main characteristics of the Phebus experiment and we describe and discuss some of the observational properties of the 18 solar hard X-ray/γ-ray events detected during the first semester of the Phebus operation. It is found that: (i) events of a few minutes duration, detected above ∼ 100 keV, systematically show subsecond time variations; (ii) longer duration events (>5 min) do not exhibit fast time variations and generally consist of 1-min peaks superimposed on a less intense, sometimes harder, slowly varying component. In addition to these general trends we discuss in more detail three events for which significant count-rates have been detected above 10 MeV.

Solar hard X-ray and γ-ray bursts observed by the sigma anti-coincidence shield aboard granat

The anti-coincidence shield of the SIGMA telescope aboard GRANAT provides observations of solar X-ray and γ-ray bursts in the energy range 200 keV–15 MeV. The characteristics of the experiment are presented here as well as the observations of the solar bursts detected during the first year of operation. Among these events, two of these bursts associated respectively with a limb and a disk flare were observed with significant flux above 10 MeV. Advantage is taken of the experiment large detector area to examine fine time structures even at high energies (≥ 10 MeV).

The mark II Nançay radioheliograph

We describe the new Nançay Radioheliograph (version II) which is a multicorrelator system. The main characteristics of this instrument are high space and time resolutions and flexible on-line data processing. At present it produces one-dimensional images at 169 MHz. This instrument is mainly intended for solar observations.An accurate calibration of the instrument has been carried out leading to a precise fringe stopping and to a proper knowledge of the grating response. Preliminary solar results are presented: existence of apparent motions in noise storms with a velocity of the order of 5000 km s−1; observations of radiobursts as narrow as 0.7′ (Kerdraon, 1977); observations of spikeburst activity.

CORRELATED FAST TIME STRUCTURES AT MILLIMETER WAVES AND HARD X-RAYS DURING A SOLAR BURST

We present an analysis of the time profiles detected during a solar impulsive flare, observed at one-millimeter radio frequency (48xa0GHz) and in three hard X-ray energy bands (25–62, 62–111, and 111–325xa0keV) with high sensitivity and time resolution. The time profiles of all emissions exhibit fast time structures of 200–300xa0ms half power duration which appear in excess of a slower component varying on a typical time scale of 10xa0s. The amplitudes of both the slow and fast variations observed at 48xa0GHz are not proportional to those measured in the three hard X-ray energy bands. However, the fast time structures detected in both domains are well correlated and occur simultaneously within 64xa0ms, the time resolution of the hard X-ray data. In the context of a time-of-flight flare model, our results put strong constraints on the acceleration time scales of electrons to MeV energies.

Time-extended acceleration of energetic particles in the corona during the 19 October 1989 GLE

Abstract The broad injection cones and long durations of large solar energetic particle events (SEPs) have been recently considered as evidence for particle acceleration in the shock wave generated by a coronal mass ejection. Radio observations of an energetic flare on 19 Oct 1989 show that energetic electrons radiate both near the flare site and several tens of heliocentric degrees away. This is evidence for rapid particle propagation from the flare site or else for the simultaneous activation of widely separated sites of acceleration. At some places energetic electron signatures persist over at least several hours after the flare. We argue that even if the CME-associated shock wave may contribute to the acceleration, it competes with processes in the low and middle corona which may supply energetic particles to interplanetary space over several hours and with injection cones comparable with the width of CMEs.

Radio and X-ray/gamma-ray observations of two solar flares

Abstract This paper deals with a preliminary study of two flares observed by the PHEBUS instrument and by the Nancay Multifrequency Radioheliograph in the hard X-ray/gamma-ray and in the decimeter-meter radio wave domains respectively. For both flares the radio emission associated with individual hard X-ray/gamma-ray peaks show different characteristics. This is discussed in terms of temporal changes of the large scale magnetic structure involved in the region of particle acceleration.

Temporal characteristics of solar hard X-ray/gamma-ray bursts observed by PHEBUS

Abstract The PHEBUS observations show that short duration hard X-ray/gamma-ray bursts detected above ⋍ 70 keV generally exhibit subsecond time structures. On the contrary, long duration events do not show fast time variations and usually consist in ⋍ one minute duration peaks superposed on a less intense and slowly varying component.

The mark III Nanay radioheliograph The Radioheliograph Group

A north-south array has been added to the Mark II Nançay Radioheliograph (Radioheliograph Group, 1977). This instrument gives at 169 MHz two one-dimensional images of the Sun in the east-west and north-south directions including measurement of the circular polarization ratio. The main performances of this instrument are high space and time resolutions, flexible on line data processing and the possibility of off-line interactive data reduction. The method of calibration is briefly described. A few observations are presented.

Dynamics of coronal magnetic fields inferred from multi-frequency radio observations of a solar flare

Abstract We analysed multi-frequency radio and hard X-Ray observations of an M1.5 solar flare that occurred on November 5, 1998. The event was observed at various discrete radio frequencies from decimetric to microwave wavelengths, covering a wide range of coronal altitudes. Using combined imaging radio and hard X-Ray emissions it was possible to follow the dynamics of solar magnetic fields from the low corona up to about 0.5 solar radius above the photosphere. The radio spectra show different characteristics during event evolution, suggesting the appearance of new sources. The imaging analysis at hard X-Rays and decimetric/metric waves confirmed the presence of various and distinct emitting sources during the event, suggesting a dynamic and complex magnetic topology, probably associated with different injection of nonthermal electrons.