Photographs of the Sun in the XUV-region

X-ray photographs obtained with a zone plate camera on October 3, 1967 in the wavelength band 49.5–52.5 Å have been investigated photometrically.The most intense X-ray emission corresponds with active regions in H and Ca ii. About one quarter of the total solar flux is emitted by the three brightest X-ray sources (A, E and J). X-ray emission from quiet regions is also observed. Limb brightening is found, also at the poles, which indicates a higher electron density at the poles than during solar minimum.The brightest X-ray regions have a very small core of the order of 20. No relation to magnetic field strengths of sunspots has been found. However, a correlation with active prominences cannot be ruled out. X-ray source A is related either to prominence activity or to flare activity. One X-ray region (J) is probably related to flare activity.Assuming an electron temperature of 3 × 10⁶K to 5 × 10⁶K for coronal active regions an emission measure of a few times 10⁴⁹ cm^–3 is derived, which yields an electron density of a few times 10¹⁰ cm^–3.     

Diagnostic application of highly ionized iron lines in the XUV spectrum of a solar flare

Recent atomic data have been used to analyze a solar flare spectrum obtained with the Goddard Space Flight Center's grating spectrometer on the OSO-5 satellite. There exist in the wavelength region 90–200 Å strong lines from each of the ions Fe xviii-Fe xxiv. The Fe xxi lines can be used as an electron density diagnostic for the 10⁷ K plasma. From our analysis of a particular flare, we find a steep positive slope in the emission measure between 10^6.5 and 10^7.2 K and an electron density of 4 × 10¹¹ cm^–3 at 10⁷ K. We emphasise the need for high spectral and spatial resolution observations of solar flares in this wavelength region, which has to date been largely neglected.     

71 GHz (4.2 mm) solar radio bursts in the period July 1967 to December 1969

The results of 2 1/2 years (July 1967 – December 1969) monitoring of solar radio bursts at 71 GHz ( = 4.2 mm) at the Radio and Space Research Station, Slough are presented. During this period only seven events were positively identified as 71 GHz bursts. One of these events (6 July, 1968) is among the largest solar bursts ever recorded anywhere in the microwave-millimetre wave band (47000 × 10^–22Wm^–2Hz^–1), and the associated magnetic field may possibly have exceeded 7200 G. Another event (27 March, 1969) has demonstrated that bursts at 71 GHz can be both intense (4700 × 10^–22Wm^–2Hz^–1) and complex. On other occasions, the absence of any detectable event at 71 GHz helps to define the high frequency spectrum of the burst, this being an important factor in determining the initial energy distribution of the electrons ejected by the associated flare. On one such occasion (21 March, 1969) the derived energy distribution index is 8, in contrast with the more usual values of 2–4.1969–1970 NCR-OAR Senior Post-Doctoral Research Associate at Air Force Cambridge Research Laboratories, L. G. Hanscom Field, Bedford, Mass., U.S.A.     

Temporal evolution of the chromospheric electron density in the flare of January 5, 1992

The analysis of the dynamic evolution of the chromospheric electron density during solar flares is fundamental for the testing of solar flare models. For this purpose we developed a digital imaging spectrograph for the observation of higher Balmer lines below 400 nm with a time resolution of 1 s and an algorithm for the determination of the electron density from the observed line profiles. On January 5, 1992 a M1/1N flare was observed in H, H and Caii H and the temporal evolution of the electron density was determined. The chromospheric electron density rises several times from less than 3 × 10¹⁹ to 1 × 10²⁰ m^–3 during the hard X-ray peaks.     

Electron beams in the low corona

Selected high-resolution spectrograms of solar fast-drift bursts in the 6.2–8.4 GHz range are presented. The bursts have similar characteristics as metric and decimetric type III bursts: rise and decay in a few thermal collision times, total bandwidth 3% of the center frequency, low polarization, drift rate of the order of the center frequency per second, and flare association. They appear in several groups per flare, each group consisting of some tens of single bursts. Fragmentation is also apparent in frequency; there are many narrowband bursts randomly scattered in the spectrum. The maximum frequency of the bursts is highly variable.The radiation is interpreted in terms of plasma emission of electron beams at plasma densities of more than 10¹¹ cm^–-3. At this extremely high frequency, emission from the plasma level even at the harmonic is only possible in a very anisotropic plasma. The scale lengths perpendicular and parallel to the magnetic field can be estimated. A model of the source region and its environment is presented.Paper presented at the 4th CESRA Workshop in Ouranopolis (Greece) 1991.     

A single loop of 21 January 1974 flare

A single loop associated with a flare of 21 January 1974 was studied with NRL spectroheliograms in order to understand the phenomenon of evaporation. The loop seen in the emission lines of Fe xv reached its maximum brightness 15 min after the onset. The loop is different from a flare loop because of the time sequence in which it appeared and is different from a post-flare loop prominence system because of its morphology. The electron density in the loop increases gradually to 4 × 10¹⁰ cm^–3. The material of the loop is thought to be supplied from the lower atmosphere of the chromosphere or the photosphere. The loop is an associated phenomenon of the main flare event distinguished by a longer rise time (15 min) and a lower peak temperature (2 × 10⁶ K).     

Electron–Whistler Interaction in Coronal Loops and Radiation Signatures

Interaction of the 30–300 keV electrons with whistlers in solar coronal loops is studied using a quasi-linear approach. We show that the electron–whistler interaction may play a dominant role in the formation of fast electron spectra within the solar flare loops with the plasma temperature 10⁷ K and plasma density 10¹¹ cm^–3. It is found that Landau damping of whistlers provides weak and intermediate pitch-angle diffusion regimes of fast electrons in coronal loops. The level of whistler turbulence in the weak diffusion regime under flare conditions is estimated as 10^–7 of the energy density in the thermal particles. The `top – footpoint' relations between the hard X-ray flux densities and spectra are derived. The reason for a `broken' spectrum of the flare microwave emission is discussed.     

A broadband spectrometer for decimeter and microwave radio bursts

Observations of solar microwave bursts with high temporal and spectral resolution have shown interesting fine structures (FSs) of short duration and small bandwidth which are usually superimposed on the smooth continuum. These FSs are very intense (up to 10¹⁵ K) and show sometimes a high degree of circular polarization (up to 100%). They are believed to be generated by electron cyclotron maser emission (ECME) in magnetic loops. Another type are the microwave type III bursts, which are drifting microwave FSs, and are probably the signatures of travelling electron beams in the solar atmosphere. The exact emission mechanisms for these phenomena, in particular the source configuration, the plasma parameters and the distribution of radiating electrons are not clear. For a detailed study of these problems new observations of intensity and polarization with high resolution in time and in frequency in decimeter and microwave wavebands are essential. In order to investigate these features in greater detail, spectrometers with high temporal and spectral resolution are being developed by the solar radio astronomy community of China (Beijing Astronomical Observatory (BAO), Purple Mountain Observatory (PMO), Yunnan Astronomical Observatory (YAO), and Nanjing University (NJU)). The frequency range from 0.7 to about 12 GHz is covered by about five spectrometers in frequency ranges of 0.7–1.4 GHz, 1–2 GHz, 2.4–3.6 GHz, 4.9–7.3 GHz, and 8–12 GHz, respectively. The radiospectrometers will form a combined type of swept-frequency and multi-channel receivers. The main characteristics of the solar radio spectrometers are: frequency resolution: 1–10 MHz; temporal resolution: 1–10 ms; sensitivity: better than 2% of the quiet-Sun level. We pay special attention to the sensitivity and the accuracy of polarization. Now, the 1–2 GHz radiospectrometer is being set up. The full system will be set up in 3–4 years.Presented at the CESRA-Workshop on Coronal Magnetic Release at Caputh near Potsdam in May 1994.     

10–100 keV electron acceleration and emission from solar flares

We present an analysis of spacecraft observations of non-thermal X-rays and escaping electrons for 5 selected small solar flares in 1967. OSO-3 multi-channel energetic X-ray measurements during the non-thermal component of the solar flare X-ray bursts are used to derive the parent electron spectrum and emission measure. IMP-4 and Explorer-35 observations of > 22 keV and > 45 keV electrons in the interplanetary medium after the flares provide a measure of the total number and spectrum of the escaping particles. The ratio of electron energy loss due to collisions with the ambient solar flare gas to the energy loss due to bremsstrahlung is derived. The total energy loss due to collisions is then computed from the integrated bremsstrahlung energy loss during the non-thermal X-ray burst. For > 22 keV flare electrons the total energy loss due to collisions is found to be 10⁴ times greater than the bremsstrahlung energy loss and 10² times greater than the energy loss due to escaping electrons. Therefore the escape of electrons into the interplanetary medium is a negligible energetic electron loss mechanism and cannot be a substantial factor in the observed decay of the non-thermal X-ray burst for these solar flares.We present a picture of electron acceleration, energy loss and escape consistent with previous observations of an inverse relationship between rise and decay times of the non-thermal X-ray burst and X-ray energy. In this picture the acceleration of electrons occurs throughout the 10–100 sec duration of the non-thermal X-ray burst and determines the time profile of the burst. The average energy of the accelerated electrons first rises and then falls through the burst. Collisions with the ambient gas provide the dominant energetic electron loss mechanism with a loss time of 1 sec. This picture is consistent with the ratio of the total number of energetic electrons accelerated in the flare to the maximum instantaneous number of electrons in the flare region. Typical values for the parameters derived from the X-ray and electron observations are: total energy in > 22 keV electrons total energy lost by collisions = 10^28–29 erg, total number of electrons accelerated above 22 keV = 10³⁶, total energy lost by non-thermal bremsstrahlung = 10²⁴erg, total energy lost in escaping > 22 keV electrons = 10²⁶erg, total number of > 22 keV electrons escaping = 10^33–34.The total energy in electrons accelerated above 22 keV is comparable to the energy in the optical or quasi-thermal flare, implying a flare mechanism with particle acceleration as one of the dominant modes of energy dissipation.The overall efficiency for electron escape into the interplanetary medium is 0.1–1% for these flares, and the spectrum of escaping electrons is found to be substantially harder than the X-ray producing electrons.Currently at Tokyo Astronomical Observatory, Mitaka, Tokyo, Japan.     

Flare fragmentation and type III productivity in the 1980 June 27 flare

We present observations of the solar flare on 1980 June 27, 16:14–16:33 UT, which was observed by a balloon-borne 300 cm² phoswich hard X-ray detector and by the IKARUS radio spectrometer. This flare shows intense hard X-ray (HXR) emission and an extreme productivity of (at least 754) type III bursts at 200–400 MHz. A linear correlation was found between the type III burst rate and the HXR fluence, with a coefficient of 7.6 × 10²⁷ photons keV^–1 per type III burst at 20 keV. The occurrence of 10 type III bursts per second, and also the even higher rate of millisecond spikes, suggests a high degree of fragmentation in the acceleration region. This high quantization of injected beams, assuming the thick-target model, shows up in a linear relationship between hard X-ray fluence and the type III rate, but not as fine structures in the HXR time profile.The generation of a superhot isothermal HXR component in the decay phase of the flare coincides with the fade-out of type III production.Universities Space Research Associates.ST Systems Corporation.     

A high-energy solar flare burst complex and the physical properties of its source region

We discuss a solar flare microwave burst complex, which included a major structure consisting of some 13 spikes of 60 ms FWHM each, observed 21 May, 1984 at 90 GHz (3 mm). It was associated with a simultaneous very hard X-ray burst complex. We suggest that the individual spikes of both bursts were caused by the same electron population: the X-bursts by their bremsstrahlung, and the microwave bursts by their gyrosynchrotron emission. This latter conclusion is based on the evidence that the radio turnover frequency was 150 GHz. It follows that the emission sources were characterized by an electron density of about 10¹¹ cm^–3, a temperature of 5 × 10⁸ K and a magnetic field of about 1400–2000 G. They had a size of about 350 km; if the energy release is caused by reconnection the sources of primary instability could have been smaller and in the form of thin sheets with reconnection speed at a fraction of the Alfvén velocity and burst-like energy injections of 10²⁷ erg during about 50 ms each. The energized plasma knots lost their injection energy by saturated convective flux (collisionless conduction) in about 30 ms.     

Electron density in flares II: Results of measurement

Measurements of the electron density in 16 flares are summarized and discussed. For 13 of them the electron density has been determined by the halfwidth method discussed in Part I of this paper. In the flash phase of all disk flares of importance 1 + and higher the electron density exceeds 10¹³ cm^–3 and increases with the flare importance. In the maximum of large flares the electron density exceeds 3 × 10¹³ cm^–3 and declines to 10¹³ cm^–3 and to lower values in about 20 minutes after the flash phase. In limb flares, i.e. higher than 5000 km above the solar limb, the electron density is lower than 5 × 10¹² cm^–3. This shows a decrease of the electron density in the flare elements situated in higher parts of the chromosphere. On the other hand, however, at least in some flares the electron density remains fairly constant within a wide range of height in the upper chromosphere and the low corona.     

Nonlinear treatment for solar radio spikes

We emphasize that a nonlinear treatment is required to realize the diagnostic potentiality of solar spiky emission. The observational constraints including the latest data on the harmonic structure, degree, and sense of polarization are discussed. A set of coupled equations for energy density of high-frequency normal modes of a magneto-active plasma involving the most important nonlinear effects within the three-wave approximation is deduced. The equations include both previously known and new effects. The qualitative evaluations of the equations obtained have provided a few new findings: (i) quasi-linear relaxation of fast electrons on quasi-potential waves (_) occurs in a characteristic time scale of the order of 10 ms if the frequency,f, is about 1 GHz; (ii) the stimulated scattering of the transverse waves on the background plasma particles is shown to be important if the brightness temperature of the spiky emission exceeds 10¹⁵–10¹⁶ K; (iii) the Raman scattering of the transverse waves on background plasma density inhomogeneities may suppress the electron cyclotron maser instability if n _e ² /n _e ² 3 × (10^–4–10^–5).     

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.     

Radio observations of the solar neutron flare of 3 June, 1982

In this paper the solar neutron and white-light flare is studied on the basis of radioastronomical observations. It is shown that the 3 June, 1982 flare had an impulsive character. A strong shock wave (M _A 2.9) was observed unusually soon after the impulsive phase of the flare. The radio spectrum of this event shows that the primary acceleration process probably occurred in the region with an electron density of n _e = 4.4 × 10¹⁵ m^–3. The pulsations of the type IV radio burst, observed 15 min after the mass ejection process, manifest the reconnection process in the post-flare stage.Proceedings of the Workshop on Radio Continua during Solar Flares, held at Duino (Trieste), Italy, 27–31 May, 1985.     

Observations of a post-flare radio burst in X-rays

More than six hours after the two-ribbon flare of 21 May 1980, the hard X-ray spectrometer aboard the SMM imaged an extensive arch above the flare region which proved to be the lowest part of a stationary post-flare noise storm recorded at the same time at Culgoora. The X-ray arch extended over 3 or more arc minutes to a projected distance of 95 000 km, and its real altitude was most probably between 110 000 and 180 000 km. The mean electron density in the cloud was close to 10⁹ cm^–3 and its temperature stayed for many hours at a fairly constant value of about 6.5 × 10⁶ K. The bent crystal spectrometer aboard the SMM confirms that the arch emission was basically thermal. Variations in brightness and energy spectrum at one of the supposed footpoints of the arch seem to correlate in time with radio brightness suggesting that suprathermal particles from the radio noise regions dumped in variable quantities into the low corona and transition layer; these particles may have contributed to the population of the arch, after being trapped and thermalized. The arch extended along the H = 0 line thus apparently hindering any upward movement of the upper loops reconnected in the flare process. There is evidence from Culgoora that this obstacle may have been present above the flare since 15–30 min after its onset.     

A search for energetic neutrons emitted during solar flares

On the basis of solar flare forecasts, balloon flights were made from Hyderabad, India (vertical geomagnetic threshold rigidity of 16.9 GV), to detect the possible emission of high energy neutrons during solar flares. The detector comprised of a central plastic scintillator, completely surrounded by an anticoincidence plastic scintillator shield. The instrument responds to neutrons of about 15–150 MeV and gamma rays of about 5–30 MeV with about the same efficiency. The detector was flown to an atmospheric depth of 25 g cm^-2 on February 26, 1969; while the balloon was at ceiling a flare of importance 2B and one of 1N occurred. No perceptible flare associated increase in the counting rate was observed. Using the observed counting rates, an upper limit of 1.2 × 10^-2 neutrons cm^-2 sec^-1 is obtained for the first time for a flare of importance 2B for neutrons of energy 15–150 MeV. The corresponding upper limit for gamma rays of energy 5–30 MeV is found to be 10^-2 photons cm^-2 sec^-1. The neutron flux limits are compared with the recent calculations of Lingenfelter.     

Evolution of the high-temperature plasma in the 15 June 1973 flare

The analysis of the high temperature plasma in Fe xxiii–xxiv in the 15 June 1973 flare is presented. The observations were obtained with the NRLXUV spectroheliograph on Skylab. The results are: (1) There was preheating of the active region in which the flare occurred. In particular, a large loop in the vicinity of the flaring region showed enhanced brightness for many hours before the flare. The loop disappeared when the flare occurred, and returned in the postflare phase, as if the energy flux which had been heating the large loop was blocked during the flare and restored after the flare was gone. The large magnetic fields did not change significantly. (2) The flare occurred in low-lying loop or loops. The spatial distribution of flare emission shows that there was a temperature gradient along the loop. (3) The high temperature plasma emitting Fe xxiii and xxiv had an initial upward motion with a velocity of about 80 km s^–1. (4) There was large turbulent mass motion in the high temperature plasma with a random velocity of 100 to 160 km s^–1. (5) The peak temperature of the hot plasma, determined from the Fe xxiii and xxiv intensity ratio, was 14 × 10⁶ K. It decreased slightly and then, for a period of 4 min, remained at 12.6 × 10⁶ K before dropping sharply to below 10 × 10⁶ K. The density of the central core of the hot plasma, determined from absolute intensity of Fe xxiv 255 Å line, was of the order of 10¹¹ cm^–3.The persistence of the high level of turbulence and of the high temperature plateau in the decaying phase of the flare indicates the presence of secondary energy release. From the energy balance equation the required energy source is calculated to be about 3 to 7 ergs cm^–3 s^–1.Ball Brothers Research Corporation.     

Estimation of an upper limit for the solar neutron emission during large flares

Solar neutron emission during large flares is investigated by using neutron monitor data from the mountain stations Chacaltaya (Bolivia), Mina Aguilar (Argentine), Pic-du-Midi (France) and Jungfraujoch (Switzerland). Registrations from such days on which large flares appeared around the local noon time of the monitor station are superimposed with the time of the optical flare as reference point.No positive evidence for a solar neutron emission was found with this method, However, by using an extrapolation of the neutron transport functions given by Alsmiller and Boughner a rough estimation of mean upper limits for the solar neutron flux is possible. The flux limits are compared with Lingenfelter's model calculations.From the Chacaltaya measurements it follows: N ₀2.8 × 10^{–3 N} cm^–2 s^–1 per proton flare, E > 50 MeV, if P0 = 125 MV N ₀1.4 × 10^{–2 N} cm^–2 s^–1 per proton flare, E > 50 MeV, if P ₀ = 60 MV and from Pic-du-Midi measurements: N ₀6.7 × 10^{–3 N} cm^–2 s^–1 per proton flare, E > 50 MeV, if P ₀ = 125 MV N ₀4 × 10^{–2 N} cm^–2 s^–1 per proton flare, E > 50 MeV, if P ₀ = 60 MV P ₀ = characteristic rigidity of the producing proton spectrum on the Sun.The flux limits estimated for some special proton flares are consistent with Lingenfelter's predictions for the acceleration phase but are too small for the slowing down phase. Therefore it is believed that Lingenfelter's assumption of isotropic proton emission from the flare region is not fulfilled.     

Multiple loop activations and continuous energy release in the solar flare of June 15, 1973

The spatial and temporal evolution of the high temperature plasma in the flare of 1973 June 15 has been studied using the flare images photographed by the NRL XUV spectroheliograph on Skylab.The overall event involves the successive activations of a number of different loops and arches bridging the magnetic neutral line. The spatial shifts and brightenings observed in the Fe xxiii–xxiv lines are interpreted as the activation of new structures. These continued for four or five minutes after the end of the microwave burst phase, implying additional energy-release unrelated to the nonthermal phase of the flare. A shear component observed in the coronal magnetic field may be a factor in the storage and release of the flare energy.The observed Fe xxiii–xxiv intensities define a post-burst heating phase during which the temperature remained approximately constant at 13 × 10⁶ K while the Fe xxiv intensity and 0–3 Å flux rose to peak values. This phase coincided with the activation of the densest structure (N _e = 2 × 10¹¹ cm^–3). Heating of higher loops continued into the decay phase, even as the overall temperature and flux declined with the fading of the lower Fe xxiv arches.The observed morphology of individual flaring arches is consistent with the idea of energy release at altitude in the arch (coincident with a bright, energetic core in the Fe xxiv image) and energy flow downward into the ribbons. The Doppler velocity of the Fe xxi 1354 Å line is less than 5 km s^–1, indicating that the hot plasma region is stationary.The relation of this flare to the larger class of flares associated with filament eruptions and emerging magnetic flux is discussed.