Densities and mass motions in transition-zone plasmas in solar flares observed from Skylab

We studied the EUV line spectra of three flare observed with the NRL slit spectrograph on Skylab. The electron densities in the flare transition-zone plasmas are determined from density-sensitive lines of Si iii and O iv. The electron densities in all three flares studied were greatest during the flare maximum with values of the order of 10¹² cm^–3. The density decreases by a factor of 2 to 3 in the decay phase of the flares. The intensities of EUV lines from the flare chromospheric and transition-zone plasmas all are greatly enhanced. In contrast to lines for Oi, Ci, Feii and other chromospheric ions, the lines of Oiv and Nv and other transition-zone lines are not only enhanced but also very much broadened.Fitting of the N v 1242 Å line with a two-gaussian model shows that for two of the flares studied, there is a red-shifted component in addition to an unshifted component. The shifted component in the N v line profiles is interpreted as due to a dynamic and moving plasma with a bulk motion velocity of 12 km s^–1 for one flare and more than 70 km s^–1 for the other. The broadened line profiles indicate that there are large turbulent mass motions with random velocities ranging from 30 to 80 km s^–1.Ball Corporation. Now with NASA/Marshall Space Flight Center.     

Analysis of the emission line spectra of a solar flare observed from Skylab

The EUV emission spectra in the wavelength range 110–1900 Å of the 5 September 1973 flare observed with the NRL slit spectrograph on Skylab are studied. The results are: (1) The chromospheric and transition-zone lines are greatly enhanced during the flare. In particular, the allowed lines are enhanced more than the intersystem lines. The Ni ii and P ii lines show the greatest enhancement with a factor of 800 increase in intensity. Other lines such as O i, C i, Si iii, S iii, S iv, O iv, O v, and N v show increases in intensity 10–100 times during the flare. (2) The chromospheric lines, although greatly enhanced during the flare, maintain their sharp and gaussian profiles and are not appreciably broadened. The transition zone lines, on the other hand, show a red-shifted component during the initial phase of the flare. The deduced downward velocity in the transition zone is 50 km s^–1. In addition, there are large turbulent mass motions. The downward mass motion is probably caused by the pressure imbalance between the flare hot plasma at 13 × 10⁶ K and the cooler plasma at 10⁵ K. (3) The density of the 10⁵ K flare plasma, as deduced from density-sensitive lines, is greater than 10¹² cm^-3. The depth of the 10⁵ K plasma in the flare transition zone is only of the order of 0.1 km, giving a steep temperature gradient. Consideration of the energy balance between the conductive flux and the radiative energy losses shows that, indeed, the high density in the transition zone requires that its thickness be very small. This is a consequence of the maximum radiative efficiency at the temperature around 10⁵ K in the solar plasma.Ball Brothers Research Corporation.     

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.     

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.     

Satellite lines in the solar X-ray spectrum

Observations of solar X-ray line emission using crystal spectrometers during a large chromospheric flare have provided a list of wavelengths with a precision of 0.003 Å in first order of diffraction and correspondingly better in higher orders. In addition to the resonance, intersystem (1 ¹ S ₀-2 ³ P ₁) and forbidden (1 ¹ S ₀-2 ³ S ₁) transitions of ions of the Hei isoelectronic sequence, we have recorded satellite lines arising from ions in the Lii, Bei and Bi isoelectronic sequences. These satellite features are most prominent in the iron spectrum. Apparent decreases in the ratio of forbidden and intersystem line intensities of Mgxi and Sixiii during the flare are used to derive electron densities possibly as high as 1 × 10¹³ cm^–3 in the Mgxi emitting region and 1 × 10¹⁴ cm^–3 in the Sixiii region during the event. A search for satellite lines on the long-wavelength side of the Lyman-alpha line of Hi-like ions has yielded no positive identifications.     

Extreme ultraviolet spectrum of the solar flare of 2114 UT March 27, 1967

Scanning spectrometer measurements in the range 1310–270 Å, observed from the satellite OSO 3, are reported for the solar flare of 2114 UT March 27, 1967. This flare was a long lasting sequence of bursts with EUV spectra consisting of enhanced lines and recombination continua normally emitted from the chromosphere and chromosphere-corona transition region, with unusually small increases in lines normally emited from the corona. An EUV flare spectrum is presented and suggested as one example for interpreting broadband observations of EUV bursts. Any broadband continuum other than known recombination continua contributed less than 6 % of the meassured line and hydrogen recombination continua in the range 270–1310 Å. The ratio of photon flux of Ciii 1176 Å to that of Ciii 977 Å was 0.86, which suggests an ambient density in the region of emission greater than 10¹² cm^-3 at temperatures near 60000 K.     

Extreme ultraviolet spectra of solar active regions and their analysis

Extreme ultraviolet spectra of several active regions are presented and analyzed. Spectral intensities of 3 active regions observed with the NRL Skylab XUV spectroheliograph (170–630 Å) are derived. From this data density sensitive line ratios of Mg viii, Si x, S xii, Fe ix, Fe x, Fe xi, Fe xii, Fe xiii, Fe xiv, and Fe xv are examined and typically yield, to within a factor of 2, electron pressures of 1 dyne cm^–2 (n _e T = 6 × 10¹⁵ cm^–3 K). The differential emission measure of the brightest 35 × 35 portion of an active region is obtained between 1.4 × 10⁴ K and 5 × 10⁶ K from HCO OSO-VI XUV (280–1370 Å) spectra published by Dupree et al. (1973). Stigmatic EUV spectra (1170–1710 Å) obtained by the NRL High Resolution Telescope and Spectrograph (HRTS) are also presented. Doppler velocities as a function of position along the slit are derived in an active region plage and sunspot. The velocities are based on an absolute wavelength scale derived from neutral chromospheric lines and are accurate to ±2 km s^–1. Downflows at 10⁵ K are found throughout the plage with typical velocities of 10 km s^–1. In the sunspot, downflows are typically 5 to 20 km s^–1 over the umbra and zero over the penumbra. In addition localized 90 and 150 km s^–1 downflows are found in the umbra in the same 1 × 1 resolution elements which contain the lower velocity downflows. Spectral intensities and velocities in a typical plage 1 resolution element are derived. The velocities are greatest ( 10 km s^–1) at 10⁵ K with lower velocities at higher and lower temperatures. The differential emission measure between 1.3 × 10⁴ K and 2 × 10⁶ K is derived and is found to be comparable to that derived from the OSO-VI data. An electron pressure of 1.4 dynes cm^–2 (n _e T = 1.0 × 10¹⁶ cm^–3 K) is determined from pressure sensitive line ratios of Si iii, O iv, and N iv. From the data presented it is shown that convection plays a major role in determining the structure and dynamics of the active region transition zone and corona.     

Large-Scale Electric Fields in Solar Flare Regions

A method of separating electric field in the flare region in the potential and vortex (induced) parts is discussed. According to the proposed model, the motion of flare ribbons from the central line of the flare region is caused by the vortex component of the coronal electric field, while the motion of bright spots within the flare region towards the central line is driven by the potential component of that field. The intensity of both the components of the flare region electric field is estimated to equal approximately 1–3 V cm^–1, which provides the input of the electromagnetic energy into the active region at a rate of about 10¹⁰ erg cm^–2 s^–1.     

Evidence of redshifts in the average solar line profiles of C iv and Si iv from OSO-8 observations

Measurements of the C iv 1548 Å and Si iv 1393 Å lines made with the University of Colorado Ultraviolet Spectrometer on board OSO-8 show that the mean profiles are redshifted at disk center. Assuming these lines to be optically thin, we measure an apparent average downflow of material in the 50 000 to 100 000 K temperature range which is weighted by the emission measure in these lines. The magnitude of the redshift varies from 6–17 km s^–1 with a mean of 12 km s^–1 and is persistent at least on the order of months, which is the time covered by the observations presented in this paper. Pneuman and Kopp (1978) have demonstrated that the flux of material associated with this downflow is of the same order of magnitude as the flux of material being carried upward in spicules. Thus, it is possible that material observed to be downflowing in C iv and Si iv has its origins in the upward moving spicule material.     

Solar X-ray spectra observed from the ‘Intercosmos-4’ satellite and the ‘Vertical-2’ rocket

Results are given of the detailed analysis of fourteen Fe xxv-xxiii lines ( = 1.850–1.870 Å) in the spectra of a solar flare on 16 Nov. 1970. The spectra were obtained with a resolution of about 4 × 10^–4 Å, which revealed lines not previously observed and allowed the measurement of line profiles. The measured values of the wavelengths and emission fluxes are presented and compared with theoretical calculations. The analysis of the contour of the Fe xxv line ( = 1.850 Å) leads to the conclusion that there is unidirectional macroscopic gas motion in the flare region with the velocity (projection on the line of sight) ± 90 km s^–1.Measurements of the 8.42 Å Mg xii and 9.16 Å Mg xi lines in the absence of solar flares indicate prolonged existence of active regions on the solar disk with T _e = 4–6 × 10⁶K and emission measure ME 10⁴⁸ cm^–3. The profile of the Mg xii line indicates a macroscopic ion motion with a velocity up to 100 km s^–1.     

Extreme ultraviolet flashes of solar flares observed via sudden frequency deviations: Experimental results

Properties of solar-flare EUV flashes measured via a type of ionospheric event, called a sudden frequency deviation (SFD), are presented. SFD's are sensitive to bursts of radiation in the 1–1030 Å wavelength range. He ii 303.8 Å, O v 629.7 Å, HL 972.5 Å and C iii 977.0 Å have essentially the same impulsive time dependence as the 1–1030 Å flash responsible for SFD's. Soft X-rays (2–20 Å) and certain EUV lines have a much slower time dependence than the 1–1030 Å flash. Most SFD's have some fine structure, but marked quasi-periodicity in EUV flashes is quite rare. EUV flashes are closely associated with hard X-ray bursts, white-light emission, microwave radio bursts and small bright impulsive kernels in the H flare. The intensity of EUV flashes depends on the central meridian distance of the H flare location; the intensity decreases at the limb. The total energy radiated in the 10–1030 Å flash for the largest events observed is about 10³¹ ergs.     

Energetics of a double flare on November 8, 1980

Here we complete an energy balance analysis of a double impulsive hard X-ray flare. From spatial observations, we deduce both flares probably occur in the same loop within the resolution of the data. For the first flare, the energy in the fast electrons (assuming a thick-target model) is comparable to the convective up-flow energy, suggesting that these are related successive modes of energy storage and transfer. The total energy lost through radiation and conduction, 2.0 × 10²⁸ erg, is comparable to the energy in fast electrons 2.5 × 10²⁸ erg. For the second flare, the energy in the fast electrons is more than one order of magnitude greater than the energy of the convective up-flow. Total energy losses are within a factor of two lower than the calculated fast electron energy. We interpret the observations as showing that the first flare occurred in a small loop with fast electrons heating the chromosphere and resulting in chromospheric evaporation increasing the density in the loop. For the second flare most of the heating occurred at the electron acceleration site. The two symmetrical components of the Ca xix resonance line and a high velocity down-flow of 115 km s ^–1 observed at the end of the second hard X-ray burst are consistent with the flare eruption (reconnection) region being high in the flare loop. The estimated altitude of the acceleration site is 5500 km above the photosphere.     

The November 22, 1977 solar flare: Evidence for 2.23 and 4.43 MeV line emission from the signe 2MP experiment

The November 22, 1977 solar flare was observed at energies up to 4.9 MeV by French-built gamma ray detectors aboard the Soviet Prognoz-6 satellite. The data show evidence for 2.23 and 4.43 MeV line emission, with the 2.23 MeV emission occurring about 3 min after the flare onset in hard X-rays. The line intensities, 0.11 cm^–2s^–1 and 0.06 cm^–2s^–1 for the 2.23 and 4.43 lines, respectively, are roughly comparable to intensities observed in other events. Particle detectors aboard the Prognoz-6 satellite, however, recorded a proton flux much lower than that observed for the 4 August 1972 event. It is shown that this may be taken as evidence for a thick target interpretation of the proton interactions in the solar atmosphere.     

Coronal survey in X-rays of O vii and Ne ix

We report some results of a rocket experiment flown on 29 April, 1971. A survey of the solar corona was carried out with a pair of collimated Bragg spectrometers to study the resonance, intersystem and forbidden line emission from the helium-like ions O vii (22 Å) and Ne ix (13 Å). In the direction of dispersion the collimator provided a field of view of 1.7. Also, the continuum radiation near 3 Å was monitored by a collimated proportional counter within a view angle of 4.2. The observed X-ray emission came from the general corona, seven plage regions, and one dynamic feature- the late stage of a small flare. From the intensity of the O vii and Ne ix resonance lines the electron temperature and emission measure of the individual emitting regions are derived on the basis of two models, one (a) in which the region is assumed to be isothermal and another (b) in which the emission measure decreases exponentially with increasing temperature. The latter model, which is the most adequate of the two, yields for the electron temperature of the time-varying feature 2–3 × 10⁶ K, for the other active regions 1.5–2.5 × 10⁶ K, and for the general corona 1.3–1.7 × 10⁶ K. The Ne ix emitting regions are about 1.5 times as hot as the O vii regions. The emission measure ranges from 0.4–2.3 × 10⁴⁸ cm^–3 for all active regions and is about 2 × 10⁴⁹ cm^–3 for one hemisphere of the general corona above 10⁶ K. From an analysis of the ratio, R, of the forbidden and intersystem lines of O vii we conclude that none of the regions producing these lines at the time of the rocket flight had electron densities exceeding about 3 × 10⁹ cm^–3. Our data demonstrate a dependence of R upon temperature in agreement with the theory of Blumenthal et al. (1971). The wavelengths for the intersystem, the 1s ²2s ² S ^e–1s2p2s ² P ⁰ satellite, and the forbidden transition show in the case of Ne ix improved agreement with predictions. The observed strength of the satellite lines for both O vii and Ne ix agrees with the predictions of Gabriel's (1972) theory, which attributes their formation to dielectronic recombination.We are saddened to report the death of A. J. Meyerott on 13 November, 1971.     

Helium emission from model flare layers

A theoretical study is made of the visible and UV line radiation of He i atoms and He ii ions from a plane-parallel model flare layer. Solutions are obtained of the statistically steady state equations for a 30 level He i-ii-iii model, with parametric representation of the line and continuum radiation fields. Optical depths and some line intensities are presented for a 1000 km thick layer. Results are given for electron temperatures 10⁴ to 5 × 10⁴ K and electron densities 10¹⁰ to 10¹⁴ cm^–3.Work sponsored by the NASA, Marshall Space Flight Center, Alabama under contract NAS8-27988.     

Anisotropy characteristics of low energy cosmic ray population of solar origin

Comparison of Explorer 34 observations on solar protons in the energy range 0.7–55 MeV with similar observations from other spacecrafts show that the large field aligned anisotropies which are observed during the rise time of a flare event change to an equilibrium anisotropy coming radially from the sunward direction due to the convective removal of the solar particles. At very late times during the decay (T 4 days) the anisotropy is observed to be from a direction 45° E of the satellite-Sun line which is interpreted as indicative of positive density gradient of solar cosmic ray population. The dependence of both types of equilibrium anisotropies on the energy and the velocity of the particles and on plasma velocity are shown to be in agreement with the theoretical predictions. The amplitude of the large field aligned anisotropies observed earlier in the event is found to be independent of the rise time of the event and to vary as (Vt)^–1.Interplanetary magnetic sector crossings during a flare event, cause abrupt changes in both the amplitude and phase of the non-equilibrium anisotropy whereas they have no significant effect on equilibrium anisotropy. The effect of azimuthal density gradients on the decay time constants of flare enhancements are also examined in an attempt to understand the complicated structures, often observed, in the time intensity profiles at low energies.Part of this work was done while the author was at the University of Texas as Dallas, U.S.A.Now at the National Academy of Sciences, Washington, D.C.     

The Evaporation Regime in a Confined Flare

We studied the evolution of a small eruptive flare (GOES class C1) from its onset phase using multi-wavelength observations that sample the flare atmosphere from the chromosphere to the corona. The main instruments involved were the Coronal Diagnostic Spectrometer (CDS) aboard SOHO and facilities at the Dunn Solar Tower of the National Solar Observatory/Sacramento Peak. Transition Region and Coronal Explorer (TRACE) together with Ramaty High-Energy Spectroscopic Imager (RHESSI) also provided images and spectra for this flare. Hα and TRACE images display two loop systems that outline the pre-reconnection and post-reconnection magnetic field lines and their topological changes revealing that we are dealing with an eruptive confined flare. RHESSI data do not record any detectable emission at energies ≥25 keV, and the observed count spectrum can be well fitted with a thermal plus a non-thermal model of the photon spectrum. A non-thermal electron flux F ≈ 5 × 10¹⁰ erg cm⁻² s⁻¹ is determined. The reconstructed images show a very compact source whose peak emission moves along the photospheric magnetic inversion line during the flare. This is probably related to the motion of the reconnection site, hinting at an arcade of small loops that brightens successively. The analysis of the chromospheric spectra (Ca II K, He I D₃ and Hγ, acquired with a four-second temporal cadence) shows the presence of a downward velocity (between 10 and 20 km s⁻¹) in a small region intersected by the spectrograph slit. The region is included in an area that, at the time of the maximum X-ray emission, shows upward motions at transition region (TR) and coronal levels. For the He I 58.4 and O v 62.97 lines, we determine a velocity of ≈−40 km s⁻¹ while for the Fe XIX 59.22 line a velocity of ≈−80 km s⁻¹ is determined with a two-component fitting. The observations are discussed in the framework of available hydrodynamic simulations and they are consistent with the scenario outlined by Fisher (1989). No explosive evaporation is expected for a non-thermal electron beam of the observed characteristics, and no gentle evaporation is allowed without upward chromospheric motion. It is suggested that the energy of non-thermal electrons can be dissipated to heat the high-density plasma, where possibly the reconnection occurs. The consequent conductive flux drives the evaporation process in a regime that we can call sub-explosive.     

Absolute redshifts in the C iv 1548 Å line in the transition region of the quiet Sun

Observations with the UVSP instrument on the SMM spacecraft were made at the polar limb and disk center for the accurate determination of Doppler shifts of the Civ 1548 Å emission line formed at 10⁵ K in the transition region of the quiet Sun. Individual data points representing 3 arc sec square pixels yield both redshifts and blueshifts, but the mean values from four different days of observations are toward the red. The mean redshifts are in the range 4–8 km s^-1 and are produced by nearly vertically directed flows; the uncertainty associated with the mean values correspond to ±0.5 km s^-1. The redshift increases with brightness of the Civ line.     

Dynamic evolution of recurrent mass ejections observed in Hα and Civ lines

During a coordinated SMY program, the consecutive formation of two new active centers merging together within AR 2646 was observed from 28 August, to 5 September, 1980. The two preceding spots compressed an inverse polarity spot on 1 September 1980, causing recurrent ejecta of matter with time intervals around 10 min. The observations of the MSDP spectrograph operating in H at the Meudon Solar tower and of the UVSP spectrometer on SMM in the Civ 1548 Å line show that cold and hot material had the same projection, although the upward Civ velocity structure was more extended than the H one. We present evidence that observed contrasts of the H absorbing structure can be interpreted in terms of a dynamic cloud model overlying the chromosphere. H matter follows a magnetic channel with upward velocity around 20–30 km s^–1 in the first phase of the event and with downward velocity ( - 40 km s^–1) in the second phase. The stored energy is not sufficient to trigger a flare, nor even to propulse matter along the full length of an arch, because of the periodic reorganisation of the magnetic field.     

Some considerations on the non-detection of interstellar Li and B towards the SN1987a

A very low upper limit of 0.15 mÅ for the interstellar 6707 Å Lii line has been recently derived towards the SN1987a by Baade and Magain (1988). This value corresponds toN(Li)<1.4×10¹¹ cm^–2 and gives [Li/H]<5.4×10^–11 assumingN(Hi)=2.6×10²¹ cm^–2 for the hydrogen column density in the LMC towards SN1987a. This value is lower than the Li abundance found in the Population II stars and lower than the minimum abundance allowed in the framework of the standard Big-Bang theory. We indirectly estimate the Li depletion usingKi observations and show that a depletion of 1.2 dex is plausible. Therefore, an interstellar abundance [Li/H] as high as 0.8×10^–9 cannot be excluded. Any improvement in the above-mentioned upper limit will place important constraints on current theories for lithium nucleosynthesis.High-resolution IUE spectra of the SN1987a have been analysed in search for IS 1362 ÅBii resonance lines. A minimum detectable equivalent width of 22 mÅ has been found, that impliesN(B)<1.2×10^–12 cm^–2 and [B/H]<4.7×10^–10 cm^–2, i.e., comparable to the solar value of [B/H]=4×10^–10. This limit is the most stringent derived so far for an external galaxy, and suggests that the rate of spallation processes in the LMC has not been higher than in our own Galaxy.