λ 5303 Fe XIV density models of the inner solar corona

The Sacramento Peak Observatory's 40 cm coronagraph was used with an emission line photometer to observe the distribution of 5303 Fe XIV brightness as a function of position angle, height above the limb, and time. These data were used to construct models of the volume emissivity as a function of solar latitude and longitude. These models in turn yield estimates of the distribution of electron density in the lower solar corona as a function of latitude and longitude for several specific periods in 1973 and 1975. Three observational results are obtained. An upper limit for the inferred electron density in coronal hole regions is set at log N _e = 7.4 for an altitude of 1.15R . Density models from late 1973 demonstrate an evolutionary trend toward a rather regular four-lobed appearance of coronal material; models from 1975 suggest that this characterization persisted for at least 27 solar rotations. A decrease in the total integrated 5303 intensity of a factor of 2.9 is inferred to have taken place between 1973 and 1975.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

A review of solar type Ⅲ radio bursts

Solar type Ⅲ radio bursts are an important diagnostic tool in the understanding of solar accelerated electron beams. They are a signature of propagating beams of nonthermal electrons in the solar atmosphere and the solar system. Consequently, they provide information on electron acceleration and transport, and the conditions of the background ambient plasma they travel through. We review the observational properties of type Ⅲ bursts with an emphasis on recent results and how each property can help identify attributes of electron beams and the ambient background plasma. We also review some of the theoretical aspects of type Ⅲ radio bursts and cover a number of numerical efforts that simulate electron beam transport through the solar corona and the heliosphere.     

CALLISTO facilities in Peru: spectrometer commissioning and observations of type Ⅲ solar radio bursts

The Astrophysics Directorate of CONIDA has installed two radio spectrometer stations belonging to the e-CALLISTO network in Lima, Peru. Given their strategic location near the Equator, it is possible to observe the Sun evenly throughout the whole year. The receiver located at Pucusana, nearby the capital city of Lima, took data from October 2014 until August 2016 in the metric and decimetric bands looking for radio bursts. During this period, this e-CALLISTO detector was unique in its time-zone coverage. To asses the suitability of the sites and the performance of the antennas, we analyzed the radio ambient background and measured their radiation pattern and beamwidth. To demonstrate the capabilities of the facilities for studying solar dynamics in these radio frequencies, we have selected and analyzed type Ⅲ Solar Radio Bursts. The study of this kind of burst helps to understand the electron beams traversing the solar corona and the solar atmospheric density. We have characterized the most common radio bursts with the following mean values: a negative drift rate of –25.8 ± 3.7 MHz s~(-1), a duration of 2.6 ± 0.3 s and 35 MHz bandwidth in the frequency range of 114 to 174 MHz. In addition, for some events, it was possible to calculate a global frequency drift which on average was 0.4 ± 0.1 MHz s~(-1).     

Type II–IV radio bursts and compact and diffuse white-light clouds in the outer corona of December 14, 1971

The radio observations of type II–IV bursts on December 14, 1971 are analyzed. These radio events were associated with a H-spray or eruptive prominence, and later followed by several compact moving clouds observed with the NRL white-light coronagraph aboard OSO-7. There was also observed a diffuse expanding cloud behind the compact moving clouds.From the comparison of the interferometer observation of the bursts with the optical observation, it is strongly suggested that the compact moving clouds were likely to be the optical counterparts of the sources of moving type IV radio emission. This fact suggests that the magnetic bubbles were really produced in the flare process. The frequency-drift of the first group of type II bursts was so rapid, that we could neither identify the type II shock with the leading edge of the diffuse expanding cloud nor interpret it as the piston-driven shock of the latter. Because of the uncertainty of the velocities of the compact clouds due to the projection effect, the possibility that the type II shock was the piston-driven shock of the compact clouds cannot be excluded. Nevertheless we suggest that the type II shock was a blast type MHD shock and had no direct physical relation to the flare-associated mass-ejection processes. The relation between the type II–IV bursts and the interplanetary shock is also discussed.     

Possible radio precursors/signatures of the CMEs onset:radio type Ⅲ bursts and fine structures in the centimeter-metric wavelength region

Seventy-one occurrences of coronal mass ejections(CMEs) associated with radio bursts,seemingly associated with type Ⅲ bursts/fine structures(FSs),in the centimeter-metric frequency range during 2003-2005,were obtained with the spectrometers at the National Astronomical Observatories,Chinese Academy of Sciences(NAOC) and the Culgoora radio spectrometer and are presented.The statistical results of 68 out of 71 events associated with the radio type III bursts or FSs during the initiation or early stages of the...     

Rotational characteristics of the green solar corona: 1964–1989

Fe XIV 5303 coronal emission line observations have been used for the estimation of the rotation behaviour of the green solar corona. A homogeneous data set, created from measurements carried out within the framework of the world-wide coronagraphic network, has been examined with a correlation analysis to reveal the averaged synodic rotation period as a function of latitude and time over the epoch from 1964 to 1989.The values of the synodic rotation period obtained for the epoch 1964–1989 for the whole range of latitudes and for a latitude band ±30° are 28.18±0.12 days and 27.65±0.13 days, respectively. The differential rotation of the green solar corona was confirmed, together with local maxima of the rotation period at latitudes 45° and -60° and a minimum at the equator, but no clear cyclic variation of the rotation has been found for the epoch examined.     

Faint Hα emission in the solar corona: Morphological,situational and hydromagnetic analysis

Very faint H emission in the solar corona registered on over-exposed photographs made by a coronagraph and an H filter is studied. The over-exposed filtergrams have been processed by a Joyce Loebl automated microdensitometer and the two-dimensional scans have been analysed by the residual image method. A classification of the faint H emission objects, revealed on the isodensity maps, is proposed and the latitudinal distribution, the morphology, and the location with respect to the other active phenomena are analysed. Taking into account some possible plasma effects that could be caused by coronal magnetic field changes, a hydromagnetic interpretation of the faint H emission is proposed.     

Is the enhancement of type Ⅱ radio bursts during CME interactions related to the associated solar energetic particle event?

We investigated 64 pairs of interacting-CME events identified from simultaneous observations by the SOHO and STEREO spacecraft from January 2010 to August 2014, to examine the relationship between large SEP events in the energy range of ～25 to～60 MeV and properties of the interacting CMEs.We found that during CME interactions, the large SEP events in this study were all generated by CMEs with the presence of enhanced type Ⅱ radio bursts, which also have wider longitudinal distributions compared to events without a type Ⅱ radio burst or its enhancement(almost always associated with small SEP events).It seems that the signature of type Ⅱ radio burst enhancement is a good discriminator between large SEP and small or no SEP event producers during CME interactions. The type Ⅱ radio burst enhancement is more likely to be generated by CME interactions, with the main CME having a larger speed(v), angular width(WD), mass(m) and kinetic energy(Ek), and taking over the preceding CMEs. The preceding CMEs in these instances have higher v, WD, m and Ekthan those in CME pairs missing type Ⅱ radio bursts or enhancements. Generally, the values of these properties in the type-Ⅱ-enhanced events are typically higher than the corresponding non-type-Ⅱ or non-type-Ⅱ-enhanced cases for both the main and preceding CMEs. Our analysis also revealed that the intensities of associated SEP events correlate negatively with the intersection height of the two CMEs. Moreover, the overlap width of two CMEs is typically larger in type-Ⅱ-enhanced events than in non-type-Ⅱ or non-type-Ⅱ-enhanced events. Most type-Ⅱ-enhanced events and SEP events are coincident and are almost always made by the fast and wide main CMEs that sweep fully over relatively slower and narrower preceding CMEs. We suggest that a fast CME with enough energy completely overtaking a relatively narrower preceding CME, especially at low height, can drive a more energetic shock signified by the enhanced type Ⅱ radio bursts. The shock may accelerate ambient particles(likely provided by the preceding CME) and lead to large SEP events more easily.     

Association of type II solar radio bursts with coronal structures above Hα filament channels

A study of type II solar radio bursts recorded at 160 MHz by the Culgoora radioheliograph during 1980 to 1982 shows that the radio emission occurs above H filaments rather than above H flares. This suggests that the type II radio emission most probably originates from within a coronal helmet streamer overlying the filament channel.     

The Fe xii 195.1 Å/1242 Å emission line ratio in the solar corona

Recent R-matrix calculations of electron impact excitation rates in Fe xii are used to derive the theoretical emission line ratio R ₁ = I(195.1 Å)/I(1242 Å), which is potentially a useful electron density diagnostic for the solar inner corona (r 1.05 61-01). These results are found to be significantly different from the earlier estimates of Withbroe and Raymond (1984), but are in good agreement with the observed values of R ₁, for the quiet Sun and an active region. Adoption of the R-matrix atomic data for the 1242 Å line in the coronal iron abundance determination removes an existing discrepancy between results derived from the EUV transition and other iron lines in the solar XUV spectrum. The R-matrix calculations confirm the prediction of Withbroe and Raymond that the earlier discrepancies in R ₁ and the iron abundance were due to the 1242 Å line excitation rates being underestimated by a factor of ~2. Withbroe and Raymond's paper is, therefore, an excellent example of how astronomical observations can be used to accurately predict atomic physics data.     

Is there a common explanation for scattering of type III radio bursts and solar radar?

The apparently common source heights of type III fundamental and harmonic components and the source height of the solar 38 MHz radar echoes may all refer to scattering at a coronal level where (radio wavenumber) × (electron gyroradius) 1, that is, where radio frequency = (2 to 5) × plasma frequency.     

Refractory forsterite in primitive meteorites: Condensates from the solar nebula?

Abstract— All groups of chondritic meteorites contain discrete grains of forsteritic olivine with FeO contents below 1 wt% and high concentrations of refractory elements such as Ca, Al, and Ti. Ten such grains (52 to 754 μg) with minor amounts of adhering matrix were separated from the Allende meteorite. After bulk chemical analysis by instrumental neutron activation analysis (INAA), some samples were analyzed with an electron microprobe and some with an ion microprobe. Matrix that accreted to the forsterite grains has a well‐defined unique composition, different from average Allende matrix in having higher Cr and lower Ni and Co contents, which implies limited mixing of Allende matrix. All samples have approximately chondritic relative abundances of refractory elements Ca, Al, Sc, and rare‐earth elements (REE), although some of these elements, such as Al, do not quantitatively reside in forsterite; whereas others (e.g., Ca) are intrinsic to forsterite. The chondritic refractory element ratios in bulk samples, the generally high abundance level of refractory elements, and the presence of Ca‐Al‐Ti‐rich glass inclusions suggest a genetic relationship of refractory condensates with forsteritic olivine. The Ca‐Al‐Ti‐rich glasses may have acted as nuclei for forsterite condensation. Arguments are presented that exclude an origin of refractory forsterite by crystallization from melts with compositions characteristic of Allende chondrules: (a) All forsterite grains have CaO contents between 0.5 and 0.7 wt% with no apparent zoning, requiring voluminous parental melts with 18 to 20 wt% CaO, far above the average CaO content of Allende chondrules. Similar arguments apply to Al contents. (b) The low FeO content of refractory forsterite of 0.2‐0.4 wt% imposes an upper limit of ～1 wt% of FeO on the parental melt, too low for ordinary and carbonaceous chondrule melts, (c) The Mn contents of refractory forsterites are between 30 to 40 ppm. This is at least one order of magnitude below the Mn content of chondrule olivines in all classes of meteorites. The observed Mn contents of refractory forsterite are much too low for equilibrium between olivine and melts of chondrule composition, (d) As shown earlier, refractory forsterites have O‐isotopic compositions different from chondrules (Weinbruch et al., 1993a). Refractory olivines in carbonaceous chondrites are found in matrix and in chondrules. The compositional similarity of both types was taken to indicate that all refractory forsterites formed inside chondrules (e.g., Jones, 1992). As refractory forsterite cannot have formed by crystallization from chondrule melts, we conclude that refractory forsterite from chondrules are relic grains that survived chondrule melting and probably formed in the same way as refractory forsterite enclosed in matrix. We favor an origin of refractory forsterite by condensation from an oxidized nebular gas.     

Different time constants of solar decimetric bursts in the range 100–1000 MHz

Between 1980, January 1 and 1981, December 31 a total of 664 decimetric pulsation events, abbreviated DCIM, were observed with the Zürich spectrometers in the frequency range 100 to 1000 MHz. All of these events were recorded on film, allowing an effective resolution in time of 0.5 s, and 5 MHz in frequency. Some of these events were also recorded digitally with higher time and frequency resolution.The class of DCIM bursts can be divided into two groups depending on their duration and thus reflecting different physical mechanisms. Each of the two groups can be further divided into small and large bandwidth subgroups, reflecting differences in the source parameters. Short decimetric events ( 1s) are most abundant in this frequency range. They may be caused by fast transients in the solar atmosphere. The half-power bandwidth of the shortest DCIM bursts, the millisecond spikes, were found to be 6 to 12 MHz. A single event may consist of more than 1000 individual spikes. The long lasting DCIM bursts (5 s to 300 s) exhibit a gradual and smooth time profile. Such long lasting events indicate the presence of trapped particles in magnetic fields. In some events decimetric gyrosynchrotron radiation was observed below 1000 MHz as a continuation of corresponding microwave events.Some of the decimetric events exhibit very large drift rates (2000 MHz s^-1). Such large values request either a drastic reduction of the effective scale height of the active region in the beam model or a different explanation than the conventional beam model.     

Physical properties of the quiet solar chromosphere–corona transition region

The physical properties of the quiet solar chromosphere–corona transition region are studied. Here the structure of the solar atmosphere is governed by the interaction of magnetic fields above the photosphere. Magnetic fields are concentrated into thin tubes inside which the field strength is great. We have studied how the plasma temperature, density, and velocity distributions change along a magnetic tube with one end in the chromosphere and the other one in the corona, depend on the plasma velocity at the chromospheric boundary of the transition region. Two limiting cases are considered: horizontally and vertically oriented magnetic tubes. For various plasma densities we have determined the ranges of plasma velocities at the chromospheric boundary of the transition region for which no shock waves arise in the transition region. The downward plasma flows at the base of the transition region are shown to be most favorable for the excitation of shock waves in it. For all the considered variants of the transition region we show that the thermal energy transfer along magnetic tubes can be well described in the approximation of classical collisional electron heat conduction up to very high velocities at its base. The calculated extreme ultraviolet (EUV) emission agrees well with the present-day space observations of the Sun.     

Diagnosing physical conditions near the flare energy-release sites from observations of solar microwave type Ⅲ bursts

In the physics of solar flares, it is crucial to diagnose the physical conditions near the flare energyrelease sites. However, so far it is unclear how to diagnose these physical conditions. A solar microwave type Ⅲ burst is believed to be a sensitive signature of primary energy release and electron accelerations in solar flares. This work takes into account the effect of the magnetic field on the plasma density and develops a set of formulas which can be used to estimate the plasma density, temperature, magnetic field near the magnetic reconnection site and particle acceleration region, and the velocity and energy of electron beams.We apply these formulas to three groups of microwave type Ⅲ pairs in an X-class flare, and obtained some reasonable and interesting results. This method can be applied to other microwave type Ⅲ bursts to diagnose the physical conditions of source regions, and provide some basic information to understand the intrinsic nature and fundamental processes occurring near the flare energy-release sites.     

Occurrence of bidirectional type III bursts in solar flares

The 13 pairs of type III bursts with the bidirectional drift structures recorded with the spectrograph in the frequency ranges of 230–300 MHz and 625–1500 MHz at the Yunnan Observatory and 2600–3800 MHz at the Beijing National Astronomical Observatories are analyzed in this present article and the outstanding characteristics of these events are obtained. These bursts respectively reveal that the separatrix frequency between the bursts with positive and negative drifts comes between 250 MHz and 3420 MHz, with a gap being between 0.6 MHz and 110 MHz; the duration is 53 ms–1880 ms and the frequency drift rate is between 45 MHz/s and 56000 MHz/s. The drift rate at metric wavelengths is relatively low, only a few decades of MHz while it is comparatively high at microwave wavelengths, reaching 56000 MHz/s. The qualitative explanation of these events is given in this paper.     

Are short-time variations of the solar S-component emission identical with microwave bursts?

Extended time series (time resolution about 2–3 min) of spatially resolved observations (≫ 17 arcsec) in one dimension of solar S-component sources obtained at the Siberian Solar Radio Telescope (SSRT) at 5.2 cm wavelength allow the detection of evolutional features of the growth and decay of active regions in the solar corona. Characteristic slow flux variations with timescales of about 1–2 hours occurring during the decay phase of the radio emission in the low corona above plages and sunspots are compared with recently detected step-like flux increases on timescales of about 10–20 min followed by quasi-constant periods appearing in the initial phase of the development of active regions. Superimposed on this basic behaviour, also fluctuations at shorter timescales (or even periodic oscillations) have been observed. As it is well known from emission-model calculations, the variations of the S-component radiation can be due to variations of the magnetic field and/or changes of the energy of the radiating particles, which is basically the same emission mechanism as for microwave bursts. Since the “S-component” is originally defined by its long timescale behaviour derived from whole-Sun flux density measurements, the presently detected small-timescale features in S-component sources require either a revised definition of S-component emission or must be considered as “burst-like”.     

1–4 GHz type II-like radio bursts and pinch processes near the transition region

Analyzing 205 radio bursts observed by the Ondejov radiospectrograph in the 1–4 GHz frequency range during 1992 and 1993, we found 6 examples of type II-like radio bursts coinciding with impulsive phases of solar flares. These bursts were interpreted as radio manifestations of MHD (shock) waves generated during impulsive phases of flares in the vicinity of the transition region. Assuming a magnetic-field perturbation origin of these waves, we studied pinch processes in the current sheet near the transition region. In the 2-D MHD numerical model of this current sheet we demonstrated that 2-D pinch processes induced by radiative losses can trigger the impulsive phase of some flares and so generate the observed high-frequency type II-like radio bursts.     

Does the ion cyclotron exist in the inner corona?

The present work is about the interpretation of the linear polarization of the O VI D₂ (λ1032) coronal line observed by SUMER/SoHO. We take into account the effect of the Doppler redistribution due to the scattering ions motion. We consider the cases of isotropic and anisotropic velocity field distributions. The latter can be interpreted by the ioncyclotron effect that affects heavy ions in the solar corona. The comparison of the numerical results with the observations yields constraints on the solar wind outflow speed and on the velocity field distribution of the O⁵⁺ ions at low coronal altitudes in the polar holes.