Supernova remnant 1987A: The latest report from the Chandra X-ray Observatory

We continue monitoring supernova remnant (SNR) 1987A with the Chandra X-ray Observatory. As of 2004 January, bright X-ray spots in the northwest and the southwest are now evident in addition to the bright eastern ring. The overall X-ray spectrum, since 2002 December, can be described by a planar shock with an electron temperature of ∼2.1 keV. The soft X-ray flux is now 8 × 10⁻¹³ ergs cm⁻² s⁻¹, which is about five times higher than four years ago. This flux increase rate is consistent with our prediction based on an exponential density distribution along the radius of the SNR between the HII region and the inner ring. We still have no direct evidence of a central point source, and place an upper limit of L_X = 1.3 × 10³⁴ ergs s⁻¹ on the 3–10 keV band X-ray luminosity.     

Compact X-ray sources in nearby galaxy nuclei

We have found compact, near-nuclear X-ray sources in 21 (54%) of a complete sample of 39 nearby face-on spiral and elliptical galaxies with available ROSAT HRI data. ROSAT X-ray luminosities (0.2 – 2.4 keV) of these compact X-ray sources are ∼10³⁷ – 10⁴⁰ erg s⁻¹. The mean displacement between the location of the compact X-ray source and the optical photometric center of the galaxy is ∼390 pc. ASCA spectra of six of the 21 galaxies show the presence of a hard component with relatively steep (Γ ≈ 2.5) spectral slope. A multicolor disk blackbody plus power-law model fits the data from the spiral galaxies well, suggesting that the X-ray objects in these galaxies may be similar to a black hole candidate (BHC) in its soft (high) state. ASCA data from the elliptical galaxies indicate that hot (kT ≈ 0.7 keV) gas dominates the emission. The fact that the spectral slope of the spiral galaxy sources is steeper than in normal type 1 active galactic nuclei (AGNs) and that relatively low absorbing columns (N_H ≈ 10²¹ cm⁻²) were found to the power-law component indicates that these objects are somehow geometrically and/or physically different from AGNs in normal active galaxies. The X-ray sources in the spiral galaxies may be BHCs, low-luminosity AGNs, or possibly X-ray luminous supernovae. We estimate the black hole masses of the X-ray sources in the spiral galaxies (if they are BHCs or AGNs) to be ∼10²–10³ M_⊙. The X-ray sources in the elliptical galaxies may be BHCs, AGNs or young X-ray supernova also.     

X-ray study of accretion flow in narrow-line Seyfert 1 galaxies

We present the results of a systematic study of narrow-line Seyfert 1 galaxies (NLS1s) observed with XMM-Newton. The 2–12 keV X-ray spectra of NLS1s are well represented by a single power law with a photon index Γ ∼ 2. When this hard power law continuum is extrapolated into the low energy band, we found that all objects in our sample show prominent soft excess emission. This excess emission is well parameterized by the thermal emission expected from an optically thick accretion disk, and we found the following three peculiar features: (1) The derived disk temperatures are significantly higher than the expectation from a standard Shakura-Sunyaev accretion disk, if we assume a central mass of a black hole to be 10^6–8M_⊙. (2) The temperatures are distributed within narrow range (ΔkT ∼ 0.08 keV) with an average temperature of 0.18 keV in spite of the range of four orders of magnitude in luminosity (10^41–45 erg s⁻¹). (3) We found a peculiar temperature–luminosity relation, where the luminosity seems to be almost saturated in spite of the significant change in temperature, during the observations of the most luminous NLS1 PKS 0558-504. These results strongly suggest that the standard accretion disk picture is no longer appropriate in the nuclei of NLS1s. We discuss a possible origin for the soft excess component, and suggest that a slim disk may be able to explain the observational results, if the photon trapping effect is properly taken into account.     

Response of the mid-latitude D-region ionosphere to the total solar eclipse of 22 July 2009 studied using VLF signals in South Korean peninsula

In this paper, we analyze VLF signals received at Busan to study the the D-region changes linked with the solar eclipse event of 22 July 2009 for very short (∼390 km) transmitter–receiver great circle path (TRGCP) during local noon time 00:36–03:13 UT (09:36–12:13 KST). The eclipse crossed south of Busan with a maximum obscuration of ∼84%. Observations clearly show a reduction of ∼6.2 dB in the VLF signal strength at the time of maximum solar obscuration (84% at 01:53 UT) as compared to those observed on the control days. Estimated values of change in Wait ionospheric parameters: reflection height (h′) in km and inverse scale height parameter (β) in km⁻¹ from Long Wave Propagation Capability (LWPC) model during the maximum eclipse phase as compared to unperturbed ionosphere are 7 km and 0.055 km⁻¹, respectively. Moreover, the D-region electron density estimated from model computation shows 95% depletion in electron density at the height of ∼71 km. The reflection height is found to increase by ∼7 km in the D-region during the eclipse as compared to those on the control days, implying a depletion in the Lyman-α flux by a factor of ∼7. The present observations are discussed in the light of current understanding on the solar eclipse induced D-region dynamics.     

Hard X-ray continuum from lunar surface: Results from High Energy X-ray spectrometer (HEX) onboard Chandrayaan-1

The High Energy X-ray spectrometer (HEX) on Chandrayaan-1 was designed to study the photon emission in the range of 30–270 keV from naturally occurring radioactive decay of ²³⁸U and ²³²Th series nuclides from the lunar surface. The primary objective of HEX was to study the transport of volatiles on the lunar surface using radon as a tracer and mapping the 46.5 keV line from ²¹⁰Pb, a decay product of ²²²Rn. HEX was tested for two days during the commissioning phase of Chandrayaan-1 and performance of all sub systems was found to be as expected. HEX started collecting science data during the first non-prime imaging season (February–April, 2009) of Chandrayaan-1. Certain anomalies persisted in this data set and the early curtailment of Chandrayaan-1 mission in August, 2009, did not allow any further operation of HEX. Despite these issues, HEX provided the first data set for 30–270 keV continuum emission, averaged over a significant portion of the lunar surface, including the polar region.     

Evolution of an electron beam pulse influenced by coulomb collision effects in the solar corona

Electrons accelerated in the corona during solar activity give rise to radio emission events that can be observed over a wide range of frequencies. Among different finer-scale structures in the dynamic spectra observed in the radio range, fast transients with extents of some milliseconds known as solar radio spikes are observed accompaning the background continuum emission. Fundamental to the generation of radio spikes is a propagating electron beam and following its evolution allows us to understand the physical processes occurring in the solar corona. With the use of a numerical Fokker–Planck code we follow a previous numerical study to simulate the propagation of an electron beam pulse injected in a small region at the top of a magnetic field and outwards the solar corona under typical flare conditions. It was found that in large ambient densities of ${10}^{10}$ cm⁻³ at the injection point, Coulomb collision effects have an important effect on the propagation of the electrons, causing that the injected electrons thermalize faster in a time of $0.1$ and $0.4$ s for an electron distribution with a low-energy cut off of 16 and 7 keV respectively and a spectral index of 3. For a tenous ambient medium of density ${10}^9$ cm⁻³ thermalization occurs only for an electron distribution with smaller low-energy cut off (7 keV) with a duration of $\approx$1.5 s, while for a larger low-energy cut off (16 keV) the loss of accelerated electrons is very slow, regardles of the spectral index ($3,7$). The electron loss time by Coulomb collisions, which depends on the low boundary ambient density, might be an important parameter that influences the generation of radio spikes due to the formation of instabilities in the corona.     

The ROSAT mission

A primary scientific objective of the ROSAT mission is to perform the first all-sky survey with an imaging X-ray telescope leading to an improvement in sensitivity by several orders of magnitude compared with previous surveys. A large number of new sources (? 10⁵) will be discovered and located with an accuracy of 1 arcmin or better. These will comprise almost all astronomical objects from nearby normal stars to distant quasistellar objects. After completion of the survey which will take half a year the instrument will be used for detailed observations of selected sources with respect to spatial structure, spectra and time variability. In this mode which will be open for guest observers ROSAT will provide substantial improvement over the imaging instruments of the Einstein observatory.The main ROSAT telescope consists of a fourfold nested mirror system with 83 cm aperture having three focal plane instruments. Two of them will be imaging proportional counters (0.1 – 2 keV) providing a field of view of 2°, an angular resolution of ≈ 30″ in the pointing mode and a spectral resolution ^ΔE/E ≈ 45% FWHM at 1 keV. The third focal instrument will be a high resolution imager (≈ 3″). The main ROSAT telescope will be complemented by a parallel looking Wide Field camera which extend the spectral coverage into the XUV band.     

Gamma-ray observations of explosive nucleosynthesis products

In this review I discuss the various γ-ray emission lines that can be expected and, in some cases have been observed, from radioactive explosive nucleosynthesis products. The most important γ-ray lines result from the decay chains of ⁵⁶Ni, ⁵⁷Ni, and ⁴⁴Ti. ⁵⁶Ni is the prime explosive nucleosynthesis product of Type Ia supernovae, and its decay determines to a large extent the Type Ia light curves. ⁵⁶Ni is also a product of core-collapse supernovae, and in fact, γ-ray line emission from its daughter product, ⁵⁶Co, has been detected from SN1987A by several instruments. The early occurrence of this emission was surprising and indicates that some fraction of ⁵⁶Ni, which is synthesized in the innermost supernova layers, must have mixed with the outermost supernova ejecta.Special attention is given to the γ-ray line emission of the decay chain of ⁴⁴Ti (⁴⁴Ti → ⁴⁴Sc → ⁴⁴Ca), which is accompanied by line emission at 68, 78, and 1157 keV. As the decay time of ⁴⁴Ti is ∼86 yr, one expects this line emission from young supernova remnants. Although the ⁴⁴Ti yield (typically 10⁻⁵–10⁻⁴M_⊙) is not very high, its production is very sensitive to the energetics and asymmetries of the supernova explosion, and to the mass cut, which defines the mass of the stellar remnant. This makes ⁴⁴Ti an ideal tool to study the inner layers of the supernova explosion. This is of particular interest in light of observational evidence for asymmetric supernova explosions.The γ-ray line emission from ⁴⁴Ti has so far only been detected from the supernova remnant Cas A. I discuss these detections, which were made by COMPTEL (the 1157 keV line) and BeppoSAX (the 68 and 78 keV lines), which, combined, give a flux of (2.6 ± 0.4 ± 0.5) × 10⁻⁵ ph cm⁻² s⁻¹ per line, suggesting a ⁴⁴Ti yield of (1.5 ± 1.0) × 10⁻⁴M_⊙. Moreover, I present some preliminary results of Cas A observations by INTEGRAL, which so far has yielded a 3σ detection of the 68 keV line with the ISGRI instrument with a flux that is consistent with the BeppoSAX detections. Future observations by INTEGRAL-ISGRI will be able to constrain the continuum flux above 90 keV, as the uncertainty about the continuum shape, is the main source of systematic error for the 68 and 78 keV line flux measurements. Moreover, with the INTEGRAL-SPI instrument it will be possible to measure or constrain the line broadening of the 1157 keV line. A preliminary analysis of the available data indicates that narrow line emission (i.e., Δv < 1000 km s⁻¹) can be almost excluded at the 2σ level, for an assumed line flux of 1.9 × 10⁻⁵ ph cm⁻² s⁻¹.     

Circulation of energetic ions of terrestrial origin in the magnetosphere

Energetic ion composition measurements have now been performed from earth orbiting satellites for more than a decade. As early as 1972 we knew that energetic (keV) ions of terrestrial origin represented a non-negligible component of the storm time ring current. We have now assembled a significant body of knowledge concerning energetic ion composition throughout much of the earth's magnetosphere. We know that terrestrial ions are a common component of the hot equatorial magnetospheric plasma in the ring current and the plasma sheet out to ? 23 R_E. During periods of enhanced geomagnetic activity this component may become dominant. There is also clear evidence that the terrestrial component (specifically O⁺) is strongly dependent on solar cycle. Terrestrial ion source, transport, and acceleration regions have been identified in the polar auroral region, over the polar caps, in the magnetospheric boundary layers, and within the magnetotail lobes and plasma sheet boundary layer. Combining our present knowledge of these various magnetospheric ion populations, it is concluded that the primary terrestrial ion circulation pattern associated with enhanced geomagnetic activity involves direct injection from the auroral ion acceleration region into the plasma sheet boundary layer and central plasma sheet. The observed terrestrial component of the magnetospheric boundary layer and magnetotail lobes are inadequate to provide the required influx. They may, however, contribute significantly to the maintenence of the plasma sheet terrestrial ion population, particularly during periods of reduced geomagnetic activity. It is further concluded, on the basis of the relative energy distributions of H⁺ and O⁺ in the plasma sheet, that O⁺ probably contributes significantly to the ring current population at energies inaccessible to present ion composition instrumentation (? 30 keV).     

FIGARO: An experiment for pulsar and variable source studies in the MeV range

We present a large area, balloon borne, NaI(Tl) detector for low-energy gamma rays with temporal signature : FIGARO.The main detector is a mosaic of 12 NaI(Tl) tiles 22.5 × 15 × 5 cm, for a total geometric area of 4050 cm².In the energy band 140 keV - 6 MeV, the expected background counting rate at float altitude is in the range of two to three thousands counts per second.For pulsar analysis the expected 3δ sensitivity for 5 hours exposition time is 2.5 10^?4 ph/cm².s.MeV (150–500 keV) 1.5 10^?4 ph/cm².s.MeV (1–6 MeV). This performance, together with the large effective area and the relatively short duration of a balloon flight, make FIGARO particularly suitable for the identification of sources by means of temporal analysis.For objectives in the Northern sky, including the Crab pulsar, a transmediterranean flight is planned for the summer of 1982 ; a Southern mission is scheduled in Brazil for the fall of 1983 (Vela, PSR 1822-09).     

Obscured QSOs and the X-ray Background

The question of whether there exists a large population of dust obscured QSOs is currently very controversial. In favour of this hypothesis are models for the origin of the X-ray Background (XRB) and also the Unified Model of AGN which both invoke large populations of obscured QSOs. For example, Madau et al. (1994) suggest a population of QSOs with N_H ∼ 10²⁴ cm⁻² or A_V = 1000^m to improve the fit to the XRB between 1 < E < 100 keV. Arguments contradicting this theory include those of Boyle & di Matteo (1995) who claim that the tight X-ray/optical flux ratio relation for QSOs precludes the existence of a large population of objects obscured by significant amounts of intrinsic dust. Here, we follow Madau et al. (1994) and Comastri et al. (1995) to make fits to the XRB using obscured QSO populations and investigate whether selection effects may allow a tight distribution of X-ray/optical ratios to be maintained. We find that even for a flat distribution of absorbing columns, reasonable fits to the XRB can be obtained while both optical and X-ray absorption combine to produce the tight observed X-ray/optical correlation.     

Laboratory simulation of the injection of energetic electron beams into the ionosphere—Ignition of the beam plasma discharge

Experiments, which somewhat simulate the injection of monoenergetic (several keV) electron beams into the ionosphere, have been performed in the very large (17 m × 26 m) vacuum chamber at Johnson Space Center. Typical operating ranges were: Beam current, I (0–130 mA), beam energy, E (0.5–3 kV), magnetic field, (0.3–2 G), path length, L (10–20 m), and injection pitch angle, α(0–80°). Measurements were carried out in both steady state and pulsed modes. In steady state and for constant V, B, p, L, α, the beam plasma discharge (BPD) is abruptly ignited when the beam current is increased above a critical value; at currents below critical, the beam configuration appears grossly consistent with single particle behavior. If it is assumed that each of the experiment parameters can be varied independently, the critical current required for ignition obeys the empirical relationship at p < 2 × 10^?5 torr:

$\text{I}\textcolor{red}{}\text{E}{}^{\mathrm{3/2}}\text{B}{}^{\mathrm{<mtext>0.7</mtext><mtext>pL</mtext>}}$

The BPD is characterized by 1) a large increase in the plasma production rate manifested in corresponding increases in the 3914 Å light intensity and plasma density, 2) intense wave emissions in a broad band centered at the plasma frequency and a second band extending from a few kHz up to the electron cyclotron frequency, 3) scattering of the beam in velocity space and 4) radial expansion and pitch angle scattering of the primary beam leading to the disappearance of single particle trajectory features.Measurements of the BPD critical current have been carried out with an ion thruster (Kaufman engine) to provide a background plasma, and these indicate that the presence of an ambient plasma of typical ionospheric densities has little effect on the critical current relation.Measurements of wave amplitudes over a large frequency range show that the amplitude of waves near the plasma and electron cyclotron frequencies are too small to cause or sustain BPD, and that the important instabilities are at much lower frequency (～ 3 kHz in these measurements).     

Particle and Joule heating of the neutral polar thermosphere in cusp region using Atmosphere Explorer-C satellite measurements

Simultaneous measurements taken by instruments on the Atmosphere Explorer - C satellite were used to compare electron and proton particle energy deposition, Joule heating, and neutral density perturbations in the region of the cusp.Altitude profiles of Joule heating, electron energy deposition, and electron density are derived using measurements taken by the satellite as input to a computer model. Electric fields are calculated using ion drift measurements. Figures are presented for a representative orbital pass.A peak Joule heating rate of 0.059 Wm^?2 occurred in the cusp region with a peak of 0.025 Wm^?2 in the evening auroral electrojet. Peak volume heating rates corresponding to these regions were 1.4 × 10^?6Wm^?3 and 7.10^?7 Wm^?3, both occurring at an altitude of 115 km. Particle energy deposition was about an order of magnitude less than Joule heating. Large neutral density perturbations are related to regions of heating.     

A topside investigation over a mid-latitude digisonde station in Cyprus

We examine the systematic differences between topside electron density measurements recorded by different techniques over the low-middle latitude operating European station in Nicosia, Cyprus (geographical coordinates: 35.14^oN, 33.2^oE), (magnetic coordinates 31.86^oN, 111.83 ^oE). These techniques include space-based in-situ data by Langmuir probes on board.European Space Agency (ESA) Swarm satellites, radio occultation measurements on board low Earth orbit (LEO) satellites from the COSMIC/FORMOSAT-3 mission and ground-based extrapolated topside electron density profiles from manually scaled ionograms. The measurements are also compared with International Reference Ionosphere Model (IRI-2016) topside estimations and IRI-corrected NeQuick topside formulation (method proposed by Pezzopane and Pignalberi (2019)). The comparison of Swarm and COSMIC observations with digisonde and IRI estimations verifies that in the majority of cases digisonde underestimates while IRI overestimates Swarm observations but in general, IRI provides a better topside representation than the digisonde. For COSMIC and digisonde profiles matched at the F layer peak the digisonde systematically underestimates topside COSMIC electron density values and the relative difference between COSMIC and digisonde increases with altitude (above hmF2), while IRI overestimates the topside COSMIC electron density but after a certain altitude (~150 km above hmF2) this overestimation starts to decrease with altitude. The IRI-corrected NeQuick underestimates the majority of topside COSMIC electron density profiles and relative difference is lower up to approximately 100 km (above the hmF2) and then it increases. The overall performance of IRI-corrected NeQuick improves with respect to IRI and digisonde.     

Energetic particles in the night-time middle- and low-latitude ionosphere

Data are presented on the zones of energetic particle precipitation at middle and low latitudes observed during and after magnetic storm injection events. Satellite measurements of the equatorial zone ion flux (～ 10³ - 10⁴ cm^?2 s^?1 sr^?1 for E > 45 keV at 240 km) are consistent with the development of a temporary low altitude ion radiation belt at the magnetic equator. In the midlatitude ion zone the flux (～ 10³ - 10⁵ ions cm^?2 s^?1 sr^?1 for E > 45 keV at 220 km) is directly related to magnetic activity while the midlatitude electron zone flux has a delayed response (～ 4 days).     

Charging of dust grains by anisotropic solar wind multi-component plasma

In this paper we study the charging process of small grain particles by anisotropic multi-component solar wind plasmas (electrons, protons and heavy ions), versus two-component (electron/proton) plasmas. We are focusing attention on the important characteristics of the charging process, namely the charging time, floating potential and current content as functions of plasma parameters such as He⁺⁺/H⁺ (α/p) number density and T_α/T_p temperature ratios of alpha particles to protons, as well as plasma streaming velocity v₀. Measured statistical properties of solar wind plasma parameters at 1 AU show considerable variations in α/p-temperature ratios from 1 to 10, in α/p-number density ratio from 0.01 to 0.35, as well as in values of streaming velocity v₀ from 200 km/s to 1000 km/s and more. Periods of these variations could last for several days each, leading to significant variability in the charging process, according to newly derived general analytical expressions. Numerical calculations performed for protons/alphas plasmas showed large disparity in the charging characteristics. For example, in anisotropic plasma, grain charging time varies up to 90% depending on α/p-particles temperature and number density ratios, whereas changes in floating potential are up to 40%. In contrast, in isotropic plasma, charging characteristic for grains do not change very much for the same plasma parameters variations, with charging time varying about 12% and floating potential only varying about 4%. It is also shown that in highly anisotropic plasma, with all ballistic electrons and ions, dust grains could not hold their charges, and characteristic discharged time is calculated. We note that the analysis is equally applicable to any sized body immersed in solar wind plasma.     

Venus EUV measurements of hydrogen and helium from Venera 11 and Venera 12

Lyman α and 58.4 nm HeI radiations resonantly scattered were observed with EUV spectrophotometers flown on Venera 11 and Venera 12. The altitude distribution of hydrogen was derived by limb observations from 250 km (exobase level) to 50,000 km. In the inner exosphere (up to ? 2,000 km of altitude) the distribution can be described by a classical exospheric distribution with T_C = 275 ± 25 K and n = 4_?2⁺³ × 10⁴ atom. cm^?3 at 250 km. The integrated number density from 250 to 110 km (the level of CO₂ absorption) is 2.1 × 10¹² atom. cm^?2, a factor of 3 to 6 lower than that predicted by aeronomical models. This number density decreases from the morning side to the afternoon side, or alternately from equatorial to polar regions. Above 2,000 km a “hot” hydrogen population dominates, which can be simulated by T = 10³K and n = 10³ atom. cm^?3 at the exobase level.The optical thickness of helium above 141 km (the level of CO₂ absorption for 58.4 nm radiation) was determined to be τ_o = 3, corresponding to a density at 150 km of 1.6 × 10⁶ cm^?3. This is about 3 times less than what was obtained with the Bus Neutral Mass Spectrometer of Pioneer Venus, and about twice less than ONMS measurements, but is in agreement with earlier EUV measurement by Mariner 10 (2 ± 1 × 10⁶ cm^?3).     

Particle and optical measurements aboard the Aureol 3 spacecraft (ARCAD 3 project): A — The particle experiments B — Optical measurements

The CESR Toulouse - IKI Moscow particle instrument package aboard the AUREOL-3 satellite consists of a complete set of charged particle spectrometers which measure electron and ion fluxes from 15 eV to 25 keV in 128 steps and in 11 directions. In addition, 4 channel spectrometers (2 electron and 2 ion channels in parallel) allow high time resolution measurements (up to 10 msec) with onboard calculation of auto and cross correlation functions. For higher energies (40 – 280 keV), solid-state spectrometers are used to measure electron and proton fluxes in 4 channels in parallel. In addition, two Geiger counters are used for the determination of the trapping boundaries. Two mass-energy ion spectrometers (1 to 32 A.M.U., 0.02 – 15 keV) are placed with viewing angles which allow a distinction between nearly isotropic auroral proton precipitation and conical beams accelerated in the auroral ionosphere. Auroral and airglow photometry is performed aboard the AUREOL-3 satellite by a set of 3 parallel directed photometers with tiltable interference filters for 6300 Å, 4278 Å and Doppler shifte H_β emissions. Various modes of energy, angular and mass scanning, correlation function calculation and various Soviet and French telemetry regimes provide the possibility of choosing the sequences of measurements according to particular experimental programs along the orbit. Finally, examples of data from inflight measurements using the above instruments are presented and briefly discussed, showing several interesting features.     

A new inversion algorithm for HF sky-wave backscatter ionograms

HF sky-wave backscatter sounding system is capable of measuring the large-scale, two-dimensional (2-D) distributions of ionospheric electron density. The leading edge (LE) of a backscatter ionogram (BSI) is widely used for ionospheric inversion since it is hardly affected by any factors other than ionospheric electron density. Traditional BSI inversion methods have failed to distinguish LEs associated with different ionospheric layers, and simply utilize the minimum group path of each operating frequency, which generally corresponds to the LE associated with the F₂ layer. Consequently, while the inversion results can provide accurate profiles of the F region below the F₂ peak, the diagnostics may not be so effective for other ionospheric layers. In order to resolve this issue, we present a new BSI inversion method using LEs associated with different layers, which can further improve the accuracy of electron density distribution, especially the profile of the ionospheric layers below the F₂ region. The efficiency of the algorithm is evaluated by computing the mean and the standard deviation of the differences between inverted parameter values and true values obtained from both vertical and oblique incidence sounding. Test results clearly manifest that the method we have developed outputs more accurate electron density profiles due to improvements to acquire the profiles of the layers below the F₂ region. Our study can further improve the current BSI inversion methods on the reconstruction of 2-D electron density distribution in a vertical plane aligned with the direction of sounding.     

Observational evidence for chromospheric footpoint penetration of nonthermal electrons during two well-observed flares

Recent advances have enabled simultaneous Hα and X-ray observations with substantially improved spatial, spectral, and temporal resolution. In this paper we study two events observed as part of a coordinated observing program between the Solar Maximum Mission and Sacramento Peak Observatory: the flares of 1456 UT, 7 May 1980 and 1522 UT, 24 June 1980. Using recently-developed physical models of static flare chromospheres, and corresponding theoretical Hα line profiles, we can distinguish effects of intense nonthermal electron heating from those of high conduction and pressure from the overlying flare corona. Both flares show the signature of intense chromospheric heating by fast electrons, temporally correlated with X-ray light curves at E > 27keV, and spatially associated with X-ray emission sites at E >62; 16 keV. Interpreting the Hα line profile observations using the theoretical Hα line profiles, we infer values of the thick-target input power contained in nonthermal electrons that are observationally indistinguishable (within a factor of 2–3) from those inferred from the X-ray data. Although these events are small, the energy flux values are large: of order 10¹¹ ergs cm^?2 s^?1 above 20 keV.