Energy Passbands Research Articles

Nanoflare Heating of the Solar Corona Observed in X-Rays

The existence of the million-degree corona above the cooler photosphere is an unsolved problem in astrophysics. Detailed study of the quiescent corona that exists regardless of the phase of the solar cycle may provide fruitful hints toward resolving this conundrum. However, the properties of heating mechanisms can be obtained only statistically in these regions due to their unresolved nature. Here, we develop a two-step inversion scheme based on the machine-learning scheme of Upendran & Tripathi (2021a) for the empirical impulsive heating model of Pauluhn & Solanki (2007), and apply it to disk integrated flux measurements of the quiet corona as measured by the X-ray solar monitor on board Chandrayaan-2. We use data in three energy passbands, viz, 1–1.3, 1.3–2.3, and 1–2.3 keV, and estimate the typical impulsive event frequencies, timescales, amplitudes, and the distribution of amplitudes. We find that the impulsive events occur at a frequency of ≈25 events per minute with a typical lifetime of ≈10 minutes. They are characterized by a power-law distribution with a slope α ≤ 2.0. The typical amplitudes of these events lie in an energy range of 1021–1024 erg, with a typical radiative loss of about ≈103 erg cm−2 s−1 in the energy range of 1–2.3 keV. These results provide further constraints on the properties of subpixel impulsive events in maintaining the quiet solar corona.

Variability in the Position of the IBEX Ribbon over Nine Years: More Observational Evidence for a Secondary ENA Source

Abstract The ribbon of enhanced energetic neutral atom flux, discovered by the Interstellar Boundary Explorer (IBEX) in 2009, has redefined our understanding of the heliosphere’s interaction with the local interstellar medium (LISM). Yet, its origin continues to be a topic of scientific debate. The ribbon is circular and traces the region where the putative LISM magnetic field (B LISM) is perpendicular to the radial direction from the Sun. Using nine years of IBEX-Hi observations, we investigate the ribbon circularity and location as functions of time and energy. We provide updated locations of the ribbon center at five energy passbands (centered at 0.7, 1.1, 1.7, 2.7, and 4.3 keV) in ecliptic coordinates [longitude, latitude]: [217.°41 ± 0.°95, 44.°36 ± 0.°93], [219.°72 ± 0.°95, 41.°50 ± 0.°87], [220.°51 ± 1.°19, 39.°96 ± 1.°00], [218.°08 ± 1.°66, 38.°44 ± 1.°24], and [214.°68 ± 1.°48, 34.°13 ± 1.°19] respectively. The weighted mean center location over all energies and all years is [218.°33 ± 0.°68, 40.°38 ± 0.°88] and its radius is 74.°81 ± 0.°65. As viewed by IBEX at 1 au, we find that (1) the ribbon is stable over time, with distinct centers at each energy; (2) ribbon centers exhibit small temporal variations, likely caused by the solar wind (SW) speed and density variations; and (3) ribbon location in the sky appears to be driven by (i) the inherent alignment of the ribbon centers along the plane connecting the presumed B LISM and the heliospheric upwind direction, and (ii) the variable SW structure along the heliographic meridian, further emphasizing that the ribbon source is outside the heliosphere.

DISTANCE TO THE IBEX RIBBON SOURCE INFERRED FROM PARALLAX

ABSTRACT Maps of energetic neutral atom (ENA) fluxes obtained from observations made by the Interstellar Boundary Explorer (IBEX) revealed a bright structure extending over the sky, subsequently dubbed the IBEX ribbon. The ribbon had not been expected from the existing models and theories prior to IBEX, and a number of mechanisms have since been proposed to explain the observations. In these mechanisms, the observed ENAs emerge from source plasmas located at different distances from the Sun. Since each part of the sky is observed by IBEX twice during the year from opposite sides of the Sun, the apparent position of the ribbon as observed in the sky is shifted due to parallax. To determine the ribbon’s parallax, we found the precise location of the maximum signal of the ribbon observed in each orbital arc. The apparent positions obtained were subsequently corrected for the Compton–Getting effect, gravitational deflection, and radiation pressure. Finally, we selected a part of the ribbon where its position is similar in the different IBEX energy passbands. We compared the apparent positions obtained from the viewing locations on the opposite sides of the Sun, and found that they are shifted by a parallax angle of 0.°41 ± 0.°15, which corresponds to a distance of AU. This finding supports models of the ribbon with the source located just outside the heliopause.

SIMULATING THE COMPTON-GETTING EFFECT FOR HYDROGEN FLUX MEASUREMENTS: IMPLICATIONS FORIBEX-HiAND -LoOBSERVATIONS

The Interstellar Boundary EXplorer (IBEX), launched in 2008 October, has improved our understanding of the solar wind-local interstellar medium interaction through its detection of neutral atoms, particularly that of hydrogen (H). IBEX is able to create full maps of the sky in six-month intervals as the Earth orbits the Sun, detecting H with energies between ∼0.01 and 6 keV. Due to the relative motion of IBEX to the solar inertial frame, measurements made in the spacecraft frame introduce a Compton-Getting (CG) effect, complicating measurements at the lowest energies. In this paper we provide results from a numerical simulation that calculates fluxes of H atoms at 1 AU in the inertial and spacecraft frames (both ram and anti-ram), at energies relevant to IBEX-Hi and -Lo. We show theory behind the numerical simulations, applying a simple frame transformation to derived flux equations that provides a straightforward way to simulate fluxes in the spacecraft frame. We then show results of H energetic neutral atom fluxes simulated at IBEX-Hi energy passbands 2-6 in all frames, comparing with IBEX-Hi data along selected directions, and also show results simulated at energies relevant to IBEX-Lo. Although simulations at IBEX-Hi energies agree reasonably well with the CG correctionmore » method used for IBEX-Hi data, we demonstrate the importance of properly modeling low energy H fluxes due to inherent complexities involved with measurements made in moving frames, as well as dynamic radiation pressure effects close to the Sun.« less

A THREE-COORDINATE SYSTEM (ECLIPTIC, GALACTIC, ISMF) SPECTRAL ANALYSIS OF HELIOSPHERIC ENA EMISSIONS USINGCASSINI/INCA MEASUREMENTS

In the present study, we use all-sky energy-resolved energetic neutral atom (ENA) maps obtained by the Ion and Neutral CAmera (INCA) instrument on board Cassini that correspond to the time period from 2003 to 2009, in four discrete energy passbands (~5.4 to ~55 keV), to investigate the geometrical characteristics of the belt (a broad band of emission in the sky). The heliospheric ENA emissions are mapped in three different coordinate systems (ecliptic, Galactic, and interstellar magnetic field (ISMF)), and spectral analyses are performed to further examine the belt's possible energy dependence. Our conclusions are summarized as follows: (1) the high flux ENA belt identified in the energy range of 8-42 keV is moderately well organized in Galactic coordinates, as the ENA minima appear in the vicinity of the north and south Galactic poles; (2) using minimization criteria ( B · R ~ 0), the deviation of the ENA emissions from the equator is effectively minimized in a rotated frame, which we interpret as ISMF, where its north pole points toward 190° ecliptic longitude and 15° ecliptic latitude; (3) ENA spectra show a power-law form in energy that can be fitted with a single function presenting higher spectral slopes in the belt region and lower outside (3.4 35 keV); and (6) in the low count rate regions, the long-term ENA count rate profiles do not match the measured cosmic ray profiles, indicating that even the minimum ENA emissions detected by INCA are foreground ENAs.

CIRCULARITY OF THEINTERSTELLAR BOUNDARY EXPLORERRIBBON OF ENHANCED ENERGETIC NEUTRAL ATOM (ENA) FLUX

As a sharp feature in the sky, the ribbon of enhanced energetic neutral atom (ENA) flux observed by the Interstellar Boundary Explorer (IBEX) mission is a key signature for understanding the interaction of the heliosphere and the interstellar medium through which we are moving. Over five nominal IBEX energy passbands (0.7, 1.1, 1.7, 2.7, and 4.3 keV), the ribbon is extraordinarily circular, with a peak location centered at ecliptic (λRC, βRC) = (219.°2 ± 1.°3, 39.°9 ± 2.°3) and a half cone angle of C = 74.°5 ± 2.°0. A slight elongation of the ribbon, generally perpendicular to the ribbon center-heliospheric nose vector and with eccentricity ~0.3, is observed over all energies. At 4.3 keV, the ribbon is slightly larger and displaced relative to lower energies. For all ENA energies, a slice of the ribbon flux peak perpendicular to the circular arc is asymmetric and systematically skewed toward the ribbon center. We derive a spatial coherence parameter δC ≤ 0.014 that characterizes the spatial uniformity of the ribbon over its extent in the sky and is a key constraint for understanding the underlying processes and structure governing the ribbon ENA emission.

On multiple classes of gamma-ray bursts, as deduced from autocorrelation functions or bivariate duration/hardness ratio distributions

Autocorrelation functions (ACFs) of 119 gamma-ray bursts (GRBs) monitored by the Burst Alert Telescope (BAT) on Swift are calculated. Contrasting with previous results for smaller numbers of bursts from other missions, the widths of the ACFs are not bimodally distributed. Although the distribution appears slightly asymmetrical, underlying mixtures of distributions can also probably be ruled out. Factors contributing to differences between the results presented here, and those in the literature, may include the differences in energy passbands used, and the superior sensitivity of the BAT instrument (which affects e.g. the redshift distribution of the detected GRB). The second part of the paper is concerned with the fitting of mixtures of bivariate Gaussians to the joint duration/hardness ratio data of 325 GRBs. A careful analysis confirms that a three-component mixture is the statistically most acceptable, but it is shown that the implied marginal distribution of the hardness ratios does not fit the data very well. It is also stressed that mixture components cannot automatically be assumed to represent different classes of GRBs. The point is illustrated by showing two substantially different, but statistically almost equivalent, mixture models for the distribution of 571 BAT-determined GRB durations.

Energy pass bands in a ribbed cylindrical shell with periodic asymmetries

The addition of periodically spaced structures to a cylindrical shell causes energy to propagate at additional wave numbers via a phenomenon known as Brillouin folding. As a result, a load representing one axial mode on a cylindrical shell with circumferentially spaced discontinuities, for example stringers, has distinct energy pass bands and stop bands. Similarly, an excitation representing one circumferential mode on a cylindrical shell with axially spaced discontinuities, for example ribs, also has distinct energy pass bands and stop bands. Discontinuities in both directions lead to energy pass bands when a point force is applied. In this paper, periodically spaced point masses in the axial and circumferential directions are added to a submerged, ribbed, finite cylindrical shell and analyzed via the finite element method. It is seen that Brillouin folding occurs and, in certain circumstances, the energy is split into pass bands and stop bands. For the frequency and circumferential modes where the subsonic energy is most narrowly focused in the axisymmetric shell, a distinct peak in radiated power occurs for the shell with point masses. The energy in this peak is about the same for several examples of different amounts of nonaxisymmetric mass.

The formation of energy pass bands from periodically spaced point masses on a ribbed cylindrical shell

An infinite cylindrical shell has bending energy that is able to propagate down the shell at all frequencies with little attenuation other than from material damping and the surrounding medium. In contrast, making the shell finite causes bending resonances to occur only at discrete frequencies. The addition of periodically spaced circumferential stiffeners causes the bending wave dispersion curves to be seen at additional axial modes, which is referred to as Brillouin folding. The stiffeners also cause energy stop bands to occur when the load is a single Fourier mode. In this presentation, periodically spaced point masses in the circumferential direction are added to a ribbed finite cylindrical shell and analyzed via the finite element method. It is seen that Brillouin folding occurs circumferentially. Also, energy pass bands are created in a certain pattern. Analysis of the axisymmetric version of this shell combined with information on the type of aperiodicity yields a prediction of the energy pass band behavior. This provides a qualitative approach toward understanding the motion of nonaxisymmetric shell structures. [Work supported by ONR.]

Spatial variations of 0.2 to 5 MeV protons in the 1–5 AU in‐ecliptic region from Ulysses, Voyager 1 and 2, and IMP 8 gradient studies

Protons with energies ranging from 0.3 to 2 MeV are nearly always present in the environment of Earth. Solar flare events and interplanetary shock waves can be identified as producing or enhancing these fluxes. However, interplanetary particles are observed even in the absence of solar flares. Explaining the presence of these proton fluxes in the interplanetary medium and accounting for their variations is a major problem in space physics. Observations of interplanetary proton fluxes have been made continuously at 1 AU with the Interplanetary Monitoring Platform (IMP) 8 from 1973 to the present and in the 1–5 AU range by Voyagers 1 and 2 (1977–1978) and Ulysses (1990–1991). Daily averaged proton fluxes of IMP 8, Voyagers 1 and 2, and Ulysses have been carefully interpolated to matching energy passbands so that fluxes in the same passbands at two radial distances could be compared. These daily averaged fluxes were then compared as ratios and were autocorrelated and cross‐correlated as functions of time delay. The radial gradient, the energy spectra, and the distribution of these proton fluxes were also examined. The results showed that protons in the 0.3 to 5 MeV energy range using the Voyager 1/IMP 8, Voyager 2/IMP 8, and Ulysses/IMP 8 paired observations in the 1 to 5 AU in‐ecliptic region tend to “decorrelate” with increasing radial separation and become uncorrelated by about 4 or 5 AU. Higher‐energy fluxes decorrelate less rapidly, and lower‐energy proton fluxes have more dominantly positive radial gradients than those of higher energy. The radial gradient of 0.3 to 0.5 MeV proton fluxes is dominantly positive for 1–2 AU, while that of 2 to 4 MeV proton fluxes is negative. In contrast, from 2 to 4 AU the radial gradient of 0.3 to 0.5 MeV proton fluxes is weakly negative, and that of 2 to 4 MeV protons is weakly positive. This finding suggests that 0.3 to 0.5 MeV protons are more subject to interplanetary acceleration than are 2 to 4 MeV protons in the 1–2 AU range. The results also showed that radial gradients are robust and persist, evert if all heliolongitude coherence is purposely removed by shuffling the time order within 120‐day time intervals.

Low-energy neutral atom emission from the Earth's magnetosphere

Imaging of the terrestrial magnetosphere is possible through the detection of low-energy neutral atoms (LENA5) produced by charge exchange between magnetospheric plasma ions and neutral atoms of the Earth's geocorona. We present calculations of both hydrogen and oxygen line-of-sight LENA fluxes expected on orbit for various plasma regimes as predicted by the Rice University Magnetospheric Specification Model. To decrease the required computation time, we are in the process of adapting our code for massively parallel computers. The speed gains achieved from parallel algorithms are substantial, and we present results from computational runs on the Connection Machine CM-2 data parallel supercomputer. We also estimate expected image count rates and image quality based on realistic instrument geometric factors, energy passbands, neutral atom scattering in the instrument, and image accumulation intervals. The results indicate that LENA imaging instruments will need a geometric factor ( G ) on the order of 0.1 cm 2 sr eV/eV to be capable of imaging storm time ring currents, and a G of 1.0 cm 2 sr eV/eV in order to image the quiet time ring current fluxes, ion injections from the tail, and subsequent ion drifts toward the dayside magnetopause.

Recent findings on angular distributions of dayside ring current energetic ions

Angular distributions of dayside ring current energetic ions are studied with the Medium Energy Particle Analyzer (MEPA) on the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer spacecraft. The anisotropy index n for pancake distribution during 10 geomagnetically quiet days is examined for the first time in the intermediate energies (50–1000 keV) for which compositional information is now available. It is shown that n for protons and helium ions increases with L whereas n for CNO ions appears to be independent of L. The empirical formula n(E) = no+k log10 (E(MeV)), for the energy dependence given by Fritz and Spjeldvik (1982), is extended to these energies and ion species. The parameter no is found to increase generally with ion mass and L, except for CNO ions which show no noticeable L dependence. The parameter k appears to be independent of L and ion mass. We have also surveyed the varieties of pitch angle distributions. Two unusual distributions were detected during the September 4–7, 1984, geomagnetic storm. The first is a net field‐aligned transport of ring current energetic ions at 6 ≲ L ≲ 9 and 1030–1300 MLT. Such a signature is shown most prominently by the heavy ions (mostly oxygen; E > 137 keV), to a lesser extent by helium ions (E > 72 keV), and only slightly by protons (E > 56 keV) which are the dominant ionic species. The second new feature is a sharp intensity enhancement over a pitch angle range centered at 90° which occurs in all ionic species and energy passbands monitored by MEPA at 3 ≲ L ≲ 4 and 1700–1800 MLT. Its characteristics suggest that it represents the addition of a freshly injected ring current population in the main phase development of the storm, and not due to L shell splitting or wave‐particle interaction.

Simultaneous quiet time observations of energetic radiation belt protons and helium ions: the equatorial α/p ratio near 1 MeV

Simultaneous monitoring of energetic helium ions and protons in the earth's radiation belts has been carried out with the satellite Explorer 45 in the immediate vicinity of the equatorial plane. Protons were measured from < 1 keV to 1.6 MeV in differential energy detector channels and above 3.3 MeV in a quasi‐integral channel responsive up to 22 MeV; helium ions were monitored in three differential energy passbands in the range 910 keV to 3.15 MeV and in a differential channel with double passband: 590–910 keV and 2.0–3.99 MeV. The measurements permit determination of the equatorial helium ion to proton flux ratios (α/p) both at equal energy per nucleon and equal energy per particle in the heart of the radiation belts on L shells below the spacecraft apogee at L ∼ 5.25. The flux ratios are found to vary significantly with energy and location in the radiation belts. At equal energy per nucleon a range of variability for α/p from 10−4 to well above 10−3 is found, and at equal energy per ion the corresponding variability is from 10−3 to above 10. The latter findings emphasize the relative importance of the very energetic helium ions in the overall radiation belt ion populations. Comparison is made with theoretical equatorial predictions of the α/p ratios that were made by Cornwall (1972) based on the radial diffusion, charge exchange, and Coulomb interactions; the observational findings support many of the theoretical results.