Steep Energy Spectrum Research Articles

Adiabatic acceleration of suprathermal electrons associated with dipolarization fronts

Recent observations in the inner magnetotail have shown rapid and significant flux increases (usually an order of magnitude of increase within seconds) of suprathermal electrons (tens of keV to hundreds of keV) associated with earthward moving dipolarization fronts. To explain where and how these suprathermal electrons are produced during substorm intervals, two types of acceleration models have been suggested by previous studies: acceleration that localizes near the reconnection site and acceleration that occurs during earthward transport. We perform an analytical analysis of adiabatic acceleration to show that the slope of source differential fluxes is critical for understanding adiabatic flux enhancements during earthward transport. Observationally, two earthward propagating dipolarization fronts accompanied by energetic electron flux enhancements observed by the THEMIS spacecraft have been analyzed; in each event the properties of dipolarization fronts in the inner magnetosphere (XGSM ≈ −10RE) were well correlated with those further down the tail (XGSM ≈ −15RE or XGSM ≈ −20RE). Coupled with theoretical analysis, this enables us to estimate the relative acceleration that occurred as the electrons propagated earthward between the two spacecraft. During the two events studied, the differential fluxes of supra thermal electrons had steep energy spectra with power law indices of −4 to −6.These spectra were much steeper than those at lower energy, as well as those of the supra thermal electrons observed before the fronts arrived. A compression factor of 1.5 as the electrons propagated earthward induced a flux increase of suprathermal electrons by a factor of 7 to 17. Provided these steep spectra, we demonstrate that adiabatic acceleration from the betatron and Fermi mechanisms simultaneously operating can account for these flux increases. Since both analytical analysis and data from the two events show that adiabatic acceleration during earthward transport does not significantly change the power law indices, the steep spectra were likely to be traced back to their source region, presumably near the reconnection site.

Decaying vortex and wave turbulence in rotating shallow water model, as follows from high-resolution direct numerical simulations

We report results of direct numerical simulations of decaying turbulence in an inviscid rotating shallow water model. We use a new-generation high-resolution well-balanced shock-capturing finite-volume scheme with several types of initializations: “classical” ones with random velocity and/or height fields, or an initialization with randomly oriented coherent vortex dipoles. Together with “full” turbulence simulations we also perform pure wave-turbulence ones, starting from an initial random wave field of small amplitude with zero potential vorticity anomaly and a given initial spectrum. Statistical properties of the rotating shallow water turbulence, as well as the development of coherent structures and their interactions are studied in detail. For all “full” turbulence simulations we find a tendency to form coherent structures with clear cyclone-anticyclone asymmetry and very steep energy spectra, with exponents close to −6. We also observe a decorrelation of the vortex and wave fields in time, even at significant Rossby numbers. However, we do not observe a universal power law in the evolution of coherent vortices, predicted by the “universal decay” theory for the 2D turbulence. A clear sensitivity to the initial conditions is thus established. For wave-turbulence simulations we observe a tendency to form very steep spectra different from the predictions of the so-called weak turbulence, and of both the turbulence of cusped nonlinear waves and the shock turbulence.

Constraining cosmic rays and magnetic fields in the Perseus galaxy cluster with TeV observations by the MAGIC telescopes

Galaxy clusters are being assembled today in the most energetic phase of hierarchical structure formation which manifests itself in powerful shocks that contribute to a substantial energy density of cosmic rays (CRs). Hence, clusters are expected to be luminous gamma-ray emitters since they also act as energy reservoirs for additional CR sources, such as active galactic nuclei and supernova-driven galactic winds. To detect the gamma-ray emission from CR interactions with the ambient cluster gas, we conducted the deepest to date observational campaign targeting a galaxy cluster at very high-energy gamma-rays and observed the Perseus cluster with the MAGIC Cherenkov telescopes for a total of ~85 hr of effective observing time. This campaign resulted in the detection of the central radio galaxy NGC 1275 at energies E > 100 GeV with a very steep energy spectrum. Here, we restrict our analysis to energies E > 630 GeV and detect no significant gamma-ray excess. This constrains the average CR-to-thermal pressure ratio to be <= 1-2%, depending on assumptions and the model for CR emission. Comparing these gamma-ray upper limits to predictions from cosmological cluster simulations that include CRs constrains the maximum CR acceleration efficiency at structure formation shocks to be < 50%. Alternatively, this may argue for non-negligible CR transport processes such as CR streaming and diffusion into the outer cluster regions. Finally, we derive lower limits on the magnetic field distribution assuming that the Perseus radio mini-halo is generated by secondary electrons/positrons that are created in hadronic CR interactions: assuming a spectrum of E^-2.2 around TeV energies as implied by cluster simulations, we limit the central magnetic field to be > 4-9 microG, depending on the rate of decline of the magnetic field strength toward larger radii.

Non-local energy transfers in rotating turbulence at intermediate Rossby number

AbstractTurbulent flows subject to solid-body rotation are known to generate steep energy spectra and two-dimensional columnar vortices. The localness of the dominant energy transfers responsible for the accumulation of the energy in the two-dimensional columnar vortices of large horizontal scale remains undetermined. Here, we investigate the scale-locality of the energy transfers directly contributing to the growth of the two-dimensional columnar structures observed in the intermediate Rossby number ($\mathit{Ro}$) regime. Our approach is to investigate the dynamics of the waves and vortices separately: we ensure that the two-dimensional columnar structures are not directly forced so that the vortices can result only from association with wave to vortical energy transfers. Detailed energy transfers between waves and vortices are computed as a function of scale, allowing the direct tracking of the role and scales of the wave–vortex nonlinear interactions in the accumulation of energy in the large two-dimensional columnar structures. It is shown that the dominant energy transfers responsible for the generation of a steep two-dimensional spectrum involve direct non-local energy transfers from small-frequency small-horizontal-scale three-dimensional waves to large-horizontal-scale two-dimensional columnar vortices. Sensitivity of the results to changes in resolution and forcing scales is investigated and the non-locality of the dominant energy transfers leading to the emergence of the columnar vortices is shown to be robust. The interpretation of the scaling law observed in rotating flows in the intermediate-$\mathit{Ro}$ regime is revisited in the light of this new finding of dominant non-locality.

On the correlation between spectra of solar microwave bursts and proton fluxes near the Earth

Studies of the extreme solar proton event of January 20, 2005 intensified the contest over of a long-standing problem: are solar cosmic rays arriving at the Earth accelerated by solar flares or by shocks preceding rapidly moving coronal mass ejections? Among the most important questions is the relationship between the energy spectra of the solar cosmic rays and the frequency spectra of flare microwave bursts. Some studies of previous solar-activity cycles have shown that such a relationship does exist, in particular, for protons with energies of tens of MeV. The present work analyzes this relation using data for 1987–2008. For flare events observed in the western half of the disk, there is a significant correlation between the index δ, which is equivalent to the power-law index of the integrated energy spectrum of 10–100 MeV protons detected near the Earth’s orbit, and radio burst parameters such as a ratio of peak fluxes S at two frequencies (for example, at 9 and 15 GHz) and a microwave peak frequency f m . Proton fluxes with hard (flat) energy spectra (δ ≤ 1.5) correspond to hard microwave frequency spectra (S 9/S 15 ≤ 1 and f m ≥ 15 GHz), while flares with soft radio spectra (S 9/S 15 ≥ 1.5 and f m ≤ 5 GHz) result in proton fluxes with soft (steep) energy spectra (δ ≥ 1.5–2). It is also shown that powerful high-frequency bursts with the hardest radio spectra (f m ≈ 30 GHz) can point at acceleration of significant proton fluxes in flares occurring in strong magnetic fields. These results argue that solar cosmic rays (or at least their initial impulses) are mainly accelerated in flares associated with impulsive and post-eruptive energy release, rather than in shocks driven by coronal mass ejections.

Large-scale dynamics in two-dimensional Euler and surface quasigeostrophic flows

The large-scale dynamics in classical two-dimensional Euler and surface quasigeostrophic flows are studied by examining the evolution of the mean-square stream function ⟨ψ2⟩ and of the Fourier mode ψ̂(k,t) for small wave number k=∣k∣. Upper bounds for ⟨ψ2⟩ and ∣ψ̂(k,t)∣2 are derived. The growth of ⟨ψ2⟩ is at most quadratic in time t and nearly quadratic in time for surface quasigeostrophic and Euler flows, respectively. At the modal level, it is found that ∣ψ̂(k,t)∣2≤ck2t2 and ∣ψ̂(k,t)∣2≤c′t2, where c and c′ are constant, for the surface quasigeostrophic and Euler cases, respectively. These bounds imply a steep energy spectrum at small k respectively, k5 and k3. The latter is consistent with previous statistical predictions and numerical results.

The Irradiation Origin of Beryllium Radioisotopes and Other Short‐lived Radionuclides

Two explanations exist for the short-lived radionuclides (T1/2 ≤ 5 Myr) present in the solar system when the calcium-aluminum-rich inclusions (CAIs) first formed. They originated either from the ejecta of a supernova or by the in situ irradiation of nebular dust by energetic particles. With a half-life of only 53 days, 7Be is then the key discriminant, since it can be made only by irradiation. Using the same irradiation model developed earlier by our group, we calculate the yield of 7Be. Within model uncertainties associated mainly with nuclear cross sections, we obtain agreement with the experimental value. Moreover, if 7Be and 10Be have the same origin, the irradiation time must be short (a few to tens of years), and the proton flux must be of order F ~ 2 × 1010 cm-2 s-1. The X-wind model provides a natural astrophysical setting that gives the requisite conditions. In the same irradiation environment, 26Al, 36Cl, and 53Mn are also generated at the measured levels within model uncertainties, provided that irradiation occurs under conditions reminiscent of solar impulsive events (steep energy spectra and high 3He abundance). The decoupling of the 26Al and 10Be observed in some rare CAIs receives a quantitative explanation when rare gradual events (shallow energy spectra and low 3He abundance) are considered. The yields of 41Ca are compatible with an initial solar system value inferred from the measured initial 41Ca/40Ca ratio and an estimate of the thermal metamorphism time (from Young et al.), alleviating the need for two-layer proto-CAIs. Finally, we show that the presence of supernova-produced 60Fe in the solar accretion disk does not necessarily mean that other short-lived radionuclides have a stellar origin.

Foreground contributions to the cosmic microwave background

A detailed search has been made for evidence of residual foreground contributions to the cosmic microwave background (CMB) in the map generated by the Wilkinson Microwave Anisotropy Probe (WMAP), a map that has been (nominally) cleaned for the foreground already. We find positive results in that various features relate to Galactic properties. For example, on the largest angular scales we find significant differences between the power in the fluctuations for positive and negative Galactic latitudes and between the four Galactic quadrants. There are also differences between the power spectrum at latitudes within 10 degrees of the plane and at higher latitudes. The mean temperature shows similar variations. An explanation in terms of Galactic effects seems inescapable. In an effort to find the origin of these Galactic-style effects we have examined evidence from Galactic gamma-rays, specifically from the EGRET instrument. We are mindful that the CMB maps examined have already been 'cleaned' (for CR and other effects) in a rather complex way, but, in our view, the cleaning has left some potentially serious 'contaminations'. A correlation is found between gamma-ray intensities and the CMB and other CR indicators. For example, regions of the Galaxy having (line-of-sight) steep CR energy spectra have low mean CMB temperatures, and the important Loop I edge region, where the CR intensity is high, has a high mean temperature. Most of the large-scale Galactic asymmetries (e.g. north-south difference and quadrant variations) have analogues in CR asymmetries and also in some other Galactic properties, such as the column density of gas. Thus, it is possible to hypothesize about direct CR-induced contributions, although it may be that CRs are simply the indicators of Galactic 'conditions' which are influencing the residual CMB fluctuations. Irrespective of the actual cause of the correlations we have endeavoured to extrapolate to the situation where the residual foreground is minimized. The effect on the usually derived cosmological properties is briefly examined. The least that can be said is that the 'error' in some of these properties has been underestimated.

Intrinsic, Observed, and Retrieved Properties of Interstellar Turbulence

We generate synthetic observations of three-dimensional, self-gravitating MHD simulations of the interstellar medium (ISM) to evaluate the ability of principal component analysis (PCA) to measure the scale dependence of turbulent velocity fluctuations in molecular clouds. Scaling exponents, α, observationally obtained from the coupled characteristic scales for line profile variability in velocity, δv, and in space, L, where δv ∝ Lα, are compared with the intrinsic scaling exponents of the MHD velocity fields. We determine the approximate structure function order at which PCA operates in order to then verify a previously established calibration of the PCA method. We also analyze the statistical properties of projected velocity line centroid fields, including effects of intermittent velocity fluctuations, density inhomogeneity, and opacity, and examine the relationship of the projected two-dimensional statistics to the intrinsic three-dimensional statistics. Using PCA, we infer steep three-dimensional energy spectra in the molecular ISM, generally steeper than can be accounted for by Kolmogorov turbulence or possibly even shock-dominated turbulence.

Comment on “MeV magnetosheath ions energized at the bow shock” by S.‐W. Chang et al.

[1] From a study of the 4 May 1998 storm event, Chang et al. [2001] (hereinafter referred to as CETAL01) suggested that ‘‘ions are accelerated at the quasi-parallel bow shock to energies as high as 1 MeVand subsequently transported into the magnetosheath during this event’’ and mentioned that ‘‘This is confirmed by a comparison of energetic ion fluxes simultaneously measured in the magnetosheath and at the quasi-parallel bow shock when both regions are likely connected by the magnetic field lines’’ (see their Abstract). After an inspection of the measured energetic ion data, however, one finds that CETAL01 have misplotted the observed ion energy spectrum in the ‘‘magnetosheath’’ (near the cusp) to lower energy which brings it in closer agreement to the flux measured near the quasi-parallel bow shock, making their analysis suspect. In fact, simultaneous measurements at this time indicate that (1) the energetic ion flux near the cusp was about one order of magnitude higher than that near the quasi-parallel bow shock, (2) the energetic ion time signatures were seen first near the cusp then near the bow shock, and (3) the energetic ion flux observed near the bow shock was independent of bow shock geometry. Each of these three facts is sufficient to demonstrate that the quasi-parallel bow shock was not the main source of the energetic ions near the cusp during this event. [2] CETAL01 stated that ‘‘A comparison of Interball and Polar ion spectra can potentially falsify our bow shock source hypothesis and is now the focus of our analysis.’’ In Figure 11 of CETAL01, accordingly, they compared the energetic ion flux measured by Interball near the quasiparallel bow shock with that measured by Polar near the cusp during the interval 1101–1142 UT on 4 May 1998, where their Polar/CEPPAD energetic ion data (open circles in their Figure 11) were taken only from the ion sensor that was looking 90 from the Polar spin axis. Our Figure 1 replots the Interball data (stars) and the Polar/CEPPAD data (open squares) for the same time interval. Comparing Figure 11 of CETAL01 to our Figure 1, we find that they have misplotted the CEPPAD ion energy spectrum near the cusp to the lower energies which reduces the difference between Interball and Polar ion fluxes. A closer inspection of their Figure 11 suggests that for each energy interval (channel) near the cusp they used the lower energy threshold to represent it without taking into account the effective energy passband [McKinnon and Fritz, 1976] of the steep energy spectrum, and the resulting energy spectrum near the cusp shown in their Figure 11 is thus lower than the actually observed spectrum as shown in our Figure 1. [3] This is not the only case where CETAL01 misplotted the observed ion energy spectrum around the cusp to lower than the actually observed spectrum, for in an earlier paper, Chang et al. [1998] (hereinafter referred to as CETAL98) misplotted the MICS (Magnetospheric Ion Composition Sensor) lower energy limit from 1 keV/e to 0.6 keV/e which brought the cusp fluxes into better alignment with ‘‘bow shock ion spectra,’’ and in addition, misplotted the HIT (Heavy Ion Telescope) helium data point in the cusp below the actual observed value of 0.22 He ions (cm-srs-keV/e) 1 at 0837:40-0845:00 UT on 27 August 1996 [CETAL98, Figure 3]. Based upon such misplotted data, CETAL01 stated in their introduction that CETAL98 ‘‘showed that cusp energetic ion spectra (<300 keV e ) matched very well with a large body of bow shock ion spectra.’’ [4] Yet even after misplotting the observed CEPPAD ion spectrum near the cusp to below the actual observed one, the Interball ion flux near the bow shock was still lower than the Polar ion flux near the cusp as shown in their Figure 11, so CETAL01 made another spectral adjustment by increasing the measured Interball ion flux near the bow shock by about 300% to match the repositioned Polar ion flux to obtain their Figure 13. They called it a ‘‘distance correction’’ and made it central to their argument stating that ‘‘an important piece of evidence for the bow shock source is demonstrated in Figure 13.’’ However, it has been reported [Lee, 1982] that the proton flux at 7 RE from the bow shock is almost the same as that at the bow shock at energies larger than 60 keV; that is, no correction is needed JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. A8, 1311, doi:10.1029/2002JA009634, 2003

Heavy ion measurement on LDEF.

A stack of CR-39 track detectors was exposed on the NASA satellite LDEF and recovered after almost 6 years in space. The quick look analysis yielded heavy ion tracks on a background of low energy secondaries from proton interactions. The detected heavy ions show a steep energy spectrum which indicates a radiation belt origin.

Energy spectra and coherent structures in forced two-dimensional and beta-plane turbulence

Results from a wide range of direct numerical simulations of forced-dissipative, differentially rotating two-dimensional turbulence are presented, in order to delineate the broad dependence of flow type on forcing parameters. For most parameter values the energy spectra of simulations forced at low wavenumbers are markedly steeper than the classical k−3 enstrophy inertial-range prediction, and although k−3 spectra can be produced under certain circumstances, the regime is not robust, and the Kolmogorov constant is not universal unless a slight generalization is made in the phenomenology. Long-lived, coherent vortices form in many cases, accompanied by steep energy spectra and a higher than Gaussian vorticity kurtosis. With the addition of differential rotation (the β-effect), a small number of fairly distinct flow regimes are observed. Coherent vortices weaken and finally disappear as the strength of the β-effect increases, concurrent with increased anisotropy and decreased kurtosis. Even in the absence of coherent vortices and with a Gaussian value of the kurtosis, the spectra remain relatively steep, although not usually as steep as for the non-rotating cases. If anisotropy is introduced at low wavenumbers, the anisotropy is transferred to all wavenumbers in the inertial range, where the dynamics are isotropic.For those simulations that are forced at relatively high wavenumbers, a well resolved and very robust k−5/3 energy inertial range is observed, and the Kolmogorov constant appears universal. The low-wavenumber extent of the reverse energy cascade is essentially limited by the β-effect, which produces an effective barrier in wavenumber space at which energy accumulates, and by frictional effects which must be introduced to achieve equilibrium. Anisotropy introduced at large scales remains largely confined to the low wavenumbers, rather than being cascaded to small scales. When there is forcing at both large and small scales (which is of relevance to the Earth's atmosphere), energy and enstrophy inertial ranges coexist, with an upscale energy transfer and downscale enstrophy transfer in the same wavenumber interval, without the need for any dissipation mechanism between forcing scales.

Heavy ion measurement on LDEF

Stacks of CR-39 track detectors were exposed on different locations on the NASA satellite LDEF and recovered after almost 6 years in space. The preliminary analysis yielded low energy heavy ion tracks on a background of low energy secondaries from proton interactions. The detected heavy ions show a steep energy spectrum and a spatial confinement close to the mirror plane in the South Atlantic Anomaly. We interpret this as evidence for a radiation belt origin.

Spectral laws for the enstrophy cascade in a two-dimensional turbulence

The author uses a generalised Von Karman-Heisenberg-von-Weizsacker-type model for the inertial transfer to give a generalised spectral law for the enstrophy cascade in a two-dimensional turbulence that exhibits a steeper energy spectrum for large wavenumbers and reduces to the well known k-3 spectrum at the other end of the spectrum. For very high wavenumbers, this spectrum is, in fact, an arbitrarily steep power law. Nonetheless, it is possible to give an even more rapidly decaying spectrum for this range, using a continuous spectral cascading model. He then discusses the intermittency aspects of the departures from the Batchelor-Kraichnan scaling law and shows that while the intermittency corrections within the framework of the beta model of Frisch et al. (1984) are in qualitative agreement with the predictions made by the generalised spectral law given in this paper, intermittency by itself is unable to account fully for the steeper spectra at large wavenumbers observed in the numerical experiments. The author discusses further fractal aspects of the enstrophy cascade, and shows that for the enstrophy cascade, the fractal dimension rules not only the manner in which the cascading proceeds but also the point where it stops.

The interaction of heavy ions from comet P/Giacobini‐Zinner with the solar wind

The encounter between the ICE spacecraft and Comet P/Giacobini‐Zinner was characterised in the solar wind by intense fluxes of heavy ions, measurable over a region 6 × 106 km in extent. The ions are observed with highly anisotropic angular distributions, steep energy spectra, and a change in the energy spectrum at around 80 keV, consistent with a composition predominantly of the water group. Flux versus time profiles follow a general fall off with increasing distance from the comet but with a marked inbound/outbound asymmetry. This asymmetry is due to the higher solar wind velocity on the outbound pass, giving rise to an increased energy gain of the pick‐up ions. The flux versus time profiles are strongly modulated by the rapid changes in the direction of interplanetary magnetic field. We present correlated observations of energetic ions, the interplanetary magnetic field and the solar wind, and compare these observations with theoretical predictions of the ion pick‐up process.

Solar filament eruptions and energetic particle events

The 1981 December 5 solar filament eruption that is associated with an energetic (E greater than 50 MeV) particle event observed at 1 AU. The eruption was photographed in H-alpha and was observed by the Solwind whitelight coronagraph on P78-1. It occurred well away from any solar active region and was not associated with an impulsive microwave burst, indicating that magnetic complexity and a detectable impulsive phase are not required for the production of a solar energetic particle (SEP) event. No metric type II or IV emission was observed, but an associated interplanetary type II burst was detected by the low-frequency radio experiment on ISEE 3. The December 5 and two other SEP events lacking evidence for low coronal shocks had unusually steep energy spectra (gamma greater than 3.5). In terms of shock acceleration, this suggests that shocks formed relatively high in the corona may produce steeper energy spectra than those formed at lower altitudes. It is noted that the filament itself maybe one source of the ions accelerated to high energies, since it is the only plausible coronal source of the He(+) ions observed in SEP events.

Observations of the interactions of heavy ions from Comet P/Giacobini-Zinner with the solar wind

During the encounter between the ICE spacecraft and Comet Giacobini-Zinner, intense fluxes of energetic heavy ions were observed at distances up to 4 × 10 6 km from the comet. These ions were observed with steep energy spectra and highly anisotropic angular distributions, and are consistent with a composition comprising mainly ions from the water group. The flux versus time profiles have a general fall-off with increasing distance from the comet, but are modulated by both changes in the magnetic field direction and the solar wind velocity, the magnetic field variations being mainly responsible for variations on a time scale of minutes, and the solar wind velocity variations being responsible for much larger time-scale modulations, such as the inbound/outbound asymmetry of the intensity profile. In this paper we present correlated observations of heavy ions, the solar wind velocity and the magnetic field direction, and compare the observations of the ions with the theoretical predictions for their variations with distance from the comet, with the solar wind velocity and with the magnetic field direction.

Energetic particle events (≥30 keV) of Jovian origin observed by Voyager 1 and 2 in interplanetary space

Short‐lived and long‐lived ion flux increases (E ≥30 keV) of Jovian origin have been observed by the low energy charged particle (LECP) instrument on the Voyager 1 and 2 spacecraft. The short‐lived events are observed more than 860 RJ upstream and more than 1500 RJ downstream of Jupiter. Observations of long‐lived events appear to be confined to ≲200 RJ upstream of Jupiter. The short‐lived events last from a few minutes to a couple of hours, while the long‐lived events last from 8 to 21 hours. Both types of events have sharp onsets and decays, are usually confined to energies below 1 MeV total energy, and show a large general enrichment of Z ≥6 particles relative to proton and helium particles when compared with energetic particle events of solar or interplanetary origin. Many of the events have a noticeable peak in the energy spectrum above 100 keV after the main portion of the event. In addition, the short‐lived events (upstream and downstream) are (1) extremely anisotropic with a flow direction consistent with flow away from Jupiter, (2) display no noticeable velocity dispersion, and (3) display initially steep energy spectra that flatten with time. The peak flux level at the lowest energies in the magnetosheath is similar to that observed during long‐lived events. We conclude that a significant fraction of the particles observed during Jovian ion events originate from within the magnetosphere of Jupiter and simply leak out into the magnetosheath. If the interplanetary magnetic field favorably connects to the bow shock, particles can leak out into the interplanetary medium. Together with the necessary leakage model, the observations presented here cannot rule out the existence of a wave particle acceleration region located immediately upstream of the bow shock.

High Energy Gamma Rays from Pulsars

We present here the results from observations of pulsed gamma ray emission from the Crab and Vela pulsars for energies above 100 GeV using the atmospheric Cerenkov technique. The results suggest a very steep energy spectrum for gamma rays emitted from pulsars at high energies. Our observations over the last 4 years suggest also that the flux is highly variable with time.

Ions of Jovian origin observed by Voyager 1 and 2 in interplanetary space

Burst‐like and long‐lived ion fluxes (E ≳ 30 keV) of Jovian origin have been observed in interplanetary space by the LECP instrument on the Voyager 1 and 2 spacecraft. Burst (few minute duration) events are observed at distances greater than 0.6 AU (1200 RJ) from Jupiter. These events are highly anisotropic and possess steep energy spectra, while long‐lived (&gt;8 hour duration) events have relatively steady fluxes at low energies, strong anisotropies that decay with time, and a variable high energy component. Both types of events usually display simultaneous onsets and sharp cutoffs for all energies, an excess of atomic number Z ≥ 6 particles compared to solar and interplanetary events, and particle flow directions pointed away from Jupiter along the local interplanetary magnetic field. The origin for the long‐lived events appears to be inside the bow shock of the planet.