B. J. Kellett

Ring current ion composition during solar minimum and rising solar activity: Polar/CAMMICE/MICS results

Abstract : This report shows statistical results of the ring current ion composition and its variability as a function of solar cycle and magnetospheric activity. Spin-averaged energetic particle (1-200 keV) measurements from the POLAR/CAMMICE/MICS instrument are combined with geomagnetic indices as well as solar wind and IMF observations from the WIND spacecraft during a period from September 1996 to March 1999. The statistics are performed both for time-averaged values for all periods as well as for peak flux values during geomagnetic storms (defined as Dst < -50 nT) that occurred during this period. The average O(+) energy density increases by about a factor of 5 during the rising phase of the solar cycle from the minimum values in 1996, while the average values of H(+) and He show variability but no consistently increasing trend. The O(+) flux is small (below 10%) compared to the hydrogen flux, and the average energy density ranges from a few percent at solar minimum to about 10% at high solar activity time in early 1999. The O(+) flux is typically smaller than the He(+) flux, reaching comparable values only during the latter part of the period when the solar activity increased. Analogously, the energy densities of O(+) and He(+) are about equal during 1996 and 1997, whereas the O(+) energy density is about twice the He(+) energy density during the higher solar activity period in 1998 and early 1999.

Entry of plasma sheet particles into the inner magnetosphere as observed by Polar/CAMMICE

Statistical results are presented from Polar/CAMMICE measurements of events during which the plasma sheet ions have penetrated deeply into the inner magnetosphere. Owing to their characteristic structure in energy-time spectrograms, these events are called “intense nose events.” Almost 400 observations of such structures were made during 1997. Intense nose events are shown to be more frequent in the dusk than in the dawn sector. They typically penetrate well inside L = 4, the deepest penetration having occurred around midnight and noon. The intense nose events are associated with magnetic (substorm) activity. However, even moderate activity (AE = 150–250 nT) resulted in formation of these structures. In a case study of November 3, 1997, three sequential inner magnetosphere crossings of the Polar and Interball Auroral spacecraft are shown, each of which exhibited signatures of intense nose-like structures. Using the innermost boundary determinations from these observations, it is demonstrated that a large-scale convective electric field alone cannot account for the inward motion of the structure. It is suggested that the intense nose structures are caused by short-lived intense electric fields (in excess of ∼1 mV/m) in the inner tail at L=4–5.

The D-CIXS X-ray mapping spectrometer on SMART-1

The D-CIXS Compact X-ray Spectrometer will provide high quality spectroscopic mapping of the Moon, the primary science target of the ESA SMART-1 mission. D-CIXS consists of a high throughput spectrometer, which will perform spatially localised X-ray fluorescence spectroscopy. It will also carry a solar monitor, to provide the direct calibration needed to produce a global map of absolute lunar elemental abundances, the first time this has been done. Thus it will achieve ground breaking science within a resource envelope far smaller than previously thought possible for this type of instrument, by exploiting two new technologies, swept charge devices and micro-structure collimators. The new technology does not require cold running, with its associated overheads to the spacecraft. At the same time it will demonstrate a radically novel approach to building a type of instrument essential for the BepiColombo mission and potential future planetary science targets.

Energetic electrons produced by lower hybrid waves in the cometary environment and soft X ray emission: Bremsstrahlung and K shell radiation

Electron acceleration by lower hybrid waves is a phenomenon well known in fusion studies (e.g., the so-called lower hybrid current drive), and it is also responsible for electron energization in many space environments. In the latter case, lower hybrid wave turbulence is of a self-generated type. It can be produced by the modified two-stream instability (MTSI) of the counterstreaming ion populations resulting from the mass-loading effect, for example, contamination of the solar wind flow or interstellar plasma with the newly born ions. In the cometary case analyzed in this paper these ions are produced by photoionization of the neutral gas outflow evaporated from the cometary nucleus. The interesting feature about lower hybrid oscillations is that they can be in simultaneous Cherenkov resonance with unmagnetized slow ions and fast but magnetized electrons, and owing to this, serve as a powerful tool for energy coupling leading to electron energization. In situ measurements at Comet Halley revealed the existence of intense lower hybrid turbulence inside the cometary bow shock. The quasi-linear theory of the MTSI evolution constructed in this paper demonstrates a consistency of these wave measurements with the large flux of energetic electrons also measured during the encounter with Comet Halley. Energetic electrons penetrating inside the cometary atmosphere produce soft X ray emission by a combination of bremsstrahlung and line radiation of oxygen atoms. Theoretical results are found to be in good agreement with six independent observations of the X ray cometary emission by the Rontgen satellite and Beppo-SAX telescopes.

Coherent cyclotron maser radiation from UV Ceti

Recent images in the radio of UV Ceti show two intense emission regions above the magnetic poles of the star. The emission is overwhelmingly right hand circularly polarized with 100% right hand polarization at 3 cm and 6 cm wavelengths during flares. This high degree of polarization rules out gyrosynchrotron emission. In this article we propose that such emission can be produced by a coherent cyclotron maser driven by field aligned electrons moving into an increasing magnetic field. Conservation of the first adiabatic invariant causes the electron beam to increase its pitch angle. This produces a broad region on the distribution function and the possibility of producing a positive slope in the perpendicular velocity space. Such distribution functions are common in magnetic mirror type geometries and are possible sources of free energy leading to electromagnetic emission by a cyclotron maser instability mechanism.

X-ray emission from comet Hale-Bopp

The discovery of X-ray emission from comets has created a number of questions about the physical mechanism producing the radiation. There are now a variety of explanations for the emission, from thermal bremsstrahlung of electrons off neutrals or dust, to charge exchange induced emission from solar wind ions, to scattering of solar X-rays from attogram dust, to reconnection of solar magnetic field lines. In an effort to understand this new phenomenon, we observed but failed to detect in the X-ray the very dusty and active comet C/Hale-Bopp 1995 O1 over a two year period, September 1996 to December 1997, using the ROSAT HRI imaging photometer at 0.1–2.0 keV and the ASCA SIS imaging spectrometer at 0.5–10.0 keV. The results of our Hale-Bopp non-detections, when combined with spectroscopic imaging 0.08–1.0 keV observations of the comet by EUVE and BeppoSAX, show that the emission has the same spectral shape and strong variability seen in other comets. Comparison of the ROSAT photometry of the comet to our ROSAT database of 8 comets strongly suggests that the overall X-ray faintness of the comet was due to an emission mechanism coupled to gas, and not dust, in the comet’s coma.

Numerical simulation of auroral cyclotron maser processes

Results are presented from a numerical investigation of radiation emission from an electron beam with a horseshoe-shaped velocity distribution. This process is relevant to the phenomenon of auroral kilometric radiation (AKR) which occurs in the polar regions of the Earths magnetosphere. In these regions of the auroral zone, particles accelerated into the increasing magnetic field of the Earths dipole develop a horseshoe-shaped velocity distribution through conservation of magnetic moment. It has been shown theoretically that this distribution is unstable to a cyclotron maser instability. A 2D particle-in-cell (PIC) code model was constructed to simulate a scaled laboratory experiment in which an electron beam subject to significant magnetic compression may be studied and brought into resonance with TE modes of an interaction waveguide. Results were obtained for electron beam energies of 75-85 keV, magnetic compression factors of up to 30 and electron cyclotron frequencies of 4.42 and 11.7 GHz. At 11.7 GHz, beam-wave coupling was observed with the TE03 mode and an RF output power of 20 kW was obtained corresponding to an RF conversion efficiency of 1.3%. At 4.42 GHz, excitation of the TE01 mode was observed with an RF output power of 35 kW for a cyclotron-wave detuning of 2%. This corresponds to an RF conversion efficiency of 2.6%. In both cases PiC particle velocity distributions show the clear formation of a horseshoe-shaped velocity distribution and subsequent action of a cyclotron maser instability. The RF conversion efficiencies obtained are also comparable with estimates for the AKR generation efficiency. (Abstract from: http://iopscience.iop.org/0741-3335/50/7/074011/)

Cyclotron maser radiation from astrophysical shocks

One of the most popular coherent radio emission mechanisms is electron cyclotron maser instability. In this article we demonstrate that electron cyclotron maser emission is directly associated with particular types of charged particle acceleration such as turbulence and shocks commonly inferred in astrophysical plasmas.

A volume-limited ROSAT survey of extreme ultraviolet emission from all nondegenerate stars within 10 parsecs

We report the results of a volume-limited ROSAT Wide Field Camera (WFC) survey of all nondegenerate stars within 10 pc. Of the 220 known star systems within 10 pc, we find that 41 are positive detections in at least one of the two WFC filter bandpasses (S1 and S2), while we consider another 14 to be marginal detections. We compute X-ray luminosities for the WFC detections using Einstein Imaging Proportional Counter (IPC) data, and these IPC luminosities are discussed along with the WFC luminosities throughout the paper for purposes of comparison. Extreme ultraviolet (EUV) luminosity functions are computed for single stars of different spectral types using both S1 and S2 luminosities, and these luminosity functions are compared with X-ray luminosity functions derived by previous authors using IPC data. We also analyze the S1 and S2 luminosity functions of the binary stars within 10 pc. We find that most stars in binary systems do not emit EUV radiation at levels different from those of single stars, but there may be a few EUV-luminous multiple-star systems which emit excess EUV radiation due to some effect of binarity. In general, the ratio of X-ray luminosity to EUV luminosity increases with increasing coronal emission, suggesting that coronally active stars have higher coronal temperatures. We find that our S1, S2, and IPC luminosities are well correlated with rotational velocity, and we compare activity-rotation relations determined using these different luminosities. Late M stars are found to be significantly less luminous in the EUV than other late-type stars. The most natural explanation for this results is the concept of coronal saturation -- the idea that late-type stars can emit only a limited fraction of their total luminosity in X-ray and EUV radiation, which means stars with very low bolometric luminosities must have relatively low X-ray and EUV luminosities as well. The maximum level of coronal emission from stars with earlier spectral types is studied also. To understand the saturation levels for these stars, we have compiled a large number of IPC luminosities for stars with a wide variety of spectral types and luminosity classes. We show quantitatively that if the Sun were completely covered with X-ray-emitting coronal loops, it would be near the saturation limit implied by this compilation, supporting the idea that stars near upper limits in coronal activity are completely covered with active regions.

Atomic data for modelling fusion and astrophysical plasmas

Trends and focii of interest in atomic modelling and data are identified in connection with recent observations and experiments in fusion and astrophysics. In the fusion domain, spectral observations are included of core, beam penetrated and divertor plasma. The helium beam experiments at JET and the studies with very heavy species at ASDEX and JET are noted. In the astrophysics domain, illustrations are given from the SOHO and CHANDRA spacecraft which span from the solar upper atmosphere, through soft x-rays from comets to supernovae remnants. It is shown that non-Maxwellian, dynamic and possibly optically thick regimes must be considered. The generalized collisional-radiative model properly describes the collisional regime of most astrophysical and laboratory fusion plasmas and yields self-consistent derived data for spectral emission, power balance and ionization state studies. The tuning of this method to routine analysis of the spectral observations is described. A forward look is taken as to how such atomic modelling, and the atomic data which underpin it, ought to evolve to deal with the extended conditions and novel environments of the illustrations. It is noted that atomic physics influences most aspects of fusion and astrophysical plasma behaviour but the effectiveness of analysis depends on the quality of the bi-directional pathway from fundamental data production through atomic/plasma model development to the confrontation with experiment. The principal atomic data capability at JET, and other fusion and astrophysical laboratories, is supplied via the Atomic Data and Analysis Structure (ADAS) Project. The close ties between the various experiments and ADAS have helped in this path of communication.