Pitch-angle scattering of energetic protons in the magnetotail current sheet as the dominant source of their isotropic precipitation into the nightside ionosphere

Characteristics of the nightside isotropic precipitation of energetic protons during a period of 4 quiet days has been studied using data from the ESRO 1A satellite. The observed features of the equatorward precipitation boundary (its thickness, energy dependence, dynamics, dependence of its latitudinal position on the magnetic field at the geosynchronous orbit, etc.) were found to be in good agreement with calculations based on recent magnetospheric magnetic field models. We argue that the mechanism of non-adiabatic pitchangle scattering in the equatorial current sheet is a dominant source of isotropic precipitation of energetic protons observed in the nightside auroral zone. Observations of the isotropic precipitation boundary can be used for monitoring the changes in the magnetotail current intensity.     

On the energy dependence of the ring current proton precipitation

Low altitude satellite measurements of protons in the 1–100 keV range indicate two energy dependent proton precipitation boundaries. At low invariant latitudes mostly below 60° there is a region of moderately weak proton precipitation. The poleward boundary of this region tends to be at higher latitudes for the high energy protons than for the low energy protons. At high invariant latitudes there is a region where both the low and high energy protons precipitate with an isotropic pitch-angle distribution. The equatorward boundary of this region tends to be at lower latitudes for protons with energy more than 100 keV than for those in the 1–6 keV range. This region with isotropic pitch-angle distribution is located well outside the plasmapause both for the 1–6 and 100-keV protons.Between these two precipitation zones there is a region where the proton pitch-angle distribution is highly anisotropic with almost no protons in the loss cone. This region tends to be wider and more pronounced in the 1–6 than in the 100-keV protons.These findings lend further support to the mechanism of ion-cyclotron instability as the cause of proton pitch-angle diffusion in the low and intermediate regions. The process responsible for the strong diffusion at auroral latitudes has not yet been identified.     

Coincident observations of ionospheric troughs and the equatorial plasmapause

Explorer 45 traversed the plasmapause (determined approximately via the saturation of the d.c. electric field experiment) at near-equatorial latitudes on field lines which were crossed by Ariel 4 (~600km altitude) near dusk in May 1972 and on field lines which were crossed by Isis II (~1400km altitude) near midnight in December 1971 and January 1972. Many examples were found in which the field line through the near-equatorial plasmapause was traversed by Explorer 45 within one hour local time and one hour universal time of Ariel and Isis crossings of the same L coordinate. For the coincident passes near dusk, the RF electron density probe on Ariel detected electron density depletions near the plasmapause L coordinates when Ariel was in darkness. When the Ariel passes were in sunlight, however, electron depletions were not discernable near the plasmapause field line. On the selected near-midnight passes of Isis II, electron density depressions were typically detected (via the topside sounder) near the plasmapause L coordinate. The dusk Ariel electron density profiles are observed to reflect O⁺ density variations. Even at the high altitude of Isis near midnight, O⁺ is found to be the dominant ion in the trough region whereas H⁺ is dominant at lower latitudes as is evident from the measured electron density scale heights. In neither local time sector was it possible to single out a distinctive topside ionosphere feature as an indicator of the plasmapause field line as identified near the equator. At both local times the equator-determined plasmapause L coordinate showed a tendency to lay equatorward of the trough minimum.     

Effects of convection electric field on the distribution of ring current type protons

The topology of the boundaries of penetration (or inversely the boundaries of the forbidden regions) of 90° pitch angle equatorial protons with energies less than 100 keV are explored for an equatorial convection E-field which is directed in general from dawn to dusk. Due to the dependence of drift path on energy (or magnetic moment) complex structural features are expected in the proton energy spectra detected on satellites since the penetration distance of a proton is not a monotonically increasing or decreasing function of energy. During a storm when the convection E is enhanced, model calculations predict elongations of the forbidden regions analogous to plasmatail extensions of the plasmasphere. Following a reduction in the convection field, spiral-structured forbidden regions can occur. Structural features inherent to large scale convection field changes may be seen in the noselike proton spectrograms observed near dusk by instrumentation on the satellite Explorer 45 (S³) (Smith and Hoffman, 1974). These nose events are modelled by using an electric field model developed originally by Volland (1973). The strength of the field is related to K_p through night-time equatorial plasmapause measurements.     

IN-SITU observations of irregular ionospheric structure associated with the plasmapause

Additional studies of the ion composition results obtained from the OGO-6 satellite support earlier observations of irregularities in the distribution of H⁺ and He⁺ within the light ion trough near L = 4, which has been associated with the plasmapause. These irregularities are in the form of sub-troughs superimposed upon the major mid latitude decrease of the light ions. In the sub-troughs, ionization depletions and recoveries of as much as an order of magnitude are observed within a few degrees of latitude, usually exhibited in a pattern which changes significantly with longitude as the Earth rotates beneath the relatively fixed satellite orbit. The location and properties exhibited by these sub-troughs appear to be consistent with the concept of a plasmasphere distortion in the form of “plasmatails” resulting from the combined effects of magnetospheric convection plus corotation. Like the light ion trough, the “plasmatail” irregularity in H⁺ may be obscured on the day side by the dominant topside distribution of O⁺. Consequently, these light ion irregularities are seen as an important factor for studies of plasmapause-trough relationships.     

Pitch-angle diffusion of ring current protons caused by electrostatic ion loss-cone waves

Pitch-angle diffusion of ring current protons is investigated, assuming that the scattering is caused by resonant interaction with electrostatic ion loss-cone waves. Bounce averaged diffusion coefficients are presented for different wave energy distributions. It is found that wave amplitudes about 1 mV/m are required to give strong diffusion of protons with average energy.     

Radiation production and energy deposition by ring current protons precipitated into the mid-latitude upper atmosphere

In the present paper the radiation production and energy deposition by ring current protons precipitated along magnetic field lines into the mid-latitude upper atmosphere is investigated. Specifically, we are interested in protons lost from the ring current by plasma instabilities. We first determine the magnitude and sharpness of the atmospheric loss cone. We then study the behavior of the precipitated hydrogen particles in the denser atmosphere using a Monte Carlo calculation. It is found that the energy deposition and radiation production will critically depend on how far the ring current protons diffuse into the loss cone before being neutralized in the atmosphere; this in turn will depend on the strength of the plasma turbulence in the ring current belt region.     

Field line topology in the dayside cusp region inferred from low altitude particle observations

Dayside low altitude satellite observations of the pitch angle and energy distribution of electrons and protons in the energy range 1 eV to 100 eV during quite geomagnetic conditions reveal that at times there is a clear latitudinal separation between the precipitating low energy (keV) electrons and protons, with the protons precipitating poleward of the electrons. The high energy (100 keV) proton precipitation overlaps both the low energy (keV) electron and proton precipitation. These observations are consistent with a model where magnetosheath particles stream in along the cusp field lines and are at the same time convected poleward by an electric field.The electrons with energies of a few keV move fast and give the “ionospheric footprint” of the distant cusp. The protons are partly convected poleward of the cusp and into the polar cap. Here the mirroring protons populate the plasma mantle. Equatorward of the cusp the pitch angle distribution of both electrons and protons with energies above a few keV is pancake shaped indicating closed geomagnetic field lines. The 1 keV electrons, penetrate, however, into this region of closed field line structure maintaining an isotropic pitch angle distribution. The intensity is, however, reduced with respect to what it was in the cusp region. It is suggested that these electrons, the lowest energies measured on the satellite, are associated with the entry layer.     

Global distribution, structure, and solar wind control of low altitude current sheets at Mars

We present the results of the first systematic survey of current sheets encountered by Mars Global Surveyor in its ∼400 km mapping orbit. We utilize an automated procedure to identify over 10,000 current sheet crossings during the ∼8 year mapping mission. The majority of these lie on the nightside and in the polar regions, but we also observe over 1800 current sheets at solar zenith angle <60°. The distribution and orientation of current sheets and their dependence on solar wind drivers suggests that most magnetotail current sheets have a local induced magnetospheric origin caused by magnetic field draping. On the other hand, most current sheets observed on the day side likely result from solar wind discontinuities advected through the martian system. However, the clustering of low altitude dayside current sheet crossings around the perimeters of strongly magnetized crustal regions, and the smaller than expected rotations in the IMF draping direction, suggest that crustal magnetic fields may also play an indirect role in their formation. The apparent thicknesses of martian current sheets, and the characteristics of electrons observed in and around the current sheets, suggest one of two possibilities. Martian current sheets at low altitudes are either stationary, with thicknesses of a few hundred km and currents carried by low energy (<10 eV) electrons, or they move at tens of km/s, with thicknesses of a few thousand km and currents carried by ions.     

Some newly discovered coronal emission lines from high altitude infrared observations of the 7 March, 1970, solar eclipse

A survey of the infrared coronal spectrum between 1 and 3 was made from a high altitude aircraft during the 7 March, 1970, solar eclipse. The observations were made with a Fourier transform spectrometer and were confined to the outer chromosphere and inner corona. In addition to well known chromospheric lines of Hi and Hei, nine additional lines were seen. Evidence is presented for the tentative assignment of these lines to forbidden transitions in highly ionized atoms of magnesium, aluminum, silicon, sulphur, and chromium.     

Ground-based observations of the Jovian ring and inner satellites

Ground-based 0.9-μm observations of the Jovian ring and inner satellites are reported. The ring observations substantially confirm those obtained by the Voyager spacecraft. The first ground-based detection of 1979J2 suggests a geometric albedo of ～0.10 and a new value for its orbit period of 16 hr 11 min 23.5 ± 0.5 sec.     

X-ray observations of non-magnetic CVs in the ASCA era and beyond

Our ability to study the hard X-ray emission in non-magnetic CVs has improved dramatically within the last decade. We review the important results on X-ray eclipses and dips, the current state-of-the-art in the modeling of X-ray spectra of CVs, then present some recent results on the hard X-ray emission from high accretion rate non-magnetic CVs. These results point to potential areas of conflict with accretion disk theories: the fate of the inner disk in quiescence, and the amount of material above the orbital plane. We conclude with some speculations on where near-future X-ray missions might take this field over the next decade.     

A note on the lifetime of the ring current particles

Characteristics of the time variations of Dst during magnetic storms are discussed in the context of several energizing and loss processes thought to be important for the production and decay of the storm time ring current. The energy input rate U_R may be more accurately evaluated if the predominant lifetime for ring current particle loss is taken as a few hours during main phase energizations, and a few tens of hours for particles remaining during recovery phases, and allowing for populations with a range of lifetimes during recurrent episodes of energization.     

EXOSAT observations of the ring nebula NGC 6888 and HD 192163

A 25 hr exposure of the ring nebula NGC 6888 was obtained with the EXOSAT X-ray Observatory. X-ray emission of the nebula was not found. Taking all instrumental effects and the large nebular area into account, a conservative upper limit of 1×10^–12 erg s^–1 cm^–2 (0.05–2 keV) is derived. This is about an order of magnitude less than predicted from braking the stellar wind of the central star HD 192163. Two point sources were serendipitously found in the field, HD 192163 and HD 192020.     

Keck observations of the 2002-2003 jovian ring plane crossing

We present new observations of Jupiter's ring system at a wavelength of 2.2 μm obtained with the 10-m W.M. Keck telescopes on three nights during a ring plane crossing: UT 19 December 2002, and 22 and 26 January 2003. We used conventional imaging, plus adaptive optics on the last night. Here we present detailed radial profiles of the main ring, halo and gossamer rings, and interpret the data together with information extracted from radio observations of Jupiter's synchrotron radiation. The main ring is confined to a 800-km-wide annulus between 128,200 and 129,000 km, with a ∼5000 km extension on the inside. The normal optical depth is 8×¹⁰⁻⁶, 15% of which is provided by bodies with radii a?5 cm. These bodies are as red as Metis. Half the optical depth, τ≈4×¹⁰⁻⁶, is attributed to micron-sized dust, and the remaining τ≈3×¹⁰⁻⁶ to grains tens to hundreds of μm in size. The inward extension consists of micron-sized (a?10 μm) dust, which probably migrates inward under Poynting-Robertson drag. The inner limit of this extension falls near the 3:2 Lorentz resonance (at orbital radius r=122,400 km), and coincides with the outer limit of the halo. The gossamer rings appear to be radially confined, rather than broad sheets of material. The Amalthea ring is triangularly shaped, with a steep outer dropoff over ∼5000 km, extending a few 1000 km beyond the orbit of Amalthea, and a more gradual inner dropoff over 15,000-20,000 km. The inner edge is near the location of the synchronous orbit. The optical depth in the Amalthea ring is ∼5×¹⁰⁻⁷, up to 20% of which is comprised of macroscopic material. The optical depth in the Thebe ring is a factor of 3 smaller.     

Propagation of low energy protons associated with the 24 January 1969 solar flare

This paper presents directional low energy solar proton measurements together with inter-planetary magnetic field measurements. Propagation of 1 to 13 MeV solar protons is discussed in terms of the relative importance of field-aligned streaming compared to convection of the proton population in the solar wind. Evidence is presented to show that protons associated with the January 24, 1969 solar flare were stored near the Sun for at least 90 minutes. It is also shown that under favourable conditions solar protons can be accelerated near the Earth's bow shock. The decay of solar protons is shown to be mainly convective; however, there are indications that in smooth field regimes convection of 1 MeV solar protons can be greatly reduced. Finally, it is pointed out that the effect of adiabatic deceleration can be quite important.     

Low altitude observations of the energetic electrons in the outer radiation belt during isolated substorms

The low energy (1–20 keV) detector registering particles onboard the polar-orbiting low altitude (~ 850 km) DMSP-F2 and -F3 satellites also records high energy electrons penetrating the detector walls. Thus we can study the dynamics of this electron population at L = 3.5, during isolated periods of magnetospheric substorms identified by the indices of auroral electrojet (AE), geomagnetic (K_p) and ring current (D_st). Temporal changes in the electron flux during the substorms are observed to be an additional contribution riding over the top of the pre-storm (or geomagnetically quiet-time) electron population ; the duration of the interval of intensity variation is observed to be about the same as that of the enhancement of the AE index. This indicates the temporal response of the outer radiation belt to the substorm activity, since the observation was made in the “horns” of the outer radiation belt. The observed enhanced radiation at low altitude may associate with the instantaneous increase and/or dumping of the outer radiation belt energetic electrons during each isolated substorm activity.     

Decay of the magnetic storm ring current by the charge-exchange mechanism

Equatorial charge-exchange lifetimes of ring current protons are recalculated, and the decay of a collection of ring current protons trapped on an L-shell by the charge-exchange mechanism is determined using recent models of the hydrogen geocorona. Observational results pertaining to the decay of ring current energy are briefly discussed, as are a number of competing loss mechanisms. Since charge exchange is a simple physical process which is very efficient in removing ring current energy from L-shells near to the Earth (say, L < 4), it is suggested that it may well be the dominant loss mechanism in this region.     

Pitch angle dependence of the charge-exchange lifetime of ring current ions

Previous work has parameterized the pitch angle dependence of the charge-exchange lifetime τ of ring current ions in terms of γ, the power of the cosine of the mirror latitude λ_m of the particles, such that $\text{τ(λ}{}_{\mathrm{m}}\text{)}\text{τ(0) ≌}\text{cos}{}^{\mathrm{\gamma }}\text{λ}{}_{\mathrm{m}}$ at given L. Using the atomic hydrogen density model of Johnson and Fish, previous authors have suggested values of γ = 5 or 6. We here evaluate γ as a function of λ_m and L using the more recent Chamberlain density models, and show that γ = 3?4 is more appropriate over most of the pitch angle and L range. Consequently, ion distributions in the ring current decay phase are expected to become rather less anisotropic in pitch angle due to chargeexchange than previously believed. We have also investigated the use of several other simple approximate analytic forms for $\text{τ(λ}{}_{\mathrm{m}}\text{)}\text{τ(0)}$, one of which gives far better agreement with the numerical results than the cos^γ λ_m, variation, and should hence be used in future studies.     

Low-frequency observations of the Vela supernova remnant and their implications

We have studied the Vela supernova remnant in the light of the 34.5 MHz observations made with the GEETEE low frequency array. The flux densities of Vela X and YZ at 34.5 MHz are estimated to be 1800 and 3900 Jy respectively. These values, along with those from earlier observations at higher frequencies, imply spectral indices (S∞Ν^α) of-0.16 ± 0.02 for Vela X and -0.53 ± 0.03 for Vela YZ. This situation is further substantiated by the spectral-index distribution over the region obtained between 34.5 and 408 MHz. The spectral-index estimates, along with other known characteristics, strengthen the earlier hypothesis that Vela X is a plerion, while Vela YZ is a typical shell-type supernova remnant. We discuss the implications of this result.