Wave-particle interactions in the ulf range: GEOS-1 and -2 results

Electromagnetic waves in the frequency range 0.2–11 Hz have been detected onboard the GEOS-1 and -2 satellites. The purpose of this paper is to report on these observations. The three orthogonal magnetic sensors allow us to determine the polarization of the waves. Two kinds of waves are commonly observed, which can easily be distinguished by their polarization.

(1) Waves with a magnetic field aligned with the DC magnetic field. These events often present a typical harmonic structure. The fundamental—which is not always observed—is often in the neighbourhood of the proton gyrofrequency F_H+. These waves are generally observed above F_H+. We will show that these emissions can be interpreted as magnetosonic waves destabilized by energetic protons (E 15 keV) with a ringlike distribution function.

(2) Waves with a magnetic field in a plane perpendicular to the DC magnetic field. These emissions are identified as Ion Cyclotron Waves (ICW's). These waves can, under certain conditions, propagate along the line of force of the magnetic field and reach the ground. They can be identified with the well-known Pcl oscillations, which generally have a clear periodic structure. In contrast these periodic structures are seldom observed onboard the satellites. At the geostationary orbit, these emissions exist in limited frequency domains, which are well organized by the helium gyrofrequency F_He+.     

Development of an auroral absorption substorm: Studies of substorm related absorption events in the afternoon-early evening sector

We discuss the effects in ionospheric absorption of particle precipitation observed in the afternoon-early evening sector during substorms with onset in the midnight sector. All events considered here occurred during magnetically disturbed periods, K_p > 3. For many of the substorm events a smooth southward moving absorption bay is seen in the midnight and evening sectors about 1 h preceeding the onset. The magnetic pulsation activity is low during this preceding bay.

After substorm onset near magnetic midnight the precipitation region may expand with a sharp onset at the front towards the West in spatially confined regions at high and low L-values separately with about equal velocities. The observations are consistent with a model of westward expansion of the energetic electron precipitation in two regions, aligned parallel to the auroral oval, at high and low L-values of about L 6 and L 4.8.

The westward expanding absorption activity correlates well with local magnetic variations. In magnetic pulsations PiB events are seen at high latitudes simultaneously with the westward moving onsets while at low latitudes IPDP pulsations are observed during the active part of the absorption events. Later in the substorm event a slowly varying absorption event (SVA) is sometimes observed at the lower L-values, L 3–4.     

Statistical characteristics of medium-latitude VLF emissions (unstructured and structured): Local time dependence and the association with geomagnetic disturbances

The purpose of the paper is to present the statistical characterictics of mid-latitude VLF emissions (both unstructured hiss and structured emissions) based on the VLF data obtained at Moshiri in Japan (geomag. lat. 35°; L = 1.6) during the period January 1974–March 1984. Local time dependence of occurrence rate and the association with geomagnetic disturbances have been studied for both types of emissions. Both types (unstructured and structured) of mid-latitude VLF emissions are found to have definite correlations with geomagnetic disturbances. Then, the time delay of the emission event behind the associated geomagnetic disturbance has enabled us to estimate the resonant electron energy for VLF hiss to be 5 keV at L = 3–4 and that for structured VLF emissions to be considerably larger, such as 20 keV at L 4. Combined considerations of these estimated resonant energies, theoretical electron drift orbits and the local time dependences, allow us to construct the following model to explain the experimental results in a reasonable way. Electrons in a wide energy range are injected during disturbances around the midnight sector, followed by the eastward drift. Lower energy ( 5 keV) electrons tend to drift closer to the Earth, resulting in the dawnside enhancement of VLF hiss within the plasmasphere. Further, these lower energy electrons are allowed to enter the duskside asymmetric plasmaspheric bulge and to generate VLF hiss there. On the other hand, higher energy (20 keV) electrons tend to drift at L shells farther away from the Earth and those substorm electrons are responsible for the generation of structured VLF emissions around dawn due to an increase of plasma density from the sunlit ionosphere. However, such higher energy electrons are forbidden from entering the duskside of the magnetosphere and so we cannot expect a duskside peak in the occurrence of structured VLF emissions, which is in agreement with the experimental result.     

The polar substorm and electron ‘islands’ in the Earth's magnetic tail

The behaviour of energetic electrons in the distant magnetosphere near the midnight meridian during polar substorms has been studied for the period March 5th–April 4th, 1965, using data from two end window Geiger counters flown on the IMP 2 satellite (apogee 15.8 Earth radii) and magnetic records from a chain of auroral zone stations around the world at magnetic latitudes equivalent to L = 7.4 ± 2.0.

When the satellite was in the distant radiation zone or in the plasma sheet which extends down the Earth's magnetic tail, sudden decreases in the horizontal magnetic field component at ground stations near the midnight meridian (negative magnetic bays) were followed by sudden increases in 40 keV electron fluxes (electron islands) at the satellite. When the satellite was at high latitudes in the magnetic tail ‘bays’ often were not followed by ‘islands.’ When the satellite was near the centre of the plasma sheet, energetic electron fluxes were observed even during magnetically quiet periods. The time delay between the sharp onset of magnetic bays in the auroral zone and the corresponding rapid increase in energetic electron intensity at the satellite, typically some tens of minutes, was least when the satellite was close to the Earth and increased with its increasing radial distance from the Earth. The delay was also a function of distance of the satellite from the centre of the plasma sheet, and of the magnitude of the intensity increase (smaller delays for larger intensity increases). We deduce that the disturbance producing the magnetic bays and associated particle acceleration originates fairly deep in the magnetosphere and propagates outward to higher L values, and down the plasma sheet in the Earth's magnetic tail on the dark side of the Earth. It is unlikely that the accelerated electrons are themselves drifting away from the Earth, because the apparent velocity with which the islands move away from the Earth decreases with increasing distance from the Earth.

It is suggested that the polar substorm and the associated particle acceleration are part of an impulsive ejection mechanism of magnetospheric energy into the ionosphere, rather than an impulsive injection mechanism of solar wind energy into the magnetosphere.     

Influence of imf sector polarity on annual and diurnal variations of absorption in the auroral zone

The effect of the polarity of the interplanetary magnetic field (IMF) on the absorption in and near the auroral zone has been studied using measurements from a meridional chain of riometers in Finland in the years 1973–1978. The measured absorption values were divided into two groups, corresponding to away and towards polarities of the IMF, and the values of absorption were examined for the two groups separately. The annual means of absorption were found to be independent of sector polarity in the whole latitude range. In the annual and diurnal variations a polarity effect could be found down to L =4.4, in the annual variation some remnants of it down to L = 3.4. In the annual variation, the towards (T) polarity is associated with a strong spring maximum and the away (A) polarity with a fall maximum in absorption. The diurnal variations are most pronounced during T polarity in the spring and during A polarity in the fall, with high daytime maxima in both cases, suggesting that the loss processes of trapped electrons in the day sector might be modulated by IMF polarity. Our analysis reveals the effect of the different components of the IMF; The annual variations show the effect of the vertical component (varying energy transfer from the solar wind into the magnetosphere) and the diurnal variations in the equinoxes the effect of the azimuthal component (shift of the absorption zone along the dawn-dusk meridian).     

On the non-adiabatic particle scattering in the earth''s magnetotail current sheet

The empirical model of disturbed magnetosphere of Tsyganenko and Usmanov (1982) and the semi-empirical model of the storm-time magnetospheric configuration of Tsyganenko (1981) are used to find the critical energy for non-adiabatic particle scattering in the midnight sector. Computed values of E_crit vs L are compared with the appropriate experimental data of Imhof et al. (1977). It is found that none of the considered models is able to reproduce the observed steep decrease of E_crit with L. The steepest slope is given by the Tsyganenko model which includes a current sheet with the finite thickness. The current sheet thickness is a crucial parameter in the non-adiabatic scattering problem. In discussion we point to natural limitations of an empirical model as far as the current sheet thickness is to be determined. Imhof et al.'s data as well as some magnetic field data sets seem to indicate that magnetosphere models incorporating a thin current sheet and allowing for the thickness dependence on the geocentric distance would probably be closer to reality than the considered models, at least during higher levels of magnetic activity.     

Ring current protons in the upper atmosphere within the plasmasphere

keV protons observed by the ESRO 1A satellite in the upper atmosphere equatorward of the main precipitation zone are described and discussed. The protons are highly anisotropic (empty loss cone) between the low-latitude boundary of the main precipitation zone and approximately L=4 during quiet and moderately disturbed conditions (K_p=0?4). Between L=4 and L=2.7 the proton flux is generally enhanced compared to that at L values somewhat above 4 and only moderately anisotropic. Substorms push the outer main precipitation zone equatorwards, but the boundaries of the innermost, moderately anisotropic region (at L=2.7 and L=4) move only when strong magnetic storms compress the plasmasphere to within L=4. It is suggested that the moderately anisotropic zone is caused by the ion-cyclotron instability for which the growth rate may have a broad maximum between about L=2.7 and L=4. For proton energies in the keV range the instability is excited only in regions with cold plasma densities above several hundred ions per cubic centimeter. It is finally concluded that the observations of low-latitude proton precipitation lend further support to the mechanism of ion-cyclotron instability as the cause of proton pitch angle diffusion, as proposed by Cornwall et al. (1970).     

High resolution method of direct measurement of the magnetic field lines'' eigen frequencies

The simultaneous observations of Pc4 geomagnetic pulsations at the two temporary stations, located along the geomagnetic meridian 50 km to the North and South from the observatory Borok (L = 2.8), have been used for the investigation of amplitude gradients of both H- and D-components of these pulsations. It has been discovered that the direction of a meridional component of the gradient H (grad_MH) depends on the frequency ƒ of a spectral component of pulsations. The grad_MD is directed more or less permanently northward independently from the frequency ƒ These results are the consequence of a local amplification of geomagnetic pulsations due to Alfvén waves resonance along the magnetic field lines. It has been demonstrated that the frequencies ƒ_R for which the northward direction of grad_MH is replaced by the southward one (with increasing ƒ) can be interpreted as the eigen frequencies of the field line which intersects the meridian in the middle between two temporary stations, i.e. in Borok.

The possible applications of a gradient method of measurement of the magnetic field lines' eigen frequencies are discussed.     

Kp index correlations with solar-wind parameters during the first and second stages of a recurrent geomagnetic storm

The dependence of geomagnetic activity during a recurrent magnetic storm on the solar-wind magnetic field and plasma parameters has been studied. According to variations of solar-wind magnetic field strength B, a recurrent magnetic storm is divided into two stages: the first proceeding during the peak of B, and the second proceeding after the return of B to quiet level. The K_p index vs solar-wind parameters scattering diagrams for stages I and II differ significantly. In particular, the random scattering for stage I is much larger than for stage II. It was found that for stage I the K_p index correlates with B, with the sign and value of northsouth field component B_z and with the magnitude ΔB of field fluctuations, the situation being similar to that during sporadic magnetic storms, though the scale of the event is smaller. For stage II, the K_p index does not correlate with B, but strongly correlates with ΔB and weaker—with B_z. So geomagnetic activity at stage II is supported mainly by solar-wind magnetic field fluctuations. The dependence of the K_p index on plasma parameters (concentration of protons n, bulk velocity v and temperature T) is weak for both stages.     

Substorm morphology of > 100 keV protons

The latitudinal morphology of > 100 keV protons at different local times has been studied as a function of substorm activity. A characteristic pattern is found: during quiet-times there is an isotropic zone centred around 67° near midnight, but located on higher latitudes towards dusk and dawn. This zone moves slightly equatorward during the substorm growth phase. During the expansive phase the precipitation spreads poleward apparently to ~ 71° near midnight. The protons are precipitated over a large local time interval on the nightside, but the most intense fluxes are found in the pre-midnight sector. A further poleward expansion, to more than 75° near midnight, seems to take place late in the substorm. Away from midnight, the expansion reaches even higher latitudes. During the recovery phase the intensity of the expanded region decreases gradually; the poleward boundary is almost stationary if the interplanetary magnetic field (IMF) has a northward component and no further substorm activity takes place. Mainly protons with energy below ~ 500 keV are precipitated in the expanded region. On the dayside no increase in the precipitation rates is found during substorm expansion, but late in the substorm an enhanced precipitation is found, covering several degrees in latitude. The low-latitude anisotropic precipitation zone is remarkably stable during substorms. A schematic model is presented and discussed in relation to earlier results.     

Electron excitation and auroral emission parameters

Auroral luminosities of the main emission lines in the aurora have been calculated for excitation by an isotopic primary electron flux with spectra of the form J(E) = AE exp (−E/E₁) + B(E₂)E exp (−E/E₁). The variation of emissions from O and N₂⁺ with height are shown, as are the variations of column integrated intensities and pertinent intensity ratios with the characteristic energy E₂, this leading to a method of estimating the electron spectrum from ground observation.     

XMM-Newton observations of the nearby brown dwarf LP 944-20

The nearby (d=5.0 pc) brown dwarf LP 944-20 was observed with the XMM-Newton satellite on 07 January 2001. The target was detected with the Optical Monitor (V=16.736±0.081), but it was not detected during the ≈48 ks observation with the X-ray telescopes. We determine a 3σ upper limit for the X-ray emission from this object of L_X<3.1×10²³ ergs·s⁻¹, equivalent to a luminosity ratio upper limit of log(L_X/L_bol)≤−6.28. This measurement improves by a factor of three the previous Chandra limit on the quiescent X-ray flux. This is the most sensitive limit ever obtained on the quiescent X-ray emission of a brown dwarf. Combining the XMM-Newton data with previous ROSAT and Chandra data, we derive flare duty cycles as a function of their luminosities. We find that very strong flares [Log(L_X/L_bol)>−2.5] are very rare (less than 0.7% of the time). Flares like the one detected by Chandra [Log(L_X/L_bol)=−4.1] have a duty cycle of about 6%, which is lower than the radio flare duty cycle (13%). When compared with other M dwarfs, LP 944-20 appears to be rather inactive in X-rays despite of its relative youth, fast rotation and its moderately strong activity at radio wavelengths.     

Gamma ray spectrum of the crab nebula in the multi TeV region

The THEMISTOCLE array of 18 Cherenkov detectors which has a 3 TeV gamma energy threshold, has detected a signal from the Crab nebula at a 5.8 standard deviation level. Information on the energy spectrum is obtained in the range 3–15 TeV. The integrated flux can be fitted with the form, Φ (> E) = (3.7 ± 0.5) × 10^-12 (E/5)-^{−1.5 ± 0.20} cm⁻² s⁻¹ (E in TeV) compatible with the extrapolation of results at lower energies. The Crab signal is used to measure the angular resolution of the multi-telescope technique. The value obtained is 2.3 mr (0.15°) in agreement with the results of simulations, and confirms the interest of this new method for multi-TeV gamma-ray detection.     

Considerations that the source of auroral energetic particles is not a parallel electrostatic field

It is shown that electrostatic fields parallel (E₁₁) to the geomagnetic field cannot be the major mechanism that accelerates charged particles to auroral energies. Principal arguments are that electron and proton precipitation occur simultaneously, and also that precipitated electrons with energies less than 100 eV are found to accompany the electrons with energies of 1–10 keV that excite auroral luminosity. It is further shown that essentially all the ambient plasma in an entire tube of flux is required to sustain this intense low-energy precipitation, and this places a severe constraint on any replenishment process. It is found that generally the upper limit to (E₁₁) throughout the auroral regions of the ionosphere and magnetosphere is of order 10 μV/m and it may be appreciably less. Relevant measurements are reviewed briefly. It is concluded that while there may occasionally be significant E₁₁ fields, they play only a minor role-if any-in auroral phenomena.     

Bidirectional distribution of energetic protons in the 18 February 1979 interplanetary shock event

Pitch angle distributions of 35–1000 keV protons in the 18 January 1979 interplanetary shock event are presented and discussed in this paper. The observed, apparently unusual distributions can be well explained by assuming a magnetic bottle configuration of the interplanetary magnetic field immediately prior to the shock passage. The implication of such a magnetic configuration on the acceleration of charged particles by shock waves is also discussed.     

Non-diffusive propagation of ultra high energy cosmic rays

We report the results of 3D simulations of non-diffusive propagation of Ultra High Energy Cosmic Rays (UHECR) (E > 10²⁰ eV) through the intergalactic and extended halo media. We quantify the expected angular and temporal correlations between the events and the sources, and the temporal delay between protons and gamma-ray counterparts with a common origin for both halo and extragalactic origins. It is shown that the proposed UHECR-supergalactic plane source associations require either extremely high values of the halo magnetic field over as much as 100 kpc length scale or a very large correlation length for the IGM, even for the largest possible values of the intergalactic magnetic field. It can be stated that the UHECR seem to point to the sources even more strongly than previously believed. The simulations also show that the calculated time delays between UHE protons and gamma-ray counterparts do not match the claimed GRB-UHECR associations for either cosmological or extended halo distance scales.     

Equatorial scintillation

Equatorial scintillations have been observed at Legon, Ghana for nearly 20 yr. The occurrence characteristics of the scintillations are reviewed, and the physical characteristics of the electron density irregularities summarized. A much more comprehensive summary of the seasonal variation of scintillation is given, and it is found to be remarkably similar to the variations in thermospheric temperature. Evidence for the suppression of scintillation during magnetic disturbances is given. Curves for the daily variation of the Faraday rotation angle φ are presented and their unusual post-sunset behaviour noted. It is suggested that this can be explained in terms of a theory presented by Rishbeth, in which the F region ionization moves with nearly the full velocity of the neutral atmospheric wind at night, after the E region conductivity has fallen to a relatively low value. This can account for the observed drift velocity of the irregularities. The rapid increase in the post-sunset horizontal velocity of the ionization together with the observed vertical rise, can account for the variations of φ. It is further suggested that the large gradients in the density and drift velocity of the ionization resulting from the mechanism suggested by Rishbeth give rise to the production of the observed F region irregularities in electron density which cause equatorial scintillation.     

Permanent flare activity in the magnetosphere during periods of low magnetic activity in the auroral zone

Auroral, magnetic variation and pulsation data from the dense network in the nearmidnight portion of the auroral zone are used together with the measurements of suprathermal particles and electromagnetic fields by the IMP-8 and ISEE-1 spacecraft within the plasma sheet to study the characteristics of activity during two magnetically quiet periods on 3 March 1976 and 23 March 1979. Contrary to existing beliefs, we found clear signatures of numerous (5–10 events per hour) transient events, characterized by plasma flows, energetic particle bursts and E−B field variations. A close association of these events in the plasma sheet with the local auroral flares (LAFs) in the conjugate sector of the auroral zone is established for many events. We conclude that LAF (local auroral arc activation with associated Pi pulsations but extremely weak magnetic bays) have the same plasma sheet manifestations (apparently, the same physics) as the individual substorm intensifications during strong substorm expansion events, which differ from the studied quiet periods mainly by the strength and number of these intensifications. These transient phenomena seem to play an important role in the energetics of the quiet time magnetotail.     

Small-scale auroral arc distortions

Small-scale spatially periodic distortions of auroral forms have been studied utilizing low-light level television observations made at various locations in the Northern and Southern Hemispheres. The most commonly observed features were folds and vortex-like curl formations. The curls, identified here with the Kelvin-Helmholtz instability due to fluid shear, invariably had a counterclockwise rotational shape and motion when viewed in a direction anti-parallel to the Earth's magnetic field. The typical measured wavelength (5 km) and measured growth rate (4.2 sec⁻¹) were used to evaluate the Kelvin-Helmholtz dispersion relation for the apparent shear ω_s = ∂ ν_x/ ∂y (28 sec⁻¹). The apparent horizontal velocities of both folds (0–5 km/sec) and curls (0–22 km/sec) were invariably observed to be counterclockwise with respect to the multiple arc centre when viewed antiparallel to B. Consistent agreement between rotational shape and rotational motion suggests that the apparent growth rate and the apparent horizontal velocities closely approximate the actual values. If the shear results from E×B drifts in a space charge field, the calculated value for ω_s, implies an unneutralized electron density 0–1 cm⁻³ and a ΔE across the arc element 500mV/m. The velocity measurements indicate that the ΔE values for individual elements can combine to produce transient electric fields at the edges of multiple arcs as high as 1000 mV/m.     

Quasilongitudinal approximation for whistler-mode waves in the magnetospheric plasma

The range of applicability of an improved quasilongitudinal approximation for whistler-mode waves in the equatorial magnetosphere (4 L 6.6) is specified based on the direct comparison between numerical solutions of the hot electromagnetic dispersion equation with the corresponding analytical quasilongitudinal solutions. It is pointed out that this approximation can be used at frequencies ω less than but not close to the electron gyrofrequency Ω (ω 0.6 Ω) and wave normal angles θ less than but not close to the resonance cone angle θ_R. At ω = 0.8 Ω the analytical results deviate considerably from numerical ones due to the strong damping of the waves, and so the quasilongitudinal solution becomes no longer valid.