Angular distributions of solar protons and electrons

High angular-resolution measurements of directional fluxes of solar particles in space have been obtained with detectors aboard OGO-5 during the cosmic ray event of 18 November 1968. This is the only case on record for which sharply-defined directional observations of protons and electrons covering a wide rigidity range (0.3 MV to 1.5 GV) are available.The satellite experiment provided data for determining pitch-angle distributions with respect to the direction of the local interplanetary magnetic field lines during the lengthy highly anisotropic phase of the event. It was found that the unidirectional differential intensities j(θ) of 3- to 25-MeV protons varied in accordance with the relationship j(θ) = b₀ + b₁cosθ + b₂cos²θ, where b₀ and b₁ ? 0, and b₂, is positive, zero or negative. Soon after onset, 79–266-keV electrons arriving from the direction of the Sun displayed an anisotropic component with the intensity varying as cos θ. Later, a double-peaked distribution appeared at the lower energies, whereas the flux at the upper end of the range covered by the experiment became isotropic. These results have been interpreted in the light of the temporal flux profiles and the state of the interplanetary medium.The observation of the unusually large and long-lasting anisotropies lead to several conclusions including: (1) If injection of the solar particles was instantaneous, the diffusion coefficient was either constant or increasing with distance from the Sun. (2) If the solar source emitted particles over an extended period, and there is evidence to that effect, there was weak scattering in the region between the Sun and the Earth and a strong scattering region beyond the Earth's orbit. (3) Solar electrons were stored near the Sun. (4) The observed angular distribution of 200-MV protons in the magnetosheath was in good agreement with that deduced in an earlier analysis of polar orbiting satellite observations and trajectory calculations.     

Excitation of whistler waves by reflected auroral electrons

Excitation of electron cyclotron waves and whistlers by reflected auroral electrons which possess a loss-cone distribution is investigated. Based on a given magnetic field and density model, the instability problem is studied over a broad region along the auroral field lines. This region covers altitudes ranging from one quarter of an Earth radius to five Earth radii. It is found that the growth rate is significant only in the region of low altitude, say below the source region of the auroral kilometric radiation. In the high altitude region the instability is insignificant either because of low refractive indices or because of small loss cone angles.     

Auroral radio absorption and the westward travelling surge

Measurements from a network of riometers during the passage of an auroral westward travelling surge are presented. These show that the energetic electron precipitation producing the radio absorption expands in an almost identical fashion to the softer precipitation associated with the visible surge, but. it is delayed by about two minutes with respect to the surge. The delay is interpreted as a hardening of the precipitating electron spectrum as the surge goes by. Simultaneous observations of electrons at synchronous orbit are shown to support this conclusion.     

Damping of electrostatic noise by warm auroral electrons

Cyclotron damping by warm electrons limits the amplitude of high frequency electrostatic waves propagating in discrete auroral arcs. The effect of this damping on whistler VLF hissupper hybrid noise and Bernstein modes is examined by calculating temporal growth rates and power flux intensities of amplified noise produced by precipitating electrons. The auroral electrons are described by a realistic distribution function. The effect of varying ionospheric conditions is also considered. Whistler mode noise is found to be less sensitive to the warm electron model than the upper hybrid mode. Bernstein modes are rapidly absorbed by the ionospheric and warm electrons. The difference in the peak power flux of the whistler and upper hybrid modes is indicative of the local value of the ratio of electron plasma frequency to electron gyrofrequency. For peak upper hybrid noise to exceed peak whistler noisethis ratio should be greater than 1. Ionospheric electron temperature has little effect on the spectrum, and intense narrow beams in the distribution function should be most effective at producing high noise levels for a given warm electron model.     

Coronal and interplanetary transport of solar energetic protons and electrons

We present a new method to separate interplanetary and coronal propagation, starting from intensity variations observed by spaceprobes at different heliolongitudes. In general, a decrease in absolute intensities is observed simultaneously with an increase in temporal delays. The coupling of these two effects can be described by Reid's model of coronal diffusion and can in principle be used to determine the two coronal time constants, diffusion time t _c and escape time A. In addition, a least-squares fit method is used to determine the parameters of interplanetary transport, assuming a radial dependence as (r) = ₀(r/1 AU)^b. The method is applied to the two solar events of 27 December, 1977 and 1 January, 1978 which were observed by the spaceprobes Helios 1, Helios 2, and Prognoz 6. Energetic particle data are analysed for 13–27 MeV protons and -0.5 MeV electrons. For the regions in space encountered during these events the mean free path of electrons is smaller than that of protons. Straight interpolation between the two rigidities leads to a rather flat rigidity dependence (P) P ⁿ with n = 0.17–0.25. This contradicts the prediction of a constant mean free path or of the transition to scatter-free propagation below about 100 MV rigidity. In three of the four cases the mean free path of 13–27 MeV protons is of the order 0.17 AU, the mean free path of electrons of the order 0.06 AU. For protons we find b - 0.7 for the exponent of the radial variation.The concept of two different coronal propagation regimes is confirmed. It is remarkable that in both regimes electrons are transported more efficiently than protons. This holds for the temporal delay as well as for the amplitude decrease. This is in contrast with the long existing concept of rigidity independent transport and puts severe limits to any model of coronal transport. For the December event all three spaceprobes are in the fast propagation regime up to an angular distance of 62°. For protons we find a finite delay even in the fast propagation region, corresponding to a coronal delay rate of about 0.8 hr rad^-1 up to 60° angular distance. In contrast, relativistic electrons may reach this distance within a few minutes.The fast transport of electrons and the different behaviour of electrons and protons is in contradiction to the expanding bottle concept. An explanation of coronal transport by shock acceleration directly on open field lines could in principle work in case of protons in the fast propagation region, but would fail in case of the electrons. The fast and efficient transport of electrons is most likely due to a region of field lines extending over a wide range of longitudes directly from the active region into interplanetary space. The much slower transport of both particle types at large azimuthal distances can neither be explained by direct access to open field lines not by the direct shock acceleration concept. A possible explanation is the loop reconnection model in a modified version, allowing for a faster lateral transport of electrons.Now at AEG, 2000 Wedel, F.R.G.     

Rocket observations of the interaction of auroral electrons with the atmosphere

Electron spectra obtained during the flight of Black Brant VB-31 on August 17, 1970 through a stable aurora to a height of 268 km have been analyzed in detail to obtain the pitch angle distributions from 25 to 155° and the electron energy distributions over an energy range of 18 keV to 20 eV through the region of atmospheric interaction down to 97 km. Backscatter ratios for 140° pitch angle range from 0.065 for 18 keV electrons to 0.22 for 1 keV electrons. Backscatter of lower energy electrons decreases with atmospheric depth below 200 km. The effect of the interactions between auroral electrons and the atmosphere is such as to give a peak in electron flux which moves progressively to higher energies with penetration depth. The secondary electron flux increases monotonically with height up to 200 km. The secondary electron spectrum can be approximated by an energy power over small energy ranges but its form is somewhat dependent on height and on the primary electron spectrum.     

Evidence of energy dependent mechanisms for the precipitation of auroral electrons

This paper discusses the experimental results on electron precipitation in a diffuse aurora obtained by a sounding rocket launched from ANDENES (L ～ 6·2) on 3 November 1968. A considerable increase in the intensity of low energy electrons, E_e ? 5 keV, followed a large precipitation of more energetic electrons E_e ? 5 keV. From the observation of angular distributions and an estimate of the diffusion coefficient (D_α ? 10^?3 (sec)^?2), it is suggested that this higher energy precipitation is induced by gyroresonant interactions of magnetospheric electrons with radiation in the whistler mode. The lower energy precipitation separated in time and/or space, shows quasi-periodic modulations in the 5–15 sec range with periods close to the bounce period. It is suggested that this precipitation is the result of bounce-resonance interactions with electrostatic waves in the equatorial plane. Finally, from a comparison between the experimental energy spectra and plasma sheet spectra it can be concluded that these electrons are injected from the plasma sheet during a substorm and are then diffused and precipitated by energy dependent mechanisms.     

Dust drift solitary waves with superthermal electrons and ions

Linear and nonlinear dust drift waves are investigated in the presence of kappa distributed electrons and ions. The dispersion characteristics of linear waves show that the phase velocity decreases with the inclusion of highly energetic particles in the tail of the distribution. In the nonlinear regime, a nonlinear partial differential equation is obtained in the long wave length limit. A stationary solution of this equation in the form of solitary waves is discussed and noticed that the amplitude of the solitary pulse decreases with the increase of superthermal particle’s effect, and its width expands. Further, it is found that speed limit of the nonlinear structures is also modified in the non-Maxwellian plasma. Theoretically obtained results are applied to Saturn’s’ dusty plasma environment. It is also pointed out that the present results can be helpful for further understanding of space plasmas.     

High latitude,outer zone boundary observations of electrons and protons

The diurnal variation of the high latitude outer zone boundary at 1400km has been determined for electrons ?140keV electrons, and for protons in two energy intervals: 0.56?E?1.1 MeV, 1.1?E?3.2 MeV, from detectors aboard the NOAA-2 satellite. The dependence of the 140 keV electron boundary on D_st has been examined as well?. A wel?l defined correlation of boundary position with D_st is found to exist during the main phase of disturbances, together with an evident local time dependence. All the boundaries are found to be consistent witn the supposition of adiabatic drift and demonstrate the stability of the boundary position over approximately ten years of comparable observation. No statistically significant hemispheric differences in boundary location were observed to occur.     

The development of the substorm in auroral radio absorption

Observations with a meridional chain of riometers in the auroral zone show that the onset of an auroral absorption substorm is usually preceded by weak activity. This activity sometimes takes the form of a weak bay-like event, moving slowly equatorward. Its velocity (typically 200–500 m/sec) is consistent with the inward convection of energetic plasma in the magnetosphere— that is, with E × B drift in an electric field of about 1.3 mV/m. The bay intensifies as it moves, indicating energization of the plasma during convection.     

Pitch angle and energy distributions of auroral electrons measured by the ESRO 4 satellite

Energy spectra and angular distributions of auroral electrons in the energy range 0.2–16 keV measured by the low-altitude polar orbiting satellite ESRO 4 are presented. The observations were made in the altitude range 800–1000 km near magnetic midnight. Energy-time spectrograms show inverted-V structures with peaked energy spectra. The inverted-V events are associated with anisotropic electron pitch angle distributions peaked at 0 deg. Frequently these distributions have a maximum also at 90 deg. Measurements of >43 keV electrons indicate that the acceleration probably occurs on closed field lines. It is found that many properties of the observed particle distributions can be explained by acceleration in an electric field parallel to the magnetic field lines, if trapping of particles under an increasing potential drop is included in the model.     

On the mechanisms producing the backscattered and the trapped electrons along auroral field lines

The production mechanisms of backscattered electrons and trapped electrons along auroral field lines are examined. The backscattered electrons are produced not only by electron-neutral collisions, but also by electrostatic turbulence predominantly in the ion-cyclotron and ion-acoustic modes. The trapped electrons can only be produced by electron wave turbulence, most likely in the whistler and upper hybrid modes known as the auroral hiss.     

A charged particle telescope to study relativistic electrons,sub-relativistic protons and muons

A bi-directional charged particle telescope has been designed to distinguish electrons, muons and protons incident on the telescope from two opposite directions and to measure the energy of particles which stop in the central total energy detector of the system. This paper describes the design, test and evaluation of the telescope using radioactive sources, cosmic rays and low energy (8 MeV) electron beam from an accelerator. This telescope has been used in an experiment to measure the flux and energy spectrum of low energy electrons in 5 to 24 MeV energy interval and protons in 70 to 110 MeV energy region in the upper atmosphere. This experiment was carried out at high altitudes, using large size scientific balloon over the city of Hyderabad, India in Dec. 1984. Ground level low energy muon flux has been observed at Ahmedabad and Hyderabad.     

The role of high-energy protons and electrons in powering the solar white-light flare emission

The temporal histories of three intense and impulsive gamma-ray flares, for which also white-light emission had been observed, are analyzed in order to test the role of high-energy particles- electrons and protons - in powering the optical continuum. By comparing the light curves at optical wavelengths and at X-ray and gamma-ray energies, we find a good correlation of the main peaks of emission, which confirms previous findings that the continuum emission is most likely associated with the energy loss of energetic particles. The power carried by the greater-than-50 keV nonthermal electrons may be sufficient to balance the optical emission. The power residing in protons or ions with energies greater than 1 MeV depends largely on the spectral shape of the particle distribution. Only if this is similar to a power law, may the energy carried by these high-energy particles be sufficient to balance the white-light flare emission.Operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. Partial support for the National Solar Observatory is provided by the USAF under a Memorandum of Understanding with the NSF.     

The absorption of energetic electrons by argon gas

The processes by which energetic electrons lose energy in a weakly ionized gas of argon are analysed and calculations are carried out taking into account the discrete nature of the excitation processes. The excitation, ionization and heating efficiences are computed for energies up to 200 eV absorbed in a gas with fractional ionizations varying up to 10^?2.     

A joint analysis of high-energy neutrons and neutron-decay protons from a flare

A joint analysis of neutron monitor and GOES data is performed to study the production of high-energy neutrons at the Sun. The main objects of the research are the spectrum of >50 MeV neutrons and a possible spectrum of primary (interacting) protons which produced those neutrons during the major 1990 May 24 solar flare. Different possible scenarios of the neutron production are presented. The high magnitude of the 1990 May 24 neutron event provided an opportunity to detect neutron decay protons of higher energies than ever before. We compare predictions of the proposed models of neutron production with the observations of protons on board GOES 6 and 7. It is shown that the precursor in high-energy GOES channels observed during 20:55–21:09 UT can be naturally explained as originating from decay of neutrons in the interplanetary medium. The ratio of counting rates observed in different GOES channels can ensure the selection of the model parameters.The set of experimental data can be explained in the framework of a scenario which assumes the existence of two components of interacting protons in the flare. A hard spectrum component (the first component) generates neutrons during a short time while the interaction of the second (soft spectrum) component lasts longer. Alternative scenarios are found to be of lesser likelihood. The intensity-time profile of neutron - decay protons as predicted in the framework of the two-component exponential model of neutron production (Kocharov et al., 1994a) is in an agreement with the proton profiles observed on board GOES. We compare the deduced characteristics of interacting high-energy protons with the characteristics of protons escaping into the interplanetary medium. It is shown that, in the 100–1000 MeV range, the spectrum of the second component of interacting protons was close to the spectrum of the prompt component of interplanetary protons. However, it is most likely that, at 300 MeV, the interacting proton spectrum was slightly softer than the spectrum of interplanetary protons. An analysis of gamma-ray emission is required to deduce the spectrum of interacting protons below 100 MeV and above 1 GeV.     

The relationship of auroral absorption to the electron energy distribution of the plasma

An auroral absorption event in the D-region of the atmosphere has been studied by simultaneous measurements of electron temperature, electron density and hyperthermal electrons with a Langmuir probe, and of radio absorption coefficient by 30 MHz riometers. The absorption of the radio waves cannot be explained only by the enhancement of the electron density but requires that the electron collision frequency v be increased above its normal value by the presence of a high energy tail in the electron distribution function. A model is used to determine the characteristics of the hyperthermal electrons in order to evaluate their contribution to the collision frequency and to the absorption coefficient. Good agreement is found between theoretical and experimental values.     

Characteristics of Saturn's polar atmosphere and auroral electrons derived from HST/STIS, FUSE and Cassini/UVIS spectra

Ultraviolet (UV) spectra of Saturn's aurora obtained with the Hubble Space Telescope Imaging Spectrograph (STIS), the Cassini Ultraviolet Imaging Spectrograph (UVIS) and the Far Ultraviolet Spectroscopic Explorer (FUSE) have been analyzed. Comparisons between the observed spectra and synthetic models of electron-excited H₂ have been used to determine various auroral characteristics. Far ultraviolet (FUV: 1200-1700 Å) STIS and UVIS spectra exhibit, below 1400 Å, weak absorption due to methane, with a vertical column ranging between 1.4×¹⁰¹⁵ and . Using the low-latitude Moses et al. [Moses, J.I., Bézard, B., Lellouch, E., Feuchtgruber, H., Gladstone, G.R., Allen, M., 2000. Icarus, 143, 244-298] atmospheric model of Saturn and an electron energy-H₂ column relationship, these methane columns are converted into the mean energy of the primary precipitating electrons, estimated to lie in the range 10-18 keV. This result is confirmed by the study of self-absorption with UVIS and FUSE extreme ultraviolet (EUV: 900-1200 Å) spectra. Below 1200 Å, it is seen that transitions connecting to the v^″<2 vibrational levels of the H₂ electronic ground state are partially self-absorbed by H₂ molecules overlying the auroral emission. Because of its low spectral resolution (∼5.5 Å), the UVIS EUV spectrum we analyzed does not allow us to unequivocally determine reasonable ranges of temperatures and H₂ columns. On the other hand, the high spectral resolution (∼0.2 Å) of the FUSE LiF1a and LiF2a EUV spectra we examined resolve the H₂ rotational lines and makes it possible to determine the H₂ temperature. The modeled spectrum best fitting the FUSE LiF1a observation reveals a temperature of 500 K and self-absorption by a H₂ vertical column of . When converted to energy of precipitating electrons, this H₂ column corresponds to primary electrons of ∼10 keV. The model that best fits the LiF2a spectrum is characterized by a temperature of 400 K and is not self-absorbed, making this segment ideal to determine the H₂ temperature at the altitude of the auroral emission. The latter value is in agreement with temperatures obtained from infrared polar spectra. Self-absorption is detectable in the LiF2a segment for H₂ columns exceeding , which sets the maximum mean energy determined from the FUSE observations to ∼15 keV. The total electron energy range of 10-18 keV deduced from FUV and EUV observations places the auroral emission peak between the 0.1 and 0.3 μbar pressure levels. These values should be seen as an upper limit, since most of the Voyager UVS spectra of Saturn's aurora examined by Sandel et al. [Sandel, B.R., Shemansky, D.E., Broadfoot, A.L., Holberg, J.B., Smith, G.R., 1982. Science 215, 548] do not exhibit methane absorption. The auroral H₂ emission is thus likely located above but close to the methane homopause. The H₂ auroral brightness in the 800-1700 Å bandwidth varies from 2.9 kR to 139 kR, comparable to values derived from FUV Faint Object Camera (FOC) and STIS images.     

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).