F. Søraas

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

Abstract 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.

Magnetospheric Ion Composition Spectrometer Onboard the CRRES Spacecraft

The magnetospheric ion composition spectrometer (MICS) in the CRRES scientific payload utilizes time-offlight and energy spectroscopy in combination with an electrostatic entrance filter to measure the mass A , energy E, and ionic charge Q of particles with energies between 1 keV/charge and 430 keV/charge. An advanced ogive design of the electrostatic filter system provides a narrow angle of acceptance and high sensitivity. Incident particles are postaccelerated prior to entering the detection segment in order to improve the resolution at the lower end of the useful energy range. The principle features of the MICS spectrometer are described in some detail. Selected data gathered in-flight are shown as an illustration of the instrument performance in the operational orbit. I. Introduction T HE magnetospheric ion composition spectrometer (MICS) in the payload of the Combined Release and Radiation Effects Satellite (CRRES) belongs to a class of advanced instruments which provide full characterization of incident ions by determining their mass A (in amu), charge Q, and velocity V (magnitude and direction) as independent parameters. The particles identity is derived from a time-offlight Tand energy E measurement, and the ionic charge Q is obtained from an electrostatic energy per charge E/Q filter which serves as the entry element of the spectrometer. Upon leaving the E/Q filter the ions energy is increased by a postaccelerating voltage to improve the instrument resolution at low particle energies. The MICS energy range extends from 1.2 keV/charge up to 426.5 keV/charge, and ion species are identified from hydrogen to iron. The obtainable mass resolution A/dA is a complex function of the particle mass and energy. For a given ion species the mass resolution increases as a function of the energy per mass ratio E/A. A typical ratio A /cL4 = 8 is obtained for oxygen ions with energies above 100 keV. Despite this only moderate mass resolution, atoms and molecules, even isobaric structures, can be discriminated by a peculiarity of the MICS detection technique: Fragmentation of swift molecules in the thin START foil of the TIE spectrometer leads to groups of particles which travel with the original velocity but each fragments energy is apportioned to its mass. The resulting statistical distribution in (E, T) space can be used to identify the presence of molecules.

A comparison between simultaneous I.P.D.P. groundbased observations and observations of energetic protons obtained by satellites

Abstract A study of simultaneous groundbased observations of I.P.D.P. (intervals of pulsation of diminishing period) magnetic field fluctuation events and satellite observations of energetic protons have been performed. Some of our results are as follows. (1) The region of I.P.D.P. occurrence is always located equatorward of the isotropic proton precipitation. (2) The I.P.D.P. generation is not connected with the poleward leap of the aurora and the poleward expansion of the precipitating protons. (3) In the evening to afternoon sector enhanced pitch angle scattering is found near L = 4 during I.P.D.P. events, earlier shown to be associated with ion cyclotron resonance. (4) I.P.D.P. events seem to be associated with increased fluxes of (40–60) keV protons injected during substorms near the plasmapause in the equatorial plane. In order to explain the observations we invoke the following model: at substorm onset ring current protons are injected deep into the nightside magnetosphere covering a certain region in L and L.T., with the inner edge of the proton population following McIlwains injection boundary. The protons drift azimuthally westward and generate ion cyclotron waves in a certain L interval at or inside the plasmapause. By taking into account the shape and position of the plasmapause and the injection boundary, the exterrt and position of the wave generating region can be determined. The frequency-time dispersion of the I.P.D.P. is largely attributed to the L-dependent drift velocity of protons in a narrow energy band. The model is able to explain the observations during several individual events. Also, the model predicts the general trends that have been found by statistical analysis of I.P.D.P. events and accounts for the constant frequency observed by satellites during I.P.D.P. events.

Proton observations supporting the ion cyclotron wave heating theory of SAR arc formation

Abstract Low altitude satellite observations of precipitated and locally mirroring protons during periods of ground-based SAR arc observations are presented. The SAR arcs are found to be located in a region with significantly enhanced proton pitch angle scattering and enhanced electron temperature, but inside the plasmapause where the proton pitch angle distribution is anisotropic. The increase in the pitch angle scattering takes place in a localized region having a width of a few tenths of a L -value. The observations can favourably be accounted for by the Cornwall et al. (1971) theory for the SAR arc formation. Using observed proton fluxes and typical energy spectra, the expected Hβ intensity in the SAR arc region is estimated to be a few Rayleighs, and the energy flux from precipitated protons above a few keV to be 10 −2 −10 −1 erg/cm 2 s. These estimates are in reasonable agreement with previously published theoretical and experimental values. Simultaneous groundbased observations of Hα emissions were found in the region of intense, isotropic proton precipitation located outside the plasmapause.

Storm time equatorial belt – an “image” of RC behavior

[1] During geomagnetic storms a well defined belt of trapped protons and ENAs (energetic neutral atoms) is observed around geomagnetic equator at low L-values. Their source is RC (ring current) protons existing at larger L-values. Through charge exchange with the geocorona RC protons become ENAs and if subjected to a new charge exchange become trapped protons. From low latitude particle observations at four different local times we follow; the RC injection region, the drift of RC-particles through the evening/afternoon into the morning sector, the RC-asymmetry and convection loss to the dayside during the storm initial and main phase, and its development into a symmetric RC in the recovery phase of the storm. INDEX TERMS: 2778 Magnetospheric Physics: Ring current; 2720 Magnetospheric Physics: Energetic particles, trapped; 2730 Magnetospheric Physics:Magnetosphere— inner; 2788 Magnetospheric Physics: Storms and substorms. Citation: Soraas, F., K. Oksavik, K. Aarsnes, D. S. Evans, and M. S. Greer, Storm time equatorial belt – an ‘‘image’’ of RC behavior, Geophys. Res. Lett., 30(2), 1052, doi:10.1029/ 2002GL015636, 2003.

Optical and particle signatures of magnetospheric boundary layers near magnetic noon: Satellite and ground‐based observations

In this paper we present a set of satellite and ground-based observations suggesting that energetic magnetospheric electrons cannot be used as an unambiguous discriminator between open and closed field lines on the dayside. Using two data sets from the Defense Meteorological Satellite Program (DMSP) F13 and NOAA 12 satellites flying through dayside Type 1 cusp aurora (both close in time and space), we reach two apparently incompatible conclusions. Cusp/mantle precipitation, stepped cusp signatures, and antisunward convection in the DMSP F13 data set strongly suggest open magnetic field lines. On the other hand, NOAA 12 observed a mixture of magnetosheath and isotropic energetic particles. Trapped energetic electrons are traditionally regarded as being on closed flux. However, in addition to earlier proposed trapping on open field lines, we suggest that transmission lines connecting merging sites near the cusp in the Southern Hemisphere with the northern auroral ionosphere can be several tens of RE long. Alfven wave transit times of several minutes may make it impossible to determine from satellite measurements in the ionosphere whether magnetic field lines threading low-latitude boundary layer (LLBL) plasmas are open or closed. New research tools will be needed to unify understanding of complementary particle measurements from the DMSP and NOAA satellites.

Precipitation of > 115 kev protons in the evening and forenoon sectors in relation to the magnetic activity

Abstract Data from a low altitude polar orbiting satellite, on auroral protons >115 keV in the evening and forenoon sectors, are presented. In the forenoon sector there is a weak but fairly steady precipitation at Λ ≈ 75° during quiet conditions. This precipitation is situated at higher invariant latitudes at local noon than at local dawn and can probably be ascribed to the high energy tail of the polar cleft protons. During moderately disturbed conditions, especially during the recovery phase of geomagnetic storms, there are some seemingly more “impulsive” precipitation events at Λ ≈ 65°. During very disturbed conditions these two precipitation zones in the forenoon sector seem to merge. In the evening sector a rather sharp equatorward boundary of the main precipitation, at Λ ≈ 69° during quiet conditions, varies fairly smoothly from pass to pass. South of this boundary, at invariant latitudes around 62°, there is a steady weak drizzle from the radiation belt. Due to a longitudinal effect this drizzle, as recorded by the satellite, shows a diurnal variation. The equatorward boundaries of the main precipitation at both local times move equatorward with increasing ring current strength. When D st gets less than about — 100 nT , the poleward boundaries are found to move equatorward too. From an attempt to reveal some of the substorm-dependent changes of the precipitation it is found that an equatorward shift of the precipitation areas takes place during, or just prior to, the substorm expansive phase, accompanied by a large intensity increase in the evening sector, whereas the recovery phase is linked with a poleward expansion of the precipitation at both local times.

Enhanced Pitch Angle Scattering of Protons at Mid-Latitudes During Geomagnetic Storms

Abstract During geomagnetic storms highly localised regions of enhanced proton (ion) precipitation in the tens to several hundred keV energy range can appear at mid-latitudes. The particle pitch angle distribution in these enhanced regions is anisotropic with maximum intensity perpendicular to the magnetic field. In a few cases, however, the distribution can approach isotropy. These regions typically have widths of a few degrees invariant latitude, but can be as narrow as 0.25°. The intensity peak is most often concentrated in a specific particle energy range, although in many cases the intensity peak at a given location is distributed over a broader energy range. During the main phase of the storms the ion enhancement is mostly observed in the highest energy protons and only in the midnight/evening MLT sector. Coincident with the ion enhancement there was often an enhancement in electrons with energies > 300 keV. In the recovery phase of the storms the ion enhancement can be observed at all local times covered by our observations and there was not any coincident enhancement in the high energy electrons. Overall the observations seem to support a picture where scattering of protons into the loss cone by cyclotron resonant wave-particle interaction occurs, while high energy electrons are parasitically scattered into the loss cone by the same ion cyclotron waves. Throughout the storm the L-dependence of the enhancements in proton fluxes is similar to the K p dependence of the location of the plasmapause. Whenever a direct comparison could be made, the SAR arc and the ion enhancement overlap. Thus the ion enhancement and SAR arc are associated, but not necessarily on a cause-effect basis.

Particle and auroral observations from the ESRO I/AURORAE satellite

Abstract The polar-orbiting satellite ESRO I/AURORAE (1968-084A) was designed to investigate auroral particles, luminosity, and associated ionospheric effects. Three groups of three consecutive passes over the auroral zone at three levels of magnetic activity were chosen for a special study. The passes occurred between midnight and 0100 hr geomagnetic time near the Scandinavian meridian. The latitude profile of the electron density at altitudes 300–400 km shows the ionospheric electron density trough (associated with the plasmapause) just equatorward of the main precipitation zone. The poleward wall of the trough is associated with a maximum in electron temperature. A broad maximum in electron density lies both poleward and equatorward of the high, structured gradients associated with the main precipitated zone. Finally, a depression in electron density approaching the pole is associated with an increase in electron temperature and a disappearance of O+. The effect of increased magnetic activity is to steepen the gradients and to extend the precipitation zone both poleward and equatorward. The latitudinal profile of 4278 A N2+ and 4861 A Hβ emissions can, in all cases, be characterized as a single, broad maximum on the equatorward side inside the trapping boundary and one or more sharp maxima usually on the poleward side of the trapping boundary. The colatitude of both regions and their separation vary directly as the magnetic activity as does the particle flux associated with them. Comparison of the auroral emissions and the electron flux equatorward of the trapping boundary gives a conversion factor of 270 Rayleighs of 4278 N2+ emission per erg net downward flux. Wide variations in the production efficiency for electrons on the poleward factor. The observed intensities referring to observations equatorward of the trapping boundary are then in fair agreement with calculations.

Substorm morphology of > 100 keV protons

Abstract 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.