Energetic neutral atom imaging at low altitudes from the Swedish microsatellite Astrid: Observations at low (≤10 keV) energies

Intense (≤106 cm−2 sr−1 s−1) fluxes of upflowing energetic neutral atoms (ENAs) from the polar cap have been observed in the energy range ∼0.1–10 keV (hydrogen assumed) by the ENA imager PIPPI (Prelude In Planetary Particle Imaging) onboard the Astrid satellite at 1000 km altitude. If a source altitude of 400 km is assumed, the ENA emissions maps to magnetic latitudes 70°–85°extending from dusk over nightside. There were no measurements in the midnight sector below approximately 80°. It is shown that the emissions cannot originate from an isotropically emitting source, which rules out any optical emissions. Local charged particle detection is ruled out on the basis of no correlation with local electron measurements and local magnetic field configuration. The ENA emissions can be explained by a conically pitch angle distributed ion source emitting ENAs in the 110°–140°pitch angle interval (source altitude 400 km). It is suggested, with support from low altitude rocket observations that the ENAs are produced by low altitude (400–600 km) ion conies of several hundreds eV charge exchanging with the upper atmosphere.

[1]  E. Roelof.  ENA emission from nearly-mirroring magnetospheric ions interacting with the exosphere , 1997 .

[2]  P. Kintner,et al.  Observations of transverse ion acceleration in the topside auroral ionosphere , 1992 .

[3]  Mats André,et al.  Sources of Ion Outflow in the High Latitude Ionosphere , 1997 .

[4]  A. Hedin.  Extension of the MSIS Thermosphere Model into the middle and lower atmosphere , 1991 .

[5]  M. Galand,et al.  Magnetic mirroring in an incident proton beam , 1999 .

[6]  Peter M. Banks,et al.  High‐latitude plasma transport: The polar wind , 1969 .

[7]  A. G. McNamara,et al.  Particle and wave observations of low‐altitude ionospheric ion acceleration events , 1983 .

[8]  B. A. Whalen,et al.  Low altitude acceleration of ionospheric ions , 1978 .

[9]  M. Torr,et al.  Escape of fast oxygen from the atmosphere during geomagnetic storms , 1974 .

[10]  J. Nordholt,et al.  High-Altitude Observations of the Polar Wind , 1997 .

[11]  Thomas E. Moore,et al.  Modulation of terrestrial ion escape flux composition (by Low‐Altitude acceleration and charge exchange chemistry) , 1980 .

[12]  C. Meng,et al.  Energy distribution of energetic O+ precipitation into the atmosphere , 1992 .

[13]  M. S. Gussenhoven,et al.  A statistical model of auroral ion precipitation , 1989 .

[14]  S. Barabash,et al.  Energetic neutral atom imaging at low altitudes from the Swedish microsatellite Astrid: Extraction of the equatorial ion distribution , 2001 .

[15]  R. Lundin,et al.  Plasma energization on auroral field lines as observed by the Viking spacecraft , 1987 .

[16]  L. Blomberg,et al.  Ion energization mechanisms at 1700 km in the auroral region , 1998 .

[17]  E. Shelley,et al.  Observation of an ionospheric acceleration mechanism producing energetic (keV) ions primarily normal to the geomagnetic field direction , 1977 .

[18]  E. Shelley,et al.  SATELLITE OBSERVATIONS OF ENERGETIC HEAVY IONS DURING A GEOMAGNETIC STORM. , 1972 .

[19]  B. A. Whalen,et al.  EXOS D (Akebono) suprathermal mass spectrometer observations of the polar wind , 1993 .

[20]  E. Roelof,et al.  Global magnetospheric imaging , 1992 .

[21]  J. Hoffman,et al.  Initial ion composition results from the Isis 2 satellite , 1974 .

[22]  S. Barabash,et al.  Energetic neutral atom imaging at low altitudes from the Swedish microsatellite Astrid: Images and spectral analysis , 1999 .

[23]  A. Nagy,et al.  Energetic O/+/ precipitation , 1982 .

[24]  E. G. Shelley,et al.  Energetic Auroral and Polar Ion Outflow at DE 1 Altitudes' Magnitude, Composition, Magnetic Activity Dependence, and Long-Term Variations , 1985 .

[25]  P. Kintner,et al.  Bursts of transverse ion acceleration at rocket altitudes , 1992 .

[26]  R. Heelis,et al.  Particle acceleration parallel and perpendicular to the magnetic field observed by DE‐2 , 1984 .

[27]  B. Mauk,et al.  ENA imaging from the Swedish micro satellite Astrid during the magnetic storm of 8 February, 1995 , 1997 .

[28]  J. Hoffman,et al.  Light ion concentrations and fluxes in the polar regions during magnetically quiet times , 1980 .