EXOS D (Akebono) observations of molecular NO⁺ and N₂⁺ upflowing ions in the high‐altitude auroral ionosphere

Abstract

We report observations of molecular upflowing ions in the high-altitude auroral ionosphere by the EXOS D (Akebono) suprathermal mass spectrometer (SMS). At Akebono altitude, the occurrence of molecular upflowing ions was rare and was often confined in latitudinal extent, in both the dayside and the nightside. In the dayside, molecular upflowing ions were observed up to about 85' A, inside and up to -10 ø poleward of the cleft. In the nightside, they were generally located at lower invariant latitudes (-60ø-75øA) and were observed both inside and equatorward of regions of energetic (1-10 keV) electron precipitation. The observed molecular ions appeared as a minor component of the upflowing ionospheric ion population, with a flux typically less than 5% but at times as high as 15% of the total. They were composed of comparable fluxes of NO* and N2* ions; at times, smaller fluxes of O2* were also observed. The observed NO*/N2* ratio ranged from 0.3 to 1.3; the observed O2*/N2* ratio was 0.1 or less. They were invariably accompanied by enhanced N* ion flux, the N*/O* ratio being 0.5-1.0. They were more intense (flux -10 s cm -2 s4), had higher density (-0.1-0.2 cm '3) and lower energy (-5 to -20 eV/q), and were more anisotropic and peaked near the upward field line direction, in the dayside than in the nightside. Data from about 440 Akebono passes in the 7-month period from November 1989 to May 1990 were surveyed for their occurrence. Molecular ions were found in 14 passes, i.e., 3% of the passes. In most but not all cases, the Kp index was 4 or higher at the time of their observation and in the preceding several hours, suggesting that they typically occur in periods of sustained auroral activity. These observations are compared with previous low-altitude observations and discussed in terms of the transit and recombination times of the observed molecular ions, the orders-of-magnitude enhancement in neutral molecular densities and the corresponding N*, N2* and NO* ion production above the F region in periods of prolonged auroral activity, and the moderate energization of the ions immediately after their production. 1. INTRODUCIION The auroral ionosphere is an important source of energetic ions in the magnetosphere (see, for example, recent reviews by Chappell [ 1988], Shelley [ 1986], and Yau and Lockwood [ 1988]). Ionospheric ion energization varies from a few electron volts in the polar wind [Nagai et al., 1984], to tens of electron volts in upwelling ions [Lockwood et al., 1985; Pollock et al., 1990] and upflowing polar cap ions [Shelley et al., 1982], to hundreds of electron volts in perpendicular ions and conics [Klumpar, 1979; Whalen et al., 1978; Yau et al., 1983], and to keV in auroral ion beams [Coffin et al., 1987]. In general, the mass distribution of accelerated ionospheric ions reflects the low-altitude source region composition with H* and O* dominating the distribution at high altitude [ Yau et al., 1985a, b; Coffin et al., 1988]. In the F region and topside polar ionosphere, extensive observations of th...

Ion upflow and downflow at the topside ionosphere observed by the EISCAT VHF radar

“…Recent observations with satellites such as Akebono (EXOS-D) and GEOTAIL have shown that thermal ions, including not only light ions H + and He + but also heavy ions O + and NO +¯o w out of the polar ionosphere and they are found in the lower magnetosphere (Watanabe et al, 1992;Abe et al, 1993a, b;Yau et al, 1991Yau et al, , 1993Yau et al, , 1995 and in the lobe magnetosphere (Mukai et al, 1994;Hirahara et al, 1996;Seki et al, 1996Seki et al, , 1998. A very comprehensive review of the terrestrial plasma source obtained from the DE mission was given by Chappell (1988).…”

First results from the hot plasma instrument PROMICS-3 on Interball-2

“…The mode used in the Akebono study measured at a maximum energy of 70 eV. Yau et al (1993) reported that typical dayside energies were 5±20 eV and that typical nightside energies were larger but no energy range was speci®ed. It is however, quite clear that very e cient energisation of the molecular ions must take place.…”

Unusual heavy ion injection events observed by Freja

“…Since the simultaneous appearance suggests that they have the same velocity at each moment, the heavy ion is most likely O+ (mass ratio 16) rather than N+ (mass ratio 14) or C+ (mass ratio 12), if the heavy ions and H+ originate from the same region. This event took place during the storm time when N+ was expected to be nearly as rich as O+ in the magnetosphere (Yau et al, 1993); yet the energy band (width in %) of injecting ions is about the same for proton and heavy ions, indicating that one species (O+) dominated among C+, N+ and O+.…”

“…Yet, some of the plasma injections into the ionosphere reported in the past (without mass spectrometers) are considered to contain substantial heavy ions. We pedagogically call them O+ but they could also contain N+, C+ or Na+ (Chappell et al, 1982;Yau et al, 1993) …”