Vibrational temperature and molecular density of thermospheric nitrogen measured by rocket-borne electron beam induced luminescence

Abstract

The vibrational temperature and molecular density of thermospheric nitrogen were probed in situ above Fort Churchill, Manitoba, Canada, on March 13, 1970. The rocket‐borne system utilized electron beam induced luminescence of the atmosphere as a diagnostic technique in effecting the measurements. The vibrational populations of N2+ ions in the B²Σu+ state were inferred from the intensities of selected transitions of the N2+ first negative (1N) band system (B²Σu+−X²Σg+) induced by a 2.5‐keV electron impact. Four narrow band photometers were used to measure the radiant intensity of the (0–1), (0–2), (1–2), and (2–4) N2+ 1N transitions. In the technique employed the vibrational population of N2(X¹Σg+) is derived from the N2+(B²Σu+, υ′ = 0 and 1) populations if it is assumed that the relative cross sections for production of the excited N2+ ions by simultaneous ionization and excitation of the neutral molecule are proportional to the appropriate Franck‐Condon factors. The inferred population of N2+(B²Σu+, υ′ = 2) is not consistent with this simple model, appearing anomalously large at low vibrational temperatures. Within the limits of uncertainty imposed by the nature of the experimental technique and by the precision realized in the measurements the inferred vibrational population ratio, [N2(υ = 1)]/[N2(υ = 0)], was not recognizably greater than that based on model atmospheric kinetic temperatures appropriate for the time of the flight (950°K exospheric temperature). An upper limit for the N2 vibrational temperature is given in the altitude range 80–175 km (1500°K at 175 km, 1200°K at 155 km, 1000°K at 135 km, and 800°K for altitudes less than 115 km). Measured N2 molecular densities are in substantial agreement (<±10% disparity) with the Jacchia (1971) model atmosphere in the 145‐ to 175‐km altitude range. At lower altitudes the measured concentrations are significantly less than the model values. For example, the measured molecular density at 120 km [2.3 × 1011 cm−3] is approximately 61% of the model value. Further departure of the results from the model at altitudes less than 120 km is attributed in part to the effects of supersonic flow about the vehicle.

N₂ Temperature of Vibration instrument for sounding rocket observation in the lower thermosphere

“…deLeeuw andDavis (1972) applied the EBF technique for in situ measurement of N 2 T r in the lower thermosphere for the first time and built an instrument onboard a sounding rocket using an electron gun to excite and ionize ambient N 2 and two channel photomultipliers to detect the fluorescence from N + 2 . Around the same time, the first in situ measurement of N 2 T v has been done in aurora by O'Neil et al (1974) using a rocket-borne instrument that had an electron gun and four photometers. More than 20 years later, Kawashima et al, (1997Kawashima et al, ( , 1999 improved the instrument of O'Neil et al (1974) and obtained a continuous spectrum using a grating spectrometer with a linear image sensor.…”

“…Around the same time, the first in situ measurement of N 2 T v has been done in aurora by O'Neil et al (1974) using a rocket-borne instrument that had an electron gun and four photometers. More than 20 years later, Kawashima et al, (1997Kawashima et al, ( , 1999 improved the instrument of O'Neil et al (1974) and obtained a continuous spectrum using a grating spectrometer with a linear image sensor. They measured T v and T r simultaneously using this instrument named "N 2 Temperature of Vibration" (NTV) onboard a sounding rocket S-310-24 over 100-160 km altitude range from Kagoshima, Japan.…”

Where does the O(¹D) energy go?

“…In the final N2(v = 1) state, however, approximately 1010 collisions are required to transfer one quantum of vibrational energy to kinetic energy [O'Neil et al, 1974]. In the final N2(v = 1) state, however, approximately 1010 collisions are required to transfer one quantum of vibrational energy to kinetic energy [O'Neil et al, 1974].…”

Role of electronic excited N2 in vibrational excitation of the N2 ground state at high latitudes

“…Although they did not include electron impact excitation of N 2 , Kummler and Bortner summarized the major mechanisms for producing vibrational excitation in N 2 . In a well conceived experiment, O'Neil et al [1974] launched a rocket into an IBC Class II aurora for the purpose of measuring the vibrational population distribution of the N 2 [X 1 Σ g + ] state in the altitude range 100–200 km (see Vallance Jones [1974] for a definition of the IBC auroral index and its relationship to other magnetic disturbance indices). Relative intensities of selected First Negative bands originating from vibrational levels 0, 1, and 2 of the upper electronic state of N 2 + were combined with a model for the direct electron impact excitation process from ground state N 2 to infer the initial vibrational population of the N 2 ground electronic state.…”

“…O'Neil et al concluded that the relative population of N 2 [X( ν ″ = 1)] “was not recognizably greater than that based on model atmospheric temperatures” but that the relative population of N 2 [X( ν ″ = 2)] was anomalously large. Eleven years later, Vlaskov and Henriksen [1985] reported results (from an analysis of ground based emission data from day and night side aurora) based on the same excitation model for N 2 + as used by O'Neil et al [1974] and relative intensities of the N 2 + 1st negative band system. The height of the dayside aurora was assumed to be 150 km and that for the night side aurora 350 km.…”