Molecular Nitrogen (N2) Density Derived from Optical Measurements During
Ionospheric Heating Experiment (or an Alternative Hypothesis for the
O(1D) Quenching Coefficient Adjustment).

High frequency (HF) experiments inducing intensification of airglow
emissions at 6300 Å; and 5577 Å (red and green lines, respectively) from
the two lowest excited states of oxygen O(1D) and O(1S) has been studied
since the early 1970s. The last generation Arecibo HF facility was
commissioned in November 2015, and since then several campaigns have
been conducted at AO. The current system consists of six transmitters,
each connected to one of six dipole elements, and each capable of
continuous wave (CW) operation at a nominal power of
~100 kW. Before the AO platform collapse on December 1,
2020, the HF transmitter system has two available frequencies, 5.125 MHz
and 8.175 MHz, with 130 kHz and 100 kHz bandwidths, respectively. In
this work we are analyzing the excitation of the red line airglow
emission (3600 Å) by high-power radio waves at ~5.125MHz
of 28 HF pulses of 5 minutes intercalated by 5 minutes of no HF
interaction. The chosen periods were the pre-sunrise and post-sunset
periods of June 5, 2016 (Figure 1). Coincidentally, a small geomagnetic
storm occurred during these observations. The first experiment started
along the initial phase of this disturbance and the second experiment at
the end of the main phase (Figure 2). Up to now, our main findings are
listed below: 1. Assuming that the modified red line comes from a narrow
height range in the vicinity of the reflection height to a first
approximation and considering that all of the excess emission comes from
a single height (equation 1) (which corresponds to the height where the
plasma frequency equals the transmitted frequency), it was detected that
the lifetime of the O(1D) varies with altitude which a peak close to the
red line emission altitudes (Figure 3). Θ_r=1/((T1+T2+T3+T4)) (1) Where
T1 is the total Einstein transition probability of the O(1D) state, T2
is the N2 concentration at the altitude of reflection times its
respective quenching coefficient (Q~5,0.10-11cm3s-1) as
well as T3 the O2 concentration times its respective quenching
coefficient (R~7,4.10-11cm3s-1). 2. Assuming a fixed
lifetime for all altitudes, we detected variation of the N2 quenching
coefficient O(1D) also varying with altitude. Such variation could be a
miss determination of the N2 neutral concentration from the NRLMSISE-00
Atmosphere Model (Figure 4) (equation 1). 3. As a practical outcome, our
study shows that the 5 minutes off is not sufficient for the excited
region to return to the previous quiet condition. Our computations show
that pulses of 3 minutes intercalated by 6 minutes off are the ones more
appropriate (Figure 5).