On the energy distribution of suprathermal electrons produced by HF heating in the F2 region

ARTICLE IN PRESS

Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 405–412 www.elsevier.com/locate/jastp

On the energy distribution of suprathermal electrons produced by HF heating in the F2 region Michael N. Vlasov, Michael C. Kelley, Elizabeth A. Gerken1 School of Electrical and Computer Engineering, 321 Rhodes Hall, Cornell University, Ithaca, NY 14853, USA Received 16 March 2004; received in revised form 13 July 2004; accepted 25 August 2004 Available online 23 December 2004

Abstract Using the High Frequency Active Auroral Research Program (HAARP) imager, measurements of intensities at 630.0 and 557.7 nm were carried out during HF heating in the F region in February 2002 (Pedersen et al., Geophysical Research Letters 30(4) (2003) 1169). Analysis of the red and green line emission observations confirms that both emissions can be observed only when suprathermal electrons are produced during HF heating. Using an artificial airglow model based on a Maxwellian energy distribution of suprathermal electrons, it is possible to determine the minimum ratio of red to green line intensity as a function of altitude. The model also facilitates estimating the effective temperatures and fluxes of fast electrons that excite artificial airglow. However, the associated energy flux is so large that the heating of background thermal electrons becomes very significant. Using this model, it is impossible to obtain agreement between the calculated and measured intensities of the green and red lines because heated thermal electrons additionally excite the O(1D) state while O(1S) excitation is negligible. If the suprathermal electron distribution is fitted by a power law instead, it is possible to decrease this disagreement. In this case, excitation of the 630.0 nm emission is provided by suprathermal electrons together with background thermal electrons heated by suprathermal electrons. r 2004 Elsevier Ltd. All rights reserved. Keywords: High-frequency heating; F2 region; Artificial airglow; Suprathermal electrons

1. Introduction The intensities of red ðl ¼ 630:0 nmÞ and green ðl ¼ 557:7 nmÞ lines of atomic oxygen were measured by the High Frequency Active Auroral Research Program (HAARP) imager during the HAARP optical campaign carried out in February 2002 (Pedersen et al., 2003). Natural airglow and artificially enhanced airglow Corresponding author. Tel.: 607 255 7425; fax: 607 255 6236.

E-mail addresses: [email protected] (M.N. Vlasov), [email protected] (M.C. Kelley), [email protected] (E.A. Gerken). 1 Also at the Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY, USA.

induced by HF heating in the F2 region were observed beginning just after sunset and well into the night. Excitation of optical emissions due to the impact of powerful electromagnetic waves (HF heating) on the ionospheric F2 region have been observed many times (Haslett and Megill, 1974; Adeishvili et al., 1978; Djuth et al., 1987; Bernhardt et al., 1989, 1991). The red and green line emissions of atomic oxygen are the main features of artificial airglow, although the green line has been detected rarely in the past. The HAARP imager, however, detected green line emissions many times during HF heating events in February 2002, with green line intensities ðI G Þ from 10 to 70 R reported by Pedersen et al. (2003). Only suprathermal electrons with energies higher than 4 eV can cause such intense green

1364-6826/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2004.08.006

ARTICLE IN PRESS M.N. Vlasov et al. / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 405–412

406

Maxwellian distribution are given by

line emissions. These fast electrons can be produced due to acceleration by Langmuir waves, upper hybrid waves, or Bernstein mode waves in the region of the ionosphere where most of the HF radiation is absorbed (Gordon and Carlson, 1974; Meltz and Perkins, 1974; Gurevich et al., 1985, 2003; Newman et al., 1998). For example, Wang et al. (1997) and Newman et al. (1998) showed that Langmuir turbulence near the critical height may produce suprathermal electrons with energies greater than 4.19 eV, which is the level necessary to excite atomic oxygen at 1S states. More recently, Gurevich et al. (2003) discussed the importance of acceleration near the altitude where the heater frequency equals the upper hybrid frequency. This may explain the airglow enhancement that occurs when the heater beam is oriented in the magnetic zenith direction, as reported by Pedersen et al. (2003). Most theoretical studies of airglow production postulate a Maxwellian distribution of suprathermal electrons characterized by a density and temperature differing from the ionospheric background. For our purposes we also initially postulate a bi-Maxwellian system. We then estimate the effective electron temperature ðT eeff Þ and density ðN eeff Þ of the suprathermal electrons using HAARP imager measurements of the red and green line emission intensities. Certain inconsistencies are found in this approach, leading us to investigate a power law distribution for suprathermal electrons.

kD;S ¼ A  1010 ðT e =1000Þ expðE=T e Þ ðcm3 s1 Þ;

(1)

1

where A ¼ 12; g ¼ 0:35; E ¼ 1:96 eV for O( D) and A ¼ 1:75; g ¼ 0:318; E ¼ 4:19 eV for O(1S) (Capitelli et al., 2000). Experiments indicate that the background electron temperature, T e ; does not exceed T e ¼ 0:3 eV ¼ 3500 K during HF heating with the Tromsø superheater (Gustavsson et al., 2001). For this temperature, kS is 2:3  1016 cm3 s1 according to formula (1). For this rate coefficient, enhancement of the green line emission induced by heated background electrons is negligible compared with the emission provided by ionospheric recombination for the same electron density profile. Also, the kD =kS ratio is higher than 10,000 for T e ¼ 3500 K: Since green line intensities are observed to be higher than the recombination level and intensity ratios of red and green lines as low as 4 have been reported by Pedersen et al. (2003), it is very unlikely that such events can be attributed to an elevated background electron temperature. For reference we provide information on the 2001 HAARP results in Table 1, where f H is a pump frequency, hH is the altitude of HF heating, and I R and I G are the red and green line intensities, respectively. Other entries in the table are defined as follows. The ratio of red and green intensities induced by fast electrons with T eeff is given by I R =I G ¼ A1 ðA3 þ A4 ÞkD =lA4 kS ;

(2)

where kD and kS depend on T eeff of the hypothesized Maxwellian suprathermal population:

2. Bi-Maxwellian model for artificial airglow

l ¼ k1 ½N2  þ k2 ½O2  þ k3 ½O þ A1 þ A2 :

The electron impact on atomic oxygen is the main enhancement source for the red and green emissions during HF heating in the F region. Rate coefficients for O(1D) and O(1S) excitations by electrons with a

(3)

1

Here l is the O( D) deactivation rate by inelastic collisions and radiation, and the other parameters are presented in Table 2.

Table 1 Measured (first four columns) and calculated (last three columns) parameters for the different observations of red and green line emissions during February 2002 No.

Date

f H (MHz)

hH (km)

N e ðcm3 Þ

I R (R)

I R =I G

N eeff =N e

T eeff (eV)

F ðeV cm2 s1 Þ

1)

02/03/02 0430 UT 02/05/02 0830 UT 02/09/02 0500 UT 02/13/02 0440 UT 0500 UT 02/17/02 0945 UT 1000 UT

5.8

280

4  105

210

10

1:25  104

1.4

5:0  1010

5.8

300

4  105

290

6

8  104

2.7

8:5  1010

5.8

300

4  105

260

13

1:7  103

1.4

6:9  1010

7.8 6.8

270 260

7:5  105 5:7  105

230 125

4 10

3  104 6:5  104

2.8 1.3

6:3  1010 3:4  1010

5.8 4.8

330 330

4  105 2:8  105

260 230

10 7

1:2  103 6:5  104

2.2 4.5

9:6  1010 1:0  1011

2) 3) 4)

5)

(1) conical scan, 151 off-zenith; (2) conical scan, 151 off-zenith; (3) magnetic zenith; (4) magnetic zenith; (5) magnetic zenith.

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Table 2 Reactions and rate coefficients Reactions

Rate coefficients ðcm3 s1 ; s1 Þ

Reference

Oð1 DÞ þ N2 ! O þ N2 Oð1 DÞ þ O2 ! O þ O2 Oð1 DÞ þ O ! O þ O

k1 ¼ 2  1011 expð107:8=T n Þ k2 ¼ 2:9  1011 expð67:5=T n Þ k3 ¼ ð3:730 þ 1:1965  101 T 0:5 n 6:5898  104 T n Þ  1012 A1 ¼ 7:1  103 A2 ¼ 2:2  103 A3 ¼ 1:215 A4 ¼ 0:076

Langford et al. (1986) Streit et al. (1976) Sun and Dalgarno (1992)

Oð1 DÞ ! O þ hn (630.0 nm) Oð1 DÞ ! O þ hn (634.4) Oð1 SÞ ! O þ hn (557.7 nm) Oð1 SÞ ! O þ hn (297.2 nm)

The associated red line intensity can be calculated by the formula

Froese Fischer and Saha (1983) Baluja and Zeippen (1988)

70 60

(4)

F ¼ N eeff V e T eeff ¼ I R lT eeff V e =ðA1 kD ½OLÞ;

(5)

where V e is the mean velocity of electrons with T eeff :

50 kD /kS

where L is the half thickness of the emission layer. Here N eeff is the number density of the hypothesized Maxwellian suprathermal population with T eeff : Although it is uncertain that such a distribution is valid, because most theoretical and experimental studies use this approach we can compare our results with other results as well to confirm the validity of this hypothesized distribution. Using formula (1), the dependence of the kD =kS ratio on T eeff is shown in Fig. 1. As can be seen, this ratio is close to a constant value of 9 for T eeff 410 eV; which is smaller by 3 orders of magnitude than the value estimated above for T e ¼ 3500 K: Using the deactivation rates calculated by the model with the neutral composition from the MSISE-90 model (Hedin, 1991) and formula (2), it is possible to estimate the I R =I G minimum values for different altitudes of the emission layer for T eeff ¼ 10 eV: Results of these calculations are shown in Fig. 2. For example, according to this model an I R =I G ratio o4 cannot be observed at altitudes above 290 km. Such a ratio would be observed only if the energy of fast electrons is higher than 30–40 eV. However, such energy seems too high for HF heating. These calculations have been made under average nighttime conditions in February, and variations in the minimum ratio may be significant due to the neutral composition changes from night to night or from sunset to midnight. Using the observed intensity and the kD value for T eeff ¼ 10 eV; it is possible to calculate, using formula (4), the concentration of fast electrons, N eeff ; corresponding to different altitudes of the emission layer. Also, by using formula (4) the energy flux of these electrons can be estimated by the formula

40 30 20 10 0

0

2

4

6

8

10 12 Teeff, eV

14

16

18

20

Fig. 1. Ratio of the rate coefficients for the excitation of red and green line emissions by electrons with different temperatures.

350 340 Teeff=10 eV

330 320 height, km

I R ¼ N eeff ½OA1 kD L=l;

310 300 FORBIDDEN RATIOS

290

ALLOWED RATIOS

280 270 260 250

2

2.5

3

3.5

4 4.5 IR/IG

5

5.5

6

6.5

Fig. 2. Height distribution of minimum values of the ratio of red and green line emission intensities.

The fluxes shown in Fig. 3 are calculated for the red line intensity of 100 R in a layer with half thickness L ¼ 15 km for T eeff ¼ 10 eV: These fluxes correspond to

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350 Model: IR /IG=10, 310 IR=100 R, L=15 km

330 320 310

300

300 height, km

height, km

Teeff, eV

320

340

290 280 270 260 250 0.6

F

Teeff

290 280 270

0.8

1

1.2 1.4 F, eV/cm2s

1.6

1.8 x 1011

260

Fig. 3. Energy fluxes of fast electrons for the excitation of a maximum green line intensity and a red line intensity of 100 R at different altitudes of HF heating.

1.5

2

2.5 3 F, eV/cm2s

3.5

4 x1010

Fig. 4. Height distributions of the temperature and energy flux of fast electrons calculated for I R =I G ¼ 10 and I R ¼ 100 R and the barometric height profiles of neutral components with the thermospheric temperature of 900 K.

Teeff, eV

320 310 300 height, km

excitation of the maximum green line emission intensity if the red line intensity is 100 R. Likewise, these fluxes yield the I R =I G ratio shown in Fig. 2. In other words, the green line intensity cannot be higher than 16 R during HF heating in the F region if the red line intensity does not exceed 100 R. The energy flux increases with altitude because the atomic oxygen concentration decreases faster with altitude than the O(1D) deactivation rate (see formulas (3) and (4)). At altitudes above 330 km the flux increase is proportional to the decrease in O concentration because the deactivation rate becomes a constant value. At altitudes below 280 km, the vibrational barrier, created by the high cross section for N2 vibrational level excitation, makes electron acceleration up to energies higher than 2 eV very difficult. This means that the energy fluxes shown in Fig. 3 must increase at lower altitudes to yield the 100 R emission intensity. A rough estimate indicates that the energy flux at 250 km must increase by a factor of 2–3. In this case, the variation of suprathermal electron energy fluxes with altitude becomes small. The height profiles of T eeff and F, calculated for the red line intensity of 100 R and the I R =I G ratio of 10, are shown in Fig. 4. A barometric height distribution of the main neutral constituents with a temperature of 900 K was used in these calculations. The results of the calculation using the neutral composition given by the MSISE-90 model at 0600 UT on February 9, 2002 are shown in Fig. 5. The height distributions of the energy fluxes calculated for the constant ratio of I R =I G ¼ 10 differ from the height distribution of the energy fluxes calculated for the minimum ratio of I R =I G at each altitude (see Fig. 3). As can be seen, the fluxes shown in Figs. 4 and 5 are much less than the fluxes

250

290

Teeff

F

280 270 260 250 1

1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 x10 F, eV/cm2s

Fig. 5. The same as in Fig. 4 but for the neutral composition given by the MSISE-90 model at 0600 UT on February 9, 2002.

corresponding to the maximum green line intensity (minimum I R =I G ratio). We conclude that suprathermal electrons with T eeff o3 eV can provide I R ¼ 100 R and I G ¼ 10 R emissions at all altitudes. The T eeff increase with altitude is a result of the kD =kS decrease, which compensates for the decrease in parameter l for a constant ratio of I R =I G (see formula (2)). The approach described above can be applied to analyzing the HAARP imager data. According to formula (2), when the emission altitude is known, the ratio of the intensities only depends on T eeff because

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Table 3 Electron gas heating rates from the energy fluxes of fast electrons, as presented in Table 2 (Qe is in eV cm3 s1 ) Date: Qe :

02/03/02 0430 UT 2  104

02/05/02 0830 UT 9:3  103

02/09/02 0500 UT 2:8  104

02/13/02 0440 UT 1:2  104

the kD =kS ratio only depends on T eeff : Simultaneously, the rate coefficient of red line excitation can be estimated by formula (1). Then, using formulas (4) and (5) and the measured intensity, it is possible to calculate the N eeff value and the energy flux corresponding to the measured intensity of the red line emission. The emission layer altitude is determined using the pump frequency, together with the electron density profile calculated by the IRI model with N m F2 and hm F2 measured by the HAARP digisonde. This approach can be applied when the observed intensity ratios are larger than the minimum ratios calculated under the expected geophysical conditions. The minimum ratio calculations were presented above and the results are shown in Fig. 2. The effective temperature, concentration, and energy flux of fast electrons thus can be inferred from the HAARP imager data on red and green line intensities. The results of these calculations are presented in Table 1. The flux value corresponds to the integrated intensity of a parabolic emission layer, which has a maximum in the radiation rate at the altitude shown in Table 1, a half thickness of 15 km, and the IR and I R =I G values shown in Table 1. The maximum value of the energy fluxes of fast electrons estimated by our model is smaller by a factor of 3 than the flux inferred by Gurevich et al. (2003) from the same HAARP imager data. Part of this discrepancy can be explained by the significant influence of the heating altitude, hH ; on the energy flux and uncertainty of the emission altitude without triangulation measurements.

3. The energy distribution of fast electrons Although we have been able to model the observed intensities of red and green line emissions using a Maxwellian distribution, we have identified a serious problem with this approach. Energy fluxes of fast electrons as high as those estimated above will heat the thermal electron gas. If the electron temperature exceeds 2500 K, the excitation of red line emissions by thermal electrons may be comparable to the excitation by fast electrons and the ratio I R =I G will rise. The fast electron heating rate can be estimated by the formula (Banks and Kockarts, 1973), Qe ¼ ð2 0:5Þ  1012 ðN e F=EÞ eV cm3 s1 ;

(6)

02/13/02 0500 UT 2:3  104

02/17/02 0945 UT 1:6  104

02/17/02 1000 UT 2:7  103

where F is the flux of fast electrons, E is the fast electron energy in electron volts, and Ebð3=2ÞkT e : Using the parameters given in Table 1 and E ¼ T eeff ; it is possible to estimate the Qe values, presented in Table 3. The maximum Qe value is 2:8  104 eV cm3 s1 at 0500 UT on February 9. This heating rate is higher by a factor of 5 than the maximum value of the heating rate in the daytime ionosphere at altitudes above 270 km. We now estimate the impact of this heating on both the ionosphere and artificial airglow. The thermal balance equation is 1:5N e kð@T e =@tÞ ¼ Qe  Le ;

(7)

where Le is the energy loss rate. Comparison of thermal electron cooling processes indicates that the elastic scattering of electrons by ions, inelastic electron collisions with atomic oxygen ðLeO Þ; and inelastic electron collisions with molecular nitrogen ðLeN 2 Þ control the electron gas cooling at altitudes around 300 km, where emissions have been observed, for example, at 0500 UT on February 9. In this case, Eq. (7) can be written as 1:5N e kð@T e =@tÞ ¼ Qe  LeN 2  3ðme =mi ÞN e knei ðT e  T i Þ  3:4  1012 N e ½Oð1  7  105 T e Þ ðT e  T n Þ=T n ;

ð8Þ

where me and mi are the electron and ion mass, respectively, k ¼ 8:63  105 eV=K; nei is the electronion collision frequency, and LeN 2 ¼ 1:3  104 N e ½N2 f1  exp½3200ðT n  T e Þ=ðT n T e ÞgA where A ¼ 2:53  106 T 0:5 for T e 42000 K e expð17; 620=T e Þ (Banks and Kockarts, 1973). A numerical solution of Eq. (8) yields an electron temperature higher than 3000 K. The values T n ¼ 1133 K; ½O ¼ 9:5  108 cm3 ; and ½N2  ¼ 2:1  108 cm3 ; given by the MSISE-90 model, were used in these calculations. In this case, formula (4) should be replaced by I R ¼ ½OA1 ½kD N eeff þ kDth N e =l;

(9) 1

where kDth is the rate coefficient of O( D) excitation by thermal electrons. The second term on the right-hand side of formula (9) takes into account red line excitation by thermal electrons. The ðkD N eeff =kDth N e Þ ratio is found to be 1.5 for T e ¼ 3000 K: Thus, the kD N eeff value must decrease by a factor of 1.66 to obtain agreement with the I R observed value. However, in this case the I G value becomes lower than the observed intensity because

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the green line excitation by thermal electrons is negligible. Thus, agreement between the calculated and measured intensities of both emissions is impossible if there is significant heating of the thermal electron gas by fast electrons during HF heating. According to the numerical solution of Eq. (8), heating up to 3000 K can occur in the cases shown in Table 1 for altitudes above 280 km, meaning that reconciling the observed intensities of both emissions with the model is impossible. The main cause of this disagreement is the kD =kS ratio using a Maxwellian energy distribution. Agreement could be reached, however, if the energy distribution (ED) of fast electrons differed from a Maxwellian ED. Note that here we consider the ED of fast electrons at altitudes higher than 280 km where the impact of N2 vibrational excitation on ED can be neglected (Vlasov et al., 2004). Wang et al. (1997) solved numerically the onedimensional Vlasov equation for conditions appropriate to the ionospheric F region during HF-heating modification. They found that the ED of suprathermal electrons induced by HF heating can be close to a power law of the form hf e iðne Þ ¼ Aðne Þ2:03 ;

(10)

where ne ¼ V e =V T e : Our calculations indicate that fast electrons with the ED given by (10) with A ¼ 2:81  102 and T eeff ¼ 0:5 eV can provide the red line intensity of 74 R and the green line intensity of 20 R at 0500 UT on February 9. The electron temperature induced by fast electrons is found to be 3100 K, and the excitation by thermal electrons can provide the red line intensity of 174 R. In this case we have good agreement between the calculated intensities and the intensities observed on February 9 (see Table 1), which are 260 and 20 R for the red and green lines, respectively. We emphasize that the kD =kS ratio for the ED (10) is about 10 for T eeff ¼ 0:5 eV; but this ratio is higher than 70 for a Maxwellian ED. The main feature of the airglow excitation by fast electrons with ED given by formula (10) is that the excitation of the 630.0 nm emission by thermal electrons heated by fast electrons is important for all entries in Table 1. This excitation provides about 70% of the 630.0 nm intensity for HF heating with a pump of 5.8 MHz on February 9 and 32% of the 630.0 nm intensity for HF heating with a pump of 7.8 MHz on February 13. Our results indicate that the heating and acceleration of electrons must take place simultaneously, in contrast with the approach presented by Gurevich et al. (2003). As mentioned above, the impact of vibrational excitation on the EED is important for the artificially modified F2 region at altitudes below 280 km. This effect was considered by Vlasov et al. (2004) under conditions when the strong increase of T e was observed during HF heating, the main feature being the absence of a green line emission. According to these results, there is a strong depletion of electrons with energies higher than

2 eV due to the energy loss in N2 vibrational excitation by electrons with energies of 2–3 eV (the vibrational barrier). The EED in the energy range of 2–3 eV is very different from MD, and EED is close to MD at energies higher than 3 eV. As can be seen from the results presented in this section, EED is very different from MD at all energies when electron acceleration occurs during artificial modification by power electromagnetic waves. Considering the mechanism of electron acceleration during HF heating, Mishin et al. (2004) concluded that the EED must differ from the Maxwellian ED and can be inverted proportional to the energy.

4. Conclusions The radiation of red and green line emissions during HF heating in the F2 region is possible if fast electrons with an energy higher than 4.19 eV are produced. The ratio of the rate coefficients of red and green line excitation by fast electrons with a Maxwellian distribution strongly varies in the electron temperature range of 1–5 eV, but becomes close to a constant value for electron temperatures higher than 10 eV. This constant corresponds to the minimum value of the kD =kS ratio. Using the minimum kD =kS ratio and the model of red and green lines excited by fast electrons, it is possible to estimate the minimum values of the I R =I G ratio at different altitudes of the emission layer. For example, our calculations indicate that the I R =I G ratio cannot be smaller than 4.5 at 300 km altitude during the night in February 2002. This means that a green line intensity higher than 22 R cannot be observed if the red line intensity does not exceed 100 R during HF heating at 300 km. Using the model of red and green line excitation and the measured intensities of these emissions, it is possible to determine the effective temperature and energy flux of fast electrons with a Maxwellian ED produced as a result of HF heating. The maximum value of this flux at each altitude corresponds to the minimum value of the I R =I G ratio. According to our estimates this maximum flux increases with altitude. The variations of the energy flux with altitude can have a minimum if the I R =I G ratio does not change with altitude. The altitude of this minimum and the shape of the height distribution are very sensitive to the neutral composition. However, in each case the energy flux increases at altitudes above 300 km for I R =I G ¼ const: The effective temperature of fast electrons also increases with altitude under any conditions. The values of the energy flux of fast electrons, inferred from HAARP imager measurements of the intensities of red and green line emissions during the experiments in February 2002, have been estimated to be in the range of ð0:5  1Þ1:1  1011 eV cm2 s1 :

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These conclusions are based on the assumption of a Maxwellian ED of fast electrons. However, the large energy fluxes of fast electrons inferred from analyzing the HAARP imager data must heat the thermal electrons, which additionally can excite the red line intensity. This in turn means that the observed red line emission does not need such large fluxes of fast electrons. However, in this case the observed green line emission cannot be explained because the excitation of this emission by thermal electrons is negligible. According to our calculations, fitting the ED of fast electrons to a power law provides better agreement between the calculated and observed intensities of both emissions. In this case, the red line emission excitation is provided both by fast electrons and by thermal electrons heated by fast electrons. It should be noted that a power law does not take into account the impact of inelastic collisions on the ED, which is important at some altitudes in the F2 region. The main conclusions of this study are as follows:  Using a Maxwellian ED of fast electrons, it is impossible to explain the observed intensities of green and red line emissions because of the strong heating of thermal electrons, which additionally excites the red line emission but cannot excite the green line emission.  Using the power law ED facilitates obtaining good agreement between the calculated and measured intensities, but in this case the electron temperature of the background electron gas sharply increases and these heated thermal electrons are, in turn, an important source of red line emission.  The region of I R =I G forbidden ratios calculated with Maxwellian ED can be used in future experimental checks of these conclusions. These results are a first step on the way to constructing an ED of the non-Maxwellian suprathermal electrons by using airglow emission data. The main deficiency of a power law ED is that it does not take into account the impact of inelastic collisions on the suprathermal electrons. The spectrum of the photoelectrons shows that these collisions are very important at altitudes of the F2 region. A future study of artificial airglow excitation would benefit from using very different pump frequencies and, hence, different emission altitudes during one night. Also, it would be useful if the series of such experiments could be carried out under different geophysical conditions and with triangulations of the emission height.

Acknowledgements Work at Cornell University was sponsored by the Office of Naval Research under grants N00014-00-10658 and N00014-03-1-0978.

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