Variation in altitude of high-frequency enhanced plasma line by the pump near the 5th electron gyro-harmonic

the 5th electron gyro-harmonic 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Jun Wu, Jian Wu, Michael. T. Rietveld , Ingemar. Haggstrom , Haisheng Zhao, Tong Xu, Zhengwen Xu National Key laboratory of electromagnetic environment, China research institute of radio wave propagation, Beijing, 102206, China EISCAT Scientific Association, 9027 Ramfjordbotn, Norway EISCAT Scientific Association, SE-981 92 Kiruna, Sweden Abstract During an ionospheric heating campaign carried out at the European Incoherent Scatter Scientific Association (EISCAT), the ultra high frequency incoherent scatter (IS) radar observed a systematic variation in the altitude of the high-frequency enhanced plasma line (HFPL), which behaves depending on the pump frequency. Specifically, the HFPL altitude becomes lower when the pump lies above the 5th gyro-harmonic. The analysis shows that the enhanced electron temperature plays a decisive role in the descent in the HFPL altitude. That is, on the traveling path of the enhanced Langmuir wave, the enhanced electron temperature can only be matched by the low electron density at a lower altitude so that the Bragg condition can be satisfied, as expected from the dispersion relation of Langmuir wave.

The enhanced Langmuir wave and ion acoustic wave are usually excited in the altitude range from the reflection altitude of the pump to the altitude where the heavy Landau effect on Langmuir wave may take place (Stubbe et al., 1992).However, the enhanced Langmuir wave and ion acoustic wave can't be observed by IS radar in the exciting altitude range, but at an altitude where the Bragg condition is satisfied (Stubbe et al., 1992;Kohl et al., 1987Kohl et al., , 1993)).Some usual observations of the ultra high frequency (UHF) radar at European Incoherent Scatter Scientific Association (EISCAT) show that the HFIL altitude is about ~ 3 km -~ 5 km higher than the HFPL altitude (Stubbe et al., 1992;Kohl et al., 1993).Additionally, the altitude extending of ~ 3 km -~ 5 km frequently appears in the power profile of the HPIL, but does not in the power profile of the HFPL (Stubbe et al., 1992;Kohl et al., 1993).Moreover, some observations at EISCAT illustrated that a descent in the altitude of the plasma turbulence took place over tens of seconds after the pump on, which was most likely attributed to the modification in electron density by the ionospheric heating (Djuth et al., 1994).UHF radar at EISCAT observed the descent in the HFIL altitude from ~ 230 km to ~ 220 km within ~ 60 s, which was also attributed to the modification in electron density (Ashrafi et al., 2006).
Although those variations in the HFPL and HFIL altitudes were attributed to the enhanced electron temperature and the modified electron density, the dominant one of which was not clearly identified (Wu et al., 2017a).Furthermore, it was identified that the enhanced electron temperature dominated over the modified electron density in the variation in the HFIL altitude (Wu et al., 2018b).As a further work, this paper examines the variation in the HFPL altitude in more detail.Indeed, the dispersion behavior of Langmuir wave is very different from that of ion acoustic wave.

Experiment and data
An ionospheric heating campaign was performed at EISCAT at 12:32:30 UT -14:30 UT (universal time) on Mar.11, 2014.The experiment arrangement has been described in more detail by Wu et al., (2016Wu et al., ( , 2017b)).Briefly, the EISCAT heater (Rietveld et al., 1993(Rietveld et al., , 2016) ) radiated the O mode pump in the frequency band of 6.7 MHz -7 MHz, and the UHF IS radar was operated as the leading diagnostic means.
The pump frequency HF f was stepped down and up in a step of 2.804 kHz with a period of 10 s as shown in those bottom panels in Figure 1, Figure 2 and Figure 3.
During the experiment, the local geomagnetic was relatively quiet.At an altitude of 200 km, the total geomagnetic varied in the range of 49202 nT -49233 nT.
Considering the variation in the intensity of ion line, we adopt a convention for the following discussion: the HF f band of 6.7 MHz -7 MHz can be divided into three daughter bands, that is, the higher band (HB, above represents the electron gyro-frequency (Wu et al., 2016(Wu et al., , 2017a(Wu et al., , 2017b(Wu et al., , 2018a(Wu et al., , 2018b(Wu et al., , 2019)).For instance, in the 1st heating cycle, the HB is set as 7 MHz -~ 6.871028 MHz, the GB as ~ 6.868224 MHz -~ 6.837383 MHz and the LB as ~ 6.834579 MHz -6.7 MHz, which temporally correspond to the time intervals of 12:30:00 UT -12:37:40 UT, 12:37:50 UT -12:39:40 UT and 12:39:50 UT -12:48:00 UT, respectively.Actually, the frequency division in each heating cycle should be somewhat different from each other due to the slight disturbance of the geomagnetic.
From the 1st panel to the 6th panel in Figure 1, the normalized plasma lines at those altitudes of 210.25 km, 207.32 km, 204.39 km, 201.45 km, 198.52 km and 195.58    T T depends on the dispersion behavior of the excited upper hybrid waves at the upper hybrid altitude (Wu et al., 2017b), where is ~ 2 km -~ 10 km below the reflection altitude of the pump (Gurevich, 2007).

Discussion
OTSI and PDI can be excited in the altitude range of (Stubbe et al., 1992) where is the reflection altitude of the pump, is the scale altitude and is does not imply that the enhanced is independent of the HFPL altitude.Indeed, on the traveling path of Langmuir wave, an remarkable enhancement in electron temperature owing to an ionospheric heating will take significant impact on .For a gyro-harmonic at EISCAT, is paid attention.The IS radar observation demonstrates that the HFPL altitude and the electron temperature behave as a function of the pump frequency.More specifically, when the pump frequency approaches the 5th gyro-harmonic from below, the electron temperature is somewhat enhanced, and the HFPL is observed at an altitude as expected.When the pump frequency sweeps above the 5th gyro-harmonic, however, the electron temperature is prominently enhanced, and the HFPL altitude slightly plunge downward.
In conclusion, the HFPL altitude is dependent on the dispersion behavior of the enhanced Langmuir wave and the Bragg condition, and is determined by the profiles of the electron density and the enhanced electron temperature.When heating above the 5th gyro-harmonic, the HFPL altitude plunge downward owing to the thermal effect of ionospheric heating on the traveling path of the enhanced Langmuir wave.In other word, when the pump sweeps above the 5th gyro-harmonic, the IS radar should observe the enhanced Langmuir wave at an lower altitude, where the low electron density can compensate the remarkably enhanced electron temperature so that the Bragg condition can be satisfied, as expected by the dispersion relation of Langmuir km are successively given, which lie in the frequency range of -6.7 MHz --7.25 MHz.One can find that those HFPLs in the GB and HB lie at frequency 4 Ann.Geophys.Discuss., https://doi.org/10.5194/angeo-2019-23Manuscript under review for journal Ann.Geophys.Discussion started: 14 March 2019 c Author(s) 2019.CC BY 4.0 License.HF 9.45kHz f  as the expected decay line from the PDI.In the GB, those strong HFPLs of up to ~ 1 occur at an altitude of 201.45 km in the 1st heating cycle, at an altitude of 210.25 km in the 2nd heating cycle and at an altitude of 207.32 km in the 3rd and 4th heating cycles respectively.In the HB, however, those strong HFPLs of up to ~ 1 descend in altitude, that is, they are located at an altitude of 198.52 km in the 1st heating cycle, at altitudes of 207.32 km and 204.39 km in the 2nd heating cycle, at an altitude of 204.39 km in the 3rd and 4th heating cycles.On the other hand, in the LB, the HFPL has not appeared at any of those altitudes due to the absence of the PDI and OTSI (Wu et al., 2019).

FrequencyFigure 1 .
Figure 1.The plasma lines versus HF f (the heating cycles), where the 1st panel is for an altitude of 210.25 km, the 2nd panel for 207.32 km , the 3rd panel for 204.39 km, the 4th panel for 201.45 km, the 5th panel for 198.52 km, the 6th panel for 195.58 km and the 7th panel for HF f (the heating cycles), successively from top to bottom.

Figure 2 T
Figure 2 gives the altitude profile of e e 0 T T as a function of HF f , where is the electron temperature and the undisturbed electron temperature.At an

Figure 2 .TFigure 3
Figure 2. e e 0 T T versus HF f (the heating cycles), where is obtained by averaging the electron temperature over the final 5 minutes of the UHF radar observations at 14:25 UT -14:30 UT. e0 T

Figure 3 N
Figure 3. e e N N 0 versus HF f (the heating cycles), where is obtained by averaging the electron density over the final 5 minutes of the UHF radar observations at 14:25 UT -14:30 UT. e0 N

FigureFigure 1
Figure 4 respectively gives the profiles of 2 2 L p e    . Discuss., https://doi.org/10.5194/angeo-2019-23Manuscript under review for journal Ann.Geophys.Discussion started: 14 March 2019 c Author(s) 2019.CC BY 4.0 License.large gradient profile of , an somewhat enhancement in may lead to anremarkable descent in the HFPL altitude.Moreover, if a small gradient profile of is considered, that is, can be approximately considered as a constant, then will be mainly determined by the profile of . in the HFPL altitude induced by the pump near the 5th