A powerful high-frequency (HF) electromagnetic wave transmitted from the ground into the ionospheric F region stimulates the strongest plasma response on long timescales in the direction antiparallel to the geomagnetic field in the Northern Hemisphere as seen from the HF transmitter. This magnetic zenith effect has been observed in several ways for a range of pump frequencies in experiments at high and mid latitudes.

In experiments with EISCAT (European Incoherent SCATter association) high-power HF facility Heating in Norway in 1999, pump-induced optical emissions were imaged unambiguously for the first time . presented tomography-like estimates of the volume distribution of the 630.0 nm emissions from these experiments and found that the emissions intensified and self-focused towards magnetic zenith during the 4 min pumping. observed that while the HF beam was directed vertically, the region of maximum optical emissions was displaced towards magnetic zenith as seen from EISCAT Heating. The authors also noted that published data of coherent HF radar scatter off geomagnetic field-aligned density irregularities tend to maximize in the magnetic field-aligned direction.

Radio tomography and scintillations using amplitude and phase measurements on the ground of VHF signals from orbiting satellites were used to study HF pump-induced electron density modifications in experiments with the mid-latitude Sura HF facility in Russia. Small-scale filamentary magnetic field-aligned plasma density irregularities were found to be strongest in magnetic zenith, both when the Sura beam was vertical and at an angle in between the vertical and magnetic zenith . Further, initial experiments with the Sura HF beam directed either 12∘ south of vertical or 16∘ both showed the strongest optical emissions at 630.0 nm near magnetic zenith at 18–19∘ south .

scanned the EISCAT Heating beam between three elevations from vertical to near magnetic zenith and found that electron temperature enhancements were almost always strongest in the magnetic zenith position. When the EISCAT UHF radar was scanned between the same positions, the strongest electron heating was always observed near magnetic zenith. In addition, optical emission at 630.0 nm was localized near magnetic zenith and HF coherent radar scatter off geomagnetic field-aligned density striations maximized when the Heating beam was in magnetic zenith. too observed the strongest field-aligned density striations when the Heating beam was in magnetic zenith.

examined the temporal evolution of the magnetic zenith effect as observed in the electron temperature measured by the EISCAT UHF radar. The beams from the Heating facility and the UHF radar were alternatively directed vertically and in magnetic zenith. Maximum temperature enhancements were observed when both the Heating and radar beams were in magnetic zenith. Further, these electron temperature enhancements reached a stationary state already within 10 s after pump-on in the 60 s on–90 s off pump cycle.

The magnetic zenith effect in optical emissions has also been observed in experiments with the HAARP (High frequency Active Auroral Research Program) facility in Alaska, USA . Further, determined the optical emission production efficiency as a function of angle by HF beam-swinging experiments. The maximum emission efficiency occurred exactly in the geomagnetic field-aligned position.

observed self-focusing of the pump beam in magnetic zenith in experiments at HAARP. The pump-induced optical emissions at 557.7 nm collapsed from a cone of approximately 22 to 9∘ within tens of seconds after pump-on while cycling the pump 60 s on–60 s off.

In the present treatment we report experimental results on the magnetic zenith effect obtained with the EISCAT Heating facility . The F-region plasma response to the HF pumping was observed with the EISCAT UHF incoherent scatter radar that was scanned in steps of either 1.0∘ or 1.5∘ around magnetic zenith to measure the electron temperature and other plasma parameter values. Nonlinear least squares analysis was used to fit electron temperature profiles obtained from integrating the electron energy equation with a parameterized heat source to measured plasma parameters, taking into account heat conduction, electron heating and cooling. The analysis gave the electron heating rate as a function of altitude and elevation angle. The results are consistent with the pump wave propagating in the L mode in magnetic zenith and in the O mode at angles deviating from zenith.

The EISCAT Heating experiments were performed during daytime in November 2014 and October 2017. The Heating facility transmitted a left-handed circularly polarized wave (LHCP, often referred to as O mode) in a beam directed towards magnetic zenith (∼78∘ elevation south) and cycling 150 s on–85 s off. The Heating beam width at -3 dB was ∼6∘. The term “left-handed” is defined with reference to the geomagnetic field direction: the electric field rotates in the opposite sense to the gyromotion of electrons.

Plasma parameter values in the F region were obtained with the EISCAT UHF incoherent scatter radar. The radar measurements utilized the Beata modulation scheme which includes a 32 bit binary alternating code with a baud length of 20 µs. The UHF radar beam was scanned in steps of 1.0∘ in the experiment in November 2014 and in steps of 1.5∘ in October 2017, between eight elevations around magnetic zenith in the plane containing the vertical and with a duration of 5 s in each position. The radar beam width was ∼0.5∘. The pump cycle of 150 s on–85 s off enabled appropriate coverage of the radar measurements throughout the pump-on time, so that after several pump cycles under stable ionospheric conditions the temporal evolution during the pumping could be obtained at all elevations. The radar data analysis provided 5 s temporal resolution and 15–20 km range resolution, depending on the range.

Figure  displays measured height profiles for the electron concentration (Nẽ), electron temperature (Tẽ) and ion temperature (Tĩ) for the experiment on 25 November 2014 (the tilde denotes measured parameters as opposed to modelled ones). For this case the pump frequency is f0=6.30 MHz, which is approximately half way between the fourth and fifth electron gyroharmonics in the F region. The transmitted power was 818 kW. For some unknown technical reason, Heating did not transmit a circularly polarized wave during this experiment: the effective radiated power (ERP) was 242 MW in LHCP and 157 MW in right-handed circular polarization (RHCP), assuming a perfectly conducting ground. However, electron heating effects from pumping with LHCP dominate over those with RHCP. estimated the height-integrated heating source for O-mode pumping to be approximately a factor of 3 larger than for X-mode pumping, for a pump duty cycle of 50 % and the O-mode pump frequency not near an electron gyroharmonic, with the X-mode frequency near a gyroharmonic (their Fig. 5), however. In addition, the ERP in our experiments for LHCP was larger than for RHCP. We therefore consider the measured heating effects to be representative of pure LHCP pumping.

The Nẽ profile in Fig. a is stable throughout the displayed time interval and does not show modulations due to the pumping. However, the Tẽ profile in Fig. b exhibits clear pump-induced modulations. The HF pumping is marked by white boxes and the red zigzag line indicates the radar elevation scan. The Tĩ shown in Fig. c only exhibits weak pump-induced modulations. The ionospheric conditions and response to the HF pumping in the experiments on 24 October 2017 were similar to those shown in Fig. . However, whereas for 2014 the ionospheric critical frequency foF2 was near 8 MHz, well above f0, foF2 was near f0 in 2017.

The used pump cycle in combination with the radar scan cycle enabled measurement of the temporal evolution of the ionospheric parameters at all radar elevation angles throughout the pumping. Figure  shows the temporal evolution of height profiles of Tẽ for the different elevation angles of the UHF radar, starting at t=5 s after pump-on for the experiments on 25 November 2014. Such measurements require reasonably stable ionospheric conditions during several pump cycles (see Fig. ), as the radar, scanning eight elevations between 75.2 and 82.2∘, samples the interaction region at a given elevation at different times after pump-on in different pump pulses. With measurements at sufficiently many pump pulses the temporal evolution can then be traced throughout the duration of pump-on (t=0–150 s) at all elevations.

As seen in Fig. , Tẽ enhancements occurred already within the first few seconds of pump-on. The high Tẽ around 300 km in the first 5 s data dump is likely not real but due to HF enhanced ion acoustic lines on the topside ionosphere. During the following few tens of seconds Tẽ was further enhanced at all elevations and in a wider altitude range. Notice also the slow conduction of electron heat toward increasing altitudes with time, up to 300–400 km altitude, as can be seen in Fig.  too. The strongest Tẽ enhancements occurred at the elevations 77.2 to 79.2∘, around magnetic zenith (∼78∘). This is also where the Tẽ enhancements extended toward the highest altitudes. Differences in the enhanced Tẽ profiles can be discerned even though the radar elevation changes by only 1.0∘.

Figure  displays Tẽ height profiles versus time for the experiment on 24 October 2017. As for Fig. , the high Tẽ around 300 km in the first 5 s data dump is likely not real. In this experiment f0=6.2 MHz, which again is approximately half way between the fourth and fifth electron gyroharmonics in the F region. The transmitted power was 734 kW and the ERP was 471 MW (LHCP). The radar was scanned in steps of 1.5∘ from 74.56 to 85.06∘, which is a larger range of elevations than that covered by the 1.0∘ steps in Fig. . Tẽ enhancements due to the HF pumping again occurred already in the first 5 s radar data integration after pump-on and Tẽ was the highest at elevations 77.56 and 79.06∘, closest to magnetic zenith (∼78∘). The gaps in the plots are because the ionospheric conditions were not stable long enough to give sufficient data to obtain the full temporal evolution at all elevations. However, the results for Tẽ are similar to those in Fig.  for the experiments on 25 November 2014.

To get information on the source that underlies the observed electron temperature enhancements, we model the electron heating rate through the fluid equations . As the measurements of the UHF incoherent scatter indicate no major pump-induced effects in Nẽ and Tĩ (Fig. ), the fluid equations can be reduced to the electron energy equation: 132NekB∂Te∂t+(ve⋅z^)∂Te∂z+NekBTe∂∂z(ve⋅z^)=∂∂zκe∂Te∂z+QHF+Qe-Le, where Te(z,t) is the modelled electron temperature, z^ is the unit vector in the direction of the geomagnetic field, kB is Boltzmann's constant, κe(Te,z,t) is the electron heat conductivity, QHF is the HF pump wave energy deposition to the electrons, Qe(z,t) is the background electron heating rate (mainly from photoelectrons), and Le(Te,z,t) is the electron cooling rate due to elastic and inelastic collisions with ions and neutrals.

With negligible plasma drift along the geomagnetic field as measured with the UHF radar, the convective terms in Eq. () can be neglected, giving 232NekB∂Te∂t=∂∂zκe∂Te∂z+QHF+Qe-Le. The heating rate of the electrons due to the electromagnetic pump wave consists of two parts: 3QHF=QΩ+QAA, where QΩ is the ohmic heating due to collisional damping of the pump wave and QAA is the heating due to the anomalous absorption of the wave associated with the excitation of, for example, upper hybrid turbulence and associated small-scale density striations. The ohmic heating rate is the time-averaged product of the pump electric field E0 and induced electric current σijE0, where σij is the conductivity tensor: QΩ=(1/2)Re[E0*⋅(σijE0)] . At the relatively high ERP levels used in the experiments, QAA gives the dominating contribution to QHF and may be several times larger than QΩ .

In the present treatment we obtain a model Te(z,t) of the observed Tẽ(z,t) by integrating the electron energy Eq. (). The electron heating rate QHF(z,t) due to the HF pumping is modelled by a one-dimensional and asymmetric Gaussian along the geomagnetic field. QHF(z,t) has its maximum Qm at range z0 and has independent upper (σu) and lower (σl) half-widths : 4QHF(z,t)=Qmexp⁡-(z-z0)2/σl2×1-exp⁡-(t-ton)/τ,z<z0,Qmexp⁡-(z-z0)2/σu2×1-exp⁡-(t-ton)/τ,z≥z0, where ton≤t≤toff is the time during which HF pumping occurs. This leads to a parameter estimation problem in the model parameters Qm, z0, σl, σu, and τ that we solved by weighted nonlinear least squares: 5parHF=argmin∑Tẽ(z,t)-Te(z,t,parHF)σTẽ2, where Te(z,t,parHF) is obtained by integrating Eq. () with QHF(z,t,parHF) and σTẽ is the standard deviation of the observed electron temperature.

When integrating Eq. () we used the observed range profiles for Tĩ and Nẽ as they evolve in time at each elevation. For example, Le depends on both Ti and Ne and both the left-hand side of Eq. () and κe depend on Ne. As the initial condition we took a smoothed Tẽ range profile measured just before pump-on. Further, we used mixed boundary conditions, taking at the lower boundary Te=Tẽ(z=150km,t) as given by the UHF radar measurements at z=150 km slightly before pump-on at t=ton and at the upper boundary ∂Te/∂z(z=500km,t)=0. The fixed temperature at the lower boundary follows from the observations with the additional theoretical justification that at such low altitudes Te and Ti are both approximately equal to the neutral temperature due to the high collision frequencies. The upper boundary condition too is based on the observations and corresponds to a balance between upward heat flux out from the ionosphere and downward heat flux from the magnetosphere into the ionosphere.

The temporal evolution of the modelled Te altitude profile for the elevation angles scanned by the radar is obtained by integrating Eq. () with the optimal parameters for QHF(z,t). The results are shown in Figs.  and , which correspond to the measurements in Figs.  and , respectively. Te(z,t) is enhanced for all elevations already within the first seconds after pump-on at t=0 s. Slow conduction of the electron heat is seen both upward and downward in altitude and Te(z,t) reaches the highest values near magnetic zenith (∼78∘). The modelling results in Figs.  and agree qualitatively with the measurements in Figs.  and , respectively.

Figure  displays the corresponding QHF versus elevation angle for the experiment on 25 November 2014 (Fig. ) in panels (a) and (b) and for 24 October 2017 (Fig. ) in panels (c) and (d). Figure 6a and c show the column-integrated QHF (blue) as well as the profile of the transmitted Heating beam (red) and Fig. 6b and d depict the altitude profiles of QHF. These modelling results are for the case after that steady state was reached in the 150 s pump-on period. The white and black lines in Fig. 6b and d show the altitudes of the plasma and upper hybrid resonances, respectively, as obtained from the ion and plasma lines. The altitude separation between the two resonances is larger in Fig. 6d for which f0 was near foF2 than in Fig. 6b for which f0 was well below foF2.

The column-integrated QHF in Fig. 6a is maximum at 78.2∘ and in Fig. 6c at 77.5∘, which are the same elevations at which the observed electron temperature reached the highest values (Figs.  and , respectively). Also, the column-integrated QHF is maximum at the elevation closest to magnetic zenith (∼78∘, labelled by MZ in the plots).

Further, as seen in Fig. a and c, the angular extent of the QHF profile is smaller than that of the Heating beam. QHF follows the profile of the Heating beam at elevations lower than magnetic zenith (∼78∘), while at higher elevations it is more confined to magnetic zenith than the Heating beam. For reference, the Spitze angle is about 6∘ from the vertical or at about 84∘ elevation.

As seen in Fig. b and d, the altitude profile of QHF is generally asymmetric, with QHF decreasing steeply with increasing altitude above the maximum and declining more gradually with decreasing altitude below the maximum. Only in Fig. 6d for 77.5∘ (at magnetic zenith) does QHF decrease more slowly toward high altitudes than toward lower altitudes. Also, QHF decreases steeply for increasing elevations beyond about 80∘, towards the vertical. This decrease in QHF with increasing elevation is steeper than what would be expected from the point view of the width of the Heating beam in vacuum (see Fig. 6a and c).

It is notable that QHF reaches larger values at the two elevations next to magnetic zenith compared to at the elevation nearest to magnetic zenith. In Fig. b, QHF is slightly higher at 77.2 and 79.2∘ than at 78.2∘, while the QHF profile is more extended in altitude at 78.2∘. Figure d shows larger differences, with QHF higher at 76.0 and 79.0∘ than at 77.5∘, while the QHF profile is more extended in altitude at 77.5∘. Thus, despite the maximum of the QHF profile being slightly lower at magnetic zenith compared to at the two nearest neighbouring elevations, the column-integrated QHF is maximum at magnetic zenith (Fig. a and c) for both experiments.

We have presented experimental and modelling results concerning electron heating and the ionospheric plasma response to HF pumping near magnetic zenith. The experiments were performed with the EISCAT Heating facility and measurements of the plasma response were done with the EISCAT UHF incoherent scatter radar. The Heating beam was tilted in the magnetic zenith direction and the UHF radar was scanned between eight positions around this direction to study the electron-heating efficiency. The electron heating rate QHF(z,t) and associated electron temperature Te(z,t) due to the HF pumping were modelled by integrating the energy Eq. () and fitting the model parameters with respect to the measurements of Nẽ, Tẽ and Tĩ.

Differences in the plasma response were observed for radar elevations differing by only 1∘ (Fig. ). The pump-induced measured Tẽ(z,t) enhancements (Figs.  and ), the modelled Te(z,t) (Figs.  and ) and the associated column-integrated QHF (Fig. ) were all found to maximize in the magnetic zenith direction (∼78∘ elevation). Further, the angular width of the QHF profile, with a full width at half maximum (FWHM) of about 4∘ around magnetic zenith, was less than that of the HF beam, which suggests that some focusing of the Heating beam occurred.

obtained the angular distribution of the optical emission production efficiency by HF beam-swinging experiments at HAARP. The optical emission production efficiency peaked at magnetic zenith with a FWHM of 7∘, for which the HAARP beam width and many other experiment variables were accounted. This FWHM of 7∘ is larger than the two cases for QHF in Fig. . The HAARP experiments used an ERP of 32.1 MW at 2.83 MHz and 42.4 MW at 3.3 MHz, thus, both lower ERP and lower f0 than in the present EISCAT experiments. It is plausible that self-focusing effects were larger at the higher ERP in the present experiments, which could give a narrower region of pump-induced enhancements.

It has been proposed that filamentary plasma density ducts can guide a transmitted LHCP wave, entering the ionosphere in the O mode, as an L-mode wave along the geomagnetic field . The L mode is an LHCP electromagnetic wave mode with the wave vector parallel or anti-parallel to the ambient magnetic field. For a homogeneous and cold magnetized plasma the refractive index (n∥) parallel to the ambient magnetic field is given by n∥2=1-fp2/f(f+fe), where fp is the electron plasma frequency and fe is the electron gyrofrequency. Whereas the O mode has a cutoff at fp=f0, the L mode has the cutoff frequency fL=-fe/2+(fp2+fe2/4)1/2, which corresponds to fp≈f0+fe/2 for fp2≫fe2 for a pump wave at frequency f=f0. Thus, an electromagnetic wave in the L mode can propagate at higher plasma densities than in the O mode.

L-mode propagation can occur when the background plasma density gradient near the plasma resonance is parallel to the geomagnetic field, instead of the density gradient for example being vertical as in a horizontally stratified ionosphere. Such a condition with the density gradient being magnetic field-aligned can occur in density ducts, either natural or pump-induced. In the L mode the pump wave can propagate upwards, passing through the plasma resonance on its way to the cutoff at fp≈f0+fe/2 if the plasma is sufficiently dense. With its perpendicular electric field, strong pumping of upper hybrid phenomena localized in small-scale density striations and related anomalous electron heating can occur at higher altitudes and deeper into the plasma compared to the case of an O-mode wave which therefore could contribute to the strong plasma response observed in magnetic zenith .

The HF pump-induced electron heating rate QHF obtained for our experiments exhibited an interesting dependence on the elevation angle near magnetic zenith (Fig. ). QHF is maximum at the elevations next to magnetic zenith. At magnetic zenith, the QHF profile is more extended in altitude, such that the column-integrated QHF is maximum in this direction. These results are consistent with the pump wave propagating in the L mode in magnetic zenith and in the O mode at angles deviating from the zenith direction. As a wave in the L mode propagates to higher altitudes than in the O mode, electron heating can occur in a more extended altitude range for L-mode propagation, thereby giving maximum column-integrated electron heating in magnetic zenith. The large difference in the QHF profile between magnetic zenith and the adjacent elevations is consistent with that the pump wave in the O mode has a much lower reflection height than in the L mode; the O-mode reflection height is well below the plasma resonance for elevations near magnetic zenith. In magnetic zenith the pump wave is guided by magnetic field-aligned density ducts in the L mode, but at the adjacent elevations the pump wave makes too large an angle to the magnetic field for trapping of the HF wave in the duct, and thus guiding, to occur so that instead the pump wave propagates in the O mode.

Evidence of L-mode propagation of the EISCAT Heating beam has previously been obtained as transionospheric propagation for f0<foF2<f0+fe/2, in which case an L-mode wave would not be reflected but pass through the ionospheric plasma density peak. This was observed by direct measurement on the CASSIOPE spacecraft and indirectly by EISCAT UHF radar observations of ion acoustic lines in the topside ionosphere .

Figure  also displays the altitude of the plasma resonance (white lines in Fig. 6b and d). The position of the QHF profile relative to the plasma resonance is not fully understood. In Fig. b, QHF is maximum slightly above the plasma resonance at magnetic zenith. This is consistent with a pump wave in the L mode being able to propagate well above the plasma resonance, whereas an O-mode wave cannot. Further, an L-mode wave has its electric field perpendicular to the geomagnetic field all the way up to its reflection height, so that pumping of upper hybrid turbulence can occur in an extended altitude range. In the O mode, on the other hand, the electric field turns to parallel to the geomagnetic field close to the reflection height, which favours excitation of Langmuir turbulence that generally causes less electron heating than upper hybrid turbulence.

However, in Fig. d all electron heating appears to occur well below even the upper hybrid resonance height (black line). In this case foF2 was near f0, whereas for Fig. 6b it was well above f0, which is consistent with that the altitude separation between the plasma and upper hybrid resonances is larger in Fig. 6d. We do not have any explanation for why electron heating seemed to occur at such low altitudes in this case.

developed a theory for self-focusing of the electromagnetic pump wave propagating in the O mode on geomagnetic field-aligned density striations. An important mechanism in the nonlinear pump beam self-focusing is the trapping of pump rays near the magnetic zenith direction in the large-scale density depletions within the beam, as previously was found in numerical studies . The results were shown to be consistent with observations of pump-induced optical emissions at HAARP . However, the possibility of propagation of the pump wave in the L mode, deeper into the plasma than what is possible in the O mode, was not considered. Whereas the nonlinear self-focusing of the pump beam is an important mechanism, particularly for guiding the pump beam in magnetic zenith, it does as it stands not seem to account for the difference that we have found in the altitude distribution of the electron heating rate in magnetic zenith compared to that just about 1∘ away from this direction (Fig. ). We therefore suggest that such theories for self-focusing are developed to include the possibility of L-mode propagation.

The EISCAT Heating facility was used to pump ionospheric F-region plasma by cycling 150 s on–85 s off with an LHCP HF beam directed in magnetic zenith. Plasma parameter values were measured with the EISCAT UHF incoherent scatter radar that was scanned in steps of 1.0∘ (November 2014) and 1.5∘ (October 2017) in elevation around magnetic zenith. The temporal evolution of the electron temperature profile was modelled by integrating the electron energy equation, which was used to fit the measured plasma parameter values with a model electron heating rate.

The observed electron temperature enhancements and the associated column-integrated electron heating rate and modelled electron temperature all exhibit maxima in magnetic zenith. In addition, the altitude range of electron heating is more extended in magnetic zenith than for elevations deviating from the zenith direction. These results are consistent with pump wave propagation in the L mode rather than purely O mode and suggest the importance of L-mode propagation for understanding magnetic zenith effects.