Presentations and discussions at the workshop highlighted the particular potential of ionospheric modification experiments to advance understanding of the mesosphere-lower thermosphere (MLT) and thermosphere regions of the atmosphere, with which the ionosphere is collocated. It was noted that the state parameters of the neutral gas in this region are difficult to measure with ground-based instruments, and the measurements that are possible are often poorly resolved in range or time, or available only within narrow altitude ranges. In particular, a workshop participant noted that neutral winds and densities in the thermosphere are poorly specified, introducing uncertainty into virtually all lines of theoretical investigation in aeronomy. Moreover, because neutral winds and densities control satellite drag, their poor specification was said to have important operational consequences.
It was noted by a participant that ionospheric modifications can also be used to explore coupling between neutral and charged species, because the radio frequency (RF) directly pumps the electrons, but neutral species are responsible for the subsequent relaxation back to radiative and chemical equilibrium. Further, it was asserted that heating experiments can explore this coupling more systematically and over a broader range of conditions than would be otherwise possible. Plasma-neutral coupling is one of the central themes of the National Science Foundation (NSF) Coupling, Energetic, and Dynamics of Atmospheric Regions (CEDAR) program.
Diagnostics generally monitor how the upper atmosphere responds to both RF heating and to its cessation. Ground-based instruments measure airglow, scattered radio signals, and stimulated radio signals from the modified volume. This information allows estimates of neutral winds, densities, temperatures, composition, plasma drifts, and rates of diffusion and cooling.
At the workshop, the utility of ionospheric modification for understanding natural aeronomic processes was said to be exemplified by research into polar mesospheric summer echoes (PMSEs). PMSE refers to coherent radar scatter from thin layers in the polar summer mesosphere where charged dust particles and ice crystals associated with polar mesospheric clouds are present. The echoes arise from fluctuations in electron density driven by neutral atmospheric turbulence. It was noted by a workshop participant that the puzzle of how electron density fluctuations at small scale sizes could be sustained in the presence of ordinary ambipolar diffusion was resolved with the help of heating experiments.
In these experiments, an overshoot in echo intensity was consistently observed after heater turn-off (Havnes et al., 2003; Havnes, 2004). The overshoot led to a belated appreciation of multi-polar diffusion, a fundamental process in multi-component plasmas. Mahmoudian et al. (2011) accounted for
1 This section includes recent research cited by one or more participants as being particularly noteworthy.
the time history of the echoes by modeling temperature-dependent multipolar diffusion, charging, and recombination processes. According to one workshop participant, the result substantially advanced understanding of mesospheric turbulence, chemistry, and transport.
Sporadic E-Layer Patches
Another example cited at the workshop in which ionospheric modification facilitated discovery science in aeronomy concerns the morphology of sporadic E ionization layers. These layers, which were evident in the earliest days of radio, are known often to be non-blanketing and patchy, but observers note that conventional remote sensing instrumentation affords no easy way to image their horizontal structure. Bernhardt et al. (2003) introduced an imaging method, termed “radio-induced aurora,” or RIA, that combines RF heating with optical imaging.
Paul Bernhardt and others at the workshop discussed how ionospheric modification induces enhanced optical emissions via induced electron impact on neutral species that can be used as a diagnostic of background aeronomic processes. Both thermal and suprathermal processes are believed to be at work, with red-line excitation through direct electron heating and green-line excitation through electron acceleration. The red-line emission has a lower excitation energy and dominates green-line emission at F-region altitudes. Since the red-line emission is collisionally quenched at E-region altitudes, however, the green line dominates there.
RIA involves emitting pump-mode radiation at a frequency below the F-region critical frequency. Where there are no sporadic E-layer patches, the radiation propagates into the F region and produces red-line emissions at the F-region interaction height. Where there are sporadic E-layer patches, however, the pump-mode radiation interacts in the E layer. Not only does this produce gaps or “shadows” in the red-line emissions, it also produces green-line emission at E region altitudes. Bernhardt et al. (2003) used this technique to reveal kilometric structure in sporadic E layers over Arecibo.
Neutral Wind and Diffusion
Presentations at the workshop included references to combining RF heating with airglow imaging as a way to measure the neutral wind and diffusivity in the thermosphere. This method involves observing clouds of metastable O(1D) atoms over the heater after heating is discontinued. During this time, these atoms drift with the neutral wind and spread under the influence of diffusion, all the time decaying by radiation and collisional quenching. Using airglow imagery of bright, distinct clouds created by ionospheric modification, Bernhardt et al. (2012) estimated the drift velocity, diffusion rate, and quenching rate of the O(1D) atoms, in effect using ionospheric modification to perform a kind of repeatable chemical-release experiment.
HAARP-Induced Ionization (“Artificial Ionization”)
As noted in Chapter 1, Todd Pedersen and colleagues (Pedersen et al., 2010) recently demonstrated the capability of the High Frequency Active Auroral Research Program’s (HAARP’s) 3.6-MW transmitter to produce significant artificial plasma in the upper atmosphere. In addition to their fundamental interest, it was pointed out that these results could have important practical implications in communication and over-the-horizon radar as they present the possibility of creating long-path propagation channels on demand that would be otherwise unavailable. The artificial “layers” (the structure of these enhanced ionization regions is an area of current study) are also highly turbulent and so could be used to inhibit as well as promote long-path radio-wave propagation, depending on the desired application.
Summarized below from workshop presentations, and with additional references added for clarity, are what some participants described as the most promising techniques for basic and applied research into the MLT, thermosphere, and ionosphere using ionospheric modification.
At the workshop, Elizabeth Kendall described NSF-funded research that has shown—using techniques that are at a research stage and not routine measurement by any means—that ionospheric modification experiments coupled with airglow observations can also be used to estimate neutral density. The long radiative lifetime of O(1D) means that it is controlled by collisional deactivation (quenching). The quenching rate, in turn, varies with the density of the neutral species in the thermosphere, so, in principle, it is possible to estimate density profiles. The technique requires optical triangulation and the ability to discern emissions from different altitudes. Gustavsson et al. (2001) used a tomographic approach to estimate the decay time of O(1D) as a function of altitude. Kalogerakis et al. (2009) went further, using the methodology to estimate density profiles of atomic oxygen, which they found dominates the quenching rate above about 200 km.
A workshop participant noted that for all of the methodologies under discussion, knowledge of the ionospheric interaction height is essential for accurate data interpretation. Ionospheric density profiles provided by an incoherent scatter radar (ISR) were said to be the most accurate source of this information. Moreover, electron temperature profiles from an ISR, being diagnostic of electron heating and acceleration, were said to provide vital context for interpreting artificial airglow data. Thus, this researcher stated, quantitatively accurate neutral density profiles at HAARP will most likely require a collocated ISR.
Winds and Temperatures
At the workshop, it was noted that several researchers have suggested a technique for studying the background ionosphere and thermosphere via artificial periodic inhomogeneities (APIs) (Belikovich et al., 1975; Fejer et al., 1984; Rietveld et al., 1996; Djuth et al., 1997; Bakhmet’eva and Belikovich, 2007). In this technique, high-power polarized (X- and O-mode) heating is used to induce very weak variations in the ionospheric index of refraction that follow the structure of the heating standing-wave pattern. Horizontally stratified, vertically periodic structure is thus induced at altitudes from the D region (starting at about 50 km) through the reflection height. Inhomogeneity is created by a combination of ponderomotive and thermal forcing and by photochemical effects, and possibly additional processes.
Once created, the ionospheric structure is diagnosed using HF sounding at frequencies calculated to match to the probe signal to the pump standing-wave pattern. Probing can be done using the same frequency and polarization as the pumping, in which case matching occurs at all heights. It can also be done with the opposing polarization at a different frequency, in which case matching only occurs over a narrow range of heights. The research results note that the advantage of the latter method is that probing and pumping can occur simultaneously, which is generally necessary in the E and F regions where the inhomogeneity decays rapidly.
The decay of the structuring is measured as a function of altitude and in the E and F regions is generally indicative of the ambipolar diffusion rate. Below that, turbulent mixing and photochemistry dominate. From the decay time constant, the electron density and temperature and the neutral density can be estimated. Research has shown that the measurement is straightforward, requires no true-height inversion, and it works in the valley region between the E and F region density maxima, as well as the
main E and F regions themselves. In the D region, electron number density can be measured, and complex photochemical and dynamical processes can be investigated.
Working at EISCAT (European Incoherent Scatter Scientific Association), Rietveld et al. (1996) also examined the Doppler shift of the API backscatter and tentatively associated it with the vertical neutral wind in the D and lower E regions. They reported small (few m/s), zero-mean winds with signs of gravity wave fluctuations. Many workshop participants agreed that this is a remarkable result that holds the promise of very accurate vertical wind measurements in daytime in over a range of altitudes not readily probed by other means. However, it was also noted that implementing the API technique at HAARP would require HF radar capability, either through the use of a collocated system or through the upgrade of HAARP itself.
Diffusion and Cooling Rates and E × B Drifts
In discussions at the workshop, it was noted that coherent radar scatter from induced plasma density irregularities provides another diagnostic of background parameters in the MLT and thermosphere. A signature feature of ionospheric-modification experiments is the generation of field-aligned plasma density irregularities (FAIs) (Hysell, 2008, p. 117). As noted in Hysell and Nossa (2009), “The irregularities are generated mainly by thermal parametric instabilities (Grach et al., 1978; Das and Fejer, 1979; Fejer, 1979; Kuo and Lee, 1982; Dysthe et al., 1983; Mjølhus, 1990) and, having entered nonlinear stages of development, by resonance instability (Vas’kov and Gurevich, 1977; Inhester et al., 1981; Grach et al., 1981; Dysthe et al., 1982; Lee and Kuo, 1983; Mjølhus, 1993)” (p. 2711). At the workshop, Herbert Carlson and others stated that these irregularities were interesting in their own right and because they also provide bright, regular targets of opportunity for study by coherent scatter radars. Most research has concentrated on F-region FAIs, although irregularities can be generated in the E region by pump waves with low enough frequency.
An example of coherent radar backscatter from E-region FAIs generated over the HAARP facility was provided by David Hysell and is shown in Figure 2.1. In this experiment, the heating pump power was ramped upward and then downward, allowing the determination of the threshold pump electric field required for initiation of thermal parametric instability. This threshold is a function of a number of parameters, including the inelastic electron cooling rate in the E region (Dysthe et al., 1983; Hysell et al., 2010). Because the altitude of the irregularities (where the heating frequency matches the local upper hybrid frequency) is well known, it is possible with this experiment to determine the electron cooling rate accurately as a function of altitude.
Whereas FAIs are created using O-mode heater emissions, research was reported at the workshop that found that simultaneous X-mode emissions can be used simply to heat the modified region. This facilitates tests of the temperature dependence of ionospheric relaxation processes. In the experiment described above (Miceli et. al.; see Figure 2.1), X-mode heating doubled the O-mode power required to generate FAIs. The doubling in this case can be attributed to enhanced absorption caused by heating the D region to approximately 2000 K. The increase in absorption is due both to the temperature dependence of the electron-neutral collision frequency and to changes in electron density arising from temperature-dependent D-region photochemistry. Both processes can be studied and quantified through ionospheric heating.
Moreover, Hysell pointed out that the diffusion rate of FAIs can be studied by monitoring how the coherent radar echoes decline in intensity after the heater is turned off. The decay timescale depends on the probe radar wavelength but is of the order of 100 ms for very-high-frequency (VHF) radars in the E region and a few seconds in the F region. Remarkably, the decay of the irregularities has been found to follow two power laws—one fast, and one slow (Hysell et al., 1996). The faster rate is consistent with ambipolar diffusion and consequently affords another estimate of the temperature, electron-neutral, and ion-neutral collision frequencies and composition at the ionospheric interaction height.
FIGURE 2.1 Range-time intensity plot of coherent echoes from heater-induced field-aligned irregularities over HAARP. The true range to the echoes is the apparent range (shown) plus 370 km. The echoes shown arose from altitudes close to 100 km. Both X- and O-mode emissions were used during the first interval whereas only O-mode emissions were used during the second. SOURCE: Adapted from: R.J. Miceli, D.L. Hysell, J. Munk, M. McCarrick, and J.D. Huba, Reexamining X-mode suppression and fine structure in artificial E region field-aligned plasma density irregularities, Radio Science 48:482-490, doi:10.1002/rds.20054, 2013. Available at http://onlinelibrary.wiley.com/doi/10.1002/rds.20054/full. Courtesy of D. Hysell.
In addition to the backscatter power, reported research found that the Doppler shift of the coherent echoes can also be measured during ionospheric modification experiments. Because the echoes have a much longer correlation time than incoherent scatter, the Doppler shifts can be measured far more accurately in a short time. In the F region, the Doppler shifts are indicative of E × B drifts. Heating experiments, consequently, afford extraordinarily accurate measurements of ionospheric electric fields using SuperDARN-class and similar radars, even where natural ionospheric irregularities are not present.
A workshop participant noted that as with airglow experiments, experiments involving coherent scatter benefit enormously from accurate knowledge of the heating interaction height. This information was said to be most gainfully provided by a collocated ISR.
When micropulsations are present, the electric fields inferred from coherent radar scatter are indicative of the electric fields of the Alfvén waves that carry them. Such micropulsations are frequently observed over the Sura heater (Belenov et al., 1997). In what one participant described as a remarkable result, Sinitsin et al. (1999) measured the Doppler shifts of heater-induced FAIs over Sura at three different places along a single magnetic flux tube and found that the three signatures could not be accounted for by a single, shear Alfvén wave. Attributing the signatures to an incident shear wave, a reflected shear wave, and a reflected magnetosonic wave, they were able to infer the Pedersen and Hall conductances at the foot of the flux tube. The authors concluded that this unique experimental capability holds great promise for magnetosphere-ionosphere coupling studies because it has the potential to provide dynamic lower boundary conditions for magnetospheric models in real time.
It was noted repeatedly at the workshop that many important instabilities in the ionosphere can only be created by injecting a large amount of power, such as that available at HAARP. Further, although other instabilities occur naturally, some participants stated that they could be investigated more systematically than is otherwise generally possible using active experiments. An example of the latter kind is the Farley-Buneman instability, which occurs naturally under conditions of strong auroral forcing and creates waves that heat the electrons and modify the ionospheric conductivity. This instability is very sensitive to diffusion and collision rates and their temperature dependencies. Thus, some participants thought it could be possible to modify or even shut off the instability by increasing the E-region temperature with a heater like HAARP.
Another important heater-induced phenomenon is Langmuir turbulence, which has been investigated at Arecibo and EISCAT, where ISRs are available for diagnostics (Isham et al., 2012). As David Hysell explained, Langmuir turbulence enhances the plasma lines seen by ISRs above background levels through a variety of mechanisms, and understanding the effects offers insights into fundamental plasma physics, much in the same way that large colliders offer insights into fundamental particle physics. Langmuir turbulence also appears to generate suprathermal electrons, which cause much of the airglow seen during heating experiments (Djuth et al., 1999).
As noted at the workshop by Mike Taylor and others, the location of HAARP at a subauroral latitude is very interesting for studying the plasmasphere boundary layer and processes that lead to electromagnetic ion-cyclotron waves. At its location, they noted that HAARP can launch waves into this region and do stimulated experiments in radiation modification. It can also be used in studies involving highly elevated electron temperature, excited neutron gas, and the effects they produce, all of which would contribute to the understanding of the mechanisms that generate space weather in subauroral geospace.
Some participants claim that heaters could also potentially produce propagating responses in the ionosphere-thermosphere system, which could then be used to test sophisticated models of the ionosphere. Three-dimensional models have been used, for example, to describe how a localized disturbance at Arecibo would propagate in terms of ion acoustic waves, thermal pulse, density pulse, and so forth, all the way to the conjugate hemisphere, that could be detected by various instruments, especially ISR, at the conjugate point.
It was noted that many questions remain about the day-to-day variations in the ionosphere-thermosphere system and that there has been an increasing appreciation of the importance of “forcing from below,” thought to account for perhaps 20 to 30 percent of variability.2 Discussions among participants pointed to the use of heaters at other latitudes and HAARP to help understand how the complex system responds to such energy inputs. It was suggested that injecting energy into the ionosphere might also clarify the degree to which the ionosphere is involved in substorms and magnetospheric processes; for example, whether there are instabilities or other phenomena in the ionosphere that limit the electric fields or currents that can be carried in the magnetosphere-ionosphere system.
2 The importance of forcing from below was emphasized by the Panel on Atmosphere-Ionosphere-Magnetosphere Interactions, whose report to the decadal survey committee comprises Chapter 8 of the 2013 report Solar and Space Physics: A Science for a Technological Society (NRC, 2013).
Testing Models and Sensor Networks
Participants at the workshop considered how HF modification experiments could be used to provide quantitative parameter assessments for ionosphere-thermosphere models. One participant noted that measuring the response to a local heat input can validate the overall model behavior. Another participant observed that comprehensive study of the global coupled atmosphere will require distributed sensor networks (NRC, 2013), and it was asserted that HAARP could be used as a testbed to provide controlled perturbations for ground-based instrument development.
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