J. Geophys. Res., 102, 11,665-11,667, 1997
(Received July 16, 1996; revised January 15, 1997; accepted March 4, 1997.)
Copyright 1997 by the American Geophysical Union.
Paper number 97JA00746
In a recent paper, Cole and Hoegy  (hereafter CH) argue that the collisional Joule dissipation of electromagnetic waves may be important for the Venus ionosphere. In particular, they argued that the amount of heating expected for the wave amplitudes measured by the plasma wave experiment on board the Pioneer Venus Orbiter (PVO) would be so large that the waves could not be attributed to electromagnetic waves that are "planetary in scale" (their phrase). CH analyze Joule dissipation rates for two wave phenomena at Venus: the nightside whistler mode radiation that has been attributed to lightning [e.g., Russell, 1991] and the 100-Hz wave signal observed above the dayside ionopause [e.g., Strangeway, 1991]. In this comment I will address the dayside wave observations in CH, as the effects of Joule dissipation in the nightside are discussed in some detail by Strangeway . I will show that the data analyzed by CH are not representative of the region where the plasma wave activity is greatest, above the ionosphere. As a consequence, the analysis in CH has little to say about dayside wave phenomena.
In discussing the plasma waves at Venus, it is not clear what CH mean by "waves of planetary scale," particularly since they only reference early work on dayside observations. Specifically, the original whistler mode hypothesis of Scarf et al. , Taylor et al. , and Scarf et al. , whereby whistler mode waves supply heat to the topside ionosphere, did not survive later detailed quantitative tests [Szegö et al., 1991; Crawford et al., 1993; Strangeway and Crawford, 1993; Strangeway and Russell, 1996]. These later papers discuss the energy flow associated with the observed waves, and several of the papers point out that very little wave energy flows into the ionosphere. This is because the waves occur above the ionosphere in a region known as the plasma mantle [Spenner et al., 1980]. The mantle is a region of mixed ionospheric and magnetosheath plasma, and both the ambient magnetic field and flow are mainly tangential to the ionopause. Depending on the wave mode, the wave energy flux will tend to be field- or flow-aligned. Electromagnetic and quasi-electromagnetic waves (whistler mode or lower hybrid waves) will have their Poynting flux field-aligned, while acoustic modes will tend to have their energy flux flow-aligned [Strangeway and Crawford, 1993]. Most of the wave energy flux is hence tangential to the ionopause. Since collisional Joule dissipation is just another mechanism for absorbing wave energy, the same arguments apply to CH, and the waves should not be a significant heat source for the dayside ionosphere, regardless of the absorption mechanism. Why then do CH come to the opposite conclusion, going so far as to suggest that the observed signals may not even be waves in the standard sense?
The answer is simply that in their analysis, CH have restricted their data to altitudes for which electron data are available within the UADS database. (UADS is a database in which data from all the PVO instruments are gathered together with a common 12-s resolution.) The PVO Langmuir probe (or OETP) is designed to measure electron density and temperature within the ionosphere, and UADS data from the Langmuir probe are generally not available for altitudes above the ionosphere. However, Strangeway and Russell , following up on the initial work of Crawford et al.  and Crawford , have shown that the wave intensities peaks at a location known as the OETP ionopause, where the Langmuir probe on PVO measures a threshold density of 100 cm. Electron data acquired when the spacecraft is in sunlight are excluded from the UADS database for densities less than 100 cm, because of the effects of photoelectrons. Also, the electron temperatures tend to be higher at higher altitudes. Lower densities and higher temperatures result in smaller Joule dissipation rates and higher conduction cooling [Strangeway, 1996].
Because the available electron data were not representative of the ambient plasma at the OETP ionopause, Strangeway  used reasonable estimates of the ambient electron parameters (n = 100 cm, T = 1 eV, where n is the electron density and T is the electron temperature) in assessing the amount of Joule dissipation expected for the dayside waves. Since the mantle is a region of mixed magnetosheath and ionospheric plasma, 1 eV may in fact underestimate the electron temperature. Heat conduction cooling could easily balance the Joule dissipation, and thus collisional Joule dissipation is unimportant for the dayside, if the heat conduction scale length (L) was about 1000 km [Strangeway, 1996].
On the other hand, on calculating the scale lengths required for heat conduction to balance Joule dissipation, CH state that many events have scale lengths < 200 km, but inspection of Figure 3 in CH shows that only 7 of ~ 170 events (4%) have scale lengths less than 200 km, whereas ~ 120 (71%) have scale lengths > 1000 km. Thus, even in an unnormalized study, such as CH, heat conduction can easily balance Joule dissipation for a large majority of the wave events shown.
Figure 1. Median wave, plasma, and heating parameter profiles above the dayside Venus ionopause. (a) The median profile of the peak 100-Hz wave amplitude (E) and ambient plasma parameters, together with the percentage of wave data points that have Langmuir probe temperature measurements associated with them (%). The plasma parameters shown are the ambient magnetic field strength (B), the electron density (n), and the electron temperature (T). (b) The derived heating rates and associated parameters. The parameters shown in Figure 1b are the Joule dissipation rate (Q), the electron collision frequency (), the electron mean free path (), the heating time constant (t), and the heat conduction scale required to match the Joule dissipation (L). In both Figures 1a and 1b the medians are calculated for 50 km altitude bins, binned with respect to the OETP ionopause altitude.
By way of presenting a normalized study, Figure 1 shows wave and plasma parameters as a function of altitude with respect to the OETP ionopause. I have used UADS data from the first three PVO dayside periapsis seasons restricted to SZA < 90°, where SZA is the solar zenith angle. For clarity, I only plot the median profiles. Additionally, electron parameters, and parameters derived from electron quantities are only determined when such data are available, while all the plasma wave and magnetic field data are used even if no electron data are available. In computing heating rates I assume a 30-Hz bandwidth. Both CH and I use the peak 100 Hz wave amplitude in estimating heating rates. In the UADS data base the peak amplitude is determined every 12 s using a 24-s window. As such, we both overestimate the amount of Joule dissipation, as the average wave amplitude is often at least an order of magnitude smaller. I also note that the median heating rate computed using the peak amplitudes often exceeds the 90 percentile heating rate using the average wave amplitude, again indicating that we are both providing an upper limit to Joule dissipation rates.
Another point to be remembered in using peak wave amplitudes with averaged plasma parameters is that the plasma can change significantly over relatively short spatial scales. A single point-by-point comparison of UADS data will therefore have some uncertainty introduced, as the peak amplitude may in fact be associated with ambient plasma parameters significantly different from the average. This is especially the case at the ionopause where both the density and measured electric field amplitude (be it signal or noise) are changing rapidly, but in opposite directions, as can be seen from Figure 1. By using median profiles we will tend to minimize such effects.
The tendency to smooth over short scale structure when using averaged data, such as UADS, is an important point to consider when assessing the relative importance of the high heating events discussed by CH. Indeed, I would argue that it is this smoothing of scales that is largely responsible for the results within CH. Orbit 9, originally cited by Scarf et al.  as an example of wave absorption at the ionopause, but now presented by CH in their Table 2 as an example of heating within the ionosphere is a case in point. In the table, CH indicate that the density profile is relatively smooth, but Table 2 is somewhat misleading. First, both inbound and outbound data are included, there is a roughly 4-min gap between the first four entries in Table 2 (inbound), and the last five (outbound). Presumably, no excessive heating events were found for lower altitude data. Second, there is a local density minimum in the outbound leg at around 248 km altitude, where the density ~ 2300 cm, which is not included in Table 2. Thus the electron density is quite variable, suggesting that there are large density gradients which are smoothed through averaging. The underlying structure of the ionosphere associated with anomalous events should be investigated with higher-resolution data than UADS before rejecting all dayside electric field observations on the basis of these events.
I wish to make it clear that it has long been recognized that the electric field data include interference signals when the spacecraft is in sunlight. In reviewing the plasma wave data, Strangeway  noted that on the dayside the strong interference made discrimination between signal and noise difficult. However, Strangeway  did conclude that the wave peak above the dayside ionosphere was likely to be due to naturally occurring waves. Because the UADS data include both signal and noise, quantitative analysis of the wave data would be better carried out using high resolution data. Such data allow for clearer separation of signal and interference. I refer the reader to Strangeway and Russell  for specific examples of high-resolution plasma wave data which include both natural signals and interference.
Keeping the limitations of UADS data in mind, Figure 1a shows the median peak wave amplitude (E) and ambient plasma parameters ordered by altitude with respect to the OETP ionopause, using 50 km altitude bins. A small amount of density data are available for altitudes above the OETP ionopause, but no electron temperature data are available. Moreover, the line labeled with a percent sign shows the percentage of 100-Hz data samples for which temperature data are available. It can be seen that at the OETP ionopause only a small fraction (~ 17%) of wave data samples also have electron temperature data. Those wave data without electron temperature data will tend to be associated with lower densities and higher temperatures, and the Joule dissipation for these data will be less than our calculated values. Thus the calculated Joule dissipation rates at the OETP ionopause overestimate the median rate at this altitude.
In producing the altitude profiles shown in Figure 1, 11,606 electron temperature data points from 386 orbits were used. While the interval is not the same as the 900 orbits analyzed by CH, this number can be used as a gauge of the relative frequency of occurrence of the anomalous heating events given by CH. As noted above, ~ 170 events are plotted in Figure 3 of CH. Thus I would estimate that such events occur < 1.5% of the time, averaged over the entire ionosphere, and have little significance. Clearly, the rate increases if the events occur in a restricted altitude range, near the ionopause (say), but then one is again presented with the question of the applicability of UADS data when gradients are present.
Figure 1b shows the electron collision frequency () and calculated heating parameters. At the OETP ionopause the conduction scale required to balance Joule dissipation (L) and electron mean free path () are both large > 1000 km. Again, the underestimate of the electron temperature biases the data at the OETP ionopause to higher Joule dissipation rates, and lower heat conduction. Because of the strong temperature dependence ( T, T, L T [Strangeway, 1996]), small changes in temperature result in large changes in the scale lengths.
At the OETP ionopause altitude the heating time constant (t) > 10 s, contradicting CH who state that the heating time constant will be smaller (~ 0.015 s) at higher altitudes because the density is lower. This statement in CH follows their equation (14), which correctly shows that t 1/n. Thus CH are clearly in error in stating that for the same wave amplitude the heating time constant is smaller for lower density. Note that the turn over in heating time constant shown in Figure 1 is due to the large increase in wave amplitude at the OETP ionopause. The relatively long heating time constant also indicates that collisional Joule dissipation is unimportant for the dayside waves.
At altitudes below the OETP ionopause the calculated Joule dissipation rate (Q) increases to about 10 W/m. However, at this altitude the wave instrument is essentially measuring background. It should be noted that we have not attempted to remove background data from the data base, as the instrument background is a function of the ambient plasma parameters. In the magnetosheath the instrument noise level is much higher than in the ionosphere, since the antenna is within the Debye sphere of the spacecraft [Strangeway, 1991; Strangeway and Russell, 1996], and part of the 100-Hz signature shown in Figure 1 is simply due to the change in instrument noise level. Crawford et al.  have attempted to remove instrument noise from some of the data, but they did not remove instrument noise at the OETP ionopause as both signal and noise were changing rapidly.
Summarizing Figure 1, all the data indicate that collisional Joule dissipation is not a significant process for the waves observed above the dayside ionopause of Venus. Quite simply, the waves occur at too high an altitude, where the plasma density is low and the temperature is high. Any heating due to wave absorption will be masked by the temperature gradients present in the plasma. Indeed the heat flux associated with these gradients is almost certainly more important than Joule dissipation in the mantle region, where the magnetic field allows heat to flow between the cold ionospheric and hot magnetosheath plasmas. The importance of heat conduction at higher altitudes was emphasized by Gan et al.  in their modeling of dayside electron temperature profiles.
In conclusion, while CH wish to show that collisional Joule dissipation is important for the dayside ionosphere of Venus, they have neglected the inherent statistical bias in their analysis, as well as the uncertainties introduced by using averaged electron data with peak wave amplitudes. The data they use are restricted to altitudes for which Langmuir probe data are available. I have shown here that the heating rate is negligible at the OETP ionopause, where the wave amplitude peaks and the Langmuir probe is near threshold, whereas CH erroneously argue that Joule heating is larger at the OETP ionopause. The relatively small number of anomalous events found by CH at lower altitudes are irrelevant to the vast majority of wave signals observed on the dayside, above the ionosphere.
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