- Top of page
- 1. Introduction
- 2. Results
- 3. Discussion
 A comprehensive explanation for the complex climatology of the so-called equatorial spread F (ESF) has eluded researchers for more than 70 years. Recently, however, a seeding hypothesis has been proposed, which appears to provide the final major piece of this puzzle. The hypothesis is based on the discovery that a direct link exists between regions of deep convective activity in the troposphere, where atmospheric gravity waves (GWs) are spawned, and the occurrence frequency of ESF during solstices. The objective here is to answer two questions that may impede the general acceptance of this hypothesis. We first show why seed plasma perturbations should develop from GW-driven neutral-wind perturbations, but only when the GW source region is located very close to the magnetic dip equator. We then reexamine this relationship using a data set on GW source regions that is better matched (in time and longitudinal coverage), than that used previously, to the data set on ESF activity used by Tsunoda (2010a). We conclude that seeding is indeed playing an important role in the development of ESF.
- Top of page
- 1. Introduction
- 2. Results
- 3. Discussion
 To summarize, three papers have now been written on a seeding hypothesis for the development of ESF; the other two are Tsunoda [2010a, 2010b]. In our view, the accumulated evidence, both theoretical and experimental, is undeniable. The fundamental theory is a neutral-ion coupling process, which requires GWBA [Klostermeyer, 1978; Huang and Kelley, 1996; Keskinen and Vadas, 2009]. But no one, until now, had shown why GWBA should occur only at the magnetic dip equator and not at finite dip latitudes. The key finding, presented herein, is that GWBA does not occur where I is finite because the GWs of interest are likely to have phase fronts with a downward tilt angle. This finding holds for phase fronts that are planar, as well as locally for phase fronts that have curvature. Concern also arose as to whether curved phase fronts necessarily lead to a shorting out of the polarization response. After all, neutral-ion coupling would not occur if an p does not appear. Hence, another key finding was that there is a substantial polarization response, even when GW phase fronts are curved [Tsunoda, 2010b]. These findings, taken together, provide a sound theoretical basis for believing that the seeding of plasma perturbations should occur, when GWs are launched from MCCs that are located near the dip equator.
 Experimentally, the evidence that seeding via the above described mechanism is playing an important role in the development of ESF is overwhelming. We have shown that the amplitude of the plasma perturbations varies with season and longitude, and that this behavior is controlled by the migration of the ITCZ in latitude with season. The migration of the ITCZ can be seen in data from any given year (shown herein), or even in data that have been averaged over as much as 17 yrs [Tsunoda, 2010a].
 Most importantly, we have been able to show that ESF activity, which varies with season and longitude [McClure et al., 1998], is basically consistent with seeding during solstices, as prescribed by the GWBA hypothesis. We have shown that both maxima and minima in ESF activity, which occur during solstices, can be explained to large extent by the seeding hypothesis. We have even uncovered an unusual situation, which occurs in the East Pacific sector, where the magnetic dip equator is located in the southern hemisphere, and the ITCZ remains in the northern hemisphere. Here, we find minima in ESF activity during both solstices, exactly as predicted by the seeding hypothesis. We have even found that ESF activity contains minor features, which appear explainable in terms of seeding effects during equinoxes. In fact, virtually all of the major features in ESF activity can be explained in terms of the STBA and GWBA hypotheses. The only exception is the behavior found in the Indonesian sector with eastward extension into the West Pacific sector.
 The notion that GWs may be more realistically represented with circular (or spherical) phase fronts is interesting, because the polarization response appears to favor longer wavelengths, which are comparable with those of GWs that reach the thermosphere [Tsunoda, 2010b]. This finding is consistent with the notion of seeding, as described, because the wavelengths of LSWS that have been observed are also comparable in length.
 An important finding that should follow from the GWBA theory is that not all GWs or MSTIDs that are observed at low latitudes are necessarily involved in the seeding of LSWS. For example, zonally propagating GWs that originate from an MCC not located on the dip equator are not expected to produce plasma perturbations. On the other hand, meridionally propagating GWs could produce MSTIDs through ion drag effects along , but they are not likely involved in the seeding process that leads to ESF. It is possible that, when a circular GW is launched at the dip equator, its zonal component could produce an LSWS, while its meridional component could excite an MSTID, if the GW propagates far enough poleward in latitude to where I is significant.
 The finding, that two distinctly different processes (STBA and GWBA) are responsible for ESF activity, is also satisfying because it is consistent with the finding that UHF scintillation tends to favor equinoxes, whereas VHF scintillation tends to favor solstices [Aarons, 1993]. Differences in ESF (or scintillation) properties should be expected, if the source mechanisms are different, and it seems reasonable that a PSSR-related (instability) process may be more capable of producing smaller-scale irregularities, hence, scintillations at UHF but not VHF, than a seeding-related process.
 We continue to be puzzled by the persistently low ESF activity in the Indonesian sector (with possible extension into the West Pacific sector). The OLR maps indicate broad regions of deep convective activity, similar to those found in HRC data presented by Waliser and Gautier . According to Figure 2, ESF activity should be high during ASO and MJJ months, but not during NDJ and FMA months. The notion that proximity to the MCC may act to depress ESF activity, which is based on expected properties of circular GWs [Tsunoda, 2010b], needs to be revisited, using more realistic models. The behavior of Fresnel zone effects should be considered by using GWs with spherical phase fronts and a dipole field.
 Much more needs to be done, but significant inroads have been made toward solving the enigma of the day-to-day variability in ESF occurrence [e.g., Tsunoda, 2005]. Clearly, we must understand ESF climatology before we can understand day-to-day variability. To make further progress, there is crucial need to measure the properties of LSWS, which reflect the effectiveness of the neutral-ion coupling process. At present, only the Communications/Navigations Outage Forecasting System (C/NOFS) satellite provides a convenient means for doing so. With C/NOFS, it is possible to measure total electron content as a function of longitude, receiving the beacon signals from C/NOFS at ground stations that are distributed in longitude [e.g., Thampi et al., 2009; Tsunoda et al., 2010].