Bondarenko M.I., Gavrik A.L. On Non-Meteoric Origin of Layers below V1 in Venusian Ionosphere. In: The Sixth Moscow Solar System Symposium (6MS3), October 5-9, 2015, ИКИ РАН. Москва , 113-ab.
Полный текст не доступен из этого репозитория.Аннотация
As more data were being collected from radio science occultation experiments on Venus and Mars (PVO, VEX, MGS, MEX), a number of ionospheric electron density (N_e) profiles were produced with additional layers, around 100-120km on Venus, below the smaller Chapman ionization layer V1, and correspondingly below M1 on Mars. Drawing an analogy with studies of Earth ionosphere, these layers were hypothesized to be “meteoric ionization layers” supposedly caused by ablation of meteoric dust, cometary ejecta, meteoric showers and other similar mechanisms in the lower ionosphere of the two planets. Numeric models developed in the early 2000s provided estimates roughly similar to or bracketing the parameters of the observed “layers”. While the existence of these signal features is indisputable, their interpretation as “meteoric layers”, cautious first, then more and more accepted as a default explanation, needs to be reexamined. Such reexamination can be started from a little known data set of 8 daytime occultations spanning the range of [-56 ÷ -82] deg latitude, obtained 14÷30 October 1983 in VENERA-15,VENERA-16 joint mission to Venus, as part of its occultation season 1, the strongest of the three in this earlier Soviet study of the planet. In contrast to the recent publications, we examined signals processed to level 02 rather than completed N_e profiles. All signals in the series up to the V1 altitudes, and possibly higher, feature wavelike variations with growing amplitudes, which exceed uncertainty levels, in one case looking spectacularly like a very well expressed sine with a near-exponential envelope. Using a Morlet wavelet transform combined with subsequent wavelet filtering, the signals were decomposed into the lower frequency component, reflecting the Chapman layers of the ionosphere (and responsible for the N_e profile shape after further processing), and what appeared to be a set of nearly-harmonic components at higher frequencies with bell-shaped envelopes growing to a certain “dissipation altitude”, then subsiding, superimposed on the lower frequency component. These higher-frequency oscillations with wavelengths approximately between 5km and 15km were further inspected as possible recordings of gravity wave activity at ionospheric altitudes. Knowing background profiles of pressure/density and temperature, it is possible to estimate GW parameters from its dissipation altitude, if it dissipates due to kinetic viscosity and thermal dumping, which affect GWs at ionospheric heights. Profiles determined in other VEX experiments were used for this estimation for both VENERA-15,16 and VEX data. This calculation for the analyzed signal components produced hypothetical GW parameters matching those of real GWs observed on UV daytime photographs of the upper cloud layer of Venus at 66km (Peralta et al, 2008), as well as GWs at 11 5-135km on images from CO2 non-LTE emissions (Garcia et al, 2009). Additionally, numeric estimates demonstrated that the real GWs identified by Peralta at the upper cloud levels would dissipate at the same altitudes as the components of our signals, if propagating upwards, and moreover that the dissipation altitudes lie at the turbopause heights. The matches conflate these separate observations from two missions into a coherent description of a physical phenomenon, which also seems to be a permanent feature of the Venusian ionosphere (VENERA-15,16 measurements and VEX mission are spaced 23-30 years apart). A new interpretation of lower ionospheric “layers” below V1 (if the above considerations are correct) immediately follows: what appears on N_e profiles as “additional layers” below V1 may in fact be the last, largest periods of GWs rising from the top of the cloud layer level (65-70km) and being on the verge of disspating at turbopause altitudes, which were registered in high-quality occultation data. Not only of non-meteoric origin, they do not seem to appear as “layers” (which would imply more stability), but according to this interpretation are dynamic and local phenomena. The overall distribution of N_e profiles with “layers” in the VEX mission, as seen in a recent recalculation of the profiles, falls geographically into higher latitudes (appr. 60-65 degrees poleward for both hemispheres, also noted in various previous works). This also appears to strengthen the GW interpretation, as these are the latitudes of the “cold collar” encircling the polar vortex formations where atmospheric parameters permit propagation of GWs in our range upwards, possibly originating from the top of the cloud layer below. Is the GW interpretation of “lower ionospheric layers” supported by the VEX VeRa data? In the more recent Western missions, radio science experiment designers standardized on much shorter wavelengths, in an attempt to emphasize atmospheric studies. The Soviet VENERA-15.16 emitted an S-band signal at 32cm and X-band at 8cm, with most power allocated to the S-band thus producing a signal with a much stronger ionospheric signature. In case of VEX VeRa (Sband = 13cm, X-band = 3.8cm, most power to the X-band) the X-band power (i.e. closed-loop AGC) signal at ionospheric altitudes is digital noise. The Soviet mission experiment design involved comparison between the derivative of the differential Doppler frequency residual (channel one) and accordingly normalized signal power (channel two), which permits separation of signal variations synchronous between the channels and noise. On signals in this form smaller variations are much more obvious. For the VEX VeRa X-band data the lack of meaningful power signal at ionospheric altitudes precludes using the same processing methods. For the VEX VeRa S-band signal the problem was further compounded by a malfunction: S-band channel power sharply dropped after the first occultation season in 2006, rendering it scientifically useless and for the rest of the mission leaving researchers with data from X-band channels only. But, even during VEX VeRa Season 1 with a valid S-band channel signal, it has been presented in the archives downsampled to 1Hz, i.e. as points 1 second apart on the time scale, which is too crude to resolve smaller signal variations. It is also possible that the rest of the occultation measurements, even when sampled at 10Hz in Level 02 processed data, were downsampled to 1Hz for the calculation of N_e profiles, also reducing resolution and possible recognition of GW patterns. THUS a set of design decisions and an unfortunate technical failure in the VEX mission (combined with a certain tradition in interpreting occultation results) probably prevented researchers from even considering the idea that GWs propagating to ionospheric altitudes may have become registered in the results of occultation experiments and subsequently seeped into N_e profiles as additional “layers” of unknown origin. HOWEVER, as a result of the above analysis of VENERA-15,16 data it becomes possible to recognize possible GW patterns in VEX VeRa data and use this data in further study of the “low-altitude ionospheric layer” phenomenon in support of the new GW interpretation. Relevance of GW interpretation to Mars studies. A preliminary and cautios optimism may be expressed that this interpretation may also account for the appearance of “meteoric layers” on Mars, where, in the absence of atmospheric superrotation, GWs (and therefore additional “layers” on N_e profiles) may be expected to show up less frequently and follow a different pattern. On the other hand, the interpretation leads us to a hypothesis that (rather than Mars orbit comet crossings and similar events, proposed by the “meteoric ion layer” theory) the appearance of “layers” may correlate with GW producing events such as dust storms. And indeed dust opacity is a parameter with which the “meteoric layers” demonstrated the highest positive correlation out of many tested in a review by P.Withers et al, 2008. This hypothesis requires further investigation. In conclusion, while leaving the possibility of ionization from meteoric ablation at heights below V1, and leaving the question of existence of layers of truly meteoric origin on Venus outside the bounds of this discussion, we propose that the signal features widely reported and branded as “meteoric layers” in recent studies may in fact be traces of GW activity at near-turbopause altitudes.
| Тип объекта: | Доклад на конференции или семинаре (Постер) |
|---|---|
| Авторы на русском. ОБЯЗАТЕЛЬНО ДЛЯ АНГЛОЯЗЫЧНЫХ ПУБЛИКАЦИЙ!: | Бондаренко М.И., Гаврик А.Л. |
| Подразделения (можно выбрать несколько, удерживая Ctrl): | 119 лаб. распространения радиоволн и дистанционного зондирования атмосферы |
| URI: | http://cplire.ru:8080/id/eprint/1920 |
 |
Изменить объект |
