A new concept radio occultation experiment to study the structure of the atmosphere and determine the plasma layers in the ionosphere. Gavrik A.L. Kotelnikov Institute of Radio Engineering and Electronics of RAS 25октября 2011 ВЕНЕРА - Д ИКИ РАН

Dual frequency radio wave sounding on the ray path Orbiter → Earth The scientific goals in the project VENERA-D T,K φ λ Variation in the Solar wind plasma. (analysis of amplitudes and phases of two coherent radio signals) 2. Monitoring the electron density on the Venus ionosphere. (analysis of amplitudes and phases of two signals during occultation) 3. Thermal and density profiles for the Venus atmosphere. (analysis of amplitudes and phases of two signals during occultation) 4. Investigate of Venus surface. (analysis of bistatic echoes from Venus surface) The electron densities in the Venusian day and night time ionosphere Changes in parameters of Solar wind plasma Variations in the temperature profiles of atmosphere Anomalous reflectivity from bistatic radar echoes Venera Vishlov et.al. Venera -15,-16 Savich et. al. Magellan Jenkins et.al Venera-15 Pavelyev et.al. h, km N e, cm -3 Day-time N(h) pass into night-time N(h) Night-time N(h)

Dual frequency VENERA-15,-16 occultation ( 4 & 1 GHz or 8 & 32 см ) Схема двухчастотного радиопросвечивания The electron density of daytime Venusian ionosphere N(h), см -3 Measured refraction attenuations induced by daytime Ionosphere and atmosphere Х СМ Х DМ 8 см 32 см Measured residual frequency in the ionosphere and atmosphere f DМ 32 см The time of occultation 2 … 20 minutes atmosphere Radio rays to the Earth ionosphere The theory of occultation experiments is based on integral equations that relate the electron density N(h) to the measured characteristics of radio signals.

Altitude, км Electron density, см -3 Altitude distributions N(h) of the electron densities in the Venus day-time ionosphere Распределения электронной концентрации N(h) в дневной ионосфере Венеры г г г Coincidence of N(h) for some days г г г Time-to-time variability г г г Coincidence of N(h) for some days г г г Time-to-time variability

about the bottom ionosphere. We can see discrepancies between the model and calculated N(h). The error can be greater than the actual value of N(h) at altitudes of h < 120 km. That is why we can see the bottom boundary of the ionosphere at altitude of h = 117 km on the experimental profile N(h). But the real influence of ionospheric plasma is observed up to 85 km in the occultation data. The traditional method to determine N(h) leads to wrong conclusions Model N(h) Calculation N(h) N(h) VENERA г. bottom part of the ionosphere Altitude, km Electron density, сm -3 Refraction attenuation

Altitude, km Temperature, K In the field of heights 80 < h < 120 km it is impossible to define atmosphere temperature precisely. It is impossible to define any parameters of Venus atmosphere for h < 35 km from occultation data because of super refraction of the radio rays. VENUS-EXPRESS M. Pätzold et al.

The following result is obtained from p(t),  (t), X(t) : The well-known relationships The ray asymptote distance Н – the altitude of straight-line ray The refractive bending angle Δf – residual frequency in the ionosphere ΔF – residual frequency in the atmosphere The refraction attenuation L – the distance between the spacecraft and point  V ┴ – the velocity of the satellite’s ingress The electron density f – the radiated frequency (1 GHz) Variations of the defocusing attenuation X(t) in the occultation experiments are proportional to the velocity of residual frequency changes.

It is necessary to determine same parameters from the experimental data: X DM (t) - the refraction attenuations of L-band (32 cm) signal. X CM (t) - the refraction attenuations of C-band ( 8 cm) signal. δf(t) = 16/15 (f DM (t) - f CM (t)/4) - the reduced frequency difference (plasma influence). Δf(t) = function [δf(t)] - frequency variation of L-band (32 cm) signal. X Δf (t) = 1 + value*d/dt[Δf(t)] - predicted refraction attenuation of the L-band signal. Coincidence between variations of refraction attenuation of the radio signal X DM (t) and variations X Δf (t) will be indicative of the influence of the regular structures of the ionosphere under investigation. The absence of this correspondence is an indication of the influence of the noise or other factors that are not taken into account. This method considerably increased the sensitivity of the radio probing method to refractive index variations and makes possible to detect small variations of electron density and atmosphere density. New method provides a possibility to distinguish the layers in the atmosphere and ionosphere during occultation.

A variations of refraction attenuation of DM signal coincide with calculated data Х ∆f (t) in the day-time ionosphere of Venus. Altitude of spacecraft-Earth straight-line h, км The refraction attenuation, Х Venera-15,-16 Gavrik A. et al. A variations of refraction attenuation of DM signal coincide with calculated data Х ∆f (t) in the night-time ionosphere of Venus. bottom ionosphere One layer night-time ionosphere Two layers night-time ionosphere

This technique will allow one to investigate wave processes in the top atmosphere and the bottom ionosphere. We observed wave processes in the top atmosphere and bottom ionosphere of Venus Refraction attenuation of a DМ-signal in the atmosphere Refraction attenuation of a CМ-signal in the atmosphere layered structure in the atmosphere Correlation between the powers of DM- and CM-signals due to the wave structure Refraction attenuation in the ionosphere calculated from the frequency of a DМ-signal Х дм and Х  f are different in the atmosphere Х10Х10 Layers in the bottom ionosphere: correlation between Х DМ and Х  f Altitude of the spacecraft-to-Earth straight line h, km

L 1 – the distance between the first spacecraft and point of ray closest to the surface of planet. L 2 – the distance between the second spacecraft and point of ray closest to the surface of planet. The method is correct for high- precision measurements of signal power and phase during dual frequency radio sounding. This method can be extended to occultation experiment Satellite → Satellite

R e s I d u a l f r e q u e n c y, Hz Altitude of radio ray straight line h, km L1= 19 cm, L2= 24 cm, Δt = 0.02 s GPS → CHAMP In these occultation experiments GPS → CHAMP we can see very high frequency fluctuations and the lack of coherence of the signals of two ranges L1 & L2. Hence, the onboard USO must be very stable on short time intervals.

Realization of informative experiments requires the development of a good on-board receiver. Small frequency fluctuations in the occultation experiments VENERA-15,-16 → Earth achieved by the high output transmitter power (100 W) and large diameter (>2m) on-board antenna. In these occultation experiments GPS → CHAMP we can see the frequency fluctuations, which exceed the influence of the ionosphere. plasma influence ВЕНЕРА-16 → Земля λ = 32 см, Δt = s λ = 19 см, Δt = 0.02 s GPS → CHAMP invalid measurements (little signal/noise) Residual frequency, Hz Altitude of radio ray straight line h, km Mean-square deviation Δf(t) from to 0.03 Hz The frequency Δf(t) in the Venus daytime ionosphere

If we choose a very long measurement interval Δt, then the effects of focusing of a signal and layered structures will not manifest themselves. Therefore, it is necessary to provide a high S/N ratio during the experiments. Altitude of radio ray straight line h, km The refraction attenuation, Х Δt = 0.06 s Δt = 0.11 s Δt = 0.23 s Δt = 0.47 s The method gives correct results for high-precision measurements during dual frequency radio sounding. Invalid data (little S/N)

High S / N ratio can be achieved if emit powerful coherent radio signals from Earth. In this case, at the same time we can perform six radio physical experiments, in addition to the work of other onboard devices. High S / N ratio give the possibility of obtaining new information concerning the structure of the planetary ionospheres and atmospheres. radar experiment bistatic radar experiment Interplanetary plasma on the two separated tracks Earth → OA and Earth → SS ОА SS Two-frequency radio sounding of the ionosphere Two-frequency radio sounding of the atmosphere signals to the ETs…

km Radio signal Echo-signal Venera D Venera It is important that the high potential allows regular bistatic location. We can determine the parameters of the Venus atmosphere near its surface from the characteristics of the echo signals. Consequently, it is possible to monitor the bottom of the Venusian atmosphere, details of which are very limited.

C o n c l u s i o n s We have shown that the new methods proposed make it possible to carry out high-quality analysis of the Venus ionosphere and atmosphere during dual-frequency occultation experiments. There are a few conditions for this investigation: 1.High-precision phase measurements. 2.High-precision power measurements with the necessary dynamic range. 3. All the measurements should be carried out within a short time interval. 4.Radiation from the Earth two coherent radio signals (that will explore the unknown properties of the atmosphere and ionosphere of Venus). Спасибо за внимание Thank you for attention Работа выполнена при частичной поддержке программы Президиума РАН №VI.15