A Comparative Study of the Influence of the Active Young Sun on the Early Atmospheres of Earth, Venus, and Mars

Because the solar radiation and particle environment plays a major role in all atmospheric processes such as ionization, dissociation, heating of the upper atmospheres, and thermal and non-thermal atmospheric loss processes, the long-time evolution of planetary atmospheres and their water inventories can only be understood within the context of the evolving Sun. We compare the effect of solar induced X-ray and EUV (XUV) heating on the upper atmospheres of Earth, Venus and Mars since the time when the Sun arrived at the Zero-Age-Main-Sequence (ZAMS) about 4.6 Gyr ago. We apply a diffusive-gravitational equilibrium and thermal balance model for studying heating of the early thermospheres by photodissociation and ionization processes, due to exothermic chemical reactions and cooling by IR-radiating molecules like CO₂, NO, OH, etc. Our model simulations result in extended thermospheres for early Earth, Venus and Mars. The exospheric temperatures obtained for all the three planets during this time period lead to diffusion-limited hydrodynamic escape of atomic hydrogen and high Jeans’ escape rates for heavier species like H₂, He, C, N, O, etc. The duration of this blow-off phase for atomic hydrogen depends essentially on the mixing ratios of CO₂, N₂ and H₂O in the atmospheres and could last from ∼100 to several hundred million years. Furthermore, we study the efficiency of various non-thermal atmospheric loss processes on Venus and Mars and investigate the possible protecting effect of the early martian magnetosphere against solar wind induced ion pick up erosion. We find that the early martian magnetic field could decrease the ion-related non-thermal escape rates by a great amount. It is possible that non-magnetized early Mars could have lost its whole atmosphere due to the combined effect of its extended upper atmosphere and a dense solar wind plasma flow of the young Sun during about 200 Myr after the Sun arrived at the ZAMS. Depending on the solar wind parameters, our model simulations for early Venus show that ion pick up by strong solar wind from a non-magnetized planet could erode up to an equivalent amount of ∼250 bar of O⁺ ions during the first several hundred million years. This accumulated loss corresponds to an equivalent mass of ∼1 terrestrial ocean (TO (1 TO ∼1.39×10²⁴ g or expressed as partial pressure, about 265 bar, which corresponds to ∼2900 m average depth)). Finally, we discuss and compare our findings with the results of preceding studies.     

Ion Acceleration and Outflow from Mars and Venus: An Overview

Solar wind forcing of Mars and Venus results in outflow and escape of ionospheric ions. Observations show that the replenishment of ionospheric ions starts in the dayside at low altitudes (??300?C800 km), ions moving at a low velocity (5?C10 km/s) in the direction of the external/ magnetosheath flow. At high altitudes, in the inner magnetosheath and in the central tail, ions may be accelerated up to keV energies. However, the dominating energization and outflow process, applicable for the inner magnetosphere of Mars and Venus, leads to outflow at energies ??5?C20 eV. The aim of this overview is to analyze ion acceleration processes associated with the outflow and escape of ionospheric ions from Mars and Venus. Qualitatively, ion acceleration may be divided in two categories:

1. Modest ion acceleration, leading to bulk outflow and/or return flow (circulation).
2. Acceleration to well over escape velocity, up into the keV range.

In the first category we find a processes denoted ??planetary wind??, the result of e.g. ambipolar diffusion, wave enhanced planetary wind, and mass-loaded ion pickup. In the second category we find ion pickup, current sheet acceleration, wave acceleration, and parallel electric fields, the latter above Martian crustal magnetic field regions. Both categories involve mass loading. Highly mass-loaded ion energization may lead to a low-velocity bulk flow??A consequence of energy and momentum conservation. It is therefore not self-evident what group, or what processes are connected with the low-energy outflow of ionospheric ions from Mars. Experimental and theoretical findings on ionospheric ion acceleration and outflow from Mars and Venus are discussed in this report.     

Mars Global MHD Predictions of Magnetic Connectivity Between the Dayside Ionosphere and the Magnetospheric Flanks

Atmospheric photoelectrons have been observed well above the ionosphere of Mars by the ASPERA-3 ELS instrument on Mars Express. To systematically interpret these observations, field lines from two global MHD simulations were analyzed for connectivity to the dayside ionosphere (allowing photoelectron escape). It is found that there is a hollow cylinder behind the planet from 1–2 R _M away from the Mars-Sun line that has a high probability of containing magnetic field lines with connectivity to the dayside ionosphere. These results are in complete agreement with the ELS statistics. It is concluded that the high-altitude photoelectrons are the result of direct magnetic connectivity to the dayside at the moment of the measurement, and no extra trapping or bouncing mechanisms are needed to explain the data.     

Planetary Magnetic Fields and Solar Forcing: Implications for Atmospheric Evolution

The solar wind and the solar XUV/EUV radiation constitute a permanent forcing of the upper atmosphere of the planets in our solar system, thereby affecting the habitability and chances for life to emerge on a planet. The forcing is essentially inversely proportional to the square of the distance to the Sun and, therefore, is most important for the innermost planets in our solar system—the Earth-like planets. The effect of these two forcing terms is to ionize, heat, chemically modify, and slowly erode the upper atmosphere throughout the lifetime of a planet. The closer to the Sun, the more efficient are these process. Atmospheric erosion is due to thermal and non-thermal escape. Gravity constitutes the major protection mechanism for thermal escape, while the non-thermal escape caused by the ionizing X-rays and EUV radiation and the solar wind require other means of protection. Ionospheric plasma energization and ion pickup represent two categories of non-thermal escape processes that may bring matter up to high velocities, well beyond escape velocity. These energization processes have now been studied by a number of plasma instruments orbiting Earth, Mars, and Venus for decades. Plasma measurement results therefore constitute the most useful empirical data basis for the subject under discussion. This does not imply that ionospheric plasma energization and ion pickup are the main processes for the atmospheric escape, but they remain processes that can be most easily tested against empirical data. Shielding the upper atmosphere of a planet against solar XUV, EUV, and solar wind forcing requires strong gravity and a strong intrinsic dipole magnetic field. For instance, the strong dipole magnetic field of the Earth provides a “magnetic umbrella”, fending of the solar wind at a distance of 10 Earth radii. Conversely, the lack of a strong intrinsic magnetic field at Mars and Venus means that the solar wind has more direct access to their topside atmosphere, the reason that Mars and Venus, planets lacking strong intrinsic magnetic fields, have so much less water than the Earth? Climatologic and atmospheric loss process over evolutionary timescales of planetary atmospheres can only be understood if one considers the fact that the radiation and plasma environment of the Sun has changed substantially with time. Standard stellar evolutionary models indicate that the Sun after its arrival at the Zero-Age Main Sequence (ZAMS) 4.5 Gyr ago had a total luminosity of ≈70% of the present Sun. This should have led to a much cooler Earth in the past, while geological and fossil evidence indicate otherwise. In addition, observations by various satellites and studies of solar proxies (Sun-like stars with different age) indicate that the young Sun was rotating more than 10 times its present rate and had correspondingly strong dynamo-driven high-energy emissions which resulted in strong X-ray and extreme ultraviolet (XUV) emissions, up to several 100 times stronger than the present Sun. Further, evidence of a much denser early solar wind and the mass loss rate of the young Sun can be determined from collision of ionized stellar winds of the solar proxies, with the partially ionized gas in the interstellar medium. Empirical correlations of stellar mass loss rates with X-ray surface flux values allows one to estimate the solar wind mass flux at earlier times, when the solar wind may have been more than 1000 times more massive. The main conclusions drawn on basis of the Sun-in-time-, and a time-dependent model of plasma energization/escape is that:

1. Solar forcing is effective in removing volatiles, primarily water, from planets,
2. planets orbiting close to the early Sun were subject to a heavy loss of water, the effect being most profound for Venus and Mars, and
3. a persistent planetary magnetic field, like the Earth’s dipole field, provides a shield against solar wind scavenging.

     

Double null points and magnetic reconnection

2D reconnection is possible only in connection with the existence of a singularity in the magnetic field line topology, associated with a magnetic null point or a current sheet. Both of these provide an X-type structure of the magnetic field where fields of opposite polarity meet and reconnect. In 3D a similar topology is found in a null point pair, when the null points are connected by a separator line. The separator is defined as the intersection line of the two null-point fan planes. This paper reports on the topological evolution of this configuration with respect to different perturbations emerging from imposed boundary velocities, using a nonlinear numerical approach.     

Adaptive Detection Algorithms for Multiple-Target Situations

The performance of a mean-level detector is considered for the case where one or more interfering target returns are present in the set of cells used in estimating the clutter-plus-noise level. A serious degradation of detection probability is demonstrated for all of the single-pulse Swerling target fluctuation models (i. e., cases 0, 2, and 4). Indeed, for fixed mean radar cross sections of the primary and interfering targets, the probability of detecting the primary target is asymptotic to values significantly less than unity as the signal-to-noise ratios of the returns approach infinity. A class of alternative adaptive detection procedures is proposed and analyzed. These procedures, based on ranking and censoring techniques, maintain acceptable performance in the presence of interfering targets, and require only a minor addition in hardware to a conventional mean-level detector.     

The Swedish Small Satellite Program for Space Plasma Investigations

The success of the Swedish small satellite program, in combination with an active participation by Swedish research groups in major international missions, has placed Sweden in the frontline of experimental space research. The program started with the development of the research satellite Viking which was launched in 1986, for detailed investigations of the aurora. To date, Sweden has developed and launched a total of six research satellites; five for space plasma investigations; and the most recent satellite Odin, for research in astronomy and aeronomy. These fall into three main categories according to their physical dimension, financial cost and level of ambition: nano-satellites, micro-satellites, and mid-size satellites with ambitious scientific goals. In this brief review we focus on five space plasma missions, for which operations have ended and a comprehensive scientific data analysis has been conducted, which allows for a judgement of their role and impact on the progress in auroral research. Viking and Freja, the two most well-known missions of this program, were pioneers in the exploration of the aurora. The more recent satellites, Munin, Astrid, and Astrid-2 (category 1 and 2), proved to be powerful tools, both for testing new technologies and for carrying out advanced science missions. The Swedish small satellite program has been internationally recognized as cost efficient and scientifically very successful.     

A Distribution-Free Doppler Processor

A distribution-free Doppler processor (DFDP) is described. This procedure is applicable to the detection of signals with unknown phase or signals which undergo a change in phase from one observation to the next. The procedure has the characteristic that the false-alarm probability is a constant, independent of the probability distribution of the received data when no signal is present. Also, an estimate of the Doppler frequency, when applied to a Doppler radar processor, is provided.