The onset of the planet encircling dust event (PEDE‐2018) started around 1 June 2018 as observed by Mars Reconnaissance Orbiter/Mars Color Imager, peaking around 7–10 July and persisting through ...mid‐October 2018. After the onset of the event, the upper atmosphere underwent significant changes in density and thermal structures. Mars Atmosphere and Volatile Evolution‐Neutral Gas and Ion Mass Spectrometer (MAVEN NGIMS) had a good opportunity to observe these changes from the first detection in the upper atmosphere and throughout the duration of the PEDE. The compositional changes included increased density at a constant altitude for CO2 and Ar, while the O decreased from the peak throughout the decay of the bulk of the PEDE.
Plain Language Summary
From June through October 2018 Mars experienced a planet encircling dust event (PEDE‐2018), a fairly rare event last observed in 2007. The dust storm grew from a local event to cover the entire planet and was opaque enough that so little sunlight reached the surface that the solar‐powered opportunity rover ceased operations and all attempts to re‐establish contact with it were unsuccessful. Meanwhile, the orbiter Mars Atmosphere and Volatile Evolution (MAVEN) was able to observe changes in the upper atmosphere in the composition as a result of this globally extensive PEDE. MAVEN observed increases in both the CO2 and Ar while also observing an unexpected reduction in the O densities.
Key Points
MAVEN/NGIMS observed increased of CO2 and Ar densities observed in the upper atmosphere corresponding to the peak of the dust event
Unexpected decrease in O densities in the upper atmosphere (160–250 km) was simultaneously observed
Comparisons between model and data results show good agreement with scale height and temperatures, further M‐GITM model revisions needed to capture circulation effects
Despite the increasing popularity of viral marketing, factors critical to such a new communication medium remain largely unknown. This paper examines one of the critical factors, namely Internet ...users' motivations to pass along online content. Conceptualizing the act of forwarding online content as a special case of a more general communication behavior, we identify four potential motivations: (1) the need to be part of a group, (2) the need to be individualistic, (3) the need to be altruistic, and (4) the need for personal growth. Using a survey of young adults, we examine the relationship between these motivations and the frequency of passing along online content. We also investigate if high trait curiosity can indirectly lead to more forwarding by increasing the amount of online content consumed. Results show that Internet users, who are more individualistic and/or more altruistic, tend to forward more online content than others.
Measurement of the dense cold thermal plasma in planetary ionospheres via orbiting spacecraft is challenging because ion energies are small (0–4 eV), densities can vary by four orders of magnitude, ...composition varies with altitude, spacecraft charging varies in time and must be measured very accurately, and instrumental effects (e.g., detector dead‐time and background) can be significant. The SupraThermal And Thermal Ion Composition instrument team has recently released a new set of data products that contain density moments of the primary ion species at Mars, including those derived at periapsis, subject to the full suite of calibration factors required. This article discusses the challenges associated with deriving these densities and provides examples of the key caveats that users of the data should be aware of. A preliminary statistical study of this new data set focuses on the structure and variability of Mars' ionosphere, demonstrating that solar zenith angle effects, the crustal magnetic fields, and electron precipitation on the nightside, drive the strongest structural features, consistent with photochemical theory and previous studies. Dayside ionospheric density profiles are highly repeatable below altitudes of 200 km, marking the region where photochemistry and collisions dominate. In the upper dayside ionosphere (altitudes >300–400 km) changes in the solar wind dynamic pressure on timescales of Mars Atmosphere and Volatile EvolutioN's orbit (hr) drive the largest (factors of 1–3) variability in ionospheric density. In contrast variability in ionospheric density peaks between 150 and 250 km altitude on the nightside (factors of 1–2), consistent with electron precipitation driving ionization in this region.
Plain Language Summary
Making accurate measurements of the ions present in planetary atmospheres via orbiting spacecraft is difficult due to the large changes in conditions encountered throughout spacecraft orbits. This paper describes the techniques implemented to address the full array of difficulties associated with measuring planetary ions at Mars using measurements made by the SupraThermal And Thermal Ion Composition instrument. These techniques are used to calculate ion densities in Mars' atmosphere and we perform a preliminary investigation of this new data set, revealing several important characteristics of how ions behave in Mars' atmosphere. In particular, we find that on the dayside of the planet, there is a transition region between the upper and lower atmosphere where planetary ions behave differently. Above this transition region planetary ions are influenced by forces that arise in the space environment about Mars, while below this transition region, planetary ions are well shielded from these effects. Our results provide insight into the processes that shape Mars' atmosphere and tell us how energy is transported through the Mars system.
Key Points
The calibration challenges and caveats of ion densities derived from SupraThermal And Thermal Ion Composition observations are discussed
Solar zenith angle effects and the crustal magnetic fields drive the strongest structural features in ionospheric density
A variety of processes drive ionospheric variability throughout different regions of the Martian ionosphere
We have analysed plasma and magnetic field data obtained from 627 orbits of Mars Atmosphere and Volatile Evolution (MAVEN) that occurred during October 2014, April 2015, September 2017 and May 2018, ...when the spacecraft was crossing the magnetic pile-up boundary of Mars. Two broad peaks and a drop are observed in this magnetic field region. The magnetic cavity boundary lies between the ionosphere and the magnetic pile-up boundary. A steep ionopause-like boundary is observed in 24 profiles of 627 orbits, when MAVEN was passing from the magnetic pile-up region during the daytime ionosphere in presence of horizontal magnetic field of high strength, while their night side ionosphere did not show such boundary in presence of horizontal magnetic field of low strength. These profiles are unique, which have not been reported earlier. We propose that the horizontal magnetic field can form such ionopause within the magnetic pile-up boundary during the daytime if time and location of the magnetic anomaly coincide with the ion and electron density measurements. The nighttime ionosphere is thought to be produced within the magnetic pile-up boundary due to transportation of plasma from dayside to nightside across the terminator by a horizontal plasma flow velocity as has been predicted by the previous studies.
Lower atmospheric global dust storms affect the small‐ and large‐scale weather and variability of the whole Martian atmosphere. Analysis of the CO2 density data from the Neutral Gas and Ion Mass ...Spectrometer instrument on board NASA's Mars Atmosphere Volatile EvolutioN (MAVEN) spacecraft show a remarkable increase of gravity wave (GW)‐induced density fluctuations in the thermosphere during the 2018 major dust storm with distinct latitude and local time variability. The mean thermospheric GW activity increases by a factor of two during the storm event. The magnitude of relative density perturbations is around 20% on average and 40% locally. One and a half months later, the GW activity gradually decreases. Enhanced temperature disturbances in the Martian thermosphere can facilitate atmospheric escape. For the first time, we estimate observationally that, for a 20% and 40% GW‐induced disturbances, the net increase of Jeans escape flux of hydrogen is a factor of 1.3 and 2, respectively.
Plain Language Summary
Atmospheric gravity waves play an important dynamical and thermodynamical role in coupling the different atmospheric layers, especially on Earth and Mars. Using data from NASA's Mars Atmosphere Volatile EvolutioN (MAVEN) mission, we study the effects of a planet‐encircling major dust storm on thermospheric gravity wave activity and observationally estimate for the first time a potential influence of gravity waves on atmospheric escape on Mars. Gravity wave activity measured in terms of relative density fluctuations increases by a factor of two during the peak phase of the storm. We show that larger‐amplitude gravity waves facilitate atmospheric escape of hydrogen from Mars' upper atmosphere. For 40% gravity wave‐induced relative disturbances of temperature, the net escape rate doubles.
Key Points
Thermospheric gravity wave activity doubles during the dust storm
Gravity wave induced density fluctuations in the thermosphere are up to 40% during the peak storm phase
Gravity waves significantly increase hydrogen escape flux by modulating temperature fluctuations
The identification of magnetic reconnection on the dayside of Mars has been elusive owing to the lack of comprehensive plasma and field measurements. Here we present direct measurements of dayside in ...situ reconnection signatures by the comprehensive particles and fields package on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft over strong crustal magnetic fields in the southern hemisphere of Mars. During a crossing of a bifurcated current sheet consisting of northward and southward magnetic fields, MAVEN recorded (i) ionospheric photoelectrons trapped on closed magneticfield lines, (ii) Hall magnetic fields and a nonzero normal field with polarity consistent with a crossing northward of the X line, and (iii) northward Alfvenic ion jets. Dayside magnetic reconnection on crustal magnetic fields could control the global configuration and topology of the Martian magnetosphere and alter the ion escape pattern from the dayside ionosphere.
The MAVEN Magnetic Field Investigation Connerney, J. E. P.; Espley, J.; Lawton, P. ...
Space science reviews,
12/2015, Letnik:
195, Številka:
1-4
Journal Article
Recenzirano
Odprti dostop
The MAVEN magnetic field investigation is part of a comprehensive particles and fields subsystem that will measure the magnetic and electric fields and plasma environment of Mars and its interaction ...with the solar wind. The magnetic field instrumentation consists of two independent tri-axial fluxgate magnetometer sensors, remotely mounted at the outer extremity of the two solar arrays on small extensions (“boomlets”). The sensors are controlled by independent and functionally identical electronics assemblies that are integrated within the particles and fields subsystem and draw their power from redundant power supplies within that system. Each magnetometer measures the ambient vector magnetic field over a wide dynamic range (to 65,536 nT per axis) with a resolution of 0.008 nT in the most sensitive dynamic range and an accuracy of better than 0.05 %. Both magnetometers sample the ambient magnetic field at an intrinsic sample rate of 32 vector samples per second. Telemetry is transferred from each magnetometer to the particles and fields package once per second and subsequently passed to the spacecraft after some reformatting. The magnetic field data volume may be reduced by averaging and decimation, when necessary to meet telemetry allocations, and application of data compression, utilizing a lossless 8-bit differencing scheme. The MAVEN magnetic field experiment may be reconfigured in flight to meet unanticipated needs and is fully hardware redundant. A spacecraft magnetic control program was implemented to provide a magnetically clean environment for the magnetic sensors and the MAVEN mission plan provides for occasional spacecraft maneuvers—multiple rotations about the spacecraft
x
and
z
axes—to characterize spacecraft fields and/or instrument offsets in flight.
Despite produced exclusively on the dayside, photoelectrons, as an important population of the Martian ionosphere, have also been observed on the nightside. Here we present a statistical survey of ...nightside photoelectrons using the suprathermal electron measurements, made by the Solar Wind Electron Analyzer onboard the Mars Atmosphere and Volatile Evolution. We find that nearly 30% of the available nightside suprathermal electron spectra show clear photoelectron signatures. Nightside photoelectrons have an occurrence rate that decreases with increasing solar zenith angle and they are characterized by a preferentially field‐aligned pitch angle distribution. Our analysis also suggests that nightside photoelectrons are less likely observed under the high solar wind condition. These observations are favorably interpreted by the scenario of photoelectron transport along cross‐terminator magnetic field lines, as supported by a simplified test particle model. Our study highlights a complex plasma environment near Mars modulated by both internal and external conditions.
Plain Language Summary
Photoelectrons are an important population of a planetary upper atmosphere that plays a crucial role in ionization, heating, radiation, and escape. Despite that photoelectrons are ideally produced by solar radiation on the dayside only, they have also been observed on the nightside of a planet such as Mars. This study is devoted to a statistical survey of photoelectrons in the nightside Martian upper atmosphere, based on the state‐of‐the‐art measurements made by the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. A surprisingly large fraction of photoelectron energy spectra is identified on the nightside of Mars, occupying nearly 30% of the available nightside measurements. Meanwhile, substantial variations in the observations of nightside photoelectrons are suggested by the data, revealing that (1) their occurrence rate declines rapidly near and beyond the terminator; (2) their velocities are more likely parallel to the ambient magnetic field lines; and (3) they are less likely observed under high solar wind dynamic pressures. These observations could be interpreted by photoelectron transport along large‐scale magnetic field lines from their dayside source regions to the nightside, a scenario that is controlled by both internal and external conditions and thus provides a useful diagnostic of the ambient magnetic field configuration.
Key Points
The photoelectron occurrence rate in the nightside Martian ionosphere decreases with increasing solar zenith angle
Nightside photoelectrons are observed to be preferentially field‐aligned, as a natural result of cross‐terminator transport
Nightside photoelectrons are more likely observed under low solar wind dynamical pressures
The MAVEN spacecraft launched in November 2013, arrived at Mars in September 2014, and completed commissioning and began its one-Earth-year primary science mission in November 2014. The orbiter’s ...science objectives are to explore the interactions of the Sun and the solar wind with the Mars magnetosphere and upper atmosphere, to determine the structure of the upper atmosphere and ionosphere and the processes controlling it, to determine the escape rates from the upper atmosphere to space at the present epoch, and to measure properties that allow us to extrapolate these escape rates into the past to determine the total loss of atmospheric gas to space through time. These results will allow us to determine the importance of loss to space in changing the Mars climate and atmosphere through time, thereby providing important boundary conditions on the history of the habitability of Mars. The MAVEN spacecraft contains eight science instruments (with nine sensors) that measure the energy and particle input from the Sun into the Mars upper atmosphere, the response of the upper atmosphere to that input, and the resulting escape of gas to space. In addition, it contains an Electra relay that will allow it to relay commands and data between spacecraft on the surface and Earth.
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