Time‐dispersed ion signatures observed in the Martian magnetosphere by MAVEN

Abstract

Mars Atmosphere and Volatile EvolutioN mission's (MAVEN) high‐cadence measurements reveal the frequent occurrence of ion energy dispersion events inside the Martian magnetosphere. The systematics of observed dispersion signatures suggest time dispersion of a broad source spectrum over a flight distance of a few thousand kilometers and disfavor mechanisms involving spatial dispersion. Pickup of heavy planetary ions in strong variable electric fields provides one potential mechanism that could produce the observed time dispersion signatures. The periodicity of many observed dispersion signatures, with frequencies near the upstream proton cyclotron frequency, suggests a possible role for low‐frequency plasma waves in accelerating the observed ions from a source near the induced magnetospheric boundary to the observation location inside the magnetospheric tail lobes. The observed dispersion signatures may provide a new way to track the flow of energy from the upstream region through the magnetosphere and the role of waves in driving the escape of ions from the atmosphere.

Solar Wind Induced Waves in the Skies of Mars: Ionospheric Compression, Energization, and Escape Resulting From the Impact of Ultralow Frequency Magnetosonic Waves Generated Upstream of the Martian Bow Shock

“…Using in situ observations from the NASA Mars Atmosphere and Voltatile EvolutioN (MAVEN) Mars Scout, we show that the impact of compressive magnetosonic waves, from the upstream solar wind, drives linearly polarized, compressive magnetosonic waves in the ionosphere, inducing a localized "ringing" in the induced magnetosphere at the local proton cyclotron frequency. We show that these waves heat and compress the atmosphere and drive time‐dispersed ions as hypothesized by Halekas, McFadden, et al (), likely enhancing ion escape from the upper ionosphere as proposed by Ergun et al (), Lundin et al (), and Dubinin and Fraenz (). Expanding on this single case study to a preliminary survey of the phenomenon, we find that no special conditions are required in the interplanetary magnetic field or solar wind for waves upstream to drive compressive waves in the ionosphere.…”

“…However, the spacecraft carried no ion spectrometer and operated in a 400‐km circular science orbit, so this could not be investigated. Recently, Halekas, McFadden, et al () discovered and mapped bursts of time‐dispersed energetic ions at Mars, with frequencies near the upstream proton cyclotron frequency, suggesting that upstream solar wind pressure perturbations may be a driver. Additionally, Fowler et al () found that electric field wave power, whose source was assumed to be within the magnetosheath, penetrates down to ∼200‐km altitude into the dayside upper ionosphere, further suggesting a non‐Martian source of waves in the Martian ionosphere.…”

Structure, dynamics, and seasonal variability of the Mars‐solar wind interaction: MAVEN Solar Wind Ion Analyzer in‐flight performance and science results

“…On the inbound segment MAVEN crossed the induced current sheet, a region surrounded by heated electrons and accelerated planetary ions. On the outbound segment, farther downstream along the flank, the transition from magnetosphere to sheath proves much less distinct, with intermixed solar wind and planetary ions, and highly accelerated heavy ion populations, a few with apparent energy‐time dispersion that may result from nonlocal acceleration of heavy ions [ Halekas et al ., ]. These accelerated ions appear in regions with a wide range of suprathermal electron fluxes, suggesting that they cross magnetic field lines, as expected given their large gyroradii.…”

“…These could represent shear‐driven instabilities such as Kelvin‐Helmholtz [ Penz et al ., ] and/or bulk loss of clouds of magnetosphere plasma [ Halekas et al ., ]. Inside the magnetosphere, energy‐time dispersed ions [ Halekas et al ., ] appear throughout the magnetosphere, suggesting acceleration by significant electric fields, possibly associated with the upstream and/or sheath dynamics. On the outbound segment, the IMF rotates away from a radial configuration, and the sheath and upstream region return to a more typical configuration.…”

Pressure and ion composition boundaries at Mars

“…The transition between the magnetosheath and the magnetic pileup region has several observational characteristics, which have been identified by different instruments on various spacecraft. One feature is a sharp increase in magnetic field strength coincident with a decrease in field fluctuations, which was observed by Phobos‐2 [e.g., Riedler et al , ], the Mars Global Surveyor (MGS) spacecraft [e.g., Vignes et al , ; Crider et al , ; Bertucci et al , ], and also the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft [e.g., Jakosky et al , , ; Connerney et al , ; Halekas et al , ; Matsunaga et al , ]. These observations resulted in the names known as the magnetic pileup boundary (MPB) or the induced magnetosphere boundary.…”