164 research outputs found
The interaction between the Moon and the solar wind
We study the interaction between the Moon and the solar wind using a
three-dimensional hybrid plasma solver. The proton fluxes and electromagnetical
fields are presented for typical solar wind conditions with different magnetic
field directions. We find two different wake structures for an interplanetary
magnetic field that is perpendicular to the solar wind flow, and for one that
is parallell to the flow. The wake for intermediate magnetic field directions
will be a mix of these two extreme conditions. Several features are consistent
with a fluid interaction, e.g., the presence of a rarefaction cone, and an
increased magnetic field in the wake. There are however several kinetic
features of the interaction. We find kinks in the magnetic field at the wake
boundary. There are also density and magnetic field variations in the far wake,
maybe from an ion beam instability related to the wake refill. The results are
compared to observations by the WIND spacecraft during a wake crossing. The
model magnetic field and ion velocities are in agreement with the measurements.
The density and the electron temperature in the central wake are not as well
captured by the model, probably from the lack of electron physics in the hybrid
model.Comment: Accepted for publication in Earth, Planets and Spac
Protons in the near-lunar wake observed by the Sub-keV Atom Reflection Analyzer on board Chandrayaan-1
Significant proton fluxes were detected in the near wake region of the Moon
by an ion mass spectrometer on board Chandrayaan-1. The energy of these
nightside protons is slightly higher than the energy of the solar wind protons.
The protons are detected close to the lunar equatorial plane at a
solar zenith angle, i.e., ~50 behind the terminator at a height of
100 km. The protons come from just above the local horizon, and move along the
magnetic field in the solar wind reference frame. We compared the observed
proton flux with the predictions from analytical models of an electrostatic
plasma expansion into a vacuum. The observed velocity was higher than the
velocity predicted by analytical models by a factor of 2 to 3. The simple
analytical models cannot explain the observed ion dynamics along the magnetic
field in the vicinity of the Moon.Comment: 28 pages, 7 figure
Dynamics of solar wind protons reflected by the Moon
Solar system bodies that lack a significant atmosphere and significant
internal magnetic fields, such as the Moon and asteroids, have been considered
as passive absorbers of the solar wind. However, ion observations near the Moon
by the SELENE spacecraft show that a fraction of the impacting solar wind
protons are reflected by the surface of the Moon. Using new observations of the
velocity spectrum of these reflected protons by the SARA experiment on-board
the Chandrayaan-1 spacecraft at the Moon, we show by modeling that the
reflection of solar wind protons will affect the global plasma environment.
These global perturbations of the ion fluxes and the magnetic fields will
depend on microscopic properties of the object's reflecting surface. This solar
wind reflection process could explain past ion observations at the Moon, and
the process should occur universally at all atmosphereless non-magnetized
objects.Comment: 12 pages, 8 figure
Shocklets and Short Large Amplitude Magnetic Structures (SLAMS) in the high Mach foreshock of Venus
Shocklets and short large-amplitude magnetic structures (SLAMS) are steepened magnetic fluctuations commonly found in Earth's upstream foreshock. Here we present Venus Express observations from the 26th of February 2009 establishing their existence in the steady-state foreshock of Venus, building on a past study which found SLAMS during a substantial disturbance of the induced magnetosphere. The Venusian structures were comparable to those reported near Earth. The 2 Shocklets had magnetic compression ratios of 1.23 and 1.34 with linear polarization in the spacecraft frame. The 3 SLAMS had ratios between 3.22 and 4.03, two of which with elliptical polarization in the spacecraft frame. Statistical analysis suggests SLAMS coincide with unusually high solar wind AlfvĂ©n mach-number at Venus (12.5, this event). Thus, while we establish Shocklets and SLAMS can form in the stable Venusian foreshock, they may be rarer than at Earth. We estimate a lower limit of their occurrence rate of âł14%
Callisto's Atmosphere and Its Space Environment: Prospects for the Particle Environment Package on Board JUICE
The JUpiter ICy moons Explorer (JUICE) of the European Space Agency will investigate Jupiter and its icy moons Europa, Ganymede, and Callisto, with the aim to better understand the origin and evolution of our Solar System and the emergence of habitable worlds around gas giants. The Particle Environment Package (PEP) on board JUICE is designed to measure neutrals and ions and electrons at thermal, suprathermal, and radiation belt energies (eV to MeV). In the vicinity of Callisto, PEP will characterize the plasma environment, the outer parts of Callisto's atmosphere and ionosphere and their interaction with Jupiter's dynamic magnetosphere. Roughly 20 Callisto flybys with closest approaches between 200 and 5,000 km altitude are planned over the course of the JUICE mission. In this article, we review the state of the art regarding Callisto's ambient environment and magnetospheric interaction with recent modeling efforts for Callisto's atmosphere and ionosphere. Based on this review, we identify science opportunities for the PEP observations to optimize scientific insight gained from the foreseen JUICE flybys. These considerations will inform both science operation planning of PEP and JUICE and they will guide future model development for Callisto's atmosphere, ionosphere, and their interaction with the plasma environment
Statistical distribution of mirror-mode-like structures in the magnetosheaths of unmagnetised planets â Part 1: Mars as observed by the MAVEN spacecraft
In this series of papers, we present statistical maps of mirror-mode-like (MM) structures in the magnetosheaths of Mars and Venus and calculate the probability of detecting them in spacecraft data. We aim to study and compare them with the same tools and a similar payload at both planets. We consider their dependence on extreme ultraviolet (EUV) solar flux levels (high and low) and, specific to Mars, on Mars Year (MY) as well as atmospheric seasons (four solar longitudes Ls).
We first use magnetic-field-only criteria to detect these structures and present ways to mitigate ambiguities in their nature. In line with many previous studies at Earth, this technique has the advantage of using one instrument (a magnetometer) with good time resolution, facilitating comparisons between planetary and cometary environments. Applied to the magnetometer data of the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft from November 2014 to February 2021 (MY32âMY35), we detect events closely resembling MMs lasting in total more than 170â000âs, corresponding to about 0.1â% of MAVEN's total time spent in the Martian plasma environment. We calculate MM-like occurrences normalised to the spacecraft's residence time during the course of the mission. Detection probabilities are about 1â% at most for any given controlling parameter. In general, MM-like structures appear in two main regions: one behind the shock and the other close to the induced magnetospheric boundary, as expected from theory. Detection probabilities are higher on average in low-solar-EUV conditions, whereas high-solar-EUV conditions see an increase in detections within the magnetospheric tail. We tentatively link the former tendency to two combining effects: the favouring of ion cyclotron waves the closer to perihelion due to plasma beta effects and, possibly, the non-gyrotropy of pickup ion distributions. This study is the first of two on the magnetosheaths of Mars and Venus.</p
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