ABSTRACT: We present evidence for time variability in the martian ionosphere due
to solar energetic particles. We also discuss possible ionospheric
effects from meteoric material, dust storms, the diurnal ionization
cycle, and crustal magnetic fields.
ABSTRACT: 1] We report observations by Mars Global Surveyor (MGS) of thousands of peaked electron energy spectra similar to terrestrial auroral electrons. They are observed on the Martian nightside, near strong crustal magnetic sources. The spectra have peak energies ranging from 100 eV – 2.5 keV, and fluxes near the peak are 10– 10000 times higher than typical nightside spectra. They occur on magnetic field lines that connect the shocked solar wind to crustal magnetic fields, and on adjacent closed field lines. Their detection is directly controlled by the solar wind, suggesting that magnetic reconnection is required for their observation. We calculate that the most energetic distributions could produce atmospheric emission with intensity comparable to that recently reported from the Mars Express (MEX) spacecraft. Half of the most energetic examples occur during the passage of space weather events past Mars, suggesting that a disturbed plasma environment is favorable for electron acceleration along magnetic field lines.
Geophys. Res. Lett. 01/2006; 33.
ABSTRACT: 1] The powerful x-class flare which occurred on the Sun on 28 October 2003 had important effects on plasma environments throughout the solar system. We present here observations of the effects at Mars from the Mars Global Surveyor (MGS) Magnetometer/ Electron Reflectometer experiment. In particular we focus on the changes in the nature of the magnetic oscillations observed at an altitude of 400 km (MGS's current orbital altitude) during the passage of the solar storm. We find that strong, regular oscillations are observed in both the B k and B ? components of the magnetic field at all solar zenith angles. We emphasize in particular the powerful, coherent oscillations observed in the normally quiet nightside region. These oscillations carry power at the proton gyrofrequency and at and below the oxygen gyrofrequency. This implies that ions of planetary origin are interacting with the solar wind plasma and raises the possibility that significant atmospheric loss may occur during the passage of large solar storms at Mars.
J. Geophys. Res. 01/2005; 110.
ABSTRACT: 1] The magnetic pileup boundary (MPB) is a sharp, thin, and well-defined plasma boundary located between the bow shock and the inner ionospheric boundary at comets, Mars, and Venus. This boundary separates the magnetosheath, a region of low magnetic fields with a conspicuous wave activity, from the magnetic pileup region dominated by strong, highly organized magnetic fields as a result of the pileup and draping of the interplanetary magnetic field. In the present paper we study the magnetic structure of the magnetic pileup boundary at Mars and Venus by means of the technique of minimum variance of the magnetic field. For each one of the crossings analyzed, we obtain a very well defined minimum variance vector. At Mars the direction of this vector agrees with the normal to the MPB fit obtained from Mars Global Surveyor crossings. The results confirm that the MPB is a well-defined plasma boundary. According to empirical criteria based on minimum variance analysis results, the Martian and Venusian MPB crossings would resemble an MHD tangential discontinuity rather than a rotational discontinuity. However, spacecraft observations suggest that the nature of the MPB could be far more complex. We compare our results with similar studies at the MPB of comets and the magnetic tail boundary of Titan, and we discuss the nature of the boundary from a general perspective.
J. Geophys. Res. 01/2005; 110.
ABSTRACT: We report observations of magnetic fields amplitude, which consist of a series of individual spikes in the Martian atmosphere.
A minimum variance analysis shows that these spikes form twisted cylindrical filaments. These small diameter magnetic filaments
are commonly called magnetic flux ropes. We examine the global characteristics of magnetic flux ropes, which are observed
on 5% of the elliptical orbits of Mars Global Surveyor. Flux ropes are more often observed in Venus' atmosphere (70% of the
orbits). In this paper we report some of the global characteristics of the flux ropes identified in the Martian atmosphere.
No flux ropes are observed in the southern hemisphere of Mars. Most of them occur at high solar zenith angles, close to the
terminator plane, and at high latitude with altitudes below 400 km. The orientation of the flux ropes appears random while
in the case of Venus the orientation is more horizontal near the terminator for altitudes greater than 200 km. We have identified
fewer flux ropes for SZA between 40 to 60 deg and for SZA lower than 20 deg, like in the case of Venus (Elphic and Russell,
1983b). Statistically, Mars' ionosphere with SZA range between 40circ to 60circ is less magnetized than near the subsolar point. As the Martian ionosphere is quite often magnetized by the magnetic components
of the crustal field, this crustal magnetic field seems to inhibit the flux ropes formation in the southern hemisphere. However,
some orbits without crustal magnetic field, called magnetic cavities, were observed without flux ropes. So the flux ropes
formation process seems to be uppressed by another factor, like the solar wind dynamic pressure for Venus (Krymskii and Breus,
Space Science Reviews 01/2004; 111(1):223-231. · 3.61 Impact Factor
ABSTRACT: Mars Global Surveyor is the sixth spacecraft to return measurements of the Martian bow shock. The earlier missions were Mariner
4 (1964), Mars 2 and 3 (1972), Mars 5 (1975) and Phobos 2 (1989) (see reviews by Gringauz, 1981; Slavin and Holzer, 1982;
Russell, 1985; Vaisberg, 1992a,b; Zakharov, 1992). Previous investigations of planetary bow shocks have established that their
position, shape and jump conditions are functions of the upstream flow parameters and the nature of the solar wind — planet
interaction (Spreiter and Stahara, 1980; Slavin et al., 1983; Russell, 1985). At Mars, however, the exact nature of the solar wind interaction was elusive due to the lack of low
altitude plasma and magnetic field measurements (e.g., Axford, 1991). In fact our knowledge of the nature of the interaction
of Mars with the solar wind was incomplete until the arrival of MGS and the acquisition of close-in magnetic field data (Acuña
et al., 1998). As detailed by a series of review papers in this monograph, the Mars Global Surveyor (MGS) mission has now shown
that the Mars environment is very complex with strong, highly structured crustal magnetic remnants in the southern hemisphere,
while the northern hemisphere experiences the direct impingement of solar wind plasma. This review paper first presents a
survey of the observations on the Martian bow shock and the upstream phenomena in the light of results from all the missions
to date. It also discusses the kinetic properties of the Martian bow shock compared to the predictions of simulations studies.
Then it examines the current status of understanding of these phenomena, including the possible sources of upstream low-frequency
waves and the interpretations of localized disturbances in the upstream solar wind around Mars. Finally, it briefly discusses
the open issues and questions that require further study.
Space Science Reviews 01/2004; 111(1):115-181. · 3.61 Impact Factor
ABSTRACT: 1] Observations are presented of low-frequency magnetic oscillations in the Martian magnetosheath, magnetic pileup region, and tail as observed by the Magnetometer/ Electron Reflectometer experiment on board Mars Global Surveyor. Within the dayside magnetosheath the oscillations are found to be predominantly compressional, elliptically polarized waves with wave vectors that have large angles relative to the mean field and dominant frequencies that are significantly below the local proton gyrofrequency. On the basis of these observations we identify these oscillations as mirror mode instabilities. In the nightside magnetosheath the oscillations are predominantly transverse and elliptical, and they propagate at smaller angles relative to the mean field at frequencies within a factor of 2–10 less than the local proton gyrofrequency. These characteristics lead us to associate these oscillations with ion/ion-resonant instabilities that arise from counterstreaming plasma populations such as the solar wind and pickup ions of planetary origin. Within the magnetic pileup region and tail the oscillations have considerably smaller amplitudes and are linearly polarized, obliquely propagating, ultralow-frequency oscillations which may be a mix of multiple wave modes.
J. Geophys. Res. 01/2004; 109.
ABSTRACT: The Electron Reflectometer (ER) onboard Mars Global Surveyor (MGS)
measures the energy and angular distributions of 10 eV - 20 keV
electrons. The ER can distinguish between solar wind electrons and
ionospheric photoelectrons on the basis of their energy spectra. During
the elliptical aerobraking orbits, a boundary separating solar wind and
ionospheric plasmas was consistently observed at altitudes ranging from
200 to 800 km and solar zenith angles >75 degrees in the northern
hemisphere (Mitchell et al., GRL 27, 1871, 2000). This boundary is
likely coincident with the ionopause. The MGS mapping altitude of ~400
km places the spacecraft close to the 380-km median ionopause altitude,
resulting in numerous boundary crossings. We have categorized more than
two million electron energy spectra from the first 15 months of the
mapping orbit (February 1999 through April 2000) to investigate
systematic variations of the ionosphere in response to crustal magnetic
fields and solar ionizing radiation. The distribution of crustal
magnetic fields is the dominant factor influencing the probability that
the ionosphere will reach the spacecraft altitude of 400 km or higher.
Over strong-field regions of the southern hemisphere, this probability
exceeds 50%, whereas over weak-field regions in the northern hemisphere
(e.g., Tharsis and Elysium) it is less than 10%. The ionosphere over
weak-field regions also shows a measureable response to variations in
solar EUV, with the probability increasing from ~1% to ~20% as the F10.7
flux (scaled and time-shifted to account for solar rotation and the
different orbital positions of Earth and Mars) increases from 30 to 80
sfu's. The probability over strong-field regions shows little response
to variations in solar EUV, probably because of the dominating influence
of the crustal fields.
ABSTRACT: Over the course of 290 orbits, the Electron Reflectometer onboard Mars Global Surveyor consistently observed a plasma boundary at a median altitude of 380 km, where electron fluxes at energies greater than ∼100 eV change abruptly by about an order of magnitude. Above the boundary, electron energy spectra are consistent with solar wind electrons that have been shocked and then cooled by impact with exospheric neutrals. Below the boundary, elec-tron energy spectra exhibit a broad feature from 20 to 50 eV, which likely results from a blend of unresolved photoion-ization peaks that have been predicted by published models of ionospheric photoelectrons at Mars. We attribute a sec-ond feature at ∼500 eV to oxygen Auger electrons. The 500-eV flux level measured below the boundary responds to variations in the solar soft x-ray flux and is consistent with a balance between photoionization and loss by impact with atmospheric neutral atoms.
ABSTRACT: The Electron Reflectometer (ER) onboard Mars Global Surveyor measures
the energy and angular distributions of 10 eV to 20 keV electrons.
During the aerobraking phase, measurements of the Martian ionosphere
were made in the northern hemisphere between solar zenith angles (SZAs)
of 45 and 115 degrees. The ionopause was crossed at altitudes ranging
from 180 km to over 1000 km, with a median of 380 km. The
400-km-altitude polar mapping orbit allows ionospheric observations at
SZAs from 25 to 155 in both the northern and southern hemispheres. The
near-planet ionosphere/magnetotail structure of the night hemisphere is
dominated by the presence of intense crustal magnetic fields, which are
located south of the dichotomy boundary. Crustal field strengths in
excess of 200 nT are measured at 400 km altitude, which are strong
enough to create miniature magnetospheres that exclude solar wind plasma
traveling up the magnetotail. When the spacecraft passes through one of
these crustal magnetospheres, the ER count rate falls to the
instrumental background, representing an electron flux drop of at least
two orders of magnitude. A map of these flux drop-outs in longitude and
latitude closely resembles a map of the crustal magnetic sources. Away
from crustal magnetic fields, ionospheric photoelectrons are typically
observed up to SZAs of 125 degrees, and solar wind electrons are seen at
ABSTRACT: 1] Mars Global Surveyor (MGS) Magnetometer (MAG) data provide constraints on magnetic morphology at Mars, including the relative importance of the solar wind and of crustal magnetic sources. We analyze MAG data to characterize the upstream interplanetary magnetic field (IMF) and confirm trends in the magnetic field expected from the solar wind interaction with a planetary atmosphere, including increases at the shock and magnetic pile-up boundary (MPB), postshock turbulence, and field line draping around the Martian obstacle. Crustal magnetic sources locally modify the solar wind interaction, adding variability to the Martian magnetic environment that depends on planetary rotation. We identify trends in the vector magnetic field with respect to altitude, solar zenith angle, and planetary location. Crustal sources influence the magnetic field to different altitudes above different regions, and the influence of the strongest source extends to 1300–1400 km. The draped IMF partially controls the field topology above crustal sources, and crustal magnetic field lines reconnect to this field in a systematic fashion that depends upon Mars' geography, IMF strength, and IMF orientation.
J. Geophys. Res. 01/1424; 108.
ABSTRACT: 1] The absence of a global-scale dynamo-generated magnetic field and the existence of an ionosphere at Venus and Mars caused many to predict that their solar wind interaction would be similar. After Pioneer Venus Orbiter (PVO) observations, it was concluded that the global aspects of the Venusian interaction could be well described by single-fluid models. According to these models, the magnetic field draping should develop progressively from the shock down to the ionopause. A recent study at Mars, where a ''Venus-like'' interaction was expected, showed that draping is prominent only inside the magnetic pileup boundary (MPB), a well-defined plasma boundary located between the shock and the ionopause first reported at comets, but never at Venus. From an identical analysis on PVO magnetometer data, we report a dramatic enhancement of draping on the dayside of Venus. Then, we deduce the existence of a Venusian counterpart of the Martian and cometary MPB.
Geophys. Res. Lett. 01/1029; 5737(30):6030-187610.
ABSTRACT: 1] During the first year of the Mars Global Surveyor (MGS) mission, 553 shock crossings have been identified from a total of 363 orbits. The shape of the shock has been determined by exami-ning the MGS spacecraft Magnetometer/Electron Reflectometer (MAG/ER) data. The location of the shock was found highly variable. The present study shows that the high crustal magnetic sources, found in the southern hemisphere, do not seem responsible for the Bow Shock (BS) variability. The present study shows that contrary to many expectations there is no obvious strong one to one correlation between the location of the highest crustal sources and the variability of the shock position. On the other hand, the shock appears farthest from Mars in the hemisphere of locally upward interplanetary electric field consistent with the idea that mass loading play a role in controlling the BS location, which confirms previous results.
ABSTRACT: The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) onboard the Mars Express spacecraft has occasionally displayed surprising features. One such feature is the occurrence of a series of broadband, low-frequency echoes at equally spaced delay times after the sounder transmitter pulse. The interval between the echoes has been shown to be at the cyclotron period of electrons orbiting in the local magnetic field. The electrons are believed to be accelerated by the large voltages applied to the antenna by the sounder transmitter. Measurements of the period of these “electron cyclotron echoes” provide a simple technique for determining the magnitude of the magnetic field near the spacecraft. These measurements are particularly useful because Mars Express carries no magnetometer, so this is the only method available for measuring the magnetic field magnitude. Using this technique, results are presented showing the large scale structure of the draped field inside the magnetic pile-up boundary. The magnitude of the draped field is shown to vary from about 40 nT at a solar zenith angle of about 25°, to about 25 nT at a solar zenith angle of 90°. The results compare favorably with similar results from the Mars Global Surveyor spacecraft. A fitting technique is developed to derive the vector direction and magnitude of the draped magnetic field in cases where the spacecraft passes through regions with significant variation in the crustal field. The magnetic field directions are consistent with current knowledge of the draping geometry of the magnetic field around Mars.
Icarus 206(1):104-111. · 3.38 Impact Factor
ABSTRACT: A substantial and complex magnetic field was discovered at Neptune
during the Voyager 2 encounter in 1989. This chapter discusses the
quantitative characteristics of the Neptunian magnetic field and its
implications with respect to the planetary interior and also the
trapping of energetic particles. Due to the several moons and rings
located within the Neptunian magnetosphere, and the large tilt of the
magnetic dipole axis, the structure of the radiation belts is quite
complicated due to the absorption of charged particles by these objects.
Finally, a comparison of Neptune's magnetic field and its implications
regarding the interior of the planet and the dynamo process with other
planetary magnetic fields is made. Neptune is like an oblique rotator
found in stellar astrophysics.
ABSTRACT: The magnetic pileup boundary (MPB) is a sharp, thin, and permanent plasma boundary reported, up to now, at comets and Mars, and located between the bow shock and the ionospheric boundary. The MPB separates the magnetosheath, a region with high wave activity, from the magnetic pileup region, where the interplanetary magnetic field piles up regularly in front of the planetary obstacle. We use magnetic field and electron plasma measurements from the MAG/ER experiment onboard the Mars Global Surveyor spacecraft to study two characteristic features of the MPB. The first feature is the sudden enhancement of the magnetic field line draping at the MPB. This new signature reveals that the MPB marks the entry into the veritable induced magnetosphere, where the magnetic field topology is more regular, in opposition to what is observed in the magnetosheath. Secondly, we study the properties and the occurrence of compressive, linearly polarized, low frequency waves, frequently observed on both sides of the boundary. An analysis of the correlation between the magnetic and the electron data reveals that on the upstream side the waves are mirror mode waves, while on the downstream side they are large amplitude, quasi-monochromatic fast magnetosonic waves. The presence of these features at other atmospheric, unmagnetized bodies can be used as indicators of the existence of a MPB.
Advances in Space Research.
ABSTRACT: The Lunar Prospector Electron Reflectometer has obtained the first global map of lunar crustal magnetic fields, revealing that the effects of basin-forming impacts dominate the large-scale distribution of remanent magnetic fields on the Moon. The weakest surface magnetic fields (<0.2 nT) are found within two of the largest and most recent impact basins, Orientale and Imbrium. Conversely, the largest concentrations of strong surface fields (>40 nT) are diametrically opposite to these same basins. This pattern is present though less pronounced for several other post-Nectarian impact basins larger than 500 km in diameter. The reduced strength and clarity of the pattern for older basins may be attributed to: (1) demagnetization from many smaller impacts, which erases antipodal magnetic signatures over time, (2) superposition effects from other large impacts, and (3) variation in the strength of the ambient magnetizing field. The absence of fringing fields stronger than 1 nT around the perimeter of the Imbrium basin or associated with craters within the basin implies that any uniform magnetization of the impact melt must be weaker than ∼10−6 G cm3 g−1. This limits the strength of any steady ambient magnetic field to no more than ∼0.1 Oe at the lunar surface while the basin cooled for tens of millions of years following the Imbrium impact 3.8 billion years ago.
ABSTRACT: A great many Martian bow shock and magnetic pile-up boundary crossings have been identified in the Phobos 2 and Mars Global Surveyor, MGS, data. From these observations the positions and shapes of the bow shock and magnetic pile-up boundary, MPB, have been derived and modelled, using curve-fitting techniques. The models thus derived separately from the Phobos 2 and MGS data sets do not differ drastically, despite the different time and space data coverages. The purpose of the paper is therefore to show the results obtained from the mixing of the Phobos 2 and MGS data bases and to compare the derived bow shock and MPB models with the ones obtained previously. The underlying objective was to see whether it was possible to determine improved bow shock and MPB models or not. The answer is definitely yes, and particularly for the MPB, thanks to the complementary nature of the observations. The boundaries crossed close to the subsolar direction or mostly far downstream by Phobos 2 indeed allow a better coverage of the Martian space environment to be considered. Nevertheless, in order to reduce the domination of the overabundant MGS data set and/or the crossings that are close to Mars (x>−4 RM, i.e. x>−13 562 km) weighting factors have been introduced.
Planetary and Space Science.