Properties of Waves At The Proton Cyclotron Frequency Upstream From
Mars
1
Norberto Romanelli *
1
César Bertucci
1
Daniel Gómez
2
Magda Delva
3
Christian Mazelle
1
Astrophysical Plasmas, IAFE, Buenos Aires, Argentina.
2
Space Research Institute, Graz, Austria.
3
Géophysique Planétaire et Plasmas Spatiaux (GPPS), IRAP, Toulouse, France.
SM41B-2216
Abstract
We present a study on the properties of electromagnetic plasma waves in the region upstream of the Martian bow shock, detected by the magnetometer and electron reflectometer (MAG / ER) onboard the Mars Global Surveyor (MGS) spacecraft during the
period known as Science Phasing Orbits (SPO). We find that a large majority of these waves is characterized - in the spacecraft frame - by a left-hand circular polarization and a frequency which coincides with the local proton cyclotron frequency. Minimum
variance analysis of the magnetic field indicates that these waves propagate almost parallel to the background magnetic field and comparison with suprathermal electron fluxes measured by ER suggests a small degree of compressibility. We also observe a
change in their occurrence rate with the highest values close to Mars’s perihelion. In addition, the spatial distribution of these waves does not seem to depend on the orientation of the solar wind convective electric field assuming a flow along the Sun-Mars line.
Also, we find that their amplitude decreases with radial distance from the planet, supporting the idea that their source is Mars. Empirical evidence and theoretical approaches suggest that most of these observations correspond to the ion-ion right hand (RH)
mode originating from the pick-up of ionized exospheric hydrogen. The left-hand (LH) mode might be present in cases where the IMF cone angle is high. We discuss these theoretical results and their implications for Mars and other atmospheric unmagnetized
objects such as Venus.
1. Properties of plasma waves in the MAG/ER measurements
I
Minimum variance analysis (MVA) / Spectral properties:
Figure 1 shows that the electromagnetic plasma waves present in the environment of Mars
are characterized, in the spacecraft frame, by a left-hand circular polarization with respect
to the mean magnetic field. MVA of the magnetic field also indicates that these waves
propagate almost parallel to the background magnetic field (θkB = 8◦, λ2/λ3 = 50.3
). These plasma waves are also characterized, in the spacecraft frame, by a frequency
which coincides with the local proton cyclotron frequency. Comparison with suprathermal
electron fluxes measured by ER suggests a small degree of compressibility.
2. Temporal occurrence and spatial distribution of the
proton cyclotron waves
Figure 3 shows the percentage of waves observed by MAG (which were determined taking into account the fourier dynamic spectrum of the transverse
component of the magnetic field) as a function of the day of the year 19971998.
Orbit P216
2
1.5
1
B2(nT)
0.5
0
−0.5
−1
−1.5
Figure 3. Percentage of waves observed by MAG as a
function of the day of the year.
−2
−2.5
−3
−2.5
−2
−1.5
−1
−0.5
0
0.5
1
Figure 5 shows the wave amplitude versus planetocentric distance for all SPO orbits.
The black curve displays the average amplitudes for 0.5 RM bins, showing a general
decrease of amplitude with increasing distance.
We find a trend which
shows an increase in the
occurrence rate of the
waves when Mars is closer
to the Sun.
It is also
observed a shift between
the day of the year corresponding to the maximum
percentage and the one
corresponding to Mars’s
perihelion (doy 7, 1998).
Figure 5. Amplitude of the PCW’s as a function of the
altitude for all SPO orbits.
Assuming that the source
for these waves are exospheric pick-up protons,
the obtained result is to be
expected for at least two
reasons: the ion density
decreases with distance,
and for smaller MGS altitudes, waves have more
time to grow as they are
convected by the Solar
Wind.
4. Results and Conclusions
1.5
B1(nT)
~ o )3 is positive and points out of the
Figure 1. Left: MVA Hodogram corresponding to 10 cyclotron periods of the orbit P216. (B
page roughly(opposite to the sense of gyration). Right: Fourier dynamic spectrum of By (orbit P216) The local proton cyclotron
frequency calculated from MAG data is plotted in black for reference.
I
3. Amplitude of the plasma waves vs planetocentric distance
MAG/ER cross correlation
Figure 2. Fluctuations in the parallel component of the
magnetic field and in the electron flux measurements
(E=116 eV) during part of the orbit P232.
Figure 2 shows the fluctuations
in the parallel component of the
magnetic field (upper panel), and
in the electron flux at 116 eV
(lower panel) during part of the
orbit P232. The cross-correlation
of the two time series shown in
the enclosed panel displays a
peak at zero displacement with
a correlation value of 0.7, falling
down to 0.44 at ±2s.
We repeat this analysis for other
events and for different energy
channels with similar results,
which is consistent with both time
series being in phase, considering an error of ±2s associated
with the cadence of the ER instrument.
We also study the spatial distribution of the PCW when they accomplish the
following criteria:
Coherence ≥ 0.7 Ellipticity ≤ −0.5 Degree of polarization ≥ 70 %
Figure 4 shows MGS trajectory segments where we observe upstream waves
at the proton cyclotron frequency in an “electromagnetic”coordinate system
where the z-axis is parallel to the convective electric field. We assume that
the solar wind velocity points radially from the Sun to Mars (X-MBE axis). We
found that their spatial distribution does not seem to be affected by solar wind
convective electric field.
I The
ULF plasma waves observed at Mars by MGS are characterized - in the
spacecraft frame - by a left-hand circular polarization and a frequency which coincides with the local proton cyclotron frequency. These waves propagate almost
parallel to the background magnetic field and they have a small degree of compressibility.
I The results also show that the occurrence rate of these waves changes with time.
The highest values are found when Mars is close to its perihelion. Also, the spatial
distribution of these waves does not seem to depend on the orientation of the
solar wind motional electric field. This lack of correlation has also been observed
in Venus [Delva et al., 2011].
I Theoretical approaches suggest that these properties are compatible with plasma
waves originating in the pick-up of ionized exospheric hydrogen. In fact, Also, the
amplitude of the waves decreases with radial distance from the planet, which also
supports the idea that their source is Mars.
References
Figure 4. Trajectory of the spacecraft on several orbits (MBE reference frame).
I Acuña, M. et al., 2001. Magnetic field of Mars: summary of results from the aerobraking and mapping orbits, J.
Geophys. Res., 106, E10, 23403-23417, 2001.
I Delva, M., C. Mazelle, C. Bertucci, M. Volwerk, Z. Vörös, and T. L. Zhang (2011), Proton cyclotron wave generation
mechanisms upstream of Venus, J. Geophys. Res., 116, A02318, doi:10.1029/2010JA015826.
I Mazelle, C. et al., 2004. Bow shock and upstream phenomena at Mars, Space Science Reviews 111, 115-181, 2004.
I http://dx.doi.org/10.1016/j.pss.2012.10.011
I Wei, H. y Russell, C.T. 2006. Proton cyclotron waves at Mars: Exosphere structure and evidence for a fast neutral
disk, J. Geophys. Res., 33, L23103, 2006.