Global Characteristics of Magnetic Flux Ropes in Mars Atmosphere !Vignes D., Acuña M., Connerney J., Crider D., NASA Goddard Space Flight Center, Greenbelt, MD, USA (
[email protected]) !Rème H., Mazelle C., Centre d'Étude Spatiale des Rayonnements, Toulouse, France !Ness N.F., Bartol Research Institute, Newark, DE, USA !Lin R.P., Mitchell D., Space Sciences Laboratory, Berkeley, CA, USA
Poster Number : P42AP42A-0533
I: What is a flux rope ? A flux rope is a small diameter twisted magnetic filaments. They are present in different locations: the geomagnetic tail, the magnetic clouds, the Venus ionosphere, on the Sun in many different forms. The nature of the flux ropes formation at Venus or at Mars is still a subject for discussion. To order the vector data, we use the minimum variance technique also called principal axis technique [Elphic and Russell, 1983]. Figure 1 [from Russell, 1990] shows the magnetic structure of a flux rope. The field is weak and azimuthal in the outer regions becoming much stronger and more axial near the center of the rope. The other part of Figure 1 shows a schematic diagram of the flight of a spacecraft through a flux rope and the resulting hodograms of the magnetic field variation in the principal axis system [Elphic et al., 1980]. The coordinates Bi, Bj and Bk refer to the maximum, intermediate and minimum variance directions, respectively. The lower left figure shows the plane of maximum variance: the width of the hodogram shows how twisted is the flux rope. The lower right figure shows the plane of minimum variance: the curvature of this hodogram indicates that the spacecraft crossed the rope off axis. Figure 1: Magnetic structure of a Flux rope & hodogram
1998−06−26 Minimum Variance Analysis 350
B1 B2 B3
0
(nT)
Altitude (km)
300
The magnetic field amplitude consists of a series of individual spike, which appears chaotic, in the Martian atmosphere (upper left panel of Figure 2).
250
−5 −10
200 −15
We order the vector data using the minimum variance technique also called principal axis technique as shown in Figure 1. Figure 2 shows an hodogram example obtained for a Martian flux rope. We use the principal axes technique for the shaded rope on the upper left corner. The upper right corner show the components of this flux rope magnetic field in the principal axes system. The lower panels shows the hodograms of the flux ropes in the plane of maximum variance and minimum variance.
150 0
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−20 20.4
20
|B| (nT)
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20.41
Time (UT)
B1 (nT)
B1 (nT)
This flux rope example is typical of those found at Venus. Using the principal axes technique, we have found 104 flux ropes in 46 different orbits over 981 orbits studied during the elliptical trajectory of MGS. The general properties of these flux ropes are summarized on the next part.
10 5
II: Hodogram of a flux rope:
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B2 (nT)
5
−15
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B3 (nT)
Figure 2: Hodogram of a Martian flux rope
20.42
Global Characteristics of Magnetic Flux Ropes in Mars Atmosphere III: Global Properties of the flux ropes
Statistical Study of Flux Ropes − Subsolar(+) Terminator(.)
Statistical Study of Flux Ropes
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=246±156 km 20
10
!50% of the FR below 200 km
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!Most of the FR near the terminator 5
!Random inclination of the FR 0 0
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N=104
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!70% FR have a FWHM inferior at 20 km
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2 j
From Figure 3:
N=104
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2 i
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Occurence
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Occurence
Properties:
=80±17
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Helicity : sqrt(si /sj )
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Helicity : sqrt(s /s )
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Magnetic Field Amplitude (nT) Solar Zenith Angle (degree)
Solar Zenith Angle (degree)
From Figure 4: =21±17 km 8
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!The strengthen FR fields are mainly found at low altitude =12 nT
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!FR half-length is highly variable near the terminator
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0 0
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Flux ropes Inclination: |sin(i)|
N=104
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!FR closer to the subsolar point appears more twisted
Flux Ropes Half−length (km)
N=104
Occurence
Occurence
!FR with weak axial magnetic field have variable helicity
10 =0.52±0.27
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Altitude (km)
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1000 800 600 400 200
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N=87
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Magnetic Field Amplitude (nT) Solar Zenith Angle (degree)
Flux Ropes Half Length (km)
Figure 3: Global properties of the flux ropes found at Mars
Figure 4: Global properties of the flux ropes found at Mars
1998 Dec 17
IV: Magnetic Cavities
40 35
Altitude versus Solar Zenith Angle for flux ropes (red) and magnetic cavities (blue)
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|B| (nT)
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We found a lot of orbits with “magnetic cavities” near the orbit periapsis. Figure 5 (right) shows an example of such a “cavity” where |B| is low and no flux ropes are observed.
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Altitude (km)
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Figure 6 (left) shows a comparison of the altitude and solar zenith angle of the location where the magnetic cavity and flux ropes are found.
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The magnetic cavities occur mainly at lower altitude than the flux ropes.
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Solar Zenith Angle (degrees)
Figure 6: Altitude distribution as a function of Solar zenith angle of the FR and the magnetic cavity
ALTITUDE (km)
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TIME
12:50
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Figure 5: Magnetic cavity without flux ropes in Mars ionosphere
V: Conclusions Flux ropes at Mars and Venus have the same characteristics: ! Flux ropes are not observed in region where the magnetic field is strong ! The magnetic structures are stationary on time according to the speed of the spacecraft ! The width of the flux ropes is of the order of a few tens kilometers ! The flux ropes have a random orientation The flux ropes at Mars have a small diameter (FWHM~20 km), with a random inclination and they are located mainly at altitude between 150 & 200 km, like those of Venus.
However, Less flux ropes are found at Mars, 5 % of the orbits, than at Venus (70 %) ⇒ A lot of magnetic cavities are found without flux ropes ⇒ Crustal field avoid the formation of the flux ropes (Figure 7)
Figure 7: FR positions projected into the Mars crustal field map
