Ion Pickup Phenomenon upstream of Mars observed by ASPERA 3

M. Yamauchi

Scientific motivationTo understand ASPERA-3 data, it is extremely important to have the magnetic field (B) measurement.

However, Mars Express (MEX) does not have it (because of beaurocracy), and we could get it only for Venus Express.

Therefore, Any method to estimate B is helpful. Use Mars Global Surveyor (MGS) data or Analyze gyromotion of ions observed by ASPERA/IMA because ion motion in velocity space is a ring! (spiral in real space)

Personal motivationAmong many interesting phenomena seen in IMA data, the ring is one of the most "beautiful" looking in Quick Look.

2005-4-27 1331-1344 UT

2005-7-12 1133-1146 UT

2005-6-3 0602-0615 UT

In-coming directions of ions

Azimuth scan Elevation scan

Electric scan

Electric scanIM

A

0 3 6 9 12 15

0 3 6 9 12 1501

2

3

4

5

6

1514

2005-4-27 (easy case)

Plot for single azimuth

= 5~15

= 4

= 3

= 2

= 1

SW

ring

H+ only (no He++) !

= 3

= 2 = 4 = 3 = 2

SW He++

Ring H+

SW H+

Motion of ring ions

V0 = 0 , Viewing from +B

Ions with V0=0 have initial perpendicular velocity to B = |VSW|cos() in SW rest frame, where sin() (= BX/B) is angle VSW & highest energy. The parallel velocity is constant in SW rest frame, and is zero in Martian frame

Selected ring data : ~ 1344~1347 UTEl Az sensor direction (X, Y, Z) E (keV) VX (km/s) VY (km/s) VZ (km/s)02 03 (0.81, -0.29, 0.51) 1.37 -414 148 -26003 03 (0.85, -0.30, 0.42) 1.86 -508 181 -25104 03 (0.89, -0.32, 0.33) 2.27 -584 208 -21805 03 (0.91, -0.33, 0.24) 2.49 -630 225 -16408 03 (0.94, -0.34, -0.05) 2.74 -680 243 3909 03 (0.93, -0.33, -0.15) 2.74 -673 241 11010 03 (0.91, -0.33, -0.25) 2.74 -659 236 18012 03 (0.85, -0.30, -0.43) 2.49 -585 210 29813 03 (0.80, -0.29, -0.52) 2.27 -529 190 34114 03 (0.75, -0.27, -0.60) 2.00 -464 167 37015 03 (0.69, -0.25, -0.68) 1.68 -392 141 382

*1) L = (0.05, -0.02, 1.00) in MSO *2) M = (0.94, -0.34, -0.06) in MSO *3) N = (0.34, 0.94, -0.003) in MSO

Distribution in velocity space

However, IMA orientation is not always ideal

2005-6-30545-0645 UT

In-coming directions of ions

Azimuth scan Elevation scan

Electric scan

Electric scanIM

A

0 3 6 9 12 15

0 3 6 9 12 1501

2

3

4

5

6

1514

Aligned in azimuthal direction

Almost 1-D alignment

Difficult to determine ring "plane"

From ring data at ~0623 UT (2005-6-3)

-750

-500

-250

0

250

-500 -250 0 250 500-750

-500

-250

0

250

-250 0 250 500 750VL (km/s) VN (km/s)

VM

(km

/s)

V0 = 0 , Viewing from +B

Ions with V0=0 have initial perpendicular velocity to B = |VSW|cos() in SW rest frame, where sin() (= BX/B) is angle VSW & highest energy. The parallel velocity is constant in SW rest frame, and is zero in Martian frame

Beam-origin (V0 ≠ 0) ring distribution

Check itemAlignment direction of the ring data by IMA ≈ Maximum variance direction (LX, LY, LZ) with LX << 1

For both V0 =0 & ≠0, B // (-1, 0, 0) x E // (0, LZ, -LY)

If L = evolution direction, sign(BZ) = - sign(LY)

Tilt toward X (=) If V0 = 0, two angles must be the same, i.e.,

* ' = angle between max energy (MAX) direction and solar wind direction , and ' = * Ratio k = VMAX/VSW= (MAX/SW)1/2 , and k = cos()

If cos(') = k , most likely ' =

Selected ring data : ~ 0608 UTEl Az energy

(keV)VX

(km/s)VY

(km/s)VZ

(km/s)count VL *1

(km/s)VM *2(km/s)

VN *3(km/s)

13 01 4771 -711 -410 485 153 -341 -401 37213 01 4352 -679 -391 464 296 -324 -362 37114 01 5712 -728 -422 616 15 -336 -495 46614 01 5225 -696 -404 590 14 -320 -453 46115 01 5712 -671 -392 696 7 -291 -497 55813 02 4771 -810 -106 492 13 -46 -470 27214 02 5225 -793 -106 596 36 -31 -521 36314 02 4771 -758 -102 569 176 -28 -476 36314 03 5225 -769 205 602 14 280 -492 33614 03 4771 -734 196 575 11 269 -449 33815 03 4771 -677 179 647 27 265 -448 43115 03 4352 -647 171 618 8 255 -406 42814 04 4352 -572 442 554 71 517 -297 383

*1) L = (0.06, 0.99, 0.14) in MSO *2) M = (0.79, 0.04, -0.61) in MSO *3) N = (0.61, -0.15, 0.78) in MSO

Changing IMF is obtained !

0608 UT, northward IMF, X tilt ~ +35°~40°, Y tilt ~ -10°

0613 UT, northward IMF, X tilt ~ +35°, Y tilt ~ -45°

0623 UT, northward IMF, X tilt ~ +20°, Y tilt ~ -30°

0633 UT, X tilt > +40°

Procedure (1) Manually select the ring distribution. (2) Apply the minimum variance method to determine L, M, and N.

If the ring data is well arranged into a partial circle, ±N // BIf not, (3) Examine |LX| < 0.3. If yes, L // -ESW. If not, remove the direction that corresponds to the lowest energy from the selected set of data and re-calculate a new L. (4) Manually obtain eMAX and '. (5) Check if MAX /4SW ≈ cos2(') is satisfied. If yes, we have ≈ ', where BX/|B| = sin().

(6) If possible, identify the evolution direction and determine the sign of L and ESW. BT is parallel to VSW x ESW.

summaryApproximate IMF orientation can be derived from ring-like distributed protons as measured by the IMA

Since the ring distribution represents a gyration trajectory (cycloid motion) of an ensemble of ions with nearly the same initial velocity (beam-like or zero velocity), the ring's plane must be perpendicular to the IMF.

However, the actual derivation of the ring plane is very complicated due to the very limited viewing directions and angular resolution of the instrument.

IMF direction derived from cycloid-like ion distributions observed by

Mars Express

M. Yamauchi, Y. Futaana, R. Lundin, S. Barabash, M. Holmström (IRF-Kiruna, Sweden)

A. Fedorov, J.-A. Sauvaud (CESR, Toulouse, France) R.A. Frahm, J.D. Winningham (SwRI, San Antonio, USA) E. Dubinin (MPS, Katlenburg-Lindau, Germany), E. Kallio (FMI, Helsinki, Finland) and ASPERA-3 team

summaryMEX/IMA often observed cycloid distributions (two-dimensional partial ring distributions in velocity phase space) of protons upstream of the Martian bow shock. Since ions are expected to gyrate around the IMF in velocity phase space when the IMF is relatively uniform on a scale larger than the gyroradius, it is in principle possible to derive the IMF orientation from the observed cycloid distributions. We show this method which has three steps: (1) Manually choose the ring data (it is one of the minor populations and is difficult to select automatically). (2) Apply the minimum variance method to the selected ring data to determine the ring plane. (3) After the quality check, use either:

(a) the minimum variance direction N as the IMF orientation or (b) maximum variance direction L as the interplanetary electric field orientation. In the latter case, the tilt angle of IMF toward the X direction from the Y-Z plane is obtained from two methods when the selected ring distribution is newly ionized hydrogen atoms: (i) angle between the velocity of the ring's maximum energy portion and the solar wind vector, and (ii) energy ratio between the solar wind and the maximum energy of the ring.

Distribution in velocity space