MESSENGER observations of magnetospheric substorm activity in ...

MESSENGER observations of magnetospheric substorm activity in ...

MESSENGER observations of tail dyna mics during substorm at Mercury Wei-Jie Sun1,2, James A. Slavin3, Suiyan Fu2, Jim M. Raines3, Gangkai Poh3, Zuyin Pu2, QiuGang Zong2, Yong Wei1, Weixing Wan1 1, Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. 2, School of Earth and Space Sciences, Peking University, Beijing, China. 3, Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI, USA. 13th ICS, 29 September 2017, Portsmouth, NH, US 1 Outline

Introduction Plasma sheet evolutions during Merc urys magnetospheric substorm Plasma waves during substorm expa nsion phase Proton energization and heating Dipolarization and magnetic reconne ction distributions 2 Mercurys magnetosphere [Slavin et al., 2009]

Mercury radius: RM 2440 km 3 Mercury possesses the sa me polarity magnetic field as the Earth. Dipole moment is 200 n T/RM3 (~ 1% of that of Eart h). The dipole is aligned to th e spin axis (< 5) of the pl anet, but has a northward offset 0.2 RM. Mercury does not have a c onducting ionosphere, but has a surface bounded ex osphere.

[e.g., Ness et al., 1974; Ip, 1986; Glassmeier, 2000; Anderson et al., 2008] MESSENGER Launched at August 3rd 2004 Inserted into Mercurys orbit at March 18th 2011 Crashed into Mercurys surface at April 30th 2015 FIP S MA G Sixteen Mercury years

data [Solomon et al., 2007] Magnetometer: 20 samples per second [Anderson et al., 2007] FIPS: < 13 keV/e ion species in 10 s [Andrews et al., 2007] 4 Loading-Unloading in the magnetotail [Huang, 2002] Earth

Earths loading-unloadin g Loading: 40 min 5 et al., 2010; Mercury [Slavin see also Sun et al., Mercurys loading- 2015] unloading Loading: 1 min Observations Three orbits of MESSENGER are shown in red (July 1st 2011), green (June 18th 2012) and blue (December 9th 2012) lines.

The dots in each orbit represent the locations where dipolarizations were observed. 6 Event I PS Event II PS Event III Lob e The magnetic field Bx (black line) is nearly the same

as the nonsubstorm pass (red dashed line) in each 7 MESSENGER locations at t ~ 0 min MESSENGER was in the plasma sheet for Event I and Event II, but was in the lobe for Event III. 8 Growth Phase Event I PS Event II

Event III Lob e Lobe constant Bx Bz decrease decrease 1 min 35 1 min 2 s red dashed s A clear Bx discrepancy between the black and 58 s

line between this time. 9 MESSENGER locations at t ~ 1 min Event I still in the plasma sheet; MESSENGER for Event II moved from plasma sheet into lobe; Event III still in the lobe. Thinning process of the plasma sheet (recession of outer edge of high-latitude plasma sheet), and magnetic flux was increased in the lobe, 10 indicating the

Expansion phase Event I PS Event II PS Event III PS Sharp decrease in Bx Dipolarizatio 30 s 40 s

1 min Sharply decrease of Bx for three events is the signature of rapid plasma sheet thickening. 11 MESSENGER locations at t ~ 2 min After dipolarization: Event I still in the plasma sheet; 12 MESSENGER for Event II and Event III moved from lobe into plasma sheet, indicating the plasma sheet thickening.

This is similar to the Earths magnetospheric expansion phase. Statistical results Locations of the 26 events in MSM XY (a) and XZ (b) plane. [Sun et al., 2015a] The duration of growth phase (c) and expansion phase 13 (d). Mercurys magnetospheric substorm Event III: Growth phase: Plasma sheet thinning,

magnetic field increasi ng. Expansion phase: Dipolarization, Magnetic field fluctuati [Sun etons, al., 2015a] 14 Magnetic field decreasi Waves during expansion phase spacecraft The beginning of substorm expansion phase, spacecraft located near the plasma sheet

15 By pulses (in red), sign atures of field-aligned currents; Field-aligned coordina te, BBperp1 is unipolar w ith BBperp2 bipolar for t he first pulse and BBper p1 is bipolar with BBperp 2 unipolar for the seco Minimum Variance Analysis (MVA) [Sonnerup and Sche ible, 1998] 1. Two By pulses, max/int < 3, int/min > 20; nmin (k) is ~ 38(or ~ 142) and ~ 9(or ~ 171) fro m the background magnetic field; 3. In the plane normal to k, the magnetic field vectors

Carrying field-aligned current, are360 circularly polarized and propagate rotated circularly by ~ with the durations of ~ nearly parallel (or antiparallel) to the background magnetic field 16 11 s and ~ 10

s Alfnic Waves 2. MESSENGER detected high observed proton densities (~1 cm-3), spacecraft entered into inner plasma sheet. 17 Four successively com pressional waves (in bl ack) following the Alfv nic waves.

fluctuated in BBt and c ompressional compone nt (BBpara). Durations: ~ 12 s, ~ 1 9 s, ~ 24 s, and ~ 12 s . The quasi-periodic dipolarizing flux bu ndles (DFB) DFBs and flow bursts are well correlated in the Earths tail. [Sundberg et al., 2012] Mercury tail: DFBs show period of 10 to 15 seconds. 18

Case observation Event I dipolarizations, field fluctuations, thin current sheet, active period 19 Before dipolarization After dipolarizatio n Green line: species w ith medium energy (Stationary Maxwelli an distribution)

Blue line: high energ y species (Kappa dist ribution) Black line: sum of th e green and blue line s. Red dots: measurem ents with one count, excluded during the fitting. Blue line: high energy species (Kappa distribution) Conclusion 1.Obvious energization for protons during dipolarization ( become small)

2.Heating for protons (only high 20 Supra-thermal particle flux Daw n Equatorial distributions of su pra-thermal proton partic le flux 1225 cases Dusk Thin current sheets: active p eriods dawn-dusk asymmetri es, with higher values in the dawnside plasma sheet Thick current sheets: quiet p eriods

21 Proton Temperature Equatorial distributions of pro ton temperature Daw n 1225 cases Thin current sheets: active per iods dawn-dusk asymmetries, with higher values in the dawnside plasma sheet Thick current sheets: quiet per iods 22 Dusk

Distribution of dipolarizations (BBzmax) We have selected out th e largest Bz increase in 5 s (Bzmax) after subtractingDaw n a 40 s moving mean bac kground magnetic field f or the 1225 plasma shee t passes. Dipolarizations were mor e frequently observed in Dusk the dawnside plasma sh eet at Mercury. 23 Distribution of tail reconnection at Mercury Magnetic

reconnection in Mercurys magnetotail occurs more frequently in the dawnside than in the duskside plasma sheet. Flux ropes Dipolarization fronts [Sun et al., 2016] 24 Near-tail reconnection at Mercury dawn dusk

Mangetic reconnection sh ows higher occurrence rat e in the dawnside plasma sheet than in the dusksid e. These plasma flows would mostly brake and initiate the substorm dipolarizatio n on the postmidnight sec tor at Mercury. 25 Summary I. II. III.

IV. V. VI. Plasma sheet evolutions were reported during Merc urys magnetospheric substorm The magnetospheric substorm growth and expansi on phases are observed to be 1min at Mercury Pi2-like pulsations were reported during Mercurys substorm expansion phase Protons could be effectively energized and heated by Mercurys substorm dipolarization Proton suprathermal particle flux and temperature display that the values on the dawnside are higher than the duskside in near-Mercury tail Mercury substorm dipolarizations, magnetic reconn ections would be more frequently observed on the dawnside tail 26

I. II. III. IV. Sun, W.-J., et al. (2015a), MESSENGER observations of magne tospheric substorm activity in Mercurys near magnetotail, Ge ophys. Res. Lett., 42, 36923699, doi:10.1002/2015GL064052 . Sun, W.-J., et al. (2015b), MESSENGER observations of Alfvni c and compressional waves during Mercurys substorms, Geo phys. Res. Lett., 42, 61896198, doi:10.1002/2015GL065452. Sun, W. J., S. Y. Fu, J. A. Slavin, J. M. Raines, Q. G. Zong, G. K. P oh, and T. H. Zurbuchen (2016), Spatial distribution of Mercur ys flux ropes and reconnection fronts: MESSENGER observati ons, J. Geophys. Res. Space Physics, 121, 75907607, doi:10. 1002/2016JA022787.

Sun, W. J., J. M. Raines, S. Y. Fu, J. A. Slavin, Y. Wei, G. K. Poh, Z . Y. Pu, Z. H. Yao, Q. G. Zong, and W. X. Wan (2017), MESSENG ER observations of the energization and heating of protons in the near-Mercury magnetotail, Geophys. Res. Lett., 44, doi:10 .1002/2017GL074276. Thank you! 27 Two projections of a map of XRS footp rint locations, plotti ng the locations as sociated with each record containing e lectron- induced flu orescence as per b oth the Si-filtered (Fig.2a and b) and Ca- Filtered (Fig.2c and d) catalogues.

[Lindsay et al. 2016] 28 Diagrams showing spatial locations and extents of all energetic electron events in this study. (a) Projection of MESSENGER orbits onto the MSO X-Y plane during times when events were detected. (b) Representation of orbit trajectory segments during electron bursts shown in a three dimensional projection. [Baker et al. 2016] 29 Dissipated Magnetic Energy Event I

At the end of expansion phase Total magnetic energy is: 7.6 1011 J At the end of growth phase Total magnetic energy 1.67 1012 J The dissipated magnetic energy is: 9.1 1011 J The typical energy loaded during theEmploying the similar process: substorm at Earth is 2.1 1015 J Event II is 6.7 1011 J [e.g., Akasofu, 1981; Tanskanen et Event III is 8.1 1011 J al., 2002], which is 3000 times larger than at Mercury. 30

Estimated Magnitude FACs Tanskanen et al. [2002] showed that By applying the net electric 30% of the magnetic energy was conductance (EC) is 1S for the dissipated in the northern FACs closure at Mercury hemisphere. [Anderson et al., 2014]. IFAC2TEX/EC = Ed. Event I is 2.7 1011 J Event II is 2.0 1011 J Event III is 2.4 1011 J TEX expansion timescale is 60 s. IFAC is 67, 58, and 63 kA for the three events, respectively.

At Earth, it is 3 MA during substorm [Iijima and Potemra, 1978]. 31 Steadystate fieldaligned currents at Mercury Average magnetic perturbations above Mercury's northern hemisphere in aberrated MSO coordinates during ascending (top) and descending (bottom) tracks for three levels of Mercury magnetic disturbance: 0 33% (left); 3367% (center); and 67100% (right). Averaged horizontal perturbations are plotted as lines from the bin center in the direction32of the averaged vector. Line lengths and colors show the magnitude (see Total currents are typically 2040 kA and exceed 200 kA during disturbed conditions. The current density and total current are two orders of magnitude lower than at Earth. An electric potential of ~30 kV from dayside magnetopause magnetic reconnection a net electrical of M.

~1Winslow, S. Anderson, B. J.,implies C. L. Johnson, H. Korth,conductance J. A. Slavin, R. R. J. Phillips, R. L. McNutt Jr., and S. C. Solomon (2014), Steady-state fieldaligned currents at Mercury, Geophys. Res. Lett., 41, 33 doi:10.1002/2014GL061677. Distribution of tail reconnection at Mercury Magnetic reconnection reconnection in Mercurys front magnetotail occurs Bz bipolar, Bz sharply

more frequently in the dawnside than in the By and Bt increase, Bt duskside plasma sheet. enhancement enhancement [Sun et al., 2016; also 34 shown in Smith et al., s region. flux rope 2017] The comparison between Earth and Mercu ry dawnsid e duskside

Revised from Imber et al. [2011], etc. The Near Mercury Neutral line: dawnside initiate dipolarization in the post-midnight sector of tail The Near Earth Neutral line: duskside initiate dip olarization in the pre-midnight sector of tail 35 Alfvnic wave The plasma waves observation during substorm expansion phase at Mercury. 36 Compressional wave

How the Pi2-pulses are generated? Mercury is a very slow rotating planet (~ 58.6 days/rotati on). Mercurys plasmasphere would be located inside ~ 0 .02 RM. Models requiring a plasmaspheric cavity are ruled out, su ch as the plasmaspheric cavity resonance and ballooning waves occurring at the inner edge of plasma sheet. Standing Alfvn Waves, unlikely [Glassmeier, 1997]

The estimated AT is 5.010.5 s, But because of the limited conductivity of the regolith in Mercurys surface [e.g., Verhoeven et al., 2009; Anderson et al., 2014], the waveform37of the bouncing Alfvn wave should be damped, which are not the case for our observ Modulated flow bursts braking fv Al n w e av compressional waves [Kepko et al., 2001] Periodic flow bursts could drive compressional waves near the equ atorial region, Alfvn wave carrying field-aligned current propagati ng toward the nightside polar region.

The resulting Pi2 has the same period as the periodic flow bursts. 38

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