Evolution of a solar wind discontinuity through its interactions with the bow shock K. Keika (1), R. Nakamura (1), W. Baumjohann (1), W. Magnes (1), K. H. Glassmeier (2), H. U. Auster (2), K. H. Fornaçon (2), D. G. Sibeck (3), V. Angelopoulos (4), E. A. Lucek (5), C. Carr (5), I. Dandouras (6) (1)Space Research Institute, Austrian Academy of Sciences, Graz, Austria / Fax: / Phone: ), (2) Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Germany, (3) Goddard Space Flight Center, NASA, MD, USA, (4) Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, CA, USA, (5) Imperial College, London, UK, (6) Centre d'Etude Spatiale des Rayonnements, CNRS/UPS, Toulouse, France.

2IWF/ÖAW GRAZ Discontinuity-bow shock interactions  June 21, 2007  A discontinuity arrived at Wind and ACE near 0910 UT. Cluster DSP/TC1 THEMIS A E C B 1 3 2

3IWF/ÖAW GRAZ Discontinuity-bow shock interactions  All S/C except THB observed a Bz increase in the discontinuity; the amplitude is >50 nT.  Rise time depends on the distance from the bow shock (BS). It is longer near BS than away from BS.  THB crossed the magnetopause into the magnetosphere at 1019 UT.  We estimate the normal direction and speed at the points indicated by three dashed lines: “ Leading Edge ”, “ Internal Part ”, and “ Trailing Edge ”. THEMIS, Cluster, TC1

4IWF/ÖAW GRAZ Discontinuity-bow shock interactions  “ Internal Part ” is slower than “ Leading Edge ” and “ Trailing Edge ”, even though they flow upstream with the same speed.  We suggest that bow shock sunward motion (caused by the Pd decrease) is responsible for the speed difference. Supported by plasma flow data from both THEMIS and Cluster. Supported by a simple calculation of Rankine-Hugoniot conditions. BS IP TE LE x t Interpretation SpacecraftPart  ( ˚ )v n (km/s) THEMIS A, C, E Leading Internal Trailing Cluster 1, 2, 3Leading Internal12316 Trailing130108

5IWF/ÖAW GRAZ Discontinuity-bow shock interactions  n decreased more gradually than in the solar wind.  n started to decrease before the discontinuity arrived. => implying that rarefaction waves carry the density.  |v x | decreased in the discontinuity, at minimum around “ Internal Part ”.  All spacecraft saw similar profiles. THEMIS particle data THA THE THC THB

6IWF/ÖAW GRAZ Discontinuity-bow shock interactions  n decreased more gradually than in the solar wind.  n started to decrease before the discontinuity arrived. => implying that rarefaction waves carry the density.  |v x | decreased in the discontinuity, at minimum around “ Internal Part ”.  All spacecraft saw similar profiles. Cluster particle data Cluster 1 Cluster 3

7IWF/ÖAW GRAZ Calculation of RH condition  For the density-decrease event, flow in the discontinuity becomes slower than that in both sides. Effect of BS motion on V sw in the magnetosheath Upstream conditions (from Wind observations) Downstream V1V1 N1N1 Pd 1 P mag1 P th1 V BS V2V2

8IWF/ÖAW GRAZ Summary  Discontinuity observed in SW by WIND/ACE and in the magnetosheath by THEMIS/CLUSTER  Rise time decreases during propagation in Magnetosheath  => Change of the internal structure  Internal part slower than edge  Outward motion of BS might be responsible for lag

Interactions between a solar wind discontinuity and the Earth’s bow shock K. Keika (1), R. Nakamura (1), W. Baumjohann (1), W. Magnes (1), K. H. Glassmeier (2), H. U. Auster (2), K. H. Fornaçon (2), D. G. Sibeck (3), V. Angelopoulos (4), E. A. Lucek (5), C. Carr (5), I. Dandouras (6) (1)Space Research Institute, Austrian Academy of Sciences, Graz, Austria / Fax: / Phone: ), (2) Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Germany, (3) Goddard Space Flight Center, NASA, MD, USA, (4) Institute of Geophysics and Planetary Physics, University of California Los Angels, Los Angels, CA, USA, (5) Imperial College, London, UK, (6) Centre d'Etude Spatiale des Rayonnements, CNRS/UPS, Toulouse, France.

10IWF/ÖAW GRAZ This Study Bx By Bz  We examine interactions between solar wind tangential discontinuities and the Earth’s bow shock, using:  THEMIS in the duskside magnetosheath,  Cluster in the dawnside magnetosheath,  DSP/TC1 in the magnetosheath around noon.  Such a large number of spacecraft enables us  to determine propagation normal and speed of transmitted discontinuities.  to study their evolution in the magnetosheath.  Two discontinuities were observed by Wind and ACE on June 21, 2007.

11IWF/ÖAW GRAZ Event 1: Density Increase - 1  Wind observations By changed from -8 nT to -2 nT. Bz increased from 3 nT to 7 nT. |B| decreased from 10 nT to 7 nT. N increased by a factor of ~1.5. Vx remains almost constant. Pd increased by a factor of ~1.5. Pth increased by a factor of ~1.5. Pmag decreased by a factor of ~2.  ACE observations By changed from -10 nT to -5 nT. Bz increased from 0 nT to 3 nT. |B| decreased from ~10 nT to 4 nT.  Normal direction is estimated to be  ~ 175˚ - 180˚, where  is the latitude in GSM coordinates. Solar wind observations

12IWF/ÖAW GRAZ Event 1: Density Increase - 2 Cluster DSP/TC1 THEMIS(A,E,and B) Geotail Cluster DSP/TC1 THEMIS(A,E,and B) Spacecraft positions

13IWF/ÖAW GRAZ Event 1: Density Increase - 3  DSP/TC1 observed a B y change from -50 nT to -20 nT and a |B| decrease from 45 nT to 20 nT at 1246 UT. => Discontinuity.  A small structure can be seen ahead of the discontinuity: B y decrease and |B| increase. => Fast forward shock generated at the bow shock.  Cluster observed similar structures followed by the discontinuity.  THEMIS A crossed the MP near 1246 UT and observed the discontinuity at 1250 UT.  The crossing is cased by inward motion of MP probably due to the generated fast forward shock. Observations in the magnetosheath

14IWF/ÖAW GRAZ Event 1: Density Increase - 4  THEMIS A observed sharp density increase at the discontinuity.  Cluster also observed the sharp density increase.  Rise time of density ( s) is shorter than that of solar wind density (~2 min).  Rise time for density is slightly shorter than rise time for B changes.  Velocity observed by Cluster increased at the fast shock. It slightly decreased after the discontinuity front arrived. THEMIS & Cluster plasma data

15IWF/ÖAW GRAZ Event 1: Density Increase - 5 SpacecraftMethod  (˚)  (˚)Ratio TC1MVA MC17929 Cluster 1MVA MC17914 Cluster 2MVA MC ‑ Cluster 3MVA MC ‑ ClusterTiming SpacecraftMethod  (˚)  (˚)v n or ratio TC1MVA CP16813 Cluster 1MVA146 ‑ CP Cluster 2MVA144 ‑ CP Cluster 3MVA143 ‑ CP ClusterTiming km/s THEMIS AMVA ‑ CP SpacecraftMethod  (˚)  (˚)v n or Ratio WindMVA178 ‑ CP ACEMVA177 ‑ CP Wind, ACE, TC1Timing km/s Normal direction and speed Discontinuity Forward fast shock MVA: Minimum Variance Analysis CP: Cross Product of B Timing: Timing Analysis  : longitude in GSM coordinates  : latitude in GSM coordinates

16IWF/ÖAW GRAZ Summary  Double Star TC1 in the dayside magnetosheath observed a fast shock (FS) at 1245:10 UT, 1 min before it saw the tangential discontinuity (TD). About two minutes later, it crossed the bow shock (BS).  Cluster in the dawnside magnetosheath observed FS at 1248:10 UT, ~2 min before they saw the TD.  Cluster observations revealed different propagation fronts; TD is ~30 deg. (~15 deg.) inclined toward dusk at Cluster (DSP/TC1), but FS is little tilted.  THEMIS A crossed the magnetopause into the magnetosheath at 1246:15 UT, because FS compressed the magnetosphere. It saw TD about 3.5 min later.  A planar front with the same normal as TD in the solar wind cannot explain time differences in the FS and TD observations between spacecraft. TD; 1318 UT?? FS; 1245:10 UT TD; 1246:10 UT BS; 1248:30 UT FS; 1248:10 UT TD; 1250:15 UT FS; 1246:15 UT TD; 1249:40 UT

17IWF/ÖAW GRAZ Conclusions What was happening? § A little inclined TD hits BS near 1245 UT. § FS is generated and then propagating anti- sunward. 1.TD keeps propagating anti-sunward in the magnetosheath. 2.BS moves anti-sunward, because of an decrease in Alfven velocity in the magnetosheath.  The FS front is not a planar, because speed of FS is faster than that of TD in the solar wind.  The TD front is not a planar, because speed of TD is slower than that of TD in the solar wind.  TD becomes steeper in the magnetosheath, probably because speed of the TD final part becomes different from that of the TD front. Is this because of bow shock inward motion?  This results in short rise time (~2 min) of SIs in the magnetosphere.  The TD front seems greatly deformed near MP. Both discontinuities compressed the magnetosphere; TD made the dominant contribution of sudden impulses (SIs) which have a front ~28° inclined toward dusk. Does this cause dawn-dusk asymmetry of SIs?

18IWF/ÖAW GRAZ Event 2: Density Decrease - 1  Wind observations Bz increased from ~0 nT to 8 nT. |B| increased from 7 nT to 12 nT. N decreased by a factor of ~2. Vx remains constant. Pd decreased by a factor of ~2. Pth decrased by a factor of 2.5. Pmag increased from 0.02 nPa to 0.06 nPa.  ACE observations Bz increased from -5 nT to 7 nT. |B| increased from 8 nT to 12 nT. Solar wind observations

19IWF/ÖAW GRAZ Event 2: Density Decrease - 2 Cluster DSP/TC1 Cluster DSP/TC1 THEMIS(A,E,and B) Spacecraft positions THEMIS A E C B 1 3 2

20IWF/ÖAW GRAZ Event 2: Density Decrease - 3  All S/C except THB observed a Bz increase in the discontinuity; the amplitude is >50 nT.  Rise time depends on the distance from the bow shock (BS). It is longer near BS than away from BS.  THB crossed the magnetopause into the magnetosphere at 1019 UT.  We estimate normal direction and speed at the points indicated by three dashed lines: “ Leading Edge ”, “ Internal Part ”, and “ Trailing Edge ”. THEMIS, Cluster, TC1

21IWF/ÖAW GRAZ Event 2: Density Decrease - 4  n decreased more gradually than in the solar wind.  n started to decrease before the discontinuity arrived. => implying that rarefaction waves carry the density.  |v x | decreased in the discontinuity, at minimum around “ Internal Part ”.  All spacecraft saw similar profiles. THEMIS particle data

22IWF/ÖAW GRAZ Event 2: Density Decrease - 5  n decreased more gradually than in the solar wind.  n started to decrease before the discontinuity arrived. => implying that rarefaction waves carry the density.  |v x | decreased in the discontinuity, at minimum around “ Internal Part ”.  All spacecraft saw similar profiles. Cluster particle data

23IWF/ÖAW GRAZ Event 2: Density Decrease - 6 SpacecraftMethod  ( ˚ )  (˚)v n or ratio Wind, ACE, TC1Timing km/s WindMVA  CP15813 ACEMVA  CP SpacecraftPart  ( ˚ )v n (km/s) THEMIS A, C, ELeading Internal Trailing Cluster 1, 2, 3Leading Internal12316 Trailing SpacecraftMethod  (˚)  (˚)Ratio THEMIS EMVA ‑ CP ‑ THEMIS CMVA ‑ CP ‑ Cluster 1MVA CP Cluster 3MVA CP Cluster 2MVA CP Normal direction and speed Discontinuity MVA: Minimum Variance Analysis CP: Cross Product of B Timing: Timing Analysis Discontinuity (each part) Solar wind

24IWF/ÖAW GRAZ Summary  Double Star TC1 in the dayside magnetosheath observed TD at 1009:40 UT, and crossed BS almost at the same time of the arrival of the TD final part.  THEMIS A, E, and C on the dusk side observed the TD front before they crossed BS. THEMIS E saw the TD front in the magnetosheath before it crossed MP.  The TD fall time is ~14 minutes.  Cluster SC observed TD in the dawnside magnetosheath.  The TD fall time is ~10 minutes.  THEMIS and Cluster observations showed that the transient region of TD has speed slower than plasma ahead of and behind it. This is probably because of bow-shock anti-sunward motion. TD_front; 1009:40 UT TD_final; ~ UT BS; 1016:20 UT TD_front; 1014:00 UT TD_final; 1024:00 UT THA TD_front; 1010:15 UT TD_final; 1021:10 UT BS; 1014:15 UT THE TD_front; 1010:40 UT TD_final; 1024:00 UT BS; 1011:40 UT THB TD_front; 1011:40 UT

25IWF/ÖAW GRAZ  “ Internal Part ” is slower than “ Leading Edge ” and “ Trailing Edge ”.  This difference results in different rise time among spacecraft. Supported by plasma flow data from both THEMIS and Cluster. Supported by a simple calculation of Rankine-Hugoniot conditions. Discontinuity-bow shock interactions BS IP TE LE x t THATHETHC SpacecraftPart  ( ˚ )v n (km/s) THEMIS A, C, ELeading Internal Trailing Cluster 1, 2, 3Leading Internal12316 Trailing130108

26IWF/ÖAW GRAZ Calculation from RH condition  For the density-increase event, flow in the discontinuity becomes faster than that in both sides. => Density increase becomes sharper.  For the density-decrease event, flow in the discontinuity becomes slower than that in both sides. => Density increase becomes less steep. A B change becomes more drastic around the trailing edge. Effect of BS motion on V sw in the magnetosheath Upstream conditions Downstream

27IWF/ÖAW GRAZ Summary

28IWF/ÖAW GRAZ Conclusions 1.A discontinuity (TD?) hits BS near 1010 UT. 2.A fast shock (FS) is not generated? or just not detectable? 3.TD keeps propagating anti-sunward in the magnetosheath. 4.BS moves sunward, probably because of an decrease in Alfven velocity in the solar wind. 5.It is likely that the BS sunward motion causes a decrease in the speed of plasma inside TD. TO DO … Estimates of the direction and speed of boundary normals Check if Rankine-Hugoniot equations are satisfied. Compare with moeling. TD BS TD BS TD BS TD BS

29IWF/ÖAW GRAZ Comparison  We are going to make comparison between TD with a density increase and TD with a density decrease, paying much attention to:  Whether or not FS is excited at the bow shock,  Propagation direction of FS and BS in the magnetosheath,  Variations of rise (fall) time and its relation with bow shock motion, Rise time of geomagnetic H-component at Kakioka:  At 1245 UT; ~2 min.  At 1010 UT; ~10 min.  and magnetospheric response. TD BS TD BS TD BS TD BS TD BS TD BS FS TD BS FS TD BS FS