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Characteristics of storm-time electric fields in the inner magnetosphere

19 th Cluster Workshop, May 2010. Characteristics of storm-time electric fields in the inner magnetosphere. H. Matsui 1 , P. A. Puhl-Quinn 2 , J. W. Bonnell 3 , C. J. Farrugia 1 , V. K. Jordanova 4 , Yu. V. Khotyaintsev 5 , P.-A. Lindqvist 6 , E. Georgescu 7 , and R. B. Torbert 1

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Characteristics of storm-time electric fields in the inner magnetosphere

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  1. 19th Cluster Workshop, May 2010 Characteristics of storm-time electric fields in the inner magnetosphere H. Matsui1, P. A. Puhl-Quinn2, J. W. Bonnell3, C. J. Farrugia1, V. K. Jordanova4, Yu. V. Khotyaintsev5, P.-A. Lindqvist6, E. Georgescu7, and R. B. Torbert1 1. University of New Hampshire 2. AER, Inc. 3. University of California, Berkeley 4. Los Alamos National Laboratory 5. Swedish Institute of Space Physics, Uppsala 6. Royal Institute of Technology 7. Max-Planck-Institut für Sonnensystemforschung 1

  2. Outline • Introduction • Data Set • Case Studies • Eveningside Event on 19 May 2002 • Nightside Event on 18 Feb. 2005 • Statistical Analysis • Electric Fields Sorted by Epoch Time • Comparison of Decay Time of Electric Fields With Those of Dst Index and IMF Bz • AC Components • Summary 2

  3. UNH-IMEF model from Cluster and ground radar data Introduction Sun Contour Interval 1kV: thin lines 5kV: thick lines • We are developing an inner-magnetospheric electric field (UNH-IMEF) model. (http://edi.sr.unh.edu/unh-imef/) • It is better to examine the electric field in more detail during storm periods. This could lead to future improvement of this model. 3

  4. Electric field data from EDI and EFW are used. • FGM data and EDI background data are used to identify dipolarization signatures and electron plasmasheet, respectively. • Interplanetary parameters and geomagnetic parameters are introduced. Data Set 4

  5. Case Study (19 May 2002) • Cluster passed through duskside inner magnetosphere around minimum Dst during a storm. • We will show Cluster data between two vertical lines. • IMF Bz is negative and approaching 0 nT. • AU index shows some activities, from which convection enhancement is expected. AL index shows a decrease at ~7 UT, indicating substorm. Time shifted 5

  6. Case Study (19 May 2002) • The electric field is outward and westward inside the electron inner edge indicated by two vertical lines, measuring SAPS and undershielding features. • After that, the electric field sporadically changes its sign due to substorm activity (oscillating signature). 6

  7. Case Study (18 Feb. 2005) • Cluster passed through postmidnight inner magnetosphere around main phase of a storm. • We will show Cluster data between the two vertical lines. • IMF Bz is largely negative after 23 UT. • AL index shows a decreasing trend. There is a sporadic decrease on top of that. • There is enhancement of Pdyn. Time shifted 7

  8. Case Study (18 Feb. 2005) • Large electric field is measured. This is accompanied by dipolarization with ~5 deg. increase of elevation angle. → Inductive E field. • After that positive Ex continues. Mean value of Ey is smaller but positive. • Spacecraft continuously stayed in the plasma sheet. 8

  9. Case Study (18 Feb. 2005) • One unique feature for Cluster is 4 spacecraft measurement. During this event each spacecraft is separated by 800-5000 km. • Spacecraft locations relative to RC1=(-4.3,-1.0,-0.4)SM Re are shown together with dipole field lines threading spacecraft. • SC3 is located at outermost L shell and then SC2, SC1, and SC4. Duskward Earthward Earthward Equatorward 9

  10. Case Study (18 Feb. 2005) • The dipolarization feature is most clear at SC3. • SC 1 measures large Ex and Ey, but not at the dipolarization. → Not high spatial coherence between spacecraft. • After the electric field related to dipolarization is observed, there is large electric field without clear magnetic signatures. Although measured at all SC, the time-series are not coherent. 10

  11. We analyze Cluster electric field data for ~7 years (71 storms). • Spatial range examined is 3.5<R<6 Re, |MLAT|<25 deg., and full MLT. • A storm is defined where Dst(min) is constant for >72 hours with Dst(min)<-50 nT. • Data between 0.2 Dst(min)>Dst>Dst(min) are picked for main and recovery phases and then visual inspection to ensure a single Dst minimum during a storm. • Dependences of electric fields on epoch time relative to Dst minimum is examined for each MLT sector divided into four (6 hour width). Statistical Analysis 11

  12. Time Sequence of Electric Field Values • 6 hour averages (in epoch time) and standard deviations are on top of 5 min values. • There is a large value at the main phase. 12

  13. Time Sequence of Electric Field Values • Again we have measured enhanced electric field around minimum Dst. 13

  14. Time Sequence of Electric Field Values • Just after the main phase, large negative Ex and positive Ey are observed. Plasma is moving toward magnetopause. • Ey is positive throughout the period, indicating possible existence of SAPS, although size is often <1 mV/m. (SC often inside the inner edge.) • There is a sporadic enhancement of E at 40 hr possible to be caused by transient phenomena. 14

  15. Red: premidnight Black: postmidnight Time Sequence of Electric Field Values • Ex is positive right around the epoch time, while Ex turns to negative after that at premidnight. • Ey component around the epoch time is on average positive but fluctuating to change signs. Not all of the phenomena are inductive. 15

  16. Electric Fields at Spacecraft Location E Ex<0 E Ex>0 • Ex>0 tends to be observed during the main phase at premidnight, while Ex<0 is observed at the recovery phase. This could be related to the variation of the location where the convection streamline is bending due to partial ring current. 16

  17. Superposed epoch analysis of 71 storms with epoch at Dst minimum. • This includes periods with Cluster data. • Each parameter shows clear maximum (or minimum) around T=0 hour. • The decay time during the recovery phase is as follows: TIMF_Bz<TEy<TDst. Superposed Epoch Analysis 17

  18. Black: 4s->5min Blue: 6 hr averages of above. Green: 5min->6 hr Red: DC components. • Higher frequency (HF) fluctuations are sometimes enhanced sporadically compared to lower frequency (LF) fluctuations. • 6 hour values of HF fluctuations are similar to those of LF fluctuations. • DC values are generally smaller than AC values. • Especially a large value could be found around minimum Dst at nightside, which is partly caused by induction field during dipolarization events or transient variations. Standard Deviation of Electric Field 18

  19. Large electric fields are observed in one event in the eveningside around minimum Dst. These are likely identified as SAPS/undershielding and substorm-related signatures. • Large electric fields are observed with or without dipolarization signatures. Spatial coherence between spacecraft is not high. • We statistically examined storm-time electric fields measured by Cluster at R=3.5-6 Re, |MLAT|<25 deg. • Electric fields are enhanced around minimum Dst at all MLT. • The decay time of electric field is shorter than that of Dst index. • AC fields tend to be larger than DC fields, possibly indicating AC fields might play some role in ring current acceleration. • Including results from these studies, we would like to improve the model. Summary 19

  20. Case Study (18 Feb. 2005) Ey<0 Ey>0 [Quinn and Southwood, 1982] • Ey is negative due to poleward motion of B field line at the off-equatorial location of SC (MLAT=-14o), while Ey is positive at the equator. • Just close to the end of dipolarization, there is a bipolar Ey features. The B field line shifted poleward moves back a little bit to the equator, which seems to be also inductive because B field variation is accompanied. 20

  21. Distribution of Data • Data are distributed throughout each plot. although there are often regions with no data. • Spatial resolution of MLT as 6 hours would be one possible solution. • The blue region indicates where the spacecraft was inside the inner edge of electron plasma sheet (from visual inspection). This is commonly located in the afternoon side. 21

  22. Data Availability • Data availability is often >80 % except a few cases. This value is defined as the number of 5 min intervals with electric field data divided by number of 5 min intervals with either EDI or EFW or both is/are in operation. 22

  23. Superposed epoch analysis of 71 storms with epoch at Dst minimum. • This includes periods with Cluster data. • Each parameter shows clear maximum (or minimum) around T=0 hour. • Average E field is small (at most 1 mV/m during the main phase). • The decay time during the recovery phase is as follows: TIMF_Bz<TAL~TIEF~TEy<TDst~TKp~TAU Superposed Epoch Analysis 23

  24. Electric fields measured by Cluster at R=3.5-6 Re, |MLAT|<25 deg. are investigated during magnetic storms. • Two case studies are performed. Both are close to Dst minimum. • Large electric fields are observed during one event in the evening sector. These are likely to be related to SAPS and undershielding. Substorm signatures are found during this event. • Large electric fields are observed during one event in the nightside. The dipolarization signature is accompanied by inductive electric fields at the outer SC, but this is not the case at the inner SC. • After the dipolarization, strong electric field is observed by all SC. • We can see variable feature of E fields, which contributes to our statistics. Summary and Future Work 24

  25. We have performed a statistical analysis. • Electric field is enhanced around Dst minimum at all MLT including morning and dayside. • The eveningside electric fields could be related to SAPS. • We have found changes of Ex signs at nightside, possibly related to bending of convection streamline. • Different decay time between Dst index and Ey at Cluster might indicate implication on coupling between these two parameters. • AC fields tend to be larger than DC fields, possibly indicating AC fields might play some role in ring current acceleration. • Following topic is not yet investigated: measurement at other spatial locations, particularly closer to the Earth. • By including results from these studies, we would like to improve the model. Summary and Future Work 25

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