M.Wiltberger NCAR/HAO

1. MHD Experiments to Isolate Different Influences on Seasonal and Solar Cycle Variations in MI Coupling M.Wiltberger NCAR/HAO CISM Seminar Series 2007

2. Outline • Overview of the LFM • Magnetospheric Model • Ionospheric Model • Electron Precipitation Model • Ways to control the magnetosphere • Vary the Earth’s magnetic field • Vary the solar wind electric field • Vary the ionospheric conductivity • Controlled Experiments with the LFM • Changes in Season • Changes in UV Ionization • Conclusions CISM Seminar Series 2007

3. LFM Magnetospheric Model • Uses the ideal MHD equations to model the interaction between the solar wind, magnetosphere, and ionosphere • Computational domain • 30 RE < x < -300 RE & ±100RE for YZ • Inner radius at 2 RE • Calculates • full MHD state vector everywhere within computational domain • Requires • Solar wind MHD state vector along outer boundary • Empirical model for determining energy flux of precipitating electrons • Cross polar cap potential pattern in high latitude region which is used to determine boundary condition on flow CISM Seminar Series 2007

4. Computational Grid of the LFM • Distorted spherical mesh • Places optimal resolution in regions of a priori interest • Logically rectangular nature allows for easy code development • Uses Partial Donor Method for advancing numerical solution • Yee type grid • Magnetic fluxes on faces • Electric fields on edges • GuaranteesB = 0 CISM Seminar Series 2007

5. LFM Ionospheric Simulation • 2D Electrostatic Model • Conservation of current • (P+H)=J|| sin(η) • J|| determined at magnetospheric BC • Conductivity Models • Solar EUV ionization • Creates day/night and winter/summer asymmetries • Auroral Precipitation • Empirical determination of energetic electron precipitation • Electric field used for flow at magnetosphere • v = -´B/B2 CISM Seminar Series 2007

6. Reflection Precipitation Auroral Electron Fluxes • Fridman and Lemaire, 1980 developed a kinetic model for the flux downward going electrons in the auroral acceleration region • Starting with conservation of total energy and adiabatic invariants you get • Integrating a isotropic distribution function over the region where precipitation occurs yields CISM Seminar Series 2007

7. Energy Fluxes • Considering the limit where the parallel potential drop is larger than the thermal energy in the source region we get the Knight relationship • Integrating the third moment of the distribution function gives the energy flux of the precipitating electrons CISM Seminar Series 2007

8. Auroral Fluxes in the LFM • Begin by computing the particle energy and number flux at the inner boundary of the LFM simulation domain • α includes effects of calculating electron temperature from the single fluid temperature known in MHD • β includes effects possible effects plasma anisotropy and loss cone filling • The initial number flux is the E||=0 case of the Flux equation which allows for the inclusion of diffuse aurora • The total energy of the particles is • The factor R allows for scaling the parallel potential drop based upon the sign of the current and account for the possibility of being outside the regime of the scaling CISM Seminar Series 2007

9. Auroral Fluxes in the LFM • The final step is to compute the flux of precipitating electrons using the flux formula in regions of upward current or downward streaming electrons • Using BI/BS = 8 for a dipole magnetic field and 2 RE gap between the source region and the ionosphere • In regions of downward current we apply • With the additional correction that the factor R is taken to be 5 time smaller in these regions • We also utilize the linearization the energy flux is simply the product of the average energy and the number flux CISM Seminar Series 2007

10. Using LFM to Study Seasonal/Cycle Effects • Ways to control the magnetosphere • Vary the Earth’s magnetic field • Earth’s dipole field flips polarity • Vary the solar wind electric field • CMEs, High Speed Streams, Orbital Effects • Vary the ionospheric conductivity • UV ionization • Auroral Precipitation • Controlled Experiments with the LFM • Constant solar wind electric field with active ionosphere • Changes in UV Ionization by altering F10.7 Flux CISM Seminar Series 2007

11. Vary the electric field • Russell & Mcpherron [1973] noted that the solar wind Parker spiral is ordered in the GSEQ coordinate system which results in a seasonal variation in the average rectified GSM Bz magnetic field • A statistical correlation exists between this variation and geomagnetic activity • “Spring towards, fall away” • Idealizing the magnetosphere as a circuit in this case means that you want to vary the driving voltage and keep the resistance constant CISM Seminar Series 2007

12. Vary the conductivity • Variation of the dipole tilt angle with season controls the amount of EUV radiation reaching the ionosphere and thus changing the amount and distribution of conductivity • They argue that auroral and geomagnetic activity depends on EUV and solar wind input • Peak in activity at equinox because aurora is in darkness for both hemispheres • Idealizing the magnetosphere as a circuit in this case means that you want to vary the resistance and keep the driving voltage constant CISM Seminar Series 2007

13. Imposing the Solar Wind Driver • We start the simulation in a SM coordinate system and then rotate it into a GSM coordinate system • We start with values for V and B which result in the same GSM values at the times of maximum dipole tilt during equinox, winter, and summer • EY has the same value for all rotation angles and all coordinate systems CISM Seminar Series 2007

14. Magnetosphere Ionosphere Configuration Equinox Winter XZ Den Ped Cond Summer CISM Seminar Series 2007

15. Summer Equinox Winter Cross Polar Cap Potential • Prior to the arrival of southward IMF Summer has a larger potential then either Winter or Equinox • This is likely due to a dipole tilt effect • After southward IMF arrival Equinox rises to the highest values and shows more variability • Seasonal variations in potential maximum have been reported by Weimer [1995] • Most pronounced durting cases with small BY • Positive IMF BY has slightly less potential in Summer hemisphere • This is opposite to what we see here CISM Seminar Series 2007

16. Summer Equinox Winter Global Field Aligned Currents • Fedder and Lyon, 1987 showed that the magnetosphere has current-voltage relationship that similar to a simple circuit of a generator with internal resistance driving an external resistor as proposed by Hill, 1984 • In order to explain the ordering of the currents we need to expand this model to consider hemispheres with different conductivities CISM Seminar Series 2007

17. Summer Equinox Winter Global Field Aligned Currents • Adding the currents flowing through each hemisphere yields some interesting results • Effectiveness of driver for northward IMF is dependent on dipole tilt • Immediately after arrival of southward IMF total currents are the same • At 5:21 the equinox case shows a clear spike in currents and difference between the systems for solstice and equinox • Simple circuit analogy implies same potential drop across each hemisphere and that is not the case in these results • Simple circuit analogy does not consider any inputs from the magnetotail CISM Seminar Series 2007

18. Summer Equinox Winter Substorm Behavior • Calculation of a AL proxy was conducted by applying Ampere’s Law to location directly beneath the maximum of horizontal current in the simulated ionosphere • The Equinox case shows a clear onset signature at 5:21ST with on a small perturbation present in the Solstice intervals • The Summer case shows a smaller feature at 05:30 ST with a smaller perturbation present in the Winter case which is not well correlated with other global diagnostics • The is also some additional activity in the solstice cases after 06:30 ST although none of it rises to the level of the equinox interval CISM Seminar Series 2007

19. Summer Equinox Winter Substorm Behavior • The total energy flux is computed by integrating the energy precipitating electrons over the entire hemisphere • Equinox case shows a clear increase in flux after the substorm onset time seen in the FAC and the simulated AL • No clear indication of abrupt increase flux present in either Winter or Summer cases • More flux is clearly flowing into Equinox • Interestingly Summer case has slightly more flux then Winter case CISM Seminar Series 2007

20. Equinox Winter Summer Ionospheric Structure • Animation of ionospheric evolution during entire interval made with the CISM-DX package • Potential contours are plotted at 10 kV intervals • Hall Conductance is background color with a range of 2 (blue) to 14 (red) mhos CISM Seminar Series 2007

21. Equinox Winter Summer Ionospheric Structure • At the beginning of the simulation interval the strong seasonal difference in the conductance is clearly present • All convection patterns clearly show the effect of IMF BY • Equinox case shows weak diffuse auroral activity • Summer case has almost diffuse auroral activity and clearly dominat dawn cell in convection pattern • Winter case scattered weak diffuse auroral activity 03:00 ST CISM Seminar Series 2007

22. Equinox Winter Summer Ionospheric Structure • This frame is from shortly after the substorm onset seen in the equinox interval • Strong enhancement of conductance seen in the Equinox case that bridges midnight and is not clearly seen Summer and only weakly present in Winter interavals • Activity occurring in region not illuminated by Sun in Equinox case • Examination the FAC structure for the Equinox case clearly indicates that the enhancement is due to an energy increase in precipitation caused by a parallel potential drop 05:30 ST CISM Seminar Series 2007

23. Equinox Winter Summer Ionospheric Structure • At the end of simulation interval significant enhancements are seen in call cases • Equinox case shows the strongest auroral zone enhancements with expansion in the predawn sector as expected • Strongest conductance are seen in Summer interval • Dusk side region 1 and dawn side region 2 current systems • Winter shows region 1 enhancement and diffuse auroral in predawn sector 08:00 ST CISM Seminar Series 2007

24. Equinox Winter Summer Ionospheric Structure • Activity diminishes as IMF remains northward • After the arrival of southward IMF two cell convection pattern grows with IMF BY effects most clear in Winter interval with displacement in cell peak locations and strengths • As IMF remains southward auroral oval expands across nights and to lower latitiudes CISM Seminar Series 2007

25. Effects of UV Ionization Level • Prolonged period of southward IMF with northward turning at the end • Seasonal Variation through date changes • Equinox 3/21 • Summer Solstice 6/28 • Cycle Variation through Year and F10.7 changes • Min 1996 – 70 • Max 2001 – 190 CISM Seminar Series 2007

26. LFM Seasonal Variation • At Solar Min • Winter potential is higher and summer • Stronger currents flowing into summer hemisphere • More Joule Heating in Summer Hemisphere • Little difference in prcecip eng. • At Solar Max • Differences between seasons are more pronounced • Especially strong in FAC • Still little diff in precp eng CISM Seminar Series 2007

27. LFM Seasonal Variation • At Solar Min • Winter potential is higher and summer • Stronger currents flowing into summer hemisphere • More Joule Heating in Summer Hemisphere • Little difference in prcecip eng. • At Solar Max • Differences between seasons are more pronounced • Especially strong in FAC • Still little diff in precp eng CISM Seminar Series 2007

28. LFM Solar Cycle Variation • Summer Hemisphere • Magnitude of variation by cycle similar to seasonal • Winter Hemisphere • All parameters show virtually no difference • Delayed onset of peak under solar min conditions CISM Seminar Series 2007

29. LFM Solar Cycle Variation • Summer Hemisphere • Magnitude of variation by cycle similar to seasonal • Winter Hemisphere • All parameters show virtually no difference • Delayed onset of peak under solar min conditions CISM Seminar Series 2007

30. Winter at solar max has similar Pedersen Conductance as Summer at solar min Comparing these two Only current shows significant difference Onset of enhanced Joule heating delayed Substorm onset affected by conductivity Dependence on Conductance CISM Seminar Series 2007

31. Future Work - Conductance • This research indicates several interesting directions for further study • Dipole tilt effects • Northward IMF case showed that while keeping solar wind electric field constant not all dipole tilt effects are removed • Planning to modify code to include zenith angle effects without tilting dipole • Active Ionospheric Feedback • Developing a version of the code which only includes the EUV ionization to study evolution of system without auroral conductance enhancements CISM Seminar Series 2007

32. Future Work – NW Currents • Neutral wind driven currents are calculated from within TING • After strong driving we see clear development of system with expected dusk/dawn asymmetry • Need to study how these parameters are affected by both season and solar cycle CISM Seminar Series 2007

33. Conclusions – Part I • Clear indication of the influence of EUV ionospheric conductance on the response of the coupled magnetosphere-ionosphere system • In Equinox case a clear substorm is seen in a variety of global parameters • In Solstice case a smaller perturbation is seen only simulated AL • Removal of RM Effect from solar wind driver and stronger activity in Equinox case supports a role for ionospheric conductance controlling geomagnetic activity CISM Seminar Series 2007

34. Conclusions – Part II • Seasonal Effects • During Southward IMF Cross polar cap potential highest in winter Hemisphere • Upward FAC strongest in summer hemisphere • Joule heating largest in summer hemisphere • Energy Precip shows little difference between winter and summer • Cycle Effects • Upward FAC increases with increasing EUV output • Stronger variation seen in summer hemisphere implies dependence on conductivity nonlinear CISM Seminar Series 2007

35. CISM Seminar Series 2007