The role of solar wind energy flux for transpolar arc luminosity

1. The role of solar wind energy flux for transpolar arc luminosity • Kullen 1, J. A. Cumnock 2,3, and T. Karlsson 2 1 Swedish Institute of Space Physics, Uppsala 2 Alfvén Laboratory, KTH, Stockholm 3 Center for Space Sciences, University of Texas, Richardson, Texas Alfven Conference, Arcachon, September 2007

2. Transpolar arc occurrence frequency Kullen et al. (2002) • Transpolar arcs (TPAs) are found to correlate best with anti-epsilon ~ vB2cos(Θ/2)4, i.e., they occur most frequently for northward IMF and high magnetic energy flux. • Why are TPAs absent in periods of low magnetic energy flux, even during northward IMF conditions ? Is this connected to the TPA luminosity ?

3. Bz By Transpolar arcs during southward IMF • TPAs are a predominantly northward phenomenon, but often IMF southturns appear during their lifetime. • TPAs occurring when IMF turns southward, are generally bright. Typically, strong luminosity variations (often in connection with auroral breakups) appear along the arc. • The dilution by auroral breakups, triggered by internal magnetotail instabilities, makes a comparison between TPA luminosity and solar wind parameters very difficult. IMF conditions:

4. Bz By Transpolar arcs during northward IMF IMF conditions: • TPAs appearing during steady northward IMF are typically very faint. The luminosity along the arc does not change much during the events. • It is known that an IMF By sign change during northward IMF triggers a TPA that moves over the entire polar cap (Cumnock, 1997; 2005).

5. Data selection The influence of the solar wind and the IMF on the luminosity of moving TPAs is examined by taking into account seasonal effects: • For a time span of 4.5 years, all cases with a single IMF By sign change during at least 5 hours steady northward IMF are selected for which global Polar UVI images of the northern hemisphere are available. • The dataset contains altogether 22 events. In 20 cases, a TPA could be identified on Polar UVI. • The luminosity of each TPA is determined when the arc has reached the noon-midnight meridian. • The luminosity of the TPAs is compared to solar wind data obtained from the OMNI-Web. Luminosity measurement along the arc when it has reached the noon-midnight meridian

6. Kullen and Janhunen (2004) Ionosphere Tail cross section at x+-30 Re Kullen et al. (2002) Transpolar arc model open-closed field line boundary Moving TPAs: The spatial evolution of TPAs occurring after an IMF By sign change during northward IMF can be sufficiently explained by the topological changes in the magnetotail. MHD simulations show: the magnetotail rotates such that near-Earth and far-tail regions are oppositely twisted, causing a bifuraction of the closed field line region that maps to a TPA. As the field-aligned currents connected TPAs do not show up in MHD, no information about FAC or aurora is gained. FAC pink dots: closed field line region

7. Seasonal dependence of TPA luminosity Geomagnetic storm cases TPAs are much brighter in the summer than in the winter (Cumnock, 2005). In the dark hemisphere, no dependence on the dipole tilt is found. In the sunlit hemisphere, a positive correlation with the dipole tilt is found. The luminosity of the winter storm case deviates strongly from the luminosity of the other winter TPAs, the summer storm case not. Quiet time cases

8. IMF dependence of TPA luminosity Time shift between luminosity measurements and IMF: 52 minutes In the sunlit hemisphere: the correlation between TPA luminosity and IMF is much higher than in the dark hemisphere the IMF influence on the TPA luminosity is much stronger than in the dark hemisphere (steeper curves) the TPA luminosity depends strongly on IMF Bz and IMF magnitude. A weak, negative correlation with IMF Bx is seen. IMF Bx IMF By IMF Bz IMF magnitude

9. Solar wind dependence of TPA luminosity In the dark hemisphere, no dependence on velocity, density or pressure is found. In the sunlit hemisphere, positive and negative correlations with solar wind velocity and density are found. The TPA luminosity is strongly controlled by the magnetic solar wind energy flux, not by the kinetic energy flux. The 2 cases where no TPA occurred have lowest energy flux values. velocity density pressure kin. energy flux mag. energy flux

10. Time delay Solar wind and IMF correlations do not change considerably during many hours. Thus the statistical results are (nearly) independent on the chosen time shift. Reason: most TPAs appear in connection with CMEs or CME-like structures due requirement of 5 hours northward IMF A first correlation maximum is reached after 50-90 min. This is the time it takes until new solar wind conditions affect the luminosity of TPAs. IMF magnitude IMF Bz IMF By IMF Bx magn. energy flux velocity kin. energy flux pressure density

11. Solar wind inter-correlations The IMF magnitude is positively correlated with IMF Bz and sw velocity. It is negatively correlated with IMF Bx and sw density. The correlations are better in the summer than in the winter. Result: IMF magnitude and TPA luminosity show similar solar wind dependencies. IMF magnitude ….…………….. IMF magnitude IMF Bx IMF By IMF Bz velocity density pressure

12. Newell et al. (1996) Liou et al. (2001) sunlight darkness Statistical studies: 1. Seasonal effect DISCRETE ARCS (electron acceleration events) DISCRETE + DIFFUSE AURORA (auroral power) Newell et al. (1996): Discrete arcs are suppressed in sunlit conditions. Explanation: feedback instability causing electron acceleration in low conductivity regions. Liou et al. (2001): Nightside aurora is brighter during the winter, dayside aurora is brighter during summer. Explanation: Nightside aurora consists mainly of discrete arcs (feedback instability). Dayside aurora contains mainly diffuse aurora with a linear dependence on the ionospheric conductivity.

13. Statistical studies: 2. solar wind dependence Correlation between auroral luminosity and solar wind parameters Baker et al. (2003): The dependence of the average auroral luminosity on solar wind parameters is similar to the TPA luminosity dependence found in our study. The similarity with several statistical studies the average auroral luminosity [Liou et al. (1998), Shue et al. (2001, 2002) and Baker et al. (2003)] indicates that our TPA luminosity results may be valid in general. Baker et al. (2003)

14. IMF Bx versus dipole tilt effect Ostgaard et al. (2003) Our study shows: dipole tilt effect dominates IMF Bx effect on TPA luminosity. Conclusions: sunlight has a stronger influence on TPA luminosity than a favorable reconnection topology. IMF Bx <0 IMF Bx > 0 (However, Ostgaard et al. (2003) observed the absence of a TPA in the summer hemisphere but for an unfavorable IMF Bx direction, i.e., IMF Bx effect dominates over dipole tilt effect.) Negative IMF Bx and positive dipole tilt are favorable for northern hemisphere high-latitude reconnection Positive IMF Bx and negative dipole tilt are favorable for southern hemisphere high-latitude reconnection

15. Summary of observations • TPAs are in general much brighter in the summer than in the winter. • The TPA luminosity is mainly controlled by the solar wind energy flux in all seasons. Separating TPAs into dark and sunlit hemisphere cases, clear solar wind and IMF dependences are found: • In the sunlit hemisphere, the TPA luminosity depends strongly on IMF magnitude and northward Bz. A positive correlation with the solar wind velocity, a negative correlation with its density, and a weak negative correlation with IMF Bx is found. • In the dark hemisphere, only a weak dependence on the IMF magnitude is observed.

16. Conclusions SEASONAL DEPENDENCE • The ionosphere does not control the spatial evolution of a TPA, but influences its brightness due to a strong seasonal dependence of the ionospheric conductivity. • In the summer, solar wind and IMF have a much stronger influence on the TPA luminosity than in the winter. • For large angles of the Earth dipole tilt, conjugate TPAs are expected to show large luminosity differences. SOLAR WIND AND IMF DEPENDENCE • MHD simulations show: The spatial evolution of an IMF By sign change triggered TPA is enforced by large-scale topological changes in the magnetotail. • This study shows: The TPA luminosity is mainly controlled by the solar wind magnetic energy flux. • Conclusion: An IMF By sign change during northward IMF leads always to the evolution of a TPA, but the TPA can be identified only when the amount of energy input into the magnetosphere is high enough for aurora to become visible on global auroral imagers, alternatively for aurora to occur at all.