“Physics of Auroral Phenomena”, Proc. XXXII Annual Seminar, Apatity, pp. 93 - 96, 2009 Polar © Kola Science Centre, Russian Academy of Science, 2009 Geophysical Institute RELATIONSHIP BETWEEN SOLAR WIND PRESSURE PULSES, PROTON AURORA FLASHES, AND Pc1 BURSTS: A STATISTICAL STUDY T.A. Popova 1 , A.G. Yahnin 1 , T.A. Yahnina 1 , and H.U. Frey 2 1 Polar Geophysical Institute, Kola Science Centre, Russian Academy of Sciences, Apatity, Russia 2 Space Sciences Laboratory, University of California, Berkeley, California, USA Abstract. Simultaneous observations of proton aurora from the IMAGE spacecraft and ground magnetic pulsations during sudden solar wind pressure increases (∆P > 1 nPa) were used to investigate the relationship between dayside proton aurora flashes and bursts of pulsations in the Pc1 frequency range. The Pc1 bursts are always registered when the observing ground station is conjugated with the region occupied by the proton aurora flash. Outside the proton aurora flash region the Pc1 bursts are typically not observed. In addition, we noted a difference in responses of the proton aurora to the pressure pulses of different origin. About 100% of the pressure increases due to interplanetary shocks are associated with proton aurora flashes in events, while the pressure increases related to tangential discontinuities correlate with the proton flashes only in about 30% of the events. Introduction A lot of geophysical phenomena correlate with sharp increases of the solar wind dynamic pressure (e.g., sudden impulses, traveling convection vortices, precipitation of particles in the auroral oval, etc.). One of such phenomena is a short-lived burst of geomagnetic pulsations in the frequency range of Pc1 (from tenths to 1-2 Hz), called by Fukunishi et al. (1981), as “hydromagnetic emission bursts”. These pulsations are typically observed on the dayside (e.g., Fukunishi et al., 1981; Kangas et al., 1986) and are indicator of electromagnetic ion-cyclotron (EMIC) waves (Anderson and Hamilton, 1993; Anderson et al., 1996). Recent observations from the IMAGE spacecraft revealed a new phenomenon related to the solar wind pressure pulses, namely, flashes of the proton aurora, which occur on the dayside equatorward of the proton aurora oval (Zhang et al., 2002; Hubert et al., 2002; Fusilier et al., 2004). The morphology of the proton flashes (such as duration and MLT distribution) is similar to that of the Pc1 bursts. Yahnina et al. (2008) and Zhang et al. (2008) showed that the proton flashes exhibit close temporal-spatial correlation with the Pc1 bursts (EMIC waves). Such correlation suggests that both proton precipitation responsible for proton flashes and geomagnetic pulsations are the result of cyclotron instability of the ring current protons that develops due to increase of the proton temperature anisotropy under magnetospheric compression. However, both Yahnina et al. and Zhang et al. papers dealt with case studies. The aim of our report is to confirm the relationship between proton flashes and Pc1 bursts statistically. Data selection and results To select necessary data for the study, the solar wind data (from OMNII data base) for 2001-2005 were searched to reveal dynamic pressure jump events. Strong enough pressure jumps (not less than 1 nPa), which duration did not exceed a few minutes, were selected. Then, those events were chosen, for which the data from the SI12 detector of the FUV instrument onboard the IMAGE spacecraft were available. This detector was designed to obtain global images of aurora, which is created solely by proton precipitation (Mende et al., 2000). Finally, 61 events were selected. Further, the data of the induction coil magnetometer in observatory Lovozero (CGMLat=64.2, MLT=UT+3) were investigated to determine the response of pulsations in the frequency range of 0.05-5 Hz to the selected solar wind pressure events. To characterize the magnetospheric compression the geomagnetic index SYM- H (e.g., Iemory and Rao, 1996) was applied. The observed responses of the pulsations to the pressure jumps can be divided into following categories: 1) the Pc1 bursts, 2) the ULF noise (like PiC/PiB), 3) the quasi-monochromatic Pc1, and 4) no change in the pulsation regime. It is worth to note that the pulsations in the Pc1 frequency range often appeared together with the ULF noise, but not vice versa. Figure 1 shows an example of the pressure jump and some associated phenomena, which took place around 0625 UT on 4 November 2003. Two upper panels represent, respectively, the interplanetary magnetic field magnitude and solar wind dynamic pressure. Third and forth panels show ground-based data: the SYM-H index and spectrogram of geomagnetic pulsation in Lovozero. On the bottom, three successive global images of the proton aurora obtained from the IMAGE spacecraft at 0625, 0627, and 0629 UT are presented. (The morning sector as well as some part of the night sector is not observed due to the camera orientation.) An arrow in the central image indicates the location conjugated with Lovozero. The magnetospheric compression in this case is confirmed by a strong increase of the index SYM-H at 0625-0627 UT. The pressure jump was associated with the proton aurora 93