Joint Astrophysics/Space Physics Seminar
"Probing the Plasma Structure of HII Regions with Faraday Rotation" by Ms. Allison Costa, Department of Physics & Astronomy, The University of Iowa
Abstract: We are involved in study concerning the modification of magnetic fields within the shells of HII regions. We report Faraday Rotation results of lines on sight through or near HII regions associated with OB associations. In the our studies of the Rosette Nebula (l = 206°, b = -1.2°), we measure positive rotation measure (RM) values in excess of +40 to +1200 rad m-2 due to the shell of the nebula and a background RM of +147 rad m-2 due to the general interstellar medium (Savage et al. 2013, ApJ, 765, 42; Costa et al. 2016, ApJ, 821, 92). We are currently completing an analysis of observations probing an addition HII region, IC 1805 (l = 135°, b = 0.9°), associated with the W4 Superbubble. We measure negative RM values across the IC 1805 region between -68 and -961 rad m-2. We find the highest RM values for lines of sight which intersect the ionized shell of the HII region for the Rosette Nebula, but in the case of IC 1805, the highest RM values are outside the bright shell of the HII region. However, we find that the magnitude of the RM between the two regions is similar. The sign of the RM across each HII region is consistent with the expected polarity of a Galactic magnetic field that follows the Perseus spiral arm in the clockwise direction, as suggested by Han et al. (2006, ApJ, 642, 868) and Van Eck et al. (2011, ApJ, 728, 14).
“Resonant Interactions Between 0.01 Hz Waves and Reflected Ions at the Moon” by Ms. Stephanie Howard, Department of Physics & Astronomy, The University of Iowa
Abstract: One generating mechanism for waves in the lunar plasma environment is through the process of resonant interactions with non-solar wind ions. A significant source of non-solar wind ions is incoming solar wind protons that are reflected by localized crustal magnetic fields on the lunar surface. This mechanism, in particular, is believed to be a main generator of waves that lie near the ion cyclotron frequency; around 0.01 Hz. Both left-handed and right-handed polarization in the spacecraft frame has been observed for waves in this frequency range. Due to the effects of Doppler shift from the solar wind, the intrinsic polarization of the waves is not necessarily the same as their polarization in the spacecraft frame. By using a combination of plasma wave analysis and trajectory analysis of reflected ions, we can examine the intrinsic properties of these waves. Using over five years of ARTEMIS data, we look at the observation rates and spatial distributions of 0.01 Hz waves around the Moon based on their polarization. We further investigate how IMF direction relates to these distributions and to the distributions of non-solar wind ions.