ALBERT

Description: For the period July 2003 to August 2010, the interplanetary coronal mass ejection (ICME) catalogue maintained by Richardson and Cane lists 106 Earth-directed events, which have been measured in-situ by plasma and field instruments on–board the ACE satellite. We present a statistical investigation of the Earth's thermospheric neutral density response by means of accelerometer measurements collected by the GRACE satellites, which are available for 104 ICMEs in the data set, and its relation to various geomagnetic indices and characteristic ICME parameters such as the impact speed ( v max ), southward magnetic field strength ( B z ). The majority of ICMEs causes a distinct density enhancement in the thermosphere, with up to a factor of eight compared to the pre–event level. We find high correlations between ICME B z and thermospheric density enhancements (≈ 0.9), while the correlation with the ICME impact speed is somewhat smaller ( ox 0.7). The geomagnetic indices revealing the highest correlations are Dst and SYM-H (≈ 0.9), the lowest correlations are obtained for k p and AE (≈ 0.7), which show a nonlinear relation with the thermospheric density enhancements. Separating the response for the shock sheath region and the magnetic structure of the ICME, we find that the Dst and SYM-H reveal a tighter relation to the B z minimum in the magnetic structure of the ICME, whereasthe polar cap indices show higher correlations with the B z minimum in the shock sheath region. Since the strength of the B z component—either in the sheath or the magnetic structure of the ICME—is highly correlated (≈ 0.9) with the neutral density enhancement, we discuss the possibility of satellite orbital decay estimates based on magnetic field measurements at L1, i.e. before the ICME hits the Earth magnetosphere. These results are expected to further stimulate progress in space weather understanding and applications regarding satellite operations.

Description: The importance of shocks in nova explosions has been highlighted by Fermi 's discovery of -ray-producing novae. Over three years of multiband Very Large Array radio observations of the 2010 nova V1723 Aql show that shocks between fast and slow flows within the ejecta led to the acceleration of particles and the production of synchrotron radiation. Soon after the start of the eruption, shocks in the ejecta produced an unexpected radio flare, resulting in a multipeaked radio light curve. The emission eventually became consistent with an expanding thermal remnant with mass 2 x 10 –4 M and temperature 10 4  K. However, during the first two months, the 10 6  K brightness temperature at low frequencies was too high to be due to thermal emission from the small amount of X-ray-producing shock-heated gas. Radio imaging showed structures with velocities of 400 km s –1 (d/6 kpc) in the plane of the sky, perpendicular to a more elongated 1500 km s –1 (d/6 kpc) flow. The morpho-kinematic structure of the ejecta from V1723 Aql appears similar to nova V959 Mon, where collisions between a slow torus and a faster flow collimated the fast flow and gave rise to -ray-producing shocks. Optical spectroscopy and X-ray observations of V1723 Aql during the radio flare are consistent with this picture. Our observations support the idea that shocks in novae occur when a fast flow collides with a slow collimating torus. Such shocks could be responsible for hard X-ray emission, -ray production, and double-peaked radio light curves from some classical novae.